Tailoring the multiscale architecture of electrospun membranes to promote 3D cellular infiltration

Controlling the architecture of engineered scaffolds is of outmost importance to induce a targeted cell response and ultimately achieve successful tissue regeneration upon implantation. Robust, reliable and reproducible methods to control scaffold properties at different levels are timely and highly important. However, the multiscale architectural properties of electrospun membranes are very complex, in particular the role of fiber-to-fiber interactions on mechanical properties, and their effect on cell response remain largely unexplored. The work reported here reveals that the macroscopic membrane stiffness, observed by stress-strain curves, cannot be predicted solely based on the Young's moduli of the constituting fibers but is rather influenced by interactions on the microscale, namely the number of fiber-to-fiber bonds. To specifically control the formation of these bonds, solvent systems of the electrospinning solution were fine-tuned, affecting the membrane properties at every length-scale investigated. In contrast to dichloromethane that is characterized by a high vapor pressure, the use of trifluoroacetic acid, a solvent with a lower vapor pressure, favors the generation of fiber-to-fiber bonds. This ultimately led to an overall increased Young's modulus and yield stress of the membrane despite a lower stiffness of the constituting fibers. With respect to tissue engineering applications, an experimental setup was developed to investigate the effect of architectural parameters on the ability of cells to infiltrate and migrate within the scaffold. The results reveal that differences in fiber-to-fiber bonds significantly affect the infiltration of normal human dermal fibroblasts into the membranes. Membranes of loose fibers with low numbers of fiber-to-fiber bonds, as obtained from spinning solutions using dichloromethane, promote cellular infiltration and are thus promising candidates for the formation of a 3D tissue.