Electrochemical Bubble Valve - Claudia Mokdad

Introduction

Microelectrochemical systems contribute to ongoing and further research towards scientific discoveries within micrototal analysis system (µ-TAS). Bubble-based actuation is a practice based on the simplicity to fabricate the technological device and bubbles are easily able to fit towards different sized microfluidic channels. These microfluidic devices are not limited to electrochemical bubbles. There are a number of different methods to generate bubbles, such as piezoelectricity, electrostatics, pneumatics, thermopneumatics, and electromagnetism.

Electrochemical valves are dependent on the hydrolysis of water based on the formation of a two-phase system. Generally these systems can be a gated, or non gated (capillary-force) through small channels.

Methods

Figure 1 The driving forces to produce H₂ bubbles where ΔV = ΔE indicating the potential of reduction/oxidation reactions from occurring at the anode, indicating lower standard potential. These equations indicate the bubbles can also be produced thermally and not limited to electrochemical systems.⁴

The main component within electrochemical bubble valves is the interaction between the cathode and anode interaction of (hydrogen-based) bubbles which can be visualized in Figure 1. Figure 1 is one representation of the cathode/anode relationship where in some cases a chlorine relationship is used. Electrodes vary based on research where a gold electrode was utilized to increase the low sensitivity of the current detection method and the electrochemically active area surrounding the object.⁵ Figure 2 represents a general schematic of electrochemical bubble system on a chip based on a silicon wafer with microfabrication techniques. This specific research indicated a need for a 15 µm neck to create a back pressure to allow adequate forces to prevent bubbles from blowing away.¹

Figure 2 Scanning electron microscope (SEM) of electrochemical bubble valve. Electrochemical bubble valve chip includes electrode pairs across the microchannel in order to produce bubble valves. Image adapted from reference [1].

Materials

To create an electrochemical system, an anode/cathode electrode pair is created with inlet and outlet reservoirs.¹ Some studies indicated the use of silicon for the microfluidic device to characterize the bubble valves while some use a glass substrate (wafers with PDMS and the use of hydrogen to produce these bubbles to generate/restrict flow through the channels.^1,2,3 A common denominator between most studies is the use of 1 pmol of salt (NaCl - Sodium Chloride) to produce these bubbles. A precursor solution and reagents are supplied from outside sources and pH indicator/dyes were obtained for visualization of the contents occurring within the device.^2,3 Salt is generally used to general salts at the micro and nano-level in minuscule amounts. In other cases, varying on research, hydroquinone and bromothymol blue was purchased to compare thermally dynamic compounds to show bubble-induced motion and electrolyte solution for pH indications based on external electric fields.⁴ Important to note that these devices can be made with silicon, glass, plastic or PDMS depending on area of research and goals to be obtained. Figure 3 displays different formation techniques of the bubble valve through a check valve and pumping from various other researchers. Both implement different materials and comments with or without the use of a bubble valve but rather a bubble pump formation outside of the various papers.

Figure 3 Bubble valves occur in different ways, referenced through the image. Based on location of the check valve to either build the bubble valve through (1) directional bubble growth based on the flow of the channel. (2) Built-in bubble transportation already implemented in the chip and (3) symmetric bubble collapsing based on liquid within the section of the channel through a potential membrane within the channel. Image adapted from reference [1].

Challenges

Various challenges occurring depending on the application of electrochemical bubble valves and the way they are implemented. While electroosmosis is currently used in biochemical molecule separation, a problem presented is the amount of power consumption and voltage required in order to create a bubble to regulate fluid flow within a channel.² This challenge varies from different applications and studies but is the most commonly reported challenge. Pumps and valves are commonly used but are still in development to improve current setups to prevent clogging of bubbles. Clogged bubbles are also a result of the transport liquid system which can be affected by the type of electrode system setup since many applications report using upwards of a 5-way electrode pair or higher.³ This can also be bypassed by incorporating a check valve but is still in research and development. Within electroosmosis, one study showed difficulty in separating acidic electrolytes from the sample solutions that obtained varying pH, but a bubble was generated between these two solutions and consistent flushing between the buffer solutions occurred.² Figure 4 displays research from one group, and how their results help interpret and readjust their current form of electrochemical bubble valves. Based on the Figure 4A-C, the varying flowrates are dependent on the amount of voltage across the electrode through the channel. With an increase in voltage (V = 4.6V), the valve closing flowrates become steeper with the increase in voltage. Figure 4D represents the valve opening characteristic where both the opening closing are dependent on the rte of collapse of the bubble valve. The bubble valve is also dependent on the hydraulic resistance present in the valve opening.

Figure 4 Figure A-C represent valve closing characteristics at varying flow rates with different voltages. These graphs display that flow regulation is controlled by tuning the voltage at a specific flowrate and cross sectional area of the channel. Figure 4D displays the valve-opening characteristic based on two different flow rates. Image adapted from reference [1].

Applications

Figure 5 Figure 5a represents a PDMS, silicon and glass substrates with electrolyte solutions to display the valve opening characteristic. Figure 5b displays the closing valve characteristic with flow and bubble location/behavior. Image adapted from reference [3].

Electrochemical bubble valves are used in a range of different applications including analytical chemistry, artificial organ (incorporates bubble traps), drug delivery, microbiology, and biotechnology in order to regulate flow.¹ Figure 5 displays general valve opening and closing characteristics displayed within research for specific applications. The research based on Figure 5 is in regards to bio and chemical sensors and the valve is supported through a sensor which it will allow the formation of a bubble valve once a specific substance is detected.

Current Studies

Denoted in most reports, bubbles valves are generated inside the microchannels in order to stop or hinder flow inside the channel. These are generally done with small durations of a voltage pulse. The bubble diameter must be slightly smaller than the channel in order to produce the best results. Flow is restored based on the hydraulic resistance of the fluid between the bubble and the channel.¹ Figure 6 displays application based on the detection of methylated DNA based on nanoelectrokinetics. Figure 6A is a schematic diagram of the methylated DNA based on electrochemical detection through a buffer channel. This included four pneumatic valves in order to allow the buffer solutions and varying concentrations to carry out through the sensing chamber. Figure 6D displays the operational aspects to the use of electrochemical valves based on a specific research compared to various other papers.

Figure 6 Figure A is the general schematic of the electrochemical detection of methylated DNA. Figure B is the view of the chip by each layer, C is the actual image of the chip. Figure D are the varying components to the valves based on sensing including preconcentration, isolate and incubation. Image adapted from reference [5].

General Trends

As studies continue, general trends are described as:

1. The ratio between the size of the object to the area inside increases with reduced dimensions.^1,3
2. Change in interfacial tension decreases the dimensions of the microfluidic channel as the bubble pressure increases.^1,3

In order to optimize the bubble valve opening and closing characteristic based on dimensions, voltage, and flowrates An important note is that valve opening is dependent on the rate of which a bubble collapses but the closing rate will increase with an increase in voltage of the system (reported that opening and closings of valves can occur within milliseconds).¹ Within the rate of collapse, it is denoted as

[math]\displaystyle{ Rate of Collapse = \frac{3 R T φ}{4 r} }[/math]

where φ is the permeability of gas-liquid interphase, RT is the gas constant and temperature and r is the radius of the spherical bubbles at a constant hydrostatic pressure.¹ The equation indicates that the smaller the bubble radius is, the rate of collapse increases compared to larger bubbles, showcasing that reduced microfluidic channels are better suited for bubble valves electrochemically produced.

Conclusion

Electrochemical systems have become increasingly popular within research, especially with bio-related aspects. Further research is looking at rather valve usage, but electrochemical bubble pumps based on lack of bubble collapse. Pumps do result in difficulty in bubble formation due to varying parameters such as pressure, flowrates, and electrical voltage applied to the system.

Electrochemical bubble valves need to be researched further due to varying reviews and papers with research. The varying degrees of electrochemical bubble valve schematics with materials also vary through the exact application with either glass, silicon or PDMS substrates and altering results from each usage.

References

1. Susan Hua, Frederick Sachs, David Yang, and Harsh Chopra. "Microfluidic Actuation Using Electrochemically Generated Bubbles." Analytical Chemistry, no. 74 (2002): 6392-6396. https://dx.doi.org/10.1021/ac0259818

2. Hiroaki Suzuki, and Rei Yoneyama. "Integrated microfluidic system with electrochemically actuated on-chip pumps and valves." Sensors and Actuators B, no. 96 (2003): 38-45. https://dx.doi.org/10.1016/S0925-4005(03)00482-9

3. Wataru Satoh, Yoshifumi Shimizu, Takeshi Kaneto, and Hiroaki Suzuki. "On-chip microfluidic transport and bio/chemical sensing based on electrochemical bubble formation." Sensor and Actuators B, no. 123 (2007): 1153-1160. https://dx.doi.org/10.1016/j.snb.2006.10.028

4. Laurent Bouffier, and Alexander Kuhn. "Design of a wireless electrochemical valve." Nanoscale, no. 5 (2013): 1305-1309. https://dx.doi.org/10.1039/c2nr32875e

5. Sung Hong, Yong-June Kim, Sung Kim, and Sung Yang. "Electrochemical detection of methylated DNA on a microfluidic chip with nanoelectrokinetic pre-concentration." Biosensors and Bioelectronics, no. 107 (2018): 103-110. https://dx.doi.org/10.1016/j.bios.2018.01.067