3D printing functional materials with extreme regulation of mechanical performances from hydrogel to engineering plastic

Polyvinyl alcohol (PVA) is a high-molecular-weight biopolymer characterized by a carbon chain backbone with hydroxyl groups, typically synthesized through the polymerization of vinyl acetate monomers. As a versatile non-ionic hydrophilic, it has emerged as a crucial component in constructing hydrogels, biomedical polymers, and other functional materials. As a widely utilized green polymer that can be completely metabolized into CO₂ and H₂O in the natural environment, PVA offers numerous advantages, including biocompatibility, non-toxicity, thermal stability, excellent mechanical strength, and flexibility. These remarkable characteristics have propelled its widespread adoption in tissue engineering [1], soft robotics, and sustainable packaging solutions [2]. Notably, the mechanical versatility of PVA-based materials spans orders of magnitude, ranging from compliant hydrogels (kPa-scale modulus) to rigid biodegradable plastics (GPa-scale modulus) [3], [4]. For example, our previous studies have reported on PVA hydrogels used for flexible sensors in extreme environments and contact lenses with remarkable lubrication, anti-protein adhesion, and biocompatibility [4], [5]. On the other hand, as one of the most important plastics, PVA can also be employed in food packaging and as a feedstock for degradable plastics [6]. Even though the significant achievements in both PVA hydrogels and PVA plastics, and the attainment of high mechanical strength in PVA hydrogels, there remains a gap in effectively bridging the properties of PVA hydrogels to those of PVA plastics, thereby enabling precise an extreme regulation of mechanical performance. Correspondingly, the functions of PVA, such as serving as a flexible sensor in its hydrogel state and tissue engineering in its plastic state, can’t be integrated into a single system. To date, significant progress has been made in enhancing and toughening PVA hydrogels through the development of double network hydrogels [7], [8], nanocomposite hydrogels [9], [10], topological hydrogels [11], [12] and double crosslinked hydrogels [13], [14]. These advancements have led to substantial improvements in their mechanical properties; however, a gap still remains when compared to anhydrous PVA plastic [15], [16]. In fact, achieving a significant transformation in mechanical properties from a single material while maintaining its initial shape offers tremendous application potential, and extensive efforts have been made to regulate the mechanical properties by testing various components [17], concentrations [18], or combinations of curing conditions [19]. For instance, Liu’s group developed a novel of functional material by integrating 3D printed hydrogel structures with enzyme-induced biomineralization, enabling the transformation of flexible hydrogels (modulus of 125 kPa) into rigid mineralized hydrogel composites (modulus of 150 MPa) [20]. Wu and co-workers reported a self-adaptive fluorogel that consists a soft fluorinated lubricating phase and a stiff yet thermal-responsive load-bearing phase, allowing its Young’s modulus to be switched from 29 kPa to 86.5 MPa [21]. Despite these significant advancements, several challenges to be addressed, including the difficult of designing and constructing shapes freely, the limitation of the mechanical properties of materials with extreme regulation between hydrogel and engineering plastic, and the singular functionality of the prepared materials.

To address these challenges, polyvinyl alcohol (PVA) hydrogel can be employed due to their ability to achieve higher mechanical strength by freezing and thawing [22], [23], chemical cross-linking [24], mechanical training [25], freeze-casting and salting out [26] and other methods. This versatility makes PVA an ideal polymer for developing hydrogels with widely adjustable mechanical, structural and physical properties. Additionally, acrylamide (AAm) enables the hydrogel to be fabricated into complex structures through light-curing 3D printing technology [27]. Moreover, hydrogen bonds and interpenetrating network structures can form between AAm and PVA, providing a foundation for the enhancement of mechanical properties [28]. In this work, inspired by the brittle ductile transition phenomenon of the discinisca tenuis [29] shell at different water contents, we utilized the dispersion and association of interlinking hydrogen bonds during the hydration and dehydration processes of PVA/AAm materials to regulate the aggregation behavior of these bonds. This approach facilitates the transition and multicable regulation of mechanical properties from hydrogel to engineering plastic. Through the simple hydration and dehydration process, we can induce changes in the hydrogen bonds between PVA and water molecules, as well as alteration in the arrangement of molecular chains and the crystallinity of PVA. Consequently, the PVA/AAm material, composed of PVA, AAm, and polyethylene glycol diacrylate (PEGDA), referred to as PAP material, exhibits a remarkable transformation from soft to rigid states, with mechanical properties varying over several orders of magnitude. In addition, the material not only demonstrates reversible regulation of mechanical properties over a wide range, but also exhibits excellent adhesion switching, shape memory behaviors, and outstanding conductivity when MgCl₂ particles were incorporated. Capitalizing on these unique properties, we designed a smart fixation system for dislocated joints by integrating the multiple functions of PAP material into a single platform. This system not only facilitates the fixation and release of dislocated joints through significant mechanical property alterations but also enables real-time monitoring of the soft-rigid transitions in PAP fixation. We believe that the innovative integration of flexible sensors with rigid engineering plastics opens up promising applications in tissue engineering and medical devices, offering a versatile solution for advanced biomedical systems.