pdms mold

10.003 Chemistry 1D Designette

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


of 16

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)


of 16

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


of 16

( ) 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.


of 16

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.


of 16

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.


of 16

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


of 16

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.


of 16

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.


of 16

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.


of 16

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)


of 16

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.


of 16

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!).


of 16

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.


of 16

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


of 16

(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.

pdms mold

Documents

Transcript of pdms mold