Organic electrochemical transistors (OECTs) are crucial for next-generation (bio-)electronic devices but are often constrained by the use of aqueous electrolytes, which introduce crosstalk, hinder miniaturization, and limit circuit integration. Here, a photo-patternable solid-state electrolyte based on iota-carrageenan (iota-CGN) and poly(ethylene glycol) diacrylate (PEGDA) is presented, enabling high-performance OECTs and complementary circuits. The iota-CGN electrolyte exhibits high ionic conductivity (>10 mS cm(-1)), comparable to a 0.1 m NaCl aqueous electrolyte, while supporting precise patterning down to 15 mu m, fast transient response times, minimal hysteresis, and excellent stability in both p- and n-type OECTs. Compact solid-state NAND/NOR gates (500 x 800 mu m(2)), 4-input NAND gates (1600 x 800 mu m(2), 8 OECTs), and half-adders (2 x 1 mm(2), 18 OECTs) are demonstrated, all exhibiting correct logic functions and low-voltage operation. To highlight its potential for implantable bioelectronics, solid-state spiking circuits, monolithically integrated with flexible cuff electrodes, are developed for vagus nerve stimulation in mice. These findings establish iota-CGN-based solid-state electrolytes as a promising platform for scalable, implantable circuits, paving the way for next-generation bioelectronic devices.