Whitepaper: Precision Micro-Braiding for Implantable Devices

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1 Whitepaper: Precision Micro-Braiding for Implantable Devices Keys to Design Success When Contemplating Medical Device Braids By: Robert Kiefer and Keith Smith

2 Braided textile structures are among the most common fabric forming methods used in medical devices because of the unique properties that they exhibit. Braided textiles, in the form of sutures have been well exploited in applications such as wound closure. Sutures may be designed from a wide variety of polymeric materials specifically optimized for strength, visibility, intended longevity and tissue dynamics. More recently, modern medical device applications utilizing braided fabrics have leveraged unique properties of Nitinol wire in a device designed to close an atrial septal defect in the heart. While these fabrics have advanced the materials technology available to biomedical engineers and device designers, they fall short of maximizing the potential benefits of a braided architecture. In recent years, leading biomedical textile firms have pushed the boundaries of traditional braiding technology to give rise to precision micro-braiding. Simply put, micro-braiding is a significant increase in the density of a braided fabric while using ever finer metallic and/or polymeric filaments. The resultant structures enable medical device companies to have biomaterial components at their disposal having very unique physical and mechanical properties for next generation implantable devices. Braiding Overview and Engineering Features Braiding is defined as the intertwining of 3 or more filaments, commonly referred to as ends, in a diagonally overlapping pattern. Most braids utilize an even number of filaments resulting in a round braid while those created with an uneven number yields a flat braid. The filaments that are used in medical braiding include a wide range of biomaterials including polymers such as polyester, polypropylene, polytetrafluoroethylene, and a growing list of various resorbable materials. Metallic filaments and alloys such as Nitinol, stainless steel, Cobalt-Chromium, Platinum Figure 1- A variety of metallic and polymeric braided fabric and others are used as braid elements as well. The engineering behavior of the braid becomes a function of numerous variables that the skilled textile designer has control over. Central among these is the density

3 of the braid filaments and the amount of intersections they make in a given unit length. Many common braid machines have a fixed capacity of carriers that direct and tension the ends during the braiding process. Braiding machines that have an end capacity of at least 144 elements are thought to be high density and capable of micro-braiding. The ends are loaded onto braider carriers and are spaced uniformly around the circumference of the braiding machine with half rotating clockwise and the remainder moving counter-clockwise in an alternating over and under pattern. This undulating effect creates the braid pattern that can be manipulated to allow for closer packing of the filaments by creating fewer filament crossings. Engineers have utilized the unique geometry of braided structures to achieve certain physical and mechanical properties when used in implantable devices. Many braids are formed over a mandrel to create a hollow lumen with a specific crosssectional shape and size. In hollow lumen and flat braids, another important variable is the braid angle. The alpha angle is the measurement of the included filament angle against the central axis of the braid. This Figure 2- Visual depiction of alpha and beta braid angle angle cannot be directly measured, unless there is a visible central axis element. The beta angle can be directly measured, as it is the angle between two intersecting filaments in the braid structure. The axial density of the braid can be changed to alter the braid angle, and the braid angle can be manipulated to demonstrate a braid s most interesting geometric feature, foreshortening. Many are familiar with the finger trap style of a child s toy whereby fingers become locked inside the toy and are unable to be removed because pulling increases inward tension on the fingers. In cylindrical, hollow-lumen braids, the ends combine to form an interdependent system that results in axial forces being translated to radial compressive forces. The inverse is also true. And so the trick of a finger-trap is to push from both ends to release your finger.

4 This engineering feature can be used in many ways in medical device design. Dynamic behaviors can be engineered that cause the structure to collapse radially for easy loading into a catheter for minimally invasive delivery. Once delivered to the correct location by a surgeon, a braided device can be unsheathed or actuated to recover its original size, effecting an in situ shape transformation not possible with many other types of implants. It is useful to explore how these braid mechanics can be utilized in high-density micro-braiding applications. Micro-Braiding in Implantable Devices The advent of micro-braiding, ultra-high density braids made from very fine wire and polymeric filaments, opens up a new realm of possibilities for implantable device design. It is now possible to produce fabrics with very small pore-sizes, essential in cardiovascular and neurovascular applications, where dense fabrics are required to slow down or stop turbulent blood flow. Another critical advantage of micro-braiding, is the use of filaments that often have a diameter of 25 microns and below. The use of these very small filaments allows for braided implants to be necked down into very small catheters, often 10F and below, and delivered through tortuous vasculature until they are deployed at the treatment site. One such surgical procedure that has been enabled through the use of microbraiding is the treatment of cerebral vascular aneurysms. Many techniques have been developed and commercialized for this application including aneurysm clipping and Platinum coil placement inside of the aneurysm sac. A recent technique for diverting the main flow of blood away from the weakened vessel wall leverages the use of a micro-braid. Tiny Nitinol filaments as small as in diameter are braided together, with end counts ranging from 144 to 288, over small diameter mandrels to create a dense metal tube that has stent-like properties. This dense mesh structure slows down blood flow and assists in facilitating clot development in the aneurysm, effectively sealing the aneurysm. The foreshortening principle of the braid geometry allows for the implant to be delivered in a low profile catheter and expanded radially once in the proper position, over the mouth of the aneurysm. The braid is oversized for the vessel that it is deployed in and relies on the radial forces generated by the helical orientation of the Nitinol wires in the fabric s architecture. The resulting microbraid is a permanent filter screen that facilitates the formation of a clot to reduce the risk of the aneurysm s rupture.

5 Another application for state of the art micro-braids comes from the ever-expanding use of transcatheter heart valve devices for aortic (TAVR) and mitral (TMVR) valve repair and replacement. A wide range of implantable fabrics are commonly used as textile skirt materials on heart valves; typically to facilitate tissue ingrowth and prevent para-valvular leakage. However, micro-braided structures have found a specialty niche during the surgical procedure used to implant some types of transcatheter heart valves. In some heavily calcified heart valves, a large amount of vulnerable plaque debris is at an increased risk of embolization during the actual TAVR implantation procedure. Some device designers have incorporated a micro-braided filter to be deployed distally from the implantation site. As before, the braid is engineered to utilize a large quantity of fine filaments to create a mesh or filter like effect. This mesh is designed to catch large particles or emboli that are sloughed off during the procedure while simultaneously remaining porous enough to allow the unrestricted passage of blood. After the implant procedure is complete, the micro-braid is necked down, using the foreshortening principle, re-sheathed and captured in a catheter for removal from the patient. Some studies have found that this highly engineered braided device is essential to preventing in-procedure strokes for high-risk patients. All micro-braided medical devices need not be porous to be effective in their end application. There are emerging device concepts in the sports medicine field that are capitalizing on micro-braiding techniques to create interdependent layers of fabric as a means to spread axial forces over a larger surface area. Through novel braided fabric design, small filaments of ultra-strong polymers can be braided in layers, concentrically, over top one another to create a core-sheath effect. This enables an increasing bundle of fibers to completely envelop the previous layer creating compressive forces through an over-braiding technique. When this technique is applied, it is possible to create a braid system that translates axial loads to the inner layers of the braid system. The use of very fine filaments at very high end counts, enables a significant amount of load bearing elements to be placed into the system. The resultant fabric system is able to withstand a significant amount of force due to an internal truss system whereby the braid layers reinforce and buttress the adjoining braids. This type of construct has application in tendon and ligament repair as well as joint reconstruction and offers an alternative in procedures where an allograft or autograft is typically utilized.

6 These few examples are but the beginning of the areas in which micro-braided structures have been explored in medical device design. There are a myriad possible combinations of braid geometry, alpha angles, end counts, raw materials and filament orientations that can be engineered to meet the significant challenges of designing a textile for implant. Often times, it is of benefit to the medical device designer to find a skilled team of engineers that understand the significance of the device challenges and possess the requisite skills to deliver the proper textile system. Biomedical Engineering and Textile Engineering Converge In the current medical device environment, it is well understood that speed is an important factor in designing and commercializing new products. Device companies can ill afford to select vendors that are unfamiliar with the plethora of challenges the industry is facing today. Qualified suppliers will also be knowledgeable about FDA requirements for selecting & qualifying raw materials for medical use. Their engineers will have extensive prior experience working with biomedical engineers to develop components for medical devices. The quality system will be certified to meet ISO requirements. A streamlined approach to vendor selection in this field is to engage the services of firms who are medical companies first and textile companies second. It is critically important that your design and technology partner is fluent in understanding the biological environment and anatomical context of the device. Bilateral communication is critical in a supply relationship where iterative design is part of the development process. In all biomedical textile processes, there is a convergence of many layers of complexity. This includes raw materials control, their quality and cleanliness and the stability of their supply for the long run. The complexities further include the fabric structure and its mechanical performance, which extends to how the component interacts in the final device. Figure 3- Micro-Braiding technology at US BioDesign

7 Navigating these challenging waters is a requirement for all device designers, but doing so with an effective partner streamlines the process. The convergence of biomedical engineering and textile engineering exists at US BioDesign, Inc. The company was formed by a group of seasoned medical device and textile industry executives who sought to launch a customer first medical company that involves the customer at every step. A team has been assembled that excels in working directly with customers as an extension of their engineering group, to facilitate a product development process that moves smoothly and swiftly. By removing the historical, natural barriers that many vendors put in place, such as restricting access to the manufacturing floor, US BioDesign is positioned to freely collaborate toward the mutual end goal of a successful device product launch. This open collaboration has proven vital in developing novel micro-braid products for US BioDesign s customers. Due to the criticality of the implantable devices used in these advanced applications, an intricately engineered biomedical textile is required. US BioDesign has assembled the team, the processes, and the advanced technology to design medical device components for all stages of the product lifecycle. For more information on how US BioDesign can accelerate your next medical device development project, please contact us at