Skin electronics need stretchable conductors that exhibit metal-like conductivity, ultrathin thickness, high stretchability, and ease of patternability. However, it is difficult to obtain these properties at the same time.
Now, researchers have designed a new float assembly technique to fabricate a nanomembrane that fulfills all these needs simultaneously. The remarkable material characteristics are attributed to its distinctive cross-sectional structure, with a monolayer of compactly assembled nanomaterials partially embedded in an ultrathin elastomer membrane.
“Skin electronics” are thin flexible electronics, which can be fitted to the skin. Although it may seem like something out of science fiction, it is expected to find applications in next-generation devices in many fields such as health diagnosis, health monitoring, human-machine interface, and virtual reality.
As predicted, developing such devices would need components that are stretchable and soft to ensure mechanical compatibility with the human skin. One of the important components of skin electronics is an intrinsically stretchable conductor that is capable of transmitting electrical signals between devices.
A stretchable conductor that displays ultrathin thickness, high stretchability, metal-like conductivity, and ease of patternability is needed to perform a reliable operation and obtain high-quality performance. Despite detailed research, it has not yet been possible to design a material that holds all of these properties at the same time. This is because they frequently have trade-offs between one another.
Researchers at the Center for Nanoparticle Research within the Institute for Basic Science (IBS) in Seoul, South Korea, headed by professor Taeghwan Hyeon and Dae-Hyeong Kim presented a new technique to fabricate a composite material in the form of a nanomembrane that possesses all the above-mentioned properties.
The new composite material comprises metal nanowires that are tightly packed in a monolayer inside an ultrathin rubber film.
This innovative material was produced through a process developed by the team, known as the “float assembly method.” The float assembly leverages the Marangoni effect, which emerges in two liquid phases with different surface tensions.
The existence of a gradient in surface tension generates a Marangoni flow away from the region with lower surface tension toward the region with higher surface tension. This implies that dropping a liquid with lower surface tension on the water surface decreases the surface tension locally. The resulting Marangoni flow leads the dropped liquid to spread thinly throughout the water surface.
The nanomembrane is developed using a float assembly technique that involves a three-step procedure. The first step involves dropping a composite solution. This is a mixture of metal nanowire ethanol and rubber dissolved in toluene on the surface of the water. The toluene-rubber phase stays above the water as a result of its hydrophobic character. The nanowire moves to the interface between the water and toluene phases.
The ethanol inside the solution combines with the water to reduce the local surface tension. This creates a Marangoni flow that moves outward and restricts the aggression of the nanowire. This organizes the nanomaterials into a nanolayer at the interface between very thin rubber/solvent film and water.
The second step involves dropping surfactant to generate the second wave of Marangoni. This causes tight compaction in the nanowires. Lastly, the third step involves toluene evaporation. This yields nanomembrane with a special structure in which a highly compacted monolayer of a nanowire is partially embedded in an ultrathin rubber film.
The special structure enables efficient strain distribution in ultrathin rubber film. This results in exceptional physical properties, such as stretchability of more than 1000% and a thickness of only 250 nm. The structure enables cold welding and bi-layer stacking of the nanomembrane onto each other. This results in a metal-like conductivity of more than 100,000 S/cm.
In addition, the researchers illustrated that it is possible to pattern the nanomembrane using photolithography. This is an important technology that is commonly used for producing advanced electronics and commercial semiconductor devices. Thus, it is assumed that the nanomembrane can act as a new platform material for skin electronics.
The impact of this research may find application beyond the development of skin electronics. Although this research demonstrated a composite material comprising silver nanowires inside styrene-ethylene-butylene-styrene (SEBS) rubber, it is also viable to utilize float assembly technique on several nanomaterials like semiconducting nanomaterials and magnetic nanomaterials, as well as other kinds of elastomers like SIS and TPU.
Thus, it is predicted that the float assembly has the potential to form the gateway to new research fields involving different types of nanomembranes with various functions.
Jung, D., et al. (2021) Highly conductive and elastic nanomembrane for skin electronics. Science. doi.org/10.1126/science.abh4357.