In vitro and in vivo comparison of bulk and surface hydrolysis in absorbable polymer scaffolds for tissue engineering

This article describes preliminary in vitro and in vivo studies comparing bulk and surface hydrolysis in absorbable polymer scaffolds proposed for tissue engineering of bone. The two polymers systems used were a bulk hydrolyzing 50:50 poly(DL-lactide-co-glycolide) (PLGA) and a surface hydrolyzing self-catalytic poly(ortho ester) (POE). Polymer scaffolds were exposed to physiological saline at body temperature and changes in polymer mass loss and inherent viscosity were monitored over time. New bone formation and local tissue response were evaluated by implanting scaffold disks of both polymer systems into non-critical-size calvarial defects in rabbits. New bone formation was determined by bone mineral density measurements, and local tissue response was determined by qualitative histology. Preliminary results confirmed that one of the main design characteristics for absorbable polymers in tissue engineering of bone, coordination of controlled polymer mass loss with new tissue formation, appeared to be achieved better using a surface hydrolyzing POE, rather than with a bulk hydrolyzing 50:50 PLGA. Bone mineral density at 6 and 12 weeks was an average 25% higher in the surface hydrolyzing scaffold. Unfortunately, the amount of bone formed was so inconsequential that this observation is of little relevance. Use of a water-soluble signaling factor such as basic fibroblast growth factor (bFGF) failed to increase bone formation. The histological response of these two polymer systems was similar and unaffected by the presence or absence of bFGF. The persistence of structural integrity for self-catalytic POE scaffolds after 6 and 12 weeks implantation, while 50:50 PLGA scaffolds had partially collapsed after 6 weeks, suggests surface hydrolyzing scaffolds may have some advantage over bulk hydrolyzing scaffolds in resisting normal in vivo stresses when used in a calvarial defect.