Research paper
A simple transparent electrode fabrication method by filling in Ag composites into scratch gap

https://doi.org/10.1016/j.mee.2020.111331Get rights and content

Highlights

  • The transparent electrode is simply fabricated by Ag into a uniform knife-scratched grids on polydimethylsiloxane (PDMS).

  • The transparent electrode are demonstrate Joule heating, which is more dependent on the current than the voltage, onto the skin arm.

  • The thin film battery onto PDMS operates by the circuit, embedded Ag composite into the uniform knife-scratched gaps.

Abstract

In this work, we demonstrate a simple and cheap process for the fabrication of transparent electrodes. We scratched the surface of polydimethylsiloxane (PDMS) to create 0.5 mm-, 1 mm-, and 2.0 mm-deep gaps to form a crossed grid; we then filled the gaps with a Ag composite and analyzed the optical and thermoelectrical properties of the system by infrared spectroscopy, optical microscopy, and scanning electron microscopy. We then plotted the current-voltage curves derived from connecting the system to a battery and assessed the electrical properties, as well as the Joule heating derived from current transmission. We discovered that the Joule heating was more dependent on the current than the voltage of the skin; this knowledge was applied by employing our circuit on a wearable heating device, which demonstrated the ability to heat the skin underneath when stimulated with a 0.6 A current and 9 V. Finally, we also demonstrated the applicability of our PDMS-supported Ag circuit by connecting it to a battery, and employing the circuit to illuminate an LED light.

Introduction

Transparent electrodes and circuits are some of the key components of wearable electronics and smart devices that can be worn as accessories or implanted into the skin; they are currently the widely employed constituents of smart textiles, watches, and small sensors [[1], [2], [3]]. The transparent circuits and electrodes used for these technological purposes must combine optimal bendabilities and stretchabilities with excellent electrical and optical properties, besides being very small in size for microtechnological applications; they also need to be attached on supports that present the same transparency. Nowadays, they are widely employed in touch panels, heating films, smart windows, and organic light-emitting diodes. [[4], [5], [6], [7]]

Polydimethylsiloxane (PDMS) is widely used as flexible and optically transparent support for smart electronics as it can be fabricated to achieve the desired flexibility [8] and can be embedded with conductive materials, such as Ag nanowires and carbon nanotubes [9]. Further, PDMS is an electric insulator with good relative permittivity [10], as well as excellent biocompatibility [[11], [12], [13], [14]]. These properties make PDMS an excellent support candidate for wearable electronics and transparent electrodes and circuits. The conductive material can be embedded on the PDMS substrate with different circuit patterns; for instance, PDMS-supported metal and carbon nanowires have been produced with random bundled networks or with percolation patterns, and they have been successfully used in health-monitoring devices [15], thermotherapy devices [16], and energy harvesting devices [17]. To embed nanomaterials onto this substrate, several methods have been investigated for a uniform, rapid, and cheap fabrication, such as electrophoretic deposition [18], doctor blade coating [19], brush coating [20], and spraying deposition [21]. Exploiting the electric transmission originating at the junction of randomly patterned nanostructures [22], transparent electrodes were can produce with good electrical properties and optical transparency. Even though their fabrication methods are relatively cheap and easy, shadow mask patterning [23] or silk-screen printing [24] are still needed to produce the complex patterns required for more demanding applications, such as in LED circuits [25] and sensors [26]. Furthermore, the cost of fabrication is higher for large-scale production as many facilities are not equipped for scale-up. Moreover, metallic and semiconducting nanomaterials such as Ag nanowires and carbon nanotubes can be expensive because of their complex fabrication processes [27] and the accuracy required to ensure that they are produced with high aspect ratios [28].

In this paper, we demonstrate the preparation of a transparent electrode created by scratching PDMS to create a crossed grid of gaps where a conductive Ag composite was percolated. This simple process can be conducted on a benchtop, and it only requires an accessible and cheap metallic material. We initially assessed the electrical properties of this new circuit, testing samples with gaps of different depths; we also evaluated their physical properties by optical microscopy and scanning electron microscopy (SEM). We demonstrated that besides being scratched and filled with Ag, the PDMS support still had appreciable transparency when analyzing its transmittance. The temperatures of the devices, which are dominantly increased by the effect of the passing current, saturated after 180 s, demonstrating that this Joule heating can be employed in practical applications. For instance, our Ag-filled PDMS device was used to simulate a skin-warming device, after covering it with a polyethylene (PET) film. Finally, we connected the circuit to a thin-film battery, and were able to illuminate an LED light.

Section snippets

Materials

PDMS was prepared by mixing SYLGARD® 184 Silicone elastomer (Dow Corning, USA) and SYLGARD® 184 Silicone elastomer curing agent (Dow Corning, USA). The Ag composite used is a conductive paint composed of 60 to 70% Ag, 20 to 30% ethyl lactate, and 5 to 15% of an acrylic resin (CANS ELCOAT P-100, Cansas, Japan). This paste had a 10−4 Ω · cm resistivity, and a 230 Pa·s viscosity at 25 °C. The polyester film (ES301238, GFM, South Korea) used for insulation had a 0.038 mm thickness. Isopropyl

Direct knife scratching method

As discussed in Section 2.2, we injected a Ag composite solution into a crossed-grid pattern engraved onto a PDMS substrate with a blade; we named this method the “knife scratching method”, which is inspired by a reported method that employed the laser direct writing of conductive structures onto PDMS [29]. Fig. 1a illustrates the fabrication process, while Fig. 1b and c are optical images of two test samples, where Fig. 1ci depicts a PDMS sheet engraved with 1 mm × 1 mm grid squares, and Fig. 1

Conclusions

In summary, we demonstrated a simple method for the fabrication of a PDMS-supported transparent electrode, in which the plastic substrate was engraved by scratching to obtain a crossed-grid pattern, which was then filled with Ag particles. We first confirmed that the transmittance of the as-prepared transparent electrodes, either with 1 mm or 2 mm-wide grids, were substantially unaltered compared to pristine PDMS. We then measured the electrical properties of the Ag composite trapped in the

Declaration of Competing Interest

None.

Acknowledgments

This work was supported by Incheon National University (International Cooperative) Research Grant in 2019.

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