Assistant Professor Qiming Wang works in the world of 3D printed materials, creating new functions for a variety of purposes, from flexible electronics to sound control. Now, working with Viterbi students Kunhao Yu, An Xin, and Haixu Du, and University of Connecticut Assistant Professor Ying Li, they have made a new material that can be manufactured quickly and is able to repair itself if it becomes fractured or punctured.
This material could be game-changing, the team adds, for industries like shoes, tires, soft robotics, and even electronics, decreasing manufacturing time while increasing product durability and longevity.
The material is manufactured using a 3D printing method that uses photopolymerization. This process uses light to solidify a liquid resin in a desired shape or geometry. To make it self-healable, they had to dive a little deeper into the chemistry behind the material.
Photopolymerization is achieved through a reaction with a certain chemical group called thiols. By adding an oxidizer to the equation, thiols (an organosulfur compound that contains a carbon-bonded sulfhydryl group) transform into another group called disulfides. It is the disulfide 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 photopolymerization behaviour becomes weaker," explained Assit Prof.Wang. "There is competition between these two behaviours. And eventually we found the ratio that can enable both high self-healing and relatively rapid photopolymerization."
In 5 seconds, they can print a 17.5-millimeter square, completing whole objects in around 20 minutes that can repair themselves in a few hours.
After being cut in half, in two hours at 60°C (four for the electronics due to the carbon used to transmit electricity) 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 to 60°C - the material can heal to almost 100 per cent," said Yu. "By changing the temperature, we can manipulate the healing speed, even under room temperature the material can still self-heal"
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.
