Energy harvesting from the environment based on electroactive polymers has been increasing in recent years. Ferroelectric polymers are used as mechanical-to-electrical energy transducers in a wide range of applications, scavenging the surrounding energy to power low-power devices. These energy-harvesting systems operate by taking advantage of the piezoelectric, pyroelectric, and magnetoelectric properties of the polymers, harvesting wasted environmental energy and converting it mainly into electrical energy. There have been developed different nano- and micro-scale power harvesters with an increasing interest for powering mobile electronics and low-power devices, including applications in remote access areas. Novel electronic devices are developed based on low-power solutions, and therefore, polymer-based materials represent a suitable solution to power these devices. Among the different polymers, the most widely used in the device application is the poly(vinylidene fluoride) (PVDF) family, due to its higher output performance.

In this work, we have prepared intergrowth of multiferroic compounds namely Bi4RTi3Fe0.7Co0.3O15-Bi3RTi2Fe0.7Co0.3O12 (BRTFCO15-BRTFCO12) (rare earth (R) = Dy, Sm, La) by solid-state reaction method. From the X-ray diffraction Rietveld refinement, the structure of the intergrowths was found to be orthorhombic in which satisfactory fittings establish the existence of three-layered (space group: b 2 c b) and four-layered compounds (space group: A21am). Analysis of magnetic measurements confirmed a larger magnetization for theSm-modified intergrowth compound (BSTFCO15-BSTFCO12) compared to Dy- and La-doped ones. The emergence of higher magnetic properties can be due to distortion in the unit cell when some Bi3+ ions are replaced with the Sm3+, bonding of Fe3+-O-Co3+ as well as a possible mixture of FexCoy-type nanoparticles that are formed generally in the synthesis of intergrowths. The changes in the magnetic state of the Aurivillius intergrowths have been reflected in the magnetoelectric (ME) coupling: higher ME coefficient (~30 mV/Cm-Oe) at lower magnetic fields and is constant up to 3 kOe. The results were corroborated by Raman spectroscopy and variation of temperature with magnetization data. The results revealed that the RE-modified intergrowth route is an effective preparative method for higher-layer Aurivillius multiferroic ceramics.

Composites consisting on poly(vinylidene fluoride), PVDF, and nano-size NiZnFe2O4 ferrites have been prepared by solution casting. The addition of NiZnFe2O4 nucleates the polar β-phase of PVDF, leading to magnetoelectric composites suitable for sensor and actuator applications. The amount of β-phase increases with increasing amount of nanoparticles up to filler concentrations of ~5%wt. For this concentration almost 90% of the crystalline phase of the polymer is in the electroactive β-phase. The nucleation occurs due to epitaxial crystallization at the nanoparticle surface, leading to variations in the crystallization kinetics and to the stabilization of the β-phase.

Thin film nanogranular composites of cobalt ferrite (CoFe2O4) dispersed in a barium titanate (BaTiO3) matrix were deposited by laser ablation with different cobalt ferrite concentrations (x). Their structural and magnetic properties were characterized. The films were polycrystalline and composed by a mixture of tetragonal-BaTiO3 and CoFe2O4 with the cubic spinel structure. A slight (111) barium titanate phase orientation and (311) CoFe2O4 phase orientation were observed. The lattice parameter of the CoFe2O4 was always smaller than the bulk value indicating that the cobalt ferrite was under compressive stress. From atomic force microscopy a broad distribution of grain sizes was observed in the nanocomposites, with a significant amount of smaller grains (<40nm) from the CoFe2O4 phase. The magnetic measurements show an increase of the magnetic moment from the low concentration region where the magnetic grains are more isolated and their magnetic interaction is small, towards the bulk value for higher CoFe2O4 content in the films. A corresponding decrease of coercive field with increasing cobalt ferrite concentration was also observed, due to the higher inter-particle magnetic interaction (and reduced stress) of the agglomerated grains.

A regeneração no sistema nervoso central (CNS) é um processo complexo de reparação ou substituição do tecido neural danificado após lesão. Ao contrário do sistema nervoso periférico, o CNS tem capacidade regenerativa limitada, com desafios significativos para a recuperação. Apesar dos obstáculos, estudos já efetuados identificaram mecanismos regenerativos e potenciais intervenções terapêuticas. As células estaminais mesenquimatosas (MSCs) têm sido apresentadas como uma abordagem terapêutica promissora para regeneração no CNS. Embora as MSCs tenham potencial para uso clínico, existem algumas limitações em relação à sua administração e segurança, bem como preocupações éticas. Por outro lado, o secretoma das MSCs, composto principalmente por fatores solúveis e vesículas, poderia servir como um substituto viável. Dado o potencial regenerativo do secretoma, têm-se desenvolvido abordagens terapêuticas inovadoras, nas quais os biomateriais têm emergido como sistemas promissores para sua a liberação no local da lesão de forma eficiente. Além disso, os biomateriais podem ser desenhados para terem propriedades específicas e interagir com células para otimizar o processo regenerativo. Os materiais piezoelétricos são uma classe promissora de materiais que podem gerar uma tensão elétrica em resposta a deformações mecânicas, influenciando de forma positiva os processos de regeneração. Desta forma, abordagens combinatórias com diferentes biomateriais e fatores biológicos como o secretoma têm emergido como sistemas promissores para terapias regenerativas. O objetivo principal desta tese consiste em desenvolver um biomaterial piezoelétrico na forma de hidrogel contendo o secretoma, e avaliar seu impacto nas populações neuronais e gliais, para avaliar o seu potencial regenerativo. Numa primeira abordagem, estabelecemos uma prova de conceito in vitro do efeito de filmes piezelétricos na promoção de diferenciação neuronal. Posteriormente, demonstrámos que microesferas magnetoelétricas afetam o crescimento e a diferenciação de neurónios, levam à ativação da microglia e melhoram a viabilidade celular após uma lesão in vitro induzida por ROS em culturas de medula espinhal. De seguida, ao incorporar essas microesferas em hidrogéis peptídicos contendo o secretoma, demonstrámos que o hidrogel recém-criado libertou o secretoma gradualmente, o que promoveu aumento da extensão das neurites e teve um efeito nas populações gliais. Em suma, este trabalho contribuiu para o desenvolvimento de novos sistemas de libertação de secretoma contendo materiais magnetoelétricos, que podem oferecer novos caminhos para abordagens terapêuticas focadas na regeneração do CNS.

In order to obtain a wide- range magnetoelectric (ME) response on a ME nanocomposite that matches industry requirements, Tb0.3Dy0.7Fe1.92 (Terfenol-D)/CoFe2O4/ P(VDF-TrFE) flexible films were produced by the solvent casting technique and their morphologic, piezoelectric, magnetic and magnetoelectric properties were investigated. The obtained composites revealed a high piezoelectric response (approximate to-18 pC N-1) that is independent of the weight ratio between the fillers. In turn, the magnetic properties of the composites were influenced by the composite composition. It was found that the magnetization saturation values decreased with the increasing CoFe2O4 content (from 18.5 to 13.3 emu. g (1)) while the magnetization and coercive field values increased (from 3.7 to 5.5 emu.g (1) and from 355.7 to 1225.2 Oe, respectively) with the increasing CoFe2O4 content. Additionally, the films showed a wide-range dual-peak ME response at room temperature with the ME coefficient increasing with the weight content of Terfenol-D, from 18.6 to 42.3 mV.cm(-1).Oe(-1).

Anisotropic magnetoelectric (ME) effect on a Fe61.6Co16.4Si10.8B11.2/PVDF Fe61.6Co16.4Si10.8B11.2laminate composite has been used for the development of a magnetic field sensor able to detect both magnitude and direction of AC and DC magnetic fields. Accuracy (99% for both AC and DC sensors), linearity (92% for the DC sensor and 99% for the AC sensor) and reproducibility (99% for both sensors) indicate the suitability of the sensor for applications. Further, the sensitivity of the Fe61.6Co16.4Si10.8B11.2/PVDF/ Fe61.6Co16.4Si10.8B11.2 anisotropic magnetic sensor, 15 and 1400 mV.Oe-1 for DC and AC fields, respectively, are the highest reported in the literature for polymer-based ME materials . Such features allied to the flexibility, versatility, lightweight, low cost and low-temperature fabrication lead to the suitability of the developed sensor for magnetic sensor applications.