3.3.1. Selection of Materials
The sustainability of a given item of wearable technology depends on the sustainability of its various components [
104]. Accordingly, designers must take into account both the sustainability of the article of clothing or bodily accessory—including fabric, fasteners, buttons, and yarn—as well as the sustainability of its integrated electronic system—such as electronic components, sensors, batteries, or displays. In addition, material choices have a direct influence on the sustainability of the whole product life cycle, including sourcing and distribution, care and maintenance practices, product durability, functionality, aesthetics and comfort, as well as on the end-of-life treatment in terms of reusability and recyclability. A study by van der Velden et al. [
22] demonstrated the importance of material selection in environmentally conscious product design. They used the method of life cycle assessment (LCA) already at the prototyping stage to analyze and compare different materials and their respective eco-costs and found that the same desired smart capabilities and design results were achieved by using alternative materials with less environmental impact. Their research shows that designers working with smart materials should consider what alternative options there are that do not interfere with the targeted user experiences [
22]. Also, sourcing recycled materials and upcycled components instead of new ones is an important design strategy, especially in small-scale production and in research and development. Designers should be encouraged to harvest and utilize electronic components from retired consumer electronics, including motors, lights, and actuators, as well as structural and mechanical parts such as buttons and hinges [
105].
True sustainable production requires that a product is designed in a way that the valuable resources inside it are not lost at the end of the product’s life cycle. Closed-loop design, also referred to as cradle-to-cradle—based on the cradle-to-cradle principle discussed in the beginning of this article—is an alternative design approach that aims to eliminate or reconceptualize the concept of waste [
20]. In this approach, the materials used should be either biodegradable, so that they can be safely returned to the environment, or reusable, so that they can be easily reused, remanufactured, or recycled into new materials and products [
20] (p. 285). Prahl’s research, which investigated the closed-loop design opportunities for wearable sensors, recommends using natural and synthetic nonwoven materials—which are suitable for reuse and recycling—along with transient electronics, which are dissolvable at the end of their life [
20]. The use of advanced biological responsive materials, such as natto cells to achieve both functionality and biodegradability, has also been researched [
106]. Other examples of biodegradable electronic concepts include biobatteries made of paper [
107] and electronic ink made from cuttlefish ink [
20,
108]. Overall, the sustainability of materials such as smart textiles requires a system-thinking approach and depends on the sustainability of its components [
109].
3.3.2. Manufacturing
The cradle-to-cradle model extends to manufacturing, as it is important to ensure that when a product reaches the end of its life, the energy used to manufacture it is preserved as much as possible. While this is relevant for the disposal phase, there are choices in the manufacturing techniques and design of the parts to enable ease of assembly as well as disassembly [
110]. The level of integration of the components for wearables directly impacts this, and when electronic components are involved, they can be either embedded within the fibers (jacquard) or removable parts. A design approach that aims for a low degree of integration will be beneficial not only for reuse and recycling, but also for washing, cleaning, and updating rapidly changing technology [
111]. An example of a low level of integration includes a shirt where the smart object is built inside a button or incorporated into pockets. However, as seamless integration has become the ultimate goal of smart textile designers, permanent and invisible integration methods—such as knitted, woven, printed, embroidered, laminated, and welded technologies—have prevailed lately on the innovation agenda [
20,
112]. In addition to levels of integration, alternative methods of disassembly should be considered. For instance, active disassembly, triggered degradation, or end-of-life unzipping of electronics are methods that enable recovery levels of over 90% of the original structure [
20] (p. 293).
There are also business models that move parts of the design, manufacturing, and assembly closer to the consumer, such as those offered by IKEA. Involving the consumer in the process also provides an opportunity for them to feel more connected to the product. In terms of wearable technologies, various toolkits are emerging that help both the designer and end user to develop functional wearable prototypes. LilyPad, which is based on the Arduino platform, is distributed through SparkFun and other channels and enables the end user to design and build wearables [
113]. Adafruit developed a line of similar wearable electronics components called FLORA. They also provide many how-to guides and tutorials to educate designers on how to make their own wearables [
114]. Developing wearables is challenging, and the toolkit approach saves time and resources to enable wearable prototype development. Another example, the FlexAbility toolkit, assists designers in developing wearable technologies for people with physical disabilities [
115]. With the rise of 3D printing, direct digital manufacturing—a process by which the consumer can print the product or parts as needed—radically changes existing manufacturing models while at the same time challenging businesses to develop successful business models [
116].
3.3.4. Use and Consumption
Laundry and maintenance create challenges for fabrics and other materials with an electronic component. However, when it comes to use practices, smart textiles are often introduced as a potential development that could render the textile industry more sustainable, since they are expected to enhance textile durability as well as to shift the way we have traditionally taken care of our clothes. One prominent stream of innovation in this regard is self-functioning textiles. Self-functioning abilities—such as a photocatalytic self-cleaning coating which removes stains, bacteria, and odor by degrading dirt particles and organic substances [
119], or a textile finish like Nano-Tex [
104,
120], which eliminates wrinkles—can help to reduce maintenance-related resource consumption including human effort, detergents, water, and energy [
119]. Furthermore, self-healing textiles—conventionally developed by using a chemical coating finish of microcapsules, hydrogels, or other polymeric matrices—can repair torn surfaces with automatic self-repairing functions and thus extend the product life cycle [
119]. While designers of smart clothes are encouraged to consider different and novel ways of reducing the environmental impact generated during the use phase, they should also consider how users can be educated about the correct maintenance, care, and repair practices, as it is important that they master these basic skills in order for the intended sustainability benefits to be fully realized.
Energy consumption is an issue through the whole life cycle of wearable technologies, from the material selection, garment production, and distribution phases to the use phase. While this energy consumption is expected to decrease due to changing care and maintenance practices, integrating electronic components into regular clothes and accessories is likely to increase it. So far, most of the wearables on the market today are powered by batteries that must be replaced and disposed of or charged frequently. In this context, the use of alternative, more sustainable energy harvesting methods has been researched. For instance, photovoltaic (PV) cells, which convert the sun’s rays into electricity, have developed to the point where they are physically flexible enough to be integrated into garments [
16,
29,
121]. Ideally, solar energy would be used to power not only the apparel itself, but also other devices, such as mobile phones [
111] (p. 326). In addition, thermoelectric generators that produce electricity from a temperature gradient represent an alternative power source to batteries [
122]. However, due to the high power consumption of most wearables, and the limited energy that can be captured by sunlight or body heat, creating truly energy-autonomous applications is challenging. One promising way to ensure an immediate, continuous, and sustainable power supply for wearable electronics is to harvest biomechanical energy [
123]. By a material- and structure-optimized triboelectric nanogenerator (TENG), human motion can be converted into electricity [
123]. Such technologies are currently under development and indicate the rapidly expanding possibilities for addressing issues of sustainable energy consumption in wearables.
Purchasing a wearable product should not increase the need for—and thus consumption of—other technological gadgets, such as tablets or laptops [
124]. Therefore, the technological independence or multi-platform usage of wearables should be assured. Furthermore, in order to avoid technological obsolescence, smart garment applications should be in constant development so that the technology is always up-to-date and compatible with the newest software, ensuring continued ease of use within the digital ecosystems of the user [
124] (p. 166).
Another issue that should be considered regarding sustainability is whether wearables meet users’ needs. Failing to meet user needs can result in early abandonment and disposal of wearables. There are many studies that have examined the main factors supporting sustained use of different types of wearables, and some of the frequently mentioned reasons for early abandonment include unattractive design, useless functions, and difficulty of use and care [
26,
27,
28]. User needs can be divided into utilitarian and hedonic needs, of which the latter is considered to be more crucial—in our modern world clothing is more likely to be worn because of social conventions rather than because of its functionality [
9,
27]. It has been claimed that functional high-performing devices may be abandoned if aesthetics are not considered [
9]. This stems from our tendency to communicate to others about ourselves through the aesthetic and expressive elements of our clothing. Thus, people do not wear devices that do not address their aspirational and style needs [
9]. Moreover, it has been found that visual attractiveness is not only about looks, but can also have an influence on usability ratings [
125]. When it comes to functionality, it is noted that smart clothing should not “interfere with the wearers’ everyday life” or require too much effort to use or maintain [
26]. Wearable technologies must be comfortable to wear and flexible in order to adapt to the movements of the body [
126]. Further, having multiple functions and use purposes instead of one was preferred among end users [
127]. In addition, wearables can have functions that help to optimize the eco-efficiency of numerous other daily activities [
1] (p. 214). As a part of smart home vision, wearables can offer an opportunity to avoid unnecessary energy losses in many areas of life, such as heating, lighting, and mobility [
128]. In that sense, wearables can be designed to include functions that can persuade their users to commit to a more sustainable behavior [
1].
In order to prolong their use, wearable technologies should be designed to enable the end users to customize and even hack the product to better fit into their lives in order to effectively meet their needs (cf. [
129]). Indeed, designers of wearables are also encouraged to consider how to allow users to change the different features of devices according to their own needs, preferences, and personal style, as it has been found that users’ requirements for wearables and aesthetical aspects change with different contexts of use [
10,
11,
130]. Furthermore, this creative freedom to modify wearables can help build the meaningful connection between the user and the device through which the product’s life span can be extended [
29,
104,
124].
In this context, an outfit-centered design approach—which argues that wearable devices should be designed in a way that they fit with users’ clothing and jewelry fashion—has been supported (i.e., [
10,
17]). In short, a long-lasting and aesthetically pleasing wearable device should be classical in style, but at the same time easily transformable according to different trends. Such transformability can be achieved, for example, through color- and pattern-changing textiles—enabled by different display technologies—or silhouette-, size-, fit-, and design-changing smart clothes that can be produced using motors or mechanical controls, shape-memory alloys, or inflatable materials [
131].
On-body placement and engagement with the product can influence a product’s longevity [
17]. Exactly what determines socially acceptable placement for wearable technology differs with respect to gender and different cultures [
9] (p. 4162), but most studies indicate that the wrist is currently the most suitable body location for placing a wearable computer since it is less intrusive and less of a social anomaly [
27,
127,
132]. A study by Smelik et al., which explored the integration of photovoltaic (PV) cells into fashion from the users’ perspectives [
2], concluded that wearables should be tailored to the body if their aim is to be socially acceptable and become sustainably integrated into users’ everyday lives.
Overall, “crowdfunding”—referred to as the newest dimension of user-involved design—is argued to be an efficient way to quickly determine which design initiatives are worth producing and distributing, based on the amount of pledges the product receives [
104] (p. 410).
3.3.5. Disposal
The amount of e-waste, which is already considered a global sustainability problem, is likely to increase when wearables reach their end of life and give rise to new disposal and recycling issues [
1]. Regular garments are difficult and costly to recycle, mainly because they are made of many different substances—from different fibers to non-textile-based components such as zip fasteners and buttons [
104]. Adding electronic components to garments will only make the recycling process even more complicated. Disassembling the different components, namely electronic systems, energy supplies, and interconnecting wires or fiber-optic cables, may be difficult or even impossible as the development of wearables heads toward seamless and permanent integration. As a result, numerous small e-waste items that contain problematic and toxic substances—such as heavy metals and halogenated organic compounds—can end up in normal household waste or find their way into recycling processes where they act as contaminants [
133]. Through competitions and public recognition, governmental initiatives have encouraged the development of responsible recycling and disposal for companies involved in production [
84], yet there is a patchwork of legislation for e-waste that is confusing for both producers and consumers [
134]. In the research literature there are attempts to develop a methodology for assessing the end of life (EOL) of household appliances and electronics with an aim toward reuse and recycling [
135].
To tackle the issue of e-waste, new solutions for products’ reuse, remanufacture and recycling have been implemented, including material innovations and Design for Disassembly like those discussed earlier. Ideally, designers should strive toward designing products that do not need to be disposed of in their entirety (modularity). Additionally, by assisting the consumer in properly disposing of or recycling parts of the product, the environmental impact of components like dangerous chemicals and batteries can be minimized. In this respect, product-service systems (PSSs) can be especially beneficial, as they help to close the loop and encourage the consumer to return pieces in exchange for another, thus increasing rates of recycling and ensuring a reduction in disposal [
87]. Deposit-refund systems that have helped the beverage industry in reducing waste plastics have also been used to incentivize consumers to properly dispose of electronics. Producers of wearables might consider ways to encourage their proper disposal or provide “trade-in” programs.