(Nanowerk Spotlight) Highly flexible and sensitive strain sensors are essential components of wearable electronic devices and graphene's piezoresistive effect, combined with its other properties such as ultra-translucency, superior mechanical flexibility and stability, high restorability, and carrier mobility, make it a very promising material for fabricating high-sensitivity strain sensors.
Potential application areas for these sensors could be found in flexible display technology, robotics, smart clothing, electronic skin, body monitoring, human-machine interfaces, in vitro diagnostics, and implantable devices.
Motivated by the high level of flexibility exhibited by spider webs, scientists in Hong Kong have developed a novel design for highly flexible and sensitive piezoresistive sensors based on an elastomer-filled graphene-woven fabric (E-GWF) structure. This technique mimics the distinct core-shell structure of spider webs. This fabrication method could also be extended to other 1D and 2D materials for many emerging practical applications.
Schematic flowchart of the fabrication procedure of an E-GWF. (Reprinted with permission by American Chemical Society)
"The combination of high sensitivity, flexibility, and stretchability makes the spider-web like wearable strain sensor suitable for mounting on the human skin," Qingbin Zheng, a Research Assistant Professor of Mechanical and Aerospace Engineering, tells Nanowerk. "In addition, the semitransparent strain sensor can improve user experience without any significant impact on daily activities."
Furthermore, the developed spider-web-like wearable strain sensor can strengthen interactions between humans and smart systems, especially in shape-conforming systems of electronic skin, elastic displays, epidermal sensors, personalized health monitoring and human-machine interfaces. These sensors should be able to maintain good functionality when conformably in contact with human skin/muscle with curvilinear surfaces.
Researchers and material scientists have long been fascinated by spider silk – ultra-strong and extensible self-assembling biopolymers that outperform the mechanical characteristics of many man-made materials, including steel.
As Zheng and his co-authors describe in their paper, a spider web is built with silk whose structure consists of a bundle of silk fibrils, known as spidroin, which are encapsulated by about 100-200 nm thick outer layers made up by lipids, glyco, and skin. The core is 2-3 µm thick and dominates the tensile strength and elasticity. The molecular structure of silk consists of regions of protein crystals separated by less-organized protein chains. The primary structural modules give rise to diverse secondary structures that, in turn, determine the functions of different silks.
Inspired by this unique architecture, the researchers developed their design for a highly flexible and sensitive piezoresistive sensor, which mimics the geometric and functional characteristics of a spider web.
Magnified SEM images of the freestanding E-GWF sensor taken at 20% tensile strain. (Reprinted with permission by American Chemical Society) (click on image to enlarge)
"It was proposed previously that there are three critical components that need to be considered when designing new functional materials, including the chemical composition, nano/microstructure, and architecture," Zheng explains. "The design of our E-GWF involves using graphene/PDMS as the main constituent to ensure high flexibility; nanoscale graphene/PDMS microfibers as the nano- and microstructural framework to provide high sensitivity; and woven fabric as the assembled architecture to guarantee structural integrity."
"These three components make E-GWFs an ideal bio-inspired hierarchical material for strain sensors arising from the multiscale structures spanning from woven fabric, PDMS microfiber to nanoscale graphene," he adds.
Although the working range of the sensor has been increased up to 20% durable strain, a much higher stretchability is required for full-range human motion monitoring, such as at the joints of the human body. The team expects that additional structural engineering such pre-straining, wrinkling and patterning will enable them to increase the sensors' stretchability significantly.