Textile electronics for wearable applications

Melt-spinning is one of the most common technologies to produce thermoplastic synthetic fibers and filaments, such as polyester, nylon, and polypropylene (PP). In the production, polymer pellets or granules are firstly fed into the screw extruder with progressively controlled heating zones. Functional fillers are also added and uniformly mixed with molten polymers at this step; the shear gradient across the undispersed fillers provided by the extruder as well as the wetting effect on individual filler surfaces by melting polymers work synergistically to obtain a homogeneous mixture. The molten then pumps through the spinneret to form filaments with specialized shapes, followed by solidification in the quenching chamber or water bath. The melt-spinning system is also equipped with an additional mechanical drawing system to stretch the as-spun filament to several times the original length, which can enhance the molecular orientation and strength of the fiber. The common approach of melt-spinning for textile electronics is compounding thermally stable and processable functional materials, such as conductive materials [52], antibacterial materials [53], and dielectric materials [54], with melt-spun polymers. Scholars have conducted many studies to fabricate conductive melt-spun fibers. The addition can be graphene [55], carbon black (CB) [56], CNTs [57] and polyaniline (PANI) [58]. However, the tenacity of these conductive fibers is relatively lower than conventional melt-spun fiber mainly due to the insufficient internal stress transfer at the interface of polymer matrix and fillers. The addition also increases polymer viscosity causing blockage and fluctuations in processing, quality, and variety [59]. Besides adding functional materials, melt-spinning also can spin filaments with specialized cross-sectional shapes. An example is the piezoelectric core-sheath fiber made by feeding polyvinylidene fluoride (PVDF) as the sheath and 10 wt% CB/polyethylene as the core into the multichannel spinneret separately, which was further drawn into the microfiber for energy harvesting, as shown in figure 2(a) [60]. Moreover, multiple functional materials can be blended and/or synthesized with the melt-spun polymer to spin composite fibers as electronic devices, for instance, a piezoelectric textile harvester was made from barium titanate nanoparticles, PVDF, and reduced graphene oxide (rGO) [61].

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Wet-spinning is normally used for the materials that can be dissolved in a suitable solvent, such as cellulose, acrylic, aramid, and conductive polymers. During the spinning process, the polymer firstly dissolves in a suitable solvent, then the dissolved polymer is pumped through the spinneret, and finally is transferred to a coagulation bath for consolidation. The key process of wet-spinning is the solidification in the coagulation bath, during which the diffusion interchange occurs between solvent and coagulant. It starts at the boundary of the polymer solution forming a sheath/core structure and slowly moves inward to the center of the fiber subsequently. Many researchers have investigated this diffusion process in the coagulation and have developed models for the wet-spinning process [64, 65]. From their studies, polymer solubility, spinning pressure, and coagulation bath under the influence of solvent composition and proportion as well as temperature are crucial elements to the solidification speed, void formation, and other as-spun fiber properties [66]. Moreover, the diffusion rate differs between different solvents and coagulants.

Using the wet-spinning method, a number of conductive fibers were demonstrated, such as the fibers made from poly(3,4-thylenedioxy-thiophene):Poly(styrenesulfonate) (PEDOT:PSS) with a conductivity of 223 S cm⁻¹ [67], meter-long MXene fiber with electrical conductivity as high as 7 713 S cm⁻¹ via wet-spinning [68] and graphene fibers [69]. Wet-spinning was explored to produce hybrid fibers composed of multiple materials. For instance, fibers containing graphene oxide and vanadium pentoxide for fiber batteries cathode are demonstrated in figure 2(b) [70]. This cathode can sustain 500 bending cycles, displaying better reliability than surface-coated fibers. Another example is a sheath-core stretchable conductive fiber with polyurethane (PU) as the sheath and GaInSn as the core [71]. Apart from conductive fibers, wet-spinning also can be used to directly fabricate other electronic devices, such as a fiber-based strain sensor, comprised of a CNT-elastomer core for strain sensing and a pure elastomer sheath for encapsulation via the wet-spinning [62].

Electrospinning has been applied to make fine fibers, yarns, and fiber webs from both molten and soluble polymers. Compared with melt-spinning and wet-spinning, it can only integrate a small amount of functional material into polymers, and like both of them, intrinsic conductive materials can also be processed via electrospinning. In the electrospinning, the polymer solution or melt is injected out as a charged droplet by the electric field. The droplet deforms from a broadly spherical shape to an elongated cone shape termed as Taylor cone. In this way, both conductive polymer composites and intrinsically conducting polymers can be processed into conductive fibrous web and porous membrane via either melt-electrospinning or solution electrospinning [72, 73]. Polymer solutions, electrospinning operation parameters, and environmental factors, such as relative humidity and temperature, are crucial in controlling the morphological properties of electrospun fibers [74, 75]. Polymer solubility, solvent concentration, solution viscosity, and solution surface tension may interact with each other to control the polymer crystallinity and morphology of fiber [74]. For example, to avoid the formation of beads and electrospraying, solution viscosity needs to be maintained over a threshold, which is related to solvent evaporation and polymer solubility. Operational factors cover the applied voltage, feed rate, distance between spinneret and collector, and types of collectors. These factors may interact to adapt fiber flight time and productivity. The priority is that the applied voltage needs to overcome a threshold of the surface tension of Taylor cone to initiate electrospinnning [76].

Two-step thermal drawing is a method for fiber optics initially and has been explored for fiber electronics with multiple materials or even embedded microelectronic chips. A preform is prepared as an enlarged precursor of the fiber. It may be composed of multiple materials or components in varied shapes, through different machining procedures, such as milling, lathing, or laser cutting [77]. Then the preform is held in a hot furnace of a drawing tower over a period of time for softening. The temperature needs to be 50 °C–100 °C higher than the materials' glass transition temperature. Next, the softened preform is pulled to form a continuous fiber. Fibers with unique textures or structures have been demonstrated by the controlling of material flow continuity. The matching of material properties is critical, for instance, the glass transition temperatures of the multiple materials should be in the same range and viscosities [78]. A piezoelectric composite fiber was reported, which was made up of P(VDF-TrFE)/BTO sandwiched between electrodes with SEBS protection. Enlarged by a fabric structure, the fiber can convert mechanical vibration to an electrical signal [50]. SEBS polymer has been used to make electronic and photonic fibers with 500% elastic deformation [79] and conductive fiber with PVA core wrapping by SEBS film [80].

Fiber printing is an additive manufacturing technique that are computer controlled to squeeze materials out of the nozzle, followed by the material curing process, to form filament structures. Among filament-based extrusion, direct ink writing (DIW) is advantageous to print complicated structures with programmed gaps because its solidification involves the rheological transition from pseudoplastic to a dilatant state or the gelation of the paste [81–83]. DIW shows promise in processing multiple materials into fiber electronics. For instance, A core-shell polyimide fiber filled with carbon paste as conductive fiber was made by DIW (figure 2(d)) [63]. When processing one functional ink by DIW. The ink viscosity, mostly characterizing with shear thinning behavior, plays a crucial role in the smoothness of flow out of the nozzle [83, 84]. In order to confirm the stable formation of as printed structures without unwanted structure deformation initiated by the gravity, the ink storage modulus need to be higher than its loss module [81, 83, 85]. To relieve above two conditions, hybrid ink plateau modules and shield yield stress need to be controlled via the addition of rheological modifiers [11]. Another concern is material incompatibility. Since some materials require an additional sintering process, in which the temperature rise may cause damage to the original printing polymer. In addition to material problem, the mechanical strength of printed product is relatively lower than other fiber spinning method, like melt spinning, due to the elimination of drawing process [81].

Yarns are the assembly of parallel bundles of fibers with or without twists to confer strength, coherence, and other properties. They are available in a variety of yarn counts, structures, and textures to meet specific end uses. The two most common types of yarns are staple spun yarns (fiber length < 60 mm) and continuous filament yarns [86]. Making yarns from intrinsic conductive fibers/filaments, such as copper wires, silver yarns, and graphene fibers, have been used as a basic component for yarn electronics [87]. Conductive yarns were made by assembling CNTs onto the cotton sliver, then followed by ring-spinning [88] or by wrapping stainless steel wires and bamboo charcoal roving for electromagnetic shielding [89].

Hollow-spindle spinning is a more frequently used method to wrap conductive materials or one-dimensional (1D) electronic circuit assemblies with textile fibers. By using a lab-made machine to encapsulate 1D flexible circuit board assembly with conventional yarns as a novel electronic yarn [90]. It can be applied to different textile products, such as bracelet and clothing. Yarn-based triboelectric nanogenerator (TENG) has been demonstrated by using this method with polyimide yarn and conductive yarn. The yarn device can then be woven as fabric [91].

Besides the development of intrinsic electronic fibers and yarns, coating insulated fibers and yarns with conductive materials is one of the facile ways to obtain conductive fibrous materials. Typically, the coating techniques can be divided into physical coating methods and chemical coating methods [92]. Physical coating methods include dip-coating [93], spray-coating [94], blade-coating [18], and electrophoretic deposition [95], while chemical coating methods include chemical vapor deposition (CVD) [88], atomic layer deposition (ALD) [96], etc. Similar to the dyeing method of commercial textile processing, dip-coating is a process the outcome of which is the application of a thin film of desired materials to the substrate surface; it can effectively cover textile materials with solution dispersible or soluble functional materials such as carbon nanomaterials [97], conductive polymers [98], and metal nanomaterials [99] in mild conditions. For example, Lee et al dip-coated PEDOT:PSS and single-walled carbon nanotubes (SWCNTs) onto to PU fibers with oxygen plasma treatment [100]. The strong interaction between PU substrate and conductive sheath enabled by hydrogen bonding guarantees the formation of uniform microscale wavy architectures upon releasing the pre-stretched fiber. Thus, the fabricated sensors demonstrated high thermal sensitivity without interference from tensile strain below 180% in a reversible manner.

Despite the facile and scalable advantages, the typical textile coating methods nowadays still suffer from low resistance to washing and are not satisfactory for high-resolution pattern coating [101]. CVD, as a non-line-of-sight vapor-based deposition method widely used in microelectronics, show the potential for overcoming these challenges [102–104]. In the CVD process, covalent bonds are formed on the interface of the resulting hybrid material and the textile surface, which improves the mechanical durability [105]. For example, PP fiber-based temperature sensors with a uniform layer of graphene grown by CVD showed no significant change in the resistance after multiple washing and bending tests [106]. The thickness of coating layer generated by CVD is limited by the slow penetration of reactive vapors into the textile structure. ALD, a variation on CVD, where the reaction occurs on the surface through sequentially exposure and purge two or more gaseous precursors, allows the growth of films on a substrate with extreme thickness control [107]. For instance, by utilizing ALD pretreatment to induce hydroxy double salt nanosheets on carbon fibers, Zhao et al proposed a functional smart fiber composite for an ultra-sensitive flexible glucose sensor [108]. In addition to good sensitivity and fast response, the prepared glucose sensors exhibited good flexibility and stable electrochemical performance under 100 cycles bending at 180 °C.

Texturing is a treatment in which flat filaments are distorted to have loops, coils, buckling, or crimps along their length to gain high bulkiness, porosity, softness, and elasticity. A typical texturing process for a filament yarn begins with a yarn distorting process like twisting that improves the crystallinity and orientation degree of fibers, followed by heating to weaken the van der Waals forces and hydrogen bonding between polymer chains and to keep the filaments crimped. After that, the distorted yarn is cooled down and enabled with new interaction between polymer chains [109]. Based on the high orientation degree of both polymer chains and crystals along the filament, Baughman et al wrapped highly twisted yarns around a mandrel and then stabilized the coiled fibers by thermal annealing to fabricate thermal-induced contraction artificial muscles with 4% strain contraction. Buckling textures with the ability to absorb large strains through flatting the wave structure are widely used in making stretchable conductive fibers which are basically fabricated by releasing pre-stretched elastic fibers coated with conductive materials as sheath [110]. Among the texture-related properties of textiles, the friction of the fibrous material surface is the main characteristic that significantly affects the output power of TENG textiles. For example, Huang et al tuned the micro-morphology of knitted textiles in a wide roughness range, and found that the resulting TENG textiles with face loops and higher stitch densities proved to be the optimum structural design for output power performance [111].

Despite progress in spinning electronic fibers and yarns, challenges remain in terms of comfort, large-scale production, and commercialization. While miniaturization has made these materials more flexible and comfortable, their mechanical performance is not yet sufficient for integration and use. Additional texturing and structuring techniques are needed to explore their comfort further.

Although low-cost spinning techniques can be scaled up, they are only suitable for a limited range of functional materials possessing unignorable risk potential for equipment and environment. Advanced manufacturing processes for fine structure fabrication inject new vitality in the field but are still expensive and not fully mature. These limitations result in a lack of diversity and functionalities of fiber/yarn electronics, which hinder the construction of electronic fabrics and the integration of textile electronics. Standardization and quantification of fiber/yarn electronics are urgently needed to confirm their quality and safety and promote large-scale production and exploration of the market for these innovative products.

Weaving interlaces two sets of yarns as weft and warp at the right angle to fabricate woven fabrics (figure 3(a)) [31]. In the woven structure, the warp and weft yarn contact each other at the cross-over points. It is applicable to produce patterns with varied numbers of weft and warp to achieve desired properties, such as porosity, strength, in-plane shear and extensibility. Weaving is a common method for fabricating textile electronics and integrating textile electronics into fabrics because of the small deformation in the weaving process and in the woven structures. The weft yarn has lower requirements on the yarn strength, elasticity, and mechanical properties. In figure 3(b), 200 µm wide commercial polyimide circuit strip was used as the weft yarn with sensing and display function [113]. In this study, the maximum strain of the yarn in plain weave is around 2.2%, which is considerably lower than that of the circuit damage threshold (10%). Moreover, the interaction between weft and warp yarns provides a potential platform to connect different devices to realize various functions as a textile electronic system. Weaving technique was utilized to integrate different electronic components, including pressure sensors, strain sensors, circuit boards, logical operation, solar cells, and batteries, as a system of medical monitoring (figure 3(c)) [114].

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Electrically conductive woven fabrics have been made from conductive yarns by automated weaving looms and sold in the market for many years. They have been used widely for large-sized applications in electromagnetic field shielding and electrodes for devices. However, for the integration of microelectronic electronic devices, several problems hinder progress. Firstly, the circuit design or device alignment needs to be at the right angle, matching the weft and warp yarn interaction. It is impossible to have an arbitrary or planar curved circuit design in weaving because this may conflict with the mechanism of weaving. On the other hand, right-angle circuit design may also have energy efficiency problems. Another critical point is that the weaving loom has strict requirements on the uniformity of warp yarn linear density [115] and the current microelectronic yarn cannot match this requirement because of the various sizes of the microelectronic chips. Thus, most current electronic fabric requires varied degrees of customizability to production techniques, limiting its yield and commercialization. In addition, the precise control of the weft yarn insertion is a challenge to confine the location of the microelectronic device. In the process, the corresponding shedding may generate a strong squeeze and undulation of yarns in each cycle. The interaction between warp and weft also generates stress to form creep. These may damage the functional material and shorten its lifespan, which might be deteriorated with higher loom speed.

Knitting is the technique of bending yarns into loops (figure 3(d)), which intermesh each other either row-by-row or column-by-column, by using of needles and control devices like sinkers and cams. In the knitting process, the yarns are fed to the needle and bent into the shape of a loop under the help of the sinker and the cam [116]. Knitting machines can be divided into circular knitting machines and flat-bed knitting, which main difference is the product format and gauge [124]. In addition to the difference in knitting beds, knitting can generally be categorized as weft knitting and warp knitting relied on the loop formation directions [124]. The loop formation of weft knitting only involves one yarn move and loops subsequently in the horizontal direction, while loop formation in warp knitting requires hundreds of yarns looping vertically. They differ in elasticity and shrinkage value and direction.

A feature of knitted fabrics is their high stretchability due to the loop structure. Knitting with intrinsic functional yarns or filaments, such as copper wires, silver conductive yarns, and polytetrafluoroethylene (PTFE) yarns, to fabricate textile electronics is beneficial and common. These materials can withstand the stress deformation from the knitting process with limited loss of functionality. Knitted structures with conductive silver coated PA yarns have been used for heating in winter garments or surface electrodes in surface EMG monitoring or electro-stimulation. The knit structure allows a high fabric deformation but yet low fiber strain or stress. For instance, a knit flexible circuit board made from PU-coated copper filament exhibits less than 1% electrical resistance variation under 300% strain (figure 3(e)) [117]. In addition, knitting can fabricate circular structures or tubular structure as bi-layers. This is advantageous to combine multiple materials in contact for device fabrication. A textile TENG was reported with nylon 66 and PTFE composite yarns in a tubular knitted structure, as demonstrated in figure 3(f) [118]. However, weft knitting is more common due to its simple processability. Above three textile electronics are fabricated via weft knitting, while warp knitting application is limited. For instance, a stretchable lithium ion battery electrode with island-bridge structure was prepared by warp knitting [125]. This electrode can withstand 1,000 cycles stretching.

Braiding is a common technique for rope, shoe laces and net-shape composite preforms. It requires at least three yarns linked to the same start point intertwining alternatively at interlacing angles in the range of 20°–75°, following a certain algorithm (figure 3(g)) [119]. The braiding machine is normally composed of multiple carriers and horn gears [126]. Carriers can hold up the bobbin as input and control the yarn tension. Horn gears in disk shape with several cuts can transmit the motion to neighboring horn gears as a carrier movement track to build up a track. When carriers move, the yarn interlaces and constructs the next piece of braid at another braiding point [127]. Braiding has been applied in fabricating TENGs because it can interlace multiple materials to form multilayers, which is consistent with the mechanism of TENGs. Figures 3(h) and (i) shows a braided TENG with high compression resilience, enhanced power output, and pressure sensitivity [120] and a core-spun TENG via coating and braiding [121], respectively. Besides TENGs, braiding also can be used to fabricate other devices. An example is the capacitive strain sensor in braid form made up of capacitive pressure yarn sensors for a miniaturized interface to recognize gestures [128].

Nonwovens refer to the assembly of fibers, continuous filaments, or/and chopped yarns as a web, and bonded together (figure 3(j)). Compared to the previous three processes, the nonwoven techniques can directly use fibers or filaments, instead of using yarns [129], endowing the non-woven fabrics with anisotropy, inhomogeneity, porosity, and permeability. Conventional nonwoven procedures involve web formation, bonding, and rolling of goods. The general web formation techniques include dry-laid, melt-spun, and wet-laid [130]. The bonding techniques consist of chemical bonding, mechanical bonding, thermal bonding, and others. The web formation and bonding processes vary depending on the types of materials used. For example, gaseous hydrogen chloride is used to chemically bonded polyamide webs together because polyamide can melt in hydrogen chloride. In addition, web formation and bonding techniques also can be combined in electrospinning and melt-brown processes.

Due to the material requirements, research on the integration and fabrication of textile electronics using the conventional non-woven technique is limited. Figure 3(k) illustrates a nonwoven fiber layer with an additional liquid metal printing the acts as a human-machine interface to control mechanical claw [122]. A conductive nonwoven composite was made from graphene nanoplatelet and polyethylene terephthalate (PET) fiber via the wet-laid method [69]. A capacitive pressure sensor comprising a dielectric layer of the electrospun ceramic non-fibrous network is shown in figure 3(l) [123]. These sensors sustained over 50 000 compressive cycles. Tissue-like mechanical properties and good permeability have been achieved by a hybrid skin conformable electronics based on an electrospinning nanofibrous substrate [131].

Fabric techniques play a crucial role in the manufacturing process of integrating fibers or yarns to provide coherence and mechanical strength to the deformation during usage. The integration of different fiber and yarn electronic components into a piece of fabric has the potential to the development of an electronic fabric system. However, there are currently limited examples due to several challenges.

One major issue is the significant variation in size, electrical performance, and format of textile electronics components, as they are fabricated using various techniques and materials. Although customized fabric techniques can address this problem, the associated costs increase accordingly. Furthermore, the resulting electronic fabric system may not meet practical standards in terms of aesthetics and comfort due to differences between the fibrous electronics. Another challenge is the need for additional connections between textile electronics components in a fabric. Fabric production arranges fibers and yarns in specialized patterns, but building electrical connections is difficult due to material mismatching and form varying. The mechanical strength of fiber or yarn electronics is another significant challenge in electronic fabric production, as it may not withstand mechanical deformation and stress during production. For example, in knitting, the yarn is deformed into a loop, requiring high flexibility of textile electronics devices in electronic knitted fabric. As a result, current electronic fabrics only involve a single or limited number of textile electronics devices, far from achieving a sophisticated electronic fabric system.

In this part, we focus on the integration and interconnection of conductive yarns during these two processes to create a fabric electrical circuit. More specifically, the construction of a network consisting of conductive and nonconductive yarns to form particular conductive patterns according to a given fabric design.

In weaving process, the fabric circuits can be developed in single layer structures including plain, twill, satin, and basket weaves with integrated conductive yarns distributed in one or both of the weft and warp directions [31]. In terms of multilayer woven configurations, some conductive yarns can travel among different layers as well as switch between warp and weft yarns; these structures accommodating a high density of conductive yarns are potential for multilayer fabric circuits board [135]. The regular grid arrangement of yarns allows interconnect at the crossing points between conductive yarns to create the track design [136], and also ensures the correct align of integrated microelectronics [137]. After that, the efficient transfer of current between conductive yarns requires electrical connection with low resistance at the crossing points of conductive yarns. This can be done with several methods, including the complex weaving techniques [138], laser ablation [137], resistive welding [135], conductive adhesives [139], and pressure contact [31]. A pioneer work of various electronic circuits made by weaving specialized fibers integrated with electronic components was reported by Bonderover and Wagner [140] Pressure contacts were adopted to ensure flexible connection between these fibers for the fabrication of a woven inverter circuit (figure 4(a)). Pressure contact brought about by axial tension in yarns is not reliable, but it might be preferred in applications where connection flexibility is the main concern. In addition to the connection of crossing points, the formation of predetermined disconnects along a conductive yarn is necessary for current control.

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Knitted fabrics are distinctive from woven fabrics in their loop structures. Given the knitted structures are relatively easy to be stretched or distorted, high flexibility and stretchability can be accommodated when routing circuits with microelectronics. These distinctive characteristics make knitted fabric circuits suitable for next-to-skin wearable electronics. Compared with weaving circuit in the form of perpendicular wiring and electrical interconnected with post-process, knitted circuit has a higher freedom degree of routing and is timesaving because standard knitted fabrics can be made entirely from a single thread [141]. For instance, a knitted fabric circuit board was reported of being stretched over 300% strain with a current capability of 15 mA [117]. Additionally, the three-dimensional (3D) looped configuration contributed to highly durable, washable, permeable features of electronic fabrics with modulus lower than human skin.

Embroidery is a type of needlework in which threads or ornamental materials (e.g. beads and sequins) are attached to fabric, generally considered as a technique of decoration [145, 146]. It plays an important role in the construction and integration of electronic components and textiles upon stitch conductive thread patterns that define circuit traces and component connection pads (figure 4(b)). An important step for seamlessly embroidering conductive paths into textiles is to find conductive yarns that are suitable for use with fabrics. Three main considerations of selecting embroidered conductive threads are the electrical conductivity, the process intactness, and mechanical stiffness for wearable comfort purposes. The high conductivity metallic threads are too stiff to be fed in embroidery machine [147]. For instance, highly-twisted plied threads made of several stainless steel yarns performed badly as embroidery threads due to the tension accumulation in filament metal fibers during sewing that caused the threads to bunch up and interrupted the sewing process [148]. While spun steel yarns are more applicable in embroidery machine, the protruding fiber may cause skin irritation to users and undergo unstable electrical connectivity during wearing. Compared with their metallic counterparts, intrinsic conductive polymer yarns and conductive polymer composite yarns dispersed with a low proportion of conductive fillers are more compatible with sewing machine, but the low conductivity might be insufficient to fulfil some textile electronic applications [31]. Thus, metal-plated yarns that provide a higher conductivity and machine-sewability are the most competitive candidate for machine embroidery. Among different types, silver-coated yarns are common embroidery threads used in textile electronics, see table 1.

Apart from the intrinsic property of conductive yarns, embroidery parameters also influence the electrical performance of embroidered conductive tracks. The traditional basic embroidery stitches are run, satin, and fill. The run stitches form linear segments, which are employed in creating fine single line conductive tracks and embroidered sensors [147, 149, 150, 155, 156]. A study revealed that silver-plated yarn embroidered tracks made of stitch 301 give the lowest resistance compared with conductive tracks constructed with 101 single-thread chain stitch, 304 zig-zag stitch, and 406 cover stitch [152]. The satin stitches are suitable for creating prominent contour that is wider than run stitched lines, or filling up a small area, showing increased reliability on the layered thick conductive tracks [147]. The fill stitch is running rows of stitches to fill up a larger area as embroider electrodes in some textile electronics [147, 149]. Besides, thread position, stitch density, machine speed, and needle thread pre-tension are factors concerned. It is known that the needle thread under tension is rubbed against metal parts of the machine, including thread guides, take-up lever, tension regulator, and needle eye. Conductive coating can damaged because of the abrasion during embroidery, leading to the resistance increase or even device malfunction. Placing the conductive thread in the bobbin as well as lowering embroidery speed and stitch length can avoid excessive coating abrasion, and therefore, guarantee the conductivity of embroidered tracks [152, 157].

In order to maintain the electrical continuity of embroidered conductive tracks, protective layer on top of tracks are required. Laminating a thermoplastic film via hot-pressing and overstitching high-density zig-zag stitches with insulating threads on the tracks are two major routes, since the polymer layer highly restricts water drying speed which negatively affects the electrical resistance, the latter one displayed better resistance to wash after 50 cycles [147].

Printing on fabrics builds flexible conductive traces made up of conductive inks, conductive polymers, or conductive solutions to textiles fixed upon on textile surface at predefined areas [158–161]. Techniques which make these possible include mask-free inkjet printing and rotary printing, that print traces directly by moving the nozzle over and rotating the perforated cylindrical screen on textile substrates, respectively. Printing circuits can also be achieved by covering predetermined masks on fabrics along with techniques such as screen printing, evaporative printing, and sputtering. Generally, conductive printing is suitable for any kind of fabrics such as woven, knitted, and nonwoven structures as long as these methods are non-corrosive and non-destructive to textile materials.

Inkjet printing is the process of appropriate inks to continuously form and deposit small droplets in the size of 100 nm—100 μm onto substrate to generate functional patterns [143]. Such a technique has attracted wide attention for the manufacturing of printed electronics, due to advantages including fully digital mask-less printability, versatile ink formulation, cost-effectiveness, low working temperature, waste-less, and feasibility for scale-up production [32, 162]. To realize stable, repeatable, and high resolution inkjet printing, it is crucial to develop high purity conductive inks in which functional materials are stable and uniformly dispersed without any aggregation [31]. They are categorized into three major groups according to the compositions of metallic solutions, conductive polymer solutions, or nanoparticle dispersed solutions together with the addition of surfactants, dispersants, thickeners, adhesion promoters, or stabilizing agents if necessary [132, 163]. Also, the viscosity and surface energy of inks are critical for the formation and deposition of droplets, respectively. Recently, Uzun et al developed an additive-free, aqueous MXene inks with tunable viscosity and surface tension upon the modification of MXene flake size from sub-micrometer scale to micrometer scale and concentration [143]. With these customizable inks, they printed various high-resolution structures such as conductive patterns and interdigitated devices via thermal inkjet printing with commercial printers (figure 4(c)), and demonstrated wearable textile electronics based on knitted and woven fabrics. In the case of conductive inks such as gold, silver, and copper based dispersions made up of metallic particle materials, a following annealing process is essential to enable the creation of continuous conductive networks upon the welding at interfaces [164]. Besides, printing with conductive ink containing dispersed metal particles could be problem due to clogging of the nozzles. Several strategies including periodical cleaning of nozzle [165], size reduction of dispersed particle [166], dilution of ink concentration [167], and using high boiling solvent [168] can be taken to avoid clogging for an efficient inkjet printing.

Screen printing is an impact printing method in which ink is applied through a fine-mesh screen onto a touched surface, except in those areas blocked by the stencil to make them ink impermeable [161]. To develop multilayer printed circuits on textile products, a screen is required for each functional ink and to define each layer's geometry. Ink characteristics, such as viscosity and curing temperature, as well as surface characteristics of the textiles, such as surface roughness and surface energy, can all affect the accuracy of pattern printing [169]. Besides the printing of conductive traces fine enough to integrate off-the-shelf electronic components, this technology fits the direct fabrication of flexible and lightweight electronic elements on textile substrate (figure 4(d)), such as patch antennas, sensors, and capacitors, for wearable electronics [170].

One of the primary obstacles in the development of fabric circuits pertains to the materials employed for constructing flexible conductors. The use of commercially available metallic fibers or yarns as circuits restricts flexibility and adds weight to textiles due to their high modulus and density. Additionally, it is vital to devise effective techniques for embedded or printed fabric circuits to withstand clothing-like handling and washing environments. Such environments expose can be detrimental to the conductive and mechanical properties of fabric circuits due to mechanical, thermal, and chemical aggressions.

In order to maximize the throughput for large-area flexible devices, roll-to-roll (R2R) manufacturing is commonly used in the industry. This process involves applying various functional materials onto a flexible rolled substrate which is continuously fed from one roller to another. The typical R2R system often consists of several rollers known as the web path to sequentially carry out a number of operations. Embroidery, inkjet printing, and screen printing allow the continuous application of materials onto a moving fabric, making them compatible with the R2R process for large-scale production of electronic circuits on textile substrates. When combined, these techniques have shown great potential for the continuous fabrication of fabric circuits in terms of low cost and high throughput [171–173].

Embroidering and printing processes can be performed on a moving fabric in a specific zone of the R2R line. Computer-controlled embroidery machine efficiently stitches conductive threads onto a continuous fabric as it moves through the zone. On the other hand, inkjet printing offers great flexibility in terms of designing and creating complex fabric circuits, as it allows for the direct printing of patterns onto fabrics without contact. Screen printing can also be adapted to R2R processing in the form of rotary screen printing by bending the flatbed screen into a cylindrical form. Multiple screens used in series allow the creation of multilayer circuits with high productivity and accuracy. However, challenges arise when these fabric circuit construction techniques are combined with R2R processing. First, the surface roughness and structural porosity of fabrics varying a lot with the tension degree and direction complicate the production of finest and consistent conductive circuits. Tension disturbances and unevenness can cause localized stretch or shrinkage, leading to distortion of the circuit or misalignment between the different layers of the circuits. Additionally, the resolution and precision of the printing circuits may be limited by the velocity of the web in the R2R process. The accuracy of the fabricated fabric circuitry is highly associated with the precise alignment of integrated machines as well as the use of sophisticated circuit construction software.

Soldering is a standard joining technology for microelectronics. Soldered connections made up of Sn alloys, such as Sn96.5-Ag3-Cu0.5 (SAC 305) and Sn96.5-Ag3.8-Cu0.7 (SAC 387) have excellent electrical conductivity but are brittle [174]. Both of them have a near-eutectic composition leading to a fixed melting point. During the soldering process, an alloy with melting temperature lower than that of the contact is heated and melts between the unjointed terminals of the microelectronic units. The soldering process completes with good connections upon the solidification of the melting solder material. The heat can be transferred via direct conduction, convection, or radiation. In terms of the integration of electronic components and textile substrates, direct contact or friction soldering [175], hot air or thermal soldering [176], ultrasonic soldering [177, 178], laser soldering [174], and infrared soldering [179] are the five most important soldering categories.

Due to the electrical conductivity of connections, soldering has been commonly used in textile electronic integration for the connection of units (figure 5) such as conductive threads [180], flexible films [28], and flexible copper wire [181]. Various microelectronic components have been soldered with textile structures, however, some issues are still unresolved: the melting solder wicked along the multi-filament direction due to the capillarity effect, which affects the durability of the joint [182]; the stiffening of textiles after the introducing of alloys whose Young's modulus is several magnitudes higher than fabric materials [29]. Modifications have been tried, such as the resistive welding at the cross-over points of orthogonal conductive yarns to improve the efficiency of current transfer [183], and the ultrasonic welding in developing textile-based transmission lines using stainless steel silver-plated conductive yarns to enhance the durability of signal lines by protecting them from probable short circuits and water contact [184].

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Comparing with soldering integration that builds a solid connection, mechanical provides a removable connection. Upon tool optimization, parameter determination, and friction effect implication [186, 187], the mechanical gripping has experienced rapid development and improvement, permeating throughout the textile electronic integration. Figure 6(a) shows the eyelets used to form vias on the textile electronic circuit board for multilayer interconnection. Snap connectors are easily detachable for the connection of textile electronics (figure 6(b)) [188], but the volume is still too large to be used for tiny fiber connections. The conductive hook and loop (figure 6(c)) were developed for detachable electrical connection [189], however the adhesive force may decrease after long time using leading to the unstable conductivity. Alternatively, the magnet connection (figure 6(d)) in a garment, relying on a magnetic force to both make and maintain a connection, is convenient for reliable and stable connection without the requirement of precise alignment [190]. But it is not suitable for circuits susceptible to interference by electromagnetic fields. All these mechanical gripping methods are based on the current connection technology. Even though the mechanical gripping is a cost-effective integration method to build easily detachable connections for textile electronics, the bulky connection structures are susceptible to the operation of end-user and the surrounding environment.

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Except the above mentioned two physical connections, the chemical substitution, conductive adhesives have also been used extensively for electrical connections in textile electronics. Conductive adhesives are generally composed of composite materials with conductive fillers dispersed in an insulating polymer matrix [30]. The conductive fillers can be normally categorized into three groups: (1) metal fillers, such as silver, gold, nickel, and copper; (2) carbon fillers, including CNTs, carbon nanofibers (CNFs), C and graphene; and (3) inherently conductive polymers, such as polypyrrole (PPy), PANI, and polythiophene (PTh). In terms of electrical connection, the dispersed conductive fillers with a concentration above the percolation threshold can form a 3D conductive network by contacting with each other to provide a path for electron transport. Meanwhile, the cured polymer matrix, as which epoxy resin is frequently used, provides the strong adhesion between the respective substrates through chemical bonding.

Besides providing conductive connection between different components, conductive adhesives display irreplaceable advantages in terms of textile electronics. These lead-free materials are eco-friendly in comparison to widely used SnPb solders, and are friendly to many temperature sensitive components due to their curability at room temperature. Also, the nature of solvent-free greatly simplifies the processing steps. Despite these advantages, the connection quality is prone to high humidity and temperature [31, 192].

The wearable textile electronics with integrated electronic components are often subjected to stresses including bending and twisting, electrical interconnects therefore require a high electrical conductivity and reliable conductance against strain. Thus the stretchable conductive adhesive was developed for printable interconnects in washable textile electronics by Ko et al [139], the conductive adhesive consisted of CNTs and silver particles in stretchable silicone adhesives. The maximum electrical conductivity of the adhesives was 6,450 S cm⁻¹, and showed little change over 3000 stretching cycles at 50% strain.

The fundamental challenge of forming these interconnects involves making them flexible, robust, and environmentally stable while ensuring adequate electrical connectivity, leading to the dilemma between preserving the natural feeling of the garment and achieving sophisticated electronic performance of the high integration degree because each electronic part has to be specifically adapted and attached to fabric structures.

Standard electronic components are separately attached to fabric circuits through electrical connections, the transition from soft to hard materials with prodigious mismatch, inevitable weak point for the device under strain, is preferred with minimum stress/strain concentration. This set a barrier to the large-scale development of reliability hybrid textile electronic systems based on the heterogeneous integration of microelectronics and textile substrate [31]. Efforts on interfacial design such as using stretchable gradient structures or materials at these fragile mechanical interfaces have the potential to improve the durability of the device [141].

In order to efficiently interconnect components of a textile electronic system, there is a need for a new, scalable, and cost-effective manufacturing technique. The existing manufacturing techniques mainly derive from electronic joining and packaging technologies, but they are not suitable for textile electrical connections. Hence, extensive research is required to develop suitable materials and methods for textile electrical connections to meet this challenge.