For many physical, chemical and biological measurements, temperature is a crucial parameter to control. In particular, the recent development of microreactors and chip-based technologies requires integrated thermostatic systems. However, the requirements of disposability and visual inspection of a device under a microscope cannot accommodate equipment such as external heaters. By exploiting a silver-filled epoxy that can be injected and solidified in a microfluidic chip, we demonstrate a simple and inexpensive design of a conductive path, which allows heating by the Joule effect of both sides of a microchannel. In addition to permitting the maintenance of a constant temperature along the channel walls, our method can control the temperature gradient across the channel, thus enabling non-equilibrium studies in a microfluidic geometry.