Physics
Computer Science-2021.gz:143412
Computer Science
92c00cf17c6117ac60bbf6745abdbc0a9abde500
Review on Data Acquisition of Electrocardiogram Biometric Recognition in Wearable Smart Textile Shirts

Electrocardiogram (ECG) wearable smart textile shirt has widely been investigated due to its high flexibility, reusability, comfort, and the possibility of being used for home-based, real-life activities and real-time measurement. ECG smart textile shirt is an embedded textile sensor inside a cloth that can capture ECG data in more convenient ways and ease user-friendly, especially for continuous and long-term ECG data acquisition outside the laboratory environment. However, the current challenge with ECG smart textile shirt is the reliability and quality of data acquired by the wearable smart textile. This review will mainly focus on the research strategies in the early stages regarding data acquisitions in ECG smart textile shirt. It will introduce researchers’ data acquisition methods in the biometric recognition system using wearable ECG smart textile. The Scopus and Mendeley databases review may help future researchers consider different parameters, which affect the reliability and data quality when selecting data acquisitions strategies in a biometric recognition system using wearable ECG smart textile shirt.


Introduction
As the world keeps advancing to digitalisation, technologies are pushed to a certain stage, where almost anything and everything can be mimicked, duplicated, hacked, replicated, and identity theft skyrocketed in terms of cases [1], [2]. Everything from economic, finance, healthcare, cybersecurity, and human turns into digitally coded for most daily transactions in this digitalisation era. The conversational identification properties such as smart cards and passwords are widely used and open to counterfeit and falsification by irresponsible people [3], [4]. Meanwhile, physiological biometric properties such as the face, iris, gait, and fingerprint face the same war of treat [5]- [8].
Uncompromised to this issue had sparked the new interest in liveness detection in human biometric recognition, and liveness detection itself is to decide if the biometric being captured is a real measure of the authorised, living person present at the time of detection [9]. Besides that, as personal biosignal properties, ECG is the heart's electrical activity, and it was proof of liveness at the point of detection [4], [10], [11]. Meanwhile, ECG smart textile is a new technology that has embarked on the market trend globally. There is an increasing number of studies that focus on this innovative technology merging between textile and ECG [12], but those studies mainly concentrate on ECG  [18]. Therefore, there is still plenty of space for this new field to be explored in the field of biometric recognition and focusing on its data acquisition.
ECG smart textile is a type of smart shirt in which tracking devices are often inserted into garments to identify, measure, track and report vital human signals such as works in [19], [20]. Textile electrodes used in ECG smart textiles are more comfortable and user-friendly, unlike traditional gelled electrodes, especially for continuous monitoring of cardiac conditions. Furthermore, the conventional gelled electrodes produce rash or skin reaction along with the electrode adhesive. [21]- [23], and it also contributes to bacterial growth [24].
Wearable ECG smart textile in data acquisitions is also a tough challenge because textile ECG electrodes' consistency depends significantly on various factors. Those factors are skin to textile surface, moisture [25], contact strain [26], positioning of electrodes, user movements, acquisition of real-life data [27], long-term ECG acquisitions [28], [29] and validity [30]. However, the appealing prospect and smart textile's bright future had gained a pool of interest in this promising field, especially in its reliability and performances as an ECG data acquisitions tool.

ECG data acquisition in wearable smart textile
Fukuma et al. in [31] stand with the belief that a t-shirt type wearable ECG system was suitable for continuous, daily long-term use among young people with high physical activity. The research had made ECG data acquisition using an electrode textile material called Hitoe embedded into a commercially available t-shirt as in Figure 1(a). The study consists of the 100-male respondent with a mean age of 52 ± 5.4 years. The study prolongs for over two months, with each participant were required to wear the smart textile for 10 hours long each day for four days a week to monitor their ECG. Even though the research produced a promising prospect in monitoring the ECG, the study only has a male subject.
Similarly, Tsukada et al. in [18] also used the same material as [31], which is the Hitoe textile material patch sewn to a t-shirt for males and brassiere for females respondent in the research. This study supported by 66 adult respondents consisting of 47 male and 19 females within the age range of 35.5 ± 10.3 years. The work performed data acquisition of the smart textile towards the respondent in supine rest, sitting and physical exercises. The results show that the signal to artefact ratio was better in male than in the female. The research was outstanding in proving the reliability of smart textile using Hitore material patch. However, the work's duration and iteration were unclear.
Meanwhile, Boehm et al. in [32] developed a 12-lead ECG smart textile t-shirt prototype using Shieldex Medtex P180 material as their sensor. The study tested the prototype in Figure 1 (b) with three male subjects under three different physiological conditions: lying down in a bed, sitting, and walking. The work also admitted that more subjects are needed to ensure the smart textile prototype's long-term functionality and stability. Even the duration of the experiment and the respondent's age was not clearly mentioned in this study.
Yu et al. in [33], the same researcher team in [32] use 12 lead ECG smart textile t-shirt prototype as in [32], moved to another level by taking the research further with five subject daily life routines at home and workplace for 422 hours of the uncontrolled environment outside the laboratory. However, there is no clear information regarding the gender and age of the subject in this study, but the researcher agrees that one lead ECG is sufficient for a wearable smart textile.
Meanwhile, Chetelat et al. in [34] worked with Long Term Medical Survey System (LTMS-S) as in Figure 1 (c), which consist of a system embedded within a vest of wearable smart textile. Out of 16 subjects in the study, which were divided into two groups with a mean age of 26.3 ± 7.3, only ten subjects were involved in the ECG data acquisition stages using LTMS-S wearable smart textile. Furthermore, the research did not specify the age group of participants engaged with ECG data acquisition. The work consists of different physiological conditions in standing, sitting, supine, running, and walking, but the study did not clearly highlight the data acquisition duration.  [31]. (b) Smart textile prototype in [32]. (c) LTMS-System in [34]. (d) Cardioskin™ in [29]. (e) Front inside view of Hexoskin in [35]. (f) HeartIn FIT T-Shirt. Furthermore, the ECG signal's quality was lower in walking condition than in the other three rest conditions demonstrated by Fouassier et al. in [29]. This study was supported by 12 lead ECG acquisition T-shirt called Cardioskin™ by Bioserinity, as shown in Figure 1 (d). The study acquired ECG from 30 subjects, 21 male and nine females within the age of 29.5 ± 7.8 years, while wearing the smart textile for 3 minutes in each of three resting positions that standing, seated, supine rest and walking. Besides that, researches in [28], [29] suggested that smart textile should be evaluated over a more extended period for ECG monitoring regarding their reliability and performance [30], [36].
Among commercially available ECG smart textile in the market and have numbers of research on its capability and performance are like Hexoskin in Figure 1(e). The manufacture descriptions show that the smart textile has the capabilities in ECG morphologies data acquisition. However, most published papers use this interesting, smart textile focusing on other modalities like heart rate and respiration [27], [28], [35], [37], [38]. On the other hand, HeartIn smart textile in Figure 1(f) was another interesting ECG smart textile capable of capturing ECG data that is commercially available. Meanwhile, in Figure 2, this study showed promising results of ECG waveform captured from both Hexoskin and HeartIn smart textile shirt for the same subject. Similarly to what had already established in the global market, Emglare smart textile is another name that can yet claim its capability in ECG data acquisition.

ECG data acquisition in biometric recognition in wearable smart textile
A few studies have been done regarding data acquisition in ECG biometric recognition using smart textile. The best available research using commercial smart textile is successfully performed by Ye et al. in [39] using VitalJacket [40], and this is followed seven years later by Pourbabaee et al. in [41] that used smart textile from OMSignal [42], [43]. Furthermore, the work in [39] showed that ECG signal capture from smart textile can be used as robust biometrics and as an automatic login solution for wearable smart textile. Extracted ECG fiducial point data from five fire-fighters over six months period was used in [39] and proved their theory.
In contrast, the study in [41] believes that fiducial point detection is an error-prone strategy affected by changes in the signal slopes, inverted or abnormal waves, noise and artefacts. The researcher works with 33-female subjects record their ECG daily for six weeks, where all the subject were required to provide at least five ECG recordings on different days using the smart textile. However, the actual effect of other physiological condition, gender differences and age of the subject is the element that both researchers can further investigate.

Future work in ECG data acquisition for wearable smart textile.
Through an analysis of the research work surveyed, it can be concluded that ECG biometrics based on wearable smart textile shirts have the potential to be one of the leading biometric traits. However, some challenges still need to be addressed, particularly concerning the acquisition strategies. It is remarkable how the ECG, which currently acquired widely through medical-grade equipment, is measurable through wearable smart textiles in seamlessly integrated systems, significantly increasing the acquisition process's comfort. Research should continue to explore seamless acquisition settings and address the possibility of non-contact ECG measurement to further enable real ECG biometric applications in smart textiles as the gap is still wide open in this field. Undoubtedly, there are a number of wearable researches in biometric [44], but in the smart textile fields, the numbers are still far left behind. Many studies in wearable smart textile still focusing on the reliability of monitoring the ECG with the smart textile [45], [46]. Little has been known to the world regarding ECG studies in the smart textile shirt as summarised in Table 1. To the best of our knowledge, only two available studies in smart textile shirt venture to biometric recognition. There are no substantial baseline numbers available on the optimum subject, duration and activity, to achieve reliability performance on ECG smart textile. For instance, many researchers favour acquiring the ECG data during supine rest, seating, and walking to simulate smart textile reliability in daily human routine performance. Some researchers prove smart textile performance outside the controlled environment focusing on human daily life routine activities in the workplaces and home. However, the actual effect of different physiological conditions, gender differences, and age is still lacking and can be further investigated in ECG smart textile.

Conclusion
In this review work, we pointed out the potential limitation present in the past published articles on wearable ECG monitoring smart textile shirt focusing on data acquisition, challenges, and issues in the field of biometric regarding the wearable ECG smart textile shirt. There is still plenty of space that can be filled regarding the stability, reliability, and performance of ECG wearable biometric smart textile shirt as a data acquisition over a more extended period and in real-life scenarios. Furthermore, there are also a lot of opportunities for ECG smart textile biometric to grow in multi-demographics studies.