Generation of oxygen gradients with arbitrary shapes in a microfluidic device

Abstract

We present a system consisting of a microfluidic device made of gas-permeable polydimethylsiloxane (PDMS) with two layers of microchannels and a computer-controlled multi-channel gas mixer. Concentrations of oxygen in the liquid-filled flow channels of the device are imposed by flowing gas mixtures with desired oxygen concentrations through gas channels directly above the flow channels. Oxygen gradients with different linear, exponential, and non-monotonic shapes are generated in the same liquid-filled microchannel and reconfigured in real time. The system can be used to study directed migration of cells and the development of cell and tissue cultures under gradients of oxygen.

Figure 1

Microfluidic device. (a) Drawing of microchannels in the device (xy-plane), with the flow channels shown in dark gray and gas channels shown in light gray. Gas inlets are marked by numbers 1 – 9. (b) Color-coded concentration of oxygen, [O₂], in the yz-cross-section of the device in the functional area (where the gas and flow channels overlap) from a 2D numerical simulation in FEMLAB. A 4×1.25 mm center bottom fragment of the 10×4 mm computational domain is shown. White rectangles near the bottom are cross-sections of the 150 μm deep gas channels, which are numbered according to the numbers of the gas inlets in (a). The boundary conditions of the simulation are [O₂] = 21% at the upper and side boundaries (atmosphere air; not shown) and insulation at the bottom of the domain (cover glass). The conditions at the walls of the gas channels are [O₂] = 0% (pure N₂) for channels 1, 6, and 7, [O₂] = 50% (1:1 O₂:N₂) for channels 2, 5, and 8, and [O₂] = 100% (pure O₂) for channels 3, 4, and 9, as in the experiment shown in Fig. 2c, curve 1. The test channels (not shown) are immediately adjacent to the bottom of the computational domain. Arrows near the bottom indicate lateral boundaries of a 2.25 mm wide internal region (between the centers of the 800 μm wide gas channels 1 and 9), in which [O₂] just above the bottom as obtained from simulations is >99.95% when all gas channels are filled with O₂ ([O₂] = 100% at all gas channel walls).

Figure 2

Concentration of oxygen, [O₂], as a function of position, x, along the 90 μm wide test channel in the microfluidic device, as evaluated from fluorescence of RTDP, with different gas mixtures flowing through the gas channels. x = 0 corresponds to the middle of gas channel 5 (the center of the gas channel array; Fig. 1); x = −0.725 and 0.725 mm correspond to the inner edges of gas channels 1 and 9, respectively. (a) Four [O₂] profiles obtained with four different linear series of [O₂] in the gas channels. (b) Three [O₂] profiles (in semi-logarithmic coordinates) obtained with three different geometrical series of [O₂] in the gas channels. Curve 1 is shifted by 0.075 mm along the x-axis for better visibility. Some undulations in the curves, with a period of ∼0.2 mm, are likely due to fluorescence background originating from the scattering of the fluorescent light by the gas channel walls, which is difficult to correct for. (c) Two [O₂] profiles (orange and cyan curves) obtained with two non-monotonic series of [O₂] in the gas channels. Thin black curve shows the result of a numerical simulation in FEMLAB (with no fitting parameters; cf. Fig. 1b). Some differences in the height of the peaks at x = −0.3 and 0.3 mm between the experiment and simulation could be due to the fact that the simulation was 2D and did not account for the presence of the test channel. (d) Two nonmonotonic [O₂] profiles obtained when the gas mixer was fed with N₂ and air and [O₂] in the gas channel varied between 0 and 21%. The [O₂] profiles appear noisier than the similar profiles in (c) because of the 5-times higher resolution in [O₂].