Hierarchical 3d structures for bioengineering applications

Abstract

Devices and scaffolds made from soft polymeric materials are being developed for various applications like filtration, catalysis, tissue engineering, and regenerative medicine. To cater to different fields, these scaffolds are designed to be of varying length scales and geometries (flat/tubular) and manufactured using diverse materials. They serve as platforms to replicate the chemical/physical processes usually carried out by natural biological systems. One of the biological system's most critical aspects is its hierarchical self-repeating nature across size scales. This research aims to develop manufacturing processes for fabricating soft polymeric scaffolds that display hierarchy across different length scales. The first type of hierarchy replicated in the polymeric scaffold is based on the morphological hierarchy of branched tubular structures that constitute the circulatory system in the human body. The second type of hierarchy is the hierarchy of the materials that comprise the microstructure of the polymeric scaffold. The first manufacturing process developed as a part of this research seeks to fabricate free-standing tubular structures that display (i) the hierarchy of sizes (internal tube diameters) and (ii) the interconnectedness of the different-sized tubes in a branching pattern. This was accomplished by developing a hybrid manufacturing process that combines alginate electrodeposition with fiber electrospinning. Alginate electrodeposition was used to fabricate tubular structures with internal diameters at the micro-scale (~100 µm). The conventional electrodeposition process was modified by incorporating a moving electrode to be able to fabricate branched/bifurcated tubular structures. Branched tubular structures were manufactured through fiber electrospinning at the macro-scale (~10mm. These tubular constructs at the two extreme ends of the scale were joined together using the electrodeposition process at a common size scale. The final structure obtained displayed a hierarchy of tube diameters. Hierarchical prototypes were fabricated with differing branching architectures and could support flow similar to the physiological circulatory system. The second manufacturing process seeks to replicate the hierarchical nature of the fibrous extracellular matrix (ECM) by combining two soft polymeric systems that are fibrous in nature and have different-sized fiber components, i.e., (i) Bacterial Cellulose (BC) fibers (~100 nm fiber diameter) and (ii) Polymeric nanofibers obtained through electrospinning (~1m fiber diameter). The integration of bacterial cellulose with the electrospun fibers was carried out using a bioreactor. The oscillatory motion bioreactor (OMB) added the BC-producing bacteria and the requisite nutrients into the electrospun fiber template in a periodic fashion. The frequency of the nutrient provision dictates the amount of BC fibers synthesized and the extent of integration with the electrospun fiber template. The resulting fibrous scaffolds showed a hierarchical microstructure with effective integration of electrospun nanofibers and BC fibers. Finally, an exploratory manufacturing process was pursued to introduce hierarchical tubular channels into a bacterial cellulose matrix using biocompatible sacrificial material. Alginate tubes (500-800 m external diameter) were incorporated into BC culture media as a sacrificial material to introduce microchannel-shaped porosity into BC. When the tubes were washed away following BC synthesis, a microstructure with a hierarchy of pore sizes, i.e., tubular pores from alginate tubes and random pores present naturally in the BC scaffold, was achieved. The tubular pores could support fluid flow and be functional. In summary, the three manufacturing processes demonstrated in this dissertation are expected to benefit both the biomedical sector and applications requiring novel thin films involving bacterial cellulose structure.