| Continuous electrospun submicron fibers (commonly known as nanofiber) have attracted considerable interest in tissue engineering and regenerative medicine community due to their capability in constructing scaffolds resembling the nanofibrous structure of natural extracellular matrix. However, whipping or bending instability during electrospinning often results in non-woven fibrous structure, which is not suitable for making biomimetic nanofibrous scaffolds for engineering the anisotropic dense connective tissues (e.g., tendon, ligament) where fiber alignment and mechanical properties are necessary and highly desired. Previously, various approaches attempting to prepare aligned electrospun fibers including the common use of a high speed rotating mandrel collector have been diversely devised. Yet these approaches are yet to meet the requirements for practical applications in terms of the degree of fiber orientation, length of fiber yarn, yield, and mechanical properties. To provide a better solution, this study aims to fundamentally control the jet stability during electrospinning process for easily preparing continuous, alligned, ultrafine and high-strength fibers to cater for the increasing demands of constructing particular structures like tendon, ligament and other tissues.In this thesis, three kinds of biodegradable polymers, i.e., chtosan (CTS), polycaprolacton (PCL), and poly-L-lactic acid (PLLA) were employed to explore the feasibility of the above objective. It was found that as long as a small amount of ultrahigh molecular weight PEO (molecular weight>5000 000 Da) was introduced to formulate an appropriate viscosity of polymer solutions, bending instabilities of the electrospinning jet could be adequately suppressed to obtain a single stable jet for fiber formation, similar to those traditional fiber forming methods (e.g., dry spinning or dry-wet spinning). This formation of ultrafine aligned polymer fibers driven by electric force can be termed as "stable jet electrospinning' (SJE). enabling to produce single fibers with a diameter of 1～10μm, fiber yarns in a length of 30 cm, and highly aligned ultrafine fibrous membranes of 30 cm by 20 cm in size. We collected these different fiber types based on a modified electrospinning setup from which controllable speeds of rotating drum and horizontally or vertically jetting can be manipulated with ease. Apart from visiable observation, real-time images taken by a high-speed camera confirmed that the jet from SJE is continuous, stable and without bending instability. By extending the SJE approach further, we were also able to prepare aligned ultrafine core-shell fibers, which will offer great possibilities on functionalization of such kinds of ultra-fine fibers.Based on the SJE approach developed above, this paper systematically investigate:(I) SJE mechanism and process control; and (2) the relationship between fiber macromolecule structure and mechanical properties. Our result showed that viscoelasty of a polymer solution is the key factor in determining whether formation of stable jet in electrospinning is possible to take place. Fiber diameter could be attenuated with increasing applied voltages and rotating speeds. Both the WAXD and polarized Fourier transform infrared spectra suggested that stable jet electrospinning faciliated polymer chain and crystalline orientation, which may have contributed to the significant improvement in the mechanical properties of thus made ultrafine fibers. The electrospun CTS, PCL and PLLA fibers by SJE possessed a modulus of 11±6 GPa,40±10 MPa and 2.13±0.73 GPa, and tensile strength of 762±93 MPa,120±30 MPa and 205.62±46.35 MPa, respectively. Furthermore, higher speed of rotating collection would further increase the polymer chain and crystalline orientation. |
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