Revealing non-crystalline polymer superstructures within electrospun fibers through solvent-induced phase rearrangements.

The design of nanofibers for biomedical applications requires a deep understanding of the fiber formation process and the resulting internal structure. In this regard, non-crystalline, mesomorphic structures play a central role in the processing of many polymers as precursors in the formation of crystalline superstructures (e.g. shish-kebab) and influence strongly the physical properties of polymers with a low degree of crystallinity. Yet, our ability to probe these relevant features is often greatly limited by their low contrast differences with the amorphous phase. We present an approach to reveal the organization of the mesomorphic superstructures within such polymeric materials, on the example of electrospun poly(l-lactide) nanofibers. Based on solvent-induced crystallization, this method employs fine-tuned solvent/non-solvent systems to enhance the contrast of these structural features by selectively triggering and controlling reorganization of the phases. Hereby, the mesomorphic regions are transformed into an α-crystalline phase, while the nanoscale spatial arrangement of the underlying superstructures is preserved. Combined with X-ray analytical techniques and electron microscopy, our approach provides detailed insights into the nanofiber's inner architecture, allowing for its direct visualization. Thereby, the influence of electrospinning parameters on the fiber formation process is explained as well as the impact of the resulting non-crystalline superstructures on single fiber mechanical properties. The method can be applied to comparable polymers for the development of materials with controlled, tailored properties.

Controlling the surface structure of electrospun fibers: Effect on endothelial cells and blood coagulation.

The influence of nano- or micron-sized structures on polymer films as well as the impact of fiber diameter of electrospun membranes on endothelial cell (EC) and blood response has been studied for vascular tissue engineering applications. However, the influence of surface structures on micron-sized fibers on endothelial cells and blood interaction is currently not known. In this work, electrospun membranes with distinct fiber surface structures were designed to study their influence on the endothelial cell viability and thrombogenicity. The thermodynamically derived Hansen-solubility-parameters model accurately predicted the formation of solvent dependent fiber surface structured poly(caprolactone) membranes. The electrospun membranes composed of microfibers (MF) or structured MF were of similar fiber diameter, macroscopic roughness, wettability, and elastic modulus. In vitro evaluation with ECs demonstrated that cell proliferation and morphology were not affected by the fiber surface structure. Similarly, investigating the blood response to the fiber meshes showed comparable fibrin network formation and platelet activation on MF and structured MF. Even though the presented results provide evidence that surface structures on MF appear neither to affect EC viability nor blood coagulation, they shed light on the complexity and challenges when studying biology-material interactions. They thereby contribute to the understanding of EC and blood-material interaction on electrospun membranes.

Catechin loaded PLGA submicron-sized fibers reduce levels of reactive oxygen species induced by MWCNT in vitro.

Reactive oxygen species (ROS) are common products of normal aerobic cellular metabolism, but high levels of ROS lead to oxidative stress and cellular damage. Therefore, effective antioxidant therapies are needed to prevent ROS overproduction. This study reports the development of poly(l-lactide-co-glycolide) (PLGA) bicomponent fibers loaded with selected amounts of the natural polyphenolic antioxidant catechin. Thereby a novel route based on emulsion electrospinning is investigated to obtain tailored and sustained release rates for chatechin. The activity of the released catechin was assessed for its influence on multi-walled carbon nanotube (MWCNT) induced formation of reactive oxygen species (ROS) in the human alveolar epithelial the cell line A549. Homogenous fiber morphologies were obtained at specified ranges of PLGA concentrations within the emulsions including the formation of a core - sheath structure localizing the drug within the fiber core. In vitro measurements of the delivery showed moderate burst release kinetics in a first phase followed by a linear and smooth release at long term. In combination with polymer degradation studies a mostly diffusion controlled release mechanism was revealed exhibiting only marginal degradation of the polymer during the time span of the drug delivery. As a proof of concept, the activity of released catechin in A549 cells stimulated with MWCNTs was determined and revealed a high reduction of ROS production in a dose dependent manner. This effect diminishes over time indicating a depletion of catechin.

Steering surface topographies of electrospun fibers: understanding the mechanisms.

A profound understanding of how to tailor surface topographies of electrospun fibers is of great importance for surface sensitive applications including optical sensing, catalysis, drug delivery and tissue engineering. Hereby, a novel approach to comprehend the driving forces for fiber surface topography formation is introduced through inclusion of the dynamic solvent-polymer interaction during fiber formation. Thus, the interplay between polymer solubility as well as computed fiber jet surface temperature changes in function of time during solvent evaporation and the resultant phase separation behavior are studied. The correlation of experimental and theoretical results shows that the temperature difference between the polymer solution jet surface temperature and the dew point of the controlled electrospinning environment are the main influencing factors with respect to water condensation and thus phase separation leading to the final fiber surface topography. As polymer matrices with enhanced surface area are particularly appealing for sensing applications, we further functionalized our nanoporous fibrous membranes with a phosphorescent oxygen-sensitive dye. The hybrid membranes possess high brightness, stability in aqueous medium, linear response to oxygen and hence represent a promising scaffold for cell growth, contactless monitoring of oxygen and live fluorescence imaging in 3-D cell models.

Fabrication of biopolymer-based staple electrospun fibres for nanocomposite applications by particle-assisted low temperature ultrasonication.

We demonstrate the fabrication of staple polymer-based fibres by the ultrasound-assisted processing of electrospun meshes. Bioabsorbable Poly-L-Lactic Acid (PLLA) was electrospun from organic solvent mixtures, yielding continuous fibres with diameters in the range of 244±78 nm. Subsequently, the obtained fibres were sonicated at low temperatures in the presence of nanoparticles in order to obtain fibres with small aspect ratios. The influence of the dispersion medium, the sonication process parameters (temperature and time) and the dimensions of the particles used on the respective length distribution of the obtained nanofibres was investigated. Hexane was identified as an optimal dispersion medium for the system studied in this work. When a cooling bath temperature of 0°C was used, a slight increase in the obtained fibres' average length and distribution was observed as compared to cooling at -80°C (54±43 µm vs 44±31 µm). Moreover, in the presence of hydroxyapatite and hydrophilic and hydrophobic TiO2 nanoparticles in the dispersion medium longer fibres were obtained (44±31 µm, 63±47 µm, and 51±52 µm). Finally, the application of the obtained PLLA-fibre-hydroxyapatite (HA) nanoparticle precursors for the fabrication of a fibre-reinforced Brushite-based cement with high compressive strength is shown. This method of obtaining nanoscaled fibre-reinforced materials opens up a wide range of perspectives for the fabrication of composites for tissue engineering applications.