Extrusion bioprinting of hydrogel scaffolds: printability and mechanical behavior
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Extrusion bioprinting (known as dispensing-based bioprinting as well) has been widely used to extrude or dispense continuous strands or fibers of biomaterials (e.g. hydrogel) and cells (such a mixture is referred to as "bioink"), layer-by-layer, to form three-dimensional (3D) scaffolds for tissue engineering. For extrusion bioprinting, one key issue is printability or the capability to print and maintain reproducible 3D scaffolds from bioink, which is typically measured by the difference in structure between the designed scaffold and the printed one. Due to the structural difference (or the difference caused by printability), the printed scaffold's mechanical properties are also different from those of the designed scaffold, notably affecting the scaffold performance as applied subsequently to tissue engineering. This dissertation aims to perform a comprehensive study on the printability and mechanical behavior of hydrogel scaffolds fabricated by extrusion bioprinting. The specific objectives are (1) to investigate the influence of design-, bioink-, and printing-related factors on the printability of hydrogel scaffolds, (2) develop an indirect printing technique to improve the printability of low-concentration hydrogels, (3) develop a numerical model representative of the elastic modulus of hydrogel scaffolds by considering the influence of printability, and (4) investigate the effect of crosslinkers on the scaffold's mechanical properties through experimental and numerical approaches. While studies on printing scaffolds from hydrogel(s) have been conducted, limited knowledge has been documented on hydrogels' printability. Current studies often consider one aspect of studying hydrogel printability (for example, bioink properties solely). The first part of this dissertation studies the multiple dimensions of printability for hydrogel scaffolds, including identifying the influence of hydrogel composition and printing parameters/conditions. Specifically, by using the hydrogels synthesized from alginate, gelatin, and methylcellulose (MC), flow behavior and mechanical properties, as well as their influence on the printability of hydrogels, were investigated. Pore size, strand diameter, and other dimensions of the printed scaffolds were examined; then, pore/ strand/ angular/ printability and irregularity were studied to characterize the printability. The results revealed that the printability could be affected by many factors; among them, the most important are those related to the hydrogel composition and printing parameters. This chapter also presents a framework to evaluate alginate hydrogel printability systematically, which can be adopted and used in the studies of other hydrogels for bioprinting. Low-concentration hydrogels have favorable properties for many cell functions in tissue engineering, but they are considerably limited from a scaffold fabrication point of view due to poor 3D printability. The second part of this dissertation is developing an indirect printing method to fabricate scaffolds made from a low-concentration of hydrogels as the second objective. This chapter briefly presents an indirect bioprinting technique to biofabricate scaffolds with low (0.5%w/v) to moderate (3%w/v) concentrations of alginate hydrogel using gelatin as a sacrificial bioink. Indirect-fabricated scaffolds were evaluated using compression, swelling, degradation, biological (primary rat Schwann cells), and morphological assessments. Results indicated that 0.5% alginate scaffolds have steep swelling changes, while 3.0% alginate scaffolds had gradual changes. 0.5% alginate demonstrated better cell viability throughout the study than 3.0% counterparts, though. It was concluded that this indirect bioprinting approach could be extended to other types of hydrogels to improve the printability of low-concentration hydrogels along with the biological performance of cells and avoid high shear stress during direct 3D bioplotting causing cell damage. One issue involved in 3D bioplotting is achieving the scaffold structure with the desired mechanical properties. To overcome this issue, various numerical methods have been developed to predict scaffolds' mechanical properties, but they are limited by the imperfect representation of scaffolds as fabricated. The third part of this dissertation is developing a numerical model to predict the elastic modulus (one important index of mechanical properties) of scaffolds, considering the penetration or fusion of strands in one layer into the previous layer as the third objective. For this purpose, the finite element method was used for the model development, while medium-viscosity alginate was selected for scaffold fabrication by the 3D bioplotting technique. The elastic modulus of the bioplotted scaffolds was characterized using mechanical testing; the results were compared with those predicted from the developed model, demonstrating a strong congruity amongst them. Our results showed that the penetration, pore size, and the number of printed layers have significant effects on the elastic modulus of bioplotted scaffolds and suggest that the developed model can be used as a powerful tool to modulate the mechanical behavior of bioplotted scaffolds. For improvement, the fourth part of the dissertation (or the fourth objective) is improving the developed model by considering the crosslinker's effect on the modeling. The use of a cation solution (a crosslinker agent such as CaCl2) is important for regulating the mechanical properties, but this use has not been well documented in the literature. Here, the effect of varied crosslinking agent volume and crosslinking time on 3D extrusion-based alginate scaffolds' mechanical behavior were evaluated using both experimental and numerical methods. Compression tests were used to measure each scaffold's elastic modulus; then, a finite element model was developed, and a power model was used to predict scaffold mechanical behavior. Results showed that crosslinking time and crosslinker volume both play a decisive role in modulating 3D bioplotted scaffolds' mechanical properties. Because scaffolds' mechanical properties can affect cell response, this study's findings can be implemented to modulate the elastic modulus of scaffolds according to the intended application. In conclusion, this dissertation presents the development of methods/models to study/represent the printability and mechanical properties of hydrogel scaffolds by using extrusion bioprinting, along with meaningful experimental and model-simulation results. The developed methods/models/results would represent an advance in bioprinting scaffolds for tissue engineering.
DegreeDoctor of Philosophy (Ph.D.)
CommitteeSarty, Gordon; Bergstrom, Donald; Zhu, Ning; Zhang, Chris
Copyright DateSeptember 2020