## 2 Literature Review  
### The Product Design and the Current Usage of Wearable Devices  
The so-called \"wearable devices\" detect and analyze physiological data through various sensors, radio frequency identifiers, and positioning systems embedded in clothing, accessories, and even human bodies. These devices are regarded as optimal electronic tools for making intelligent judgments and delivering information [4, 5]. With the increasing sophistication of technological tools, the development of wearable devices since the beginning of the 21 st century has not only benefited from the monitoring of various physiological signals but also from the integration with the Internet of Things (IoT). Wearable devices and their accompanying systems can interact to establish a mutual communication environment that is increasingly being utilized in future healthcare [6]. Furthermore, within the broader IoT framework, pervasive data can be continuously retrieved from the human body or any connected device, forming a machine-to-machine communication environment that provides personalized, networked health solutions [7].  
In the context of sustainability, wearable devices contribute by reducing the environmental impact of healthcare delivery. By enabling real-time monitoring and remote healthcare services, they reduce the need for physical hospital visits, thus decreasing the consumption of medical resources and transportation-related carbon emissions. Moreover, advancements in energy-efficient designs and the use of eco-friendly materials in the production of wearable devices further align with sustainability goals. Sustainable wearable devices, by promoting preventive healthcare, not only enhance individual well-being but also contribute to a more sustainable healthcare system.  
At present, wearable devices on the market can be categorized by functionality and product type. Functionally, wearable devices fall into three categories: life health, information consultation, and somatosensory control. Life health products, such as smart brackets, capture and analyze physiological data; information consultation products, such as smartwatches and smart glasses, provide real-time information access; and somatosensory control products, primarily designed for entertainment, include various motion-sensing controllers [1, 4, 8]. As for product types, wearable devices have become increasingly miniaturized due to advancements in microelectronics and wireless communication technologies. These wearable monitoring systems, now an integral part of everyday life, include wristband devices (e.g., watches, bracelets, gloves), head-mounted devices (e.g., glasses, helmets), electronic textiles (e.g., coats, underwear, pants), and other body-sensing devices [9, 10].  
The wearable device market has experienced significant growth in recent years. According to Mordor Intelligence (2020), Asia and Oceania are expected to be the fastestgrowing regions for wearable devices until 2024, driven by the rapid aging of the population. While Europe and the United States have shown moderate growth, the adoption in Africa and South America has been slower due to economic factors [11]. Notably, the COVID-19 pandemic in 2020 accelerated the sales of wearable devices, as the crisis heightened public awareness of health metrics. According to Statista, the sales of wearable devices reached a new high in 2020, as people purchased wearable health devices to monitor their well-being [12]. Even after the pandemic, wearable device sales continued to grow steadily, reflecting the increased focus on personal health and preventive care, which aligns with the global shift towards sustainability in healthcare.