Enzymatically in situ shell cross-linked micelles composed of 4-arm PPO-PEO and heparin for controlled dual drug delivery. 
We report a controlled dual drug delivery system using heparinized 4-arm poly(propylene oxide) (PPO)-poly(ethylene oxide) (PEO) micelles (cHTM) that are sterically stabilized by enzymatic shell cross-linking (SCL).  Tyramine (TA) was chemically conjugated to 4-arm PPO-PEO (Tetronic) and heparin, resulting in Tetronic-TA (Tet-TA) and heparin-TA (Hep-TA), respectively.  To prepare a series of cHTM, different amounts of Hep-TA were added to a micellar solution of Tet-TA, followed by addition of horseradish peroxidase (HRP) and hydrogen peroxide (H2O2) to trigger SCL between TA groups at the micellar surfaces.  Increasing the feed amount of Hep-TA led to increased heparin content of cHTM, thereby resulting in increased micelle size with more negatively charged surfaces.  All SCL micelles were found to be highly stable over 4weeks, showing negligible changes in their sizes and zeta potentials.  Dual drug-loaded cHTM containing indomethacin (IMC) and basic fibroblast growth factor (bFGF) were prepared via a one-pot procedure.  With favorable IMC loading, the loading efficiencies of bFGF into cHTM were much higher than those in the controls due to the presence of heparin on the micellar surface.  After bFGF was added to IMC loaded cHTM the surface of HTM became less negative with an increase in size, suggesting successful binding of positively charged bFGF to heparinized micelle surfaces.  In vitro release data clearly showed more sustained release of IMC and bFGF as compared with non-cross-linked micelles.  Based on these results, we suggest that cHTM can be used as a new drug delivery platform for controlled dual drug release.