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https://medicalxpress.com/news/2020-06-artificial-synapse-cells.html

In 2017, Stanford University researchers presented a new device that mimics the brain's efficient and low-energy neural learning process. It was an artificial version of a synapse—the gap across which neurotransmitters travel to communicate between neurons—made from organic materials.

Now, in a paper published June 15 in Nature Materials, they have tested the first biohybrid version of their artificial synapse and demonstrated that it can communicate with living cells. Future technologies stemming from this device could function by responding directly to chemical signals from the brain.

"The cells are happy sitting on the soft polymer. But the compatibility goes deeper: These materials work with the same molecules neurons use naturally."

While other brain-integrated devices require an electrical signal to detect and process the brain's messages, the communications between this device and living cells occur through electrochemistry—as though the material were just another neuron receiving messages from its neighbor.

The biohybrid artificial synapse consists of two soft polymer electrodes, separated by a trench filled with electrolyte solution—which plays the part of the synaptic cleft that separates communicating neurons in the brain. When living cells are placed on top of one electrode, neurotransmitters that those cells release can react with that electrode to produce ions. Those ions travel across the trench to the second electrode and modulate the conductive state of this electrode. Some of that change is preserved, simulating the learning process occurring in nature.

To test their device, the researchers used rat neuroendocrine cells that release the neurotransmitter dopamine. Before they ran their experiment, they were unsure how the dopamine would interact with their material—but they saw a permanent change in the state of their device upon the first reaction.

"We knew the reaction is irreversible, so it makes sense that it would cause a permanent change in the device's conductive state,"

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.Following dopamine oxidation, the resulting change in the charge state of the gate electrode induces ion flow in the aqueous electrolyte, thus altering the conductance of the postsynaptic channel, as explained in the literature (ref. 11). The dopamine oxidation process emulates the postsynaptic receptor binding observed in biological synapses, while the modified channel conductance emulates synaptic weight modulation by the neurotransmitter. When dopamine is present in solution, the change in the synaptic weight of the postsynaptic channel consists of both a shortand a long-term component (Fig. 1e). Indeed, the flow of ions under the applied postsynaptic gate bias results in short-term modulation of the postsynaptic channel conductance (independent of dopamine concentration) as previously reported 26, whereas dopamine oxidation results in long-term conditioning of the neuromorphic channel.

When using the cell culture media as an electrolyte (in the absence of cells and absence of dopamine), a voltage pulse at the gate electrode (Vpost) results in the reversible (short-term) change in conductance of the PEDOT:PSS neuromorphic channel due to ion flow into the channel changing its doping state (Supplementary Fig. 1)10,29. When dopamine is introduced into the electrolyte solution, the conductance change is enhanced due to the oxidation of dopamine at the postsynaptic gate electrode 

Crucially, unlike when only the cell culture media is present, the conductance change resulting from dopamine oxidation is permanent as a result of the irreversibility of this reaction

 

A biohybrid synapse with neurotransmitter-mediated plasticity, Nature Materials (2020). DOI: 10.1038/s41563-020-0703-y , www.nature.com/articles/s41563-020-0703-y