Monolithic and flexible polyimide film microreactors for organic microchemical applications fabricated by laser ablation

Article abstract of DOI:10.1002/anie.201002004

Keeping limber: A monolithic and flexible polyimide film microreactor is introduced for organic reactions and syntheses. Unlike glass microreactors, it is easy to fabricate, yet it is inert to solvents and acids under harsh conditions, unlike other polymer microreactors.

Full text of DOI:10.1002/anie.201002004

Angewandte
Chemie
DOI: 10.1002/anie.201002004
Microreactors
Monolithic and Flexible Polyimide Film Microreactors for Organic
Microchemical Applications Fabricated by Laser Ablation**
Kyoung-Ik Min, Tae-Ho Lee, Chan Pil Park, Zhi-Yong Wu, Hubert H. Girault, Ilhyong Ryu,
Takahide Fukuyama, Yu Mukai, and Dong-Pyo Kim*
There is a growing interest in innovative chemical synthesis in
microreactors owing to the potential for high efficiency,
There are many fabrication techniques available for
microfluidic devices. Of these, laser ablation is convenient
to use for polymer chips. The whole microfabrication
[1]
[6]
selectivity, and yield. In microfluidic systems, the low-
volume spatial and temporal control of reactants and
products offers a novel method for chemical manipulation
process could be completed in a few minutes instead of the
days that are needed for wet photolithography processes. It is
well known that polyimide (PI) offers advantages over other
polymers, such as excellent chemical and thermal stability and
[
2]
and product generation. Glass, silicon, poly(dimethylsil-
oxane) (PDMS), and poly(methylmethacrylate) (PMMA)
have been used for the fabrication of miniaturized devices.
Fabrication with glass or silicon substrates requires relatively
complex processes, and the fabrication costs are high.
Relatively cheap polymers such as PDMS or PMMA are
not suitable for application in organic chemical processes
[
7]
low water uptake. Recently Barrett et al. reported fabrica-
[8]
tion of PI-based microfluidic devices by laser ablation. The
devices were used for X-ray scattering experiments to yield a
better spatial resolution for structural measurements. How-
ever, there has been no attempt to demonstrate the potential
advantages of PI-based microreactors for chemical syntheses.
Herein, we introduce a monolithic and flexible PI film
microreactor for organic synthesis. Mixing units, such as a
staggered herringbone pattern on the channel surface, can
easily be built into the reactor during the fabrication. The film
microreactors were readily fabricated within several tens of
minutes by ablation with either a 193 nm excimer laser (ArF)
or a 355 nm UV laser. The excellent stability of the fabricated
microreactors was successfully demonstrated by performing
five chemical reactions under various harsh conditions. The
simple and economical laser fabrication process and the facile
adhesive sealing step facilitate mass production of the flexible
PI film microfluidic devices for various microchemical
applications. Furthermore, these devices could provide a
platform for integrating microfluidic and electronic compo-
nents that are necessary for a micro total analysis system
[3]
owing to their low chemical stability and easy swelling.
Therefore, there is a strong demand for economical organic-
solvent-resistant materials that can be used for easy fabrica-
tion of microfluidic systems with reliable durability. Our
group has recently reported novel polymer microreactors for
organic syntheses, which involve two kinds of inorganic
polymers that are manipulated in relatively simple micro-
[
4]
fabrication processes. The improved solvent resistance
allowed good performance in organic reactions, except in
chlorinated solvents such as dichloromethane. Nevertheless,
the bonding step was somewhat tricky and had a low rate of
success. Alternatively, a monolithic thin-film microreactor
can be used for applications in organic synthesis, which allows
integration of electrodes, heaters, light-emitting diodes, and
[5]
various electronics.
(
m-TAS).
[
8]
[
*] K. I. Min, T. H. Lee, Dr. C. P. Park, Prof. D. P. Kim
National Creative Research Center of
Applied Microfluidic Chemistry, Chungnam National University
Daejeon, 305-764 (South Korea)
Fax: (+82)42-823-6665
E-mail: dpkim@cnu.ac.kr
As described in the literature, photoablation could be
performed either dynamically by continuous moving and
shooting of laser pulses to generate the channel or statically
by shooting on specific spots assigned by the controlling
[
6]
program. As shown in Figure 1b, two types of microchannel
designs, ArF and UV, were fabricated using the ArF excimer
laser with a metal mask and a UV laser with a 25 mm diameter
beam spot. Finally, 4 cm long ArF-type microchannels were
ablated on the smooth PI film (ArF-1 type) or on a grooved
surface with a staggered herringbone (SH) pattern (ArF-SH
type). UV-type microchannels 53 cm long were directly
ablated along the channel (UV-1 type) or across the channel
Prof. Z. Y. Wu
Research Center for Analytical Science
Northeastern University, 110819, Shenyang (China)
Prof. H. H. Girault
Laboratoire d’Electrochimie Physique et Analytique
EPFL SB ISIC LEPA, 1015 Lausanne (Switzerland)
Prof. I. Ryu, Dr. T. Fukuyama, Y. Mukai
Department of Chemistry, Graduate School of Science
Osaka Prefecture University, Sakai, Osaka, 599-8531 (Japan)
(UV-2 type) in a repeated scanning mode, finally generating
the line-grooved microchannels. The SH pattern was also
grooved statically along the beginning part of the channel
[
**] This research was supported by the Creative Research Initiatives
(
CRI) project R16-2008-138-01001-0 (2008) funded by the Korean
[9]
(UV-SH type).
Ministry of Education, Science and Technology. We thank Valerie
Devaud for her technical support in preparation of the herringbone
mask. I.R. and T.F. thank JSPS and MCPT for funding.
In general, it is reported that laser irradiation ablates by
photochemical decomposition of chemical bonds in the
polymer as well as by photothermal evaporation or melt
expulsion. It is also known that the laser is strong enough to
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201002004.
[
6]
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communications
respect to the centerline of the channel from one region to the
[9]
next, as presented by Stroock et al.
Alternatively, UV-type microchannels with different sur-
face patterns (Figure 2c–e) were fabricated by direct ablation
of PI film using a UV laser in multiple scans. The repeated
scanning ablation with a 25 mm diameter spot generated
grooved lines on the surface; the ablation direction could be
controlled to align along the channel (UV-1, Figure 2c) or
across the channel (UV-2, Figure 2d). And the surface profile
image of Figure 2g for the UV-1 channel displayed a channel
with approximately 200 mm width and 55 mm depth and a
grooved pattern with approximately 5 mm depth in a 25 mm
period on the bottom surface. Moreover, we found that the
economical, low-energy 355 nm UV laser enabled function-
alization of the channel surface with an SH pattern. More-
over, the fast fabrication within several tens of minutes is a
great advantage over the long processing time needed with
Figure 1. a) Fabrication of PI film microreactors. b) Microchannel
designs fabricated by ArF, UV laser.
[8]
the femtosecond laser. It should also be emphasized that the
extended and complex microchannels up to 53 cm long were
readily sealed using a self-adhering PI composite film (PI core
and PI adhesive layer) at 3008C under a pressure of 10 kPa.
More importantly, the sealed 53 cm long microchannels could
break chemical bonds with little thermal damage, resulting in
smooth patterned surfaces. In particular, the 193 nm ArF
excimer laser with high energy first ablated throughout the
square metal mask in a single-shot manner to fabricate the
T-shaped microchannel on the PI film (ArF-1 channel,
Figure 2a). The surface profile image in Figure 2 f shows the
À1
endure high flow rates over 200 mLmin with no delamina-
tion. Eventually, the simple fabrication process and the facile
adhesive sealing step with high fidelity should facilitate the
mass production of flexible PI film microfluidic devices for
practical applications in a manner as highly reproducible as
PDMS microfluidics.
Chemical resistance is an essential characteristic in the
development of materials for organic microreactors. Solvent
resistance of the structural PI film material after thermal
bonding at 3008C was examined by soaking the film for 24 h
in a range of solvents at different temperatures (Table 1). The
PI sample exhibited reliable resistance to all solvents, showing
Table 1: Solvent resistance of PI film, evaluated by soaking the 1ꢀ1 cm
film in a Soxhlet extractor at various temperatures for 24 h.
[
a]
[a]
Solvent
T [8C]
W/W0
Solvent
T [8C]
W/W0
Water
100
RT
75
100
75
1.00
1.01
1.01
1.02
1.02
n-Hexane
DMSO
DCM
75
100
RT
100
100
1.00
1.01
1.02
1.02
1.00
Acetone
Ethanol
Toluene
THF
DMF
Figure 2. a–e) SEM images and diagrams of various PI film micro-
channels: a) ArF-1 type with no surface pattern, b) ArF-SH type with
staggered herringbone pattern, fabricated by ArF laser, c) UV-1 type
with line pattern along the channel, d) UV-2 type with line pattern
across the channel, e) UV-SH type with staggered herringbone pattern,
fabricated by UV laser. f,g) Cross-sectional surface profile graphs of
ArF-1 type (f) and UV-1 type (g). h) Optical images for UV-type flexible
PI microreactors.
2n H SO
2
4
[
a] Experimentally measured swelling ratio W/W , where W and W are
0
0
the weight of the PI film after and before soaking in each solvent. THF=
tetrahydrofuran, DMSO=dimethylsulfoxide, DCM=dichloromethane,
DMF=dimethylformamide, RT=room temperature.
insignificant weight change within 1–2%. And there was no
change in color or appearance even at long soaking times of
24 h. Particularly noteworthy are the excellent stability in
dichloromethane (DCM) and strongly acidic H SO solution
concave channel with approximately 200 mm width and 60 mm
depth and the smooth bottom surface with nearly vertical
walls. Furthermore, the additional ablation with different
masks rendered the grooved SH pattern with 17 mm depth and
a total of 15 mixing cycles along the channel (ArF-SH
channel, Figure 2b). Each mixing cycle in the ArF-SH
channel is composed of two sequential regions of ridges; the
direction of asymmetry of the herringbones switches with
2
4
at 1008C, which are characteristics obviously superior to those
[
4a]
of solvent-resistant poly(carbosilane),
poly(vinylsilaza-
[4a,b]
[4c]
ne),
and fluoropolymers
reported recently. On the
basis of these results, PI must be an excellent structural
polymer for microchemical devices applicable to organic
solvents as well as aqueous media in harsh conditions.
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The mixing efficiencies of the sealed microchannels were
determined by comparing the fluorescence images and the
intensity profiles when the system was subjected to laminar
flow of water or DMF. The ArF-SH microchannel presented
excellent mixing efficiency and showed an aqueous homoge-
neous fluorescent phase after only 0.28 cm flow along the SH
mixer portion. This result clearly contrasts that of the ArF-1
microchannel, for which significant mixing was observed after
inflowing 3 cm from the inlet merging point (Figure 1S in the
Supporting Information). This behavior is consistent with
previous reports that natural diffusion and linear convection
are not sufficient to mix the species in a short microchannel,
even under low flow rates in the range of microliters per
Firstly, synthesis of e-caprolactam demonstrates the
stability of this PI microreactor (Scheme 1), as the reaction
requires strongly acidic H SO solvent at elevated temper-
2
4
[
10]
ature. It is surprising that the PI microreactor (ArF-SH
Scheme 1. Synthesis of e-caprolactam in the ArF-SH microreactor.
type) was successfully operated in the extremely acidic
solvent (H SO /cyclohexanone oxime 1:1.5) without any
[9]
minute. The result indicates that the mixer-supported
microchannel remarkably enhances the mixing efficiency of
inlet solutions compared to the mixer-free microchannel, and
it promises fast chemical reactions. Figure 3 shows the
fluorescence images and intensity profiles of DMF solution
2
4
leaking and swelling problems. And under the annealing
temperature at 1308C the synthetic yield of 46% for 0.9 s
retention time in the film microreactor was 1.6-fold higher
than the 28% achieved in a batch system for 1 h reaction time.
Furthermore, the PI microreactor was sustained under the
above condition over 10 days with no delamination and no
defects, thus demonstrating the superior stability to the
inorganic polymer microreactors, which showed leakage or
deformity when exposed to a few organic solvents (THF,
[
4a,b]
DCM) and acidic conditions.
This excellent stability was
not reported with any other polymer microreactors, except
glass microreactors.
The Knoevenagel condensation reaction was carried out
in three microreactors, UV-1, UV-2, and UV-SH (each 53 cm
length, 200 mm width, 50 mm depth), in ethyl alcohol
[
11]
(
Scheme 1S in the Supporting Information). As shown in
Figure 4, the synthetic yields of product increased when
[
4a]
retention time increased, as previously reported.
As
Figure 3. Comparative fluorescence distribution images and intensity
profiles of various UV-type microchannels at 4 cm from the two inlet
merging positions.
expected, the mixer-supported UV-SH microreactor showed
the highest yields, which is more obvious at shorter retention
times. And the UV-2 reactor exhibited higher yields than the
UV-1 reactor, owing to slight breaking of the laminar flow
(Figure 3). Moreover, extending the retention time to 300 s
led to complete reaction with over 90% yield by diffusion-
dominated chemistry. In contrast, a noticeably longer reaction
time of 20 min within a batch reaction vial (ID = 1 cm,
working volume 1.57 mL) resulted in a yield of only 51%.
in UV-type microchannels with three different surface
patterns at 4 cm from the two inlet merging positions along
the microchannel. The image and the profile of the UV-SH
microchannel exhibit less laminar flow and low gradient,
respectively, when compared to those of the UV-1 and UV-2
microchannels, which maintained the laminar flow. The UV-
SH pattern demonstrated superior mixing to other UV-type
channels in the microfluidic devices, although the pattern
shape was not as well-defined as for the Ar-SH channel,
presumably owing to the thermal effect of the lasers. There-
fore, it is favorable to use an economical 355 nm UV laser as
an alternative ablation tool to an expensive 193 nm ArF
excimer laser for polymer film microchannels with functional
surface patterns.
To test the chemical durability of the PI film micro-
reactors, we selected a Beckmann rearrangement reaction
under strongly acidic conditions up to 1308C, exothermic
bromination of benzyl alcohol at room temperature (RT), and
two step-wise exothermic Vilsmeier formylations at RT and
1
008C. Furthermore, the Knoevenagel reaction and a tin
hydride mediated radical reaction were also selected to
compare the mixing effect of UV microchannels and the
performance with conventional capillary microreactors.
Figure 4. Knoevenagel condensation reaction in UV-type PI film micro-
reactors as a function of surface pattern and reaction time.
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Using a single sheet of UV-2 film microreactor with a
3 cm long microchannel, we examined exothermic bromina-
In conclusion, a monolithic and flexible film microreactor
has been introduced for organic reactions and syntheses. The
microreactor can easily be fabricated by photoablating a PI
film and then simply placing an adhesive PI film on the
patterned PI film for bonding. Mixing units that can assure
complete mixing can also be built into the reactor surface
during fabrication. Although it does not require the compli-
cated processing needed for glass microreactors, the film
microreactor is as inert, sturdy, and versatile as glass micro-
reactors. The utility of the microreactor as an all-purpose
reactor has been demonstrated with five different types of
organic chemical reactions. These include Beckman rear-
rangement under strongly acidic conditions at up to 1308C,
Knoevenagel reaction in reactors with three different surface
patterns, two highly exothermic reactions for bromination of
benzyl alcohol and Vilsmeier–Haack formylation, and tin
hydride mediated radical reaction at 1008C. The easy
fabrication should facilitate mass production of flexible PI
microfluidic devices, which in turn could provide a platform
for integrating thin-film electronics for a total micro analysis
system.
5
tion of benzyl alcohol with highly toxic PBr (Scheme 2S in
3
the Supporting Information). When the reaction was carried
out with 5 min residence time at room temperature, benzyl
bromide was obtained in 75% yield with 23% starting benzyl
alcohol.
The Vilsmeier–Haack formylation reaction (Scheme 2)
was performed in a step-wise manner at significantly different
temperatures using two serially connected sheets of UV-2
film microreactors (Figure 2S in the Supporting Information).
In batch reaction systems, the Vilsmeier reagent is usually
Scheme 2. Vilsmeier–Haack formylation reaction in the UV-2 micro-
reactor.
Experimental Section
To evaluate the mixing efficiency of the UV-1, UV-2, and UV-SH
channels, dimethylformamide (DMF) was infused at a flow rate of
À1
2
2
mLmin through one inlet of the Y-shaped microchannel, and a
prepared by careful addition of phosphoryl chloride to DMF
50 mm fluorescent rhodamine solution in DMF was provided from
the other inlet of the microchannel at a flow rate of 2 mLmin
at 08C because this step is highly exothermic; the reaction
À1
.
[
12]
mixture is then heated with electrophiles. In our system,
DMF and phosphoryl chloride were directly mixed in the first
microchannel sheet at room temperature for 10 min residence
time. Then N,N-dimethylaniline was introduced into the
second microchannel sheet and allowed to react at 1008C for
Fluorescent distribution images were taken at 4 cm from the inlet
merging position, and the intensity profiles were extracted with aid of
the Image J program.
[
14]
The Beckmann rearrangement for the synthesis of e-caprolactam
was performed within the ArF-SH microreactor, into which 20 mm
cyclohexanone oxime in dimethylsulfoxide (DMSO) was introduced
1
0 min residence time. The reaction was successful and gave
À1
from one inlet at 10 mLmin and the strongly acid H SO solution
2
4
the formylated product in 75% yield (ortho/para 15:85).
Tin hydride mediated radical reaction was carried out
using a UV-2 microreactor with one inlet to compare the
results with those obtained by the use of a stainless steel tube
(30 mm, DMSO) from the other inlet with the same flow rate. The
temperature of the microreactor was kept constant at 1308C using a
hotplate. The result was compared with that of the batch reaction.
During the Knoevenagel condensation reaction in UV-1, UV-2,
and UV-SH microreactors, 50 mm ethyl cyanoacetate mixed with
piperazine (v/v, 1:0.1) and 50 mm 4-bromobenzaldehyde in ethyl
alcohol (EtOH) were separately introduced into each inlet at an equal
flow at 608C. For comparison, this reaction was conducted within
different surface pattern microreactors. This reaction was also carried
out in a batch format using a glass vial.
[13]
reactor (Scheme 3). The reduction of 1-iodododecane with
tributyltin hydride and AIBN with 5 min residence time at
1
008C in a continuous-flow system gave dodecane in 85%
yield. Radical cyclization was also tested with N-(2-bro-
moethyl)-N-(3-methylbut-2-enyl)-p-toluenesulfonamide, and
the expected cyclization product was obtained in 59% yield.
Thus, we were gratified to know that similar efficiency was
observed with the present polymer-based microreactors.
Bromination of benzyl alcohol was performed in a UV-2 micro-
reactor; benzyl alcohol (1.25m) and PBr3 (1.88m) in THF were
À1
introduced into each inlet of the microreactor with 1.5 mLmin
injection rate each at room temperature.
For the Vilsmeier–Haack formylation, two sheets of UV-2 film
microreactors each 53 cm long were connected using a capillary tube
À1
to give a 106 cm long microchannel. DMF (1.3 mLmin ) and POCl3
À1
(
0.17 mLmin ) were allowed to react in the first part of the
microreactor. The resulting product was directly introduced into the
second microreactor to perform a final-step reaction with N,N-
À1
dimethylaniline (0.17 mLmin ).
Tin hydride mediated radical reaction was performed in a UV-2
microreactor with one inlet. Premixed toluene solution of 1-iododo-
decane (0.05m), Bu SnH (0.06m), and AIBN (0.01m) was introduced
3
À1
Scheme 3. Tin hydride mediated radical reaction in UV-2 microreac-
tors. AIBN=azobisisobutyronitrile. Ts=p-toluenesulfonyl.
with 3.2 mLmin injection rate at 1008C. In a second reaction, N-(2-
bromoethyl)-N-(3-methylbut-2-enyl)-p-toluenesulfonamide (0.05m)
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Received: April 5, 2010
Revised: April 28, 2010
Published online: June 23, 2010
Keywords: microreactors · polyimides · polymers ·
.
synthetic methods · thin films
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