Electrically conductive highly elastic polyamide/lycra fabric treated with PEDOT:PSS and polyurethane
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Conductive elastic fabrics are desirable in wearable electronics and related applications. Highly elastic conductive polyamide/lycra knitted fabric was prepared using intrinsically conductive polymer poly (3,4-ethylenedioxythiophene) (PEDOT) blended with polyelectrolyte poly (styrene sulfonate) (PSS) using easily scalable coating and immersion methods. The effects of these two methods of treatments on uniformity, electromechanical property, stretchability, and durability were investigated. Different grades of waterborne polyurethanes (PU) were employed in different concentrations to improve the coating and adhesion of the PEDOT:PSS on the fabric. The immersion method gave better uniform treatment, high conductivity, and durability against stretching and cyclic tension than the coating process. The surface resistance increased from ~ 1.7 and ~ 6.4 Ω/square at 0% PU to ~ 3.7 and ~ 12.6 Ω/square at 50% PU for immersion and coating methods, respectively. The treatment methods as well as the acidic PEDOT:PSS did not affect the mechanical properties of the fabric and the fabric showed high strain at break of ~ 650% and remained conductive until break. Finally, to assess the practical applicability of the treated fabric for wearable e-textiles, the change in surface resistance was assessed by cyclically stretching 10 times at 100% strain and washing in a domestic laundry for 10 cycles. The resistance increased only by a small amount when samples were stretched cyclically at 100% strain, and the samples showed good durability against washing.
Electronic textiles (e-textiles) are textiles that could provide electrical properties as electronics and perform physically as textiles which enable computing, digital components, and electronics which can be embedded in them. E-textiles combine electrical conductivity with flexibility, stretchability, wearability, wrapping large surface area for sensing, and making them an ideal for wearable electronics applications where the traditional rigid electronics are lacking. E-textiles cover a broad area of applications including thermal therapy , wearable energy harvesting/storage [2, 3, 4], real-time healthcare monitoring , electromagnetic interference shielding , and other flexible and portable wearable electronics applications [7, 8]. Conductive textiles are the most integral parts of e-textiles. Conductive textiles have been produced by several approaches including electroless metal deposition , inserting metal wires in the fabric , using intrinsically conductive fibers and yarns , coating with conductive polymers [11, 12, 13], in situ polymerization of conductive polymers on the fabrics [14, 15], and pyrolysis of the textile .
Producing conductive textiles by means of the coating technique with conductive polymers is quite interesting owing to its large area coating, compatibility with the current textile processing methods like dyeing and printing, and cost. Among the conductive polymers, poly (3, 4-ethylenedioxthiophene) (PEDOT) blended with polyelectrolyte polystyrene sulfonate (PSS) is quite promising due to its reasonably high electrical conductivity, aqueous processability, stability, and flexibility. Its conductivity can be significantly enhanced by using a number of processing agents including dimethyl sulfoxide, ethylene glycol, alcohols, and acids [17, 18].
There is a growing interest in deformable and wearable devices [19, 20]. For practical application of wearable electronics, in addition to flexibility, some amount of stretchability and elasticity is required to accommodate mechanical stresses and strains which arise due to intense body motions like folding and twisting. Stretchable electronics can be achieved broadly in two ways: using intrinsically stretchable conductive materials which can be obtained by compositing conductive filler with elastomer or attaching conductive materials on stretchable structure. The polyurethane (PU) elastomer has been widely used to make elastic conductive fibers and films for different applications [21, 22]. PU is highly elastic, scratch resistant, adhesive, and easy to apply to different substrates like textiles and has been in use for coating and printing of textiles .
Conductive elastic textiles can be produced either by weaving or knitting inherently conductive fiber/yarn  or coating the elastic fabric substrate with conductive polymers . Coating the elastic fabric with aqueous-based conductive polymers which is easily applicable to the current textile processing techniques is quite interesting from processing, performance and cost aspect. In this work, we prepared conductive elastic polyamide/lycra knitted fabrics, which can be stretched up to 650%, by coating and immersion methods with PEDOT:PSS and different proportions of waterborne PU. The methods are quite versatile in which larger area samples up to 900 cm2 could be produced with uniformity and reproducibility, which mimic industrial printing and the exhaust dyeing of textiles, respectively. The fabric showed low surface resistance of ~ 1.7 Ω/square rivaling previous reports. We investigated the application methods and the effect of PU content on the electrical property, stretchability and washing durability. The samples were reasonably conductive while stretching until break, cyclic stretching, and ten washing cycles. To the best of our knowledge, this is the lowest resistance and highest stretchability with cyclic and washing durability reported for elastic conductive textiles.
Materials and methods
Properties of different waterborne PU dispersions
Solid content (%)
Tensile strength (N mm−2)
E-modulus (N mm−2)
Elongation at break (%)
Textile and leather coatings
Textile, leather, and plastic coatings
Textile, leather, and plastic coatings
Hydrophobically modified ethoxylated urethane (Gel L75 N, 48 wt%) from Borchers, Germany, was used for modifying the rheology (thickener) of the paste for the coating application of the conductive paste.
PEDOT:PSS dispersion was first mixed with 0.1 vol% Zonyl surfactant and 5 vol% DMSO and vortexed to mix well and allowed to settle for several hours. Then, the required amount of PU dispersion was added dropwise using a syringe slowly while stirring the PEDOT:PSS mixture with a magnetic stirrer at 1250 rpm and stirring continued for 15 min for homogenization. The formulations were prepared with 0–90 wt% of the PU based on the solid content of the PU and PEDOT:PSS. This formulation was used for the immersion method of treatment. For the coating method, the appropriate amount of rheology modifier was added to a beaker containing the above formulation and then thoroughly and uniformly mixed with a mechanical stirrer at a speed of 650 rpm for 10 min to obtain a homogeneous conducting paste. When the amount of binder decreased, the amount of PEDOT:PSS increased in proportion and thus the viscosity decreased accordingly. This was compensated by increasing the amount of rheology modifier in amount to keep the viscosity sufficient for the coating as in Ref. .
The PA/lycra knitted fabric was immersed in the PEDOT:PSS-PU composite dispersion for 5 min at room temperature by stirring gently. The immersion was performed only once. The coating method was done using ZUA 2000 (Zehntner GmbH) coating applicator . The PEDOT:PSS-PU dispersion with the rheology modifier was dropped on the fabric surface and squeezed manually with a coating applicator resulting in a paste thickness of roughly 0.5 mm. Finally, the samples were dried at 90 °C for 30 min followed by curing at 130 °C for 5 min using a laboratory mini dryer machine to obtain the conductive knitted elastic fabric.
The electrical surface resistance was measured using an in-house designed and assembled four-point probe setup connected to a multimeter (Agilent 3401A) using Van der Pauw principle . The fabric samples were placed on flat insulating material, and the probe was placed on top of the fabric. To improve the contact between the probe and the fabric sample, a 2 kg weight was applied on top of the four-point probe equipment. Before testing, the samples were conditioned for more than 18 h under standard atmospheric condition. The surface resistance readings were taken after 1 min so that the readings were settled. The measurements were taken in both the course and wale directions four times, and the averaged results were taken.
Scanning electron microscopy (SEM) images were taken with a Leo Ultra 55 SEM equipped with a field emission gun (LEO Electron Microscopy Group, Germany) and a secondary electron detector. The acceleration voltage was 3 kV. Samples were cut by a razor blade and imaged without metal sputtering on them.
Tensile testing was performed using an Instron tensile tester (model 5565A) with 100 N load cell at a cross-head speed of 10 mm min−1 and jaw length of 20 mm. For tensile testing, a sample of 6 mm wide was cut in the wale direction by carefully hammering with a homemade spacer fitted with sharp blades. The change in resistance of the fabrics while stretching was measured using KEYSIGHT U1233B digital multimeter with flat crocodile clips by connecting to the ends of the cut fabrics (beyond the grips) with silver paste and copper tape for better contact. The resistance values were recorded at a 10% strain value difference. The change in resistance during cyclic stretching at 100% strain was recorded by interfacing the KEYSIGHT U1233B digital multimeter with a computer. To check the resistance relaxation, the samples were held for 1 min at 100% strain and again at 0% strain after each stretching cycle.
The conductivity durability of the conductive fabrics against washing was assessed by washing up to 10 times using the domestic laundry washing machine using 100% polyester ballast by following the procedures of standard washing type 3A reference washing for textile testing (ISO 6330:2012). The samples were sealed using a laundry bag so that the washing reagent can penetrate easily into the sample and then placed in a laundry machine having 20 ± 1 mL of reference detergents. Washing was carried out at normal agitation at 30 ± 3 °C and 100 mm liquor level for 15 min followed by four consecutive rinsing cycles where each cycle lasted for 2 min. The specimens were then dried at room temperature before the next washing cycle.
Results and discussion
Coating and immersion methods were performed to fabricate conductive PA/lycra knitted elastic fabrics which mimic the industrial textile processes of coating/continuous chemical processing and exhaust dyeing, respectively. The immersion treatment was also done for a short time (for only 5 min) and hence can be easily upgraded to the commercial continuous padding technique. We prepared samples up to 900 cm2. The schematic diagram of the treatment methods is shown in Fig. 1b. The PU dispersion has been intensively used in textile applications as a coating material to increase the durability and softness of coated textile [23, 28]. Recently, PU is also being used as a composite material to make elastic conducting composite films [29, 30]. Combining these two, we prepared and investigated the electromechanical properties of the conducting elastic knitted fabric treated with different proportions of the conductive polymer blend PEDOT:PSS and the waterborne PU dispersion.
Fabrics treated by the immersion method displayed surface resistance of ~ 1.7, ~ 3.7 and ~ 7.5 Ω/square, while those by coating method displayed ~ 6.4, ~ 12.6 and ~ 16.9 Ω/square for 0, 50, and 90% of U2101 in the conductive formulation, respectively. The corresponding calculated conductivities are ~ 14 and ~ 3.8 S cm−1 (taking the whole bulk of the fabric) for fabrics treated by immersion and coating methods with 100% PEDOT:PSS which are superior than previously reported values even done with multiple immersion in PEDOT:PSS [25, 26, 31]. It is expected that in the immersion method, PEDOT:PSS diffuse easily and penetrate into the structure of the fabric, yarn, as well as the fiber. Earlier reports showed that there is dyeing effect on silk when treated with PEDOT:PSS by immersion method . As both PA and Lycra contain amide functional groups, we also expect some sort of dyeing rather than a mere coating. Hence, in the immersion method PEDO:PSS easily and uniformly diffuse in the yarn and fiber pores leading to better dyeing/coating and later better conductivity and durability. Since the U3251 and U4101 binders gave aggregated particles during mixing and hence lower conductivity, we therefore chose PU2101 for further investigations.
In conclusion, we developed conductive elastic PA/lycra knitted fabric by treating with PEDOT:PSS and PU binder with facile and easily scalable methods of coating and immersion which are similar to the commercial textile processes of coating and exhaust dyeing. The immersion method gave more uniform and better conductivity than the coating method, and surface resistances as low as ~ 1.7 and ~ 6.4 Ω/square were obtained for immersion and coating methods, respectively. Acidic PEDOT:PSS treatment didn’t affect the mechanical performance of the fabric and the treated fabrics remained reasonably conductive until breakage when stretched (~ 650% strain). When the fabrics were stretched at 100% strain for 10 cycles, the resistance increased only to 2.7 times showing the practical usability of the method. The PU used didn’t affect the change of resistance during stretching. Hence, if we aim high conductivity and stretchability, immersion method without PU is more advisable. However, the PU addition improved very well the conductivity durability to laundering and maintained reasonable conductivity after 10 washing cycles using domestic laundry. The immersion method was found to be a more effective method for achieving high conductivity, durability against stretching, and cyclic stretching than the coating method. As large area sample sizes can be easily produced and the fabric maintained its original flexibility, the highly conductive elastic knitted fabrics will find promising applications for various wearable e-textiles.
This work has been financially supported by Erasmus Mundus Joint Doctorate Programme SMDTex-Sustainable Management and Design for Textile (grant number n_2015-1594/001-001-EMJD) and the European Research Council (ERC) under grant agreement no. 637624. We thank Prof. Christian Müller from Chalmers University of Technology for using his laboratory facility to do mechanical testing and SEM characterizations.
Compliance with ethical standards
Conflict of interest
The authors declare no conflict of interest in research and publication.
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