# Fabrication of Stretchable Transparent Electrode by Utilizing Self-Induced Vacuum Force
## 1 Introduction  
Stretchable electrodes have received attention because of the increasing demands of flexible and stretchable devices, such as artificial skin and muscle, smart clothing, and electronic textiles [1, 2]. Stretchable electrodes are fabricated by patterning an electric circuit on a stretchable substrate. Poly-di-methyl-siloxane (PDMS) has been widely used as an electrode substrate because of its stretchability and biocompatibility [3]. However, a main challenge for the fabrication process is in utilizing the flexibility of PDMS with electrically conductive materials, which are mostly inflexible. Therefore, numerous types of stretchable conductors have been studied. In addition, the production procedure of stretchable conductors can be categorized into two methods: The use of intrinsically stretchable conducting composites [4, 5, 6], and building the structure of the conductor such that bending and stretching are enabled [7]. Here, nanoparticle-polymer composites can be specified as intrinsically stretchable conducting composites. In nanoparticle-polymer composites, metal powder filler is widely used as the nanoparticles because of its simplicity of fabrication and high conductivity [8].  
Another challenge in the fabrication of stretchable electrodes is patterning of the electric circuit, which has microscale dimensions. Owing to the high level of precision required, the stretchable conductor pattern has been prepared using the photolithography method [9]. Furthermore, other methods, such as growing the electrode pattern using chemical etching or by performing deposition with masking, have been proposed [10, 11]. Despite achieving accurate circuit patterning, the abovementioned methods are too expensive for commercial use of fabricating stretchable electronic devices. Therefore, to overcome the cost issues, printed-electronics methods have been proposedbecause of their simplicity of use in the circuit-patterning process and cost efficiency [12]. In printed electronics methods, the conductor materials are directly transferred or injected on an elastomer substrate. For example, the gravure-offset method has been highlighted because of its cost efficiency and rapid processing [13]. Nevertheless, the lithography method is still in development, as the adhesive force between the conductor to be transferred and the substrate is not sufficient, thereby, results in poor durability of the stretchable electrode.  
In this study, we propose a fabrication method that, not only utilizes the flexibility of an electric conductor, but also improves the adhesive force between the electrode conductor and the elastomer substrate. In the proposed method, an electric conductor fills the engraved pattern on a stretchable substrate. To completely transfer the electric conductor material onto the substrate, we induced vacuum force, which thereby, resulted in full penetration of the elastomer into the electric conductor. Here, the vacuum force was self-induced, without the use of any vacuuming devices, because of the evaporation of the solvent in the conductor slurry. This self-induced vacuum force can only be achieved via the wet transferring process demonstrated in this study. An additional advantage of the proposed method is the significant reduction in the incurred cost of electrode fabrication, as the additional drying process for the electric conductor is omitted by transfer of the conductive slurry to the elastomer substrate under wet conditions (see Figure 1).