Robust biodegradable synapse with sub-biological energy and extended memory for intelligent reflexive system

The study reports a fully biodegradable multilayer artificial synapse (M-AS) based on a trilayer architecture of chitosan–guar gum (CS–GG) ion-active layers embedding NaCl mobile ions, a cellulose acetate (CA) ion-binding layer with high dipole moment, and Mg electrodes, where synaptic functionality arises from ion–dipole coupling (IDC) at the IAL–IBL interface: applied stimuli drive Na⁺/Cl⁻ migration toward CA, whose aligned dipoles trap ions beyond conventional electric double-layer effects, generating asymmetric postsynaptic currents and enabling memory retention. This mechanism allows faithful emulation of biological synaptic behaviors, including paired-pulse facilitation, short- and long-term plasticity, and multilevel conductance states controlled by pulse amplitude and number. Owing to efficient ionic transport and retention at ultralow voltages, the device achieves sub-biological energy consumption (~0.85 fJ per event) and an extended long-term memory of ~5944 s, the longest reported for biodegradable synapses, further enhanced by stacking multiple IAL–IBL layers to increase ion storage and IDC strength. Material optimization supported by DFT and dielectric analysis identifies CA as superior to other biodegradable dielectrics (e.g., PCL, PLA) for strong ion–dipole interactions, while Na⁺ provides an optimal balance between mobility and stability compared to Li⁺ or K⁺. Integrated with a thermistor sensor and actuator, the M-AS enables an intelligent reflexive system exhibiting adaptive learning and stimulus-history-dependent responses, demonstrating its applicability beyond memory storage. Overall, the work establishes a scalable, sustainable neuromorphic element that combines ultralow-energy operation, robust plasticity, and biodegradability, pointing toward environmentally friendly, bioinspired computing and interfacing platforms.

Source : https://www.nature.com/articles/s41467-025-66511-3