Cooling electronics boxes often requires extraction of high heat fluxes from closed boxes with many heat-producing components. The direct use of ventilation is sometimes limited by demands to use hermetic units or the need to extract heat from a specific place in a large and complicated system. A liquid system introduced inside of the electronic box can be used for this purpose. Unfortunately, metallic heat exchangers have a number of shortcomings in these applications, including significant weight as well as cost and space demands. Polymeric heat exchangers consisting of hollow fibers were proposed a decade ago and can be used as an alternative in such applications. Flexible polymeric hollow fiber heat exchangers were prepared and tested in liquid/air conditions. These heat exchangers use plastic capillaries with an outer diameter of 0.5-0.8 mm and a wall thickness of 10% of the outer diameter. They consist of flexible fibers and can be used in narrow slots and/or in shaped channels. These heat exchangers are effective even in natural convection applications because of their high heat transfer intensity on micro-objects. Experimentally obtained overall heat-transfer coefficients in water/air applications are up to 250 W/m2 K for forced convection and up to 80 W/m2 for natural convection. The use of plastic and non-corrosive materials is advantageous in electronic systems where high heat fluxes must be extracted safely from difficult to access spaces or from hermetically-sealed boxes.