A synthetic mimic of mussel adhesive protein, dopamine-modified four-armed poly(ethylene glycol) (PEG-D4), was combined with a synthetic nanosilicate, Laponite (Na^0.7+(Mg_5.5Li_0.3Si₈)O₂₀(OH)₄)^0.7–), to form an injectable naoncomposite tissue adhesive hydrogel. Incorporation of up to 2 wt % Laponite significantly reduced the cure time while enhancing the bulk mechanical and adhesive properties of the adhesive due to strong interfacial binding between dopamine and Laponite. The addition of Laponite did not alter the degradation rate and cytocompatibility of PEG-D4 adhesive. On the basis of subcutaneous implantation in rat, PEG-D4 nanocomposite hydrogels elicited minimal inflammatory response and exhibited an enhanced level of cellular infiltration as compared to Laponite-free samples. The addition of Laponite is potentially a simple and effective method for promoting bioactivity in a bioinert, synthetic PEG-based adhesive while simultaneously enhancing its mechanical and adhesive properties.

Due to the increasing needs for organ transplantation and a universal shortage of donated tissues, tissue engineering emerges as a useful approach to engineer functional tissues. Although different synthetic materials have been used to fabricate tissue engineering scaffolds, they have many limitations such as the biocompatibility concerns, the inability to support cell attachment, and undesirable degradation rate. Fibrin gel, a biopolymeric material, provides numerous advantages over synthetic materials in functioning as a tissue engineering scaffold and a cell carrier. Fibrin gel exhibits excellent biocompatibility, promotes cell attachment, and can degrade in a controllable manner. Additionally, fibrin gel mimics the natural blood‐clotting process and self‐assembles into a polymer network. The ability for fibrin to cure in situ has been exploited to develop injectable scaffolds for the repair of damaged cardiac and cartilage tissues. Additionally, fibrin gel has been utilized as a cell carrier to protect cells from the forces during the application and cell delivery processes while enhancing the cell viability and tissue regeneration. Here, we review the recent advancement in developing fibrin‐based biomaterials for the development of injectable tissue engineering scaffold and cell carriers.

The remarkable underwater adhesion strategy employed by mussels has inspired bioadhesives that have demonstrated promise in connective tissue repair, wound closure, and local delivery of therapeutic cells and drugs. While the pH of oxygenated blood and internal tissues is typically around 7.4, skin and tumor tissues are significantly more acidic. Additionally, blood loss during surgery and ischemia can lead to dysoxia, which lowers pH levels of internal tissues and organs. Using 4-armed PEG end-capped with dopamine (PEG-D) as a model adhesive polymer, the effect of pH on the rate of intermolecular cross-linking and adhesion to biological substrates of catechol-containing adhesives was determined. Adhesive formulated at an acidic pH (pH 5.7–6.7) demonstrated reduced curing rate, mechanical properties, and adhesive performance to pericardium tissues. Although a faster curing rate was observed at pH 8, these adhesives also demonstrated reduced mechanical and bioadhesive properties when compared to adhesives buffered at pH 7.4. Adhesives formulated at pH 7.4 demonstrated a good balance of fast curing rate, elevated mechanical properties and interfacial binding ability. UV–vis spectroscopy evaluation revealed that the stability of the transient oxidation intermediate of dopamine was increased under acidic conditions, which likely reduced the rate of intermolecular cross-linking and bulk cohesive properties for hydrogels formulated at these pH levels. At pH 8, competing cross-linking reaction mechanisms and reduced concentration of dopamine catechol due to auto-oxidation likely reduced the degree of dopamine polymerization and adhesive strength for these hydrogels. pH plays an important role in the adhesive performance of mussel-inspired bioadhesives and the pH of the adhesive formulation needs to be adjusted for the intended application.

A high strength hydrogel was fabricated by one-step copolymerization of dipole–dipole interaction-containing monomer, acrylonitrile, super-hydrophilic comonomer, 2-methacryloyloxyethyl phosphorylcholine and crosslinker, polyethylene glycol diacrylate (M_n = 575, PEGDA575). This dipole–dipole reinforced (DDR) hydrogel demonstrated intriguing combinations of properties such as withstanding several MPa tensile stress, tens of MPa compressive strength, excellent fatigue resistance and no yielding during tensile tests. The equilibrium water content and transparency of DDR hydrogels could be tuned by varying monomer concentration and monomer ratio. The gels exhibited low cytotoxicity and antifouling characteristic. Biodegradable high strength hydrogel could also be constructed by merely replacing PEGDA575 with bioreducible crosslinker. The method reported here offers a general strategy to design biocompatible high-strength hydrogels for tissue engineering scaffolds by copolymerizing monomer containing dipole–dipole pairing with other hydrophilic monomer.