By Jo Twist
BBC News Online science and technology staff
A revolutionary kind of glass that needs little cleaning could mean soap and chamois are binned for good.
Normal glass (left) and Activ (right) makes for clear views
The Pilkington Activ glass has a special nano-scale - extremely thin - coating of microcrystalline titanium oxide which reacts to daylight.
This reaction breaks down filth on the glass, with no need for detergent. When water hits it, a hydrophilic effect is created, so water and dirt slide off.
It is one of four finalists for the eminent MacRobert engineering award.
The prize is given out by the UK's Royal Academy of Engineering for technological and engineering innovation.
"Pilkington Activ is based on titanium dioxide, which is used in foodstuffs, toothpastes, and sun cream," explained Dr Kevin Sanderson, one of the team members who developed Activ at Pilkington's technical research centre.
"But usually it is a white powder which is not ideal for glass because you can't see though it.
"So we used it in a thin film form - 15 nanometres thick - so that it appears as close to normal glass as it can."
Although not strictly nanotechnology, the special coating and the chemical reactions happen at the nano-scale (one thousand millionth of a metre).
The titanium dioxide coating on the glass had two properties that made it special, said Dr Sanderson.
It absorbs sunlight - ultraviolet radiation - which causes what is called a photocatalytic effect.
Through this process, the coating reacts with light which then breaks down organic dirt.
Secondly, the coating makes the surface of the glass hydrophilic. This means that when water hits the surface of the coated glass the water droplets attract each other forming a sheet, rather than individual droplets.
"It destroys the organic dirt, and destroys it naturally, as well as reducing the 'glue' that other dirt can stick to on the surface," said Dr Sanderson.
With the photocatalytic effect of the coating constantly working in the background, the glass dries cleanly, an effect he and his team like to call the "invisible squeegee".
The environmentally friendly innovation is a result of a long process of research and development, starting from the early 1990s, into "thin film" technologies.
"When we realised we could get these properties with it, that is when it really drove through," said Dr Sanderson.
"It took five to seven years to take that from lab samples to putting it on the coating production line."
Activ glass clearly had several benefits, argued Dr Sanderson. Although he promises it will not put window cleaners out of business, it will cut down on the need to clean windows so often, and it will reduce the need for noxious detergents.
"Each time harsh chemicals are used, they are washed off into ground, which produces contamination. What we say here is that you can just spray water on top."
There was an additional safety benefit too, Dr Sanderson said.
"Each year people are killed using ladders to clean windows. What we are saying is that that should be reduced because you can use a hose pipe to reach the first storey."
Although it is slightly more expensive than conventional glass, adding about 15 to 20% to the cost of installation, the technology is already being used in sheltered housing windows where easy cleaning is important.
But Dr Sanderson said it could potentially be used to break down E. coli or other bacterial infections on surfaces.
It could also be used to naturally decompose pollutants in the air, like formaldehyde, and ground-level ozone.
The other finalists for the award include switchable 2D-3D displays, an eco-friendly fuel injection system, and software that connects multiple system computer platforms.
The winner of the £50,000 prize money, handed over by HRH the Duke of Edinburgh, is announced on 10 June.
1 - Dirt, glass, Nano-scale Activ coating containing microcrystalline titanium dioxide
2 - Sun shines on window. The UV rays trigger a chemical reaction in Activ coating, called a photocatalytic process, which breaks down dirt
3 - When water hits glass, a hydrophilic effect is created. Water spreads evenly over the surface, instead of droplets, so runs off and takes dirt with it