(J-402) Enhancing Extensibility and Toughness of Hydrogel by Internal Structure Modification

Introduction:: Hydrogels are hydrophilic polymer networks that can absorb and retain large amounts of water. Owing to their biocompatibility, hydrogels are promising materials for various biomedical applications. As mechanical weakness limits broader applications of hydrogels, methods for mechanical reinforcement have been extensively studied. However, because rigidity and extensibility tend to be mutually exclusive, achieving high toughness is still challenging. Here, we show a structure modification method for the mechanical reinforcement of hydrogels inspired by the anisotropic and hierarchical structure of the tendon. The resultant hydrogel was rigid and extensible, and consequently tough.


Materials and Methods:: The mechanical reinforcing method is comprised of two steps: deformation and fixation. As precursor hydrogel should be flexible to withstand deformation and additionally crosslinkable to fix the deformed structure, alginate/polyacrylamide hybrid hydrogel is used. For the deformation step, precursor hydrogel is stretched in the length direction and twisted. Longitudinal stretching has been used for unidirectional polymer chain alignment in many studies. In this work, we additionally applied twisting to induce characteristic structure which leads to mechanical enhancement. Twisting causes coiling and deformation to different degrees depending on the distance from the axis of rotation; the further away, the more deforms. Subsequently, the deformed hydrogel is densified and additionally crosslinked to fix the induced temporary structure. Densification is performed by immersing the hydrogel in isopropanol. Upon immersing, hydrogel shrinks as water is rapidly extracted from hydrogel and constituting polymers are aggregated. The densified structure allows dense crosslinking during the following additional crosslinking step performed by immersing in aluminum ion solution which can crosslink the alginate network. The resultant hydrogel could maintain the modified structure due to the dense additional crosslinking.


Results, Conclusions, and Discussions::

The twisted structure may allow additional interlocking between polymers. In addition, heterogeneous deformation and diagonal polymer chain alignment cause mechanical heterogeneity across the radial direction. Those factors can facilitate stress transfer. In the tensile test, non-twisted hydrogel showed clear necking and yielding peak which represents strain amplification and stress concentration. Owing to poor stress transfer, the non-twisted hydrogel fractured before undergoing strain hardening stage. Conversely, twisted hydrogel showed less clear necking and yielding peak, and significant strain hardening region right after yield point. This behavior indicates sufficient stress transfer. Consequently, twisted hydrogel exhibits higher extensibility and toughness. In addition, according to the energy-dissipating nature of the alginate network, the hydrogel showed high energy dissipation, up to 5.9 MJ m^-3, and high damping capacity, up to 90%. By utilizing high extensibility, we applied the hydrogel as a strain sensor. The hydrogel strain sensor showed considerable sensitivity, even can detect 0.001 of strain, and a wide sensing range, up to 6.25 of strain. Furthermore, high strength and damping capacity allow the potential application of the hydrogel as a sensor for human motion detection with injury prevention ability. For hierarchical assembly, multiple strands of hydrogel were assembled into a hydrogel rope. Due to the twisted structure of each strand and rope, stress transfer is facilitated, and stress distribution is even in the rope. Hence, while separated hydrogel strands fractured individually, hydrogel rope withstood higher strain and fractured together. The hydrogel rope was applied to impact reduction application. A weight was hung on a hydrogel rope and dropped with measuring applied load. Owing to the high damping capacity, hydrogel rope showed a low peak load and short equilibrium time, compared with a cotton band and rubber band. We developed a simple yet effective structure modification method for enhancing the mechanical properties of hydrogels. The hydrogel reinforced by the method has the potential to be used in various fields where both mechanical properties and biocompatibility are crucial. The method can be applied to other hydrogel systems to broaden their scope of application.