Functionalized carbon nanotubes as a filler for dielectric elastomer composites with improved actuation performance

Among the broad class of electro-active polymers, dielectric elastomer actuators represent a rapidly growing technology for electromechanical transduction. In order to further develop this applied science, the high driving voltages currently needed must be reduced. For this purpose, one of the most widely considered approaches is based on making elastomeric composites with highly polarizable fillers in order to increase the dielectric constant while maintaining both low dielectric losses and high-mechanical compliance. In this work, multi-wall carbon nanotubes were first functionalized by grafting either acrylonitrile or diurethane monoacrylate oligomers, and then dispersed into a polyurethane matrix to make dielectric elastomer composites. The procedures for the chemical functionalization of carbon nanotubes and proper characterizations of the obtained products are provided in detail. The consequences of the use of chemically modified carbon nanotubes as a filler, in comparison to using unmodified ones, were studied in terms of dielectric, mechanical and electromechanical response. In particular, an increment of the dielectric constant was observed for all composites throughout the investigated frequency spectrum, but only in the cases of modified carbon nanotubes did the loss factor remain almost unchanged with respect to the simple matrix, indicating that conductive percolation paths did not arise in such systems. An effective improvement in the actuation strain was observed for samples loaded with functionalized carbon nanotubes

Crack growth behavior of SBR, NR and BR rubber compounds: comparison of Pure-Shear versus Strip Tensile test

Fatigue crack growth experiments on different carbon black–filled rubber compounds have been carried out to evaluate the influence of pure-shear and strip tensile testing mode by using sine and pulse as waveforms. In a previous set of experimental investigations regarding the influence of both waveform and tested material, it was found that the mode I of crack opening sometimes propagates too quickly to be properly monitored in tests involving strip-tensile specimens. An alternative test methodology based on pure-shear test mode has been investigated, optimizing both the shape of the specimen and the test equipment. Data obtained from the different compound formulations were consistent with the theoretical background and resulted in similar ranking of compound crack growth resistance for the two testing modes; in addition, pure-shear mode showed a higher sensitivity to formula variations

Nanomaterials-based strategies for piezoelectric cochlear implants

Nanomaterials-based strategies for piezoelectric cochlear implant

Design, fabrication and characterization of composite piezoelectric ultrafine fibers for cochlear stimulation

Sensorineural hearing loss, primed by dysfunction or death of hair cells in the cochlea, is the main cause of severe or profound deafness. Piezoelectric materials work similarly to hair cells, namely, as mechano-electrical transducers. Polyvinylidene fluoride (PVDF) films have demonstrated potential to replace the hair cell function, but the obtained piezoresponse was insufficient to stimulate effectively the auditory neurons. In this study, we reported on piezoelectric nanocomposites based on ultrafine PVDF fibers and barium titanate nanoparticles (BTNPs), as a strategy to improve the PVDF performance for this application. BTNP/PVDF fiber meshes were produced via rotating-disk electrospinning, up to 20/80 weight composition. The BTNP/PVDF fibers showed diameters ranging in 0.160-1.325 μm. Increasing collector velocity to 3000 rpm improved fiber alignment. The piezoelectric β phase of PVDF was well expressed following fabrication and the piezoelectric coefficients increased according to the BTNP weight ratio. The BTNP/PVDF fibers were not cytotoxic towards cochlear epithelial cells. Neural-like cells adhered to the composite fibers and, upon mechanical stimulation, showed enhanced viability. Using BTNP filler for PVDF matrices, in the form of aligned ultrafine fibers, increased the piezoresponse of PVDF transducers and favored neural cell contact. Piezoelectric nanostructured composites might find application in next generation cochlear implants

Evaluation of mechanical properties of green wood of Italian stone pine (Pinus pinea L.)

Italian stone pine is a very distinct, big size tree developing a characteristic dense, flat-topped or umbrella-shaped head. It is the most widespread species along the roads of the Tuscany coast, with problems due to plant age, to summer and winter drop branches, to stem failure or tree fall and to wide extension of the root system. This study aims to contribute to a better understanding of the biomechanic parameters necessary to support the visual and analytical techniques currently used for tree safety evaluation. Some mature pine trees were chosen in different places of the Tuscany coast and submitted to a classic visual tree assessment. Stem cross sections were cut in correspondence of the instrumental analyses, wood specimens were prepared and compression tests were performed by recording the response in terms of the resistant strain. The MOE values (1.49 - 2.39 GPa) significantly differed from those observed in other species

Polyurethane unimorph bender microfabricated with Pressure Assisted Microsyringe (PAM) for biomedical applications

This paper describes a new microfabrication technique for bender-type electromechanical actuators made of an elastomeric electroactive polymer. The technique is based on a computer-controlled deposition of the active material with a microsyringe. The paper describes the developed microfabrication system and proposes a simple deposition model. The realization of solid-state unimorph bender actuators made of polyurethane as electrolyte and a mixture of carbon black and polyurethane as electrodes is presented. Prototype actuators fabricated both with the new technique were driven with electrical field of 100 V/μm and showed bending angles higher than 30°. In this way, we have demonstrated that it is possible to fabricate polyurethane based microactuators using a polyurethane/carbon black composite such as device

Synthesis of Bioactive Hydroxyapatite-Zirconia Toughened Composites for Bone Replacement

Hydroxyapatite (HAp) is a major inorganic component of human hard tissues, such as bones and teeth, and its content determines their microstructures and physical properties. Artificial HAp shows strong biocompatibility and bioactivity and thus it has found broad applications in tissue engineering for replacing damaged hard tissues. The artificial HAp, however, suffers from its intrinsic low mechanical properties, so to meet mechanical requirements, HAp can be incorporated with stiff mineral phases (mullite, zirconia, alumina). The performance and long-term survival of these biomedical devices are also dependent on the presence of bacteria surrounding the implants. In order to reduce the incidence of implant-associated infections, several treatments have been proposed, e.g. introduction of silver or fluoride in the HAp. The objective of this research is the sintering of composites based on calcium phosphate, mainly HAp supported on zirconia, for bone replacement with better microstructural features. In fact the use of zirconia can enhance the mechanical properties of bioceramics. Moreover the introduction of small amounts of silver, which should improve the antibacterial properties, will be taken into consideration since it is expected also to further toughen the whole structure

Microwave Technology Applied to a Vulcanized Model Rubber Compound: Reclamation and Recycling Ability

The objective of this work was to investigate the effects of a thermal treatment induced by microwaves (MW) and applied to a simple rubbery model system. This has to be intended as the first step of a study whose final goal is to find an effective, industrially applicable, way to induce partial devulcanization, and improve recycling capabilities, in more complex systems like end-use tyres, whose disposal constitutes a severe environmental issue. The chosen system was based on a styrene-butadiene rubber (SBR) mixed with vulcanization agents. Such “uncured rubber” (UR) was vulcanized under static load to give the reference “vulcanized rubber” (VR), and finally processed in a MW oven to obtain a “treated rubber” (TR). All the materials were characterized in terms of gel-percentage and mechanical properties. In order to evaluate the effects of MW treatment on the recycling capability of the rubber, the UR was loaded with various levels of TR, in a range from 20 to 100 phr, and then subjected to re-vulcanization. Besides the reference VR, also samples obtained by re-vulcanization of UR loaded with cryo-milled vulcanized rubber (CR) were considered for comparison. Rheological, calorimetric, stress-strain and dynamic-mechanical characterizations were performed on all the obtained compounds. By this way it was possible to assess how the addition of either TR or CR to the virgin material influences both the curing process and the properties of the final product. While loaded systems generally showed less attractive mechanical properties than the original VR, the TR-loaded compounds had better performances than those with analogous contents of cryo-milled VR. Moreover, a significant increase of the glass transition temperature was observed in the final material, indicating that the “recycled” part was still able to participate in the second vulcanization process

VISCOELASTIC CHARACTERISATION OF PIG LIVER IN UNCONFINED COMPRESSION

Understanding and modelling liver biomechanics represents a significant challenge due the complex nature of this organ. Several methods and models based on direct measurements on the liver (e.g. rheological, compressive or indentation tests) or image-based techniques (e.g. magnetic resonance or ultrasound-based elastography) are reported in literature to characterise the liver viscoelastic behaviour in-vitro or in-vivo [Marchesseau et al, 2010]. Unfortunately, there is no consensus on liver viscoelastic properties, and results are strongly dependent on adopted testing method, sample type, status and testing conditions. We focused on in-vitro unconfined bulk compressive tests for deriving liver viscoelastic parameters in the linear viscoelastic region (i.e. small strain region). We propose the use of the ε̇M (epsilon dot method) which we developed to address the major drawbacks of standard tests (e.g. step response or dynamic mechanical tests) such as long test duration and initial contact between sample and testing apparatus, that may significantly pre-stress/strain very soft and hydrated samples and alter their status [Tirella et al, submitted]. With the ε̇M, samples are characterised using standard compressive tests at different strain rates (ε̇). Stress-time series collected at various ε̇ are then fitted using a multi curve shared parameter fitting approach. Liver viscoelastic parameters estimated with ε̇M were compared to those obtained using conventional dynamic mechanical (DMA) testing systems