Generation of Concentration Gradients by a Outer-Circumference-Driven On-Chip Mixer

Abstract

The concentration control of reagents is an important factor in microfluidic devices for cell cultivation and chemical mixing, but it is difficult to realize owing to the characteristics of microfluidic devices. We developed a microfluidic device that can generate concentration gradients among multiple main chambers. Multiple main chambers are connected in parallel to the body channel via the neck channel. The main chamber is subjected to a volume change through a driving chamber that surrounds the main chamber, and agitation is performed on the basis of the inequality of flow caused by expansion or contraction. The neck channel is connected tangentially to the main chamber. When the main chamber expands or contracts, the flow in the main chamber is unequal, and a net vortex is generated. The liquid moving back and forth in the neck channel gradually absorbs the liquid in the body channel into the main chamber. As the concentration in the main chamber changes depending on the pressure applied to the driving chamber, we generated a concentration gradient by arranging chambers along the pressure gradient. This allowed for us to create an environment with different concentrations on a single microchip, which is expected to improve observation efficiency and save space.

Keywords: density control; lab on a chip; microfluidics; micromixer; pneumatically driven.

Figure A1

How to generate concentration gradient by outer-circumference-driven on-chip mixer. (a) Generation of concentration gradient by enclosure angle. Flow path was designed so that all driving chambers were equally pressurized, and the concentration was adjusted by enclosure angle. (b) Generation of concentration gradient by pressure gradient. All driving chambers had the same enclosure angle, and the concentration gradient was generated by the pressure supplied to the driving chambers.

Figure A2

Verification of pressure uniformity. (a) To generate a concentration gradient using the enclosing angle, pressure must be uniform to generate a concentration gradient using the enclosing angle. (b) Experimental results. Concentrations were not uniform.

Figure A3

Use of pressure gradient. (a) Air was supplied from the left side of the figure, and the driving chamber was deformed by air pressure. The pressure gradient was generated by the pressure drop through the flow path, which generated the concentration gradient. (b) Experimental results of (a). Each chamber was driven for 15 s, but no stirring occurred.

Figure A4

Flow path without the effect of channel width. (a) Concentration gradient was generated by the pressure gradient after removing the effect of the channel width. (b) Experimental results. Concentration gradient was generated, but concentrations in the chambers at both ends were low.

Figure A5

Wall deformation in relation to the position of the pneumatic source. Deformation differeed depending on the position of the chamber.

Figure A6

N-th to N + 2nd frame of an experiments. Main chamber repeatedly expanded and contracted, which caused rattling luminance values.

Figure A7

Time variation in mixing index until 9 s after the application of air pressure.

Figure 1

Driving principle of outer-circumference-driven mixer. The liquid in the main channel is gradually siphoned into the main chamber. Mixing speed can be changed by driving pressure.

Figure 2

Design of the microchip. (a) Overall dimensions of flow channel. Air pressure is supplied from the air source. Arrow above the driving chamber indicates the expected pressure supplied to the driving chamber. (b) Detailed dimensions of the main chamber, which were determined on the basis of previous studies [33].

Figure 3

Simulation results. (a) Main chamber model. Because the surface to which pressure is applied must be flat, the outer shape was polygonal. (b) How to apply pressure to the main chamber. Since it was not possible to simulate the change in flow owing to the deformation of the wall, it was assumed that pressure is applied. (c,d) Streamlines during expansion and contraction. Streamlines were different between expansion and contraction; therefore, the net vortex was created by repeating this process.

Figure 4

(a) Experimental setup. (b) Experimental apparatus.

Figure 5

Measured concentration. Numbering from left to right is 1, 2, …, 8. Calculations were performed using Python and OpenCV.

Figure 6

Generation of concentration gradient using an outer-circumference-driven mixer. (a) Time variation of an experiment (Chip 1, first trial). (b) Experimental results. Similar results were obtained from nine experiments.

Figure 7

Time variation of mixing index until 9 s after application of air pressure. Graph was drawn using 3 points of a simple moving average. Mixing index indicates the degree of mixing; the lower the mixing index was, the more the beads were absorbed and mixed.

Figure 8

Graph of average of concentrations after 9 s for 9 experiments. Error bars represent standard error. The concentration gradient iwass generated roughly along the pressure gradient.

Figure 9

Average of nine experiments was calculated, and graph shows the simple moving average. Final concentrations were C-1, -2, -3, -4, -5, -6, -8, and -7.

Figure 10

Time variation of experiment using colored water.

Figure 11

Time variation of mixing index in experiments using colored water. We obtained s similar result as that in experiments using 3 μm microbeads. Final mixing indices were C-1, -2, -3, -4, -5, -6, -8, -7.

Figure 12

Evaluation of pressure gradient. (a) Wall deformation during expansion. Since the amount of deformation varied with the magnitude of pressure, it was used to evaluate the pressure gradient. (b) Details of evaluation region. Pixel values in Regions A and B were used to evaluate wall deformation in Region C. Equation (4) was used for evaluation. (c) Wall deformation in each chamber. Pressure gradients were generated approximately in the order of the concentration gradient.