‘Thubber’ – Stretchable, thermally conductive rubber for heated garments and robot muscles

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.

Lisa Kulick, Carnegie Mellon University February 13, 2017

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

“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 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.

The first part of this video demonstrates the thermally conductive ‘thubber’ material (bottom, labeled LMEE) as it stretches and dissipates the heat of an embedded LED light. The plain rubber material (top) cannot dissipate the heat and ultimately breaks apart under the strain. The second part of the video shows the caudal fin locomotion of a robotic fish. Its thermal actuator is made of ‘thubber’. YouTube on Feb 10, 2017

“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 mechanical engineering. “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.

Fig. 1. Soft, thermally conductive composite. (A) Highly deformable LMEE. (scale bars, 25 mm.) (B) EGaIn alloy is liquid at room temperature and shows fluid characteristics as demonstrated by falling droplets. (Scale bar, 10 mm.) (C) Schematic illustration of the LMEE composite where LM micro-droplets are dispersed in an elastomer matrix and, upon deformation, the LM inclusions and elastomer elongate in the direction of stretching. (D) Alternating strips of LMEE and unfilled elastomer are heated with a heat gun, and the IR photo time sequence shows the LMEE dissipating heat more rapidly than the elastomer (images correspond to t = 0, 5, 10, and 15 s after the heat source is removed). (Scale bar, 25 mm.) (E) The ϕ = 50% LMEE composites described here occupy a unique region of the material properties space when comparing thermal conductivity with the ratio of strain limit to Young’s modulus. (Data points are from refs. 2, 9, 12, and 14). PNAS

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.

Fig. 3. Soft robot and stretchable electronics implementation of the LMEE composite. (A) Soft robotic fish composed of a silicone body and caudal fin connected by an LMEE-sealed SMA actuator. (B) Top-down view during forward caudal fin locomotion. (C) LMEE, unfilled silicone elastomer, and commercial thermal tape actuated at a frequency of 5 Hz. (D) LMEE actuated at 1-, 5-, and 10-Hz signal. (E) Time sequence images of the soft robotic fish swimming with a stroke frequency of 0.7 Hz. (F) XHP LED lamp mounted on an LMEE composite stretched to 400% strain with a sequence of IR images during LED operation. (G) The same experiment on an elastomer sample where the sample breaks at 60 s due to significant localized heating. (H) Temperature versus time plots for the IR image sequence, where the temperature is measured across the sample’s length. LED is turned on and off at t = 0 s and t = 70 s, respectively. (I) An XHP LED is mounted to a strip of LMEE that is wrapped around the leg and shows high brightness during (J) running and (K) cycling. PNAS

“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.”

Fig. S10. The soft swimming robot is composed of an LMEE SMA thermal actuator, silicone caudal fin, and two silicone-enclosed chambers for the body. The buoyancy is adjusted by filling the body with fluid (water or EGaIn). PNAS

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.

Source Carnegie Mellon University

References

High Thermal Conductivity in Soft Elastomers With Elongated Liquid Metal Inclusions, Michael D Bartlett, Navid Kazem, Matthew J Powell-Palm, Xiaonan Huang, Wenhuan Sun, Jonathan A Malen, Carmel Majidi. Published online before print. Proceedings of the National Academy of Sciences, PNAS. February 13, 2017, doi: 10.1073/pnas.1616377114

Also see
‘Thubber’ could revolutionise robotics and even lead to flexible phones: Radical new material can conduct heat and stretch to six times its length The Daily Mail