Reinforcement of hydroxyapatite bone cement via thin glycerol-citrate polyester infiltration: Microstructural, mechanical and in-vitro evaluation

Abstract

The development of calcium phosphate cements (CPCs) incorporating biopolymers, such as thermoplastic polylactides, is known as an efficient strategy to enhance the physico-chemical, biological, and mechanical properties of CPCs. However, the use of thermosetting polyol citrates as fillers in CPCs has rarely been reported. Hence, the main goal of the present work was to produce composite CPCs based on tetracalcium phosphate–monetite cement matrix (C) and glycerol-citrate polyester (GCA) and to thoroughly study the effect of GCA addition on microstructural, micro, and nano-mechanical as well as in-vitro cellular properties of final cement composites. The GCA polymer was synthesized by esterification of glycerol with citric acid and coated on the C cement powder matrix at a concentration of 2.5 wt% by infiltration in ethanol solution. Two systems were prepared, by drying the composite powder at 105 and 170 °C (denoted as GCA/C105 and GCA/C170), while the respective composite cements were produced by mixing the powders with the 2 wt% of NaH2PO4 liquid phase. The results showed that the GCA polyester had no significant effect on the hydrolysis and transformation of the original cement matrix, and the nanocrystalline hydroxyapatite (Hap) was found as the main hydration product in all types of cement. The cement setting time decreased slightly with the GCA addition. On the other hand, a significant increase in mechanical properties was reached by introducing the GCA polyester into the CPC matrix, achieving more than a 70 % increase in the compressive strength, and about 50 % and 20 % in micro and nanohardness values compared to that obtained for the original C cement sample. The nanoindentation analysis revealed that the improved compressive strength and hardness were due to the stronger boundaries between the platelet-like hydroxyapatite particles in GCA-doped cements and partially caused by the denser, less porous, and more compact microstructures. The results of in-vitro cytotoxicity testing revealed high proliferation activity of the osteoblastic cells in all cement extracts, providing useful information for designing novel composite cements with the potential to be applied as bone substitute materials.