Abstract
Understanding and controlling molecular transport in hydrogel materials is important for biomedical tools, including engineered tissues and drug delivery, as well as life sciences tools for single-cell analysis. Here, we scrutinize the ability of microwells—micromolded in hydrogel slabs—to compartmentalize lysate from single cells. We consider both (i) microwells that are “open” to a large fluid (i.e., liquid) reservoir and (ii) microwells that are “closed,” having been capped with either a slab of high-density polyacrylamide gel or an impermeable glass slide. We use numerical modeling to gain insight into the sensitivity of time-dependent protein concentration distributions on hydrogel partition and protein diffusion coefficients and open and closed microwell configurations. We are primarily concerned with diffusion-driven protein loss from the microwell cavity. Even for closed microwells, confocal fluorescence microscopy reports that a fluid (i.e., liquid) film forms between the hydrogel slabs (median thickness of 1.7 μm). Proteins diffuse from the microwells and into the fluid (i.e., liquid) layer, yet concentration distributions are sensitive to the lid layer partition coefficients and the protein diffusion coefficient. The application of a glass lid or a dense hydrogel retains protein in the microwell, increasing the protein solute concentration in the microwell by ∼7-fold for the first 15 s. Using triggered release of Protein G from microparticles, we validate our simulations by characterizing protein diffusion in a microwell capped with a high-density polyacrylamide gel lid (p > 0.05, Kolmogorov-Smirnov test). Here, we establish and validate a numerical model useful for understanding protein transport in and losses from a hydrogel microwell across a range of boundary conditions.
Highlights
Molecular transport through hydrogels is important across a wide range of bioengineering systems, including tissue engineering, drug delivery, and single-cell assays
Proteins diffuse from the microwells and into the fluid layer, yet concentration distributions are sensitive to the lid layer partition coefficients and the protein diffusion coefficient
The equilibrium partition coefficient, Keq, is defined as the ratio of concentration of solute in the gel to that in liquid where C is the solute mass per volume, Cgel is the solute concentration in the hydrogel, and Cliquid is the solute concentration in the liquid phase.[1,2]
Summary
Molecular transport through hydrogels is important across a wide range of bioengineering systems, including tissue engineering, drug delivery, and single-cell assays. In addition to equilibrium solute concentrations, time-dependent and diffusion-driven solute concentration gradients—both within a single material and between materials—are important. Understanding both drug delivery to the bloodstream and transport capabilities of cellular waste products out of capsules benefit from understanding these types of concentration distributions.[7,8,9,10,11,12,13] Within homogeneous hydrogels such as polyacrylamide (PA), which have mobile polymer chains, solute diffusion in hydrogels behaves according to a scaled hydrodynamic model[10] and can be empirically determined.[1,11] In hydrogels with immobile polymer chains (e.g., alginate), APL Bioeng.
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