A new injectable biphasic hydrogel based on partially hydrolyzed polyacrylamide and nanohydroxyapatite as scaffold for osteochondral regeneration

Attempts have been made to fabricate an injectable biphasic hydrogel based on partially hydrolyzed polyacrylamide (HPAM), nanocrystalline hydroxyapatite (nHAp), and chromium acetate (Cr(III)) as a crosslink agent as a novel scaffold for osteochondral repair. The HPAM water solution comprising 4 wt% HPAM and 0.2 wt% Cr(III) (HPAM3) and its counterpart containing 50 wt% nHAp (HPAM6) were shown to be optimized in a vial titling method and cell viability assays. The biphasic hydrogel demonstrated high interfacial attraction in tensile test measurements. The rheo-mechanical spectroscopy (RMS) performed on the prepared hydrogels revealed that incorporation of 50 wt% of nHAp (HPAM6) not only reinforced the mechanical strength of the final crosslinked gel, but lowered the gelation onset as well as gelling time. Atomic force microscopy (AFM) characterization together with water uptake measurement showed that an increase in crosslink density enhanced surface elasticity as well as reduced equilibrium swelling degree. The engineered constructs (HPAM3 and HPAM6) also supported cell attachment as indicated by SEM observation. Alcian blue, safranin O and dimethyl methylene blue (DMMB) indicated chondrogenic phenotype in the HPAM3 constructs with the highest amount after 21 days of culture. Chondrogenic differentiation of mesenchymal stem cells (MSCs) in the HPAM3 was evident by real-time PCR analysis for cartilage-specific ECM gene markers. Interestingly, in Real-time PCR analysis for bone-specific ECM gene markers, up-regulated expression of osteogenic genes was related to osteoblastic differentiation of encapsulated MSCs within HPAM/nHAp scaffolds, as confirmed by calcium deposition. The results suggested that our designed biphasic hydrogel has potential to be considered as a promising scaffold for osteochondral regeneration.