Ectopic mineralization and retention of heparin microparticles in subcutaneously implanted alginate constructs. 1 mg of AF647-labeled HMPs (unloaded or loaded with 5 μg of BMP-2) were mixed into alginate constructs, which were implanted subcutaneously in the backs of rats for 6 weeks (n = 6). A) Representative IVIS® image (Ex: 640, Em: 720) depicting explanted constructs with and without labeled HMPs after 6 weeks. B) Fluorescent signal of constructs, normalized to initial fluorescence, immediately prior to implantation (Week 0) and following explanting (Week 6). AF647 signal decreased over time in both unloaded and loaded HMP-containing constructs. (* = p < 0.05 as indicated) C) Micro-CT quantification of mineral volume induced in constructs. No differences were observed between groups. Safranin O/Fast Green staining of alginate constructs containing D) BMP-2-loaded HMPs, E) BMP-2 only, and F) unloaded HMPs depicted mineralization (white arrows) in BMP-2-containing constructs and the presence of intact HMPs (yellow arrows) in HMPs-containing constructs after 6 weeks in vivo.