Extended Data Fig. 1: Device fabrication and assembly.

(a) Left, schematic illustration of the circuit base of the electronic module of the device using three-layer flexible printed circuit board (fPCB). Right, images of the fPCB for the electronic module. Scale bar: 5 mm. (b) Left, schematic illustration of the assembly of electronic components on the fPCB of the electronic module using hot-air soldering. Right, images of the electronic module after hot-air soldering of electronic components. (c) Left, schematic illustration of the circuit base of the FOD using three-layer fPCB. Right, magnified view of the soldering site for the μ-ILED with solder paste. Scale bar: 500 µm. (d) Left, schematic illustration of the FOD after hot-air soldering of the μ-ILEDs. Right, magnified view of the soldered μ-ILED. (e) Left, schematic illustration of the serpentine traces of the device using a five-layer fPCB. Right, image of the serpentine traces on the fPCB. Scale bar: 5 mm. (f) Left, image of the serpentine traces after laser ablation from the fPCB substrate. Right, scanning electron microscopic (SEM) image of the serpentine traces from lateral view. The experiment was repeated independently on 3 serpentine traces. Scale bar: 300 µm. (g) Schematic illustration of the final assembly of the electronic module, serpentine traces, and FOD. (h) Image of the final device after assembly. (i) Schematic illustration of the mold used for final silicone (Ecoflex 00-30) coating after parylene-C encapsulation. (j) Photograph of the device during mold casting of silicone. (k) Image of a fully encapsulated device ready for in vivo experiments.