Skin electronics or “e-skin” is an emerging class of electronics that puts the focus on mimicking the properties of skin. As such, e-skin devices must be soft, stretchable, conformable to curved and dynamic surfaces (as it is our skin), and non-toxic. Also, they must potentially be able to cover large areas (skin is the largest organ in the human body after all).

Many efforts are on-going to equip e-skin with the sensing capabilities of natural skin, such as pressure, thermal and humidity sensors. Even more, the concept of “enhanced skin” for humans or robots also foresees including functionalities beyond those of normal human skin, such as biosensing, quick self-healing or even displaying info through a skin-like display. 

When targeting the development of e-skin for human skin replacement, the question of how to achieve communication with neurons arises. The skin serves as our interface with the world, thus it can efficiently communicate with our brain through nerves. Achieving this brain-machine interfacing capability artificially will open new ways in prosthetics, enabling for instance sensitivity to prosthesis. Imagine a prosthetic hand covered by e-skin that could seamlessly connect with our nerves, transmitting signals to our brain and eliminating the phantom limb symptom.

Due to the utilization of non-standard functional materials, the fabrication of e-skin encounters limitations when using conventional microfabrication techniques like photolithography, which requires exposing the whole system to different chemical baths; or evaporation, which is not suitable for most polymers. Hence, finding a fabrication strategy capable of patterning stacks of heterogeneous chemically-sensitive materials over large areas is key for the feasibility of e-skin development. 

In this work we have fabricated soft and stretchable field effect transistor (FET) arrays exclusively from solution by inkjet printing intrinsically stretchable materials: a stretchable ionic version of PEDOT:PSS for electrodes and interconnections; carbon nanotube networks for the channel; and ionic solid-state fluoropolymer as the gate dielectric. The devices show high mobility of 30 cm² V^-1 s^-1. Moreover, owing to the ionic nature of the gate dielectric, an electrical double layer is formed that enables low operation voltage of less than 1 V. The FETs display neuromorphic behavior, which suggests that the gate ions dynamics emulate neurotransmitters in the synaptic cleft of neurons. As such, our system could find applications in e-skin with capability to connect directly with neurons and act as an artificial neuron synapse (Figure 1). With this work, we hope to bring ourselves one step closer to the concept of e-skin with neuronal communication.

If you would like to know more, please check our paper published in Nature Communications: Inkjet-printed stretchable and low voltage synaptic transistor array (https://www.nature.com/articles/s41467-019-10569-3)