Electrospun fibers for Flexible Electronics

Advances in electrospinning have enabled this process to deposit fibers with precision of a few microns. This opens up more opportunities for electrospun nanofibers to be used in electrical and electronics applications. Such devices can be made flexible due to a combination of material selection and high aspect ratio of nanofibers. Even without precision deposition, in its nonwoven form, electrospun membrane has been used for construction of electronic devices.

There are several ways in which electrospun nanofibers may be made conductive. One of the simplest methods is by blending conductive additive such as carbon nanotubes to the polymer solution to be electrospun [Jeong et al 2006, Chronakis et al 2006]. Other methods such as coating and sintering have also been tested. Although electrospun conductive composite fiber membrane may be less conductive than film [Laforgue et al 2007, Wei et al 2005], its conductivity per unit weight has been shown to be better on nanofibrous membrane in some cases [Laforgue et al 2007].

The relative ease of forming a continuous line from point to point creates the possibility of using near field electrospinning to generate nanowires across nano-connection points. Bisht et al (2011) used low voltage, near field electrospinning to suspend nanofiber across carbon post with diameter of 30 µm and interpostal distance of 100 µm. Using a patterned silicon substrate with micro-pillars of diameter ranging from 1.6 µm to about 9 µm as collector, Zheng et al (2010) were able to successfully deposit a single strand of nanofiber across the micro-pillars thereby providing a new method for integrating nanofibers into micro/nano systems. To deposit a linear nanofiber strand across the pillar, the collector speed needs to at least match the jet spinning speed. Higher collector speed will result in a reduction in the fiber diameter due to stretching

Aligned nanofibers using near-field electrospinning of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]/polyethylene oxide solution [Camillo D D et al Nanoscale 2013; 5: 11637. doi:10.1039/C3NR03094F. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

There are several ways in which electrospinning can be used to construct conductive and flexible nanofibers. Wu et al (2010) used copper acetate/polyvinyl acetate solution for electrospinning to give copper (Cu) nanofibers. Aligned Cu nanofibers was able to show 90% transmittance at sheet resistance of 25 ohm/sq [Wu et al 2010]. Transparent and flexible electrodes have been constructed by transferring the Cu nanofiber network to poly(dimethylsiloxane) (PDMS) substrate [Wu et al 2010].

Using electrospun nanofibers as a template material metallic coatings such as chromium, gold, copper, silver and aluminum can be applied to it [Wu et al 2013]. The coated nanofiber sheet can be transferred to a silicon substrate using dropcast. With a coating thickness of 100 nm on one side, a nanotrough layer can be created and is sufficiently strong to be self-supporting after the nanofiber templates have been removed [Wu et al 2013]. A single gold nanotrough was found to have an electric conductivity of 2.2 x 105 S/cm which is slightly less than its polycrystalline bulk [Wu et al 2013]. The nanotrough network exhibit greater transparency than flat nanostrips due to its concave shape which reduces its electromagnetic cross-section with transmittance of more than 90% for Cu and Au nanotrough materials across the visible wavelengths. It is also highly bendable, stretchable and foldable without significant deterioration in electrical conductivity [Wu et al 2013].

Electrospun conductive nanofibers network with high transparency has been used as an antenna in a soft, smart contact lens for real-time detection of the cortisol concentration in tears [Ku et al 2020]. This antenna needs to occupy a large area over the soft contact lens and exhibit a low sheet resistance for wireless operation of standardized NFC chips. To construct such an antenna, a suspension of Ag nanoparticle ink in ethylene glycol was electrospun to form a network of continuous Ag nanofibers. Thermal annealing was carried out to form a conductive Ag nanofibers network. Finer Ag nanowires were subsequently electrosprayed over the Ag nanofibers network to improve the conductivity of the antenna. The constructed Ag nanofibers/ Ag nanowire antenna has an average sheet resistance(Rs) of 0.3 ohm per square and a transparency of 71% at 550 nm. To maintain conductivity of the antenna over a substantial length of time, passivation by coating the antenna with a layer of parylene elastomeric cover may be used to retard Ag oxidation.

Electrospun fibers mesh may be used as a supporting substrate for flexible electronic devices. Wang et al (2020) constructed a nanomesh organic electrochemical transistor (NMOECT) for on-skin electrodes with local amplifying function with electrospun polyurethane (PU) nanofibers mesh as the base. In preparation of the base, the PU nanofibers mesh was coated with parylene to fuse the junctions between fibers. Au was coated on the prepared PU/parylene mesh to function as the drain and source electrodes of the NMOECT and conductive wires. Another round of parylene coating was applied over the coated Au. Reactive ion etching (RIE) was used to expose the underlying Au for the drain/source electrodes. PEDOT:PSS was then deposited by spray coating over a hard mask on the exposed contact part. The constructed NMOECT was able to exhibit acquisition and local amplification of electrophysiological signals (electrocardiography) with DC-level cutoff.

(a) Schematic of the NMOECT. (b) NMOECT laminated on the skin surface of a finger with a wiring Au pad on 2 µm-thick parylene film (scale bar: 5 mm). (c) Enlarged photo of NMOECT on the skin surface (scale bar: 1 mm). (d) SEM image of the channel part of the NMOECT (scale bar: 200 µm). (e) SEM image of PEDOT:PSS on the nanomesh (scale bar: 50 µm) [Wang et al 2020].

[+]

[-] comments powered by Disqus

Content [Electronics]