Laser micro- and nanofabrication of biomaterials
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Introduction Light amplification by stimulated emission of radiation (laser) has been used in medical applications, including manipulation of human tissues as well as processing of medical device materials, for more than 50 years. In 1960, Maiman first demonstrated the operation of what was then known as an optical maser (microwave amplification by stimulated emission of radiation); stimulated optical emission at a wavelength of 694 nm from chromium in corundum (ruby) was observed.1 Shortly thereafter, Javan et al. described discharge of near-infrared light from a helium-neon gaseous mixture; Johnson demonstrated continuous operation of a CaWO4:Nd3+ laser; Bennett et al. showed continuous oscillation from noble gas (e.g., Ne-O and Ar-O) mixtures; and Patel developed the carbon dioxide laser.2–5 Physicians and surgeons rapidly recognized the utility of lasers, since xenon arc-based photocoagulation of ophthalmic lesions had been developed in the previous decade.6–9 For example, Zaret et al. utilized a pulsed ruby laser to generate thermal injury in pigmented ocular tissues; retina and iris lesions were demonstrated in a rabbit model.9 Goldman and Rockwell noted the utility of lasers for delivering light to confined regions on the order of a few micrometers.10 They described use of the laser for microscale surgical procedures involving tissues and cells; for example, laser spectroscopy of skin was used to determine the presence of calcium. It should be noted that many early studies indicated the destructive nature of laser energy on biological tissues.6 For example, Klein et al. used a ruby laser to treat
melanoma lesions in mice and noted severe lesions in tissues that were located deeper than the laser irradiation sites.11 Laser light is obtained by providing energy to a lasing medium, which results in an increase in the number of atoms in an excited state.6 When atoms fall in energy in response to photon irradiation, photons with identical energies that are in phase with the first photon are produced. Important laser properties for both laser-biological tissue interaction and laser-biomedical material interaction include beam density, exposure time, power output, and wavelength.6 Many of the medical uses of lasers do not utilize the monochromatic nature of laser energy, but instead involve localized tissue heating and cutting.13 Advantages of laser-based tissue cutting approaches include minimal tissue trauma and touch-free tissue cutting.6 For example, continuous wave lasers (e.g., carbon dioxide lasers) and optimized delivery platforms enable accurate cutting and vaporization of biological tissues; in addition, carbon dioxide lasers provide hemostasis during tissue cutting.6 Takac et al. noted that laser surgery offers better visualization and is associated with minimal postoperative swelling.14 Uses of the carbon dioxide laser include removal of tissue, sealing of small blood vessels, sealing of lymphatic vessels, and sealing of nerve endings. Applications of the argon ion laser include blood vessel coagulation for
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