Instead of throwing away your broken boots or broken toys, why not let them fix it? Researchers at the University of Southern California's Viterbi School of Engineering have developed 3-D rubber materials that can do just that.
Assistant Professor Qiming Wang works in the world of 3-D 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 with University of Connecticut Assistant Professor Ying Li, they have created a new material that can be manufactured quickly and can repair itself if it becomes fractured or perforated. This material can be revolutionary for industries such as footwear, tires, light robotics and even electronics, reducing manufacturing time, increasing the durability and longevity of the products.
The material is manufactured using a 3-D printing method that uses photopolymerization. This process uses light to solidify a liquid resin in a desired shape or geometry. To make it self-curable, they had to dive a bit more into the chemistry behind the material.
The photopolymerization is obtained through a reaction with a certain chemical group called thiols. Adding an oxidant to the equation, the thiols turn into another group called disulfides. It is the disulfide group that is able to reform when broken, leading to the capacity for self-healing. Finding the right ratio between these two groups was the key to unlocking the unique properties of the materials.
"When we gradually increase the oxidant, the self-healing behavior becomes stronger, but the photopolymerization behavior becomes weaker," explained Wang. "There is competition between these two behaviors. And finally, we find the relationship that can allow high self-cure and relatively rapid photopolymerization."
In just 5 seconds, they can print a 17.5mm square by completing whole objects in about 20 minutes that can be repaired in just a few hours. In their study, published in NPG Asia Materials, they demonstrate the capability of their material in a range of products including a shoe, a soft robot, a multiphase compound and an electronic sensor.
After being cut in half, in just two hours at 60 degrees Celsius (four for electronics because of the carbon used to transmit electricity), they healed completely, maintaining their strength and function. The repair time can be decreased only by increasing the temperature.
"We actually showed that under different temperatures – from 40 degrees Celsius to 60 degrees Celsius – the material can reach almost 100 percent," said Yu, who was the first author of the study and is studying structural engineering. "By changing the temperature, we can manipulate the speed of healing, even under room temperature, the material can still self-cure"
After conquering 3-D printable soft materials, they are now working to develop different self-repairing materials over a range of rigidity, from the current soft rubber to rigid hard plastics. These can be used for vehicle parts, composite materials and even body armor.
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