Electrolyte-Gated Synaptic Transistor for Artificial Electronic Skin Through Bioinspired Skin-Healing Snail Secretion

The sense of touch is an important way the human body interacts with the environment, enabling to accurately perceive, grasp, and manipulate a variety of objects. However, most studies for tactile sensing lack learning and memory functions, making active and direct interaction with the environment difficult. Recently, by inserting a neuromorphic computing system that mimics the functions and information processing of the biological nervous system into the conventional electronic skin (E-skin), many researchers have attempted to implement a future E-skin that can simultaneously perform cognition, learning, and memory. For the realization of artificial synapses required for neuromorphic computing systems, electrolyte-gated synaptic transistors (EGSTs) have been studied due to various advantages such as low power consumption, printability, and the opportunity to fabricate novel device architectures. However, biocompatibility must be considered for application to artificial E-skin.<br/>Herein, we propose EGSTs using snail secretion as the electrolyte, which is a biocompatible and skin-healing material suitable for natural skin for artificial E-skin. To fabricate snail secretion EGSTs, we deposited indium gallium zinc oxide (IGZO) channel by radiofrequency (RF) magnetron sputtering on a heavily boron-doped silicon wafer with a thermally oxidized 120 nm thick silicon dioxide film, as the substrate. Thereafter, indium tin oxide (ITO) electrodes as the source and drain were also deposited by RF magnetron sputtering process via shadow mask consisting of a width/length ratio of 1000/150 µm. For activation of IGZO and ITO film, they were annealed at 300°C in the air for 1 h. Finally, snail secretion was dripped on the top of the IGZO channel.<br/>Snail secretion including glycolic acid, allantoin, and mucin, which were skin-healing functional materials, contains a large number of hydroxyl groups, which can promote the transfer of protons (i.e., hydrogen ions) through temporary hydrogen bond formation. Therefore, it is suitable for the implementation of biological synapses. To verify the biological synapses, we confirmed the anticlockwise hysteresis characteristic of snail secretion EGSTs under the gate voltage (V_G) sweep ±3.0 V. A negative threshold voltage (V_th) shift from −0.32 to −0.66 V was observed. The negative V_th shift could be caused by protons trapped at the interface between the IGZO and snail secretion. It could be predicted by the synaptic weight of biological synapses. Therefore, to investigate the synaptic weights, we measured the paired-pulse facilitation (PPF) characteristics of snail secretion EGSTs. PPF means that a larger second excitatory postsynaptic current (EPSC) is triggered when the second pre-synaptic spike pulse is applied before the first EPSC is recovered to the initial value. When paired pre-synaptic spikes with a pulse width of 10 ms apply with a time interval (TI) of 10 ms, the PPF index, defined as the amplitude ratio between the second EPSC (A₂) and the first EPSC (A₁), was 168.8%. And, to confirm the learning-forgetting (L-F) characteristics of our devices, we conducted consecutive 100 positive pulses (V_G of 1 V, TI of 10 ms) and followed by 50 negative pulses (V_G of −0.75 V, TI of 10 ms). As a result, it was confirmed that our device has precise control for L-F by observing its linear characteristics. Finally, we measured the spike rate dependent plasticity characteristics of our devices, which can be judged on the ability of the artificial E-skin to be subjected to a stimulus applied to a certain abnormality. When 800 nA was set as the threshold, the 30 pulses applied at 10, 20, and 25 Hz did not exceed the threshold, but the threshold was exceeded at 50 Hz. These results could be recognized as pain by repeated and strong stimulation in the artificial E-skin. Therefore, we believe that snail secretion EGSTs will contribute to the development of improved artificial E-skin.