See comments next page - GWR

Multilayer Microfluidics_______________________________________________________________

Department of Materials Science and EngineeringUniversity of Maryland, College Park

ENMA490Fall 2003

Susan Beatty, Charles Brooks, Shawna Dean, Mark Hanna, Dan Janiak, Chen Kung, Jia Ni, Bryan Sadowski, Anne Samuel, Kunal Thaker

Special Thanks to Dr. Gary Rubloff and Theresa Valentine

INSTRUCTOR’S COMMENTS ON FINAL REPORT:

Please see the comments I have made throughout the report – I think there are important lessons to learn from them.

Introduction: This section is weak in not motivating the project very well, and in not putting enough materials information into the project. As I have reminded you before, this is a materials course despite the fact that we have concentrated a lot more on how to integrate materials into design of products and systems. I don’t think this section was critically reviewed and revised, at least not enough.

Device Design Stages: These sections are generally very good, nicely distinguishing motivation for each design, masking levels, test sites, process flow, and experiments. Constituting the bulk of the report, they reflect good progress in the project. The primary weakness in these parts are a number of places where the writing is unclear or confusing, a problem I attribute to inadequate proofreading and refinement of the report.

Modeling: This is good work and important for the project. However, the variables and units for the equations were not adequately explained, and references to where the equations came from (since most were not derived here) are missing. This section would also benefit from some brief descriptions of the meaning of the equations.

Alternative valve designs: This section is interesting, and you came up with some interesting ideas. In a sense it was too bad we didn’t have time to look at these in more depth.

Preferred design elements: This section is a little short but reasonable, although the perspective is not all that critical an analysis.

Conclusion: This section is weak. If the section on Preferred Design Elements had been more critically evaluated, then the broader perspective from it could have been put into the conclusion, or the two combined altogether.

References: The handling of referencing in your work is a SERIOUS shortcoming. At first there were basically no references (one web site). Of course this is unacceptable for any report or paper. I basically returned the report to you, and now there are 16 references, of which 4 are web sites and 2 are magazines (Industrial Physicist). This might be considered acceptable, but mediocre, particularly since I know you found and read numerous other references. However, many places in the report should have noted references but did not (see my detailed comments), so that the referencing is at best a patch job. I believe this flaw is an unanticipated consequence of inadequate planning for completion of the report. But I must admit to my disappointment after my efforts to give guidelines about referencing on the class web site at http://www.isr.umd.edu/gwrubloff/teaching/enma490fall03/referencing.htm and the time we spent a the beginning of the course discussing this. This is an issue of professionalism and engineering ethics.

Writing style: You will notice some places where imprecision in wording either makes the discussion seriously unclear, or even wrong. When it comes to explaining research results, you must try to be precise in both content and writing. The mindset should be to explain things to a novice, and thereby to consciously avoid imprecision. Furthermore, with a team project, at least a second person should be charged to read and

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revise the first draft of each portion, so that a cross-check on clarity is done. Virtually all writing has this activity somewhere along the line.

Instructor’s summary:

The accomplishments of this project are very good. You investigated many dimensions of a real materials-based technology challenge, pursued experimental and modeling work, analyzed designs and process flows, and most of you contributed actively in class. All this made a stimulating experience for me, and I think for you.

Unfortunately, the quality of the final report falls notably short of reflecting the quality of the work you did. Integration of the report - particularly proofreading with a keen eye to consistency, clarity, and logic – was inadequate. The absence of referencing was a major shortcoming, which should have been easily avoided with proper planning.

We emphasized from the beginning that there is a firm end date, so all work has to be planned and scheduled to make those deadlines. This planning was the responsibility of the class, particularly the development team, and if this had been done correctly the proofreading and referencing would have been far better. I might have required the report earlier so that there would be time for iteration and repair, but I considered this part of your responsibility as a project team.

During the course, and despite my attempts to highlight the issues, I sensed several times that the importance of project planning and the role of some kind of development team were not understood or appreciated. I believe this impression was validating by the referencing problem, the inadequate attention to proofreading and consistency in the report, and the anxiety accompanying the last-minute practice just before the final presentation. I hope your learning from this course includes an appreciation of the importance for each team member to make sure that he/she understands the overall project and how the pieces fit together (even if not in great technical detail), to help keep the project moving toward the intended goal along an appropriate timeline, and to meet the deadline with a quality product.

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Table of Contents

Introduction…………………………………………………………………………………………………...3Problem Definition……………………………………………………………………………………..3Problem Scope…………………………………………………………………………………………3Initial Materials Information…………………………………………………………………………...3Initial Literature Research Results……………………………………………………………………..3Device Design Overview………………………………………………………………………………4

Device Design Stage 1: Initial Microchannel Test Design……………………………………………….....4Device Objective……………………………………………………………………………………….4Device Logic…………………………………………………………………………………………...4Device Dimensions…………………………………………………………………………………….5Materials………………………………………………………………………………………………..6Processing method with Mask Design………………………………………………………………....6Stage 1 Summary……………………………………………………………………………………....8

Device Design Stage 2: Modified Microchannel Test Design………………………………………………8Device Objective……………………………………………………………………………………….8Device Logic…………………………………………………….…………………………………......9Device Dimensions…………………………………………………….………..……………………10Materials…………………………………………………….……………………………...…………10Processing Method with Mask Design………………………………………………………………..10Fabrication Step: SU-8 Molds…………………………………………………….…………………..13Fabrication Steps: PDMS Microchannels…………………………………………………………….13Experimental Trials…………………………………………………….………………………..……14Future Work …………………………………………………….……………………………………16 Stage 2 Summary…………………………………………………….……………………………….16

Device Design Stage 3: Pressure Actuated Valve Test Design……………………………………………16Objective……………………………………………………………………………………………...16Device Logic…………………………………………………….……………………………………17Device Dimensions…………………………………………………….…………………………..…17Materials…………………………………………………….………………………………………...18Design Problems…………………………………………………….………………………………..18Processing Method with Mask Design………………………………………………………………..18Additional Issues…………………………………………………….………………………………..19Stage 3 Summary…………………………………………………….……………………………….19

Membrane Deflection Modeling…………………………………………………….……………………...20Fluid Flow Modeling…………………………………………………….…………………………………..21Alternative Actuated Valve Designs…………………………………………………….…………………22

Piezoelectric Valves………………………………………………………………………………..…22Electrochemical Valves……………………………………………………………………………….23Thermally Activated Valves…………………………………………………….……………………24

Preferred Design Elements…………………………………………………….……………………………25Channels…………………………………………………….………………………………………...25Valves…………………………………………………….…………………………………………...25Scaling…………………………………………………….………………………………………......25

Conclusion…………………………………………………….……………………………………………...26Appendix…………………………………………………….……………………………………………….26

Gantt Chart…………………………………………………….…………………………………...…26Fluid Flow Modeling………………………………………….…………………………………...…26

References…………………………………………………………………………………………………..27

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Introduction

Problem Definition To use micro processing techniques to address the problems associated with multi-level

channel routing in bio-micro fluidic applications [1] To integrate materials application for building the layers of a multilevel micro fluidics

system To use a control system to arrange fluidic flow through the multilevel micro fluidics

Problem ScopeThe mission of this project is to create a multi-level micro-fluidics system for bio-micro fluidic

application.

The packaging of this device should be efficient, feasible and versatile because we would want the fluid flow to reach multi-levels instead of remaining on a single layer. Active control devices will control the fluid flow. To flow from one layer to another layer we would have vertical vias or interconnects from the first layer to the next. Therefore to process this we would need the basic knowledge of materials that are feasible and current research accomplished on micro fluidics. These are mentioned in the Materials Information and Literature Research section.

Due to time budget our group decided to neglect the biochemistry interactions of the fluid and the interior walls of the channels. We will only consider the fluid flow and how to transport the fluid from one reservoir to another within the system. We will be looking at many control systems that will manage the fluid flow throughout the channels and interconnects. All the control systems we will be discussing will be internally integrated within the micro fluidics system. The design of the control system will be discussed more thoroughly in stage 3 of the Devices Design Stages. Therefore the biochemistry interactions will not be discussed in our report due to time constraints, and we will not use external control systems.

Initial Materials InformationMaterials considered for our micro-fluidic design consisted of Pyrex and silicon substrates with

polydimethylsiloxane (PDMS), (SU-8), and (PMMA) layers [2]. Piezoelectric materials were also researched as possible materials for pressure actuated control valves. Our final design utilized silicon as a substrate, with PDMS to form channels and a flexible membrane layer, and SU-8 layers to fabricate rigid fluid flow control gates. We narrowed down our list of potential materials by determining the desired material properties in our design as well as the ease of manufacturing of each material.

Initial Literature Research ResultsWe divided our group into teams researching different areas of interest including microchannels and

control devices. The microchannel team researched multilayer micro-fluidic designs. The control device team researched various control valve designs.

Single level microfluidic devices are limited to fluid flow in two-dimensions. To explore the advantages of microfluidic devices having more than a single level, we examined Prof. Stephen R. Quake’s work on microfluidic multiplexors that are combinatorial arrays of binary valve patterns [3,4]. Their work focused on increasing the processing power of a network by allowing complex fluid manipulations with a minimal number of controlled inputs. The multiplexors worked as a binary tree and allowed control of n fluid channels with only 2 log2 n control channels. The integration of additional microfluidic levels was shown to overcome the limitations of single level microfluidics.

In the effort to control fluid flow in microfluidic devices, an attempt is being made to phase out check valves and other mechanisms that slow down the frequency response of the pumping system. The

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control device team researched many controlled valve designs including pressure, bubble, and PZT actuated valves [5]. The easier and cheaper the valve is to fabricate the more likely it will be used. From the literature on various valve designs, the pressure actuated valve seemed to be the most feasible design for our project.

Device Design OverviewIn each device design stages we will have have objectives, device logic, device dimensions,

materials used in the design, the processing method, additional issues, manufacturing results and experimental results.

The initial microchannel design consists of only 2 layers with interconnects. The initial design purpose is only used to test if the fluid flows through channels. Controls are neglected in this design because if fluid cannot flow through the channels then adding controls will not necessary. The second stage is the modified version of stage one which is designed to fit the packaging that will be used during testing. The third stage and final design stage consists of an actuated valve that will allow control over fluid flow. Within each stage are fabrication and experimental results that leads to transition from one stage to the next.

Device Design Stage 1: Initial Microchannel Test Design

Device ObjectiveOnce the design requirements and assumptions were finalized, the group determined that testing

preliminary designs on the path to a final design was necessary to ensure constant feedback to assess the practicality of design choices. In this path, the testing of the fluidic channels was of primary importance, as the option of including control elements would be mute moot if the fluid itself was unable to pass through the channels designed. Therefore, the group generated the Initial Microchannel Test Design. The purpose of this device was to allow the group to test the experimental capabilities available to us, as well as establish a base upon which more complicated devices could be modeled. More specifically, the device was designed to test the viability of basic multi-level micro-fluidic devices with the equipment and materials currently available.

Device LogicThe design that was chosen consisted of the simplest two-channel horizontal/lateral interconnect

layer three-dimensional channel geometry that could be constructed, and at the same time test the practicality of multi-level microfluidics. Moreover, the group decided to use sequential layers of PDMS molds to build up the desired structure. These molds were to be stacked on a bare silicon wafer, which would provide rigidity for transport and testing. There were several reasons for this choice, which included the following:

1. The PDMS layers would allow for heightened design flexibility, as the molds could be re-used and the layers created from these molds could be stacked numerous times in several different orientations.

2. The existing knowledge available to the group based on prior experimental tests done on similar processes by students in the department.

3. The known material compatibility between PDMS and many biological agents that could be used in multi-level micro fluidic devices.

4. The equipment and material constraints based on availability, or lack thereof, of potential materials and equipment for other, less common forms of microprocessing.

5. The PDMS and SU8 molding process and fabrication of isolated, patterned PDMS layer from SU-8 molds were was known to have a relatively fast turnaround time, and this was critical due to the time constraints on the semester.

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Figure 1 below shows a schematic of the proposed design. The colors (blue, green, and yellow) signify voids within the PDMS (white). The design consists of three layers: one interconnect layer, and two microchannel layers. The lower microchannel layer (shown in blue) connects to the interconnect pegs (shown in green), which in turn, connect to the top microchannel layer (shown in yellow). The circles located at the ends of the microchannels are the reservoirs which run from the top to the bottom layer, and provide top down access points to all the microchannels, thus allowing for fluid access to all microchannels to assist in testing. Fluid flow into anyone of the 12 reservoir inputs would allow the fluid to enter the device and test to ensure the fluid was able to stay within the pre-determined microchannels that were constructed, as opposed to being forcedforcing between the layers and resulting in layer delamination.

Micro-Channel Layer 1

Micro-Channel Layer2

Interconnect Layer

Figure 1: A Schematic of the Initial Microchannel Test Design (Top View)

Device Dimensions Based on the logic of the design, the group then determined some appropriate dimensions. Given that

this was the first design stage of the semester, there were only a few constraints on the dimensions that could be chosenknown or understood, so the dimensions were chosen based on the approximate sizes encountered in most of the literature. The only constraints that were considered during the device dimensioning were the overall silicon wafer size of diameter = 4 inches and the fact that the PDMS layers could not be molded to a thickness greater then approximately 100m, given past experimental use with the material. Below in Table 1 is a summary of all the critical dimensions that were determined for this Initial Microchannel Test Design. These dimensions were chosen very loosely as the purpose of this design was to test the general performance of the design proposal and not the specifics of the device geometry.

Critical Dimension ValuePDMS Layer Height 100mMicrochannel Width 150mMicrochannel Length 45mmInterconnect Width 150mInterconnect Depth 150mReservoir Diameter 300mDistance Between Channels 300m

Table 1: A Table of the critical dimensions for the Initial Microchannel Test Design

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The cross sectional shapes of the microchannels were rectangularprocess limited, as the SU8 and PDMS molding process does not easily allow for the creation of ridges or grooves that are non-rectangular. Therefore, the cross-section the microchannels were made rectangular. Given the different orientation of the reservoirs and interconnects relative to the micro channels, they could have been made any number of shapes, however, for simplicity, the interconnects were made square in vertical cross-section and the reservoirs were made circular as seen in layout viewin cross-section. These dimensions and geometry constituted, what the group thought asconsidered, the most basic design option to test the viability of multi-level micro-fluidic devices.

Materials In the introductory sections, we gave a list of materials that are candidates. In this section, we will be discussing the materials that are used, as well as why they are used.

At this stage, the materials used for fabrication of our device are silicon, SU-8 and PDMS (polydimethyl siloxane). We selected a silicon wafer as our substrate because it is cheap and convenient for most of the fabrication processes like lithography. PDMS is a soft polymer that has attractive physical properties, in addition to a low cost. Fabricating PDMS involves a lithographic process. Its physical properties include elasticity, conformality, optical transparency, etc. Devices made of PDMS can be integrated with other components, since PDMS conforms to materials like silicon or glass easily. This conformal property makes both reversible and irreversible sealing possible. It is non-toxic to biological agents, such as proteins, and it is gas permeable. Also, since it is transparent in the visible/UV region, it is compatible with many optical detection methods.

SU-8 is a negative based epoxy photo-resist consisting of 8 epoxy groups [6]. This photo-resist is photosensitive and forms a cross-linking reaction when exposed to light [7]. During developing, the SU-8 coated regions are not removed. The characteristics of this particular photoresist are the following: provides good adhesion to where it is spin-coated, near UV-sensitive, high aspect ratios (~15 for lines and 10 for trenches), and it works for a range of thicknesses (750 nm to 500 m can be coated using a conventional spin-coater). SU-8 is spin coated on a Si wafer, and after exposure and developing, can be used to create reverse mold patterns of micro channels, reservoirs and interconnects for patterning PDMS layers.

Processing Method with Mask Design Based on the initial mask design, the process requires the creation of SU-8 molds, which in turn will be used as a template for the subsequent PDMS layers. In this section, the process sequence for the initial design is discussed in detail. The initial mask design is shown in Figure 2.

Mask 1 Mask 2

Figure 2: Mask 1(Channel mask) & Mask 2(Interconnect mask)

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Our process sequence begins by coating SU-8 on a Si wafer, exposure using Mask 1 or Mask 2 to create the molds, followed by spinning PDMS on the molds, and finally stacking the PDMS layers to form the final structure. The process sequence is given below:

1. Begin with a polished Si wafer.2. Spin SU-8 (negative photoresist) on Si wafer and pre-bake at 95°C.3. Align wafer with Mask 1 (Figure 1) and expose SU-8 to ultraviolet light. Post-bake at 95°C. 4. Develop SU-8 in SU-8 developer and unexposed areas are removed. This creates Mold 1 from Mask 1. In the same way, Mold 2 is formed from Mask 2. Figure 3 shows both Mold 1

and Mold 2

Mold 1 Mold 2 SU-8 Protrusions

Figure 3: Mold 1and 2 from exposure and development of an SU-8 surface using Mask 1 and Mask 2 respectively

5. After creating the molds, spin on the PDMS less than the vertical dimension of the SU-8 protrusions. Dip the Si wafer in a sodium dodecyl sulfate(SDS) adhesion barrier and allow it to dry

naturally. Mix PDMS (Sylgard 184, Dow-Corning) 10:1 with curing agent. Spin on PDMS. Bake in box furnace for 2 h at 70°C.

6. Spin PDMS Layer 1 on Mold 1 (Bottom Fluid Layer), PDMS Layer 2 on Mold 2 (Interconnect layer) and PDMS Layer 3 on Mold 1 at 90° rotated relative to PDMS Layer 1(Top Fluid layer). Make a total of two layers from the channel mold and one layer from the interconnect mold. Figure 4 shows the PDMS Layers 1, 2 and 3.

PDMS Layer 1 (from Mold 1) PDMS Layer 2 (from Mold 2) PDMS Layer 3 (from Mold 1) rotated 90° relative to Mold 1

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Figure 4: Three completed PDMS Layers

7. Stack all three PDMS layers in the following order: channel, interconnect, channel (90° rotation from the first channel layer). The final result of the stacked PDMS Layers is shown in Figure 5.

Bottom layer Middle layer Top layer

Figure 5: The final result of three PDMS layers stacked on one another (Top View)

Stage 1 SummaryStage one was designed to be a logically simple device that met the overall objectives of the project.

The general concept of how the fluid should flow through the device and between layers appears to be accepted as a viable approach. The materials and the processing of the device also appear to be on target. For this beginning stage, it seems that the fluid flow, dimensions and arrangement of the channels will need to be modified before continuing on to the next stage. Given these limitations, the Stage 1 device was not fabricated and efforts were focused on developing a more viable Stage 2 device that would allow for effective testing of similar design concepts.

It was determined that adjusting the design to fit the existing packaging would be advantageous for testing. This translates to moving the inlet and out let reservoir holes to the same positions as the inlets and outlets on the packaging. The packaging also has some affect effect on the reservoir dimensions. The reservoir diameters will also need to be consistent with the diameters of the inlet and outlets on the packaging.

Other dimensions, not affected by the packaging may also want to be changed. For a preliminary design and testing phase ease is of great importance. The dimensions will need to be adjusted so that both ease of manufacturing and ease of testing are optimized.

Lastly, it appears that the simple grid design will need to be modified in order to more efficiently test the capabilities of the device. This may include deleting portions of the channels and possibly removing some interconnects.

Device Design Stage 2: Modified Microchannel Test Design

Device ObjectiveBased on the design of Stage 1, and the inability to fabricate and test the design because of packaging

integration problems, Stage 2 had three major objectives. The first objective was to adapt the reservoir positions from Stage 1 to locations matching the existing acrylic packaging solution. The second objective

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was to reduce the number of I/O and interconnects to produce unique flow paths to test different flow conditions and routes. Finally, the third objective was to scale up the dimensions of the device to ease fabrication and testing. The overall objective of Stage 2 is to address the shortcomings of Stage 1 to test the viability of a two level passive micro-fluidic device. Fabrication and test data from this stage will be necessary to move toward the eventual goal of a two level actively controlled micro-fluidic device.

Device LogicAs in Stage 1, the device for Stage 2 was to be constructed by stacking PDMS layers on a silicon

wafer. The PDMS layers were to be made from a SU-8 based mold. In Stage 2, this stacking sequence included two distinct micro-channel layers, one interconnect layer, and one top cover layer to provide a seal with the acrylic packaging. Also based on Stage 1, the logic of Stage 2 continues to use a simple grid pattern to move fluid within and between fluid layers. However, the locations of the reservoirs were changed to fit the existing acrylic packaging option to facilitate testing. Moreover, as can be seen in Figure 6 below, the design includes five distinct fluid paths, using a total of 11 I/O.

Figure 6: A diagram of the Stage 2 device

Each of the five fluid paths were chosen to test different and increasingly more complicated situations, culminating in Fluid path 5, which was to mimic a more realistic fluid path that is more likely to be found in micro-fluidic routing. The five fluid paths test both the logic capabilities of the design as well as the capabilities of the process used to fabricate the device. Table 2, below, outlines the five fluid paths constructed.

Fluid Path 1: This path proceeds down from the input reservoir to the bottom microchannel layer, across the wafer, and back up the output reservoir. This fluid path serves to test the ability of the device to handle simple flow through the interconnect layer.

Fluid Path 2: This path proceeds down from the input reservoir to the top microchannel layer where the fluid is directed in two sequential 90 degree turns and returns back to the I/O next to the input reservoir, where it then exits up the output reservoir. The purpose of this path was to test the ability of the device to handle direction of the fluid in more complicated fluid paths.

Fluid Path 3: This path proceeds down from the input reservoir to the top microchannel layer, where the path runs across the top layer, down to the bottom microchannel layer, across the bottom microchannel layer, and finally up the output reservoir. The purpose of this path was to test the ability of the device to handle more complicated fluid flow (as in Fluid Path 2) on two levels.

Fluid Path 4: This path proceeds down from the input reservoir to the bottom microchannel layer, where the path runs across the bottom layer, turns 90 degrees, then proceeds up to the top microchannel layer, across the top microchannel layer, down to the bottom microchannel layer, across the bottom microchannel layer, and finally up the output reservoir. This path is logically similar

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to Fluid path 3, except an additional 90-degree turn and layer change were added for additional complexity.

Fluid Path 5: This path proceeds down from the input reservoir, across the bottom microchannel layer, up to the top microchannel layer, and diverges in two possible directions, each of which leads to a different output reservoir. The purpose of this path is to test a situation where the fluid has more then one possible fluid path. Moreover, this fluid path is ideal for the testing of a valve in future design stages to direct the flow in one of the two possible directions.

Table 2: A Table summarizing the five fluid paths in the Stage 2 device

Device Dimensions Based on the dimensions of Stage 1, the dimensions of Stage 2 were scaled up to ease in fabrication

and testing. Table 3 below summarizes the critical dimensions that were chosen for Stage 2.

Critical Dimension ValuePDMS Layer Height 100m Microchannel Width 500m Interconnect Width 1000m Interconnect Depth 1000m Reservoir Diameter 0.4 cm

Table 3: A table summarizing the critical dimensions of the Stage 2 device

As in Stage 1, the dimensions were only limited by the maximum PDMS layer thickness of ~100 m and the silicon wafer diameter of 4 inches. Based on these constraints, the dimensions were chosen to make fabrication and testing as easy as possible to observe without the aid of instrumentation such as microscopes, etc. The interconnect dimensions were made twice as large as the microchannel width in case there were problems in aligning the sequential PDMS layers. This larger size interconnect was used to guarantee the two microchannel layers would be connected despite small misalignments during the layer assembly. The reservoir diameter chosen exactly matches that which was needed to fit within the existing acrylic packaging that is available to the group. Adapting the Stage 2 design to the existing package was seen as a way to facilitate a fast and efficient testing setup.

Materials The materials that we used for the fabrication of the Stage 2 device are the following (same as in

Stage 1): Silicon, PDMS and SU-8. The Silicon wafer is used as a substrate, as it is cheap and convenient for fabrication processes like lithography. PDMS is a soft polymer that has properties like elasticity, conformality, optical transparency, etc. Due to its conformal nature, devices made of PDMS can be integrated with materials like glass and silicon, so both reversible and irreversible sealing is possible. In this stage, we used PDMS to create layers using SU-8 molds as described above, as standard lithographic processes make fabrication of these layers possible. And when all the layers are stacked on the top of each other, PDMS easily conforms and makes stacking possible.

SU-8 is a negative based epoxy photo-resist consisting of 8 epoxy groups. This photo-resist is photosensitive and forms a cross-linking reaction when exposed to light. During developing, the SU-8 coated regions are not removed. SU-8 is spin coated on a Si wafer, and after developing, can be used to create reverse mold patterns of micro channels, reservoirs and interconnects.

Processing Method with Mask Design In our preliminary design, the alignment between channels and interconnects was an important issue

that was raised. The misalignment between top layer reservoirs and bottom layer reservoirs could have been

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a problem. Say input 1 causes liquid to flow through output 1 as well as through inputs 2 and 3 in the preliminary design. This causes overflow of liquid in the channels. The Stage 2 device was designed in such way that the top layer had connections with the layer on bottom. Thus, the preliminary design was modified to make connections between the inlet as well as outlet reservoirs consistent with existing package. We designed our modified masks and the modified versions of the masks are given in Figure 7 below.

Figure 7: A schematic of the modified masks for Stage 2

With this particular design of mask sets, we encountered certain questions. The questions were:

1. How many of the nine channel intersections should be used as interconnects between layers (3 or 9)?2. Should the first channel layer be open or closed on the bottom surface (PDMS or Pyrex bottom)?3. Should a top layer channel be used or should the top remain open?4. Should the size of the reservoir throughputs be the same size or smaller than the reservoirs?

Through the use of golf tees and rubber bands mounted in a wooden board, we created a three-dimensional model of the micro-fluidic device that led toenabled us to identify further modifications of the design. We modified our mask design by: re-routing the input and output channels, deleting portions of the channels and reservoirs, and removing some interconnects from the previous design. There were nine interconnects in the previous design, but in the new design, we reduced it to four interconnects. The new mask sets are given below in Figure 8 below:

Mask 1: Bottom fluid layer Mask 2: Interconnect layer Mask 3: Top fluid layer Mask 4: Top Cover layer

Figure 8: A schematic of the re-modified masks for Stage 2

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The processing method of the modified design consisted of the following: 1. Begin with four polished Si wafers.2. Spin SU-8 (negative photoresist) on Si wafer and pre-bake.3. Align wafer with Mask 1 - (Figure 3) and expose SU-8 to ultraviolet light.4. Develop SU-8 in SU-8 developer and unexposed areas are removed. This creates Mold 1 from

Mask 1. In the same way, Mold 2 is formed from Mask 2, Mold 3 from Mask 3, and Mold 4 from Mask 4.

5. After creating the molds, spin on the PDMS less than the vertical dimension of SU-8 protrusions.

6. Spin PDMS layer 1 on mold 1 (bottom fluid layer), PDMS layer 2 on mold 2 (interconnect layer), PDMS layer 3 on mold 3 (top fluid layer) and PDMS layer 4 on mold 4 (top cover layer). Make a total of four PDMS layers: two layers from the channel mold (1 & 3), one layer from the interconnect mold, and one layer from top cover mold. Figure 9 shows the PDMS layers: 1, 2, 3,4.

PDMS Layer 1: Bottom fluid layer PDMS Layer 2: Interconnect layer

PDMS Layer 3: Top fluid layer PDMS Layer 4: Top order layer

Figure 9: A schematic of the four PDMS layers of Stage 2

7. Delaminate and stack all four PDMS layers in the following order: Layer 1 (bottom fluid layer), Layer 2 (interconnect layer), Layer 3 (top fluid layer) and Layer 4 (top cover layer). The final result of the stacked PDMS layer is shown on Figure 10.

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Figure 10: A schematic of the final Stage 2 device.

Fabrication Steps: SU-8 MoldsUsing the masks created by the microchannels team, SU-8 molds were created for fabrication of the

PDMS microchannels. To create the SU-8 molds, a bare 3" silicon wafer was placed on the spinner and 2/3 of the wafer was covered with the SU-8 liquid. Next, the spinner was programmed using a recipe formulated to create a layer of SU-8 220 microns in height.

The spinning was complete within 30 seconds, and the SU-8 was then left to pre-bake on a hotplate at 95 C for 100 minutes. Once the pre-bake was complete the wafer was left to cool at room temperature for 30 minutes. The wafer was cooled slowly to stop the formation of cracks and defects in the SU-8 mold. At this point in the experiment, we were working under the assumption that cracks in the SU-8 mold would be detrimental to the final PDMS product.

Once the wafer was cooled to room temperature, it was placed in the aligner to be aligned with the mask. The exposure dose used was 900 mJ/cm2 and using an intensity meter, we measured the intensity of the light to be 26.6 mW/cm2. Dividing the dose by the intensity, we calculated the exposure time to be 33.7 seconds.

After the SU-8 had been exposed for 33.7 seconds, the wafer was placed on the hotplate to bake for 30 minutes at 95 C and then left to cool for an additional 30 minutes. Next, the wafer was put into a beaker filled with SU-8 developer and placed on a rocking table to develop. The wafer was developed for 22 minutes and then rinsed in fresh SU-8 developer and left to dry. This same process was repeated for all of the molds until a mold for each PDMS layer was fabricated.

Analysis of the SU-8 molds using the optical microscope revealed a substantial amount density of cracks that can be seen in Figure 11. However, we continued with fabrication of the device confident that the cracks were small enough so that the effect on the final device would be negligible.

Figure 11: Cracks in an SU-8 mold. This is a reservoir region.

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Fabrication Steps: PDMS MicrochannelsFabrication of the PDMS microchannel layers began by placing the previously fabricated SU-8

molds in an SDS (soap) solution and drying. The PDMS was then mixed with curing agent at a 10:1 weight ratio and poured over the SU-8 mold. The recipe used for PDMS was intended to create layers of 130 microns in height. However, similar to the SU-8 molds, time constraints prevented an accurate measurement from being obtained. Once the spinning was complete, the wafer was placed in the furnace to bake for 2 hours at 70 C.

Two sets of identical PDMS layers were fabricated, and assembly of the micro channel layers took place on two separate occasions. In both assembly trials, removing the PDMS microchannel layers from the SU-8 mold and aligning the layers in the proper sequence proved to be the most difficult part of the device fabrication. The microchannels were released from the mold by hand with the aid of razor blades and tweezers. Methanol was used as a release agent to allow the PDMS channel layers to slide easily off the mold. Aligning the layers was extremely difficult because the PDMS layers had the tendency to stick to each other when they were not coated with methanol. Alignment was further complicated because the layers became extremely slick whenever the methanol was used. Another problem arising from the fabrication of the PDMS device was the formation of wrinkles and air pockets between the layers. Eventually, the layers were crudely assembled although it was easy to observe that some of the features were not properly aligned. In addition, the interconnect layer did not provide a connection between the top and bottom layers.

During the second trial, fewer defects were observed within the layers and there were no significant problems with air bubbles or delamination. This is due to the fact that during the second assembly trial, addition of each PDMS layer to the previous layer was followed by compression of the layers with a metal rolling pin. This rolling process removed excess moisture and air from the layers, resulting in fewer defects.

Experimental TrialsThis project involves the development of a microfluidics device which would function to meet our

goals, and which could be fabricated by us. It is very important to consider the manufacturing process when working towards a final proposed design. This is important because manufacturing constraints are the largest limitation to our design. We were able to design, fabricate, and test two prototypes of the phase two designs. Both prototypes met some of our goals, and fell short of achieving others.

Our testing set up consisted of a syringe to inject liquid into the inputs of our device, and water colored with food coloring. We found that lighter shades of orange, red, and green showed up the best against the silicon wafer. For both prototypes, we injected liquid into each of the 5 inputs, and recorded observed the result. We also tried injecting liquid into the outputs and observed the results.

The first experimental prototype had several large problems, making it very difficult to test. The PDMS layers were thicker than we had anticipated, and therefore the interconnects did not transfer from the mold to the penetrate all the way through the PDMS layer. The result was that our layers were not connected to each other, and the top layer sealed the channels from the outside because the vertical interconnect in it did not pass fully through the top layer. This limited us to testing only the channels test sites that were orientedremained only in a singlethe horizontal plane direction.

This problem also made it difficult to inject liquid into the inputs, since the top layer of PDMS sealed them off. We solved this problem by poking through the top layer with the syringe, and injecting the liquid into the input, under the top layer. This technique worked, but had limitations. We observed no capillary action in the channels, meaning that the liquid would only move through them with applied pressure. This required a seal between the syringe and the top layer of PDMS, otherwise the injected liquid would flow out around the needle and not into the channels. This problem was corrected by sealing the channel with the needle in it using applied pressure from a finger. (see Fig. 12)

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Figure 12: The experimental setup with the syringe, colored water, and applied pressure from a finger sealing the channel

Once we were able to inject liquid into the channels we observed some success in moving the liquid through from input to output. Unfortunately, there were many air bubbles between the PDMS layers, causing the liquid to spread out and fill the air bubble instead of staying in the channel. (see Fig. 13) The layers were also full of defects caused by a chemical used to separate the layers from their molds. Many of the problems we encountered during fabrication and experimentation were corrected for the second prototype.

Figure 13: A picture of the first trial of stage 2. Shows air bubbles between layers, and the defects present in the layers.

The second prototype had many improvements over the first trial. Each layer was the correct thickness, allowing for interconnects between layers. The layers were aligned with good accuracy, meaning the interconnects connected the channels on both the top and bottom, and the inputs and outputs were open on the top layer. There were no air bubbles between PDMS layers. (see Fig. 14)

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Figure 14: This picture shows our second trial of stage 2. You can see that there are no air bubbles between layers, and very few defects visible in any of the layers.

With the more accurate fabrication of the design we were able to achieve several of our goals in testing. We successfully got liquid to flow in all five channels using applied pressure from the syringe. We were able to push liquid all the way through two of the five channels. We also observed two colors of liquid one on top of the other, as designed, proving that our channels were accurately fabricated.

We observed some of the same problems that we had encountered with the testing of the first prototype. We observed no capillary action, so we had to jam the needle into the end of each channel to obtain a seal. In doing this, the layers sometimes delaminated near the end of the channel. We once again corrected this problem by applying pressure behind the needle opening with a finger. Our biggest problem with getting fluid to flow through the channels occurred at the interconnects. We could not get fluid to flow vertically in any of the trials. In the channels that included vertical interconnects, the liquid would stop flowing when it reached the interconnect. To deal with this we applied more pressure to the fluid and the layers delaminated around the interconnect. This problem could have been caused by either our design or the fabrication of our prototype.

Future WorkThe majority of the processing difficulties were associated with PDMS fluidic channel alignment and

thickness of the PDMS and SU-8 layers compared to their corresponding protusions in SU-8 molds. Accurate measurements of the PDMS channel layer and SU-8 heights would be extremely useful not only for device fabrication, but also to verify the spin coating recipes and determine if modifications to the recipe are required. A new alignment technique, perhaps making use of the mask aligner was suggested and is highly desirable to achieve greater layer alignment accuracy. However, testing of such a technique was not possible due to time constraints associated with the project.

Stage 2 SummaryStage two was designed using the concepts from stage one. The new design fit the grid pattern of

stage one to the existing packaging and set up flow paths that would test the functionality of both layers individually as well the interconnects and the ability of the fluid to move through them. Because the overall function of the device stayed the same, the materials and the reasoning behind using those materials also remained.

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Stage two accomplished the goal of creating a testable two level micro-fluidic device. Moreover, a small amount of control was added to the system through the manipulation of the fluid channels and interconnects. The creation of the new fluid paths made it necessary to use an additional mold in the fabrication step. This addition mold was in effect the only change made to the processing. Because the materials remained the same, the processing steps also did not change significantly.

The functionality of the device was examined during the testing stage. Because the packaging was not available, testing proceeded using a manual approach. Testing showed that our overall design worked, but that there were some issues with the channel material, PDMS, and the flow pressure required. These results, in part, justified our decisions in stage three.

It appears that the limits of the passive system have been reached. The logical next step is to then integrate a form of valve into the channels to enable even more control over the fluid flow. The materials may need to be altered, taking into account the testing problems from stage two. The channel layout should remain as intact as possible to aid in the feasibility of testing the next stage.

Device Design Stage 3: Pressure Actuated Valve Test Design

ObjectiveThe 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 locally controlled within the microfluidic system 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 15) [8].

Figure 15: A side view of the intersection point between the fluid layers and gas layers

The overall layout of the design (Figure 16) 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.

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Figure 16: A top view of the interconnect, and dark blue is gate.

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 manufacturedfabricated.

Device DimensionsThe 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 (see modeling and discussion below). 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.

MaterialsPDMS 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 as a mold for creating the patterns for the PDMS but actually as a structural material for the valve flexing layer and the gas layer. 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.

Design ProblemsThe entire pressure actuated valve device was intended to be fabricated on a Pyrex wafer. However,

the Pyrex wafer was very thin (500 μm) and when we attempted to drill holes for inputs into the microchannel using a diamond tip blade, a substantial amount of cracks resulted around these holes. Consequently, the Pyrex wafer completely cracked during the subsequent fabrication stages, making design

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of the final device impossible. To overcome the problem of the Pyrex wafer, the pressure actuated valve device was fabricated using a bare Pyrex wafer with no holes drilled for channel inputs. While the lack of inputs prevents any testing of the device, it provides a reference for future work.

Processing Method with Mask DesignSU-8 is now a structure material in our design, and the processing steps change to include the new

gate features[9]. However, SU-8 was used to create molds for the PDMS layers in Stage 2 and we have become familiar with its processing [10]. 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 elements in the design.

1. Create the bottom fluid layer by spinning on SU-8 and exposing and developing it to mask design #12. 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 layer3. Add the flex PDMS layer. (mask #3) This is the last layer to be placed on the bottom portion of the

device, and it seals the tops of the bottom fluid layers.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 Pyrex wafer7. Align and sandwich the top and bottom portions of the device together.

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

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

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Mask #5- Top Layer

Figure 17: A schematic of the masks for Stage 3

Additional IssuesIn 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 3 SummaryThe Stage 3 design is very different than its predecessors in several ways. The most pronounced is

the inclusion of fluid control elements (Figure 18). The gas-actuated values bring us closer to the goal of a controllable microfluidic device. With the valvues incorporated into the design, 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 layout 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.

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Figure 18: A schematic of the final Stage 3 device

Membrane Deflection Modeling

In Stage 3 design there is a PDMS flex layer is the membrane for the pressure-actuated valve. Modeling for this layer was donecreated to validate that the dimensions were appropriate for the valve design and to predict needed pressures to control the device. Below is an equation for the maximum defection deflection (w) of a rectangular membrane. We used the 500x500 m for ‘a’ and

w =0.00265P(ab)2 12(1-)/(E*t3) [1]

‘b’ which are the dimensions of the membrane over the channel. The thickness is 50 m. The Young’s modulus and Poisson’s ratio used were 750 kPa and 0.5. The equation is then rearranged to solve for the pressure, P, as seen in Equation 2. Using a desired range of deflection, 30-60 um, it was a pressure between 2.8 and 5.6 kPa will beis needed, which . This roughly converts to 24-42 torr.

P= w(E*t3)/(0.0318*(ab)2(1-)) [2]

Next we need to know if the material can withstand this pressure pressure difference or deformation. Using Equation 3 we can calculate the strain for the membrane. Using the relationship between

=0.3081P(ab/t2) [3]

stress, strain, and the Young’s modulus a stress of 1.3 kPa results. In the Polymer Data Handbook PDMS is quoted to have a tensile range up to 1.5 MPa and even 9 MPa depending how the material was processed. For the calculated pressure the material will not enter the visco-elastic region typical of most many polymers, have and the membrane should behave as predicted. We will also not have to worry about deformingT the polymer will not break, because the stress the membrane will experience is under the maximum tensile strength.

Fluid Flow Modeling

The fluid flow modeling was done from a completely mechanical standpoint (neglectinged surface energies, and channel layout (e.g., turns, etc), etc. for simplicity). A literature search was done to find a commonly used fluid velocity, which was 1500cm/min [11]. From this velocity, a flow rate was determined for the microchannel dimensions in our design using Equation 4. This flow rate turned out to be approximately 0.0125cm3/sec.

v = Q/A [4]

The next part of this modeling involved figuring out the various fluidic resistances for the different sections of the design, which include the reservoir, interconnect, micro-channel, and valve. The reservoir resistance was calculated using Equation 5, and the other three sections were calculated using Equation 6. These resistances were then added together to give the total fluidic resistance of: 12,264,067g/cm*sec4.

R = 8L/(r4) [5] R = 12L/(wh3) [6]

Using this fluidic resistance and flow rate in Equation 7, a pressure gradient was calculated for the four sections. The individual pressure gradients were also totaled to give a total pressure of 115Torr, which corresponds how much pressure is needed to force fluid through the entire apparatus. What may be

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surprising about the results is that the pressure gradient for the valve’s very small constriction (assumed to be 500*100*20 m) is smaller than the pressure gradient for the microchannel. This can easily be explained, however, by the fact that length plays a major role in the calculation.

R = P/Q [7]

Another value looked at during this modeling is the Reynolds's number. This value is simply a way of estimating whether the flow of the liquid will be laminar. Though not exact, the value where laminar flow completely vanishes is around 1700. Using Equation 8, this value was calculated for all of the sections, and none of the values even went over 50. This makes it very obvious that the flow in the microchannels is laminar.

Re = (vDh)/[8]The final two calculations that were done involved velocity for individual sections using Equation 9,

and the total cycle time using Equation 10. Though we used a pre-existing value for velocity previously, it would not help to calculate the cycle time, since the velocities in various sections varies greatly due to dimensional changes. The final results for these sections, and all of the others can be seen in Table 4.

v = Q/A [9] t = (L/v) [10]

Table 4: A table of the results from the fluid flow modeling

Alternative Valve Designs

The following three sub-sections define the potential valve alternatives that were considered by the group during and after the design of the Stage 3 device. Time limitation prevented further exploration into these topics. However, these technologies hold sufficient promise that they are discussed as potential fields of inquiry for future work [12].

Piezoelectric ValvesValves utilizing piezoelectric actuation use the electrically induced mechanical deformation of a

piezoelectric material to close or open [13]. This type of valve was considered early on in 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.

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<As can be seen in Fig. 19, the lower and upper volumes are both V=A*x, where A = cross sectional area and x = vertical displacement. If we have a change in x called dx, then the volume change is dV. Since volume of an incompressible fluid is conserved, reducing the lower volume by dV=A*dx will cause the same change in dV for the upper volume. This means that the ratio dx(upper)/dx(lower) = A(lower)/A(upper). You could also calculate the pressure or force available at the upper in a similar way.>

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 offexploit principles similar to that involved in a hydraulics multiplication. This is illustrated in Figure 19, which contains images of both before and after an electric voltage potential is applied to a piezoelectric material that compresses a liquid chamber.

Figure 19: Before and after a voltage potential is applied to the piezoelectric valve

Figure 20: Image of the piezoelectric valve with the microchannel

Figure 20 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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Electrochemical ValvesElectrochemical 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[14]. 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 [15]. Figure 21 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 21: A figure of a typical Electrolysis reaction [16]

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 may be 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 422 below shows how this design could be integrated into the existing Stage 3 Final Manufactured Design.

Figure 22: A figure of the potential to integrate an electrochemical valve into the existing Stage 3 Final Manufactured Design.

Figure 22 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 22, 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.

Thermally Activated Valves The thermally activated valve is set up in a similar manner to the gas valve that was constructed. In

the gas valve, an external source controls the pressure of a gas line. The change in pressure causes a flexible PDMS membrane to deflect creating a change in the flow state of the channel. Instead of a gas line, the

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thermally activated valve uses an isolated enclosure of a volatile liquid. In lieu of the external fluidic (gas) control, this valve uses an electrically controlled resistive heater.

Figure 23: A figure of the potential to use a thermally activated valve in lieu of a gas valve for the Stage 3 device.

The valve works under the principle that an increase of heat 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 control liquid will cool, causing the gas towill 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 cylcopentane cyclopentane 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.

Preferred Design Elements

ChannelsFrom all of the designing and experimenting that was done, two main things came out about the

microchannels. First if of all, in our case, larger channels were easier to make. Since we did not have the ability to line up layers very accurately, using relatively large the larger channels, and interconnects, allowed us to have a degree of inaccuracy and still obtain decent results. In our case, there seemed to be no benefit to scaling down much farther, since there was ample room on the substrate to fit all of the channels we needed.

The other realization about the microchannels was that the surfacesy should be made more hydrophilic. While flow does occur to some degree in our channels, it could also be increased with some sort of surface treatment. This increase in flow could reduce required pressure for flow, cutting down on the cost and energy in the system. It would also reduce the effect of channels that are not opened fully, allowing for more error.

ValvesIt is hard to draw any conclusions about valves, because only one was fabricated, and it was never

tested. That being said, there does seem to be some potential with the valve types we studied. All four types of valves we looked at (pressure actuated, bubbles, both thermally formed and electrolytically formed, and PZT), could possibly be integrated into a multilevel design, though their degree of success could be questionable.

ScalingThe potential for adding more levels to the design does seem possible, but there would need to be a

lot of designing done in order to fit all of the valves into the design, since all of the designs we looked into require outside assistance to be operated. This means that for the gas actuated design we fabricated, a way to control each gas line would have to be incorporated. This would mean either adding many more inputs and

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SU-8PDMS Flex LayerPDMS Fluid LayerSU-8 Bottom Layer

Heater

outputs, or finding a way to control multiple lines with very few inputs and outputs, much like the Quake team.

A design such as the electrolytic bubbles or the PZT would seem to be better choices for adding multiple layers since the valves only need an electrical connection to be activated. This means that when multiple layers are added, a single wire run from the packaging could be used as the actuator, drastically cutting down on the amount of space needed for the valves. These kinds of designs could most likely be made very easily with the proper technology, making large scaling a very real possibility.

Conclusion

Technology for multilevel microfluidic devices has the potential to increase design flexibility with the integration of additional channel layers. By studying various materials and developing various microchannel designs, we succeeded in fabricating a two-level microfluidic circuit with vertical interconnects and valves.

Working as a team, we experienced the design, fabrication, and testing phases of a multistage project. We learned that modeling and experimental feedback are essential to evolution of design. The dynamics of working as a team were experienced and we realized that project organization and management are critical to meeting project goals.

AppendicesSee attached Gantt ChartSee attached fluid-flow Excel file

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[16] "Electrolysis: Obtaining hydrogen from water: The Basis for a Solar-Hydrogen Economy. Introduction to Electrolysis: Hydrogen from Water." 10 November 2003. < http://www.nmsea.org/Curriculum/7_12/electrolysis/electrolysis.htm>.

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