Got a hole in your shoe? A team at USC is taking steps toward self-healing sneakers.

Researchers at the University of Southern California Viterbi School of Engineering have developed a 3D-printed rubber that repairs itself when broken or fractured.

Aside from busted footwear, the material may also help to someday support electronics and soft robotics.

The rubber is manufactured via a 3D-printing method known as photopolymerization, where light solidifies the liquid resin into a desired shape.

During photopolymerization, a chemical group called thiols reacts with an oxidizer to create the special self-healing component: disulfides. The disulfide group allows the broken bonds to be reformed — when the ratio is right at least.

“When we gradually increase the oxidant, the self-healing behavior becomes stronger, but the photopolymerization behavior becomes weaker,” said USC Assistant Professor Qiming Wang. “There is competition between these two behaviors. And eventually we found the ratio that can enable both high self-healing and relatively rapid photopolymerization.”

In the team's study, published in NPG Asia Materials, the researchers demonstrated their material’s ability on a range of products, including a shoe pad, a soft robot, a multiphase composite, and an electronic sensor. (See a video demonstration below.)

Professor Wang spoke with Tech Briefs about where he wants to see the self-healing material appear next.

Tech Briefs: What inspired this idea?

Assistant Professor Qiming Wang, University of Southern California Viterbi School of Engineering: The idea is inspired by nature. Animal organs or trees can autonomously heal wounds by themselves. At the same time, they feature very complex geometries and microstructures. The existing self-healing polymers, however, cannot be easily shaped into 3D structures. This motivates us to conduct the research to search for a solution for 3D-printable self-healing materials.

Tech Briefs: How is the material able to repair itself? What are the essential components that enable this kind of capability?

Prof. Wang: The healing of the material is enabled by a chemical group called disulfide bonds. These bonds can autonomously reform after you break them.

The key is how to incorporate these bonds into a material that can also be 3D-printable. Eventually, we found a solution: we find that a chemical group called thiol can be incorporated with carbon double bonds to enable photopolymerization-based 3D printing, and the thiol group can be oxidized into disulfide groups by adding an oxidant. By playing with the oxidant, we find a way to synthesize a material that can be rapidly 3D-printed and rapidly self-healed.

Tech Briefs: In what industries and applications do you envision this being used?

Prof. Wang: This type of material may make impacts on the shoe companies, tire companies, robot industries, and flexible electronics. The major contribution of our research would be introducing a way to rapidly manufacture 3D structures that can heal themselves like human organs.

Tech Briefs: On what products have you have demonstrated this material so far?

Prof. Wang: We have demonstrated a number of fabricated samples. Some interesting examples include:

  • A shoe pad that can heal a fracture. (The shoe pad can sustain large angle twisting after healing)
  • A soft robot actuator that can lift a weight ten times of itself after healing a fatal fracture
  • A soft electronic sensor that can restore the flexibility and electric conductivity after healing a fracture.

Tech Briefs: How well did the test with the shoe go? How does the healing work exactly?

Prof. Wang: We first 3D-print a shoe pad and cut it with a blade. Then, we contact two parts with a small force and then put on a hot plate with the corresponding healing temperature (e.g., 40-60 °C). The disulfide bonds broken during the cutting process will reform around the interface to bridge the fracture interface. After healing for 2 hours, the fracture interface will be nicely healed with a smooth surface. (See Fig. 2bc in our paper for reference.)

Tech Briefs: What’s next for you and your team regarding this development?

Prof. Wang: The next step is to develop 3D-printable and self-healable rigid polymers. We want to use those materials to 3D-print soldier armors or airplane wings. Just imagine when there are damages and fractures in those structures on the battlefield; they can autonomously self-heal and refunction.

3D-printed rubber breaks and self-heals, developed by University of Southern California (USC) and Professor Qiming Wang
The "steps" of the 3D-printer rubber as it heals. (Image Credit: Prof. Wang)

Tech Briefs: What is most exciting to you about this material and its possibilities?

Prof. Wang: The most exciting thing about this material is that it provides a new paradigm to rapidly make nature-like structures. Modern engineering is always learning from nature and acquiring inspirations from nature, but does not do better than nature. The living structures like human organs and trees can feature complex functional structures and, at the same time, heal wounds. The engineering materials and structures cannot do both.

Our research shows a possibility to borrow the wisdom of nature to enable better next-generation manufacturing engineering.

Professor Wang's research is a collaboration with Viterbi students Kunhao Yu, An Xin, and Haixu Du, and University of Connecticut Assistant Professor Ying Li.

What do you think? How do you envision this kind of self-healing material being used? Share your comments and questions below.