“The research provides a new paradigm to rapidly 3D print structures that can be rapidly healed after damage”Qimin Wang
A team from the University of Southern California and University of Connecticut have developed a new rubber material that can repair itself after breaking, fracturing or being punctured, and which could lead to a new generation of self-healing products. The 3D-printed materials are able to be manufactured quickly and offer improved product durability and longevity for a range of applications, including tires, shoes, soft robotics and flexible electronics.
As described in the journal NPG Asia Materials [Yu et al. NPG Asia Mater. (2019) DOI: 10.1038/s41427-019-0109-y], the material is made by a 3D printing method based on photopolymerization, a process that utilizes light to solidify a liquid resin into a particular shape. Photopolymerization depends on the optimum combination of two chemical groups, thiols and disulfides. When an oxidizer is added to a thiol, they transform into disulfides, which can re-form when broken, offering the ability to self-heal. On slowly increasing the oxidant, the self-healing behavior of the material becomes stronger while the photopolymerization behavior becomes weaker. The team identified the ratio between the two that enables both high self-healing and relatively rapid photopolymerization.
A 17.5-millimeter square of the material was shown to be printed in only five seconds, with whole objects taking about 20 minutes, and they can repair themselves in just a few hours. This was achieved with a variety of products, such as a soft robot, a shoe pad, a multiphase composite and an electronic sensor. When cut in two, it took only two hours for the material to fully heal while retaining its strength and function.
At temperatures between 40 degrees Celsius and 60 degrees Celsius, the material can heal to nearly 100%, and by changing the temperature they can alter the healing speed, with the material able to self-heal even at room temperature. As team leader Qimin Wang told Materials Today, “The research provides a new paradigm to rapidly 3D print structures that can be rapidly healed after damage”.
While existing 3D methods of shaping self-healing polymers involve molding and direct writing, this could be the first stereolithography of self-healing polymers. As well as the obvious uses for rubber products, the researchers hope to develop other 3D-printable and self-healable materials with different stiffnesses, including rigid hard plastics that could be used in vehicle parts, composite materials and body armor. The eventual aim is to produce a library of 3D-printable and self-healable polymers for a range of engineering applications.
Self-healing of a 3D-printed shoe pad: the shoe pad can sustain a 540-degree twist. Once cut, the shoe pad is brought into contact to heal for two hours at 60 °C. The healed shoe pad can then sustain the 540-degree twist again (scale bar represents 4 mm).