Implanted medical devices such as pacemakers, defibrillators, neurostimulators, and drug pumps need reliable electrical power inside the body. A pacemaker uses a small battery and electronic circuit to monitor heart rhythm and send carefully timed pulses when needed. Long battery life matters because replacing an implant usually requires surgery.
Engineers must balance energy storage, safety, size, and biocompatibility.
Key Facts
- Electrical energy used by a device is E = P t, where P is power and t is time.
- Battery capacity is often given in ampere-hours, with charge Q = I t.
- Average power for a pulsed implant can be estimated by Pavg = E pulse x pulses per second.
- Lithium-based primary batteries are common in pacemakers because they have high energy density and stable voltage.
- Rechargeable implants often use inductive coupling, where an external coil transfers energy to an implanted coil through a changing magnetic field.
- Implant electronics must use very low current because even microampere loads add up over years.
Vocabulary
- Pacemaker
- A pacemaker is an implanted device that helps control heart rhythm by sending small electrical pulses to heart tissue.
- Electrode lead
- An electrode lead is an insulated wire that carries signals between an implant and the target tissue.
- Battery capacity
- Battery capacity is the amount of electric charge a battery can deliver, commonly measured in ampere-hours or milliampere-hours.
- Inductive charging
- Inductive charging transfers energy wirelessly between coils using a changing magnetic field.
- Biocompatible casing
- A biocompatible casing is a protective outer shell, often made of titanium, that can remain in the body without causing harmful reactions.
Common Mistakes to Avoid
- Confusing voltage with battery life is wrong because voltage tells the electrical push, while lifetime depends mainly on total stored energy and average power use.
- Ignoring standby power is wrong because implants spend most of their time monitoring, and a tiny continuous current can use significant energy over years.
- Assuming wireless charging is perfectly efficient is wrong because energy is lost as heat and through imperfect coil alignment.
- Treating all implants as having the same power needs is wrong because a pacemaker, cochlear implant, and neurostimulator can have very different pulse rates, currents, and duty cycles.
Practice Questions
- 1 A pacemaker uses an average power of 12 microwatts. How much energy does it use in 1 year? Give your answer in joules.
- 2 An implant battery has a capacity of 1.2 ampere-hours and supplies an average current of 10 microamperes. Estimate the battery life in years, assuming the full capacity is usable.
- 3 A rechargeable implant is charged through the skin using external and internal coils. Explain why coil alignment and heating are important design concerns.