Instead of throwing your broken boots or cracked toys, why not let them fix themselves? Researchers at the University of Southern California Viterbi School of Engineering have developed 3-D printed rubber materials that can do just that.
Assistant Professor Qiming Wang works in the world of 3-D printed materials that create new features for different purposes, from flexible electronics to audio control. Now they have worked with Viterbi students Kunhao Yu, En Xin and Haixu Du and University of Connecticut Assistant Professor Ying Li. They have made a new material that can be quickly manufactured and can repair itself if broken or punctured. This material can be game-changing for industries such as shoes, tires, soft robotics and even electronics, decreasing production time, while increasing product durability and longevity.
The material is made using a 3-D printing method using photopolymerization. This process uses light to solidify a liquid resin in a desired shape or geometry. To make it self-healing, they had to delve deeper into chemistry behind the material.
Photopolymerization is achieved through a reaction with a particular chemical group called thiols. By adding an oxidizer to the equation, thiols are converted to another group called disulfides. It is the disulfide group that is capable of reforming when broken, leading to the self-healing ability. Finding the right relationship between these two groups was the key to unlocking the unique properties of the materials.
"As we gradually increase the oxidant, the self-healing behavior becomes stronger, but photopolymerization behavior becomes weaker," Wang explained. "There is competition between these two behaviors. And finally, we found the relationship that can enable both high self-healing and relatively quick photopolymerization."
In just 5 seconds, they can print a 17.5 millimeter square and complete entire objects in about 20 minutes that can repair in a matter of hours. In their study, published in NPG Asia Materials, they demonstrate the ability of their materials to a variety of products, including a shoe, a soft robot, a multiphase composite, and an electronic sensor.
After being cut in half, they are cured for two hours at 60 degrees Celsius (four for the electronics due to the ball being used to transmit electricity) and they maintained their strength and function. The repair time can be reduced by raising the temperature.
"In fact, we show that under different temperatures – from 40 degrees to 60 degrees Celsius – the material can cure to almost 100 percent," said Yu, the author's first study and studying construction engineer. "By changing the temperature, we can manipulate the cure rate, even under room temperature, the material can still heal"
Having captured 3-D printable soft materials, they are now working on developing various self-adhesive materials along a variety of stiffnesses, from the current soft rubber to rigid hard plastic. These can be used for vehicle parts, composite materials and even body arms.
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