In recent years, microfluidic devices have been increasingly utilized for the study of cell behavior under well-defined chemical and mechanical conditions. A microfluidic device capable of establishing and maintaining a spatially varying chemical gradient in absence of any fluid flow will provide a robust, repeatable and well-defined chemical environment for studying cellular behavior and response. In this work, a new class of polyethylene glycol diacrylate (PEG-DA) based devices with tunable porosities was developed for controlled diffusion of chemical agents. This microfluidic device is composed of three channels fabricated using conventional microfabrication and soft-lithography methods and operates under the principle of diffusion. A solution of a chemical agent of choice is flown in one of the outer channels while a buffer solution is flown in the other outer channel. Cells (subject of study) reside in a buffer solution in the center channel. Chemical agent diffuses through the porous gel wall to the center channel, generating a spatially varying concentration field. The wall thicknesses and porosity can be designed to accommodate for the required chemical diffusion rate. COMSOL multiphysics simulations and diffusion coefficient measurement by optical spectrophotometry were employed to develop custom-devices with desired gel wall characteristics.
Sample gels were fabricated and studied to determine their diffusion coefficient. Two different gels were characterized (10% and 30% (w/v) PEG-DA gel base in DI water). The diffusion coefficient of the gels were calculated using the mean first passage time measurements2 and determined to be 8.1 10-6 cm2/s for the 10% PEG-DA and 3.5 10-6 cm2/s for the 30% PEG-DA. The diffusion coefficient enable establishment of a linear chemical attractant gradient in the center channel of the microfluidic device within minutes and maintain it for about 25 minutes. These time durations were obtained by performing a mass transport simulation of 0.1% casamino acid solution (a common bacterial chemo-attractant) using finite element analysis software package COMSOL.