In
this study, hydroxyapatite (HA)-based microspheres with the
ability to deliver bone morphogenetic protein-2 (BMP-2) were developed
for accelerating bone regeneration. The incorporation of calcium sulfate
dihydrate (CSD) in the HA matrix improved the rate of BMP-2 release
from the microspheres. Under physiological conditions, the CSD fully
degraded within 7 days and generated pore channels in the microspheres.
The porosity and pore size of the HA–CSD microspheres after
CSD degradation were 34.3% ± 4.2% and 11.5 ± 2.4 μm,
respectively, significantly larger than those of the HA microspheres
(23.9% ± 3.1% and 8.7 ± 0.9 μm, respectively). The
increased porosity directly affected the rate of BMP-2 release from
the microspheres. An in vitro experiment showed that both the BMP-2
release rate and the total amount of BMP-2 released increased considerably
when incorporating the HA microspheres with CSD. BMP-2 was released
slowly from the HA microspheres for up to 6 weeks. BMP-2 release was
notably improved in the HA–CSD biphasic microspheres compared
to the microspheres without CSD; the rate of release was 2.4-times
faster due to the pores created by CSD dissolution after 7 days. Prior
to animal testing, in vitro cell tests were performed to evaluate
the biocompatibility of the HA–CSD microspheres. During CSD
dissolution, biocompatible bone-like apatite precipitated on the cell
surfaces, and preosteoblasts grew on the microspheres. In vivo experiments
using a rabbit lateral femoral condyle defect model demonstrated that
the level of bone regeneration was significantly enhanced by mineralization
on the surface, generated additional pores as well as improved BMP-2
release behavior. The HA–CSD microspheres accelerated new bone
growth to fill the entire defect in 6 weeks, corresponding to a 170%
improvement in performance compared to the HA microspheres.