pacemaker electronics
A pacemaker is an implanted electronic conductor for a heart that has lost the beat — a battery, a tiny computer, and a wire (the 'lead') threaded into the heart muscle, all sealed in a titanium case the size of a coin. When the heart's own natural pacemaker fails to fire on time, the device delivers a small, precisely timed electrical pulse through the lead's electrode, triggering the chamber to contract. Crucially it doesn't beat blindly: it first listens. A modern 'demand' pacemaker continuously senses the heart's own biopotentials and stimulates only when a beat is missing — so it stays silent while you exercise and your own heart races, and steps in only when needed.
The defining engineering constraint is energy. The battery (a lithium-iodine cell) cannot be recharged and must last 7–10 years inside a sealed body, so every microamp counts: the circuit sleeps in nanoamp standby, the stimulus pulse is just ~2–5 V for ~0.5 ms delivered into a ~500 Ω lead, and the sensing amplifier must detect millivolt cardiac signals while ruthlessly rejecting muscle noise and external interference. It marries a low-noise biopotential front-end (to sense) with a high-voltage, charge-balanced output stage (to stimulate) — the two halves of biomedical electronics, sensing and stimulation, fused in one life-critical chip that must never glitch.
A demand pacemaker spends most of its life sensing in nanoamp standby and only occasionally fires a stimulus.
Output pulses must be charge-balanced (a stimulating phase followed by an equal, opposite recovery phase) so no net DC current flows into tissue — a sustained DC would electrolyse body fluid and corrode the electrode, the same hazard that governs all implanted neural stimulators.