One of the most remarkable things cells can do is sense chemical inputs in their surroundings and respond to them. These signals can come from outside the body—like smells and flavors—or from inside the body, such as hormones. Hormone signaling also happens in the brain. There, neurons communicate through tiny junctions called synapses and exchanging chemical messengers known as neurotransmitters. Neurons release these messengers in short, controlled bursts called action potentials.
The goal of the SYNAPS project is to mimic this type of communication in a much simpler, fully artificial system. Instead of real neurons, we form tiny artificial “cells” called liposomes—small spherical bubbles made of the same type of fat molecules that form real cell membranes. Each liposome is only about 100 nanometers across and is the simplest model of a cell. In addition, we can easily add whatever molecules we want either into their membrane or into their interior.
Our system, inspired by neuronal communication, uses two types of liposomes: senders and receivers. We also design a pair of molecules that anchor themselves in the liposome membranes and stick tightly to each other once mixed. When sender and receiver liposomes are mixed, these molecules pull together and create a narrow gap between them: an artificial version of a synaptic cleft.
Within this artificial “nanotissue”, we then study how a signal can travel from the inside of the sender liposome to trigger a response inside the receiver liposome. The process begins with shining light on the system. In the sender, a light-activated molecule absorbs this light, triggering a photochemical reaction where it captures an electron from an electron-donor molecule on the inside of the liposome. It then transfers the electron to another molecule outside the liposome, in the synaptic cleft, resulting in the formation of a special messenger compound.
This messenger molecule participates in a chain of fast chemical reactions involving other reagents we place in the cleft. We design these reactions such that, under constant light, the messenger’s concentration rises sharply and then falls again, producing a pulse that mimics the aforementioned action potential.
To sense the messenger, the receiver liposome is equipped with receptor molecules that recognize and temporarily bind the messenger. These receptors span the membrane and transmit the information that the messenger is present to the interior of the receiver, where we can read the information by measuring the color of fluorescent light.
Overall, while we shine a steady light onto the system, our aim is to make the system send light back that changes color in a repeating pattern. If successful, this means our artificial neurons are producing their own chemically driven “signals,” much like simplified synthetic neurons, responding to an exterior signal, such as light.
