Hydrogels are useful materials as scaffolds for tissue engineering applications. The solid content used for hydrogels require a balance between scaffold stiffness and nanoporosity, which impacts nutrient diffusion into cell-laden scaffolds. Using hydrogels with additive manufacturing techniques has been a challenge, due to inconsistencies in print fidelity. In this study, agarosebased hydrogels commonly used for cartilage tissue engineering were compared to Pluronic, a hydrogel with established printing capabilities. Moreover, new material mixtures were developed for bioprinting by combining alginate and agarose. We compared mechanical and rheological properties, including yield stress, storage modulus, and shear thinning, to determine parameters that may predict better extrusion-based printability and to assess their potential as a bioink for cell-based tissue engineering. We found that all gels demonstrated shear-thinning behavior, yet recovered immediately upon the absence of a shear stress. Print fidelity of agarose-based gels improved with the addition of alginate, which did not significantly alter yield strength (p > 0.1).Alginate-agarose composites prepared with 5% w/v (3:2 agarose to alginate ratio) demonstrated high print fidelity with excellent cell viability that was maintained over a 28-day culture period (>~70% cell survival at day 28). Therefore, agarose-alginate mixtures showed the greatest potential as an effective bioink for additive manufacturing of biological materials for cartilage tissue engineering.
Extracellular-matrix composition impacts mechanical performance in native and engineered tissues. Previous studies showed collagen type I-agarose blends increased cell-matrix interactions and extra-cellular matrix production. However, long-term impacts on protein production and mechanical properties of engineered cartilage are unknown. Our objective was to characterize the effect of collagen type I on matrix production of chondrocytes embedded in agarose hydrogels. We hypothesized that the addition of collagen would improve long-term mechanical properties and matrix production (e.g., collagen and glycosaminoglycans) through increased bioactivity. Agarose hydrogels (2% w/v) were mixed with varying concentrations of collagen type I (0, 2, 5 mg/mL). Juvenile bovine chondrocytes were added to the hydrogels to assess matrix production over 4 weeks through biochemical assays, and mechanical properties were assessed through unconfined compression. We observed a dose-dependent effect on cell bioactivity, where 2 mg/mL of collagen improved bioactivity, but 5 mg/mL had a negative impact on bioactivity. This resulted in higher modulus for scaffolds supplemented with lower collagen concentration as compared to the higher collagen concentration, but not when compared to the control. In conclusion, the addition of collagen to agarose constructs provided a dose-dependent impact on improving glycosaminoglycan production but did not improve collagen production or compressive mechanics.
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