E-tattoos, electronic tattoos or epidermal electronics are a form of electronics that can be worn and be placed directly on your skin. They are soft, thin and lightweight, which allows them to be mounted on a human body for noninvasive sensing and high-fidelity use. E-tattoos are made for controlling your smartphone and they are touch-sensitive buttons, they can be placed on your skin as a stick-on dosimeter to measure graphene-based skin biosensors, UV exposure, or lab-on-skin for a health monitoring equipment that is wearable. 

During the placement of electronic tattoos or E-tattoos, they are exposed constantly to different types of mechanical inputs such as bending, cutting, pressing and twisting. This may lead to malfunction as it could cause mechanical damage to your electronic tattoo.  

"We developed our self-healing, biocompatible and multifunctional e-tattoo by incorporating graphene with silk fibroin/Ca2+ films," said Yingying Zhang, Associate Professor in the Department of Chemistry and the Center for Nano and Micro Mechanics at Tsinghua University. "We show that custom-designed and highly flexible e-tattoos can be facilely prepared through screen printing or direct writing of a graphene/silk fibroin/Ca2+ suspension."

The graphene flakes found on the tattoo are distributed in the matrix form a path that is electrically conductive and it is responsive to all sorts of environmental changes like temperature, variations, humidity and strain. This endows the tattoo with increased sensitivity to multi-stimuli.  

What is great about this invention is that the tattoo can be healed immediately, in fact it has an efficiency rate of 100% even after fracturing for 0.3 seconds. The tattoo can be healed immediately by wetting it with a drop of water because the hydrogen on water and the coordination of the tattoo can help reform it and bond the fracture of the interface. 

"Based on the superior capabilities of our e-tattoos, we believe that such skin-like devices hold great promise for manufacturing cost-effective artificial skins and wearable electronics," notes Zhang.

The researchers used silk fibroin, this is a natural protein material that is found in silkworms. It is the best material for electronic gadgets and applications that can be worn because it is mechanically durable, it has good biocompatibility, it has all-aqueous processing and it has a tunable secondary structure. 

Even though the silk fibroin has a massive potential to use on wearable electronics such as e-tattoos, the application of this material in electronics that can be worn are limited as it is used as a substrate and not as the active material on the electronic that is functional. 

The graphene in the tattoo shows great potential in the application because it is stable and very strong, electrochemically and mechanically and it is biocompatible. The material can be used through numerous physical and chemical approaches, giving a lot of space for tailoring the performance and the properties of the sensors of the electronic that are graphene-based. 

"The integration of graphene materials and silk has been explored previously, but the majority of studies focuses on mechanical enhanced materials," notes Zhang. "We found that silk/graphene composite materials are highly promising for emerging epidermal electronics."