10.4.1 Shape changing under stimuli
Many man-made and natural materials are naturally shape changing. A classic example is cotton, which expands when exposed to humidity and shrinks
Intelligent textiles and clothing 170
back when dried. Such behaviour has not been used for aesthetic effects because the changes, though physical, are in general not noticeable to the naked eye. In the case of shape changing textiles and clothing, a key requirement is to have a noticeable, visible shape changing effect, be it surface-bound, or form-bound. This consequently leads to two essential criteria: the efficacy of the material and the ability for the effect to be triggered as and when required.
In a woven, knitted or non-woven textile, the interactions between yarns and fibres contribute to the strength and mechanical properties of the fabric.
With blended SMMs, these interactions can also act as a resisting force, to constrict the effect of the SMM. Hence for example, Chan and Stylios (2003a) found that in the case of wrap-spun shape memory polymer yarns, the twist of conventional fibres around a core shape memory filament prevented the core from performing its shape-changing function. The situation was further exacerbated when the yarn was turned into fabric. It is therefore essential that the shape changing ‘force’ is higher than the resisting forces in the textile structure.
The second criterion is the ability to be stimulated to the right level when required, or under normal conditions of use. Temperature-sensitive shape memory alloys and polymers can be programmed to respond at specific temperatures, which falls within a broad range. Programming however requires that the maximum effect be achieved under the temperatures of use. Other stimuli can also be used, e.g., electrical currents, magnetic fields or UV lights. Wang et al. (2004) found that the shape change in nickel-titanium alloys largely depended on the magnitude of the electrical current. For SMPs, the challenges of making the polymer conducting current are still to be met, but work is already in progress (Yang et al., 2005). As for magnetic shape memory responses in alloys, they have been found to be highly variable, therefore posing a challenge for their applications (Heczko, 2005).
10.4.2 Problems associated with processability
Incorporating shape memory materials into textiles has been tried for over 15 years now, but many of the difficulties of turning the materials into textiles and clothing still exist. Alloys, due to their low strains, are difficult to weave and knit, because a certain amount of stretch in the material is required in order to interweave and inter-loop. With respect to aesthetic requirements, some novel woven or knitted structures require added flexibility for ease of production and suitability. Polymers, which have higher strains and lend themselves more to textile processing, are easier to turn into yarns, fabrics and clothing (Oohira, 1990; Terada, 1990). The textile assembly consisting of the shape memory materials must be able to be programmed, i.e., treated to train the material to remember a shape. If high temperatures
Engineering textile and clothing aesthetics 171 are required (e.g. for some alloys), it may not be possible to treat the whole textile assembly, so the alloy has to be programmed prior to its inclusion into the textile structure. This can pose problems associated with the creation of specific effects. Chan and Stylios (2003a) have successfully pre-programmed and used SMA in textile fabrics as well as spun and programmed SMP into various textile woven structures for the first time.
10.4.3 Durability of shape changing effects
Durability of effects during use includes requirements for resistance to abrasion caused by ageing, wear and tear, but also good performance in cyclic repetition of the shape memory effect over a period of time. It is well known that shape memory alloys such as Nitinol can undergo deformation and shape reversing many times, but much less is known in the area of SMPs. Hysteresis and fatigue in cyclic loading of SMMs have been studied to some extent for metal alloys (Eggeler et al., 2004). Among other parameters, temperature, microstructure and surface quality of the material inevitably affect their fatigue behaviour. Bhuniya et al. (2005) reported that the addition of small amounts of titanium to SMAs reduces their microstructural degradation caused by ageing. This is of interest for textile applications, where shape memory responses and reactions must be efficient for at least the estimated lifespan of the product in order to justify the costs.
10.4.4 Aesthetic degradation
Aside from performance, aesthetically, shape changing textiles should aim at having minimum visual and tactile degradation over time. This is particularly the case for products that will undergo harsher environmental conditions, e.g., interior textiles such as window blinds exposed to strong sunlight and environmental pollution. Nickel-titanium alloys are generally resistant to the environment and will not undergo corrosion. In the case of shape memory polymers, little is known about their long-term visual and tactile effects.
Many polymers often experience brittleness, stiffness and colour change with ageing, in particular if exposed to harsh environmental conditions. This could impact on the visual effects, handle, comfort and drape of the textile characteristics which are particularly important for clothing.
10.4.5 Requirements for fashion and clothing
The requirements for fashion and clothing are slightly higher than for non- wearable textiles, in particular if the material is to be worn next to the skin and has to be washed. Chan and Stylios (2003a) and Winchester and Stylios (2003) highlighted the relatively harsh handle of the alloy and bulkiness of
Intelligent textiles and clothing 172
the shape memory polymer filaments, but also showed that this is not in itself a deterrent to the use of such material in textiles, as they can be used sparsely, but with significant visual and functional impact. They have also shown how blending with conventional or specialised yarns like elastane, can improve handle, tactile and recovery properties. Alloys pose a particular challenge for designers as they will provide the most dramatic shape memory effects, but also adversely affect drape, handle, touch and comfort.