Highly Stretchable Textile Knitted Interdigital Sensor for Wearable Technology Applications

Wearable technology applications have experienced remarkable development and advancements, with soft and stretchable strain sensors playing a significant role in this progress. Despite the promising potential of combed‐shaped interdigital capacitive strain sensors in wearable electronics, several challenges exist, including limited stretchability, universal mass fabrication, and seamless integration into diverse clothing parts. This study presents a textile knitted interdigital capacitive sensor that incorporates stretchable conductive yarn, produced using textile twisting technology, to achieve stretchability and adaptability, allowing seamless conformation to human body movements and textile materials. The fabrication process involves embedding the interdigital electrodes and interconnections directly into the fabric through textile knitting technology, ensuring robust integration. Furthermore, this work presents opportunities for commercializing the stretchable interdigital strain sensor through a low‐cost and mass production strategy. Electromechanical characterization demonstrates exceptional performance with high stretchability (≈230%), excellent linearity (R2 = 0.997), a gauge factor (GF) of −0.68 representing relative capacitance change, and a rapid response time of 66 ms. To validate the usability of sensors in wearable technology, a knee brace application is employed to investigate capacitance changes during walking and cycling exercises. This approach will accelerate the accessibility of wearable stretchable interdigital sensors for all.


Introduction
Interdigital capacitive sensors have emerged as a technology with various applications, ranging from human motion detection [1][2][3][4][5][6][7][8] DOI: 10.1002/adsr.202300121 and touch sensing [9] to soft tactile robotics [10] and industrial automation. [11]hey have a fast dynamic reaction, allowing them to detect and respond to capacitance changes quickly. [12]They are useful in applications that require real-time measurements and quick sensing capabilities. [11]Moreover, their sensitivity allows them to detect even the most minute variations in the measured quantity, enhancing accuracy and precision. [4]These characteristics make interdigital capacitive sensors suitable for applications requiring precise measurements or the detection of motion changes in human activities.Nevertheless, the fabrication of flexible interdigital capacitive sensors has challenges and limitations that require effective solutions to achieve seamless integration in wearable technologies. [13]he production of mechanically flexible interdigital capacitive sensors faces several key challenges, primarily concerning the employed fabrication strategies.The selection of electrode materials assumes paramount importance in this context. [14]Also, achieving precise and uniform electrode patterns on flexible substrates poses a significant obstacle.For instance, flexible interdigital capacitive sensors often incorporate materials with distinct and limited stretchable mechanical properties, such as conductive inks or thin metal films on polymer substrates.[17][18][19][20][21] Generally, these sensors have been manufactured on limited flexible substrates, typically mounted on flat surfaces.However, with the increasing demand for wearable electronics, there is a need for stretchable and repeatedly bendable sensors.][24][25][26][27][28][29][30][31] This not only enhances user comfort but also allows the sensors to mimic natural mobility. [3]Furthermore, their stretchable property empowers them to be applied to various shapes and surfaces, enabling direct integration into garments or attachment to body parts, thereby offering design flexibility for numerous wearable electronic applications.
The development of stretchable sensors involves a multitude of intricate challenges that demand careful consideration.Among these challenges, the primary focus is on ensuring the durability and long-term performance of these sensors.They must demonstrate the ability to withstand repetitive stretching and deformation without compromising their functionality or precision, particularly in situations involving continuous mechanical strain or stress. [32]The selection of materials that can maintain their electrical or sensing properties while undergoing stretching and deformation poses a substantial obstacle.These materials must exhibit the qualities of flexibility, stretchability, and durability to endure repeated stretching without experiencing degradation.Additionally, it is other important challenge to achieve cost-effective and scalable manufacturing of these sensors.35][36][37][38][39] Nonetheless, embedding sensing elements on flexible and stretchable substrates presents significant technical challenges.[42][43][44][45][46][47][48][49][50][51][52] Interdigital capacitive sensors have been integrated using various textile production methods, such as weaving [53] and embroidery [54,55] technologies.These interdigital sensor production approaches, however, do not impose stretchability constraints and completely support the different body proportions and movement requirements of wearable technologies.Furthermore, these approaches are insufficiently rapid and cost-effective for high-volume production.As a result, efficient and stable textile manufacturing methods for large-scale manufacture of stretchy interdigital strain sensors are required.
In this study, we address these challenges by presenting novel textile knitted interdigital capacitive strain sensors in wearable technologies.By incorporating stretchable conductive yarn manufactured with twisting technology into the sensors, we achieve high stretchability and adaptability in the interdigital capacitive strain sensor, enabling them to seamlessly conform to body movements and the natural flexibility of textile materials.Leveraging the capabilities of textile knitting technology, we seamlessly embed these electrodes of sensor and connection lines directly into textile products, integrating them into the fabric itself.This strategy allows users to effortlessly and comfortably utilize the sensors in their everyday lives, bringing additional functionalities to their daily experiences.Furthermore, the utilization of knitting technology enables mass production, ensuring rapid and costeffective manufacturing of high quantities of interdigital sensors.With increased accessibility, these stretchable conductive yarns, produced through the direct twist method, coupled with the integration of knitting technology, emerge as an effective approach for the fabrication of interdigital capacitive sensors in wearable technologies.

Fabrication of Stretchable Conductive Yarn and Knitted Interdigital Sensors
The first step in creating a textile knitted interdigital capacitive sensor was to make stretchy conductive yarn.To achieve this, we employed the DirecTwist2C twisting machine, as depicted in Figure 1a, to wrap electrically conductive yarn around an elastomeric core yarn.The elastomeric core provided the yarn with its stretchable properties, while the conductive yarn enabled its electrical conductivity.
During the direct twisting process, the elastomeric core and conductive yarns are carefully aligned and subjected to twisting.During the direct twisting process, the conductive yarn was wrapped around the elastomeric core, and the twisting action involved rotating the yarns around their longitudinal axes (Figure S1, Supporting Information).This process securely bounds the conductive yarns with the elastomeric core, creating a stretchable continuous conductive yarn.
The direct twisting process demanded precise control over various twisting parameters to achieve the desired characteristics of the conductive yarn.These parameters directly influence the stretchability, mechanical strength, and electrical conductivity of twisted stretchable conductive yarn.To determine the optimal production parameters, we conducted multiple trial sets, aiming to achieve an optimum cover ratio of the conductive yarn while minimizing tension within the elastomeric core.These trials were documented in Table S2 and Figure S2, Supporting Information.
The number of turns per meter (Tr m −1 ) played a crucial role in determining the cover ratio of the conductive yarn around the elastomeric core.Similarly, the elastane feeding ratio (E%) defined the proportion of elastane relative to the cover yarn during the direct twist process, allowing us to comprehend its influence on the inner tension of the twisted yarn and its overall mechanical properties.Ensuring a sufficient covering ratio and yarn tension were critical to ensuring the usability of the stretchable conductive yarn for creating sensors in knitting technology.By optimizing the parameters of the direct twisting yarn fabrication, we successfully produced stretchable conductive yarn, which serves as a key component in our knitted interdigital capacitive sensor.
After the successful production of stretchable conductive yarn, the manufacturing stage of the knitted interdigital sensor has been initiated.Various knit pattern designs were created using the APEX4 software of flat weft knitting technology to fabricate comb-shaped interdigital capacitive sensors.The interlock knit structure (Figure S3, Supporting Information) was chosen as the basis for the knitted interdigital sensor design.The interdigital sensing design consisted of a combination of two rows of comb electrode areas and eight rows of separator dielectric areas of weft knitted loops.This design was preferred to obtain a high capacitance value of the sensor in the unstretched state because the separator dielectric areas created a smaller area where the electrodes were close to each other, hence increasing the capacitance.In addition, the separator dielectric areas improved the reliability of the sensor by helping to prevent the electrodes from touching each other.The production of knitted interdigital capacitive sensors was carried out on the Shima Seiki N.SVR 122 flat weft knitting machine.Fabricated stretchable conductive yarn was used for the conductive electrodes, while 140/70 × 2 denier Polyamide 6.6/elastane was employed for the dielectric layers of the capacitive sensor.These yarn materials were carefully selected for their compatibility with the knitting process and their ability to provide the necessary electromechanical properties for the functionality of sensor.The flat knitting production process of the interdigital capacitive sensor, showcasing the successful integration of the stretchable conductive yarns and dielectric layers into the final sensor design, is presented in Figure 1b and Video S1, Supporting Information.By incorporating interlock knit pattern design and stretchable conductive and dielectric yarn materials, the production of comb-shaped interdigital capacitive sensors using knitting technology was accomplished.These sensors not only exhibit high functionality and stretchability but also can be seamlessly integrated into various wearable electronics and smart textile applications.

Electromechanical Theory
The fabricated sensor is characterized as a planar 2D interdigitated capacitive sensor, which calculates the capacitance using Equation (1).
In this equation, C L represents the measured capacitance value. and  r correspond to the permittivity of free space and the dielectric layer, respectively.W represents the overlapped length of electrodes, d denotes the distance between the two electrodes, t indicates the thickness of the conductive electrode, and n signifies the number of interdigital electrodes within the structure.
For our knitted sensor, there are 24 electrodes with an overlapped length W of 50 mm, a thickness t of 1.4 mm, and a distance d of 2.4 mm, and the relative permittivity of free space  r is set to 8.85 × 10 −12 F m −1 .The capacitance value obtained from Equation (1) provides essential information about the electrical behavior of interdigital sensor.It reflects the interplay between various geometric parameters, such as the number of electrodes, their dimensions, and the distance between them.Moreover, the dielectric properties of the yarn material surrounding the electrodes influence the capacitance reading.Understanding these factors helps in designing and optimizing stretchable interdigital knitted sensors that are both sensitive to stretching and tailored to meet specific capacitance requirements.
The reason for selecting a separation distance of d = 2.4 mm between the electrodes was to ensure that they could be positioned as close to each other as possible without causing a short circuit.This specific value of 2.4 mm was determined based on the results of fabrication trials, where it was found to correspond to 8 loops set during the weft knitting process.Maintaining this separation distance was crucial because anything less than 2.4 mm in an interlock structure would result in the electrodes coming into contact with each other when the sensor was stretched.By choosing this design parameter, the sensor was able to preserve its functionality and capacitance even when subjected to stretching, ensuring reliable performance.
Our sensor is used in a knee brace application, and because of the dynamic nature of knee movements during various activities in the knee region, accurately detecting knee bending and motions is of paramount importance.Therefore, the deliberate choice of designing the sensor with a 50 mm width specific to this region is aimed at ensuring precise detection of these movements.
Figure 2a displays the structural configuration of the textile knitted interdigital strain sensor and its response under transversal and longitudinal strains.Additionally, Figure 2b illustrates the electrical field between loop-based knitted electrodes in both relaxed initial and strain-induced states.Knitted textile sensors, comprising interconnected loops, have the capability to modulate capacitance, allowing the sensor to be tailored for different applications and optimized according to the specific parameter to be measured.Each loop acts as a fundamental unit, significantly impacting the overall capacitance of the sensor.The spacing between the loops plays a critical role in capacitance modulation, with closer arrangements fostering enhanced electrical interac-tions and resulting in amplified capacitance levels.The dielectric permittivity directly impacts capacitance values.The structure and geometry of the sensor are critical factors affecting capacitance variations.
The direction of tensile strain influences the capacitance, which changes as negative or positive in a stretchy interdigital knitted sensor.Stretching the 2D knitted interdigital sensor in two directions, the course and wale directions, is possible.The horizontal rows of loops in a knitted fabric are referred to as the course direction.The wale direction in a knitted fabric refers to the vertical columns of loops.Tensile force in the course direction of the stretchable electrodes causes the dielectric loops between them to compressed, reducing the distance between them while stretching and elongating the electrodes, resulting in an increase in their effective surface area.Conversely, when a force is applied in the wale direction, the dielectric loops between the electrodes are decompressed, increasing the distance between the electrodes.The ability to modulate the capacitance change of a stretchable interdigital knit-based sensor by applying tensile strain direction is a valuable property for a variety of applications.

Electromechanical Performance of Sensor
To facilitate the electromechanical performance of the knitted interdigital sensor, we utilized a specially designed tensile test rig and graphical user interface (GUI) (Figures S4 and S5, Supporting Information).After evaluating various knit structures, the interlock knit structure was selected as the most suitable choice due to its excellent recovery properties after being stretched.We fabricated an interdigital sensor design with dimensions of 6 cm in width and 8 cm in length, consisting of a total of 24 electrodes.The conductive electrodes included 2 rows of knitted loops, while the dielectric area comprised 8 rows.
During the strain testing, the sensor was evaluated in two directions, termed the "positive" and "negative" samples.The positive sample was stretched in the transversal (course) direction, whereas the negative sample was stretched longitudinally (wale).This enabled us to evaluate the effect of testing direction on capacitance variations.A strain level of 50% was chosen to fulfill the requirements for sensing human body posture and movement.Each positive and negative sample consisted of three replicates, and the tests were conducted for 100 cycles, reaching a strain level of 50%.
The sensor exhibited greater stretching capability (≈230%) in the transversal direction, as demonstrated in Figure S6, Supporting Information.When the sensor is extended in the positive direction in the course direction, the dielectric yarn loops between the stretchable electrodes are squeezed.When the distance between these electrodes is shortened, capacitance increases.Also, the knitted electrodes are stretched and extended, increasing their effective surface area and contributing to the increase in capacitance.The dielectric loops between the electrodes, on the other hand, are decompressed in the negative direction.This effect causes the distance between the electrodes to rise, resulting in a decrease in capacitance.Furthermore, the knitted electrodes may experience compression and contraction, resulting in a decrease in capacitance.Sensitivity is a crucial characteristic that determines the performance of a sensor in measuring the intensity of tested physical phenomena.Sensitivity is typically quantified using the gauge factor, which represents the relative change in the reported signal compared to the initial stationary value.For capacitive sensors, the GF is calculated as (∆C/C n )/, where ∆C is the capacitance change, C n is the initial capacitance value, and  is the applied strain.To determine the GF of different sensor designs, the sensors were extended up to 45 mm for the negative design and 35 mm for the positive design, equivalent to a 50% strain, surpassing the required range of ≈35%-45% for human applications. [1]igure 3 illustrates the GF and working range of the sensors.The positive sample in Figure 3a achieved a peak GF of 0.46.In Figure 3b, the negative sample exhibited a peak GF of −0.68, consistent with previous studies on capacitive-based strain sensors ranging from 0.7 to 1. [56] Both sensor types maintained their peak GF values within the strain levels required for human applications, demonstrating their suitability for practical use.The results provide valuable insights into the GF and optimal working range of the sensor, highlighting its high sensitivity to dimensional deformation.
Another important characteristic of the sensor is its response time, which determines the speed and accuracy of detecting changes in the tested physical phenomena.A quicker response time ensures higher accuracy in detecting shifts.On the other hand, signal drift is an undesirable feature where the signal magnitude fluctuates under constant intensity.The response time of the negative design was measured to be 66 ms (Figure 4a), while the positive design exhibited a response time of 68 ms (Figure 4b).These response times indicate the delay between mechanical stimulation and the sensor signal when it rises three times above the standard deviation of the baseline signal magnitude.Comparing these response times to the motion frequencies of the human body, the sensor can effectively monitor human movements since human limb reflexes rarely exceed frequencies of 10 Hz.In the next section, the successful monitoring of knee movements will be demonstrated to further validate the suitability of the sensors for human applications.
The linearity of the negative design, as shown in Figure 4c, was calculated to be R 2 = 0.997, consistent with a previous study using a different interdigital design.Likewise, the linearity of the positive design was calculated as R 2 = 0.994 (Figure 4d), demonstrating a high degree of linearity.
To assess the drift values of the sensor signal, strain levels of ɛ = 0.125, 0.25, 0.375, and 0.5 were applied and held for 10 s to observe the effect of static loading on signal drift.Figure 5a illustrates the electrical response changes of the negative design  under constant strain values.The drift values for the negative sample were measured as 2.4%, 1.8%, 1.3%, and 1.6% for ɛ = 0.125, 0.25, 0.375, and 0.5 strain levels, respectively.Similarly, the drift values for the positive design were determined as 1.8%, 1.6%, 1.6%, and 1.3% for the corresponding strain levels (Figure 5b).These values can be attributed to the relaxation of the knitted loops in a stressed position.
In wearable electronic applications, the durability of sensor structures is a crucial characteristic.Figure 5c presents the outcomes of a cyclic strain test performed on a negative sample, where the sample was subjected to 50% strain and then relaxed for 100 cycles.Similarly, Figure 5d displays the results of the positive sample subjected to the same test.These results indicate that both designs exhibit negligible changes in the ∆C/Cℴ ratio, signifying the exceptional stability of the knitted interdigital structure under repetitive stretching.This remarkable durability can be attributed to the uniformly interconnected loops that form a secure conductive network within the knitted fabric structure.

Knee Brace Application
To assess the applicability of the textile knitted interdigital sensor in human motion applications, we modified the knitted base fabric of the sensor to tightly enclose the knee area of the subject, as demonstrated in Figure S7, Supporting Information.This adap-tation enabled a comfortable fit on the human body while facilitating the collection of data related to knee movement.
To enable wireless data collection in real-life scenarios, we utilized the DFRobot Beetle BLE device from Shichuan, China, along with an Arduino Uno equipped with Bluetooth 4.0.By measuring the relative change in capacitance, we captured the data wirelessly, eliminating the need for cumbersome wired connections.The compact circuitry design required a small portable battery and consumed minimal energy while efficiently collecting and transmitting data through low-power Bluetooth technology.
During the experiment, which included consecutive movements of walking and cycling exercises (Figure 6a-c and Video S2, Supporting Information), we recorded the real-time data.The walking tests were performed at 1, 3, and 6 km h −1 speeds.The recorded data revealed a clear relationship between the intensity of the exercises and the corresponding sensor signal magnitude, as shown in Figures 6b,d.Specifically, we observed increasing capacitance values during both walking and cycling exercises.This indicates that our stretchable knitted interdigital capacitive sensor effectively captures and quantifies the dynamic changes associated with different levels of physical activity.These findings highlight the versatility and potential applications of our capacitive sensor technology in monitoring and analyzing various types of human knee motion, providing valuable insights for gait analysis, sports performance monitoring, and rehabilitation.Although interdigital sensors are adaptable in a wide range of applications, it is important to recognize that the environmental circumstances in these applications are dynamic.However, the characteristics of the sensor remain unchanged after a 6-month period, with no electrical changes (Figure S8, Supporting Information).In terms of exhibiting the same electrical sensor characteristics in both negative samples from 6 months ago and more recent ones, it indicates the long-term stability and reliability of the sensor.This shows the sensor's durability under environmental conditions and variables.Considering that factors like temperature and humidity did not adversely affect the performance of the sensor, it can be confidently stated that this sensor can be used in various application areas.

Conclusion
This study has introduced a universal mass fabrication methodology for advancing wearable technology applications through the utilization of textile knitted interdigital capacitive strain sensors.The successful fabrication and thorough characterization of these sensors have demonstrated their exceptional stretchability, high linearity, and rapid response time.Through the incorporation of stretchable conductive yarn with textile knitting technology, these sensors have exhibited outstanding adaptability and wear-comfort to knee movements.The direct integration of knitted sensors into fabric has ensured a robust and fully functional integration.Furthermore, the utilization of knitting tech-nology has facilitated the rapid, reliable, reproducible, and economical production of interdigital capacitive sensors.The capacitance change performance of the sensor has been effectively demonstrated through its application in knee braces during walking and cycling exercises, showcasing remarkable repeatability across multiple cycles.These obtained results serve as concrete proof of the enhanced electromechanical performance and functionality of interdigital capacitive strain sensors in wearable technologies.In summary, this study offers a substantial contribution to the realm of wearable technology through an effective approach for the advancement of stretchable interdigital capacitive sensors in a wide range of applications such as healthcare monitoring, sports performance analysis, soft robotics, entertainment applications, and human-machine interfaces.Future studies may focus on exploring additional functionalities, employing sustainable textile materials, and exploring novel applications.

Experimental Section
Materials: The homogeneity of the structure of a knitted sensor played a critical role in achieving the desired sensor characteristics, including high sensitivity and durability, while also expanding its potential applications.The uniform stretchability along the sensor structure directly impacted its linearity, signal stability, and operating range.To ensure homogenous stretchability, it was necessary to match the elastic cores of the hybrid conductive and dielectric yarns, including their yarn counts and materials (140 denier Spandex for both cores).The stretchable conductive yarn production involved using a twisting machine (DirecTwist 2C, Agteks, Istanbul, Turkey) with silver-plated nylon conductive yarn (235/36 dtex HC+B, Shieldex, Germany) and Spandex monofilament (140 denier, Creora, Turkey) based on the specifications provided in Table S1, Supporting Information.The interdigital sensors were manufactured using the N.SVR 122 V-bed flat knitting machine (Shima Seiki, Sakata Wakayama, Japan).The dielectric base stretchable yarn (140/70 × 2 denier spandex/nylon, Combas, Turkey) was selected as the equivalent to the elastomeric core of the hybrid conductive yarn.The sensor structure exhibited course density of 48 courses per inch and wale density of 40 wales per inch (wpc), resulting in a stitch density of 1920 stitches per square inch (spi) with a weight of 12.5 grams.The manufacturing parameters of the sensor on the Shima Seiki N.SVR 122 V-bed flat knitting machine have been provided in Figures S9-S11, Supporting Information.
Electromechanical Characterization: A test rig for conducting strain tests was constructed, as depicted in Figure S4, Supporting Information.The design of the test rig involved the utilization of two clamps to secure the knitted specimens on a platform.One clamp remained fixed at one end, while the other clamp was movable.The movable clamp was attached to a threaded power shaft connected to a stepper motor.Once the specimens were securely positioned, strain test data was collected based on the number of rotations driven by the motor, which were controlled by commands from a microcontroller (Arduino Uno Atmega328P, Microchip, USA).Simultaneously, the capacitance value was measured using a capacitive touch sensor (Arduino MPR121, NPX, Netherlands).

Figure 1 .
Figure 1.Fabrication of stretchable conductive yarn and interdigital capacitive knitted sensor.a) Twisting of elastic core yarn and conductive cover yarn to fabricate stretchable conductive yarn.b) Flat weft knitting of stretchable conductive yarn and PA 6.6/elastane yarn to fabricate interdigital knitted sensor.

Figure 2 .
Figure 2. a) Photographs of the states of the knitted interdigital sensor under longitudinal and transversal strains.b) Visualization of the electrical field between loop-based knitted electrodes relaxed and strain induced states.

Figure 4 .
Figure 4. Response times of interdigital capacitive sensors.a) Positive strained sensor.b) Negative strained sensor.c) Linearity of capacitance values for the positive strained sensor as a function of strain.d) Linearity of capacitance values for the negative strained sensor as a function of strain.

Figure 5 .
Figure 5. Drift signals under constant strain levels.a) Positive strain direction sensor.b) Negative strain direction sensor.Durability test with 50% cyclic strains.c) Positive strain direction sensor.d) Negative strain direction sensor.

Figure 6 .
Figure 6.a) Photograph of walking activity on a treadmill.b) Sensor capacitance signals corresponding to walking at different speeds.c) Photograph of cycling exercise.d) Sensor capacitance signals corresponding to cycling exercise.