MODIFYING A CONTROLLED DIFFUSION MICROFLUIDIC DEVICE THROUGH APPLICATIONS OF TUNABLE WIDTH CHANNELS BASED ON PDMS NANOCRACKS

The study of B-Cells and their migration is vital in cancer research and recent advancements in microfluidics have greatly improved these studies.One such study is currently at a roadblock due to limitations created by the microfluidic device being used (Figure 1).This device is molded from polydimethylsiloxane (PDMS) and its purpose is to create a diffusion gradient in the center chamber by allowing the solutions in the two flanking chambers to diffuse into the center through connecting channels patterned into the device with heights of 10m.This design failed to create a proper diffusion gradient necessary for the cell migration study to progress.Our proposed modification to this device replaces the traditionally patterned connecting channels with crack-based microchannels (Figure 2).The advantage of crack-based channels is that the width of channels can be varied, ranging from fully-closed to fully-open, by changing the amount of tensile load applied to the device [1-2].When a load is applied, the cracks open and act as channels for diffusion into the center chamber and the rate of diffusion can be varied depending on the width of the crack-based channels.The cracks are created by patterning triangular notches on the edges of each of the flanking chambers.These triangular notches act as stress concentration points for cracks to form and extend until they reach the center chamber, and therefore forming a crack-based channel that fluidically connects the flanking channel to the central chamber.Currently, creating the crack-based channels for the diffusion device is being investigated using two different fabrication methods.The first method utilizes air/oxygen plasma to create a thin surface oxide layer on the PDMS.This oxide layer is brittle and susceptible to cracking, which allows the controlled fabrication of the crack-based channels on PDMS.The second fabrication method utilizes a thin layer of hard polydimethylsiloxane (hPDMS), which is a higher modulus formulation of PDMS, that is spin coated onto a PDMS substrate.An advantage of this hPDMS fabrication method compared to plasma oxidation is that, by controlling the speed and duration of the spin coating, the height of the hPDMS film is far more controllable, and therefore so is the height of the crack-based diffusion channels.Conversely, during the plasma oxidation process, it is both difficult to control the thickness of the PDMS oxide layer [3] and to achieve oxide thicknesses matching the required height of the diffusion channel necessary for the desired migration studies (Figure 3).Therefore, finite element analysis (FEA) was used to further investigate the use of hPDMS for creating crack-based channels for this application (Figure 4).FEA determined a maximum stress of 3MPa concentrated at the notch tips is greater than the yield strength of hPDMS, 0.5MPa, and less than that of PDMS, 5MPa, which indicates the cracks should be contained in the hPDMS [4].The FEA results show that cracking should occur at the notch locations (Figure 4) using the hPDMS method and will therefore be demonstrated to create this microfluidic device to study effects of diffusion.