Hydrogel iontronic devices can emulate biological functions and communicate with living matter. But the fabrication of miniature, soft iontronic devices according to modular designs has not been achieved. In this work, we report the use of surfactant-supported assembly of freestanding microscale hydrogel droplets to construct various iontronic modules, circuits, and biointerfaces. Chemical modifications of silk fibroin produced a pair of oppositely charged hydrogels. Microscale assembly of various combinations of hydrogel droplets produced iontronic diodes, npn- and pnp-type transistors, and diverse reconfigurable logic gates. Through the incorporation of poly(amino acid)s, we have demonstrated a droplet-based synthetic synapse with ionic polymer-mediated long-term plasticity. Further, our iontronic transistor can serve as a biocompatible sensor to record electrophysiological signals from sheets of human cardiomyocytes, paving a way to the building of miniature bioiontronic systems.