Electrically Activated Membrane Valves - Kaleb Seifert

General Overview

Electrically activated membrane valves are an important type of valve in a microfluidic device. These valves are electrically activated, and utilize a membrane, typically made of PDMS, to bend or deform into or out of the channel, effectively allowing or stopping the flow of fluid.^[1] There are a few different types of membrane-based electrically activated valves, such as electrostatic valves, electrochemical valves, and piezoelectric valves. Each of these valves have their own advantages and disadvantages, and their own different types of applications, which will be discussed throughout this page.

Types of Electrically Activated Membrane Valves

Electrostatic Valves

Figure 1: Normally closed electrostatic valve (Left) vs. Normally open electrostatic valve (Right).^[1]

Electrostatic valves are generally comprised of a valve-opening and closing electrode, and a flexible membrane, typically made from either polydimethylsiloxane (PDMS), but other materials have been used such as copper foil.^[1] These valves are typically normally closed, but have also been used in open microvalves. There is no real major difference between the two except for the original shape of the membrane layer. Typically, for normally open valves, the membrane is concave, while for normally closed valves, the membrane is flat.^[1] These are similar to pneumatic valves, which are talked about extensively here. These valves are operated by controlling the voltage between the electrodes, which is applied through the membrane, causing the membrane to bend, either opening or closing the valve. These valves have been shown to have fast response time and low energy consumption when controlling gasses.^[2] It is important to note though, when controlling liquids, the valves require a higher applied voltage, and due to this, these types of valves are typically only used to control airflow.^[2]

The equation of the electrostatic force between the electrodes are as follows:^[3]

[math]\displaystyle{ F_{e} = (\epsilon_{0}\epsilon_{a}AV^{2})/(2g^{2}) }[/math]

where:

[math]\displaystyle{ \epsilon_{0} }[/math] is the vacuum permittivity

[math]\displaystyle{ \epsilon_{a} }[/math] is the dielectric constant of the medium between the electrodes

[math]\displaystyle{ A }[/math] is the area of the electrode

[math]\displaystyle{ V }[/math] is the voltage across the electrodes

[math]\displaystyle{ g }[/math] is the total distance between the electrodes

Current Studies

Figure 2: A) Voltage off. B) Voltage just turned on, the valve starts to close. C) Voltage on, valve closed.^[4]

There is a lot of work in this current field of improving and redesigning different types of electrostatic valves. One current method is by Anjewierden et al. which utilized electrostatically controlled pneumatic valve.^[4] This microvalve was assembled by depositing a chrome layer on poly(methyl methacrylate), and used a copper foil as its flexible membrane.^[4] This valve was built as a normally-open valve, and when a voltage was applied between the copper and chrome, the electrostatic force pulled the foil closed against the chrome to prevent airflow. According to the researchers, the valve would be layered below the entire device to remove bulk or an off-device control system. In their simplistic model, they had demonstrated a leak-proof seal, but suffered from a 12 second actuation time to open the valve, and a 6 second actuation time to close the valve, taking significantly longer than other microfluidic valves. The reason for this is due to the materials used, which increases the dielectric charging time greatly.^[4]

Electrochemical Valves

Electrochemical based valves are comprised of a compact electrochemical actuated membrane that is typically made of PDMS or a SU-8 cantilever, similar to electrostatic activation.^[1] Furthermore, similarly to electrostatic activation, it also contains electrodes, but these electrodes are used to electrolyze solution instead of causing the membrane to deform.^[1] These types of valves are strictly normally-open valves. The ways these valves work is similar in fashion to that of a pneumatic valves, where electrolyzing fluids produces hydrogen bubbles, which increase a pressure gradient in a microchamber above or below the membrane. Mainly aqueous sodium chloride is used as the electrolyzing fluid due to the voltage being lower to actuate because of the ions present in the fluid and the cheap cost of sodium chloride. This pressure deforms the membrane, and causes it to close the microfluidic channel.^[1] Actuation of this device can be achieved if the device is disrupted from its resting potential, which results in either bubble formation or consumption.^[3] This can be mathematically modeled through the Nernst equation:^[3]

[math]\displaystyle{ E_{e} = E^{0}+\frac{RT}{NF}*ln\frac{C_{0}}{C_{R}} }[/math]

Where:

[math]\displaystyle{ E_{e} }[/math] is the equilibrium potential

[math]\displaystyle{ E^{0} }[/math] is the standard redox potential

[math]\displaystyle{ R }[/math] is the universal gas constant

[math]\displaystyle{ T }[/math] is the temperature of the solution

[math]\displaystyle{ N }[/math] is the amount of moles

[math]\displaystyle{ F }[/math] is the Faraday's constant

[math]\displaystyle{ C_{0} }[/math] is the oxidized concentration

[math]\displaystyle{ C_{R} }[/math] is the reduced concentration

Due to the precise control of electrolyzation of fluids to control pressure, electrochemical valves can make precise adjustments to the device as necessary. But, because of the need of a microchamber, and the need for the electrolyzation of fluids, these devices tend to be complex and have a slower operation speed compared to the other valves.^[3]

Piezoelectric Valves

Figure 3: Normally closed piezoelectric valve.^[2]

Piezoelectric valve is slightly different from the other two methods of electrically activated valves. The structure of these microvalves involves a piezo-actuator, the valve membrane which is also known as the valve stopper, and the valve seat. This valve operates by a piezoelectric crystal stretching from the electric field applied on it, causing the valve membrane to deform into the microchannel stopping fluid flow.^[2] There are multiple different types of piezoelectric devices, but looking at Figure 4 is an example of a normally closed piezoelectric valve with a stopper. How the normally closed valve operates it by sending an electric signal to the piezoelectric vibrator, causing it to deform down, decreasing fluid pressure in the chamber, and deforming the driving medium, opening the valve.^[2] From research on different types of piezoelectric valves, it has been determined that the opening time is shorter than 10 ms, it had low power consumption, and there is continuous and precise control over the valve. But also suffers from a high-voltage needed for actuation.^[1]

References

1. Qian, J.-Y.; Hou, C.-W.; Li, X.-J.; Jin, Z.-J. Actuation Mechanism of Microvalves: A Review. Micromachines (Basel) 2020, 11 (2), 172. https://doi.org/10.3390/mi11020172
2. Chen, S.; Lu, S.; Liu, Y.; Wang, J.; Tian, X.; Liu, G.; Yang, Z. A Normally-Closed Piezoelectric Micro-Valve with Flexible Stopper. AIP Adv. 2016, 6 (4), 045112. https://doi.org/10.1063/1.4947301
3. Lee, D. E.; Soper, S.; Wang, W. Design and Fabrication of an Electrochemically Actuated Microvalve. Microsyst. Technol. 2008, 14 (9–11), 1751–1756. https://doi.org/10.1007/s00542-008-0594-3
4. Anjewierden, D.; Liddiard, G. A.; Gale, B. K. An Electrostatic Microvalve for Pneumatic Control of Microfluidic Systems. J. Micromech. Microeng. 2012, 22 (2), 025019. https://doi.org/10.1088/0960-1317/22/2/025019
5. Kaigala, G. V.; Hoang, V. N.; Backhouse, C. J. Electrically Controlled Microvalves to Integrate Microchip Polymerase Chain Reaction and Capillary Electrophoresis. Lab Chip 2008, 8 (7), 1071–1078. https://doi.org/10.1039/b802853b
6. Yoshida, K.; Tanaka, S.; Hagihara, Y.; Tomonari, S.; Esashi, M. Normally Closed Electrostatic Microvalve with Pressure Balance Mechanism for Portable Fuel Cell Application. Sens. Actuators A Phys. 2010, 157 (2), 290–298. https://doi.org/10.1016/j.sna.2009.11.030