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New Kirigami-Based Method to Develop Damage-Resistant Wearable Sensors

With the increased prevalence of wearable sensors, the demand for a material with resistance to damage caused by the stress and strains of a person’s natural body movements becomes even more crucial.

Using kirigami to cut and fold graphene allows wearable sensors to better conform with the natural movement of the body. (Image credit: University of Illinois at Urbana-Champaign)

To meet that requirement, scientists at the University of Illinois at Urbana-Champaign have formulated a technique in which they adopted kirigami architectures to help materials gain more strain tolerance and become more adaptable to movement.

Like origami, the more famous art of paper-folding, kirigami involves cutting as well as folding. The team headed by SungWoo Nam, associate professor of Mechanical Science and Engineering, and Keong Yong was successful in applying kirigami architectures to graphene, an ultra-thin material, to develop sensors ideal for wearable devices.

To achieve the best sensing results, you don’t want your movement to generate additional signal outputs. 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.

SungWoo Nam, Associate Professor of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign

The researchers achieved these results by carrying out several simulations in collaboration with Narayana Aluru, professor of Mechanical Science and Engineering. They also created online software on a nanomanufacturing node, the first of its kind to be ever developed. The online software platform allows scientists to carry out simulations before building the real devices or platforms.

As soon as the researchers developed a design that was effective in simulation, it was immediately tested. Graphene looked promising as a potential material due to its ability to withstand major deformation and breaking than metals and other traditional materials.

Since graphene is an atomically thin material, the researchers were able to encapsulate the graphene layer between two polyimide layers (the same material used to safeguard foldable smartphones). After creating the “sandwich,” they engineered kirigami cuts to improve the material’s stretchability.

Because graphene is sensitive to external changes, yet also a flexible semimetal conductor, people are very interested in creating sensors from it. This sensitivity is well suited for detecting what is around you, such as glucose or ion levels in sweat.

SungWoo Nam, Associate Professor of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign

The researchers learned that the application of kirigami architecture rendered the graphene stretchable as well as strain-insensitive and devoid of motion artifacts. This means that even as it was deformed, there was no variation in the electrical state. They also learned that the graphene electrodes displayed strain-insensitivity up to 240% uniaxial strain, or 720° of twisting.

The study outcomes have been reported in Materials Today.

What’s interesting about kirigami is that when you stretch it, you create an out of plane tilting. That is how the structure can take such large deformations.

SungWoo Nam, Associate Professor of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign

In their design, the scientists placed the active sensing element on an “island” between two “bridges” created from kirigami graphene. The graphene did not lose any electrical signal regardless of the bending and tilting, but it still handled the load from the straining and stretching, allowing the active sensing element to stay connected to the surface.

As such, kirigami has the distinctive ability to redistribute stress concentrations, thus realizing improved directional mechanical qualities.

Although the researchers have been successful in proving the basic technique, they have already started their efforts toward optimization in version 2.0, with the likelihood of finally commercializing the technology.

The team also achieved positive outcomes using polydimethylsiloxane (PDMS) as the sandwich layers. Therefore, they consider that, apart from graphene, the design could also be used with other atomically-thin materials like transition metal dichalcogenides.

Uniaxial Stretching of Kirigami Inspired Strain Insensitive Graphene Based Electrode

(Video credit: University of Illinois at Urbana-Champaign)


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