For Shape-Changing Smart Materials, Applications Abound
Researchers are developing new and groundbreaking smart materials that can react to light or heat, change shape, and heal themselves.
Smart materials are so named because they are designed to react to environmental changes, such as variances in temperature, moisture, electric or magnetic fields, or physical stress. Examples of smart materials include self-healing materials, which have the innate ability to repair damage due to normal usage and thus extend their lifetime; photomechanical materials, which can change shape under exposure to light; and shape-memory materials (SMMs), in which large deformations can be induced or recovered by stress or temperature changes. But now, for the first time ever, researchers at Washington State University have been able to combine several disparate properties, such as light-activated movement, shape memory, and self-healing behavior, into one multifunctional smart material.
“Fold and unfold, remember and heal”
The WSU researchers have filed a provisional patent on their work, which also has been published in the scientific journal ACS Applied Materials & Interfaces. The research team is led by Michael Kessler, Berry Family director and professor in the WSU School of Mechanical and Materials Engineering (MME), and Yuzhan Li, MME staff scientist, in collaboration with Orlando Rios, a researcher at Oak Ridge National Laboratory.
As described in an article by Tina Hilding for WSU News, the research team worked with liquid crystalline networks (LCNs): a class of long-chain molecules with the ability to direct the material and imbue it with unique properties. By taking advantage of the way that the material changes in response to heat, the researchers induced a unique three-way shape-shifting behavior. They also added groups of atoms that react to polarized light and used dynamic chemical bonds to improve the material’s reprocessing abilities. The resulting material is novel in that it “reacts to light, remembers its shape as it folds and unfolds, and can heal itself when damaged.”
Think of a razor blade scratching across the material. Instead of the material being irreparably damaged, it can be fixed by applying ultraviolet light. From there, Hilding writes, “the material’s movements can be preprogrammed and its properties tailored.”
According to Kessler, he and his team “knew these different technologies worked independently and tried to combine them in a way that would be compatible.” Fortunately, their hard work panned out.
A multiplicity of applications
Smart materials that can react to external stimuli, such as heat and light, attracted the researchers at the outset. But they ran up against some steep barriers. As Hilding reports, smart materials have not come into widespread use for two major reasons: they are generally difficult to make and usually can only perform one function at a time. Other researchers had struggled for years to reprocess such materials so that their special properties could continually repeat themselves, to little or no avail. Nevertheless, the WSU research team succeeded in developing a material that not only allows multiple functions at once, but also includes the potential to add more.
“One of the potential applications of the material we developed is smart fabrics,” Li “Taking advantage of the shape memory property of the material, it is possible to make fabrics with self-adjustable texture. For example, when the body temperature of the wearer reaches a specific value, the texture of the fabrics could respond to the temperature change and open up small pores, allowing the heat and moisture to be expelled away from the body.”
Another application of the material is shape memory foam. “If our material is processed into foam,” Li explains, “its shape memory properties would allow the porous structure to be reconstructed by heating or other forms of stimuli, and thereby restore performance and increase durability of the cushions. Epoxies are also frequently used as coating materials.”
The self-healing property of the material is of particular import. Li says it could be used to design smart coatings that allow scratches to be healed upon the application of external stimuli.
“The material can also be potentially used as orthopedic devices,” Li continues. “Many orthopedic devices have to be customized and only can be used once. If using shape memory materials, the shape of the devices can be finely adjusted to fit each patient. After each use, the original shape of the device can be restored for next use, which could save a lot of materials and time.”
4D-printing: The next “smart” frontier
On the heels of the WSU researchers’ discovery, a research team from Lawrence Livermore National Laboratory (LLNL) has made strides in the nascent field of 4D-printing by creating additively manufactured structures that can change shape, as well as fold and unfold, when exposed to heat or electricity. The 3D-printing news site 3ders.org reports that the LLNL team is the first to combine 3D-printing and subsequent folding with conductive smart materials to make complex structures, including conductive and medical devices. For example, the team has demonstrated the ability to straighten a bent electrical conductor by exposing it to electricity and heat. They also have taken a collapsed stent and expanded it to its proper shape by simply exposing it to heat.
The LNNL research, published in the journal Scientific Reports, builds upon the method of 4D-printing. Though still in its early stages, 4D-printing has the ability to expand the capabilities of additive manufacturing by first 3D-printing an object that can then be programmed to change its shape and/or reassemble itself in relation to environmental factors.
“It opens up a whole new property set,” says Lab staff scientist James Lewicki, in reference to the development of 3D-structured smart materials that can morph in response to stimuli while also retaining a memory of their previous structure
“If you can print with these polymer composites, you can build things and electrically activate them to unfold,” Lewicki continues. “Instead of a dumb lump, you are left with this sentient, responsive material.”