Medical electrodes are the contact points that connect living tissue to electronic medical devices. They are used to record signals in tests such as ECG and EEG, and they can also deliver controlled stimulation in therapies such as nerve or muscle stimulation. Good electrode design matters because the body produces very small voltages that can be hidden by noise, motion, or poor contact.
A reliable electrode helps turn tiny biological activity into useful medical information.
A typical surface electrode has a conductive metal layer, a gel or adhesive layer, and a lead wire that connects to an instrument. Ions carry charge in body fluids, while electrons carry charge in the wire, so the electrode interface must convert between ionic and electronic current. For recording, the device measures voltage differences between electrodes without drawing much current.
For stimulation, the device applies a carefully controlled current or voltage so charge enters tissue safely and predictably.
Key Facts
- Voltage is measured between two electrodes: V = V1 - V2.
- Ohm's law relates voltage, current, and resistance: V = IR.
- Electrode contact impedance should be low and stable to reduce noise and signal loss.
- ECG signals are typically about 0.5 mV to 5 mV, so small interference can matter.
- Current density is current per area: J = I/A.
- For stimulation safety, delivered charge is charge per pulse: Q = It.
Vocabulary
- Electrode
- An electrode is a conductive contact that transfers electrical signals between a device and the body.
- Bioelectrical signal
- A bioelectrical signal is a voltage or current produced by activity in cells such as heart, brain, nerve, or muscle cells.
- Contact impedance
- Contact impedance is the opposition to electrical signal flow at the electrode skin interface.
- Electrolyte gel
- Electrolyte gel is a conductive gel that improves contact by carrying ions between skin and electrode.
- Stimulation pulse
- A stimulation pulse is a brief controlled electrical output delivered by a device to activate tissue.
Common Mistakes to Avoid
- Treating an electrode like a simple wire is wrong because the skin electrode interface has impedance, chemistry, and ion to electron conversion.
- Ignoring skin preparation is wrong because dry skin, oil, hair, or loose adhesive can greatly increase noise and contact impedance.
- Using only one electrode to measure a signal is wrong because most medical recordings measure a voltage difference between two points and often use a reference or ground.
- Assuming stronger stimulation is always better is wrong because high current density or excessive charge per pulse can cause pain, burns, or tissue irritation.
Practice Questions
- 1 An ECG electrode pair measures V1 = 2.3 mV and V2 = 0.8 mV. What voltage difference does the device record?
- 2 A stimulation electrode delivers a current of 12 mA for 200 microseconds. What charge is delivered in one pulse in microcoulombs?
- 3 Explain why adding electrolyte gel can improve an electrode recording even if the metal electrode itself is already a good conductor.