O o d h e a d p u b L i s h I n g L i m I t e d

13.4
Current applications of wool in
intelligent textiles
At the 1999 Tissu Premier exhibition in Lille, a speaker from the Institut
Textile de France suggested that the properties of wool would endear it to
developers of emerging intelligent textile technologies (Anon., 1999). However,
the following years have seen only a trickle of wool-based intelligent textiles.
This may be due to the overall slower-than-expected commercialisation of
smart textile technologies (Wilkinson, 2004), with wool’s prominence in the
intelligent textiles field simply being a reflection of its prominence in textiles
globally. Therefore it is perhaps not surprising that much of the research and
product development involving wool in intelligent textile systems has taken
place in the countries where wool has greater prominence: Australia and
New Zealand.
13.4.1 Wool and inherently conductive polymers
Electrical conductivity is an important property of fibres used in many
intelligent textile systems. Conductive fibres have the potential to be used in
a number of ways in intelligent textile systems, including as sensors (where
the sensor stimulus is detected as change in electrical properties, such as
resistance) and in wearable electronics (discussed in the following section).
Therefore, a key initial stage in the development of electronic sensing and
responsive mechanisms for wool is to provide the fibre with useful electrical
conductivity. The technique used by many researchers has been to incorporate
a conductive coating onto wool by the deposition of inherently conductive
polymers (ICPs) on the fibre surface. One particular reason for interest in the
use of ICPs as a means of making textiles conductive is their potential to be
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chemical sensors (via interaction of the species to be sensed with the chemical
structure of the polymer), and potentially even electro-active materials.
One question that should be addressed is the level of conductivity that is
required to make a wool fibre useful in an intelligent textile system. This
depends on the application being targeted. For example, in many sensing
applications the actual level of conductivity is almost irrelevant, as the most
important characteristics are how conductivity changes in response to the
stimulus being detected, and how reproducible the electrical behaviour is
over time and with repeated use. On the other hand, in a wearable electronic
application it might be necessary to have a high conductivity to allow power
to be efficiently transferred, or a signal to/from a device to be accurately
transmitted/received. In the former case, resistance levels into the mega-ohm
range may be acceptable, while in the latter case ohm-order levels or lower
will be required. The level of conductivity achievable in ICPs can be very
high, depending on their composition and the synthesis technique used.
Generally though, the conditions of polymerisation onto a textile substrate
are such that the polymer formation is imperfect, and the conductivity
diminished well below theoretical maximums.
Alongside considerations of an appropriate level of conductivity is the
consideration of suitable ICP loading on the fibres, i.e. the ICP ‘add-on’.
Generally, putting more ICP onto a substrate will make it more conductive
(as would be expected), but this relationship is not universal. For example,
in work carried out by this author, the conductivity of a fabric with a thiophene
variant polymerised onto it did not increase with increasing ICP add-on
(Collie, 2007). In fact there was an optimum add-on level, above and below
which conductivity decreased. This was probably due to better-ordered polymer
formation under conditions that led to lower add-on, and when multiple
treatments at the optimum level were used there was good correlation between
the amount of ICP present and conductivity. It should be noted that the ICP
add-on is often not reported by researchers, but is critical to considerations
of the economic efficiency of the deposition process, and in all likelihood
influences the intrinsic textile mechanical properties, such as flexibility and
extensibility.
As discussed elsewhere in this volume there is ongoing interest in exploiting
the chemical functionality of the wool fibre surface for the attachment of
functional finishes, and conductive polymers are no exception. Conductive
polymers (most commonly polypyrrole and polyaniline) are deposited onto
textile substrates by two main techniques: in situ oxidative polymerisation of
the ICP on the fibre surface (and often also in the fabric interstices), or
deposition of a soluble form of the ICP. ‘Doping’ of the ICP is an essential
process to make it fully conductive, and is simultaneous with polymerisation
when ionic salts are used as oxidants. A very commonly used reagent is
iron(III) chloride, where the iron(III) cation is the oxidant species, and the
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chloride anion is incorporated into the ICP as the dopant. Also, aromatic
sulphonic acids (such as p-toluene sulphonic acid or naphthalene disulphonic
acid) are used as sources of dopant anions. Iodine vapour or hydrochloric
acid are also commonly used to dope ICPs after they have been polymerised.
A discussion of ICP deposition techniques as they have been applied to wool
follows.
A well-known and widely used deposition method is an in situ polymerisation
technique developed by Milliken Research Corporation (Gregory et al., 1989;
Kuhn and Kimbrell, 1989). In some publications Milliken researchers suggest
that fibre type has minimal influence on the conductivity of the substrate
(Kuhn and Kimbrell, 1989; Kuhn and Child, 1997), which of course means
that wool would offer no advantage (or disadvantage) over other fibre types.
This is somewhat surprising, as in an adsorption reaction (as this deposition
technique is), it is logical that the chemical and physical properties of the
substrate onto which reagents are being adsorbed will be relevant. Other
researchers have examined the effect of substrate fibre type, and several
have included wool in their comparisons. For example, the interaction between
the wool fibre and the polymerisation environment was discussed by Kaynak
et al. (2002), with the polymer–dopant system thought to attach to molecular
groups on the wool fibre by ionic and hydrogen bonds. For this reason, they
concluded that wool is an ideal substrate to use in the preparation of textile-
conductive polymer composites. The research found that treated yarns showed
increased tenacity, due to the reinforcing effect of the polypyrrole coating,
but that the initial modulus was reduced; this was attributed to the reduction
in surface friction between fibres, causing more inter-fibre slippage during
extension. In related work, the same research team confirmed that the ICP
coating reduced the coefficient of friction of the wool fibres and reduced the
differential friction effect caused by fibre scales by coating and ‘smoothing’
the fibre surface (Wang et al., 2005). Tensile properties of the individual
fibres were largely unchanged. In an interesting off-shoot they found that the
thermal conductivity of polypyrrole-coated wool was increased compared
with that of uncoated wool (Wang et al., 2006).
Hirase  et al. (2004) examined reaction parameters for deposition of
polyaniline by the Milliken technique onto wool and other fibre types. In