The present disclosure describes devices useful for microscale fluid purification, separation, and synthesis. Such devices generally 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. Examples of devices made according to the present invention included an oxygenator, a dialyzer, microheat exchangers, etc.