A recently published review in Advanced Science details the primary research routes towards high-performance electronic skin, with a focus on the most recent technologies and performance requirements.
Our skin plays a crucial role for survival. Not only does it provide a barrier to the elements, it is a highly flexible multifunctional sensor that allows our bodies to adapt to the environment, healing itself when damaged. Its complexity sets the standard for electronic skin (e-skin), which is a major area of interest for artificial intelligence and human-machine interactive electronics. Amongst other uses, e-skin and its related technologies deliver important medical applications, for example, in health monitoring or for use with prostheses.
The sensory capabilities of e-skin must be high resolution and highly sensitive with a rapid response. Current transduction methods have produced flexible and stretchable pressure arrays based on piezoresistivity, capacitance and piezoelectricity. Thinner substrates can typically be employed for greater flexibility, however achieving a stretchable electronic sensor is more complex. Recent investigations to improve stretchability include bonding methods, where conductive materials are attached to rubber or elastic substrates. Another method involves assembling the devices using intrinsically stretchable conductors.
An important consideration for e-skin in real applications is its scalability. Transistor arrays are an important route towards large scale e-skin production. With ideal signal transduction and amplification, these arrays enable pressure mapping across large areas. Current avenues to further improve the flexibility, resolution, and power consumption of these devices are discussed in the review article. In order to achieve a sensitivity that equals or indeed surpasses human skin, high resolution sensitivity is extremely important. While piezoresistive or capacitive sensors suffer from a decrease in signal output quality with the device size, this is avoided with piezopotential signals. Piezoelectric devices are therefore good candidates for high resolution, miniaturized sensors.
Beyond the basic pressure-sensing functions, e-skin is being developed with additional functionalities in mind: self-power for continuous operation, the ability to self-heal, to simultaneously differentiate between multiple stimuli (for example, temperature, humidity, pressure, and strain). Combining these developments with wireless technologies enables data and energy transmission, allowing for real-time monitoring or control of sensors, which could be used in clinical applications for remote in vivo treatments such as drug delivery.
Full mimicry of human skin requires complex engineering to achieve multifunctionality. E-skin must not just be highly tactually sensitive, but must also adapt to the surrounding environment. While self-healing and self-powered sensors have been developed, it remains a challenge to couple these abilities with the high electrical conductivity and pressure sensitivities key for practical applications of e-skin in robotic and prosthetic applications. For comprehensive details and relevant citations for the main developments towards high performance e-skin, and the challenges that remain, readers are referred to the engaging review article by X. Wang, C. Pan and co-workers.
Advanced Science is a new journal from the team behind Advanced Materials, Advanced Functional Materials, and Small. The journal is fully Open Access and is free to read now at www.advancedscience.com.