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2015 MIE Graduate Student Conference
April 18, 2015
CHARACTERIZATION OF THERMOPLASTIC FUSION BONDING OF MICRO
CHANNELS USING PRESSURE ASSISTED BOILING POINT CONTROL SYSTEM
Kavya Dathathreya
M.S. Candidate
Faculty Advisor: Michael C. Murphy
ABSTRACT
An innovative method of thermoplastic fusion
bonding using a pressure assisted boiling point control
(PABP) system is being characterized to determine the
optimum parameters for bonding polymethyl methacrylate
(PMMA) components containing microchannels and thin,
250 m cover sheets. The PABP system enables precise
control of the temperature boundary condition by
immersing the components being bonded in boiling water
and the applied pressure by varying the vapor pressure.
Test structure geometries containing microchannels of
four different aspect ratios were designed: 1:10 (Depth:
10µm, Width: 100 µm and Depth: 5µm, Width: 50 µm),
1:50 (Depth: 10µm, Width: 500µm and Depth: 5µm,
Width: 250 µm), 1:100 (Depth: 10µm, Width: 1000µm and
Depth: 5µm, Width: 500µm) and 1:200 (Depth: 10µm,
Width: 2000µm and Depth: 5µm, Width: 1000µm)
Microchannels were hot embossed using micro-milled
brass mold inserts. Bonding conditions are being
optimized by observing microchannel deformation under a
microscope. Different grades of PMMA are being
evaluated. Bonded samples will be rupture and leak tested
to determine the integrity and strength of the bonds.
INTRODUCTION
Microfluidics is one of the rapidly progressing areas of
research with new innovations and numerous applications in
the biomedical field. Lab-on-a-chip microdevices have the
potential to assist in rapid, precise diagnoses for a wide
variety of diseases, such as stroke and cancer, much faster
than traditional labs leading to more precise medical
treatments tailored for each individual. These microdevices
contain enclosed microchannels formed by bonding two
polymer sheets which makes bonding is critical step in the
manufacturing process. Bonding can be broadly classified
into two types: direct and indirect bonding.
Indirect bonding, uses an adhesive layer like a glue or
laminate sheet, to bond two polymers.2 As the adhesive is
often of a different material, it poses problems like forming
dead volumes, clogging of the channels and contamination
of the biological samples. In direct bonding methods, the
polymers are bonded without using any external agent by
heating the polymer to its glass transition temperature and
by applying pressure. Since no external material is used,
direct bonding methods results in homogeneous surface
properties. Different methods of bonding are well
summarized and compared in the review article.2
Among the different bonding methods, thermoplastic
fusion bonding (TFB) is most widely used because of high
bond strength and homogeneous properties. Polymers are
heated to high temperature under pressure which results in
diffusion of polymer chains at the surface creating the
bond.3 However, the applied pressure should be evenly
distributed and the temperature precisely controlled for
good bonding. Park, et al. ensured uniform pressure and
precise control of temperature by using the boiling water
based PABP system.1 More experiments are being
conducted to identify the optimum conditions for bonding
with minimum deformation and the effect of the material,
device aspect ratio and duration of bonding process.
PROCEDURE
In this method, pressure and temperature obtained by
boiling water at controlled pressure was used to bond the
polymer samples. The relationship between the pressure and
boiling point of a liquid is governed by Clausius-Clapeyron
equation given by
where TB = boiling temperature at a given pressure [K], Tnb
= the normal boiling point [K], R = ideal gas constant,
8.314 [JK21 mol], Pg = gauge pressure [Pa] and ΔHvap =
heat of vaporization of the liquid [J mol21]. The heat of
vaporization of water is 40.65 kJ/mol.
Polymer that was used in the experiments was
Polymethyl methacrylate (PMMA) because its glass
transition temperature, about 100 oC, is realizable at
relatively low pressures . Microchannels of aspect ratios
1:10 (Depth: 10µm, Width: 100 µm and Depth: 5µm,
Width: 50 µm), 1:50 (Depth: 10µm, Width: 500µm and
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Depth: 5µm, Width: 250 µm), 1:100 (Depth: 10µm, Width:
1000µm and Depth: 5µm, Width: 500µm) and 1:200
(Depth: 10µm, Width: 2000µm and Depth: 5µm, Width:
1000µm) were designed. Brass mold insert of 3” diameter
was fabricated using a micro milling machine (MMP,
KERN Micro- und Feinwerktechnik GmbH & Co. KG,
Eschenlohe, Germany). Samples were hot embossed in
2.5mm thick PMMA sheets (Acrylite, Eplastics, San Diego,
CA, USA) using Jenoptik HEX 02 (Jena,Germany). Impact
modified PMMA sheets (Goodfellow, Oakdale, PA, USA)
of 250 ms were used as cover slips. Samples were scanned
using an optical profilometer (Nanovea ST400, Irvine, CA,
USA) to ensure that dimensions were acceptable. The
samples were then cleaned using a soap solution and 5%
isopropyl alcohol and dried overnight in an oven at 80°C to
remove all the moisture. This is a very important step to
ensure good bonding result.
Thermoplastic fusion bonding was carried out using a
modified commercial pressure cooker (8-quart, Philippe
Richard, China) as a pressure vessel Pressure gauge
(McMaster-Carr 4088K29, Cleveland, H,USA) is connected
to the cooker with a brass relief valve (McMaster-Carr
48935K25, Cleveland, OH, USA) to control the pressure
manually. A portable butane gas burner (Sun Star,
Geumsan, Chungcheongnam, South Korea) was used to
provide heat to boil the water to the required temperature.
The experimental set up is shown in Fig. 2(a).1
(Torr 353444, Torr Technologies, Inc., Auburn, WA,USA)
through which thermocouple (K-type,OMEGA Engineering,
Inc., Stamford, CT, USA) was inserted to measure the
temperature. A rubber tube also acted as a vent tube and
thermocouple was connected to a data logger (Omegaette
HH306, OMEGA Engineering, Inc., Stamford, CT, USA)
which recorded the temperature throughout the process.1
The polymer bag with the samples are then immersed in
boiling water and vapor pressure acts evenly on the polymer
samples and helps in achieving good bonding between the
two polymers. The required temperature and in turn
pressure can be obtained from relief valve.
RESULTS AND DISCUSSIONS
Three samples for each channel size were bonded at
temperatures between 105°C to 107°C and pressures
between 0.5psi – 2 psi. Bonding was done for duration of 15
mins after which the heat supply was disconnected. Fairly
good bonding results were obtained for high aspect ratio of
1:10 shown in Fig.3(a). However, in some of the samples,
slight deformation of the channel was observed. For low
aspect ratio of 1:100 and 1:200, most of the samples had
channel deformation. The reasons for this are being
investigated. A different grade of PMMA is being tested to
see its effect. Samples will be leak and rupture tested to
evaluate the bond strength.
Fig.3(a) Microscopic image of bonded sample of depth
10µm width 100µm at 105°C and 0.5psi (b) Microscopic
image of bonded sample of depth 5µm and width 500µm at
105.5°C and 0.6psi
(a)
(b)
Fig.2(a) Experimental set up1 (b) Polymer samples in the
sealing bag with connector
Dried PMMA samples along with the cover slips are fixed
against a glass plate which provides a flat reference. This
assembly is placed inside a sealing bag (Foodsaver T15000011-002, Sunbeam Products, Neosho, MO, USA) to
protect the samples from the boiling water as shown in
Fig.2 (b). Polymer bags containing the samples were sealed
using vacuum sealer (Foodsaver V2830, Foodsaver
Advanced Design, Sunbeam Products, Neosho, MO, USA)
to avoid the mixing of water with the samples. The sealed
bag was then connected to the rubber tube using a connector
ACKNOWLEDGMENTS
I thank Department of Mechanical Engineering for
support. I also thank Dr. Daniel S. Park for his guidance,
Mr. Jason Guy for brass mold fabrication and the staff of
Center for Advanced Microstructures and Devices for hot
embossing polymer samples at Louisiana State University.
REFERENCES
1 T. Park, I. -H. Song, D. Park, B. H. You and M. C.
Murphy, Lab Chip, 2012, 12, 2799-2802
2 C. -W. Tsao and D. L. DeVoe, Microfluid Nanofluid, 2009,
6, 1-16
3 Y. H. Kim and R. P. Wool, Macromolecules, 1983, 16,
1115-1120
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