Concept Generation and
Selection Document
Microfluidic Impedance Spectroscopy
Project
Matthieu Giraud-Carrier
Kyra Moon
February 16, 2011
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Table of Contents
Table of Contents................................................................................................................. 1
Introduction ......................................................................................................................... 2
Overview .......................................................................................................................... 2
Purpose............................................................................................................................. 2
Procedure ......................................................................................................................... 2
Body of Facts........................................................................................................................ 3
Proposed Design .................................................................................................................. 4
Concept Selection & Scoring ............................................................................................... 5
Material Selection ............................................................................................................ 5
Leakage Prevention Design .............................................................................................. 7
Signal Path Geometry ....................................................................................................... 8
Conclusion ......................................................................................................................... 10
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Introduction
Overview
Our goal is to build a microfluidic impedance spectroscopy system. In other words, we
will build a chip with a small channel which can be filled with fluids (primarily protein
solutions) and then analyzed. There are several possible geometries and
implementations that will meet our design specifications. Each solution has both
advantages and disadvantages.
Purpose
The purpose of this document is to lay out all possible design concepts and decisions
and to document the reasoning for our eventual concept selection. While many of our
decisions have little effect on the overall operation of our final product, a few decisions
are critical to proper operation. These concepts are considered in greater detail and
explained in greater depth. This document is designed to be easily understandable and a
good reference for anyone trying to familiarize themselves with this project and the
decisions that led to our particular implementation of the microfluidic impedance
spectroscopy system.
Procedure
The simplest way to outline the decisions to be made and the reasons they are made in
certain ways is by using decision matrices. They enable us to take into consideration a
variety of weighted factors and put a numerical value on each solution. The matrices in
this document list the proposed concepts for a few of the key components needed for
microfluidic spectroscopy. In order to assess the nature and weight of each factor in the
leftmost column we refer to our technical advisors and customer (Agilent). The matrices
make clear the reasons for each major choice made in the design process.
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Body of Facts
Specifications

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Bandwidth: 100MHz to 7GHz
Volume of channel: 0.5mm X 1mm X 2cm (10uL)
Temperature stability: Within 1 degree
Final outputs: Conductivity and Permittivity (from S-Parameters)
Develop a numerical model for the waveguide
Interface between Network Analyzer and Plotting Mechanism (LabView, Matlab)
Sufficient reservoir volume for a 5-minute test
Known Facts
 The microfluidic channel on the board will allow for the study, characterization
and experimentation of many liquids (particularly protein solutions).
 We know that we will be using SMA connectors and Nanoport pressure ports.
 We will have to use LabVIEW, HFSS and Matlab to interface with our Network
Analyzer and to accurately model our system.
 Proteins are expensive, so we a need cheap and small system.
 The microfluidic channel will be set up parallel to the waveguide.
 We know that the behavior of the protein solutions will not be linear.
Assumptions
 We are assuming that PCB will bond to itself easily and well so that we can get a
tight seal around the channel.
 We are assuming that there is actually a way of getting a temperature-stable
system.
 We are making many assumptions about how quickly we will be able to learn
new languages, fabrication processes and technical vocabulary and background
knowledge.
 We are assuming of course that the software and hardware involved will function
as expected and that we will not encounter glitches or broken machines.
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 We are assuming that the cohesive properties of these liquids are still intact in
our small rough-edged channel.
 We are assuming an evaporation rate low enough that we will have time to take
our measurements.
 We assume that the network analyzer is specific and powerful enough to give
valuable data even while working with tiny systems.
Proposed Design
The design constraints are placed on our microfluidic chip. Below is a block diagram of
the chip operation and the other necessary instrumentation as well as the connections
we use. Those connections outline the most basic system requirements. Size and
geometry will be discussed in the following sections.
Figure 1: Overall Design Block Diagram
The most basic need for our system is a small channel that the liquid passes through;
this will need to be close to a waveguide. All of the materials must be relatively cheap
and easy to use. Besides those few set things, there is a lot of flexibility in design. For
that reason, we select the following concept areas to develop our final design plan.
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 Material Selection: Decide what PCB type material will be optimal for our system.
 Leakage Prevention: Design the board so that the protein solution can be
confined properly to the channel.
 Signal Path Geometry: Consider how to align the channel and waveguide and
how to fit all the connections on the board.
We focus on these three because they are the essential concepts; all other decisions do
not affect the system operation and can therefore be made fairly easily based on
convenience and other trivial factors. Those include the actual size of the device and the
width of the boards, amongst others.
Concept Selection & Scoring
Material Selection
There are a variety of materials that we can use to create our chip. We will use two
pieces with the same dimensions and then bond them together to create a channel
between them. The material we choose will affect our fabrication process as well as the
functionality of our final chip, so it is definitely a critical point.
Concept Definitions
Clear Acrylic – Simple clear solid plastic. Quite easy to obtain and some of our team
members are familiar with it already due to prior research.
FR-2 PCB – Low grade standard PCB material. This is the kind you would get in a cheap
PCB kit at Radioshack for example.
FR-4 PCB – Higher grade standard PCB. The typical FR-2 material is woven with fibers of
glass to make it more resistant and solid. This is more standard In applications
throughout engineering and is the kind we will readily find in the department shop.
Concept Evaluation
Bonding – It is critical that we be able to bond the material to itself AND to copper (the
waveguide traces). If it does not bond, we cannot create a channel and therefore we
cannot create a microfluidic system at all.
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Easy to cut and form – We will be using milling machines or laser cutter to shape the
material, the easier it is to cut and form with these machines, the faster we can
fabricate the chips and begin testing.
Transparency – While transparency is not critical, it will be very convenient during
testing to be able to actually see fluid flow down the channel. It would also be useful to
check for leaking problems.
Customer Approval – Obviously, whatever material we use has to be approved by our
supervisors as well as by Agilent. If they do not like it for some reason or it is an
unfamiliar material to them, it is probably a poor choice.
Concept Scoring Matrix
Weighing Factors – Once we have established what factors are involved in this particular
concept, we can rank them using a weight system. We realize that the most important
factors to consider are how well the material can bind and how easy it is to cut and
form. Since neither of these is really more important than the other, we assign them the
same weight of 30. Transparency is quite important also but not at all as important as
the previous two ideas. We assign it a value of 20. Finally, customer approval is given a
weight of 20. We chose this because even though customer approval is actually quite
important, we could obtain customer approval quite easily on any of our choices if we
had a good reason to present to our supervisors and customer.
Note: For all design matrices, the scores range from 1-10, 10 being the best score.
Clear Acrylic
Weighted
Score
Weighted
Bonding
30
1
30
10
300
10
300
Easy to cut and form
30
8
240
8
240
8
240
Transparency
Customer Approval
Total
20
20
100
10
7
200
140
8
9
160
180
7
8
140
160
Score
Score
Weighted
Value
FR-4 PCB
Weighted
Concept Scoring
Material Selection
FR-2 PCB
610
880
840
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Analysis of Scoring
We assigned acrylic the lowest score in the bonding category because it doesn’t bond to
itself. Since it is clear, it received high marks in the transparency category. It is easy to
cut and form, so we assigned it an 8 out of 10. Since the customer offered no opinion
on its use, we assigned it 7 out of 10.
Both FR-2 and FR-4 can bond to themselves which is why they both received 10 out of
10s in that category. They are both easy to cut and form so they both received 8 out of
10 in that category. FR-2 is more transparent than FR-4, though both are transparent
enough for the needs of the channel. Since the professors we are working preferred FR2 over FR-4, we gave FR-2 a 9 out of 10 and FR-4 an 8 out of 10.
Result
We see that our best option is FR-2 PCB. This is quite easy to obtain and can be cut using
both the milling machine and the laser cutter. FR-4 is not too far behind and is also a
reasonable option. It is only inferior to FR-2 in the less critical areas. However, it cannot
be cut with the laser cutter because the glass fibers in it reflect the laser. The
transparency difference is not too important either. Acrylic would have ideal
transparency but the fact that it doesn’t bond to itself totally removes it from the realm
of possibilities in our design.
Leakage Prevention Design
Concept Definitions
Multilayer Board – Multilayer boards allow for a lot of flexibility in design. However, we
do not have the necessary equipment to make these boards so we would have to send
our designs to an outside company which adds time and cost.
Adhesive Method – A variety of adhesives could be used to help in the binding process
and to reduce leakage. These would be complicated to use because they may fill the
channel or get into other undesired areas.
Pressure and Heat Method – This is by far the cheapest and easiest method. We have all
that we need in our lab to bind using this technique. However, it is also the least reliable
one. This is a fairly typical trade-off situation.
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Concept Evaluation
Cost – The cost of our system must be low, but none of these methods would be
particularly expensive so it remains a fairly low priority.
Reliability – The method must do a very good job of avoiding leakage. Leaking will
become an even bigger factor when our system is pressurized.
Complexity – The easier the method, the faster we can get it done and move on to
testing and fine-tuning the device. Since we will likely be repeating the process a lot, it is
a very good thing to be able to do it quickly.
Concept Scoring Matrix
Weighing Factors – The single most important factor in this case is reliability. The device
has to be leak proof. For this reason, we assign a weight of 40 to that factor. While
complexity is not as critical as reliability, it will affect several aspects of our project. If
the leakage prevention is something that we can do quickly and simply without the use
of many machines, we will save a lot of time in the long run. Because of how much it
affects our overall project, we assign complexity with a weight of 40 also. Finally, we
give a value of 20 to cost. It is always a good thing to minimize cost so it definitely has its
place as a factor. However, none of the methods we would use as leakage prevention
will ever be very costly so it is reasonable to not make it as important as the other
factors.
10
200
5
100
1
20
Reliability
40
8
320
5
200
5
200
Complexity
Total
40
100
2
80
6
240
10
400
600
540
Weighted
20
Score
Cost
Score
Weighted
Weighted
Value
Pressure &
Heat Method
Score
Concept Scoring
Switch Selection
Adhesive
Method
Weighted
Multilayer
Board
620
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Analysis of Scoring
A multilayer board would be reliable because the additional layers would prevent the
liquid from leaking. Thus we assigned it an 8 out of 10. Since the multilayer board would
have to be fabricated, this would add to the time and complexity of the project. Hence,
we assigned it a low complexity score.
Adhesive methods would also be inexpensive, but since they would be more expensive
than the multilayer board due to additional materials, we assigned the cost of this
method 5 out of 10. We presume that the adhesive method would be reliable enough
for our needs, though not as reliable as a multilayer board. Hence this was also assigned
5 out of 10. Since we could add the adhesive within our own shops, this method is less
complex than the multilayer board method. Hence we assigned this a 6 out of 10.
We assume that the pressure and heat is as reliable as the adhesive method because
heat and pressure should keep the board sealed together as well as with an adhesive.
Thus we assigned this a 5 out of 10. Since heat and pressure require no extra materials,
we assigned this a 10 out of 10 for complexity.
Result
The heat and pressure method seems to be the best from the matrix. This is definitely
convenient for us. Although the multilayer board comes quite close numerically, the
added time it would take to send the design out and have the board fabricated is too
much to be worthwhile.
Signal Path Geometry
There are several ways to implement our chip. All we need to meet our specifications is
for the channel to run parallel to the waveguide for two centimeters and for the two
ends of the copper waveguide to meet the edge of the board at some point. Two
alternate geometries are shown below. An additional one (not pictured) would include
vias and imply pass the copper to the other side of the board at the point where they
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bend in the first picture.
Concept Definitions
90 degree angle – The 90 degree angles in the copper lines were our initial design idea.
The channel and reservoirs are very easy to mill and everything fits well on the chip.
Using Vias – Vias are a good option for leakage problems because they eliminate the
need for the copper traces to pass under the channel wall, thus creating holes.
However, they are significantly more complicated to mill.
Straight Traces – Another idea is to bend the channel instead and keep the copper
traces straight. This may improve the functionality of our waveguide and also makes our
system slightly easier to model.
Concept Evaluation
Reliability – In general this means how reliable the chip will be if we use this method.
This includes how well it deals with leaking problems and how well the waveguide will
work in the given geometry.
Complexity – How complicated it is to actually fabricate.
Customer Approval – Obviously, the geometry again needs to be approved by our
customer. They may for example prefer the SMA connectors in a specific location on the
chip.
Concept Scoring Matrix
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Weighing Factors – Some of our factors in this analysis are the same as those we used in
our earlier concept selections and will be weighed similarly for the same reasons.
Reliability is our number one factor; if the chip does not work, there is no point
considering other factors at all. We give reliability a weight of 45 and place it as the
most important factor. Next, we have complexity. For the same reason as in our leakage
prevention discussion, complexity is quite important. It is not part of the definition of
the concept we are working on but it will greatly affect the time and effort put into the
project. Complexity is assigned a weight of 35. Finally, we again have customer approval.
As we have said before, we are not allowed to proceed without customer approval but
feel it is easy to obtain provided we have a good understanding of why we selected the
given method. A value of 20 seems reasonable for our customer approval weight.
45
5
140
8
360
8
360
Complexity
Customer Approval
Total
35
20
100
8
8
240
160
6
7
210
140
10
8
300
160
665
710
Weighted
Reliability
Score
Score
Using Straight
Traces
Weighted
Using Vias
Score
Weighted
Value
Weighted
Concept Scoring
Signal Path Selection
Keeping 90
degree turn
angles in
copper
820
Analysis of Scoring
Since we are not confident in the success of a waveguide with 90 degree bends, we
assigned the first option a 5 out of 10. We have already demonstrated that it is easy to
fabricate, so we assigned it an 8 out of 10. We assume that as long as it works properly,
the customer will be satisfied so we assigned the customer approval an 8 out of 10.
We expect that vias will be reliable since will prevent leakage by eliminating the need
for copper traces to pass under the channel wall. Thus we assigned vias an 8 out of 10
for reliability. They are difficult to fabricate and would likely be sent elsewhere for
fabrication, so we assigned them a 6 out of 10 for complexity. Again, as long as the
device works properly, the customer will be satisfied so we assigned customer approval
a 7 out of 10 (a bit lower due to the increased complexity in the finished product).
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Since we anticipate that the waveguide will work better when it is completely straight,
we assigned the reliability of straight traces an 8 out of 10. They are the simplest to both
fabricate and model so we assigned them a 10 out of 10 for complexity. As before, we
assigned customer approval an 8 out of 10.
Result
Straight traces are our best option. It turns out to be almost equally as reliable as both
other geometries but is simpler to fabricate and convenient for the user. This is the least
critical of our concept selections and could be modified if the modification led to an
improvement in another area. For example, if another geometry ended up severely
affecting leakage.
Conclusion
We have chosen to implement our system using FR-2 PCB material. We will bind it using
the heat and pressure method and will adopt a geometry that uses straight copper
traces on a single side of the board. The reasons for these decisions are clearly outlined
in this document.
This design is still very flexible and as outlined above, many of the concepts can be
changed with only minor effects on the overall final system. The object of this document
is not to outline the only good way to design a microfluidics impedance spectroscopy
system as there are many ways to do so. It is simply to provide a solid background and
reasoning for the option we have chosen to follow.
If changes are to be made in the design later on in the design process, this document
will be a good resource in order to evaluate the pros and cons of such a decision. It
contains all the critical information needed to assess the trade-offs of making important
adjustments.