Customizable 3D Microelectrode Arrays Enabled by Two-Photon Polymerization and Liquid Metal Integration for Neural Interface Applications.

Conventional silicon-based microfabrication with both 2D and 3D geometries has enabled increasingly sophisticated neural microelectrode arrays; however, the inherent rigidity of silicon fundamentally limits both the mechanical compliance necessary for chronic implantation in soft neural tissues and the geometric customizability achievable through microfabrication processes. Here, we present a silicon-free strategy in which soft polymeric 3D neural microelectrode arrays are fabricated via customizable two-photon polymerization (2PP) and readily functionalized as electrodes by simple injection of liquid-phase gallium (Ga) into the printed hollow channels. The liquid Ga confinement in the 3D-printed hollow microneedle is characterized by a newly defined "retention number (Rt)" enabling the prediction of stable Ga confinement under physiological conditions. Also, to overcome the high impedance arising from the natively formed oxide on Ga, the microelectrode surface was modified with gold nanoparticles and then poly(3,4-ethylenedioxythiophene) with tetrafluoroborate dopants (BF4) (PEDOT:BF4), resulting in an approximately 3-order-of-magnitude decrease in impedance and improved electrochemical stability. Lastly, in vivo recordings in an invertebrate model successfully captured neural spiking activity with no Ga electrode leakage from the microelectrodes. These results demonstrate an effective strategy for integrating liquid metal electrodes into customizable 2PP 3D-printed structures, offering a versatile platform for next-generation soft bioelectronic devices.