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. The fluids can both be liquids, gases, or a liquid and a gas, such as may be used for gas absorption into a liquid. 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. Examples of devices made according to the present invention included an oxygenator, a dialzyer, microheat exchangers, 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. In order to make the nanocrystalline cellulose-polymer composite, a new method was devised for making an organic dispersion of nanocrystalline cellulose. The method comprised first forming an aqueous dispersion of nanocrystalline cellulose. A mixture was then formed comprising the aqueous dispersion and an organic liquid having a boiling point higher than water. The water was then selectively removed to form a second mixture comprising the nanocrystalline cellulose and the organic liquid.