This paper describes a microfluidic approach to perform multiplexed nanoliter-scale experiments
by combining a sample with multiple different reagents, each at multiple mixing ratios. This approach employs
a user-loaded, equipment-free SlipChip. The mixing ratios, characterized by diluting a fluorescent dye,
could be controlled by the volume of each of the combined wells. The SlipChip design was validated on an
∼12 nL scale by screening the conditions for crystallization of glutaryl-CoA dehydrogenase from Burkholderia
pseudomallei against 48 different reagents; each reagent was tested at 11 different mixing ratios, for a
total of 528 crystallization trials. The total consumption of the protein sample was ∼10 µL. Conditions for
crystallization were successfully identified. The crystallization experiments were successfully scaled up in
well plates using the conditions identified in the SlipChip. Crystals were characterized by X-ray diffraction
and provided a protein structure in a different space group and at a higher resolution than the structure
obtained by conventional methods. In this work, this user-loaded SlipChip has been shown to reliably handle
fluids of diverse physicochemical properties, such as viscosities and surface tensions. Quantitative
measurements of fluorescent intensities and high-resolution imaging were straighforward to perform in
these glass SlipChips. Surface chemistry was controlled using fluorinated lubricating fluid, analogous to
the fluorinated carrier fluid used in plug-based crystallization. Thus, we expect this approach to be valuable
in a number of areas beyond protein crystallization, especially those areas where droplet-based microfluidic
systems have demonstrated successes, including measurements of enzyme kinetics and blood coagulation,
cell-based assays, and chemical reactions
