Pressure Actuated Valve Test Design

Pressure Actuated Valve Test Design

Objective

The pressure actuated valve design was made to see if simple control mechanisms could be incorporated into a three-dimensional microfluidic system. While the two previous designs both contained elements of multi-dimensional flow, they lacked the ability to have this flow controlled to any real degree. Since the main purpose of this project was to eventually be able to make liquids flow in any hole and out any other, the valves were crucial for any real success.

Device Logic

The beauty of this valve design is the simplicity of the mechanism that is used. A thin layer of PDMS is put over a layer that has lines filled with gas instead of fluid. Above the thin layer, where the gas line crosses under the fluid layer, a small gate is added that stops the flow of fluid when the gas line is pressurized. When the pressure is lowered in the lines, the thin layer flexes down, creating a gap for the liquid to flow through (Figure __).

The overall layout of the design (Figure __) was chosen to fit with the preexisting packaging that was available. The T-section that was present in the second design was used because it offered a place where the fluid could flow in two different directions, and controlling the flow at this point would the first step in showing that fluid control could be achieved. The other line put into this design, which simply runs across the top layer, and contained two valves was added to show as a way of showing, if the other section failed, whether the valves were to blame.

This design is useful for several reasons, first of which was the aforementioned ease of design, and ease of understanding. This design is far easier to understand and use than the other designs that were looked into as possible choices. The other main reason this design was chosen was because it was the only one that we has that tools and materials to make with the time and monetary restraints that were present in the class. While some of the other designs may have been more advanced, and may have worked better, this design could actually be manufactured.

Device Dimensions

The basic dimensions for the third device design were preserved from earlier stages. Values for channel width, reservoir dimensions, and channel layer thickness were conserved. The new dimensions of design to take into account were the thickness of the flexible membrane separating the gas channel from the fluid channel and the gate responsible for the closing of the valve. The flexible membrane thickness needed to be thick enough to allow fabrication while still being thin enough to be able to deflect sufficiently under pressure. The thickness of the PDMS flexible membrane layer was decided to be 50 µm. The gate length was designed to be across the entire 500 µm of the channel and to have a width of 100 µm with a thickness identical to that of the SU-8 layer it is a part of, 100 µm. The gas channels were designed with the same attributes as the fluid layers. The gas channels, like the fluid layers, were designed with a height of 100 µm and a width of 500 µm.

Materials

PDMS and SU-8 were the materials decided upon to make the microchannels and structure of the device at this stage. SU-8 is used not only for creating the patterns for the PDMS but actually as a structural material. Both PDMS and SU-8 were selected because of the different requirements of the design. The material used for the actual channel structure was not as selective as the material needed for the flexible membrane and gate. The flexible membrane was designed to make use of the flexibility of PDMS. The gate needed to be more rigid than the PDMS membrane to enable adequate closing of the valve. The gate was designed to make use of the rigidity of SU-8. Because SU-8 is a photoresist and due to the current valve design, it then became necessary to make use of two substrates to allow the fabrication of the design at this stage. The bottom substrate was decided to be silicon, as in earlier stages. The top substrate was decided to be pyrex so that the device would remain visible because pyrex is optically transparent.

Processing Method with Mask Design

SU-8 is now a structure material in our design, and the processing steps change to include the new gate features. However, SU-8 was used to create molds for the PDMS layers in Stage 2 and we have become familiar with its processing. Since a detailed description of processing conditions, such as temperatures, were give in Stage 2 this section will only focus on the construction of the device with regards to the new design.

Mask #1- Bottom Fluid Layer Mask #2-Gas Layer

Mask #3- Thin Flex Layer Mask #4 – Top Fluid Layer

Mask #5- Top Layer

1. Create the bottom fluid layer by spinning on SU-8 and exposing and developing it to mask design #1
2. Add the PDMS gas layer. This layer was created by using a SU-8 and Si mold described in Stage 2’s processing method section. Mask design #2 correlates with the PDMS gas layer
3. Add the flex PDMS layer. (mask #3) This is the last layer to be placed on the bottom portion of the device
4. Next, make the top layer of device by spinning and exposing (mask #5) SU-8 on a Pyrex wafer. But do not develop the SU-8, wait for the next layer.
5. Spin on another SU-8 layer and expose it using mask #4. This will be the top fluid layer.
6. Develop the SU-8 on the Pryex wafer
7. Align and sandwich the top and bottom portions of the device together.

Additional Issues

In the first two stages the design is passive and the team did not have to heavily consider the mechanical behavior of the chosen materials. With the addition of the gas actuated valves we now need to predict the mechanical response of the PDMS membrane. Two models were created for this purpose. One model calculates the needed pressure that will result in a desired deflection to adequately open the value. The other model takes into account the fluid resistance at specific sections in the channel to predict back up pressure. Detailed explanations and calculations can be found the Project Results section.

Stage Summary

The Stage 3 design is very different than its predecessors in several ways. The most pronounced is the inclusion of fluid control elements. The gas actuated values bring us closer to the goal of a controllable microfluidic device. With the values pathway options are increased, as is timing control for reaction testing.

Another larger difference between Stage 3 and Stage 2 is the use of SU-8 in the microfluidic system. By integrating this material on both top and bottom, the device is no longer built layer by layer in sequential order. Now the system is built by creating two separate sections and adjoining them. SU-8 was frequently used in Stage 2 as a mold for the PDMS layers and including it directly to the design does not create a new process method, but a new series of process steps.

Though Stage 3’s design is advanced with the addition of fluid control valves, its test site lay-out is not extensive compared to Stage 2. The experimental design only tests two active sites, a portion of a bottom channel, one top channel, and one interconnect. Stage 2 was able to test many more channels, interconnects, and many alternative pathways. There are many directions a future design could go in; a change in logic, or in actuation method.