|M.Sc Student||Kesselman Dafna|
|Subject||Investigating the Effect of Dynamic Cellular Remodeling on|
the Bulk Material Properties on Biological and
Biosynthetic Composite Scaffolds for
|Department||Department of Biomedical Engineering||Supervisor||Professor Dror Seliktar|
|Full Thesis text|
The field of tissue engineering provides an opportunity to generate tissues using synthetic or biological material scaffolds and living cells. It is therefore critically important to understand the structure-function relationship between cells and tissues. Since natural tissues posses a three dimensional architecture, it has been recognized that structural and mechanical characteristics must be included in the design of a tissue engineered construct. Towards this goal, we have developed a novel biosynthetic hybrid hydrogel material that can encapsulate and cultivate cells in a three dimensional (3D) manner for extended durations. The system is composed of a synthetic polymer component, poly(ethylene glycol)-diacrylate (PEG-DA), covalently conjugated to a fibrinogen backbone. The material is polymerized into a 3D hydrogel network that limits cell-mediated bulk degradation by proteolysis. The PEG-Fibrinogen (PF) can be injected into the body and polymerized in situ with or without prior cell cultivation [2-3]. In this thesis we investigated the local cellular remodeling effects on bulk mechanical properties of cell-seeded constructs using rheometry. PEG-fibrinogen precursor solutions were prepared with neonatal human dermal fibroblasts in suspension, polymerized using UV photo-initiation, and cultivated for several days. Using a unique rheometry technique, the shear modulus (G’) of the cell-seeded hydrogels was measured for a one-week period. A similar protocol was used with fibrin constructs for comparison. Acellular enzymatic degradation was characterized in Trypsin. The results showed that the PF samples increased significantly in mechanical strength over a one-week period, reaching almost 3 times the initial modulus of the scaffold. In comparison, fibrin scaffolds showed a slight increase in modulus only when Aprotinin (a protease inhibitor) was added, reaching above 1.5 times the initial stiffness. In the absence of the protease inhibitor, the cell-seeded fibrin scaffolds deteriorated by their second day of cultivation. These results underscore the importance of proteolytic degradation and structural remodeling in the development of the PF and fibrin-based tissue engineered constructs. When measuring the proteolytic degradation rates of acellular PF and fibrin constructs, we determined that fibrin gels have a characteristic half-life (t50) in Trypsin (a non-specific protease) that is twice that of the PF gels. We speculate that the molecular structure of the PF - a highly amorphous and dense matrix - contributes to its unique degradation and remodeling profile when compared to fibrin, which is highly fibrillar and porous. We therefore conclude that controlling the structural features of the PF gels provides a means of better controlling their remolding in various tissue-engineering applications.