Small Wonders

Imagine minute particles that can seek out tumors and destroy them in a burst of heat, or tiny machines that circulate throughout your body on the lookout for disease. These are just some of the possibilities when nanotechnology meets medicine.

How Small Can You Go?

A nanometer (nm) is one billionth of a meter (10^-9 m). To give you a sense of perspective, a single atom is 0.1 to 0.2 nm wide, a strand of DNA is two nm wide, a red blood cell has a diameter of 7,000 nm, a single human hair is 80,000 nm wide, and a pinhead is a whopping one million nm wide.

Nanotechnology uses materials that are 0.1 to 100 nm in size. These nanomaterials are not only minute, but can also have special physical and chemical properties. Common nanomaterials include buckyballs, carbon nanotubes, and metal oxide nanoparticles. Buckyballs are hollow spheres made from 60 or more carbon atoms, and their hollow center and unique properties make them attractive candidates for drug delivery and other applications. Carbon nanotubes are cylindrical rods that are more durable than diamond, but flexible as a human hair. Metal oxide nanoparticles consist of small clusters of oxygen and other atoms.

Disease Diagnosis and Treatment

Nanomaterials have already made their way into a number of consumer goods. Sunscreens rely on nanoparticles of titanium dioxide or zinc oxide to absorb UV rays. Silver nanoparticles have antibacterial properties, and have been used in everything from clothing to kitchenware. Carbon nanotubes are used as backlights for LCD screens, and can make sports equipment like baseball bats and tennis rackets lightweight yet strong. In medicine, nanotechnology has the potential to improve diagnosis and treatment for a wide range of diseases.

For example, nanotechnology offers new hope for the early detection of Alzheimer's disease. One of the earliest signs of Alzheimer's is an increase in the amount of amyloid-beta-derived diffusible ligands (ADDLs), tiny proteins found in cerebrospinal fluid. In a process called bio-barcoding, researchers used a combination of magnetic and gold nanoparticles to pick out ADDLs from the soup of proteins present in cerebrospinal fluid, creating a screening procedure that is thousands of times more sensitive than conventional tests.

Nanotechnology also promises new and improved ways to treat diseases. One example is the creation of drug-delivering contact lenses, which have nanosized pockets that can release glaucoma medication or other drugs in precisely controlled doses. Another ingenious device is an implantable capsule for diabetics, which produces insulin whenever the body requires it. The capsule, which has shown great promise in diabetic rats, consists of animal pancreatic cells housed in a silicon box that's one-tenth of a millimeter wide. The box is dotted with holes 20 nm wide, which are large enough to allow glucose to enter and cause the pancreatic cells to produce insulin, but small enough to prevent the body's immune cells from entering and rejecting the animal cells.

Further down the road, we may see nanomachines that live inside our blood and cells, constantly monitoring our health and dispensing treatment as needed. According to an article from the October 6, 2001 issue of New Scientist, researchers have already created some components that could power these nanomachines, such as a motor that runs on ATP and a fuel cell that produces electricity from glucose and oxygen. On the treatment side, researchers have developed a minute lock that could only be opened by a certain DNA sequence, which could one day form part of a smart pill box that dispenses the appropriate medication whenever it encounters certain mutations. Swedish scientists have created a microrobotic arm that can move objects the size of a single cell, and may one day be used in diagnosis or surgery. Some researchers want to use nanotechnology to switch on genes within specific cells, and instruct the cells to produce whatever proteins, hormones, or antibodies that the body might need.

Nanotechnology Versus Cancer

Much nanotechnology research has focused on cancer diagnosis and treatment. Want to know if a tumor is about to spread? Try poking it with a very, very small finger. Previous research showed that cancer cells were softer than normal cells. Therefore, researchers put cell samples under a powerful atomic force microscope, and poked individual cells with a probe only 400 nm wide. The nanoprobe could easily distinguish cancerous and healthy cells that were nearly identical in appearance, and found that metastatic cancer cells were more than 70% softer than healthy cells.

Scientists from Rice University in Houston, Texas, have developed gold-coated silica nanospheres that can seek out and destroy tumors. Since blood vessels around tumors are leakier than those in healthy tissue, more nanospheres will leak out of the bloodstream and accumulate around cancer sites. Shining low-intensity infrared light makes the nanospheres light up and reveal the cancer sites, and shining a near-infrared laser causes the nanospheres to heat up and kill the cancer cells. This noninvasive procedure can remove tumors while preserving healthy tissue, and has the potential to reveal hidden tumors and greatly speed up the cancer treatment process. Nanospectra Biosciences has commercialized the protocol as the AuroLase™ Cancer Therapy, and the company has applied to the FDA to begin human trials.

Nanoparticles can also deliver anticancer drugs to tumors while minimizing damage to healthy tissues. In one system, researchers coated nanoparticles with small bits of nucleic acid that only bind to cancer cells. After entering the cell and delivering the anticancer drug, the nanoparticles signal their success by turning fluorescent.

Proceeding with Caution

Safety testing procedures for nanomaterials are still under development, and there are three main challenges to overcome. First, a standard method for naming and identifying nanomaterials must be established so that researchers can be certain that they're testing and comparing identical materials. Second, because unlike standard chemicals the behavior of a nanomaterial is determined not only by its chemical composition and concentration, but also its particle size, crystal structure, and interactions with other particles, any toxicity tests will have to take all of those factors into account. Finally, nanomaterials need to be followed throughout their lifecycles to ensure that they never reach harmful concentrations in the human body, other organisms, or the environment.

Thus, more testing will be required to ensure the safety of nanomaterials before they can be widely used in medical applications. A few red flags have already been raised. Certain types of buckyballs have shown moderate toxicity, killing water fleas and causing brain damage in fish. Both buckyballs and carbon nanotubes have caused cell death in the lab, and inhaling carbon nanotubes can lead to lung damage in mice. However, we're already gaining some insights into which hazards to watch out for and how to modify nanomaterials to make them safer. For example, researchers were able to make buckyballs harmless to cells by attaching hydroxyl groups to the carbon spheres. The toxicity of buckyballs could also be deliberately increased, making them more effective drug-delivery vehicles against cancer or bacteria.

But despite these cautions, it is clear that in our ongoing battle against disease, nanomaterials are tiny wonders that hold great promise for the future.

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