Bone loss affects millions of patients in the United States annually and can be caused by traumatic injury, inflammatory and infectious diseases, congenital malformation or oncologic resection. Conventional treatment often involves using autologous or allograft bone tissues, which is limited by donor site morbidity, insufficient donor tissue supply, and potential immunogenicity. Stem cell-based therapy offers a promising approach for the repair of bone loss such as cranial and long bone defects. However, a critical barrier to progress in the field is the lack of suitable cell carriers that can support stem cell survival, and guide vascularized and mineralized bone formation in situ without the addition of supraphysiological concentration of growth factors. To address the above challenges, my proposed multidisciplinary approach aims to validate the efficacy of microribbon-based scaffolds as a novel type of cell carrier for enhancing stem cell survival and mineralized bone matrix deposition in vivo using a mouse critical size cranial defect model. Our RB- like hydrogels were fabricated by wet-spinning gelatin (digested collagen), the most abundant ECM matrix protein, into RB-like structures, which can be crosslinked into a macroporous scaffold. The resulting hydrogels combine the injectability and cell-encapsulation ability of standard hydrogels with the macroporosity that facilitates faster vascular ingrowth, cell proliferation, adhesion, migration, and ECM deposition. This injectable system empowers minimally invasive surgery for a broad range of diseases. In our preliminary studies, RB-based matrices markedly enhanced the survival of human adipose-derived stromal cells (ASCs) and accelerated tissue regeneration in vivo. Here I propose to further enhance the osteoinductivity and osteoconductivity of RB-like hydrogels by tuning RB stiffness, bulk stiffness and hydroxyapatite coating. I hypothesize that osteogenic differentiation of human mesenchymal stem cells (MSCs) in microribbon-based scaffolds can be enhanced by increasing the RB stiffness and hydroxyapatite coating of microribbons. Furthermore, I hypothesize that increasing bulk stiffness of the scaffold will result in reduced matrix deposition due to decreasing pore size. Overall, the macroporosity of microribbon-based scaffolds would lead to enhanced cell survival, faster vascularization and enhanced bone tissue formation in vivo compared to convention biomaterials. The outcomes of the proposed work would lead to the development of new tissue engineering therapy for treating craniofacial and other large bony defects, and correspondingly reduce the associated socio-economical burden on society.