Introduction to Biomedical Sensors

1. Introduction to Biomedical Sensors BME 301 BiomedicalSensors LectureNote 1 BME 301 Biomedical Sensors - Ali Işın 2014

2. What is a Biomedical Sensor? • Any instrumentation system can be described as having three fundamental components: • a sensor, • a signal processor, • and a display and/or storage device. BME 301 Biomedical Sensors - Ali Işın 2014

3. Although all these components of the instrumentation system areimportant, the sensor serves a special function in that it interfaces theinstrument with the system being measured. • In the case of biomedicalinstrumentation, a biomedical sensor is the interface between the electronicinstrument and the biological system. BME 301 Biomedical Sensors - Ali Işın 2014

4. Important Concerns • There are some general concerns that are important for any sensor in aninstrumentation system interface function especially for biomedicalsensors: 1. The sensor can affect the system, for that sensors must be designedto minimize their interaction with the biological host. It is importantthat the presence of the sensor does not affect the variable beingmeasured in the vicinity of the sensor via interaction between thesensor and the biologic system. This may change the quantity beingsensed in thevicinity. BME 301 Biomedical Sensors - Ali Işın 2014

5. 2. The biological system can affect the performance of the sensor. Theforeign body reaction might cause the host’s system to attempt tobreak down the materials of the sensor in order to remove it. Thismay, in fact, degrade the sensor package so that it can no longerperform in an adequate manner. So the material of package must be proper. BME 301 Biomedical Sensors - Ali Işın 2014

6. 3. Sensors that are implanted in the body are not accessible forcalibration. Thus, they must have extremely stable characteristics sothat frequent calibrations are not necessary. BME 301 Biomedical Sensors - Ali Işın 2014

7. Classification of BiomedicalSensors • Biomedical sensors can be classified according to how they areused with respect to the biological system: 1. Noninvasive biomedical sensors do not even contact thebiological system being measured. Sensors of radiant heat orsound energy coming from an organism are examples of noncontactingsensors. Noninvasive sensors can also be placedon the body surface like Skinsurface thermometers, biopotentialelectrodes, and strain gauges placed on the skin. BME 301 Biomedical Sensors - Ali Işın 2014

8. 2. Indwelling sensors (minimally invasive sensors) :are those thatcan be placed into a natural body cavity that communicateswith the outside. Examples: oral–rectal thermometers,intrauterine pressure transducers, and stomach pH sensors. BME 301 Biomedical Sensors - Ali Işın 2014

9. 3. Invasive sensors: are those that need to be surgically placed andthat require some tissue damage associated with their installation. BME 301 Biomedical Sensors - Ali Işın 2014

10. We can also classify sensors in terms of the quantities that they measure: • 1. Physical sensors: are used in measuring physical quantities suchas displacement, pressure, and flow. BME 301 Biomedical Sensors - Ali Işın 2014

11. 2. Chemical sensors: are used to determine the concentration ofchemical substances within the host. A sub-group of thechemical sensors that are concerned with sensing the presenceand the concentration of biochemical materials in the host. 3. Bio-analytical sensors or biosensors: used to measure someinternalquantitieslikeenzymes. BME 301 Biomedical Sensors - Ali Işın 2014

12. 1. Bio-analytical Sensors: • A special class of sensors of biological molecules has evolved inrecent years. These bioanalytical sensors take advantage of one of the following biochemical reactions: (1) enzyme–substrate. (2) antigen–antibody. (3) ligand–receptor. BME 301 Biomedical Sensors - Ali Işın 2014

13. The advantage of using these reactions in a sensor is that they are highlyspecific to a particular biological molecule, and sensors with highsensitivity and selectivity can be developed based upon these reactions. BME 301 Biomedical Sensors - Ali Işın 2014

14. The basic structure of a bio-analytical sensor; BME 301 Biomedical Sensors - Ali Işın 2014

15. There are two principal regions of the sensor. The first contains onecomponent of the biological sensing reaction such as the enzymeor the antibody, and the second region involves a means of detectingweather the biological reaction has taken place. • This second portion of a bioanalytical sensor is made up of either aphysical or chemical sensor that serves as the detector of the biologicalreaction. This detector can consist of an electrical sensor such as used inelectrochemical sensors, a thermal sensor, a sensor of changes incapacitance, a sensor of changes in mass, or a sensor of optical properties. BME 301 Biomedical Sensors - Ali Işın 2014

16. Example bio-analytical sensor: • One example of a bioanalytical sensor is a glucose sensor. The firstportion of the sensor contains the enzyme glucose oxidase. Thisenzyme promotes the oxidation of glucose to glucuronic acid andhydrogen peroxide while consuming oxygen in the process. • Thus,by placing a hydrogen peroxide or an oxygen sensor along with theglucose oxidase in the bioanalytical sensor, one can determine theamount of glucose oxidized by measuring the amount of hydrogenperoxide produced or oxygen consumed. BME 301 Biomedical Sensors - Ali Işın 2014

17. Stability is important for bioanalytical sensors, especially those that areusedforlong-termmeasurements. • The stability issues are also related to preservation of the biologicalmolecules used in the first portion of the sensor. These molecules canoften be degraded or destroyed by heat or exposure to light.. Thus, animportant issue in dealing with bioanalytical sensors is the preservation ofthe biochemical components of the sensor. BME 301 Biomedical Sensors - Ali Işın 2014

18. 2.Chemical Sensors: • There are many biomedical situations where it is necessary to know theconcentration or chemical activity of a particular substance in a biologicalsample. Chemical sensors provide the interface between an instrumentand the specimen to allow one to determine these quantities. • These sensors can be used on a biological specimen taken from thehost and tested in a laboratory, noninvasive or invasive sensors. BME 301 Biomedical Sensors - Ali Işın 2014

19. The below table indicates the most famous biomedicalsensors, the most commonthree types of the biomedical sensors areelectrochemical, optical and thermal biomedical sensors:- BME 301 Biomedical Sensors - Ali Işın 2014

20. A Clark amperometric electrode for sensing oxygen; BME 301 Biomedical Sensors - Ali Işın 2014

21. It consists of an electrochemical cell separated from the specimenbeing measured by an oxygen-permeable membrane. The cell is biased ata fixed potential of 600 mV, and under these conditions the followingreaction occurs at the noble metal cathode: O2 + 2H2O → 4e- + 4OH- BME 301 Biomedical Sensors - Ali Işın 2014

22. Thisreaction involves the reduction of molecular oxygen that diffuses into the cell through the oxygen permeable Membrane. Since the othercomponents of the reaction are in abundance, the rate of the reaction islimited by the amount of oxygen available. Thus, the rate of electronsused at the cathode is directly related to the available oxygen. In otherwords, the cathode current is proportional to the partial pressure ofoxygen in the specimen being measured. BME 301 Biomedical Sensors - Ali Işın 2014

23. The electrochemical cell is completed by the silver anode. Thereaction at the anode involves forming the low-solubility salt, silverchloride,from the anode material itself and the chloride ion contained inthe electrolytic solution. The cell is designed with these materials inabundance so that their activity does not affect the sensor performance. • This type of sensor is an example of an amperometric electrochemicalsensor that is the current in the cell is proportional to the concentration ofthe analyte; in this case, oxygen. BME 301 Biomedical Sensors - Ali Işın 2014

24. 3. Physical Sensors • Physical variables measured include temperature, strain, force, pressure, displacement, position, velocity, acceleration, optical radiation, sound, flow rate, viscosity, and electromagnetic fields. BME 301 Biomedical Sensors - Ali Işın 2014

25. TemperatureSensors; • Temperature is an important parameter in many control systems, mostfamiliarly in environmental control systems. Several distinctly differenttransduction mechanisms have been employed to measure temperature. • The mercury thermometer is a temperature sensor which produces a non electronic output signal. The most commonly used electrical signalgenerating temperature sensors are thermocouples, thermestors, andresistancethermometers. BME 301 Biomedical Sensors - Ali Işın 2014

26. Thermocouples; • Thermocouples employ the Seebeck effect, which occurs at the junctionof two dissimilar conductors. A voltage difference is generated betweenthe hot and cold ends of the two conductors due to the differences inthe energy distribution of electrons at the two differenttemperatures. Thevoltage magnitude generated depends on the properties of the conductor,e.g., conductivity and work function, such that a difference voltage will bemeasured between the cold ends of two different conductors. The voltagechanges fairly linearly with temperature over a given range, depending onthechoice of conductors. BME 301 Biomedical Sensors - Ali Işın 2014

27. To minimize measurement error, the cool end of the couple mustbe kept at a constant temperature and the voltmeter must have high input impedance. Acommonly used thermocouple is made of copper and constantan wires. BME 301 Biomedical Sensors - Ali Işın 2014

28. A thermocouple is an ‘‘auto-generator,’’ i.e., it produces a usable outputsignal, in this case electronic, directly in response to the measurandwithout the need for auxiliary power. BME 301 Biomedical Sensors - Ali Işın 2014

29. Resistancethermometer; • The resistance thermometer relies on the increase in resistance of a metalwirewithincreasingtemperature. • As the electrons in the metal gain thermal energy, they move about morerapidly and undergo more frequent collisions with each other and theatomic nuclei. These scattering events reduce the mobility of theelectrons, and since resistance is inversely proportional to mobility, theresistance increases. • Resistance thermometers typically consist of a coil ofthin metal wire. Platinum wire gives the largest linear range of operation.The resistance thermometer is a ‘‘modulator’’ or passive transducer. Inorder to determine the resistance change, a constant current is suppliedand the corresponding voltage is measured (or vice versa). • A measure of the sensitivity of a resistance thermometer is itstemperaturecoefficient of resistance (TCR). BME 301 Biomedical Sensors - Ali Işın 2014

30. Thermistors; • Thermistors are resistive elements made of semiconductor materialsand have a negative temperature coefficient of resistance. Themechanism governing the resistance change of a thermistor is the increasein the number of conducting electrons with increasing temperature, due tothermal generation, i.e., the electrons that are the least tightly bound to thenucleus (valence electrons) gain sufficient thermal energy to break away(enter the conduction band) and become influenced by external fields. • Thermistors are measured in the same manner as resistance thermometers,but thermistors have up to 100 times higher TCR values. BME 301 Biomedical Sensors - Ali Işın 2014

31. DisplacementandForceSensors; • Many types of forces can be sensed by the displacements they create. • Forexample, the force due to acceleration of a mass at the end of a spring willcause the spring to stretch and the mass to move. Its displacement fromthe zero acceleration position is governed by the force generated by theacceleration (F =m · a) and by the restoring force of the spring. BME 301 Biomedical Sensors - Ali Işın 2014

32. Anotherexample is the displacement of the center of a deformable membrane dueto a difference in pressure across it. • Both of these examples requiremultiple transduction mechanisms to produce an electronic output: a • primary mechanism which converts force to displacement (mechanicalto mechanical) and then an intermediate mechanism to convertdisplacement to an electrical signal (mechanical to electrical). BME 301 Biomedical Sensors - Ali Işın 2014

33. Applications Of BiomedicalSensors: • Biomedical research: • One of the application fields of the biomedical sensors is using them incontinuous biomedical research. So, these sensors can be used to improve thequality of the biomedical product). BME 301 Biomedical Sensors - Ali Işın 2014

34. Patient care applications: Sensors are used as a part of instruments thatcarry out patient monitoring bymaking measurements such as blood pressure, oxygen saturation, body temperature and ECG. BME 301 Biomedical Sensors - Ali Işın 2014

35. Specimenanalysis: • This can include analyses that can be carried out by the patients themselves intheir homes such as it is done with home blood glucose analyzers. • Sensors also are a part of large, multi-component, automatic blood analyzersused in the central clinical laboratories of medical centers. BME 301 Biomedical Sensors - Ali Işın 2014