A team in the US has developed 3D-printed rubber materials that can repair themselves, an advance that could prove transformative to manufacturers of shoes, tyres, or soft robotics.
Severed 3D-printed shoe pad repairing itself (Pic: An Xin and Kunhao Yu)
Developed by researchers from the University of Southern California Viterbi School of Engineering (USC Viterbi) and University of Connecticut, the new material is said to have the added advantage of being quick to manufacture as well as being durable.
The material is manufactured using photopolymerization-based 3D printing. Photopolymerization – a process that uses light to solidify a liquid resin – is achieved through a reaction with thiols, which are organosulphur compounds that can be made into disulphides with the addition of an oxidiser. It is the disulphide group that can reform when broken, leading to the self-healing ability. Finding the right ratio between these two groups was the key to unlocking the materials’ properties.
“When we gradually increase the oxidant, the self-healing behaviour becomes stronger, but the photopolymerization behaviour becomes weaker,” said Assistant Professor Qiming Wang, USC Viterbi. “There is competition between these two behaviours. And eventually we found the ratio that can enable both high self-healing and relatively rapid photopolymerization.”
They can print a 17.5mm square in five seconds, completing whole objects in around 20 minutes that can repair themselves in a few hours. In their study, published in NPG Asia Materials, they demonstrated their material’s ability on a range of products, including a shoe pad, a soft robot, a multiphase composite, and an electronic sensor.
In two hours after being cut in half the items healed completely and retained their strength and function. The process took four hours for an electronic component. 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 USC Viterbi student Kunhao Yu. “By changing the temperature, we can manipulate the healing speed, even under room temperature the material can still self-heal”
The team – including University of Connecticut Assistant Professor Ying Li and USC Viterbi students An Xin and Haixu Du – is now looking at the development of 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.