A Fluidic Interface with High Flow Uniformity for Reusable Large Area Resonant Biosensors

Abstract

Resonant biosensors are known for their high accuracy and high level of miniaturization. However, their fabrication costs prevent them from being used as disposable sensors and their effective commercial success will depend on their ability to be reused repeatedly. Accordingly, all the parts of the sensor in contact with the fluid need to tolerate the regenerative process which uses different chemicals (H₃PO₄, H₂SO₄ based baths) without degrading the characteristics of the sensor. In this paper, we propose a fluidic interface that can meet these requirements, and control the liquid flow uniformity at the surface of the vibrating area. We study different inlet and outlet channel configurations, estimating their performance using numerical simulations based on finite element method (FEM). The interfaces were fabricated using wet chemical etching on Si, which has all the desirable characteristics for a reusable biosensor circuit. Using a glass cover, we could observe the circulation of liquid near the active surface, and by using micro-particle image velocimetry (μPIV) on large surface area we could verify experimentally the effectiveness of the different designs and compare with simulation results.

Keywords: biosensor; fluidic interface; micro-machining; microengineering; planar flow.

Figure 1

Exploded view of an acoustic biomedical microelectromechanical systems (BioMEMS) sensor, based on the assembly of an array of functionalized piezoelectric membranes with a microfluidic interface, for real time monitoring of specific molecules capture.

Figure 2

Simulated microfluidic interface topologies with 1 cm² chambers (scale bar is 10 mm).

Figure 3

Numerical simulation of flow lines in the microfluidic interfaces of Figure 2.

Figure 4

Numerical simulation of the mean velocity of fluid in the microfluidic interfaces of Figure 2 (mm/s) (the scale has been limited to v < 1 mm/s to better show the velocity inside the chamber).

Figure 5

Numerical simulation of the profile of velocity across the middle of the chamber in the microfluidic interfaces of Figure 2 (the velocity is normalized to the velocity at the center of the chamber).

Figure 6

Layout of the compensation structures and definition of parameters: (a) layout Q for intersection of perpendicular channels; (b) layout I for intersection of channel with cavity. Dotted line and gray area shows the projected etch profile. The dark gray area is the open area of the etching mask.

Figure 7

Flow chart of the microfabrication process for the microfluidic interface test.

Figure 8

Optical microscopy image of the intersections of channels obtained using several sets of compensation structures, depth etch = 77 μm (a) case of set Q1; (b) case of set Q2; (c) case of set Q3; (d) case of set I1.

Figure 9

Optical microscope image of the microfluidic circuits after micromachining. Structure (a) C4 single input and output; (b) C5 single or distributed input and output; (c) C6 distributed input and output (central inset showing a zoomed view of the intersection between channels and with the chamber).

Figure 10

Schematic of the experimental set up used to study velocity flow in the microfluidic interface.

Figure 11

Flow velocity field with streamlines in the chamber measured using micro-particle image velocimetry for the microfluidic interface structure (left) C4, (center) C5, and (right) C6.

Figure 12

Profile of the mean flow velocity (mm/s) in the middle of the chamber measured using particle image velocimetry for the microfluidic interface configuration (left) C4, (center) C5, and (right) C6.

Figure 13

Flush sequence (shown as a function of liquid volume to facilitate comparison) in configurations (C4), (C5), and (C6) (red tinted water is entering from the left) (see movie for C5 configuration in online additional materials).