A method of preparing hydrogel particles includes applying a solution including a plurality of hydrogel particles to a stir cell. A retentate side of a filter defines a lower surface of the stir cell. The filter has the retentate side and a permeate side. The method further includes, while stirring the solution within the stir cell, dispensing a buffer solution at a first flow rate to the stir cell and drawing a permeate from the permeate side of the filter using a pump at a second flow rate, the permeate including a subset of the plurality of hydrogel particles.