TOP ENGINEERING ESSENTIALS FOR MEDICAL DEVICE TECHNOLOGY

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1 LIBRARY FOCUS ON: TOP ENGINEERING ESSENTIALS FOR MEDICAL DEVICE TECHNOLOGY A compendium of popular medical device technology articles from the editors of Machine Design Sizing a Canted Coil Spring Coil width Spring Lead Coil height Wire diameter Groove height Copyright 2018 by Informa. All rights reserved.

2 FOCUS ON: TOP ENGINEERING ESSENTIALS FOR MEDICAL DEVICE TECHNOLOGY STEPHEN MRAZ Senior Editor Machine Design INTRODUCTION Biomedical engineers, more than any other type of engineers, needs to ensure the devices they design will do little or no damage to the doctors that install them or the patients on or in which they are used. In fact, medical devices must be proven to improve the lives of patients. For that reason, biomedical engineers also need to reduce the risks and hazards in their designs and follow strict FDA guidelines that help them do so. The first two articles address this issue and offer some advice on how to best follow the FDA rules. The second group of articles looks at a variety of technologies used in many biomedical devices. These include coatings and one- and two-part epoxies that protect devices from fluids and chemicals in the body while also preventing the circuits or stray voltages from affecting the patient. There are also a pair of articles addressing the electrical connectors used in implants and medical equipment to carry power and data or control signals. Seemingly simple cables must be designed to be safe, easy to use and clean, and reliable. Finally, the last three articles look at biomedical engineering approaches to three different medical problems: Macular degeneration and loss of sight, loss or degradation of hearing, and prosthetic hands that have a sense of touch. TABLE OF CONTENTS CHAPTER 1: HOW TO DESIGN SAFE MEDICAL PRODUCTS... 2 CHAPTER 2: CONTROLLING RISK IN MEDICAL DEVICES... 7 CHAPTER 3: THE NEXT GENERATION OF MEDICAL CONTACTS FOR MEDICAL IMPLANTS CHAPTER 4: DESIGNING CONNECTORS FOR PORTABLE MEDICAL EQUIPMENT CHAPTER 5: EPOXIES FOR MEDICAL DEVICE APPLICATIONS CHAPTER 6: PROTECTING MEDICAL DEVICES WITH PARYLENE CHAPTER 7: IMPLANTED TELESCOPE HELPS PATIENTS OVERCOME MACULAR DEGENERATION CHAPTER 8: TECHNOLOGY ADDS THE SENSE OF TOUCH TO PROSTHETIC HANDS CHAPTER 9: TECH ADVANCES UPGRADE HEARING AIDS MORE RESOURCES FROM MACHINE DESIGN REGISTER: machinedesign.com 1

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4 TOP ENGINEERING ESSENTIALS FOR MEDICAL DEVICE TECHNOLOGY CHAPTER 1: HOW TO DESIGN SAFE MEDICAL PRODUCTS It isn t enough to design products that meet industry standards. Engineers must use a formal method that identifies and mitigates risks. PETER HAVEL, Senior Vice President Global Head of Medical & Health Services, Director U. S. Medical Health Service Div., TUV SUD America Inc. Designers new to the medical field sometimes are surprised to discover the lengths to which they must go before their medical devices can be considered safe for use. Young engineers, for example, frequently don t expect that it takes five to ten times more effort to develop a device that is safe and complies with regulations than to develop a laboratory prototype. A device can only be considered safe after undergoing tests that prove its safety. So the safety discussion starts by devising the right tests that provide that proof. Safety engineering principles emphasize that there are three aspects of making designs safe that are particularly important. They apply to the hardware, the software, and the user interface. Medical hardware uses a functional safety approach where two independent failures are not allowed to harm the patient. There are rules for designing software so the chances of harm arising from bugs are acceptably low. Finally, user interface design should employ usability rules that make the man-machine interface as safe as possible. Engineers also are surprised to find that designing equipment to medical industry standards isn t enough to guarantee that it is safe. It is understandable why this is so when you examine how standards come about. Standards are set by committees of experts. The standard-setting process is a political event; some committee members want strong requirements, some want weaker ones. It generally takes a long time to agree on specifics, so many standards are outdated by the time they publish. All in all, standards can t hope to cover all risks. So designers must make up for the areas standards don t cover by conducting a comprehensive risk analysis. Risk management is actually a combination of several risk analysis methods that should let designers identify all relevant risks. In the medical field, ISO 14971:2007 specifies a process by which RISK MANAGEMENT IN A NUTSHELL Risk Management Decision database Risk evaluation Mitigating measures Risk control Acceptable risk A flowchart can express the principle of managing risks in medical products. Each risk gets evaluated both as a part of the development process and based on evidence from public data on device failures. Developers build in mitigating measures, then analyze the result to determine whether the result is an acceptable risk. REGISTER: machinedesign.com 2

5 CHAPTER 1: HOW TO DESIGN SAFE MEDICAL PRODUCTS ISO RISK MANAGEMENT Severity of damage TYPICAL SAFETY SYSTEM Equipment under control Patient Probability of occurrence Risk reduction Motor Analyze Set safe state Sensor Control Actor Safety system The ISO standard for product risk management spells out a method for categorizing risks according to their chance of occurrence and severity. The goal is to mitigate risks such that they all lay in the bottom of the matrix below the main diagonal. manufacturers can identify the hazards associated with medical devices, including in-vitro-diagnostic (IVD) medical devices, to estimate and evaluate the associated risks, to control these risks, and to monitor the effectiveness of the controls. The requirements of ISO 14971:2007 are applicable to all stages of the life cycle of a medical device. However, medical devices have safety requirements that are less stringent than those for certain other product categories. For example, there are more requirements for making an airplane safe than for a medical device. The reason is that a medical device usually can only kill one person at a time while a commercial aircraft that isn t safe might kill hundreds. Similarly, though medical devices must have designs that prevent two independent hardware failures from harming a patient, elevator designs must be safe in the event of three independent failures. All in all, the possibility of multiple lost lives brings with it stiffer requirements for safety. Once developers have identified the risks in their product design, their next step is to define safety measures to mitigate them. Risk and safety The task of developing medical devices that are safe boils down to identifying risks and then establishing the measures that give the confidence to say the risks are acceptable. Developers must judge the severity of potential harm and the probability that the harm occurs. Once developers have identified the unacceptable risks, their next step is to define safety measures to mitigate them. For example, consider the case of an infusion pump. Its main function is to pump fluid. Potential hazards related to the pumping function include a wrong flow rate, infusing the wrong volume, an unintended start or stop of infusion, a buildup of excessive pressure, an infusion of air, and a reverse in the direction of flow. Designers would consider all these factors during development so the device could cause no harm in the case of a breakdown. Fortunately there are standards that provide guidance on safety. The IEC pertains specifically to infusion pumps. Other standards pertain to other kinds of widely used medical equipment. An example is IEC , which pertains to dialysis equipment. But, as stated before, standards aren t enough to make devices safe. Designers must also conduct a formal risk analysis to determine requirements for the design of the device. In that regard, ISO is a standard that details how manufacturers should conduct risk management to determine the safety of a medical device during the product life cycle. Such activity is required by higher level regulations and other quality standards such as ISO The main standard for medical-device safety is IEC Medical electrical equipment Part 1: General requirements for basic safety and essential performance. Some countries also deviate from the standard under certain circumstances and sometimes use different versions of it. For example, the European and Canadian versions of the standard are identical to the IEC standard, but the U.S. version of the standard (ANSI/AAMI HA ) excludes nursing homes from coverage. It also emphasizes usability requirements. Devices typically mandated to use the new standard include oxygen concentrators, body-worn nerve and muscle stimulators, beds, sleep-apnea monitors, and associated battery chargers prescribed for use at home. One of the principles of IEC is that a medical device must be safe in the case of a single fault. It defines a single fault as a failure of a safety system. Thus, one facet of designing a safe device REGISTER: machinedesign.com 3

6 CHAPTER 1: HOW TO DESIGN SAFE MEDICAL PRODUCTS is to imagine how a first failure in a safety system could endanger the patient, and then implement a safety system that still makes the device safe even in the event of a first failure. One complicating factor is that the safety system has its own reliability level. Developers must establish what this level is. One approach to make safety systems reliable is to either use two redundant safety systems or use one system that is tested periodically to see if it is still functioning. The basic approach is to go through every component in the device and figure out what happens if it fails. Each possible failure is acceptable if it is obvious and an operator can stop operations before the device can harm someone. For example, assume a safety system fails but the device continues to function properly. Designers must anticipate what happens in the event of a second safety-system failure after a certain time. When the safety system fails silently it no longer protects; still, the patient must be safe. In the same vein, failures can be either systematic or random. Systematic failures are basically built-in design flaws. Examples include errors in the PCB layout, components used outside their specification, or unanticipated environmental conditions. All software bugs are systematic failures; there are no random software failures. The effect of software failures might be random. For example, when a programmer doesn t initialize a variable, its content on first use is random. This can cause a random effect at power on. The fact that the variable is not initialized is a systematic failure. Other examples of systematic failures in software include errors in the software specification, and errors in the operating system or compiler. Systematic errors in both hardware and software can be prevented through use of a robust development process. Random errors, on the other hand, happen even though the design is correct and production is flawless. Random hardware errors can t be predicted individually, they can only be described statistically. The general approach to controlling random errors is with PROTECTION ON THE SYSTEM LEVEL Position accuracy Self-test Sensor 1 Control 2 Patient C1 C2 C3 C4 F1 F2 F1 F2 P1 P2 P1 P2 IDENTIFYING THE SIL LEVEL W W W S I L L E V E L C RISK CONSEQUENCE C1: Minor injury C2: Serious permanent injury to one or more people or death of one person C3: Death to several people C4: Many people killed F EXPOSURE TIME IN HAZARDOUS ZONE F1: Rare to more often F2: Frequent to permanent P POSSIBILITY OF AVOIDING THE HAZARDOUS EVENT P1: Possible under certain conditions P2: Almost impossible W PROBABILITY OF THE UNWANTED OCCURRENCE W1: Very small W2: Small W3: Relatively high The IEC standard lays out a method of categorizing each fault condition in terms of a specific safety integrity level or SIL. redundant features or by adding self-testing or by adding a safety system that reacts in the event of a random failure. (Readers should note that a failure of a software storage medium is, in fact, a hardware and not a software error.) Designers typically use both redundancy and diversity as safety features. Redundancy is simply duplicating the same feature while diversity is the use of two different methods to deliver the same function. (The classic example is that of a seat belt and airbag protecting a car occupant from hitting the dashboard.) Diversity protects against random hardware errors as well as against some systematic failures. Redundancy, on the other hand, protects only against random hardware failures. One of the questions that designers must decide is how much protection they must build in against random hardware failures. The answer depends on such factors as whether a first failure is a hazard and whether designers should assume there is a possibility of a second, third, or even more numerous failures. The main logic here is that hazards potentially able to kill multiple people PROTECTION ON THE COMPONENT LEVEL Self-test Safety 1 Safety 2 It s possible to build in a high degree of safety into the architecture of a product. Designers typically treat each function as a black box, then determine whether or not a specific black box is safe or whether it can be made safe by the introduction of safety systems. Safety systems Patient When designers can t determine the safety of a system or subsystem, they must go deeper, perhaps down to the level of sensing and providing for problems with the operation of individual components. REGISTER: machinedesign.com 4

7 CHAPTER 1: HOW TO DESIGN SAFE MEDICAL PRODUCTS at one strike demand more attention. The potential for harm will give guidance on how many independent failures designers must consider in the lifetime of the device. The potential for harm also determines the amount of redundant/diverse safety systems necessary. Normally medical devices kill one person, so designers must consider a maximum of two independent failures. In general, designers must consider the possibility of ever-more unlikely events, the higher the risk of harm. For electrical medical devices, the IEC standard specifies that a combination of two independent failures should not be life threatening. This mandate expresses the concept of the single fault condition for medical devices. The principle is that a first failure should not cause a hazard. If the first failure is obvious to the operator, the operator stops using it and has it repaired. If the first failure can t be detected, the designers must assume that a second failure will arise sometime later. They must also arrange the design so a combination of the first and second failures won t cause a hazard. Unfortunately the term single-fault condition can be misleading in the context of medical safety standards. It can suggest that designers need only assume that the device experiences only one failure. This is not correct. Usually there is a time period after the first failure where the combination of a first and second failure is not allowed to be a hazard. For example, suppose the safety system has a first failure and undergoes a self-check within 24 hr that reveals the safety system is dead. That level of safety is acceptable in many medical systems. The assumption is that two independent safety-system failures would not arise within 24 hr of each other. Conversely, there is an unacceptable level of hazard if there is no self-check in the 24 hr after the first failure. In this case, the device either needs a self-check routine or a second safety system. There is a progressive procedure for analyzing each hardware failure that could be dangerous. It is a risk graph spelled out in the IEC standard. It categorizes each fault condition in terms of a specific safety integrity level or SIL. Designers usually start by dividing risk consequences into four categories ranging from minor injury, serious injury, several deaths, and many deaths. They further subdivide risks according to the amount of exposure time to the hazard and the possibility of avoiding the hazardous event. Finally, they categorize the probability of the unwanted occurrence as very small, small, or relatively high. Designers often start the safety analysis with a functional diagram of the product. This is an appropriate starting point because it s possible to build in a high degree of safety on the level of system architecture. Designers typically treat each function as a black box. They then try to determine whether or not a specific black box is safe, or if it can be made safe by the introduction of safety systems. When they can t determine the safety of a black box, they then open the box and go deeper, perhaps down to the level of individual components. Fortunately designers in the U.S. need not rely on their own analysis of product functions to note safety red flags. The U.S. FDA maintains a market surveillance system that can give designers a heads-up on potential problem areas in medical devices. Any time a medical device has a failure, the manufacturer must report the details to the FDA. Alarms are raised if a specific device has a failure rate exceeding a certain threshold. Thus, medical-device engineers can consult this database to see what kinds of failures similar devices are experiencing. Other countries have similar databases of medical device failures. However, their data tends to be less useful than that in the U.S. simply because individual countries each collect their own information. There is no central repository as yet for tabulating System RISK-BASED V-MODEL Decomposition Decomposition DECOMPOSITION EXPLAINED Software requirement Subsystem design SPECIFICATION VERIFICATION A Software Create a box Create a cardboard box Create a cardboard box with dimensions x, y, z and thickness d The opening should be Is it a box (yes/no) Is it made out of cardboard? (yes/no) Check the dimensions x, y, z, d Check opening dimensions The principle used for determining whether software functions properly and safely is that of decomposition: Each software function is defined precisely enough to make possible some kind of check of its properties that will reveal whether or not the software has done what has been designed to do. The decomposition process is often illustrated with the example of verifying the construction of a cardboard box. B C Software-safety analysis frequently employs a so-called V-model that is analogous to that widely used for visualizing the progression of system-development tasks. The model is named for its graphic depiction of how designers should decompose software requirements into ever-more-detailed specifications, then test and validate from the detailed levels up through to the system level. REGISTER: machinedesign.com 5

8 CHAPTER 1: HOW TO DESIGN SAFE MEDICAL PRODUCTS worldwide results. The safety of software Software for medical devices has its own standard, IEC It specifies life-cycle requirements for the development of medical software and software within medical devices. The standard spells out a risk-based decision model and defines testing requirements. The primary principle for verifying software is to describe the function it is supposed to perform, then devise a test that verifies the software works as planned. The key lies in devising a test that is specific enough to identify all functions. Unfortunately, the U.S. medical-device industry is not as advanced as it should be when it comes to implementing such procedures. In many cases, software descriptions tend to be ambiguous, and this condition causes several harmful side effects. For example, software engineers may develop something that has unintended functions. Equally bad, poor descriptions often prevent designers from devising all the tests that will expose harmful software bugs. Additionally, many software engineers seem to have a too-high opinion of their own work. They seem to forget that numerous studies have shown software developers typically create between five and 10 bugs daily, a statistic that illustrates why an accurate description of software functions is essential. Software safety analysis frequently employs a V-model named for its graphic depiction of how designers should decompose software requirements into ever-more-detailed specifications, then test and validate from the detailed levels up through to the system level. The software risk model defines A, B, and C levels of safety. Level A software is harmless if it fails. Level C software that fails can injure or kill someone. If software is neither A nor C, then it is level B by default. Most V-models divide decomposition steps into three levels. Top level is termed the system level. The lowest level represents the smallest software unit that can be 100% tested. The V-model represents the principle that designers must clearly describe software tasks in several levels of detail. There must be tests at every level that check that the software delivers its intended functions and identifies any bugs. The software risk model defines three different levels of safety. Level A software is nonharmful if it fails. Level C software that fails can injure or kill someone. If software is neither A nor C, then it is level B by default. Categorizing software into one of the three levels helps determine how much testing is appropriate. Level A software just needs a system test. Level B tests must be detailed enough to check individual software modules. And as you might expect, most safety-system software in medical devices is at level C, which tests subsets of software code at the unit level. The human element It is no secret that appliances and instrumentation of all kinds are getting more complicated to operate. And medical instrumentation that is complicated is also prone to operator errors with potentially tragic consequences. Complicated instrumentation puts a demand on the operator s intellectual ability. But users aren t getting cleverer. Unfortunately, standards for human-factor usability aren t well developed. For example, one such document is IEC Annex D 1.4. It is weak in that it only supplies general guidelines about the steps necessary to identify risks of usage problems. It basically requires designers to analyze how users will interact with the device and implement risk-mitigating measures to avoid erroneous usage. It can be quite expensive for manufacturers to analyze the ergonomics and usage of their equipment. One widely used technique is to round up 10 users, then ask them to say out loud what they are thinking as they operate the device. The whole set of interactions get recorded on video. A typical finding in sessions like this is that only about 10% of instrument functions get used daily. The other 90% get used rarely or not at all. But it is not unusual to find often-used features buried in complicated menu structures with a huge potential for accidentally making an error. However, there is no standard that requires an ergonomic analysis or user studies. These practices are simply recommended practices among firms that have experience developing medical equipment. BACK TO TABLE OF CONTENTS REGISTER: machinedesign.com 6

9 TOP ENGINEERING ESSENTIALS FOR MEDICAL DEVICE TECHNOLOGY CHAPTER 2: CONTROLLING RISK IN MEDICAL DEVICES Applying everyday principles to product development can reduce hazards and increase patient safety. P.J. TANZILLO, Embedded Software Product Manager & Biomedical Segment Lead National Instruments, Austin, Tex Engineers developing medical equipment may not have much control over the hazards inherent in devices they are designing. X-ray imagers and anesthesiology equipment will always pose some risks to patient safety. But if a design fails, consequences can be catastrophic. So one of every medical designers tasks is to reduce the risks and probabilities of failure. And almost every design decision can affect these two factors. Here are some strategies that may help reduce risk and failures in medical equipment. (And the same concepts can be applied to all forms of engineering.) Gathering requirements According to some estimates, nearly half of all project costs stem from rework to correct inadequate features or add ones that were left out of the design process. Missing and inadequate requirements also account for an estimated 75% or more of the software bugs in medical equipment. So it is important the design team start with a list of valid, must-have requirements. Interviews are the most common way design teams gather requirements. Interviews let all stakeholders quickly provide the various bits of information designers use to construct a list of requirements. For example, doctors can explain traditional methods and the range of measurements or outputs expected from the equipment. Patients can provide feedback on the comfort and convenience of using the device. And health-care managers should be able to address costs and market scope. Although interviews are an important first step, they can be useless if pertinent questions aren t put to the right people. Another problem with interviews is that those being interviewed may not know what they want until after seeing and understanding a set of options. This is where prototypes can play a helpful role. Focus groups are useful because they let people discuss opinions with their peers, and the group is usually reacting to a relatively fleshed-out concept or set of options. However, as with interviews, focus groups may not capture all the requirements needed for a successful device. For example, focus-group members typically won t give negative feedback if they believe the moderator is involved with the design. They don t want to hurt anyone s feelings. Software modeling lets design teams simulate devices so end users can evaluate the controls and outputs. This lets users more fully understand what the designers have in mind so they can give more informed feedback as to what is wrong, what works well, and what might need to be changed before taking on the expense of building a physical prototype. Teams often then take the next step and develop a physical mock-up of the controls and display panels so users have a more realistic experience with the latest design. Functional prototyping, the next logical step after software modeling, involves a working model built using off-the-shelf development tools. It lets users operate the device in its normal environment. Prototypes should use as few custom parts as possible to reduce development costs and time. After all, because the design team is still looking for user feedback and gathering requirements, the design will likely change. There s no need to spend much time REGISTER: machinedesign.com 7

10 CHAPTER 2: CONTROLLING RISK IN MEDICAL DEVICES refining features that may need significant rework. Design process Design inputs and outputs should not only be clearly defined in requirements documents. These documents should also be mapped to source code. This ensures that all requirements are covered by code, and that all code is mapped to requirements. Often, requirements that have not been implemented are simple to detect. It can be more difficult to find implementations not covered by requirements documentation. Such gaps in requirements may lead to incorrect assumptions and miscommunications between engineering groups. Well-mapped requirements also ensure traceability so that when a requirement changes, there is a clear mapping to affected source code. Why FPGAs are easier to validate than microprocessors When developing an embedded medical device, validating and verifying it can take longer than the time it took to develop the firmware. And even after testing each component, a completed microprocessor-based device needs to be put through extensive testing to demonstrate the safety of the device as a whole. This is necessary because seemingly independent subsystems in software can conflict and cause catastrophic failures through common bugs like resource contention. That s because a processor can execute only one instruction at a time, so resources such as memory, peripherals, and registers must be shared by several processes to handle any type of multitasking between parallel processes. On FPGAs, however, independent subsystems are truly independent because true parallelism is possible on FPGAs. Each tick of the clock can result in latching many parallel registers and executing many paths of combinatorial logic. Therefore, tested FPGA code is traditionally deemed more reliable than tested processor code. OptiMedica Corp., Santa Clara, Calif., discovered this when it developed an FPGA-based photocoagulator. Management found that FPGA chips provide the reliability of hardware and does not require the same level of code reviews as processor-based devices when obtaining FDA approval. Design reviews are extremely important at all phases of development. Requirements, architecture, and specifications should go through a formal review process, but source-code reviews by peers are also essential to produce high-quality code. Best practices include having developers walk through the code for an audience of their peers. For lower-risk items, tools for static-code analysis can also be used for automated code review. The importance of a smooth design transfer is sometimes overlooked, and miscommunications in this process can be one of the sources of software bugs. Most design teams consist of domain experts responsible for algorithm and concept development as well Courtesy of Thinkstock REGISTER: machinedesign.com 8

11 CHAPTER 2: CONTROLLING RISK IN MEDICAL DEVICES as implementation engineers responsible for converting the design into a form that can be commercialized. The transition from one team to another is typically done with specification documents, but source code may also be transferred if a prototype has been developed, By using high-level design tools like state charts and other graphical representations of code, design teams can deliver executable specifications used to derive final implementations. Design changes should be tracked, justified, and validated against the entire system. To ensure small code changes do not have large and unintended effects, design teams should have an automated test suite in place that runs as an acceptance test against any code changes. In addition, regardless of the size of your design team, you should set up source-code control systems to track history and changes. System architecture Another way to reduce risk in medical devices is by choosing system architectures with various layers of redundant protection for the most-hazardous elements. For example, designers can choose whether control elements will be carried out by software or hardware. Dedicated hardware is considered more reliable but also more difficult to design for complex tasks. Software can be easier to put in place and update. And software is well suited for features such as networking and data storage. But software bugs can be difficult to identify and correct. When designing complex digital or mixed-signal hardware, application-specific integrated circuits (ASIC) are commonly chosen for mass-produced devices. They provide the reliability of hardware circuits without the complexities of manufacturing and assembly. However, fabricating ASICs can be prohibitively expensive, so unless mass production is a certainty, use field-programmable gate arrays (FPGA) instead. FPGAs have the reliability of ASICs and are almost as easily changed as software. And although unit costs are higher compared to ASICs, overall production costs are lower for most designs. In addition, FPGAs can be repeatedly reprogrammed, making them a good choice for designs with requirements likely to change. When it comes to executing software, complex code is more likely to contain bugs than simple code. This often makes 8-bit microcontrollers the more reliable choice. These controllers are usually programmed in C or assembly and almost never run operating systems. Instead, they carry out simple tasks such as updating a display or monitoring buttons. Though they re useful and relatively easy to program, the scope of what 8-bit chips can do is limited by their relatively small memory. More-complex systems often call for cooperative multitasking, communications drivers, and other high-end features. This means they need more-powerful processors with more memory. Most often, these systems use 32-bit processors with real-time operating systems (RTOS) containing drivers and middleware like TCP/IP stacks and file systems. But with these features comes more complexity and additional risk of failure. Most designers add watchdog timers and other failure mitigation techniques to detect system failures and then recover gracefully. The most-complex systems, those that need computationally intense algorithms or extremely rich user interfaces, require desktop-computing capabilities. Although desktop-computing failures like or browser crashes can be common and inconvenient, users need only reboot the computer and continue to work in most cases. But this is not nearly the level of reliability designers need for critical medical devices. Therefore, if a desktop PC is needed, designers should add hardware that will monitor and correct for failures to minimize patient risk. For example, a touch-panel display running a desktop-operating system can connect via Ethernet to a 32-bit processor running an RTOS. The RTOS checks for failure and adds reliability. Including an FPGA in this signal path would further improve reliability. The FPGA could monitor signals to ensure nothing went outside of the safe and acceptable operating range. With this third layer of protection, simply powering the device ensures outputs remained within ranges specified in hardware. Verification and validation Although designers need to test all aspects of their code, they should focus their most rigorous software testing on high-risk areas. High-risk code can be identified several ways. Code complexity analysis, for example, can help determine which code is statistically most likely to fail. When coupled with code coverage tools, it ensures testing all paths of the most complex code. In addition, coding situations identified as high risk for failure should undergo the most-rigorous testing. Some high-risk areas, for example, concern user interfaces (keys pressed too quickly), kernel-driver data transfers (buffer over and under flows), data conversions (pointer casts and loss of precision), and multithreaded portions of code. Good designers reuse parts of the design process in validating and verifying code. The simplest way to do this is to construct the test based on the requirements documents rather than the code. In fact, it is best if someone outside the design team puts together the tests. Models or prototypes developed during the design process can be used as comparison for acceptance tests. Furthermore, any models used to design algorithms can be used in a hardwarein-the-loop (HIL) setup to serve as a verification tool for device acceptance. BACK TO TABLE OF CONTENTS REGISTER: machinedesign.com 9

12 TOP ENGINEERING ESSENTIALS FOR MEDICAL DEVICE TECHNOLOGY CHAPTER 3: THE NEXT GENERATION OF ELECTRICAL CONTACTS FOR MEDICAL IMPLANTS MARK RUSSELL, Global Market Manager for Medical Electronics, Bal Seal Engineering Inc. Medical devices that alter brain, spine, or nerve activity for therapeutic benefit are a fast-growing segment of the healthcare industry, notes a recent report from Life Science Alley, a Minnesota-based medical device trade association. The 2015 report, Sector Landscapes: Neuromodulation, projects that the strong global market for neuromodulation devices will double by 2018, reaching $6 billion to $7 billion. This article examines the electrical contact technologies that are driving this market toward the next generation of device development. The global neurostimulation/neuromodulation market is surging as an aging population faces diseases such as Alzheimer s, epilepsy, spinal-cord injury, and Parkinson s disease. The therapies for these diseases are dominated by implantable neurostimulation devices, which make up the largest part of this market with a 96% share of the total. According the Life Science Alley report, neuromodulation devices are registered in more than 1,000 ongoing FDA-regulated clinical trials worldwide in which more than 1,300 indications, including metabolic disorders, inflammation, migraine, and psychiatric disorders are being investigated. The report also notes that one of the key developments within this growing market is the emergence of closed-loop systems that are capable of sensing ongoing brain or nerve activity and incorporating it into stimulation parameters for optimized therapeutic efficacy in real-time. This closed-loop functionality in active implantables requires optimal power efficiency and signal isolation, Here is a cutaway view of a canted coil-spring assembly that serves as an electrical contact. The male connector (shown) goes through the middle and makes contact with many of the spring s coils. and much of this is dependent upon the performance of electrical contacts and contact systems used to connect leads to batteries and electronics in these devices. Highly conductive and space-efficient components, such as the Bal Conn electrical contact and the Sygnus implantable contact system, are designed to support the signal-isolation requirements of closed-loop, high-connector-count arrays. With the use REGISTER: machinedesign.com 10

13 CHAPTER 3: NEXT GENERATION OF ELECTRICAL CONTACTS FOR MEDICAL IMPLANTS of closed-loop devices becoming more widespread, the ability to support smaller footprint connections for active implantables in a proven and reliable design will be increasingly critical. Unpacking the Technology For over two decades, the Bal Conn electrical contact has been helping manufacturers improve implantable medical-device performance and push the technology envelope. OEMs developing each generation of active implantables have relied on electrical connections that use canted coil springs as contacts. It s estimated that canted coil spring based contacts have been used in more than a million pacemakers, defibrillators, neurostimulators, and other active implantables that deliver life-improving therapies to patients worldwide. These electrical contacts have also dramatically simplified the process surgeons use to connect leads to implantables across the therapy spectrum, including cardiac healthcare, pain management, and sensing therapies, ultimately shortening procedure times. Developed in cooperation with a medical electronics manufacturer seeking to reduce package size while increasing reliability, the Bal Conn electrical contact is an electrically conductive component consisting of a precision-engineered canted coil spring retained in a metal housing. The contact, which is molded into a device header, facilitates a uniform, consistent electrical connection between the lead and the battery in active implantable devices. Coils can be made in a variety of sizes, such as these from Bal Seal Engineering Inc. Sizing a Canted Coil Spring Spring Lead Groove height Coil height Coil width Wire diameter Inside diameter The Bal Conn s individual spring coils provide multipoint conductivity, adjusting individually to maintain maximum contact with electrodes on the lead that is inserted into the device header. The contact s compact size allows for greater connector density where space is limited, and its unique design eliminates the need for tools during the connection process. Due to its redundant contact points, the Bal Conn offers low contact resistance, and its canted coils provide excellent resistance to fatigue. With its ability to offer low insertion force and exceptional electrical conductivity, it is ideal for use in devices with high connection counts such as neurostimulators. In a typical neuromodulation device, the lead diameter ranges from 0.9 to 1.4 mm. The Bal Conn is sized so that its spring coils exert a measured, consistent force, also called the breakout force, on the lead when it is first inserted. The force needed to slide the lead further in, the running force, is as predictable and reliable as the breakout force, and engineers can adjust these forces through design. These forces are low enough to make it easy for surgeons to insert and remove the lead. Applications requiring light forces have a theoretical breakout force as low as 0.5 N and a running force as low as 0.2 N, depending on design parameters. For those applications requiring heavy forces for latching, the theoretical breakout forces are as high as 15 N and the running forces are as high as 4 N with a single spring. The spring material for neuromodulation applications is typically platinum iridium. Platinum iridium is selected for its biocompatibility, durability, electrical conductivity, and radiopacity. The housing for neuro applications can be made from a variety of materials, including MP35N, 316L stainless steel, medical-grade titanium, or platinum iridium. Within the contact, both the coils of the spring and the spring itself have precise diameters, but there is no standard size for neuromodulation devices. Each Bal Conn is custom designed to fit the particular unit s lead interface, and the lead is held in place via frictional fit. A set screw or locking detent, located at either end of the channel into which the lead slides, anchors the lead in place. Once the electrodes are aligned with each Bal Conn, the device becomes operational. The spring electrically connects the implantable pulse generator to the lead through all of its individual coils. REGISTER: machinedesign.com 11

14 CHAPTER 3: NEXT GENERATION OF ELECTRICAL CONTACTS FOR MEDICAL IMPLANTS Applications Neuro (bi-directional) IS-1 (bi-directional) IS-4/DF-4 (uni-directional) IS-4/DF-4 (bi-directional) Lead diameter (mm) 1.35 Tech SpecS for coiled Spring connectors Force category Max. breakout force (N) Running force range (N) Medium to 0.4 Light Using the spring as an electrical contact provides redundant paths for power and information signals, and the redundant contact points provide for low contact resistance. Their canted coil design helps ensure that they resist compression set and fatigue, providing consistent performance over a long operational life, one that typically exceeds that of the implantable device itself. The springs compensate for imperfections on the surface of leads, accommodating a certain amount of misalignment. Engineers at Bal Seal Engineering work with medical OEMs to ensure that they have the right design for the application. With its long history of designing for active implantables, the company has developed both uni-directional and bi-directional contacts, which enables OEM integrators to choose which design works best for their particular requirements. With uni-directional versions, the lead must be inserted from one specific side of the contacts. With bi-directional contacts, the lead can be inserted from either side. For neuro devices, the contacts are always bi-directional. Both Housing material Spring material Static dry contact resistance (mω) MP35N Platinum/iridium 70±20 316L Platinum/iridium 600±200 Medical-grade titanium Platinum/iridium 350±150 MP35N Platinum/iridium 80±30 Platinum/iridium Platinum/iridium 40± Medium to L MP35N 100± Heavy to 1.0 MP35N MP35N 80± Medium to 0.75 MP35N platinum/ iridium Platinum/iridium 40±20 VAD 3.2 Medium to 0.75 Platinum/iridium Platinum/iridium 30±20 types are available for other active implantables; however, once integrated into the IPG header, the distinction is no longer significant and has no impact on the surgeon or device functionality. Simplifying Design for OEMs The Sygnus implantable contact system, which combines electrical contacts and isolation seals in a standardized, platform-ready stack configuration, is designed to improve speed to market and eliminate the need for procurement and testing of individual components. It lets OEMs concentrate on the design and function of the implantable rather than worrying about the intricacies and technology behind the connectors. It also creates greater connector density with pitches, or distances between contacts. Implantable-grade silicone isolation seals keep the contacts electrically separated, as well as separating the different segments of the lead. The seals help prevent signal leakage that can disrupt the active implantable s function. The result is a densely packed connector stack that accommodates leads with diameters down to 0.7 mm. A 0.64 mm version is currently under development. Sygnus can be modified to suit device-manufacturer requirements for size and number of connections, as long as they are within design limits. The seal design and dielectric materials were tested in accordance to industry standards by submerging them in saline, a good substitute for the conditions inside the human body. The goal of the testing is to measure impedance between connectors in a simu- Neuro applications are usually neurostimulators; the IS-1 and IS-4 connectors are for cardiac pacemakers. IS-4 I is replacing IS-1 as the standard. VAD stands for ventricular assist device, a pump inserted into the heart to keep cardiac patents alive until a donor heart becomes available, though it is often left in patients long-term. This cutaway view of a Sygnus connector shows a lead segmented into three sections inserted to mate with three canted coil spring contacts. The table lists the functional and physical attributes of a Sygnus multi-contact connector used for neurostimulation. The connector lets OEMs save space while increasing the functionality. REGISTER: machinedesign.com 12

15 CHAPTER 3: NEXT GENERATION OF ELECTRICAL CONTACTS FOR MEDICAL IMPLANTS lated body condition. The seals are cleaned to 10K cleanroom standards and packaged in antistatic bags to prevent contamination. Evergreen Medical Technologies has used Sygnus technology in its device development, including in its Encompass Lead-Interconnect System. In the Encompass, the Sygnus system is combined with a pre-molded 16-channel header. By combining the header, connector, sealing components, and lead into a single, reliable system, the system lets companies developing neurostimulators shorten the time needed to design, develop, source, and test the final device. Although it was originally designed for implanted neurostimulator devices, the system could find use in other implantables as well. Preparing for the Next Generation Future developments are being driven by the need for higher connection counts, more compact footprints, closed-loop configurations, and high levels of manufacturability. One reason for such improvements, especially the move toward smaller lead interfaces, is to give OEMs the platform on which to build cutting-edge treatments for hearing and vision loss. One of the engineering challenges they must overcome is that as the connectors get smaller, tolerances become more crucial and manufacturability becomes more complex. With the use of closed-loop devices becoming more widespread, the ability to support smaller-footprint connections for active implantables in a proven and reliable design will become increasingly critical. To address this, Bal Seal Engineering has developed a new sub-assembly concept to help engineers dramatically improve device functionality while reducing overall package size. The concept, which can incorporate a cap or cover design, uses vertically positioned pins or rods of varying diameters to double or triple the amount of connections available to the device lead. It can be engineered to minimize insertion force issues that present challenges in serial arrays with small leads, and it offers designers new opportunities for improved contact density. Essentially a build up, not out approach that challenges the established linear contact/header layout, the high-density vertical array was conceived to help OEMs pack more functionality into active implantables and shrink the size of the electrical connectors in medical devices. The array combines one or more canted coil-spring contacts, a non-conductive polymer interface, and implant-grade silicone. Some of the uses envisioned for the vertical array are in implantables where small size is critical, such as cochlear implants and deep-brain simulators. Life Science Alley s report points out that the complex nature of neurodegenerative disorders has set the stage for numerous potential physiological targets, low competition in treatment-resistant diseases, and significant drive for innovative treatments and novel technology. As active implantables in neurotechnology continue to advance and prove to be the gold standard for therapy, there will be increased adoption of these life-changing devices. And, as advances such as closed-loop configurations lead to the need for higher connection counts and more compact footprints, design engineers will be faced with the challenge of making the required number of connections in less space with proven underlying technologies. BACK TO TABLE OF CONTENTS The Encompass implantable multi-contact connector accepts two leads, each with eight separate contacts or signal paths. The bottom one is rendered so that you can see the inner workings. REGISTER: machinedesign.com 13

16 TOP ENGINEERING ESSENTIALS FOR MEDICAL DEVICE TECHNOLOGY CHAPTER 4: DESIGNING CONNECTORS FOR PORTABLE MEDICAL EQUIPMENT As medical equipment becomes more portable and is used in small clinics and homes, connectors must be safe, simple, and reliable. CARL BUNKE, Project Engineer, Advanced Technology ITT Interconnect Solutions, White Plains, N.Y. Engineers designing connectors for portable medical equipment must take several factors into consideration, including size and insertion force, shielding from interference, and preventing electrical shocks. Here s a closer look at mechanical, electrical, and safety requirements, as well as customer concerns. Mechanical considerations Connectors must often be small but have high pin counts. Customers are also demanding more mating cycles (connect-disconnect) along with consistent and reliable connections. In many cased, customers also want zero-insertion-force (ZIF) designs. To meet demands for smaller connectors with more pins, designers are working to cut pin spacing to less than 1 mm. This can shrink the size of the connector by more than 60% compared to older connectors with the same number of pins. As a result, some connectors boast 260 contacts, each with a 0.8-mm contact pitch, in PC-board mountings that measure just mm. Having more pins lets engineers choose from a variety of grounding schemes to maintain signal integrity, and this makes these connectors well suited for portable ultrasound machines, patient monitors, and test equipment. Engineers have devised several approaches to extending connector life, including new materials and entirely new designs. For example, modern connectors with rugged nickel-plated aluminum housings can have minimum rated lives of 20,000 mating cycles with no performance loss. Such connectors can also be mated and unmated in less than 2 sec and cost 25% less per mated line than high-density rack-and-panel versions. Designers have also come up with a quick-disconnect breakaway connector that includes a simple push/pull mating mechanism rated at more than 5,000 cycles. And the coupling mechanism s canted spring cuts the time it takes to hook up medical gear. Breakaway features that remove the danger of tearing connectors off equipment or out of walls are another recent innovation. Breakaway connectors are often used in portable medical imaging and diagnostic equipment because they are rugged and reliable enough to withstand field use. Some breakaway connectors feature a spring probe pin and pad contacts for durability and to withstand harsh environments. The probe pin in the plug connector works across multiple sizes. An internal clip ensures individual pins and sockets remain electrically connected and accommodate misalignments. The spring probe lets the connector receptacle house individual touch-pad areas, providing reliable electrical contacts. Further, the spring probe and touch pads make connectors easy to clean in the field. The individual touch pads, for instance, contain no crevices that let contaminants accumulate. ZIF connectors, besides being easy to engage and disengage, also rate high in terms of mating cycles, durability, and minimizing cross-talk. This lets them serve well in patient monitors and portable imaging equipment like ultrasound devices. ZIF connectors often use landed contacts, which eliminate engagement forces and reduce wear on the contacts to the short time they are pressed together and lightly wiped past each other during cam-and-lock operations. As a result, contacts in the plug and receptacle do not REGISTER: machinedesign.com 14

17 CHAPTER 4: DESIGNING CONNECTORS FOR PORTABLE ELECTRICAL EQUIPMENT touch each other while connector halves are being engaged. Not only do these connectors have minimum rated lives of 10,000 mating cycles, they can be mated in less than 2 sec. Electrical considerations Once engineers determine a connector s mechanical characteristics, electrical issues come into play, including contact resistance and shielding requirements. Contact resistance impedes current flowing through the connector. One way to decrease this resistance is by choosing the right material. For example, gold plating over contacts made of high-conductivity copper alloys lowers resistance. If strength is a concern, consider using beryllium copper as the base material. Beryllium copper also has low stress relaxation which boosts the number of mating cycles the connectors will withstand. Spring-probe-and pin-pad designs mentioned earlier also reduce electrical resistance, thanks to the internal clip that always provides a highly conductive path. Tools for getting the perfect connector Design-failure-mode-effects analysis (DFMEA) and process-failure-mode-effects analysis (PFMEA) play significant roles in meeting the mechanical and electrical design challenges of building the right connector for a specific application. DFMEA explores ways products might fail during real-world use, while PFMEA investigates whether manufacturing process will be able to handle a given design. Another tool, 3D modeling, often via stereo-lithography (SLA) or selective-layered sintering (SLS), is also crucial to successful medical-connector designs. It has become the preferred way to make connector prototypes. Manufacturers can also drop a 3D model of a connector into a model of the customer s equipment to verify that it will meet design specifications. Shielding against EMI and RFI signals, another consideration, is critical for devices such as pacemakers and patient monitors. Signal noise can affect a pacemaker s operation and corrupt data in monitors. In these applications, it is also vital that connectors use nonmagnetic materials because magnetic emissions degrade image clarity and increase signal noise. As a result, connector manufacturers rely on stainless steel, alloys, and brasses, as they offer non or low-magnetic fields, thus keeping EMI/RFI from interfering with equipment. Shielding effectiveness lets some connector manufacturers offer EMI performance greater than 85 db at frequencies from 40 MHz to 10 GHz. Another method of minimizing effects of EMI/RFI is to overmold the connector cable. This is often accomplished by attaching a stainless-steel shield over the shell (the shielding lies between the wires and connector jacket), and then premolding or overmolding the end of the cable to the connector. So when there is EMI/RFI, it is absorbed by the overmolded cable, thus minimizing insertion loss and any electrical variations. The overmold also adds tensile strength to the cable. Some connectors use springs and shield-locking mechanisms to ensure pressure around the perimeter of the mated connector is uniform and creates an EMI/RFI shield. By eliminating EMI/RFI disruptions, signal noise can t affect pacemakers, nor can it corrupt data or images traveling from or stored in patient monitors and diagnostic equipment. And shield cans placed on PC boards protect circuits from signal interference. Filter connectors also play a critical role in managing and controlling EMI and RFI. Some connectors have standard filtering features, including individual isolated-pin filtering for high-frequency noise, built-in ground plane barriers in connector inserts, and shield cans on PC boards to protect circuits from signal interference. But the filter-design approach is more effective. It lets engineers define and change individual circuit capacitance, ground, and electromagnetic-pulse (EMP) performance during development. To ensure medical devices work, especially in critical applications, engineers must design interconnects that are reliable and maintain signal fidelity. This can be done by using breakaway connectors, EMI-shielding, and grounding-electronics cables. Such designs allow for shell-to-shell grounding at less than 10 mω, as well as EMI performance of greater than 85dB at frequencies from 40 MHz to 10 GHz. Performance is further enhanced by termination processes which allow for 360 shield/connector coverage. Complex EMI/RFI electronic issues have driven connector manufacturers to develop higher-performance and more-cost-effective EMI-suppression methods, including spring-probe contacts a chip-on-flex (CoF) filter. CoF filters, using a flex circuit with chip capacitors, are surface-mounted to a pad on the feed-thru contact. This replaces traditional planar-array block capacitors and while provides reliable filtering. In addition, the filters perform well despite thermal shocks and vibrations. Safety considerations From a safety aspect, portable medical devices need finger protection and touchproof connectors. IP2X, a finger-protection standard, requires that a connector s live or electrified parts cannot be touched by a human finger. Because portable medical equipment may need to be repaired in the field, touchproof connectors prevent health-care professionals and patients from getting shocked when they touch a connector. Touchproof construction often involves placing a plastic plunger over male pins, letting only the female contacts touch the male pin. Making pins on the active side of the connector touchproof using any means eliminates the risk of shocks. BACK TO TABLE OF CONTENTS REGISTER: machinedesign.com 15

18 TOP ENGINEERING ESSENTIALS FOR MEDICAL DEVICE TECHNOLOGY CHAPTER 5: EPOXIES FOR MEDICAL DEVICE APPLICATIONS Biocompatible epoxies let designers affix subcomponents to medical devices that meet USP Class VI guidelines. LINDSEY FRICK, Contributing Editor One-component epoxies find use in a wide variety of medical products. Part of the reason is that the materials can be specially formulated to resist chemicals including multiple cycles of autoclaving while meeting rigorous biocompatibility standards. One-part epoxies also adhere well to metals, plastics, glass, and other substrates used in medical devices. Medical-grade one-part epoxies commonly serve in disposable and reusable devices such as catheters and surgical instruments. They can also be found in orthopedic devices and in diagnostic equipment such as MRI machines and ultrasound devices. Both one- and two-part epoxy-based systems can be used in bonding, sealing, coating, and encapsulating applications. But one-part systems have a few added benefits for medical applications. Before one can understand the nature of onepart systems, it is important to first understand two-part systems. Two-part systems, the most basic of epoxies, are formed through the polymerization of two starting compounds: a resin and a curing agent. The curing process takes place when the reactive constituents of the resin and curing agent combine. As this reaction proceeds, an exotherm develops, enhancing the cross-linking of the two components. The speed of the reaction, plus the ultimate cured properties, both depend on the nature of the resin and the curing agent. Pictured here is a central vein puncture and catheterization in an operating room. The catheter and guidewire assembly uses onecomponent adhesives that meet USP Class VI and ISO biocompatibility standards. Most of these one-part adhesives, developed by Tangent Industries, Winsted, Conn., cure in fractions of a second under LED or UV light. Applications for these adhesives include manifold bonding and thermistor potting. The majority of industrial-grade epoxies are two-part systems, which offer a comprehensive range of properties. Two-part systems require mixing and have a corresponding open time. Open time can be defined as the maximum time between mixing and the time REGISTER: machinedesign.com 16

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20 CHAPTER 5: EPOXIES FOR MEDICAL DEVICE APPLICATIONS when the product can no longer be applied. The time it takes to fully cure a two-part system can range from as little as a few hours, to as much as two weeks. Often, heat can be added, thereby accelerating curing, enhancing cross-linking, and optimizing performance properties. With two-part systems, designers must consider a number of processing issues. First and foremost, all two-part systems have a mix ratio. Some ratios are simple one-to-one weight or volume ratios. Others are more specific and unforgiving 100-to-7 by weight, for example. The process of mixing also requires a certain level of skill, especially when trying to ensure the two components are fully mixed. Complete mixing is as critical as avoiding air entrapment. Often, technicians mix more epoxy than is needed for the job thereby wasting epoxy. One-part epoxies are more efficient from a processing point of view. In a one-part epoxy, the resin and curing agent are premixed with inhibitors that prevent a reaction. The reaction only takes place when heat is added because heat eliminates these inhibitors. But it is important to heat the epoxy for the recommended cure time. Otherwise, it will only cure partially. Most one-part systems require a cure temperature of 257 to 302 F, although, there are now specialty grades for a 176 F cure. This lower curing temperature is useful for bonding heat-sensitive substrates. Another benefit of one-part epoxies is quick curing usually in just a couple of minutes or several hours at most. Yet, there s often a trade-off between curing time and performance. Long-cure epoxies often offer more-advanced properties than quick-cure versions including better electrical insulation and higher physical strength. A point to note is storage temperature. A range of 40 to 75 F is safe for most one-part epoxies, but refrigeration is desirable in most cases. Higher storage temperatures (above 75 F) can degrade the system. Custom-formulated one-part epoxies Most epoxies can be modified through appropriate selection of resins and hardeners and through the addition of fillers, diluents, and other components to realize a variety of processing conditions and performance properties. While two-part epoxy systems offer the broadest array of properties and applicability among all adhesive families, one-part epoxies are also highly multifunctional. A number of resins can be used in one-part epoxies including conventional, high temperature, and flexible, as well as low-ionic versions. The selection of a suitable curing agent is more restricted. Unlike two-part systems, which have a huge array of curing agent options, the number of compounds available to function as a curing agent in a one-part epoxy is limited. All curing agents for one-part epoxies require heat activation and continuation of heating. If the heat is withdrawn prior to full cure, the curing process stops and the epoxy remains partially cured. In contrast, for a two-part room-temperature curing system, one can initiate curing by adding heat. This is primarily done to fixture parts together quickly. If heat is withdrawn the system will continue to cure at ambient conditions. This will not happen with a one-part system. So why choose one-part epoxies for medical applications? Fully cured one-part epoxies offer important properties including high bond strength to a wide variety of substrates, high-performing mechanical and physical strength properties, robust temperature resistance, and the ability to withstand chemicals. One-part epoxies can be formulated in almost any viscosity imaginable. However, a paste consistency is often preferred for bonding applications to avoid material flow when heat is applied for curing. Chemical resistance, the ability to withstand high temperatures, electrical properties, optical clarity, thermal conductivity, and cure speed can also be modified to meet specific medical applications. For example, formulations are available to withstand cryogenic conditions, or to pass USP Class VI biocompatibility standards. One significant application issue regarding one-part epoxies is that they are more exothermic than two-part systems, and in most situations, are limited by curing depths 0.25 in. or less. Shown here is a breathing circuit that was assembled using LED-curable adhesives. The adhesives are made by Tangent Industries and are capable of bonding multiple substrates including acrylic, PVC, polyethylene, polycarbonate, and styrene. This wide range of substrates lets engineers connect various components to build products like anesthesia masks, breathing bags, tubing and connectors, and laryngeal mask airways. REGISTER: machinedesign.com 17

21 CHAPTER 5: EPOXIES FOR MEDICAL DEVICE APPLICATIONS Delicate devices One-part systems are used in delicate applications like medical devices with electronics and optoelectronics. A typical use is as an underfill system, where the epoxy is designed to protect and support delicate flip chips. Important considerations would be good flow and superior dimensional stability upon curing. Ultralow-viscosity one-part epoxies can also be used in vacuum-impregnation applications, ranging from transformer coils to carbon-fiber composites. On the other side of the spectrum, thixotropic one-part epoxies can be used as glob tops. Glob tops can be applied to circuitry, wire bonds, and even to delicate electronic components in medical equipment. One unusual iteration of these one-part systems is epoxy films. Films are becoming increasingly used in medical applications, especially where uniform bond-line thicknesses are needed. There are a host of other applications where they are being used lid sealing, the bonding of electronic components to heat sinks, and attaching substrates in microelectronic packages. In addition to determining the level of biocompatibility needed, it s important to look at several factors before choosing a one-part epoxy. Factors that may influence the selection process include cured properties, especially bonding and physical strength, chemical resistance, thermal and electrical properties, temperature resistance, thermal-cycling capabilities, and color. The handling and processing characteristics such as viscosity, exotherm, curing conditions, and temperature also play a role. North America is the United States Pharmacopeia (USP) Class VI. Any polymeric material that meets Class VI requirements is suitable for direct and indirect patient contact. Before granting Class VI certification, scientists test the materials by injecting and implanting material samples into subjects and then monitoring the subjects for signs of reactivity and toxicity. Epoxies with Class VI certification are suitable for both disposable and reusable medical devices, including endoscopes and infusion pumps. These medical-grade epoxies are also useful in prosthetics and diagnostic equipment. However, just because a material complies with USP Class VI doesn t mean it can go in a medical device. That s because a plethora of other FDA regulations also apply to medical devices. For example, designers that submit a device for FDA approval must also ensure that its materials undergo International Organization for Standardization (ISO) certification, which includes tests to verify compatibility with blood and bodily fluids. Tip for engineers: If a device assembly requires the use of an epoxy to hold subcomponents together, identify the right Class VI-approved epoxy before submitting the medical device to the FDA. The industry categorizes adhesives by the number of components they mix together and how they cure. However, each type of epoxy changes with the appropriate selection of resins and hardeners and the addition of fillers, diluents, and other components. BACK TO TABLE OF CONTENTS USP Class VI-approved epoxies Medical devices must be biocompatible, which means they must be made of nontoxic materials that don t irritate or damage living human tissue and don t cause physiological or immune-system reactions. One stringent certification of material biocompatibility in Tangent Industries has developed a series of one-part LED light-curable adhesives that bond stainless-steel cannula into hubs of various substrates and configurations. The biocompatible adhesives are ideal for high-volume production of disposable hypodermic needles, syringes, safety syringes, pen needles, winged infusion sets, and biopsy needles. REGISTER: machinedesign.com 18

22 TOP ENGINEERING ESSENTIALS FOR MEDICAL DEVICE TECHNOLOGY CHAPTER 6: PROTECTING MEDICAL ELECTRONICS WITH PARYLENE This biocompatible conformal-coating material protects medical devices against fluids, chemicals, and stray electrical charges LONNY WOLGEMUTH, Senior Medical Market Specialist Specialty Coating Systems Inc. Many medical devices and their electronics need protection from moisture, chemical contamination, electrical charges, and body fluids. Otherwise, patients and healthcare providers may be put at risk. One way biomedical engineers provide this protection is by encapsulating devices in a conformal coating, one made of a dielectric, or poor conductor of electricity, such as silicone, acrylic, urethane, or epoxy. But one of the best materials for this purpose is parylene. Parylene basics Parylene is the generic name for a series of organic polymers poly(para-xylylene) polymers used as coatings. They are polycrystalline and linear in nature, optically clear, and colorless. Parylene coatings have useful dielectric and barrier properties and are chemically inert. Three different types give engineers a range of dielectric and other properties from which to choose. The coatings contain no fillers, stabilizers, solvents, catalysts, or plasticizers, so they are not subject to any leaching, outgassing, or extraction issues. Parylene coatings are also compatible and stable in the presence of bodily fluids and tissues, critical factors in the medical-device industry. Parylene provides dryfilm lubricity with coefficients of friction similar to that of PTFE (Teflon), and dielectric strengths REGISTER: machinedesign.com 19

23 CHAPTER 6: PROTECTING MEDICAL ELECTRONICS WITH PARYLENE up to 7,000 V at a mil (25 microns) of coating thickness. No other material can be applied as thinly as parylene and provide the same levels of protection. Parylene withstands all common sterilization methods steam, ethylene oxide, electron beam, hydrogen peroxide plasma, and gamma radiation. It can be applied to most vacuum-stable materials, including plastics, metals, ceramics, fabrics, paper, and even granular materials. For example, parylene coatings could be applied to microspheres or moisture-absorbent powders. Parylene can be selectively removed with plasma, lasers, or strong abrasion, for instance, to repair devices. Parylene is not soluble in harsh detergents and chemicals; in fact, it protects components from such chemicals. Parylene is not a hard coating, so excessive abrasion will remove it. However, most components coated with parylene do not abrade or rub against other parts. If an application does include abrasive contact, it is not a good candidate for parylene. Parylene deposition Parylene coatings are applied using vapor- deposition polymerization (VDP) in a vacuum chamber at room temperature. Film deposition actually takes place on the molecular level, with the coating literally growing one molecule at a time. This lets parylene penetrate and coat small cracks, crevices, and openings, and protect even hidden surfaces in areas where other coating methods such as sprays and brushes cannot reach. Coating thickness is uniform, even on irregular surfaces. And VDP is a clean, self-contained process that uses no additional chemicals. Parylene is deposited as a vapor, so it surrounds the target and perfectly follows its contours, literally encapsulating it. Parylene coatings are ultrathin and pinhole-free. The only raw material used in the coating process is known as dimer. Technicians place the powdered double-molecule dimer into the vaporizing chamber at one end of the coating machine. The dimer is heated, sublimating it directly to a vapor, and then heated again until the dimer cracks into a monomeric vapor. This vapor flows into an ambient-temperature deposition chamber kept at a medium vacuum (0.1 torr) where it spontaneously polymerizes onto all surfaces, forming an ultrathin, uniform film. No curing or additional steps are required. The size of the coating chamber may be an issue if products are too large to fit inside. For example, medical wire on a reel that needs to be coated as one continuous piece may not be suitable for parylene. However, if wires are precut to various lengths, hundreds of pieces might fit into one chamber. Because there is never a liquid phase in VDP, there is no meniscus or pooling. There is also no bridging or blocking of small openings, which can happen when applying a liquid coating. The thickness of a parylene coating can range from 500 Å to 75 microns, so it does not significantly change the coated device s dimensions or mass. In many medical devices, such as intraocular and cochlear implants, maintaining minimal dimension and mass are critical to the device s performance. An added benefit of parylene is its ability to strengthen delicate wire bonds by an estimated factor of 10. The preparation and coating processes vary from device to device. Typical turn times are five to 10 business days, but that can be negotiated. Times may be extended if parts require extra inspection, pretreatment, or masking and demasking. Many medical-device manufacturers send parts to coating-service providers due to the art and complexity of parylene coating process. Also, medical-device manufacturers typically do not want to become experts in a coating process they may use on only one or two product lines. Some device manufacturers do, however, purchase VDP equipment and bring the process in-house. Parylene variants The parylene family includes several members. Parylene N, for example, is nonchlorinated poly(paraxylylene) that has a low dissipation factor, high dielectric strength, and a dielectric constant that doesn t vary with the frequency of the electrical current. Parylene N also performs well when it comes to penetrating and coating into a device s small crevices and spaces. Parylene C is produced from the same dimer used to make Parylene N, but it is modified by a chlorine atom attached to the molecule s benzene ring. It has a useful combination of electrical and physical properties, plus a low permeability to moisture, fluids, and corrosive gases. Its ability to provide pinholefree conformal barriers makes it the coating of choice for many critical medical electronic assemblies. Parylene HT is the newest commercially available parylene. It carries fluorine atoms on the benzene ring instead of hydrogen atoms. It has the lowest dielectric constant and dissipation factor of all the parylenes, as well as the highest continuous service temperature (350 C). It also maintains its properties despite exposure to UV light. The other two parylenes are susceptible to damage by UV light. All three parylene formulations are biocompatible and biostable, as confirmed by ISO and USP Class VI biological evaluations. Parylene applications As noted, parylene coatings protect devices from moisture, biofluids, and biogases that can cause assemblies to fail prematurely. This protection extends product life, prevents costly repairs and, most importantly, reduces the risk of failure. Parylene has also been helpful in tackling challenges raised by new regulations. Metallic whiskers, for example, are one of the unintended by-products of removing lead from solder as part of RoHS regulations. These whiskers can lead to reliability problems for electronic assemblies. Parylene coatings suppress the formation of metallic whiskers. Another benefit is parylene s dry-film lubricity, which makes it an ideal release agent for molds. Being solid and inert, parylene REGISTER: machinedesign.com 20

24 leaves no residue to contaminate molded products. And parylene s lubricity extends the life of forming tools such as wire mandrels by eliminating flaking and delamination. Coating costs The cost of coating a product with parylene depends on several factors, including: Complexity of the item being coated. Do one or more areas need to be masked so that parylene does not coat them? How thick a coating is needed? This depends on the coating s intended function. Will it be used to protect electronics, add lubricity, be a tie-layer for other coatings, or be an elution-control layer for drugs? What type of parylene is required? N, C, or parylene HT? How many parts are to be coated at one time? It is obviously less expensive to coat hundreds of parts in a large chamber than 10 or 20 parts in a smaller chamber. While some elastomeric O-rings can be coated for less than a penny each, a single, large, complex, military circuit board can cost hundreds of dollars to coat. In general, parylene is competitive with other coatings given the right production volumes, complexity, and other variables. And although it may be more costly than some other coatings, rylene may be the only option for the protection needed by a given device. For the sophisticated microdevices being developed for medical implants, parylene provides longterm biocompatibility and device protection. The available parylene formulations, coupled with newly developed adhesion-enhancement technologies, let parylene coating perform on medical-device components, circuits, and equipment, regardless of their size, configuration, or material. CHAPTER 6: PROTECTING MEDICAL ELECTRONICS WITH PARYLENE A short history of parylene In 1947, Michael Szwarc was pursuing his academic career in physical chemistry at the Univ. of Manchester, England. His interest in the strength of individual chemical bonds led him to investigate a class of aliphatic carbon-hydrogen bonds in which the carbon was directly attached to a benzene ring. While doing so, he heated gases of the simplest compounds having both benzene and carbon toluene and the xylenes to high temperatures. He monitored both the decomposition products and rates of decomposition as a function of temperature. With p-xylene only, a tan-colored deposit formed in the cooler reaches of his glassware. The material has been described as a thin, imsy, tube-shaped mass, the skin of a small snake. Szwarc correctly deduced that this lm had been formed by polymerizing reaction products of the p-xylene, called p-xylylene. He also noticed the new polymer s physical properties and chemical inertness. This serendipitous polymerization was the world s rst vapor deposited poly(paraxylyene). Today its purer colorless form is called parylene N. A few years later, William Franklin Gorham at Union Carbide Corp. continued the research on parylene. By 1967, this work led to the availability of a new polymeric coating. Parylenes was the term used to describe both a new family of polymers and the vacuum-deposition process for applying them. In fact, Union Carbide developed over 20 types of parylene, but only three were deemed commercially viable. BACK TO TABLE OF CONTENTS REGISTER: machinedesign.com 21

25 TOP ENGINEERING ESSENTIALS FOR MEDICAL DEVICE TECHNOLOGY CHAPTER 7: IMPLANTED TELESCOPE HELPS PATIENTS OVERCOME MACULAR DEGENERATION Implanting some magnifying optics into patients eyes lets them salvage what s left of their vision. STEPHEN MRAZ, Technology Editor Age-related macular degeneration (AMD) is the leading cause of vision loss and affects 10 million Americans, more than those suffering from cataracts and glaucoma combined. That number could grow to over 20 million by 2020 as the U.S. aged population grows. Worldwide, the number afflicted in 2020 could be as high as 196 million. There currently is no cure and doctors could do little more than prepare patients for the inevitable loss of vision in one or both eyes. Now, however, patients have an option, thanks to researchers at VisionCare who have developed an implantable telescope that can preserve a patient s vision. The Telescope The implantable telescope consists of two lenses within a glass tube. It is about the size of a pencil eraser (3.6-mm in diameter and 4.4 mm) and uses bi-convex and bi-concave convergent and divergent micro-lenses coupled with air lenses, according to VisionCare. Details on the micro-optics are proprietary, but the outcome is that the implant acts like a fixed telephoto lens that works with the cornea to project images onto the retina that are enlarged by a factor of 2.7. The iris is also left in place, but the implant is longer than it is deep, so the end of the telescope protrudes through the inactive iris. Although the macula of the retina is partially destroyed and useless, the magnified image overlaps the diseased section to stimulate undamaged rods and cones to partially return central vision The implantable telescope from VisionCare improves the central vision of patients suffering from advanced macular degeneration. It gets implanted in one eye so that it can detect central vision (where the person is looking); the other eye then picks up the task of peripheral vision. to the patient. The implant also has a polymethylmethacrylate (PMMA) carrier and a blue PMMA restrictor. The sealed optical components snapfits into the carrier, which includes shaped projections that hold the REGISTER: machinedesign.com 22

26 CHAPTER 7: TELESCOPE HELPS OVERCOME MACULAR DEGENERATION The implanted telescope sends visual information to areas outside the damaged macula (the dark red spot). When implanted, the telescope is behind the undamaged cornea. The restrictor, the blue components, prevents too much light from entering around the edges of the telescope and washing out the image. implant in place. These are similar to those found on intraocular lenses implanted in patients who have had cataract-removal surgery in which part of their natural lens is removed. The projections, called haptics by VisionCare, are snugged into the capsular bag, a smooth transparent membrane that surrounds the natural lens. THE THREAT: AGE-RELATED MACULAR DEGENERATION AGE-RELATED MACULAR DEGENERATION (AMD) is an incurable eye disease that damages the macula, the small area near the center of the retina that contains a high concentration of light receptors (rods and cones). The macula is responsible for sharp, central vision and lets people see objects they are looking directly at. AMD is a slow-working disease and those with it might not notice any symptoms for years. Over time, however, the person s center of vison in the affected eye(s) becomes increasingly blurry. The blurred spots grow and blind spots can develop in the eye s field of vision over time. Eventually, central vision is lost altogether, rendering the person legally blind. They still might be able to see objects in their peripheral vision, but they can no longer read, see faces, drive safely, or do close work. There are two types pf AMD, dry and wet. The dry version, which accounts for 90% of all AMD cases, results from the slow breakdown of light-sensitive cells in the retina and the supporting tissue beneath the macula. The other type, wet AMD, accounts for 10% of AMD patients, and is also known as late-stage AMD. In This image simulates what a person with AMD sees: The central portion of the image is totally unusable, but the peripheral vision is still available (though blurry). this stage, abnormal blood vessels grow underneath the macula. The vessels often leak fluids and blood, leading to swelling and damage to the macula. The damage is severe and happens quickly, unlike the slower pace of dry AMD. But not everyone who gets dry AMD develops wet AMD. People with early AMD in only one eye have a 5% chance of developing late-stage AMD within 10 years. These with early AMD in both eyes run a 14% chance of developing late-stage AMD in at least one year after 10 years. There is another form of macular degeneration called Stargardt disease. It is found in younger patients and is caused by a recessive gene. There is no cure for AMD, although highdose vitamins and minerals and a healthy diet have been known to slow its progression. It s also recommend that those with AMD stop smoking. (Smoking doubles the chances a person will contract AMD.) Other risk factors for contracting AMD include genetics; AMD does run in families, but researchers have identified 20 genes that affect the risk of developing AMD, and many more are suspected. That s why there are currently no genetic tests that reliably predict if someone will come down with it. AMD is also more common in Caucasians than among African-Americans, Asians, or Hispanics. But the largest risk factor is age: the older you get, the more likely you are to be afflicted by AMD. REGISTER: machinedesign.com 23

27 CHAPTER 7: TELESCOPE HELPS OVERCOME MACULAR DEGENERATION Over time, the membrane grows up and around the haptics, securing the implant in place. But patients are still warned to avoid situations in which their head or eyes are exposed to trauma so they don t damage or dislodge the implant. They are also told to refrain from rubbing their eyes too forcefully. The blue-tinted restrictor, a washer-shaped component, surrounds the implant and reduces the amount of light that can enter the eye from the periphery so it does not wash out images coming in through the implant. The optics are designed and built to have an optimal focusing distance of about 11.5 ft., assuming an average-sized eyeball. Patients are prescribed glasses if distance or near-viewing corrections are needed. There are no moving parts or electronics in the implant. The device is implanted behind the iris in one eye during an outpatient surgical operation that also involves removing the eye s natural lens. The implant is sterilized by the manufacturer using ethylene oxide (EtO). This gas infiltrates packaging and kills germs. EtO is often used on medical devices and components that need to be sterilized but cannot withstand conventional high-temperature steam sterilization. After sterilization, the implant is packaged and then not opened until inside the clean operating room. Post-Op Results After the operation, which usually lasts one to one-and-a-half hours, patients are given eye exercises and go through some training to get the most out of the implant. For example, they practice tracking objects with the new implant, as well as watching TV and reading. The exercises, which can last six to 12 weeks, also help reprogram the optical cortex of the brain and how it processes inputs from the eyes. This is needed because the patients are now using their eyes in a completely new way. In fact, vision gradually improves and it can take a few months before all the benefits are realized. In post-op patients, the eye with the implant provided their brain with visual details of what they are looking directly at while the other eye provides peripheral vision. AMD does not affect peripheral vision, a low-resolution form of vision humans rely on for detecting objects near or nearing them and those moving in their field of vision. So instead of using two parts of the same eye, the patients (and their brains) need to switch between eyes to get the same information. The implant has been shown to improve a person s ability to identify what they are looking at, to look someone in the eye during conversation, and to see facial expressions. The implant can t completely restore a person s natural vision. However, in clinical tests, 50% of the people with the implant could read two to three lines lower on the standard eye chart, and 90% reported improved vision. Patients might also still require a magnifying glass to see fine details or small print. The implant is practically unnoticeable to others because it is totally inside the eye and behind the iris, the colored portion of the eye. The implant, which costs about $15,000, is approved by the FDA for patients 65 and older, and is covered by Medicare. It is designed to last the life of the patient. BACK TO TABLE OF CONTENTS REGISTER: machinedesign.com 24

28 TOP ENGINEERING ESSENTIALS FOR MEDICAL DEVICE TECHNOLOGY CHAPTER 8: TECHNOLOGY ADDS THE SENSE OF TOUCH TO PROSTHETIC HANDS Biomedical researchers at the Veterans Administration are exploring options and developing techniques for directly stimulating nerves to let prosthetic users feel sensations in their artificial hands. STEPHEN MRAZ, Technology Editor Losing an arm is a traumatic, life-changing event. Too many military personnel suffer that loss. Even in peacetime, as many as 20 armed services members endure upper or lower limb amputations annually due to noncombat accidents. To help wounded warriors return to active and healthy lifestyles, the Department of Veterans Affairs not only provides medical assistance, prosthetics, training, and other assistance, it also funds R&D into better prosthetics. At the Advanced Platform Technology Center (APTC) at the VA Medical Center in Cleveland, one team of biomedical researchers is working to develop technology that would give veterans (and civilians) with upper-limb losses a sense of feeling in their prosthetics. They are installing sensors on the artificial hand portion of the prosthetic, then processing and routing those sensor signals to the user s brain via the nerves that once Users with prosthetics that give them a sense of touch and feel are more proficient at locating and picking up objects like these cubes. served the missing hand. The goal of the project, according to team member Assistant Professor Dustin Tyler, Director of Engineering, Quality, and Regulatory Affairs at APTC and a faculty member in nearby Case Western Reserve University s Biomedical Engineering Dept., is determining what kind of sensations the sensors on prosthetics can provide users, how many sites can be given this artificial sense of touch, and what technologies can best do the job. Sensors The sensors we currently use are thin-film, force sensor resistors (FSRs) from Tekscan Inc., says Tyler. An FSR consists of two electrodes separated by a thin sheet of material. Applying pressure moves the electrodes closer together and changes the sensor s resistance as a function of pressure. FSRs are simply taped to the artifical hand, a classic three-jaw hand prosthesis from REGISTER: machinedesign.com 25

29 CHAPTER 8: TECHNOLOGY ADDS THE SENSE OF TOUCH TO PROSTHETIC HANDS Ottobock with one degree of freedom the index and middle finger move together toward and away from the thumb. The center s current prosthetic has four FSRs that measure the finger opening or span between the thumb and fingers, as well as pressure at the tip of the thumb, index finger, and middle finger. These let users grasp and release objects and give an indication of how much force they are exerting in their grasp. But in the future, sensors need to be more rugged to withstand daily use without breaking down. And they will be mounted so they are protected. But there are some drawbacks to hardening sensors, notes Tyler. One medical-device manufacturer puts sensors on the internal structural metal parts of its prosthetics. This provides a level of protection, but limits the range of forces they can detect, Tyler says. Down the line, as we try to detect forces distributed over artificial hands, we will need to find mounting locations that let sensors detect surface sensations and pressure with While this setup works to give people better resolution, and not just a gross output who have the instrumented prosthetic of total forces but better spatial resolution, hand a sense of touch, it would have to says Tyler. We will always want to detect be miniaturized and made more rugged feelings from fingers, so sensors will likely before it could be economically produced mount on the prosthetic s fingertips, which in volume and widely used. means they will have to be protected against wear and liquids. In APTC s current setup, an exterior battery powers the sensors. Actual prosthetics, however, would not tether users to external batteries or power supplies. Batteries are getting more capable, and motors and sensors are becoming more efficient, so the ideal would be to have a single-charged battery pack last all day, says Tyler. But until then, biomedical engineers will likely design artificial hands and arms with quick-change-out battery packs holding a minimum of 4 hours of use. new combinations of stimulation pulses. That s one reason the stimulator and PC will likely remain separate even in later versions, says Tyler. In the future, users might be able to control some gain parameters so that the hand is more sensitive for doing delicate work and less sensitive for heavier-duty work, explains Tyler. But it would likely be nothing more sophisticated than a volume knob. The relatively simple stimulator, the product of biomedical engineers at the VA s Functional Electrical Stimulation Center (FESC) in Cleveland, creates three current-controlled pulse trains. The three stimulation signals get sent to the three major nerves that usually carry sensory signals from the hand to the brain: the median, ulnar, and radial nerves. This stimulation is ac in nature and biphasic with balanced current flow in successive negative and positive impulses. Biomedical engineers long ago discovered that long-term monophasic stimulation with all positive or all negative pulses creates chemical and charge imbalances that break down nearby blood vessels and muscle tissues. The APTC project s goal is to eventually replace the PC with an embedded The PC and nerve stimulator Raw sensor data gets amplified, filtered, digitized, and then sent to the PC at no more than 100 Hz, which is enough bandwidth for an artificial hand s sense of touch. The PC, a standard desktop model running Windows, records all the sensor data and performs some numerical processing on it using Matlab (from Mathworks). In general, the PC maps the sensor information to stimulation patterns for nerves that will generate the desired sensations in users. Currently, only researchers can adjust or change settings on the PC and stimulator, letting them update programming and try The sense of touch lets prosthetics users adjust their grips strength on the fly so they can pick up or manipulate delicate objects like these grapes. REGISTER: machinedesign.com 26

30 CHAPTER 8: TECHNOLOGY ADDS THE SENSE OF TOUCH TO PROSTHETIC HANDS processor that would be small enough to mount inside the prosthetic. It would pull in sensor data, do some processing, and match incoming sensor data with the proper nerve stimulation signal (pulse, timing, and amplitude). One long-term approach is to use prosthetic-mounted sensors that detect the raw haptic data, which will be processed in a module attached to the artificial hand, perhaps no larger than a wristwatch, says Tyler. This would communicate with a stimulator implanted in users like a pacemaker but send sensory signals to the proper nerves. The prosthetic would also record the muscle activity (electromyogram, or EMG) of the user trying to control his missing hand. These EMG signals would get processed and be used to control the prosthetic s drive motors that move the digits and hand. The sense of touch lets prosthetics users adjust their grips strength on the fly so they can pick up or manipulate delicate objects like these grapes. The right stimulation One of the most difficult but intriguing parts of the APTC project is determining what stimulation to apply given a specific sensor input. Biomedical engineers started by copying the paradigm used to control the muscles of people with spinal-cord injuries. It relies on consistent trains of pulses sent to muscles via efferent nerves (those that go from the spinal cord to muscles). The higher the pulses amplitude, the more muscle fibers get excited, so the muscle generates more force, Tyler says. But this doesn t work on afferent nerves (which carry information from sense organs to the brain). Users felt sensations, but they weren t natural. Instead, they got that pins and needles feeling, like their fingers or hand had fallen asleep. This wasn t too useful, but to people who had never felt anything from their prosthetics, it was better than nothing, he explains. These results were not surprising. Sensation is much more complex than muscle movement. For example, muscles do little if any processing on incoming nerve signals, while the brain does a host of processing on incoming sensory nerve signals. That s because sensory signals include more information, which could be encoded in the signal s frequency, the pulse duration and amplitude, the space between pulses, or the interference or combinational effects of signals travelling along the same and nearby nerves. Or the information might be a contained in a permutation of all these parameters, notes Tyler. Over time, Tyler and his team have gone far beyond letting users just feel how tightly they are grasping an object. In fact, we ve elicited feelings and sensations in as many as 19 different places in the users missing hands, says Tyler. This includes a couple places in the palm, the fingertips, a few places on the back of the hand and wrist, and several down the outside edge of the pinky. So our technology for sensors on the prosthetic hand is behind in terms of the number of places we can elicit sensations in users. The APTC haptic system currently uses a pair of implanted eight-site electrodes like this one. Each runs along a nerve, delivering different stimuli to eight different portions of the nerve. The strings attached to it are sutures used to secure the electrode in place inside the user s forearm. The system also uses a four-site spiral electrode that wraps around one of the user s nerves. We would also like to find the right stimulation that would recreate shear, which would let users feel when something is sliding through their grasp, says Tyler. Another sensation amputees really miss is warmth, especially the warmth of a child s or spouse s touch, says Tyler. Unfortunately, that would add another type of sensor to a space-limited prosthetic. And nerves that relay heat and cold to the brain are smaller than those that carry pressure information. Smaller nerves are more difficult to locate, isolate, and stimulate, and special electrodes would have to be developed. So giving prosthetics a sense of hot and cold is not our focus. Connectors and electrodes In the APTC setup, different stimulation signals are sent to 20 nerve sites accessed by three implanted electrode cuffs, each on a different nerve. There is a pair of eight-contact cuffs, and a four-contact spiral cuff wraps around one of the nerves, making 20 sites. On the eight-contact electrode, for example, each of the eight wires or leads is connected to a small piece of platinum foil in contact with the nerve. Platinum is biocompatible and provides good electrical contact with the nerve tissue. Each of the 20 leads terminate in pins, which attach to 20 corresponding transcutaneous leads via a matched-pin connector. This connector uses a spring to connect two lead pins. Mating pins are inserted in either end of the spring, which is wound around each pin in the opposite direction of its spiral, a time consuming process. Pulling the connected leads apart tightens the spring around the pins, much like a Chinese finger trap. The spring and pins are then covered with a silicone sleeve to isolate and insulate the electrical connection from body fluids. The benefit of this somewhat arcane connector, which will be replaced in future, refined versions of the system, is that it creates solid elec- REGISTER: machinedesign.com 27

31 CHAPTER 8: TECHNOLOGY ADDS THE SENSE OF TOUCH TO PROSTHETIC HANDS wider variety of different stimulus combination to give users more sensations. The APTC system will be upgraded to use a flat interface nerve electrode (FINE). It opens so surgeons can place the nerve inside of it. When the housing closes, the nerve is held flat, making it easy to access more nerve fibers. The nerve is held loosely in place so blood flow is not interrupted and the nerve is not damaged. trical connections and is replaceable. These second leads run under the skin up the arm to the user s shoulder, where they exit the user s body, leaving 2 or 3 in. of wires terminating in a multilead jack protruding from the skin. Tyler uses flat interface nerve electrodes (FINEs) in the haptic system. Such electrodes have a two-part housing that opens like a clamshell to let surgeons place a nerve longitudinally down its center. Closing the housing holds the nerve in place with minimal force, ensuring it is not damaged and blood flow to it is not interrupted. In future versions, the bulk of the electrode will be reduced by using high-performance polymers, such as PEEK, to give the electrode good mechanical properties, as well as more precise alignment of the more closely positioned electrical contacts. Eventually, the electrodes will include thin-film componenets containing small and precisely postioned contacts, thanks to photolithographic manufacturing, the same processes used to make ICs. Larger nerves are much like ropes in that they are made up of smaller fibers or fassicles. Fassicles are about 0.5 mm in diameter, so the typical 10-mm-wide nerve likely has at least 16 of them going from side to side. With these more refined and precise FINEs, Tyler plans to increase the number of contacts from eight to 32, or 16 on each side of the nerve. Eventually, FINEs could each have 64 contacts. This would let us stimulate more areas in the brain that correlate to more places on the prosthetic hand, says Tyler. Or if we combined thin-film technology with FINE, we could add rows of electrodes along the nerve, stimulating more nerve sections using a Do Prosthetic Feet Need Feelings? Prosthetic lower limbs, especially those for people with amputations below the knee, have been very successful at replicating the needed biomechanics to get users up and walking, even running in many cases. Yet none of them give users any sense or feeling in their artificial limbs. So do prosthetic feet need haptic technology? Yes, the mechanics of lower-limb prosthetics are spectacular, but they are usually being used on flat, level surfaces, notes Assistant Professor Dustin Tyler, Director of Engineering, Quality, and Regulatory Affairs at the Advanced Platform Technology Center (APTC) at the Veterans Center in Cleveland and faculty member in nearby Case Western Reserve University s Engineering Department. It becomes more challenging on uneven surfaces when the artificial foot moves unpredictably because the user cannot feel the ground. So users probably could benefit from receiving foot position data, which the sense of touch should be able to provide, Tyler says. Climbing steps and hills also present problems to people with prosthetic feet. For example, they have to watch carefully to know whether their entire artificial foot is on a step or just a portion, and they don t know which portion, the toe or the heel. Giving them the sense of pressure on the bottom of their feet would give them better capability and confidence in climbing steps, says Tyler. Plus, receiving sensation from an electromechanical device, foot or hand, lets people consider the device as part of themselves rather than just a tool hanging on the end of their limb, says Tyler. BACK TO TABLE OF CONTENTS REGISTER: machinedesign.com 28

32 TOP ENGINEERING ESSENTIALS FOR MEDICAL DEVICE TECHNOLOGY CHAPTER 9: TECH ADVANCES UPGRADE HEARING AIDS Advances in MEMs and signal processing let hearing aids provide understandable speech and more natural, comfortable sounds to the hearing impaired. STEVE MRAZ, Technology Editor Technological developments in hearing aids (HAs) have always been driven by efforts to miniaturize hardware and give those with hearing problems an estimated 35 million in the U.S. the ability to better hear and understand the people and environment around them. Miniaturization lets engineers pack more amplification and signal processing power in small, wearable housings. It also lets hearing-impaired people use HAs small enough to go largely unnoticed. Advancements in HAs have always been spurred by developments in electronics and MEMS and their manufacturing processes, and HAs are often the first commercial devices to make use of those developments. For example, the first consumer use of transistors was in a 1952 HA from Sonotone, according to John Dzarnoski, director of technology development at Starkey Hearing Technologies. It used a transistor and two vacuum tubes for amplification in a device that measured in., small enough to fit in a shirt pocket. That is the story of hearing aid development, says Dzarnoski. It uses new technologies ahead of other industries to build smaller, more capable devices. And that story continues. The Pure hearing aid from Sivantos, a division of Siemens, is a behind-the-ear model that features dual microphones for better directionality and a telecoil, which lets telephones and facilities equipped with an induction loop such as churches and theaters beam sounds directly to the user s hearing aid. Here s a look at some of the other ways biomedical engineers are using new technologies to improve the lives of the hearing impaired. Microphones Two types of microphones are commonly used to pick up the sounds that will be amplified by HAs: electret and MEMS. Both have sensitivities of between -32 to -35 db, according to Eric Branda, product manager for Sivantos Inc., the hearing-aid division of Siemens. Electrets, long used in HAs, are electrostatic capacitor-based microphones. They measure about mm and use a Teflon or polymer diaphragm. These diaphragms are sensitive to temperature, humidity, and aging, which can be a problem. MEMS microphones are made of silicon using the processes similar to those that make ICs. Thus, they do not have the electrets material-related problems. They are also smaller than electrets, measuring mm, so engineers can fit several on a single HA. Their downside is that HA need a bandwidth of about 8,000 Hz to detect all the sounds needed for speech and normal environmental noises. MEMS microphones currently have a difficult task doing this, though they are fine for cell phones where their limited bandwidth is REGISTER: machinedesign.com 29

33 CHAPTER 9: TECH ADVANCES UPGRADE HEARING AIDS The Insio in-the-ear hearing aid from Sivantos is said to be the first such device to employ wireless technology for exchanging data with another Insio in the user s other ear to synchronize settings and exchange audio signals. It can also be adjusted by the user through an iphone or Android device. not a problem, says Dzarnoski. Companies often place two microphones on HAs, especially behind-the-ear and in-the-ear types, to let the wearer tell what direction the sound is coming from (directionality). Inputs from these microphones, both of which are omnidirectional, can be electronically manipulated or processed (after being converted to digital signals) to give users a sense of directionality, says Branda. The inputs also help in identifying and tracking unwanted noise, letting the HA reduce amplification of those sounds. Directionality and selective amplification also helps users understand speech when it comes from behind them. Processing Today s HAs have about the same processing power as a 386 chip and can handle about 5 million instructions per second, which is a lot of computational power to be running on a small battery, Dzarnoski points out. Some of that processing power goes to reduce or eliminate feedback. Feedback happens when sound from the speaker leaks back to the microphone and gets repeatedly amplified into a loud squeal. The approach taken by Starkey to reduce feedback is to decorrelate or change the output from the input by frequency shifting the output b t5 to 20 Hz. A 20- Hz shift represents an aggressive feedback prevention scheme while a 5-Hz shift is used when listening to music, says Dzarnoski. Digital processors also carry out sound compression, reducing the dynamic range of sounds so that it better matches the users hearing capability. At Sivantos, some HAs use digital signal processing for SpeechFocus. It s a feature that selects the best directional pattern or amplification scheme for incoming microphone signals based on where a speaker is located. First it uses spectral analysis to recognize speech by its frequency spectrum. Then it boosts amplification to the microphone best positioned to pick up that sound. Sivantos and other HA manufacturers also use digital algorithms to reduce transient and impulsive noises such as rustling paper or clanging dishes without affecting speech signals. Sivantos version, called Sound Smoothing, relies on spectral and temporal analysis to attenuate only the transient frequencies, usually high or low frequencies, within one millisecond of their onset. One of the major goals HA designers have been working on for years is to use signal processing to let HA wearers hear and understand speech from one individual in a room full of people talking, such as in a crowded restaurant or bar. The ability of hearing aids to recognized unwanted sounds is still primitive, and the top complaint for years has been their inability to recognize speech in the presence of noise, even other speech, notes Dzarnoski. Receivers The receivers in HAs, or what those outside the industry might call speakers, are electro-acoustic balanced-armature magnetic The Lyric in-the-ear hearing aid from Phonak is so small, it can be placed deeply in the ear canal, only 4 mm from the ear drum. It is said to be the first extended-use device that can be continuously worn for months at a time. To bring Lyric to maker, Phonak engineers had to come up with a low-power circuit, a long-life battery, and materials and a design that could safely reside in the ear canal for long periods of time. The Lyric in-the-ear hearing aid. REGISTER: machinedesign.com 30

34 CHAPTER 9: TECH ADVANCES UPGRADE HEARING AIDS transducers. They consist of a magnetic armature precisely centered between a pair of electromagnets. Electric current sent through the magnets coils induces force which moves the armature back and forth. This mechanical force is transferred to a membrane that converts the motion to air-pressure differences or sound, explains Branda. Receivers vary in size from mm for small devices and moderate amplification to mm for larger, more powerful receivers. How much power the receivers require is a function of the amplification needed. HA designers are coming with smaller receivers, letting them fit in smaller housing that sit nearer to the ear drum. Some designers are also adding two or more receivers, each optimized for a specific range of frequencies. This is much like hi-fi speakers that contain tweeters for high frequencies and woofers for the low tones. Crossover circuitry routes the sounds to the proper receiver. Batteries Zinc-air batteries are the power source of choice for HAs based on their safety and energy densities. Zinc-air batteries are very safe, says Dzarnoski, And they will not get hot enough to burn a person even if the positive and negative side of the battery is shorted out. They generate 1.35 to 1.45 V of electricity using metallic zinc and oxygen pulled from the air. These disc-shaped batteries have storage capacities that depend on how much zinc they contain. They range in size from 6 Ñ 2mm up to 11 Ñ 5 mm and last from four to 10 days, depending on how much they are used and how much amplification the HA provides. They cost about $0.60 to $1.60 and are not rechargeable. The Lyric, an in-the-canal HA from Phonak, uses extremely little power, thanks to ultra-low power electronics and the fact it is so deep within the ear canal, the receiver is only 3 to 4 mm from the eardrum. It operates for months on a single battery. In fact, users can keep Lyric in their ear for up to six months before a technician removes and replaces it. Sivantos, however, does make HAs that use rechargeable batteries, the only major HA maker to do so. Rechargeable batteries hold less capacity than regular batteries, says Branda. Only HAs that are sufficiently energy efficient, such as some Sivantos makes, can use rechargeables and still last a full 16-hr before needing recharged. Protection Biomedical engineers consider the ear canal a hostile environment when it comes to materials and electronics. The ear canal is loaded with wax-generating glands that remove foreign material from the canal naturally, explains Dzarnoski. When hair follicles in the canal sense something in the canal, it stimulates pores to secrete a chemical that results in earwax. So putting a hearing aid in the The Ponto BAHA. canal can lead to excess wax creation, which plugs up ports and holes, and is somewhat corrosive. In most cases, the tight plastic housing that goes in the canal keeps out most debris and wax. Some manufacturers, such as Starkey, add a screen over the microphone. When it gets clogged, it is replaced. Starkey also uses molecular vapor deposition to put a proprietary nanocoating called HydraShield on its housing and exposed components. The 100-nm thick layer The Ponto bone anchored hearing aid (BAHA) from Oticon takes sound form the outside words, processes it, and then sends it through a titanium screw implanted just behind the user s ear. The screw penetrates the skins and is embedded in the user s skull. Users hear the sound through bone conduction, the same way you can hear yourself chewing or biting. Because using BAHAs involve surgery, the price for one can range from $15,000 to $25,000. Nevertheless, they are a last option for patients who have occluded ear canals or lack them entirely, or have some other physical restriction that prevents them from using more traditional hearing aids. REGISTER: machinedesign.com 31

35 CHAPTER 9: TECH ADVANCES UPGRADE HEARING AIDS makes oil and water to bead up and run off the coated parts. Sivantos makes a completely water and dust proof HA, Aquaris. It has been tested to IP68 standards and can be fully immersed in water. The technologies which make that possible include a bottle housing concept, extra microphone and receiver protection, a waterproof battery door with a membrane seal, and acoustic vents. The Wireless Advantage Designers are now taking advantage of wireless innovations to improve the usability and function of HAs. A common application lets HA wearers adjust the controls on their devices - while they are wearing them - from a smart phone or PDA. They can also key in various profiles, a group of settings for low-noise environments, speech in a noisy room, or for listening to music, among others. Starkey has a similar feature they call Geo Tagging. Wearers use their iphone and an app to adjust their HA s controls for a particular setting, which could be a theater, classroom, or subway platform. The app stores the settings, as well as the GPS-derived latitude and longitude for the place. The next time the person goes to that place, the iphone recognizes it from the GPS coordinates and automatically switches the settings to those saved for that location. Wireless technology has also made it easier to use and wear binaural HAs in which users put a HA in each ear. Just as natural hearing combines inputs from both ears and process it in the brain, outputs from one HAs gets sent to the other to maximize directionality and increase the ability to pull sound out of noise. The main wireless platform used by HA seems to be the Apple iphone and its proprietary 2.4-GHz protocols, according to Dzarnoski. Companies are building Android-based devices, but Android is not a single standard. There are many different types and versions, he says. That means companies have to develop HA apps and systems for each one, making development more complex. Apple also forbids anyone from using its protocols with Android. But Apple does work with hearing-aid developers and iphones seem to have the best functionality. All this technology does not come cheap. HAs cost from $2,500 to $7,000, depending on the level of technology, but this cost includes an audiologist who helps patients choose the proper device and helps fit and adjust it. Lyric, from Phonak, is sold as a subscription that costs about $2,400 per year, which includes up to seven replacements and visits with an audiologist. BACK TO TABLE OF CONTENTS TYPES OF HEARING AIDS THERE ARE FIVE BASIC TYPES of hearing aids, and which ones patients choose depends on their level of hearing impairment, their physiology and health, and their lifestyle. Behind-the-ear (BTE): These are the more traditional devices that consist of a durable plastic housing that rests behind the outer ear connected via a thin plastic tube that snakes into the ear canal. The tube carries sound from a receiver in the housing to the tympanic membrane in the ear, so there is no sound directionality. Its relatively large size lets it provide more amplification and larger batteries, so it is well-suited for those with severe hearing loss. They can also contain more processing power, so audiologists, the medical professionals who fit patients with hearing aids, have more options in tailoring the sound. BTE units can have amplification or gains of 35 to 82 db. In-the ear (ITE): These devices contain the microphone and speaker, as well as the battery and processor, and fit into the ear. Its size accommodates two microphones for directionality, as well as larger batteries, and makes it is easier to insert and remove for older persons whose hands aren t as nimble as they once were. ITE hearing aids provide 40 to 70 db of gain. In-the-canal (ITC): Smaller than ITE hearing aids, this fits directly into the canal. Being smaller makes is less noticeable, but it is only good for mild to moderate hearing loss. Users do, however, get better sound localization because the microphone takes advantage of the outer ear s sound-reflecting properties. ITC devices can amplify sound by 35 to 50 db. Completely-in-the-canal (CIC): The smallest of all hearing aids, CIC versions are almost invisible to others. There are also more compatible with talking on the phone and they, like ITC models, use the outer ear to reduce wind noise and help in sound localization. Its size limits battery capacity and how much amplification it can provide, but CIC devices do not need much amplification because they place the receiver so close to the tympanic membrane. They are uncomfortable for some patients due to their small ear canals and how deeply the HA must be inserted. It can also take several visits to the audiologist to get the device to remain comfortably in the ear. Bone anchored hearing aids (BAHA): Some patients suffering hearing loss have an occluded or non-existing ear canal, or their ear canal secretes too much wax. For these people, traditional hearing aids aren t an option. To get around their limitations, engineers have developed a hearing aid that picks up and amplifies sound, then sends it to the patient s hearing centers in the brain using bone conduction though a titanium anchor inserted in the skull just behind the ear. REGISTER: machinedesign.com 32

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