Science in pics: A next level pacemaker, smaller than a grain of rice!
It has a galvanic cell as battery and could be injected into the body to regulate heartbeats for up to 3 weeks. After it´s job is done, it simply dissolves into the body. It is not yet FDA approved.

https://www.scientificamerican.com/article/tiny-injectable-pacemaker-runs-on-light-and-then-dissolves/