The search for biocompatible, stretchable, and wearable electronics has been a major materials science goal of the 21st century and is expected to be the pivot for the internet-of-things network. The concept of electronic skin (e-skin) embodies a class of materials that can mimic many of the skin functions while retaining some of its fundamental properties. Such materials must be biocompatible, stretchable, self-healing, and able to provide haptic feedback. As such, they should be able to detect small deformations through changes in their electrical resistance. The requirement of biocompatibility levies an additional constraint of hydrophilicity on this material. To this end, hydrogels have been explored owing to their viscoelastic properties. Protein–hydrogel crystals and conductive nanofiller-based hydrogels are recent highlights, although commercial applications have been limited due to long-term sensing reliability.   

Now researchers from King Abdullah University of Science and Technology (KAUST), Saudi Arabia have introduced a hydrogel composite that meets the necessary criteria for an efficient e-skin. The research team headed by Husam Alshareef employed a class of two-dimensional (2D) metal carbides, called MXenes, as a conductive filler in crystal clay, a commonly used hydrogel, as reported in a recent issue of Science Advances. The team utilized MXenes’ superlative strain sensitivity and hydrophilic nature to good effect and demonstrated hydrogel composites that could be stretched to 3400% of their original length with excellent self-healing properties. Without the introduction of MXenes (Ti₃C₂T_x), crystal clay, the pristine hydrogel, showed much inferior stretchability and self-healability. The difference was ascribed to the unique polymer–clay networked structure and increased cross-linking by hydrogen bonds between MXene nanosheets and the polyvinyl alcohol (PVA) network of the hydrogel.

The mechanical deformation of the MXene-hydrogel was found to accompany change in electrical resistance. The hydrogel exhibits much higher sensitivity under compressive strains than under tensile strains. This asymmetrical strain sensitivity coupled with the viscoelastic properties is exploited to add new dimensions to the sensing capability of hydrogels. Consequently, both the direction and speed of motions on the hydrogel surface can be detected conveniently, demonstrating their worth as smart sensors. This key finding led the researchers to demonstrate advanced sensing applications such as motion detection, handwriting recognition, voice detection, and sensing facial expressions. As an illustration of one such application, the researchers found that when stuck to the forehead, the sensor could detect, through changes in electrical resistance, facial expressions such as smiling and frowning.

Yuri Gogotsi, a pioneer in the field of MXenes who heads the Nanomaterials Group at Drexel University and was not involved in this study, is excited and feels this is very promising work. “MXenes are extremely strong and highly conducting (more than solution-processed graphene and other 2D materials). As a result, the authors’ hydrogel composites outperformed all reported hydrogels for sensing strain.” Gogotsi believes that the availability of a vast variety of MXenes and polymers will open the door to a slew of opportunities at tailoring properties of composites following this very simple, but highly efficient approach.

Explaining the main idea behind the finding, Yi-Zhou Zhang, the first author on the article, says, “MXenes possess a negatively charged 2D nanosheet morphology with abundant surface functional groups which can form hydrogen bonding. The combination of these structural characteristics can form a [three-dimensional] networked structure within the hydrogel between MXene nanosheets, the polymer chains, and water, leading to enhanced mechanical properties including stretchability and self-healability.” More importantly, Zhang says, strains can modify the 3D networked structure leading to the asymmetrical strain sensitivity transforming the traditionally disadvantageous viscoelastic property of hydrogels into an advantage for sensing.

Alshareef believes these results are a first step into his vision of “gel electronics.”

By incorporating other functional materials like magnetic, light-emitting, or energy storage materials into hydrogels, it might, one day, be possible to create new and interesting properties different from their solid-state counterparts. “Gel-circuits” might be realized by a judicious combination of p-type and n-type gels. “Given that hydrogels are stretchable, soft, and conformable to the human body, ‘gel electronics’ can be a potential bridge between electronics and humans, thus creating a paradigm shift in the electronics industry,” Alshareef says.

Read the article in Science Advances.