A biomimetic self-powered electronic skin

Date: 7th July 2020

Article in brief:

The skin is the largest organ in the human body and it creates a physical and biological barrier to the environment. It is also an important somatosensory system for humans to perceive, interact, and communicate with the environment. Now scientists have created a breathable, biodegradable, antibacterial, and self-powered electronic skin with highly sensitive capabilities.

Mimicking the comprehensive functions that the human skin possesses is crucial for accelerating the development of human-machine interaction, artificial intelligence and creating the next-generation of wearable medical applications.

Whilst there has been much interest in developing electronic skin (eskin), current systems have in the main only been able to reproduce the major functions of the skin, excluding factors such as comfort, environmental friendliness, and antibacterial activity.

Now scientists from Georgia Institute of Technology, US, the Inner Mongolia University of Technology, and the Chinese Academy of Sciences, China, have developed a breathable, biodegradable, and antibacterial e-skin based on all-nanofibre triboelectric nanogenerators creating a flexible and stretchable eskin that harvests energy from motion, allowing self-powered whole-body physiological signal monitoring.

Triboelectric nanogenerator (TENG), is a relatively new energy-harvesting technology; which converts mechanical energy into electricity based on the coupling effect of contact electrification and electrostatic induction caused by friction.

To begin here, the team used TENG to build an e-skin that would attached to almost anywhere on the human body. The eskin consisted of three functional layers; the top polylactic-co-glycolic acid (PLGA) for contact electrification and water proofing, a middle layer containing silver nanowire (Ag NW) electrodes for conducting electricity and also an antibacterial agent, and a bottom layer comprising polyvinyl alcohol (PVA) for flexibility and attachment.

• The PLGA and PVA nanofibre layers were constructed by electrospinning - a fibre production method which uses electric force to draw charged threads of polymer solutions or polymer melts to form a nanofibre mat.
• The resulting eskin was light (80 mg), thin (120 μm) and could be stretched to 100% strain level demonstrating high flexibility.
• The breathability of the eskin enabled the exchange of air and moisture between the environment and the skin, and was vital to allow regulation of temperature and humidity.
• The breathability of e-skin was ~120 mm s⁻¹, as a comparison the breathability of a pair of commercial available jeans was much lower at ~10 mm s⁻¹.
• Ambient humidity could potentially weaken the air permeability of e-skin, however, at 80% humidity, air permeability was still 40 mm s⁻¹ verifying the eskin’s thermal-moisture stability and comfort.
• The mechanical durability of the e-skin was assessed: 50,000 cycles of repeated loading-unloading under pressure was performed. The eskin voltage profile displayed no noticeable fluctuation during these repetitive pushing tests confirming the e-skin was stable for long-term use.
• Antimicrobial activity was extremely important for the safety of the eskin as it is placed directly on the skin. Silver is known to have a broad-spectrum biocidal properties and this was demonstrated by inhibited growth of gram-negative coli and gram-positive S. aureus.
• To assess the biodegradation of the eskin, it was subjected to in vitro biodegradation tests for 50 days. Whilst the layers degraded at different rates, the eskin started to degrade in the first few days, and lost 50% of its weight by day 50.
• The real test for the device was to determine whether the eskin could perform whole-body physiological and motion monitoring.
• The device was placed in areas all over the body and the resulting voltage signals were monitored.
• The eskin was able to monitor facial expressions such as blinking, frowning and smiling – conveying human emotion.
• Respiration in the clinic is a primary vital sign, this was monitored by placing the eskin on the vent of a conventional mask, or the chest or abdomen of a human and could distinguish four distinctive breathing states.
• The skin could also be used to detect the pulse, determine sound and speech patterns, detect limbs bending and flexing, or feet moving.

Conclusions and future applications:

The team here have presented a highly sensitive, self-powered, biodegradable, breathable, antibacterial eskin which was developed for the detection of physiological characteristics and movement states of the whole body.

The team envisage the eskin will have a far-reaching range of medical applications, from personal health monitoring, to rehabilitation of patients, to the enhancement of intelligent prostheses. It may also offer hope in communicating with those that are paralysed, or play a role in medical diagnosis. It seems this tech has a great potential to integrate into many clinical areas.

We are currently in a digital revolution and as such we are seeing an increasing drive of mobile device health apps to detect disease such as heart defects, or skin cancer, and to track fitness and lifestyle it is not hard to imagine the potential benefits the eskin monitor could bring to such tech outside the clinic and into our everyday lives.

This work presented here provides a previously unexplored strategy for multifunctional e-skins with excellent practicability and biocompatibility.

Peng, X., K. Dong, C. Ye, Y. Jiang, S. Zhai, R. Cheng, D. Liu, X. Gao, J. Wang and Z. L. Wang (2020). “A breathable, biodegradable, antibacterial, and self-powered electronic skin based on all-nanofiber triboelectric nanogenerators.” Science Advances 6(26): eaba9624.

https://doi.org/10.1126/sciadv.aba9624