Development and Washing Reliability Testing of a Stretchable Circuit on Knit Fabric

applied sciences

Article Development and Washing Reliability Testing of a Stretchable Circuit on Knit Fabric

Paula Veske *, Pieter Bauwens , Frederick Bossuyt, Tom Sterken and Jan Vanﬂeteren Centre for Microsystems Technology (CMST), Interuniversity Microelectronics Centre (IMEC), Ghent University, Technologiepark 126, 9052 Gent, Belgium; [email protected] (P.B.); [email protected] (F.B.); [email protected] (T.S.); Jan.Vanﬂ[email protected] (J.V.) * Correspondence: [email protected]

Received: 24 November 2020; Accepted: 14 December 2020; Published: 18 December 2020

Abstract: The smart textiles and wearable technology markets are expanding tirelessly, looking for efficient solutions to create long-lasting products. The research towards novel integration methods and increasing reliability of wearables and electronic textiles (e-textiles) is expanding. One obstacle to be tackled is the washability and the endurance to mechanical stresses in the washing machine. In this article, different layering of thermoplastic polyurethane (TPU) films and knit fabrics are used to integrate three different designs of stretchable copper-based meander tracks with printed circuit boards. The various combinations are washed according to the ISO 6330-2012 standard to analyze their endurance. Results suggest that one meander design withstands more washing cycles and indicate that the well-selected layer compositions increase the reliability. Higher stretchability together with greater durability is accomplished by adding an extra meander-shaped TPU film layer.

Keywords: wearables; packaging; knitted-fabric structures; e-textiles; electronics; washing reliability

1. Background and Introduction Electronic textiles (e-textiles) can be categorized as wearable technology, finding applications in many appliances, such as interactive clothing or wearable sensors for sports and healthcare. McLuhan [1] mentioned in 1964 how “the computer is the most extraordinary of man’s technological clothing” by suggesting that it can be “extension of our nervous-system”. Mark Weiser [2] discussed further in 1991, where the author stated “the most profound technologies are those that disappear”. Both authors’ common idea was how technology “weaves itself into the fabric of everyday life until they are indistinguishable from it”. Smart clothing possibilities were already proposed and discussed with some examples (e.g., shoe to measure impact on the leg) by Steve Mann and MIT late 20th century [3–5]. The main idea of wearable computing was to bring everyday technology, such as cellphone, personal music system (e.g., Walkman, CD-player), etc., to a single unit, possibly improving personal health monitoring with sensors (e.g., heart rate check, respiration rate, etc.) and decreasing crime by being “a seamless extension of the body and mind” [5]. Refining from that, now, smart textiles (including e-textiles) are most commonly defined as “fabrics or apparel products that contain technologies, which sense and react to the conditions of the environment they are exposed to, thus allowing the wearer to experience increased functionality” [6]. Post and Orth [7] in their 1997 conference proceedings bring out how metal-based yarns have been used in clothing for centuries already for decorative purposes. With computers growing smaller towards the end of the 20th century, these conductive textiles could also be used as sensors and powering and data distribution tracks. The authors also create demonstrators by soldering surface-mount components on conductive (copper) textiles or using conductive textiles as capacitive sensors. Moreover, the same

Appl. Sci. 2020, 10, 9057; doi:10.3390/app10249057 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 9057 2 of 13 work is enhanced a few years later by embroidery technique where in several e-textile demonstrators mainly only textile-based materials were used [8]. In the early 2000s, Leah Buechly and Michael Eisenberg together with SparkFun Electronics created The Lilypad Arduino—a toolkit that could help to develop and prototype e-textiles quickly and efficiently [9–11]. The toolkit included easily sewable sensors and microcontroller(s), which simplified the e-textile prototype creations extensively. However, as the core material of e-textiles is, and should be, textile, these applications will need to be washed regularly by the user. This could cause reduced reliability and shortened lifespan of products and is one of the main shortcomings for entering the mass-market. Earlier works describe how the electronics could be removed (e.g., with the help of snap buttons) [7]. However, as wearable computing or wearables have developed extremely fast during the last years (smartwatches and smartphones), the end-users will be expecting easy-maintenance and reliability also from textile-based wearables. That could be achieved if no or a minimum amount of parts are needed to be removed for washing and everyday maintenance. In more recent literature, different materials (e.g., conductive threads and structured copper foils) are tested with various encapsulation methods. The research towards washability is mostly based on the ISO 6330 (International Organization for Standardization) washing procedure [12]. However, details about the used materials and integration methods are often left out. Tao et al., 2017, integrated LEDs with various types of conductive threads onto fabric, encapsulating it with thermoplastic polyurethane (TPU) film and comparing those results with samples without encapsulation. The study proved the value of TPU encapsulation as a method for keeping the electrical resistance of the tracks within acceptable values [13]. However, only one TPU film was used with one layering method. Rotzler et al., 2020, studied several variations of ISO 6330 standard washing programs using TPU encapsulation for two different conductive materials: structured copper foil (SCP) and conductive fabric (Tex-Printed Circuit Board) [14]. Both materials also laminated to the fabric. While TPU type and thickness is not mentioned, the washing program variations were thoroughly investigated, and a standardized washing test was used. The TPU film encapsulation method has proven to be beneficial in the e-textiles field to endure washing [13–17]. Nevertheless, to benchmark these results, it would be necessary to know the lamination parameters and the thickness of the films used. Moreover, textiles have several variables to consider while analyzing the results, e.g., knit fabric weight, the composition of the materials and their treatment procedures, end-user wearing habits, etc. This study focuses on improving the integration of flexible and stretchable electronics in knit fabrics by lamination with TPU films. The stretchability of the circuits is obtained by using extensible meander shaped Cu interconnections. The work aims to increase the washing reliability of the integrated circuit by evaluating three different copper-meander designs and three TPU film thicknesses. Additionally, two different knit fabrics are used in the first round of experiments. The prepared samples are washed according to domestic washing and drying procedures standard (ISO 6330-2012 [12]). This work aims to determine the most reliable and stretchable combination of integration materials and design, with a focus on its use in wearable sports and healthcare applications.

2. Materials and Methods The correct choice of textile materials for creating e-textiles is critical for their usability: since these materials are the main carriers of the electronics in the product the lifetime of the e-textiles may be strongly reduced by a mismatch between the material properties of the textile materials and those of the electronics. In total 2 knit fabrics, 2 different types of TPU (thermoplastic polyurethane) film and 3 copper-based meander designs were tested. Different TPU film thicknesses were also combined and layered. The two different knit fabrics used were Eurojersey Sensitive® Fit (68% microfiber polyamide, 32% elastane, weight 213 g/m2) and Eurojersey Sensitive® Seric Plus (80% microfiber polyamide, 20% elastane, LYCRA®/elastane/spandex 120 g/m2). These materials are produced by Eurojersey S.P.A. Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 14

In total 2 knit fabrics, 2 different types of TPU (thermoplastic polyurethane) film and 3 copper-based meander designs were tested. Different TPU film thicknesses were also combined and layered.Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 14 The two different knit fabrics used were Eurojersey Sensitive® Fit (68% microfiber polyamide, In total 2 knit fabrics, 2 different types of TPU (thermoplastic polyurethane) film and 3 32% elastane, weight 213 g/m2) and Eurojersey Sensitive® Seric Plus (80% microfiber polyamide, 20% copper-based meander designs were tested. Different TPU film thicknesses were also combined and ® 2 elastane,Appl. Sci.layered.2020 LYCRA, 10 , 9057/elastane/spandex 120 g/m ). These materials are produced by Eurojersey S.P.A.3 of in 13 Caronno Pertusella,The two different Italy. Theknit TPUfabrics films used used were for Eurojersey laminating Sensitive circuits® Fit on (68% the microfibertextile are polyamide,listed in Table 1. 32% elastane, weight 213 g/m2) and Eurojersey Sensitive® Seric Plus (80% microfiber polyamide, 20% in Caronno Pertusella, Italy. The TPU films used for laminating circuits on the textile are listed in elastane, LYCRA®/elastane/spandex 120 g/m2). These materials are produced by Eurojersey S.P.A. in TableCaronno1Table. 1. Pertusella, Overview Italy.and properties The TPU filmsof the used different for laminating thermoplastic circuits polyurethane on the textile (TPU) are listedfilms used.in Table 1. TableMaterial 1. Overview Type and properties of theThickness different thermoplastic polyurethane Young’s Modulus (TPU) films (Ey used.) TableBemis 1. Overview3914 and properties of the 100 different μm thermoplastic polyurethane2.2 (TPU)Mpa [18]films used. Material Type Thickness Young’s Modulus (Ey) Prochimir TC5011 150 μm Material Type Thicknessµ Young’s28 Modulus Mpa [19] (E y ) Prochimir TC5011Bemis 3914 250 μ 100m m 2.2 Mpa [18] BemisProchimir 3914 TC5011 100 150μm µm 2.2 Mpa [18] 28 Mpa [19] ProchimirProchimir TC5011 TC5011 150 250μm µm 28 Mpa [19] The electronicsProchimir in theTC5011 test system consist 250 of μ ma flexible printed circuit board, which is processed as described in the following paragraph (Figure 1). Starting from base polyimide-copper sandwich The electronics in the test system consist of a flexible printed circuit board, which is processed Upisel-N SEThe 1220 electronics (UBE inEXSYMO the test system CO., LTD.,consist Tokyo, of a flexible Japan), printed the coppercircuit board, tracks which are patterned is processed using as described in the following paragraph (Figure1). Starting from base polyimide-copper sandwich dry-filmas described lithography, in the followed following by paragraph a CuCl2 -based(Figure etching1). Starting step. from In baseour experiments,polyimide-copper the sandwichcopper layer Upisel-N SE 1220 (UBE EXSYMO CO., LTD., Tokyo, Japan), the copper tracks are patterned using had a Upisel-Nthickness SE of 1220 18 μ (UBEm, on EXSYMO top of a CO., 50 μ LTD.,m polyimide Tokyo, Japan), base. Afterthe copper the etching tracks are procedure, patterned the using circuit dry-film lithography, followed by a CuCl -based2 etching step. In our experiments, the copper layer was laminateddry-film lithography, with the followed ShengYi by aSF305C CuCl2 -based coverlay etching (with step. Ina ourthickness experiments, of 25 the μ copperm), which layer has had ahad thickness a thickness of 18 of µ18m, μm, on on top top of of a a 50 50µ μmm polyimide polyimide base. After After the the etching etching procedure, procedure, the circuit the circuit perforations allowing contacting the copper and perform measurements. Lastly, the tracks were cut was laminatedwas laminated with thewith ShengYi the ShengYi SF305C SF305C coverlay coverlay (with a(with thickness a thickness of 25 µ ofm), 25 which μm), haswhich perforations has from the sheet using a laser, allowing one to shape the polyimide into a desired stretchable design. allowingperforations contacting allowing the copper contacting and the perform copper measurements. and perform measurements. Lastly, the tracks Lastly, were the tracks cut from were the cutsheet Design 1 and 1-1 have a wave-shaped polyimide left while lasering to enhance the durability of the usingfrom a laser, the allowingsheet using one a laser, to shape allowing the polyimideone to shape into the apolyimide desired stretchable into a desired design. stretchable Design design. 1 and 1-1 design.Design 1 and 1-1 have a wave-shaped polyimide left while lasering to enhance the durability of the have a wave-shaped polyimide left while lasering to enhance the durability of the design. design.

Figure 1. The copper processing flow chart. FigureFigure 1. 1.The The coppercopper processing flow ﬂow chart. chart. Three different stretchable designs were patterned in the polyimide, based on the meandering ThreeThree diﬀ erentdifferent stretchable stretchable designs designs were were patternedpatterned in in the the polyimide, polyimide, based based on the on themeandering meandering designsdesigns used usedin previous in previous studies studies and and application application trialstrials [20–24]. Meanders Meanders dimensions dimensions are areseen seen in in designs used in previous studies and application trials [20–24]. Meanders dimensions are seen in FigureFigure 2 and 2 Tableand Table 2. In 2. Figures In Figures 3 and 3 and 4 the4 the resulting resulting meandersmeanders are are shown. shown. Figure2 and Table2. In Figures3 and4 the resulting meanders are shown.

Figure 2. Dimensions of meanders (polyimide outline). FigureFigure 2. 2. DimensionsDimensions of of meanders meanders (polyimide (polyimide outline). outline).

Table 2. Dimensions of meanders.

Meander Mark Description Measurement Unit Design l1 Width of the meander bus 11.4 mm The length of the straight part I2 4.8 mm connecting the top and bottom arc Total width of meander including 4 w1 0.9 mm Cu tracks Design 1 Half of the period between the middle d1 5 mm top and middle bottom part Appl. Sci. 2020, 10, 9057 r1 Meander radius 2.45 mm 4 of 13 a1 Meander angle 90 ° Spacing between copper tracks 0.1 mm Table 2.CopperDimensions track ofwidth meanders. 150 μm

Meander Designl2 Mark The total length Description of the sample Measurement 9.4 Unit mm l Total widthWidth of sample of the meander including bus 4 Cu 11.4 mm w2 1 0.9 mm The length of the straight part connecting the I tracks 4.8 mm 2 top and bottom arc w TheTotal period width ofbetween meander including the middle 4 Cu tracks top 0.9 mm d2 1 3.5 mm Design 1 Half of the period between the middle top Design 2 d and middle bottom part 5 mm 1 and middle bottom part r2 Meander radius 2.55 mm r1 Meander radius 2.45 mm a2 a1 MeanderMeander angle angle 90 45 ◦ ° SpacingSpacing between between copper copper tracks tracks 0.1 0.1 mm mm Copper track width 150 µm Copper track width 150 μm l2 The total length of the sample 9.4 mm l3 w2 Total The width total of length sample includingof the sample 4 Cu tracks 0.9 5.9 mm mm The period between the middle top and d Total width of sample including 4 Cu 3.5 mm 2 middle bottom part Design 2 w3 0.9 mm r2 Meandertracks radius 2.55 mm a2 The period betweenMeander anglethe middle top 45 ◦ d3 Spacing between copper tracks 0.15 mmmm Design 3 and middleCopper trackbottom width part 150 µm

r3 l3 TheMeander total length radius of the sample 5.9 2.45 mmmm w Total width of sample including 4 Cu tracks 0.9 mm a3 3 Meander angle 90 ° The period between the middle top and d 5 mm 3 Spacing betweenmiddle bottom copper part tracks 0.1 mm Design 3 r3 CopperMeander track radius width 2.45150 mmμm a3 Meander angle 90 ◦ Spacing between copper tracks 0.1 mm Copper track width 150 µm

(a) (b)

Appl.Figure Sci.Figure 2020 3. ,( 3.a10) ,( Dixa )FORﬀ Differenterent PEER meander REVIEW meander designs. designs. From From the top:the top: Meander Meander type type 1, type 1, 2type and 2 type and 3 type and Meander3 and5 of 14 typeMeander 1-1, 2-2 andtype 3-3.1-1, (b2-2): Dummyand 3-3. PCB(b): (printedDummy circuitPCB (printed board) thatcircuit was board) soldered that to was the centralsoldered interposer. to the central interposer.

(a) (b)

FigureFigure 4. 4.(a ():a): Meander Meander typetype 11 close-up.close-up. ( (bb):): Meander Meander type type 1-1 1-1 close close up. up.

The testing-circuit consists of two I2C bus tracks, power supply track, ground track and three interposer islands. A dummy PCB (Figure 3b) was soldered to the central interposer island for failure analysis. To ensure a good solderability, the PCB was finished with NiAu coating, thus soldering to the copper on the meander. The function of the PCB was to electrically connect to each meander on the circuit. The sample was considered failed if the electrical resistance of any of the 4 tracks in between wash cycles grew above 10 times the nominal resistance before washing, or it stopped conducting at all. The most straight-forward design of adding 4 tracks to a polyimide meander is shown in Figure 4a: here all tracks are evenly spaced and centered on the polyimide meander. However, as our initial results clearly indicated that failures mainly occur at the top of the meander, we also introduced a secondary design, where this region is reinforced with additional copper tracks [20] (Figure 4b). The additional track is not connected to the circuit but merely added to support the durability of connected copper tracks. This was done for all three designs presented in Figure 3. The corresponding designs are labeled as “1-1”, “2-2” and “3-3”. By combining the above-mentioned options in materials, textiles and designs, the first set of 24 samples was composed and tested according to the washing procedure of Table 3. Next, based on the initial results, the second set of 9 samples was constructed and tested. This set comprises the aforementioned reinforced meanders on the one hand but also differs from the first set in the way the layering was composed.

Table 3. Washing test procedure overview.

Nr Washing Phase Time, min Temperature, °C Spin, rpm Water Volume, L 1 Main wash 15 40 49 15 2 Rinse 1 3 Coldwater 49 19 3 Drain 1 8 Coldwater 49 - 4 Rinse 2 3 Coldwater 49 19 5 Drain 2 8 Coldwater 49 - 6 Rinse 3 3 Coldwater 49 19 7 Drain 3 8 Coldwater 49 - 8 Rinse 4 3 Coldwater 49 19 9 Drain 4 8 Coldwater 49 - 10 Spin 5 Coldwater 1100 -

Methodology and Layering In general, the method of the study was set up in 7 steps, as applied to samples 1–24: Appl. Sci. 2020, 10, 9057 5 of 13

The most straight-forward design of adding 4 tracks to a polyimide meander is shown in Figure4a: here all tracks are evenly spaced and centered on the polyimide meander. However, as our initial results clearly indicated that failures mainly occur at the top of the meander, we also introduced a secondary design, where this region is reinforced with additional copper tracks [20] (Figure4b). The additional track is not connected to the circuit but merely added to support the durability of connected copper tracks. This was done for all three designs presented in Figure3. The corresponding designs are labeled as “1-1”, “2-2” and “3-3”. By combining the above-mentioned options in materials, textiles and designs, the first set of 24 samples was composed and tested according to the washing procedure of Table3. Next, based on the initial results, the second set of 9 samples was constructed and tested. This set comprises the aforementioned reinforced meanders on the one hand but also differs from the first set in the way the layering was composed.

Table 3. Washing test procedure overview.

Nr Washing Phase Time, min Temperature, ◦C Spin, rpm Water Volume, L 1 Main wash 15 40 49 15 2 Rinse 1 3 Coldwater 49 19 3 Drain 1 8 Coldwater 49 - 4 Rinse 2 3 Coldwater 49 19 5 Drain 2 8 Coldwater 49 - 6 Rinse 3 3 Coldwater 49 19 7 Drain 3 8 Coldwater 49 - 8 Rinse 4 3 Coldwater 49 19 9 Drain 4 8 Coldwater 49 - 10 Spin 5 Coldwater 1100 -

Methodology and Layering In general, the method of the study was set up in 7 steps, as applied to samples 1–24:

The new set of samples (25–33), on the other hand, was constructed in a similar fashion, however using the layering as given by Figure5b. For both groups, the PCBs were added onto the centre interposer island, by first dispensing SAC305 solder paste, aligning the PCBs, and consecutively reflowing the solder in the vapor phase reflow oven IBL SLC 300 (IBL, Königsbrunn, Germany) at a temperature of 250 ◦C. The finished circuits were laminated to the textile swatches. A fabric piece of 30 cm 22 cm × was laminated with TPU film. The three different meander designs were placed on the fabric in the warp direction, and another layer of TPU was laid on top to laminate again. Lamination parameters ® used were: 170 ◦C with 4 bar pressure for 40 s. Lamination was done on a Hotronix Air Fusion IQ® (Hotronix®, Carmichaels, PA, USA) heat press. Before cutting the samples into 3 test strips, Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 14

1. Preparation of the meander design through the etching process based on the standard lithography technique; 2. Laminating the designs with ShengYi SF305C coverlay; 3. Laser-cutting the designs in the shape of the circuits; 4. Assembly of the dummy PCB (printed circuit board) to centre interposer on the meander. Soldering was done in a vapor phase oven using SAC305 solder; 5. Lamination of fabric swatches with base TPU film; 6. Lamination of meander designs between these base fabric + TPU film (step 3) and the top TPU film layer, using layering seen in Figure 5a; 7. Conducting washing tests according to ISO 6330-2012—domestic washing and drying procedures standard. The new set of samples (25–33), on the other hand, was constructed in a similar fashion, however using the layering as given by Figure 5b. For both groups, the PCBs were added onto the centre interposer island, by first dispensing SAC305 solder paste, aligning the PCBs, and consecutively reflowing the solder in the vapor phase reflow oven IBL SLC 300 (IBL, Königsbrunn, Germany) at a temperature of 250 °C. The finished circuits were laminated to the textile swatches. A fabric piece of 30 cm × 22 cm was laminated with TPU film. The three different meander designs were placed on the fabric in the warp direction, and another layer of TPU was laid on top to laminate again. Lamination parameters used were: 170 °C with 4 bar pressure for 40 s. Lamination was done on a Hotronix® Air Fusion IQ® (Hotronix®, Carmichaels, PA, USA) heat press. Before cutting the samples into 3 test strips, each including one meander design, the laminated pieces were set aside for 12 h to let the TPU film set and become more durable [18,19]. Each sample was then numbered by sewing. Appl.Two Sci. 2020 different, 10, 9057 layering systems were used: one TPU layer (Figure 5a) or two TPU layers under6 of 13 and over the circuits (Figure 5b). In the latter option, the second TPU layer was structured in the shape of the corresponding polyimide track to create more protection for the circuit while not losing each including one meander design, the laminated pieces were set aside for 12 h to let the TPU ﬁlm set the overall stretchability. and become more durable [18,19]. Each sample was then numbered by sewing.

(a) (b)

Figure 5. Layering systemssystems ofof samples. samples. ( a():a): one one TPU TPU layer layer is usedis used on on and and under under the circuit.the circuit. (b): two(b): TPUtwo TPUlayers layers are used are used on and on underand under the circuit. the circuit.

TheTwo testing diﬀerent of layeringthe samples systems consists were of used: alternat oneively TPU performing layer (Figure washing5a) or two tests TPU and layers electronics under resistanceand over the measurements. circuits (Figure The5b). washing In the latter tests option, were theperformed second TPUin an layer Electrolux was structured W465H in(Electrolux, the shape Stockholm,of the corresponding Sweden) professional polyimide track washing to create machin moree. protectionNo detergent for thewas circuitused, whileand every not losing washing the cycleoverall additionally stretchability. included 2 kg of microfiber towels (Nabaji, 88% PES, 12% PA). The sample sets did notThe have testing any of other the samples protection, consists e.g., of they alternatively were not performing washed in washing a separate tests bag. and The electronics washing resistance cycles measurements. The washing tests were performed in an Electrolux W465H (Electrolux, Stockholm, Sweden) professional washing machine. No detergent was used, and every washing cycle additionally included 2 kg of microfiber towels (Nabaji, 88% PES, 12% PA). The sample sets did not have any other protection, e.g., they were not washed in a separate bag. The washing cycles were conducted according to ISO 6330-2012—“Textiles-Domestic washing and drying procedures for textile testing” standard at 40 ◦C. The electricalAppl. Sci. resistance 2020, 10, x FOR PEER of each REVIEW track was measured after every washing cycle7 of 14 until 5 washes were performed.were After conducted that, according measurements to ISO 6330-2012—“Text were takeniles-Domestic after everywashing otherand drying washing procedures cycle. To verify the inﬂuence offor the textile PCB testing” connection standard at 40 and °C. tracks durability, the resistance was measured of (1) four The electrical resistance of each track was measured after every washing cycle until 5 washes separate trackswere until performed. the middle After that, interposer measurements were on thetaken oneafter every hand othe andr washing (2) cycle. four To separate verify the tracks of the whole circuit (seeinfluence inFigure of the PCB6) onconnection the other and tracks hand. durabi Datality, the was resistance collected was measured manually of (1) four using a Keithley separate tracks until the middle interposer on the one hand and (2) four separate tracks of the whole 2400 SMU (Sourcecircuit Measure (see in Figure Unit) 6) on (Textronix, the other hand. Beaverton, Data was collected OR, manually United using States) a Keithley to collect 2400 SMU 4-point resistivity measurements.(Source The measurementsMeasure Unit) (Textronix, were Beaverton, taken in OR, ohms United and States) calculated to collect 4-point into mOhmresistivity/ cm to have an objective averagemeasurements. of the whole The measurements sample. This were way, taken it in was ohms clear and calculated if any tracks into mOhm/cm or connections to have an on the sample objective average of the whole sample. This way, it was clear if any tracks or connections on the were damaged orsample not. were damaged or not.

Figure 6. FigureResistance 6. Resistance measuring measuring points points on the on sample. the sample.

To gain further insight into the failure modes of the samples, a visual inspection was performed using a stereo microscope Zeiss Stemi 2000C (Carl Zeiss AG, Oberkochen, Germany), in order to study the cracks in the tracks or disconnections of any kind.

3. Results The purpose of the experiments was to determine the most stable, reliable and stretchable combination of the materials used. Thus, the experimental work was carried out with 33 different sample sets (Table 4), which included 66 samples in total.

Table 4. Overview of experimental work set-up.

Nr TPU Meander Style Fabric with Weight 1 Bemis 100 μm 1 Eurojersey Sensitive® Fit 213 g/m2 2 Bemis 100 μm 2 Eurojersey Sensitive® Fit 213 g/m2 3 Bemis 100 μm 3 Eurojersey Sensitive® Fit 213 g/m2 4 Bemis 100 μm 1 Eurojersey Sensitive® Seric Plus 120 g/m2 5 Bemis 100 μm 2 Eurojersey Sensitive® Seric Plus 120 g/m2 6 Bemis 100 μm 3 Eurojersey Sensitive® Seric Plus 120 g/m2 7 Prochimir 150 μm 1 Eurojersey Sensitive® Fit 213 g/m2 8 Prochimir 150 μm 2 Eurojersey Sensitive® Fit 213 g/m2 9 Prochimir 150 μm 3 Eurojersey Sensitive® Fit 213 g/m2 10 Prochimir 150 μm 1 Eurojersey Sensitive® Seric Plus 120 g/m2 11 Prochimir 150 μm 2 Eurojersey Sensitive® Seric Plus 120 g/m2 12 Prochimir 150 μm 3 Eurojersey Sensitive® Seric Plus 120 g/m2 13 Prochimir 250 μm 1 Eurojersey Sensitive® Fit 213 g/m2 14 Prochimir 250 μm 2 Eurojersey Sensitive® Fit 213 g/m2 Appl. Sci. 2020, 10, 9057 7 of 13

3. Results The purpose of the experiments was to determine the most stable, reliable and stretchable combination of the materials used. Thus, the experimental work was carried out with 33 diﬀerent sample sets (Table4), which included 66 samples in total.

Table 4. Overview of experimental work set-up.

Nr TPU Meander Style Fabric with Weight 1 Bemis 100 µm 1 Eurojersey Sensitive® Fit 213 g/m2 2 Bemis 100 µm 2 Eurojersey Sensitive® Fit 213 g/m2 3 Bemis 100 µm 3 Eurojersey Sensitive® Fit 213 g/m2 4 Bemis 100 µm 1 Eurojersey Sensitive® Seric Plus 120 g/m2 5 Bemis 100 µm 2 Eurojersey Sensitive® Seric Plus 120 g/m2 6 Bemis 100 µm 3 Eurojersey Sensitive® Seric Plus 120 g/m2 7 Prochimir 150 µm 1 Eurojersey Sensitive® Fit 213 g/m2 8 Prochimir 150 µm 2 Eurojersey Sensitive® Fit 213 g/m2 9 Prochimir 150 µm 3 Eurojersey Sensitive® Fit 213 g/m2 10 Prochimir 150 µm 1 Eurojersey Sensitive® Seric Plus 120 g/m2 11 Prochimir 150 µm 2 Eurojersey Sensitive® Seric Plus 120 g/m2 12 Prochimir 150 µm 3 Eurojersey Sensitive® Seric Plus 120 g/m2 13 Prochimir 250 µm 1 Eurojersey Sensitive® Fit 213 g/m2 14 Prochimir 250 µm 2 Eurojersey Sensitive® Fit 213 g/m2 15 Prochimir 250 µm 3 Eurojersey Sensitive® Fit 213 g/m2 16 Prochimir 250 µm 1 Eurojersey Sensitive® Seric Plus 120 g/m2 17 Prochimir 250 µm 2 Eurojersey Sensitive® Seric Plus 120 g/m2 18 Prochimir 250 µm 3 Eurojersey Sensitive® Seric Plus 120 g/m2 19 Prochimir 250 µm doubled 1 Eurojersey Sensitive® Fit 213 g/m2 20 Prochimir 250 µm doubled 2 Eurojersey Sensitive® Fit 213 g/m2 21 Prochimir 250 µm doubled 3 Eurojersey Sensitive® Fit 213 g/m2 22 Prochimir 250 µm doubled 1 Eurojersey Sensitive® Seric Plus 120 g/m2 23 Prochimir 250 µm doubled 2 Eurojersey Sensitive® Seric Plus 120 g/m2 24 Prochimir 250 µm doubled 3 Eurojersey Sensitive® Seric Plus 120 g/m2 25 Prochimir 250 µm doubled 1-1 Eurojersey Sensitive® Fit 213 g/m2 26 Prochimir 250 µm doubled 2-2 Eurojersey Sensitive® Fit 213 g/m2 27 Prochimir 250 µm doubled 3-3 Eurojersey Sensitive® Fit 213 g/m2 28 Bemis 100 µm + Prochimir 250 µm 1-1 Eurojersey Sensitive® Fit 213 g/m2 29 Bemis 100 µm + Prochimir 250 µm 2-2 Eurojersey Sensitive® Fit 213 g/m2 30 Bemis 100 µm + Prochimir 250 µm 3-3 Eurojersey Sensitive® Fit 213 g/m2 31 Bemis 100 µm doubled 1-1 Eurojersey Sensitive® Fit 213 g/m2 32 Bemis 100 µm doubled 2-2 Eurojersey Sensitive® Fit 213 g/m2 33 Bemis 100 µm doubled 3-3 Eurojersey Sensitive® Fit 213 g/m2

The Design-of-Experiments that was performed compared the influence of four parameters—meander shape, TPU-stack, fabric type and whether or not the meander is reinforced. In the first analyses, given in Figure7, the percentage of surviving tracks is plotted as a function of the number of washing cycles, without taking into account the TPU stack or the fabric type. A track of which the resistance stayed under 50 Ohm, due to washing or other treatment, is hereafter called a “surviving track”. From this graph, it can be seen that the meander design 2 was less resilient to the high forces induced by the washing tests, both in terms of ﬁrst occurring failure (1 washing cycle) as in terms of the total number of losses at the end of the experiment. Meander style 1 shows an earlier failure compared to meander style 3. While Type 1 and 3 seem statistically evenly reliable, the isolated data showed that type 1 survived more cycles in more cases, when comparing similar conditions, e.g., sample 16 (type 1) survived longer than sample 18 (type 3), resulting in a higher number of survivors after 25 washing cycles. Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 14

The reinforced meander types seem to outperform the designs 1,2,3, although it can not be concluded whether this is a result of the reinforced design or a consequence of the novel stack as shown in Figure 5b. Although the failure of design 2-2 seems to confirm the earlier conclusion on the worse robustness to washing cycles of this meander design, a closer inspection of this failed sample revealed a failure at the interconnection between the (flexible) track and the (rigid) interposer PCBs: a small crack was found next to the interposer transition to the tracks. One of the reasons for that specific crack could be the failure of encapsulation layering and meander design together. Although the number of samples used in this experiment was too small to make sound Appl.statistical Sci. 2020 conclusions,, 10, 9057 Figure 7 illustrates the importance of a well-balanced meander design, tuned8 of 13 to the specific mechanical load applied in the use-case (such as machine washing in this paper).

100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0%

Percentage of survived samples Percentage Design 1 Design 2 Design 3 Design 1-1 Design 2-2 Design 3-3

Figure 7. Survival graph of the experiments, per meandermeander style. Design 1-1 and Design 3-3 overlap.

TheAs mentioned reinforced in meander the previous types paragraph, seem to outperform all measurements the designs of track 1, 2, resistances 3, although were it can averaged not be concludedper sample, whether and the this overall is a result resistance of the of reinforced the track designper bridged or a consequence distance was of calculated the novel stack (mOhms/cm). as shown inThe Figure bridged5b. Althoughdistance is the the failure distance of designin between 2-2 seems two islands to confirm of components the earlier conclusionthat needs onto be the bridged worse robustnessby the tracks, to washingwhile the cycles resistance of this measured meander design,will depend a closer on inspectionthe shape of the this meander. failed sample Based revealed on the ageometric failure at parameters the interconnection given in betweenTable 2, the the track (flexible) length track per and unit the cell (rigid) (cfr. Figure interposer 2) and PCBs: the bridged a small crackdistance was can found be calculated, next to the resulting interposer in transition a geometrical to the form tracks. factor: One of the reasons for that specific crack could be the failure ofFF encapsulation = Track length layering per unit and cell/bridged meander designdistance together. per unit cell Although the number of samples used in this experiment was too small to make sound statistical conclusions,As all copper Figure 7tracks illustrates are designed the importance with the of sa ame well-balanced width (w = meander0.15 mm) design, and have tuned the to same the specificthickness mechanical t = 18 μm, loadthe resistance applied in per the bridged use-case distance (such as can machine be calculated washing as: in this paper). As mentioned in the previous paragraph,ro all × FF measurements of track resistances were averaged R= (t × w) per sample, and the overall resistance of the track per bridged distance was calculated (mOhms/cm). The bridgedWith ro distancebeing the is bulk the distanceresistivity in of between the copper two layer. islands of components that needs to be bridged by theIn tracks, Figure while 8, the the average resistance values measured of the measured will depend copper on the tracks shape are of given the meander. for all designs—as Based on the a geometricfunction of parameters the washing given cycles, in Tableand the2, the maximum track length and perminimum unit cell values (cfr. Figurethat were2) and measured. the bridged This distancegraph confirms can be calculated,the validity resulting of the measured in a geometrical and calculated form factor: resistances for meander types 1,2 and 3, as the measured average values at the start of the experiment relate to each other as the form factors do in Table 5. ThereFF is= howeverTrack length a discrepancy per unit cell in/bridged between distance the values per unit of the cell standard meanders, compared to those of the reinforced meanders 1-1, 2-2 and 3-3: it seems that these measured values, althoughAs all their copper geometrical tracks are shapes designed were with similar, the same were width 83.5% (w smaller= 0.15 mm) than and the haveoriginal the samedesigns. thickness When µ tinvestigating= 18 m, the the resistance copper per tracks bridged under distance a microscope can be calculated, it was seen as: that the track width was wider where the extra copper track was added (Figurero FF9), a process variability, which might have R = × originated from an overetching of the standard meander(t w) types during production. × With ro being the bulk resistivity of the copper layer. In Figure8, the average values of the measured copper tracks are given for all designs—as a function of the washing cycles, and the maximum and minimum values that were measured. This graph confirms the validity of the measured and calculated resistances for meander types 1, 2 and 3, as the measured average values at the start of the experiment relate to each other as the form factors do in Table5. There is however a discrepancy in between the values of the standard meanders, compared to those of the reinforced meanders 1-1, 2-2 and 3-3: it seems that these measured values, although their geometrical shapes were similar, were 83.5% smaller than the original designs. When investigating the Appl. Sci. 2020, 10, 9057 9 of 13 Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 14 copper tracks under a microscope, it was seen that the track width was wider where the extra copper track was added (Figure9), a process variability, which might have originated from an overetching of the standard meander types during production. Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 14

Figure 8. Average, minimum and maximum resistance per bridged cm, per type of meander.

Table 5. Relevant geometrical parameters related to the electrical properties of the meanders.

Meander Type 1 Meander Type 2 Meander Type 3 FigureFigure 8. 8. Average, minimum and maximum resistance per bridged cm, per type of meander. Track lengthAverage, per minimumunit cell and maximum 24.99 mm resistance per 24.03 bridged mm cm, per type of15.4 meander. mm BridgedTableTable 5. distance5.Relevant Relevant per geometrical geometrical unit cell parameters parameters 9.8 relatedmmrelated toto thethe electricalelectrical 6.9 mm properties of the 9.8 meanders. mm FF 2.55 3.48 1.57 Meander Meander Type Type 1 1 Meander Type Type 2 2 Meander Meander Type Type 3 3 FigureTrack 8 alsoTrack length illustrates length per unit per that, cell unit independent cell 24.99 24.99of mm the mm lifetime of the 24.03 24.03 track, mm mm the resistance 15.4 15.4 did mm mm not degrade significantlyBridgedBridged distance as distancea function per unit per cellof unit washing cell cycles: 9.8 mm 9.8 mmmost tracks, just 6.9 6.9 mmbefore mm failure, still 9.8 9.8 mmshow mm a similar FF 2.55 3.48 1.57 conductance as beforeFF the experiment, with only2.55 a minor increase3.48 in variance. 1.57

Figure 8 also illustrates that, independent of the lifetime of the track, the resistance did not degrade significantly as a function of washing cycles: most tracks, just before failure, still show a similar conductance as before the experiment, with only a minor increase in variance.

(a) (b) (c)

Figure 9. Comparison of copper track width on meander design 1 ( a), meander design 1-1 (b) and meander design 1 middle part (c).). (a) (b) (c) Figure 810 also presents illustrates the standard that, independent deviation based of the on lifetime the average of the number track, the of resistancewashes performed did not Figure 9. Comparison of copper track width on meander design 1 (a), meander design 1-1 (b) and degradeon a sample signiﬁcantly before failure. as a functionOn average, of washing 10 washing cycles: cycles most could tracks, be performed just before per failure, sample still that show was a meander design 1 middle part (c). similarmade from conductance lighter fabric as before and the13 experiment,washes from with a heavier only a fabric. minor increaseThe standard in variance. deviation of heavier Figure 10 presents the standard deviation based on the average number of washes performed fabric Figurewas 26% 10 smaller.presents Based the standard on that, deviation the heavier based fabric on was the averagechosen for number the second of washes experiment performed part on a sample before failure. On average, 10 washing cycles could be performed per sample that was (numberon a sample 25–33). before failure. On average, 10 washing cycles could be performed per sample that was made from lighter fabric and 13 washes from a heavier fabric. The standard deviation of heavier fabric was 26% smaller. Based on that, the heavier fabric was chosen for the second experiment part (number 25–33). Appl. Sci. 2020, 10, 9057 10 of 13 made from lighter fabric and 13 washes from a heavier fabric. The standard deviation of heavier fabric was 26% smaller. Based on that, the heavier fabric was chosen for the second experiment part

(numberAppl. Sci. 2020 25–33)., 10, x FOR PEER REVIEW 11 of 14 Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 14

Figure 10. Comparing fabric weight as a parameter for lifetime. Figure 10. Comparing fabric weight as a parameter for lifetime. The failed samples were investigated with stereo microscope Zeiss Stemi 2000C (Carl Zeiss AG, AG, The failed samples were investigated with stereo microscope Zeiss Stemi 2000C (Carl Zeiss AG, Oberkochen, Germany). The The failure failure points points were were alwa alwaysys on on one one of the copper tracks and not on the Oberkochen, Germany). The failure points were always on one of the copper tracks and not on the connections between PCB and the circuit.circuit. Figure Figure 1111 displaysdisplays typical typical cracks cracks that that occurred on the connections between PCB and the circuit. Figure 11 displays typical cracks that occurred on the failed samples. The The cracks cracks and and the track track ruptures ruptures always occurred on the top and bottom part of the failed samples. The cracks and the track ruptures always occurred on the top and bottom part of the meanders, which was also tested in previous works (Gonzalez et al., 2007). From sample sets with an meanders, which was also tested in previous works (Gonzalez et al., 2007). From sample sets with an extra track (nr 25–33), only one sample set (from nr 32 one sample out of two)two) failed after the 23rd extra track (nr 25–33), only one sample set (from nr 32 one sample out of two) failed after the 23rd washing cycle. As As mentioned mentioned earlier, earlier, this this failure failure originated from from a a lack of robustness of the tracks, washing cycle. As mentioned earlier, this failure originated from a lack of robustness of the tracks, and not from the inconsistency at the connection withwith thethe PCB-interposer.PCB-interposer. and not from the inconsistency at the connection with the PCB-interposer.

Figure 11. Typical cracks in samples after washing tests. From left: sample 10, 14 and 32. FigureFigure 11. 11.Typical Typical crackscracks inin samplessamples afterafter washingwashing tests. From left: sample sample 10, 10, 14 14 and and 32. 32.

FigureFigure 12 12 compares compares thethe lifetimelifetime ofof thethe trackstracks as a function of of the the thickness thickness of of the the TPU TPU that that was was usedused (single (single layer). layer).layer). It It It can can can be be be seenseen seen thatthat that therethere there was was little little difference diﬀerence inin thethe in the performanceperformance performance ofof the the of Bemis theBemis Bemis 100 100 100μμmm µthick thickm thick TPU TPU TPU as as compared compared as compared to to the the to 150150 the μμ 150mm TPUTPUµm TPUas provided as provided by Prochimir. by Prochimir. However,However, However, there there was therewas a a wasclear clear a clearimprovementimprovement improvement when when whenthe the thicker thicker the thicker versionversion version ofof thethe oflatterlatter the was latter used. was The used. experimentalexperimental The experimental resultsresults of resultsof the the tracks, tracks, of the tracks,whichwhich featured whichfeatured featured thethe adaptedadapted the adapted meandersmeanders meanders andand the and do theuble-layered double-layered TPU stack TPUstack stack werewere were omittedomitted omitted fromfrom from thethe thegraph,graph, graph, as as most asmost most trackstracks tracks survivedsurvived survived thethe the fullfull full setset set of of25 25 wash wash cycles. cycles. In In general,general, general, aa atrendtrend trend cancan can bebe be seen seen that that thickerthicker layers layers of of TPU, TPU, or or aa doubledouble stackstack ofof TPU,TPU, significantly improved thethe robustnessrobustness of of the the tracks tracks toto standard standard washing washing cycles, cycles, independentlyindependently ofof thethe meander design, whichwhich waswas used.used. Appl. Sci. 2020, 10, 9057 11 of 13 thicker layers of TPU, or a double stack of TPU, signiﬁcantly improved the robustness of the tracks to standardAppl. Sci. 2020 washing, 10, x FOR cycles, PEER independentlyREVIEW of the meander design, which was used. 12 of 14

FigureFigure 12.12. ComparisonComparison ofof thethe lifetimelifetime ofof aa tracktrack asas aa functionfunction ofof TPUTPU thickness.thickness.

4.4. Discussion TheThe design-of-experiment design-of-experiment that wasthat proposed was proposed in the previous in paragraphthe previous includes paragraph 3 parameters—meander includes 3 style,parameters—meander TPU type and fabric style, weight TPU and type based and on these fabric observations weight and and based results, on the these 4th mix observations of materials andand designsresults, wasthe proposed,4th mix of which materials outperformed and designs the first wa sets ofproposed, tracks. which outperformed the first set of tracks.By looking at each parameter independently, the initial conclusions can be drawn, although we are awareBy looking that the at dataset each parameter on which theseindependently, conclusions the are initial built conclusions is rather small, can and be drawn, the results although will need we toare be aware confirmed that the using dataset a larger on dataset.which these conclusions are built is rather small, and the results will needThe to be etching confirmed process using showed a larger places dataset. of nonuniformity, especially in design 1 and 1-1 middle area whereThe the etching copper process tracks widthshowed varied places a of lot nonuniformit and were they, smallest especially (Figure in design9c). The1 and results 1-1 middle could area be improvedwhere the bycopper adding tracks extra width copper varied to these a lot areas and (copperwere the filling smallest technique). (Figure 9c). Moreover, The results laser-cutting could be influencesimproved by the adding end-shape extra of copper the whole to these circuit. areas (c Thus,opper the filling alignment technique). during Moreover, the process laser-cutting should beinfluences improved. the end-shape of the whole circuit. Thus, the alignment during the process should be improved.Furthermore, while studying the different parameters independently, we disregarded the mutual interplayFurthermore, between thesewhile parameters: studying the maybe different an individual parameters meander independently, style fails in we combination disregarded with the a thinmutual TPU, interplay but by changingbetween these a few parameters: geometrical maybe parameters an individual it could meander perform asstyle good fails as in the combination best track thatwith was a thin tested. TPU, Further but by analysischanging of a thefew underlying geometrical failure parameters mechanisms it could is perform needed, as not good only as based the best on theoreticaltrack that andwas numericaltested. Further analysis analysis but also of confirmedthe underlying by real-life failure experiments. mechanisms For is example,needed, not to try only to avoidbased bucklingon theoretical in higher and mechanical numerica stressl analysis in the but washing also process.confirmed by real-life experiments. For example, to try to avoid buckling in higher mechanical stress in the washing process. 5. Conclusions and Future Work 5. ConclusionsThe lack of reliabilityand Future of Work e-textiles has been often caused by the high resistance of circuits or failure of materialThe lack strength of reliability after mechanical of e-textiles stress has andbeen washing often caused procedures. by the Thishigh study resistance set out of tocircuits increase or thefailure washing of material reliability strength of the after integrated, mechanical flexible stress circuit and in washing knit fabrics procedures. considering This various study materials,set out to e.g.,increase three the di ffwashingerent TPU reliability films. The of the main integrated, result was flexible determining circuit thein knit most fabrics durable considering copper-meander various design,materials, together e.g., withthree stretchable different andTPU robust films. TPU The layering. main result The doubled was determining meander-shaped the most TPU layeringdurable alsocopper-meander enhanced the durabilitydesign, together of the circuit with while stretc nothable losing and the robust stretchability TPU oflayering. the integration. The doubled meander-shaped TPU layering also enhanced the durability of the circuit while not losing the stretchability of the integration. The study should be repeated using functional PCBs, including different components and sensors. More work will need to be done to determine the effect of various fabrics in the integration process. This experiment used only two knit fabrics, and the results were inconclusive at the end. Appl. Sci. 2020, 10, 9057 12 of 13

The study should be repeated using functional PCBs, including different components and sensors. More work will need to be done to determine the effect of various fabrics in the integration process. This experiment used only two knit fabrics, and the results were inconclusive at the end. Woven and non-woven fabrics and also the weft direction were not considered and should be investigated further. It would also be worth considering adding extra protection to the e-textile samples or products during the washing program by putting them to a textile bag. It could decrease additional mechanical stress from the drum of the washing machine and increase the durability of electronics. Moreover, laundry detergents will be added in future washing tests to see and compare their effect on the reliability.

Author Contributions: Conceptualization, P.V.,F.B., T.S. and J.V.;methodology, P.V.,F.B., and J.V.; writing—original draft preparation, P.V.; writing—review, P.V., F.B., P.B., T.S. and J.V. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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