Practical Fundamentals of Biomedical Engineering

1. Practical Fundamentals of Biomedical Engineering Sensors

2. Introduction • Importance: • Sensors are the main building blocks of diagnostic medical instrumentation • Research Motivation: • Lower healthcare costs • Optimising efficiency • Patient self-testing • Instantaneous diagnosis at point of care

3. Sensors are used to measure • Electrolytes, enzymes and others in blood • Pressure, flow and gas concentration • Typically gases like O2 and CO2 • First design step • Asses in vitro: • Accuracy • Operating Range • Response Time • Sensitivity • Resolution • Reproducibility • In vivo testing

4. Sensor Classification • Physical • Electrical • Chemical • Biosensors • Biological recognition element • Supporting structure to convert biochemical reaction to recognizable signal

5. Sensor Packaging • Very important for in vivo applications • Long operational lifetime & biocompatibility • Host affecting sensor Protein absorption and cellular deposits leading to drift, sensitivity and stability loss • Sensor affecting host • Nontoxic • Nonthrombogenic • Covering to prevent toxin leaching

6. Biopotential Measurements • Electrodes coupling ionic potentials in body to electronic instrument

7. Electrolyte/Metal Electrode Interface • Metal in electrolyte solution creates charge distribution causing half-cell potential at interface

8. Typically two similar electrodes of same metal used to measure biopotential • E.g. two similar electrodes taped to chest to measure electrical potentials generated by heart (electrocardiogram – ECG)

9. ECG Electrodes • Mylar • Disposable snap-type Ag/AgCL

10. EEG Electrodes • Electroencephalographic signals • Signals from the brain • Cup electrodes • Platinum or Tin cup • Subdermalneedles • Platinum or Stainless Steel needle electrode

11. Microelectrodes • Ultra-fine tapered tip to be inserted into individual cells • E.g. recording action potentials in neurophysiological studies • Capillary glass microelectrode

12. Electromagnetic flow transducer • Measure fluid flow , e.g. blood flow in Aorta • Potential difference V over vessel proportional to blood velocity • V = B x l x u • B = Uniform magnetic field • l = Diameter of vessel • u = Uniform velocity in vessel • Clips around vessel

14. Potentiometer • Converts linear or angular displacement to voltage by sliding a contact along a surface of a resistive element • Elastic resistive transducer around chest

16. Strain gauges • Measures strain of object as result of applied force • Gauge factor G indicator of sensitivity • G = ( ΔR / R ) / ( ΔL / L ) • Bonded • Unbonded

18. Piezoelectric transducer • Contains piezoelectric crystal that generates a small electric potential if mechanically strained • Uses • Listening to heart sounds in cardiology • Automated blood pressure measurements • Generating ultrasonic waves for blood measuring

19. Airflow Transducers • Fleisch pneumatachometer • Uses • Monitoring volume, flow and breathing of patients on mechanical ventilators

20. Temperature Measurement • Temperature very important in healthcare • Measuring areas • Armpit • Bodily cavities • Thermistor • Small enough to ensure rapid response

21. Non-contact Thermometer • Measures body core temperature inside auditory canal • Measures near tympanic membrane in ear canal

23. Blood Gases and pH Sensors • Arterial blood gases (pO2 and pCO2) and pH frequently measured on critically ill patients under operation or in ICU. • Why? - Physicians use information to adjust mechanical ventilation systems / administering of pharmacological agents. - Information on respiratory and metabolic imbalances in the body. - Reflects adequacy of blood oxygenation and CO2 elimination.

24. Traditionally: Blood withdrawn from peripheral artery and sample analyzed in laboratory. Presently: Continuous noninvasive blood gas monitoring - allows physicians to follow trends in patient’s condition and receive immediate feedback on therapeutic methods employed.

25. Oxygen Measurement • Two ways to measure blood oxygen or oxygenation: 1. Polarographic pO2 Sensors 2. Optical Oximeter (oxygen saturation)

26. Measures oxygen saturation, SO2, relative amount of oxygen carried by hemoglobin in erythrocytes. • Measurements of arterial blood SaO2 and venous blood SvO2 made in vivo or in vitro. • Method (oximetry) is based on light absorption properties of blood. • Specifically interested in relative concentration of Hband HbO2, since deoxygenated blood is blue and fully oxygenated blood is bright red. Optical Oximeters

27. Measurement Method: • Measurement performed at two specific wavelengts: red wavelength (λ1) and near-infrared region of the spectrum (λ2). • λ1 – large difference between light absorbance of Hb and HbO2 – 660nm. • λ2 – either isobestic (around 805 nm where absorbencies of Hb and HbO2 are equal) or around 940 – 960 nm where absorbance of Hb is slightly smaller than HbO2.

29. In vivo – Uses noninvasive pulse oximetercontaining • optical sensors for measuring SaO2. • Optical sensors consists of pair of small inexpensive • LED’s. • LED’s typically red (R) – 660 nm and infrared (IR) – 940 – 960 nm. • Highly sensitive silicon photodetector. • Components mounted inside reusable spring-loaded • clip or disposable adhesive wrap. • Electronic circuits inside oximeter switches LED’s • on and off in sequential manner and synchronously • measure photodetectoroutput.

31. Pulse oximetry relies on detection of photoplethysmographic signal. • Signal caused by: • Changes in arterial blood volume with systolic heartbeat. • Magnitude of signal depends on: • Amount of blood ejected from heart into peripheral vascular bed. • Optical absorption of blood. • Composition and color of skin and underlying tissues. • Wavelengths used to illuminate blood.

32. Optical Biosensors • Play important role in development of highly • sensitive and selective methods for • biochemical analysis. • Fundamental principle employed based on • change in optical properties of biological or • physical medium. • Change produced can be result of changes in • absorbance, reflectance, scattering, fluorescence, • polarization or refractive index of biological medium.

33. Optical Fibers • Optical fibers used to transmit light from one location to another. • Fibers typically made from two concentric and transparent glass or plastic materials. • Center piece is known as the core and outer layer, serving as the coating, is called the cladding. • Core and cladding have different index of refraction, n.

34. Case study: Fibre optic sensor • Measure the stress in the knee’s patellar tendon.

37. Case study: Fibre optic sensor • The posterior region of the proximal patellar tendon is subjected to greater tendinous forces than the corresponding anterior region • Jumping and deep squat exercises expose the patellar tendon to very large forces • This should be taken into account during training and rehabilitation

38. Sport technology – eccentric/concentric • Eccentric exercise is commonly recommended for the treatment of patellar tendinopathy • 8 subjects • Calibration • Squat exercise

39. calibration

40. concentric phase caused greater loads on the patellar tendon than the eccentric phase end