Clarisse Ribeiro

Influence of processing conditions on polymorphism and nanofiber morphology of electroactive poly(vinylidene fluoride) electrospun membranes

Poly(vinylidene fluoride) (PVDF) electrospun membranes are obtained from a 20% w/w solution of the polymer in dimethyl formamide. Processing parameters are systematically changed to analyze their influence on fiber morphology, fiber orientation, and on the crystallinity and crystal phase of the membranes. The PVDF concentration and travelling distance from the needle to the collector are kept fixed while the influence of the flow rate, needle diameter, applied voltage, and rotating collector speed is considered. The study allows concluding that those parameters leading to a higher stretching of the jet or straining of the fibrils during collection favor the formation of the electroactive β-phase while the total crystalline fraction depends only slightly on the electrospinning conditions.

Piezoelectric polymers as biomaterials for tissue engineering applications

Tissue engineering often rely on scaffolds for supporting cell differentiation and growth. Novel paradigms for tissue engineering include the need of active or smart scaffolds in order to properly regenerate specific tissues. In particular, as electrical and electromechanical clues are among the most relevant ones in determining tissue functionality in tissues such as muscle and bone, among others, electroactive materials and, in particular, piezoelectric ones, show strong potential for novel tissue engineering strategies, in particular taking also into account the existence of these phenomena within some specific tissues, indicating their requirement also during tissue regeneration. This referee reports on piezoelectric materials used for tissue engineering applications. The most used materials for tissue engineering strategies are reported together with the main achievements, challenges and future needs for research and actual therapies. This review provides thus a compilation of the most relevant results and strategies and a start point for novel research pathways in the most relevant and challenging open questions.

Tailoring the morphology and crystallinity of poly(L-lactide acid) electrospun membranes

Biodegradable poly(L-lactic acid) (PLLA) microfibers were prepared by electrospinning by varying the applied potential, solution flow rate and collector conditions. PLLA fibers with smoothly oriented and random morphologies were obtained and characterized by scanning electron microscopy. The optimum fiber orientation was obtained at 1000 rpm using a 20.3 cm diameter collecting drum, while for higher and lower drum rotation speeds, the rapid random motion of the jets resulted in a random fiber distribution. The deformation of the jet with rapid solidification during electrospinning often results in a metastable phase. PLLA electrospun fibers are amorphous but contain numerous crystal nuclei that rapidly grow when the sample is heated to 70-140 °C. In this way, the degree of crystallinity of the fibers can be tailored between 0 and 50% by annealing. Infrared transmission spectra revealed that the processing conditions do not affect the PLLA samples at the molecular level and that the crystallinity of the samples is related to the presence of α-crystals.

Proving the suitability of magnetoelectric stimuli for tissue engineering applications

A novel approach for tissue engineering applications based on the use of magnetoelectric materials is presented. This work proves that magnetoelectric Terfenol-D/poly(vinylidene fluoride-co-trifluoroethylene) composites are able to provide mechanical and electrical stimuli to MC3T3-E1 pre-osteoblast cells and that those stimuli can be remotely triggered by an applied magnetic field. Cell proliferation is enhanced up to ≈ 25% when cells are cultured under mechanical (up to 110 ppm) and electrical stimulation (up to 0.115 mV), showing that magnetoelectric cell stimulation is a novel and suitable approach for tissue engineering allowing magnetic, mechanical and electrical stimuli.

Electrosprayed poly(vinylidene fluoride) microparticles for tissue engineering applications

Poly(vinylidene fluoride) (PVDF) microparticles have been produced by electrospraying as a suitable substrate for tissue engineering applications. The influence of the polymer solution concentration and processing parameters, such as electric field, flow rate and inner needle diameter, on microparticle size and distribution has been studied. Polymer concentration is the most influential parameter on PVDF microparticle formation. Higher concentrations promote the formation of fibers while dilute or semi dilute concentrations favor the formation of PVDF microparticles with average diameters ranging between 0.81 ± 0.34 and 5.55 ± 2.34 μm. Once the formation of microparticles is achieved, no significant differences were found with the variation of other electrospray processing parameters. The electroactive β-phase content, between 63 and 74%, and the crystalline phase content, between 45 and 55%, are mainly independent of the processing parameters. Finally, MC-3T3-E1 cell adhesion on the PVDF microparticles is assessed, indicating their potential use for biomedical applications.

Fibronectin adsorption and cell response on electroactive poly(vinylidene fluoride) films.

Due to the large potential of electroactive materials in novel tissue engineering strategies, the aim of this work is to determine if the crystalline phase and/or the surface electrical charge of electroactive poly(vinylidene fluoride), PVDF, have influence on the biological response in monolayer cell culture. Non-polar α-PVDF and electroactive β-PVDF were prepared. The β-PVDF films were poled by corona discharge to show negative or positive electrical surface charge density. It has been concluded that hydrophilicity of the PVDF substrates depends significantly on crystalline phase and polarity. Furthermore, by means of atomic force microscopy and an enzyme-linked immunosorbent assay test, it has been shown that positive or negative poling strongly influences the behavior of β-PVDF supports with respect to fibronectin (FN) adsorption, varying the exhibition of adhesion ligands of adsorbed FN. Culture of MC3T3-E1 pre-osteoeblasts proved that cell proliferation depends on surface polarity as well. These results open the viability of cell culture stimulation by mechanical deformation of a piezoelectric substrate that results in varying electrical charge densities on the substrate surface.

Enhancement of adhesion and promotion of osteogenic differentiation of human adipose stem cells by poled electroactive poly(vinylidene fluoride)

Poly(vinylidene fluoride) (PVDF) is a biocompatible material with excellent electroactive properties. Nonelectroactive α-PVDF and electroactive β-PVDF were used to investigate the substrate polarization and polarity influence on the focal adhesion (FA) size and number as well as on human adipose stem cells (hASCs) differentiation. hASCs were cultured on different PVDF surfaces adsorbed with fibronectin and FA size and number, total adhesion area, cell size, cell aspect ratio and FA density were estimated using cells expressing vinculin fused to enhanced green fluorescent protein. Osteogenic differentiation was also determined using a quantitative alkaline phosphatase assay. The surface charge of the poled PVDF films (positive or negative) influenced the hydrophobicity of the samples, leading to variations in the conformation of adsorbed extracellular matrix proteins, which ultimately modulated the stem cell adhesion on the films and induced their osteogenic differentiation.

Piezoelectric poly(vinylidene fluoride) microstructure and poling state in active tissue engineering

Tissue engineering strategies rely on suitable membranes and scaffolds, providing the necessary physicochemical stimuli to specific cells. This review summarizes the main results on piezoelectric polymers, in particular poly(vinylidene fluoride), for muscle and bone cell culture. Further, the relevance of polymer microstructure and surface charge on cell response is demonstrated. Together with the necessary biochemical cues, the proper design of piezoelectric polymers can open the way to novel and more reliable tissue engineering strategies for cells in which electromechanical stimuli are present in their environment.

Osteoblast, fibroblast and in vivo biological response to poly(vinylidene fluoride) based composite materials

Electroactive materials can be taken to advantage for the development of sensors and actuators as well as for novel tissue engineering strategies. Composites based on poly(vinylidene fluoride), PVDF, have been evaluated with respect to their biological response. Cell viability and proliferation were performed in vitro both with Mesenchymal Stem Cells differentiated to osteoblasts and Human Fibroblast Foreskin 1. In vivo tests were also performed using 6-week-old C57Bl/6 mice. It was concluded that zeolite and clay composites are biocompatible materials promoting cell response and not showing in vivo pro-inflammatory effects which renders both of them attractive for biological applications and tissue engineering, opening interesting perspectives to development of scaffolds from these composites. Ferrite and silver nanoparticle composites decrease osteoblast cell viability and carbon nanotubes decrease fibroblast viability. Further, carbon nanotube composites result in a significant increase in local vascularization accompanied an increase of inflammatory markers after implantation.

Thermal Properties of Electrospun Poly(Lactic Acid) Membranes

Poly(lactic acid) (PLA) electrospun membranes were obtained by electrospinning and characterized by scanning electron microscopy (SEM) and thermal analysis. The polymer membranes showed a random fiber distribution with a mean diameter of 1 μm (±300 nm). Differential scanning calorimetry (DSC) experiments showed that the membranes had a glass transition, cold crystallization, and melting temperatures of 54, 90, and 151°C, respectively. The kinetic study of thermal degradation of PLA under a nitrogen atmosphere was performed by means of thermogravimetry (TGA). It was found that the thermal decomposition kinetics of PLA could be interpreted in terms of a multi-step degradation mechanism. Several theoretical models were applied to the TGA data. The activation energies obtained by the Broido and Ozawa–Flynn–Wall (OFW) models were in good agreement with the value of the activation energy calculated by the Kissinger method.