Fig. 17. Hydrogels for delivering cargo biomolecules (therapeutic proteins, growth factors, and drugs) with pre-programmed or user-defined release behavior. (A) A degradable silk fibroin hydrogel for pre-programmed release was formed in the presence of BMP-2, using a sonication-induced gelation process, and was injected into a bone defect. (B) Enhanced bone formation was observed in vivo in SASCO Sprague Dawley rats injected with the silk hydrogel containing BMP-2 as compared to the control group (no growth factor) (week 12, X-ray radiography). A, B reprinted from Diab et al. ³³⁸ with permission from Elsevier. Copyright (2011). (C) Photodegradable hydrogel microparticles were synthesized by an inverse suspension polymerization of a PEG-diphotodegradable acrylate with a PEG tetrathiol (PEG4SH) via base-catalyzed Michael addition. (D) A model cargo protein (Annexin V) was loaded (right), and its release to 3T3 cells (left) was triggered by cytocompatible irradiation (1 min of 13.5 mW cm^–2 at 365 nm). C, D reprinted from Tibbitt et al. ³³⁹ with permission from John Wiley and Sons publishing. Copyright (2012).