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Generally, materials will degrade over time due to fatigue, environmental conditions, or damage incurred during operation. Repair of certain materials during their lifecycle (especially those with structural functions such as concrete) can be very expensive and labour intensive. Therefore Scientists began developing self-healing materials , those artificial (synthetic) substances that automatically repair themselves without any overt diagnosis of the problem or intervention by a human being. Self-healing materials could have applications from bridges and buildings that repair their own cracks to car fenders made from shape memory polymers that automatically flex back to shape after low-speed collisions.
Military systems are becoming ever more complex, and so are the materials used to build them. One of the potential applications of advanced materials being explored include the potential of self-healing materials. Lightweight armors utilizing polymer matrix composite materials have demonstrated the potential to significantly reduce the weight and increase the mobility and transportability of U.S. Army ground vehicles. However, one of the primary limitations of these composite armors is the logistical requirements associated with maintenance and repair. In conventional composites, this damage is typically repaired by removing large areas of composite, or replacing the composite part completel. y These repair steps are often expensive, time consuming, and require a highly skilled composites technician.
Military is also looking for next generation wearable armor systems which could self heal themselves. “For defense applications, smart clothing can be used to detect bullet wounds and to monitor vital signs during combat conditions,” said Ryan Harbison, an analyst at ABI Research, a technology research firm located in Oyster Bay, New York. “Medics, for example, can use the data generated by smart sensors to find and better treat injured soldiers.”
In 2019, Army researchers and their partners at Texas A&M reported have developed a reversible cross-linking epoxy that is 3-D-printable and is self-healing at room temperature without any additional stimulus or healing agent. The unique chemistry of the material even enables it to be programmed to morph shape when stimulated with temperature. Army researchers are exploring whether these materials could create reconfigurable Army platforms of the future that could morph shapes on-demand.
In 2020, By tweaking the chemistry of a single polymer, researchers at Texas A&M University and the U.S. Army Combat Capabilities Development Command Army Research Laboratory reported to have created a family of synthetic materials that range in texture from ultra-soft to extremely rigid. The researchers said their materials are 3D printable, self-healing, recyclable and naturally adhere to each other in air or underwater. Their findings are detailed in the May 2020 issue of the journal Advanced Functional Materials.
“We have made an exciting group of materials whose properties can be fine-tuned to get either the softness of rubber or the strength of load-bearing plastics,” said Svetlana Sukhishvili, professor in the Department of Materials Science and Engineering and a corresponding author on the study. “Their other desirable characteristics, like 3D printability and the ability to self-heal within seconds, make them suited for not just more realistic prosthetics and soft robotics, but also ideal for broad military applications such as agile platforms for air vehicles and futuristic self-healing aircraft wings.”
Synthetic polymers are made up of long strings of repeating molecular motifs, like beads on a chain. In elastomeric polymers, or elastomers, these long chains are lightly crosslinked, giving the materials a rubbery quality. However, these crosslinks can also be used to make the elastomers more rigid by increasing the number of crosslinks. Although previous studies have manipulated the density of crosslinks to make elastomers stiffer, the resulting change in mechanical strength was generally permanent. “Crosslinks are like stitches in a piece of cloth, the more stitches you have, the stiffer the material gets and vice versa,” Sukhishvili said. “But instead of having these ‘stitches’ be permanent, we wanted to achieve dynamic and reversible crosslinking so that we can create materials that are recyclable.”
The researchers focused their attention on the molecules involved in the crosslinking. First, they chose a parent polymer, called prepolymer, and then chemically studded these prepolymer chains with two types of small crosslinking molecules — furan and maleimide. By increasing the number of these molecules in the prepolymer, they found that they could create materials stiffer. In this way, the hardest material they created was 1000 times stronger than the softest.
However, these crosslinks are also reversible. Furan and maleimide participate in a type of reversible chemical bonding. Put simply, in this reaction, furan and maleimide pairs can “click” and “unclick” depending on temperature. When the temperature is high enough, these molecules come apart from the polymer chains and the materials soften. At room temperature, the materials harden since the molecules quickly click back together, once again forming crosslinks. Thus, if there is any tear in these materials at ambient temperatures, the researchers showed that furan and maleimide automatically re-click, healing the gap within a few seconds.
A research team based at Pennsylvania State University reports that it has created a way to produce more durable flexible wearable systems embedding circuits that can heal themselves after breaking. The technology could allow troops operating in the harshest and stressful environments to access communications, information and delivery tasks without the need to carry bulky external devices.
