## 6 Challenges and Future Directions  
Several challenges need to be addressed for the widespread adoption of RISs. The current challenges of RISs are design complexity, fabrication complexity, integration with existing systems, power consumption, robustness and reliability, and regulatory and safety issues, which are detailed next. The design of RIS structures can be complex and time-consuming, requiring advanced simulation tools and techniques. The optimization of RIS structures for specific applications is also challenging, and there is a need for more efficient and automated design methods [74]. The fabrication of RIS structures can also be challenging, requiring advanced techniques such as nanofabrication or 3D printing. The scalability and cost-effectiveness of RIS fabrication methods are also essential considerations [75]. RISs need to be integrated with existing wireless communication or sensing systems, which can be challenging due to compatibility issues or the need for additional hardware. RISs also need to be compatible with different wireless communication standards and protocols, which can add to the complexity of system integration [76]. The operation of RIS structures requires power, and the power consumption can be significant, especially for large-scale RIS applications. There is a need for more efficient power management methods and low-power RIS designs [32]. RIS structures can be susceptible to environmental factors such as temperature, humidity, and mechanical stress, affecting their performance and reliability. There is a need for more robust and reliable RIS designs that can withstand different environmental conditions [70]. RISs may need to comply with regulatory standards related to electromagnetic interference and safety. There is a need for more research and development on the safety and regulatory aspects of RIS applications [61]. There are several potential solutions to the current challenges facing RIS technology. One solution is to improve the design and optimization of RIS structures. This can be achieved by using advanced simulation tools and optimization algorithms that can accurately model the behavior of electromagnetic waves and optimize the RIS structure for specific applications. Machine learning and artificial intelligence techniques can also be used to develop intelligent RIS systems that can adapt to changing environmental conditions and optimize their performance [77]. Another potential solution is to improve the fabrication techniques for RIS structures. This can be achieved by developing new materials and manufacturing processes that can produce complex and high-performance RIS structures at a low cost. 3D printing and nanofabrication techniques can also produce customized RIS structures with high precision and accuracy [78]. In addition, the collaboration between researchers in different fields can help to address the current challenges facing RIS technology. Collaboration between electromagnetics, material science, and manufacturing experts can lead to developing new RIS structures and fabrication techniques optimized for specific applications [79]. Collaboration between researchers and industry can also help bridge the gap between research and commercialization, leading to the development of practical RIS systems for various applications [80]. Finally, government support and funding can be critical in developing and commercializing RIS technology. Government support can provide researchers and companies with the resources needed to develop and test new RIS systems and support for developing standards and regulations that can help ensure RIS technology's safety and reliability. Government funding can also help accelerate the commercialization of RIS technology by providing financial support for research and development and the production and deployment of RIS systems in real-world applications [81]. Future RIS research can be categorized into two main directions: improving the performance and capabilities of RISs and exploring new applications and domains for RISs.