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Permanent Hydrophobic Surface Treatment Combined with Solvent
Vapor-Assisted Thermal Bonding for Mass Production of Cyclic
Olefin Copolymer Microfluidic Chips
Tianyu Guan, Sineenat Yuket,
§
Hengji Cong, Douglas William Carton, and Nan Zhang*
Cite This: ACS Omega 2022, 7, 20104−20117
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ABSTRACT: A hydrophobic surface modification followed by solvent vapor-
assisted thermal bonding was developed for the fabrication of cyclic olefin
copolymer (COC) microfluidic chips. The modifier species 1H,1H,2H,2H-
perfluorooctyl trichlorosilane (FOTS) was used to achieve the entrapment
functionalization on the COC surface, and a hydrophobic surface was developed
through the formation of a Si−O−Si crosslink network. The COC surface coated
with 40 vol % cyclohexane, 59 vol % acetone, and 1 vol % FOTS by ultrasonic
spray 10 and 20 times maintained its hydrophobicity with the water contact angle
increasing from ∼86 to ∼115°after storage for 3 weeks. The solvent vapor-assisted
thermal bonding was optimized to achieve high bond strength and good channel integrity. The results revealed that the COC chips
exposed to 60 vol % cyclohexane and 40 vol % acetone for 120 s have the highest bond strength, with a burst pressure of ∼17 bar,
which is sufficient for microfluidics applications such as droplet generation. After bonding, the channel maintained its integrity
without any channel collapse. The hydrophobicity was also maintained, proved by the water contact angle of ∼115°on the bonded
film, as well as the curved shape of water flow in the chip channel by capillary test. The combined hydrophobic treatment and solvent
bonding process show significant benefits for scale-up production compared to conventional hydrophilic treatment for bonding and
hydrophobic treatment using surface grafting or chemical vapor deposition since it does not require nasty chemistry, long-term
treatment, vacuum chamber, and can be integrated into production line easily. Such a process can also be extended to permanent
hydrophilic treatment combined with the bonding process and will lay a foundation for low-cost mass production of plastic
microfluidic cartridges.
1. INTRODUCTION
In recent years, microfluidics with hydrophobic surfaces have
attracted much interest and have been widely used in diverse
applications, including the modulation of protein adsorption
and cell adhesion,
1
the reduction of flow resistance in
microfluidic chip channels,
2
and the water-in-oil droplets
generation in droplet-based microfluidics.
3
The demand for
hydrophobic microfluidics necessitates the fabrication, bond-
ing, and surface modification of thermoplastic devices.
4
Thermoplastic polymers, such as cyclic olefin copolymer
(COC), cyclic olefin polymer (COP), poly(methyl methacry-
late) (PMMA), and polycarbonate (PC), have been widely
investigated as the substrates for microfluidic devices,
5
due to
their precise replication of micropatterns with high-quality
surfaces. They are also suitable for mass production through
injection molding and hot embossing at a low cost. Compared
with other thermoplastics, COC has apparent advantages, such
as good optical transparency, low water absorption, low
autofluorescence, high chemical resistance, and good thermal
resistance.
6
However, native COC cannot provide sufficient
hydrophobicity in some applications, including the formation
of water-in-oil droplets, which requires a hydrophobic surface
treatment on COC substrate. Nevertheless, the hydrophobic
surface treatment will lead to a lack of wettability between two
mating surfaces during bonding, hindering the sealing of COC
microfluidics.
Bonding and hydrophobic surface treatment are two
essential processes for fabricating microfluidic devices with
hydrophobic surfaces. A wide variety of bonding techniques
have been investigated for thermoplastic microfluidics, such as
adhesive bonding, thermal fusion bonding, and solvent
bonding. Adhesive bonding is simple to operate for sealing
thermoplastic microfluidics. Liquid adhesives are typically
applied on the chip surface, which could be cured after
ultraviolet (UV) exposure or solvent evaporation.
7
However,
this method may cause channel clogging, which requires the
removal of uncrosslinked adhesive trapped in the channel by
Received: March 30, 2022
Accepted: May 19, 2022
Published: May 31, 2022
Article
http://pubs.acs.org/journal/acsodf
© 2022 The Authors. Published by
American Chemical Society 20104
https://doi.org/10.1021/acsomega.2c01948
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organic solvent,
8
making it difficult to carry out on a large
scale. In thermal fusion bonding, thermoplastic chips are
heated to a temperature around/above their glass-transition
temperature (Tg) and compressed by a hold pressure. This
bonding method often results in higher bond strength as the
complete diffusion of polymer chains occurs between two
mating surfaces. However, microchannels are prone to collapse
once applied with high temperature and pressure, making it
challenging to maintain channel integrity.
9
Meanwhile, to
reduce the surface energy of bonding surfaces, UV/ozone or
oxygen plasma treatment is typically applied to make surfaces
more hydrophilic,
10
resulting in a processing time often longer
than 1 h.
7
During solvent bonding, thermoplastic substrates are
dissolved in organic solvents with similar solubility.
11
After the
polymer surfaces are solvated, the polymer chains are mobile
and can easily diffuse across the solvation layer, forming an
entanglement layer and resulting in a high bond strength.
12
However, the immersion of polymer into the solvent is not an
easily controllable process: the excessive solvent absorption in
the polymer substrates could cause severe channel deformation
once being mated under pressure.
7
Various surface treatment and modification methods have
been reported to obtain a highly hydrophobic surface of
thermoplastics, including plasma treatment, graft polymer-
ization, and entrapment functionalization. Ghosh et al. exposed
the thermoplastic chips to the mixture of CF4and O2for
plasma treatment to have a hydrophobic surface.
13
However,
this treatment is expensive and difficult to scale up. Meanwhile,
treating the thin surface layer without changing the bulk
properties of the thermoplastics is also a challenge for plasma
treatment.
14
Industrially, plasma-enhanced chemical vapor
deposition is used to deposit hydrophobic polymers into the
microchannels. However, due to the smaller channel size, such
coating requires a longer diffusion time and is not so reliable
Figure 1. Optical image of high-precision tool steel mold insert (a) and injection-molded chip with micropatterns (b). (c−f) 3D and two-
dimensional (2D) images of micropatterns on the mold.
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due to restricted diffusion. Robust coatings can be produced by
graft polymerization, with tunable chemical properties and
precise control of local definitions.
6
This surface modification
typically involves two steps: surface activation and graft
polymerization.
15
Due to the lack of chemically reactive
functional groups on thermoplastic surfaces, the activation
process through UV treatment, plasma treatment, or high-
energy electrons is required to generate reactive sites for
further grafting processes. Then, the functional molecules with
reactive groups will be covalently coupled to the surface.
15
The
whole operation process is relatively time-consuming and
complex, making the modification process slow. During
entrapment functionalization, the surface of thermoplastic is
immersed in a solvent containing modifier species and the
polymer surface will swell due to the interaction with the
solvent. The polymer chains become mobile and entangled
with modifier species during surface swelling. Then, the
polymer surfaces deswell in the water moisture and the
modifier species are firmly embedded on the polymer surface
with the evaporation of solvent, and the surface properties are
modified accordingly.
In most studies, bonding and hydrophobic treatment are
conducted sequentially, as the surface energy required for
bonding and surface treatment is the opposite. For the bonding
process, the surface energy should be increased to improve the
wettability and adhesion of two mating surfaces. In contrast,
for hydrophobic surface treatment, the surface energy is
decreased to reduce the wettability of microchannels. There-
fore, modifying the channel wall to hydrophobicity after
bonding becomes a challenging and time-consuming task due
to this process sequence. It is difficult to quickly achieve the
microfluidics bonding and hydrophobic surface treatment for
scalable production. Su et al. prepared the solvent mixture with
1.0 vol % 1H,1H,2H,2H-perfluorooctyl trichlorosilane (FOTS)
as a surface modifier, acetone and n-pentane as solvent
bonding solution to bond and modify the hydrophilic paper
chromatography (PC) chips to hydrophobicity in one step,
and the bond strength was 3.8 MPa with the water contact
angle of ∼117.8°on the modified and bonded PC substrate.
16
However, they did not identify the scalability for applying the
hydrophobic modifier. In all, laboratory treatment using
surface grafting and other techniques involves nasty chemistry
and takes a much longer time. Industrial surface treatment
using plasma-enhanced chemical vapor deposition requires
vacuum and is difficult for reliable treatment for tiny channels.
It is still a significant challenge to develop a scalable, low-cost,
and reliable process for the integration of microfluidic chips by
combining surface treatment and bonding process.
In this work, a hydrophobic surface modification by
ultrasonic spray coating on COC 8007 substrates followed
by solvent vapor-assisted thermal bonding was developed to
fabricate microfluidic chips. This combined method offers a
cost-effective alternative for the large-scale production of chip
assemblies with highly hydrophobic surfaces. Moreover, the
surface treatment and bonding method were optimized to
achieve a long-term stable hydrophobicity and high bond
strength of the bonded COC chips. Each treatment was
analyzed in terms of water contact angle, attenuated total
reflection Fourier transform infrared (ATR-FTIR) spectra, and
surface roughness to achieve the optimum surface treatment
condition for high hydrophobicity and good optical clarity.
After bonding, a leakage test followed by a burst pressure test
was carried out to identify the bonding parameters that could
achieve the highest bond strength. The channel integrity was
also analyzed, and the stability of surface hydrophobicity was
further proved by capillary test and water contact angle
measurement after bonding. This work will lay a foundation for
large-scale accessible surface treatment and bonding processes
for other thermoplastics in microfluidic applications.
2. MATERIAL AND METHODS
2.1. Materials. Cyclic olefin copolymer (COC) pellets
(Grade 8007) for chip fabrication and films (Grade 8007, 150
μm) were obtained from Topas (TOPAS Advanced polymers
GmbH, Frankfurt, Germany). Cyclohexane (≥99.9%) and
acetone (≥99.9%) were purchased from Sigma-Aldrich.
1H,1H,2H,2H-Perfluorooctyl trichlorosilane (FOTS; 97%)
was obtained from Fischer Scientific. All reagents were used
as received.
2.2. Microfabrication. The COC microfluidic chips with
micropatterns were fabricated using an injection molding
machine (FANUC ROBOSHOT S-2000i15B) with a high-
precision tool steel mold (shown in Figure 1), and the mold
insert was characterized by a three-dimensional (3D) micro-
scope (Keyence VHX-5000). The COC-8007 pellets (glass-
transition temperature 78 °C) were used as raw materials. The
dimensions of the chips were 60 mm (length) ×40 mm
(width) ×1.70 mm (height). The microfluidic channels on the
chips were ∼100 μm deep and ∼100 μm wide at the smallest
section. The mold temperature was set at 80 °C, and the
nozzle temperature was 230 °C. The injection velocity was set
as 100 mm/s with a shot size of 43 mm, and the holding
pressure was 62 MPa for 5 s. The injection-molded COC chips
had uniform surfaces without any defects. After injection
molding, both COC chips and prepared COC films were
washed with isopropanol in an ultrasonic bath for 15 min and
dried with compressed air before bonding.
2.3. Surface Modification and Bonding. The mixture
solution of different concentrations of FOTS, cyclohexane, and
acetone was used to modify the surface of both chips and films
by ultrasonic spray coating for different coating times (as
shown in Table 1 and Figure 2). The spray coating speed was
0.5 mL/min with an air pressure of 0.3 MPa and an ultrasonic
current of 0.05 A. For each round of coating, there was 0.02
mL of the solution deposited on the chip surface, which
contained 1% of FOTS. In our previous study, various
concentrations of FOTS have been tried for surface treatment.
However, the lower concentration could not achieve desired
hydrophobicity, while the higher concentration would not
allow bonding or gave a low bonding strength. Therefore, 1%
FOTS was selected as the optimized concentration for surface
treatment. In this study, 0.2 μL of FOTS were deposited onto
the surface per round. For each sample, coatings were
performed 5, 10, and 20 times.
Subsequently, the modified films were exposed to the vapor
of the prepared solvent mixture of 60% cyclohexane (30 mL)
and 40% acetone (20 mL) for 60 and 120 s in a glass Petri dish
Table 1. Surface Modification Parameters
vol %
cyclohexane vol %
acetone vol %
FOTS coating times for each
sample
20 79 1 5, 10, 20
30 69 1 5, 10, 20
40 59 1 5, 10, 20
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with a lid (shown in Figure 2). The treated film was attached
to the Petri dish lid by magnetic metals, and the distance
between film and solvent liquid level was fixed as 1 cm. After
exposure, the film cover was aligned in parallel with the
modified chip in the holder in the hot embossing machine. The
temperature and holding pressure were applied to the film and
chip. As shown in Figure 2b, the bonding temperature was set
at 72 °C, which is below the Tgof COC 8007 (78 °C), to
maintain the integrity of the microstructure of the chip. Finally,
a beaker of water (∼20 mL) was placed in an oven with the
modified chips overnight to let the solvent evaporate in the
atmospheric environment at room temperature. For the
bonding process mentioned before, the unmodified COC
film and chip were also bonded under the same solvent vapor
bonding conditions and were characterized to compare the
effect of surface modification on subsequent bonding.
2.4. Surface Analysis. To prove the performance of
hydrophobic treatment, the modified chips were evaporated
under the same condition as mentioned in Section 2.3 before
the surface characterization.Watercontactangleswere
measured on the surfaces of modified and unmodified chips
using a contact angle goniometer (Ossila) at five different
positions on each chip at five different time points (from
before treatments to 10 days after the treatments). The
ultrapure water was used for contact angle measurement, and
the droplet volume was controlled at 10 μL by a pipette.
Attenuated total reflection Fourier transform infrared (ATR-
FTIR) spectroscopy was used to investigate the mechanisms of
the surface modification on the COC surface. The surface
roughness change of the COC chip before and after the
modification was detected by an optical 3D profilometer
(NPFlex). The modified film cover was placed on a pattern to
compare the optical clarity with and without the hydrophobic
surface treatment. Finally, to investigate the effect of coating
times on the optical transmission properties of the COC film,
the transmission of the native and coated COC film was
carried out using an ultraviolet−visible (UV−vis) spectropho-
tometer (Agilent Technologies Ireland Ltd., Dublin, Ireland)
in the range of 200−800 nm.
2.5. Bond Strength Analysis. 2.5.1. Leakage and Burst
Test. A leakage test was performed before bonding character-
ization to ensure the films, and the chips were bonded
successfully without any leakage. A drill was used to punch
inlet and outlet holes on the chip side, and those holes enabled
the fluid infusion in the microchannels. Silicone tubes were
inserted into the inlets, and the blue ink solution was injected
through the tubes into the microchannels by a syringe pump
with a flow rate of 10 mL/min for better visualization of the
leakage test.
The burst pressure test of the bonded chips was performed
in four different channels to evaluate the bonding strength.
During the test, the outlet hole of the channel was blocked by a
Figure 2. Schematic diagram of ultrasonic spray coating and subsequent solvent vapor-assisted thermal bonding process (a) and thermal bonding
parameters (b).
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plug. The burst pressure was tested by pumping water into the
inlet hole of the channel with a manual test pump (EGA
MASTER Manual Test Pump 60005) and by monitoring the
pressure at which the bonded chips were disassembled.
2.5.2. Channel Integrity Characterization. The channel
integrity of the bonded chip was then investigated by the cross-
sectional analysis under the microscope (Am scope led-144s).
The chip was cut by saw, ground, and polished using P320,
P600, P2500, and P4000 sandpaper with cooling water for
smooth surface finishing before observation. The cross section
of the coated and bonded chip was compared with the COC
chip without treatment and bonding.
2.5.3. Water Contact Angle Measurement and Capillary
Effect Test. After bonding, the water contact angle was
measured on the excess film cover to detect the effect of
bonding on the hydrophobicity of the film. The capillary effect
was performed to observe the flow shape affected by the
surface modification and chip bonding. The deionized (DI)
water was injected into the channel slowly until the injection
flow was stopped at a certain point inside the channel to
observe the interface between air and water. The flow shape at
the interface was observed under the microscope (Am scope
led-144s) to show the capillary effect inside the chip channel
with and without hydrophobic modification.
3. RESULTS AND DISCUSSION
3.1. Mechanism of Surface Modification and Bond-
ing. 3.1.1. Mechanism of Solvent Vapor-Assisted Thermal
Bonding. The solvent vapor-assisted thermal bonding was
achieved using the combination of cyclohexane and acetone in
the solution. It is possible that acetone acted as a sacrificial
solvent while cyclohexane acted as a solvating solvent, which
could be explained by the following reasons: first, COC has
similar solubility with nonpolar organic solvents, such as
hydrocarbons; the solubility parameter (δ) of COC is 17.7 [(J/
cm3)1/2], and the δof cyclohexane is 16.7 [(J/cm3)1/2], while
the δof acetone is 20.4 [(J/cm3)1/2].
12
Therefore, COC
prefers to dissolve in cyclohexane, and it is almost insoluble in
polar organic solvent acetone.
7
Second, acetone has a boiling
point of 56.10 °C, while that of cyclohexane is 80.75 °C. After
solvent vapor exposure, the COC cover film and substrate were
aligned in the preheated thermal press machine.
17
Once
heated, acetone evaporated faster than cyclohexane, increasing
the concentration of cyclohexane. After the COC chip and film
were thermally pressed, all of the solvent evaporated, causing
the mobilized COC polymer chains to entangle with each
other on the film cover and chip substrate, leading to a strong
bonding (shown in Figure 3a).
11
3.1.2. Mechanism of Hydrophobic Surface Modification
through Entrapment Functionalization. During the solvent
vapor-assisted thermal bonding process, the hydrophobic COC
surface transformed into a more hydrophobic state. As shown
in Figure 3b, during the surface treatment process, the modifier
FOTS was immobilized on the COC surface.
16
After the COC
film cover and chip substrate were fed into the thermal press
machine, the concentration of cyclohexane temporarily
increased due to the higher evaporation rate of acetone,
causing COC polymer chains to become mobile in the solvent
vapor. When the COC surface material was swelling, it allowed
the FOTS molecules to be embedded into its surface and
entangled with these modifier molecules. After solvents were
evaporated, these FOTS molecules were firmly embedded on
the COC surface. The next step was the deswelling by water in
the air so that the FOTS species could be fixed on the COC
surface.
18
During the deswelling process, the Si−Cl groups of
the FOTS modifier would be gradually hydrolyzed by water in
Figure 3. Mechanism of (a) solvent vapor-assisted thermal bonding and (b) hydrophobic surface modification by entrapment functionalization.
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the air to become Si−OH groups.
19
The Si−OH groups of
adjacent FOTS molecules dehydrated spontaneously with each
other, forming a Si−O−Si crosslinked network, which could
further fix the FOTS molecules on the COC surface.
20
3.2. Surface Treatment Optimization and Character-
ization. 3.2.1. Water Contact Angle Measurement. The
relationship between COC substrates’hydrophobicity and
cyclohexane concentration, coating times was investigated. The
wettability of the COC surface was determined by measuring
the water contact angle (WCA) of its surface. The hydro-
phobic surface plays an essential role in some applications,
including water-in-oil droplet generation. From Su’s work, the
chips that show stable monodisperse droplet generation have a
WCA of ∼115°.
16
In this study, the WCA of the native COC
substrate is ∼86°, as shown in Figure 3. The COC substrate
coated 20 times with 40% cyclohexane and 1% FOTS show the
highest WCA of ∼116.08°(shown in Figure 4c), and its WCA
remains unchanged after 3 weeks, which proves the
effectiveness and stability of hydrophobic treatment.
The COC substrates treated with 20% cyclohexane and 1%
FOTS show the WCA of ∼93.12 to ∼95.17°in Figure 4a,
which increases slightly compared with the native COC surface
(86°). The coating times and the time after the hydrophobic
surface treatment reveal little significance in changing the
WCA of the COC surface. The possible reason is that 20%
cyclohexane is insufficient to make COC polymer chains fully
mobile, thus causing fewer mobile chains to entangle with the
FOTS molecules. Generally speaking, the more coating times
represent more FOTS modifiers sprayed onto the COC
substrate, indicating a higher amount of FOTS molecules
spread onto the COC substrate. However, the low
concentration of cyclohexane disables the COC polymer
chains to move. Under this condition, the coating times and
the time after treatment have little effect on the WCA.
For the treatment of COC substrate with 30% cyclohexane
and 1% FOTS, all samples show a higher WCA value
compared with the samples treated with 20% cyclohexane
and 1% FOTS (shown in Figure 4b). This result proves the
effectiveness of 30% cyclohexane in dissolving the COC
polymer surface and causing polymer chains to be mobile and
thus entangled with the FOTS molecules. The WCA increases
from 86°before treatment to ∼103.15°for 5 times coating,
∼106.36°for 10 times coating, and ∼105.19°for 20 times
coating at 10 min after the surface treatment. For 5 times
coating, the WCA increases to ∼105.13°after 2 days of storage
and remains almost unchanged (∼106.75°) during 3 weeks of
storage. For 10 times coating, the WCA of the COC surface
remains ∼106.85°after 2 days of storage and keeps increasing
to ∼112.26°at 3 weeks after the treatment. For 20 times
coating, the WCA increases to ∼108.85°at 2 days and ∼112.9°
at 3 weeks after the treatment. The increase of WCA indicates
that the hydrophobic surface is developed by both molecular
entanglement between the COC polymer chains and the
FOTS molecules, with the assistance of cyclohexane, and by
the formation of a Si−O−Si crosslinked network after 2-day or
even longer-time storage.
16
The COC substrate treated with 40% cyclohexane and 1%
FOTS shows the highest WCA value after 1 week of storage,
with ∼111.49°for 5 times coating, ∼114.00°for 10 times
coating, and ∼115.76°for 20 times coating (shown in Figure
4c). And the WCA remains stable after 3 weeks of storage,
with ∼111.81°for 5 times coating, ∼115.37°for 10 times
coating, and ∼116.08°for 20 times coating. This result proves
the effectiveness of a high concentration of cyclohexane in
causing COC polymer chains to be more mobile and thereafter
entangled with FOTS molecules. The coating times 10 and 20
ensure a sufficient amount of FOTS species embedded onto
the COC surface. All treated COC substrates show a delay in
surface hydrophobicity, represented as the WCA continuously
increasing after the treatment and stabilizing after 1 week. The
delay of the hydrophobicity effect proves the two-step
mechanism of hydrophobic surface formation, which is
molecular entanglement followed by Si−O−Si network
formation. To conclude, the COC surface coated with 40%
cyclohexane and 1% FOTS for 10 and 20 times has a WCA
Figure 4. Changes in water contact angle on COC surface over time,
after the surface treatment with (a) 20%, (b) 30%, and (c) 40%
cyclohexane and 1% FOTS with different coating times.
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higher than 115°, which is potential for hydrophobic
microfluidics applications, especially for water-in-oil droplet
generation.
3.2.2. FTIR Analysis. The ATR-FTIR analysis was used to
identify the chemical bonds on the native COC surface and the
change of chemical bonds on the modified surfaces. Figure 5
shows the FTIR spectra of different COC substrates. For the
native COC substrate, the bands of (2855 and 2915 cm−1)
represent the stretching vibration modes of CH2and CH3, and
the band of (1453 cm−1) is attributed to the CH3wagging
mode of the polymer backbone (illustrated by the green line in
Figure 5a).
For the surfaces treated with 20% cyclohexane and 1%
FOTS for 20 times coating, the characteristic absorption bands
are identified as Si−O−Si stretching vibration (1071 cm−1)
and O−Si−O bending vibration (480 cm−1) (illustrated by the
blue line in Figure 5a), which have also been proved in others’
work.
16
Meanwhile, the absorption bands can be identified as
the CF2group (1191 cm−1) and CF3group (1110 and 1350
cm−1). The presence of Si−OH is detected at the band (3400
cm−1), which occurs during the crosslinking of silane.
However, the surfaces treated with 20% cyclohexane and 1%
FOTS for 5 times and 10 times coating have the same
spectrum as the native COC substrate (illustrated by the black
and red lines in Figure 5a). It could be explained that a lower
concentration of cyclohexane (20%) with fewer coating times
is not sufficient to dissolve the COC surface and form the
entrapment layer. Therefore, the COC substrate barely
interacts with the low concentration of cyclohexane at low
coating times.
ThespectraoftheCOCsurfacestreatedwith30%
cyclohexane and 1% FOTS have the features of both COC
and FOTS crosslinked networks since 5 times coating
(illustrated by the black line in Figure 5b). Similar spectra
features have also been found in the COC substrates coated 10
and 20 times (illustrated by the red and blue lines in Figure
5b), which suggests this concentration is sufficient to dissolve
the COC surface and form the entrapment layer since 5 times
coating.
Similarly, the COC surfaces treated with 40% cyclohexane
and 1% FOTS for 5, 10, and 20 times show the spectra with
the features of both COC and FOTS crosslinked networks
(illustrated by the black, red, and blue lines in Figure 5c).
Therefore, the above chemical bond analysis could verify the
effectiveness of the hydrophobic surface treatment. The time
delay of the hydrophobic effect could be explained by the
formation of Si−OH, which is the result of the interaction
between FOTS and water. The Si−OH molecules could
further trap FOTS on the COC surface and cause the CF
groups to modify the surface chemistry. Similar results have
been found in other work, which proved that the fluorinated
silane changed the wetting ability of the polymer surface by
lowering its surface energy.
21
The FTIR result also confirms
that the entrapment of FOTS occurs only with a sufficient
concentration of cyclohexane, which is capable of dissolving
the COC surface and causing COC polymer chains to become
mobile and entangled with FOTS molecules. This is the reason
why almost no difference could be observed for the surfaces
treated with 20% cyclohexane and 1% FOTS for 5 and 10
times coating.
From water contact angle measurement and FTIR analysis,
it could be concluded that the COC substrates treated with
40% cyclohexane and 1% FOTS 10 and 20 times successfully
generate hydrophobic surfaces with the WCA of ∼115°, which
could be used for water-in-oil droplet generation, as also
proved in Su’s work: the stable monodisperse droplets were
generated in the PC chip with the WCA of ∼115°.
16
In Liu’s
work, the water-in-oil droplets could be formed in the
polydimethylsiloxane (PDMS) microfluidic chip with a WCA
Figure 5. FTIR spectra of the COC substrates treated with (a) 20%,
(b) 30%, and (c) 40% cyclohexane and 1% FOTS with different
coating times.
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of ∼112°.
22
Therefore, chips treated under this condition were
selected for other characterizations.
3.2.3. Surface Roughness Measurement. The COC
substrates treated with 40% cyclohexane and 1% FOTS for
Figure 6. 3D images of the surface roughness for COC chips from the profilometer: (a) native COC, (b) COC substrate coated by 40%
cyclohexane and 1% FOTS for 10 times, and (c) COC substrate coated by 40% cyclohexane and 1% FOTS for 20 times. (d) Surface roughness
value of native COC substrate and COC substrates coated by 40% cyclohexane and 1% FOTS for different times.
Figure 7. Influence of the coating times on the optical clarity of the COC films: (a) Original pattern; (b) native COC film; (c) COC film coated
with 40% cyclohexane and 1% FOTS for 10 times; (d) COC film coated with 40% cyclohexane and 1% FOTS for 20 times; and (e) UV−vis
transmittance spectra of native COC film and COC film treated with 10 and 20 times coating.
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10 and 20 times were characterized by a profilometer to detect
the effect of surface treatment on the surface roughness (Sa)of
the substrate. The Savalue is ∼39.17 nm for the untreated
COC surface. After hydrophobic treatment, Saincreases to
∼156.54 and ∼178.42 nm for the substrate coated 10 and 20
times, respectively (Figure 6d). The exposure to 40%
cyclohexane roughens the COC surface, causing its surface
to swell and form a sticky bonding layer. It is possible that each
time the cyclohexane was applied, the COC surface was
dissolved layer by layer, which may lead to uneven texture
when the surface was dried. Therefore, the more coating times
were applied, the thicker sticky bonding layer was developed,
16
resulting in a higher Savalue. More importantly, it could be
observed from the 3D image that the surface texture is uniform
due to the application of ultrasonic spray coating, which
confirms the effectiveness of ultrasonic spray coating.
3.2.4. Optical Clarity Characterization. As shown in Figure
7, the effect of coating times on the optical clarity of the COC
film was investigated. The pattern covered by native COC film
shows a distinct border (shown in Figure 7b), representing the
good optical charity of native COC film. The optical charity of
COC film can be maintained by the proper surface treatment
with 40% cyclohexane and 1% FOTS for 10 times coating. The
border of the pattern gradually becomes indistinct with the
increase of the coating times (shown in Figure 7d), indicating
that the COC film coated with 40% cyclohexane and 1%
FOTS for 20 times have lower optical clarity compared with
the film coated for 10 times. Figure 7d shows the UV−vis
optical transmittance of native COC film, and the COC film
treated with different coating times. Compared with native
COC film, the transmittance decreased slightly (0−8%) for 10-
times-coated film. While for 20-times-coated film, the decrease
of transmittance is more significant (13−23%). The optical
clarity change is possibly due to the increment of the surface
roughness, as it has been proved in others’work that the
change of surface roughness directly influences the trans-
parency of the polymer microfluidic devices.
23
It should be
noted that, the 10-times-coated COC film has the trans-
mittance of 93.8−97.5% in the visible domain (400−700 nm),
indicating that this coating technique is applicable to the
optical COC devices in diverse applications, as also discussed
in other study.
24
Therefore, proper treatment times and
concentration optimization is important to attain the required
transparency for detection.
In summary, the COC substrates treated with 40%
cyclohexane and 1% FOTS 10 and 20 times achieved desired
hydrophobicity, which were selected for bonding optimization.
In practical applications, coating conditions could be
customized according to the desired optical requirements.
3.3. Solvent-Assisted Thermal Bonding Optimization.
After the COC film and chip were treated with 40%
cyclohexane and 1% FOTS 10 and 20 times, they were
prepared for bonding. To investigate the effect of solvent
exposure time on the bonding effect and bonding strength, we
prepared the chips with different exposure time (60 and 120 s)
in solvent-assisted thermal bonding, while the composition of
solvent was fixed at 60% cyclohexane and 40% acetone and the
bonding temperature was set at 72 °C, which is below the Tgof
COC 8007 (78 °C). Our previous study found that when the
exposure time was less than 60 s, the film and chip cannot be
bonded due to the lack of mobile polymer chains; when the
exposure time exceeded 120 s, the channels deformed after
chips and films were bonded. Therefore, 60 and 120 s were
selected for bonding optimization. Similarly, the bonding
temperature was optimized to be 72 °C, as lower temperatures
led to insufficient bonding while higher temperatures caused
channels’deformation.
3.3.1. Leakage Test. A leakage test was performed by
injecting blue-colored water into the chip channel to determine
whether the chips could remain functional after the surface
treatment and bonding. For untreated chips, all chips were
successfully bonded with 60% cyclohexane and 40% acetone,
and all channels functioned well without leakage after bonding.
For the chips treated with 1% FOTS 10 times, the 60 s
exposure achieved a lower bonding strength, as some channels
did not function after bonding. However, when the film was
exposed to a solvent mixture for 120 s, the chip was
successfully bonded and remained functional after bonding
(Figure 8a). Figure 8b shows the microchannel remained
functional after the hydrophobic treatment and bonding
process. Under this condition, the 120 s exposure could
cause the COC chains on the upper layer to become mobile
and fully entangled with the COC chains on the top surface of
the chip substrate during thermal bonding. When the coating
times increased to 20 times, some channels lost function after
bonding, and leakage was observed for moderate syringe
pressure (∼300 kPa). This is because a large amount of FOTS
molecules are entrapped on the top layer of the COC film
cover and chip, which could increase the hydrophobicity of the
interface and potentially reduce the bonding effect, as also
proved in other research.
16
3.3.2. Bonding Strength Characterization. The bonding
strength was evaluated by testing the burst pressure of four
different channels in each bonded chip. As shown in Figure 9a,
Figure 8. (a) Images of blue dye flowing through the chip channels and (b) the microchannel filled with the blue dye under the optical microscope.
The chip was coated with 40% cyclohexane and 1% FOTS 10 times, followed by 120 s solvent exposure during bonding. No leakage was observed
at the pressure of ∼500 kPa.
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four different channels were selected for bonding strength
characterization. Figure 9b shows that the position of the
channels on the chip does not affect the bonding strength
significantly, indicating that the bonding effect across the
whole chip is uniform. Moreover, for both treated and
untreated samples, the burst pressure increases with the
increase of solvent exposure time. The COC chips with 120 s
exposure always have higher burst pressure than those with 60
s exposure under the same surface treatment condition (shown
in Figure 9b). This is because more solvent exposure can
dissolve more COC chains on the top surface of the film,
causing more COC chains to be mobile and diffuse across the
surface, thus resulting in a higher bonding strength. Mean-
while, the surface treatment also affects the bonding strength.
For the chips exposed to the solvent mixture for 60 s, the burst
pressure decreases with the increase of coating times,
represented as the average burst pressure decreasing from
16.08 bar for nontreated chips to 12.83 bar for 10-times-
treated chips and 10.41 bar for 20-times-treated chips.
Similarly, for the chips exposed for 120 s, the average burst
pressure decreases from 20.33 bar for nontreated chips to
17.41 bar for 10-times-treated chips and 12.04 bar for 20-
times-treated chips. It could be concluded that the hydro-
phobic treatment reduces the bonding strength. The surface
modification proves to fix the FOTS modifier on the surface of
the chip and film. FOTS molecules may obstruct the bonding
process, which requires exposing cyclohexane to the COC
chains and fusing the COC chains between the film cover and
the COC chip. Additionally, more coating times will spray too
much cyclohexane on the COC surface, resulting in a thicker
entrapment layer.
16
This thick entrapment layer is not as dense
as the native COC substrate, reducing the bonding strength
between two COC surfaces.
It should be noted that, among all treated chips, the chip
that demonstrates the maximum burst pressure (17.41 bar) is
treated with 40% cyclohexane and 1% FOTS 10 times,
followed by 120 s solvent exposure. This bonding strength is
sufficient for droplet generation, as it is higher than the typical
pressure required for droplet generation.
25
Therefore, the
ultrasonic spray surface treatment with 40% cyclohexane and
1% FOTS for 10 times coating followed by 120 s solvent
exposure was selected as the optimized surface treatment and
bonding conditions for subsequent cross-section analysis.
3.3.3. Cross-Section Analysis. The chips treated with 40%
cyclohexane and 1% FOTS for 10 times coating followed by
120 s solvent exposure was selected for cross-section analysis.
As shown in Figure 10, after bonding, the coated and bonded
chip maintained its integrity compared with the original COC
chip. The holding temperature during the thermal bonding is
72 °C, below the Tgof COC 8007 (78 °C). Therefore, the
COC substrates should remain in the solid state without any
deformation.
In summary, the optimized treatment and bonding
conditions for this study are that the COC chip and film are
Figure 9. Schematic of four selected channels on the bonded chip (a) and the relationship between different surface treatment conditions and
solvent exposure time and the burst pressure of the microfluidic chips (b). The average burst pressure of four channels is labeled at the top of each
group of columns.
Figure 10. Optical images of microchannels on (a) nonbonded COC chip and (b) COC chip coated with 40% cyclohexane and 1% FOTS 10
times followed by exposure to 60% cyclohexane and 40% acetone for 120 s and then thermal bonded at 72 °C. The bottom width of the channel is
∼100 μm, and the height is ∼100 μm. The channel’s sloped wall results from the draft angle of the micro structures on the stainless-steel mold.
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coated with 40% cyclohexane and 1% FOTS 10 times, then
exposed to 60% cyclohexane and 40% acetone for 120 s and
then thermal bonded at 72 °C. The chips bonded under this
condition show the highest bonding strength while maintaining
the channel integrity without collapse.
3.4. Characterization of Chips Fabricated by Hydro-
phobic Treatment and Solvent-Assisted Thermal Bond-
ing. Considering the surface treatment and bonding character-
ization, the chips treated with 40% cyclohexane and 1% FOTS
for 10 and 20 times develop hydrophobic surfaces; the cover
films exposed to the solvent mixture for 120 s generate a
relatively higher bonding strength after bonding. Therefore,
chips and films treated and bonded under these conditions
were selected for hydrophobicity characterization.
3.4.1. Water Contact Angle Measurement. After surface
treatment and bonding, the water contact angle (WCA) on the
excess film cover was measured to evaluate the stability of
hydrophobic surface treatment. It is expected that the
hydrophobicity was not affected by solvent exposure as well
as the heat and pressure applied during solvent-assisted
thermal bonding. As shown in Table 2, the chip bonded
without surface treatment has the WCA of 86 ±1.7°, while the
chips first treated with 40% cyclohexane and 1% FOTS for 10
and 20 times and then bonded have the WCAs of 115 ±1.2
and 115 ±1.8°, respectively. This result proves the
effectiveness and stability of hydrophobic surface treatment.
3.4.2. Capillary Effect Evaluation. To evaluate the hydro-
phobicity of channels after treatment and bonding, the DI
water was injected into the chip channel, and the shape of the
water-air interface was observed under a microscope. As shown
in Figure 11, the untreated COC chip has a flat flow interface,
indicating the wettability of the channel is good. As for the
COC chips treated with 40% cyclohexane and 1% FOTS 10
and 20 times and then bonded, the shape of the water flow is
more curved due to the lack of wetting in the microchannel
surfaces.
26
Therefore, the surface hydrophobicity inside the
channel is well maintained after bonding.
3.5. Summary of Bonding Techniques. For the mass
production of microfluidic chips, four prevalent bonding
techniques are used: laser welding, ultrasonic welding, thermal
diffusion bonding, and solvent-assisted thermal bonding.
7,17
Table 3 compares the advantages and limitations of each
method. Bonding itself is a challenging task for the fabrication
of microfluidic chips. Combining bonding and surface
treatment is even more challenging and largely influences the
microfluidic cartridge cost since they account most of cycle
time of chip production. Hydrophilic treatment such as oxygen
plasma and UV/ozone can be conducted before bonding and
benefits to increase the wettability of the chips, reducing the
surface energy and improving the bonding strength.
12
However, most bonding techniques are not applicable with
hydrophobic surface treatment due to the difficulty of mating
surfaces with decreased surface tension after the hydrophobic
treatment.
16
Laser welding and ultrasonic welding require
plasma-enhanced chemical vapor deposition (PECVD) as
hydrophobic surface treatment to modify the wettability of
microchannels in the sealed chip.
27
However, it is difficult to
coat narrow channels and holes with PECVD, and the coatings
produced from PECVD may result in a poor uniformity,
28
due
to limited diffusion from bonded channels with small size after
bonding. In this study, ultrasonic spray coating is applied to
uniformly coat the modifier onto the chip and film surface
before bonding, which provides a more effective and less time-
consuming hybrid surface hydrophobic treatment and bonding
technology. The nanodroplets from the ultrasonic nozzle are
Table 2. Comparison of Hydrophobicity among the Bonded
Native COC Film, COC Film Treated with 40%
Cyclohexane and 1% FOTS for 10 and 20 Times and then
Bonded
a
a
During the bonding process, the exposure time for all samples was
fixed at 120 s.
Figure 11. Capillary effect of injected DI water; (a) bonded COC chip without surface treatment, (b) COC chip treated with 40% cyclohexane and
1% FOTS for 10 times and then bonded, and (c) COC chip treated with 40% cyclohexane and 1% FOTS for 20 times and then bonded. During
the bonding process, the exposure time for all samples was fixed at 120 s.
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capable of covering the small features in the chip, thus ensuring
the uniformity of the coating. The solvent-assisted thermal
bonding provides the high bonding strength, and the 2-day
post-treatment requires no labor work, which enables the cost-
effective scale-up of the chip production from injection
molding to surface treatment and bonding.
4. CONCLUSIONS
In summary, a hydrophobic surface treatment followed by
solvent-assisted thermal bonding was successfully developed to
modify and bond COC chips for microfluidic applications,
including water-in-oil droplet generation. The mechanism for
entrapment functionalization and solvent-assisted thermal
bonding was analyzed. The COC chips coated with 40%
cyclohexane and 1% FOTS by ultrasonic spray 10 and 20 times
have a water contact angle of ∼115°, and this hydrophobicity
could be maintained after the storage of 3 weeks. The FTIR
results confirmed the surface functionalization by revealing the
formation of Si−OH and Si−O−Si groups, indicating the
crosslinking of silane after the surface treatment. The surface
roughness of 10 times coated chips is lower, presenting better
optical clarity. After the surface treatment, the solvent-assisted
thermal bonding was optimized to yield a bonding strength
over 17 bar (represented as the burst pressure of bonded
chips), which is sufficient for droplet generation. The COC
chip under 120 s exposure to 60% cyclohexane and 40%
acetone shows a maximum bonding strength and maintains its
channel integrity without any water leakage during the
characterization. The chips were uniformly bonded, repre-
sented as the similar burst pressure of different channels on the
chip. The hydrophobicity was maintained after the bonding
process, with the water contact angle of ∼115°on the film
cover and the curved shape of water flow inside the channel,
which is due to the lower wettability of the channel wall after
bonding.
This work provides a repeatable and scalable hydrophobic
surface modification process, which entraps FOTS molecules
on the polymer surface through ultrasonic spray coating.
Meanwhile, a solvent vapor-assisted thermal bonding method
is developed in a controllable way to achieve a high bonding
strength and meanwhile maintain the channel integrity.
Compared to other assembly processes requiring hydrophobic
surface modification, these two methods can be combined
sequentially for large-scale production of plastic microfluidic
cartridges with shorter fabrication time and lower cost. If
extended, such a process also has a high potential for
microfluidic applications with hydrophilic coatings.
■AUTHOR INFORMATION
Corresponding Author
Nan Zhang −Centre of Micro/Nano Manufacturing
Technology (MNMT-Dublin), School of Mechanical &
Materials Engineering, University College Dublin, Dublin 4
Dublin, Ireland; MiNAN Technologies, NovaUCD, Dublin 4
Dublin, Ireland; orcid.org/0000-0001-7849-3974;
Email: nan.zhang@ucd.ie
Authors
Tianyu Guan −Centre of Micro/Nano Manufacturing
Technology (MNMT-Dublin), School of Mechanical &
Materials Engineering, University College Dublin, Dublin 4
Dublin, Ireland
Table 3. Comparison of Different Bonding Techniques for the Mass Production of Microfluidic Devices
bonding
method bonding time
bonding strength
(represented as
burst pressure) advantages limitations applicable surface treatment references
laser welding ∼10 s up to 10 bar (me-
dium)
low temperature, pure, and strong
bond, localized bonding
can be costly for complex microfluidic
structures; need a nontransparent part
for bonding
plasma-enhanced chemical vapor deposition (PECVD),
bonding is conducted before PECVD, thus difficult to treat
small channels after sealing
29
ultrasonic
welding
several to 30 s up to 10 bar (me-
dium)
low temperature, localized bonding,
rapid welding process, no curing, or
solvent involved
not accurate for very small and intricate
features; energy director is required
PECVD, difficult to treat small channels after sealing 30,31
thermal diffu-
sion bond-
ing
several to 30 min 5−10 bar (medium) low cost, simple operation, no adhe-
sive clogging
heating temperature higher than the Tgof
polymer, prone to cause channel dis-
tortion
can be treated with UV/ozone or plasma to acquire
hydrophilic surfaces, but hard to be combined with
hydrophobic treatment
32,33
solvent-assis-
ted thermal
bonding
several to 30 min 10−100 bar (high) simple operation, low temperature,
low cost
the process requires optimization to
prevent the channel from collapsing due
to polymer softening
can be combined with spray coating as the surface treatment,
which takes at least 2 days as the post-treatment
16,30,33
our bonding
technique
16 min for solvent-assisted bonding,
2 days for post-treatment (no labor
work needed)
∼17 bar (high) simple operation, low cost, low
temperature, large-scale production
the post-treatment for the generation of a
stable hydrophobic surface takes
∼2 days
combined with ultrasonic spray coating prior to bonding,
ensuring the uniformity of coating and maintaining a high
bonding strength
ACS Omega http://pubs.acs.org/journal/acsodf Article
https://doi.org/10.1021/acsomega.2c01948
ACS Omega 2022, 7, 20104−20117
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Sineenat Yuket −Centre of Micro/Nano Manufacturing
Technology (MNMT-Dublin), School of Mechanical &
Materials Engineering, University College Dublin, Dublin 4
Dublin, Ireland
Hengji Cong −Centre of Micro/Nano Manufacturing
Technology (MNMT-Dublin), School of Mechanical &
Materials Engineering, University College Dublin, Dublin 4
Dublin, Ireland
Douglas William Carton −Centre of Micro/Nano
Manufacturing Technology (MNMT-Dublin), School of
Mechanical &Materials Engineering, University College
Dublin, Dublin 4 Dublin, Ireland; MiNAN Technologies,
NovaUCD, Dublin 4 Dublin, Ireland
Complete contact information is available at:
https://pubs.acs.org/10.1021/acsomega.2c01948
Author Contributions
§
S.Y. contributed equally to the first author. T.G. contributed
to conceptualization, methodology, data analysis, and writ-
ingoriginal draft. S.Y. involved in characterization and
software. H.C. and D.W.C. performed review & editing. N.Z.
contributed to conceptualization, resources, review & editing,
and supervision.
Notes
The authors declare no competing financial interest.
■ACKNOWLEDGMENTS
The authors gratefully acknowledge the support from China
Scholarship Council (CSC), the Science Foundation Ireland
(SFI) (nos. 15/RP/B3208 and 20/FIP/PL/8741), and
Enterprise Ireland (CF-2021-1635-P).
■REFERENCES
(1) Ayala, R.; Zhang, C.; Yang, D.; Hwang, Y.; Aung, A.; Shroff, S.
S.; Arce, F. T.; Lal, R.; Arya, G.; Varghese, S. Engineering the cell−
material interface for controlling stem cell adhesion, migration, and
differentiation. Biomaterials 2011,32, 3700−3711.
(2) Suzuki, K. Flow resistance of a liquid droplet confined between
two hydrophobic surfaces. Microsyst. Technol. 2005,11, 1107−1114.
(3) Shang, L.; Cheng, Y.; Zhao, Y. Emerging droplet microfluidics.
Chem. Rev. 2017,117, 7964−8040.
(4) Teh, S.-Y.; Lin, R.; Hung, L.-H.; Lee, A. P. Droplet microfluidics.
Lab Chip 2008,8, 198−220.
(5) Zhang, N.; Srivastava, A.; Kirwan, B.; Byrne, R.; Fang, F.;
Browne, D. J.; Gilchrist, M. D. Manufacturing microstructured tool
inserts for the production of polymeric microfluidic devices. J.
Micromech. Microeng. 2015,25, No. 095005.
(6) Perez-Toralla, K.; Champ, J.; Mohamadi, M. R.; Braun, O.;
Malaquin, L.; Viovy, J.-L.; Descroix, S. New non-covalent strategies
for stable surface treatment of thermoplastic chips. Lab Chip 2013,13,
4409−4418.
(7) Tsao, C.-W.; DeVoe, D. L. Bonding of thermoplastic polymer
microfluidics. Microfluid. Nanofluid. 2009,6,1−16.
(8) Zhang, Y.; Gao, K.; Fan, Y. Application of a new UV curable
adhesive for rapid bonding in thermoplastic-based microfluidics.
Micro Nano Lett. 2019,14, 211−214.
(9) Sun, Y.; Kwok, Y. C.; Nguyen, N.-T. Low-pressure, high-
temperature thermal bonding of polymeric microfluidic devices and
their applications for electrophoretic separation. J. Micromech.
Microeng. 2006,16, 1681.
(10) Uba, F. I.; Hu, B.; Weerakoon-Ratnayake, K.; Oliver-Calixte,
N.; Soper, S. A. High process yield rates of thermoplastic nanofluidic
devices using a hybrid thermal assembly technique. Lab Chip 2015,
15, 1038−1049.
(11) Ogilvie, I.; Sieben, V.; Floquet, C.; Zmijan, R.; Mowlem, M.;
Morgan, H. In Solvent Processing of PMMA and COC Chips for Bonding
Devices with Optical Quality Surfaces, 14th international conference on
miniaturized systems for chemistry and life sciences, 2010; pp 1244−
1246.
(12) Tsao, C. W.; Hromada, L.; Liu, J.; Kumar, P.; DeVoe, D. Low
temperature bonding of PMMA and COC microfluidic substrates
using UV/ozone surface treatment. Lab Chip 2007,7, 499−505.
(13) Ghosh, S.; Kamalakshakurup, G.; Lee, A. P.; Ahn, C. H. A mass
manufacturable thermoplastic based microfluidic droplet generator on
cyclic olefin copolymer. J. Micromech. Microeng. 2019,29, No. 055009.
(14) Sarmadi, M. In Advantages and Disadvantages of Plasma
Treatment of Textile Materials, 21st International Symposium on
Plasma Chemistry (ISPC 21), Sunday, 2013.
(15) Ruckenstein, E.; Li, Z. Surface modification and functionaliza-
tion through the self-assembled monolayer and graft polymerization.
Adv. Colloid Interface Sci. 2005,113,43−63.
(16) Su, S.; Jing, G.; Zhang, M.; Liu, B.; Zhu, X.; Wang, B.; Fu, M.;
Zhu, L.; Cheng, J.; Guo, Y. One-step bonding and hydrophobic
surface modification method for rapid fabrication of polycarbonate-
based droplet microfluidic chips. Sens. Actuators, B 2019,282,60−68.
(17) Keller, N.; Nargang, T. M.; Runck, M.; Kotz, F.; Striegel, A.;
Sachsenheimer, K.; Klemm, D.; Länge, K.; Worgull, M.; Richter, C.;
et al. Tacky cyclic olefin copolymer: a biocompatible bonding
technique for the fabrication of microfluidic channels in COC. Lab
Chip 2016,16, 1561−1564.
(18) Guo, H.; Ulbricht, M. The effects of (macro) molecular
structure on hydrophilic surface modification of polypropylene
membranes via entrapment. J. Colloid Interface Sci. 2010,350,99−
109.
(19) Badv, M.; Jaffer, I. H.; Weitz, J. I.; Didar, T. F. An omniphobic
lubricant-infused coating produced by chemical vapor deposition of
hydrophobic organosilanes attenuates clotting on catheter surfaces.
Sci. Rep. 2017,7, No. 11639.
(20) Matsumoto, N. Overview of silicon-based materials. Jpn. J. Appl.
Phys. 1998,37, 5425−5436.
(21) Subramanian, B.; Kim, N.; Lee, W.; Spivak, D. A.; Nikitopoulos,
D. E.; McCarley, R. L.; Soper, S. A. Surface modification of droplet
polymeric microfluidic devices for the stable and continuous
generation of aqueous droplets. Langmuir 2011,27, 7949−7957.
(22) Liu, F.; Xu, T.; Liu, W.; Zheng, X.; Xu, J.; Ma, B. Spontaneous
droplet generation via surface wetting. Lab Chip 2020,20, 3544−
3551.
(23) Roy, E.; Pallandre, A.; Zribi, B.; Horny, M. C.; Delapierre, F.
D.; Cattoni, A.; Gamby, J.; Haghiri-Gosnet, A. M. Overview of
materials for microfluidic applications. Adv. Microfluid.: New Appl.
Biol., Energy, Mater. Sci. 2016.
(24) El Fissi, L.; Vandormael, D.; Houssiau, L.; Francis, L. A. Surface
functionalization of cyclic olefin copolymer (COC) with evaporated
TiO2 thin film. Appl. Surf. Sci. 2016,363, 670−675.
(25) Schuler, F.; Trotter, M.; Geltman, M.; Schwemmer, F.; Wadle,
S.; Domínguez-Garrido, E.; López, M.; Cervera-Acedo, C.;
Santibánez, P.; von Stetten, F.; et al. Digital droplet PCR on disk.
Lab Chip 2016,16, 208−216.
(26) Long, J.; Weng, Q.; Hong, W.; Cao, Z.; Zhou, P.; Xie, X. Fast
water flow in laser micromachined microgrooves with nonuniform
surface wettability. Exp. Therm. Fluid Sci. 2019,103,9−17.
(27) (a) Lee, K. S.; Ram, R. J. Plastic−PDMS bonding for high
pressure hydrolytically stable active microfluidics. Lab Chip 2009,9,
1618−1624. (b) Taylor, L. C.; Lavrik, N. V.; Sepaniak, M. J. High-
aspect-ratio, silicon oxide-enclosed pillar structures in microfluidic
liquid chromatography. Anal. Chem. 2010,82, 9549−9556.
(28) Hosseini, S.; Ibrahim, F.; Djordjevic, I.; Koole, L. H. Recent
advances in surface functionalization techniques on polymethacrylate
materials for optical biosensor applications. Analyst 2014,139, 2933−
2943.
(29) (a) De Pelsmaeker, J.; Graulus, G.-J.; Van Vlierberghe, S.;
Thienpont, H.; Van Hemelrijck, D.; Dubruel, P.; Ottevaere, H. Clear
to clear laser welding for joining thermoplastic polymers: A
ACS Omega http://pubs.acs.org/journal/acsodf Article
https://doi.org/10.1021/acsomega.2c01948
ACS Omega 2022, 7, 20104−20117
20116
comparative study based on physicochemical characterization. J.
Mater. Process. Technol. 2018,255, 808−815. (b) Acherjee, B. Laser
transmission welding of polymers−A review on process fundamentals,
material attributes, weldability, and welding techniques. J. Manuf.
Processes 2020,60, 227−246. (c) Brettschneider, T.; Dorrer, C.;
Brundel, M.; Zengerle, R.; Daub, M. Wafer-level packaging and laser
bonding as an approach for silicon-into-lab-on-chip integration. J.
Micromech. Microeng. 2013,23, No. 055005.
(30) Ereku, L.; Mackay, R.; Balachandran, W.; Ajayi, K. Review of
commercially available microfluidic materials and fabricating
techniques for point of care testing. Sens. Transducers 2016,202,
1−24.
(31) Zhang, Z.; Luo, Y.; Wang, X.; Zheng, Y.; Zhang, Y.; Wang, L. A
low temperature ultrasonic bonding method for PMMA microfluidic
chips. Microsyst. Technol. 2010,16, 533−541.
(32) Jena, R.; Chester, S.; Srivastava, V.; Yue, C.; Anand, L.; Lam, Y.
Large-strain thermo-mechanical behavior of cyclic olefin copolymers:
application to hot embossing and thermal bonding for the fabrication
of microfluidic devices. Sens. Actuators, B 2011,155,93−105.
(33) Sivakumar, R.; Lee, N. Y. Microfluidic device fabrication
mediated by surface chemical bonding. Analyst 2020,145, 4096−
4110.
ACS Omega http://pubs.acs.org/journal/acsodf Article
https://doi.org/10.1021/acsomega.2c01948
ACS Omega 2022, 7, 20104−20117
20117
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