Solventless adhesive bonding using reactive polymer coatings

Article abstract of DOI:10.1021/ac800341m

A novel solventless adhesive bonding (SAB) process is reported, which is applicable to a wide range of materials including, but not limited to, poly(dimethylsiloxane) (PDMS). The bonding is achieved through reactions between two complementary polymer coatings, poly(4-aminomethyl-p-xylylene-co-p- xylylene) and poly(4-formyl-p-xylylene-co-p-xylylene), which are prepared by chemical vapor deposition (CVD) polymerization of the corresponding [2.2]paracyclophanes and can be deposited on complementary microfluidic units to be bonded. These CVD-based polymer films form well-adherent coatings on a range of different substrate materials including polymers, glass, silicon, metals, or paper and can be stored for extended periods prior to bonding without losing their bonding capability. Tensile stress data are measured on PDMS with various substrates and compared favorably to current methods such as oxygen plasma and UV/ozone. Sum frequency generation (SFG) has been used to probe the presence of amine and aldehyde groups on the surface after CVD polymerization and their conversion during bonding. In addition to bonding, unreacted functional groups present on the luminal surface of microfluidic channels provide free chemical groups for further surface modification. Fluorescently labeled molecules including rhodamine-conjugated streptavidin and atto-655 NHS ester were used to verify the presence of active functional groups on the luminal surfaces after bonding.

Full text of DOI:10.1021/ac800341m

Anal. Chem. 2008, 80, 4119–4124
Solventless Adhesive Bonding Using Reactive
Polymer Coatings
Hsien-Yeh Chen,† Arthur A. McClelland,| Zhan Chen,§, and Joerg Lahann*,†,‡,§
Departments of Chemical Engineering, Materials Science and Engineering, Macromolecular Science and Engineering,
Applied Physics, and Chemistry, University of Michigan, Ann Arbor, Michigan 48109
A novel solventless adhesive bonding (SAB) process is
reported, which is applicable to a wide range of materials
including, but not limited to, poly(dimethylsiloxane)
(PDMS). The bonding is achieved through reactions
between two complementary polymer coatings, poly(4-
aminomethyl-p-xylylene-co-p-xylylene) and poly(4-formyl-
p-xylylene-co-p-xylylene), which are prepared by chemical
vapor deposition (CVD) polymerization of the correspond-
ing [2.2]paracyclophanes and can be deposited on comple-
mentary microfluidic units to be bonded. These CVD-
based polymer films form well-adherent coatings on a
range of different substrate materials including polymers,
glass, silicon, metals, or paper and can be stored for
extended periods prior to bonding without losing their
bonding capability. Tensile stress data are measured on
PDMS with various substrates and compared favorably
to current methods such as oxygen plasma and UV/ozone.
Sum frequency generation (SFG) has been used to probe
the presence of amine and aldehyde groups on the surface
after CVD polymerization and their conversion during
bonding. In addition to bonding, unreacted functional
groups present on the luminal surface of microfluidic
channels provide free chemical groups for further surface
modification. Fluorescently labeled molecules including
rhodamine-conjugated streptavidin and atto-655 NHS
ester were used to verify the presence of active functional
groups on the luminal surfaces after bonding.
between biology, materials science, and chemistry that has
provided novel materials and advanced microfluidic device fabrica-
tion processes, which can be precisely tailored toward specific
biotechnological requirements. Along with the introduction of
novel materials and device architectures came a series of chal-
lenges, many of which are related to device integration. For the
fabrication of miniaturized devices with use in biotechnology,
effective bonding of independently designed substructures has
emerged as a major challenge. Current bonding methodologies
include direct bonding, adhesive bonding, anodic bonding, solder
or eutectic bonding, as well as thermo-compression bonding, but
these methods are often limited to a specific substrates or involve
the application of additional bonding chemicals or require harsh
process conditions.7,8 For instance, bonding of poly(dimethylsi-
loxane) (PDMS) is widely used for low-cost biotechnological
applications but requires oxidative pretreatment, such as oxygen
plasma9–12 or UV/ozone13 activation. Upon exposure to highly
oxidative environments, silanol groups are created, which can
result in strong intermolecular bonding.12 In most of the cases,
the oxidative activation effects are temporary due to hydrophobic
recovery,15 and the oxidation reaction must be conducted im-
mediately prior to bonding. Hydrophobic recovery may be slowed
down by storage in water or extraction of low molecular weight
components.7,13,14 In addition, these approaches are limited to a
small group of substrates, such as PDMS, silicon, or glass,
impeding the use for more complex devices that may be
constructed of a number different materials. To date, further
progress has been limited by the lack of precision in the chemical
reactions used for bonding, which often require empirical process
optimization. These challenges have not only been witnessed in
biotechnological applications but also in many chemical and
electronic device applications.
Rapid progress in biotechnology and related fields has fueled
the need for progressively miniaturized devices with increasing
biological complexity.1–6 This trend has led to a cross-fertilization
* To whom correspondence should be addressed. E-mail: lahann@umich.edu.
† Chemical Engineering.
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10.1021/ac800341m CCC: $40.75 2008 American Chemical Society
Published on Web 05/06/2008
Analytical Chemistry, Vol. 80, No. 11, June 1, 2008 4119

As a consequence, tailored surface reactions have recently
generated intense attention in modifying the surfaces of micro-
fluidic devices and for protein immobilization.16–18 Among the
most promising candidates are chemical processes that ensure
selective conversions and high thermodynamic driving forces,
while minimizing the risk of undesired side reactions. A prime
example of a highly efficient and orthogonal reaction is the so-
called click chemistry19,20 that has been rapidly embraced by the
materials community.19–22 We now propose an alternate strategy
for chemical bonding, the solventless adhesive bonding (SAB)
process, which relies on reactive polymer coatings with precisely
engineered surface chemistries and is applicable to a wide range
of materials including, but not limited to, PDMS. In addition, we
elucidate the exact molecular mechanism of the SAB process
using sum frequency generation (SFG) spectroscopy. Because of
the high selectivity of the chemistry exploited for SAB, the process
lends itself to simultaneous bonding and surface modification.
Bn ) 0.01 for polymer 1 and An ) 1.76, Bn ) 0.01 for polymer
2 using a Cauchy model and software module integrated with
the system. SFG spectra were recorded by using a pulsed visible
laser beam and a tunable pulsed infrared beam that are overlapped
spatially and temporally on sample surfaces at incident angles of
60° and 45°, with pulse energies of ∼200 and ∼100 µJ, respectively.
Unless otherwise specified, all SFG spectra were recorded by ssp
(s-polarized sum frequency output, s-polarized visible input, and
p-polarized IR input) polarization combination and were normal-
ized by the intensities of input visible and IR beams. The details
of the SFG setup and experimental geometry were detailed
elsewhere.24–28
Bonding Process and Tensile Stress Test. The SAB process
was performed by first coating substrates with polymers 1 or 2.
After coating, samples were brought into contact and were then
placed in an oven at 140 °C for 3 h. The resulting samples were
tested and stored at room temperature (20 °C). UV/ozone bonding
was performed by using a UVO cleaner (model 342, Jelight Co.)
treating the substrates for 30 min. The resulting samples were
then cured at 120 °C for 20 min. Oxygen plasma bonding was
performed by using a plasma etcher (SPI Plasma-Prep II, SPI
Supplies/Structure Probe, Inc.). Treatment was done by using 10
W of energetic oxygen plasma under 200-300 mTorr pressure
for 30 s. The plasma-treated samples were then cured at 60 °C
for 10 min or 120 °C for 10 min. For all samples, tensile stress
was tested using a Bionix 100 mechanical tester (MTS, Co.)
equipped with a 10 N load sensor. Samples were prepared in a
cross-sectional area of 10 mm × 10 mm, and the measurement
was recorded at a displacement rate of 0.05 mm/min.
Surface Immobilization. PDMS microchannels were fabri-
cated using standard photolithography procedures described
elsewhere29 and were coated with polymer 2. Commercially
available cover glasses (25 mm × 25 mm, Fisher) were used as
received and were coated with polymer 1. The CVD coated PDMS
and cover glass were bonded via the SAB process. After bonding,
the devices were first incubated with atto-655 NHS ester (50 µg/
mL, Fluka) in phosphate-buffered saline (PBS, pH 7.4) for 120
min and subsequently rinsed several times with PBS containing
0.1% (w/v) bovine albumin and Tween 20 (0.02% (v/v)). The
resulting devices were then incubated with 20 mM biotin-hydrazide
solution (Pierce Biotechnology, Inc.) in acidic condition (pH 3)
for 5 min. Deionized water and PBS solution was used to separate
unreacted biotin-hydrazide. The rinsed devices were incubated
with rhodamine (TRITC)-conjugated streptavidin (50 µg/mL,
Pierce) in PBS containing 0.1% (w/v) bovine albumin and Tween
20 (0.02% (v/v)) for 90 min. Finally, samples were thoroughly
rinsed with PBS containing 0.1% (w/v) bovine albumin and Tween
20 (0.02% (v/v)).
EXPERIMENTAL SECTION
Materials. PDMS samples were prepared by uniformly mixed
PDMS prepolymer and curing agent (Sylgard 184, Dow Corning)
at a ratio of 10:1 and were cured at 70 °C for 1 h.13 Glass slides
(Fisher), poly(tetrafluoroethylene) (PTFE) films (0.01 mm, Good-
fellow), and stainless steel foils (AISI 316LsFe/Cr18/Ni10/Mo3,
annealed, 0.1 mm, Goodfellow) were used as received. Gold
samples were prepared on silicon wafers (Silicon Valley Micro-
electronics, Inc.) by e-beam deposition, with 3 nm of titanium
followed by 80 nm of gold.
Chemical Vapor Deposition Polymerization of Poly(p-
xylylenes). Poly(4-aminomethyl-p-xylylene)-co-(p-xylylene) (1)
and poly(4-formyl-p-xylylene-co-p-xylylene) (2) were synthesized
via chemical vapor deposition (CVD) polymerization in a
custom-made CVD polymerization system.23 The starting
materials, 4-aminomethyl[2.2]paracyclophane or 4-formyl[2,2]
paracyclophane, were sublimed under vacuum and converted
by pyrolysis into the corresponding quinodimethanes, which
spontaneously polymerized upon condensation to the cooled
substrate surface, which was maintained at 15 °C. Throughout
CVD polymerization, a constant argon flow of 20 sccm and a
working pressure of 0.5 mbar were maintained. The pyrolysis
temperature was set to be 700 °C, and sublimation temperatures
were between 90 and 110 °C under these conditions. CVD
polymerization spontaneously occurred on samples placed on
a rotating, cooled sample holder.
Surface Characterization. Film thicknesses were measured
using a multiwavelength rotating analyzer ellipsometer (M-44, J. A.
Woollam) at an incident angle of 75°. The data were analyzed
using WVASE32 software. Thickness measurements were re-
corded by fitting the ellipsometric ψ and δ data with An ) 1.65,
Fluorescence and Confocal Microscopy. Fluorescence im-
ages were visualized using a Nikon TE 200 fluorescent micro-
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4120 Analytical Chemistry, Vol. 80, No. 11, June 1, 2008

Figure 1. Schematic illustration of the solventless adhesive bonding (SAB) process. During SAB, formation of a strong adhesion layer is
achieved by bonding of two complementary CVD reactive coatings 1 and 2.
scope, 10× or 4× dry objectives were used, and a digital interline
CCD camera (Cascade 512F, Roper Scientific) was controlled by
the Metavue software (version 6.1r6). Three dimensional fluores-
cence images were captured by using a confocal laser scanning
microscope (CLSM) (TCS SP2, Leica Microsystems, U.S.A.) on
an inverted microscope (DMIRE2, Leica Microsystems, U.S.A.).
A GreNe laser (wavelength 543 nm) and a HeNe laser (wavelength
633 nm) were used to excite tetramethylrhodamine isothiocyanate
(TRITC)-labeled streptavidin and atto-655 NHS ester, respectively
(www.atto-tec.de). The emission was confined to 560-595 nm for
TRITC-labeled streptavidin and 650-700 nm for atto-655 NHS
ester upon observation. Confocal z-stack imaging scanning was
performed using a 2.5 µm pixel resolution in the z-direction.
Table 1. Experimental Tensile Stress Data
bonding
method
material
strength (MPa)
SABa
SAB
PDMS-PDMS
PDMS-PTFE
>2.44 ± 0.15b
1.21 ± 0.35
>2.44 ± 0.15
>2.44 ± 0.15
>2.44 ± 0.15
>2.44 ± 0.15
0.02 ± 0.11
0.19 ± 0.09
0.78 ± 0.08
2.34 ± 0.27
SABa
SABa
SABa
SABa
PDMS-stainless steel
PDMS-silicon wafer
PDMS-glass
PDMS-gold
physical contact
heat (140 °C)
UV/ozone
oxygen plasma
(cured at 120 °C)
oxygen plasma
(cured at 60 °C)
PDMS-PDMS
PDMS-PDMS
PDMS-PDMS
PDMS-PDMS
PDMS-PDMS
1.15 ± 0.18
a CVD coatings of 1 and 2 were used as examples. b Fracture
strength of PDMS was 2.44 ± 0.15 MPa compared to 2.24 MPa reported
in the Polymer Data Handbook (Mark, J. E. Polymer Data Handbook;
Oxford University Press: New York, 1999).
RESULTS AND DISCUSSION
As shown in Figure 1, the SAB process uses two vapor-
deposited reactive polymer coatings with complementary func-
tional groups, which are amino and aldehyde groups in this
example. The ultrathin adhesion layers are conformally applied
to the substrates of choice using a coating technology recently
developed in our group.30,31 Under the influence of moderately
increased temperatures (about 140 °C), the polymer coatings react
with each other with high affinity. In contrast, both coatings are
relatively inert when stored at room temperaturesallowing for
bonding on demand. This feature is distinct from traditional PDMS
bonding techniques, where the energy treatment has to be
conducted immediately prior to bonding. In addition, CVD-based
polymer films form well-adherent coatings on a range of different
substrate materials including polymers, glass, silicon, metals, or
paper and can be stored for extended periods prior to bonding
without losing the bonding capability.32–36 In fact, reactive coatings
used herein, poly(4-aminomethyl-p-xylylene-co-p-xylylene)37,38 (1)
and poly(4-formyl-p-xylylene-co-p-xylylene)33 (2), are stable under
dry conditions until brought in contact with each other at elevated
temperatures. Prior to mechanical testing, the chemical structures
of all polymer films were confirmed by X-ray photoelectron
spectroscopy (XPS). Film thicknesses were in the range of 40-80
nm as measured by ellipsometry. Because this novel approach
uses solventless polymer coatings, it eliminates the need for
solvent-based adhesives as well as pretreatment steps that need
to be applied immediately prior to bonding.
In a typical bonding experiment, a substrate coated with
polymer 1 and a second substrate coated with polymer 2 were
brought into conformal contact without applying additional pres-
sure. The contacting samples were then placed in a 140 °C oven
for at least 3 h. Adhesion was measured using a standard tensile
stress experiment (Table 1),39 using a Bionix 100 mechanical
tester (MTS, Co.) equipped with a 10 N load sensor. For many of
the tested substrate combinations described in Table 1, high
bonding strengths were measured. When bonding two PDMS
substrates coated with either polymer 1 or 2, tensile stresses as
high as 2.44 MPa were measured, which is identical to the fracture
strengths of PDMS. Accordingly, rupture consistently occurred
within one of the PDMS substrates and not at the bonding
interface. Similar results were obtained when PDMS was bonded
to stainless steel, silicon, or glass. When using the dry bonding
approach to bond PDMS to PTFE (Teflon), bonding strengths
averaged 1.21 MPa. These bonding strengths are comparable to
those of samples bonded by plasma plus heat treatment, which
was included in the study as reference, but higher than bonding
data obtained without reactive coatings or simple plasma or UV/
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Analytical Chemistry, Vol. 80, No. 11, June 1, 2008 4121

Figure 2. SFG spectra of polymers 1 and 2 before and after bonding. (a) SFG spectra of polymer 1 showing the presence of the characteristic
NH2 bending (coupled with the aromatic ring stretching) peak at 1635 cm-1 before bonding, which disappears after bonding. (b) SFG spectra
of polymer 1 showing the presence of the characteristic N-H stretching peak at 3325 cm-1 before bonding. The signal disappears after bonding.
(c) SFG spectra of polymer 2 showing the presence of characteristic CdO stretching peak before bonding.
ozone activated bonding. Although the group of reference materi-
als included in this study is not exhaustive, it becomes evident
that adhesion obtained with the SAB process clearly falls within
a range required for many microfluidic applications. The use of
CVD-based polymer coatings also effectively suppresses restruc-
turing of otherwise quite dynamic PDMS surfaces. For example,
a PDMS-based microchannel bound to a silicon substrate using
the SAB methodology showed excellent adhesion after storage
for more than 1 year under ambient conditions (air, medium
humidity, varying temperatures between 20 and 25 °C).
basis of these experiments, we concluded that the strong bonding
is due to the chemical reaction between amino and aldehyde
groups.
In addition to bonding, an important aspect of biological devices
is the need for precise modification of the luminal surfaces of
microchannels. CVD polymerization results in homogeneous
coating of the entire surface of a device substructure. During
thermal bonding, however, only functional groups that come in
direct contact with the complementary surface chemistry will be
consumed. Functional groups that are located within the micro-
channels and therefore are not exposed to complimentary func-
tional groups remainsat least in principlesavailable for surface
modification.
To address the question of whether polymer films deposited
within microchannels still maintain their typical reactivity after
the curing process, we conducted a series of immobilization
studies in previously bonded devices. For this purpose, PDMS-
based microchannels were fabricated using a standard procedure
described previously.29 Poly(4-formyl-p-xylylene-co-p-xylylene) (2)
was deposited onto the resulting PDMS microchannels via CVD
polymerization. Commercially available glass slides were coated
with poly(4-aminomethyl-p-xylylene-co-p-xylylene) (1) via CVD
polymerization. In this setting, the reactive coatings provide dual
function: (i) they chemically bound the microchannels, where
glass and PDMS are brought in direct contact to each other, and
(ii) they provide free chemical groups for further surface modifica-
tion on the luminal surfaces of the microchannels, which are
conserved under the conditions of thermal bonding (Figure 3).
Whereas polymer 1 provides primary amino groups for coupling
with activated carboxyl groups (amide formation), polymer 2 has
aldehyde groups that can react with hydrazines or hydrazides.
After CVD modification, the PDMS microchannels and glass
coverslips were brought in contact and bonded following the SAB
process, as described before. To assess the chemical activity of
the coating after exposure to the SAB process, hydrazide-derived
biotin ligands were allowed to react with the functional groups
present within the microchannels. The biotin ligand was chosen
To further elucidate the underlying mechanism of the bonding
process at the molecular level, we used SFG spectroscopy. SFG
provides details about the buried interface involving polymers24–28
and is well suited for investigating the adhesion layer formed
between the two substrates. On the basis of a series of control
experiments with CVD coatings that had different side groups,
we hypothesized that covalent binding via imine bonds formed
between CHO and CH2NH2 groups of the two polymer coatings.
When a PDMS substrate coated with polymer 1 is initially brought
in contact with a quartz substrate coated with polymer 2,
characteristic signals related to the primary amino groups of
polymer 1 were detected at 1630 cm-1 (NH2 bending coupled
with aromatic ring stretching, Figure 2a) and 3325 cm-1 (N-H
stretch, Figure 2b). Moreover, the characteristic CdO stretches
(Figure 2c) of the aldehyde groups stemming from polymer 2
were detected at 1725 cm-1. Prior to exposure to sufficiently high
temperatures, the complementary chemical groups continue to
coexist without detectable chemical conversion. After the substrate
temperature was raised to 140 °C, however, both amines and
aldehyde groups were simultaneously consumed as indicated by
the disappearance of the characteristic signals at 1630 and 3325
cm-1, on the one hand, and 1725 cm-1 on the other. Interestingly,
these effects were not observed when the substrates were brought
in contact without heating (Figure 2a) or when one coating alone
was heated (Figure 2c), suggesting that the observed changes in
the SFG spectra indeed correlate with the bonding event. On the
4122 Analytical Chemistry, Vol. 80, No. 11, June 1, 2008

Figure 3. Process of dry adhesive bonding on microchannels by using CVD reactive coatings. The fluorescence micrograph (enlargement) is
showing the biotin/TRITC-streptavidin modification on CVD polymer 2 within sealed microchannels after the dry adhesive bonding process.
because it undergoes nearly quantitative conversion with aldehyde
groups and its interactions with streptavidin are known to result
in tight confinement of streptavidin on the biotin-modified
surface,33,40 which can be exploited for visualization of ligand
binding. In a second step, we allowed TRITC-conjugated strepta-
vidin to bind to the biotin immobilized onto the luminal surfaces
of the microchannel. After rinsing with PBS buffer, the surfaces
were inspected by fluorescence microscopy. The fluorescence
micrograph of Figure 3 shows surface-modified microchannels
that were coated with polymer 2 on the top and two side walls
(PDMS-based surfaces) as well as polymer 1 on the bottom
surface (glass slide) and then subjected to the biotin-streptavidin
protocol. Homogenous distribution throughout the entire micro-
channel was observed, indicating that aldehyde groups were
available throughout the entire luminal surface area. Because of
the limited z-resolution of fluorescence microscopy, cross-sectional
analysis was challenging, and this experiment cannot decidedly
clarify whether the binding of biotin-streptavidin is specifically
occurring on areas modified with polymer 2. Reference experi-
ments without biotin immobilization suggest that there are only
low levels of nonspecific streptavidin adsorption in the absence
of immobilized biotin, which is in accordance with previously
reported data on patterned surface structures.32,34,36
a follow-up experiment involving two consecutive immobilization
cycles.41 First, the microchannels were reacted with atto-655 NHS
ester allowing covalent amide formation with polymer 1 modified
surface (glass substrate, bottom). After a thorough rinse with
buffer solution, subsequent biotin-streptavidin immobilization was
performed according to a previously described procedure.40
Confocal laser scanning microscopy was used to examine the
resulting microchannels in details. As shown in Figure 4, top and
side surfaces were associated with green color due to immobilized
TRITC-labeled streptavidin present on surfaces modified with
polymer 2. The bottom surface had a red color due to covalent
immobilization of a red-fluorescent atto-655 NHS ester via amino
groups that originated from reactive coating 1. The CLSM image
reveals clearly distinguishable colors on the luminal surfaces
associated with PDMS and glass, respectively. It should be noted
that in spite of the additional complexity stemming from using a
cascade of immobilization steps, excellent homogeneity through-
out the entire channel, minimal nonspecific adsorption between
different molecules (based on confocal microscopy), and excellent
bonding strength (based on tensile stress testing) were observed.
CONCLUSIONS
In summary, we report a novel, solventless bonding approach
for microfluidic devices that relies on a well-defined chemical
reaction between two reactive polymer coatings with complemen-
In order to address the simultaneous immobilization of two
different biomolecules on top and bottom surfaces, we performed
(40) Chen, H. Y.; Elkasabi, Y.; Lahann, J. J. Am. Chem. Soc. 2006, 128, 374–
(41) Chen, H. Y.; Rouillard, J. M.; Gulari, E.; Lahann, J. Proc. Natl. Acad. Sci.
380
.
U.S.A. 2007, 104, 11173–11178.
Analytical Chemistry, Vol. 80, No. 11, June 1, 2008 4123

Figure 4. Left: illustration showing the distribution of fluorescence signals according the ligands and dye used. The green color comprises
TRITC-labeled streptavidin, which was immobilized according to the biotin-hydrazide presented on polymer 2 modified surfaces. The red surface
in the bottom was formed by covalent immobilization of a red-fluorescent atto-655 NHS ester on the amino groups presented on the bottom
surface that was modified by polymer 1. Right: confocal micrograph showing the according fluorescence image of a sealed microchannel. The
image was recorded by continuous image collection in the z-direction.
tary chemical groups. The polymeric reaction partners converted
with high selectivity and close to quantitative yields and also
showed excellent storage stability and low tendencies to undergo
side reactions. Macromolecular reactants equipped with either
amino or aldehyde groups were deposited onto substrates using
a solventless CVD polymerization process that can be applied to
a wide range of different substrate materialssthereby directly
addressing substrate independence, a major need of many bio-
technological applications. Moreover, bonding results in excellent
adhesion of PDMS to a wide range of substrates including PDMS,
glass, silicon, stainless steel, and PTFE with tensile strengths
being higher than those measured for substrates bonded with
plasma or UV/ozone treatment. The currently exploited temper-
atures of 140 °C are compatible with PDMS, PTFE, and many
other polymers but clearly provide a challenge when materials
such as polymethacrylate or polycarbonate ought to be used.
Reducing the process temperature through process optimization
and/or exploration of alternate bonding chemistries will therefore
be an important direction of future work.
Because this dry adhesion process is (i) independent of the
chemical composition of the substrate, (ii) well-defined with
respect to the bonding chemistry, (iii) can be controlled on
demand, and (iv) can provide a biologically distinct binding
platform, it may have important technological implications for
miniaturized bioanalytical systems as well as microelectrome-
chanical systems (MEMS) fabrication, where a substantial diver-
sification of process materials is currently observed.1,17,42
ACKNOWLEDGMENT
J.L. gratefully acknowledges support from the NSF in the form
of a CAREER Grant (DMR-0449462) and funding from the NSF
under the MRI program (DMR 0420785). This work is further
supported by NSF (CHE-0449469, to Z.C.). We thank Professor
Shuichi Takayama and Yaokuang Chung, University of Michigan,
for the assistance with oxygen plasma bonding. We thank
Professor Kotov, University of Michigan, for use of the ellipsom-
eter. We gratefully acknowledge Dr. Shuji Ye for the ab initio
calculation results.
Received for review February 18, 2008. Accepted March
31, 2008.
(42) Beebe, D. J.; Mensing, G. A.; Walker, G. M. Annu. Rev. Biomed. Eng. 2002,
4, 261–286
.
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