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.
Highlights
The advantages of microfluidics over conventional lab-scale techniques—reduced reagent consumption, faster reactions, smaller instrument size, enhanced automation, higher throughput, and so on—have enabled applications for microfluidic instruments in fields as diverse as health care, environmental monitoring, and space exploration
We demonstrated that the orientation of a microfluidic chip can be used to precisely control the flow of fluids inside the chip
By using the orientation of a chip to control fluid flow instead of on-chip valves or off-chip pumps and regulators, our technique can eliminate a substantial portion of the cost, size, and power-consumption of a microfluidic assay or instrument
Summary
The advantages of microfluidics over conventional lab-scale techniques—reduced reagent consumption, faster reactions, smaller instrument size, enhanced automation, higher throughput, and so on—have enabled applications for microfluidic instruments in fields as diverse as health care, environmental monitoring, and space exploration. To control the mixing ratio of two fluids in a microfluidic chip, two off-chip pumps (pressure regulators or syringe pumps) are typically required. Electrical methods for controlling fluids, such as dielectrophoresis [3, 4] and electrowetting, [5] require off-chip electrical power supplies and complicate the fabrication of the microfluidic chip. In each of these approaches, the offchip hardware required to control on-chip fluid flow can cost thousands of dollars, consume hundreds of watts of electrical power, and contribute significant bulk to the instrument
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