Heat-sensitive smart gels with rapid response could have applications in soft robotics and temperature-controlled valves.
Hydrogels are a soft, squishy and wet material that consist mainly of water. They are characterised by a their response to changes in temperature: when heated, the material shrinks. When cooled, the gels expand back to their original size.
Lead researcher at the University of Wollongong’s Institute for Superconducting and Electronic Materials (ISEM), Dr Zhenguo Huang says hydrogels have seen limited application due to their relatively slow response to temperature changes.
“The effectiveness of these applications is highly dependent on the hydrogel’s response to changes in temperature. In most cases it needs to be very fast. The problem is that the heat does not spread rapidly through the gel,” Dr Huang explained.
The researchers found that they could improve the response times of the hydrogels by adding boron nitride to the mix.
Boron nitride is structurally similar to graphene on the atomic scale, but with boron and nitrogen atoms in place of the carbon atoms of graphene. Atom-thick sheets of boron nitride have outstanding thermal conductivity, are simple to make, low cost, and free of harsh chemicals.
When combined with the hydrogel, the boron nitride sheets create continuous pathways that allow heat to travel rapidly and evenly throughout the material, leading to rapid response times. The addition of the boron nitride does not compromise the mechanical strength of the hydrogel.
After the initial discovery, the researchers encountered difficulties trying to functionalise the boron nitride. Because of the material’s chemical and thermal stability, it was did not readily disperse uniformly through the hydrogel.
The researchers developed a way to build boron nitride nanosheets using water steam. By flowing hot steam over hexagonal boron nitride bulk powder, the researchers created a type of boron nitride nanosheet that could effectively bond with water, and thus be incorporated into the hydrogel.
The resulting hydrogel boron nitride mix was almost 50% more thermally conductive than the hydrogel without additives.
The quick response to temperature changes could be observed by eye, as the hydrogel changes its opacity when responding to changing temperatures.
Because the hydrogel boron nitride mix is non-toxic and biocompatible, the enhanced hydrogel can be used in medical applications, such as drug delivery.
“The key features of shape change and the dye release show this material is really promising for use in medical applications such as drug delivery as well as soft robotics that change shape in soft environments, eliminating the need for mechanical contact,” said Dr Huang