Biocompatible Nanostructured Solid Adhesives for Soft Tissues

Objectives: General adhesives are polymers, and it is difficult to glue hydrogels (including soft tissues) together with polymer adhesives. Herein we develop a novel type of solid adhesives consisting of biocompatible hydroxyapatite (HAp) nanoparticles, and the adhesion performance was evaluated using model hydrogels.
Methods: Aqueous dispersions of HAp nanoparticles with different morphologies were prepared by wet precipitation methods under different pH and temperature conditions. The morphologies of HAp nanoparticles were observed with a scanning electron microscope (SEM). Poly(dimethylacrylamide) (PDMA) hydrogels (water content, 90 wt.%) were used as an in vitro soft tissue model and prepared by polymerization with a 0.1 mol% crosslinker, N,N’-methylene bisacrylamide. The gluing of two PDMA gels (overlap area, 5x5 mm²) was achieved by inserting the HAp dispersions, and a constant pressure was applied on the overlap area. Lap-shear adhesion tests were performed on a universal testing machine at a loading rate of 150 mm/min. The adhesion strength of two PDMA gels without HAp insertion was also measured as a control group. Four samples per group were tested.
Results: In the case of control group without HAp insertion, the adhesion strength between two PDMA gels was 1.8±0.3 kPa. The adhesion strength increased significantly by inserting HAp nanoparticle dispersions and reached up to 5.0±0.4 kPa, which almost corresponded to commercially available fibrin glues. The increased concentration of HAp dispersion led to higher adhesion strength, whereas the morphology of HAp did not significantly affect the adhesion strength. The SEM observation after the adhesion tests revealed cohesive failure of HAp nanoparticle aggregates on PDMA gels.
Conclusions: Strong and rapid adhesion between two hydrogels can be achieved by inserting HAp nanoparticle dispersions on the gel surfaces. The nanostructured solid adhesives consisting of HAp will be used as a novel type of biocompatible adhesives for soft tissues.