title: Controlled fabrication of microfibers using microfluidic devices
Creators: name: Hofmann, Eddie
Publisher: name: University of Bayreuth
Year: 2020
Subjects: subject: microfibers
subject: microfluidics
subject: small‐angle X‐ray scattering
subject: solution blow spinning
subject: fluid dynamics
subject: 540

description: 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 blow spinning (μSBS). Here, the spinneret is replaced by a microfluidic nozzle device, which allows to produce ultrafine fibers of virtual endless length having precise diameter control. The gas dynamic virtual nozzle (GDVN) principle is applied to generate a fine liquid jet by three-dimensional air focusing of the spinning solution. When the polymer solution is ejected from the nozzle, the solvent evaporates, and the solid polymer fiber remains. The ejected fibers can either be sprayed directly onto a substrate as a nonwoven mesh or collected on a rotating spool as filament yarn. From hydrodynamic considerations and mass balance, equations were derived which allow to quantitatively predict and control the diameter of the jetted polymer solution and the resulting fiber. In the last part of this thesis, micro solution blow spinning is combined with small- and wide-angle X-ray scattering (SAXS, WAXS) to directly relate the macroscopic spinning conditions to the molecular structure of the resulting fibers. Having precise control of the jet diameter and velocity also gives excellent control of the fiber diameter and the internal macromolecular alignment. Using the software Scatter, 2D-SAXS patterns were simulated and compared with the measured ones to determine the orientational order parameter. It was shown that the elongation rate is the decisive parameter that transduces the macroscopic flow properties to the local macromolecular structure and orientation and thus determines the mechanical properties of the resulting fiber. The well-defined shish–kebab crystal structure of the fluorinated terpolymer THV transforms into an extended chain crystal structure upon plastic deformation in tensile tests. In summary, this thesis contributes to the methodical advancement of microfluidic devices for the purpose of spinning microfibers and the fundamental understanding of structure formation in the process of fiber spinning.