Adhesion Between Biodegradable Polymers and Hydroxyapatite: Relevance to Synthetic Bone-like Materials and Tissue Engineering Scaffolds

Many studies are currently underway on the quest to make synthetic bone-like materials with composites of polymeric materials and hydroxyapatite (HA). In the present work, we use wetting experiments and surface tension measurements to determine the work of adhesion between biodegradable polymers and HA, with specific reference to the role of humid environments. All the polymers are found to exhibit low contact angles (<or=60 degrees) on the ceramic with work of adhesion values ranging between 48Jm(-2) for poly(epsilon-caprolactone) and 63Jm(-2) for polylactide; these values are associated with physical bonding across the organic/inorganic interface. The corresponding mechanical fracture strengths, measured using four-point bending tests of HA-polymer-HA bonds, scale directly with the results from the wetting experiments. Short-time aging (up to 30h) in a humid environment, however, has a dramatic influence on such HA/polymer interfacial strengths; specifically, water diffusion through the organic/inorganic interface and degradation of the polymer results in a marked decrease, by some 80-90%, in the bond strengths. These results cast doubt on the use of biodegradable polymers/ceramic composites for load-bearing synthetic bone-like materials, as desired optimal mechanical properties are unlikely to be met in realistic physiological environments.