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10.003 Chemistry 1D Designette
Page 1 of 16
Design and Fabrication of Microfluidic Device for Lab-on-a-Chip Chemistry
Microfluidics, the manipulation of fluid streams in micro-scale dimensions, is of growing
technological interest because of its diverse applications, including medical diagnostics and
environmental monitoring of contaminants. Microfluidic devices are becoming increasingly
important because (i) they require only small reaction and hence reagent volumes (nano- to
picoliters) and (ii) their narrow channel diameters facilitate efficient analytical systems. For
example, microfluidic channels have been used to deliver samples for DNA manipulation
and protein analysis; such samples usually come only in small volumes due to their high
cost. Furthermore, microfluidic networks built on lap-on-chip devices are considered to be
high-throughput, miniaturized, mobile laboratories.
A key feature of solutions in a microfluidic device that is different from solutions in bulk
analog is that the fluid flow within microfluidic channels is laminar. This means that parallel
fluid streams in the same channel will flow independently of each other with little or no
mixing at the interface. Practically, however, diffusion across adjacent streams allows
reagents from each stream to react. A microfluidic channel therefore, provides a useful
platform for performing confined chemical reactions and demonstrating the dynamics of fluid
flow in small volumes.
In this 3-session laboratory exercise, we are going to learn:
(1) how chemistry knowledge is used to design materials with desirable physical and
chemical properties;
(2) how different materials are used in a design sequence to prototype a device for certain
function: in this case, a microfluidic device with Y-shaped channel for interfacing two
fluids;
(3) to visualize chemical reactions under laminar flow in the microfluidic device and compare
the results to those in bulk solutions.
Background
Let’s have a brief introduction of Polymer Chemistry before going into the details of the
laboratory process. A rational design and synthesis procedure could help us tailor the
structure and architecture of a polymer which would impact its various physical (e.g.,
flexibility) and chemical (e.g., surface adhesion) properties.
Polymers are macromolecules formed by linking large numbers of much smaller
molecules called monomers. These chemicals have a large molecular weight and many
repeating units or chemical structures. Polymerization refers to the reactions by which the
monomers are combined. ―Curing‖ is another common terminology used to describe
polymerization or the ―hardening‖ process. Two types of classifications can be used to
group the polymers. One classification is based on polymer structure: condensation and
10.003 Chemistry 1D Designette
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addition. The other classification is based on polymerization mechanism: step and chain
polymerizations. Generally, most condensation polymers are produced by step
polymerizations and most addition polymers are produced by chain polymerizations. The
most important difference in chain and step polymerizations is in the identities of the species
that can react with each other. As the name suggests, step polymerizations proceed by the
stepwise reaction between the functional groups of reactants as in reactions, for example
COOH with NH2, or COOH with OH. In chain polymerization, an initiator is used to produce
an initiator species R* with a reactive center which may be a free radical, cation, or anion.
Chain polymerization occurs by the propagation of the reactive center by the successive
additions of large numbers of monomer molecules in a chain reaction. (George Odian,
Principles of Polymerization, Wiley-Interscience, 2004)
The process of polymerization can be controlled by manipulating reaction conditions,
including temperature, choice of compounds involved in building the polymer, and the ratio
of the compounds involved. These reaction
parameters are important in controlling
polymerization rates, polymer molecular
weight, and architecture such as branching
and crosslinking. Polymers can have a wide
range of properties that are dependent on their
molecular structure and architecture (see
Figure 1). Depending on the properties,
polymers have extensive uses ranging from
"wash and wear" clothing to rubber tires and
even protective enamels and paints.
Innovations in polymer chemistry constantly
bring both improved and entirely new
technological applications for polymers. We
will now discuss about the four polymers that would be used for the three successive
laboratory sessions. You will be learning how structure and architecture of a polymer can
influence its physical and chemical properties
(1) polydimethylsiloxane (PDMS)
PDMS is one of the most common materials used for fluid delivery in
microfluidic chips. It can be molded into different shapes during the
polymerization process, and can serve as an elastic stamp, which enables
transfer of patterns onto glass, silicon or other polymer surfaces. The
chemical formula for PDMS is CH3[Si(CH3)2O]nSi(CH3)3, where n is the number of repeating
monomer [SiO(CH3)2] units. The long Si—O bond linkage in PDMS offers flexibility to the
Figure 1. Increased cross-linking in the right-hand
polymer sample compared to the left-hand sample
(cross-links shown in red) results in a stiffer polymer.
(http://mrsec.wisc.edu/Edetc/LEGO/PDFfiles/bookcha
p3.PDF)
10.003 Chemistry 1D Designette
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polymer backbone, whereby PDMS can behave as a very flexible rubber. The -CH3 group
makes the polymer surface hydrophobic.
(2) polyurethane (PU)
(urethane groups:
— NH-(C=O)-O—)
As mentioned earlier, the polymerization process can be controlled through manipulation
of reaction conditions (e.g., temperature). For some polymers such as adhesives or
coatings, the polymerization (i.e., curing, or hardening) process can be induced and
controlled by UV light. These UV curable polymers offers several unique advantages: lower
energy consumption compared to thermally cured polymers, wider range of formulation with
varying viscosities for convenient processing and application on different surfaces. One of
the prime application areas for UV curable polymers is to provide clear coatings on
substrates ranging from metals and wood to floors and paper. Each of these substrates
demands a different set of properties from the UV curable coating. For instance, coatings on
metallic substrates must have good adhesion and be able to resist deformation induced by
stresses experienced during use.
All UV curable systems have four basic components which must be included in order to
develop a successful coating. They are the photoinitiator(s), oligomer(s), monomer(s), and
additive(s). Light emitted from a suitable UV source causes the photoinitiator to fragment
into reactive species. These fragments subsequently initiate a rapid polymerization process
with monomers and oligomers in the systems to form a crosslinked, durable polymer. The
oligomers provide the photocured polymer with its basic physical properties. Polyurethane
(PU), which contains urethane-based oligomers, is usually abrasion resistant, tough, and
flexible. These properties are ideal for coatings on floors, paper, printing plates, and in
packaging materials. Monomers (e.g., acrylates) are used in UV curable systems to provide
final film properties and viscosity control of the polymer. They are also important in
determining the speed of curing, crosslink density, and final surface properties. Additives
are included to act as dispersants for pigment dispersion, color pigments, and stabilizers for
both thermal and UV protection.
(3) 5-min® Epoxy
If every monomer forming a polymer has only two reactive sites, then only chains or
rings can be made. However, if some or all of the monomers in a polymer have three or
more reactive sites, then they can be cross-linked to form sheets or networks. One important
example of cross-linking is 5-min® Epoxy, which is formed from two different chemicals (i.e.
―copolymer‖), termed as the "resin" and the "hardener". The resin contains an epoxide group
10.003 Chemistry 1D Designette
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( ) at either end. This can be seen from the structure of a commercially available
resin EPON 862 in Figure 2. The hardener consists of polyamine monomers, for example,
triethylenetetramine (TETA) whose structure is shown in Figure 2. When these compounds
are mixed together, the amine groups react with the epoxide groups to form a covalent bond.
Each -NH group can further react with an epoxide group, which results in a polymer which is
heavily cross-linked, and is thus rigid and strong.
Figure 2: Reaction schematic using EPON 862 and TETA. Source: Polymer, Vol 48, pg 2174–
2178, 2007
(4) Polyvinyl chloride (PVC)
This is the material used in 3D printer and you can find detailed information about this
polymer from the textbook, Oxtoby, page 1105-1110.
10.003 Chemistry 1D Designette
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Overview of Procedure
Now that we know how the chemical structures result in polymers with different physical
and chemical properties, we are going to use these building blocks in different steps of a
molding process to prototype a microfluidic device. The Y-shaped channel dimensions are
shown in Figure 3. The second set of dimensions should provide a similar fluid flow
configuration, but its wider channel provides a better visibility of the fluid.
At the beginning of your first lab session, each group will be given a PDMS master mold
made by your instructor in advance. You will use this PDMS master mold to make your own
microfluidic device. The fabrication process overview of a PDMS master mold is shown in
Figure 4. The initial channel master can be made with a Y-shaped PVC block (prepared
using a 3D rapid prototyping printer) placed in a Petri dish, based on which the PDMS
master mold are formed.
Figure 3. Schematics of the channel dimensions for the microfluidic device.
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With the PDMS master mold given to you, each group is going to achieve the following
goals (also shown in Figure 4.):
Figure 4. Schematics of fabrication of the PDMS master mold.
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Lab Session 1. Fabrication of microfluidic channel master (week 5)
1.1 Formation of daughter master (material: PU) of the Y-shaped template from the PDMS
master mold
1.2 Formation of inlets and outlets master (material: 5 min® Epoxy)
1.3 Fabrication of PDMS microfluidic channel from the microfluidic channel master
Lab Session 2. Generation of microfluidic device (week 6)
2.1 Assembly of the final product by (1) adding inlet and outlet holes and tubing, and (2)
sealing the PDMS channel against a glass slide
Lab Session 3. Chemical reactions at the fluid-fluid interface (week 6)
Two scenarios are used to demonstrate how the microfluidic device manipulates fluid flow
and the chemical reactions involved.
3.1 A visualization of laminar flow at the interface between two dye-colored fluid streams
3.2 An acid-base reaction at the fluid-fluid interface
10.003 Chemistry 1D Designette
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Lab Session 1 Manual
A. Materials and equipment:
PDMS Master Mold (made by instructors beforehand), glass slides, Petri dish, disposable
cups, stirring stick, balance, UV light source, vacuum, oven, desiccator, camera
B. Chemicals:
Norland optical adhesive 81 (polyurethane), 5 min® Epoxy, Sylgard 184 elastomer kit
(poly(dimethylsiloxane)), curing agent, ethanol
C. Procedure:
1.
2.
Clean a glass slide with water, then ethanol, and leave it
to dry in air, Label your group’s slide with (a) cohort
number (b) group number.
Place the Norland optical adhesive (PU) in a line on the
slide. Put down the expiration date information in your
notebook. (This step needs good ventilation)
3. Place the PDMS Master Mold face-down in contact with
the adhesive. Apply pressure by placing your finger on
the top of the mold to drive the air bubbles out.
Tip: The adhesive should spread to form a layer under
the entire area of the stamp.
4. Pre-cure1 the adhesive under the UV lamp for ~10 min.
The pre-cure cross-links the polyurethane in the optical
adhesive, forming a robust PU Daughter Master. The
UV lamp should be positioned an inch or so above the
substrate.
(Caution: Please don’t put your hands under the UV
lamp while it’s turned on)
5
Peel the PDMS mold off.
1 Pre-cure is the industry terminology for the cross-linking step. The full cure is accomplished in 70°C
oven for <12 h. The full curing process is not necessary, but may be used if either the PU or PDMS
prepolymer does not fully solidify.
10.003 Chemistry 1D Designette
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6.
7.
8.
Form the inlet and outlet wells by applying three large
drops of 5 min® Epoxy at the end of the channels. (This
step needs good ventilation)
Tip: Ensure that inlets/outlet are not placed too close to
the edge of the slide; otherwise, the adhesive will
prevent adequate sealing of the device. Also note that
epoxy flows along channel length due to adhesive
forces.
Cure channel system + inlets/outlet under UV light for
30-45 min to cure the optical adhesive fully. Take a
photograph of your product, if possible
Place the structure made in a Petri dish and mold
against PDMS using the following method:
a. Weight the PDMS pre-polymer (35 g) in a
disposable cup, add in curing agent (3.5 g), and
mix for 5min.
b. Place the cup under vacuum for 20 minutes so
that most of the bubbles are removed from
PDMS
c. Place the master in a petri dish and pour PDMS
mixture over it.
d. Place the petri dish under vacuum for 1h.
Tip: step b and c are crucial to obtaining a
transparent PDMS mold for clear visualization
under microscope in Lab Session 3.
e. Cure PDMS in oven at 70°C overnight. (This
step will be done by your instructor.)
9. Label your group’s petri dish with (1) cohort number, (2) your group number.
10. Clean up and have your instructor sign on your pre-lab question section before
you leave.
Steps 8(a) and 8 (b) should be
carried out while you wait your
product from step 6 to be cured.
10.003 Chemistry 1D Designette
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Lab Session 2 Manual (Week 6)
A. Hazards
Sharps – Ensure safe use and disposal of razor blades and needles. Keep its cap in safe
place when the needle is in use; cap the needle at all time when not in use.
B. Materials and equipment:
21-gauge needle, glass slide, polyethylene tubing PE 10 (inner diameter: 0.28mm, outer
diameter 0.61mm), tweezers, razor blade, paper binders (x2), transparent tape, tweezers (x1
pair), razor blade (x1), syringe (5mL, x1), camera
C. Chemicals:
5 min® Epoxy, ethanol and deionized water.
D. Procedure:
1.
2.
3.
a.
Cut out the PDMS stamp using a razor blade.
Note: Make the stamp as large as possible to reduce
damage to it when peeling it from the glass slide. Trim
the stamp to leave approximately 0.5 cm between the
edge of the stamp and the channel structure.
The resulting PDMS channel must have its wells
punctured by a 21-gauge needle with a blunt tip (made
by filing or sanding it down as prepared for you by your
instructor).
Assembly of microfluidic device using the following
method:
Place the PDMS channel against the glass slide and
press until the PDMS is sealed to the glass. If this is
difficult, clean both the channel and glass slide with
ethanol, and dry it in air, and try again.
10.003 Chemistry 1D Designette
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d. To ensure the sealing between PDMS and glass, use either paper binders or
transparent tape
e. Assemble syringe with syringe needle.
f. Insert syringe needle into the end of the outlet tubing. Caution: use tweezers to
guide the end of the outlet tubing; be careful not to get fingers hurt.
4. To ensure the sealing is sufficient, fill ~1 – 2 mL of deionized water into the syringe.
Push the syringe steadily, and allow the water to flow into the Y-shaped channel to
check for leaks and blockage.
5. In case leaks are detected, think about different ways of troubleshooting and try
flowing in water again If the channel is blocked, take apart the device and clean it
with water and ethanol
6. Label your group’s slide with (1) cohort number, (2) your group number. Take a
photo of your product, if possible.
7. Clean up and have your instructor sign on your pre-lab section before you leave.
b
c
.
Insert three ~4 cm, ~4 cm and ~3 cm lengths of tubing
into the inlets and outlet. It is suggested to cut the
tubing end at 45o before inserting into the PDMS. The
sharpened tip is easier to insert.
(Optional)To prevent leakage of fluids, apply 5 min®
Epoxy to the tubing over the holes. Allow the epoxy to
cure for 5 min. (this step needs good ventilation)
10.003 Chemistry 1D Designette
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Lab Session 3 Manual (Week 6)
In the first two lab sessions, we have fabricated the microfluidic channel master and
generated microfluidic device with a Y-shape channel. A key feature of solutions in
microfluidic device that is different from its bulk analog is that the fluid flow within microfluidic
channels is laminar. This means that parallel fluid streams in the same channel will flow
independently of each other with little or no mixing at the interface. Practically, however,
diffusion across adjacent streams allows reagents from each stream to react. A microfluidic
channel therefore provides a useful platform for demonstrating the dynamics of fluid flow in
small volumes and performing confined chemical reactions.
In Lab Session 3, we are going to have hands-on experience on how the microfluidic
device manipulates fluid flow and the chemical reactions involved. Specifically, the following
two scenarios are tested: (1) a visualization of laminar flow at the interface between two dye-
colored fluid streams, (2) an acid-base reaction at the fluid-fluid interface.
A. Hazards
Sharps – Ensure safe use and disposal of razor blades and needles. Keep its cap in safe
place when the needle is in use; cap the needle at all time when not in use.
Chemicals – HCl (aq) and NaOH (aq) are corrosive and can cause burns to any area of
contact. Wear lab-coat, goggles and gloves.
B. Materials and equipment:
Syringe (5mL, x1), 1.5mL vials (x6), pH paper, camera
C. Chemicals:
0.05 M HCl solution with phenolphthalein (C20H14O4) indicator, 0.1 M NaOH solution, water
with red food coloring dye, water with blue food coloring dye, water with yellow food coloring
dye and deionized water.
10.003 Chemistry 1D Designette
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D. Procedures:
1. Visualization of laminar flow
a. Fill vial 1 with red/yellow solution.
b. Fill vial 2 with blue solution.
c. Pull the syringe connected to the outlet
tubing, and allow the solutions from
vial 1 and 2 to flow into the Y-shaped
channel simultaneously.
-- Observe the laminar flow through
the entire length of the channel, and
note the mixing only occurs at the
outlet.
Figure 1. Mixing of food coloring. Images were
captured with a standard digital camera
through the eyepiece of a compound
microscope.
d. Stop pulling the syringe and disconnect it (leaving the needle in) from the outlet tubing.
Observe what happens to the solution remaining in the channel. Consider the reason for
this change.
e. Wash the syringe and needle by drawing in and pushing out deionized water for a few
times. Re-connect the syringe and needle to the outlet tubing with tweezers (Proceed
with caution!).Clean the channel with at least 5 ml of water.
f. Disconnect the syringe again from the outlet tubing , but leave the needle in.
2. Acid-base reaction at the fluid-fluid interface
Although laminar flow results in physically separated fluid streams, some diffusion
occurs at the interface between the streams. The interaction of reactants at this interface can
be demonstrated with an acid-base reaction.
Note: for better visualization and photo taking, you can flip the device upside down, so that
the glass slide is facing up. In this case, make sure you hold the glass slide horizontally (do
not tilt it sideways—think about why).
a. Fill vial 1 with an acid solution (HCl, 0.05 M) with phenolphthalein (C20H14O4) indicator.
Note the color of the solution.
b. Fill vial 2 with a basic solution (NaOH, 0.1 M). Note the color of the solution.
c. Re-connect the syringe and needle to the outlet tubing with tweezers (Proceed with
caution!).
10.003 Chemistry 1D Designette
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d. Pull the syringe steadily, and allow the
solutions from vial 1 and 2 flow into
the Y-shaped channel simultaneously.
-- Observe the color change in the
channel as the flow continues.
-- Check whether there is any
difference between the region near
the inlet and that toward the outlet.
-- Consider the reasons for the color
change and the difference between
regions if there is any.
Figure 2. Acid-base reaction. Images were
captured with a standard digital camera
through the eyepiece of a compound
microscope.
e. Stop pulling the syringe and disconnect it from the outlet tubing. Observe what happens to
the solution remaining in the channel. Consider the reasons for this change.
f. Take a piece of pH paper, push out a few drops of solution from the syringe onto the pH
paper and record down the pH value.
g. Rinse and clean the syringe, syringe needle and vials with DI water. Cap the syringe
needle.
3. Clean up and have your instructor sign on your pre-lab section before you leave.
Reference:
This lab for Chemistry 10.003 is adapted from the paper published in Journal of Chemical
Education: Chemistry in Microfluidic Channels, by Matthew C. Chia, Christina M. Sweeney,
and Teri W. Odom (Vol. 88, pp. 461, 2011). It is only for internal use, and please do not
distribute outside of this course.
10.003 Chemistry 1D Designette
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Lab Report Format and Requirements
Before and during each Lab Session
Please complete the Pre-Lab questions for each lab session (attached at the end of this
handout) and then have your handout signed by one of the instructors before you leave .
Lab Report
You are required to write and submit an individual lab report (maximum of 10 pages, font
size 12) which is due 1st July, 2013. The lab report must be typed using Microsoft Word or
an equivalent word processing software, and it should include the following information:
(0) Your full name, group number, group members’ names.
(I) Objectives (Please write down the objectives for each lab session. Don’t write it in
one sentence (e.g. ―To make a microfluidic device) 5 pts
(II) Methods (procedures) 20 pts
(III) Observations along the procedures 20 pts
(IV) Results and Discussion 20 pts
Note: Sections (III) and (IV) should not be compiled as one section in your lab report. In
Section III, you simply write down your observations, whereas in section (IV), you need
to write down your interpretations based on your observations/findings from the three lab
sessions.
(V) Summary 5
e.g. conclusions, what worked, what did not work, what would you do differently? pts
(VI) References (if any)
(VI) Please also include your answers to the following post-lab questions and attach them
with your lab report. (10 pts)
(a) If we do not have 3D printer, how would you create the Y-shaped block which the
PDMS master mold is made? 2 pts
(b) Let’s take a closer look at the UV curable adhesive PU. The wavelength of the UV
light that can initiate PU curing step is 380 nm and below. Use quantum theories
that you have learnt in Unit 1 of Chemistry 10.003 to briefly explain this. For the
Norland Optical Adhesive 81 (PU) that we used, according to the manufacturer
specfications, ―2 Joules/sq. cm of energy is required to fully cure the material‖. If
the wavelength of UV light is 360nm, and the PU area is 2 cm by 4 cm, how
many reaction sites in PU are activated by this 2 J/cm2 of light (assuming one
photon activate one reaction site in PU)? 4 pts
10.003 Chemistry 1D Designette
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(c) What are the advantages of UV curable process? 2 pts
(e) Describe the observations during the HCl (aq) - NaOH (aq) reaction (step 2d-2f)
and explain the reason. 2pts
(Can also attach your photo to explain your answer)
Grading
These three sessions of labs contributes to 100 points out of the total 850 points of
chemistry (refer to the Syllabus found on e-dimension). The lab will be evaluated based on
the following:
Lab handouts (i.e. pre-lab questions, notes taken during lab): All the lab handouts must
be submitted with the lab report (15 pts)
Lab reports including the post lab questions (80 pts))
Conduct during lab classes (e.g. punctuality, observing safety rules, housekeeping)(5
pts)
A note on plagiarism: Please make sure to write the report in your own words. Do
not copy directly from your lab manual. If making use of web-based resources, please cite
the references accordingly.