Easy-to-use osmosis-based microfluidic chip for protein crystallization: application to a monoclonal antibody

We developed a disposable microfluidic chip which mimics the vapor diffusion method for exploring protein crystallization conditions but at the nanoliter scale. This device exploits the permeation of water through a thin poly(dimethylsiloxane) (PDMS) layer separating droplets stored in the chip and containing a mixture of proteins and precipitants from an open microfluidic reservoir. The water chemical activity fixed by the reservoir makes it possible to modify the volume of the droplets in a controlled way (reduction or increase). Because PDMS is only permeable to water, the imposed water activity therefore increases or decreases the concentration of the solutes present in the droplet and consequently the supersaturation of the solution. The specificity of our approach, in addition to the low volumes ensuring controlled mass transport conditions, is that the concentration of all the solutes is known at any time, thus allowing the extraction of quantitative information on the crystallization process. We exploit these chips on a protein of therapeutic interest: the full-length monoclonal antibody anti-CD20. Our experiments allow us in particular to estimate both the solubility of this protein and the width of the metastable zone with Na2SO4 and PEG400 as crystallizing agents. We also show that the fine-tuning of the permeation rate makes it possible to perform crystallization/dissolution cycles to selectively dissolve small crystals and increase the mean size of the remaining anti-CD20 crystals.