[1][2] The broad class of materials often contain sensing abilities that are intended to reproduce the capabilities of human skin to respond to environmental factors such as changes in heat and pressure.[5] The successful combination of flexible and stretchable mechanical properties with sensors and the ability to self-heal would open the door to many possible applications including soft robotics, prosthetics, artificial intelligence and health monitoring.[1][5][6][7] Recent advances in the field of electronic skin have focused on incorporating green materials ideals and environmental awareness into the design process.[10] They found that incorporating 2,6-pyridine dicarboxamide (PDCA) into the polymer backbone could impart self-healing abilities based on the network of hydrogen bonds formed between groups.[4] Thin film with poly(N,N-dimethylacrylamide)-poly(vinyl alcohol) (PDMAA) and reduced graphene oxide have shown high electrical conductivity and self-healing properties.[8] The e-skin developed by the group consists of a network of covalently bound polymers that are thermoset, meaning cured at a specific temperature.The ability of electronic skin to withstand mechanical deformation including stretching and flexing without losing functionality is crucial for its applications as prosthetics, artificial intelligence, soft robotics, health monitoring, biocompatibility, and communication devices.[10] Oh et al. developed a stretchable and flexible material based on 3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP) and non-conjugated 2,6-pyridine dicarboxamide (PDCA) as a source of hydrogen bonds (Figure 4).[13] Skin is composed of collagen, keratin, and elastin fibers, which provide robust mechanical strength, low modulus, tear resistance, and softness.To achieve conformability, it is preferable for devices to match the mechanical properties of the epidermis layer when designing skin-based stretchy electronics.The aforementioned approach was used to create devices composed of 100–200 nm thick Si nano membranes deposited on thin flexible polymeric substrates.[17] In the case of island interconnect, the rigid material connects with flexible bridges made from different geometries, such as zig-zag, serpentine-shaped structures, etc., to reduce the effective stiffness, tune the stretchability of the system, and elastically deform under applied strains in specific directions.[1] The self-healing conductive composite synthesized by Tee et al. (Figure 2)[7] investigated the incorporation of micro-structured nickel particles into a polymer host.[3][23] The Bao Group at Stanford University have designed an electrochromically active electronic skin that changes color with different amounts of applied pressure.[29] Scientists at the University of Glasgow have made inroads in developing an e-skin that feels pain real-time, with applications in prosthetics and more life-like humanoids.
Figure 1. Polymerization scheme for formation of polyimine-based self-healing electronic skin.
Figure 2. Self-healing material based on hydrogen bonding and interactions with micro-structured nickel particles.
Figure 3. Recycling process for conductive polyimine-based e-skin.
Figure 4. A stretchable and self-healing semiconducting polymer-based material.
