Influence of cyclic compressive forces in bone tissue engineering
Date of Issue2017
School of Chemical and Biomedical Engineering
Bone is one of the most transplanted tissues in the world and the current clinical strategies are unable to meet the growing demand for bone grafts. Therefore, bone tissue engineering (BTE) has become one of the potential alternatives to produce highly cellularized and mineralized bone grafts to meet the growing demand in clinics. Despite the advances in BTE, the current in vitro methods have not successfully generated viable, voluminous and mineralized 3D constructs. In this thesis, we hypothesized that application of physiologically relevant mechanical forces would enable generation of highly osteogenic constructs. Influence of mechanical stimulation and fluid flow on in vivo bone remodeling has been extensively studied in the past. However, there is lack of a systemic study to understand the effect of physiological strains on in vitro bone formation. To fill this gap, we developed a multi-chamber compression bioreactor system to apply cyclic compressive forces on cell loaded scaffolds under controlled environment to evaluate osteogenesis and mineralization of cellular grafts. A parametric optimization study conducted by applying a range of strains on Mesenchymal Stem Cells (MSC) seeded Polycaprolactone-β-Tricalcium Phosphate (PCL-TCP) scaffolds demonstrated that physiologically relevant strains resulted in enhanced osteogenic differentiation. Further, a long term study with optimized strain values indicated cyclic compressive loading induced upregulation of osteogenic gene expression. Osteogenic enzyme activity was also highly elevated by the end of two weeks in comparison to the static cultures. These results suggested the potential of cyclic compressive loading for generating mineralized tissues. Although, long term cyclic compression of cell seeded PCL-TCP scaffolds resulted in increased osteogenesis and mineralization, stable cellular adhesion was not achieved prior to cyclic mechanical loading and lack of media exchange resulted in poor cellular viability. It was hypothesized that dynamic culture with fluid flow can result in tissues that possess higher cell numbers and hence enhanced matrix deposition, which in turn would enable better mechanotransduction to the cells under cyclic loading. Hence, we developed a multimodal bioreactor system that allows application of physiologically relevant compressive strains on premature bone grafts that are cultured under biaxial rotation conditions. The bioreactor system was integrated with sensors for dissolved oxygen and pH that allow real-time, non-invasive sensing of the culture parameters. The system was used as a platform to study the independent and combined effects of fluid flow and cyclic mechanical stimulation on cellular proliferation and osteogenic differentiation of stem cells on 3D bone grafts. MSC seeded PCL-TCP scaffolds were cultured in the multimodal bioreactor for a period of 2 weeks under four different modes including static, cyclic compression, biaxial rotation and multimodal modes (combination of cyclic compression and biaxial rotation). The combined effects of optimal fluid flow conditions and cyclic compression led to the upregulation of osteogenic genes. The multimodal culture also resulted in higher cellular proliferation in comparison to the static groups. We have demonstrated the potential of sensor integrated bioreactor system. In summary, we have developed a platform to engineer bone tissues using a multimodal bioreactor system and demonstrated its potential advantages for generation and maintenance of bone grafts under physiologically relevant conditions.