More than 25 years ago, Mirtchi and Lemaître discovered brushite cements and were the pioneers in the development of this biocompatible calcium phosphate cement system. In contrast to hydroxyapatite, brushite can be resorbed under physiological conditions at a pH-level of 7.4 . Based on the idea that bone is a composite material of inorganic mineral calcium phosphate reinforced with an elastic matrix, we established a dual setting system of a degradable poly(ethylene) glycol (PEG) based-hydrogel incorporated in a brushite cement phase.
In this study, we investigated the mechanical properties and especially the compressive strength. Furthermore, the Young’s modulus of both, the pure hydrogel and the composite material, was analyzed.
The composites were produced by dissolving the monomers with a concentration of 25 wt% and 50 wt% in a 0.5 M citric acid solution. An amount of 5 wt% hydrogen peroxide solution (30 %) was added to the liquid phase. The raw powder consisted of β-tricalcium phosphate (β-TCP) and monocalcium phosphate anhydrous (MCPA) in an equimolar ratio with the addition of 5 wt% ascorbic acid as radical initiator for the polymerization. After mixing, the cement setting reaction and the polymeric gelation were initiated simultaneously. The samples were fabricated in silicone molds and stored in phosphate buffered saline with a pH of 7.4.
We demonstrated extremely flexible and ductile specimens with a homogenous phase composition of the hydrogel in the cement matrix. Though the polymeric content was doubled from 25 wt% to 50 wt%, no difference in stress could be observed. With the set inorganic phase, a network of crystals was formed that gave a certain stability to the system. This also explained the increase of Young’s modulus up to 150-300 kPa in comparison to the pure hydrogels with 25-30 kPa. Additionally, XRD-measurements were performed where we could notice an influence of the polymeric matrix on the conversion from β-TCP and MCPA to brushite. Investigations concerning the compressive strength of the composites showed a more ductile behavior with the addition of high-molecular weight PEG-based hydrogels. Furthermore, the samples relaxed into the original shape after removal of the force.