(Right) Schematic illustration of the graphene kirigami device. (Left) Highly conformal graphene kirigami structure on the surface of a human wrist under extension articulation.
(Right) Schematic illustration of the graphene kirigami device. (Left) Highly conformal graphene kirigami structure on the surface of a human wrist under extension articulation.

Two-dimensional nanomaterials like graphene possess a variety of useful properties but are vulnerable to small strains. While their thinness is attractive for flexible devices like wearable electronics, their physical weakness cannot withstand the high strains that on-skin structure can experience. But now researchers have found that using kirigami-inspired design allows graphene-based devices to withstand large strains [Yong et al., Materials Today (2019) https://doi.org/j.mattod.2019.08.013].

“We adopted kirigami features to engineer strain-insensitive two-dimensional (2D) material-based wearable sensors capable of withstanding high strains and with the desired signals decoupled from signal artifacts,” explains SungWoo Nam of the University of Illinois at Urbana-Champaign, who led the work.

Kirigami, the ancient Japanese art of paper cutting, employs a pattern of slits and notches in flat materials to create three-dimensional (3D) shapes and structures. Nam and his team used this approach to cut patterns, notches, and bridges into sheets of monolayer graphene sandwiched between ultrathin layers of polyimide using reactive ion etching. The active sensing part of the device is positioned in a stiff central island between two kirigami bridges. When the kirigami device is stretched, the cuts and notches initially widen, leaving the active device unaffected on its island. Further stretching rotates the kirigami beams on their hinges before the structure ultimately fails.

“Our [approach] was to adopt kirigami and island-bridge sensor architectures that specifically redistributes strain away from the 2D material and active sensing element,” says Nam. “This both prevents mechanical and electrical failures up to large strains and additionally decouples the effects of body movement from the desired sensor output signals.”

The kirigami architecture enables the sensor devices to withstand stretching of up to 240% and two full twists (i.e. 720° torsional twisting). Not only do the devices not fail physically but also the deformation results in negligibly small changes in resistance.

“The strain tolerance of the resulting devices is unmatched for 2D material-based wearable sensors,” claims Nam. “The enhanced stretchability implies applications in the field of wearables.”

The researchers demonstrated a graphene-based field-effect transistor sensor, which could be used for a wide range of biosensing uses from monitoring glucose to pH, applied to a wearer’s wrist. The sensor successfully withstood repeated flexing and extending without degrading performance.

“There are still challenges to integrating these sensors with the other required components (e.g. battery, display, controller) to produce fully stretchable devices,” points out Nam. “We are currently pursuing more complex designs using the same island-bridge and kirigami principles to allow us to perform under more extreme strain conditions. Our team is also considering various polymer scaffolds and extending [our approach] to other atomically-thin materials such as molybdenum disulfide.”