Intrinsic and Extrinsic Tunability of Double-Network Hydrogel Strength and Lubricity

Abstract

Double-network (DN) hydrogels are promising materials for tissue engineering due to their biocompatibility, high strength, and toughness, but understanding of their microstructure−property relationships still remains limited. This work investigates a DN hydrogel comprising a physically crosslinked agarose, as the first network, and a chemically crosslinked copolymer with a varying ratio of acrylamide and acrylic acid, as the second network. The charge, intrinsic to most DN hydrogels, introduces a responsive behavior to chemical and electrical stimuli. The DN strengthens agarose hydrogels, but the strengthening decreases with the swelling ratio resulting from increasing acrylic acid content or reducing salt concentration. Through careful imaging by atomic force microscopy, the heterogenous surface structure and properties arising from the DN are resolved, while the lubrication mechanisms are elucidated by studying the heterogeneous frictional response to extrinsic stimuli. This method reveals the action of the first (agarose) network (forming grain boundaries), copolymer-rich and poor regions (in grains), charge and swelling in providing lubrication. Friction arises from the shear of the polymeric network, whereas hydrodynamic lift and viscoelastic deformation become more significant at higher sliding velocities. We identify the copolymer-rich phase as the main source of the stimulus-responsive behavior. Salt concentration enhances effective charge density and reduces viscoelastic deformation, while electric bias swells the gel and improves lubrication. This work also demonstrates the dynamic control of interfacial properties like hydrogel friction and adhesion, which has implications for other areas of study like soft robotics and tissue replacements.

Hydrogel-based treatments for spinal cord injuries

“…As a natural polymer, AG is hydrophilic, and its molecular chain contains many hydrogen bonds, which can give the material a water stimulation response through the breaking and recombination of hydrogen bonds. Furthermore, the low melting point of AG allows for the material to have a thermal stimulation response [ 90 ]. Cryogel technology can be used to easily prepare AG-based scaffold materials with macroporous structures, which may be useful for biomedical applications [ 91 ].…”

Hydrogels for Lubrication: Synthesis, Properties, Mechanism, and Challenges

“…The structure of hydrogel lubricants has been thought to be a crucial influence on their tribological properties. Regulating the microstructure of hydrogel networks has been demonstrated to be one of the efficient routes to improve their mechanical and tribological properties [61,62]. Polymer hydrogels with both physical and chemical crosslinks, named dual-network polymeric hydrogels, incorporate the advantageous features of rigid and tightly cross-linked by covalent bonds as well as soft and loosely cross-linked through supramolecular interactions, making them promising material for lubrication.…”

Lubricating Polymer Gels/Coatings: Syntheses and Measurement Strategies

“…The sliding friction between sample and tip is represented by the half width of the friction loop in the vertical direction. Using a UV-triggered FRP approach, Lee et al [ 32 ] prepared an agarose/poly(acrylamide-co-acrylic acid) (AgP(Am-co-AAc)) double network (DN) hydrogel. The height, adhesion, and frictional images of the DN hydrogel were obtained from LFM measurement, as shown in Figure 2 d. Sader’s method [ 33 ] was employed to assess the normal spring constant, meanwhile a noncontact thermal noise method was applied for lateral calibration.…”

“…The Y axis corresponds to the lateral signal on the photodetector as the tip scans on the sample surface [ 30 ]. ( d ) QI images of adhesion, height, and friction maps of AgP(Am-co-AAc) DN hydrogel in water [ 32 ]. ( a ) Reproduced with permission from Ref.…”