text: Microfluidic diffusional sizing (MDS) is a method to measure the size of particles based on the degree to which they diffuse within a microfluidic laminar flow. It allows size measurements to be taken from extremely small quantities of material (nano-grams) and is particularly useful when sizing molecules which may vary in size depending on their environment - e.g. protein molecules which may unfold or become denatured in unfavourable conditions.