# A 60-GHz Ultra-Thin and Flexible Metasurface for Frequency-Selective Wireless Applications
## 2 Materials and Methods  
The physical implementation of the MMW metasurface on flexible materials is highly critical due to the optimization required for the parameters which determine the precision, robustness and reliability of the printed pattern. Type of a flexible substrate is important in radio-frequency (RF) circuit design and the selection is usually made among the available options based on the electrical, mechanical and thermal properties of the material. Recyclable and eco-friendly plastics, polyesters, polyimides and polymers have been widely investigated in developing flexible electronics for the wearable and conformal applications. For instance, polypropylene, a thermoplastic polymer which can withstand temperatures up to 160 \({}^{\circ}\)C, has been used with aluminum metallization to implement an RFID antenna [30], and MMW metasurface constituting split-ring and complementary split-ring resonators [31]. Moreover, liquid crystal polymer (LCP) is highly anticipated for MMW flexible antennas and RF circuitries due to low dielectric loss, moisture repellent, chemicals resistant, and can handle up to 300 \({}^{\circ}\)C [32]. In addition, polyethylene terephthalate (PET) by DuPont (Melinex ST504) is a crystal-clear, high-gloss, heat-stabilized and widely available thermoplastic polymer film from the polyester family. Due to its cost-effectiveness and eco-friendly recycling qualities, it is produced in bulks and has been consumed in disposable packaging and in the manufacture of several resins [33]. Melinex films with varying composition, permittivity and thickness are commercially available and have been deployed in many reported flexible printed circuits [34; 35]. In order to perform patterning by means of a suitable ink or paste, the Melinex sheet is preheated at one side to improve adhesion. Also, polyimide film produced by DuPont (Kapton) is suitable for flexible devices because of outstanding features of flexibility, mechanical, chemical and thermal stability as well as constitutes an inert and hydrophobic surface [36]. In this research, Melinex and Kapton films have been selected as substrates for the metasurface design and printing due to their high-performance attributes. A fabrication technique capable to produce precise small-scale dimensions and ensures high-speed and low-cost manufacturing is desired to produce large panels of metasurface which can be deployed as wallpapers. Several state-of-the-art processes have been considered in this regard. For instance, flexible films with a copper/aluminum cladded metal surface can be deployed for laser prototyping, though a costly solution as it requires an expensive laser milling machine [37]. Inkjet printing is an additive fabrication technique which involves a controlled layer-by-layer deposition of ink for precise patterning on flexible films by an inkjet printer [38]. The number of nozzles used for printing determines the speed of the process, and this can be extended to a reel-to-reel process for mass production. Flexography is a roll-to-roll process, comparatively faster than inkjet printing though has a limited resolution [39]. Photolithography deposits a pattern on a substrate by means of light masking and involves a layer-by-layer growth sequence, which requires a sophisticated clean-room [40]. The screen printing process, on the other hand, avoids the expensive facilities of laser prototyping, inkjet printing or a clean-room, and ensures a reasonable pattern accuracy and time-efficiency [41]. The screen stencil is typically a fine mesh constructed from silk or steel and is partially covered with an ink-blocking coating to define the print pattern. The pattern on the screen allows the conductive paste to be transferred on the substrate. In this research work, the screen printing process has been chosen for metasurface realization due to the advantages of low equipment cost, reasonable printing quality and high conductivity. Several types of conductive inks and paste are commercially available which can be deployed for this process.