Smart Textile

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1		Abdella Simegnaw Ahmmed	--	2513	2022-05-05 18:41:31	\|
2	format correction	Dean Liu	-25 word(s)	2488	2022-05-06 04:40:18	\|

This entry is adapted from the peer-reviewed paper 10.3390/ma15082892

The integration of electronic components in/onto conductive textile yarns without compromising textile qualities such as flexibility, conformability, heat and moisture transfer, and wash resistance is essential to ensuring acceptance of electronic textiles. One solution is creating flexible and stretchable conductive yarns that contain tiny surface-mounted electronic elements embedded at the fiber level.

smart textile wearable textile conductive yarn E-yarn surface mounted devices light emitting diode e-textile integration

1. Smart Textile

Smart textiles are a new generation of textile that play critical roles in a wide range of technical applications. Smart and intelligent textiles refer to a new generation of fibers, fabrics, and materials that can detect and react to changes in the environment, such as mechanical, chemical, thermal, magnetic, electrical, and optical changes, in a predetermined way ^[1]^[2]^[3]. For practical appliances, smart textiles have five purposes. They can operate as sensors, data processing elements, actuators, power storage devices, and communication devices ^[4].

Smart textiles can be constructed in numerous forms. Most commonly, smart textiles have been created in the form of electronic textiles (e-textiles) ^[5]. E-textiles are textile yarns or textile fabrics in which electronics and circuits are implanted. The oldest technique for embedding conductive materials into a textile substrate to form conductive fiber networks or circuits is to incorporate tiny conductive metals, e.g., by twisting them into the textile yarns ^[6]. However, thin metals cannot withstand abrasion, especially during laundry cycles, so more solutions that are robust are required.

Integration of electrical components such as LEDs, resistors, thermistors, capacitors, and inductors on or in conductive fabrics is a major application area of wearable e-textiles and has been extensively researched in recent decades ^[7]. For the integration, it is necessary to construct reliable textile-based transmission tracks during the first phase of the e-textile structure construction. Researchers have tried several mechanisms of interconnection of electronic device with textile substrates, as discussed in the following section.

2. Methods of Integration

Several researchers have conducted studies on joining methods for electrical components and textile fabrics using various techniques, by highlighting their merits and drawbacks. These techniques included mechanical interconnection, stitching and sewing, embroidering, soldering and adhesives ^[8]^[9].

2.1. Mechanical Interconnection

Various methods of mechanical interconnection of electronics and textiles have been used, such as snapper plugs, zippers, hook and loop connectors, and magnetic connectors ^[10]. These either use rigid parts or are not suitable for many connections due to their bulk or weight, thereby becoming burdensome. Furthermore, due to the reduced dimensional rigidity of the components, the freedom of motion by a fabric substrate is restricted ^[11].

2.2. Sewing and Embroidery

Functional smart textiles have been developed by embedding SMDs using a Brother PR-650e embroidery machine ^[12]. The results showed that failure occurred at the connection point between the stitched thread and the SMD, which resulted in poor performance of the product. Researchers have effectively integrated SMD onto a textile fabric in the form of bendable electronic sheets in sequin ^[13]. In addition, stitching has been used to attach SMDs onto a fabric substrate ^[14]. However, the uninsulated conductive thread was susceptible to corrosion during washing, and subsequently the added plastic insulation base altered the material’s feel. Furthermore, the LEDs were prone to breaking off during twisting. In addition to stitching, subtractive technologies and 3D printing have been used to make flexible and functional sequins for handheld embroidery textile applications ^[15]. Electronic sequins can be embroidered or stitched onto textile fabrics with electrically conductive thread. A conductive thread, however, is susceptible to mechanical abrasion.

2.3. Adhesives

Electronic components have been attached onto a textile circuit with a non-conductive glue using pressure force to transfer the glues and make a secure mechanical bond and electrical connection. The thermoplastic adhesive adhered the electronics to the fabric by applying pressure and heat ^[16]. Li and Wong ^[17] worked with conductive glues to replace solder in electronic packaging applications. However, this type of technology is not widely available and has a number of drawbacks, including poorer electrical conductivity, little conductivity fatigue resistance, and a limited lifespan.

3D Printing

3D printing has also been used to make electrical and mechanical contacts between SMD electronic parts and textiles ^[18]. However, the contact resistance between 3D-printed conductive tracks and conductive yarns was too high for practical usage and should be lowered. Furthermore, the contact between the 3D printed area and the electronic components that were afterwards inserted was still not satisfactory.

2.4. Soldering

Buechley and Eisenberg ^[19] studied the technique of integrating fabric and PCBs via soldering using silver crimping bead attachments onto the terminals of surface-mounted LEDs with lead-free solder. However, needs further processing steps are needed for the LED sequins before they can be sewn onto the textile fabric with conductive thread. The crimp beads can be soldered to the SMD perpendicularly, allowing a thread to move through it and be crimped. In addition, the interconnection of electronic components with a smart textile has been done using pulsed laser soldering techniques ^[20]. The results showed that the textile-integrated, varnish-insulated copper strings had successful contacts. However, the nearby polyester knitted fabric was heated and melted. From this, it can be seen that the interconnection of electronic devices and textile substrates, especially textile fabrics, can be successfully performed. However, the integration still suffers from inconsistencies between smooth, elastic, pliable textile fabrics and stiff electronic components, particularly because sequins are already larger than the pure SMD. This has a significant impact on the final design and properties of the textile fabric.

In order to accomplish higher degrees of incorporation and user comfort, scholars have developed and studied stretchable and bendable electronic parts for smart textile applications ^[21]. In addition, the development of E-yarns by integrating miniaturized electronics into textile fibers or yarns has also been carried out. E-yarn with integrated electrical components can be used as a building block for smart textiles, allowing for the creation of electronically functional wearable textiles, for instance, wearable sensors, wearable heaters, and wearable color changing displays ^[22]. The difficulty with these is the correct positioning of the E-yarn so that the component is in the correct position, as the component is present in the yarn before the yarn is placed in the circuit, as opposed to the methods presented which form the circuit first and then attach a component at each necessary position.

3. Integration of Electronics at Yarn Level

A recent development by Rein et al. ^[23] developed LEDs integrated into copper metallic wires or high-melting-temperature tungsten during the extrusion process of drawing a fiber. Due to the impossibility of extracting the filaments quickly after extrusion without destroying the copper wire connections, the extrusion technique produces a yarn with low tensile strength ^[24]. This has an impact on the copper wire’s capacity to be processed in normal textile manufacturing procedures. Moreover, it is limited to conductive materials for co-drawing, and it is also limited to electronic devices with two terminals. This will limit the number of electronic devices that can be used and the functionality of E-yarn.

A method for embedding SMDs on 2 mm wide flexible plastic strips has been devised by the ETH Zurich wearable computing laboratory ^[25]^[26]. In the weft direction, the components were weaved into a cloth. However, the degree of bending of the strips was limited due to the use of a typical bare die, and the strips were not ideal for knitting or embroidery. Combinations of electronics on stretchy plastic strips with textiles, during the weaving process, have also been investigated.

The E-thread^® ^[27] was developed by the PASTA project, in which the die was embedded into two conductive wires and the die and interconnects were masked with a fibrous blanket. This E-thread^® was not appropriate for washing, and the chips were not enclosed. Only two-pin electronic devices could be used in the E-Thread^® manufacturing, and this limited the range of electronic devices and thus the functions that could be incorporated.

An E-yarn was developed consisting of a conductive core of multi-stranded copper wire to which semiconductor dies were attached by direct soldering ^[28]. The E-yarn was then covered in a mastic micro-pod, which was successively surrounded by a textile casing that also protected the copper wires. Due to their copper conductors, they either were not bent at a 90° angle or had a limited number of bending cycles ^[29]. In turn, the thickness of the sealing material led to an increase in the diameter of the E-yarn. Large yarn diameters reduce the comfort of the textile. They can also hinder moisture and heat transfer, which are critical performance properties ^[30].

The development and electro-mechanical characterization of e-conductive yarns are important for their subsequent applications in wearable e-textiles ^[31]^[32]. The electrical resistance of a conductive yarn is a critical design factor for E-yarns ^[33]. Besides investigating their electro-conductive properties, there is a need to investigate their electro-mechanical behavior for different applications of smart textiles ^[34]^[35].

In e-textile, low linear resistance E-yarns are used to transfer electrical signals. However, these yarns are subjected to significant strains during production or usage, which can lead to conductive track failure. Therefore, for the design and fabrication of safe and reliable e-textiles, electrically conductive textiles made from conductive yarns require thorough characterizations of their properties. For this reason, the physical and electromechanical behavior of the E-yarn should be investigated.

The physical and mechanical properties (tensile strength and elongation) and electrical properties of conductive textile yarns were studied in ^[36]^[37]. Furthermore, the electromechanical properties of textile structures for wearable sensors made of silver-coated conductive Statex^® yarn were also studied ^[38]^[39].

Overall, the research done so far is limited, and therefore, there are inadequate data on the electrical and electromechanical analyses of E-yarns, especially flexible, conductive yarns with SMD light emitting diode (LED) electronics embedded, which are more complex components to characterize than resistors.

Development Process of Stainless Steel Yarn with Embedded SMD Components

The SMD electronic components were selected to be integrated into stainless steel conductive thread. For connecting the SMD with a SS conductive thread, a soldering method was applied. However, direct soldering of stainless steel yarn is difficult due to the existence of a thick passive oxide (Cr₂O₃) film that blocks the melted solder paste from sticking to the surface of the SS textile thread ^[40]. Therefore, other additional alternative techniques of direct soldering were required.

A unique integration method was developed. First, a surface preparation of the stainless-steel yarn was performed. The tips of the SS conductive yarn were heated via 50 °C hot air for 2 min and a small drop of 85% concentrated H₃PO₄ paste (phosphoric acid-based paste) was applied. The surface preparation was used to polish the thick oxide layer at the surface of SS thread. In addition, it helps with activating the SS thread in order to connect and stick them together for the next step. A proper connection of the SMD and conductive SS yarn with the 1.5 mm silver crimp beads was performed by applying a mechanical force by using pliers on the bead after inserting the SS tip. Furthermore, the final integration of the SMD into the stainless steel conductive thread was done by using a four-in-one 909 D hot air gun soldering rework station, immediately after cleaning. Thereafter, to create a protective layer on the solder pad, a 1.5 mm heat shrink tube was transplanted over at the connection joints. The steps used to create the E-yarn are shown in Figure 1.

Figure 1. Steps to perform soldering.

The soldering steps in Figure 1:

Two pieces of 15 cm stainless steel yarn to be connected at both ends into crimp beads were cut.
The tip of the stainless steel thread was bent to form a loop, which was inserted into the 1.5 mm cylindrical crimp beads. The loop at the tip improved the connection in the crimp bead.
The crimp bead was pressed securely by using flat nose pliers, to flatten and hold the yarn firmly.
The terminals of the SMDs were connected to the tip of flat crimp beads by adhesive tape.
Solder paste was applied on the connection point of the SMD and crimp beads.
Controlled temperature was applied with hot air onto the soldering paste, which melted without damaging the component, thereby connecting the crimp beads to the pads of the SMD.
As an optional extra step, a 2 mm heat shrink tube was placed over the joints, and hot air was applied to shrink them and create an insulating layer over the solder joints.

After the development of LED SMD embedded stainless steel electronic yarn, the physical, the electrical, and mechanical properties such as

the length dependence on electrical resistance,
the effects of stress on the electrical resistance,
the total electrical resistance of the electronic yarn,
the effects of washing and laundry action on the electrical resistance,
the power dissipated from the conductive yarn and electronic yarn, and
the tensile strength of the electronic yarn

were investigated.

In addition, the functionality of the developed electronic yarn was proven as shown in Figure 2. This proof of concept and all the above results regarding electrical conductivity and other E-yarn properties showed that the resulting E-yarn is promising for flexible, wearable textile sensors and actuators in items that do not require frequent washing.

Figure 2. The actual prototype of LEDs integrated into a stainless steel yarn (A) and lit E-yarn (B)

Generally, the integration of light-emitting surface-mounted devices into stainless steel conductive thread with a combination of 1.5 mm crimp beads was performed. The techniques for insertion of SMDs into SS conductive yarn involved using the hot air soldering method without damaging the SS conductive thread and the tiny SMDs. The investigation of electro-mechanical characteristics of the selected conductive yarns was performed. The influences of clamping gauge length and the strain of the conductive yarn on the electrical properties were studied for both the conductive yarn and SMD-integrated E-yarns. It is clear that as the clamping length increases, the resistance of the conductive yarn increases. The dependence of electrical resistance on clamping length showed that the SS conductive yarn has fairly linear behavior. According to the experimental data, the SS conductive yarn was built in a uniform manner resulting in no irregularities structurally. Furthermore, the effects of tensile strain on the electrical resistance of SS conductive threads and SMD-embedded E-yarn was presented. The analytical finding showed that, due to the elastic deformation of the SS conductive yarn under strain, its cross-section decreased and the electrical resistance grew proportionally. In addition, the experimental results showed that breakage of all samples of E-yarn occurred at the connection point between the SMD terminal and crimp beads. This occurred likely due to poor mechanical bond formation during the soldering process. All SMD-integrated E-yarns remained functional after twenty cycles of machine washing. The E-yarn, on the other hand, had a substantially greater failure rate before 20 washing cycles if its solder pad was not enclosed by a heat shrink tube. E-yarns’ ability to be washed is critical to their future use in the e-textile sector for wearable applications. This method enhanced the capacity to create E-yarns needed for the development of prototype electronic textiles, but washing resistance should be improved. One option might be using special washing techniques, but this would make electronic textiles less consumer-friendly.

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Update Date: 06 May 2022

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