Silicon microfabrication has produced increasingly sophisticated 2D and 3D neural microelectrode arrays, but silicon’s rigidity limits both the mechanical compliance needed for chronic implantation in soft neural tissue and the geometries that can be made. The team took a silicon-free route: soft polymeric 3D microneedle arrays are printed by customizable two-photon polymerization (2PP), then turned into electrodes simply by injecting liquid-phase gallium into the printed hollow channels. The researchers define a ‘retention number’ to predict whether the liquid gallium stays confined inside the microneedle under physiological conditions.
The oxide that forms natively on gallium raises impedance. To counter it, the electrode surface was modified with gold nanoparticles and then with PEDOT doped with tetrafluoroborate, which the authors report cut impedance by approximately 3 orders of magnitude and improved electrochemical stability. In vivo recordings in an invertebrate model captured neural spiking activity with no gallium leaking from the electrodes. The authors describe the approach as a versatile platform for integrating liquid-metal electrodes into customizable 2PP-printed structures for next-generation soft bioelectronic devices.