In general, heating food on Shabbat involves two Torah prohibitions: Turning the equipment on, which is an act of "burning" (heating up the electrical elements), and cooking.
The microwave, which in recent years has been widely used in the home, can help solve the halachic problems in a hospital, allowing small quantities of food to be heated when necessary. Most rabbis agree that use of a microwave does not involve the prohibition of "cooking" on Shabbat (since it does not use a flame or hot electrical elements but rather radiation). Therefore, a microwave is to be preferred for heating on Shabbat as compared to other devices such as an electric oven.
For people who are ill, The Zomet Institute provides microwave equipment that operates by a mechanism of gramma (indirect action), which can be used on Shabbat. It is turned on with a simple switch or a rotating dial, without any possibility of complex digital programs.
For more details (in Hebrew) about the halachic approach to cooking on Shabbat with a microwave oven, see: Rabbi Prof. Zeev Lev, "The Microwave in Halacha," Techumin, Volume 8, page 21.
For details and to place an order, contact The Zomet Institute.