We present photo-crosslinkable hybrid organic-inorganic composites of poly(propylene fumarate) (PPF), an injectable and biodegradable polyester, and methacryl-polyhedral oligomeric silsesquioxane (mPOSS), which has eight methacryl groups tethered with a cage-like hybrid inorganic-organic nanostructure, for bone tissue engineering applications. Two PPF samples, one with a higher molecular weight (HPPF) and one with a lower molecular weight (LPPF), were blended with mPOSS at various mPOSS compositions. We emphasize on the role of mPOSS in modifying the photo-crosslinking kinetics of PPF and the physical properties of crosslinked products, and in regulating mouse pre-osteoblastic MC3T3-E1 cell behavior on the crosslinked substrates. The viscoelastic behavior of uncrosslinked PPF/mPOSS and the mechanical, degradation, and surface characteristics of photo-crosslinked PPF/mPOSS samples have been investigated. The crosslinking kinetics of PPF/mPOSS was characterized using the gel fraction and rheological properties such as storage modulus and viscosity of the samples crosslinked for 0.5-5 min. Blending mPOSS with PPF was found to decrease the viscosity of PPF and expedite the photo-crosslinking process, both of which are important in formation of PPF networks. In addition, tensile modulus was increased by crosslinking PPF with mPOSS up to the mPOSS composition of 15%, above which phase separation occurred. The degradation rate in 1 N NaOH solution could also be regulated by varying the mPOSS composition, while crosslinked PPF/mPOSS samples did not show detectable degradation in one week in phosphate buffered saline (PBS) and mPOSS did not significantly alter surface wettability and serum protein adsorption. MC3T3 cell adhesion, spreading, proliferation, differentiation, and gene expression were examined on crosslinked PPF/mPOSS disks. Addition of mPOSS did not introduce cytotoxicity or influence cell behavior evidently. It is interesting to note that crosslinked HPPF/mPOSS showed distinct physical properties and promoted MC3T3 cell functions compared with their LPPF counterparts possibly because of higher crosslinking densities. (c) 2011 Elsevier Ltd. All rights reserved.