Self-healing materials are capable of performing autonomous repair when damaged. Since the first concept of self-healing materials using capsule based composites¹ were proposed by White et al. in 2001, many research groups have proposed several new material strategies to achieve self-healing functions.  

     In conjunction with the growth of soft, stretchable electronics² that begin to develop from early 2000s, interest in making elastomeric materials and devices grew. At the same time, the scientific community began to explore several intrinsically self-healing polymeric materials. Of note, Leibler et al. first proposed using hydrogen bonding to create a supramolecular assembly³. The rubbery material could repeatably self-repair mechanical damages, even at room temperature. As a then graduate student in Zhenan Bao’s research group working on electronic skins^4–6, I wondered if it is possible to create artificial skins that can self-repair, similar to human skins.

     Inspired by the work of many self-healing materials pioneers^7,8, we started researching possible strategies to make materials that repeatably self-heal and has electronic conductivities for electronic skins. In 2012, we developed a composite using metal particles that utilizes quantum tunneling⁹. Although functional, the self-healing conductor was opaque, and would not self-heal under exposure to water molecules that negatively impact the hydrogen bonding sites needed for healing to occur.

     In our latest work, Chao Wang and my research group set out to develop a transparent conductor that works under various aqueous environments. We drew inspiration from jellyfishes: transparent, sensitive sea creatures that communicate optically and survive under many different aquatic conditions. Such a material could be useful for emerging soft robotics applications.

       One obvious choice would be to use hydrogels, but an aqueous environment makes them swell as they absorbed water. Chao has previously worked on stretchable ionogels¹⁰ that could self-heal mechanically, but none exhibited high electrical conductivity nor retained their physical properties when submerged in water.

     The eureka moment came when we wondered if we could utilize hydrophobic ionic-dipole interactions with the right type of ionic liquid and elastomer. After screening many different types of ionic liquids, we found one that works with a fluoro-elastomer. And the GLASSES (Gel-Like, Aquatic, Stretchable and Self-healing Electronic Skin) ionic composite was born. We were very surprised that the material retained self-healing capabilities and physical properties despite being submerged and damaged in water. Using Density Functional Theory (DFT) simulations done by our collaborator Dr. Yongqing Cai, we found that the strong interactions between the ionic liquid and fluoro-elastomer were responsible for the hydrophobicity and consequent water-resistance.

Artificial 'jellyfish' made from our GLASSES material

           The GLASSES material is printable. To demonstrate potential use of the material in systems, we printed compliant ionic circuit boards and demonstrated  touch and pressure sensing functions. In addition, we made an artificial ‘jellyfish’ that could communicate optically with small crustaceans. Perhaps someday, this material can be used in underwater soft robots or in flexible mobile screens that automatically self-repair.

Printed Ionic Circuit Boards from GLASSES material

For more details on our work, please read the paper at: https://www.nature.com/articles/s41928-019-0206-5

This work was enabled by the collaborative effort of our research teams cutting across multi-disciplines, aided by the highly constructive peer review process throughout.

References

1.       White, S.R. & Geubelle, P.H. Self-healing materials: Get ready for repair-and-go. Nat. Nanotechnol. 5, 247–248 (2010).

2.       Bauer, S. et al. 25th Anniversary Article: A Soft Future: From Robots and Sensor Skin to Energy Harvesters. Adv. Mater. 26, 149–162 (2014).

3.       Cordier, P., Tournilhac, F., Soulié-Ziakovic, C. & Leibler, L. Self-healing and thermoreversible rubber from supramolecular assembly. Nature 451, 977 (2008).

4.       Mannsfeld, S.C.B. et al. Highly sensitive flexible pressure sensors with microstructured rubber dielectric layers. Nat. Mater. 9, 859–64 (2010).

5.       Lipomi, D.J. et al. Skin-like pressure and strain sensors based on transparent elastic films of carbon nanotubes. Nat. Nanotechnol. 6, 788–792 (2011).

6.       Tee, B.C.K. et al. A skin-inspired organic digital mechanoreceptor. Science (80-. ). 350, 313–316 (2015).

7.       Chen, X. et al. A thermally re-mendable cross-linked polymeric material. Science 295, 1698–702 (2002).

8.       Tan, Y.J., Wu, J., Li, H. & Tee, B.C.K. Self-Healing Electronic Materials for a Smart and Sustainable Future. ACS Appl. Mater. Interfaces 10, 15331–15345 (2018).

9.       Tee, B.C.-K.K., Wang, C., Allen, R. & Bao, Z. An electrically and mechanically self-healing composite with pressure- and flexion-sensitive properties for electronic skin applications. Nat. Nanotechnol. 7, 825–32 (2012).

10.     Cao, Y. et al. A Transparent, Self‐Healing, Highly Stretchable Ionic Conductor. Adv. Mater. 29, 1605099 (2017).