Cellulose Fiber Tortuosity as a Bioinspired Design Strategy for Light-Driven, Self-Powered Ionotronic Synapses

The drive toward energy-efficient, brain-inspired computing has spurred interest in ionotronic synapses that mimic ionic signaling in neurons. However, most light-driven synaptic devices overlook the influence of ion transport architecture, an essential aspect of biological learning and memory, which is shaped by the structural complexity of neural tissue. Here, we present a two-terminal ionotronic synaptic device constructed on cellulose fiber substrates, such as thread, cloth, and paper, offering tunable tortuosity to mimic complex ion pathways. Carbon electrodes patterned on these fibers are separated by a gap (∼1 mm) filled with the ionic liquid 1-ethyl-3-methylimidazolium acetate (EMIM: OAc), while lanthanum hexaboride (LaB₆) nanoparticles are selectively coated near the gap on one electrode. The device operates in a self-powered mode, utilizing LaB₆-induced light-to-heat conversion and thermodiffusion in EMIM: OAc, and enables light-modulated synaptic responses. Our findings reveal that upon illumination (λ_laser = 638 nm, ∼500 mW), increasing substrate tortuosity reduces the excitatory post-synaptic potential (EPSP) amplitude (∼127 mV, ∼109 mV, and ∼26 mV) when illuminated for 1 s and enhances the retention time (∼245 s, ∼270 s, and ∼4224 s) under a 2.5 Hz light frequency with 10 s on and 20 s off cycles, corresponding to thread, cloth, and paper, respectively. In addition to EPSP, the device emulates paired-pulse facilitation (PPF) and spike-dependent plasticity by modulating optical parameters such as frequency, intensity, duration, and pulse number with operation spanning 450-808 nm. Demonstrations of optically driven Morse code and binary-to-hexadecimal data encryption further showcase the potential for secure communication. This work highlights fiber tortuosity as a previously underexplored yet powerful design variable for developing light-responsive, flexible ionotronic systems for neuromorphic vision and communication technologies.