Multilayer Microfluidics
_______________________________________________________________
Department of Materials Science and Engineering
University of Maryland, College Park
ENMA490
Fall 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
Introduction
Problem Definition:
 To use micro processing techniques to address the problems associated with multi-level
channel routing in bio-micro fluidic applications
 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 Scope:
The 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 Information:
Materials considered for our micro-fluidic design consisted of Pyrex and silicon
substrates with polydimethylsiloxane (PDMS), (SU-8), and (PMMA) layers. 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 Results:
We 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 Stephen R.
Quake’s work on microfluidic multiplexors that are combinatorial arrays of binary valve
patterns. 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 control device team researched many controlled valve designs including
pressure, bubble, and PZT actuated valves. 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 Overview:
In each device design stages we will have objectives, device logic, device dimensions, materials
used in the design, the processing method, additional issues, manufacturing results and experimental results.
The initial micro-channel 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 Objective:
Once 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 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 Logic:
The design that was chosen consisted of the simplest two-channel 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 was known to have a relatively fast turnaround time, and
this was critical due to the time constraints on the semester.
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 forcing 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 chosen, 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 100m, 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
PDMS Layer Height
Microchannel Width
Microchannel Length
Interconnect Width
Interconnect Depth
Reservoir Diameter
Distance Between Channels
Value
100m
150m
45mm
150m
150m
300m
300m
Table 1: A Table of the critical dimensions for the Initial Microchannel Test Design
The cross sections of the microchannels were process 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 cross-section and the reservoirs were made circular in
cross-section. These dimensions and geometry constituted, what the group thought as, 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 for this project and an
overview of their electrical and mechanical properties. 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. It’s 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. 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. 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 developing, can be used to create reverse mold patterns of micro
channels, reservoirs and interconnects.
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)
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
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)
Conclusion
Stage 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.
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 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 Objective:
Based 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
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 shortcoming 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 Logic:
As 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 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 microfluidic 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
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
PDMS Layer Height
Microchannel Width
Interconnect Width
Interconnect Depth
Reservoir Diameter
Value
100m
500m
1000m
1000m
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, 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 photoresist is photosensitive and forms a cross-linking reaction when exposed to light. During developing, the SU8 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
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.
Modified Masks :
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:
 How many of the nine channel intersections should be used as interconnects between layers (3 or
9)?
 Should the first channel layer be open or closed on the bottom surface (PDMS or Pyrex bottom)?
 Should a top layer channel be used or should the top remain open?
 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, we created a three-dimensional model of the micro-fluidic
device that led to 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
The processing method of the modified design consisted of the following:
1.
2.
3.
4.
Begin with four polished Si wafers.
Spin SU-8 (negative photoresist) on Si wafer and pre-bake at 95°C.
Align wafer with Mask 1 - (Figure 3) and expose SU-8 to ultraviolet light. Post-bake at 95°C.
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.
 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), 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 3: Top fluid layer
PDMS Layer 2: Interconnect 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.
Figure 10: A schematic of the final Stage 2 device.
Stage 2: Modified Microchannel Test Design
Fabrication Steps: SU-8 Molds
Using 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 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.
Fabrication Steps: PDMS Microchannels
Fabrication 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 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 Trials: Phase 2
This 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
design. Both prototypes met some of our goals, and fell short of achieving others.
Our testing set up consisted of a (size?) 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 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 PDMS layer. The result was that our layers were not connected to each other, and the top layer
sealed the channels from the outside. This limited us to testing the channels that were oriented only in the
horizontal 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.
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. Many of the
problems we encountered during fabrication and experimentation were corrected for the second prototype.
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.
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 Work
The majority of the processing difficulties were associated with PDMS fluidic channel alignment and
thickness of the PDMS and SU-8 layers. 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 Summary
Stage 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.
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.
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 12).
Figure 12 - A side view
of the intersection point
between the fluid layers
and gas layers
The overall layout of the design (Figure 13) 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.
Figure 13 - This is a top view of the
overall gas line setup. Green is top fluid,
red is bottom fluid, brown is gas, yellow
is 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 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.
Stage 3: Pressure Actuated Valve Test Design
The 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
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.
[INCLUDE STAGE 3 PICTURES HERE]
Fluid Flow Modeling
The fluid flow modeling was done from a completely mechanical standpoint. A literature search was
done to find a commonly used fluid velocity, which was 1500cm/min. From this velocity, a flow rate was
figured for the microchannel dimensions in our design using Equation 1. This flow rate turned out to be
approximately .0125cm3/sec.
[1] v = Q/A
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 2, and the other three sections were calculated using Equation 3.
These resistances were then added together to give the total fluidic resistance of 12264067g/cm*sec4.
[2] R = 8L/(r4)
[3] R = 12L/(wh3)
Using this fluidic resistance and flow rate in Equation 4, 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
surprising about the results is that the pressure gradient for the valve’s very small constriction 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.
[4] R = P/Q
Another value looked at during this modeling is the Reynold’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 5, 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.
[5] Re = (vDh)/
The final two calculations that were done involved velocity for individual sections using Equation 6,
and the total cycle time using Equation 7. Though we used a pre-existing value for velocity previously, it
would not help to calculate the cycle time, since the sections varied so greatly in time. The final results for
these sections, and all of the others can be seen in Figure 14.
[6] v = Q/A
[7] t = (L/v)
Alternative Actuated Valve Designs
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
thermally activated valve uses an isolated enclosure of a volatile liquid. In lieu of the external control, this
valve uses an electrically controlled resistive heater.
SU-8
PDMS Flex Layer
PDMS Fluid Layer
SU-8 Bottom Layer
Figure 14
Heater
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 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 (www.wimserc.org/Downloads/bioAR.pdf) 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.
Preferred Design Elements:

Channels:
From all of the designing and experimenting that was done, two main things came out about the
microchannels. First if all, in our case, larger channels were easier to make. Since we did not have the
ability to line up layers very accurately, 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 they should be made more hydrophilic. While
flow does occur to some degree in our channels, it could also be increased with some sort of 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.

Valves:
It 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.

Scaling:
The 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
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.
Appendix