An underlay intended to produce heat through chemical reaction typically used to warm and/or maintain the body temperature of neonates or infants. It is typically designed with a soft outer casing (e.g., a soft plastic) that contains a chemically-reactive gel activated by the user (e.g., by breaking its inner enclosure) to provide a heat of approximately normal body temperature (e.g., 38? Celsius) for a limited period. This device is typically used to maintain the body temperature of infant patients during transportation and may be x-ray translucent and magnetic resonance imaging (MRI) compatible. This is a single-use device.