Fig. 4: System assembly.

a Schematic diagram of back-to-back alignment of two sub-arrays with two needles inserted through the identical round holes into a PDMS layer. The inset shows the two sub-arrays assembled together using epoxy dispensed at reserved positions. b Two flexible printed circuit boards (FPC-1 and FPC-2) hot-pressed to the contact pads of the micro-LED array and microelectrode array, respectively. c Bond of two FPCs close to the interfaces with a PI tape. d Side view of the assembled device with hybrid recording and stimulating capabilities