Microfluidic Microdialysis: Spatiotemporal Control over Solution Microenvironments Using Integrated Hydrogel Membrane Microwindows

We present a powerful and versatile technique that enables exquisite spatial and temporal control over local solution chemistry in microfluidic devices. Using a microscope and a UV lamp, we use projection lithography to photopolymerize thin ($10–25\mu \mathrm{m}$) hydrogel membrane “microwindows” (HMMs) into standard microfluidic devices. These microwindows are permeable to solute and solvent diffusion and to electric fields, 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 a HMM using standard microfluidic techniques, provoking changes in solution conditions on the other side without active mixing, stirring, or diluting. We highlight three paradigmatic experimental capabilities enabled by HMMs: (1) rapid dialysis and swapping of solute and/or solvent, (2) stable and convection-free localized concentration gradients, and (3) local electric permeability. The functional versatility of hydrogel microwindow membranes, coupled with the ease and speed of their fabrication and integration into simple microchannels or multilayer devices, will open a variety of novel applications and studies in a broad range of fields.

Popular Summary

Microfabricated fluidic devices with $10–100\mathrm{\text{−}}\mu \mathrm{m}$ channels and chambers enable small fluid samples to be manipulated quickly and carefully. Nonetheless, the ability to rapidly change or impose a microchemical environment has remained elusive. The two-step strategy of the human circulatory system provides inspiration: Gas, nutrients, and waste are first distributed throughout the body via blood flow and then diffuse to cells and tissues that lie close to blood vessels. Here, we present an analogous strategy that gives exquisite control, in both space and time, over the chemical and electrical microenvironment in microfluidic systems. Our technique enables solutions to be rapidly swapped, strong and stable gradients to be imposed and superposed, and electric fields to be locally sculpted.

Specifically, we present “hydrogel membrane microwindows” (HMMs). These are thin ($10–20\mathrm{\text{−}}\mu \mathrm{m}$) hydrogel walls that can be integrated into precise locations within standard microfluidic devices. The hydrogel pores are small enough that the HMM acts like a rigid wall to macroscopic fluid flow, yet large enough to allow the diffusive passage of solutes and solvents. HMMs thus provide microscale dialysis membranes: Standard microfluidic manipulations establish a “reservoir” solution on one side of a HMM, whose contents diffusively cross the HMM to locally and rapidly change the “sample” environment on the other side.

HMMs enable powerful new capabilities for microchemical control. Mere seconds are required to change the salt, fluorophore, solvent, and pH buffers in $10–100\mathrm{\text{−}}\mu \mathrm{m}$ sample chambers. Electric fields can be imposed and sculpted locally, without introducing bubbles or reaction products. Crystallization and dissolution can be triggered using HMMs to introduce and remove antisolvents. Strong, stable gradients of both solute and solvent can be rapidly imposed, and the resulting phoretic motion of colloidal particles can be visualized directly.

HMMs require only standard laboratory equipment to synthesize and can be integrated into specific locations in even complex microfluidic networks. Versatile chemistry can be adapted if required for particular systems. We envision a broad range of applications where chemical probing of microscale features in fluidic systems is required: cell signaling, differentiation, migration, and response to drugs or toxins, combinatorial screens of solubility, synthesis or reaction conditions, the transient response of soft matter, and many more.

Figure 1

Fabrication of HMMs. (a) A UV-polymerizable solution of poly(ethylene-glycol)-diacrylate and photoinitiator initially fills a three-channel microfluidic device (b), with gaps in the walls between channels. Patterned UV light is used to cross-link the hydrogel in two specified rectangular regions (width: $10–20\mu \mathrm{m}$) filling the gaps in the walls. (c) After flushing the unreacted solution, hydrogel microwindow membranes remain (phase contrast, scale $\mathrm{bar}=20\mu \mathrm{m}$). (d) Simplified microscope schematic for fabricating HMMs. The microfluidic device is placed on the microscope stage, and a photomask is inserted into the microscope field aperture. A computer-controlled mechanical shutter exposes the PEG-DA solution to UV irradiation for the desired exposure time. Inset: Natural fluorescence of NOA-81 optical glue enables the patterned UV light to be aligned.

Figure 2

(See Video 1.) HMMs are permeable to small-molecule solutes, yet admit no measurable hydrodynamic flow. Here, we use the three-channel device shown in Fig. 1b and specifically show the same close-up view as Fig. 1c. All channels contain a fluorescent pH indicator at the same concentration, and each channel is buffered at a different pH: 8.1 (left, bright), 6.3 (center, intermediate), and 4.7 (right, dark), with ionic strengths balanced. Fluorescent tracer particles (bright lines) travel in straight lines past the center channel, indicating no convective inflow through either the left or right HMM. Diffusion of pH buffers through the membrane is clearly evident. Insets: Contrast-enhanced and enlarged images showing tracer particle paths. Scale bar: $20\mu \mathrm{m}$.

Figure 3

Antisolvent diffuses from the reservoir channels across HMMs into a saturated aqueous salt solution in the sample channel, depicted in the three-channel device of Figs. 1b, 1c. (a-c) Upon stopping KCl flow in the sample channel, ethanol diffuses through the HMMs. As ethanol concentration progressively increases near the HMM, KCl solubility decreases, triggering nucleation and growth of KCl crystals. (d-f) Resuming convective delivery of fresh aqueous KCl solution flushes away the ethanol, restoring KCl solubility to its bulk aqueous value, and dissolving the crystals. (See Videos 2 and 3.) (Phase contrast, scale $\mathrm{bar}=20\mu \mathrm{m}$.)

Figure 4

(a) In the three-channel device [Figs. 1b, 1c, 2], the amount of convection admitted by HMMs at large transmembrane pressure drops, here $\Delta P_m=1.5\mathrm{bar}$, is lower for a highly cross-linked HMM (50% v/v, left) than for a lightly cross-linked HMM (20%, right). Tracer particles are focused towards the center of the channel by transmembrane flow from the side channels. (b) The diffusive flux through a lightly cross-linked membrane (20% v/v, right) is nearly identical to that through a highly cross-linked (50% v/v, left) membrane. (c) After diffusing through each HMM, the pH buffer is convected downstream, forming a boundary layer whose thickness decreases with Peclet number (flow velocity) like ${\mathrm{Pe}}^{1/2}$, as expected for 2D convection-diffusion. Scale bars: $20\mu \mathrm{m}$.

Figure 5

(a) Device geometry for prompt reservoir solution swapping using PDMS valves and rapid dialysis in the sample channel using an HMM. Panel (b) and Video 4: Swapping between 250-mM-NaCl (bright) and 0-mM-NaCl (dark) solutions in a $150\mathrm{\text{−}}\mu \mathrm{m}$-wide sample channel, with a fluorescent NaCl indicator. (c) Swapping time is greatly decreased by decreasing sample-channel width (here, $23\mu \mathrm{m}$ wide). Inset graph: Semilog plot of HMM intensity shows exponential decay of salt concentration in HMM. Panel (d) and Video 5: Observed dynamics of NaCl concentration along the $x$ axis of the sample channel are well predicted by the one-dimensional diffusion equation.

Figure 6

(a) An ac potential applied between reservoir channel inlets drives an electric field that only crosses the sample channel through the HMMs. Fluorescent colloids in the sample channel oscillate electrophoretically, tracing local electric-field lines much like iron filings align with magnetic fields. Panels (b-c) and Video 6: Continuous-flow electrophoretic separations of negatively charged colloids by applying a dc electric field E transverse to the flow. Scale bars: $20\mu \mathrm{m}$.

Figure 7

Concentration gradients created locally within microchannels. (a) Direct gradient visualization with Oregon Green fluorescent dye at 0 (black), 10 (red), and 20 (blue) seconds after stopping flow. Panel (b) and Video 7: Diffusiophoretic motion of fluorescent colloids up a NaCl gradient. Panel (c) and Video 8: Solvophoretic motion down an ethanol gradient. Scale bar: $20\mu \mathrm{m}$.