Defining Flexibility and Sewability in Conductive Yarns
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Defining Flexibility and Sewability in Conductive Yarns Margaret Orth, PhD International Fashion Machines, Inc. 32R Essex Street Cambridge, MA 02139 ABSTRACT In order for electronic textiles to truly qualify as textiles, they must maintain one of the intrinsic qualities of textiles, flexibility, or the ability to resist permanent deformation under bending, lateral stress and strain. Flexibility will allow electric textiles to be intimate, soft, wearable, conformable and durable. Unfortunately, flexibility is poorly understood by many researchers who come from a traditional electronics background. This paper presents some common terminology of textiles, and different approaches to understanding flexibility in fibers and yarns. Because one of the most mechanically stressful textile manufacturing process is machine sewing and embroidery, this paper defines the necessary properties of machine sewable yarns and demonstrates a formal Curl Test for judging the sewability and flexibility of stainless steel yarns. This paper also examines flexibility in yarns and fibers, historically and based on a mathematical model and more qualitative properties. INTRODUCTION The idea of machine sewing an electrical trace element is highly appealing for many practical reasons. Machine sewing allows sewn electrical elements of almost any shape and size to be easily and quickly placed. Machine sewing allows continuous electrical traces to be sewn across the seams in a garment; this means that electrical continuity between two separate panels can be achieved. Machine embroidering electrical traces allows for the CAD control of the placement, size, texture and design of electrodes and other circuit elements. To make embroidered or machine sewn electrical elements requires using a thread that is machine sewable, electrically conductive and maintains that conductivity under the mechanical stress of machine sewing. Over the course of our research, we have empirically observed that most conductive threads are NOT machine sewable. They simply cannot stand up to the compound mechanical stress of machine sewing through the needle. Conductive coatings can rarely withstand the forces of passing through the needle, and conductive fibers and yarns often jam the machine or simply break too easily. One solution to this problem would be to create a new sewing machine. But this solution is flawed for two reasons. Practically, it is attractive to use the standard sewing equipment and facilities of the already huge and established textile industry. In addition, equipment that is used for sewing standard textile materials assumes that those materials have certain mechanical properties, including flexibility or the ability to resist permanent deformation under bending. These properties are not just suited to the machinery. They are essential to the appropriateness of a fiber or yarn for its use any textile that is intended for or human wear. Thus, the ability of a conductive fiber to be sewn or used in existing textile machines says a lot about how soft
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