The actuator for the caudal fin on this robotic fish is sealed with liquid metal embedded elastomer (LMEE), or “thubber.” (Image courtesy of Proceedings of the National Academy of Sciences.)
Engineers have developed a thermally conductive, stretchable rubber (dubbed “thubber”) that could open new doors in the pursuit of flexible electronics
Thubber is a soft-matter material that consists of liquid metal microdroplets embedded within a deformable silicon elastomer. This composite nature allows thubber to achieve its dual properties of high elasticity and metal-like thermal conductivity, as the liquid metal can deform along with the surrounding rubber to maintain pathways for heat dispersion.
Typically, there’s a trade-off between the thermal and mechanical properties of soft dielectric materials: the more stretchy the material, the less its thermal conductivity. This trade-off arises from the kinetic theory of phonon transport. However, in the case of thubber, this constraint is bypassed because the thermal conductivity of the liquid metal droplets is dominated by electrons, not phonons, and the droplets can deform with the surrounding matter.
In this way, thubber demonstrates what the researchers claim is an unprecedented balance of thermal and mechanical properties. Thubber can stretch to over six times its initial length, while achieving a thermal conductivity of up to fifty times the base polymer alone. Plus, despite the liquid metal droplets, thubber is also electrically insulating.
Thubber’s liquid metal microdroplets deform in the direction of stretching. (Image courtesy of Proceedings of the National Academy of Sciences.)
These properties are promising for a number of applications, according to the researchers.
"Our combination of high thermal conductivity and elasticity is especially critical for rapid heat dissipation in applications such as wearable computing and soft robotics, which require mechanical compliance and stretchable functionality," said Carmel Majidi, associate professor of mechanical engineering at Carnegie Mellon University.
The engineers demonstrated the potential efficacy of thubber in two ways: building a robotic fish with an artificial muscle of thubber powering its caudal fin, and mounting an LED light on a thubber strip to act as a safety lamp worn around a jogger’s leg. The heat dissipation of thubber prevented the LED light from burning the jogger.
"Until now, high power devices have had to be affixed to rigid, inflexible mounts that were the only technology able to dissipate heat efficiently," said Jonathan Malen, an associate professor of mechanical engineering at CMU. "Now, we can create stretchable mounts for LED lights or computer processors that enable high performance without overheating in applications that demand flexibility, such as light up fabrics and iPads that fold into your wallet."
The fields of soft robotics and flexible electronics are growing, according to Majidi, who added that the need for thubber-like materials will grow as well. However, there are numerous other uses for such materials. “Efficient thermal transport is critical for applications ranging from electronics and energy to advanced manufacturing and transportation,” reads the team’s paper in Proceedings of the National Academy of Sciences.
To learn more about the applications of thubber-like materials, read A Better Way to Make Artificial Muscles.