Dielectric behavior characterization of functional fibrous-ceramic/polymer nanocomposites

This study is mainly focused on forming fibrous-ceramic/polymer nanocomposites and characterizing their dielectric behavior. The fibrous-ZnO/PVDF nanocomposite is prepared in two steps. First, a network of nano-scale zinc oxide (ZnO) fibers is produced by sintering electrospun PVA/Zinc Acetate fibers. Second, the ZnO fibrous non-woven mat is sandwiched between two polyvinylidine fluoride (PVDF) thermoplastic polymer films by hot-press casting. Referring to the extensive literature search within the thesis, this work is the first demonstration of the use of electrospinning to secure the dispersion and distribution of a network of inorganic fillers. Moreover, processing a fibrous-ZnO/PVDF flexible composite facilitate material handling and enable dielectric property measurement, in contrast to that on a fibrous mat of pure ZnO. Due to the high surface area of the short ZnO fibers and their polycrystalline structure, interfacial polarization is pronounced in the nanocomposite film. The dielectric constant is enhanced significantly—up to a factor of ten at low frequencies compared to the dielectric constant of constituent materials (both bulk ZnO and PVDF), and up to a factor of two compared to a bulk- ZnO/PVDF composite. Similar effort is also presented for the fibrous-PZT/polyvinylester nanocomposite. Nanofibers are obtained by electrospinning a sol-gel based solution and polyvinyl pyrrolidone (PVP) polymer, and subsequent sintering of the electrospun precursor fibers. The average diameter of the precursor PZT/PVP green fibers is increased with the aging of the precursor solution along with an increase in the viscosity. Preparation of 3-3 nanocomposites by infusion of polyvinylester into the nanofiber mat facilitates successful handling of the mats and enables measurements of dielectric properties

Cooperativity in the enhanced piezoelectric response of polymer nanowires

We provide a detailed insight into piezoelectric energy generation from arrays of polymer nanofibers. For sake of comparison, we firstly measure individual poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFe)) fibers at well-defined levels of compressive stress. Under an applied load of 2 mN, single nanostructures generate a voltage of 0.45 mV. We show that under the same load conditions, fibers in dense arrays exhibit a voltage output higher by about two orders of magnitude. Numerical modelling studies demonstrate that the enhancement of the piezoelectric response is a general phenomenon associated to the electromechanical interaction among adjacent fibers, namely a cooperative effect depending on specific geometrical parameters. This establishes new design rules for next piezoelectric nano-generators and sensors.Comment: 31 pages, 11 figures, 1 tabl

Focal, remote-controlled, chronic chemical modulation of brain microstructures

Direct delivery of fluid to brain parenchyma is critical in both research and clinical settings. This is usually accomplished through acutely inserted cannulas. This technique, however, results in backflow and significant dispersion away from the infusion site, offering little spatial or temporal control in delivering fluid. We present an implantable, MRI-compatible, remotely controlled drug delivery system for minimally invasive interfacing with brain microstructures in freely moving animals. We show that infusions through acutely inserted needles target a region more than twofold larger than that of identical infusions through chronically implanted probes due to reflux and backflow. We characterize the dynamics of in vivo infusions using positron emission tomography techniques. Volumes as small as 167 nL of copper-64 and fludeoxyglucose labeled agents are quantified. We further demonstrate the importance of precise drug volume dosing to neural structures to elicit behavioral effects reliably. Selective modulation of the substantia nigra, a critical node in basal ganglia circuitry, via muscimol infusion induces behavioral changes in a volume-dependent manner, even when the total dose remains constant. Chronic device viability is confirmed up to 1-y implantation in rats. This technology could potentially enable precise investigation of neurological disease pathology in preclinical models, and more efficacious treatment in human patients. Keywords: brain; drug delivery; substantia nigra; neural implant; PETNational Institutes of Health (U.S.) (Grant R01 EB016101)National Institute of Biomedical Imaging and Bioengineering (U.S.) (Grant R01 EB016101)National Cancer Institute (U.S.) (Grant P30-CA14051

Ferroelectric/piezoelectric flexible mechanical energy harvesters and stretchable epidermal sensors for medical applications

Multifunctional sensing capability, ‘unusual’ formats with flexible/stretchable designs, rugged lightweight construction, and self-powered operation are desired attributes for electronics that directly interface with the human body. The collective results in this dissertation suggest utility in a variety of sensors and energy harvesting components, with lightweight construction, attractive mechanical properties and potential for implementation over large areas, with promising application in unusual bio-integrated electronics, such as self-powered cardiac pacemakers, skin-mounted blood pressure sensors, modulus sensors and skin cancer detection bio-patches. For these and related applications, unusual electronics provide the capability of intimate and conformal integration with a variety of substrates on biological tissues. These systems can be twisted, folded, stretched/flexed and wrapped onto curviliniar surfaces without damage or significant alteration in operation. In this dissertation, the application of ferroelectric/piezoelectric materials and patterning techniques for ‘unusual’ electronics, with an emphasis on bio-integrated systems were demonstrated. Overall, the results suggest that the various sensor capabilities could be valuable for a range of applications in continuous self-powered health/wellness monitoring and clinical medicine

Dielectric properties of ZnO/PVDF flexible composites

The focus of this study is to make flexible ZnO/PVDF fibrous composites and to investigate their dielectric behavior. ZnO fibrous network is first produced by calcination and sintering of the precursor PVA/Zinc Acetate electrospun fiber mats. Composite making includes hot-pres melt-casting of ZnO fibrous nonwoven mat after sandwiched between solution cast PVDF films. SEM images of the nanocomposite show that fibrous network is affected during casting and turned into the ZnO short fibers, but remained well distributed/dispersed into PVDF. Processing the ZnO/PVDF flexible composite film facilitates successful handling, and enables measurements for dielectric properties, not practical on sole ZnO fibrous mat. Existence of the ZnO short fibers in the composite film increases the dielectric constant significantly while slight penalty on dielectric loss is measured compared to PVDF film alone

Pb(Zr,Ti)O3 nanofibers produced by electrospinning process

Lead zirconate titanate (PZT) nanofibers are obtained by electrospinning a sol-gel based solution and polyvinyl pyrrolidone (PVP) polymer, and subsequent sintering of the electrospun precursor fibers. The PVP content of the precursor solution is critical in the formation of the fully fibrous mats. Scanning electron microscope (SEM) is used to examine the morphology of the precursor fibers and annealed PZT nanofibers. The diameter of the precursor PZT/PVP green fibers have increased with the aging of the precursor solution along with an increase in the viscosity. The viscosity of 500 mPa results in successful fibrous mats, yielding green PZT/PVP fibers with a diameter of 400 nm. Thefiber mats are then sintered at 700C. X-ray diffraction (XRD) pattern of the annealed PZT fibers exhibits no preferred orientation and a pure tetragonal perovskite phase. Preparation of piezocomposites by infusion of epoxy into the nanofiber mat facilitates successful handling of the fragile mats and enables measurements of dielectric properties