The Importance of New Processing Techniques in Tissue Engineering
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lar weight (MW), MW distribution, degree of crystallinity, and environmental conditions such as temperature and pH.3 Mechanical loading of the scaffold may affect its degradation.4 Porosity, pore size, and pore structure are important factors that are associated with nutrient supply to transplanted and regenerated cells. To regenerate highly vascularized organs such as the liver, porous scaffolds with large void volume and large surface-area-to-volume ratio 5 are often desirable. In that way, the scaffolds provide adequate space for cell seeding, growth, ECM production, and vascularization.6 Small-diameter pores are preferable to yield high surface area per volume as long as the pore size is greater than the diameter of a cell in suspension (typically 10 μ-m). In the case of bone regeneration, there is an optimal pore size for maximum tissue ingrowth 7 ranging from 200 to 400 μ-m. The rate of fibrovascular tissue growth in porous biodegradable polymers also depends on the pore size.8 Compared to the isolated pore structure, the interconnected pore network structure enhances the diffusion rates to and from the center of the scaffold and facilitates vascularization, thus improving oxygen and nutrient 9 supply and waste removal. Most organ-cell types are anchoragedependent and require the presence of a suitable substrate to retain their ability to proliferate and perform differentiated functions. 9 Strong cell adhesion and spreading often favor proliferation while rounded cell morphology is required for cell-specific function.10 Thus a polymer scaffold must act as a suitable substrate to maintain differentiated functions without hindering proliferation. In the
case of epithelial cells that are polar and have unique apical, basal, and lateral surfaces, proper orientation of the cells is 11 essential to their function. A polymer scaffold should therefore provide proper surface chemistry and surface microstructure for optimal cell-substrate interaction and act as a template to direct and organize cell growth and ECM formation. Besides cell morphology, the function of many cells is dependent on the threedimensional spatial relationship of cells and ECM, as in the case of liver regen12 eration. The shape of a hard tissue such as bone or cartilage is also critical to its 1314 function. A polymer scaffold should be easily and reproducibly processed into the desired shape and structure that can be maintained after implantation. In some other cases, a scaffold with unique three-dimensional geometry is required to fit in an irregular defect. The regenerated tissue is therefore expected to take the shape of the initial scaffold. Mechanical properties are also of crucial importance in polymer-scaffold design. For load-bearing tissues such as bone, the scaffolds should be strong enough to withstand physiological 15 stresses. Their degradation rates should be adjustable to match the rate of tissue regeneration since strength decreases as a polymer degrades over time. In some instances, the scaffolds are required to be pliable enou
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