الفهرس 
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Abstract
In this study, injectable photo-curable hyaluronic acid hydrogels loaded with nano-hydroxyapatite and chitosan were prepared using a new photoinitiating system composed of riboflavin 5′-monophosphate sodium salt hydrate as a photoinitiator, dimethylaminoethyl methacrylate as a coinitiator and diphenyl iodonium chloride as an accelerator. The irradiation time to form stable hydrogels was 10 seconds.
Scanning electron microscope images revealed that the addition of nano-hydroxyapatite led to the formation of interconnected polyhedral pores which mimic bone trabecular structure that is essential for cell growth and osteogenesis while the addition of chitosan led to the formation of a filamentous structure which is essential for cell attachment. Addition of nano-hydroxyapatite and chitosan simultaneously increased significantly the mechanical properties of the formed hydrogel by three folds, while the addition of nano-hydroxyapatite alone increased the mechanical properties by two folds only, compared to the group without any additions.
Addition of nano-hydroxyapatite didn’t affect degradation rate compared to the control group while the addition of chitosan increased the degradation rate significantly, due to forming a semi-interpenetrating polymer network, so this can be used to control degradation rate. The prepared hydrogels were biocompatible and didn’t affect cell viability as evident by MTT assay.
Histological observations and histomorphometric analysis results revealed that the use of nano-hydroxyapatite with photo-cured hyaluronic acid hydrogels increased the osteogenic potential in rabbits by one and half folds while the use of nano-hydroxyapatite and chitosan
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simultaneously increased the osteogenic potential by two folds and resulted in the formation of interconnected mature lamellar bone trabeculae.
Thus, it can be concluded that the prepared injectable photo-curable hyaluronic acid hydrogel loaded with bioactive nano-hydroxyapatite and chitosan using the new photoinitiating system based on riboflavin as a photoinitiator is a promising biomaterial to be used in bone regeneration applications.