US Army Researchers Create Self-Healing 3D Printing Materials

Researchers from Texas A&M University and the U.S. Army Research Laboratory (ARL) developed a new material that can autonomously heal in air and underwater. This first-of-its-kind, 3D printable and stimuli-responsive polymer is expected to be critical to building realistic prosthetic limbs, as well as enable massive reconfigurability in future military programs, opening new opportunities for morphing unmanned air vehicles, self-healing helicopter blades and robotic platforms.

By tweaking the chemistry of a single polymer, the team of researchers created a family of synthetic materials that range in texture from ultra-soft to extremely rigid. Led by aerospace engineer and principal investigator of this work for the ARL, Frank Gardea, and study co-author and professor of materials science and engineering at Texas A&M University, Svetlana Sukhishvili, the study resulted in 3D printable materials that can self-heal if damaged, morph into another shape, and even naturally adhere to each other in air or underwater.

The new discovery detailed in the journal Advanced Functional Materials May 2020 issue reports how a family of 3D printable dynamic covalent polymer networks with widely variable mechanical properties and self-healing capabilities could be fabricated with “on-demand” mechanical characteristics.

According to the study, polymers are made up of repeating units, like links on a chain. For softer polymers, these chains are only lightly connected to each other through crosslinks, yet the more crosslinks between chains, the more rigid the material becomes. Gardea explained that most crosslinked materials, especially those that are 3D printed, tend to have a fixed form, meaning that once the part is manufactured, the material cannot be reprocessed or melted. Instead, the new polymers have a dynamic bond that allows them to go from liquid to solid multiple times, which permits it to be 3D printed and recycled. Furthermore, the scientists said the dynamic bonds introduce a unique shape memory behavior, in which the material can be programmed and triggered to return to a remembered shape.

“Crosslinks are like stitches in a piece of cloth, the more stitches you have, the stiffer the material gets and vice versa,” suggested Sukhishvili. “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 flexibility introduced to the polymer chain allows it to be fine-tuned, in unprecedented ways, to get either the softness of rubber or the strength of load-bearing plastics, claimed the experts. Associate Chief Scientist for the ARL’s Vehicle Technology Directorate, Bryan Glaz, described how much of the previous work on adaptive materials was for materials systems that are either too soft for structural applications or otherwise not suitable for platform development, so turning to epoxies could be considered groundbreaking. He also suggested that the research team’s scientific advancement marks “a first step along a very long path toward realizing the scientific possibility for deep future platforms.”

A new family of materials can be made softer or harder by changing the number of cross-linking molecules. (Image courtesy of Texas A&M University)

Gardea detailed that “by modifying the hardware and processing parameters in a standard 3D printer, we were able to use our materials to print complex 3D objects layer by layer. The unique advantage of our materials is that the layers that make up the 3D part can be of vastly different stiffness.”

As the 3D part cools to room temperature, the different layers join seamlessly, precluding the need for curing or any other chemical processing, the expert explained. Consequently, the 3D printed parts can easily be melted using high heat and then recycled as printing ink. Additionally, the researchers noticed that their materials are reprogrammable, in other words, after being set into one shape, they can be made to change into a different shape using just heat. In the future, they plan to increase the functionality of their new materials by amplifying their multifaceted properties.

“Right now, we can easily achieve around 80% self-healing at room temperature, but we would like to reach 100%. Also, we want to make our materials responsive to other stimuli other than temperature, like light,” Gardea outlined. “Further down the road, we’d like to explore introducing some low-level intelligence so that these materials know to autonomously adapt without needing a user to initiate the process.”

Fast mending of fractured surfaces in the air at 25, 50, and 75 °C and underwater. (Image courtesy of Texas A&M University/U.S. Army Laboratory/Wiley Online Library)

For the study, samples were additively manufactured by using a Makerbot Replicator 2X, incorporating a syringe-based extruder following the custom design 3D bioprinting technique called freeform reversible embedding of suspended hydrogels (FRESH), developed by Carnegie Mellon University researchers. The modification allowed for the rotary motion of the original stepper motors from the thermoplastic extruder to be converted to linear motion to drive the syringe plunger.

Soft materials with widely tailorable mechanical properties throughout the material’s volume can shape the future of soft robotics and wearable electronics, impacting both consumer and defense sectors. This study is in fact part of an exploratory research program led by the ARL to look at new scientific developments that may disrupt current scientific and technological paradigms 30 to 50 years from now.

Gardea described that the ARL wants “a system of materials to simultaneously provide structure, sensing, and response,” as army researchers envision a future platform, suitable for air and ground missions, with the reconfiguration characteristics of the T-1000 android character in the Hollywood film Terminator 2. The science fiction aspirations of the research continue to advance, and as the project matures, the material is expected to have the ability for massive reconfigurability and have embedded intelligence allowing it to autonomously adapt to its environment without any external control.

Researchers expect to embed newly developed materials in intelligent systems, allowing the material to autonomously adapt to its environment without external control. (Image courtesy of Texas A&M University )

A feature like this could become a powerful means to create mechanically diverse objects with tunable self-healing and reprogrammable shape memory behavior using 3D printing processes. Beyond its potential in defense systems and prosthetics, the new material addresses several performance and durability issues and the exploratory research supports challenges highlighted by the U.S. Army by improving knowledge of material behaviors that may be capable of introducing multi-functionality into far-future Army platforms.