Ultrasensitive 3D Printed Self-Healing Ionic Hydrogels for Wearable Multifunctional Sensing

The demand to connect the human and technology worlds has increased dramatically in recent years. Softness, multi-sensing capability, self-healing ability, and adaptability are all characteristics of living tissues. Robots and machines, on the other hand, are generally stiff and unable to self-repair. As a result, devices with intermediate qualities are required to connect the two worlds. Tactile or smart sensors, which are devices that can feel external stimuli such as deformations, temperature, wetness, and light, and are inspired by the human skin, which is the greatest sensor tissue of living humans, could be an ideal option. The key difference between standard sensors and these ones is that they are made of flexible materials to be more adaptable to the uneven surfaces of soft tissues and robots. Tactile sensors can be used in a variety of disciplines, including wearables [1], e-skin [2], prosthetics [3], and soft robotics [4], because to this capability. As a result of these advancements, the global market for soft tactile sensors has seen incredible growth in recent years. In particular, despite its relevance in biomedicine, reproducing haptic sensation remains a task that has yet to deliver consolidated results: flexible strain sensors are great candidates to overcome this difficulty.<br/>Because of their softness and water-rich environment, hydrogels are attractive options for the production of sensing-active materials, as they may be able to overcome mechanical incompatibilities between humans and electronics. Hydrogels are water-based materials that can be easily manufactured and have great transparency and stretchability without losing conductivity.<br/>Herein, an extremely flexible strain sensor based on a 3D printable self-healing hydrogel is presented. The photocurable ink was prepared by mixing an aqueous solution of Poly (vinyl alcohol) (PVA) with acrylic acid (AAc), Poly (ethylene glycol) diacrylate (PEGDA), a water-compatible photoinitiator and sodium chloride (NaCl) to endow ionic conductibility. The device were then printed with a commercial Digital Light Processing printer. The sensor, exploitable both as piezoresistor or piezocapacitor, presents high sensitivity to external stimuli, together with an extreme stretchability. The sensors show self-healing ability at room temperature without any stimuli and are able to restore both mechanical properties and strain sensitivity after the self-healing procedure and also after a drying and water restoring process. The very high strain sensitivity and the 3D printing capability enables the hydrogel to be implemented into complex shape wearable strain sensors to monitor various human motion and physiological data, making it a viable solution for the creation of easily manufactured and conformable devices.<br/><br/>[1] Huang F, Wei W, Fan Q, Li L, Zhao M, Zhou Z. Super-stretchable and adhesive cellulose Nanofiber-reinforced conductive nanocomposite hydrogel for wearable Motion-monitoring sensor. <i>J Colloid Interface Sci</i>. 2022;615:215-226.<br/>[2] Lei Z, Wang Q, Sun S, Zhu W, Wu P. A Bioinspired Mineral Hydrogel as a Self-Healable, Mechanically Adaptable Ionic Skin for Highly Sensitive Pressure Sensing. <i>Adv Mater</i>. 2017;29(22):1-6.<br/>[3] Kim J, Lee M, Shim HJ, et al. Stretchable silicon nanoribbon electronics for skin prosthesis. <i>Nat Commun</i>. 2014;5.<br/>[4] Pang Y, Xu X, Chen S, et al. Skin-inspired textile-based tactile sensors enable multifunctional sensing of wearables and soft robots. <i>Nano Energy</i>. 2022;96(January).