Designing an interface between thetextile and electronics usinge-textile composites

Matija VargaETH Zurich,Wearable Computing LabGloriastrasse 35, Zurichmatija.varga@ife.ee.ethz.ch

Gerhard TrosterETH Zurich,Wearable Computing LabGloriastrasse 35, Zurich

AbstractA design concept for textile-electronics integration ispresented. The design describes utilization of textilecomposites for building textile circuits. Customizedelectronic blocks are placed between two e-textile layers.Textile circuits are formed by contacting conductivethreads and the unit blocks, without modifying thee-textile material. Routing of textile circuits using theproposed approach is shown in two examples.

Author KeywordsSmart textiles; Textile circuits; E-textile composites;Wearable computing

ACM Classification KeywordsH.5.2 [Information interfaces and presentation (e.g.,HCI)]: Miscellaneous.

IntroductionIn this work we present a design concept fortextile-electronics integration using textile composites. Weconsider using stacked layers of e-textile and plasticelectronics to build textile circuits. The resultinge-composite will have embedded sensing functionality.

While building e-composites, the idea is to follow theparadigm of conventional high volume production where

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measurable parameters and formal descriptions are usedto describe how to manufacture a certain product. Forexample, in the printed circuit board (PCB) productionprocess, manufacturers are provided with standard filesdescribing the board substrate, components arrangement,top/bottom masks etc., which they use to develop theend product. A point that will be investigated here is howto develop a textile-integration method that can also besuitable for computer aided design of e-composites. Theexpected result would be to develop e-composites using acomputer aided design tool that produces a set ofdescription files (as in PCB example), which can be usedto assemble the final smart garment.

E-textile material considered in this work is manufacturedusing regular weaving technique. Regular structure of thematerial enables measuring as well as fine tuning of textileparameters (e.g. fiber diameters, fiber alignment, share ofmetallic fibers in a textile). In previous work, weavingconductive fibers (metallic fibers/yarns) was proven to bea reliable and well-defined process [7].

The main contributions in this paper:

• We propose a separation of textile and electronicsmanufacturing processes.

• We show a design approach for simplifiedtextile-electronics integration using textilecomposites.

Related workTextile-electronics integration assumes modification of thetextile material. In the recent review by Castano et.al. [4],modifications of textile materials are split into four levels.Level one modification includes construction of conductivefibers for e-textile manufacturing, level two includesreplacing conventional threads in a textile with conductive

threads, level three describes modifications on the fabricmaterial (e.g. conductive surface coating) and level fourrefers to the use of multiple fabrics to build a textilecomposite.

In the work by Brun et.al. [2], level one modifications aremade by integrating the chip inside the yarn. Specializedmachines are used to modify conventional thread.E-textile is then manufactured using a standard weavingprocess. Ability to route electronics inside the resultinge-textile is limited due to the weaving process. In the workby Bonderover et.al. [1] and Zysset et.al. [9], level twomodification is presented. In the latter, textile circuits areformed by replacing conventional threads with strips ofelectronics on a flexible substrate. During themanufacturing process, weaving machine is stopped forthe strip to be inserted. In the work by Locher et.al [7]textile circuits are formed on the textile (level threemodification). Metallic threads in textile are used as theinterconnection infrastructure. Due to the mismatchbetween the textile orthogonal structure and thecomponent footprint, customized interposers are used.Similar modifications are described in the work by Simonet.al. [8] where printed circuit boards are crimped onto anorthogonal textile grid. A total of 9 steps are needed tocomplete the textile-electronics integration. We would liketo propose an approach that does not interfere with theweaving process of e-textiles and does not require multiplemodification steps to make e-textile compliant toelectronics.

Level four modifications are shown in [3] where textilecircuits (e.g. lines and pads) are laser cut and placed on anonconductive textile substrate. Nonconductive textile isalso used as an insulator on line crossings. Screen printingis presented by Kim. Y. et.al. [5] with the line resolution

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of 200 µm and 100 µm for sputtering process. System ona chip is directly bonded on the textile and molded formechanical stability. Routing of circuits on multiple layersis not shown. In contrast to that, we would like to proposea design that enables routing on both layers without usingvias, holes and additional insulating material.

Textile composite designPrevious work by Locher et.al. [7] investigates how textilecircuits can be realized on a single e-textile layer. Cuttingconductive lines, removing isolation and protecting thecuts are some of the steps that create overhead in overallprocess of textile electronics integration. Textilecomposites built in our work consist of two components:electronic unit blocks and textile layers with conductivethreads. Regarding the textile material, the goal is todevelop a general interconnection infrastructure that isindependent from the end application. This also impliesthat very little knowledge about textile technology isneeded to realize electrical part of the system. Usually,design of the electrical part is tightly coupled with thetextile technology used. The goal of our approach is toreduce the coupling and constraints imposed on theelectrical part of the textile composite.

Unit blocks are flexible electronic devices customized fortextile-electronics integration. They consist of flexibleelectronics and miniature off-the-shelf components.Depending on the embedded functionality, unit blocks canbe divided in three groups: sensors/actuators,interconnects/insulators, and processing units. From thetextile technology perspective, unit blocks are beneficialbecause they do not require from textile technologyexperts to work with electrical part of the system. Fullfunctionality of the textile circuit is achieved by arrangingand contacting unit blocks with the e-textile grid.

Arrangement of the blocks on the grid is defined by anelectrical engineer in the design phase. Separating thetextile processing (e.g. bonding, cutting, sewing) andelectrical routing is achieved using unit blocks.

Figure 1: Three layers of the textile composite structure: Blueand green threads are regular threads and grey threads aremetallic (conductive). Orange block represents the electronicunit block.

Figure 2: Graphical representation of the resulting textilecomposite patch with 16 unit blocks.

Textile composite is assembled by placing the unit blocks

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between the two layers of textile. Unit blocks are glued tothe textile to assure electrical contact. In the work byLenz et.al. [6], bonding using nonconductive adhesiveswas presented as a reliable technique for contacting thee-textile. To ensure mechanical stability, textile layers aresewn together. Figure 1 shows layers of the composite.Conductive threads are represented by grey coloredthreads, while orange blocks represent electronic unitblocks. In Figure 2 unit blocks are embedded in the textilecomposite and connected with the conductive textileinfrastructure.

Examples of integrationUse of textile composites for textile-electronics integrationis described in this chapter. The first example describesthe use of textile composite for routing analog signalswhile the second shows how the I2C and SPI enableddevices are interconnected using the described concept.

Multiplexing analog signalsThis example shows how analog signals can be routedusing the proposed textile integration approach. Blockdiagram of a switch sensor is given in the Figure 3.Sensors are powered with Vdd at the input. Each switchactivates one output of the sensor block. A multiplexer isused to read the output of each sensor in a circuralfashion. The multiplexer is a unit block that interconnectsan A/D converter and the sensor output.

Figure 3: Block diagram of sensors (grey) connected to themultiplexer (orange).

This example shows the worst case routing scenariobecause each sensing unit block uses all threads in athread group, thus total of 4 independent threads perblock must be reserved for each sensor (Figure 4). Oncethe block is connected to 4 threads, they are not availablefor other blocks. Therefore, the multiplexer unit blockmust be extended to reach over four thread groupcrossings. White unit block is used as interconnectionbetween the top and the bottom layer. For the integrationof additional 2 sensors, multiplexer unit block shouldexpand over two additional thread group crossings.

From Figure 4 it is obvious that the size of the multiplexerunit block grows with the complexity of the system. If themultiplexer unit block size is fixed, textile material withmore than 4 threads in a thread group is used. Size ofunit blocks and number of threads in a thread group canbe extracted from the design as integration parameters.

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Figure 4: E-composite patch with two sensors (grey),multiplexer (orange) and one routing block (white).

Integrating I2C and SPI enabled sensors in textilesIn contrast to the previous example, unit blocks here sharemost of the threads in a thread group. This exampleemphasizes a very good utilization of the orthogonalstructure. The block diagram in Figure 5 shows multipleslave SPI enabled sensors connecting to one master.

Figure 5: Block diagram of the SPI bus.

In this example, sensors are arranged in a 3x3 matrix array(Figure 6). White unit blocks are used as interconnectsthat link the master with slaves. In case of I2C bus, allsensors use the e-textile infrastructure together and thewhole system scales without increasing sizes of unitblocks.

Figure 6: Matrix array of sensing unit blocks enabled with theSPI protocol. Layout of the I2C enabled matrix array is givenfor comparison.

If SPI protocol is used instead of I2C, the layout ofsensing blocks remains unchanged, but somemodifications are needed. To enable the slave addressing,sensing unit blocks with the additional electronics areused - an OR gate is implemented inside the blocks.Moreover, additional interconnecting unit blocks areadded. Master unit block extends over two crossings toenable addressing of columns and rows of the matrix. Asin the first example, increasing the number of threads in athread group could compensate for the size of the masterblock. In this setting sensors can be arbitrarily arrangedon the grid and addressed by the master device. Even ifsensor layout is changed during the e-composite designprocess, only the software in the master device requiresmodification. The e-textile infrastructure is not modified.

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OutlookTo quantify the approach described in this paper,parameters of the resulting textile composite will beextracted. The regular (orthogonal) structure of thetextile composite is suitable for parameter extraction.Examples of parameters are: share of conductive threadsin a material, overall textile area occupied by the device,share of the plastic substrate in the overall composite areaand number of threads in a thread group. Distancebetween thread groups determines the distance betweensensing unit blocks. Therefore, it is a parameter that isimportant for setting the resolution of the resultinge-composite. Number of threads in a thread group isdetermined by the complexity of unit blocks, i.e. numberof input and outputs of the unit block. If the orthogonalstructure is considered as a model and all parameters aretunable and easy to extract, optimization can beconducted during the design phase of the device.

AcknowledgmentThis work was supported by the collaborative projectSimpleSkin under contract with the European Commission(# 323849) in the FP7 FET Open framework. Woventextile material for realizing the concept presented in thiswork is provided by the project partner SEFAR AG.

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[9] Zysset, C., Kinkeldei, T., Munzenrieder, N., Petti, L.,Salvatore, G., and Troster, G. Combining electronicson flexible plastic strips with textiles. Textile ResearchJournal 83 (2013), 1130–1142.

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