The broad size distribution of biological particles requires redesigning and reoptimizing inertial microfluidic separation devices. The fixed separation range has been identified as the most pressing obstacle in the field of inertial microfluidics. The present work demonstrates the proof of concept of a novel stretchable microfluidic device that controls the length using a stretching platform. Changing the channel dimensions allows the device to be used for various particle sizes and flow rate ratios. We employed this approach for the separation of a mixture of particles. Stretching the device produce significant improvements in focusing and separation efficiency of the particles.