Bioelectric signals and electrodes

In short

A bioelectric signal is the voltage that appears outside living cells because excitable membranes pump ions unequally and then let them move: the resting membrane potential and the action potential are its source, and ECG, EEG, EMG, EOG, ERG and EGG are its measurable surface forms. An electrode is the transducer that converts this ionic current in tissue into an electronic current in a wire, so its half-cell potential, impedance and artifact behaviour decide whether a 50 µV EEG wave survives at all. This topic is the foundation of every biomedical instrumentation paper, carries direct GATE (BM) marks on the Nernst/Goldman equations, electrode models and signal specifications, and is asked in interviews at BPL Medical Technologies, Schiller Healthcare India, Allengers, Skanray, Wipro GE, Philips, Siemens Healthineers, Nihon Kohden India and hospital biomedical-engineering departments. In practice it governs electrode selection, skin preparation, lead placement and the safety limits written into IEC 60601 as adopted by BIS and enforced through the Medical Devices Rules, 2017.

Explained

Origin of bioelectric potentials

• Resting membrane potential: the cell interior is 60-90 mV negative to the exterior, maintained by the Na⁺-K⁺ ATPase pump (3 Na⁺ out for 2 K⁺ in) and by the membrane being far more permeable to K⁺ than to Na⁺.
• Typical mammalian concentrations (mmol/L): K⁺ 140 in / 4 out; Na⁺ 12 in / 145 out; Cl⁻ 4 in / 110 out. Hence E_K ≈ −95 mV, E_Na ≈ +67 mV, E_Cl ≈ −88 mV at 37 °C.
• Nernst potential gives the equilibrium voltage for one ion; Goldman-Hodgkin-Katz combines several with their permeabilities and predicts the real resting value (≈ −70 to −90 mV).
• Action potential phases: (1) stimulus raises the potential to threshold (≈ −55 mV); (2) voltage-gated Na⁺ channels open - depolarization to about +20 to +40 mV; (3) Na⁺ channels inactivate, K⁺ channels open - repolarization; (4) hyperpolarization (undershoot) then return to rest.
• All-or-none law: a sub-threshold stimulus gives no action potential; above threshold the amplitude is fixed and information is carried by frequency, not size.
• Refractory period: absolute (no new AP possible, ≈ 1 ms in nerve) and relative (a stronger stimulus is needed). Duration ≈ 1 ms in nerve, 5-10 ms in skeletal muscle, 200-300 ms in cardiac muscle (the long plateau from Ca²⁺ influx prevents tetanus in the heart).
• Propagation: continuous in unmyelinated fibres; saltatory (node to node across myelin) in myelinated fibres, raising velocity to 100-120 m/s against ~1 m/s.
• Volume conductor: the body's fluids conduct, so the summed activity of many cells appears at the skin as a much attenuated, low-pass-filtered version - this is why surface ECG is millivolts while a single cell's AP is ~100 mV.

The bioelectric signals and their specifications

• ECG - heart; 0.5-4 mV at the limbs, 0.05-150 Hz (diagnostic), 0.5-40 Hz monitoring; P, QRS, T waves.
• EEG - brain; 2-200 µV (typically 10-100 µV), 0.5-100 Hz; rhythms delta (0.5-4 Hz), theta (4-8), alpha (8-13), beta (13-30), gamma (>30).
• EMG - skeletal muscle; 0.05-5 mV surface and up to 10 mV with needles, 10-2,000 Hz (dominant 20-500 Hz).
• EOG - eye movement (corneo-retinal standing potential); 50-3,500 µV, DC-50 Hz - needs DC-coupled amplification.
• ERG - retina under light flash; 0-900 µV, 0.2-200 Hz.
• EGG - stomach; 10-1,000 µV, 0.01-1 Hz; ENG nerve action potential, 5 µV-10 mV, up to 10 kHz.
• All are low amplitude, low frequency, high source impedance signals in a noisy environment - hence the need for high input impedance, high CMRR differential amplifiers (covered under instrumentation and signal conditioning).

Electrode-electrolyte interface and half-cell potential

• At a metal in electrolyte: M ⇌ Mⁿ⁺ + n e⁻ (oxidation) and the reverse (reduction). Net charge separation forms an electrical double layer - a capacitance in the µF/cm² range.
• Half-cell potential is the equilibrium voltage of that interface against a reference. The standard hydrogen electrode is defined as 0 V; Ag/AgCl is +0.223 V, Ag/Ag⁺ +0.799 V, Cu +0.340 V, Pt ≈ +1.2 V, Zn −0.763 V.
• Two electrodes of the same metal still differ by tens of millivolts - this offset potential (and its slow drift) can be far larger than the signal and is why ECG machines are AC-coupled, while EOG/DC recordings demand carefully matched electrodes.
• Polarization: current flow shifts the interface away from equilibrium (over-potential), composed of ohmic, concentration and activation components. Polarization raises impedance, adds noise and causes baseline wander.
• Perfectly polarizable (capacitive, no charge crosses - e.g. platinum, stainless steel): behaves as a capacitor, poor for DC and low-frequency work but inert, used for stimulation.
• Perfectly non-polarizable (resistive, charge crosses freely - Ag/AgCl): low, stable, nearly frequency-independent impedance, low noise and low drift. The practical standard for ECG, EEG and EMG.
• Why Ag/AgCl works: the sparingly soluble AgCl layer fixes Cl⁻ activity, so the interface potential is stable and reversible with the Cl⁻ in the electrode gel. Made by electrolytic chloriding or by sintering Ag/AgCl powder.
• Equivalent circuit (Warburg model): half-cell EMF E_hc in series with the double-layer capacitance C_d in parallel with the faradaic leakage resistance R_d, all in series with the electrolyte/lead spreading resistance R_s.
  • At DC and low frequency: |Z| → R_s + R_d (high). At high frequency: C_d shorts R_d, |Z| → R_s (low). So electrode impedance falls with frequency - always quote the test frequency (10 Hz or 20 Hz for ECG electrodes).
• Skin adds its own network: stratum corneum behaves as a parallel R-C (R_e ~ 100 kΩ-1 MΩ at low frequency) in series with the epidermis EMF (10-30 mV, changes with stretch) and sweat-gland path. Abrading, cleaning with alcohol/spirit and conductive gel can cut skin impedance to a few kilohms.

Types of electrodes and their classification

1. Surface (macro) electrodes - record from the skin.
   • Metal plate: German-silver or nickel-plated limb plates with rubber straps and electrolyte paste; cheap, reusable, prone to motion artifact.
   • Suction cup (Welch) electrode: precordial ECG, no strap needed.
   • Floating / recessed electrode: the Ag/AgCl disc never touches skin; a gel-filled cavity isolates it, so mechanical disturbance of the skin does not disturb the metal-electrolyte interface - the standard anti-motion-artifact design.
   • Disposable pre-gelled foam/tape electrodes with sintered Ag/AgCl and solid hydrogel - the ward and ICU default; snap or tab connection.
   • Flexible / textile / conductive-polymer electrodes for neonates, long-term Holter and wearables.
   • Dry and capacitive (insulated) electrodes: no gel; capacitive coupling through an insulator with a very high input-impedance buffer. Convenient for wearables but noisy and motion-sensitive.
   • Ear-clip, scalp-cup (collodion/paste-fixed), bridge and spray electrodes for EEG and ERG.
2. Needle and wire (intramuscular) electrodes - EMG and nerve conduction studies.
   • Monopolar needle (insulated except the tip, against a surface reference), concentric (coaxial) needle - the clinical standard, picks up a small territory, bipolar concentric (two wires in one cannula, most selective), fine-wire hooked electrodes for kinesiology.
3. Microelectrodes - tip 0.5-5 µm, record a single cell's membrane potential.
   • Glass micropipette filled with 3 M KCl, with an Ag/AgCl wire; resistance 1-100 MΩ, tip potential and high noise are its drawbacks.
   • Metal microelectrodes: electrolytically etched tungsten or stainless steel, insulated to the tip; mainly capacitive impedance.
   • Solid-state / MEMS arrays: silicon multi-site probes, Utah and Michigan arrays, microelectrode arrays (MEA) for in-vitro cardiac and neural work.
4. Stimulating electrodes (pacing leads, defibrillator pads, TENS, deep-brain stimulation) - must be charge-balanced and inert (Pt, Pt-Ir, Ir oxide, titanium nitride) to avoid tissue damage and electrode dissolution.
5. Internal / indwelling - catheter-tip, oesophageal, intracardiac (EP mapping), cortical strip/grid for electrocorticography.

Placement, lead systems and reference schemes

• ECG: Einthoven's triangle of RA, LA, LL (with RL as the driven reference). Bipolar limb leads I = LA−RA, II = LL−RA, III = LL−LA; Einthoven's law II = I + III. Augmented unipolar aVR, aVL, aVF, each taken against the average of the *other two* limb electrodes (Goldberger's modification); the Wilson central terminal - the average of the three limb electrodes through 5 kΩ resistors - is the reference for the chest leads. Six chest leads V1-V6; standard sites V1 and V2 in the 4th intercostal space either side of the sternum, V4 at the 5th space on the mid-clavicular line, V3 between V2 and V4, V5 anterior axillary line, V6 mid-axillary line, all level with V4. Standard 10 electrodes give the 12-lead ECG; 25 mm/s, 10 mm/mV.
• EEG: the international 10-20 system - 21 electrodes located at 10% and 20% fractions of the nasion-inion and inter-aural distances. Prefixes Fp, F, C, P, O, T (frontopolar, frontal, central, parietal, occipital, temporal); odd numbers left, even right, z (zero) on the midline; extended 10-10 system for higher density. Montages: bipolar (chains), common reference (ear/A1-A2), and average reference.
• EMG: surface pair placed along the muscle fibre direction over the belly, 20 mm apart, reference on a bony landmark; needle electrodes for motor unit action potentials and nerve conduction velocity.
• Right-leg drive: a driven reference (common-mode feedback through ~5 MΩ safety resistance) rather than a hard ground - lowers common-mode voltage without raising leakage current.

Noise, artifacts and remedies

• Motion artifact - mechanical disturbance of the double layer and of the skin EMF; the largest low-frequency error. Remedies: floating/recessed electrodes, Ag/AgCl, gel, firm strain-relieved leads, skin abrasion, 0.5 Hz high-pass for monitoring.
• 50 Hz mains interference - capacitive coupling to the body and leads (displacement current ~0.1-1 µA) plus magnetic loop pickup. Remedies: high CMRR instrumentation amplifier, shielded and twisted leads, guard/shield driving, right-leg drive, balanced low electrode impedance, notch filter only as a last resort (it distorts the QRS).
• Electrode impedance imbalance converts common-mode into differential voltage - the dominant practical cause of 50 Hz on one lead; always check and reduce ΔZ.
• Baseline wander from respiration, sweat and offset drift; sweat changes Cl⁻ concentration and hence half-cell potential.
• Half-cell / offset potential saturating a DC-coupled front end; polarization after defibrillation - electrodes must recover quickly.
• Other artifacts: EMG contamination of ECG and EEG, ECG contamination of EEG, eye-blink and pulse artifacts in EEG, cross-talk between adjacent muscles in EMG, and 1/f electrochemical noise plus thermal noise of R_s and R_d.

Standards, safety and Indian practice

• IEC 60601-1 (general safety and essential performance; adopted in India through the BIS IS 13450 series) plus the collateral 60601-1-2 for electromagnetic disturbances. Particular standards: 60601-2-25 diagnostic ECG, 60601-2-27 ECG monitoring, 60601-2-26 EEG, 60601-2-40 EMG and evoked response, 60601-2-47 ambulatory ECG.
• Applied-part classification: Type B, BF (floating) and CF (cardiac floating). Indicative 60601-1 patient leakage limits are 100 µA normal / 500 µA single-fault for BF and 10 µA / 50 µA for CF a.c.; verify against the current edition and amendments before quoting them in a test report.
• Microshock - as little as about 10 µA through a catheter in contact with the heart can cause ventricular fibrillation, which is why intracardiac equipment is CF and isolated.
• ANSI/AAMI EC12 for disposable ECG electrodes is the usual performance reference: a.c. impedance, d.c. offset (typically ≤ 100 mV for an electrode pair), defibrillation overload recovery and bias-current tolerance. AAMI EC53 covers cables and lead wires.
• India: electrodes and ECG/EEG/EMG machines are regulated devices under the Medical Devices Rules, 2017 administered by CDSCO; all medical devices are notified as drugs under the Drugs and Cosmetics Act, with risk-based classes A to D (surface electrodes are low-risk, Class A/B). Licensing and QMS requirements follow the Rules and the associated essential-principles checklist; check CDSCO notifications for the current class and licence route of a specific product. The Electricity (Measures relating to Safety and Electric Supply) Regulations and hospital earthing/equipotential practice govern the installation side.
• ISO 10993 biological evaluation applies to gels and skin-contact materials (irritation and sensitisation); ISO 13485 is the QMS standard for manufacturers and ISO 9001:2026 the general edition.

Key points

• Bioelectric signal - extracellular voltage produced by ion movement across excitable cell membranes.
• Resting potential - −70 to −90 mV inside, set by K⁺ permeability and the Na⁺-K⁺ pump.
• Nernst equation - equilibrium potential of one ion from its concentration ratio; 26.7 mV per natural log unit at 37 °C.
• Goldman equation - weights several ions by permeability; predicts the actual resting potential.
• Action potential - all-or-none, ~100 mV, 1 ms in nerve, 200-300 ms in cardiac muscle; information is coded in frequency.
• Refractory period - absolute then relative; the cardiac plateau prevents tetanic contraction.
• Signal levels - ECG ~1 mV / 0.05-150 Hz, EEG ~50 µV / 0.5-100 Hz, EMG ~1 mV / 10-2,000 Hz, EOG DC-50 Hz.
• Electrode - a transducer converting ionic current to electronic current, not a passive contact.
• Half-cell potential - interface EMF; Ag/AgCl = +0.223 V vs SHE; mismatch gives the offset potential.
• Polarizable vs non-polarizable - capacitive (Pt, steel) vs resistive and stable (Ag/AgCl); Ag/AgCl is used for recording.
• Electrode model - E_hc + (R_d ∥ C_d) + R_s; impedance is high at DC and falls to R_s at high frequency.
• Floating (recessed) electrode - gel column isolates the metal from skin movement; the main motion-artifact fix.
• Skin impedance - stratum corneum dominates; abrasion, cleaning and gel reduce it from hundreds of kΩ to a few kΩ.
• Impedance imbalance - converts common-mode mains voltage into a differential error; keep ΔZ small.
• Microelectrode - tip <5 µm, 1-100 MΩ, records single-cell membrane potential; needs a very high input impedance amplifier.
• Lead systems - Einthoven triangle and Wilson central terminal for 12-lead ECG; 10-20 system for EEG.
• Right-leg drive - common-mode feedback reference that improves rejection without lowering safety.
• Standards - IEC 60601 family (IS 13450 series), AAMI EC12 for electrodes, CDSCO Medical Devices Rules, 2017 in India.

Formulas and examples

Example 1 - Nernst potential for potassium. [K]_in = 140, [K]_out = 4 mmol/L, T = 310 K, z = +1.

• RT/F = (8.314 × 310)/96,485 = 2,577.3/96,485 = 0.02671 V = 26.71 mV.
• ln(4/140) = ln(0.02857) = −3.555.
• E_K = 26.71 × (−3.555) = −95.0 mV (inside negative). Repeating for Na⁺ (145 out, 12 in): ln(12.083) = 2.492 → +66.6 mV; for Cl⁻ (110 out, 4 in, z = −1): −26.71 × ln(27.5) = −26.71 × 3.314 = −88.5 mV.

Example 2 - resting potential from the Goldman equation. P_K : P_Na : P_Cl = 1 : 0.04 : 0.45, same concentrations.

• Numerator = (1 × 4) + (0.04 × 145) + (0.45 × 4) = 4 + 5.8 + 1.8 = 11.60.
• Denominator = (1 × 140) + (0.04 × 12) + (0.45 × 110) = 140 + 0.48 + 49.5 = 189.98.
• Ratio = 0.06106, ln = −2.796 → V_m = 26.71 × (−2.796) = −74.7 mV. Note it sits close to E_K because K⁺ dominates the permeability.

Example 3 - electrode impedance against frequency. R_s = 1 kΩ, R_d = 50 kΩ, C_d = 100 nF.

• f_c = 1/(2π × 50,000 × 100 × 10⁻⁹) = 1/(0.03142) = 31.8 Hz.
• At DC: |Z| = 1 k + 50 k = 51 kΩ. At 10 kHz: C_d shorts R_d, |Z| ≈ 1 kΩ.
• At 50 Hz: X_C = 1/(2π × 50 × 100 nF) = 31.83 kΩ; R_d ∥ (−jX_C) = 14.42 − j22.65 kΩ; adding R_s gives 15.42 − j22.65 kΩ, |Z| = 27.4 kΩ. Hence an electrode quoted as "2 kΩ" at 10 kHz may be 25-50 kΩ in the ECG band - always state the test frequency.

Example 4 - why unequal electrodes put 50 Hz on one ECG lead. V_cm = 2 V at 50 Hz on the body, Z_in = 10 MΩ per input, electrode impedances 10 kΩ and 30 kΩ (ΔZ = 20 kΩ).

• V_diff ≈ 2 × (20 × 10³ / 10 × 10⁶) = 2 × 0.002 = 4 mV - four times a 1 mV QRS, so the trace is unusable.
• Prepare the skin so both electrodes read 5 kΩ (ΔZ ≈ 1 kΩ): V_diff ≈ 2 × 10⁻⁴ = 0.2 mV. Raise Z_in to 1,000 MΩ as well: 2 µV, now negligible. This is the numerical argument for high input impedance *and* good skin prep.

Example 5 - amplifier CMRR and microelectrode noise. (a) CMRR = 100 dB, V_cm = 1.5 V: error = 1.5/10⁵ = 15 µV, i.e. 1.5% of a 1 mV ECG but 30% of a 50 µV EEG wave - EEG front ends need ≥ 110-120 dB.

• (b) Johnson noise of a 10 MΩ microelectrode over 0-10 kHz at 300 K: V_n = √(4 × 1.38 × 10⁻²³ × 300 × 10⁷ × 10⁴) = √(1.656 × 10⁻⁹) = 4.07 × 10⁻⁵ V = ≈ 41 µV rms - significant against a small extracellular spike, which is why micro-recording bandwidth is restricted and the amplifier is placed at the headstage.

Where it is used

• Diagnostic equipment manufacturing: ECG, EEG, EMG/NCV and multi-parameter monitors at BPL Medical Technologies, Schiller Healthcare India, Allengers (Chandigarh), Skanray, Recorders & Medicare Systems, Clarity Medical, and the India R&D centres of Philips, Wipro GE, Siemens Healthineers and Nihon Kohden.
• Consumables: disposable Ag/AgCl electrodes, hydrogels and defibrillation pads - a large Indian consumables market; electrode offset and impedance testing to AAMI EC12 is a routine QC job.
• Hospital biomedical engineering (AIIMS, PGIMER, Apollo, Fortis, CMC Vellore): lead placement training, artifact troubleshooting, electrical-safety and leakage-current testing of ICU and cath-lab equipment.
• Implants and neuromodulation: pacemaker and ICD leads, cochlear implants, DBS and FES electrodes; IIT and SCTIMST (Thiruvananthapuram) groups work on electrode materials and testing.
• Wearables and telemedicine: dry/textile-electrode single-lead ECG patches and ambulatory Holter devices in Indian health-tech start-ups; signal quality is almost entirely an electrode problem.
• Job roles: biomedical design engineer (analog front end), test and validation engineer for IEC 60601, regulatory-affairs associate for CDSCO submissions, clinical application specialist, and research posts in neuroengineering and brain-computer interfaces.

Interview questions

• Q: Why must a biopotential electrode be treated as a transducer? A: Current in tissue is carried by ions and in the lead by electrons; the electrode-electrolyte interface performs that conversion through a redox reaction, and it adds its own EMF, impedance and noise.
• Q: Why is Ag/AgCl preferred over platinum for recording? A: It is non-polarizable - charge crosses freely through the reversible AgCl/Cl⁻ reaction, giving low, almost frequency-independent impedance and a stable half-cell potential with little drift or motion artifact.
• Q: What is the half-cell potential and why does it matter? A: The equilibrium interface voltage (+0.223 V for Ag/AgCl vs SHE); a mismatch between two electrodes appears as a DC offset of tens of millivolts, far larger than a 1 mV ECG, so recording systems are AC-coupled or offset-corrected.
• Q: Explain the electrode equivalent circuit and its frequency behaviour. A: E_hc in series with R_d parallel C_d and a series R_s; impedance is R_s + R_d at DC and falls to R_s above the corner 1/(2πR_dC_d), so quoted impedance is meaningless without a frequency.
• Q: A patient's ECG shows 50 Hz on lead II only. What do you check first? A: Electrode impedance imbalance and a loose or dry electrode on that pair - the mismatch converts common-mode mains voltage into a differential signal; re-prep the skin before touching any filter.
• Q: How does a floating electrode reduce motion artifact? A: The Ag/AgCl disc is recessed and contacts the skin only through a gel column, so skin movement does not mechanically disturb the metal-electrolyte double layer where the potential is generated.
• Q: Why is the resting potential closer to E_K than to E_Na? A: The resting membrane is roughly 25-100 times more permeable to K⁺, and the Goldman equation weights each ion by its permeability.
• Q: What is the difference between the Nernst and Goldman equations? A: Nernst gives the equilibrium potential of a single ion at zero net current; Goldman gives the steady-state membrane potential when several ions with different permeabilities move simultaneously.
• Q: Why do microelectrodes need an amplifier with input impedance above 10¹² Ω? A: Their source impedance is 1-100 MΩ, so a lower input impedance would load the source, attenuate the signal and distort its frequency response.
• Q: What does the right-leg drive circuit do, and why not simply earth the patient? A: It feeds the inverted common-mode signal back to the body through a large resistance, actively reducing common-mode voltage; a direct earth connection would create a low-impedance fault path and violate leakage-current limits.
• Q: Which standards would you test an ECG electrode and machine against in India? A: AAMI EC12 for electrode performance (impedance, DC offset, defib recovery), IEC 60601-1 with 60601-2-25/-2-27 for the machine (adopted in the IS 13450 series), ISO 10993 for gel biocompatibility, under CDSCO's Medical Devices Rules, 2017.
• Q: Why is the required bandwidth 0.05-150 Hz for diagnostic ECG but 0.5-40 Hz for monitoring? A: Diagnostic reading needs faithful ST segments (low-frequency fidelity) and sharp QRS detail; monitoring trades that away to suppress baseline wander and muscle noise during movement.

Common mistakes

• Treating the electrode as a plain resistor and ignoring the half-cell EMF and double-layer capacitance - then being unable to explain baseline drift or saturation.
• Using the wrong sign or forgetting z = −1 for chloride in the Nernst equation, or inverting the chloride term incorrectly in the Goldman equation.
• Mixing 26.7 mV (natural log) with 61.5 mV (log₁₀) in the same calculation, or using 25 mV (20 °C) for body temperature.
• Quoting an electrode impedance without its test frequency, and assuming a "1 kΩ" electrode is 1 kΩ in the ECG band.
• Blaming the amplifier for 50 Hz interference when the real cause is electrode impedance imbalance or poor skin preparation; reaching for a notch filter that then distorts the QRS.
• Confusing polarizable with non-polarizable, or recommending stainless steel/platinum for low-frequency recording and Ag/AgCl for high-current stimulation.
• Confusing the EEG 10-20 naming convention (odd = left, even = right, z = midline) and placing V1-V6 chest electrodes by eye instead of by intercostal landmarks.
• Writing ECG amplitude in microvolts and EEG in millivolts - an order-of-magnitude error that interviewers pick up immediately.

Related topics

Biomedical instrumentation · ECG, EEG and EMG instrumentation · Signal conditioning and data acquisition · Biomedical signal processing · Patient monitoring and life-support equipment · Electrical safety and medical device standards.