Electrospun poly(vinylidene fluoride-trifluoroethylene)/zinc oxide nanocomposite tissue engineering scaffolds with enhanced cell adhesion and blood vessel formation

Piezoelectric materials that generate electrical signals in response to mechanical strain can be used in tissue engineering to stimulate cell proliferation.Poly (vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)),a piezoelectric polymer,is widely used in biomaterial applications.We hypothesized that incorporation of zinc oxide (ZnO) nanoparticles into the P(VDF-TrFE) matrix could promote adhesion,migration,and proliferation of cells,as well as blood vessel formation (angiogenesis).In this study,we fabricated and comprehensively characterized a novel electrospun P(VDF-TrFE)/ZnO nanocomposite tissue engineering scaffold.We analyzed the morphological features of the polymeric matrix by scanning electron microscopy,and utilized Fourier transform infrared spectroscopy,X-ray diffraction,and differential scanning calorimetry to examine changes in the crystalline phases of the copolymer due to addition of the nanoparticles.We detected no or minimal adverse effects of the biomaterials with regard to blood compatibility in vitro,biocompatibility,and cytotoxicity,indicating that P(VDF-TrFE)/ZnO nanocomposite scaffolds are suitable for tissue engineering applications.Interestingly,human mesenchymal stem cells (hMSCs) and human umbilical vein endothelial cells cultured on the nanocomposite scaffolds exhibited higher cell viability,adhesion,and proliferation compared to cells cultured on tissue culture plates or neat P(VDF-TrFE) scaffolds.Nanocomposite scaffolds implanted into rats with or without hMSCs did not elicit immunological responses,as assessed by macroscopic analysis and histology.Importantly,nanocomposite scaffolds promoted angiogenesis,which was increased in scaffolds pre-seeded with hMSCs.Overall,our results highlight the potential of these novel P(VDF-TrFE)/ZnO nanocomposites for use in tissue engineering,due to their biocompatibility and ability to promote cell adhesion and angiogenesis.     

Electric poling and electromechanical characterization of 0.1-mm-thick sensor films and 0.2-mm-thick cable layers from piezoelectric poly(vinylidene fluoride-trifluoroethylene)

Piezoelectric polymers have been proposed for many sensor and actuator applications. Among these, piezoelectric polymer films with thicknesses between several tens and a few hundreds of micrometers as well as coaxial cables with piezoelectric polymer layers are highly suitable and attractive for the detection of mechanical loads. In addition to good piezoelectric properties, materials for such sensors should have high mechanical strength. Therefore, the most common materials are nonporous piezoelectric polymers, such as polyvinylidene fluoride (PVDF) or its copolymer with trifluoroethylene (P(VDF-TrFE)). Here, P(VDF-TrFE) polymer films as well as the operating principle and the geometry of piezoelectric polymer cables are described. As active piezoelectric cable layer, P(VDF-TrFE) (76/24) was employed. After electrical poling with one or more point-to-cable corona discharges, the polarization in the P(VDF-TrFE) layer was investigated. Poling parameters, such as electric field and poling time, were varied. The resulting polarization was characterized with measurements of polarization profiles across the film or the cable-layer thickness as well as with the determination of other electromechanical quantities. The optimized poling process yields good piezoelectric properties in the piezoelectric polymer layers as well as useful sensor properties of the piezoelectric polymer cable, which are assessed and discussed.     

Enhanced ferroelectric and dielectric properties of the P(VDF-TrFE)/Ag nanoparticles composite thin films

The scope of the present work was the synthesis of homogeneously dispersed silver (Ag) nanoparticles (NPs) in P(VDF-TrFE) polymer by N,N-dimethylformamide’s reducing reaction on silver nitrate and the study on the surface micromorphology, crystalline phases, electrical and optical properties of the P(VDF-TrFE)/Ag NPs composite thin films. The results demonstrate that incorporating appropriate concentration of Ag NPs improve the ferroelectric and dielectric properties with an increase of 38 % in the remanent polarization and 47 % in dielectric constant respectively compared with the pristine P(VDF-TrFE) films. The reasons for the improved properties are explained by the effective compensation to the bounding charges provided by the appropriate amount of the Ag NPs fillers and Ag NPs’ acting as micro capacitors in P(VDF-TrFE) matrix. Furthermore, the surface plasmon resonance absorption in the composite films thin films is observed at the wavelength of ~415 nm, whose intensity is dependent on the density of the Ag NPs.     

Properties of triglycine sulfate/poly(vinylidene fluoride-trifluoroethylene) 0-3 composites

Composites of triglycine sulfate (TGS) powder dispersed in a poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE), 70/30 mol%) copolymer matrix have been prepared using solvent casting followed by compression moulding. The composites have been characterized by means of scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). Three groups of samples have been prepared: unpoled composites, composites with only the TGS phase poled and composites with both phases poled. The observed relative permittivity of these composites are consistent with the predictions of the Bruggeman model. When the TGS and copolymer phases are poled in the same direction, the piezoelectric activities of the two phases partially cancel each other while the pyroelectric activities reinforce. The pyroelectric coefficients exhibit good agreement with the effective-medium model. These composites have high pyroelectric figures of merit which increase as the volume fraction of TGS increases.     

Piezoelectric and pyroelectric properties of PZT/P(VDF-TrFE) composites with constituent phases poled in parallel or antiparallel directions

Composites of lead zirconate titanate (PZT) powder dispersed in a vinylidene fluoride-trifluoroethylene copolymer [P(VDF-TrFE)] matrix have been prepared by compression molding. Three groups of polarized samples have been prepared by poling: only the ceramic phase, the ceramic and polymer phases in parallel directions, and the two phases in antiparallel directions. The measured permittivities of the unpoled composites are consistent with the predictions of the Bruggeman model. The changes in the pyroelectric and piezoelectric coefficients of the poled composites with increasing ceramic volume fraction can be described by modified linear mixture rules. When the ceramic and copolymer phases are poled in the same direction, their pyroelectric activities reinforce while their piezoelectric activities partially cancel. However, when the ceramic and copolymer phases are poled in opposite directions, their piezoelectric activities reinforce while their pyroelectric activities partially cancel.     

PCLT/P(VDF-TrFE) 0–3 nanocomposite thin films for pyroelectric applications

 Nanocrystalline calcium and lanthanum modified lead titanate (PCLT) powder was prepared by a sol-gel process followed by annealing at 850°C. 0–3 nanocomposite thin films of PCLT powder dispersed in a vinylidene fluoride /trifluoroethylene (P(VDF-TrFE)) copolymer matrix were fabricated on glass substrates using the spin-coating technique. The permittivity, pyroelectric coefficient, specific heat and pyroelectric figures-of-merit were measured as functions of the volume fraction (φ) of ceramic for φ up to 0.16. Single-element pyroelectric sensors with PCLT/P(VDF-TrFE) nanocomposites as the sensing elements were fabricated and characterized. The voltage and current responsivities R_v and R_i of the sensors were measured as functions of frequency and found to agree well with the calculated values. Since the pyroelectric coefficients of PCLT and P(VDF-TrFE) have like signs while the piezoelectric coefficients have opposite signs, the poled composite has high pyroelectric but low piezoelectric activity, thereby reducing the vibration-induced electrical noise in pyroelectric sensor applications. Received: 21 July 1998 / Reviewed and accepted: 22 October 1998     

The effect of processing conditions on the morphology, thermomechanical, dielectric, and piezoelectric properties of P(VDF-TrFE)/BaTiO3 composites

In this study (0–3) P(VDF-TrFE)/BaTiO₃ composites containing up to 60 vol% of ceramic phase were prepared by solvent casting or compression molding. Their thermomechanical, dielectric, and piezoelectric properties were investigated, and discussed in the light of the properties of the basic components, the processing route and the resulting morphology. The crystalline structure of the P(VDF-TrFE) matrix was found to be highly dependent on the processing route, while the structure of BaTiO₃ was not affected by any of the processing steps. The mechanical properties of the solvent cast materials showed a maximum at 30 vol% BaTiO₃, while they increased monotonically with BaTiO₃ content for compression molded materials. This difference was attributed to a higher amount of porosity and inhomogeneities in the solvent cast composites. Permittivity as high as 120 and piezoelectric coefficient d ₃₃ up to 32 pC/N were obtained for compression molded composites, and the observed decrease in d ₃₃ with aging time was attributed to the effect of mechanical stress release in the polymer matrix.     

Residual stress in piezoelectric poly(vinylidene-fluoride-co-trifluoroethylene) thin films deposited on silicon substrates

The residual stress and its evolution with time in poly(vinylidene-fluoride-co-trifluoroethylene) (P(VDF-TrFE) (72/28)) piezoelectric polymer thin films deposited on silicon wafers were investigated using the wafer curvature method. Double-side polished silicon wafers with minimized initial wafer warpage were used to replace single-side polished silicon wafers to obtain significantly improved reliability for the measurement of the low residual stress in the P(VDF-TrFE) polymer thin films. Our measurement results showed that all the P(VDF-TrFE) films possessed a tensile residual stress, and the residual stress slowly decreased with time. Our analysis further indicates that the tensile stress could arise from the thermal mismatch between the P(VDF-TrFE) film and the silicon substrate. Besides possible viscoelastic creep mechanism in thermoplastic P(VDF-TrFE) films, microcracks with widths in the range of tens of nanometers appeared to release the tensile residual stress.     

Design considerations for piezoelectric polymer ultrasound transducers

Much work has been published on the design of ultrasound transducers using piezoelectric ceramics, but a great deal of this work does not apply when using the piezoelectric polymers because of their unique electrical and mechanical properties. The purpose of this paper is to review and present new insight into seven important considerations for the design of active piezoelectric polymer ultrasound transducers: piezoelectric polymer materials selection, transducer construction and packaging requirements, materials characterization and modeling, film thickness and active area design, electroding selection, backing material design, and front protection/matching layer design. Besides reviewing these design considerations, this paper also presents new insight into the design of active piezoelectric polymer ultrasonic transducers. The design and fabrication of an immersible ultrasonic transducer, which has no adhesive layer between the active element and backing layer, is included. The transducer features direct deposition of poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer onto an insulated aluminum backing substrate. Pulse-echo tests indicated a minimum insertion loss of 37 dB and -6 dB bandwidth of 9.8 to 22 MHz (71%). The use of polymer wear-protection/quarter-wave matching layers is also discussed. Test results on a P(VDF-TrFE) transducer showed that a Mylar/sup TM/ front layer provided a slight increase in pulse-echo amplitude of 15% (or 1.2 dB) and an increase in -6 dB pulse-echo fractional bandwidth from 86 to 95%. Theoretical derivations are reported for optimizing the active area of the piezoelectric polymer element for maximum power transfer at resonance. These derivations are extended to the special case for a low profile (i.e., thin) shielded transducer. A method for modeling the non-linear loading effects of a commercial pulser-receiver is also included.     

Determination of particle size distributions and the degree of dispersion in nanocomposites

Objective of this study was the investigation of measurement techniques to determine the quality of the dispersion process of nanoparticles in polymer composites. In order to prepare the matrix suspension, alumina nanoparticles were dispersed applying shear mixing techniques in a high performance laboratory kneader. The product quality in liquid state was determined by means of dynamic light scattering (DLS) and centrifugal sedimentation analysis (CSA). However, particle measurements in carrier fluids like epoxy resin are complex and challenging. Measuring values like particle size distribution and grade of homogeneousness are strongly influenced by the sample preparation and adjustments of the measuring device. Within this study the machine settings and the formulation was analysed systematically. Hereby an identification of the key parameters and an optimisation of the measuring process were possible. Additionally, the composite was cured and analysed by scanning electron microscopy (SEM). Finally all measuring techniques were evaluated and compared among each other. Thus, DLS is the fastest method to measure spherically particles in the liquid matrix, CSA allows a certain deviation from the spherical shape and SEM gives a qualitative impression of the final particle size in cured composite condition.     

Investigation of the characteristics of platinum group metal modified alumina nanofibers

Alumina nanofibers containing either platinum or rhodium crystalline nanoparticles have been successfully fabricated by electrospinning a solution of polyvinylpyrrolidone mixed with platinum or rhodium chloride and subsequent calcination and hydrogen reduction. Transmission electron microscopy images indicate that the platinum and rhodium nanoparticles are well dispersed on the electrospun alumina nanofibers. X-ray diffraction results demonstrate that the platinum and rhodium nanoparticles are crystalline, while the alumina matrix is amorphous. Furthermore, X-ray photoelectron spectroscopy was used to investigate the chemical nature of these nanofibers containing noble metals before and after calcination and hydrogen processing.     

Piezoelectric and pyroelectric properties of (Bi0.5Na0.5)0.94Ba0.06TiO3/P(VDF-TrFE) 0–3 composites

Bismuth sodium barium titanate [(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ or BNBT] ceramic powder have been incorporated into a polyvinylidene fluoride-trifluoroethylene [P(VDF-TrFE) 70/30 mol%] copolymer matrix to form 0–3 composites. With the composition near the MPB region, BNBT has relatively high piezoelectric and dielectric properties. P(VDF-TrFE) ferroelectric copolymer films can be poled to give piezoelectric and pyroelectric performance without prior mechanical stretching. The composites were prepared using solvent casting to disperse the ceramic powder homogeneously in the copolymer matrix. Composites with BNBT volume fraction ϕ ranging from 0.05 to 0.30 were fabricated using a hot-press method. The piezoelectric and pyroelectric coefficients of the composites were studied as a function of ϕ under different poling conditions. As BNBT has a low relative permittivity, so it is relatively easy to pole the BNBT ceramic inclusion. Hence, the BNBT 0–3 composites were found to have better pyroelectric properties than that of the lead zirconate titante PZT/P(VDF-TrFE) 0–3 composites.     

Zirconium tungstate/cyanate ester nanocomposites with tailored thermal expansivity

The dimensional stability of polymer matrix composites can be enhanced by reducing the mismatch in the coefficient of thermal expansion (CTE) between the high CTE polymer matrix and low CTE fiber reinforcements, which leads to development of residual stresses and matrix microcracking. A potential strategy to diminish these residual stresses involves development of polymer nanocomposites with well dispersed nanoparticles that reduce the extent of mismatch in CTE. In this work, we explore the potential for development of bulk polymer nanocomposites with tailored thermal expansivity through incorporation of zirconium tungstate nanoparticles that are characterized by a negative CTE in a unique low viscosity bisphenol E cyanate ester (BECy) thermosetting polymer matrix. Incorporation of up to 10 vol.% whisker-like nanoparticles, synthesized by a hydrothermal method, results in a 20% reduction in the CTE of the polymer matrix. However, the nanoparticles exert a dramatic catalytic effect on the cure reaction of BECy resin and subsequently decrease the onset temperature of the glass transition for the cured polymer network, at high filler loadings.     

The role of epoxy matrix occlusions within BaTiO3 nanoparticles on the dielectric properties of functionalized BaTiO3/epoxy nanocomposites

In this work, the effects of controlled nanoparticles aggregations of barium titanate (BaTiO₃) on the dielectric properties of epoxy nanocomposites are investigated in detail with respect to different experimental parameters like frequency, ceramic content and temperature. Dispersing silanized BaTiO₃ nanopowder under ultrasonic and stir, nanocomposites of epoxy-amine matrix with different morphologies are obtained. The nanoparticles silane functionalization containing amine end groups effectively improve the compatibility of the nano-BaTiO₃ and the epoxy matrix. Storage modulus, glass transition temperature, tensile and flexural properties of nanocomposites and dielectric properties are increased until 10% by weight of nano-BaTiO₃ loading, well dispersed in the matrix. Above 10 wt.% of nano-BaTiO₃, scanning electronic microscopy and thermal analysis showed that agglomeration of nanoparticles occurs. Rheological and mechanical nanocomposites properties were evaluated and matrix occlusion behaviors were identified. In light of the specific behavior of the occluded polymer, the dielectric properties, especially dielectric loss are discussed.     

Fabrication of mesoporous titania membrane of dual-pore system and its photocatalytic activity and dye-sensitized solar cell performance

We report the fabrication of a novel titania membrane of the dual-pore system that is strategically designed and prepared by a two-step replication process and sol-gel reaction. The primary nanoporous channel structure is fabricated by the cage-like PMMA template (CPT) obtained from the nanoporous alumina membrane and the secondary mesoporous structure is formed by the sol-gel reaction of the lyotropic precursor solution within the CPT. Furthermore the mesoporous titania membrane (MTM) frame consists of the titania nanoparticles of 10-12?nm in diameter. Morphology and structural properties of the MTM are investigated by field emission scanning electron microscopy, high resolution transmission electron microscopy, x-ray diffraction and Brunauer-Emmett-Teller surface area. The photocatalytic activity and the solar energy properties of the MTM are characterized by UV-vis spectrophotometer, spectrofluorometer and photoinduced I-V measurement. The photocatalytic test indicates that the MTM has higher efficiency than the commercial P25 with a good recyclability due to its large-scale membrane style and the preliminary result on the solar cell application shows a solar energy conversion efficiency of 3.35% for the dye-sensitized solar cell utilizing the MTM.     

Hierarchical organization of Au nanoparticles in a poly(vinyl carbazole) matrix for hybrid electronic devices

We report a novel one-step method for the preparation of hierarchically patterned Au nanoparticles in a conducting polymer matrix by controlling the interface properties between Au nanoparticles and the conducting polymer matrix. The terminal group of capping molecules for the Au nanoparticles was modified to change the interface properties, not to change the size of the Au nanoparticles which affects their intrinsic properties. By modulating the interface properties, it is possible to construct Au nanoparticle-conducting polymer composites with two different structures: one presents a triple layer in which the conducting polymer layer is sandwiched between Au nanoparticle layers at the top and bottom; the other exhibits a form like a raisin cake in which Au nanoparticles are homogeneously organized in the conducting polymer matrix. High-resolution transmission electron microscopy was used to study the morphology and patterning of Au?nanoparticles in the conducting polymer matrix.     

Morphology and mechanical properties of polyacrylonitrile/attapulgite nanocomposite

The major objective of this work is to understand the effects of attapulgite (AT) on the mechanical properties of polyacrylonitrile (PAN)/AT nanocomposite film. The well dispersed but irregularly distributed AT nanoparticles in the matrix was observed by scanning electron microscopy (SEM) and UV–vis spectra. The mechanical properties were investigated by means of tensile tests and dynamic mechanical analyses (DMA). The results showed that the incorporation of AT significantly improved the tensile strength and modulus of the PAN matrix. The fracture morphologies analysis has further suggested that small amount of AT nanorods may slide and orient along the tensile direction, resulting in homogenous stress transfer, thus increase the toughness of the PAN. However, the nanorods network formed in high AT content sample probably hindered the deformation of the matrix and generated the stress concentration points, leading to the remarkable increase of embrittlement of the samples. Following this concept, the volume of the constrained polymer chains was also calculated with DMA data and showed the good correlation with conclusion drawn in the tensile tests.     

Tribological properties of polymer nanohybrids containing gold nanoparticles obtained by laser ablation

We have studied polystyrene (PS)+Au particles nanohybrids. Approximately spherical gold nanoparticles with the average diameter of 15 nm were obtained by laser ablation in a liquid environment. Thus any chemical residue on the particles was eliminated. Focused ion beam (FIB) milling plus scanning electron microscopy (SEM) observation show that Au particles are fairly well dispersed inside the polymer matrix, better than when PS is simply dissolved in a nanoparticle solution. The Au particles concentration as low as 0.15 wt% results in dramatic changes in tribological properties, namely dynamic friction and pin-on-disk wear. Both wear and dynamic friction results are explained in terms of high brittleness of PS, abrasion of Au particles against a ceramic indenter, and also effects of density of filler particles in the matrix on tribological properties. Effects of varying normal load on friction are small.     

Fatigue behaviour of nanoclay reinforced epoxy resin composites

Nanoparticle filling is a feasible way to increase the mechanical properties of polymer matrices. Abundant research work has been published in the last number of years concerning the enhancement of the mechanical properties of nanoparticle filled polymers, but only a reduced number of studies have been done focusing on the fatigue behaviour. This work analyses the influence of nanoclay reinforcement and water presence on the fatigue behaviour of epoxy matrices. The nanoparticles were dispersed into the epoxy resin using a direct mixing method. The dispersion and exfoliation of nanoparticles was characterised by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Fatigue strength decreased with the nanoclay incorporation into the matrix. Fatigue life of nanoclay filled composites was significantly reduced by the notch effect and by the immersion in water.     

Mechanical properties of tri-modal Al matrix composites reinforced by nano- and submicron-sized Al2O3 particulates developed by wet attrition milling and hot extrusion

Among all metal matrix composites, aluminum is the most widely used matrix due to its low density coupled with high stiffness. In this study, aluminum matrix composites reinforced by two sizes of alumina particles (35 nm and 0.3 μm) are prepared by wet attrition milling and hot forward extrusion processes. The effect of the ratio of the nano- to submicron-sized particles (2:8, 3:7, 4:6, 5:5, and 6:4 in weight percent) on mechanical properties of the composites is evaluated by micro-hardness and tensile tests. It is found that by increasing the nanoparticles content, the hardness and strength of the composites first increase and then decrease when the amount of the nanoparticle exceeds 4 wt.%. The tensile fracture surfaces are also observed by scanning electron microscopy.