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Microfluidics, Could Soon Cool Your Clothes

THE idea came to Ralph Liedert while he was sweltering in the Californian sunshine, having been standing with his daughter for over an hour in a queue for a ride at Disneyland. What, he thought, if his T-shirt had a cooling system he could switch on, at the tap of a smartphone app, when he needed it. No doubt similar thoughts have crossed the minds of many a parent in such circumstances. They, though, did not have the means to make their dream reality. Mr Liedert does, for he works at the VTT Technical Research Centre of Finland, as one of a team there studying the burgeoning field of microfluidics.

Cooling vests already exist (they are sometimes used by racing drivers, motorcyclists and people such as furnace operators, who work in hot conditions). But the tubes through which the cooling water is being pumped, and the vests’ need to be connected to external units that chill this water, make them bulky and unwieldy. Mr Liedert thought VTT’s microfluidics department could do things better.

As its name suggests, microfluidics is the art of building devices that handle tiny amounts of liquid. Inkjet-printer cartridges are a familiar example. Less familiar, but also important, are “labs-on-a-chip”. These are tiny analytical devices that transport fluids such as blood through channels half a millimetre or less in diameter, in order to carry them into chambers that hold analytical reagents. Sensors, either in the chip itself or in a machine into which the chip is inserted, then detect the resulting reactions and provide an instant analysis of a sample. Designing labs-on-a-chip is the VTT microfluidics department’s day job. One of its chips, for example, can tell whether water is contaminated with the bacteria that cause Legionnaires’ disease.

The department’s biggest contribution to the field, though, is to have developed a way of printing microfluidic channels onto large rolls of thin, flexible plastic, which can be cut up into individual devices. This process, called hot embossing, is faster and cheaper than conventional ways of making labs-on-chips, such as photolithography of the sort employed to manufacture computer chips. It works by passing the plastic between two heated rollers, one of which contains raised outlines of the required channels. As the rollers squeeze the plastic they create a pattern of channels recessed into one surface. A second plastic film is then fused over the top as a cover. This process might, thought Mr Liedert, be suitable for printing a microfluidic fabric that was thin enough and pleasant enough to wear as a cooling vest.

The group’s first prototype demonstrated that such a material could indeed be made and used to circulate chilled water. The initial idea was to put the material into a jacket, but the team found that it worked much better when in direct contact with the skin. They are therefore making a second prototype which covers the wearer’s neck and shoulders, and can be clipped inside a sports shirt.

They are also looking at ways the water being circulated through the microchannels might be cooled. They have identified two. One uses a small heat-exchanger, the details of which they are keeping secret at this stage. The other employs evaporation. It thus works in the same way that heat from circulating blood is removed by the evaporation of sweat. (The vest also permits such natural sweating, via small holes in the fabric.)

Whichever cooling system prevails, the electronics needed to power and control it would be shrunk into a small package contained on the back of the vest. This could be operated manually or, as Mr Liedert originally envisaged in his Californian queue, by a wireless link to a smartphone. Moreover, what can cool down can also, if run in reverse, warm up. In Finland, where winter temperatures fall as far as -50°C, that might be the technology’s killer app.

Source: Economist