Think ``tiny medicine,'' and you probably think Fantastic Voyage, the 1966 movie (and Isaac Asimov novel) about a minuscule medical crew, pictured above, traveling through a patient's circulatory system in a submarine. In fact, some nanomedicine experts foresee a day when invisibly small robots will cruise the body looking for signs of disease - albeit without the added attraction of a neoprene-clad Raquel Welch.

``Nanobots'' remain imaginary for now, but a number of other futuristic nanodevices are already proving their potential in animal and human experiments. More than 60 drugs and drug delivery systems based on nanotechnology, and more than 90 medical devices or diagnostic tests, are already being tested, according to NanoBiotech News, a weekly newsletter. These examples, drawn from recent scientific publications, offer a glimpse of just how small the field of medicine is getting.

Quantum dot diagnostics

Quantum dots, also known as ``qdots,'' are bits of material - silicon, for example - so tiny they are in some cases just a few atoms across. Illuminated by ultraviolet light, they glow very brightly with a specific hue that depends on their size: qdots with diameters of about two nanometers (billionths of a meter) glow bright green, for example; five-nanometer dots glow brilliant red.

Scientists are already using quantum dots as research tools to help them understand how proteins, DNA and other biological molecules catch rides on the various transportation systems inside cells. First they coat some qdots with a material that makes the dots attach specifically to the molecule they want to track, then they inject those coated dots into cells growing in laboratory dishes.

Once the dots grab their targets, researchers simply watch the trails of colored light to see where they go.

Qdots shine brighter and longer than conventional dyes used to illuminate the inner workings of cells. And by coating different size qdots so each attaches to a different kind of molecule, scientists can track the movements of many substances in a cell at once by following the various color trails.

Now scientists are developing qdots not just for basic research but to diagnose diseases.

There are scores of proteins and other substances in the body that are early indicators of disease but that are difficult to detect with existing technologies. While qdots and other nanomaterials have not been proved safe for use in the body, they are clearly capable of spotting diseases in blood or tissue specimens. Qdots that bind to proteins unique to cancer cells, for example, can literally bring tumors to light.

Nursing neurons with nanogels

Injured nerves do not regenerate easily, and the little healing that does occur is often inhibited by scar tissue formation. Samuel Stupp and John Kessler at Northwestern University in Chicago are using nanotechnology to overcome those hurdles.

They made tiny rod-like molecules called amphiphiles, each of which is capped by a cluster of amino acids known to spur the growth of neurons and prevent scar tissue formation. The molecules are designed to remain suspended in a few drops of liquid until they come in contact with living cells. At that point they spontaneously arrange themselves like spokes in a wheel, and then further assemble into spaghetti-like nanofibers a few thousandths the thickness of a human hair. The nerve-healing amino acids end up arranged nicely on the fibers' surface.

The nanofibers turn the liquid in which they are suspended into a therapeutic gel, which in experiments with cultured cells spurred neuron growth and inhibited scar formation. Moreover, rats and mice that got injections of the liquid a day after spinal cord injuries were more likely to recover the ability to walk than untreated animals. The team also made self-assembling nanofibers bearing amino acids involved in bone healing, which sped the recovery of rodents with severe bone injuries.

Blood test in a nanotube

Among the more curious creations of nanotechnology are carbon nanotubes - hollow tubes about 1/100,000th the diameter of a human hair, made of interwoven carbon atoms. Because the laws of physics get strange at those scales, they display bizarre electrical and optical properties.

Michael Strano of the University of Illinois and his colleagues are among many scientists developing biomedical applications for nanotubes. They coated the tubes with an enzyme that, in the presence of sugar, makes hydrogen peroxide, which in turn triggers a flow of electrons into the tiny tubes. The electrons make the tubes glow when they are exposed to infrared light - a reaction unique to nanotubes.

Thousands of these nanotubes can be packed into a hair-like capsule the size of a splinter, which can be painlessly implanted under the skin. The result: a quick and easy way to measure blood sugar. Simply shine infrared light on the tiny implant and then measure, with a handheld device, the intensity of the glow. Strano envisions coated nanotubes being used as implantable biosensors to get continuous readings of a number of medically important measures, such as cholesterol or hormone levels, without ever having to get a drop of blood from the body.

Frying tumors

One of the best ways to destroy a tumor is to burn it. But that is difficult to do without frying nearby healthy tissue, especially when the tumor is deep in the body. Enter ``photo-thermal nano-shells,'' little creations of Jennifer West at Rice University in Houston.

The shells are gold-coated spheres about 130 nanometers in diameter, which means about 15,000 of them could line up across the head of a pin. Metallic spheres of that size are very good at absorbing ``near infrared'' light - a variant of the kind of light emitted by television remote controls - that can harmlessly penetrate several centimeters into the body.

When West and her colleagues infused her nanoshells into the bloodstreams of mice with cancer, the spheres traveled through the circulatory system and then concentrated around the animals' tumors - a fortuitous result of the fact that blood vessels tend to be leaky near tumors. Then the team exposed the animals to the near infrared light. The nanospheres quickly absorbed that energy and heated up to about 50 degrees Celsius, cooking the tumors but leaving surrounding tissues unharmed. Months later, the animals were still cancer-free.

``We can easily get them even hotter than that,'' West said of the spheres, which later get eliminated by the immune system.

With nearly US$10 billion (HK$78 million) slated for investment in nanotech research this year, nanomedicine is sure to get hotter as well.