By manipulating the way light passes through tissue, a biomedical engineering research team at The University of Texas at Austin has taken a step toward what was once the realm of science fiction. The researchers have made tiny bits of flesh transparent for brief periods of time in the lab.
In effect, the engineers temporarily can create a window of tissue, allowing doctors to see up to five times deeper than they can at present. Although it has not yet been tested on human skin, the engineers say the process could have wide applications for medical diagnosis and new therapies.
Using injections of substances such as glycerol, the UT Austin researchers have succeeded in making small areas of rat or hamster skin nearly transparent for a period of 20 minutes or more. Glycerol is hygroscopic alcohol, which means it pulls water out of tissue. Glycerol often is used in cosmetics and for medical applications.
Dr. Ashley J. Welch, a professor of biomedical engineering and chief investigator on the project, said: "When we injected glycerol into the skin of a hamster, we could actually see a blood vessel which had not been visible." Welch is the Marion E. Foreman Centennial Professor in Engineering.
If the procedure works on human tissue, it has the potential to lead to improvements in a variety of diagnostic and therapeutic medical procedures ranging from laser surgery to the treatment of tumors, Welch said. At present, therapeutic procedures that depend on accurately focusing lasers on a particular area -- such as retinal surgery and the accurate targeting of tumors and other laser surgeries -- are limited by the fact that a beam of light cannot pass directly through overlying tissue, Welch explained.
The new process has to do with refraction -- or the fact that light is scattered as it moves through skin.
Welch explained that the optical process could be compared to searching for an object obscured by a dense fog. When a searchlight is directed at the fog, the light is scattered in all directions, creating a glow about the remnants of the light beam.
The degree of light scattering depends upon the size of the small water droplets and their index of refraction relative to the index of refraction of the surrounding air, Welch said.
Just as each water droplet in the fog scatters light, so small components of tissue also scatter light. This can be demonstrated by placing a penlight against the thin tissue between the fingers.
"This scattering prevents us from seeing blood vessels near the surface of the skin, because none of the light passes directly through the skin to reflect from the blood vessel back to our eyes. It is all multiply scattered, destroying the image," Welch explained.
Light travels straight through transparent objects because they have similar or identical degrees of refraction throughout, Welch explained. Different components in skin and tissue scatter or refract light differently, and that is why skin and tissue are opaque.
Glycerol briefly can change the degree of scattering through small areas of tissue by two different mechanisms. First, glycerol shrinks tissue because of an outflow of water. When glycerol is added to cells, the imbalance in pressure causes water to flow from the cell. Glycerol enters at a slower rate. Over time, the process reverses and water begins to enter the cell again. Researchers believe the shrinkage may bring certain tissue components, such as collagen fibers, closer together, modifying the way light is scattered.
Secondly, it changes the optical composition of tissue because the glycerol that replaces the water within the tissue has an index of refraction that matches tissue components such as collagen.
The discovery was made by two graduate students at UT Austin's College of Engineering. They are Dr. Eric Chan, currently employed by Indigo Medical in Cleveland, Ohio, and Dr. Jennifer Barton, currently an assistant professor of biomedical engineering at the University of Arizona.
Doctoral candidate Gracie Vargas is continuing the research. UT Austin has applied for a patent of the process. This peer-reviewed research is supported by grants from the National Science Foundation, the Texas Higher Education Coordinating Board Advance Research Program, and the Albert W. and Clemmie A. Caster Foundation.
Researchers said the next step would be to study various other agents, which may have similar effects on tissue, looking for the safest agents to use. The effects of glycerol and other agents will be studied on different types of tissue. Scientists also wish to learn more details of the processes by which tissue can become optically clear as well as medical applications of these processes.
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