Mimicking the human brain is a dream of many scientists, both the computer world, which is envious of the energy efficiency and learning character of our brain, and the medical world, which seeks solutions to neural problems. Scientists at the Eindhoven University of Technology, the Italian Institute of Technology, and  Stanford University have succeeded in developing artificial synapses that can actually communicate with living cells. This system may be used in the future to ‘connect’ prostheses to the brain. The results are published in Nature Materials.

The brain consists of nerve cells that send electrochemical signals (neurotransmitters and ions) to each other. These cells talk to each other through two synapses and a narrow cleft in between that serves, as it were, the means to transport the signals. Each time a signal passes through the synaptic cleft, the connection becomes stronger and the transmission expends less energy. This is because the signals permanently adjust the conductivity of the receiving synapse. This strengthening of the path is the way the brain learns. The receiving synapse not only processes the signals, but it also has a memory function. All this makes it an ultra-efficient and learning system.

In 2017, researcher Yoeri van de Burgt, then a postdoc at Stanford University, successfully developed an artificial synapse made of organic materials. With the same research team, he has now been able to make this synapse actually communicate with living cells that resemble nerve cells. Van de Burgt: “Just like a real brain, our system appears to have a learning and a memory function. This brings us one step closer to an adaptive connection with the brain, which makes advanced prostheses and regenerative medicine possible.”

Yoeri van de Burgt, researcher microsystems, Mechanical Engineering, TU Eindhoven, photo © Bart van Overbeeke

“Most research groups that work on measuring brain activity and brain-machine interfaces are only able to measure electrical signals. But those signals are only a derivative of the processes in the synapse. We can really mimic the process. We work, just like the brain itself, with electrochemical signals. That makes our approach more efficient but also more relevant,” says Van de Burgt.

The system of two synapses and a synaptic cleft was reconstructed by the researchers using two conductive electrodes consisting of a soft polymer and an electrolyte solution in between. Subsequently, the researchers were able to stick the living cells on top of the first electrode and feed them via a culture medium.

Prostheses

The next step for Van de Burgt is to really apply his system to problems in medicine. For example, to develop better prostheses that can really communicate with the body and the brain or to repair non-functioning parts of the brain with adaptive material.

But Van de Burgt also dreams about applications that are even more difficult. For example, patients with a severed spinal cord as a result of an accident. You would rather not reconnect them 1-on-1, because then the ability to learn disappears. If instead, Van de Burgt can put an adaptive system in between, the nerves will be able to communicate with each other again. Van de Burgt: “But that’s still a distant future dream because at that point in the spinal cord you have no synapses nearby. And that’s what our current system is built on.”

Main picture: The artificial synapse made of organic materials. The electrical probes (metal pieces) measure the conductivity. The microfluidic system (tubes above) feed the living cells and restore the synapses to their original state. Photo: Yoeri van de Burgt.

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