## 4 RIS Implementation  
RISs are typically fabricated using traditional microfabrication techniques and novel material synthesis methods. Next, we describe some of the most commonly used fabrication techniques for RIS. Electron beam lithography (EBL) is a high-resolution patterning technique used to create the complex geometries required for RIS devices. A beam of electrons is focused onto a substrate coated with a resist material, and the pattern is defined by selectively exposing the resist material to the electrons. The exposed resist is then etched away, leaving behind the desired pattern [46]. Photolithography is a widely used patterning technique in microfabrication. It involves using light to transfer a pattern from a photomask onto a photosensitive substrate. The pattern is then etched into the substrate using wet or dry etching techniques [47]. Chemical vapor deposition (CVD) is a technique used to deposit thin films of materials onto a substrate. The process involves introducing precursor gases into a reaction chamber, which then reacts to form a solid film on the substrate. CVD is commonly used to deposit metal and dielectric films onto RIS devices [48]. Atomic layer deposition (ALD) is a thin film deposition technique that precisely controls film thickness and composition. The process involves sequentially exposing a substrate to alternating pulses of precursor gases. Each pulse chemically adsorbs onto the substrate, allowing precise control of the deposited film thickness [49]. Inkjet printing is a printing technique that deposits small ink droplets onto a substrate. In the case of RIS devices, inkjet printing is used to deposit conductive and dielectric inks onto the substrate to create the required RIS structure [50]. Three-dimensional printing is an additive manufacturing technique that can fabricate RIS devices with complex geometries. This technique creates a 3D model of the RIS's structure, and the printer deposits the material layer by layer in order to create the final structure [51]. Overall, the choice of RIS fabrication technique depends on the specific requirements of the RIS device. Each technique has its advantages and disadvantages regarding cost, complexity, scalability, and performance. In general, the choice of fabrication technique depends on the specific application requirements and constraints [51]. Printed circuit board (PCB) fabrication is a popular and mature technique that is relatively low-cost and can produce high-performance RISs with good reproducibility [51]. However, it is limited in terms of design flexibility and scalability. Metamaterial-based techniques, such as photolithography and nano-imprint lithography, offer high design flexibility and scalability, but are generally more complex and expensive. Three-dimensional printing is a relatively new technique gaining interest in RIS fabrication due to its high design flexibility and rapid prototyping capabilities. However, the material properties and performance of 3D-printed RISs may not be as good as those produced by other techniques [51]. Overall, the choice of fabrication technique depends on several factors, including the required performance, design flexibility, scalability, and cost. Experimental implementations of RISs have been conducted in various applications, ranging from wireless communication systems to sensing and imaging. One common approach for experimental RIS implementations is to use a prototype system that consists of a transmitter, an RIS, and a receiver. The RIS is usually designed and fabricated based on a specific application requirement, and its performance is evaluated experimentally by measuring the signal quality and strength at the receiver [23]. RISs have a significantly improved signal quality and coverage area in wireless communication systems. For example, an experimental RIS system was used to create a virtual environment with multiple signal paths that can improve the performance of a wireless network in indoor environments [52].