(Nanowerk Spotlight) The field of epidermal electronics, which involves flexible electronic systems that interface with the skin, has attracted growing interest in recent years for potential applications ranging from health monitoring to human-machine interaction. However, widespread adoption has been hindered by limitations in existing technologies, including complex manufacturing, high costs, and difficulties achieving a combination of strong adhesion, durability, and adaptability to the dynamic nature of human skin.
Overcoming these challenges requires innovation in the fundamental materials used to construct epidermal electronics. Conventional approaches like thin electronic patches offer limited customization, while newer concepts like electronic tattoos and electronic inks face issues with complicated fabrication or exposure to sweat.
Researchers have increasingly turned to hydrogel materials, which combine water with networks of polymers, as a promising alternative. Hydrogels’ soft, flexible nature and biocompatibility make them well-suited for interfacing with biological tissue. Specific hydrogels known as “silly putty-like” materials have garnered particular interest due to their unique viscoelastic properties that allow them to be easily reshaped and molded.
However, developing hydrogels that simultaneously possess the necessary electrical conductivity, mechanical strength, adhesion, and self-healing abilities for robust epidermal electronics has proven difficult. High-performance conductive materials like silver nanowires are costly, while increasing the concentration of conductive fillers can degrade mechanical properties. Existing conductive hydrogels also tend to have limited reusability. These constraints have restricted the practical viability of hydrogel-based epidermal electronics despite their theoretical promise.
In a potential breakthrough, researchers from City University of Hong Kong, have now developed a new PVA-based hydrogel they call “electronic slime” or “E-slime” that takes inspiration from the remarkable adaptability of amoebas. Amoebas, as single-celled organisms, can rapidly alter their shape by extending their elastic outer membrane to form temporary arm-like protrusions called pseudopods. This flexibility allows amoebas to navigate surfaces and adapt to their environments. The scientists sought to replicate this dynamic shapeshifting capability in a conductive material suitable for epidermal electronics.
The schematics of fabrication, crosslinking structure, conductive mechanism, and characterizations of E-slime. a) The schematic preparation of E-slime. b) The crosslinking structure of E-slime. c) The bonding structure of E-slime. d) The conductive mechanism of E-slime. (Adapted from DOI:10.1002/adfm.202402393, CC BY) (click on image to enlarge)
To create E-slime, the team started with a biocompatible PVA-glycerol hydrogel and incorporated adhesive plant-based tannic acid. Inexpensive graphite flakes and carbon black powder were then added as conductive fillers. The components are simply mixed and treated with a borax solution to trigger gelation, resulting in a sticky, flexible conductive material through a scalable fabrication process.
Notably, the carbon fillers are dispersed into a highly stretchable “island bridge” structure where carbon black particles link graphite flakes. This allows E-slime to maintain high conductivity even when heavily deformed.
The simplicity and scalability of the fabrication process are significant. E-slime can be rapidly synthesized from abundant raw ingredients using basic equipment, which hints at the feasibility of mass production. Moreover, the fabrication method is environmentally friendly and low-cost, involving simple steps like mixing PVA, glycerol, tannic acid, graphite, and carbon black, followed by treatment with a borax solution.
Comprehensive testing validated E-slime’s impressive capabilities. It can be stretched to 25 times its original length and restore high conductivity within seconds after being severed. The material adheres strongly to skin with no need for adhesives, maintaining an adhesive strength of ≈3 kPa even on highly textured or moving skin surfaces. Remarkably, E-slime can be peeled off undamaged and reused over 100 times while preserving its conductivity and strain sensitivity.
To demonstrate E-slime’s functionality as an epidermal sensor, the researchers adhered it to various locations on the body including the fingers, face, limbs, joints, neck, and wrist. It detected a wide range of large and subtle motions, from full-range joint movement to minute vibrations from speech, coughing, and pulse. Machine learning analysis could discern distinct signal patterns from different facial expressions, gestures, and writing motions. This versatility highlights E-slime’s potential for human-computer interaction, health monitoring, and activity recognition.
The properties and applications of amoeba-inspired E-slime. a) The optical photos of amoeba with shape-morphing pseudopodia. b) The full biological reshapability of amoeba from different morphologies. c) The easily customized and fully reusable properties of E-slime on skin. d) The instant draw of E-slime on skin with ultra-deformable properties. e) The ultra-conformal properties of E-slime on skin. f) Schematic of the E-slime on skin subject to stretch and compression. (Adapted from DOI:10.1002/adfm.202402393, CC BY) (click on image to enlarge)
E-slime exhibits a unique combination of properties that surpass other “Silly Putty-like” hydrogels, with stretchability up to 2600%, a self-healing time of ≈1 s, and high sensitivity with a gauge factor of 2.95. These attributes, combined with its biocompatibility and low-cost production, position E-slime as a superior alternative for epidermal electronics.
By taking inspiration from the adaptability of living organisms, the material pushes the boundaries of what’s possible in stretchable, self-healing electronics that can seamlessly merge with the body. With further refinement, E-slime’s low-cost and environmentally friendly composition could enable a new generation of multifunctional epidermal sensors for healthcare, gaming, athletics, and more.
The simplicity and scalability of the fabrication process is also significant. Being able to rapidly synthesize E-slime from abundant raw ingredients using basic equipment hints at the feasibility of mass production. The ability for users to cut and reshape the material on demand for different wearing contexts is equally crucial. Together, these attributes bring personalized and accessible epidermal electronics a step closer to reality.
Ultimately, E-slime represents an exciting confluence of concepts from materials science, electronics, and biology to solve persistent challenges in wearable technology. Though still an early-stage research prototype, it lays a strong foundation for a new class of multifunctional epidermal electronics. With further development guided by this innovative bioinspired design approach, E-slime and future materials modeled on its adaptive properties could soon be a practical and ubiquitous part of our digital lives.
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