(403a) Rapid Dialysis in Microfluidic Devices Using Hydrogel Membrane Micro-Windows: Phoretic Migration Under Imposed Gradients

We have developed a simple microfluidic technique to synthesize in-channel hydrogel structures with precise, high resolution microscale geometries.  Using a microscope and UV lamp, we use projection lithography to photopolymerize thin (W=10-25µm) hydrogel membrane ‘microwindows’ (HMMs) into standard microfluidic devices. These microwindows are permeable to solute and solvent diffusion, yet act as rigid walls from the standpoint of fluid flow. Reservoirs of solution may thus be rapidly imposed, switched and maintained on one side of the HMM using standard microfluidic techniques, provoking changes in solution conditions on the other side without active mixing, stirring, or diluting to the sample-containing solution.

Here, we investigate steady and dynamic salt transport within HMMs.  By imposing steady concentration gradients across the HMMs, comparisons of permeability properties of HMMs may be obtained.  In particular, we control the effective pore size within the HMM and investigate the effect on the diffusive and Darcy permeability of the hydrogel.  Dynamics of salt transport are also investigated by quickly switching reservoir solutions and visualizing the HMM and sample channel response.  Fast switching is demonstrated by complete switching of salt solutions (0 to 250 mM NaCl) in a flow-free microfluidic channel in under 6 seconds.  We investigate the salt transport mechanisms within the hydrogel which sets this HMM response time, and exploit this rapid transient time to investigate the phoretic motion of suspended colloids.