*Proceeding Paper* **Development of a Knitted Strain Sensor for Health Monitoring Applications †**

**Beyza Bozali \*, Sepideh Ghodrat and Kaspar M. B. Jansen**

Faculty of Industrial Design Engineering, Delft University of Technology, 2628 CE Delft, The Netherlands

**\*** Correspondence: b.bozali@tudelft.nl

† Presented at the 4th International Conference on the Challenges, Opportunities, Innovations and Applications in Electronic Textiles, Nottingham, UK, 8–10 November 2022.

**Abstract:** As an emerging technology, smart textiles have attracted attention for rehabilitation purposes to monitor heart rate, blood pressure, breathing rate, body posture and limb movements. Compared with traditional sensors, knitted sensors constructed from conductive yarns are breathable, stretchable and washable, and therefore, provide more comfort to the body and can be used in everyday life. In this study, knitted strain sensors were produced that are linear with up to 40% strain, sensitivity of 1.19 and hysteresis of 1.2% in absolute values, and hysteresis of 0.03 when scaled to the working range of 40%. The developed sensor was integrated into a wearable wrist-glove system for finger and wrist monitoring. The results show that the wearable was able to detect different finger angles and positions of the wrist.

**Keywords:** knitted strain sensor; health monitoring applications; smart textiles; wearable textiles

#### **1. Introduction**

Flexible and wearable sensors have gained attention in recent years for a variety of applications, including human–device interfaces and the monitoring of health indicators such as respiration rate, heart rate and body position [1–3]. Conventional sensors are often integrated into structures as an external element or attached to the surface, but these create discomfort for the user due to the bulky and rigid nature of electronic devices such as IMUs for health monitoring purposes [4]. In this context, textile-based strain sensors offer a new generation of devices that combine wearability, lightness, comfort and stretchability with strain-sensing functionality. They can be comfortably worn and sense a wide range of body strains for a vast number of health monitoring applications, thus making them a good alternative to traditional bulky electronic sensors and making wearable systems more feasible [5].

By using textile-based strain sensors, it is possible to investigate and identify the ideal rehabilitation posture by analyzing the physiological properties of finger and wrist movements. For these wearable sensors, several materials and methods have been investigated to monitor the different parts of the body such as the finger, wrist, arm and leg for rehabilitation purposes. Ryu et al. investigated the performance of the knitted strain sensor in a glove by using silver-plated yarns to distinguish the finger movements, and the electrical responses of the compressive strain demonstrated strong stability and linearity through various finger rolling angles [6]. Lee et al. also concluded that the developed glove might be useful to amputees as a tool that allows them to rehabilitate or regulate a myoelectric prosthesis by putting the sensing elements into the glove and producing the whole-garment knitting technique for ease of commercialization [7]. Isaia et al. evaluated the performance of strain sensors knitted with various conductive yarns in terms of their sensing properties, hysteresis and comfort for joint motion-tracking applications during repetitive flexion–extension cycles [8,9]. Textile-based and knitted strain sensors have been proposed in many studies but have been less useful in practical applications due to

**Citation:** Bozali, B.; Ghodrat, S.; Jansen, K.M.B. Development of a Knitted Strain Sensor for Health Monitoring Applications. *Eng. Proc.* **2023**, *30*, 10. https://doi.org/ 10.3390/engproc2023030010

Academic Editors: Steve Beeby, Kai Yang, Russel Torah and Theodore Hughes-Riley

Published: 29 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the differences in measured strain during loading and unloading (hysteresis). In recent work [10], however, we were able to develop knitted strain sensors with extremely low hysteresis values. In this study, we integrated those sensors into a wrist-to-finger wearable demonstrator, the performance of which was tested.

#### **2. Materials and Methods**

In the previous study [10], a textile-based strain sensor was developed with a specific knitting technique, and its electromechanical performance was tested and reported as a hysteresis of 0.03 and a gauge factor of 1.19; in the following study, this newly developed strain sensor was integrated into a wrist-to-finger wearable system. A 1 × 1 rib-knit design was chosen for the strain sensor design and knitted on a Stoll CMS 530 machine using conductive yarn from Shieldex with a yarn count of dtex 235 and initial resistance of ≤600 Ω/m, and elastic yarn from Yeoman of Nm 15. Utilizing the plating technique, knitted strain sensors were produced by positioning the conductive yarn inside and the elastic yarn on the outside of the knitted structure (See Figure 1). In the second stage of the study, this developed knitted strain sensor was integrated into the wrist-to-finger wearable system to monitor the movements of the finger and wrist, as shown in Figure 2. Apart from the knitted sensor part in the wearable system, woven cotton fabric was selected for the rest of the design to be able to have a non-elastic and adjustable structure. A Bluetooth Arduino Nano and a power supply were integrated into the wrist-to-finger design using conductive yarns. According to the finger and hand size, the design can be easily modified, and this helps with ease of manufacturing in the later stages. The electromechanical performance of the knitted strain sensor was tested in the course-wise direction by performing four test cycles at 30 mm/min, using a custom-made tensile tester, and the resistance response during tensile extension–relaxation tests was assessed. The performance of the wrist-toglove design during finger and wrist movements was evaluated and the movements were recorded.

**Figure 1.** (**a**) The developed knitted strain sensor; (**b**) optical images of the sensing region which shows the conductive yarns (yellow) positioned inside and elastic yarns outside (white); and (**c**) illustration of the conductive and elastic yarn positioning within the knitted structure.

**Figure 2.** The wearable sensor-glove system with a knitted strain sensor to monitor (**a**) finger movements and (**b**) wrist movement.

#### **3. Results**

#### *3.1. Electromechanical Performance of the Knitted Strain Sensor*

The electromechanical performance of the knitted strain sensor was investigated and is illustrated in Figure 3. The sensor works linearly over a range of 40% with a hysteresis value and gauge factor of 0.03 and 1.19, respectively. The hysteresis along the strain axis amounts to 1.2% (absolute value) and 0.03 when scaled to the working range of 40% [10]. Because of the high linearity and low hysteresis, this developed sensor is found to be promising for monitoring finger or wrist movement, and the sensor is integrated into the wearable system to test its performance.

**Figure 3.** The developed knitted strain sensor graphs under four cyclic tests: (**a**) relative resistance change versus strain and (**b**) resistance versus time.

#### *3.2. Performance Evaluation of Wrist-to-Finger Monitoring System*

Bending the finger and wrist deforms the fabric, causing the sensor to change resistance. In this way, finger and wrist movements can be directly detected and monitored. Figure 4a shows the results of finger motion detection at different bending angles by the wrist-to-finger wearable system, targeted at 30◦, 60◦ and 90◦. This shows that with the increase in the bending angle of the finger joint from 30◦ to 60◦, and then to 90◦, the change in the relative resistance value increases. To investigate the wrist monitoring, the same test was applied to the wrist-to-finger wearable system, and the plot also provides distinguishable patterns in the flexed and unbent positions, as shown in Figure 4b. Note that tests were performed to demonstrate the effects, and the bending angles were not well controlled. However, the observed signal changes were seen to be measurable.

**Figure 4.** The wrist-to-finger wearable system: (**a**) finger movement monitoring at different angles of 30, 60 and 90◦, and (**b**) wrist monitoring.

#### **4. Discussion**

For applications such as posture monitoring, VR and rehabilitation, it is desired to have integrated, soft and stretchable strain sensors. We used a novel knitted sensor to construct a wrist-to-finger demonstrator. The measurement showed that it is possible to distinguish between the bending angles and loads of the fingers and the wrist [10]. When the sensor is implemented into the wrist-to-finger system, overall, it performs linearly. The small differences observed in peak values during the tests can be attributed to both the slipping of the garment with respect to the skin and the reproducibility errors inherent to manual movements. In a future study, these irregularities could be amended by fixing the wearable system to the joints of the body.

#### **5. Conclusions**

We produced a linear knitted strain sensor with low hysteresis and a working range of at least 40%. The developed knitted sensor can easily be utilized as a part of a wearable system to monitor finger and wrist movement without interfering with the existing fabric performance and appearance. The adjustable wearable system demonstrates the usefulness of the newly developed sensors and has the potential to be used for rehabilitation purposes in health monitoring applications.

**Author Contributions:** B.B. conceived the study and carried out the testing; S.G. and K.M.B.J. supervised and reviewed the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** Beyza Bozali was funded by the Turkish Ministry of National Education under the study abroad program for her PhD studies.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


**Disclaimer/Publisher's Note:** The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

## *Proceeding Paper* **Smart Sock Feasibility Study †**

**Lucie Hernandez**

Touch Craft Ltd., Penryn TR10 8NT, UK; luciefhernandez@gmail.com

† Presented at the 4th International Conference on the Challenges, Opportunities, Innovations and Applications in Electronic Textiles, Nottingham, UK, 8–10 November 2022.

**Abstract:** Touch Craft Ltd completed a feasibility study to understand the market position, technologies and user requirement for a smart sock sensing system designed to help monitor foot problems for people living with diabetes. The proposed system would involve a non-invasive, low-cost approach to gathering and measuring skin and foot data considering parameters such as pressure, temperature, and activity levels. The smart sock sensing system would enable both clinicians and patients to have a broader view of foot health and assist in improving the quality of life for patients and their carers.

**Keywords:** wearable technology; sensing system; diabetic foot; healthcare; biofeedback

#### **1. Introduction**

This paper describes a feasibility study undertaken by Touch Craft Ltd to develop a smart sock sensing system, which was supported by the Challenge Fund awarded by eHealth Productivity and Innovation in Cornwall and the Isles of Scilly [1]. The study was established to understand three key areas: the market position, technologies and user requirements for a smart sock sensing system that would be designed to help monitor foot problems.

The perspectives of clinical staff were critical in defining those people living with neuro-diabetes and foot problems as the patient population that would most benefit from a smart sock sensing system. The proposed system would help monitor foot problems in those people at high risk to help prevent ulcers and skin lesions as well as providing a foot management system to prevent re-ulceration. Healthcare professionals acknowledge the benefits of reducing foot pressures as a mechanism by which foot ulceration can be prevented [2]. Monitoring the temperature of the foot is an evidence-based preventive practice for patients at risk of diabetic foot complications [3]. The functionality of a smart sock would need to involve a non-invasive, low-cost approach to gathering and measuring skin and foot information involving pressure and temperature data.

#### **2. Materials and Methods**

The feasibility study adopted Public and Patient Involvement and Engagement as a method of engaging patients and public in the research, referencing values and principles that should be followed to reflect good practice and guide project development [4]. Additionally, interviews with medical specialists and healthcare professionals addressed requirements for a smart sock from a clinical perspective. There was agreement from clinicians around the type of data to capture, adherence and compliance issues and the identification of monitoring and rehabilitation as significant applications for assessing and treating patients.

The researcher used PPIE methods including questionnaires, interviews and group discussion to collect data from the Lay Panel at Barts Hospital in London. The group consists of 25 members made up of patients, nurses, voluntary representatives, and clinicians that meet quarterly to improve research for the benefit of people with diabetes, giving valuable on-line feedback to researchers and refining study design and delivery [5].

**Citation:** Hernandez, L. Smart Sock Feasibility Study. *Eng. Proc.* **2023**, *30*, 11. https://doi.org/10.3390/ engproc2023030011

Academic Editors: Steve Beeby, Kai Yang, Russel Torah and Theodore Hughes-Riley

Published: 29 January 2023

**Copyright:** © 2023 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

#### *2.1. Preliminary Prototypes*

Touch Craft Ltd worked with Marina Toeters from By-Wire.Net [6] to design and produce innovative textile prototypes as an initial proof of concept that demonstrated core functionality (see Figure 1). They were designed to collect, process and display pressure and temperature body data using off-the-shelf technologies to illustrate the possibilities of a wearable 'smart sock' to physiotherapists and orthopaedic clinicians.

**Figure 1.** Preliminary prototypes.

Further funding would support the development of more refined, robust prototypes if the project were supported in future studies. At this stage, the preliminary prototypes are presented as rough sketches using 'lo-fidelity' technology to prompt discussion and help outline more specific features and functionality. A feasibility study was not extensive enough to test all the different fabrics and sensors that could be used to construct a smart sock. Prototyping helped the team understand the possibilities, limitations and outcomes that could then be experienced by clinicians and patient groups.

#### *2.2. Printed Electronics*

Printed electronics are part of an emerging market for flexible and printed sensors and components that are flexible and offer seamless integration. Printed electronics are usually screen-printed in multiple layers and fused onto fabric substrates. Touch Craft Ltd worked with Marina Toeters from the fashion and technology company By-Wire.Net [6] to demonstrate the viability of printed electronic components that could be included in future prototypes (see Figure 2).

**Figure 2.** Printed electronics.

#### **3. Results**

A summary of themes that emerged from interviews and questionnaires are presented below and have been organised into sections depending on their overall theme.

#### *3.1. Remote Consultations*


#### *3.2. Foot Health, Mobility and Compliance*


#### *3.3. Practical Considerations, Design, Ergonomics and Fit*


#### *3.4. System Design, Technical Features and Data Transmission*


#### *3.5. Funding, Development and Future Studies*

• Respondents raised concerns around the project being a beneficial investment and requested more information about methods of funding smart sock development.

#### **4. Discussion**

Many of the findings outlined themes that could be explored in future studies but were outside the scope of this limited feasibility study. Patient and clinical perspectives emphasized technical as well as practical elements, which emerged as strong themes and demonstrated a concern with the design of the sock to maximise comfort, fit and performance. More details were requested regarding the technical features and how these could be more seamlessly embedded into the sock. Much of the future work in this area would concentrate on integrating printed electronics that are discussed in Section 3.2.

What emerged from the feedback was that many of the respondents were interested in the idea of collecting foot information and biofeedback to indicate issues around foot health. Patients were particularly interested in the idea of exploring further the idea of developing a 'digital health assistant' that could support patients in managing their own foot care.

#### **5. Conclusions**

Patient feedback indicated that a future smart sock should not replace in-person consultations but would complement the personal, reassuring meetings with health professionals. This would be enhanced with data gathered from home monitoring and self-management of foot problems. This might include conducting virtual foot checks as well as regular manual foot checks to monitor changes and risk factors.

To conclude, it is worth noting that more robust, hi-fidelity prototypes would not have been produced at this stage in the development cycle. The feasibility study aimed to present information to support the proposition for the smart sock concept being technically and clinically viable and identifying patient support for the idea. The study was successful in determining a clinical and patient need for a smart sock sensing system and suggesting that further funding and research could progress the idea into a robust and beneficial product.

**Funding:** This project received a feasibility grant awarded by EPIC, eHealth Productivity and Innovation in Cornwall and the Isles of Scilly, funded by the European Regional Development Fund.

**Institutional Review Board Statement:** Ethical review and approval were waived for this study due to the restricted nature of the research methods adopted.

**Informed Consent Statement:** Patient consent was waived as it was categorised as involvement rather than research because it asked opinion to inform what was proposed rather than collecting study data.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The data are not publicly available due to the conditions of the funder.

**Acknowledgments:** The author would like to thank Joanne Paton, School of Health Professions, Faculty of Health: Medicine, Dentistry and Human Sciences, University of Plymouth.

**Conflicts of Interest:** The author declares no conflict of interest.

#### **References**


**Disclaimer/Publisher's Note:** The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

## *Proceeding Paper* **Effect of Bandage Materials on Epidermal Antenna †**

**Irfan Ullah 1,\*, Mahmoud Wagih 2, Abiodun Komolafe <sup>1</sup> and Steve P. Beeby <sup>1</sup>**


**Abstract:** This study explores the effect of different types of bandages on the performance of an epidermal antenna. Three identical dipole antennas are designed on three different types of bandages, and the measured reflection coefficients, *S*11, show that the antennas resonate at the same frequency despite the different types of fabric bandages. However, the antennas resonance frequency shifts to a lower frequency when the antennas are mounted on the body. The transmission coefficient, *S*21, over a 60 cm link with a standard RFID antenna is at least −30 dB, and −34 dB in free space and on the body, respectively, demonstrating that the antenna is suitable for communication and wireless RF power transfer in wearable applications.

**Keywords:** epidermal antenna; dipole; smart bandage; e-textiles; wearable application

#### **1. Introduction**

Smart bandages incorporate various types of sensors to continuously monitor of wound−related parameters, such as temperature, moisture level, pH level, and wound oxygenation, in chronic wound care and management [1,2]. They provide wound data to health practitioners, which allows them to remotely assess the healing of chronic wounds without removing the bandage. The smart bandage requires a power source for embedded electronics, and an antenna for wireless data transmission to an external device. An antenna design is critical in the development of a wireless smart bandage since it can be used to transmit data and harvest RF energy.

Several antenna designs for smart bandages have been presented in the literature. In [3], a via free planar antenna, similar to an adhesive bandage, for medical telemetry service is proposed. However, the antenna includes a ground plane, which increases the thickness of the antenna and, therefore, is less suitable for wearable applications. Similarly, in [4], a planar rectangular loop antenna is implemented in a battery−powered smart bandage for wireless monitoring of wounds. The antenna is small in size but operates at a higher frequency of about 2.4 GHz. Furthermore, near−field communications (NFC) antennas are also being investigated for wireless smart bandages [5]. Such bandages have a very low reading range, and need the bandage to be in close proximity to the reader, which is especially undesirable for applications requiring continuous monitoring.

In this study, we proposed an all−fabric epidermal antenna operating at 915 MHz for smart bandages in healthcare applications. The study also explored the effect of different types of bandages on the epidermal antenna resonance frequency. Three identical dipoles were designed on three different types of bandage materials. The performance of the antennas in terms of the reflection coefficient in free space and in the presence of body were investigated. Measurements demonstrate that the different types of bandage material have no discernible effect on the antenna performance.

**Citation:** Ullah, I.; Wagih, M.; Komolafe, A.; Beeby, S.P. Effect of Bandage Materials on Epidermal Antenna. *Eng. Proc.* **2023**, *30*, 12. https://doi.org/10.3390/ engproc2023030012

Academic Editors: Steve Beeby, Kai Yang, Russel Torah and Theodore Hughes-Riley

Published: 29 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

#### **2. Antenna Design**

The epidermal antenna design is based on an electrical dipole with bending radiating arms to reduce the antenna size. The radiating arms of the antenna were tuned to resonate at 915 MHz in the presence of human tissue. The antennas were made of silk coated Litz wires with a diameter of 0.36 mm. A PFAFF creative 3.0 sewing machine was used to embroider Litz wires into three types of bandages: (i) cotton crepe bandage made of cotton, (ii) self−fixing cohesive support bandage made of cotton/elastane with latex, and (iii) adjustable cohesive bandage made of polypropylene and elastane, as shown in Figure 1.

**Figure 1.** Manufactured prototype of the embroidered all−fabric epidermal antennas: (**a**) cotton crepe bandage; (**b**) adjustable bandage; and (**c**) self−fixing bandage.

#### **3. Measurements and Results**

#### *3.1. Reflection Coefficient*

The reflection coefficient, *S*11, of the antennas were measured with a vector network analyser (VNA) in free space and in the presence of the body, Figure 2a. Figure 2b shows that the antennas resonate at around 1.10 GHz in free space, and 915 MHz when the antennas are mounted on the body. It is observed that the different types of bandages have no significant effect on the antenna resonance frequency, as shown in Figure 2b. However, the resonance frequency shifts to a lower frequency of 915 MHz in the presence of the body. This is due to the high dielectric constant and conductivity of human tissue.

**Figure 2.** (**a**) All−fabric epidermal antenna mounted on the human arm; (**b**) measured reflection coefficient, *S*11, of the three identical antennas in free space and on the body.

#### *3.2. Transmission Coefficient*

The experimental setup, Figure 3a, was used to measure the transmission coefficient, *S*21, between the fabric antenna and an external antenna. A circularly polarized antenna, with 8.2 dBi gain and operating at 915 MHz, was placed about 60 cm away from the bandage antenna. Both antennas were connected to a vector network analyser (VNA) and the transmission losses were measured. The measured *S*<sup>21</sup> frequency responses are depicted in Figure 3b for both cases with and without human tissue. The results show that *S*<sup>21</sup> is about −30 dB in free space, and −34 dB when mounted on the body. This shows that for a 30 dBm RF input power, at least −4 dBm will be received by the receiver antenna, indicating that the bandage antenna is suitable for RF power harvesting over a short distance at the UHF band.

**Figure 3.** (**a**) Experimental setup for measuring transmission performance; (**b**) measured transmission coefficient, *S*21, between the all−fabric antenna and an external antenna, showing that the transmission losses for the antenna in free space and on the body are −30 dB and −34 dB, respectively.

#### **4. Conclusions**

All-fabric epidermal antenna fabricated on fabric bandages is demonstrated in this paper. The resonance frequency of the antenna shifts to a lower frequency when it is mounted on the body due to the high relative permittivity and conductivity of human tissue. The measured results show that the different types of bandages have no significant effect on the antenna resonance frequency. Transmission losses, *S*21, of the antenna is −34 dB in the presence of human tissue when the external antenna is 60 cm away from the arm on which the antenna is mounted. This means that the receiver antenna will receive at least −4 dBm for an input power of 30 dBm. The antenna is flexible, lightweight, easy to fabricate, and comfortable to the body, and, therefore, can be used to develop wireless and battery−free smart bandages. The objective of our future work is to closely investigate all fabric dipole array for RF information and RF power transfer for wearable applications.

**Author Contributions:** Conceptualization, I.U. and M.W.; methodology, I.U.; software, I.U.; validation, I.U., M.W., A.K., and S.P.B.; formal analysis, I.U.; investigation, I.U.; writing—original draft preparation, I.U.; writing—review and editing, M.W. and A.K.; supervision, S.P.B.; project administration, S.P.B.; funding acquisition, S.P.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by European Regional Development Fund (ERDF) via its Inter-reg V France (Channel) England programme: Smart Textile for Regional Industry and Smart Specialisation Sectors (SmartT). The work of Steve Beeby was supported by the Royal Academy of Engineering under the Chairs in Emerging Technologies Scheme.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** We thank Yixuan Sun for helping in the fabrication of antennas on the bandages.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


**Disclaimer/Publisher's Note:** The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.
