Inside the human brain, billions of individual neurons connect to form a vast network capable of rapidly processing information and electrical signals. A team of researchers and engineers at Penn State is taking inspiration from these natural computers to build soft networks of water-based droplets, connected by membranes like those found in the walls of living cells. Their networks lay the foundation for a new kind of soft computer built for wet environments, capable of sensing and processing information with ions much like silicon chips do with electrons, but at far lower energy cost.
The team immerses tiny lipid-containing droplets in oil, where lipid molecules naturally arrange themselves around each droplet. When neighboring droplets touch, these lipids form a thin, two-layer membrane between them. The soft surfaces contain voltage-sensitive channels that allow electrical signals to pass between droplets through the movement of charged atoms or molecules called ions. The membranes store charge, while the channels respond to present and recent signals. When combined, these properties allow the droplets to form a synthetic, interconnected network that the researchers call a "neuromorphic tissue."
They detailed these tissues' properties in a paper published in Science Advances, using laboratory measurements and computer simulations to show that interconnecting the droplets' membranes can significantly improve the material's ability to process complex and unpredictable signals.
According to Joseph Najem, associate professor of mechanical engineering, associate director of the Convergence Center for Living Multifunctional Materials System and co-corresponding author on the paper, integrating a computer interface or sensor in the body, for example, is not as simple as implanting circuitry because even though both electronics and biology process signals and compute, they carry information differently. Conventional electronics work by moving charge carriers like electrons through conductive materials such as metal wires, while the electrical signaling that happens in the body relies on ions moving through fluids and across cell membranes.
"Scientists and developers must create interfaces in devices that use traditional silicon computing to process and translate between electronic and ionic signals," said Najem, who holds an additional affiliation in the Intercollege Graduate Degree Program in Materials Science and Engineering at Penn State. "However, these interfaces can delay and distort signals, while adding significant energy costs. To address this problem, we must investigate creating new computing platforms that go beyond just electronics."
Ionic computing explores approaches to processing information using ions instead of the traditional charge carriers used in electronics. Neuromorphic computing is an approach to computing that draws inspiration from the brain's structure and information processing functionality. The team's new approach uses aqueous compartments, lipid membranes and peptide ion channels - structures that mimic the natural architecture of the brain - to bring both the structure and functionality of these ideas together, drawing on the nodes' interconnectivity to create electrical feedback within the material.
With this architecture, the researchers can use reservoir computing, in which a changing input produces a pattern of responses that retains traces of earlier inputs. In this network, membrane charging and ion-channel activity generate responses, and the connections allow the droplets to influence one another. Coupled nodes can accumulate information in the form of electric charge, passing it to the next node in the chain. This behavior, which also occurs in neurons, allows electricity to rapidly transfer through each node without losing overall signal clarity. The membranes between the droplets give the network short-term, fading memory, so its response to a new input depends partly on recent inputs, Najem explained.
"In traditional systems, memory and processing are separated, which consumes a lot of energy to make those actions work together," Najem said. "In the brain, information processing and memory occur in the same place. That's why we are seeing interest in this idea of neuromorphic computing, as it could significantly reduce the amount of energy needed for operation."
Joseph explained that individual droplets can connect to as many as six neighbors in a structure reminiscent of a honeycomb, noting that various arrangements provide different patterns of connectivity. To measure the dynamics of a droplet chain, the team introduced an electrical current, recording voltages at individual nodes. These measurements were compared with a physics-based model of the network, allowing the researchers to explore how connections and differences between droplets influence overall computational performance.
