Hydrodynamic energy harvesting using an ionic polymer-metal composite stack for underwater applications

Ionic Polymer Metal Composites (IPMCs) are a class of Electro-Active Polymers (EAPs) consisting of a base polymer (usually Nafion), sandwiched between thin films of electrodes and an electrolyte. Apart from fuel cell like proton exchange process in Nafion, these IPMCs can act both as an actuator and a sensor. Typically, IPMCs have been known for their applications in fuel cell technology and in artificial muscles for robots. However, more recently, sensing properties of IPMC have opened up possibilities of mechanical energy harvesting. In this paper, we consider a bi-layer stack of IPMC membranes where fluid flow induced cyclic oscillation allows collection of electronic charge across a pair of functionalized electrode on the surface of IPMC layers/stacks. IPMCs work well in hydrated environment; more specifically, in presence of an electrolyte, and therefore, have great potential in underwater applications like hydrodynamic energy harvesting. Hydrodynamic forces produce bending deformation, which can induce transport of cations via polymer chains of the base polymer of Nafion or PTFE. In our experimental set-up, the deformation is induced into the array of IPMC membranes immersed in electrolyte by water waves caused by a plunger connected to a stepper motor. The frequency and amplitude of the water waves is controlled by the stepper motor through a micro-controller. The generated electric power is measured across a resistive load. Few orders of magnitude increase in the harvested power density is observed. Analytical modeling approach used for power and efficiency calculations are discussed. The observed electro-mechanical performance promises a host of underwater energy harvesting applications.

On the toughness enhancement in hydroxyapatite-based composites

Among various biologically compatible materials, hydroxyapatite (HA) has excellent bioactivity/osteointegration properties and therefore has been extensively investigated for biomedical applications. However, its inferior fracture toughness limits the wider applications of monolithic HA as a load-bearing implant. To this end, HA-based biocomposites have been developed to improve their mechanical properties (toughness and strength) without compromising biocompatibility. Despite significant efforts over last few decades, the toughness of HA-based composites could not be enhanced beyond 1.5-2 MPa m(1/2), even when measured using indentation techniques. In this perspective, the present work demonstrates how spark plasma sintering can be effectively utilized to develop hydroxyapatite titanium (HA-Ti) composites with varying amounts of Ti (5, 10 and 20 wt.%) with extremely high single edge V-notch beam fracture toughness (4-5 MPa m(1/2)) along with a good combination of elastic modulus and flexural strength. Despite predominant retention of HA and Ti, the combination of critical analysis of X-ray diffraction and transmission electron microscopy investigation confirmed the formation of the CaTi4(PO4)(6) phase with nanoscale morphology at the HA/Ti interface and the formation of such a phase has been discussed in reference to possible sintering reactions. The variations in the measured fracture toughness and work of fracture with Ti addition to the HA matrix were further rationalized using the analytical models of crack bridging as well as on the basis of the additional contribution from crack deflection. The present work opens up the opportunity to further enhance the toughness beyond 5 MPa m(1/2) by microstructural designing with the desired combination of toughening phases. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Fretting wear behaviour of hydroxyapatite-titanium composites in simulated body fluid, supplemented with 5 g l(-1) bovine serum albumin

Damaged articulating joints can be repaired or replaced with synthetic biomaterials, which can release wear debris due to articulation, leading to the osteolysis. In a recent work, it has been shown that it is possible to achieve a better combination of flexural strength/fracture toughness as well as in vitro bioactivity and cytocompatibility properties in spark plasma sintered hydroxyapatite-titanium (HA-Ti) composites. Although hydroxyapatite and titanium are well documented for their good biocompatibility, nanosized hydroxyapatite (HA) and titanium (Ti) particles can cause severe toxicity to cells. In order to address this issue, fretting wear study of HA-Ti composites under dry and wet (1x SBF, supplemented with 5 g l(-1) bovine serum albumin (BSA)) condition was performed to assess the wear resistance as well as wear debris formation, in vitro. The experimental results reveal one order of magnitude lower wear rate for HA-10 wt% Ti (7.5 x 10(-5) mm(3) N-1 m(-1)) composite than monolithic HA (3.9 x 10(-4) mm(3) N-1 m(-1)) in simulated body fluid. The difference in the tribological properties has been analyzed in the light of phase assemblages and mechanical properties. Overall, the results suggest the potential use of HA-Ti composites over existing HA-based biocomposites in orthopedic as well as dental applications.

Electromagnetic Characteristics of Polyaniline/SWCNT Composites

Electromagnetic field interactions with the composites made up of polyaniline (PANI) and single wall carbon nanotube (SWCNT) are simulated using the discrete dipole approximation. Recent observations on polymer nano-composites explain the interface interactions between the PANI host and the carbon nanostructures. These types of composite have potential applications in organic solar cell, gas sensor, bio-sensor and electro-chromic devices. Various nanostructures of PANI is possible in the form of nanowires, nanodisks, nanofibers and nanotubes have been reported. In the present study, we considered two types of composite, one is the PANI wrapped CNT and the other is CNT immersed in PANI nanotube. We use Modified Thole's parameters for calculating frequency dependent atomic polarizability of composites. Absorption spectra of the composites are studied by illuminating a wide range of electromagnetic energy spectrum. From the absorption spectra, we observe plasmon excitation in near-infrared region similar to that in SWCNTs reported recently. The interactions between the PANI and CNT in the composite, resulting electromagnetic absorptions are simulated.

Multifunctional Properties of Multistage Spark Plasma Sintered HA-BaTiO3-Based Piezobiocomposites for Bone Replacement Applications

This work reports the processing-microstructure-property correlation of novel HA-BaTiO3-based piezobiocomposites, which demonstrated the bone-mimicking functional properties. A series of composites of hydroxyapatite (HA) with varying amounts of piezoelectric BaTiO3 (BT) were optimally processed using uniquely designed multistage spark plasma sintering (SPS) route. Transmission electron microscopy imaging during in situ heating provides complementary information on the real-time observation of sintering behavior. Ultrafine grains (0.50m) of HA and BT phases were predominantly retained in the SPSed samples. The experimental results revealed that dielectric constant, AC conductivity, piezoelectric strain coefficient, compressive strength, and modulus values of HA-40wt% BT closely resembles with that of the natural bone. The addition of 40wt% BT enhances the long-crack fracture toughness, compressive strength, and modulus by 132%, 200%, and 165%, respectively, with respect to HA. The above-mentioned exceptional combination of functional properties potentially establishes HA-40wt% BT piezocomposite as a new-generation composite for orthopedic implant applications.

Improved magneto-electric response in Na0.5Bi0.5TiO3-MnFe2O4 composites

Magneto-electric composites comprising Na0.5Bi0.5TiO3 (NBT) and MnFe2O4 (MFO) were fabricated using their fine powders obtained via sol-gel method. X-ray diffraction and scanning electron microscopy results confirmed the single-phase formation of NBT and MFO and the composite nature when these were mixed and sintered at appropriate temperatures. The dielectric constant (epsilon(r)) and dielectric loss (D) decreased with increase in frequency (40-110 MHz). Room temperature magnetization measurements established these composites to be soft magnetic. Further, the nature of these composites were established to be magneto-electric at 300 K. The highest ME response of 0.19 % was observed in 30NBT-70MFO composite. The ME coefficient (alpha) was 240 mV/cm Oe for the same composition. The present study demonstrated the effectiveness of NBT/MFO as a lead-free multiferroic composite and provides an alternative for environment-friendly ME device applications.

Bactericidal effect of silver-reinforced carbon nanotube and hydroxyapatite composites

Bacterial infection remains an important risk factor after orthopedic surgery. The present paper reports the synthesis of hydroxyapatite-silver (HA-Ag) and carbon nanotube-silver (CNT-Ag) composites via spark plasma sintering (SPS) route. The retention of the initial phases after SPS was confirmed by phase analysis using X-ray diffraction and Raman spectroscopy. Energy dispersive spectrum analysis showed that Ag was distributed uniformly in the CNT/HA matrix. The breakage of CNTs into spheroid particles at higher temperatures (1700 degrees C) is attributed to the Rayleigh instability criterion. Mechanical properties (hardness and elastic modulus) of the samples were evaluated using nanoindentation testing. Ag reinforcement resulted in the enhancement of hardness (by similar to 15%) and elastic modulus (similar to 5%) of HA samples, whereas Ag reinforcement in CNT, Ag addition does not have much effect on hardness (0.3 GPa) and elastic modulus (5 GPa). The antibacterial tests performed using Escherichia coli and Staphylococcus epidermidis showed significant decrease (by similar to 65-86%) in the number of adhered bacteria in HA/CNT composites reinforced with 5% Ag nanoparticles. Thus, Ag-reinforced HA/CNT can serve as potential antibacterial biocomposites.

Synthesis and Characterization of CoFe2O4/Polyaniline Nanocomposites for Electromagnetic Interference Applications

The Cobalt ferrite (CoFe2O4) powders were synthesized by Co-precipitation method. The as prepared ferrite powders were incorporated into a polyaniline matrix at various volumetric ratios. The as prepared composites of ferrite and polyaniline powders were characterized using X-ray diffraction (XRD), transmission electron microscope (TEM). The particle size of CoFe2O4 is found to be 20 nm. The saturation magnetization (M-s) of all the composites was found to be decreasing with decrease of ferrite content, while coercivity (H-c) remained at the value corresponding to pure cobalt ferrite nanopowders. The complex permittivity (epsilon' and epsilon `') and permeability (mu' and mu `') of composite samples were measured in the range of 1 MHz to 1.1 GHz. The value of epsilon' and mu' found to be increased with ferrite volume concentration.

Amine-functionalized multiwall carbon nanotubes impart osteoinductive and bactericidal properties in poly(epsilon-caprolactone) composites

Poly(epsilon-caprolactone) (PCL) is an aliphatic polyester widely used for biomedical applications but lacks the mechanical properties desired for many load-bearing orthopedic applications. The objective of this study was to prepare and characterize PCL composites incorporating multiwall carbon nanotubes (MWNTs) with different surface functional groups. PCL composites were prepared by melt-mixing with three different types of MWNTs: pristine (pMWNT), amine functionalized (aMWNT), and carboxyl functionalized (cMWNT). Melt rheology and scanning electron microscopy indicated good dispersion of the nanotubes in the matrix. Tensile strength and elastic modulus of the polymer was significantly increased by the incorporation of MWNTs and further enhanced by favorable interactions between PCL and aMWNTs. Thermal analysis revealed that MWNTs act as heterogeneous nucleation sites for crystallization of PCL and increase polymer crystallinity. Incorporation of functionalized MWNTs increased the surface water wettability of PCL. Osteoblast proliferation and differentiation was significantly enhanced on functionalized composites. aMWNT composites also exhibited the best bactericidal response. This study demonstrates that surface functionalization of MWNTs profoundly influences the properties of PCL and amine-functionalization offers the optimal combination of mechanical properties, osteogenesis and antimicrobial response. These results have important implications for designing nanocomposites for use in orthopedics.

In vitro osteogenic cell proliferation, mineralization, and in vivo osseointegration of injection molded high-density polyethylene-based hybrid composites in rabbit animal model

The present work reports the biocompatibility property of injection molded HDPE-HA-Al2O3 hybrid composites. In vitro cytocompatibility results reveal that osteogenic cell viability and bone mineralization are favorably supported in a statistically significant manner on HDPE-20% HA-20% Al2O3 composite, in comparison to HDPE-40 wt.% HA or HDPE-40 wt.% Al2O3. The difference in cytocompatibility property is explained in terms of difference in substrate wettability/surface energy and importantly, both the cell proliferation at 7 days or bone mineralization at 21 days on HDPE-20% HA-20% Al2O3 composite are either comparable or better than sintered HA. The progressive healing of cylindrical femoral bone defects in rabbit animal model was assessed by implantation experiments over 1, 4 and 12 weeks. Based on the histological analysis as well as histomorphometrical evaluation, a better efficacy of HDPE-20% HA-20% Al2O3 over high-density polyethylene (HDPE) for bone regeneration and neobone formation at host bone-implant interface was established. Taken together, the present study unequivocally establishes that despite the presence of 20% Al2O3, HDPE-based hybrid composites are as biocompatible as HA in vitro or better than HDPE in vivo.

Perovskite ceramic nanoparticles in polymer composites for augmenting bone tissue regeneration

There is increasing interest in the use of nanoparticles as fillers in polymer matrices to develop biomaterials which mimic the mechanical, chemical and electrical properties of bone tissue for orthopaedic applications. The objective of this study was to prepare poly(epsilon-caprolactone) (PCL) nanocomposites incorporating three different perovskite ceramic nanoparticles, namely, calcium titanate (CT), strontium titanate (ST) and barium titanate (BT). The tensile strength and modulus of the composites increased with the addition of nanoparticles. Scanning electron microscopy indicated that dispersion of the nanoparticles scaled with the density of the ceramics, which in turn played an important role in determining the enhancement in mechanical properties of the composite. Dielectric spectroscopy revealed improved permittivity and reduced losses in the composites when compared to neat PCL. Nanofibrous scaffolds were fabricated via electrospinning. Induction coupled plasma-optical emission spectroscopy indicated the release of small quantities of Ca+2, Sr+2, Ba+2 ions from the scaffolds. Piezo-force microscopy revealed that BT nanoparticles imparted piezoelectric properties to the scaffolds. In vitro studies revealed that all composites support osteoblast proliferation. Expression of osteogenic genes was enhanced on the nanocomposites in the following order: PCL/CT>PCL/ST>PCL/BT>PCL. This study demonstrates that the use of perovskite nanoparticles could be a promising technique to engineer better polymeric scaffolds for bone tissue engineering.

Supercapacitors based on composites of PANI with nanosheets of nitrogen-doped RGO, BC1.5N, MoS2 and WS2

Performance of supercapacitors based on 1:1 (by weight) composites of polyaniline (PANI) with nanosheets of nitrogenated reduced graphene oxide (NRGO), BC1.5N, MoS2 and WS2 has been investigated in detail. The highest specific capacitance is found with the 1:1 NRGO-PANI composite, the value being 561 F/g at a current density of 0.2 A/g. All the 1:1 nanocomposites show good cyclability. Increasing the PANI content increases the specific capacitance and the highest value found being 715 F/g at a current density of 0.5 A/g in the case of the 1:6 NRGO-PANI composite. However, all the 1:6 composites show a marked decrease in specific capacitance with increase in current density. The energy density of 1:6 NRGO-PANI is similar to 25 Wh/Kg at 0.5 A/g and 1:1 NRGO-PANI is similar to 19 Wh/Kg at 0.2 A/g. NRGO-PANI composites clearly stand out as viable materials for practical applications. (C) 2014 Elsevier Ltd. All rights reserved.

Dynamic compression behavior of reactive spark plasma sintered ultrafine grained (Hf, Zr)B-2-SiC composites

This paper reports the dynamic compression behavior of ultrafine grained (Hf, Zr)B-2-SiC composites, sintered using reactive spark plasma sintering at 1600 degrees C for 10 min. Dynamic strength of similar to 2.3 GPa has been measured using Split Hopkinson Pressure Bar (SHPB) tests in a reproducible manner at strain rates of 800-1300 s(-1). A comparison with competing boride based armor ceramics, in reference to the spectrum of properties evaluated, establishes the potential of (Hf, Zr)B-2-SiC composites for armor applications. (C) 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Tailoring the interface in graphene/thermoset polymer composites: A critical review

With the emergence of scientific interest in graphene oxide (GO) in recent times, researchers have endeavored to incorporate GO in thermoset polymeric matrix to develop composites with extraordinary set of properties. The current state of research in graphene/thermoset polymer composites is highlighted here with a focus on the role of interface in dictating the overall properties of the composites. Different strategies like covalent and non-covalent functionalization of GO have been discussed with respect to improvement in mechanical, electrical, thermal and rheological properties. In addition, future prospects have been outlined. By assessing the current state of research in graphene/thermoset composites, it is obvious that graphene derivatives are promising materials in enhancing the structural properties of the nanocomposites at extremely low levels of filler loading. This opens new avenues in designing lightweight composites for myriad applications and by tailoring the interfacial adhesion with the polymer, ordered structure can be achieved at macroscopic processing scales. (C) 2015 Elsevier Ltd. All rights reserved.

Twinning induced enhancement of fracture toughness in ultrafine grained Hydroxyapatite-Calcium Titanate composites

Despite being highly bioactive and biocompatible, the limitations of monolithic hydroxyapatite (HA) include extremely low fracture toughness, poor electrical conductivity. While addressing these issues, the present study demonstrates how CaTiO3 (CT) addition to HA can be utilized to obtain a combination of long crack fracture toughness (1.7 MPa m(1/2) SEVNB technique) and flexural strength of 98-155 MPa (3-point bending) and a moderate tensile strength (diametral compression) of 17-36 MPa. The enhancement in fracture resistance in spark plasma sintered HA-CT composites has been explained in reference to the observed twin morphology. TEM reveals the presence of twins in CT grains due to 1800 rotation about 101]. The measured properties along with our earlier reports on biocompatibility and electrical properties make HA-CT suitable for bone tissue engineering applications. When compared with other competing HA-based biocomposites, HA-CT composites are found to have a better combination of properties useful for medium load bearing implant applications. (C) 2015 Elsevier Ltd. All rights reserved.