Push it good

This ChemSci Pick of the Week features an actuator made from a single organic crystal, which is capable of pushing more than 10,000 times its own weight.

An actuator is a widely-used part of a system that is responsible for moving and controlling. For example, the muscles in our bodies are a type of soft, organic actuator that pull our legs as we walk, or our fingers as we type.

A specific type of actuator – a linear actuator – provides movement in a straight line, such as that needed by a car jack. At the smaller scale, linear actuators can be used for applications like focusing camera lenses at a fine resolution.

Now, a team of scientists has opened the doors towards research for a new class of actuator, made from expanding crystals. The material is organic and flexible, presenting the possibility for bio-compatible applications, and can exert forces 10,000 times greater than its own weight.

"Crystals are normally thought be hard, brittle and inert objects: and we enjoy challenging that misconception by showing people that crystals can jump, twist, heal, bend, curl – and in this case –  push," said Panče Naumov, associate professor of chemistry at New York University Abu Dhabi. "This article is one of the first to actually show how these simple yet powerful materials can be useful in devices."

The crystal was formed of a simple organic compound called hexamethylbenzene, which has only very weak molecular interactions. When heated, the crystal structure was transformed into another solid phase, in a similar way to when water turns to ice.

"The difference is that water expands by changing from a liquid to a solid, whereas our compounds expand by changing from a solid to a slightly longer solid. In its essence, this process is a process of conversion of energy from heat into mechanical work," Naumov said.

The expansion exerted a force that was demonstrated by the authors to push a ball bearing, which then collapsed a series of glass block in a domino-type sequence. Compared to complex natural actuators, such as those employed by ants, the forces exerted were found to be ten times stronger.

Notably, the study offered a bridge between the "language barriers" faced between engineers and chemists. As well as demonstrating that the small molecule organic crystals can exhibit mechanical properties, the authors clearly measured this using common performance indices such as strain and stress.

Naumov said: "The study provides, for the first time, a comparison between the performance of a molecular crystal and other classes of actuators used in engineering or everyday life. Once engineers know how well organic crystals perform, we might be able to see them used in everyday devices."

The authors hope that their work could lead to advances in several fields, including for soft robotics, micro and nano-scale (opto)electronics, and biomaterials.

This article is free to read in our open access, flagship journal Chemical Science: Liang Li, Patrick Commins et al., Chem. Sci., 2019, Advance Article. DOI: 10.1039/C9SC02444A. You can access our 2019 ChemSci Picks in this article collection.