In recent years, hydrogels have been highly studied to develop stretchable electronics for soft robots and wearables. However, regular hydrogels are easily dehydrated, unavoidably frozen at sub-zero temperature, and have weak surface adhesion.
In a recent study, a team at the University of Toronto’s Faculty of Applied Science & Engineering revealed how a hydrogel-based ionic skin (iSkin) has superior anti-freezing capabilities, high toughness, high stretchability, excellent ambient stability, and strong surface adhesion.
The novel material combines the flexibility of human skin with improved conductivity and tolerance of temperatures as low as -93°C (-135.4°F)! This means it has great potential of enhancing a wide range of technologies.
The iSkin is demonstrated for the following applications:
- Strain sensing on the human body and a winter coat
- Human-machine interaction
- Motion/deformation sensing on a soft gripper
- A soft robot in icy conditions
This work offers a new model for developing high-performance artificial skins for wearable electronics and soft robotics.
Binbin Ying, who led the design of the material while working as a visiting Ph.D. student in U of T Engineering Professor Xinyu Liu‘s lab, explained:
Hydrogels are cross-linked polymers that can hold a lot of water within their chemical structures. Many tissues in our bodies are hydrogels, so they are often used in applications where biocompatibility is important such as cosmetics or tissue engineering. But if we want to use them in soft, flexible, or wearable electronics, we need to add in new functionalities such as mechanical stretchability and electrical conductivity.
During the unveiling of an earlier iteration of iSkin last year, Ying and Prof. Liu showed off some of its capabilities: it can stretch up to 400% of its original size, it’s nontoxic and self-powered. Most significantly, bending the material proportionally alters its conductivity, enabling it to convert physical movement into a corresponding electrical signal.
Prof. Liu said:
A physiotherapist could stick it on your knee or your elbow to measure when and by how much your joint is moving. We’ve also coated it on a glove, enabling us to measure and track hand movements, which, in turn, can be used to control a robot. So it’s a very versatile way to facilitate all kinds of human-machine interactions.
Earlier this year, engineers in Hong Kong applied a version of the ionic skin to artificial limbs to alert users if they’ve damaged their prosthetics by bruising up.
Ying and Prof. Liu are exploring further applications of iSkin, with the help of undergraduate students Runze Zuo, Ph.D. candidate Zhanfeng Zhou, and Ryan Chen. For example, applying patches of the material to a mechanical gripper could provide feedback signals that are unique to each item being grabbed. Analyzing the combinations of those signals can enable the robot to “feel” what it’s picking up. In addition, when integrating artificial intelligence algorithms, the robot can learn to single out items that are soft versus hard, round versus cubic, etc. – and sort them appropriately.
Up till now, iSkin had a significant downfall common to all hydrogels; when the water within it freezes, the resulting ice crystals seriously damage the complex polymer matrix. In addition, cool, dry air can draw the remaining liquid water out of the hydrogel.
To address this problem, Ying and his team added glycerol – a nontoxic chemical used in everything from hair gel to foods – into the gel. After testing hundreds of different recipes, they finally developed the perfect one – a new iSkin formulation that increases cold tolerance without compromising the material’s valuable properties. The team published their study in Advanced Functional Materials.
As a bonus, the new formulation enables the hydrogel to adhere more effectively to the skin, clothing, and other similar materials. “We stuck it to the outside of a jacket and walked out into a Toronto winter, where it was 10 degrees below zero. We were able to take the same kinds of measurements as we did in the lab,” said Ying.
Improved adhesion and cold tolerance further increase the list of possible applications for this material. For example, in addition to sorting, the mechanical gripper could function in a freezing storage facility where it would be very uncomfortable for a human to carry out the work. Other possibilities could include soft robots crawling over rough terrain in arctic environments.
The team plans to continue developing and improving the material and possibly miniaturize it too.
