Materials Design of Functional Gels to Realize 3D Artificial Skin Device based on Extrusion-Based 3D Printing

Due to unique properties such as biocompatibility, flexibility, and conductivity, hydrogels are widely investigated in various applications, including tissue engineering, E-skin, wearable, and ionic devices. Especially, in the field of E-skin devices, hydrogels are recognized as one of the promising materials due to their flexibility and capability to realize various sensing functionalities. Still, there are challenges that need to be addressed: the structural freedom of devices. Most E-skin devices have been demonstrated with simple structures such as blocks or films due to the demand for complicated multi-layer structures or limitations in the fabrication process. Especially, because body parts with various shapes and movements (e.g., fingers, elbows, and shoulders) are under massive fatigue by strains in various directions and amounts, it is important to realize artificial skin devices with complex 3D structures that are optimized for the targeted body part.<br/>To realize complex 3D structures of hydrogel via extrusion-based 3D printing, we systemically investigated the correlation between materials design and their characteristics including gelation properties, rheological characteristics, and 3D printing processability. From the investigation, we successfully presented the material design window based on the rheological parameter (G` &lt; 2500 Pa and tan δ &lt; 0.2) to prepare target-oriented 3D structures of the hydrogel via extrusion-based 3D printing.<br/>Based on the defined material design window, we successfully prepared functional hydrogel 3D structures to demonstrate novel artificial skin devices that can detect accurate touch points and spontaneously heal mechanical damage. Ring-shaped and fingertip-shaped artificial skin devices have been successfully prepared to fit the finger model. Furthermore, like human skin, artificial skin devices provide accurate location information for arbitrary touch points without complex device fabrication or data processing.<br/>In hydrogel-based 3D e-skin devices, it is also important to address the inherent drying and freezing problems of hydrogel for practical application. As an approach to these problems, we presented organohydrogel systems via solvent displacement and investigated the correlation between solvent system design and the material properties of organohydrogels. By investigating the rheological properties and ion conductivity of the gels, we provided insights for preparing optimal organohydrogels for various processes and applications. Based on the defined solvent system design, we prepared organohydrogels that show excellent drying resistance even over 1,000 hours and exhibit superior freezing resistance by showing no phase transition down to -60°C. Furthermore, the 3D artificial skin devices are demonstrated based on the organohydrogels, and the artificial skin devices successfully provide the exact position of touchpoints over time and show excellent operation capability even at temperatures below 0°C without losing flexibility.