Backed membranes cast onto a nonporous, polyester film offer two advantages. First, these membranes are easier to handle. The film provides a high level of mechanical strength. Secondly, the film acts as a barrier to chemical contamination from adhesives that are used to mount the membrane on cards during test strip manufacture.
The most common backings are 2-mil and 4-mil polyester films. Membranes on these backings perform similarly. Please be aware that film thickness is dictated by constraints of the test strip manufacturing process and the tolerances of the intended space in the housing. Unbacked membranes, in comparison, are extremely fragile and can be very difficult to work with during test strip assembly.
