Thermally Activated Valves

ALTERNATIVE VALVE DESIGNS

The pressure valve from stage three appeared to be effective. It was however desirable to have an electrically actuated valve. The following valve designs are all set up under similar conditions. These conditions stemming from the gas valve.

All of the designs maintain the PDMS flexible membrane, as this is desirable to prevent any interactions. Below the membrane, instead of the gas line, an isolated enclosure is used. The contents of the enclosure change depending on the design, however each contains some type of fluid. The activation of the valve will then cause the membrane to deflect, which will push against an SU-8 peg thereby closing the valve. To open the valve the electrical actuation is removed and the membrane returns to level

THERMALLY ACTIVATED VALVES:

The thermally controlled valve works under the principle that an increase of heat to an enclosed liquid will cause the liquid to boil. The boiling will then cause an increase in pressure as the gas forms and expands. The increased pressure will push on all walls of the enclosure equally, however because the top of the enclosure is the thin membrane, the top will flex upward hitting the stopper and therefore closing the channel. To open the channel all that needs to be done is for the heater to be turned off. Once no more heat is being entered into the system, the existing energy will leave and the gas will condense back into liquid form.

To reduce the amount of heat required to heat the liquid to boiling, a highly volatile substance should be used. In a similar experiment [1] cylco pentance was used. For that experiment only a 7o rise was needed to create a 6.5 Kpa pressure change. Assuming a similar heat to pressure ratio was attainable, the thermally activated valve would be a feasible option for electrical actuation.

ELECTROCHEMICAL VALVES:

Electrochemical valves operate on the principle that the addition of a potential to an electrolytic solution, such as water, will force a phase change reaction that can be used to deflect membranes for use in micro-valves. The flowing reaction characterizes the electrolysis of water to produce both oxygen and hydrogen gas: Energy (electricity) + 2H2O -> O2 + 2H2. This is similar to reactions used to generate hydrogen gas on the macro-scale for use in fuel cell technology. The current Stage 3 Final Manufactured Design uses a pressure differential to flex a PDMS membrane; however, this requires the input of multiple gas lines. To alleviate this problem and make packaging of the final device more streamlined, it is more convenient to use imbedded electrical lines to direct the logic of the valves. To this end, electrochemical valves are one of the three classes of alternative design options that were considered in this investigation. Figure 1 below shows the theoretical basis for this design option. The addition of a potential on the water reservoir using metal, typically Pt, Cu, or Ag, electrodes forces the breakdown of water into hydrogen and oxygen gas. If the voltage is reversed, then the constituent gases combine to form water. The reversibility of this reaction provides a means to effectively open and close valves based on electrical input.

Figure 2: A figure of a typical Electrolysis reaction [2]

Given the PDMS used in the existing design is a dielectric, transmission of the voltage only to the water for phase change should be relatively easy. Moreover, since this phase change reaction operates on the use of electrical rather then thermal actuation, there are fewer problems in terms of heat dissipation and material breakdown due to thermal cycling. The amount of chemical change in the liquid is proportional to the amount of current introduced to the system according to the following equation: V = (R*I*T*t)/ (F*P*z), where V = volume, R = gas constant, I = current, T = temperature (K), F = Faraday constant, P = pressure and z = # excess electrons. Given this design option, Figure 2 below shows how this design could be integrated into the existing Stage 3 Final Manufactured Design.

Figure 3: A figure of the potential to integrate an electrochemical valve into the existing Stage 3 Final Manufactured Design.

Figure 2 shows how two sets of electrodes and necessary routing can be patterned on the Silicon wafer prior to the addition of the subsequent SU-8 and PDMS layers. Though the details of the device dimensions and processing sequence have not been determined, based on Figure 2, it can be seen that integrating this device option with the existing design should be relatively straightforward. This is a potential valve alternative that could be considered for future developments on this project, or other similar projects trying to design, fabricate and test multi-level controllable micro-fluidic devices.

PIEZOELECTRIC VALVES:

Valves utilizing piezoelectric actuation use the electrically induced mechanical deformation of a piezoelectric material to close or open. This type of valve was considered early on the design process.

Because incorporating piezoelectric valves requires the deposition of a piezoelectric material, the fabrication process for the device would have been of higher complexity. The main problem with using piezoelectric valves was that the mechanical deformation required for a device with the dimensions being used was unreasonably large. To address this problem it is possible to create a larger volume of piezoelectric than the active valve area would allow and then, through a deformation amplification system making use of a constant volume liquid chamber, the resultant deformation is enough to properly close the valve.

The area between the piezoelectric and one surface of the liquid chamber is larger than the surface between the opposite surface of the liquid chamber and the flexible membrane. This allows for a smaller deformation from the piezoelectric to be translated into a much larger deformation of the flexible membrane due to the conservation of volume in the liquid chamber. This valve would work off principles similar to a hydraulic multiplication. This is illustrated in Figure 1, which contains images of both before and after an electric voltage potential is applied to a piezoelectric material that compresses a liquid chamber.

Figure 4: Before and after a voltage potential is applied to the piezoelectric valve

Figure 5: Image of the piezoelectric valve with the microchannel

Figure 2 is an image of the piezoelectric material, isolated liquid chamber, microchannel, and gate. The purpose of the gate is to aid in the closing of the valve. The flexible membrane at the interface between the liquid chamber and the fluid microchannel will be pressed and cause the microchannel to close when the membrane comes into contact with the gate.

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