Orientation-Based Control of Microfluidics

Abstract

Most microfluidic chips utilize off-chip hardware (syringe pumps, computer-controlled solenoid valves, pressure regulators, etc.) to control fluid flow on-chip. This expensive, bulky, and power-consuming hardware severely limits the utility of microfluidic instruments in resource-limited or point-of-care contexts, where the cost, size, and power consumption of the instrument must be limited. In this work, we present a technique for on-chip fluid control that requires no off-chip hardware. We accomplish this by using inert compounds to change the density of one fluid in the chip. If one fluid is made 2% more dense than a second fluid, when the fluids flow together under laminar flow the interface between the fluids quickly reorients to be orthogonal to Earth's gravitational force. If the channel containing the fluids then splits into two channels, the amount of each fluid flowing into each channel is precisely determined by the angle of the channels relative to gravity. Thus, any fluid can be routed in any direction and mixed in any desired ratio on-chip simply by holding the chip at a certain angle. This approach allows for sophisticated control of on-chip fluids with no off-chip control hardware, significantly reducing the cost of microfluidic instruments in point-of-care or resource-limited settings.

Fig 1. Using the orientation of a microfluidic chip to control the mixing ratio of fluids on-chip.

Two fluids (yellow and blue) flow into the chip; the blue fluid includes an additive (sucrose) that makes the blue fluid 2% more dense than the yellow fluid. When the two fluids flow together in the chip, the fluids rotate to orient the more-dense blue fluid toward Earth’s gravity. When the channel then splits, the amount of each fluid flowing in each direction is precisely controlled by the angle of the chip. By using this approach, any desired mixing ratio of the yellow and blue fluids can be obtained simply by holding the chip at a certain angle; no off-chip control hardware is needed.

Fig 2. Principle of orientation-based control of microfluidics.

These simulations show fluid flowing inside a simple microfluidic channel network containing two inlets and two outlets. Inlet 1 contains a less-dense yellow fluid and Inlet 2 contains a more-dense blue fluid. When the chip is oriented such that Inlet 2 is aligned with the Earth’s gravitational force (A), the yellow and blue fluids remain unperturbed and exit the chip in the same directions from which they entered (yellow at Outlet 1 and blue at Outlet 2). However, when the chip is rotated 180° (B), the force of gravity causes the fluids to swap places in the horizontal channel, placing the more-dense blue fluid on the bottom of the channel and the less-dense yellow fluid on the top. Consequently, the two fluids exit in the opposite directions from which they entered: Outlet 1 contains blue fluid and Outlet 2 contains yellow fluid. When the chip is oriented at −90° relative to gravity (C), the fluids rotate 90° clockwise to orient the more-dense blue fluid on the bottom of the channel and the less-dense yellow fluid on the top. When the channel splits into two outlets, each outlet receives an identical mixture containing 50% yellow fluid and 50% blue fluid. Finally, when the chip is oriented at 90° (D), the fluids rotate 90° counterclockwise to once more orient the more-dense blue fluid on the bottom of the channel, and again both outlets contain identical mixtures containing 50% yellow and 50% blue. In this manner, the orientation of a microfluidic chip may be used to route fluids in different directions on-chip without using any off-chip control hardware.

Fig 3. Photographs of a microfluidic mixer chip oriented at different angles θ relative to gravity.

In each case Inlet 1 contains a less-dense yellow fluid (water; density ρ = 1.00 g/mL) and Inlet 2 contains a more-dense blue fluid (sucrose solution; ρ = 1.07 g/mL). When θ = 0° (A) the arrangement of yellow and blue fluids in the horizontal channel remains unchanged, and Outlet 1 contains yellow fluid and Outlet 2 contains blue fluid. However, at θ = 90° (B) gravity causes the more-dense blue fluid to move to the bottom of the horizontal channel and the less-dense yellow fluid to move to the top. This twists the contents of the horizontal channel by 90° and causes both outlets to contain identical mixtures containing ∼50% blue and ∼50% yellow. Finally, at θ = 180° (C) the gravity-induced repositioning of the fluids in the horizontal channel causes the fluids to twist by 180°, effectively swapping places in the channel. As a result, the two fluids exit the chip in directions opposite from where they entered, with Outlet 1 containing blue fluid and Outlet 2 containing yellow fluid.

Fig 4. Concentrations of yellow and blue dyes in the mixer chip’s two outlets as the angle of rotation of the mixer chip is varied from 0° to 180°.

Also shown are photographs of the fluid collected at each outlet and an illustration of the chip’s orientation at each angle of rotation. N = 3 measurements for each point; error bars indicate ±1 standard deviation. These results show that mixture composition is a function of the angle of rotation of the chip, and any desired mixture can be generated simply by orienting the chip and the required angle.

Fig 5

(A) Concentrations of yellow and blue dyes at Outlet 1 in mixer chips held at 0°, 90° and 180°, for chips with circular, square, and rectangular channel cross-sections. Chip orientation can be used to control on-chip fluids in devices with a variety of channel cross-sectional shapes, although performance deteriorates somewhat in the rectangular channel. These results suggest that microfluidic channels with aspect ratios close to one are best suited for orientation-based control. (B) Outlet fluid concentrations measured at Outlet 1 while orienting the mixer chip at 0°, 90° and 180° for four different fluid densities at Inlet 1. The density of fluid at Inlet 2 was kept constant at 1.00 g/mL. When there is no difference in fluid densities between the two inlet fluids, fluid reorientation is not observed and orientation-based control cannot be used (Control case). However, if the fluid density in Inlet 1 is just 2% greater than the fluid density in Inlet 2, then the flowing fluids reorient with respect to gravity and orientation-based control is possible.