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Friday, 09 February 2018

Conclusions of the FLEXOFABRIC Project

Project FLEXOFABRIC has studied the viability of PVDF fabrics for flexoelectric smart textiles. A flexoelectric material is one that generates electricity when bent. Polivinilidene di-fluoride (PVDF) is a well-known flexoelectric and piezoelectric polymer; films of this material have been reported to have large piezoelectric and flexoelectric coefficients and therefore a large capacity to generate electricity under mechanical stress. Our purpose was to see whether such good electromechanical properties could be transferred to a textile form for smart wearables.

Polisilk is a large-scale textile factory based in Sant Salvador de Guardiola (Barcelona), with a capacity to produce industrial quantities (square metres) of PVDF fabric. They provided the samples. At ICN2, a collaboration was set between the Oxide Nanophysics Group of ICREA Prof. Gustau Catalá and the NanoBioelectronics and Biosensors Group of ICREA Prof. Arben Merkoçi. The scientists in charge of the research were, respectively, Irene Sanchez and ICN2 technician Marc Balsells: they deposited on the fabric electrodes of various Materials (Silver Paint, conductive carbon, carbon fibre) and by different methods (screen-printing, stamping, heat-pressing), and measured the electromechanical performance of the resulting devices.

The main results are summarized below:

  1. The as-made fabrics have a large pore size and treatment is required in order to prevent shorting between electrodes on opposite sides of the fabric.
  2. Shorting can be mitigated if the fabric is heated (which shrinks the pore size) and folded (which doubles its thickness) before depositing electrodes.
  3. The most effective electroding system, not requiring pre-treatment of the PVDF fabric, is attaching pre-made carbon-fibre conductive fabric. Samples of these conductive carbon fabrics were loaned to us by Juan José Vilatela and Alfonso Monreal from IMDEA Materials in Madrid.
  4. The Factory-made PVDF fabric is not piezoelectric/ferroelectric (β-phase). It is 60% non-piezoelectric α-phase and only 40% of ferroelectric β-phase. Attempts at poling this residual ferroelectric phase were unsuccessful.
  5. Despite the lack of piezoelectricity, the fabrics are flexoelectric. However, their flexoelectric performance is disappointing: the flexoelectric coefficient is only of the order of 0.1nC/m, which is 105 times smaller than reported in the literature for continuous PVDF films.
  6. This flexoelectric coefficient is insufficient to generate a useful amount of electricity in a wearable device.
    The main conclusion is therefore that current industrial PVDF fabrics are not usable as a source of electricity in smart textiles. Steps that might redress this negative result should include: (a) changing the fabrication recipe so that PVDF is in the more electromechanically active β-phase. (b) increasing the yarn density so as to make the fabric less porous and less prone to shorting.

Project FLEXOFABRIC (ref. 2016 LLAV 00050) has been promoted with FEDER funds by the Llavor programme of the Secretariat for Universities and Research of the Department of Business and Knowledge of the Catalan Government.