Researchers in Japan have developed a flexible paper-based sensor that operates like the human brain.

The optoelectronic synaptic device, using physical reservoir computing, exhibits synaptic behavior and supports cognitive tasks at a timescale suitable for health monitoring,” says Dr. Takahashi Ikuno, senior author of a paper published online in the journal Advanced Electronic Materials.

An AI-based health monitoring and biological diagnosis device

To achieve low-power consumption for prolonged use, the AI-based health monitoring and biological diagnosis device uses a standalone sensor that operates independently, with no need for constant connection to a central server, explains Ikuno.

“It should also be capable of handling rapidly changing biological signals for real-time monitoring, be flexible enough to attach comfortably to the human body, and be easy to make and dispose frequent replacements for hygiene reasons.”

Artificial synapse design

To achieve those goals, the researchers fabricated a photoelectronic artificial synapse device, comprising gold electrodes on top of a 10 µm transparent film consisting of zinc oxide (ZnO) nanoparticles and cellulose nanofibers (CNFs).

The transparent film serves three main purposes:

It allows light to pass through, enabling it to handle optical input signals representing various biological information.

The cellulose nanofibers impart flexibility and can be easily disposed by incineration.

Synapse-like design

The zinc oxide nanoparticles are photoresponsive and generate a photocurrent when exposed to pulsed UV light at a constant voltage. This photocurrent mimics the responses transmitted by synapses in the human brain, enabling the device to interpret and process biological information received from optical sensors.

Notably, the film was able to distinguish 4-bit input optical pulses and generate distinct currents in response to time-series optical input, with a rapid response time on the order of subseconds. This quick response is crucial for detecting sudden changes or abnormalities in health-related signals.

When exposed to two successive light pulses, the electrical current response was stronger for the second pulse. This behavior termed post-potentiation facilitation contributes to short-term memory processes in the brain and enhances the ability of synapses to detect and respond to familiar patterns.

To test this, the researchers converted MNIST images (a standard dataset of handwritten digits) into 4-bit optical pulses. They then irradiated the film with these pulses and measured the current response. Using this data as input, the neural network was able to recognize handwritten numbers with an accuracy of 88%.

Wearable sensors for health monitoring

Remarkably, this handwritten-digit recognition capability remained unaffected, even when the device was repeatedly bent and stretched up to 1,000 times, demonstrating its ruggedness and feasibility for repeated use. 

“This study highlights the potential of embedding semiconductor nanoparticles in flexible CNF films for use as flexible synaptic devices for PRC,” concludes Ikuno. The research paves the way for wearable sensors in health monitoring applications, he suggests.

Citation: Komatsu, H., Hosoda, N., Kounoue, T., Tokiwa, K., & Ikuno, T. Disposable and Flexible Paper-Based Optoelectronic Synaptic Devices for Physical Reservoir Computing. Advanced Electronic Materials, 2300749. https://doi.org/10.1002/aelm.202300749 (open-access)

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