Page 412 - Mechatronics for Safety, Security and Dependability in a New Era

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wearing comfort, usability, and safety aspects of electrical devices, hi smart clothing design these soft
values are as important criteria as technical functions. Consequently, we need to develop solutions that
are easy to use and maintain their clothing-like properties as well as technical reliability.
In smart clothing applications we usually utilise a distributed electronics architecture, which ensures
that any single electronic module is not straining too much textiles and the clothing itself feels good to
wear. This distribution of electronics to several pieces of clothing or to several locations in a piece of
clothing create needs for communication between different electronic modules. Ordinary wires could
be used, since they provide straightforward, inexpensive, and reliable communication medium.
However, long wires inside the clothing may cause rigidity and separate connectors are needed
between the different pieces of clothing. Wireless solutions are the most practical for that, since extra
connections of wires could impair dressing and undressing. On the other hand, wireless solutions can
be too complex and e.g. in demanding industry environment the simplest wireless solutions such as
inductive coupling may suffer from environment interference. To overcome these difficulties we have
studied the use of conductive fibres as one of the key building elements of smart clothing. As a
communication medium conductive fibre yarns are as straightforward solutions as ordinary cables.
Furthermore, they are lighter and softer to wear than plastic shielded cables, which make them more
clothing-like and comfortable. In addition to communication, versatile electrically conductive fibres
can also be used as sensing elements.

This paper introduces usage of electrically conductive fibres (ECFs) in smart clothing applications
concentrating on the reliability of the connection mechanisms between the fibre and the printed wiring
boards (PWBs).

CONDUCTIVE FIBRES

In general, ECFs are used in the prevention of electrostatic shock. Same fibres can also be adapted for
wire replacement usage in clothes. To be able to take care of also power transfer between different
electronic modules, low resistance and wire shielding is required. The first requirement reduces power
consumption and the latter requirement protects users against electrical shock and ensures proper
functioning of the system. These were also our main starting points while choosing suitable ECF
materials. Conductive fibres become useful for us when several thin filaments form a twisted yarn,
which then resembles one core metal cable. Tn our smart clothing applications, so far, we have mostly
used metal clad aramid fibres (DuPont data sheet), which are intended for braided electromagnetic
interference shielding in cables and harnesses and for electrical conductions in specialised applications.
These fibre yarns have better specific gravity and tensile strength properties than copper offering also
flexibility more typical for clothing than metal. Fibres are coated with silver, copper, or nickel metals.
The latter may cause allergy reactions while being attached directly to skin. Therefore, only silver or
copper coating materials are acceptable. Due to better availability we have chosen to use silver clad
fibre yarns.
ECF Usage Experiments

We have employed metal clad aramid fibre yarns to replace plastic shielded cables in lightweight
underclothes. One of them is a sensor shirt, which measures user's body surface temperature,
respiration rate, and skin's electrodermal activity (Rantanen at al. 2001). Altogether nine temperature
sensors are attached to the shirt so that they are in close contact to skin allowing skin surface
temperature measurements. Long ECF yarns connecting these sensors are unnoticeable for users while
plastic shielded cables so close to skin could cause rigidity and feel uncomfortable (Rantanen at al.
2001). The most challenging has been the implementation of reliable joints. When connecting yams to
temperature sensors by knotting yarns around component's pins and covering the joints with shrinking