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Introduction to Biomedical Engineering

Kung-Bin Sung5/21/2007

Biomedical sensors

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Outline• Chapter 9: Biomedical sensors

– Biopotential measurements– Physical measurements– Chemical measurements

• Blood gases and pH sensors– Biosensors

• Affinity biosensors: enzymatic and antibody-based

• Direct and indirect detection• Fiber optic biosensors

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Introduction• Transducer is a device that converts energy

from one form to another• In sensors, a transducer converts an observed

change into a measurable signal• Integrated with other parts to “read” out the

signal (electrically, optically, chemically)• Some are used in vivo to perform continuous,

invasive or non-invasive monitoring of critical physiological variables

– pressure, flow, concentration of gas• Some are used in vitro to help clinicians in

various diagnostic procedures– electrolytes, enzymes, metabolites in blood

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Introduction• in vivo: inside a living body (human or animal)• ex vivo: outside the living body• in vitro: in a test tube• in situ: right in the place where reactions

happen (could be in the cells, tissue, test tube, etc.)

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Physical measurements

• Displacement– Inductive– Resistive– Capacitive– Ultrasonic

• Air flow• Temperature

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Displacement transducersLinear Variable Differential Transformer (LVDT)

Induced voltage ∝ Displacement

The primary coil P is excited by an AC currentThe induced potentials at the 2 secondary coils are canceled due to the opposite polaritiesWhen the core moves toward one coil, the induced potential in the coil increases and the voltage in the other coil decreases

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Displacement transducers

Potentiometer: resistance is proportional to positionIf the current through the resistor is constant, the displacement (linear or angular) will be proportional to

RIVO ∆=∆

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Displacement transducers

llS ∆

=

AlR ρ=

2//

=∆∆

=llRRG

Strain gauge: measures a small change in the length of an object as a result of an applied force

Resistance of a conductive materiel with length l and cross-sectional area A. ρ is a constant (resistivity)

The fractional change in length of an object is called strain

Now consider a metal wire as a strain gauge

Stretched

Original

l+∆l

lR

ll

Al

llAlllR ∆

≈−∆+

∆+=∆ 2

)/(ρρ

The gauge factor G

For silicon strain gauges, G > 100 (much more sensitive than metal)

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Displacement transducersStrain gauge examples

Bonded to a semiflexiblebacking material

Unbonded configuration for measuring blood pressure

The change in resistance is quite small ⇒ amplifiersStrain gauges are very sensitive to temperature ⇒ temperature compensation

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Displacement transducersCapacitive: change in distance between two parallel plates (an insulating material sandwiched in the middle) results in a change in capacitance

dAC rεε 0=

where ε0 is the permittivity of vacuum = 8.85×10-12 F/mεr is dielectric constant of the insulating material

A: aread: distance between two conductors

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Displacement transducersPiezoelectric transducer: certain crystal (such as quartz) generates a small electric potential when it is mechanically strained. Thecharge Q induced at surface is proportional to the applied force F

CQV =

kFQ = Then the voltage across the crystal is

Not used for measuring DC (static) displacement due to internal leakage resistance that dissipates the surface chargeThe inverse reaction can also happen ⇒ apply an voltage across the crystal, and the crystal expands or contractsIn fact, piezoelectric transducers are often used to generate high frequency vibrations such as ultrasonic pulse transducers and resonant oscillators

(k is a constant and specific to the material)

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Air flow

P∆∝Flow rate

The screen obstruction provides some resistance to the air flow and therefore generates pressure drop across the screen

Fleish pneumotachometer

Pressure is measured at both sides of the resistive screen

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Blood flow

dVqqEqvB ==

)( BvqFrrr

×=

dVE =

rF

B E

Apply a uniform magnetic field B across blood vessel

If velocity of blood flow is v, F is force experienced by charged particles in bloodThis force causes movement of charges ⇒ distribution of charges generates an electric field E

(blood flows into the screen)

d: diameter of blood vessel

BdVv =⇒

For charged particles, there is a second force qE, at equilibrium:

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Temperature transducer - thermocoupleThermocouple: two different metal wires welded together

Seebeck (1821) effect: - Thermal to electrical- An electromotive force (emf) exists across the junction and is temperature dependent

)( 21 TTI −∝ when the temperature difference is small

If we use two such junctions, one is at a known temperature and the other is at the sample

Nickel

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Temperature transducer - thermocoupleThe reference is typically in ice bath ⇒ 0°C

Thermocouples:- very rapid response- stable over wide range of temperatures (up to hundreds or even 1000)- can be made quite small ⇒ in 30 gauge needle (outside diameter = 0.3mm)- disadvantage: need a reference (ice bath)

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Temperature transducer – thermistorThermistor: semiconductor; the resistance is a function of temperature )/1/1(

00TT

T eRR −= β

where R0 is the resistance at a reference temperature, T0, and RT is the resistance at temperature T. β is a material-specific constant. Both temperatures are expressed in degrees K

The size and mass of a thermistor probe must be small to produce a rapid response to temperature variations

Common shapes of thermistors

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Temperature transducer – thermistorThermistors:- have high sensitivity (<<1°C)- range is not as great as thermocouples (-50°C – 100°C), but suitable for biological/physiological measurements- need calibration (R vs. temperature curve)- can also be made very small

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Cardiac output measured by thermodilution

- Inject cold saline into the right atrium (intravenous catheter)- Measure temperature at the pulmonary artery over time- Conservation of energy: The total heat content of the injected saline will be

∫ ∆⋅=∆T

bbbiiii dttTflowccTV0

)(ρρ

Average blood flow (m3/s)

ρ: density (kg/m3)c: specific heat capacity (J/kgK)Vi: injection volumeT: temperature

Catheter with multiple ports

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Chemical measurementsImportant analytes and their normal ranges in blood, which indicate the physiological status of the body: gas pressure and related parameters, electrolytes, and metabolites

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Electrochemical cells as sensors- Typically used for measurements of ion or gas-molecule concentrations- The sensor consists of two electrodes and an ionic conductive material called an electrolyte- Operation of electrochemical sensors is based on reactions and their equilibrium at the electrode surfaces (interface between electronic and ionic conductors)

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Blood oxygen measurement

−+ +↔ eAgAg

Measuring arterial blood gases pO2: in operating room and intensive care unit to monitor respiratory and circulatory condition of a patient

Clark electrode: measures partial pressure of O2

↓↔+ −+ AgClClAg

bias voltage ~0.7 VMeasured current ∝concentration of O2−− ↔++ OH4e4OH2O 22

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Transcutaneous pO2 measurement

Mostly used on newborn babies in the ICU because their skin is thinner

The skin is heated to 43°C to increase local blood flow and enhance diffusion of O2 through the skin

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Blood oxygenationOxygen saturation (% of oxygenated hemoglobin) can be measured and used to represent blood oxygenationRelationship between arterial blood oxygen saturation and partial pressure of O2

Note different curves at different pH values

mm Hg

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Oxygen saturation[ ]

[ ] [ ] %100HbOHb

HbO

2

2O2

×+

=S

Oximetry: (color) measures light absorbance at one wavelength where there is a large difference between Hb and HbO2 and at another wavelength (or more wavelengths)

Saturation of O2

Absorbance

)()(S

2

1O2 λ

λAAyx +=

A: absorbancex and y are constants depending on optical properties of blood

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Oxygen saturationPulse oximetry: use the pulsatile (AC) component to extract oxygen saturation information and the non-pulsatile (DC) signal as a reference for normalization

Change in arterial blood volume associated with periodic contraction of the heart

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pH measurements

K+⋅= + ]H[log059.0V 10

Membrane permeable to H+ is located at the tip of the active electrodeThe potential across the membrane (Nernst equation)

]H[logpH 10+−=

K is a constant

ion-selective membrane The glass membrane is

highly selective to H+

V

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pCO2 measurements

−+ +↔↔+ 33222 HCOHCOHCOOHFor pCO2 measurement

)PCO(][CO 22 a= a is a constant

23 COlogloglog]log[HCOpH Pak −−−= −

It can be shown that

Linear relationship over range of 10-90 mmHg

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Biosensor – introduction• Biosensor: is a sensor using a living

component or a product of a living thing for measurement or indication. Generally a biosensor consists of two parts

– A biological recognition element (enzyme, antibody, receptor) to provide selectivity to sense the target of interest (referred to as the analyte)

– A supporting element which also acts as a transducer to convert the biochemical reaction into “signal” that can be read out

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Biosensor – introduction“Receptor” provides selective molecular recognitionFor example: enzymes, antibodies, receptors, nucleic acids, polypeptides, etc.

Transducer types in biosensors: calorimetric, electrochemical, optical, etc.

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Enzymatic biosensors

ES + ES

Enzymes are catalysts for biochemical reactions; large macromolecules consisting mostly of protein, and usually containing a prosthetic group (metals)Substrate: the target molecule of an enzymatic reaction

PE +

S: substrate; E: enzyme; ES: enzyme-substrate complex;P: product

Reaction rate vs. substrate concentration at constant enzyme concentration

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Enzymatic biosensors• Advantages

– Highly selective– Enzymes are catalytic, thus improving the

sensitivity– Fairly fast

• Disadvantages– Expensive: cost of extraction, isolation and

purification– Activity loss when enzymes are immobilized– Enzymes tend to be deactivated after a

relatively short period of time

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Enzymatic biosensors- In an enzymatic biosensor, the enzyme is immobilized as the “receptor”- Enzymes are specific to their substrates which can be the analyte

Example: glucose sensors

22oxidase glucuse

22 OHacid gluconicOHOGlucose +⎯⎯⎯⎯ →←++

permeable to O2 only

Glucose O2

Measures the amount of O2 consumption

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Enzymatic biosensors

−+ ++→ e2O2HOH 222

Alternatively, the reaction product hydrogen peroxide can also be measured through reduction of H2O2

Use a Clark-type electrochemical cell similar to the one for measuring O2, but at a positive bias voltage of 0.65V

Problems: 1. The reduction/oxidation potentials are fairly high ⇒ other materials will interfere2. The concentration of O2 needs to be kept constant, which is difficult since O2 is involved in the reactions

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Electron mediators in enzymatic biosensors

−+ ++→ e2O2HOH 222

22oxidase

2 OHPOS +⎯⎯ →⎯+In the membrane:

At the anode:

Oxygen is consumed and later regenerated, therefore it only serves to transfer electron ⇒ can be replaced by a mediator

Example: multi-cycle chemical electrochemical reaction chain for detecting glucose

GO: glucose oxidaseM: mediator

(ox)(red)

(ox)

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Immunosensors

Structure of antibody

Immunosensors: based on highly specific antigen-antibody reactions

Antibody: works against the foreign bodyImmunoglobin: globular proteins that participate in the immune response

The Fc end of an antibody can bind to other immune cells and the binding activate these immune cells

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Immunosensors

Direct detection: the binding of the analyte (antigen) to the “receptor”(antibody) is detected directly through the presence of the analyte

However, most are indirect due to lack of signal from the analyte itself

Detection of the reaction (binding/association) can be either direct or indirect (labeled)

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ImmunosensorsIndirect detection: signal is provided by an exogenous reporter

competitive sandwich

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Immunosensors

[ ][ ][ ]BA

ABkkK

d

aA ==

The interaction between antigen and antibody can be expressed as

[ ][ ][ ]AB

BAkkK

a

dD ==

BA+ ABka

kb

A: free antigen; B: free antibodyka: association rate constant; kb: dissociation rate constant

At equilibrium, the affinity constant (or association constant)

And the dissociation constant

(units M-1)

(units M)

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ImmunosensorsAntigen-antibody interactions are reversible and the interaction half-life is determined by the magnitude of the kinetic rate constant

Dissociation rate constant kd and half-life of an antigen-antibody binding complex

Kinetic rate constants of antigen-antibody interactions can be obtained with biosensors based on surface plasmon resonance

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ImmunosensorsThe surface density of antigen molecules bound to the sensor vs. the bulk concentration of antigen

K: binding constant of the antibody-antigen interactionΓmax: maximum surface density of antigen molecules

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Indirect detection• Radioimmunoassay (RIA)

– The labels are radioactive isotopes (for example 131I)– Highly sensitive and specific but inconvenient and

expensive– Less popular now due to safety issues

• ELISA (Enzyme-Linked Immunosorbent Assay)– Enzymes are used for labeling and signal is provided by

the reaction product– Simple in terms of procedure and equipment

• Fluorescent immunoassay– Labeled with fluorescent dyes

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ELISA

Capture antibody

analyte

detection antibody

secondary detection antibody

substrate and

product

Typically colorimetric measurement (absorbance at certain wavelength ∝ analyte concentration)

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Direct detection – Surface plasmon resonanceA surface plasma wave (SPW) is an electromagnetic wave (due to charge density oscillation) which propagates along the boundary between a dielectric (e.g. glass) and a metal

The magnitude of the surface plasmon wave decreases exponentially into both dielectric and metal media

Electric field is parallel to the plane of incidence

Magnetic field parallel to the interface

n1

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SPR biosensorsA surface plasmon wave can be excited optically: when the propagation constant β of the incident light and the SPW are equal in magnitude and direction for the wavelength

This corresponds to a certain angle of incident light at a fixedwavelength ⇒ the reflected light reaches its minimum

Binding induced change in propagation constant of SPW is proportional to the refractive index change in the sample ⇒ the angle of incident light also changes

reflectivity

Incident light with various angles

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SPR biosensorsDifferent parameters are measured in SPR biosensors to indicate the refractive index change in the sample (due to binding)

Angle of incident light Wavelength of incident light

Using monochromatic light Using fixed incident and reflection angles

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SPR biosensorsThe kinetics of binding events can be monitored continuously -sensorgram

BSA: bovine serum albumin, used to non-specific bindingAnalyte: SEB (staphylococcal enterotoxin B)a-SEB: antibody against SEB ⇒sandwich binding

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SPR biosensors• Direct detection ⇒ no need for labeling• High sensitivity• Fast ⇒ real-time monitoring• Used with sample in liquid ⇒ important

for biological samples• Relatively simple device, which makes

multichannel parallel detection easier ⇒high throughput

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SPR commercialization - BIACOREBIACORE optical system: light from different angle of reflection is imaged onto different position of a photo-detector array

Multiple flow-cells ⇒ simultaneous detection of reflection intensity

2D photo-detector array

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BIACORE products

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BIACORE features• Specificity: different molecules interact with a

single partner immobilized on a sensor surface• Kinetics: rates of complex formation (ka) and

dissociation (kb)• Affinity: the level of binding at equilibrium (KD,

KA)• Concentration: can be determined for purified

molecules or for molecules in complex mixtures such as serum (needs a calibration curve)

• Multiple interactions during complex formation

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Fiber optic based biosensorsOptical fibers are flexible and compact which are important features for applications that need miniaturization such as in vivomeasurements

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Optical fiberTypically made of glass SiO2, and consisting of a core and a cladding layerTransmits light through the core (total internal reflection)Diameter can be very small ~µm ⇒ very flexibleThe angle of light rays that go into the fiber core and can be accepted (transmitted) by the fiber is dependent on the core and cladding materials

NA of an optical fiber = sinθ = 22

21 nn −

n1

n2

θθt

Incident angle of light rays going into a fiber must be less than θ

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Optical fiber sensor – examples

Total internal reflection inside the fiber

• Sensing area located at distal end surface of an optical fiber

• Sensing area along side of an optical fiber

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Optical fiber sensor – examplesA fiber optic (imaging) bundle is used for detecting multiple analytes simultaneously- Each latex sphere can specifically binds to one analyte- The analytes can be fluorescently labeled- Use of reference channels could increase the reliability of detection (e.g. spheres without recognition element still bind to some analyte molecules non-specifically, so the measured fluorescence from the reference spheres is considered “background” and should be subtracted)

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Trends in biomedical sensors

Centralized laboratory Point of care

Patients’ home

in vitro in vivo

Key technologies for miniaturization of biomedical sensors and realization of biochips: microfluidics, semiconductor fabrication processes, microelectromechanical systems (MEMS)Equally important are: the development of more stable and effective biomolecules used for recognition; search for new target analytes that are of diagnostic and/or biological significance

Single test Multiple tests in a single run

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Biochips

• Miniaturized and highly integrated• Provide analytical or diagnostic function• Substrates can be glass, silicon, polymer,

plastic, etc• Desired features: high sensitivity and

specificity, high speed, small size, small sample requirement, multi-channel and parallel analyzing, reliable, cost-effective

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References• Medical Instrumentation: Application and

Design, edited by John G. Webster• Chemical sensors and biosensors, by Brian R.

Eggins• Sensors in Biomedical Applications:

Fundamentals, Technology & Applications, by Gábor Harsányi

– Ch7: Biosensors• Information about glucose meters

http://www.diabetesmonitor.com/meters.htm#fcnim

• Biomolecular sensors, edited by Electra Gizeli& Christopher R. Lowe