Sensors : Introduction

Transcription

1 GBM5320 Dispositifs Médicaux Intelligents Sensors : Introduction Mohamad Sawan, Professor Benoit Gosselin, and Louis-Francois Tanguay, Ph.D. Candidates Laboratoire de neurotechnologies Polystim mohamad.sawan@polymtl.ca M February 2008 Biomedical microsensors : Course outline Microsensors - Overview - Definitions Microsensors types: - Strain - Pressure - Displacement - Temperature - Gas (Electrode-based) - Chemical sensors (ISFET, CHEMFET) Biosensors Lab-on-chip technology GBM Dispositifs Médicaux Intelligents 2 Microsensors? Microsensors are small devices that convert physical or chemical signals to electrical signals. They enable objects to interface to the real world; Implantable microsensors enables monitoring biological parameters. They could allow real-time measurement of temperature, pressure, ph, oxygen and nitric oxide concentrations in vivo; They allow to help the medical research community in learning about the progression of diseases and assess degree of response to treatment; More & better access to measurement sites - Do not perturb the system under test - More accurate measurements and less invasive - Less psychological trauma & feedback More functionality, better portability, and lower cost. Pressure sensor Gas sensor GBM Dispositifs Médicaux Intelligents 3

2 Definitions Microsensor A microdevice that transforms a signal in measured/analyte format in an electrical signal. Direct sensor Signal to be measured is directly transformed to electrical signal. Example: photo-conductor converts light to change of resistance. Indirect sensor Signal to be measured is first converted to some other variable that is then converted to an electrical signal Example: acceleration sensor converts acceleration to strain which is then sensed. Biosensor A microsensor dedicated for medical implantable and cellular devices. GBM Dispositifs Médicaux Intelligents 4 Sensor Performance Characteristics Transfer Function: The functional relationship between physical input signal and electrical output signal. Sensitivity: The sensitivity is the ratio between a small change in electrical signal resulting from a small change in the physical signal to be measured. Dynamic Range: The range of input physical signals which may be converted to electrical signals by the sensor. Signals outside of this range are expected to cause unacceptably large inaccuracy. Linearity: The maximum deviation from a linear transfer function over the specified dynamic range. Accuracy: Generally defined as the largest expected error between actual and ideal output signals. Resolution: The resolution of a sensor is defined as the minimum detectable signal fluctuation. GBM Dispositifs Médicaux Intelligents 5 Sensor Performance Characteristics Hysteresis: Some sensors do not return to the same output value when the input stimulus is cycled up or down. The width of the expected error in terms of the measured quantity is defined as the hysteresis. Noise: All sensors produce some output noise in addition to the output signal. The noise of the sensor limits the performance of the system based on the sensor. Noise is generally distributed across the frequency spectrum. Bandwidth: All sensors have finite response times to an instantaneous change in physical signal. In addition, many sensors have decay times, which would represent the time after a step change in physical signal for the sensor output to decay to its original value. The reciprocal of these times correspond to the upper and lower cutoff frequencies, respectively. The bandwidth of a sensor is the frequency range between these two frequencies. GBM Dispositifs Médicaux Intelligents 6

3 Microsensors: General architecture A generalized architecture of a microsensor system: Inputs Sensor/Actuator Array Signal Conditioners ( Digital (Analog + Drivers Embedded Controller (Calibrate-measure, process & compress, store & forward) Comm. Interface GBM Dispositifs Médicaux Intelligents 7 Strain sensors - Resistive Resistance is related to length and area of cross-section of the resistor and resistivity of the material as By differentiating both sides, the equation becomes Piezoresistance Dimensional Strain gage component can be related by poisson s ratio (v) as Length Transfer Function : Input is strain, output is dr. Webster, Medical Instrumentation GBM Dispositifs Médicaux Intelligents 8 Strain sensors - Resistive Gage Factor of a strain gage G is a measure of sensitivity ε = dl/l Put mercury strain gage around an arm or chest to measure force of muscle contraction or respiration, respectively Used in prosthesis or neonatal apnea detection, respectively. Webster, Medical Instrumentation GBM Dispositifs Médicaux Intelligents 9

4 Piezoelectric Sensors What is piezoelectricity? Strain causes a redistribution of charges and results in a net electric dipole q = k f & V = q / C where q = charge, f = force k = 2.3 pc/n for quartz = 140 pc/n for Barium Different transducer applications: - Accelerometer, - Microphone. group27imaging.com/respiratorysensor.aspx GBM Dispositifs Médicaux Intelligents 10 Displacement Sensor - LVDT An LVDT (Linear Variable Differential Transformer) is used as a sensitive displacement sensor: for example, in a cardiac assist device or a basic research project to study displacement produced by a contracting muscle. Inductive displacement sensors: - Self inductance; - Mutual inductance; - Differential transformer. Signal Conditioning Electronics Muscle LVDT GBM Dispositifs Médicaux Intelligents 11 Capacitance-based Sensors Variable Area Mode Variable Dielectric Mode Differential Mode GBM Dispositifs Médicaux Intelligents 12

5 Acceleration sensor Accelerometer for displacement monitoring - Surface micromachined, capacitive sensor - Sensor + Electronics on same substrate= smart g Analog Devices ADXL-50 C1 C2 GBM Dispositifs Médicaux Intelligents 13 Pressure sensors Miniature Passive Pressure Transensor for Implanting in the Eye Measurement of intraocular and other physiological pressures. Displacement transducer contained in a small distensible pillbox. This passive resonant transensor absorbs energy from an oscillating detector coil outside of the animal at a frequency dependent upon the pressure in the eye. Collins Miniature Passive Pressure Transensor for Implanting, GBM Dispositifs Médicaux Intelligents 14 Pressure sensors 15 Flexible Wireless Passive Pressure Sensors for Biomedical Applications. The sensor consists of a cavity, bounded on 2 sides by capacitor plates interconnected with inductance. The value of capacitor change with pressure due to the deflectable diaphragm. This variation change the resonant frequency of the LC circuit and is measured wirelessly. Allen, GA Tech, PTFE = Polytetrafluoroethylene FEP = Fluorinated Ethylene Propylene Ceramic chamber GBM Dispositifs Médicaux Intelligents 15

6 Micromachined pressure sensors Pressure Sensor -Resistive / capacitive based measurements -Thin Silicon Membrane deforms with pressure -Piezoresistors change with strain induced by bending membrane -Packaging requires sealing to maintain pressure differential. High sensitivity capacitive strain sensor. GBM Dispositifs Médicaux Intelligents 16 Temperature sensors Temperature sensors have become common elements in wide range of modern integrated circuits The main parameters of temperature sensors are: temperature range, sensitivity, output range, linearity, accuracy Types of integrated temperature sensors: - Resistance based : Thermistors, RTDs - Thermocouples & CMOS PTAT references. GBM Dispositifs Médicaux Intelligents 17 Thermistors Thermistors are made from semiconductor material Generally, they have a negative temperature coefficient (NTC), that is NTC thermistors are most commonly used Ro is the resistance at a reference point (in the limit, absolute 0), B is material constant, and T and T0 are absolute and reference temperatures. Webster, Medical instrumentation GBM Dispositifs Médicaux Intelligents 18

7 Thermocouples A conductor generates a voltage when subjected to a temperature gradient. To measure this voltage, one must use a second conductor material which generates a different voltage under the same temperature gradient. So, Thermocouples measure temperature differences and need a known reference temperature to yield the absolute readings. When a pair of dissimilar metals are joined at one end, and there is a temperature difference between the joined ends and the open ends, thermal electromotive force (emf) is generated, which can be measured in the open ends. There are three major effects involved : the Seebeck, Peltier, and Thomson. Webster, Medical Instrumentation GBM Dispositifs Médicaux Intelligents 19 CMOS temperature sensor The voltage difference between the two diodes, operated at a different current density, is used to generate a Proportional To Absolute Temperature (PTAT) current. This voltage difference is PTAT with a temperature coefficient of mv/ C at room temperature. V BE 2!V BE1 = "V BE (T) = kt q ln # pi & E % (! kt $ I S2 ' q ln # I & E % ( = kt $ I S1 /r' q ln[ p ) r] Pertijs et al, Precision Temperature Measurement using, IEEE Sensors, v4, GBM Dispositifs Médicaux Intelligents 20 CMOS temperature sensor : Complete PTAT circuit V X! V Y V R1 = V Y!V Z " V X!V Z V R1 = V EB1!V EB 2 = kt q ln # A & 1 % ( $ A 2 ' I R1 = I 2 = V R 1 = 1! kt R 1 R 1 q ln " A % 1 $ ' # A 2 & The current mirrored at the output is PTAT: I PTAT = I 5 =W 5 /W 4 *I 2 GBM Dispositifs Médicaux Intelligents 21

8 ph Electrodes Glass electrodes develop a gel layer with mobile hydrogen ions when dipped into an aqueous solution; ph changes cause ion diffusion processes generating an electrode potential. Lithium-rich glasses are well suited for this purpose; The potential is measured in comparison to a reference electrode which is usually an Ag/AgCl system; The electric circuit is closed via a diaphragm separating the reference electrolyte from the solution. Sonnleitner, Bioanalysis and Biosensors for Bioprocess Monitoring, Springer, GBM Dispositifs Médicaux Intelligents 22 Oxygen Partial Pressure (po 2 ) electrode A membrane through which oxygen must diffuse separates the measuring solution from the electrolyte Oxygen is reduced by electrons coming from the central platinum cathode which is surrounded by a glass insulator. This design, a so-called polaro-graphic electrode, needs an external power supply. For oxygen, the polarization voltage is in the order of 700 mv and the typical current for atmospheric po 2 is in the order of 10 7 A. Sonnleitner, Bioanalysis and Biosensors for Bioprocess Monitoring, Springer, Clark-type oxygen partial pressure (po2) electrode GBM Dispositifs Médicaux Intelligents 23 Carbon Dioxide Partial Pressure (pco 2 ) electrode CO 2 diffuses through the membrane into or out of the electrolyte where it equilibrates with HCO 3 thus generating or consuming protons. The respective ph change of the electrolyte is sensed with a ph electrode and is logarithmically proportional to the pco 2 in the measuring solution. Sonnleitner, Bioanalysis and Biosensors for Bioprocess Monitoring, Springer, GBM Dispositifs Médicaux Intelligents 24

9 ISFET/CHEMFET sensors Ion-Sensitive Field Effect Transistors (ISFETS and CHEMFETs) are basically metal oxide semiconductor field-effect devices. The construction of an ISFET differs from the conventional MOSFET devices, in that the gate metal is omitted and replaced by a membrane sensitive to the ions of interest. Shepherd, Weak Inversion ISFETs Sensing, S&A B, v107, GBM Dispositifs Médicaux Intelligents 25 ph ISFET equivalent model ph-isfet Macromodel The drain current for the weak inversion ISFET in saturation is given by: Shepherd & Toumazou, Weak Inversion ISFETs for Ultra-Low Power Biochemical Sensing, Sensors and Actuators B (Chemical), v107, GBM Dispositifs Médicaux Intelligents 26 Classification of biosensors Ferrari et al, BioMEMS and Biomedical Nanotechnology: Vol IV: Biomolecular Sensing, Processing and Analysis, Springer, GBM Dispositifs Médicaux Intelligents 27

10 Biosensors A bioreceptor is a biological molecular species (e.g., an antibody, an enzyme, a protein, or a nucleic acid) or a living biological system (e.g., cells, tissue, or whole organisms) that utilizes a biochemical mechanism for recognition The sampling component of a biosensor contains a bio-sensitive layer. The layer can either contain bioreceptors or be made of bioreceptors covalently attached to the transducer. The most common forms of bioreceptors used in biosensing are based on: - Antibody/antigen interactions - Nucleic acid interactions - Enzymatic interactions - Cellular interactions (i.e. microorganisms, proteins) - Interactions using biomimetic materials (i.e., synthetic bioreceptors). GBM Dispositifs Médicaux Intelligents 28 Example : Glucose Sensors Enzymatic Approach Makes use of catalytic (enzymatic) oxidation of glucose The setup contains an enzyme electrode and an oxygen electrode and the difference in the readings indicates the glucose level. The enzyme electrode has glucose oxidase immobilized on a membrane or a gel matrix*. Glucose Plastic membrane Platinum electrode Glu coseoxidase Glu cose + O!!!!!!! " GluconicAcid + H O O2 H2O2 O Gluconic acid O2 Silver anode *In the enzyme electrode, when glucose is present it combines with O2, so less O2 arrives to the cathode. Webster, Medical Instrumentation GBM Dispositifs Médicaux Intelligents 29 Example : Glucose Sensors Affinity Approach (Optical) This approach is based on the immobilized competitive binding of a particular metabolite (glucose) and its associated fluorescent label with receptor sites specific to the metabolite and the labeled ligand. This change in light intensity is then picked up. Hollow dialysis fiber Excitation Emission Optical Fiber Glucose 3 mm Measure of glucose concentration by detecting changes in fluorescent light intensity caused by competitive binding of a fluorescein-labeled indicator. 0.3 mm Schultz et al, Affinity sensor : A new technique, Diabetes Care, GBM Dispositifs Médicaux Intelligents 30