The effect of hydroxyapatite particle size on viscoelastic properties and calcium release from a thermosensitive triblock copolymer

Well-defined “smart” injectable hydrogel based on hydrophilic poly(ethylene glycol) (PEG) and hydrophobic poly(lactic acid-co-glycolic acid) (PLA/PGA) copolymer (PLGA-PEG-PLGA) gelling at the body temperature was modified by bioactive hydroxyapatite (HAp) in the form of micro-, nano-, and core-shell particles (μ-HAp, n-HAp, and CS-x, respectively) to be applicable as calcium delivery system in bone regeneration. Viscoelastic moduli increased with HAp content as expected. Whereas systems containing μ-HAp or CS-x particles maintained two sol-gel and gel-sol phase transitions, the n-HAp containing system showed only one sol-gel phase transition due to the strong interactions between polymer chain and the n-HAp surface. In vitro, studies proved the controlled uniform release of calcium cations from both CS-x and n-HAp over the 9-day period without any initial burst release.