Sensors and Bioinstrumentation: Transducers, Signal Conditioning, Biopotentials, Physiological Measurement, Medical Equipment and Electrical Safety
2. Signal conditioning, and the LASER in sensing and therapy
A resistive sensor is read in a Wheatstone bridge. With excitation V_s and gauge factor GF, a small strain ε gives an output of about V_s·GF·ε/4 for one active arm (quarter bridge), V_s·GF·ε/2 for two and V_s·GF·ε for four (full bridge); the half and full bridges also cancel temperature drift, because the same change appears in adjacent arms. The bridge output feeds an instrumentation amplifier, then filtering, isolation and an ADC.
- A piezoelectric sensor is a charge source with a large internal capacitance, read by a charge amplifier: V_o = −q/C_f, independent of cable capacitance, with a low-frequency cut-off 1/(2πR_fC_f) set by the feedback resistor.
- An LVDT or capacitive bridge is excited by an ac carrier and demodulated by a phase-sensitive detector, which recovers both the magnitude and the sign of the displacement.
- A thermistor is linearised by a parallel resistor, and a photodiode is read by a transimpedance amplifier, V_o = −I_pR_f.
A LASER (light amplification by stimulated emission of radiation) needs a gain medium, a population inversion maintained by pumping, and an optical resonator; its light is monochromatic, coherent and highly collimated. In sensing it is the source for laser Doppler flowmetry of skin and tissue perfusion, for optical coherence tomography and for fibre-optic sensors. In therapy the effect depends on wavelength and power density: the CO₂ laser at 10.6 µm is absorbed by water and cuts and vaporises tissue; the Nd:YAG laser at 1064 nm penetrates deeper and coagulates; the argon laser at 488 and 514 nm is absorbed by haemoglobin and used for retinal photocoagulation; the ArF excimer laser at 193 nm ablates the cornea with little heat in refractive surgery; and photodynamic therapy uses a laser to activate a photosensitising drug.
3. Biopotentials and their measurement
Excitable cells hold a resting potential of tens of millivolts, inside negative, and fire action potentials; the ionic currents of many cells firing together flow through the body as a volume conductor and appear as small potentials at the skin. An electrode converts that ionic current into electronic current at an electrode–electrolyte interface with its own half-cell potential. A silver/silver-chloride electrode is close to non-polarisable — charge crosses the interface freely, so motion artefact and dc offset are small — whereas a noble-metal electrode is polarisable and behaves more like a capacitor.
| Signal | Source | Typical amplitude and band | How it is recorded |
|---|---|---|---|
| ECG | depolarisation and repolarisation of the heart | about 1 mV; 0.05–100 Hz diagnostic, narrower for monitoring | limb and chest electrodes, 12-lead system |
| EEG | summed postsynaptic activity of the cortex | tens of µV; delta < 4, theta 4–8, alpha 8–13, beta 13–30 Hz | scalp electrodes in the international 10–20 placement |
| EMG | motor-unit action potentials of skeletal muscle | up to a few mV; tens of Hz to several hundred Hz (surface) | surface electrodes, or needle electrodes for single units |
| ERG | retinal response to a light flash | hundreds of µV; a- and b-waves | corneal contact-lens electrode with a flash stimulus |
| EOG | corneo-retinal standing potential that moves with the eye | varies roughly linearly with gaze angle; near dc | electrodes beside and above/below the eyes |
| GSR (electrodermal activity) | sweat-gland activity under sympathetic control | a change of skin conductance, not a generated voltage | small constant voltage or current across two finger electrodes |
| PCG | heart sounds — S1 from closure of the AV valves, S2 from the semilunar valves | acoustic, roughly 20 Hz to several hundred Hz | piezoelectric or electret microphone on the chest |
ECG lead geometry. Einthoven’s bipolar limb leads are I = V_LA − V_RA, II = V_LL − V_RA, III = V_LL − V_LA, so II = I + III (Einthoven’s law). The Wilson central terminal averages RA, LA and LL and is the reference for the unipolar chest leads V1–V6; Goldberger’s augmented leads aVR, aVL and aVF use the average of the other two limbs, are 1.5 times the corresponding unipolar limb potentials, and sum to zero. Three bipolar, three augmented and six chest leads make the 12-lead ECG. On standard paper at 25 mm/s one small (1 mm) square is 0.04 s and one large (5 mm) square 0.2 s, so heart rate = 60/RR(s) = 1500/(RR in small squares). A driven right-leg circuit feeds the inverted common-mode voltage back to the patient to reduce mains interference, through a current-limiting resistor.
4. Blood pressure, temperature, flow, respiration and cardiac output
Blood pressure. Indirect measurement occludes the brachial artery with a cuff and lets it down slowly. In the auscultatory method the Korotkoff sounds begin (phase I) at systolic pressure and disappear (phase V) at diastolic pressure. The oscillometric method records the cuff-pressure oscillations: their maximum occurs close to the mean arterial pressure, and systolic and diastolic are estimated from empirical ratios of the oscillation amplitude. Direct measurement uses an arterial catheter coupled through saline to a strain-gauge transducer; the fluid column makes a second-order system whose natural frequency and damping decide how faithfully the waveform is reproduced. Mean arterial pressure is approximately diastolic + (systolic − diastolic)/3.
Temperature is measured by thermistor or thermocouple probes (oral, rectal, oesophageal, skin) and by infrared thermometers that read the thermal radiation from the tympanic membrane or forehead. Flow and volume: the electromagnetic flowmeter puts a field B across a vessel, and blood moving at mean velocity v through diameter d induces e = Bdv (Faraday); ultrasonic Doppler and transit-time flowmeters measure velocity acoustically; plethysmography tracks volume changes — venous-occlusion plethysmography measures limb inflow, impedance plethysmography follows the conductance change as blood volume changes, and photoplethysmography follows the optical absorption change.
Respiration. A spirometer records lung volumes and timed expiration (FEV₁, FVC); it cannot reach the residual volume, which needs gas dilution or body plethysmography. A pneumotachograph passes the airflow through a small fixed resistance — a Fleisch tube of parallel capillaries keeps the flow laminar — so the pressure drop across it is proportional to flow, and integrating flow gives volume. Capnography measures expired CO₂ by infrared absorption.
| Method | Principle | Formula |
|---|---|---|
| Fick | oxygen uptake equals blood flow times the arteriovenous O₂ difference | CO = V̇O₂/(C_aO₂ − C_vO₂) |
| Indicator (dye) dilution | a known mass of indicator is diluted by the flow; area under the first-pass concentration curve (recirculation excluded) | CO = m/∫c dt (Stewart–Hamilton) |
| Thermodilution | cold saline injected in the right atrium, temperature drop sensed by a thermistor in the pulmonary artery | same form, with heat in place of dye mass |
| Impedance cardiography | thoracic impedance changes with each stroke | stroke volume from dZ/dt, non-invasively |
5. Operating principles of the named medical equipment
| Instrument | How it works | Points examiners like |
|---|---|---|
| Sphygmomanometer | an inflatable cuff, a pump with a release valve and a manometer (mercury, aneroid or electronic); pressure read at the Korotkoff events or from oscillations | a cuff too narrow for the arm overestimates pressure; deflate at about 2–3 mmHg per second |
| Ventilator | delivers positive-pressure breaths, volume-controlled (set tidal volume) or pressure-controlled (set pressure), in controlled, assist-control, SIMV or pressure-support modes | PEEP keeps alveoli open at end-expiration; triggering senses the patient’s own effort |
| Cardiac pacemaker | a pulse generator (lithium-iodine cell, sensing amplifier, timing logic) delivers stimuli through leads to the atrium and/or ventricle | fixed-rate (asynchronous) against demand; three-letter code: chamber paced, chamber sensed, response — VVI paces and senses the ventricle and is inhibited by a sensed beat |
| Defibrillator | a capacitor charged to kilovolts is discharged through the chest via paddles or pads, depolarising the myocardium at once so that the SA node can resume | stored energy E = ½CV²; damped-sine or biphasic truncated-exponential waveforms; cardioversion is synchronised to the R wave to avoid the vulnerable T wave |
| Pulse oximeter | red (about 660 nm) and infrared (about 940 nm) LEDs and a photodiode across a finger or earlobe; deoxyhaemoglobin absorbs more red, oxyhaemoglobin more infrared | ratio of ratios R = (AC/DC)_red/(AC/DC)_IR uses only the pulsatile arterial part, and SpO₂ comes from an empirical calibration of R |
| Haemodialyser | blood and dialysate flow counter-currently on opposite sides of a semipermeable membrane (hollow fibres); urea and creatinine diffuse out down their concentration gradients | excess water is removed by ultrafiltration driven by transmembrane pressure; heparin prevents clotting in the circuit |
6. Electrical isolation, safety and regulation
Current through the body, not voltage, does the harm, and its effect depends on the path. For a 50/60 Hz macroshock applied at the skin, roughly 1 mA is perceptible, around 10 mA or more prevents a subject from letting go, and currents of the order of 100 mA across the chest can cause ventricular fibrillation. A microshock delivered directly to the heart through a catheter or pacing lead bypasses the skin and the body’s bulk, and fibrillation can follow from currents of the order of 100 µA — which is why leakage limits for cardiac-contact equipment are set around 10 µA.
- Isolation breaks every conductive path between the patient-connected front end and the mains-referenced circuitry. Optical isolation passes the signal as light from an LED to a photodetector (an optocoupler); electrical isolation passes it through a transformer on a modulated carrier or through a small capacitive barrier. Power crosses by an isolated dc-dc converter.
- IEC 60601-1 classes equipment by protection against shock — Class I (protective earth), Class II (double or reinforced insulation), internally powered — and applied parts by how closely they touch the patient: B, BF (floating) and CF (cardiac floating, the strictest, for direct cardiac application, with patient leakage limited to 10 µA in normal condition).
- Hazards are reduced by an equipotential ground for all equipment around a patient (to prevent ground loops), ground-fault (residual-current) interrupters, isolated power systems in critical areas, and current-limiting resistors in every patient lead.
Regulation classifies devices by risk and scales the evidence demanded accordingly. The US FDA uses Classes I, II and III; in India the Medical Devices Rules, 2017, administered by the Central Drugs Standard Control Organisation (CDSCO), use Classes A (low risk) to D (high risk); the European Union regulates under its Medical Device Regulation. International standards carry the technical content: ISO 13485 for the quality-management system of a device maker, ISO 14971 for risk management, the IEC 60601 family for the electrical safety and essential performance of medical electrical equipment, and ISO 10993 for biological evaluation.
Key takeaways
- Quarter bridge V_s·GF·ε/4, full bridge V_s·GF·ε; Pt100 R = 100(1 + 0.00385T); NTC thermistor R = R₀eβ(1/T − 1/T₀).
- Einthoven: I = LA − RA, II = LL − RA, III = LL − LA and II = I + III; at 25 mm/s, HR = 1500/(RR in mm).
- Korotkoff onset is systolic, disappearance diastolic; the oscillometric maximum is near mean pressure.
- Fick CO = V̇O₂/(C_a − C_v); dilution CO = m/∫c dt; EM flowmeter e = Bdv; defibrillator E = ½CV².
- Microshock thresholds are about a thousand times below macroshock; CF applied parts, isolation and equipotential grounding exist for that reason.
Practice questions (18)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
The gauge factor of a strain gauge is defined as
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Answer: A — (ΔR/R)/ε
GF is the fractional change in resistance per unit strain, (ΔR/R)/ε, about 2 for metal foil gauges and far higher for semiconductor gauges. ΔR/ε has units of ohms and depends on the gauge’s nominal resistance, which the definition is meant to remove.A single active strain gauge (GF = 2) in a quarter Wheatstone bridge excited at 5 V experiences 500 µε. Using the small-strain approximation, the bridge output (in mV) is ____.
Numerical answer — type the value.
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Answer: 1.25
V_o ≈ V_s·GF·ε/4 = 5 × 2 × 500 × 10⁻⁶/4 = 1.25 mV; the exact bridge equation gives 1.249 mV, the difference being the small non-linearity of a quarter bridge. Leaving out the 1/4 gives 5 mV, the full-bridge figure.A full bridge of four active gauges (GF = 2.1), two in tension and two in compression each at 200 µε, is excited at 10 V. The output (in mV) is ____.
Numerical answer — type the value.
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Answer: 4.2
With all four arms active and correctly placed, V_o = V_s·GF·ε = 10 × 2.1 × 200 × 10⁻⁶ = 4.2 mV, four times the quarter-bridge output of 1.05 mV. Putting the tension gauges in adjacent rather than opposite arms would cancel the signal instead of adding it.A Pt100 RTD (100 Ω at 0 °C, α = 0.00385 /°C) is at body temperature, 37 °C. Its resistance (in Ω), to two decimal places, is ____.
Numerical answer — type the value.
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Answer: 114.25
R = R₀(1 + αT) = 100(1 + 0.00385 × 37) = 100 × 1.14245 = 114.245 Ω, which rounds to 114.25 (114.24 if truncated). Using α = 0.0039 gives 114.43 Ω; treating α as per kelvin with T = 310 K gives 219 Ω, which confuses a temperature difference with an absolute temperature.An NTC thermistor has R = 10 kΩ at 25 °C and β = 3500 K. Its resistance at 37 °C (in kΩ), to two decimal places, is ____. Take 0 °C = 273.15 K.
Numerical answer — type the value.
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Answer: 6.35
R = R₂₅ exp[β(1/T − 1/T₂₅)] = 10 exp[3500(1/310.15 − 1/298.15)] = 10 exp(−0.4542) = 10 × 0.635 = 6.35 kΩ. Putting Celsius temperatures into the exponent gives an absurdly small resistance, and dropping the minus sign — treating the device as PTC — gives 15.7 kΩ.At one instant of an ECG, lead I reads 0.5 mV and lead III reads 0.7 mV. Lead II at that instant (in mV) is ____.
Numerical answer — type the value.
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Answer: 1.2
Einthoven’s law: II = I + III, because (LA − RA) + (LL − LA) = LL − RA. So II = 0.5 + 0.7 = 1.2 mV. Subtracting the two (0.2 mV) mixes up Einthoven’s law with the lead definitions.On ECG paper running at 25 mm/s, successive R peaks are 20 mm apart. The heart rate (in beats per minute) is ____.
Numerical answer — type the value.
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Answer: 75
RR = 20 mm/(25 mm/s) = 0.8 s, so HR = 60/0.8 = 75 bpm — equivalently 1500/20. Dividing 300 by 20 treats the millimetres as large squares and gives 15; dividing 60 by 20 gives 3.In the auscultatory method of blood-pressure measurement, systolic pressure is read at
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Answer: A — the first appearance of the Korotkoff tapping sounds as the cuff deflates
When cuff pressure first falls below the systolic peak, blood spurts through the partly open artery each beat and the first tapping sounds appear. Their disappearance marks diastolic pressure, and the oscillation maximum corresponds to mean arterial pressure in the oscillometric method.A subject consumes 250 mL of O₂ per minute; arterial blood carries 200 mL O₂ per litre and mixed venous blood 150 mL O₂ per litre. The cardiac output by the Fick principle (in L/min) is ____.
Numerical answer — type the value.
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Answer: 5
CO = V̇O₂/(C_a − C_v) = 250/(200 − 150) = 250/50 = 5 L/min. Dividing by the arterial content alone gives 1.25 L/min, which ignores that venous blood returns still carrying most of its oxygen.In an indicator-dilution measurement 5 mg of dye is injected, and the area under the first-pass concentration–time curve, with recirculation removed, is 50 mg·s/L. The cardiac output (in L/min) is ____.
Numerical answer — type the value.
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Answer: 6
Stewart–Hamilton: CO = m/∫c dt = 5 mg/(50 mg·s/L) = 0.1 L/s = 6 L/min. Leaving the answer as 0.1 forgets the seconds-to-minutes conversion; including the recirculation hump in the area would make the curve larger and the output falsely low.A defibrillator’s 32 µF capacitor is charged to 5 kV. The energy stored (in J) is ____.
Numerical answer — type the value.
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Answer: 400
E = ½CV² = ½ × 32 × 10⁻⁶ × (5000)² = ½ × 32 × 10⁻⁶ × 25 × 10⁶ = 400 J. Forgetting the ½ gives 800 J, and CV = 0.16 C is the stored charge, not energy. Not all of the stored energy reaches the patient, because the internal and electrode resistances take a share.An electromagnetic flowmeter applies B = 0.03 T across an artery of internal diameter 5 mm in which the mean blood velocity is 0.4 m/s. The induced voltage (in µV) is ____.
Numerical answer — type the value.
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Answer: 60
e = Bdv = 0.03 × 0.005 × 0.4 = 6 × 10⁻⁵ V = 60 µV — a very small signal, which is why these flowmeters use careful carrier techniques. Using the radius instead of the diameter gives 30 µV.Which statements about the pulse oximeter are correct?
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Answer: A — it uses red (about 660 nm) and infrared (about 940 nm) light; B — deoxyhaemoglobin absorbs more red light than oxyhaemoglobin; C — normalising the pulsatile (AC) signal by the steady (DC) level at each wavelength removes the contribution of tissue and venous blood
Two wavelengths either side of the isosbestic point let the ratio of AC/DC ratios track the fraction of oxygenated haemoglobin, and only the pulsatile part comes from arterial blood. What it reports is saturation, SpO₂, found through an empirical calibration; pO₂ needs a blood-gas electrode such as the Clark electrode.Which statements about electrical safety of patient-connected equipment are correct?
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Answer: A — microshock applied directly to the heart can cause fibrillation at currents far below macroshock thresholds; B — type CF applied parts are intended for direct cardiac application and have the strictest leakage limits; C — optical isolation removes the galvanic path between the patient circuit and mains-referenced circuitry
Current delivered inside the heart through a catheter bypasses the skin, so microshock thresholds are of the order of 100 µA against about 100 mA for macroshock, hence the 10 µA CF limit and isolated front ends. A direct earth on the patient offers any fault a low-resistance path through the body; a driven right leg works through a current-limiting resistor instead.A pacemaker is described by the code VVI. It
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Answer: A — paces the ventricle, senses the ventricle and is inhibited by a sensed ventricular beat
The letters are chamber paced, chamber sensed, response to sensing: V, V, I — a ventricular demand pacemaker that stays silent while the heart beats on its own. Fixed-rate dual-chamber pacing would be DOO, atrial tracking is VDD or DDD, and shock delivery is the job of an implantable defibrillator.The Clark electrode determines blood pO₂ by
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Answer: A — the current produced when oxygen is reduced at a polarised platinum cathode
It is amperometric: oxygen diffusing through the membrane is reduced at a cathode held at about −0.6 to −0.7 V, and the resulting current is proportional to pO₂. The glass-membrane arrangement is the Severinghaus pCO₂ electrode, and the two-wavelength method is pulse oximetry.A piezoelectric pressure sensor is read by a charge amplifier with feedback capacitance 1 nF and feedback resistance 100 MΩ. The low-frequency −3 dB cut-off of the measurement (in Hz), to two decimal places, is ____.
Numerical answer — type the value.
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Answer: 1.59
f_L = 1/(2πR_fC_f) = 1/(2π × 10⁸ × 10⁻⁹) = 1/(2π × 0.1) = 1.59 Hz; below this the charge leaks away, which is why a piezoelectric sensor cannot hold a static pressure. Forgetting the 2π gives 10 Hz, the corner in rad/s.Which laser is used to ablate the cornea in refractive (LASIK-type) surgery?
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Answer: A — the argon-fluoride excimer laser at 193 nm
Deep-ultraviolet photons at 193 nm break molecular bonds in a thin surface layer and remove corneal tissue with very little heating. The CO₂ laser cuts by heating water, Nd:YAG penetrates and coagulates, and the argon laser is absorbed by haemoglobin for retinal photocoagulation.