Artificial synapses are an important step towards emulating the supercomputer that is the human brain. Now scientists have successfully bridged the gap between organic and artificial, with biohybrid synapses that let living cells communicate with electronic systems, not with electrical signals but with neurotransmitters like dopamine.
In the brain, neurons pass signals back and forth across gaps called synapses. This connection gets stronger every time it’s called upon, which is the basis for how we learn. The fact that information is processed and stored in the same part of the brain drastically speeds up recall.
That gives the organic brain a huge advantage over traditional computers, which process and store information in separate places. It makes sense then that emerging computer systems are beginning to mimic the structure of the brain, using artificial neurons and synapses.
The researchers first designed their artificial synapse in 2017. It worked a bit like a transistor, involving three terminals surrounded by a salty water electrolyte. In this way, the terminals act like neurons, sending electrical signals across the water (the synapse) to each other.
For the new study, the team has taken things a step further by creating a biohybrid artificial synapse. This time, the device is made up of two soft polymer electrodes, again separated by an electrolyte solution. But the key difference is that living cells were then placed on top of one of the electrodes, which were able to communicate with the other electrode across the synapse.
When these living cells released their neurotransmitters, they reacted with the electrode below them. That induces the electrode to produce ions, which then travel through the electrolyte to the other electrode, changing its conductive state.
This is a remarkable breakthrough, the team says. Similar devices usually still communicate using electrical signals, but this biohybrid artificial synapse is using the same electrochemical signals that an organic brain uses.
“This paper really highlights the unique strength of the materials that we use in being able to interact with living matter,” says Alberto Salleo, co-senior author of the study. “The cells are happy sitting on the soft polymer. But the compatibility goes deeper: These materials work with the same molecules neurons use naturally.”
In tests, the team placed rat neuroendocrine cells on top of the electrodes. These cells release dopamine, which influences an animal’s behavior by motivating them towards a goal. It does this by permanently changing the conductivity of neurons.
The team wasn’t sure if the chemical would have the same effect on the artificial neurons. But sure enough, when they tried it out the device was permanently changed after just one reaction.
“We knew the reaction is irreversible, so it makes sense that it would cause a permanent change in the device’s conductive state,” says Scott Keene, co-lead author of the study. “But, it was hard to know whether we’d achieve the outcome we predicted on paper until we saw it happen in the lab. That was when we realized the potential this has for emulating the long-term learning process of a synapse.”