Proportional Microvalve Using a Unimorph Piezoelectric Microactuator

Abstract

Microvalves are important flow-control devices in many standalone and integrated microfluidic applications. Polydimethylsiloxane (PDMS)-based pneumatic microvalves are commonly used but they generally require large peripheral connections that decrease portability. There are many alternatives found in the literature that use Si-based microvalves, but variants that can throttle even moderate pressures (1) tend to be bulky (cm-range) or consume high power. This paper details the development of a low-power, normally-open piezoelectric microvalve to control flows with a maximum driving pressure of 1, but also retain a small effective form-factor of 5x5x1.8. A novel combination of rapid prototyping methods like stereolithography and laser-cutting have been used to realize this device. The maximum displacement of the fabricated piezoelectric microactuator was measured to be 8.5 at 150. The fabricated microvalve has a flow range of 0-90 at 1 inlet pressure. When fully closed, a leakage of 0.8 open-flow was observed with a power-consumption of 37.5. A flow resolution of 0.2- De-ionized (DI) water was measured at 0.5 pressure.

Keywords: microvalve, microactuator, piezoelectric, unimorph, stereolithography, 3D-printing.

Figure A1

Schematic of the piezoelectric unimorph microactuator: Design parameters are the PZT diameter ($d_1$), PZT thickness ($h_p$), epoxy thickness ($h_e$), steel membrane diameter ($d_2$), and thickness ($h_m$). Positive (V${}^{+}$) and ground terminals are indicated. The dashed arrows refer to the inward lateral motion of the PZT. The dashed line refers to the downward bowing motion of the actuator.

Figure A2

(a) Transparent view of the microchannels: Internal channels are visible. The microchannel part is large (15 mm) to enable easy manual handling. (b) Cross-section of microchannels along A-A’ with spacer and microactuator: Design parameters are spacer thickness ($t_s$), device inlet diameter ($d_{di}$), device outlet diameter ($d_{do}$), distance between device orifices ($l_{io}$), chamber inlet diameter ($d_{ci}$), chamber outlet diameter ($d_{co}$), internal channel thickness ($t_c$). Static resistances are represented as $R_i$.

Figure A3

Fractional reduction in flow-rate at different pressures.

Figure 1

(a) Schematic of the microvalve: It consists of a circular piezoelectric unimorph microactuator, a spacer, and 3D-printed microchannels. The microactuator is placed on top of the microchannels with an intermediate spacer and the entire assembly is clamped. Red arrows indicate the direction of the clamping force. Black dashed lines indicate the membrane deformation when actuated. (b) Exploded 3D view of the microvalve.

Figure 2

Optimization for maximum actuator displacement: (a) Central displacement (shown as color-bar) plotted for different Lead Zirconate Titanate (PZT) parameters using the analytical formulation. The yellow cross is the optimal set of parameters and the black cross is the set of fabricated parameters. Steel diameter = 5 mm, Steel thickness = 50 μm. (b) Dependence of microactuator displacement on adhesive thickness using COMSOL Multiphysics v5.3: PZT radius = 2 mm, PZT thickness = 127 μm [12], Voltage = 190 V.

Figure 3

Flow rate dependence on spacer thickness with a 1000 mbar pressure differential.

Figure 4

(a) Fabricated microactuator with a laser-cut PZT part bonded to a stainless steel membrane using conductive epoxy. The region where the actuator is clamped by the holder is shown in grey. (b) 3D printed buried microchannels with inlet and outlet connections. The channels become visible when illuminated with a bright light source underneath. (c) Top view of microchannels (d) Bottom view of microchannels.

Figure 5

(a) Assembled microvalve holder with constituent parts. Terminal wires are copper with 0.5 mm diameter. (b) Bottom view of the holder (c) First step: O-rings placed in O-ring grooves, second step: microchannels placed in groove, third step: spacer placed over microchannels, fourth step (not shown): actuator is placed and then assembly is clamped.

Figure 6

(a) Schematic of the test set-up for measuring flow rate and displacement of the microvalve: Green dashed arrows are signal wires, red and black arrows are terminal wires of the microvalve, blue arrows are microfluidic tubing, and the black dashed line is tubing from the pressure source. Double-sided green arrows indicate information transmitted both ways. (b) Photograph of the test-setup.

Figure 7

(a) Comparison of measured and predicted displacement for a 4 mm diameter actuator in the unimorph piezoelectric microactuator (UPM) holder. (b) Displacement of different actuators at 150 V. The predicted displacements are plotted for comparison. Error bars indicate one standard deviation of three measurements in each sample for both graphs.

Figure 8

Comparison of predicted and measured flow rate in the microvalve assuming a valving chamber height of 3 μm. Error bars indicate one standard deviation of three measurements.

Figure 9

(a) Proportional control of flow rate at different pressures. Error bars indicate one standard-deviation of three measurements. (b) On-off behaviour and leak-rate of the microvalve at different pressures. Leak-rate is the ratio of closed and open flow rates.

Figure 10

(a) Fine control of the microvalve at 500 mbar pressure. (b) Reproducibility of flow rate behaviour at 1000 mbar with three microvalves. The flow sensor has a limit of 90 μL min⁻¹.