Design of Wearable Devices for “Wearability”: A Scoping Review

1. Introduction

Wearable devices, including smartwatches, fitness trackers, smart clothing, smart eyewear, wearable cameras, physiological and biochemical monitors, are becoming increasingly integrated into daily life. There is a growing interest in utilizing these devices for clinical monitoring, recording, and decision-making. However, this interest is accompanied by several operational, legal, and reimbursement challenges that must be addressed to facilitate their use in clinical care [1,2,3,4,5]. Significant research efforts are focused on improving the performance and user acceptance of wearable devices. This includes advancements in wireless communications, optimization of operating systems, reduction of size, and enhancements in sensing capabilities. Improvements in sensing and transducer technologies will enable wearable devices to accurately monitor vital signs and other health metrics, providing valuable insights for healthcare and wellness applications. Wearable sensing technologies are poised to transform medical practice by offering continuous real-time monitoring of biomarkers associated with various health conditions, including diabetes, stress, inflammation, heart disease, gout, and fertility [6]. Numerous research initiatives highlight the latest advancements in wearable biosensing technology development, healthcare applications, and manufacturing concerns [7,8,9].

Despite technological advancements, the comfort of wearable devices remains a crucial design consideration. For long-term monitoring applications, the devices must be comfortable enough for continuous wear: they must be “wearable to be worn.” Therefore, a balanced approach that combines technological innovation with user-centric design principles is imperative for the successful development and adoption of wearable health devices [10].

Wearable comfort is directly related to user acceptance of wearable devices and can potentially affect the performance of the products in a variety of aspects such as safety, sensing accuracy, reliability, user dependency, adherence and compliance. The terms “adherence” and “compliance” are often used interchangeably, but they have distinct meanings. Compliance refers to the extent to which a patient follows the prescriber’s advice [2], implying obedience to the physician’s authority [3,4,5]. In contrast, adherence signifies a collaborative effort between the patient and physician to improve the patient’s health, integrating the physician’s medical opinion with the patient’s lifestyle, values, and preferences for care [6,7,8]. Moreover, the comfort level of wearable devices can be subject to numerous factors: wearing methods and regions, supportive device materials, mechanical configurations, and products appearance. Therefore, appropriate categorizations and definitions of wearable comfort are important for research on investigating comfort assessment. An extensive description has been given by Slater defining comfort as a pleasant state of physiological, psychological, and physical harmony between a human being and the environment [11]. Although there is no comprehensive and widely accepted definition of wearable comfort, attempts have been made to define it in either a holistic perspective or based on specific devices. Among various classifications of wearable comfort, physical comfort and psychological comfort are typically referred to as two major categories. As the names indicate, physical comfort mostly comes from physical contact between the human body and wearable devices, whereas psychological comfort is more about inner sensory perception such as emotional concerns about the safety and reliability of the devices. To assess the comfort levels of wearable devices, researchers have proposed various approaches in different application scenarios, which could be roughly categorized into subjective and objective assessment methods. Most of the subjective assessment methods are based on self-report scales including Visual Analogue Scales (VAS), Numeric Rating Scales (NRS), Verbal Rating Scales (VRS), and Likert Scales, by which comfort levels of subjects are evaluated in a straightforward way based on cognitive recognition of one’s feeling and comprehension. By contrast, objective assessments focus on measurement methodologies using quantifiable physiological signals and only indirectly suggest wearability.

This study focuses on reviewing the literature to explore the relationship between wearability and the design of wearable devices. It aims to identify how wearability is conceptualized and to examine studies that provide insights into how devices can be designed to enhance wearability. The objective is to understand the factors influencing wearability and to identify design principles validated by clinical trials that optimize user experience, comfort, and usability. This review seeks to contribute to a deeper understanding of the interplay between wearability and design, offering valuable guidance for the development of future wearable technologies for clinical use.

1.1. Wearable Devices

The terms “wearable devices”, “wearable technologies”, or simply “wearables” refer to the smart electronic devices worn on the human body that can sense, record, transmit, and analyze physiological or biochemical signals in a real-time manner and/or assist users to perform desired tasks and/or actuate certain physical activities. Ideally, these functions should be executed effortlessly to the person who wears the devices. Because of the portability, intelligence and convenience, wearable devices such as smart-watches, fitness trackers, smart clothing, smart eyewear, wearable cameras, and wearable medical devices are currently used in a variety of areas. They include medical health monitoring, human motion detection, interactive gaming, physical therapy and rehabilitation, sports performance monitoring, and so forth.

Wearability of biosensors remains unexplored and represents real-world usability. To align with emerging definitions of terminology used for digital health and wearable products, we define “utility” as whether a product has the features that users need, and “usability” as how easy and pleasant those features are to use. Fundamentally, the more wearable a device is, the more people will wear and adhere to using it [12]. Conversely, poor adherence to using a wearable biosensor will limit the ability to predict adverse events [13]. Wearability is the concept describing the characteristics of an effective wearable biosensor, spanning sensor accuracy, comfort, battery life, aesthetics, form factor, method of attachment to the patient, and more [14,15,16]. User-centered design acknowledges the intimate relation between the human body and wearable technology [17,18], and while critical to patient compliance, but many devices are simply not designed for comfortable or long-term use, even for users who may be more tolerant to design flaws [19,20,21].

1.2. Design Perspective

The adoption of new technology is often driven by its ability to address unmet patient or clinical need [22]. Two key checkpoints in the design process are verification and validation [23]. Figure 1 illustrates the distinction between design validation and clinical validation within the overall design of a wearable system. A critical first step is the translation of clinical needs (T1) into design input specifications, which guide the device design process. Subsequently, the design output is translated (T2) into the presentation of the new wearable system. Verification testing ensures that the technology is designed correctly. However, if the design outcomes are not validated to meet clinical needs, they will have little impact on clinical decision-making. The design process can be executed proficiently and verified, but biosensor technology can still be refined without being validated against clinical needs. Efforts to optimize performance and quality improvement of emerging technology are necessary, but the translation into clinical practice is hindered if, at the outset, T1 fails to convert user functionality specifications into a comprehensive set of design input specifications.

This study focuses on contemporary commercial wearable devices, akin to those commonly used for health monitoring, which have gained popularity through the “quantified self” movement. By examining the literature on these devices, the current work to explore device design elements in which validation efforts link design and wearability, thus providing design guidance that can inform standards [17,24,25] leading to devices designed to meet the needs of users. Through an analysis of existing literature, the study aims to uncover published research efforts that can inform the design and development of future wearable devices for health monitoring and beyond.

1.3. Research Questions

In this scoping review, the objective is to identify key concepts and determine the scope or coverage of the literature. While specific research questions are not required, having a set of general research questions helps guide the review process. Here are three general research questions for exploring the “wearability” of wearable devices in our scoping review:

• What are the key factors influencing the wearability of wearable devices?
• What methodologies and measures are used to assess the wearability of wearable devices?
• What are the reported user experiences and satisfaction levels regarding the wearability of different types of wearable devices?

1.4. Design Considerations

Translating functional specifications into design specifications is a well-known challenge in design [26,27] and for the present work, Table 1 highlights 12 key parameters that assist in understanding the landscape of potential user needs and related design criteria. Our scoping survey aligns with the Stanford Biodesign process by defining the problem space broadly to capture a wide range of user needs, not constrained by well-defined pre-existing functional specifications [23]. The parameters in Table 1 collectively ensure that the wearable device is practical, comfortable, and effective for use in a clinical trial setting. These design considerations are central to the critical appraisal of research studies.

Table 2 offers a rubric for evaluating papers based on their relevance to user needs outlined in Table 1. Scores range from 1 to 10, where 10 indicates the paper provides extensive data, examples, and critical analysis, making it highly relevant. Conversely, a score of 1 signifies that the paper either does not mention or only briefly touches on the topic without detail. This scoring system helps in assessing how well a paper meets the specified user requirements

2. Materials and Methods

A scoping approach was taken when collecting all relevant papers for the literature review. Pilot testing of literature searches across multiple databases revealed a limited number of studies examining design guidance derived from wearable validation studies. Given the lack of specific questions and concepts for critical appraisal, a mixed-methods (qualitative and quantitative data) scoping review was deemed more appropriate than a systematic review. While a systematic review allows for the examination of practice based on the quality of evidence, specific deficiencies, and gaps in evidence to inform future research, the scoping review approach was chosen to better map the breadth and scope of the existing literature. A scoping review aligns with the current work in which we seek to identify evolving or emerging topics prior to undertaking a systematic review, the latter of which could meet the criteria for registration with Cochrane [28,29].

2.1. Search Strategy

Our mixed-methods scoping literature review search strategy employed PubMed, Scopus, Google Scholar, ScienceDirect, Cochrane, and ClinicalTrial.gov to identify scholarly works linking the concept of wearability with specific device design features and guidance. Preliminary work focused on articles, papers, reviews and studies that explored the relationship between wearability and the design characteristics of wearable devices. By utilizing these comprehensive databases, we aimed to gather a diverse range of academic perspectives and findings on how design features influence the wearability of such devices.

The complexity of this topic became apparent in the need to identify slightly different sets of key words when searching a specific domain. In a way, this slightly decomposes the original topic question into more focused searches to enable capturing relevant literature from each domain. This is especially relevant in interdisciplinary topics where different fields may approach the topic from distinct perspectives. This approach allowed us to access a breadth of relevant literature and insights, facilitating a thorough analysis of the interplay between wearability and device design. However, it’s important to maintain coherence and relevance across the different search outcomes to ensure that the overall objectives of the review are met.

In this scoping review, we included references that were systematic reviews or mini-reviews, as these sources typically encompass citations that could be informative of experimental data or clinical trials. These references were explored with the belief they may provide valuable insights into the scope of design criteria used in wearable device design. Additionally, we included references that described experiments with the potential to include or lead to clinical trial work, further enriching our understanding of the design and application of wearable devices.

Figure 2 illustrates word search results from six databases, yielding 130 manuscripts as the foundation for further scrutiny in the current work. Figure 3 illustrates the manuscript review process in a PRISMA flow diagram, identifying the final number of manuscripts from the database search and other sources that were subject to detailed review.

2.2. Eligibility Criteria and Screening

Inclusion criteria for our scoping review required a focus on the qualitative aspects of wearability concerning wearable devices. This was to ensure inclusion of studies examining user experiences, perceptions, and subjective evaluations of device comfort, usability, and overall satisfaction. Papers meeting this criterion provide valuable insights into the qualitative dimensions of wearability, highlighting factors influencing user acceptance and adoption.

Additionally, our review targeted papers reporting on quantitative pilot studies or clinical trials that specifically examined device wearability-related outcomes described in Table 1. Such studies employ quantitative methodologies to assess objective measures of wearability, such as ergonomic design features, physiological impacts, and usability metrics.

By incorporating quantitative data, these papers contribute to a more comprehensive understanding of the effectiveness and efficacy of wearable device designs in enhancing user experience and usability. Furthermore, manuscripts intended for inclusion had to report a link between wearability and device design. These papers elucidated how specific design features or interventions influence the wearability of wearable devices, providing empirical evidence of the relationship between design and user experience. By examining this link, these manuscripts offer valuable insights for designers, engineers, and researchers seeking to optimize wearable device design to maximize user comfort, usability, and overall wearability.

Manuscripts that did not meet these eligibility criteria were excluded from the pool of candidate studies for detailed review.

3. Results

Table 3 lists the 19 literature search outcomes. Although this is a fairly small subset of papers for further study, it nonetheless represents the current state of the art in research related to wearable device design for comfort and wearability. Despite the extensive body of literature on wearable technology, our specific criteria—focusing on qualitative wearability assessments, quantitative evaluations, and the relationship between design features and user experience—resulted in a significantly narrowed pool.

Many of the papers identified during our review did not meet our inclusion criteria due to their primary focus on sensing materials, new sensor designs, or data analytics. While these papers contribute valuable insights to the broader field of wearable technology, they did not directly address wearability aspects or the impact of design features on user experience—the central themes of our study.

Despite the limited quantity of qualified papers, the selected subset exhibits diversity in methodologies, perspectives, and findings. These papers frequently represent cutting-edge research efforts aimed at enhancing wearable device comfort and wearability, showcasing innovative approaches, emerging trends, and relevant challenges. Although the literature search outcomes featured specific design data, our focused analysis in the next section aims to uncover key insights and implications that can inform future research and contribute to the ongoing evolution of wearable technology, ultimately optimizing user experience and satisfaction.

Each of the 19 papers were reviewed to determine whether it included one or more of the user-need design criteria. The scores presented in Table 4 are based on the scoring rubric outlined in Table 2. After evaluating each paper’s ability to address the user-need design criteria, an average score was calculated based on an unweighted average of the 12 design criteria; this is shown in Table 4 as “average score.”

Table 5 was then used to make an approximate assessment of the Cross-reference for the evaluation of the potential impact a reviewed manuscript may have as a data source for the ‘design for wearability’ process. This created three categories of papers as a way to summarize the abundance or absence of design data that would have an impact on an effort to design for wearability.

Table 5 enables the results of Table 4 to be prioritized for the potential impact a reviewed manuscript may have as a data source for the ‘design for wearability’ process. Table 6 provides the impact scale along with a brief summary description of each of the papers reviewed.

4. Discussion

It seems logical that the greater the wearability of a device, the more likely people are to consistently use it or comply with long-term usage recommendations. The current work finds that while numerous wearable technologies have been proposed and some have entered the commercial market, very little scholarship prevails on how to design for wearability. While the dominant use of wearables is for fitness applications, comprehensive studies on wearability and its impact on usage and accuracy in clinical contexts remain scarce. Validated design standards and recommendations for designing for wearability are virtually absent.

In our review of 19 research papers related to wearable devices, Table 3, Table 4 and Table 6, succinctly present a summary description for each of these papers, capturing their key findings, methodologies, and contributions. However, mere summaries do not suffice; we sought to distill their relevance further.

To achieve this, we devised a ranking system that led to a cross-reference for the potential impact a reviewed manuscript may have as a data source for the ‘design for wearability’ process. This assessment considered factors such as alignment with our research focus, methodological rigor, and impact on the field. Approximately 90% of the papers were ranked as providing low or medium impact in terms of the ability to inform the design for wearability process.

Although some studies emphasize user-centered design, objective metrics are often missing, and the concept of long-term wearability does not always align with patient needs. Notably, compliance with wearables tends to decline over time during research studies. For instance, in an adult study involving wristwatch-style wearable biosensors, mean daily data collection hours decreased from 13.3 to 6.3 hours over a 6-month period.[44]. In another study workers were asked how comfortable they found a temperature measurement sensor worn for an 8-hour work shift, though data on the impact of design on wearability was not collected [45]. Wearable devices that study self-powered systems [18,46] center on the technology to power a device but not on specific design elements for long-term wear. Recent research studying continuous monitoring of fall detection utilizing machine learning has the potential to tease out wearability classification, but the study design did not explore wearability [47].

With the Stanford BioDesign process in mind, user need is a critical starting point in design, and demographics matter [23]. For instance, consider the unique case of wearables for children. For applications in children, sensing modalities are limited due to size and sensitivity requirements. Currently, studies are limited to recordings of ECG, audio, and accelerometer signals. There have been no studies of pO2, blood pressure, respiration or other important biological signals. Assessments examine (a) increased compliance due to good comfort and interesting features [48] and (b) decreased compliance related to devices being uncomfortable, [48,49], embarrassing, noisy, or falling off a lot [50].

Ensuring wearability is crucial for real-world adoption of wearable devices. However, despite the growing interest in wearable biosensors, many studies fail to address wearability [15,51]. Unfortunately, our understanding of wearability has seen limited progress over the past several years [52]. Enhancing wearability assessment is essential for advancing translational research and promoting wider adoption of well-designed wearable biosensors.

4.1. Limitations

Our review has two main limitations.

• Despite providing insight into the limited data available to support design for wearability, the sample size for the scoping review was small and the search terms may not have been adequate to tease out design data. The field of digital medicine continues to evolve rapidly, and investigators may use terms in their studies that we did not use in our search.
• Our search was limited to peer-reviewed literature. It seems reasonable that we are unaware of many usability studies are undertaken by technology manufacturers - clinical trials we reviewed that were sponsored by industry appeared to be more for the goal of collecting marketing data, not providing design insight.

5. Conclusions

In our comprehensive literature search, we have identified three critical limitations in prior research within the wearable technology domain:

• Lack of Standardized Assessment Methods: The absence of an accepted and standardized method for assessing wearability has resulted in inconsistent evaluations across studies. Researchers often rely on subjective criteria, leading to variability in how wearability is measured and reported.
• Qualitative Nature of Assessments: Most existing assessments of wearability remain qualitative, lacking objective metrics for rigorous analysis. While self-report scales provide valuable insights, they fall short of quantifying wearability in a consistent and comparable manner.
• Limited Utility for Design: Despite the wealth of existing studies, their qualitative nature and lack of quantifiable data hinder their practical utility in designing wearable devices. Insights gleaned from these studies do not directly inform design decisions or address the specific needs of users.

To advance the field, it is imperative to address these limitations. Researchers must collaborate to establish robust methodologies for assessing wearability objectively. By doing so, we can enhance the design, usability, and overall impact of wearable technologies, ultimately benefiting users and advancing the field.