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. 2017 Sep 30;9(10):479.
doi: 10.3390/polym9100479.

Biocompatible Silk/Polymer Energy Harvesters Using Stretched Poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) Nanofibers

Raghid Najjar  1 , Yi Luo  2 , Dave Jao  3 , David Brennan  4 , Ye Xue  5 , Vince Beachley  6 , Xiao Hu  7   8 , Wei Xue  9
Affiliations

Biocompatible Silk/Polymer Energy Harvesters Using Stretched Poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) Nanofibers

Raghid Najjar et al. Polymers (Basel). .

Abstract

Energy harvested from human body movement can produce continuous, stable energy to portable electronics and implanted medical devices. The energy harvesters need to be light, small, inexpensive, and highly portable. Here we report a novel biocompatible device made of poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) nanofibers on flexible substrates. The nanofibers are prepared with electrospinning followed by a stretching process. This results in aligned nanofibers with diameter control. The assembled device demonstrates high mechanical-to-electrical conversion performance, with stretched PVDF-HFP nanofibers outperforming regular electrospun samples by more than 10 times. Fourier transform infrared spectroscopy (FTIR) reveals that the stretched nanofibers have a higher β phase content, which is the critical polymorph that enables piezoelectricity in polyvinylidene fluoride (PVDF). Polydimethylsiloxane (PDMS) is initially selected as the substrate material for its low cost, high flexibility, and rapid prototyping capability. Bombyx Mori silkworm silk fibroin (SF) and its composites are investigated as promising alternatives due to their high strength, toughness, and biocompatibility. A composite of silk with 20% glycerol demonstrates higher strength and larger ultimate strain than PDMS. With the integration of stretched electrospun PVDF-HFP nanofibers and flexible substrates, this pilot study shows a new pathway for the fabrication of biocompatible, skin-mountable energy devices.

Keywords: electrospinning; energy harvester; piezoelectricity; polyvinylidene fluoride (PVDF); silk; stretching.

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Conflict of interest statement

The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

Figures

Figure 1
Figure 1
Schematics of (A) the electrospinning system to produce random nanofibers and (B) the electrospinning system to produce aligned and stretched nanofibers.
Figure 2
Figure 2
(A) Schematic of the device design and fabrication. (B) A sample PDMS/PVDF-HFP/PDMS device. PVDF-HFP = poly (vinylidene fluoride-co-hexafluoropropylene); PDMS = polydimethylsiloxane.
Figure 3
Figure 3
The mechanical-electrical measurement system for the energy harvester. DAQ = data acquisition system.
Figure 4
Figure 4
(A–I) SEM images showing the surface morphology of (A–C) pure silk fibroin, (D–F) silk fibroin with 20% glycerol content, and (G–I) pristine PDMS films at 1000× (scale bar of 80 µm), 5000× (scale bar of 10 µm), and 10,000× (scale bar of 8 µm), respectively. (J–R) SEM images showing the cross-sections of (J–L) pure silk fibroin, (M–O) silk fibroin with 20% glycerol content, and (P–R) pristine PDMS at 1000× (scale bar of 80 µm), 5000× (scale bar of 10 µm), and 10,000× (scale bar of 8 µm), respectively.
Figure 5
Figure 5
(A) FTIR spectra of the PDMS, pure silk fibroin (SF), and SF-glycerol composite films in the range of 4000–450 cm−1. (B) FTIR spectra of the pure SF and SF-glycerol composite in the range of 1800–1450 cm−1.
Figure 6
Figure 6
Stress-strain curves of the PDMS, pure SF, and SF-glycerol composite films.
Figure 7
Figure 7
SEM images of (A) traditionally prepared electrospun PVDF-HFP nanofibers and (B) stretched PVDF-HFP nanofibers. Diameter distribution histograms of (C) random nanofibers and (D) stretched nanofibers. Nanofiber orientation distribution of (E) random nanofibers and (F) stretched nanofibers.
Figure 8
Figure 8
(A) Electrical output of a PDMS/PVDF-HFP/PDMS energy harvester using random nanofibers. (B) Electrical output of a device using aligned nanofibers.
Figure 9
Figure 9
Normalized FTIR spectra of a cast PVDF-HFP film, random nanofibers, and aligned nanofibers.
See this image and copyright information in PMC

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