Hydrogels are polymeric materials that consist of three-dimensional cross-linked networks swollen by large amount of water. Due to their high water content, large network porosity, and extraordinary biocompatibility, hydrogels have become appealing biomaterials for tissue engineering. However, most hydrogels are nearly incompressible and barely mitigate mechanical energy under hydrostatic loading condition, which greatly impedes their applications. To address this challenge, we propose to embed hydrophobic nanoporous particles (HNPs) into the polymeric network of hydrogels. Our hypothesis is that water molecules confined in hydrogels can be activated and forced into the nanopores enclosed in HNPs under hydrostatic loading to mitigate energy. Due to the extremely large specific surface area of HNPs, substantial external energy can be mitigated at the intruded liquid-nanopore interface.To verify the above hypothesis, HNP-hydrogel is synthesized by incorporating chloro(dimethyl)octylsilane (C8) treated silica nanoporous particles in polyacrylamide (PAM) hydrogels. To improve and stabilize the dispersion of HNPs, surfactant Pluronic® F127 is added into the aqueous mixture. The network structure of the synthesized HNP-hydrogels is characterized by scanning electron microscope (SEM). The results show successful incorporation of HNPs in hydrogel networks. Due to the hydrophobic nature of HNPs, hydrogel polymeric pockets impregnating HNPs exhibit increased size. This ensures that the water molecules confined in the polymeric pockets are sufficient to fulfill the nanopores, and thus mitigate large amount of energy. The density of HNPs in polymeric pockets is proportional to the loading fraction of HNPs in the resulted hydrogel.The energy mitigation capacity of HNP-hydrogels is evaluated using a lab-customized compression chamber under hydrostatic loading condition. It has been demonstrated that the HNP additives change the non-hysteretic neat PAM hydrogel to highly hysteretic HNP-hydrogel. With each 1 wt% addition of nanoporous particles, the energy mitigation capacity of the HNP-hydrogel is increased by 300%. This additional energy mitigation capacity is correlated with the interfacial tension of the F127 solution-HNP combination.In summary, a hydrogel system that is capable of mitigating energy under hydrostatic compression has been developed by the incorporation of HNPs into PAM hydrogel network. Compare to the neat hydrogel, which has negligible energy mitigation capacity, the HNP-hydrogels exhibit much enhanced energy mitigation capacity under hydrostatic compression. This enhancement originates from activating the ultrahigh amount of liquid molecules confined in the hydrogel polymeric network. This new energy mitigation mechanism holds great promise to improve the mechanical properties and durability of the next generation biomaterials for tissue regeneration.

Functionalization of Water Molecules Confined in Hydrogels for Energy Mitigation

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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