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This technology is a MDDS device enabling both sample concentration and separation in a single device with a one inlet configuration that requires no additional sheath flow. The MDDS device is composed of sequentially connected two spiral channels having two different dimensions. The sample is injected through the inlet, and all the target particles become focused into the inner wall side as they pass through the first smaller-dimension spiral channel due to the strong net lift force. The focused stream in the first spiral channel directly feeds to the outer wall side of the second larger-dimension spiral channel through the S-shaped transition region. In the second spiral channel, due to the increased channel dimension, the net lift force toward the inner wall side become weaker so that target particles have different equilibrium positions depending on their size, resulting in their separation; in general, as particle size increases, the equilibrium position gets closer to the inner wall side. This MDDS device provides high separation resolution and efficiency for a wide variety of particle sizes due to the initial focusing in the first smaller-dimension spiral channel. For example, a blood sample can be loaded into this device to achieve accurate separation of red and white blood cells.