Design of functional hydrogels to improve 3D printability, anti-drying and anti-freezing characteristics, and biodegradability

Abstract

Hydrogels, owing to their distinctive attributes such as biocompatibility, flexibility, and conductivity, are extensively explored across diverse applications, including tissue engineering, drug delivery systems, biosensors, wearable technologies, E-skin, and soft robotics. Still, some challenges need to be addressed: the structural freedom of devices, drying & freezing issues of devices, and degradation control. Most hydrogel-based devices have been demonstrated with simple structures like blocks or films, primarily because of either the need for complex multi- layered structures or constraints within the fabrication processes. In particular, since body parts like fingers, elbows, and shoulders endure considerable strain from diverse directions and magnitudes, it is critical to realize artificial skin devices featuring intricate 3D structures tailored and optimized for specific body parts. To realize structural freedom of hydrogel-based devices through extrusion-based 3D printing, the correlation between materials design and their properties such as gelation characteristics, rheological properties, and 3D printing processability were systemically investigated. From the investigation, the material design window is defined in terms of the rheological parameter (G` < 2500 Pa and tan δ < 0.2) for the realization of target-oriented 3D structures of the hydrogel through extrusion-based 3D printing. Based on the defined material design window, the functional hydrogel 3D structures were achieved and the novel artificial skin devices were demonstrated by showing the detection of accurate touchpoints and spontaneously healing mechanical damage. Through extrusion-based 3D printing, artificial skin devices were prepared in the form of ring-shaped and fingertip- shaped tailored to fit the finger model. In addition, these artificial skin devices mimic human skin by precisely identifying touch locations without requiring elaborate device manufacturing processes or intricate data analysis. In hydrogel-based devices, it is also significant for treating the inherent drying and freezing issues for their practical application, and these challenges need to be addressed to ensure the functionality and reliability of the devices. As an approach to these issues, the solvent displacement method was chosen for the preparation of organohydrogel systems and the correlation between solvent system design and the material properties of organohydrogels was systemically investigated. Based on the exploration of the rheological properties and ion conductivity of these gels, insights were presented for fabricating organohydrogels that are optimized for a wide array of processes and applications. From the defined solvent system design, the organohydrogels were prepared and they show superior drying resistance even over 1,000 hours and exhibit excellent freezing resistance by showing no phase transition down to -60°C. Moreover, organohydrogel-based 3D artificial skin devices are demonstrated and can detect the precise positioning of touchpoints over time and exhibit outstanding operational performance even in temperatures below 0°C, all while retaining their flexibility. Beyond the realization of structural freedom and drying and freezing issues, controlling hydrogel degradation in biomedical applications is worthy of attention. Because hydrogel remaining in the body after treatment can cause inflammation, research on controlling the degradability of hydrogel is continuously needed. As an approach to this problem, oxidized alginate hydrogel was selected to fabricate injectable and degradable photothermal therapy (PTT) hydrogels, and inexpensive PANI:PSS was selected as a photothermal agent. PANI:PSS nanoparticles were incorporated into oxidized alginate hydrogel and various material properties of the designed hydrogel were systematically investigated. The prepared PTT hydrogel showed excellent photothermal effects with a temperature increase of up to 53 ℃ in 5 minutes, and degradation of the hydrogel was controlled depending on the degree of oxidation. Furthermore, the proposed PTT hydrogel has very low toxicity, and the controlled degradability showed its applicability as a drug release carrier.