Combining electronics on flexible plastic strips with textiles

Abstract

Electronic textiles are an enabling technology for a variety of applications in health care, rehabilitation or sports. In this paper, three methods to combine flexible plastic strips with textiles are discussed. Flexible plastic strips serve as carriers for electronic devices. The first approach uses a roving frame to wrap flexible plastic strips with cotton fibers. For the second method, flexible plastic strips are woven into textiles using a narrow fabric loom and a rapier loom. In the third approach, flexible plastic strips are embroidered on a shuttle embroidery machine. Due to the flexible plastic strips, electronic devices mounted on the strips are exposed to strain with the potential to damage the electronics. For the three presented methods, strain on the surface of a flexible plastic strip is investigated: roving causes strain >25%, embroidery approximately 6% and weaving 15%. Weaving flexible plastic strips is investigated in more detail because it offers possibilities to embed thin-film devices, surface mount devices and integrated circuits into textiles. Conductive yarns within the textile are used to contact the electronics. Weaving flexible plastic strips is first applied to fabricate a textile patch with woven flexible plastic strips carrying thin-film temperature sensors. In a second textile patch, a flexible plastic strip carrying an accelerometer is woven in the weft direction and connected with conductive yarns in the warp direction. The textile-integrated accelerometer can have an angular deviation of up to 10° from the textile surface. Nevertheless, applications in rehabilitation and health care can benefit from textile-integrated accelerometers.

The combination of electronics with textiles offers potential for applications in a variety of scenarios. Clothing provides a suitable platform to monitor physiological signals with textile-integrated sensors and thus supply potentially crucial information to the wearer. Examples of potential applications are sports, health care, rehabilitation and high-risk professions.^1–3 An imperative requirement for electronic textiles, besides reliable electronic functionality, is an unobtrusive and seamless integration of electronics into textiles.

Besides applications in clothes, electronic textiles are advantageous for technical textiles such as carpets, car interiors or curtains.⁴

A common approach to combine electronics into textiles is attaching a printed circuit board (PCB) with microprocessors, sensors, communication modules, data storage units, etc. to a textile substrate.^5–9 The rigid nature of PCBs causes a local rigidity in the electronic textile and reduces the ability of the textile to wrap around a body. To establish interconnects among several PCBs on a single textile substrate, different methods are demonstrated: insulated copper wires are integrated into textile substrates during weaving;^7,8 silver paste is screen-printed onto textiles;⁹ copper wires are glued onto textiles;⁶ and conductive yarns are embroidered.¹⁰ In Kallmayer and Simon,¹¹ Vervust et al.,¹² Vieroth et al.¹³ and Gonzalez et al.,¹⁴ a method to integrate stretchable electronics into textiles is presented. Meandering copper interconnect lines and contact areas are embedded in a stretchable polymer matrix and subsequently attached to textile substrates. In Vieroth et al.¹³ a dress with light-emitting diodes is presented based on this technology.

In a different approach, electronic devices such as single transistors or resistors are integrated into yarns.^15–17 This method allows the fabrication of electronic textiles that are bendable, but the circuit complexity is at the level of single transistors and basic logic gates.

Within the last few years, one area with increasing interest in electronics research and development is flexible electronics, mainly driven by the vision of flexible displays.^18,19 Flexible electronics on flexible plastic substrates offer the potential of integrating a vast variety of devices. These devices range from thin-film transistors, to organic light-emitting diodes (OLEDs), to entire systems (display with decoder electronics) on a single flexible plastic substrate, to the convergence of silicon-based integrated circuits in small outline packaging on flexible plastic substrates.²⁰ In addition to a wide range of electronic building blocks, electronics processed on flexible plastic substrates offer large-scale fabrication techniques, such as roll-to-roll processing.²¹

In order to advance electronic textile development, the goal of this paper is to combine the advantages of flexible electronics with textile fabrication processes. In several experiments, flexible plastic strips, which served as carriers for electronic devices, were merged into yarns and textiles. Strips with widths of 2 mm or less were cut from flexible plastic substrates. In Figure 1(a) flexible plastic strips were wrapped with cotton fibers on a roving frame. Figure 1(b) shows strips woven into textiles using a loom, and in Figure 1(c) strips were embroidered.

Figure 1.

(a) Flexible plastic strip wrapped with cotton fibers. (b) Flexible plastic strip woven into textile. (c) Embroidered flexible plastic strips on a felt substrate.

Figure 2.

Lift-off process: (a) structured photo-resist on a 50 µm thick Kapton substrate; (b) evaporation of a 100 nm thick gold layer; (c) remaining photo-resist is dissolved to structure the gold layer; (d) structured gold layer on a flexible Kapton foil.

In this paper, the fabrication of electronics on flexible plastic strips is discussed first with a focus on mechanical strain. Bending or twisting a flexible strip induces mechanical strain, which can severely damage single material layers or entire electronic devices. Then, the approaches to manufacture the twisted rove in Figure 1(a) and the two textiles in Figures 1(b) and (c) are described. For the three methods, mechanical strain is evaluated. In the final section, weaving flexible plastic strips, as shown in Figure 1(b), is applied to weave thin-film devices and integrated circuits in the form of accelerometers into textiles.

Flexible plastic strips

Using flexible plastic substrates, which are cut into strips as carriers for electronic devices, offer several advantages for electronic textiles.

Plastic strips are bendable. This is beneficial for the combination with textiles and yarns because the strips do not cause a local rigidity.

Plastic strips provide a platform for a variety of electronic components. On the one hand, thin-film devices, such as transistors or sensors, can be fabricated directly on the substrate. On the other hand, chips without packaging or surface mount devices (SMDs) can be attached to flexible plastic strips.

Using roll-to-roll techniques allows large-scale processing of flexible plastic substrates resulting in a down-scaling of production costs.²¹

Strip fabrication

Fabricating electronics on flexible plastic strips involved two different methods: the first method was used to deposit and structure a thin-film layer for thin-film devices. For the second method, a material layer was structured and then an integrated circuit was attached.

Thin-film devices structured from a 100 nm thick gold layer were fabricated on a 50 µm thick Kapton E flexible foil substrate.²² Kapton E was used because of its high glass transition temperature, T_g, of ≈300℃ (PET T_g ≈ 76℃, PEN T_g ≈ 120℃²³), which allowed processing with micro-machining tools such as lithography and evaporation.

Method 1: the deposition and structuring of a 100 nm thick gold layer comprises the following steps. A 7.6 cm × 7.6 cm wide Kapton substrate was cleaned by sonicating it in acetone and isopropanol (IPA) for 10 min. Trapped solvents in the Kapton E foil were removed by annealing the substrate for 24 h at 200℃ in a vacuum oven.²⁴ To further increase the adhesion of the gold layer on the flexible plastic foil, the surface of the Kapton substrate was treated with oxygen plasma for 2 min. The 100 nm thick gold layer was deposited and structured using a lift-off process. Therefore, photo-resist was spin-coated and structured using photo-lithography (Figure 2(a)). A total of 100 nm of gold was then deposited using electron beam evaporation (Figure 2(b)). Dissolving the remaining photo-resist concluded the lift-off process, resulting in a structured gold layer on the Kapton substrate (Figure 2(c)). Figure 2(d) shows a schematic cross-section through a Kapton strip with a gold layer on top fabricated with the described process steps.

Method 2: to solder integrated circuits or SMDs, the following process applies. Photo-resist was deposited and structured on top of an 18 µm copper layer on a 25 µm thick Kapton substrate (Figure 3(a)). With an etchant (solution of HCl, H₂O₂, H₂O), the copper layer was structured as shown in Figure 3(b). After removing the remaining photo-resist, a chip with solder paste on its contact areas was placed on the structured copper layer (Figure 3(c)). The chip was then attached to the copper layer on the Kapton substrate by applying heat, causing the solder to melt and establish a solder joint, as depicted in Figure 3(d). Alternatively, instead of copper, gold can be used.

Figure 3.

Attaching a chip: (a) photo-resist is structured on a 25 µm thick Kapton substrate with an 18 µm copper layer on top; (b) an etchant is used to structure the copper layer; (c) after removing the photo-resist, a chip with solder is placed on the structured copper layer; (d) by applying heat the solder is molten and joins the chip with the copper layer.

For both methods, the substrate is then separated into 1 and 2 mm wide strips using a wafer saw. This technique allows the fabrication of strips as narrow as 50 µm.²⁵

Mechanical strain

Merging flexible plastic strips with textile substrates or yarns causes mechanical strain on any layer due to bending of the strip. Excessive strain in a metal layer on a flexible strip leads to cracks in the metal layer, reducing the electrical conductivity and ultimately destroying the metal layer. To illustrate bending in textiles, Figure 4 shows a cross-section through a woven textile with an integrated flexible plastic strip with a 100 nm thick gold layer on top. A minimum bending radius of 165 µm of the plastic strip was measured. The cross-section shown in Figure 4 was obtained by molding the textile in epoxy resin and abrading the hardened epoxy until the strip was visible.

Figure 4.

Cross-section of a woven textile with an integrated flexible plastic strip. Bending radii of the flexible plastic strip as small as 165 µm were measured.²⁴

To calculate the strain in a single metal layer on a flexible plastic substrate, the following formula developed from Suo et al.²⁶ applies:

ɛ_{top} = \frac{d_{s} + d_{f}}{2 R} \times \frac{1 + 2 η + χ η^{2}}{(1 + η) (1 + χ η)}

(1)

Here, ɛ_top denotes the strain at the surface of the metal layer when the flexible plastic strip is bent to a radius of R. d_s is the thickness of the substrate, d_f the thickness of the metal layer, η = d_f/d_s and χ = Y_f/Y_s with Y_f and Y_s the Young’s moduli of the metal layer and the substrate, respectively. As an example, a 100 nm thick gold layer (d_f) on a 50 µm thick Kapton substrate (d_s) with the Young’s moduli of Y_s = 5.4 GPa and Y_f = 79 GPa, bent to a radius of 165 µm, results in a strain at the surface of ɛ_top = 14.8%.

To investigate the influence of strain exerted by bending, flexible plastic strips with a 100 nm thick gold layer were subjected to bending. While decreasing the bending radii, ohmic resistance of the gold layer was monitored. This test directly indicates the influence of the applied bending radius (and hence strain) on the ohmic resistance of metal layers and shows at which bending radius the metal layer is destroyed. Resulting normalized resistance versus bending radius is shown in Figures 5(a) and (b). The resistance increased linearly with decreasing bending radius and hence increased the length of the gold layer down to a radius of ≈0.5 mm. For bending radii <0.5 mm, the resistance increased drastically. At bending radii <100 µm (corresponding to a strain of ≈24%) the gold layer started to crack, which is indicated by a drastic increase of the resistance. Therefore, a safe operation of a 100 nm thick gold layer on a flexible strip (thickness 50 µm) can be guaranteed to bending radii as small as 100 µm. The dashed gray line at 165 µm in Figure 5 indicates the minimum bending radius measured in the woven textile shown in Figure 4.

Figure 5.

(a) Measured normalized resistance versus bending radius. With decreasing bending radius, strain is increased, which causes the resistance to rise. (b) The normalized resistance for bending radii <0.5 mm. The vertical dashed line indicates 165 µm bending radius corresponding to the minimum bending radius measured in the woven textile shown in Figure 4.

The plastic strips with a width of 2 mm serving as substrates for metal layers withstood tensile strain of up to 50% and required at least a force of 34 N to tear. The obtained values were measured using a Zwick Roell extensometer.

Cleaning and pre-treating the surface of the Kapton substrate allows the fabrication of gold layers (100 nm thick) on Kapton that can withstand bending radii of the substrate as small as 100 µm. Similar results were obtained for 500 nm thick copper layers on Kapton exposed to bending radii as small as 130 µm.²⁴

Wrapping flexible plastic strips with cotton fibers

Flexible plastic strips were wrapped with cotton fibers to alter the cross-section of the strips from a rectangular to an elliptical shape. Flexible plastic strips show a thickness of 50 µm and widths of 1–2 mm, whereas textile filaments have an elliptical or circular cross-section.

Three meter long and 1 mm wide plastic strips, similar to the one shown in Figure 6, were wrapped with cotton fibers using a Rieter roving frame F15. The roving frame was set to introduce 175 turns/m into the resulting rove to achieve a mechanically stable twisted rove. The set turn rate resulted in a twisted rove with a diameter of ≈1 mm.

Figure 6.

A 3 m long and 2 mm wide flexible plastic strip.

Figure 7 shows the working principle of the employed roving frame. The cotton fibers first pass through a stretching unit consisting of three sets of rollers that rotate at different speeds. Then, a twist is added to the rove by the flyer, which rotates around the bobbin.

Figure 7.

Schematic of the roving frame. Flexible plastic strips were introduced into the rove in front of the last roller of the stretching unit. The flexible strip, together with cotton fibers, is then twisted by the flyer and wound onto the bobbin.

To wrap a strip with cotton fibers, the roving frame was set to a working speed of ≈4.5 meters of produced twisted rove per minute. Then, the flexible plastic strip was inserted into the machine in front of the last roller of the stretching unit, as depicted in Figure 7. The resulting twisted rove was then wound onto the bobbin by the flyer.

Figure 8(a) shows the resulting twisted rove without a flexible plastic strip. The twisted rove has a diameter of ≈1 mm. Figures 8(b)–(d) show different portions of the resulting twisted rove with a wrapped plastic strip. The photographs of the twisted rove show, with several different results, how flexible plastic strips and cotton fibers were joined: parts of the strip were covered with cotton fibers and not twisted (Figure 8(b)). Parts with coverage and without twisting extended to lengths of up to 10 cm. In other parts, however, the strip was twisted, as in Figures 8(c) and (d). While in Figure 8(c) the strip was wound around the cotton fibers, the strip in Figure 8(d) was partially covered with cotton fibers. Both twisted parts exhibit lengths of up to 10 cm. Results, as shown in Figure 8(b), mainly occurred at the beginning and at the end of the flexible plastic strip when the strip was only clamped either by the roller of the stretching unit or the flyer. This allowed the flexible strip to relax the introduced torsion due to twisting. However, when both the beginning and the end were clamped by the roller of the stretching unit and the flyer, the flexible strips could not relax, leading to results as shown in Figures 8(c) and (d).

Figure 8.

(a) Resulting twisted rove without a flexible plastic strip.(b) Flexible plastic strip covered with cotton fibers after roving. (c), (d) Twisted flexible plastic strip.

As described in the Flexible plastic strips section, strain caused by bending flexible plastic strips affects electronics on the strips and can even lead to destruction. The bending radii of the twists, as shown in Figure 8(c), were ≈0.5 mm. According to Equation (1), a gold layer experiences strain in the range of 1.5%. The observed twists occur due to the flyer rotating around the bobbin.

In the twisted parts of the flexible plastic strips, folds were observed (Figure 9). Folding a strip can correspond to a bending radius smaller than 100 µm, which leads to the possible destruction of a thin-film gold layer on strips.

Figure 9.

Folded flexible plastic strip covered with cotton fibers using a roving frame.

The pilot test of wrapping flexible plastic strips with cotton fibers on a roving frame lead to the following finding: in general, it is possible to fabricate a twisted rove that consists of a flexible plastic strip and cotton fibers. The analysis of the resulting twisted rove showed that the strips can be twisted or even folded when incorporated into cotton fibers. Twists introduce strain of approximately 1.5% into material layers on flexible plastic strips and folds cause the strain to rise to values as large as 25%, which leads to the destruction of material layers on the strips. To reduce the occurrence of folds, further optimization of the presented method is necessary by, for example, adapting the length of strips to prevent simultaneous clamping in the roller of the stretching unit and the flyer.

Embroidering flexible plastic strips

To investigate the potentials of embroidering flexible plastic strips, the 3 m long and 2 mm wide strips were embroidered on an industrial shuttle embroidery machine (Saurer Era). Figure 10(a) shows a photograph of the embroidery head while a plastic strip was inserted. In Figure 10(b), a schematic cross-section of the embroidery head and a felt substrate is shown while the needle stitches through the substrate to bring the upper yarn to the back side where it is retained by the shuttle yarn.

Figure 10.

(a) Photograph of the embroidery head of the Saurer Era machine with a flexible plastic strip, the needle and the upper yarn. (b) A schematic cross-section through the embroidery head and the felt substrate when the needle brings the upper yarn to the back side of the felt substrate and the upper thread gets retained by the shuttle yarn.

Flexible plastic strips were embroidered to a felt substrate by using a zigzag stitch for the upper yarn. The distance between the stitches on the left-hand side of the strip and on the right-hand side was set to 3.3 mm to avoid stitches going into the strip. Figure 11 shows a flexible strip embroidered onto a felt substrate.

Figure 11.

Flexible plastic strips embroidered onto a felt substrate using a shuttle embroidery machine. The upper yarn stitches were placed beside the flexible plastic strip to avoid stitches going into the flexible plastic strips.

Since flexible plastic strips are embroidered planar onto the felt substrate, no bends occur. However, during the integration, the strip is subjected to a 90° turn in the embroidery head, as can be seen in Figure 10(b).

To estimate strain in a flexible plastic strip during embroidery caused by the 90° turn in the embroidery head, the following experiment was performed. For the strain estimation, thin-film strain gauges in the longitudinal and transversal direction were fabricated on a flexible plastic strip in accordance with the first method to deposit and structure a material layer. Figure 12(a) shows the strain gauges made of a 100 nm thick gold layer. Line width and line spacing of the strain gauges was 40 µm. In Figure 12(b), an embroidered flexible plastic strip with strain gauges is shown. By measuring the resistance of the strain gauges before and after embroidering, the strain that is permanently induced into the strip can be calculated:²⁷

ɛ = \frac{R_{a} - R_{i}}{k R_{i}}

(2)

where ɛ is the strain, R_i the resistance before embroidery and R_a after embroidery. k is the gauge factor, a material parameter that corresponds to approximately two (2) for gold.

Figure 12.

(a) Transversal and longitudinal strain gauges integrated on flexible plastic strips. (b) Strain gauges on flexible plastic strips embroidered onto felt substrate.

The resistance measurements before and after embroidery yielded a maximum resistance change of 2%, which corresponds to a strain of 1%. To infer the strain during the integration process from permanently induced strain by plastic deformation of the flexible plastic substrate caused by the 90° turn, strain was applied to flexible plastic strips with a gold line on top. In the next step, strain was released and the resistance of the gold line without externally applied strain was measured after a relaxation time of 10 min. Strain caused by plastic deformation of the flexible plastic substrate lead to increased resistance values of the gold line. In Figure 13, the relation between applied strain and the resistance change after relaxing the applied strain is shown. According to the dependence in Figure 13, a 2% resistance change due to the 90° turn on an embroidery machine corresponds to an applied strain of ≈6% during the embroidery process.

Figure 13.

A gold line on a flexible plastic strip is strained once and then relaxed. On the y-axis the resistance change in the relaxed state is measured. Up to about 5% applied strain, the deformation of the flexible plastic strip is reversible and hence no resistance change is measured in the relaxed state. The blue dashed line (color online only) indicates the measured persistent resistance change for embroidered flexible strips.

Embroidering flexible plastic strips onto textile substrates leads to the following finding: gold layers on plastic strips were subjected to a strain of ≈6% due to the 90° turn in the embroidery head during the embroidery process. The strain during the embroidery process introduced a persistent strain in the flexible strip of 2%. In addition, the strips were embroidered planar and without twists. Plastic strips with strain gauges demonstrated the possibility to embroider strips with thin-film devices onto textile substrates without destroying the devices using standard manufacturing equipment.

In the case that thin-film devices are damaged during embroidery, one possibility is to sandwich the devices between two flexible plastic strips. The sandwich structure moves the neutral bending plain (the region where no strain is present, although the strip is bent) to the thin-film device between the two plastic layers.²⁸ In addition, the thin-film device layers are protected against mechanical abrasion.

Weaving flexible plastic strips

Weaving flexible plastic strips into textiles was performed on two different weaving machines: one machine was a rapier loom with a weaving width of 1.6 m and the second machine was a narrow fabric loom with a weaving width of 4.5 cm.

Rapier loom

On the rapier loom (type Dornier PTV8), polyester monofilament yarns with 40 µm diameter were woven with a weft yarn density of 1.2 yarns/mm and a warp yarn density of 3.3 yarns/mm into a plain weave. The woven textile had a cover factor of ≈17%.²⁹ Three meter long and 2 mm wide flexible plastic strips (see Figure 6) were woven in the weft direction. To avoid neighboring weft yarns crossing the strips, three weft yarns were omitted by disabling the weft yarn introduction to generate space for the strip. The controllable positive rapiers allowed a fully automated introduction of the strips into the woven textile using a weaving speed of five introduced wefts per minute for strip insertion. The strips were presented to the rapier on a bobbin. Figure 14(a) shows a section of the resulting textile and Figure 14(b) shows a close-up view on the integrated plastic strip. In Figure 14(c), a cross-section of the integrated strip is shown. The plastic strips were not bent and are therefore not exposed to strain.

Figure 14.

(a) Textile with integrated flexible plastic strips woven on a Dornier PTV8 rapier machine. (b) Close-up view of the integrated strips. (c) Micro-graph of the cross-section of the textile along the dashed line in (b).

Weaving on a rapier loom leads to the following findings: the rapiers allowed a fully automated introduction of flexible plastic strips into woven textiles with a width of 1.6 m. The used weaving pattern and filament types did not cause strain in the woven flexible plastic strips.

Narrow fabric loom

The employed narrow fabric loom with a weaving with of 4.5 cm was a Müller Frick NFREQ 42. Cotton yarns with an average diameter of 330 µm were woven in weft and warp directions with weaving densities of 2.5 yarns/mm into a 7/1 twill weave pattern. The resulting textile had a cover factor of ≈97%. Due to the smaller weaving width of the narrow fabric loom, 6 cm long and 2 mm wide flexible plastic strips were used. The strips were introduced in the weft direction: first, cotton yarns were woven with a speed of 15 weft yarns/min. Then, five weft yarns were omitted by blocking the weft yarn feed to open a space of 2 mm for the plastic strip. The strip was inserted manually into the textile. Until the next strip was woven into the narrow band, cotton yarns were woven with a speed of 15 weft yarns/min. The resulting textile is shown in Figures 15(a) and (b). A cross-section of a textile with a woven flexible plastic strip on a narrow fabric loom is shown in Figure 4. As indicated, bending radii as small as 165 µm were measured, which cause strain of 14.8% in a 100 nm thick gold layer on the woven plastic strip.

Figure 15.

(a) Woven textile band with integrated flexible plastic strips. (b) Close-up view of the woven band.

Weaving strips on the narrow fabric loom did show that the integration of flexible plastic strips is feasible. However, woven plastic strips were exposed to bending radii as small as 165 µm, which corresponds to a strain of 14.8%. Although the resistance of a 100 nm thick gold layer is increased by 25%, the line is not destroyed.

The discrepancy of bending radii of woven flexible plastic strips on a rapier loom and on a narrow fabric loom, illustrated in Figures 4 and 14(c), is explained by the combination of several factors: yarn density, form and size of the yarn cross-section and materials of the yarns result in different cross-sections of woven fabrics, as described by Hu.³⁰

Weaving flexible plastic strips with electronic devices

Two woven demonstrators with electronics integrated on flexible plastic strips were manufactured. For the first demonstrator, strips with integrated thin-film devices were woven, while the second demonstrator consists of strips with accelerometers in the form of integrated circuits.

Thin-film devices

As a first demonstrator, flexible plastic strips, 6 cm long and 0.5 mm wide, with a thin-film resistive temperature sensor (RTD) per strip were fabricated. The RTDs consisted of a 100 nm thick gold layer, structured with a lift-off process, as shown in Figure 2 (method 1). A 20 µm line width and spacing between the lines of the meander structure of the RTDs was used. To contact the RTDs, four interconnect lines leading to both ends of the flexible strip were structure to enable a four-wire Kelvin measurement. In Figure 16(a), the strip with the RTD and the four interconnect lines is shown.

Figure 16.

(a) Sensor part of the flexible plastic strip with four interconnect lines. (b) Textile prototype with integrated flexible plastic strips containing thin-film resistive temperature devices. The inset shows a close-up view of the integrated RTD in the textile.

A total of seven flexible plastic strips with RTDs were woven on a Müller Frick NFREQ 42 narrow fabric loom with a spacing of ≈1 cm between two adjacent strips. The resulting textile is shown in Figure 16(b) and, in the inset, a close-up view of a textile-integrated RTD is depicted.

The functionality of the RTDs on the woven strips was tested by measuring the temperature response before and after weaving by contacting the interconnect lines at both ends of the plastic strips with standard copper wires. The temperature response was determined in a climate chamber at temperatures between 30℃ and 90℃ in steps of 5℃, while the humidity was held constant at 50% relative humidity (RH). Figure 17 shows the normalized resistance R/R₀ (R₀ at 30℃) versus the applied temperature. For both measurements, the RTDs show a linear behavior, indicating that integrating thin-film RTDs into a woven textile is possible without degradation of sensor performance.^25,31

Figure 17.

Normalized resistance of resistive temperature sensors (RTDs) versus temperature before and after weaving. A linear RTD behavior is observed.³¹.

Integrated circuits on woven flexible plastic strips

For the second demonstrator, a flexible plastic strip with a three-axes accelerometer in the form of an integrated silicon device (Freescale Semiconductor MMA7660FC) with a footprint of 3 mm × 3 mm was woven in the weft direction. The accelerometer was equipped with the I²C bus protocol to transmit the measured acceleration values digitally. In the warp direction, four conductive yarns were woven to connect the accelerometer. Textile fabrication was based on the technology presented by Zysset et al.³²

Strips to mount accelerometers were fabricated following the second method shown in Figure 3. First, an 18 µm thick copper layer on a 5 cm × 5 cm Kapton substrate was structured by etching. The structured copper layer provided a landing pattern for the accelerometer chip, and four contact pads, one for supply, one for ground, one for the clock signal and one for the data signal of the I²C protocol . The landing pattern of the chip and the contact pads were connected with 200 µm wide interconnect lines. The Kapton substrate was then separated into 3 mm wide and 5 cm long strips using a wafer saw.²⁵ As a final step, accelerometers were soldered onto the strips. An exemplary strip is shown in Figure 18(a).

Figure 18.

(a) Flexible plastic strip with an accelerometer. Four contact pads were required to supply power, connect ground, clock signal and data signal. (b) The strip with the accelerometer woven into a textile on a narrow fabric loom. In the warp direction, four conductive yarns were integrated to contact the accelerometer.

The strips with accelerometers were woven on a narrow fabric loom. To contact the strips in the woven textile, a total of four conductive yarns (Bekinox VN 14/1 x) were woven in the warp direction and integrated into the warp beam prior to weaving. To weave a strip, five cotton weft yarns were omitted to open space for the stip. Then, the weaving machine was stopped and a plastic strip was inserted manually and aligned against the conductive yarns to ensure that the conducting yarns cross the contact pads on the strip. Finally, weaving was continued with cotton yarns.

After weaving, the contact pads on the flexible strips were connected to the conductive yarns using conductive epoxy (Epo-Tek H20E). The conductive epoxy was cured in an oven at 80℃ for 24 h. The resulting textile with a woven strip, four conductive yarns and the contacts is shown in Figure 18(b).

The resulting textile was connected to a computer by attaching crocodile clips to the conductive yarns at the edge of the textile. This allowed one to supply the accelerometer with power and to read-out the acceleration data for all three axes with a sample rate of 10 Hz. Figure 19 shows the acceleration signals of the textile-integrated accelerometer: the textile was rotated in a way that every axis was exposed to the gravity.

Figure 19.

Acceleration signals of the textile-integrated three-axes accelerometer. The drawings on top of the graph indicate how the textile-integrated accelerometer was exposed to gravity. The white time frames in between two consecutive ways of exposing the accelerometer show the rotation of the textile.

In the resulting graph in Figure 19, it is clearly visible which axis was exposed to gravity and also whether in the positive or negative direction (1 and –1 for the corresponding axis). However, the recorded acceleration signals showed that the two axes which were not exposed to gravity were not always zero. This is due to the fact that the x–y plane of the accelerometers can deviate up to 10° from the x–y plane of the textile because of the flexibility of the plastic strip and the textile itself.

Nevertheless, from textile-integrated accelerometers, applications in health care and rehabilitation, such as body posture monitoring, can profit.⁶ Even given the fact that the flexibility of the textile and the plastic strip can decrease the accuracy of the textile-integrated accelerometers, it is still possible to classify, for example, a body posture with accuracies of over 85%.³³

Conclusion

In this paper, three approaches to combining electronics on flexible plastic strips with textiles or yarns were presented. A roving frame was employed to wrap flexible plastic strips with cotton fibers, strips were embroidered onto felt substrates and strips were woven into textiles, using in all cases industrial machines and equipment. For the three methods, strain was measured, because excessive strain in material layers on top of the strips can damage the material layer. Wrapping strips with cotton fibers caused 1% strain; in cases the strip is folded strain reaches >25%; during embroidery strain of approximately 6% was present; after embroidering 2% of persistent strain remained in the strip; for weaving, strain of up to 15% was measured.

Weaving flexible plastic strips was applied to fabricate two demonstrators: the first demonstrator was a woven textile patch with thin-film resistive temperature devices on strips. The measurements showed that weaving does not affect the performance of the sensors when woven. The second demonstrator consisted of a plastic strip carrying an accelerometer in the form of an integrated circuit. In the warp direction, four conductive yarns were woven to supply power and read-out the sensor. The textile-integrated accelerometer was able to measure acceleration for three axes, allowing one to distinguish the different orientations of the textile. Such a textile-integrated accelerometer is beneficial for applications of smart textiles in health care and rehabilitation.

Using a rapier loom as demonstrated allows weaving flexible plastic strips into textiles as wide as 1.6 m. Roll-to-roll fabrication will enable large-scale processing of flexible plastic substrates and strips.

However, flexible plastic strips carrying electronic devices are affected by water and chemical detergents used for washing, as shown by Zysset et al.³² To resolve this issue, flexible plastic substrates with improved barriers against moisture and other chemicals have to be developed.

Nevertheless, we believe that weaving flexible plastic strips is one possibility to further enable smart textile development. Due to the on-going miniaturization of electronic devices, the size of flexible plastic substrate can be decreased and will enable an unobtrusive integration into woven textiles. To enable the further development of weaving plastic strips, additional electronic devices, such as flexible polymer-based sensors and flexible transistors will be developed, as well as more complex electronic systems, for example array structures or sensor systems with textile-integrated control and communication electronics. In addition to the electronics, the weaving machines have to be adapted to allow a fully automated weaving of flexible plastic strips by altering the weft insertion mechanism.

Footnotes

Acknowledgment

The authors would like to thank Dr J Zimmermann from Forster-Rohner AG, R Bühler, G Miersch and H Schwippl from Rieter AG, Dr I Locher from Sefar AG and J Egli and S Nicoli from the Swiss Textile College.

Funding

This work was supported in part by the Swiss Confederation and in part by the Nano-Tera.ch initiative.

References

Van Langenhove

Herteleer

. Smart clothing: a new life. Sci Technol 2004; 16: 63–72.

Paradiso

Loriga

Taccini

. WEALTHY, a wearable health-care system: new frontiers on e-textiles. J Telecommun Inf Technol 2005; 4: 105–113.

Mattmann C, Amft O, Harms H, et al. Recognizing upper body postures using textile strain sensors. In: proceedings of the 2007 IEEE international symposium on wearable computers (ISWC 2007), p.36.

Savio D and Ludwig T. Smart carpet: a footstep tracking interface. In: proceedings of the 21st international conference on advanced information networking and applications workshop, 2007, pp.754--760.

Park S, Mackenzie K and Jayaraman S. The wearable motherboard: a framework for personalized mobile information processing (PMIP).. In: proceedings of the 39th annual design automation conference, 2002, p.174. ACM.

Harms H, Amft O, Tröster G, et al. Smash: a distributed sensing and processing garment for the classification of upper body postures. In: proceedings of the ICST 3rd international conference on body area networks, p.22 2008.

Locher

Tröster

. Fundamental building blocks for circuits on textiles. IEEE Trans Adv Packag 2007; 30: 541–550.

Martin T, Jones M, Chong J et al. Design and implementation of an electronic jumpsuit. In: proceedings of the 2009 IEEE international symposium on wearable computers (ISWC 2009), p.157.

Locher

Tröster

. Screen-printed textile transmission lines. Textil Res J 2007; 77: 837–842.

10.

Linz

Gourmelon

Langereis

. Contactless EMG sensors embroidered onto textile. 4th international workshop on wearable and implantable body sensor networks (BSN) 2007, pp. 29–29.

11.

Kallmayer C and Simon E. Large area sensor integration in textiles. In: proceedings of the 9th international multi-conference on systems, signals and devices, 2012, pp.1--5.

12.

Vervust T, Buyle G, Bossuyt G, et al. Integration of stretchable and washable electronic modules for smart textile applications. J Textil Inst 2012; 103: 1127–1138.

13.

Vieroth R, Löher T, Seckel M, et al. Stretchable circuit board technology and application. In: proceedings of the 2009 IEEE international symposium on wearable computers (ISWC 2009), p.33.

14.

Gonzalez

Axisa

Bulcke

. Design of metal interconnects for stretchable electronic circuits. Microelelectron Reliab 2008; 48: 825–832.

15.

Bonderover

Wagner

. A woven inverter circuit for e-textile applications. IEEE Electron Device Lett 2004; 25: 295–297.

16.

Hamedi

Forchheimer

Inganäs

. Towards woven logic from organic electronic fibers. Nat Mater 2007; 6: 357–362.

17.

Abouraddy

Bayindir

Benoit

. Towards multimaterial multifunctional fibres that see, hear, sense and communicate. Nat Mater 2007; 48: 336–347.

18.

Chen

Kazlas

. Flexible active-matrix electronic ink display. Nature 2003; 423: 136–136.

19.

Burns

Reynolds

Reeves

. A scalable manufacturing process for flexible active-matrix e-paper displays. J Soc Inf Disp 2005; 13: 583–586.

20.

Reuss

Chalamala

Moussessian

. Macroelectronics: perspectives on technology and applications. Proc IEEE 2005; 93: 1239–1256.

21.

Service

. Patterning electronics by the cheap. Science 1997; 278: 383–384.

22.

DuPont, Kapton datasheet, http://www2.dupont.com/Kapton/en_US/index.html (accessed 23 May 2012).

23.

Polymer Processing, http://www.polymerprocessing.com/index.html (accessed 1 June 2012).

24.

Cherenack

Kinkeldei

Zysset

. Woven thin-film metal interconnects. IEEE Electron Device Lett 2010; 31: 740–742.

25.

Kinkeldei T, Zysset C, Cherenack K, et al. Development and evaluation of temperature sensors for textile integration. In: Proceedings of IEEE Sensors, 2009, pp.1--4.

26.

Suo

Gleskova

. Mechanics of rollable and foldable film-on-foil electronics. Appl Phys Lett 1999; 74: 1177–1179.

27.

Hoffmann J. Handbuch der Messtechnik, Hanser: München, 2004.

28.

Kinkeldei

Münzenrieder

Zysset

. Encapsulation for flexible electronic devices. IEEE Electron Device Lett 2011; 32: 1743–1745.

29.

Hamilton

. A general system of woven-fabric geometry. J Textil Inst Trans 1964; 55: 66–82.

30.

. Structures and mechanics of woven fabrics, Woodhead Publishing: Cambridge, 2004.

31.

Kinkeldei

Zysset

Cherenack

. Textile electronics for monitoring the room climate. In: Plastic electronics 2010.

32.

Zysset C, Cherenack K, Kinkeldei T, et al. Weaving integrated circuits into textiles. In: proceedings of the 2010 IEEE international symposium on wearable computers (ISWC 2010), pp.1--8.

33.

Harms

Amft

Tröster

. Estimating posture-recognition performance in sensing garments using geometric wrinkle modeling. IEEE Trans Inf Technol Biomed 2010; 14: 1436–1445.