How were radioisotope thermoelectric generators used in cardiac pacemakers?

In the 1970s, radioisotope thermoelectric generators (RTGs) were developed and utilized as power sources for cardiac pacemakers. These devices leveraged the heat produced by the nuclear decay of specific radioactive isotopes to generate the electrical energy required to power the implants.12

Specifically, the isotope plutonium-238 was used for this application due to its favorable heat-producing properties and long half-life. This technology provided a reliable, long-lasting power solution for patients before the subsequent availability of high-efficiency chemical batteries, which eventually superseded the use of nuclear-fueled generators in pacemakers.1

RTGs function by utilizing a temperature differential between a radioactive heat source and a heat sink to produce direct current through a thermoelectric converter, a process known as direct power conversion. Because these generators are highly dependable and capable of operating for long periods without maintenance, they were ideal for remote or unattended applications, including medical implants during that era.231