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Graphene Material Designed for Wearable Devices

Graphene illustration

(Maxpixel.net)

23 Oct. 2019. An engineering lab devised techniques for better adapting the material graphene to handle the physical stresses imposed by wearable devices. Researchers from University of Illinois in Urbana describe their process in the 30 September issue of the journal Materials Today (paid subscription required).

A team led by Illinois engineering professor SungWoo Nam is seeking ways to better use advanced nanoscale materials in the proliferation of wearable devices, particularly for health care and fitness. These devices, worn on the body or woven into fabrics must be able to bend, twist, and stretch while maintaining ability to sense activity in the body or environment, as well as collect and transmit the data.

For this project, Nam and colleagues worked with graphene, a material with many desirable qualities for a range of industries. The material is very light, strong, chemically stable, and only one atom in thickness, arrayed in a hexagonal pattern. Graphene can conduct both heat and electricity, with many applications in electronics, energy, and health care.

To find their solution, the Illinois team turned to the traditional Japanese paper art form known as kirigami, which combines simple paper folds and cuts to make intricate designs. Kirigami is similar to origami that involves only folding paper, not cutting. The researchers hypothesized kirigami-style cuts could withstand the strains of movement, without distorting the electrical signals.

“To achieve the best sensing results, you don’t want your movement to generate additional signal outputs,” says Nam in a university statement. “We use kirigami cuts to provide stretchability beyond a material’s normal deformability. This particular design is very effective at decoupling the motion artifacts from the desired signals.”

With the help of fellow engineering colleague Narayana Aluru, the researchers performed a series of computer simulations testing different kirigami cuts to find the optimum designs. The simulation software, called Gamian, is available online, and makes it possible to pre-test various kirigami cut designs with different kinds of materials.

The team used the results of their simulations to design graphene sensors for wearable devices. They encased the graphene sensors between layers of polyimide, a strong, heat- and chemical- resistant polymer material used in foldable smartphones. In lab tests, the sensors show they can withstand combinations of stretching, twisting, and shearing, including push-and-pull strains of up to 240 percent. The sensors could also maintain their electrical properties after twisting 720 degrees.

The tests show that despite the stretching and strains, the kirigami-designed sensors remained adhered to the target surface, and redistributed the stress concentrations. “What’s interesting about kirigami,” notes Nam, “is that when you stretch it, you create an out-of-plane tilting. That is how the structure can take such large deformations.”

The researchers plan to continue developing their kirigami-designed graphene sensors for eventual commercialization. The work includes testing the sensors with more materials, including polydimethylsiloxane, or PDMS, a widely used bio-compatible polymer found in a number of products and used more recently in fabricating biomedical micro-electromechanical systems, and with other types of one-atom thick nanoscale materials.

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