Photodynamic therapy (PDT) is a new approach to treating cancers. Patients are injected with drugs that bind dyes to the rapidly dividing cells. Narrow-band lasers then excite the dyes, inducing a toxic reaction which kills the cells. It is used to treat esophageal cancer, Kaposi’s sarcoma, an AIDS related condition, and overgrowth of blood vessels in the eye (macular degeneration), which afflicts seven million people in North America alone.
The problem, explains Egyptian-born, Brown University Professor Nabil M. Lawandy, is that "it requires the use of expensive and temperamental lasers. Dye lasers for photodynamic therapy cost in the order of US$ 75,000." Lawandy, who obtained his doctorate in chemical physics when only 22, has developed a coating that can replace the expensive dye lasers.
A New Shine
For nearly 30 years lasers have required a precise mirror arrangement to amplify the light emitted and create a beam. Lawandy has developed a mixture of fluorescent molecules and light-scattering particles — one example is rhodamine mixed with titanium dioxide (found in ordinary white paint). The mixture can be suspended in a liquid, fixed in a transparent plastic, or incorporated into a gel. Most important, when hit by a light beam it emits very pure light. Lawandy’s patented technology is trademarked as LaserPaint™.
"The light emerging from LaserPaint™ material is in all respects laser light,” emphasises Lawandy. "The material exhibits all the salient features of lasers in terms of its energy input-output behaviour and its spectral purity.” In fact, its purity is even higher than some of the conventional dye lasers used in PDT.
Currently, PDT needs at least two lasers. One produces a secondary light which is conducted via an optical fibre to the diseased tissue. The light is scattered on the tissue by a special tip. This second laser is a complex collection of tubing and pumps to inject dye between precisely aligned mirrors.
Lawandy aims to bypass this costly second laser and conduct the light from the first laser directly to the tissue, using LaserPaint™ on the tip of the fibre to simultaneously convert and scatter the laser light at the right wavelength.
As a result of his discovery, he foresees the development of "low-cost, coin-size substitutes for the expensive dye lasers required for photodynamic therapy". During the last few months, he and his company, Spectra Science Corporation, have developed LaserPaint™ catheters that will produce the wavelengths that are required to activate important PDT photosensitiser drugs such as Photofrin.
LaserPaint™’s other potential applications in the electronic, pharmaceutical and defence industries have already been recognised for visual displays, improving the resolution of optical HIV tests, for industrial bar coding, even for locating downed aircraft and lost ships, or differentiating between friend and foe on the battlefield.
"It turns on like a laser. It powers up like a laser. It is narrow-line like a laser. But when it is off, it looks like paint," enthused a photonics journal.
Lawandy’s company is in the process of forming several business relationships to commercialise the technology. But he stresses: "Forwarding the LaserPaint™ discovery into the areas of medicine and health care is a personal goal of mine. Growing up as a child in Egypt, I was exposed to the poverty which afflicts many parts of the world and in particular its medical ramifications. Lasers, like many other medical accessories, can now be treated as low-cost disposables. LaserPaint™ will greatly lower the costs of photodynamic therapy, making it more accessible to patients in countries where economics often make lasers impractical."
Published in 1996