The invention pertains to flexible devices used for zero-heat-flux, deep tissue temperature measurement, especially to disposable temperature measurement devices. Such a device is constituted of a flexible substrate. An electrical circuit is disposed on a side of the substrate. The electrical circuit includes first and second thermal sensors disposed, respectively, on first and second substrate layers. A heater trace is disposed on the first substrate layer with the first thermal sensor. The first and second substrate layers are separated by a flexible layer of insulation disposed between the first and second substrate layers. The heater trace defines a heater with a central portion that operates with a first power density and a peripheral portion around the central portion that operates with a second power density greater than the first power density.