Textiles rely on air trapped between the fibres for thermal insulation. The volume of air, and consequently the thermal insulation properties of fabrics, depends on thickness and density - hence we use lightweight cotton in summer and heavy wools in winter. But the weather and degrees of activity change during the day. The fashion industry has been looking for some time for a lightweight fabric that can be worn comfortably in all seasons. Outlast may be one solution.
Outlast was recently demonstrated at the Techtextil Fair in Frankfurt. The material uses a Phase Change Material (PCM), which responds to changes in body temperature and can either store or release heat. Nasa did some early research into these chemicals, listing more than 500 in a handbook published in 1971.
The principle that materials absorb or give out heat when they change is known as "latent heat". Water is an example of a PCM: when water turns to ice at 0C it gives off heat.
The type of PCM chosen depends on the intended use. For insulation purposes, any compound that changes from liquid to solid might be used. In theory, this could include water, but the low temperature at which it changes phase makes this impractical. Most of the PCMs used for insulation change phase at between 32 and 38C, allowing them to absorb and retain heat as it is released from the body. Outlast incorporates different PCMs according to the uses to which the garments will be put.
The PCM is encapsulated as particles at a microscopic level before being incorporated into the fabric, either as a coating on the finished fabric or through the fibres using a wet-spinning process. One application being looked at is acrylic hosiery, where little insulation has previously been available. Thick wool tights or white legs may soon be a thing of the past if these stockings can provide warmth in winter and coolness in summer.
Curiously, this technique of microencapsulation was invented in the mid- Fifties by the National Cash Register Company in Ohio, originally for use in carbonless paper. More recently, it has been used in the textile industry to create perfumed fabrics for clothing and also car upholstery.
There has already been considerable interest in "smart" clothing from manufacturers in Europe and America that see potential uses in sports and casual wear, as well as in the protective clothing industry for workers operating in refrigerated conditions. The American Tempo-Shain Corporation is planning Outlast's first application in a range of footwear for sports and outdoor pursuits. No launch date has been set for the products so it is unlikely they will be in the shops this winter.
In architecture, some designers have started to use membranes - tented structures - often for roofing systems, as seen in the canopy over the stands at Lord's cricket ground and also part of the new Inland Revenue building at Nottingham. Architectural membranes offer low thermal insulation in comparison with conventionally insulated buildings. There are ways around this, such as the use of a double-layer membrane, but this eliminates some of the natural light that is one of the main reasons for using a membrane in the first place. It is very early days for testing the use of Outlast in membrane systems, but William Taylor, of Michael Hopkins & Partners, is enthusiastic about the possibilities.
"I think some of the more immediate uses for Outlast will be in wall and roof insulation, where space-saving would be useful," Mr Taylor says. "Of course, there would need to be a lot of research done before it could be ready to use in a membrane expected to withstand all weathers for 30 years."
It may take be some time before we are woken in the morning by the sun shining through translucent rooftops. But by next year, manufacturers predict that the fabric will be used in much of the sports and casual wear in our high-street shops.