A photo of the thin-film transistor arrays on the paper substrate

Brain-mimicking paper electronics revealed

A paper-based electronic device that mimics electrochemical signalling in the human brain has been designed by researchers from China.

The thin-film transistor (TFT) has been designed to imitate the junction between two neurons, known as a synapse, and could help develop artificial neural networks, which could be used in a range of fields from robotics to computer processing.

And the device was even able to mimic a phenomenon known as synaptic plasticity, by which the connection between two neurons become stronger as more and more neurotransmitters are passed across a synapse, which forms the basis of how humans learn and memorise things.

“A paper-based synapse could be used to build lightweight and biologically friendly artificial neural networks, and, at the same time, with the advantages of flexibility and biocompatibility, could be used to create the perfect organism–machine interface for many biological applications,” said Qing Wan, from Nanjing University and corresponding author of the study published in journal Nanotechnology.

The device is also the latest device to be fabricated on paper, making the electronics more flexible, cheaper to produce and environmentally friendly.

The artificial synaptic TFT consisted of indium zinc oxide (IZO), as both a channel and a gate electrode, separated by a 550-nanometre-thick film of nanogranular silicon dioxide electrolyte, which was fabricated using a process known as chemical vapour deposition.

The design was specific to that of a biological synapse – a small gap that exists between adjoining neurons over which chemical and electrical signals are passed. It is through these synapses that neurons are able to pass signals and messages around the brain.

All neurons are electrically excitable, and can generate a ‘spike’ when the neuron’s voltage changes by large enough amounts. These spikes cause signals to flow through the neurons which cause the first neuron to release chemicals, known as neurotransmitters, across the synapse, which are then received by the second neuron, passing the signal on.

Similar to these output spikes, the researchers applied a small voltage to the first electrode in their device which caused protons—acting as a neurotransmitter—from the silicon dioxide films to migrate towards the IZO channel opposite it.

As protons are positively charged, this caused negatively charged electrons to be attracted towards them in the IZO channel which subsequently allowed a current to flow through the channel, mimicking the passing on of a signal in a normal neuron.

The researchers also found their device demonstrated synaptic plasticity as when two short voltages were applied to the device in a short space of time, the second voltage was able to trigger a larger current in the IZO channel compared to the first applied voltage, as if it had ‘remembered’ the response from the first voltage.

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