The synthesis and characterization of nano-hydroxyapatite (nHAP)-g-poly(lactide-co-glycolide)-g-collagen polymer for tissue engineering scaffolds

Bone grafts, commonly performed to augment bone regeneration from autologous or alleogeneic sources, carry an enormous cost, estimated at upwards of 21 billion dollars per year. Hydroxyapatite (HAP) bio-ceramic has been widely used in clinic as a bone graft substitute material due to its biocompatibility and the similarity of its structure and composition to bone mineral. However, its applications are limited due to its lack of strength and toughness. Researchers have attempted to overcome these issues by combining HAP bio-ceramics into resorbable polymers to improve their mechanical properties. However, poor bonding between the HAP and the polymer caused separation at the polymer-filler interface. To overcome this, short chains of polymers were grafted directly from the hydroxyl groups on the surface of nanocrystalline HAP. Collagens, being the most abundant proteins in the body, and having suitable properties such as biodegradability, bioabsorbability with low antigenicity, high affinity to water, and the ability to interact with cells through integrin recognition, makes them a very promising candidate for the modification of the polymer surface. In this study, a novel method of synthesizing nano-hydroxyapatite (nHAP)-g-poly(lactide-co-glycolide)-g-collagen polymer was introduced. The synthesis process was carried out in several steps. First, poly (lactide-co-glycolide) (PLGA) polymer was directly grafted onto the hydroxyl group of the surface of n-HAP particles by ring-opening polymerization, and subsequently coupled with succinic anhydride. In order to activate the co-polymer for collagen attachment, the carboxyl end group obtained from succinic anhydride was reacted with N-hydroxysuccinimide (NHS) in the presence of dicyclohexylcarbodiimide (DCC) as the cross-linking agent. Finally, the activated co-polymer was attached to calf skin collagen type I, in hydrochloric acid/phosphate buffer solution and the precipitated co-polymer with attached collagen was isolated. The synthesis was monitored by ¹H NMR and FTIR spectroscopies and the products after each step were characterized by thermal analysis (TGA and DSC). These composite materials will be tested as potential scaffolds for tissue engineering applications.