Researchers at University of Southern California Viterbi School of Engineering have developed a 3D printed rubber materials that can repair itself all on its own if it becomes fractured or punctured. The material could be game-changing for industries like shoes, tires, soft robotics, and even electronics, decreasing manufacturing time while increasing product durability and longevity.
A severed 3D-printed shoe pad repairing itself - Image: An Xin and Kunhao Yu
The material is manufactured using a 3D printing method that uses photopolymerization, which uses light to solidify a liquid resin in a desired shape or geometry. Photopolymerisation is achieved through a reaction with a certain chemical group called thiols. By adding an oxidiser to the equation, thiols transform into another group called disulphides. It is the disulphide group that can reform when broken, leading to the self-healing ability.
“When we gradually increase the oxidant, the self-healing behaviour becomes stronger, but the photopolymerisation behaviour becomes weaker,” explained said Qiming Wang, an assistant professor at University of Southern California. “There is competition between these two behaviours. Eventually we found the ratio that can enable both high self-healing and relatively rapid photopolymerisation.”
When doing tests using photopolymerisation, in just five seconds the scientists were able to print a 17.5mm square, completing whole objects in around 20 minutes that can repair in just a few hours. In their study, they demonstrated the material’s ability on a range of products, including a shoe pad, a soft robot, a multiphase composite and an electronic sensor.
After the products were cut in half, within just two hours (four hours for the electronics, due to the carbon used to transmit electricity) at a temperature of 60°C the subjects had self-repaired, retaining their strength and function. Furthermore, the repair time can be decreased just by raising the temperature.
“We actually show that under different temperatures – from 40°C to 60°C – the material can heal to almost 100 per cent,” said structural engineering student Kunhao Yu, who was first-author of the study and is studying structural engineering. “By changing the temperature, we can manipulate the healing speed. Even under room temperature the material can still self-heal.”
The team are now working to develop different self-healable materials along a range of stiffnesses, from the current soft rubber, to rigid hard-plastics. These could be used for vehicle parts, composite materials and body armour.