Figure 1: Principles and example of the ionoprinting process.

(a) By applying a potential to a patterned Cu anode, Cu²⁺ ions complex locally with anionic hydrogels; (b) side view of an ionoprinted pNaAc gel using a 3.2-mm diameter copper cylinder at various time steps (5 V). The current determines the amount of copper bound to the hydrated network, which controls the depth of imprinting and degree of gel curvature as a result of ionic crosslinking. Scale bar=3 mm; (c) ionoprinting current and imprinted depth versus time based on triplicate experiments (5 V). The error bars represent s.d. The depth of the copper-imprinted layer was measured by micrographs of cross-sectional slices of the hydrogel. The superimposed blue line is only used to guide the eyes. Using inert graphite electrodes results in a background current of ~15 mA cm⁻² for the natural ionic conduction of the hydrogel.