A Composite Biomaterial: Poly 2 (Hydropoxyethyl) Methacrylate / Alkaline Phosphatase Initiates Mineralization In Vitro

Bone substitutes are nowadays largely used in orthopedic surgery but they lack osteoinductive properties. Poly (2-hydroxyethyl methacrylate) (pHEMA) has numerous biomedical applications. Alkaline phosphatase (AlkP), an ectoenzyme elaborated by osteoblasts, initiates bone mineralization by hydrolyzing organic phosphates before calcium-phosphorus deposition. We have immobilized AlkP in pHEMA in a copolymerization technic. Histochemical study revealed that AlkP has retained its biological activity . Image analysis of sections using a tessellation method showed a lognormal distribution of the area of tessels around AlkP particles thus confirming an homogeneous distribution of the enzyme in the polymer. Pellets of pHEMA and pHEMA + AlkP were incubated with synthetic body fluids containing either inorganic or organic phosphates (,6-glycerophosphate). Mineral deposits with a round shape (calcospherites) were obtained on pHEMA + AlkP pellets incubated in the presence of organic phosphates. No deposits were observed on pHEMA in either incubating conditions or on pHEMA + AlkP incubated with inorganic phosphates. Calcospherites were observed by transmission and scanning electron microscopy. They appeared composed of minute single tablets packed together. X-ray microanalysis showed a Ca/P ratio of 1.42 and X-ray diffraction identified hydroxylapatite. AlkP entrapped in an hydrogel is able to initiate mineralization in vitro by a mechanism that closely mimics the cartilage/bone mineralization in vivo .