Research in miniaturization of microfluidic devices and total analysis systems for chemical and biological applications has been active for more than a decade. Sample size and processing time have decreased dramatically, while sensitivity has increased, along with the ability to run multiple tests in parallel. However, sensitivity depends in most cases on the purification or enrichment of the sample prior to loading onto a device. Filtration is performed either prior to sample loading or through membranes adhered to the substrate. Currently, these membranes cannot be patterned to micrometer resolution and the adhesion process may be incompatible with the fabrication and/or may introduce contaminants to the process. We have developed a process to incorporate filtration onto microfluidic devices that is compatible with microfabrication methods and is suitable for biological applications. We cast cellulose acetate membranes directly onto silicon wafers without the use of adhesives and studied the filtration properties of these membranes using compounds with molecular weights ranging from 100 to 900. We also have varied the casting conditions and studied the effects of these variations on the membrane's rejection characteristics. Cellulose acetate membranes, cast directly onto silicon wafers, adhered well to the substrate and had high structural integrity. We have developed membranes with four different molecular weight cut-offs; 300, 350, 600, and 700Da. Parameters such as solubility and charge were also investigated for their contribution to the rejection characteristics.