Methods for filtering particles from a fluid are disclosed, wherein an array of microstructures defining respective microchannels having respective minimum widths are used to separate the fluid from particles to be filtered. The fluid flows through the minimum widths into the microchannels defined between adjacent microstructures. The particles to be filtered are prevented from passing through the respective minimum widths, resulting in filtration of those particles from the fluid. The microchannels can be provided with gradient characteristics to separate particles in the fluid according to size.