Freeform fabrication processes for soft polymeric biomaterials

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Nowak, James
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Electronic thesis
Mechanical engineering
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Soft, polymeric biomaterials are critical towards developing our understanding of the human anatomy, and the realization of tissue engineering scaffolds that achieve the grand vision of “organ-on-demand” regenerative medicine. In light of this pressing need, the overall objective of this research is to advance the processing capabilities of soft, polymeric biomaterials to meet the requirements of biomedical applications. Two types of biomaterials are targeted in this work, viz. (i) fiber-reinforced hydrogels, and (ii) bacterial cellulose (BC).For fiber-reinforced hydrogels, two novel electrospinning printheads, viz., (I) A direct-write printhead; and (II) an air-assisted, dual-polarity printhead were created to realize freeform geometries that are currently not attainable with traditional processing techniques. The direct-write electrospinning printhead was used to create laminated fiber-reinforced hydrogels and three-dimensional fiber-reinforced hydrogels that contained continuous fiber reinforcement to mimic the extracellular matrix seen in soft tissues. The direct-write electrospinning printhead used guiding electrodes to manipulate the electric field and extend the stability region seen in the electrospinning process. Fiber contours were produced using this printhead and high-precision linear motion stages. These contours were then stacked, to create a fiber preform, and infused with hydrogel solution. Significant improvements in the mechanical strength were observed in the laminated fiber-reinforced hydrogels in comparison to neat (pure) hydrogels. In a follow-on investigation, this printhead was successfully adapted to realize cell-laden, three-dimensional fiber-reinforced hydrogels with continuous fiber reinforcement. This was achieved using simultaneous deposition of electrospun fibers (using the direct-write electrospinning printhead) and cell-laden hydrogel (using gravimetric syringe deposition). The process was shown to be tunable to varying fractions of fibrous reinforcement within the final composite. Biologically-relevant freeform fiber-reinforced hydrogel geometries were realized using the air-assisted, dual-polarity electrospinning printhead. This printhead converges high-strength electric fields, with low-velocity air flow to remove the collector dependency seen with traditional far-field electrospinning setups. The use of this printhead, in conjunction with different configurations of deformable collection templates, has resulted in the production of three classes of prototype freeform geometries, viz. (i) tubular geometries including bifurcations and meso-scale texturing, (ii) hollow, non-tubular geometries including single- and dual-entrances, and (iii) flat geometries with varying fiber density. All three classes of prototype geometries were mechanically characterized, and displayed properties that were in line with those observed in living soft tissues. On the bacterial cellulose (BC) front, a template-assisted bioreactor was developed to facilitate the conformal coating and growth of BC at the air-liquid interface. The novel continuous oscillatory motion bioreactor (COMB) alternately introduced oxygen and nutrients through a combination of rotational and translation motion applied to a 3D-printed thermoplastic template. Fluid simulations indicate that the combined motion introduced a higher degree-of-mixing, which prolonged the BC production process. In addition, an anisotropic morphology was produced as a result of the COMB, which resulted in an improvement in mechanical properties. The COMB realized freeform geometries and internal meso-scale channels that were previously unattainable using existing BC bioreactors. Overall, this body of work spanning fiber-reinforced hydrogels and bacterial cellulose has resulted in the development of new manufacturing pathways critical to the biomedical research community. Clinically-relevant future adaptations of these processes can be expected to be transformative for discoveries within the medical field and health care sector.
August 2021
School of Engineering
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Rensselaer Polytechnic Institute, Troy, NY
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