Enzymatically gellable gelatin improves nano-hydroxyapatite-alginate microcapsule characteristics for modular bone tissue formation

To maintain gelatin (Gel) as adhesive motifs inside alginate microcapsule as building blocks of modular approach, phenol moiety (Ph) was introduced into gelatin (Gel Ph). Addition of Gel Ph to alginate (Alg-Gel Ph) dramatically altered the physical properties of alginate-based hydrogels as compared to unmodified gelatin (Alg-Gel) addition. Alg-Gel Ph hydrogels revealed a dramatically lower swelling ratios (63%) as compared to Alg-Gel hydrogels (150%). Moreover, Gel Ph decreased 40% degradation rate of alginate-based hydrogels after 72 hr, while increasing compressive modulus 3.5-fold as compared to Alg-Gel hydrogels. Introducing nano-hydroxyapatite (nHA) to Alg-Gel Ph hydrogel (Alg-Gel Ph-nHA) could reduce degradation rate to 41.5% and improve compressive modulus of hydrogels significantly, reaching to 294 +/- 2.5 kPa. The microencapsulated osteoblast-like cells proliferated considerably and showed more metabolic activities (two times) in Alg-Gel Ph-nHA microcapsules during a 21-day culture period, resulting in more calcium deposition and alkaline phosphatase (ALP) activities. The subcutaneous microcapsules could also be identified readily without complete absorption and signs of toxicity or any untoward reactions and viable osteoblast-like cells were seen as red colored areas in the central regions of cell-laden microcapsules after 1 month. The study demonstrated Alg-Gel Ph-nHA microcapsule as a promising 3D microenvironment for modular bone tissue formation.