In 1980, Heinlein recalled in Expanded Universe that: "I designed the waterbed during years as a bed patient in the middle thirties; a pump to control water level, side supports to permit one to float rather than simply lying on a not very soft water filled mattress. Thermostatic control of temperature, safety interfaces to avoid all possibility of electric shock, waterproof box to make a leak no more important than a leaky hot water bottle rather than a domestic disaster, calculation of floor loads (important!), internal rubber mattress and lighting, reading, and eating arrangements—an attempt to design the perfect hospital bed by one who had spent too damn much time in hospital beds." Although Heinlein made no attempt to build his invention, modern waterbeds would be developed and they appeared on the market in 1971.
Robert Heinlein, Stranger in a Strange Land. New York: Putnam, 1961