Tissue engineering has emerged as a well-investigated research area that aims to ultimately create biological substitutes, ranging from skin replacement to artificial blood vessels, for regenerative medicine applications. It is a multidisciplinary field that integrates knowledge and advancements from biology, materials, chemistry, and many others. Although the goal of tissue engineering is to engineer tissues with structure and function that closely mimic those of native tissues for not only diagnostic and drug testing applications but also repair and replacement of diseased and injured tissues, it provides in-depth understanding of tissue development and morphogenesis, including the intricate biological systems in the body that together operate to direct growth, repair, and death.
The general approach of tissue engineering is to use cells, biomaterials, bioactive molecules, or a combination thereof to best recapitulate the properties of native tissues and reconstitute function. Specifically, biomaterial scaffolds play a key role in that their main function is to serving as a basis to interact with and support cells, promoting their attachment and migration while providing a porous and biomimetic microenvironment for mass transport and integration of bioactive factors.
Therefore, in order to fabricate implantable scaffolds with properties that emulate the natural extracellular matrix (ECM) in terms of structure and organization, we utilize and implement electrospinning technology. Electrospinning is a versatile technique with immense potential to create cell-instructive scaffolds possessing ECM-like fibrous structures. Because of the ability to tune scaffold structure with respect to physical (i.e. geometry and mechanical properties) and chemical (i.e. retention and presentation of bioactive molecules) features, electrospun scaffolds, which are made from synthetic, natural, or a combination of the two polymers, have demonstrated functional success in an array of tissue engineering applications. In fact, the fibrous network, especially in the nanoscale, possesses high surface-area-to-volume ratio that is favorable for surface modifications to facilitate cell attachment as well as immobilization and covalent conjugation of bioactive molecules. However, one major shortcoming of electrospun scaffolds as a result of such densely packed network of fibers is their small pore size, limiting cell infiltration and tissue ingrowth essential for desired angiogenesis and tissue integration. Thus, electrospun scaffolds with dense fibrous structure and small pore size have not had enormous success in the regeneration of large, more complex tissues that require abundant vascularization and mass transport of oxygen and nutrients.
In this dissertation, we attempt to address both the physical and chemical elements of electrospun scaffolds as we engineer novel ways to modulate their structural features and surface chemistry for their utilization in different cell and tissue engineering applications. We focus primarily on micro- and nanostructures, topographical cues, and chemical modifications and their role in enhancing cell infiltration and promoting better tissue integration in terms of angiogenesis in and overall functional performance of implanted scaffolds as well as vascular graft. Specifically, we detail the employment of an ultrafast, femtosecond (FS) laser system as a promising post-fabrication technology to pattern structural features (i.e. through-holes) that influence cell behavior and improve the integration electrospun scaffolds in vivo. We also demonstrate that changes in biophysical factors, such as increasing pore size and porosity via incorporation and removal of sacrificial fibers from composite scaffolds, can not only improve cell infiltration but more importantly regulate stem cell differentiation. Lastly, we show that we can fabricate small-diameter, nanofibrous vascular graft made of Carbosil®, a commercially available polycarbonate-urethane (PCU), via electrospinning and subsequently manipulate its surface properties through plasma treatment and reductive amination to effectively end-point immobilize heparin to reduce surface thrombogenicity while enhancing bioactivity and short-term in vivo performance.
In summary, we offer novel design and fabrication of electrospun scaffolds with large pore size and highly porous structure, and vascular grafts with tailored surface chemistry and bioactivity optimal for their biological performance. We demonstrate through our studies that physical and chemical features of not only electrospun scaffolds but implantable biomaterials in general can be engineered and manipulated using post-fabrication processing (i.e. FS laser and tunable surface chemistry) and unique fabrication approaches (i.e. multiple polymers with sacrificial components). Only with ample vascularization and tissue integration will such biomaterial scaffolds regenerate native tissue function and demonstrate success in both simple, such as skin substitutes, as well as complicated, such as artificial heart, tissues and organs for applications in regenerative medicine.
