A LED (Light Emitting Diode) is made of semiconductor crystals that light up when an electric current passes through them. Such lights exist in various colors (the most common are green and red), and they are most commonly used for signaling and control. LEDs use about one-tenth of the electric power as other types of bulbs with the same intensity, and they give off very little heat. This means that they have advantages for use on Shabbat, since many prominent rabbis (such as Rabbi S.Z. Auerbach) ruled that on Shabbat forming an electric circuit that does not cause a glow is not included in the prohibition of "burning" and is therefore only prohibited by a rabbinical decree ("molid - creating a new current").
Medical equipment often has a built-in control mechanism that cannot be separated from the main device. For example, a signal to mark the direction of movement (forward or backward) of a Shabbat scooter is a necessary factor in preventing accidents. The fact that such a scooter can be used on Shabbat by the handicapped is partially based on the fact that they are provided with LED signal lights, thus only violating a rabbinical decree.
Recently, high intensity LED lights have been developed which can be used as a light source (and not only for signaling), by creating a LED cluster. When this technology will be well developed, it will without a doubt find a respectable place in halachic-technological solutions in the fields of medicine and security (for example, a LED-headlight on a Shabbat scooter for traveling in the dark, a flashlight for looking into the ear, etc).