Controlled fabrication of microfibers using microfluidic devices

Abstract

Microfibers are of great interest in a wide variety of research fields because of their high surface-area-to-volume ratio and unique mechanical properties. Accordingly, they are basis of diverse applications in tissue engineering, biomedicine, filtration, and sensor technology. The multidisciplinary field of microfluidics deals with the behavior and manipulation of fluids confined to such small dimensions that surface forces, energy dissipation, and diffusive mixing start to dominate the system. Microfluidics has already proven its potential in various research areas such as modern medicine, biology and chemistry. The scope of this thesis is to explore the options, select suitable approaches and exhaust the possibilities of utilizing microfluidic devices for spinning of microfibers. Microfluidics offers some key advantages associated with laminar flow and provide unique control over the entire spinning process. Two different methods of conventional fiber spinning were identified and adapted for microfluidic spinning of microfibers. Both approaches, which are variants of wet and dry spinning, have in common that a spinning solution of a natural or synthetic polymer is ejected through a spinneret. When the solvent is removed or exchanged by the surrounding medium, this causes the polymer to solidify and form a mechanically stable fiber. The macromolecules are aligned within the nozzle by shear and elongational forces. When collecting the fiber on a rotating spool, the mechanical properties can be further enhanced by additional stretching. Microfluidics offers a high degree of control of all relevant spinning parameters and the possibility to optimize the nozzle design. Computer-aided design software allows to design almost any channel geometry, which can be created using lithographic techniques. This allows not only to fabricate fibers of uniform diameter and endless length in a steady and controlled process, but also to gain insights on the formation of fibrous microstructure by applying suitable characterization methods. Collagen microfibers are in the focus of biomedical research projects. In this thesis it could be shown that microfibers can be produced from pure type I collagen in a microfluidic wet spinning process using hydrodynamic flow focusing and an asymmetric channel architecture. Irreversible clogging of the channels by the assembling collagen could be prevented by reducing wall adhesion with an elaborate channel geometry, which ultimately results in a continuous and adjustable process. These microfluidically produced collagen fibers stand out due to their exceptional small diameter, while their tensile strength and Young’s modulus exceed that of classical wet-spun fibers and even natural tendon. Cell culture tests showed directional axon growth of neuronal NG108-15 cells along the microfiber axis, which qualifies these fibers for a potential application in peripheral nerve repair. The second approach for microfluidic fiber spinning is a special variant of dry spinning, which is called micro solution…