Step 1: identification of user needs
State-of-the-art overview
The findings from the mixed-methods systematic review [28] showed that accelerometer data from wrist worn sensors have mainly been studied to detect epileptic seizures and to quantify upper extremity activity and movement patterns in stroke and PD. Usability had only been evaluated in a handful of studies [28]. The readiness to use wearables was strongest for unnoticeable devices resembling ordinary items like watches and clothing, since the appearance in public was considered an important factor for willingness to use the device [29,30,31,32,33]. As expressed by the users, it was important that the sensors could be removed at any time [30]. In studies using straps to fasten the sensors, discomfort and difficulties to take the sensors on and off were described as problematic [29, 32]. For long-term use, sensors embedded into clothing or ordinary items were preferred to attaching the sensors directly on the skin [1, 33]. Other factors that were considered to influence the usability were comfort, how easy it was to take the wearable on and off, what the required wearing time was and whether the design was user friendly. Anticipated or experienced concerns included using the wearables incorrectly, confronting technical failures, and receiving insufficient support and feedback.
Clinical user needs
To monitor and characterize movements was the first priority for all three neurological conditions. Upper limb movements below the elbow along with trunk movements were considered central. Recordings of heart rate, heart rate variability, blood pressure, oxygen saturation and perspiration were considered important for better detection and differentiation of epileptic seizures and for monitoring of autonomic dysfunction in PD. Taking into account the technology readiness and shared priorities for all three conditions, it was concluded that movement data from arms and trunk together with heart rate was the first priority. Collection of data during free out of hospital everyday activities was highly prioritized, although validation of measurements in more controlled environments might be needed prior to free-living testing.
User perceptions of wearables
The focus group study revealed that simple, attractive, discreet design, comfortable to wear and water resistance were considered as facilitators of usability [34]. Aspects considered as barriers were: bulk, weight, and difficulty to take the garment on and off. Also the aspect of appearance was a barrier when the sensors were different from ordinary items or could not be covered when needed. Motor and cognitive disability was thought to hinder the usability when fine motor dexterity, memory and problem solving skills were required to handle the wearables. Complex systems were considered to increase the risk for using the wearables incorrectly or encountering technical problems. The potential benefits for improved treatment effect were, in general, valued more than the possible inconvenience of wearing the sensors. In addition to usability preferences, both patients and health professionals in the focus groups were confident that data from wearables can improve clinical decision making, but the health professionals also saw a potential risk for self-adjustment of medication. The use of wearables was, in general, perceived as cost-effective, but it was unclear by whom and how the data would be analysed and interpreted. It was a common understanding that privacy issues need to be addressed and it needs to be clear what is registered, why it is registered and how the output data will be used.
Step 2: garment requirements
Based on the identified user needs it was considered important that the garment should be aesthetic and appealing to users and not cause stigmatization or unwanted attention. It should be safe to use and comply with personal integrity. The garment should allow use by people with disabilities and the design should be simple and intuitive. It should be easy to take on and off and it should not interfere with movements and common daily activities. The sensor components should be as small and light as possible.
The sensor zones around the torso and forearms needed to be tight to maximize skin contact and minimize motion artefacts. The sensors and garment needed to be robust, tolerate normal handling and washing using gentle machine wash cycle (the battery should be detached during washing). The time and effort for production needed to be reasonable. The results from the small pilot testing of fabric preferences showed that characteristics such as elasticity, breathability, not too warm, too cold, too thick or too thin were considered important. Some participants disliked the polyester fabric for everyday use and the color black was preferred. Men favored a v-neckline and women a more open u-neckline.
Movement data and heart rate (HR) were prioritized. A garment for the upper body with three measurement zones, one located centrally on the trunk and one on each forearm was designed (Fig. 2). The garment was designed with ¾ -length sleeves for two main reasons. First, the arm movements were considered important for detection of epileptic seizures as well as for quantification of dyskinesia in PD. The input from the distal part of the arm is also informative for quantification of the use of upper limb and symmetry quantification in stroke and PD. Second, the ¾ -length sleeve was preferred to long-sleeve since it would be more practical during the summer season and would be easier to use under other clothes.
Step 3: garment development and production
Development of electronics
The electronics, sensor units and a central unit, consisting of conventional electronics were mounted on Printed Circuit Boards (PCB, Figs. 3 and 4). The sensor units on each arm’s sensor zone include a motion sensor (accelerometer and gyroscope) and an optical sensor (photoplethysmograph, PPG) for detection of pulse. The sensor units on the trunk sensor zone consist of a motion sensor and two textile silver coated electrodes for heart rate registration (electrocardiogram, ECG). The heart rate reference electrode is placed in the left arm’s sensor zone. All sensor units incorporate a flash memory and a microcontroller. The central unit consists of a microcontroller and a signal switching device (multiplexer, MUX), battery, flash memory, three LEDs and a low energy bluetooth unit. The central unit controls the measurement, communication and read out of data. An 1800 mAh battery was used that allowed approximately 24 h continuous recording at 100 Hz sampling rate, but individual configuration of the sensors including the sampling rate was possible. The start and stop of the measurement and the transfer of data required connection to a PC using a terminal application.
Development of garment and integration with electronics
First a small series of prototypes of viscose fabric with standard sizes for men and women was developed and produced at the Swedish School of Textiles. For the interconnections between sensors, the central unit and battery, a solution with coupling wires running into a channel with a meandering path between the sleeves and torso was used to allow unrestricted movements of the arm and trunk and to avoid mechanical strain on the interconnections (Figs. 3 and 5). The electronics were integrated into the garment during the sewing process, whereas the final connections to the central unit were carried out manually, after the assembly of the garment.
Subsequently, following iterative changes to the production process, a new series of 10 prototypes (5 for women, 5 for men) was produced. One important change was that the electronics were added in the final production step to reduce the risk of damaging the cables during sewing. The textile electrodes for heart rate were made from commercially available electrically conducting silver plated fabric (Shieldex), sewn into the garment over the chest and padded to add pressure to improve skin contact. Different technical solutions were tested to ensure that the electronics and connections were water resistant. For this prototype a transparent epoxy was used to seal the electronics. The transparency was necessary for the optical sensor to function, although the hardness of the epoxy was a risk to the cables. The textile electrodes and the electronics were connected with a thin tubing of conductive fabric (Shieldex) attached to the electrode during manufacturing. Part of the coupling wire from the electronics was wrapped around the tubing, and the connections were then secured and sealed with different dimensions of heat shrinking tubing. The weight of the prototype with all sensor components except the battery was 280 g.
Algorithm development
The parallel work with algorithm development made it possible to define the core features for each targeted neurological condition. In epilepsy, the accelerometer data from wrist sensors was used to develop machine learning algorithms based on standard binary classifiers in combination with a custom post-processing stage trained to discriminate the characteristic progression of tonic-clonic seizures. Evaluation showed that more complex non-linear classifiers (k-nearest neighbours, support vector machines and random forest) improved the algorithm generalizability and robustness for detection of epileptic seizures [35, 36]. In PD, both time and frequency domain representations of the accelerometer signal from wrist sensors formed the foundation for future development of algorithms quantifying movement-related signs, such as dyskinesia, bradykinesia and tremor. In stroke, several standard accelerometer features were evaluated as measures of activity levels. The measurement data indicated that the Signal Magnitude Area (SMA) and corresponding asymmetry indices (SMA ratio between more affected and less affected arm/leg) were most sensitive to variations in activity [37].
Step 4: evaluation of garment design, functionality and usability
Evaluation of the garment functionality and washability
Garment functionality was evaluated in the first series of 10 prototypes (Fig. 5) by 14 healthy individuals who wore the garment for 2–48 h. The battery life-time for the system was approximately 12 h. The limited battery life-time was mainly caused by the LEDs in the two PPG sensors on the arms. An error in data storage on the memory flash card was identified as a critical problem and occurred in 8 out of 10 prototypes at some point after 1–3 measurement sessions. All electronic components passed the washing test, although in two cases the connection between the sensor and cables broke close to the hardened epoxy edge during washing or use. The textile silver plated electrodes conducted a clear electrical signal when slightly damp. As anticipated, the ECG and PPG signals were also sensitive to motion induced signal artefacts. The most robust signal was obtained from accelerometer and gyroscope sensors.
Evaluation of the usability and garment design
The experiences and perceptions of the users (individuals with PD and healthy adults) after wearing the garment for 2 days were analysed using qualitative content analysis methodology. Three main categories emerged from the interviews reflecting acceptance in daily life, concerns and worries and personal preferences.
Acceptance in daily life comprised acceptance for continuous use, acceptance in daily activities and acceptance for health and disease benefits. The garment was, in general, well accepted for continuous use over 2 days. Some participants expressed reluctance wearing the same garment for the next day after a night’s sleep. Taking a shower or washing in the morning was important to feel clean and fresh after a night’s use. Wearing time of 2 days was acceptable to most and a 4 to 14 days measurement was presumed to be acceptable if the garment could be changed or washed. A longer wearing time was considered meaningful when the results were important to one’s health or quality of life. Similarly, the aesthetic aspects, such as colour and design, were considered less important when the measurement would provide important information for disease management. The garment did not hamper movements during daily activities and participants in general felt comfortable when using it in front of other people. They described that wearing the garment was perceived as a positive experience and that others were interested in the technology and it was fun to talk about it. Many described that they forgot that they were wearing a special garment.
Concerns and worries reflected concerns with the garment fit and worries about electronics and technology. Participants described that it may be difficult to find a good fit that will suit people with different body sizes. Several participants perceived that the measurement zones (chest and arms) were too tight, especially when taking the garment on and off. Some reported that the tight arm cuffs pulled the neckline down too much. Most participants were aware of the battery, and a few found it troublesome at night. A smaller and lighter battery that could be changed without taking off the garment was a common wish. Half of the participants with PD received help with changing the battery and taking the garment on and off, but they also described that this was not different to the help they usually needed. Some participants worried that the electronics might break when the garment is taken on and off. They also felt unsure where they could get a solid grip when taking the garment on and off. Some participants reflected over the feeling of being monitored and that specific activities could be seen in the data. They also wanted to be able to get some indication on the garment, whether it is measuring or not. To see the results of the measurement was also of interest to many. Participants were conscious about the measurement in the beginning, but only a few commented that they were reflecting about the integrity and what can be seen in the data.
Personal preferences comprised aspects regarding comfort, style and use. Many described that the personal feeling of the garment was important. They found the garment to be comfortable, nice and pleasant. Men described that they felt unfamiliar with the neckline, which was perceived to be too wide and they also felt that it was unusual to use a ¾ long sleeve shirt under other clothes. To use a long-sleeve garment at night was also considered unusual by some. Black was preferred by most and the viscose material was perceived as pleasant. The breathability of the material and not feeling sweaty were described as pluses. The garment was predominantly used under other clothes and was not considered suitable for strenuous exercise when getting sweaty, during hot summer days or festive occasions.
Identification of improvements for the next prototype
The main improvement areas for the next prototype will include improved connection between electronics and cables to avoid disruption of the cables during use and improved water protection of the electronics and connections. The moulding of the electronics was done manually with clear epoxy resin. This process was time consuming and provided varying results. A more standardized and unified method will be used for encapsulating the electronic parts to ensure the washability and general robustness. Some minor changes in the hardware will be made, e.g. exchange of the flash memory cards, using smaller programming contacts and adding a magnetic switch for hardware reset. The design of the garment will also be upgraded with respect to comfort, design and robustness in accordance with the knowledge gained from the literature, focus groups and user interviews. The cables and connections will be strengthened to tolerate the pulling and strain of normal handling e.g. when taking the garment on and off. The location and design of the battery pocket will be altered, making it easier to change battery. A more elastic fabric will be used for the measurement zones and the neckline will be designed less wide. To ensure that the garment will be easy to take on and off by individuals with impairments, a changed design that allows loosening and tightening of the measurement zone over the chest will be implemented.