Living cells respond to threats in their environment. What if materials could do the same? Using a similar pressure-regulating mechanism to that found in cells, scientists created an artificial cell that responds to a sudden and possibly catastrophic change in its surroundings. They created the artificial cell using self-assembled surfactants (chemicals that lower the surface tension between different materials, for example, detergents), sugar, and water.
For those creating materials with new properties, such as greater strength or self-healing, the artificial cells ability to sense a possibly damaging event and respond shows an option for changing material structures in response to stress.
In biology, cells use a variety of mechanisms to deal with sudden changes in their surroundings. For instance, when the level of dissolved nutrient molecules (e.g., sugar) in the cell's watery environment drops, water flows into the cell through osmosis. Protein channels in the cell's membrane release the excess water, preventing catastrophic expansion and bursting of the cell. While the biological process is reasonably well understood, translating this resiliency into synthetic materials remains a major challenge. Scientists at the University of California, Davis, led a team with Nanyang Technological University that demonstrated a similar type of mechanism for managing osmosis with simply constructed surfactant capsules.
They found that giant capsules formed from mixtures of lipids (i.e., soap-like fat molecules that make up the capsule membrane) responded to a sudden drop in the amount of sugar in the surrounding water. The response involved reorganizing the membrane molecules to open a hole for less than a second, through which the excess water escaped. Interestingly, this process operated via a pulsating pattern mechanism. With each pulse, a bit of the excess contents were released and a cyclical breathing-like change in the artificial cell's texture was produced.
This autonomous ability of sensing an environmental change and regulating structure in a feedback loop in simple surfactant capsules is quite surprising. Further, the research demonstrated that far-from-equilibrium self-assembly processes involving energy and molecular exchanges between a structure and its local environment were possible for a simple composite formed from surfactants, water, and sugar. From the vantage point of developing predictive design and formation approaches for synthetic materials, the observations suggest how chemical energy stored across adjacent regions with unequal distributions of dissolved molecules can use processes derived from biology to drive structural reorganizations for advanced functions.
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