Soft electronics that can be directly laid on the skin surface provide a conformal way to extract physiological and physical information of the body for facile interaction with the human body and long-term monitoring capabilities. The human body’s physical, chemical, biological, and environmental status could be monitored by various flexible sensors with high efficiency and minimum artifacts. Epidermal electronic technology, which mimics the physical features of human skin, can offer unique properties inaccessible to conventional, wafer-based electronics, such as extreme levels of deformability, conformal mechanics, lightweight construction, compatibility with soft tissues, biodegradability, and self-healing capability, among others.

Nowadays, some flexible on-skin devices have been reported and demonstrated their potential, such as elastic patches, electronic tattoos, and electronic ink; however, several inherent issues restrict them from practice at the current stage. Preparing complex and personalized epidermal electronics in a cost-effective and quickly is still challenging, not to mention simplifying the usage process to a point where no specialist knowledge or skills are required. Here we propose electronic inks made from carbon-based materials and two different preparation methods using them to create highly customizable epidermal devices. These devices can be used to detect a wide range of biosignals and have additional features that greatly enhance their environmental suitability.

Firstly, carbon-based materials are commonly used as conductive fillers for epidermal electronic systems. We have developed two types of conductive fillers, graphite-carbon black hybrid conductive powder and graphene oxide aqueous solution, based on the requirements in different application scenarios. The graphite-carbon black hybrid conductive powder improves conductivity by combining two carbon-based materials with complementary microstructures to form a composite island-bridge structure; this structure can also amplify resistance changes when used as a sensor to improve sensitivity. The use of reduced graphene oxide in an aqueous solution reduces the cost of graphene sheets and improves the flexibility of use. These graphene sheets are compatible with solution processing techniques such as spraying, dip coating, spin coating, and direct painting for deposition on customizable, flexible substrates for direct use in epidermal electronics applications.

Secondly, natural gums and cellulose can be used as substrate modules for constructing of electronic devices due to their excellent biocompatibility and environmental properties. The mechanical properties, the water resistance, and the directional distribution of the natural materials are used to give these functions to electronic devices. At the same time, the various functional groups attached to these natural polymers, such as -COOH, -OH, etc., allow for extended functions such as high humidity response and self-healing. In addition, to improve the physical and chemical properties of the natural materials, we also try to use modified natural materials or add additives to adjust the indicators further to obtain a substrate that meets the requirements of the application.

Thirdly, the design and fabrication of an epidermal electronic system based on carbon-based conductive materials are presented. Simplifying manufacturing and usage methods is one of the main challenges in making devices that can be widely used. Using modified natural materials, our electronic ink can be molded directly onto the skin surface or attached to a substrate and then adhered to the skin surface. We have designed 3D printed stamps in different shapes as well as paper cut structures to enable rapid prototyping of complex shapes. In addition, direct scribing with a traditional brush provides a flexible complement to connecting external devices and adjusting resistors.

Finally, we tested electronic ink based epidermal electronic systems on volunteers for both biophysical/bioelectrical signal sensing. The electronic ink can be used directly as a sensor material for humidity and stretch sense; as a sampling electrode for EMG and ECG acquisition; and as a wire for blood oxygen sensing connected to other devices, all demonstrating high accuracy sensing performance, resistance to motion artifacts and reliability over long periods of time. The application in thermal therapy has also been demonstrated using a high-resolution infrared camera to observe the electronic ink heater on the surface of the pig skin. The high heating performance, fast heat dissipation, and high customizability of shape/temperature make it feasible and effective for use as a heater. The results of these tests were compared with existing commercial devices and the performance was fully validated.

In conclusion, electronic ink is one kind of scalable and easy-to-use bioelectronics material that can be directed transferred to the skin or combined with other materials to construct an epidermal electronic system. The use of modified natural polymers gives them superior mechanical behavior. Moreover, the electronic ink based electronic devices exhibit comparable electrical properties as the commercialized devices and can be applied to monitor respiratory, ECG/EMG, oxygen saturation, and customized hyperthermia therapy. This research could pave a new path for developing health monitoring, low-cost epidermal devices in various application scenarios.