The present disclosure describes devices useful for microscale fluid purification, separation, and synthesis devices. Generally, such devices comprise a fluid membrane that separates two or more fluids flowing through plural microchannels operatively associated with the membrane. Often, the membrane is a semipermeable membrane, such as might be used with a filtration device, such as a dialyzer. Devices of the present invention can be combined with other microscale devices to make systems. For example, the devices may be coupled with one or more microchemical microfactories, one or more micromixers, one or more microheaters; etc. Particular materials had to be developed for use with certain embodiments of the device disclosed herein. For example, a new composite material was made comprising nanocrystalline cellulose filler and a polysulfone polymeric material. A dialyzer comprising the composite membrane also is disclosed.