Understanding and tailoring the multiscale architectural and mechanical properties of electrospun membranes for tissue engineering applications
II An approach for the investigation of non-crystalline, mesomorphic superstructures of PLLA fibers was developed based on solvent-induced crystallization. The fiber mesomorphic architectures, which are challenging to investigate due to their low contrast with the surrounding amorphous phase using most of the analytical tools, were revealed after post-treatment in tailored acetone-water blend systems. In this process, the architecture of the superstructures was preserved, while the underlying
... sophase were ordered in an α-crystalline phase, enhancing thus their contrast. With this method in place, we demonstrated that a fast evaporation of the electrospinning solvent during the fiber formation favors the growth of fibrillar superstructures, which, in turn, resulted in a higher fiber stiffness. These findings describing the physical properties of single fibers and the influence of the fabrication parameters were used to inform a 3D predictive numerical model of electrospun network, in collaboration with the group of Prof Dr. E. Mazza at ETHZ. These models are of high importance to further understand and predict the mechanical behavior of nanofibrous scaffolds and can assist their development. The virtual networks were compared to real ones by simulating, respectively performing uniaxial tensile testing. In addition, methods to visualize the deformation of real networks upon stretching were developed using a custom-built tensile stage and electron microscopy. These comparisons revealed good agreements between the virtual and real networks. What is more, the numerical models helped elucidating the auxetic behavior of nanofiber networks, which was found to be generated by the buckling of the fibers oriented transversally to the stretching forces and compressed by the lateral contraction of the membrane. In the last part, the methods developed for structural analyses at the fiber and network levels, and the knowledge acquired during the thesis, e.g. concerning the fiber formation process, were applied to closely investigate the multiscale architectures of membranes, from the nano-to the macroscale level. This approach allowed us to demonstrate that the influence of fiber-to-fiber junctions on the macroscopic membrane stiffness can overcome the one from the single fiber Young's modulus. These results draw the attention on important aspects to be accounted for during the development of scaffolds for tissue engineering applications. In conclusion, this thesis provides important points of reference for the production of tailored electrospun PLLA fibers, including by the use of pilot plant needleless equipment, and an extensive understanding of the fiber formation process by the electrospinning procedure. Moreover, a set of methods were established for the characterization of the multiscale architectures of nanofibrous scaffolds. This work contributed as well to the development of a predictive numerical model that already demonstrated its potential toward the understanding of fiber network properties. The newly acquired knowledge will contribute to refine the development of advanced electrospun scaffolds with tailored structural and mechanical properties e.g. for tissue engineering applications.