Sight and insight

If you thought contact lenses were just for correcting eyesight, think again. E&T delves into some of the ideas to wire them up to the latest technology.

You can tell a lot about a person from the eyes. In poetry and novels, lovers will gaze into each others' eyes and see into the depths of their loved ones' souls. In contrast, the eyes of fictional villains often reveal them to be cruel and cold.

It's not just romantics who can learn about people from their eyes. Scientists are also interested in far more than correcting vision problems.

For one thing, this tiny part of the body can reveal a surprising amount about the state of people's health. "The surface of the cornea is covered with live cells. These cells are in direct contact with the rest of body, so you can get a lot of information from them," explains Babak Parviz, an assistant professor at the University of Washington, USA.

This information might be on cholesterol, glucose or an infectious disease. And finding out does not necessarily require constant visits to eye specialists or staring into bulky pieces of equipment. There are a range of plans afoot to monitor what is going on in the eye in a way that is already part of many people's everyday routine - by putting in contact lenses.

Tracking diabetes

For Parviz and his colleagues, the idea of putting biochemical sensors on contact lenses is a way of demonstrating nanofabrication and microfabrication on unconventional substrates.

They have developed a nano--scale electrochemical sensor to monitor glucose, an indicator of diabetes. The sensor includes an enzyme that reacts with glucose and generates a by-product that can be measured electronically. The sensor would typically occupy an area of just a few square micrometres on the contact lens. In addition, the contact lens would include a radio chip and power supply - both built on the nanoscale. The group is doing lab tests on these sensors prior to putting them onto contact lenses.

The Washington team is not the only one interested in using contact lenses to monitor glucose levels in the eye. In fact, the idea has been around for many years. Back in 1979, for example, a paper in the journal Transactions of the American Society for Artificial Organs suggested using an oxygen-permeable contact lens to position lasers, detectors and a power supply on the eye in order to optically monitor glucose concentrations.

More recently, Chris Geddes and colleagues at the University of Maryland, USA looked into embedding fluorescent probes into disposable contact lenses. When their probes, based on boronic acid-containing fluorophores, bind with the very low concentrations of glucose in tears, the colour of their fluorescence changes. Patients could look in the mirror and see if their glucose levels are raised simply by comparing the colour of the spots on their contact lenses with a colour chart.

Geddes' group is developing the colour-spot contact lens to monitor cholesterol, sodium, potassium and lithium levels as well as body core temperature. These are also being extended to military applications where sensor spots could change colour in response to biological agents.

Another approach, which has been developed by Christopher Lowe at Cambridge University in the UK and is being taken further in a spin-out company Smart Holograms, also employs optical techniques. Lowe's solution, which can be used in contact lenses, exploits 'smart holograms'. These contain a thin polymer layer with a receptor designed to bind with a particular molecule, such as glucose. When this happens, the light reflected by the hologram is changed and that part of the contact lens changes colour.

Such approaches offer obvious benefits over today's more invasive blood tests. However, there are challenges. In glucose monitoring, for example, the sensors need to be 100 to 1,000 times more sensitive that the ones used for blood samples because the glucose concentrations in tear fluid are much lower than in the blood. Glucose also shows up in blood before it appears in tears.

Healthy eyes

As well as looking at overall health, sensors in contact lenses could help to check that the eye itself is well. This is something that Tingrui Pan and colleagues at UC Davis in the USA are interested in. They have turned their expertise in nanocomposite materials to the problem of glaucoma, where optic nerves are damaged by increased pressure in the eye. This is one of the leading causes of blindness around the world.

Current methods to detect glaucoma, such as the air puff, look at the eye for a very short period. Recent studies have questioned whether this is the best way to diagnose the disease.

The alternative being developed at UC Davis involves an electrical circuit, based on powdered silver, deposited onto a composite known as polydimethylsiloxane (PDMS). According to Pan, this material combines the benefits of both polymers and metals: it is flexible, soft and inexpensive - like polymers, but also thermally and electrically conductive - like metals. It detects pressure by exploiting the piezoresistive effect where a material's electrical properties change accordingly.

There is an added benefit of the material, too: silver has anti-microbial properties. This has obvious advantages for something intended to be used in the eye. Similarly, Pan recognises that, in practice, such sensors need to be disposable, so that there is no cross-contamination between patients.

For this reason, the sensor lenses need to be cheap and easy to make. The researchers have developed a way to produce thousands of these sensors at the same time. "The electronics are added by photolithography onto the same silicone substrate as soft contact lenses. We can mould them into shape at the end," explains Pan. "We wanted to show that we could make a nanocomposite that was as easy to fabricate as a computer chip. It would probably cost a few dollars per contact lens with the packaging and sterilisation that would be required, but the technology to make the contact lenses themselves is very cheap - just a few cents," he points out.

The applications go beyond glaucoma: this is a way to comfortably apply electricity to the eye. "It means you could do other measurements such as monitoring the flow of ocular fluid or even therapeutic applications such as drug delivery," says Pan.

Like many of the other smart contact lens projects, Pan and colleagues aim for the components on the lens to communicate wirelessly, but this is still a future goal. One challenge that has already been resolved is making the contact lenses transparent, although Pan says that this has yet to be made public.

Pan and colleagues hope to get approval to test these lenses on humans and this is where Pan expects the biggest challenges to arise. "It is much easier at the research bench because you don't have to worry about issues such as comfort," he observes.

Personal displays

Health applications are not the only ideas being investigated for smart contact lenses. Over the past year, technology enthusiasts on the Internet have been excitedly blogging about the research of Babak Parviz and colleagues at the University of Washington. These researchers have demonstrated a contact lens that includes an electronic circuit and red light-emitting diodes (LEDs) that are just one-third of a millimetre across. The components are constructed away from the delicate organic materials used for contact lenses and then reconstructed on the lens using self-assembly.

Their aim is to make tiny displays that people would wear in their eyes. The idea is that people might eventually use their mobile phones without having to look at the displays on their handsets. It could also enable computer games fans to immerse themselves in play as never before. And as these displays move into the realms of augmented and virtual reality, it's easy to imagine many more applications for business, personal and military use.

Of course, such applications require very sophisticated displays. Parviz and colleagues are initially aiming for more modest goals - such as text in one colour. They plan to increase the pixel numbers over time though, in a similar way to that of conventional displays.

Some of the challenges in building these devices are similar to those of conventional displays but there are issues that are more important and difficult for contact-lens ones. These include powering them, making sure that the images are in focus and systems integration.

Another task is to ensure that the wearers can still see. According to Parviz, the system would be semi-transparent. "We are trying to make the pixel units as small as possible to avoid blocking too much light. It would be like the effect of wearing sunglasses," he explains.

Also, these devices must not damage the eyes when put into them. This means that all the materials must be biocompatible. Parviz and colleagues have tested their contact lenses on rabbits for up to 20 minutes, and the animals showed no adverse effects. Light intensity and heat have to be watched carefully too. The surface of the eye cannot tolerate much heat.

Perhaps this is an area where the display and sensor possibilities of smart contact lenses could be combined. Maybe, the ideal lenses would be able to display your emails and Web pages while at the same time making sure that your eyes stay healthy?

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