07 March 2017

Temperature-sensitive technology for artificial skins: smart structures

Researchers around the world are in the midst of developing artificial skins essential to modern robotics, prosthetic limbs, and other applications. Designed to emulate the most practical properties of human skin, some artificial skin technologies have managed to surpass the sensory capabilities of living tissues. One such technology is a temperature-sensitive electronic film, which has paralleled the record performance of the world's most sensitive heat-detecting organism, the Crotalinae, commonly know as the pit viper.

While in the process of fabricating materials for synthetic wood, a team of researchers discovered a film made of pectin, a sugar molecule responsible for the temperature sensitivity in plants, could exhibit an electrical response to changes in temperature when enriched with positively-charged calcium ions. This finding led to the study "Biomimetic temperature-sensing layer for artificial skins" by senior author Chiara Daraio, et al., which was published in the February issue of Science Robotics. The transparent and flexible pectin films under examination were incorporated into artificial skins made from elastic materials such as silicon rubber, then tested for sensitivity.

Pectin is considered a bionic material; a class of materials utilized to preserve, enhance, and exploit properties of living systems for engineering purposes. Until now, bio-engineering synthetic materials that reproduce or surpass the performance of natural materials has been intangible. Fabricating the synthetic pectin material by combining carbon nano-particles in a matrix of plant cells, has resulted in new temperature sensors with record-breaking responsivity.

Flexible pectin film
Photo: Caltech/Science Robotics

When temperatures rise, molecules of pectin detach from one another, releasing calcium ions that can be detected by electrodes embedded in the film. Artificial skins containing the pectin film can precisely map temperature variations across its surface. From the gentle touch of a finger to warm objects–a teddy bear microwaved to 37° C–a meter away, the films maintain their sensitivity even after being warmed and cooled over 215 times, as well as experiencing physical deformations, a highly desirable trait for artificial skins.

The pectin films “are extremely easy to fabricate and extremely low cost—you can buy pectin at your local supermarket to make gelatin, jams, or jellies,” said Daraio, Professor of mechanical engineering and applied physics and materials scientist at the California Institute of Technology. “We think it’s pretty straightforward to scale up to large-scale production if needed."

Daraio will share her team's research in a plenary presentation titled "Plant nanobionic materials for thermally active, soft, artificial skins" at SPIE Smart Structures/NDE in Portland, Oregon. Spanning 25–29 March, the symposium offers a unique collaboration between engineers who develop advanced materials and researchers who use sensor networks and non-destructive evaluation methods to monitor the health of structural and biological systems.

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