Wearable Solutions: Design, Durability, and Electrical Performance of Snap Connectors and Integrating Them into Textiles Using Interconnects

Abstract

Electronic textiles (e-textiles) merge textiles and electronics to monitor physiological and environmental changes. Innovations in textile functionalities and diverse applications have propelled e-textiles’ popularity. However, challenges like connection with external devices for signal processing and reliable interconnections between flexible textiles and rigid electronic circuits persist. Wearable connectors enable the effective communication of e-textiles with external devices. Factors such as electrical functionality and mechanical durability along with textile compatibility are crucial for their performance. Merging the rigid connectors on the flexible textiles requires conductive and flexible interconnects that can bridge this gap between soft and hard components. This work focuses on designing two-part detachable mechanical snap connectors for e-textiles. The textile side connectors are attached to the data transmission cables within the textiles using three interconnection techniques—conductive epoxy, conductive stitches, and soldering. Three types of connectors were developed that require three detaching or unmating forces (low, medium, and high). All connectors were subjected to 5000 mating–unmating cycles to evaluate their mechanical durability and electrical performance. Connectors with low and medium unmating forces exhibited a stable performance, while those with high unmating forces failed due to wear and tear. Conductive stitches maintained better conductance as compared to conductive epoxy and soldering methods.

1. Introduction

In recent times, the allure of wearable devices has surged dramatically. From sleek smartwatches to dynamic fitness trackers, these innovations have seamlessly integrated into our daily routines. Yet, what precisely defines a wearable device [1]? E-textiles, as per the standard ISO/TR 23383:2020, ref. [2] are garment or textile products that contain embedded electronics, whether the circuitry is made of textile components or more conventional electronic circuitry [3]. E-textiles offer unconventional functionalities like physiological or ambient sensing, as well as power and signal transmission. Due to these advanced functionalities, e-textiles have been widely adopted for healthcare, military, and entertainment purposes.

Integrating these electronic functionalities into conventional textiles is challenging as it alters the physical properties of the textiles [4]. While wearable sensors are widely researched, they are currently designed in a way to plug into computers or output data processing devices through rigid and bulky connectors. Therefore, there is a need to develop a wearable connecting interface [3].

Two distinct technical components are configured within e-textile systems—the connectors and the interconnects. Connectors link e-textiles and external electronics, while interconnects create electrical paths between the connectors and the e-textiles. With numerous contact points, even a single malfunction can disrupt the entire system. Wearable products rely on connectors for various functions, including antennas, sensors, power connections, battery links, board connections, and memory cards [5].

Detachable connectors in e-textiles typically comprise a plug (male connector) and a receptacle (female connector), facilitating electrical connection upon mating [3]. They can enable power and data transmission to and from e-textiles [6] and allow garment washing [7].

Notable form factors of wearable connectors include snaps, buckles, hook and loop (Velcro^®), and zippers [8]. Snaps, also known as snap fasteners, are a type of closure that can be used to fasten clothing, bags, and other items. They are made up of two or four interlocking metal or plastic parts that fit together to create a secure closure. Snap buttons are versatile connectors that are compatible with textile materials and electronic components, facilitating integration into various systems [9,10,11]. Coated or conductive snap buttons serve as electrical connectors in smart textiles, offering easy disconnection and compact sizes (as small as 1 cm) [7]. Another common form factor seen in textiles is buckles. Buckle connectors have a familiar and easy to use form factor. They have a low profile for snag avoidance and comfort [12] and can provide durability and conformability [12,13]. Hook and loop mechanisms by Velcro^® provide ease of use, strong fastening, reusability (washed and drycleaned [12]), and low maintenance needs [3]. Conductive versions of Velcro^® have offered mechanical stability [14]; however, they are not widespread in e-textile applications [7]. This is because conductive Velcro^® is known to be inconsistent in electrical connection since the number of hooks and loops encountering each other can vary when the two parts are connected [3]. Zippers can function as electrical connectors either by using the two sides of a metallic zipper as contacts themselves or by using a plastic zipper to press together two contacts made from a conductive textile [3]. Zipper-style connectors have been proposed and refined over recent decades [7], appearing in patents and commercial products [3].

However, these connectors are expected to provide a robust mechanical, as well as an electrical, connection. They must also endure intensive cycles of mating–unmating without hampering the connector itself and the interconnections within the e-textiles [15]. Mating the detachable connectors provides external pressure to facilitate electrical connections between connector components. Pogo pins, commonly utilized in electronics, are adapted for e-textiles to link rigid circuit modules with flexible circuitry. Their small size (typically 1–2 mm diameter [7]) allows for numerous connections in a confined area. Pogo pins provide compact, easy to use, reliable, and low-resistance electrical connections, compensating for surface irregularities. Their spring-loaded design accommodates misalignments and contact pressure variations, ensuring stable connections that are capable of handling high-current and high-frequency signals [3,16].

In e-textiles, challenges stem from strain during wear and washing, particularly at the interfaces between rigid electronics and soft textiles, where failure is probable [17]. Interconnectivity is crucial among electronic components within complex e-textile systems, creating electrical paths and linking rigid electronics to soft textiles. In wearable devices, interconnects must endure stresses due to textile stretching [8] and maintain low and uniform electrical resistance at the interconnect points. Various methods such as soldering, welding, and bonding are utilized, with low temperature soldering being increasingly favored for textile integration. However, the hardened solder makes the connection rigid in nature. The lack of flexibility may be overcome by making conductive stitches or embroidery using conductive yarns [18]. Conductive stitches or embroidery offer flexibility but may risk the stability of the electrical connection due to stitches coming loose. This draws attention towards conductive adhesives, which are highly conductive, moderately flexible, highly durable, nontoxic, and facilitate strong mechanical and electrical connections between rigid components and flexible substrates [15].

Connectors on e-textiles integrate circuitry seamlessly into wearable garments, ensuring they maintain the appearance and feel of regular clothing rather than electronic devices [19]. The choice of housing material, such as plastic [3,20] or metal, depends on the application-specific requirements encompassing mechanical strength, electrical conductivity, environmental resilience, and user comfort. Plastics offer lightweight properties and ease of manufacturing, while metals provide a superior mechanical robustness [21].

Choosing how e-textiles and connectors communicate with external electronics (for data transfer or power) is crucial for design [22,23]. The communication protocol determines the number of wires needed per connector, affecting connector size [22]. Ongoing research aims to shrink connector size, while enhancing performance to handle higher bandwidth signals and increased power demands [24]. Communication protocols govern how information is structured, transmitted, and received, facilitating communication with e-textiles [25,26]. Serial communication is a basic method for data exchange over long distances and is suitable for situations with limited cables [25]. Various communication standards exist, including SPI, CAN bus, Firewire, USB, I2C bus, and RS-232 [22]. SPI and 1-wire interface have limited data transfer rates [22].

E-textile connectors and interconnects must endure daily wear and fabrication stresses, to ensure reliability throughout the device’s lifespan. Fatigue resistance, vital for sustaining electronic functions and structural integrity through repeated loading–unloading cycles, is critical [27]. When choosing connectors, considering the mating cycle rating is essential to prevent unreliable products [28]. Existing standardized reliability testing protocols for e-textiles are insufficient [17]. Established organizations like ISO, ASTM, CEN, and AATMC have published standards for textile examination that can be consulted if necessary [29]. Characterizing the electrical properties of e-textiles can be challenging, emphasizing the importance of maintaining low and stable electrical resistance. Variations in mechanical deformation can impact conductivity, underscoring the significance of electrical resistance [29].

Both connections (connectors and interconnects) need to be functional and dependable [18]; however, they are tricky to design. This is because they not only exist between two distinct environments—textiles and electronics—but are also expected to merge the contrasting conditions, being flexible and rigid, respectively. Having extreme changes in stiffness, together with a large degree of freedom in movement, the interconnects (where the connectors are integrated into soft substrates) are under intense daily wear and tear and become notorious for being electrically vulnerable. Electrical attenuation on interconnected points is accelerated by the continuous plugging (or mating) and unplugging (or unmating) motions of the connectors as well. This makes it extremely important for the connectors as well as interconnects to withstand the repeated mating–unmating cycles to maintain robust electrical conductivity. Even though the overall functionality of the complex e-textiles system requires robust connectivity and interconnectivity, there is a lack of research on this.

This work focuses on developing a modular wearable detachable connector that will allow users to customize (attach, remove, and reconfigure) their wearable devices as per their needs. The low-profile snap connectors intend to blend seamlessly with the garment, enhancing the overall appearance and esthetics of the wearable device. The snap connectors of varying unmating forces were integrated into the textile using different interconnect methods. All the connector samples were subjected to repeated mating–unmating cycles for up to 5000 cycles, to evaluate the overall mechanical performance and electrical reliability. The USB 2.0 protocol was chosen to demonstrate the technical viability of producing textile-based components for a point-to-point wearable personal area network.

2. Materials and Methods

In this paper, we developed mechanical snap connectors. Despite snap popularity, limited research has been carried out on the technical capability of snap fasteners as electronic connectors. Three metal studs manipulate the unmating force of the detachable connector. The three interconnect technologies used were conductive epoxy, conductive stiches, and soldering. They were selected considering the diverse levels of conductivity and flexibility, as depicted in Figure 1. These various designs were subjected to 5000 mating–unmating cycles to evaluate the electrical performance of the connectors for use in the standard USB 2.0 communication protocol (Figure 2).

2.1. Snap Connector Fabrication

Connector bodies were 3D modeled using Autodesk Fusion 360 software (San Francisco, CA, USA). The designs were exported to Standard Tessellation Language or Standard Triangle Language (STL) files and were sent to a print preparation software named PreForm 3.21.0 by Formlabs. Polymer resin (RS-F2-GPGR-04) supplied by Formlabs was used for 3D printing the connector body due to its high resolution and high strength properties (ultimate Tensile Strength: 65 MPa; tensile modulus: 2.8 GPa) [30]. The Formlabs Form 3+ printer (Somerville, MA, USA) was used to 3D print the connectors using low force stereolithography (SLA) technology. The printed parts were cleaned using 90% Isopropanol (IPA) and were cured using the Form Cure (FH-CU-01) curing station from Formlabs for post processing the parts with 405 nm UV light at 60 °C for 60 min following the supplier’s recommendations [30].

Snap connectors consist of two parts—one on the textile side and the other on the non-textile side—as shown in Figure 3 and Figure 4. These parts were fabricated by assembling and joining conductive and non-conductive components together. The configuration of each connector part is described in detail in the following sections. The connector attached to the textile side was meant to be as small and lightweight as possible as it must be worn on the human body, whereas the non-textile side connector did not demand much in terms of its wearability. A basket-woven cotton polyester blend fabric of 201.38 g/m² with 0.510 mm thickness was used to mount the textile side connector before performance evaluation.

Mechanical resistance standards exist for garment fasteners, whereas the number of mating–unmating cycles they can survive as an electrical connector still needs to be explored [5]. Therefore, the mating–unmating force of the snaps was manipulated using three different Romefast^® studs (Milford, CT, USA), as shown in

Table 1. Romefast^® metal studs and sockets with measured average unmating forces to separate the studs and socket.

Force	Stud	Eyelet	Socket	Cap/Eyelet	Avg Unmating Force (N)
Low	77EZ	77	76	77	9.98
Medium	76	73C	76	73C	17.27
High	77K	77	76	77	26.61

and Figure 5, corresponding to the three unmating force levels. Three force levels were selected to evaluate the effect of unmating force levels on the conductance of the connector. Fastener strength has never been quantified for apparel uses, but in general, the studs with lower unmating forces are used in innerwear and higher unmating forces are used in outerwear. Studs were crimped, using the snap fastener attaching machine (Figure 5), to the textile side connector using 76 and 77 die sizes, whereas sockets were crimped to the non-textile side connector. The average unmating force of the three Romefast^® studs and sockets was measured using the IMADA motorized test stand with a force gauge. An electrical connection was made by contacting pogo pins on the non-textile side connector to the PCB traces on the textile side connector.

The three Romefast studs with low, medium, and high force levels, as shown in the figure, were crimped to the plastic of the textile side connector. PCB was added to this assembly. The Romefast^® eyelets were crimped to the non-textile side connector. Sleeve and plunger spring-loaded pogo pins (P/N: 0985-1-15-20-71-14-11-0) from Digi-Key electronics (Thief River Falls, MN, USA) were soldered into the non-textile connector to provide reliable and repeatable electrical connections. The pogo pins on the non-textile side connector were designed to contact the PCB traces on the textile side connector. See Figure 3 for snap connector schematic and Figure 4 for the fully assembled snap connector.

2.2. Interconnect Methods

The three interconnect methods used were conductive epoxy, stitches, and solder. Epoxied interconnects were achieved using low-temperature conductive silver epoxy (S-CEP7-SF4) supplied from Sunray Scientific S-CEP7-SF4 (Eatontown, NJ, USA). This is a two-part, silver-filled epoxy-based, electrically conductive adhesive designed by Sunray Scientific. It forms a strong bond with the substrate and circuitry, while maintaining exceptional flexibility. It can offer reduced silver migration properties with our anti-silver migration additive. Madeira HC-40 highly conductive embroidery thread (Freiburg, Germany) with 117/2 dtex linear density and <300 ohm/m resistance was used to make the conductive stitch interconnects. A Juki DDL-8700 industrial sewing machine (Tokyo, Japan) was used to create stitches under the needle thread tension of 1.75 N and the bobbin thread tension of 0.25 N. MG Chemicals 60/40 rosin core leaded solder (Burlington, Ontario, Canada) with a 0.032” diameter and a melting temperature of 183 °C enabled soldered interconnects. The three interconnect materials were applied to pre-arranged holes (marked with orange arrows in Figure 6) of the connector body and PCB on the connectors. Conductive stitches were repeated four to five times, and each interconnection hole was filled with the epoxy and solder, respectively. Additional holes were made on the connector body to sew the connector body to the textiles using non-conduction stitches (marked with black arrows in Figure 6).

A textile data cable from WEEL Technology (Greensboro, NC, USA) was used to construct the electric infrastructure for power and data transmission on the fabric surface. The cable was 1.00 mm-wide and 0.70 mm-thick woven tape having a polyester sheath and four copper (Cu) wires inside. The four-wire design was used for the USB 2.0 protocol, as shown in Figure 2. Each Cu wire had 14 filaments with a resistance of approximately 1.5 ohm/m. One end of the cable was unraveled to attach to the hook-up wires. Four Cu wires on the other end were inserted into four pre-arranged holes on the connector body and PCB. These four holes are individually connected to the four concentric conductive electrodes on the PCB, respectively. This makes each Cu wire connected to each conductive ring on PCB using three different interconnect technologies. These concentric electrodes on the PCB are meant to contact the pogo pins on the non-textile side of the connector to make an electrical connection. The metal stud and socket were used for mechanical connection. A circular/ring design was adopted for the non-directional mating of the textile and the non-textile side of the connector. In the case of the sewn interconnect, the conductive yarn was stitched from the hole of the connector body to a hole adjacent to it on the PCB, in which the Cu wire was encased. The conductive epoxy and solder were placed right on the hole of the PCB. As mentioned above, this assembly of the textile side connector was mechanically sewn to the fabric using a regular sewing thread (non-conductive interconnect), 27-tex cotton-poly spun yarn (marked with black arrows in Figure 6).

The MTS Q-test machine (Berlin, Germany) was used to characterize the mechanical strength of the three interconnects. Under the standard ASTM D2261—Fabric Tongue Tear Test setting [31]—the textile side connector was pulled away from the fabric substrate and the textile cable until separation. For the accurate characterization of interconnect strength, no means of mechanical attachment was added. The test was performed in an angle same to the snap mating–unmating direction. It was observed that the interconnect using conductive stitches showed the highest pull force—99.45 N for snap. For the other two interconnect methods, the pull force stayed around 9–10 N, regardless of connector types. The force seemed to be recorded when the textile cable broke, indicating the strength of the Cu wires before damaging the interconnect itself.

2.3. Measurement and Analysis

An Imada ZTS-11 digital force gauge (Figure 7a) (Aichi, Japan) with a push/pull capacity up to 499 kg was mounted on an Imada MH2 motorized test stand (Figure 7b). As the motorized test stand mated and unmated the connectors, the force gauge recorded the mating–unmating peak force. Three snap connector samples, one of each force level, were subjected to 5000 mating–unmating cycles (the most used number of cycles for wearable detachable connectors) at a speed of 300 mm/min (approximately 10 cycles per minute). Hook-up wires were soldered on the textile data cable at the opposite end of the interconnect attachments on the textile side connector to obtain reliable readings from resistance and impedance meters. Hook-up wires were also soldered to the non-textile side connector. During the measurement, the connectors were mated, and electrical readings were taken from the hook-up wires attached to the textile data cable and the corresponding channels on the non-textile side connector. Figure 8 demonstrates a connector sample mounted on the test stand in unmated position (marked by the red circle) ready for the mating–unmating cyclic test.

The Keysight U1231A multimeter (Figure 9a) (Santa Rosa, CA, USA) was used to measure the resistance of the snap connectors. At a set time, one alligator clip was connected to the hook-up wire attached to the textile data cable and the other was clipped onto the corresponding channels on the non-textile side connector. This was repeated for all four wires on each connector. Resistance was measured at 0 cycles and after every 1000 mating–mating cycles.

The impedance magnitude as a function of the frequency was measured for the connector samples using the Gamry device Potentiostat (Figure 9b) (Philadelphia, PA, USA). The device was first calibrated and, like the resistance readings, alligator clips of the Gamry were appropriately attached to the hook-up wires on the textile side connector and its corresponding channels on the non-textile side connector. The impedance was measured across frequencies from 100 kHz to 1 MHz for the USB 2.0 and I2C communication protocol. The USB 2.0 protocol was chosen due to its widespread usage, familiarity, and compatibility with various devices [32]. The device recorded 200 points of data across these frequencies from each sample. Impedance was measured at 0 and 5000 cycles with the connectors in the mated position.

Analysis of Variance (ANOVA) was conducted to examine the effects of independent variables on the dependent variable. Full factorial design factors included force level, mating–unmating cycles, interconnect methods, and the response ‘resistance’ in ohms. The resistance values were converted to conductance values. The statistical analysis aimed to assess main effects, interaction effects, and the overall impact of the independent variables on the response. The data were analyzed using JMP software (Cary, NC, USA) and significance levels were set at 0.05.

3. Results and Discussion

Samples with the highest mating–unmating force levels achieved failure before reaching the 1000th mating–unmating cycle, as shown in Figure 10, while samples with low and medium force levels endured 5000 cycles. It is postulated that the connectors with higher mating–unmating forces create a dampened force that is propagated through both the plastic housing as well as the interconnects themselves. Repeated mating–unmating can cause the samples to experience friction against several mated surfaces and jerks, causing mechanical vibrations. These mechanical vibrations reduce the mechanical stability, leading to wear and tear and hence, the premature failure of the connectors.

This failure happened regardless of the interconnection method. Conductive-stitched samples have a better attachment between the connector and the fabric due to the presence of four non-conductive and four conductive stitches, whereas the conductive epoxy and solder samples only have four non-conductive stitches. In the case of conductive epoxy and solder samples, the metal socket and stud detached from the connector, respectively. In the case of conductive stitches, the unraveling of the non-conductive stitches was observed. This can be due to the mating–unmating force exerted on the connector being much higher for the four non-conductive stitches that hold the connector to the fabric. The conductive stiches samples also showed cracks and the breakage of the plastic housing. The excessive force exerted during mating–unmating can cause damage to the connector housing or casing (in this case, the plastic body). Figure 10c shows cracks, fractures, and the breakage of the plastic housing material. Therefore, a very high mating–unmating force can compromise the structural integrity and protection of the connector components.

3.1. Resistance

It is important to test the mating–unmating force that a connector can endure depending on the materials used for connector composition, as well as the number of mating–unmating cycles the connector needs to endure. In this study, the low and medium unmating force samples for the three interconnects survived 5000 mating–unmating cycles. The below sections show results for the low and medium mating–unmating samples.

Table 2. Effect tests for snap connector conductance.

Source	DF	Sum of Squares	F Ratio	Prob > F
Force Level	1	0.9900	16.0728	0.0001 **
Interconnect Method	2	0.6009	4.8782	0.0094 **
Force Level × Interconnect Method	2	0.9644	7.8288	0.0007 **
Mating–Unmating Cycles	5	0.3935	1.2778	0.2788
Force Level × Mating–Unmating Cycles	5	0.0110	0.0357	0.9993
Interconnect Method × Mating–Unmating Cycles	10	0.5099	0.8279	0.6027
Force Level × Interconnect Method × Mating–Unmating Cycles	10	0.0740	0.1202	0.9995

** indicateds statistical significance.

shows the ANOVA results for the snap connector. The factors of the snap-connecting interface that had a significant effect on the conductance were Force Level (main effect), Interconnect Method (main effect), and Force Level × Interconnect Method (interaction effect). The overall trend in the conductance from force levels (low and medium) across the three interconnect methods (conductive epoxy, conductive sewing, and solder) at every 1000 mating–unmating cycles starting from 0 to 5000 for the snap type of connector can be found in Figure 11.

The force level had a significant impact on the conductance, as per Table 2. From Figure 11, the medium force level showed a lower conductance in interconnects as compared to the low force level. This could be due to the mechanical wear and tear, which is expected to be higher in the medium unmating force level as opposed to the low force level. The medium unmating forces may be impacting the quality of the interconnect. For example, in conductive stitches, the medium unmating force level might be pulling or unraveling the conductive yarn more so compared to the low force level, thereby lowering the conductance. As mentioned above, higher force levels could not survive even the first 1000 cycles. Therefore, this behavior must have been observed in medium and low forces; however, both connectors endured all 5000 cycles.

The interconnect method had a significant impact on the conductance, as per Table 2. From Figure 11, a significant difference in the three interconnects can be seen. This could be because of the nature of the interconnect. Conductive epoxy is a flexible adhesive so it can make a secure conductive connection at any point while maintaining some flexibility. Conductive stitches are mechanically stronger and provide flexibility; however, the conductance can be affected based on the moving, unraveling, or breaking of the conductive yarns. And finally, solder, which is highly conductive, but at the same time, is not as flexible as others.

After this comes the interaction effect between force level and interconnect method, which had a significant impact on conductance.

Table 3. Tukey’s HSD connecting letter report of Force Level x Interconnect Method.

Level				Least Sq Mean
Low, Conductive Epoxy	A			0.5225
Low, Conductive Sewing	A	B		0.4337
Medium, Conductive Sewing	A	B	C	0.3333
Low, Solder		B	C	0.2304
Medium, Solder			C	0.2241
Medium, Conductive Epoxy			C	0.1316

shows the Tukey HSD analysis comparing between different combinations of force level and interconnect methods. Levels connected by the same letter indicate homogeneous groups without significant differences in the resulting conductance. The highest conductance was observed in conductive epoxied samples at low unmating force levels. This could be because the low mating force is not impacting the conductive epoxy on the interconnect. However, the conductance in these samples was not significantly different from conductance in the stitched samples of both low and medium unmating force levels. This shows that the conductive stitches interconnect is mechanically very robust; however, electrical conductance fluctuates depending on conductive yarns. This fluctuation could be, again, because of the nature of this interconnect. From the wear and tear during mating–unmating cycles, these yarns tend to move, unravel, or break, hence impacting the conductance.

The least conductance was shown in conductive epoxied samples of medium unmating force levels. This might have been due to the conductive epoxy not being able to withstand the medium unmating force level like conductive stiches. This shows that the conductive epoxy works great for lower unmating forces, but not at the medium unmating force level. As the force level increased, the difference between interconnect methods became ambiguous. The force level seems to have impacted the solder interconnect. This could be because of the rigid nature of the solder interconnect as well. The overall conductance at both unmating force levels was low. This could be because of the rigid nature of the solder, which could have led to cracks or the breaking of the solder. Solder showed a relatively lower conductance; however, it did not perform significantly differently than the conductive epoxy and conductive stitches at the medium force level. Therefore, higher force and mating–unmating cycles with solder might not be the best option for the interconnect method.

On the other hand, mating–unmating cycles did not have a significant impact on the conductance. This could be because the 5000 mating–unmating cycles can be endured by this housing material and the interconnects as well. Some ups and downs in the bar chart from Figure 11 can be seen; however, they are not significant. It should be noted that for the highest force level, the interconnect samples could not even withstand the first 1000 cycles due to very high unmating forces disassembling or breaking the connector.

Within the low unmating force samples, the snap connector with a conductive epoxy interconnect did not show much change in conductance from 0 to 5000 cycles. The conductive stitches interconnect performed better than conductive epoxy samples only until the first 2000 cycles. Solder consistently showed lower conductance values as compared to the conductive epoxy and conductive stitches interconnects. For example, for soldered samples, the conductance varies between 0.1 and 0.3 siemens. Similarly, for conductive stitched samples, the conductance reduces from 3000 mating–unmating samples onwards; however, this reduction is not significant, as per the statistical analysis.

As the unmating force increased to the medium level, most snap connectors showed lower conductance values as compared to the samples with low unmating forces. Despite these lower values, the epoxied interconnect did not show much change in conductance from 0 to 5000 mating–unmating cycles. Snap connectors with conductive stitches showed higher conductance values as compared to the other two interconnect methods. As the number of mating–unmating cycles increased, the conductance lowered; however, the difference was not significant, as per the statistical analysis. The soldered interconnect samples did not show a trend in conductance with increases mating–unmating cycles, but seemed to be fluctuating.

3.2. Impedance

Figure 12a,b show the impedance magnitude between 100 kHz (or 100,000 Hz) and 1 MHz (or 1,000,000 Hz) frequencies, across the three interconnect methods (conductive epoxy, conductive sewing, and solder) at 0 and 5000 mating–unmating cycles respectively. It can be observed that the impedance of the soldered snap connectors stayed lower as compared to the conductive-epoxied and conductive-stitched samples. At 0 cycles, conductive-stitched interconnect samples had a slightly higher impedance as compared to the conductive-epoxied samples. However, after 5000 cycles, it was observed that the impedance in the conductive-epoxied samples was slightly higher than the conductive-stitched samples. Overall, impedance increased with frequency, but the active increase started from 100 kHz to 1 MHz frequency. This increase was less than 2 ohms.

As the frequency of the alternating (AC) current flowing through the circuit increases, the opposition to the flow of current also increases. This opposition to the current flow is termed impedance. The impedance of an AC circuit increases with frequency due to the capacitance and inductance. In a circuit with capacitance (the ability of a capacitor to hold electrical energy), at higher frequencies, the reactance (opposition to the flow of AC current due to capacitance) decreases. However, the impedance of a capacitor increases with frequency because impedance is a combination of resistance and reactance. At higher frequencies, the decrease in reactance dominates, leading to an increase in impedance. In a circuit with inductance (ability of the inductor to store energy in the form of a magnetic field when a current flows through it), at higher frequencies, the reactance (opposition to the flow of AC current due to inductance) increases. This increase in reactance contributes to the overall impedance of the circuit. Therefore, as the frequency increases, the impedance of an inductor also increases. The net effect is that in an AC circuit containing both capacitance and inductance, as the frequency increases, the impedance tends to increase due to the combined effects of these components. This phenomenon is fundamental in the analysis and design of AC circuits, particularly in applications such as filters, transmission lines, and power electronics.

3.3. Application Example of the Wearable Snap Connector

Figure 13a shows a prototype of the example system implemented with the proposed connector. A temperature sensor was stitched to the inner side of a forearm sleeve to be in contact with skin and to monitor the skin temperature. The textile data cable was soldered to the sensor to carry the data from the sensor to the textile side snap connector (plug). A textile data cable was also soldered to the PCB on the snap connector. The snap connector was attached to the forearm sleeve with non-conductive stitches. Figure 13b shows that the temperature signal was read at the non-textile side snap connector (receptacle) when mated with the textile side connector. This evidences that the snap connector is a miniaturized wearable device that can integrate seamlessly into a garment and look aesthetically pleasing due to its snap form factor. In future, to evaluate the reliability of this connector, it can be compared to a conventional connector under a more controlled experimental setting.

4. Conclusions

Wearable connectors are critical components of wearable systems as they perform the important task of transferring monitored physiological or environmental signals to electronics for data amplification or display. In this study, we designed and developed detachable mechanical connectors that can be integrated into a garment in the form of snaps. Results showed that unlike the connectors of low and medium unmating forces, which could withstand all 5000 cycles, the connector with a high mating–unmating force level failed before 1000 mating–unmating cycles were complete. Conductors mating–unamating at low force levels gave the highest overall conductance in all interconnects. Epoxy interconnects performed the best over 5000 cycles at the low force level. For the medium force level, conductive stiches showed a higher conductance as they are mechanically stronger and more flexible than the other two interconnect types. Overall, the conductance of all the interconnects was low (less than 1 siemens). The empirical data evidence how important it is to engineer materials and to design interconnect methods, mating–unmating forces, and mating–unmating cycles for a particular application of wearable connectors.

There are a few additional considerations for improvement. The body of the current snap connector was rigid and could be brittle for applications where high mating–unmating forces or high mating–unmating cycles are associated. This could be reconsidered by making a connector supple with flexible resin and/or flexible PCB, which can provide additional buffers between soft textiles and rigid electronics. New materials and form factors can be looked at for making the connectors durable and reliable. Due to the off-the-shelf metallic studs and sockets, it was required for wearers to align the connectors and apply force to mate and unmate the connectors. This can be improved by adopting a magnetic force to mate the connectors. A magnetic force provides a quick and effortless way to connect and disconnect devices, ensuring secure and stable connections. The magnetic force guides the connectors into alignment, thereby enhancing user experience (e.g., consumer electronics, military, and disabled people) due to its universal design. While magnetic connection offers ease of connection, it is essential to consider factors such as the required holding force, environmental conditions, and specific application when choosing between magnetic and mechanical connectors. Mechanical connectors may still be preferred in scenarios where a secure connection is necessary or where specific mechanical properties are crucial. In this case, attention must be given to the design of the connection, the materials used to make the connector, and the application requirements to make sure the connector can endure the required number of mating–unmating cycles at the desired mating–unmating forces.