Flexible multimode electrochromic artificial synapses for visualized health monitoring

Artificial synapses have attracted growing attention in artificial intelligence and neuromorphic computing. Currently available synaptic devices often adopt electrical parameters as their synaptic weight, whereas electrochromic artificial synapses with optical synaptic weight, providing unique visual functionality, are highly desirable yet challenging. We report a universal strategy to construct flexible multicolored hydrogel devices with different viologen derivatives. Using transmittance as their optical postsynaptic potential, these viologen-based artificial synapses demonstrate synaptic plasticity in coloration, dependent on the amplitude, duration, number, and frequency of electrical stimulating pulses. The synaptic features of the electrochromic devices, such as paired-pulse facilitation and short-term memory, can be regulated readily by selecting different viologen molecules. Typically, in response to a low-voltage stimulus of 2.0 V at 1.0 Hz, the transmittance of the viologen-based device is significantly reduced by 84 % after 100 pulses, with a short-term retention exceeding 300 s. By leveraging a simple combination of two different wearable electrochromic synapses with distinct levels of synaptic plasticity, electrocardiogram signals are able to be converted into intuitive color changes, offering real-time visualized health feedback. This work not only develops a versatile approach to electrochromic artificial synapses, but also expands their application scenarios to health monitoring through straightforward physiological information visualization.