Fig. 1: Electrically driven hydrogel microactuator fabrication, mechanism and devices.

a, (1) Optimization of hydrogel pore size at the nanometre scale using TPP-based 3D printing. Top, TEM image of the 3D-printed hydrogel. Bottom, schematic showing enhanced ion flux and flow from increased EDL overlap in a nanometre-scale hydrogel channel under an electric field. (2) Working mechanism of the hydrogel microactuator. The hydrogel network is divided into region 1 (near the cathode) and region 2 (near the anode). In DI water, dissociated H⁺ ions from the –COOH groups dominate. Under an electric field, concentrated H⁺ in region 1 convert fixed –COO⁻ groups to –COOH, reducing repulsion and shrinking the network, bending the microactuator towards the cathode. In physiological saline, Na⁺ ions dominate; concentrated Na⁺ in region 1 attracts water, swelling the network and bending towards the anode. (3) Schematic of the electrically driven gel microcilia array and AAc-co-AAm hydrogel network structure. b, Gel microcilia array on a polyimide-based microelectrode substrate. (1) Microelectrodes on glass. (2) Flexible substrate with microelectrodes on a human hand. (3) Gel microcilia array composed of four actuation cells, with each cell surrounded by four electrodes. (4) SEM image of the gel microcilia. c, Kinematics of a gel cilium in physiological saline (diameter 2 µm, height 18 µm; 5 Hz). The electrode polarity is marked by + and −. (1) Unidirectional bending along the y-direction. (2) Non-reciprocal 3D anticlockwise rotation. (3) Unidirectional bending along the x-direction. (4) Non-reciprocal 3D clockwise rotation. T denotes one motion cycle. d, Main devices used in this work. (1) 25 gel microcilia (cilium diameter 10 µm, height 90 µm). Each cilium has four surrounding electrodes for individual control. (2) 625 gel microcilia (cilium diameter 10 µm, height 90 µm). Each actuation cell with 25 cilia exhibits synchronized motion (inset, one cell). (3) 10⁶ gel microcilia fabricated by micromoulding, partially shown in this image (cilium diameter 5 µm, height 35 µm; inset, one cell). Scale bars, 100 nm (a (1)); 2 cm (b (1), (2)); 200 µm (b (3), (4)); 6 µm (c (4)); 40 µm (d (1)); 200 µm (d (2)); 100 µm (d (2) inset); 300 µm (d (3)); 60 µm (d (3) inset).