Researchers engineer 'thubber,' a stretchable rubber that packs a thermal conductive punch

February 13, 2017
A nano-CT scan of "thubber," showing the liquid metal microdroplets inside the rubber material. Credit: Carnegie Mellon University

Carmel Majidi and Jonathan Malen of Carnegie Mellon University have developed a thermally conductive rubber material that represents a breakthrough for creating soft, stretchable machines and electronics. The findings were published in Proceedings of the National Academy of Sciences this week.

The new material, nicknamed "thubber," is an electrically insulating composite that exhibits an unprecedented combination of metal-like , elasticity similar to soft, biological tissue, and can stretch over six times its initial length.

"Our combination of 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 Majidi, an associate professor of mechanical engineering.

Applications could extend to industries like athletic wear and sports medicine—think of lighted clothing for runners and heated garments for injury therapy. Advanced manufacturing, energy, and transportation are other areas where stretchable electronic material could have an impact.

"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 Malen, an associate professor of . "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 key ingredient in "thubber" is a suspension of non-toxic, liquid metal microdroplets. The liquid state allows the metal to deform with the surrounding rubber at room temperature. When the rubber is pre-stretched, the droplets form elongated pathways that are efficient for heat travel. Despite the amount of metal, the material is also electrically insulating.

To demonstrate these findings, the team mounted an LED light onto a strip of the material to create a safety lamp worn around a jogger's leg. The "thubber" dissipated the heat from the LED, which would have otherwise burned the jogger. The researchers also created a soft robotic fish that swims with a "thubber" tail, without using conventional motors or gears.

Navid Kazem (left), Jonathan Malen (center), and Carmel Majidi (right) demonstrate the elasticity of a strip of 'thubber,' a thermally conductive rubber material that represents a breakthrough for creating soft, stretchable machines and electronics. Navid is a Ph.D. student and Malen/Majidi are associate professors of mechanical engineering at Carnegie Mellon University. Credit: Lisa Kulick

"As the field of flexible electronics grows, there will be a greater need for materials like ours," said Majidi. "We can also see it used for artificial muscles that power bio-inspired robots."

Majidi and Malen acknowledge the efforts of lead authors Michael Bartlett, Navid Kazem, and Matthew Powell-Palm in performing this multidisciplinary work. They also acknowledge funding from the Air Force, NASA, and the Army Research Office.

Explore further: Breakthrough soft electronics fabrication method is a first step to DIY smart tattoos

More information: High thermal conductivity in soft elastomers with elongated liquid metal inclusions, Proceedings of the National Academy of Sciences, www.pnas.org/cgi/doi/10.1073/pnas.1616377114

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RealScience
5 / 5 (1) Feb 13, 2017
From the paper:
a ∼25× increase in thermal conductivity (4.7 ± 0.2 W⋅m−1⋅K−1) over the base polymer (0.20 ± 0.01 W⋅m−1·K−1) under stress-free conditions and a ∼50× increase (9.8 ± 0.8 W⋅m−1·K−1) when strained.

4.7 W/mK is impressive for a flexible compound, and 9.8 W/mK when strained is even more so, but it is not as good as most metals (even the poor metals, bismuth, mercury, plutonium and manganese are ~8 W/mK, and copper is almost 400 W/mK.

So "metal-like thermal conductivity" is just barely true if one picks the worst metals to compare to... Pretty cool material, though!
ACoffeeDrinker
5 / 5 (1) Feb 14, 2017
I wonder if this could be useful in space suits, with regards to thermal control? As I understand it, right now they use a water filled tube system to regulate the heat in a suit?
I might be completely of the mark though.
dirk_bruere
5 / 5 (1) Feb 14, 2017
Since it almost certainly contains Gallium it is potentially very corrosive to metal when it frays
EyeNStein
5 / 5 (2) Feb 14, 2017
How's about using it to bond semiconductors to their heat sinks as an application.
Its about 5 times as good as a typical heat sink compound. (~1W/mK)

The PC overclockers community would buy it just for the bragging rights.
RealScience
5 / 5 (2) Feb 14, 2017
@EyeNStein - Yes, that's why I looked at the linked article to find the actual thermal conductivity. Try Arctic Silver 5 - it is in the same range (~8 to ~9 W/mK) as even stretched 'thubber'.
Captain Stumpy
5 / 5 (1) Feb 15, 2017
I wonder if this could be useful in space suits, with regards to thermal control? As I understand it, right now they use a water filled tube system to regulate the heat in a suit?
@ACoffeeDrinker
maybe - it would still require a source of heat for the astronaut as the outside temp in space is a mite chilly
as they state the material
is especially critical for rapid heat dissipation
- so it would have to address the temperature difference with either some kind of non-thermally conductive barrier between the body and the space temps or use a series of existing heating elements
as noted in the article here:
think of lighted clothing for runners and heated garments for injury therapy
this may well be something to consider, but also as noted by RealScience, there is already known material being used that is quite functional and perhaps better suited
pun intended

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