Most TPV systems include additional components such as concentrators, filters and reflectors. Hair dryers are available with attachments, such as diffusers, airflow concentrators, and comb nozzles. Host computers then connect as single-attached devices to the routers or concentrators. There also are other medical uses for oxygen concentrators, including COPD and other respiratory diseases. Much current interest involves designing invisibility cloaks, event cloaks, field concentrators, or beam-bending waveguides. Medical oxygen concentrators are used in hospitals or at home to produce oxygen for patients. In contrast, most complex eyes use refractive ray concentrators (lenses) and a concave retina. The FAA has approved the use of portable oxygen concentrators on commercial airlines. Other airlines allow passengers to carry on approved portable concentrators. Usually, "demand" or pulse-flow oxygen concentrators are not used by patients while they sleep. Since the early 2000s, a number of companies have produced portable oxygen concentrators. Medical oxygen concentrators were developed in the late 1970s. Although simple, these solar concentrators are quite far from the theoretical maximum concentration. Clients connect to these communication concentrators, which act as proxies for the clients. As of November 2006, most airlines permit the use of oxygen concentrators.