can be selected. The finest frequency selection may also be obtained through the use of an etalon. In practice, together with isotopic substitution, this means that a continuous comb of frequencies separated by around 1 cm−1 (30 GHz) can be used that extend from 880 to 1090 cm−1. Such "line-tuneable" carbon-dioxide lasers are principally of interest in research applications. The laser's output wavelength is affected by the particular isotopes contained in the carbon dioxide molecule, with heavier isotopes causing longer wavelength emission. Applications Industrial (cutting and welding) Because of the high power levels available (combined with reasonable cost for the laser), CO2 lasers are frequently used in industrial applications for cutting and welding, while lower power level lasers are used for engraving. It is also used in the additive manufacturing process of Selective laser sintering (SLS). Medical (soft-tissue surgery) Carbon-dioxide lasers have become useful in surgical procedures because water (which makes up most biological tissue) absorbs this frequency of light very well. Some examples of medical uses are laser surgery and skin resurfacing ("laser facelifts", which essentially consist of vaporizing the skin to promote collagen formation). CO2 lasers may be used to treat certain skin conditions such as hirsuties papillaris genitalis by removing bumps or podules. CO2 lasers can be used to remove vocal-fold lesions, such as vocal-fold cysts. Researchers in Israel are experimenting with using CO2 lasers to weld human tissue, as an alternative to traditional sutures. The 10.6 μm CO2 laser remains the best surgical laser for the soft tissue where both cutting and hemostasis are achieved photo-thermally (radiantly). CO2 lasers can be used in place of a scalpel for most procedures and are even used in places a scalpel would not be used, in delicate areas where mechanical trauma could damage the surgical site. CO2 lasers are the best suited for soft-tissue procedures in human and animal specialties, as compared to laser with other wavelengths. Advantages include less bleeding, shorter surgery time, less risk of infection, and less post-op swelling. Applications include gynecology, dentistry, oral and maxillofacial surgery, and many others. The CO2 laser at the 9.25–9.6 μm wavelength is sometimes used in dentistry for hard-tissue ablation. The hard-tissue is ablated at temperatures as high as 5,000 °C, producing bright thermal radiation. Other The common plastic poly (methyl methacrylate) (PMMA) absorbs IR light in the 2.8–25 μm wavelength band, so CO2 lasers have been used in recent