R. Gonçalves

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.

Development of magnetoelectric CoFe2O4 /poly(vinylidene fluoride) microspheres

Magnetoelectric microspheres based on piezoelectric poly(vinylidene fluoride) (PVDF) and magnetostrictive CoFe2O4 (CFO), a novel morphology for polymer-based ME materials, have been developed by an electrospray process. The CFO nanoparticle content in the (3–7 μm diameter) microspheres reaches values up to 27 wt%, despite their concentration in the starting solution reaching values up to 70 wt%. Additionally, the inclusion of magnetostrictive nanoparticles into the polymer spheres has no relevant effect on the piezoelectric β-phase content (≈60%), crystallinity (40%) and the onset degradation temperature (460–465 °C) of the polymer matrix. The multiferroic microspheres show a maximum piezoelectric response |d33| ≈ 30 pC N−1, leading to a magnetoelectric response of Δ|d33| ≈ 5 pC N−1 obtained when a 220 mT DC magnetic field was applied. It is also shown that the interface between CFO nanoparticles and PVDF (from 0 to 55%) has a strong influence on the ME response of the microspheres. The simplicity and the scalability of the processing method suggest a large application potential of this novel magnetoelectric geometry in areas such as tissue engineering, sensors and actuators.

Novel Anisotropic Magnetoelectric Effect on δ-FeO(OH)/P(VDF-TrFE) Multiferroic Composites

The past decade has witnessed increased research effort on multiphase magnetoelectric (ME) composites. In this scope, this paper presents the application of novel materials for the development of anisotropic magnetoelectric sensors based on δ-FeO(OH)/P(VDF-TrFE) composites. The composite is able to precisely determine the amplitude and direction of the magnetic field. A new ME effect is reported in this study, as it emerges from the magnetic rotation of the δ-FeO(OH) nanosheets inside the piezoelectric P(VDF-TrFE) polymer matrix. δ-FeO(OH)/P(VDF-TrFE) composites with 1, 5, 10, and 20 δ-FeO(OH) filler weight percentage in three δ-FeO(OH) alignment states (random, transversal, and longitudinal) have been developed. Results have shown that the modulus of the piezoelectric response (10-24 pC·N(-1)) is stable at least up to three months, the shape and magnetization maximum value (3 emu·g(-1)) is dependent on δ-FeO(OH) content, and the obtained ME voltage coefficient, with a maximum of ∼0.4 mV·cm(-1)·Oe(-1), is dependent on the incident magnetic field direction and intensity. In this way, the produced materials are suitable for innovative anisotropic sensor and actuator applications.

Magnetoelectric CoFe2O4/polyvinylidene fluoride electrospun nanofibres

Magnetoelectric 0-1 composites comprising CoFe2O4 (CFO) nanoparticles in a polyvinylidene fluoride (PVDF) polymer-fibre matrix have been prepared by electrospinning. The average diameter of the electrospun composite fibres is ∼325 nm, independent of the nanoparticle content, and the amount of the crystalline polar β phase is strongly enhanced when compared to pure PVDF polymer fibres. The piezoelectric response of these electroactive nanofibres is modified by an applied magnetic field, thus evidencing the magnetoelectric character of the CFO/PVDF 0-1 composites.

Understanding nucleation of the electroactive β-phase of poly(vinylidene fluoride) by nanostructures

β-Poly(vinylidene fluoride) (PVDF) is of large technological relevance due to its piezoelectric, pyroelectric and/ferroelectric properties. In this way, a variety of methods have been developed to obtain such electroactive β-phase, being the addition of fillers one of the most popular, upscalable and innovative methods. The electrostatic interaction between negative charged fillers with the CH2 groups having a positive charge density has been the most widely accepted mechanism for the direct formation of polar β-phase on nanocomposites. Nevertheless some controversy remains in this matter as the dominating crystallization into the β-phase within PVDF is sometimes attributed to the interaction between the positively charged surfaces of the fillers and the CF2 dipoles in PVDF. In order to clarify such a controversial issue, this work uses two types of nanostructures, Fe3O4 nanorods and Fe3O4 nanoparticles, with distinct sizes and surface charges to study, isolate and evaluate the effects of the different ion–dipole interactions and shapes on the crystalline structures of PVDF. As a result it is shown that in the case of positive ion–CF2 dipole based β-phase nucleation, and beyond the effect of the intermolecular interactions, the rod-shape optimizes the crystallization in the electroactive conformation, thus promoting current development in PVDF-based electroactive devices.

Electroactive poly(vinylidene fluoride)-based structures for advanced applications

Poly(vinylidene fluoride) (PVDF) and its copolymers are the polymers with the highest dielectric constants and electroactive responses, including piezoelectric, pyroelectric and ferroelectric effects. This semicrystalline polymer can crystallize in five different forms, each related to a different chain conformation. Of these different phases, the β phase is the one with the highest dipolar moment and the highest piezoelectric response; therefore, it is the most interesting for a diverse range of applications. Thus, a variety of processing methods have been developed to induce the formation of the polymer β phase. In addition, PVDF has the advantage of being easily processable, flexible and low-cost. In this protocol, we present a number of reproducible and effective methods to produce β-PVDF-based morphologies/structures in the form of dense films, porous films, 3D scaffolds, patterned structures, fibers and spheres. These structures can be fabricated by different processing techniques, including doctor blade, spin coating, printing technologies, non-solvent-induced phase separation (NIPS), temperature-induced phase separation (TIPS), solvent-casting particulate leaching, solvent-casting using a 3D nylon template, freeze extraction with a 3D poly(vinyl alcohol) (PVA) template, replica molding, and electrospinning or electrospray, with the fabrication method depending on the desired characteristics of the structure. The developed electroactive structures have shown potential to be used in a wide range of applications, including the formation of sensors and actuators, in biomedicine, for energy generation and storage, and as filtration membranes.

Influence of solvent properties on the electrical response of poly(vinylidene fluoride)/NaY composites

Different solvents were used for the preparation of poly(vinylidene fluoride), PVDF, and NaY zeolite composites by solvent casting and melt crystallization. Solvents like N,N-dimethylformamide (DMF), dimethylsulphoxide (DMSO) and triethyl phosphate (TEP) were chosen as they present different dielectric constants and can be encapsulated in the porous structure of NaY zeolite introduced in the PVDF/zeolite composites. The solvent molecules encapsulated induce variations in the dielectric response of the composite films according to the solvent dielectric constant. In this way, the solvent with the higher dielectric constant, DMSO, results in the composite with higher dielectric constant, while the opposite happens with TEP. The solvent molecules modify the distribution of intra zeolite cations increasing the dielectric constant of the composite. The zeolite also contribute to the increase of the d.c. conductivity, which is characterized by a double regime indicated by a breaking voltage, which value decreases when the dielectric constant of the solvent increases.

Synthesis and size dependent magnetostrictive response of ferrite nanoparticles and their application in magnetoelectric polymer-based multiferroic sensors

This work shows that it is possible to synthesize, chemically tailor and optimize the magnetostriction of ferrite nanoparticles and improve the magnetoelectric (ME) response of their multiferroic composites. Thus, Fe3O4 nanoparticles with different sizes have been synthesized by a solvothermal procedure and an oxidative hydrolysis method. The first method results in nanoparticles with 9 nm average size and 167 ppm magnetostriction; the second one in nanoparticles with average sizes of 30 nm and 50 nm and magnetostriction of 17 ppm and 26 ppm, respectively. Furthermore, Fe3O4/P(VDF-TrFE) multiferroic nanocomposites were produced with those nanoparticles, showing ME voltage coefficients (α31) of 920 μV cm−1 Oe−1, 100 μV cm−1 Oe−1 and 150 μV cm−1 Oe−1, respectively, for samples fabricated with nanoparticles with 9 nm, 30 nm and 50 nm average size. The high magnetostrictive response and the biocompatibility of ferrite nanoparticles, together with their anhysteretic magnetic response, enhance their application potential for biomedical applications as well as their use in high performance ME materials for sensors and actuators.

Magnetically Controlled Drug Release System through Magnetomechanical Actuation

A drug release system is developed capable to modulate the drug release kinetics by the application of a magnetic field. Thus, this work reports on the production, characterization, and release kinetics of a poly(l-lactic acid) (PLLA) microporous membrane containing a zeolite (Faujasite) and a magnetic stimuli-sensitive component, magnetostrictive Terfenol-D (TD), for the release of ibuprofen (IBU) as drug model. For membranes containing IBU-loaded zeolites and TD without an applied AC magnetic field, the release kinetics is characterized by a first order release. On the other hand, the application of an AC magnetic field modifies the release profile of the membrane, leading to an increase of the release rate by more than 30%, the magnetically driven release being characterized by a super case-II within the Korsmeyer-Peppas model, indicating a release mainly driven by a swelling or erosion mechanism, induced by the magnetostrictive particles under the applied magnetic field. The increase of the TD w/w from 10% to 20% has as a consequence a decrease in the quantity of IBU released from 79% to 70%; on the contrary, increasing the H AC intensity from 100 to 200 mT promotes an increase on the percentage of IBU released from 67% to 75%.

Silk Fibroin Separators: A Step Toward Lithium-Ion Batteries with Enhanced Sustainability.

Battery separators based on silk fibroin (SF) have been prepared aiming at improving the environmental issues of lithium-ion batteries. SF materials with three different morphologies were produced: membrane films (SF-F), sponges prepared by lyophilization (SF-L), and electrospun membranes (SF-E). The latter materials presented a suitable porous three-dimensional microstructure and were soaked with a 1 M LiPF6 electrolyte. The ionic conductivities for SF-L and SF-E were 1.00 and 0.32 mS cm-1 at 20 °C, respectively. A correlation between the fraction of β-sheet conformations and the ionic conductivity was observed. The electrochemical performance of the SF-based materials was evaluated by incorporating them in cathodic half-cells with C-LiFePO4. The discharge capacities of SF-L and SF-E were 126 and 108 mA h g-1, respectively, at the C/2-rate and 99 and 54 mA h g-1, respectively, at the 2C-rate. Furthermore, the capacity retention and capacity fade of the SF-L membrane after 50 cycles at the 2C-rate were 72 and 5%, respectively. These electrochemical results show that a high percentage of β-sheet conformations were of prime importance to guarantee excellent cycling performance. This work demonstrates that SF-based membranes are appropriate separators for the production of environmentally friendlier lithium-ion batteries.