Finger-Actuated Microfluidic Concentration Gradient Generator Compatible with a Microplate

Abstract

The generation of concentration gradients is an essential part of a wide range of laboratory settings. However, the task usually requires tedious and repetitive steps and it is difficult to generate concentration gradients at once. Here, we present a microfluidic device that easily generates a concentration gradient by means of push-button actuated pumping units. The device is designed to generate six concentrations with a linear gradient between two different sample solutions. The microfluidic concentration gradient generator we report here does not require external pumps because changes in the pressure of the fluidic channel induced by finger actuation generate a constant volume of fluid, and the design of the generator is compatible with the commonly used 96-well microplate. Generation of a concentration gradient by the finger-actuated microfluidic device was consistent with that of the manual pipetting method. In addition, the amount of fluid dispensed from each outlet was constant when the button was pressed, and the volume of fluid increased linearly with respect to the number of pushing times. Coefficient of variation (CV) was between 0.796% and 13.539%, and the error was between 0.111% and 19.147%. The design of the microfluidic network, as well as the amount of fluid dispensed from each outlet at a single finger actuation, can be adjusted to the user's demand. To prove the applicability of the concentration gradient generator, an enzyme assay was performed using alkaline phosphatase (ALP) and para-nitrophenyl phosphate (pNPP). We generated a linear concentration gradient of the pNPP substrate, and the enzyme kinetics of ALP was studied by examining the initial reaction rate between ALP and pNPP. Then, a Hanes⁻Woolf plot of the various concentration of ALP was drawn and the V_max and K_m value were calculated.

Keywords: 96-well microplate; Hanes–Woolf plot; concentration gradient; enzyme kinetics; finger actuation; pneumatic valve; power-free microfluidics.

Figure 1

Design and working principle of the finger-actuated concentration gradient generator. (A) Drawing of the device design. (B) Operation principle of the pumping unit. The red dotted box in panel A indicates the single pumping unit. (C) Six concentrations with a linear gradient are generated from the device. Scale bar = 1 cm. (D) The picture of the device operation on the 96-well microplate.

Figure 2

Fabrication processes of a finger-actuated microfluidic device.

Figure 3

Estimation of concentration gradient generation. (A) The optical density of the dispensed solutions from each outlet was compared between the manual pipetting method and the finger-actuated concentration gradient generator. The error bars represent the standard deviation of three replicates. (B) The dispensed volume from each outlet was compared with respect to the number of the pushing times. The error bars represent the standard deviation of three replicates.

Figure 4

The concentration of p-nitrophenol was measured for 1 min to determine the initial reaction rate for the various enzyme concentrations. (A) 100 U/L. (B) 150 U/L. (C) 200 U/L.

Figure 5

The results of the enzyme kinetics study. (A) The Hanes–Woolf plot for the various concentrations of ALP. The error bars represent the standard deviation of three replicates of the assay. (B) V_max and K_m value of each enzyme unit were obtained from the linear fitting of panel A.