A low friction, biphasic and boundary lubricating hydrogel for cartilage replacement

Fluorescence microscopy visualization of the roughness-induced transition between lubrication regimes

We investigate the transition between different regimes of lubrication and directly observe the thickness of nanometric lubrication films with a sensitivity of a single molecular layer at a multi-asperity interface through fluorescence microscopy. We redefine specific film thickness as the ratio of the lubricant film thickness and the surface roughness measured only at those regions of the interface where the gap is "minimal." This novel definition of specific film thickness successfully captures the transition from full elastohydrodynamic lubrication to mixed and boundary lubrication. The transition can be triggered by increasing the surface roughness and is accurately predicted by using the new film thickness definition. We find that when the liquid carries part of the load, its apparent viscosity is greatly increased by confinement, and show how the transition between different lubrication regimes is well described by the viscosity increase and subsequent glass transition in the film.

Zwitterionic Hydrogel Incorporated Graphene Oxide Nanosheets with Improved Strength and Lubricity

Graphene oxide (GO) has been evaluated as a multifunctional cross-linker or reinforcement agent in composite hydrogels. In this study, a nanocomposite hydrogel consisting of GO nanosheets and zwitterionic poly(sulfobetaine methacrylate) (PSBMA) was synthesized in an aqueous system via chemical and physical cross-linking effects. GO nanosheets were well dispersed in the hydrogels and effectively cross-linked into the sulfobetaine methacrylate (SBMA) polymer chains through the electrostatic interactions. The PSBMA hydrogel exhibited a significant enhancement in the compressive stress (close to a 5-fold increase) and a remarkable reduction in the coefficient of friction (COF) (corresponding to a decline of 52-76%) after the embedding of GO nanosheets. These improvements indicate the existence of synergetic interaction and good compatibility between GO nanosheets and the PSBMA hydrogel matrix, which results in an intertwined network structure with higher load-bearing capacity and better lubrication properties. This study provides potential in the development of new graphene-polymer composites, which is beneficial for cartilage replacement with high mechanical properties and excellent lubrication characteristics.

Creating an Interface: Rendering a Double-Network Hydrogel Lubricious via Spontaneous Delamination

Hydrogels engineered with specific surface chemistries and architectures have found myriad applications in electronics, biofouling, biolubrication, and biomedical devices. Free-radical polymerization is frequently employed to construct covalently bonded networks in hydrogels, and any inhibition of the radical reactions by oxygen at the surface of the reaction mixture is generally undesirable. The internal stress caused by the resulting gradient in the cross-linking density during polymerization can give rise to a physical deformation of the surface, resulting in wrinkles, creases, or cracks. However, this oxygen-inhibition effect can be positively utilized to create finely controlled surface structures. We describe a two-step cross-linking strategy for the fabrication of a P(AAm-AMPS)/alginate double-network hydrogel in the presence of air, which enables greater independent control over surface chemistry and functionality than homogeneously processed conventional double-network hydrogels. An alginate-rich "skin" layer is spontaneously delaminated due to the mechanical instability and osmotic mismatch between the swollen double-network hydrogel matrix and the rigid "skin" layer. Removal of the "skin" layer results in a lubricious surface with coefficients of friction as low as 0.02 against glass in aqueous solutions. The proposed strategy can be generalized to develop soft functional materials with unique structures and properties and precise control over the surface chemistry.

Modern Strategies To Achieve Tissue-Mimetic, Mechanically Robust Hydrogels

Hydrogels are frequently used biomaterials due to their similarity in hydration and structure to biological tissues. However, their utility is limited by poor mechanical properties, namely, a lack of strength and stiffness that mimic that of tissues, particularly load-bearing tissues. Thus, numerous recent strategies have sought to enhance and tune these properties in hydrogels, including interpenetrating networks (IPNs), macromolecular cross-linking, composites, thermal conditioning, polyampholytes, and dual cross-linking. Individually, these approaches have achieved hydrogels with either high strength (σ f > 10 MPa), high stiffness (E > 1 MPa), or, less commonly, both high strength and stiffness (σ f > 10 MPa and E > 1 MPa). However, only certain unique combinations of these approaches have been able to synergistically achieve retention of a high, tissuelike water content as well as high strength and stiffness. Applying such methods to stimuli-responsive hydrogels has also produced robust, smart biomaterials. Overall, methods to achieve hydrogels that simultaneously mimic the hydration, strength, and stiffness of soft and load-bearing tissues have the potential to be used in a much broader range of biomedical applications.

Biomaterials to Mimic and Heal Connective Tissues

Connective tissue is one of the four major types of animal tissue and plays essential roles throughout the human body. Genetic factors, aging, and trauma all contribute to connective tissue dysfunction and motivate the need for strategies to promote healing and regeneration. The goal here is to link a fundamental understanding of connective tissues and their multiscale properties to better inform the design and translation of novel biomaterials to promote their regeneration. Major clinical problems in adipose tissue, cartilage, dermis, and tendon are discussed that inspire the need to replace native connective tissue with biomaterials. Then, multiscale structure-function relationships in native soft connective tissues that may be used to guide material design are detailed. Several biomaterials strategies to improve healing of these tissues that incorporate biologics and are biologic-free are reviewed. Finally, important guidance documents and standards (ASTM, FDA, and EMA) that are important to consider for translating new biomaterials into clinical practice are highligted.

Polymer grafted graphitic carbon nitrides as precursors for reinforced lubricant hydrogels

Carbon nitride-based hydrogels are formed in a two-step procedure and feature significant toughness, compressibility and lubricant properties.