In Vitro and In Vivo Characterization of PLLA-316L Stainless Steel Electromechanical Devices for Bone Tissue Engineering-A Preliminary Study.

Bone injuries represent a major social and financial impairment, commonly requiring surgical intervention due to a limited healing capacity of the tissue, particularly regarding critical-sized defects and non-union fractures. Regenerative medicine with the application of bone implants has been developing in the past decades towards the manufacturing of appropriate devices. This work intended to evaluate medical 316L stainless steel (SS)-based devices covered by a polymer poly (L-lactic acid) (PLLA) coating for bone lesion mechanical and functional support. SS316L devices were subjected to a previously described silanization process, following a three-layer PLLA film coating. Devices were further characterized and evaluated towards their cytocompatibility and osteogenic potential using human dental pulp stem cells, and biocompatibility via subcutaneous implantation in a rat animal model. Results demonstrated PLLA-SS316L devices to present superior in vitro and in vivo outcomes and suggested the PLLA coating to provide osteo-inductive properties to the device. Overall, this work represents a preliminary study on PLLA-SS316L devices' potential towards bone tissue regenerative techniques, showing promising outcomes for bone lesion support.

Optical properties of hydrothermally synthesised and thermally annealed ZnO/ZnO2 composites.

ZnO/ZnO2 composites grown by hydrothermal synthesis at low temperature (180 °C) and thermally annealed at 300 °C were fully analysed by morphological, structural and optical techniques. X-ray diffraction patterns (XRD) and Raman spectroscopy clearly evidence the presence of both crystalline phases in the ZnO/ZnO2 sample. The differential scanning calorimetry analysis and thermogravimetric profiles indicate an exothermic event with a peak temperature ca. 225 °C, which is accompanied by a 8.5% weight loss, being attributed to the crystallization of ZnO from ZnO2. Upon a thermal annealing treatment at 300 °C the ZnO2 phase was completely converted into ZnO, as measured by XRD and Raman spectroscopy. Photoluminescence investigations reveal that the emission is dominated by a broad band recombination in both samples, due to the overlapping of different emitting centres, and that the peak position of the PL emission is dependent on the excitation density. The ZnO/ZnO2 sample exhibits a widening of the bandgap when compared to the one only containing ZnO, likely related to the presence of the additional ZnO2 phase and suggesting a bandgap energy of ~3.42 eV for this compound. Surface analysis revealed that the sample exhibits a surface area of 90 m2 g-1, which decreases to 30 m2 g-1 after the thermal annealing and the full conversion into ZnO. This difference in the surface area showed particular relevance in the stability of the measured optical properties. Particularly, the intensity of the photoluminescence signal was seen to be higher in the ZnO/ZnO2 sample and strongly dependent on the measurement atmosphere, highlighting its potential to be employed in the fabrication of optical-based sensing systems for environmental applications, namely in gas sensors.

NMR metabolomics to study the metabolic response of human osteoblasts to non-poled and poled poly (L-lactic) acid.

Untargeted nuclear magnetic resonance (NMR) metabolomics was employed, for the first time to our knowledge, to characterize the metabolome of human osteoblast (HOb) cells and extracts in the presence of non-poled or negatively poled poly-L-lactic acid (PLLA). The metabolic response of these cells to this polymer, extensively used in bone regeneration strategies, may potentially translate into useful markers indicative of in vivo biomaterial performance. We present preliminary results of multivariate and univariate analysis of NMR spectra, which have shown the complementarity of lysed cells and extracts in terms of information on cell metabolome, and unveil that, irrespective of poling state, PLLA-grown cells seem to experience enhanced oxidative stress and activated energy metabolism, at the cost of storage lipids and glucose. Possible changes in protein and nucleic acid metabolisms were also suggested, as well as enhanced membrane biosynthesis. Therefore, the presence of PLLA seems to trigger cell catabolism and anti-oxidative protective mechanisms in HOb cells, while directing them towards cellular growth. This was not sufficient, however, to lead to a visible cell proliferation enhancement in the presence of PLLA, although a qualitative tendency for negatively poled PLLA to be more effective in sustaining cell growth than non-poled PLLA was suggested. These preliminary results indicate the potential of NMR metabolomics in enlightening cell metabolism in response to biomaterials and their properties, justifying further studies of the fine effects of poled PLLA on these and other cells of significance in tissue regeneration strategies.

Growth of BiFeO3 thin films by chemical solution deposition: the role of electrodes.

BiFeO3 (BFO) thin films were grown by chemical solution deposition on a range of electrodes to determine their role in controlling the phase formation and microstructure of the films. The crystallization on oxide electrodes followed the sequence: amorphous → Bi2O2(CO3) → perovskite, while those on Pt crystallized directly from the amorphous phase. IrO2 electrodes promoted perovskite phase formation at the lowest temperature and LaNiO3 additionally induced local epitaxial growth. All compositions exhibited fully coherent Fe-rich precipitates within the grain interior of the perovskite matrix, whereas the incoherent Bi2Fe4O9 second phase was also observed at the grain boundaries of BFO grown on Pt electrodes. The latter could be observed by X-ray diffraction as well as transmission electron microscopy (TEM) but coherent precipitates were only observed by TEM, principally evidenced by their Z contrast in annular dark field images. These data have pronounced consequences for the extended use of BFO films under an applied field for actuator, sensor and memory applications.

Alkali Niobate and Tantalate Perovskites as Alternative Photocatalysts.

Alkali tantalates and niobates are listed as important photocatalysts for the development of renewable energy technologies and environmental remediation. Herein, the photocatalytic degradation of methylene blue dye in aqueous solution by using highly crystalline particles with perovskite-like structures, LiTaO3 , LiNbO3 , NaTaO3 , NaNbO3 , KNbO3 , and KTaO3 , is investigated. It is demonstrated that ferroelectric KNbO3 is the most efficient photocatalyst of those tested because it combines an electronic band structure that can respond successfully to UVA light with a relatively high surface energy that enhances the catalytic properties. Additionally, the built-in electric field due to internal polarization of ferroelectric particles may contribute to the unique properties of this functional photocatalyst. This work provides an ideal platform for the rational design of more efficient ferroelectric-based photocatalytic devices.

Unleashing the Full Sustainable Potential of Thick Films of Lead-Free Potassium Sodium Niobate (K0.5Na0.5NbO3) by Aqueous Electrophoretic Deposition.

A current challenge for the fabrication of functional oxide-based devices is related with the need of environmental and sustainable materials and processes. By considering both lead-free ferroelectrics of potassium sodium niobate (K0.5Na0.5NbO3, KNN) and aqueous-based electrophoretic deposition here we demonstrate that an eco-friendly aqueous solution-based process can be used to produce KNN thick coatings with improved electromechanical performance. KNN thick films on platinum substrates with thickness varying between 10 and 15 µm have a dielectric permittivity of 495, dielectric losses of 0.08 at 1 MHz, and a piezoelectric coefficient d33 of ∼70 pC/N. At TC these films display a relative permittivity of 2166 and loss tangent of 0.11 at 1 MHz. A comparison of the physical properties between these films and their bulk ceramics counterparts demonstrates the impact of the aqueous-based electrophoretic deposition (EPD) technique for the preparation of lead-free ferroelectric thick films. This opens the door to the possible development of high-performance, lead-free piezoelectric thick films by a sustainable low-cost process, expanding the applicability of lead-free piezoelectrics.

Pyrochlore and perovskite potassium tantalate: enthalpies of formation and phase transformation.

Alkali niobates and tantalates are currently important lead-free functional oxides. The formation and decomposition energetics of potassium tantalum oxide compounds (K2 O-Ta2 O5 ) were measured by high-temperature oxide melt solution calorimetry. The enthalpies of formation from oxides of KTaO3 perovskite and defect pyrochlores with K/Ta ratio of less than 1 stoichiometry-K0.873 Ta2.226 O6 , K1.128 Ta2.175 O6 , and K1.291 Ta2.142 O6 -were experimentally determined, and the values are (-203.63 ± 2.92) kJ mol(-1) for KTaO3 perovskite, and (-339.54 ± 5.03) kJ mol(-1) , (-369.71 ± 4.84) kJ mol(-1) , and (-364.78 ± 4.24) kJ mol(-1) , respectively, for non-stoichiometric pyrochlores. That of stoichiometric defect K2 Ta2 O6 pyrochlore, by extrapolation, is (-409.87 ± 6.89) kJ mol(-1) . Thus, the enthalpy of the stoichiometric pyrochlore and perovskite at K/Ta=1 stoichiometry are equal in energy within experimental error. By providing data on the thermodynamic stability of each phase, this work supplies knowledge on the phase-formation process and phase stability within the K2 O-Ta2 O5 system, thus assisting in the synthesis of materials with reproducible properties based on controlled processing. Additionally, the relation of stoichiometric and non-stoichiometric pyrochlore with perovskite structure in potassium tantalum oxide system is discussed.

Electrophoretic deposition on nonconducting substrates: a demonstration of the application to microwave devices.

Through the use of a sacrificial carbon layer, this work reports a method of performing electrophoretic deposition (EPD) of thick films on fully nonconducting substrates, overcoming the restricting requirement for EPD of a conducting or partially conducting substrate. As a proof of concept, the method was applied to the development of microwave-thick films on insulating alumina substrates. The key parameter to be controlled is the thickness of the sacrificial carbon layer; this is expected to be a general result for the application of the processing method. The method allows direct patterning of the structure and leads to the potential use of EPD in a far wider range of electronic applications (multilayer ceramic capacitors (MLCCs), low-temperature cofired ceramics (LTTCs), and biotech devices). Furthermore, in conjunction with work reported elsewhere, the development of specific BaNd2Ti5O14 (BNT) thick-film microwave dielectrics opens up a technology platform for a range of high-quality factor (Q) devices. More specifically, 100-µm-thick BNT layers were achieved with a dielectric constant of 149 and Q of 1161 (10 GHz). These materials can now be integrated with tunable dielectrics or dielectrics on metal substrates to provide a platform for devices in the front end of communication systems and cellular base stations.

Unveiling the Role of CNTs in the Phase Formation of One-Dimensional Ferroelectrics.

Carbon nanotubes (CNTs) have the potential to act as templates or bottom electrodes for three-dimensional (3D) capacitor arrays, which utilize one-dimensional (1D) ferroelectric nanostructures to increase the memory size and density. However, growing a ferroelectric on the surface of CNTs is nontrivial. Here, we demonstrate that multiwalled (MW) CNTs decrease the time and temperature for the formation of lead zirconium titanate Pb(Zr1-xTix)O3 (PZT) by ∼100 °C commensurate with a decrease in activation energy from 68 ± 15 to 27 ± 2 kJ/mol. As a consequence, monophasic PZT was obtained at 575 °C for MWCNTs/PZT, but for pure PZT, traces of pyrochlore were still present at 650 °C, where the PZT phase formed due to homogeneous nucleation. The piezoelectric nature of MWCNTs/PZT synthesized at 500 °C for 1 h was proven. Although further work is required to prove the concept of 3D capacitor arrays, our result suggests that it is feasible to utilize MWCNTs as templates/electrodes for the formation of 1D PZT nanoferroelectrics.

Stimuli-Responsive Oligourea Molecular Films.

We have designed and synthesized a helical cysteamine-terminated oligourea foldamer composed of ten urea residues featuring side carboxyl and amine groups. The carboxyl group is located in proximity to the C-terminus of the oligourea and hence at the negative pole of the helix dipole. The amine group is located close to the N-terminus and hence at the positive pole of the helix dipole. Beyond the already remarkable dipole moment inherent in oligourea 2.5 helices, the incorporation of additional charges originating from the carboxylic and amine groups is supposed to impact the overall charge distribution along the molecule. These molecules were self-assembled into monolayers on a gold substrate, allowing us to investigate the influence of an electric field on these polar helices. By applying surface-enhanced infrared reflection-absorption spectroscopy, we proved that molecules within the monolayers tend to reorient themselves more vertically when a negative bias is applied to the surface. It was also found that surface-confined oligourea molecules affected by the external electric field tend to rearrange the electron density at urea groups, leading to the stabilization of the resonance structure with charge transfer character. The presence of the external electric field also affected the nanomechanical properties of the oligourea films, suggesting that molecules also tend to reorient in the ambient environment without an electrolyte solution. Under the same conditions, the helical oligourea displayed a robust piezoresponse, particularly noteworthy given the slender thickness of the monolayer, which measured approximately 1.2 nm. This observation demonstrates that thin molecular films composed of oligoureas may exhibit stimulus-responsive properties. This, in turn, may be used in nanotechnology systems as actuators or functional films, enabling precise control of their thickness in the range of even fractions of nanometers.

Advanced Cellulose-Nanocarbon Composite Films for High-Performance Triboelectric and Piezoelectric Nanogenerators.

Natural polymers such as cellulose have interesting tribo- and piezoelectric properties for paper-based energy harvesters, but their low performance in providing sufficient output power is still an impediment to a wider deployment for IoT and other low-power applications. In this study, different types of celluloses were combined with nanosized carbon fillers to investigate their effect on the enhancement of the electrical properties in the final nanogenerator devices. Cellulose pulp (CP), microcrystalline cellulose (MCC) and cellulose nanofibers (CNFs) were blended with carbon black (CB), carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs). The microstructure of the nanocomposite films was characterized by scanning electron and probe microscopies, and the electrical properties were measured macroscopically and at the local scale by piezoresponse force microscopy. The highest generated output voltage in triboelectric mode was obtained from MCC films with CNTs and CB, while the highest piezoelectric voltage was produced in CNF-CNT films. The obtained electrical responses were discussed in relation to the material properties. Analysis of the microscopic response shows that pulp has a higher local piezoelectric d33 coefficient (145 pC/N) than CNF (14 pC/N), while the macroscopic response is greatly influenced by the excitation mode and the effective orientation of the crystals relative to the mechanical stress. The increased electricity produced from cellulose nanocomposites may lead to more efficient and biodegradable nanogenerators.

Nanoporous piezo- and ferroelectric thin films.

Nanoporous barium titanate and lead titanate thin films (∼100 nm calculated from ellipsometric data) are prepared starting from sol-gel solutions modified with a commercially available block-copolymer and evaporation-induced self-assembly methodology. The tuning of the thermal treatment followed by in situ ellipsometry allows the decomposition of the organic components and of the structuring agent leading to the formation of porous tetragonal crystalline perovskite structures as observed by XRD, HRTEM, SEM, and ellipsoporosimetry. Both nanoporous barium titanate and lead titanate thin films present local piezoelectric and ferroelectric behavior measured by piezoresponse force microscopy (PFM), being promising platforms for the preparation of the generation of new multifunctional systems.

Low-Temperature Dielectric Response of Strontium Titanate Thin Films Manipulated by Zn Doping.

The voltage dependence of the dielectric permittivity ε' and the low dielectric loss tanδ of incipient ferroelectrics have drawn vast attention to the use of these materials for the development of tuning elements in electronics and telecommunications. Here, we study the DC electric field dependence of low-temperature ε' in ~320 nm thick sol-gel-derived SrTi1-xZnxO3-δ thin films with x = 0.01 and 0.05, deposited on Pt/TiO2/SiO2/Si substrates. Incorporation of Zn onto Ti sites is found to decrease ε' compared to undoped SrTiO3 films, while increasing the relative tunability nr up to ~32.9% under a DC electric field of 125 kV/cm at low temperatures. The hysteresis-free variation in ε' with electric field and tanδ values below 0.6% observed for SrTi1-xZnxO3-δ film with x = 0.01 make this compound more attractive for tunable device applications.

High Q Dielectric Titanium Tellurite Thick Films on Alumina Substrates for High Frequency Telecommunications.

The vital role of high-quality-factor (Q) high-frequency (f) dielectric resonators in the growing microwave telecommunication, satellite broadcasting and intelligent transport systems has long motivated the search for new, small size, and lightweight integrated components and packages, prepared by low cost and sustainable processes. One approach is replacing the currently used bulk ceramic dielectrics by thick films of low-sintering-temperature dielectrics fabricated by affordable processes. Here we demonstrate the fabrication of high-Q TiTe3O8 thick films directly on low loss Al2O3 substrates by electrophoretic deposition using sacrificial carbon layer. Nineteen-micrometre-thick TiTe3O8 films on Al2O3 sintered at 700 °C are found to have a relative permittivity εr of 32 and Q × f > 21,000 GHz. Being thus able to measure and provide for the first time the microwave dielectric properties of these films, our results suggest that TiTe3O8 films on Al2O3 substrates are suitable for microlayer microstrip array applications.

Atmosphere-Assisted FLASH Sintering of Nanometric Potassium Sodium Niobate.

The request for extremely low-temperature and short-time sintering techniques has guided the development of alternative ceramic processing. Atmosphere-assisted FLASH sintering (AAFS) combines the direct use of electric power to packed powders with the engineering of operating atmosphere to allow low-temperature conduction. The AAFS of nanometric Potassium Sodium Niobate, K0.5Na0.5NbO3, a lead-free piezoelectric, is of great interest to electronics technology to produce efficient, low-thermal-budget sensors, actuators and piezo harvesters, among others. Not previously studied, the role of different atmospheres for the decrease in FLASH temperature (TF) of KNN is presented in this work. Additionally, the effect of the humidity presence on the operating atmosphere and the role of the compact morphology undergoing FLASH are investigated. While the low partial pressure of oxygen (reducing atmospheres) allows the decrease of TF, limited densification is observed. It is shown that AAFS is responsible for a dramatic decrease in the operating temperature (T < 320 °C), while water is essential to allow appreciable densification. In addition, the particles/pores morphology on the green compact impacts the uniformity of AAFS densification.

Flash Sintered Potassium Sodium Niobate: High-Performance Piezoelectric Ceramics at Low Thermal Budget Processing.

Alternative sintering technologies promise to overcome issues associated with conventional ceramic sintering such as high thermal budgets and CO2 footprint. The sintering process becomes even more relevant for alkali-based piezoelectric ceramics such as K0.5Na0.5NbO3 (KNN) typically fired above 1100 °C for several hours that induces secondary phase formation and, thereby, degrades their electrical characteristics. Here, an ability of KNN ceramics to be of high performance is successfully demonstrated, using an electric field- and current-assisted Flash sintering technique at 900 °C only. Reported for the first time, Flash sintered KNN ceramics have room-temperature remnant polarization Pr = 21 µC/cm2 and longitudinal piezoelectric coefficient d33 = 117 pC/N, slightly superior to that of conventional ones due to the reduced content of secondary phases. High-performance KNN ceramics Flash sintered at a low-thermal budget have implications for the development of innovative low carbon technologies, electroceramics stakeholders, and piezoelectric energy harvesters.

Mechanical writing of electrical polarization in poly (L-lactic) acid.

Stimuli responsive materials are found in a broad range of applications, from energy harvesters to biomolecular sensors. Here, we report the production of poly (L-lactic acid) (PLLA) thin films that exhibit a mechanical stress responsive behaviour. By simply applying a mechanical stress through an AFM tip, a local electrical polarization was generated and measured by Kelvin Probe Force Microscopy. We showed that the magnitude of the stress generated electrical polarization can be manipulated by varying the thickness or crystallization state of the PLLA thin films. Besides exhibiting a mechanical stress-response behaviour with potential for energy harvesting and sensor applications, we show by AFM that these platforms react to mechanical forces with physiological relevance: interaction forces as low as a cell sheet migrating over a substrate or larger ones as the fluid induced stresses in bone tissue. In living tissues, as most mechanical stimuli are transduced as strain gradients for the anatomical structures, these mechanically responsive substrates can be used as ex vivo platforms to study the protein and cells response over a large range of electrical stimuli amplitude. As a proof of concept, selective adsorption of a human fibronectin was demonstrated by local patterning of the stimuli responsive PLLA films. STATEMENT OF SIGNIFICANCE: Bioelectricity is inherent to the formation and repair of living tissues and electrical stimulation has been recognized for promoting regeneration. Given the proven beneficial effects of electric fields and the absence of a suitable method of stimulation, there is a clinical need for smart substrates, which can generate a polarization (charges) to promote tissue regeneration without the need of external devices. In this work, we report the fabrication of poly(L-lactic) acid platforms that exhibit a mechanical stress responsive behaviour when subjected to physiologically relevant forces. This behaviour can be tailored by varying the thickness or crystallization state of the PLLA films. We further demonstrate the biofunctionality of such platforms by exploiting the mechanically-induced charge for adhesion protein adsorption.

Polar Phonon Behaviour in Polycrystalline Bi-Doped Strontium Titanate Thin Films.

Strontium titanate-based materials with ferroelectric or relaxor-like properties have drawn vast attention as polar dielectrics for electronics and telecommunications. Here, we study the lattice dynamics in sol-gel-derived Sr1-1.5xBixTiO3 thin films with x = 0.0053 and 0.167, deposited on Al2O3 substrates, using a variable-temperature far-infrared spectroscopy in a transmittance mode. Bi doping, known to induce a low-frequency dielectric relaxation in SrTiO3 (ST) ceramics and films, due to off-centre dopant ion displacements generating electric dipoles, is shown to affect the polar phonon behaviour of thin films. We show that in weakly Bi-doped films, the low-frequency polar TO1 mode softens on cooling but less than in undoped ST. In heavily Bi-doped ST films, this mode displays no significant frequency variation with temperature from 300 to 10 K. The polar phonon behaviour of polycrystalline Bi-doped ST thin films is comparable with that of Bi-doped ST ceramics, which exhibit dielectric relaxations and harden soft-mode behaviour instead of the ferroelectric phase transition.

Dielectric Relaxation, Local Structure and Lattice Dynamics in Mn-Doped Potassium Tantalate Ceramics.

Alkaline niobate and tantalate perovskites have attracted attention as polar dielectrics for electronics and telecommunications. Here, we studied the polar behaviour, lattice dynamics, and local structure in conventionally processed K0.985Mn0.015TaO3±Î´ ceramics using a combination of variable-temperature dielectric and Raman spectroscopies, and X-ray absorption fine structure (XAFS) measurements, respectively. Mn doping induces a low-frequency dielectric relaxation in KTaO3 (KT), which follows the Arrhenius law with an activation energy U ≈ 105 meV and the characteristic relaxation time τ0 ≈ 4.6 × 10-14 s. Our XAFS results support preferential Mn occupancy of the cuboctahedral sites as Mn2+, with these cations strongly off-centred in the oversized oxygen cages. Such disordered Mn displacements generate electric dipoles, which are proposed as the source of the observed dielectric relaxation. We show that in Mn-doped ceramics, the low-frequency polar TO1 mode softens on cooling and, at low temperatures, exhibits a higher frequency than in undoped KT. This mode displays no detectable splitting, which contrasts with Li-doped KT that also contains off-centred Li+ species on the cuboctahedral sites. Therefore, we conclude that the coupling between the Mn displacements and the lattice is weaker than in the Li case, and Mn-doped KT therefore exhibits a dielectric relaxation but no ferroelectric transition.

Particle Characteristics' Influence on FLASH Sintering of Potassium Sodium Niobate: A Relationship with Conduction Mechanisms.

The considerable decrease in temperature and time makes FLASH sintering a more sustainable alternative for materials processing. FLASH also becomes relevant if volatile elements are part of the material to be processed, as in alkali-based piezoelectrics like the promising lead-free K0.5Na0.5NbO3 (KNN). Due to the volatile nature of K and Na, KNN is difficult to process by conventional sintering. Although some studies have been undertaken, much remains to be understood to properly engineer the FLASH sintering process of KNN. In this work, the effect of FLASH temperature, TF, is studied as a function of the particle size and impurity content of KNN powders. Differences are demonstrated: while the particle size and impurity degree markedly influence TF, they do not significantly affect the densification and grain growth processes. The conductivity of KNN FLASH-sintered ceramics and KNN single crystals (SCs) is compared to elucidate the role of particles' surface conduction. When particles' surfaces are not present, as in the case of SCs, the FLASH process requires higher temperatures and conductivity values. These results have implications in understanding FLASH sintering towards a more sustainable processing of lead-free piezoelectrics.