Zeagler et al. find that their silver-based printed conductive tracks exhibit cracks after six washing cycles, which results in increased resistance. They reason that the swelling of the hydrophilic cotton substrate, as well as bending occurring during washing, are responsible. In some samples, the printed ink is not only cracked, but also flaking off [55]. Kim et al. wash aramid textiles coated with a graphene and polyurethane (PU) composite. After repeated washing, the graphene is (partially) lost and the fibers of the underlying textile substrate loosened and entangled (see Figure 1a). The collision of the samples with other materials during the washing process is suspected to be the reason [30]. When testing the washing reliability of differently formulated, metal based conductive coatings, Malm et al. observe a loss of and cracks in coating, with the severity of damages—and subsequent increase in resistance—being strongly dependent on the formulation and thickness of the coating (see Figure 1c) [38]. In woven fabric that is coated with graphene based ink, Afroj et al. observe a loss of graphene flakes after washing, which results in an increase in resistance (see Figure 1d) [9]. Shahariar et al. report crack formation of varying severity in their printed-on silver-based paste, as well as a delamination of the paste from the substrates [45]. Other sources also suspect the loss of conductive coating or ink as the reason for increased resistance after washing [10,48]. In a larger study with two types of silver-based conductive ink printed onto 14 different woven textiles, Kazani et al. observe a lost ink and resulting increased resistance for all samples, with differing severity dependent on the material combinations (see Figure 1b) [29]. Screen printed antennas in another publication by Kazani et al. also exhibit cracks after washing [67].

Failures that are similar to those described in the previous section occur when washing metallized yarns and textiles. Cai et al. measure the silver content of the washing water of their metallized textile and observe an increase after several cycles, indicating a gradual loss of the silver coating [64]. Dhanawansha et al. also observe lost silver, but find that, with an increasing number of washing cycles, the amount of silver lost decreases, which suggests that silver loss is greatest during the first few washing cycles [65]. In one publication, the loss of metallization is suspected to be the reason for increased resistance after washing, but not further investigated [53]. Rotzler observes a loss of the metallization layer for knitted silver coated nylon fabric, which is dependent on the employed washing program (see Figure 2a) [42]. Similar observations are made by Gerhold [20] and Foerster, who supposes the friction between the test samples and other textiles during washing is responsible for the loss of metallization [18]. Other studies with similar damages to metallized yarns include Tao et al. and Gaubert et al. They observe that the amount of lost metallization increases with the number of washing cycles (see Figure 2b) [49], and it is dependent on the washing parameters (see Figure 2d) [19]. Lee et al. find the amount of metal lost, and related increase in resistance, is dependent on the combination of involved materials [34]. uz Zaman et al. presume that mechanical wear is responsible for peeled off and abraded metal layers in metallized nylon fabric [50,51,52]. When washing gold and copper coated yarns, Schwarz observes cracks in and a loss of metallization, also suspecting mechanical forces as the main cause [43]. Depending on the washing conditions, cracks in the metallization can even occur when the conductive textile is covered by a protective layer of thermoplastic polyurethane (TPU) (see Figure 2c), as a study by Rotzler et al. shows. The authors reason that, due to the conductive textile being covered by a protective layer, mechanical wear and abrasion are not the main cause of damages. Cyclic temperature changes during washing are more damaging to the silver layer, due to a mismatch of the thermal expansion coefficient in the involved materials [1].

When washing strips of flexible circuit board woven into textiles, Komolafe et al. observe cracks in the copper tracks, located only at the transition zone between tracks and bond pads, identifying these zones as weak points of the circuit [57]. Hardy et al. wash different e-yarns of similar composition—wires that are wrapped around a textile core, with and without added functional components. They see wire breakages along the whole length of yarns, as well as broken and torn off wires at the added components, see Figure 3c [22]. Rotzler et al. conduct a washability study with different types of conductive tracks. In tracks made from meander shaped copper foil embedded in thermoplastic polymer (so-called stretchable circuit boards, SCB), they observe breakages in the copper mostly at the transition from conductive tracks to contacting pads, see Figure 3b, as well as near the added dummy interposer. Torn litz wires occur in conductive yarn, embroidered onto the textile substrates using the tailored fibre placement (TFP) technique, see Figure 3a. Similar to the results for the SCB tracks, the torn wires occur at a larger number at the contacting points and the interposer, and to a lesser extent along the length of the tracks [1]. SCB tracks that were washed by Veske et al. also exhibit breakages in the tracks, located exclusively at the top and bottom of the meander shapes of the tracks, see Figure 3d [62]. Tao et al. register an increase in broken litz wires corresponding to the number of wash cycles. Another failure mode occurring are broken contacts between the embroidered and interwoven tracks and added LEDs [49].

When washing PU coated copper wires that were knitted into textiles, Li et al. observe breakages of the wires, leading to an increase in the resistance or complete electrical failure. The authors suspect the mechanical strain during washing and resulting deformation of the wires to be responsible [35]. Molla et al. find that their embroidered tracks exhibit breakages of the metallized yarn—but the number of breakages is very low when compared to the number of samples with a decreased conductivity, indicating that not only yarn breakages occur, but also a loss of silver coating [39].

Wires embroidered onto a knitted textile using the TFP technique show breakages only at the soldered interconnections to a flexible circuit board in research by Vervust et al. Added LEDs on the board still show function, indicating the board itself and its functional components did survive the washing, only the solder connections are not stable enough [61]. Similar observations are made by Gui et al., their liquid metal tracks are still fully intact after washing, with the only defects presenting at the contacting points between the tracks and added components [56]. Others also report damaged interconnections between tracks and components, circuit boards, or connectors [1,22,32,39,60]. In polydimethylsiloxane (PDMS) encapsulated, textile integrated circuits, Ojuroye et al. detect wires that were torn from, as well as chips pulled from, solder joints after repeated washing, being attributed to abrasion, bending, and twisting affecting the samples during the cycles, see Figure 4d [58].

Not only can the conductive materials and components of e-textiles get damaged during washing, but also protective layers or elements. Once these protective structures are corrupted, the underlying conductive or electronic components are more vulnerable to damages themselves [39]. Berglund et al. observe the delamination of the protective PU film in their stitched stretch sensors, with the magnitude of delamination corresponding with an increase in resistance [13]. When washing SCB type conductive tracks, delamination of the protective PU layer occurs along the copper foil tracks in the research conducted by Rotzler. The extent of the damages coincides with the harshness of the washing program, which indicates that frictional forces acting on the samples will lead to thinning and eventual breaks of the PU layer along the edges of the copper tracks (see Figure 4a) [42]. Textile-based radio-frequency identification (RFID) tags in a publication by Wang et al. exhibit a steady decrease in read range when subjected to washing cycles. Cracks in the protective coating of the tags, see Figure 4c, which results from mechanical stresses during washing, are suspected to be among the reasons for this reduced functionality [71]. Tao et al. also report the delamination of a TPU layer after washing, leading to an ingress of water and, thus, possibly responsible for increased resistance [49]. When testing different protective films and tapes on embroidered tracks, Molla et al. observe delamination for some of the materials. The exposed tracks and components experience greater damage than intact samples [39]. Komolafe reports peeled-off glob-top and encapsulation layers after washing (see Figure 4b). Another failure mode that is connected to the protective layer occurring in the same publication is the encapsulation peeling off the underlying printed silver layer from an embedded component. The author suggests this is due to a stiffness difference, which makes the contact vulnerable to bending [32]. The textile integrated touch sensors by Ojuroye et al. exhibit an increasingly limited functionality after each washing cycle. A microscopic analysis reveals cracks in the PDMS encapsulation (see Figure 4d), which enables water to damage the encapsulated components [58]. Even reversible effects on the protective layers can impact washability. Parkova et al. argue that their silicone coating will stretch during washing, which leads to damages and function loss of embedded LEDs [40], while Gerhold observes the swelling of the PU layer on conductive textiles and suspects that this could lead to decreased conductivity [20].

When looking at e-textile washability in its entirety, maintaining not only electronic, but also textile, integrity should be considered, even though being less relevant for the continued functionality of the e-textile system. Several sources report changes and damages to the textile substrate. Ojuroje et al. observe wrinkling in their test samples [58], Rotzler pilling [42]—with varying severity depending on washing program and substrate, Quandt et al. entangling of fibers [69], and Martinez-Estrada et al. the loss of fabric pre-treatment after washing [68]. Du et al., Kim et al. (see Figure 1a) and Zhao et al. notice an increase in yarn or textile surface roughing [15,30,72]. Xu et al. observe a shrinkage in the wale direction of a knitted metallized textile, which results in a decrease in resistance after one wash cycle [53]. Zhao et al. wash conductive yarn up to 20 times, with shrinkage occurring only during the first two cycles [72], similar findings are made by Hardy et al. [22].

Tao et al. report corrosion on some of their washed conductive yarns, as well as an oxidized battery in an ECG-device [49]. The metallized textiles washed by Gaubert et al. show a darkening of the surface, indicating the oxidation of the silver coating [19]. Rotzler sees a loss of interposers and Ojuroye et al. detached chips [42,58]. Gerhold stops wash testing at the first incidence of detached LED modules from textile based conductive tracks [20].

Washing can lead to numerous issues in e-textiles, as this overview of failure modes shows. These failures can occur at specific points in the design or allover, depending on the type and composition of the e-textile. Weak points are contacts between different materials and components—especially if there is a high stiffness gradient—as well as transition areas within the same material (e.g., from conductor track to contact pad). In conductive yarn or textiles without some form of protection, damages can occur throughout the structure. Because added components in the researched publications are mostly encapsulated or otherwise protected, they fail less often than the conductive tracks connecting them or the contacts between tracks and components. For the same reasons, lost or detached components do not occur often. If the protective elements are themselves damaged during washing, though, the components are also vulnerable to damages, e.g., through water ingress. On the other hand, many of the cited sources do not include added components at all, which makes the findings of this paper concerning components less reliable.