(196a) Microfluidic Droplet Dehydration For Separation and Purification Of Biomolecules

Droplets in microfluidic devices have proven useful as picoliter reactors for biochemical processing operations such as polymerase chain reaction, protein crystallization, and the study of enzyme kinetics.  Although droplets are typically considered to be self-contained, constant volume reactors, there can be significant transport between the dispersed and continuous phases depending on solubility and other factors.  In the present talk, we show that water droplets trapped within a microfluidic device for tens of hours slowly dehydrate, concentrating the contents encapsulated within.  We use this slow dehydration along with control of the initial droplet composition to influence gellation, crystallization, and phase separation processes.  For example, we show that a common aqueous two-phase system consisting of a PEG-Dextran mixture can be used to generate initially homogeneous droplets.  Upon dehydration, the concentrated mixture phase separates, first into a microemulsion of Dextran droplets within the PEG phase, and later coalescing into two distinct droplets of nearly pure PEG and Dextran.  The interfacial tensions of each phase with the continuous oil phase promote complete de-wetting of the two droplets, such that the PEG and Dextran phases separate into distinct, isolated droplets.  Since DNA and proteins preferentially partition into PEG or Dextran, this phenomenon offers a useful method for on-chip separation and purification of samples containing these important biomolecules.