## 2 Wireless Communication Technologies
#### Smart Wearable Devices
Future implantable sensors need to use as little power as possible, for example, getting power from the body's movement, heat, or other in situ energy sources such as biofluid [91; 103]. In [107], flexible MEMS-type sensor devices were proposed. For example, MEMS technologies have been used to produce such flexible electronics, they need equipment for vacuum deposition, photolithography, and wet and dry etching, as shown in Figure 3d. Various advanced flexible electrochemical sensor devices have been used for various applications, such as in fitness and biomedicine [108]. With significant advancements in wearable devices, there has been an exploration of near-infrared spectroscopy (NIRS) electronics, which hold potential for numerous healthcare applications [109]. Several unique shapes of wearable body-worn sensing devices are proposed in the literature. In general, there are two types of sensing and monitoring systems: contact-based and contactless wearable sensor devices. They include optical and RF signal-based devices. Vital signals are frequently monitored by wearable sensors; nevertheless, there are drawbacks to this technology, including the high costs, dependence on user compliance, and the potential for disease transmission if not adequately disinfected. In situations where hygiene is a concern, like hospitals, contactless systems, especially those that use RF signals in the range of 30 kHz to 300 GHz, offer a viable alternative by enabling non-intrusive monitoring without direct touch or line-of-sight. Wi-Fi signals, which range in frequency from 2.4 GHz to 7.2 GHz, have become an effective means of tracking activity and identifying vital signs. These signals find use in fall detection and real-time health monitoring. By integrating these Wi-Fi-based devices with IoT frameworks, healthcare providers can receive useful data, and smart-home features like automatically altering environmental parameters depending on vital signs can be improved. But compared to wearable and visual-based systems,  
Figure 3: Smart devices: (**a**) laser-engraved [92], (**b**) flexible [105], (**c**) multi-materials devices [106], and (**d**) MEMS sensor [107]. Wi-Fi and other contactless technologies have some limitations in terms of precision and dependability, notwithstanding their potential. It is anticipated that advancements in big data and machine learning, as well as the development of sophisticated semiconductor technologies like CMOS and Bi-CMOS, would lead to improvements in wireless sensing for medical applications. Technologies, including mHealth, 6 LoWPAN, and Institute of Electrical and Electronics Engineers (IEEE 11073, Health informatics--Medical,/health device communication), are being investigated for remote health monitoring in smart-city frameworks as IoT solutions in healthcare progress from basic designs to more elaborate smart systems [110]. Furthermore, new technologies like NFC, RF transmitters, and carbon nanotubes are opening new possibilities for data extrapolation in smart healthcare applications. With the help of multidisciplinary research, these technologies should continue to progress, opening new ways for contactless, affordable healthcare monitoring, especially when issues with accessibility and dependability are resolved [110; 111; 112]. Recently, wearable sensors for various purposes, including energy storage and health monitoring, have been transformed by recent developments in biodegradable and superhydrophobic materials-based and MXenes-based devices [24; 86; 113]. Furthermore, multimodal sensing e-skins fall into three categories, such as systems that combine chemical, electrophysiological, and physical sensors, and are investigated to simulate human skin for next-generation smart technology [5]. Recently, self-powered Triboelectric Nanogenerator (TENG) sensors monitoring ethanol during simulated exhalation and humidity using copper-oxide and polyethylene-oxide nanofibres have been investigated, which are suitable for non-invasive breath analysis applications.