tutorial transductores

GBM5320GBM5320Dispositifs Mdicaux IntelligentsDispositifs Mdicaux Intelligents

Sensors : IntroductionSensors : IntroductionMohamad Sawan, Professor

Benoit Gosselin, and Louis-Francois Tanguay, Ph.D. CandidatesLaboratoire de neurotechnologies Polystim

http://www.cours.polymtl.ca/gbm5320/[email protected]

M5418

9 February 2008

GBM5320 - GBM5320 - Dispositifs Mdicaux IntelligentsDispositifs Mdicaux IntelligentsGBM5320 - Dispositifs Mdicaux Intelligents 2

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


Microsensors?

Microsensors are small devices that convert physicalor chemical signals to electrical signals. They enableobjects to interface to the real world;

Implantable microsensors enables monitoringbiological parameters. They could allow real-timemeasurement of temperature, pressure, pH, oxygenand nitric oxide concentrations in vivo;

They allow to help the medical research community inlearning about the progression of diseases and assessdegree 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 sensorhttp://www.nist.gov/public_affairs/techbeat/tb2003_0910.htm


Definitions

MicrosensorA microdevice that transforms a signal in measured/analyte format inan electrical signal.

Direct sensorSignal to be measured is directly transformed to electrical signal.Example: photo-conductor converts light to change of resistance.

Indirect sensorSignal to be measured is first converted to some other variable that isthen 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.


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 inelectrical signal resulting from a small change in the physical signal tobe measured.

Dynamic Range: The range of input physical signals which may beconverted to electrical signals by the sensor. Signals outside of thisrange are expected to cause unacceptably large inaccuracy.

Linearity: The maximum deviation from a linear transfer function overthe specified dynamic range.

Accuracy: Generally defined as the largest expected error betweenactual and ideal output signals.

Resolution: The resolution of a sensor is defined as the minimumdetectable signal fluctuation.


Sensor Performance Characteristics

Hysteresis: Some sensors do not return to the same output valuewhen the input stimulus is cycled up or down. The width of theexpected error in terms of the measured quantity is defined as thehysteresis.

Noise: All sensors produce some output noise in addition to theoutput signal. The noise of the sensor limits the performance of thesystem based on the sensor. Noise is generally distributed across thefrequency spectrum.

Bandwidth: All sensors have finite response times to aninstantaneous change in physical signal. In addition, many sensorshave decay times, which would represent the time after a step changein physical signal for the sensor output to decay to its original value.The reciprocal of these times correspond to the upper and lowercutoff frequencies, respectively. The bandwidth of a sensor is thefrequency range between these two frequencies.


Microsensors: General architecture

A generalized architecture of a microsensor system:

Sensor/ActuatorArray

Signal Conditioners(Analog + Digital)

Embedded Controller(Calibrate-measure, process & compress, store & forward)

Drivers Comm.Interface

Inputs


Strain sensors - ResistiveResistance is related to length and area of cross-section of theresistor and resistivity of the material as

By differentiating both sides, the equation becomes

DimensionalPiezoresistance

Strain gage component can be related by poissons ratio (v) as

LengthTransfer Function : Input is strain, output is dR.

Webster, Medical Instrumentation


Strain sensors - Resistive

Gage Factor of a strain gage

G is a measure of sensitivity

Put mercury strain gage around an arm orchest to measure force of musclecontraction or respiration, respectively

Used in prosthesis or neonatal apneadetection, respectively.

= dL/L

Webster, Medical Instrumentation www.microstrain.com/


Piezoelectric Sensors

What is piezoelectricity ?

Strain causes a redistri-bution of charges andresults in a net electricdipole

where q = charge, f = force

k = 2.3 pC/N for quartz = 140 pC/N for Barium

Different transducerapplications:- Accelerometer,- Microphone.

q = k f & V = q / C

www.ipodlinux.org/group27imaging.com/RespiratorySensor.aspx


Displacement Sensor - LVDT

LVDT

www.pages.drexel.edu/~pyo22/mem351-2004/lecture04/pp062-073lvdt.pdf

An LVDT (Linear Variable Differential Transformer) is used as asensitive displacement sensor: for example, in a cardiac assist deviceor a basic research project to study displacement produced by acontracting muscle.

Signal ConditioningElectronics Muscle

Inductive displacement sensors:- Self inductance;- Mutual inductance;- Differential transformer.


Capacitance-based Sensors

DifferentialMode

Variable Dielectric ModeVariable Area Mode


Acceleration sensor

Accelerometer fordisplacement monitoring- Surface micromachined,

capacitive sensor- Sensor + Electronics on same

substrate= smart

Analog Devices ADXL-50

C1 C2

g


Pressure sensors

Collins Miniature Passive Pressure Transensor for Implanting, 1967.

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 outsideof the animal at a frequency dependent upon the pressure in the eye.


15

The value of capacitorchange with pressuredue to the deflectablediaphragm. Thisvariation change theresonant frequency ofthe LC circuit and ismeasured wirelessly.

Pressure sensors

Allen, GA Tech, 1999-2002.

PTFE = PolytetrafluoroethyleneFEP = Fluorinated Ethylene Propylene

Ceramic chamber

Flexible Wireless Passive PressureSensors for Biomedical Applications.

The sensor consists of a cavity,bounded on 2 sides by capacitorplates interconnected with inductance.


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.

www.dolphin.fr/flip/mems/mems_cps.htmlwww.memstouch.net/

High sensitivity capacitivestrain sensor.


Temperature sensors

Temperature sensors have become common elements in wide range ofmodern 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.

www.singleiteration.com/


Thermistors

Thermistors are made fromsemiconductor material

Generally, they have a negativetemperature coefficient (NTC), that isNTC thermistors are most commonlyused

Ro is the resistance at a reference point(in the limit, absolute 0), B is materialconstant, and T and T0 are absolute andreference temperatures.

Webster, Medical instrumentation


Thermocouples

A conductor generates a voltage when subjected to a temperature gradient. Tomeasure this voltage, one must use a second conductor material whichgenerates a different voltage under the same temperature gradient. So,Thermocouples measure temperature differences and need a known referencetemperature to yield the absolute readings.

When a pair of dissimilar metals are joined at one end, and there is atemperature difference between the joined ends and the open ends, thermalelectromotive force (emf) is generated, which can be measured in the openends. There are three major effects involved : the Seebeck, Peltier, andThomson.

Webster, Medical Instrumentationwww.efunda.com/.../images/thermocouple_A.gif


CMOS temperature sensor

VBE 2 !VBE1 = "VBE (T)

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The voltage difference between the two diodes, operated at a differentcurrent density, is used to generate a Proportional To AbsoluteTemperature (PTAT) current.

This voltage difference is PTAT with a temperature coefficient of+0.085 mV/C at room temperature.

Pertijs et al, Precision Temperature Measurement using ,IEEE Sensors, v4, 2004.


IPTAT = I5 =W5/W4*I2

CMOS temperature sensor : Complete PTAT circuit

The current mirrored at theoutput is PTAT:

VX!V

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VR1 =VY !VZ "VX !VZ

VR1 =VEB1 !VEB 2 =kT

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pH Electrodes

Glass electrodes develop a gel layerwith mobile hydrogen ions when dippedinto an aqueous solution;

pH changes cause ion diffusionprocesses generating an electrodepotential. Lithium-rich glasses are wellsuited for this purpose;

The potential is measured in comparisonto a reference electrode which is usuallyan Ag/AgCl system;

The electric circuit is closed via adiaphragm separating the referenceelectrolyte from the solution.

Sonnleitner, Bioanalysis and Biosensors for Bioprocess Monitoring,Springer, 1999.


Oxygen Partial Pressure (pO2) electrode

A membrane through which oxygenmust diffuse separates the measuringsolution from the electrolyte

Oxygen is reduced by electronscoming from the central platinumcathode which is surrounded by aglass insulator.

This design, a so-called polaro-graphicelectrode, needs an external powersupply.

For oxygen, the polarization voltage isin the order of 700 mV and the typicalcurrent for atmospheric pO2 is in theorder of 107 A.


Clark-type oxygenpartial pressure (pO2)

electrode


Carbon Dioxide Partial Pressure (pCO2) electrode

CO2 diffuses through the membrane into or out of the electrolyte whereit equilibrates with HCO3 thus generating or consuming protons.

The respective pH change of the electrolyte is sensed with a pHelectrode and is logarithmically proportional to the pCO2 in themeasuring solution.



Ion-Sensitive Field Effect Transistors (ISFETS and CHEMFETs) arebasically metal oxide semiconductor field-effect devices.

The construction of an ISFET differs from the conventional MOSFETdevices, in that the gate metal is omitted and replaced by amembrane sensitive to the ions of interest.

ISFET/CHEMFET sensors

www.sentron.nl/nieuw/index.php?id=4Shepherd, Weak Inversion ISFETs Sensing , S&A B , v107, 2005.


pH ISFET equivalent model

The drain current for the weak inversion ISFET in saturation is given by:

Shepherd & Toumazou, Weak Inversion ISFETs for Ultra-Low PowerBiochemical Sensing , Sensors and Actuators B (Chemical), v107, 2005.

pH-ISFET Macromodel


Classification of biosensors

Ferrari et al, BioMEMS and Biomedical Nanotechnology: Vol IV:Biomolecular Sensing, Processing and Analysis, Springer, 2006.


Biosensors

A bioreceptor is a biological molecular species (e.g., an antibody, anenzyme, a protein, or a nucleic acid) or a living biological system (e.g.,cells, tissue, or whole organisms) that utilizes a biochemicalmechanism for recognition

The sampling component of a biosensor contains a bio-sensitive layer.The layer can either contain bioreceptors or be made of bioreceptorscovalently attached to the transducer.

The most common forms of bioreceptors used in biosensing are basedon:- Antibody/antigen interactions- Nucleic acid interactions- Enzymatic interactions- Cellular interactions (i.e. microorganisms, proteins)- Interactions using biomimetic materials (i.e., synthetic bioreceptors).


Example : Glucose Sensors

Enzymatic Approach

Glu e O GluconicAcid H OGlu eOxidase

coscos

+ ! "!!!!!! +2 2 2

Makes use of catalytic (enzymatic)oxidation of glucose

The setup contains an enzymeelectrode and an oxygen electrode andthe difference in the readings indicatesthe glucose level.

The enzyme electrode has glucoseoxidase immobilized on a membrane ora gel matrix*.

Platinumelectrode

Plasticmembrane

Glucose

O2

Gluconicacid

Silveranode

O2

H2O2O2

*In the enzyme electrode, when glucose is present it combines with O2, so less O2 arrives to the cathode.

Webster, Medical Instrumentation


Example : Glucose SensorsAffinity Approach (Optical)

This approach is based onthe immobilized competitivebinding of a particularmetabolite (glucose) and itsassociated fluorescent labelwith receptor sites specific tothe metabolite and thelabeled ligand. This changein light intensity is thenpicked up. 3 mm

0.3 mm

Hollow dialysis fiber

Excitation

Emission

OpticalFiber

Glucose

Schultz et al, Affinity sensor : A new technique, Diabetes Care, 1982.

Measure of glucose concentration by detecting changes in fluorescent lightintensity caused by competitive binding of a fluorescein-labeled indicator.

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