Scientists have 3D-printed rubber material that can repair itself if it becomes fractured or punctured, paving the way for self healing car tires or shoes. The material, developed by researchers from University of Southern California in the US, could be game-changing for soft robotics and even electronics, decreasing manufacturing time while increasing product durability and longevity. It is manufactured using a 3D-printing method that uses photopolymerisation. This process 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, these transform into another group called disulphides. It is the disulphide group that is able to reform when broken, leading to the self-healing ability. Finding the right ratio between these two groups was the key to unlocking the materials’ unique properties. “When we gradually increase the oxidant, the self-healing behaviour becomes stronger, but the photopolymerisation behaviour becomes weaker,” said Qiming Wang, an assistant professor at University of Southern California in the US. “There is competition between these two behaviours.
And eventually we found the ratio that can enable both high self-healing and relatively rapid photopolymerisation,” Wang said. In just 5 seconds, they can print a 17.5-millimetre square, completing whole objects in around 20 minutes that can repair themselves in just a few hours. In their study, published in NPG Asia Materials, they demonstrate their material’s ability on a range of products, including a shoe pad, a soft robot, a multiphase composite, and an electronic sensor. After being cut in half, in just two hours at 60 degrees Celsius they healed completely, retaining their strength and function. The repair time can be decreased just by raising the temperature.
“We actually show that under different temperatures — from 40 degrees Celsius to 60 degrees Celsius — the material can heal to almost 100 per cent,” said Kunhao Yu, a student at University of Southern California. “By changing the temperature, we can manipulate the healing speed, even under room temperature the material can still self-heal,” said Yu. After conquering 3D-printable soft materials, they 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 even body armour.