PRINTED FLEXIBLE HEATERS

Printed flexible heaters are constructed using several elements including positive-temperature-coefficient (PTC) materials for delivering heat. Such constructions can be designed to operate in a steady-state or limiting modes. In the latter mode the final temperature is bounded by the limiting resistance of the PTC material. Temperatures up to 80°C can be achieved by allowing the heater to draw a small amount of current at a fixed potential. At the start of the heating the current draw is typically a few microamperes, but as the heater approaches equilibrium, the current requirement is diminished to a level that is necessary to maintain the limiting temperature.

Critical parameters for heater construction include physical and chemical characteristics of the electrodes and the applied voltage. PTC material can be deposited using standard screen-printing techniques in a wide range of thickness. As the deposit thickness increases, its resistance decreases and the observed temperature decreases. Electrode spacing as small as 250 microns (0.010") can be achieved. Typical spacings are in the range of 0.75-1.5 mm. Heating temperatures at a fixed potential increase as the electrode spacing decreases. The temperature response as function of applied potential is always positive. Applied voltages are usually in the range of 3-12 VDC.

Substrates are typically PET in the range of 0.125-0.356 mm. More flexible materials such as Tyvex® can be used for applications where extreme flexibility or conformability is required.