Scientists Create Shape-Changing Peptide Crystals

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A team of researchers from the United States and the United Kingdom has created nanoporous tripeptide crystals that directly convert evaporation energy into mechanical motion.

Piotrowska et al developed tripeptide crystals which feature aqueous pores that expand and contract in response to humidity changes and use evaporation to create an effective mechanical actuator. Image credit: Tony Wang.

Piotrowska et al developed tripeptide crystals which feature aqueous pores that expand and contract in response to humidity changes and use evaporation to create an effective mechanical actuator. Image credit: Tony Wang.

“We essentially created a new type of actuator, which is driven by water evaporation,” said first author Roxana Piotrowska, a Ph.D. student in the Advanced Science Research Center at the Graduate Center of the City University of New York.

“By observing its activity we’ve been able to identify the fundamental mechanisms of how water-responsive materials can efficiently convert evaporation into mechanical energy.”

Different from traditional crystals that are usually stiff and brittle, the tripeptide crystals developed by the team have the ability to change their shapes, enabled by their molecular architectures.

The crystals are comprised of a pattern of small pores that is interspersed with connecting flexible domains that are repeated throughout the crystal structure.

The pores that run throughout the crystals strongly bind to water molecules.

“When evaporation causes water to be removed from the pores, this results in a forceful deformation of the entire crystal through a network-like connection,” said co-lead author Dr. Xi Chen, a researcher in the Department of Chemical Engineering and the Advanced Science Research Center at the Graduate Center of the City University of New York.

“The resulting shape-change is reversed when water vapor is reintroduced.”

“Our peptide crystals allow the direct observation of water-material interactions at the molecular level by using existing crystallographic, spectroscopic and computational methods.”

“The revealed actuation mechanisms are applicable more generally for the designs of materials or structures that efficiently harness evaporation.”

By using a combination of laboratory-based experiments and computer simulations, the scientists were able to identify and study the factors that control the actuation of these crystals.

This approach resulted in new insights that inform the design of more efficient ways to use evaporation for a variety of applications, which may include robotic components or mechanical micro- and nanomachines that are powered by water evaporation.

“Our work enables the direct observation of materials’ evaporation-driven actuation at the molecular scale,” Dr. Chen said.

“By learning how to efficiently extract energy from evaporation, and turn it into motion, better and more efficient actuators can be designed for many applications, including evaporation energy harvesting devices.”

“Importantly, our designed crystals are produced from the exact same building blocks that proteins are made of, but they are radically simplified and as a result, their properties can be precisely tuned and rationally optimized for this application,” said Dr. Rein Ulijn, a researcher in the Department of Chemistry and Biochemistry and the Advanced Science Research Center at the Graduate Center of the City University of New York.

“The beauty of using biological building blocks to create this new technology is that the resulting morphogenic crystals are biocompatible, biodegradable, and cost-effective.”

The team’s work was published in the journal Nature Materials.

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R. Piotrowska et al. Mechanistic insights of evaporation-induced actuation in supramolecular crystals. Nat. Mater, published online September 14, 2020; doi: 10.1038/s41563-020-0799-0

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