I don’t know how exactly this one will be done specifically but I have a (much larger) passive implant that was inserted via a catheter in a vein in my leg and guided up to my atrium, with an endoscope down my throat to see it as it was positioned. My atrial septal defect occluder was not only precisely positioned inside my heart but unwrapped from a shape that can fit down a vein to a stacked flat disc shape that clamps on two side of the heart wall to hold itself in place.

This technique is being expanded to robotic catheters that can carry out the precise surgery automatically and there are the simpler “deployable stabilization devices” that are used to stabilize the heart muscle. If this is really small enough to be injected, it should work with either of those methods rather noninvasively.

I know basically nothing about this area so take all this with a large grain of salt (or rice), but my understanding is they can go through the femoral or brachial vein to inject things directly into the heart.

https://en.wikipedia.org/wiki/Peripherally_inserted_central_...

They mention the use case of temporary pacing after surgery (specifically children where the desolving of the device is a great feature that leaves less abandoned litter behind as the grow).

I imagine if it's useful for adults, something that small will probably be put in using a catheter (similar to how stents often are placed).

Mine was done under sedation and inserted under my pec (for protection as I lift a lot) with the wires fed into my heart (I assume).

A cardiac electrophysiologist advances a catheter into the right ventricle and then deploys this directly onto the heart muscle. This is only a little bit smaller than existing leadless pacemakers which are in wide use.

I've access to the article, this snippet explains it (taken directly from the article, but formatted for HN).

"The device exploits a self-powered mechanism, where the battery electrodes are the pacing electrodes. Specifically, an active, bioresorbable magnesium (Mg) alloy AZ31(Mg 96 Al 3 Zn 1) foil or a zinc (Zn) composite (1.6 mm × 1.6 mm) serves as the anode, and a more electropositive bioresorbable molybdenum trioxide (MoO3) composite (1.6 mm × 1.6 mm) serves as the cathode 18–20.

The cardiac tissue and associated biofluids act as the electrolyte to form a galvanic cell/battery (Mg–MoO3 or Zn–MoO 3). As a demonstration, the pacemaker utilizes the Mg–MoO3 pair if not otherwise specified. The two electrodes electrically interconnect through a silicon (Si) bipolar junction phototransistor (Supplementary Figs. 1 and 2), designed to respond at tissue-penetrating wavelengths in the near-infrared (NIR) range. This component provides an optical mechanism to control the operation of the device with an external light source. In particular, the anode and cathode connect to the emitter and the collector terminals of the phototransistor, respectively, using a biodegradable conductive paste (Candelilla wax/tungsten (W) powder)21. A bioresorbable formulation of polyanhydride 22 or wax 21 encapsulates the entire structure, leaving regions of the electrodes exposed to the interface with the cardiac tissue. These unusual materials, components, and operating mechanisms serve as the basis for the pacing technology"

Indeed there's no comment about the "bioresorbability" of silicon, possibly it's such an small quantity it doesn't matter.

I think the pacing/logic part is in the patch worn on the skin surface. From what I understand the rice grain part converts an optical signal it sends into an evoked potential.

Does this grain of rice make me look fat?

Perhaps these will eventually be simple pills, and the nanobot finds its way through the body, either on its own or perhaps with some external electromagnetic field.