High-sensitivity microfluidic calorimeters
for biological and chemical applications
Wonhee Lee, Warren Fon, Blake W. Axelrod, and Michael L. Roukes
1
Kavli Nanoscience Institute, California Institute of Technology, Pasadena, CA 91125
Edited by George M. Whitesides, Harvard University, Cambridge, MA, and approved July 22, 2009 (received for review February 9, 2009)
High-sensitivity microfluidic calorimeters raise the prospect of
achieving high-throughput biochemical measurements with mini-
mal sample consumption. However, it has been challenging to
realize microchip-based calorimeters possessing both high sensi-
tivity and precise sample-manipulation capabilities. Here, we re-
port chip-based microfluidic calorimeters capable of characterizing
the heat of reaction of 3.5-nL samples with 4.2-nW resolution. Our
approach, based on a combination of hard- and soft-polymer
microfluidics, provides both exceptional thermal response and the
physical strength necessary to construct high-sensitivity calorim-
eters that can be scaled to automated, highly multiplexed array
architectures. Polydimethylsiloxane microfluidic valves and pumps
are interfaced to parylene channels and reaction chambers to
automate the injection of analyte at 1 nL and below. We attained
excellent thermal resolution via on-chip vacuum encapsulation,
which provides unprecedented thermal isolation of the minute
microfluidic reaction chambers. We demonstrate performance of
these calorimeters by resolving measurements of the heat of
reaction of urea hydrolysis and the enthalpy of mixing of water
with methanol. The device structure can be adapted easily to
enable a wide variety of other standard calorimeter operations;
one example, a flow calorimeter, is described.
calorimetry
lab-on-a-chip
nanocalorimetry
biosensor
F
luidic calorimeters provide the ability to characterize the
thermodynamics of chemical processes completely without
labeling or analyte immobilization. This flexibility is widely used
for the study of biomolecular interactions, intramolecular struc-
tural changes, and enzyme kinetics (1–3). However, measure-
ment protocols with existing calorimeters involve relatively large
sample volumes, typically on the scale of hundreds of microliters,
and long measurement times, typically on the order of tens of
minutes.
There is growing need for high-throughput, small-volume
fluidic calorimeters for both fundamental scientific research and
applications in technology. Recent developments in microfabri-
cation now make it possible to build ‘‘chip calorimeters’’ capable
of assaying volumes ranging from the microliter to the tens of
picoliter scale (4–14). In the future, scale-up to array-based
operations will enable high measurement throughput with the
reduced sample volumes necessary to make calorimetric screen-
ing of large analyte libraries cost effective. Although existing
chip calorimeters suggest the feasibility of such future possibil-
ities, practical development and deployment of this technology
have been hampered by low device sensitivity and the lack of
reliable sample handling down to picoliter volumes.
Chip calorimeters can be classified into two categories ac-
cording to the configuration of their measurement/reaction
chambers. Open-chamber chip calorimeters are built by using
thermally isolated wells or platforms onto which samples are
spotted as droplets by micropipette or inkjet printing (4–10).
Closed-chamber chip calorimeters, by contrast, use microfluidic
channels to access closed measurement chambers, into which the
samples are introduced and reactions are monitored (11–14).
Although open-chamber chip calorimeters provide fairly good
thermal isolation, they generally suffer from critical limitations
arising from evaporation and awkwardness in sample handling.
These can readily lead to erroneous measurements. On the other
hand, closed-chamber chip calorimeters generally have greater
thermal conductance to their surroundings compared with open-
chamber designs. This can result in significant heat loss, which,
in turn, can limit device sensitivity. In addition, their sensitivity
typically suffers from a larger device heat capacity than that
common to open-chamber devices.
We report here the fabrication and operation of microchip-
based, closed-chamber calorimeters based on a novel configu-
ration providing greatly enhanced sensitivity. We embed the
calorimeter within a thin-film parylene microfluidic system that
is thermally isolated from its surroundings by on-chip vacuum
encapsulation. Sample handling at 1 nL and below is provided by
interfacing these calorimeters with soft pneumatic microfluidics
(valves, pumps, and flow channels) for easy and accurate reac-
tion control. This technology can be readily scaled up to array
architectures capable of high-throughput calorimetric assays for
a wide range of applications in chemistry, the life sciences, and
medicine.
Results and Discussion
The calorimeters we have developed comprise three principal
components: microfluidics, thermopiles, and vacuum encapsu-
lation (Figs. 1 and 2). The microfluidics include a measurement
chamber, flow channels, and pneumatic flow-control compo-
nents comprising valves and pumps. Integrated within the mi-
crofluidic chamber are thermopiles for the local measurement of
temperature. The microfluidic channels and measurement cham-
ber are enclosed within vacuum encapsulation (Fig. 1
B
and
C
).
The principal calorimeter components, including the mea-
surement chamber and thermometer, are built on a thin, trans-
parent parylene-C polymer membrane, as shown in Fig. 1
D
.
Parylene-C, a member of the polyxylylene polymer series, is used
as the structural material of this membrane and the principal
microfluidic components. In this device, these consist of four
channels and the reaction/measurement chamber. The chamber
is located at the center of the membrane, and it connects to
fluidic channels used to inject and purge the sample. In a typical
measurement protocol, two different sample solutions are in-
jected into the chamber from two separate channels. After the
measurement, buffer solution from a third channel flushes the
chamber, and its contents are evacuated through a fourth
channel.
In developing a chip-based calorimeter, it is both critical and
challenging to engineer a small device heat capacity relative to
that of the sample itself. The unique vapor-phase deposition of
parylene allows it to form very thin, conformal layers. The
parylene microfluidic structures in our device have
2-
m-thick
walls that permit a very significant reduction in the device heat
Author contributions: W.L., W.F., and M.L.R. designed research; W.L. performed research;
B.W.A. contributed new reagents/analytic tools; W.L., W.F., B.W.A., and M.L.R. analyzed
data; and W.L., W.F., B.W.A., and M.L.R. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
1
To whom correspondence should be addressed. E-mail: roukes@caltech.edu.
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pnas.0901447106
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September 8, 2009
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APPLIED PHYSICAL
SCIENCES
capacity compared with that of prior closed-chip calorimeter
implementations. Parylene is also an excellent insulating mate-
rial, which provides substantial additional improvement of the
calorimetry chamber’s thermal isolation.
Parylene has many additional properties, including its chem-
ical resistance and biocompatibility; both are favorable for
microfluidic applications (15–17). However, to date, no simple
approach has existed to incorporate valves or pumps into
parylene microfluidics (18). In this work, to realize a straight-
forward solution to this problem, we interfaced soft polydim-
ethylsiloxane (PDMS) microfluidics, incorporating valves and
pumps, with the ‘‘harder’’ parylene microfluidic components
(Figs. 1
B
and 2). PDMS is a gas-permeable silicone elastomer
with Young’s modulus three orders smaller than parylene;
components made from it provide fluidic control with facility,
accuracy, and the potential for great complexity, whereas the
parylene microfluidic components have the requisite attributes
to serve as the principal calorimeter structures. Because
parylene coatings are conformal, the top surfaces of our com-
pleted calorimeters were not flat. Therefore, we planarized the
structures by using SU-8, a negative photoresist. We used this
material to fabricate a thick overlayer that facilitates optimal
mating between the parylene calorimeter and PDMS microflu-
idic structures. A good fluidic seal between the different mi-
crofluidic components is obtained by first aligning partially
cured PDMS fluidic channels to the parylene microfluidic inlets
and outlets, then further curing the mechanically joined struc-
ture to achieve a robust bond.
Our device architecture permits variable control of the sample
injection volume by sequentially closing four adjacent PDMS
valves to facilitate peristaltic pumping. Each valve permits
reproducible injection of a 700-pL fluidic volume with
50-pL
accuracy. The overall operational protocol of the calorimeter
can be modified readily by replacing the PDMS microfluidic
system with a variety of alternative designs. The PDMS-to-
parylene microfluidic interface enables easy integration of cal-
orimeter functions with other forms of lab-on-a-chip devices.
The low thermal conductivity of parylene significantly reduces
the heat loss in our devices down to a level comparable to that
of the best open-chamber chip calorimeters demonstrated to
date. We obtained further enhancement of the thermal isolation
of the reaction chamber through a combination of on- and
off-chip vacuum encapsulation (Fig. 1
B
and
C
). This configu-
ration proves crucial for achieving maximum sensitivity in
closed-chamber, chip-based devices, given their high surface-to-
volume ratio. Indeed, for the parylene devices presented here,
90% of the heat loss at ambient pressure is through air.
Parylene has very low gas permeability and high mechanical
strength, and these properties permit us to apply vacuum across
the thin microfluidic walls. Table 1 provides a comparison of the
thermal conductance of our vacuum-encapsulated parylene
closed-chamber calorimeters with state-of-the-art open and
closed chip-based calorimeters.
A
B
C
D
E
Fig. 1.
Parylene-polymer-based microfluidic calorimeter chip. (
A
) Device chip (3
3 cm) mounted on a vacuum chuck. The sensor leads (
Left
) and the PDMS
microfluidic control tubing (
Right
) are displayed. (
B
) Device schematic (cross-sectional view). The parylene membrane is suspended and thermally isolated by
vacuum.(
C
)Optical-microscopyimageofthemeasurementchamberregionofthechip,includingtheboundaryofon-chipvacuumspace.Thevacuumevacuation
is through the purple hole visible at the top left. (
D
) The parylene microfluidics, the thermopile, and the heater on the parylene membrane (
1.5-mm square).
The thermopile has a meandering shape to increase its longitudinal thermal resistance. The suspended parylene membrane is transparent; its reddish
color arises
mainly from light reflection. (
E
) Magnified view of the 3.5-nL parylene measurement chamber (200-
m diameter) and connecting channels (35
m wide)
surrounded by vacuum. The reaction chamber swells slightly because of pressure difference (compared with
D
) when the vacuum chamber is evacuated.
calorimetry chip
PDMS control chip
membrane
Fig. 2.
Microfluidic layout. Pneumatically controlled PDMS microfluidics
chip mated to parylene microfluidics on the calorimeter chip. The PDMS
control layer (red) contains valves and peristaltic pumps. The PDMS flow layer
(blue) is connected to the parylene channel (black) through an SU-8 via. Four
injection pumps (containing large area valves) are built in the right upper
corner.
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The vacuum encapsulation comprises two regions. The region
above the parylene membrane (the on-chip vacuum space) is
defined by SU-8 sidewalls and a top glass coverslip sealed to the
calorimeter with UV-curable epoxy. The glass cover facilitates
optical imaging of the calorimeter chamber and its contents (Fig.
1
B
). The SU-8 structure is patterned to achieve both planariza-
tion and the vacuum encapsulation in a single step. The region
below the parylene membrane (the off-chip vacuum space) is
sealed conventionally with a small o-ring within a custom
vacuum chuck. The two vacuum spaces are linked by a pumping
port etched through the wafer, as depicted in Fig. 1
C
.
A thin, metal film thermopile, fabricated directly on the
parylene membrane, enables measurement of temperature
changes within the calorimeter chamber during reactions (Fig.
1
D
). Thermopower-based sensors provide the most common
form of thermometry used in calorimeters. Their principal
advantage is that they provide zero-power sensing; that is,
sensing uncompromised by additional heat dissipation from the
measurement process itself, as encountered with electrically
biased resistive sensors. A further advantage over resistive
sensing is the circumvention of sensor 1/f noise; however,
straightforward thermopower measurements can be degraded by
1/f noise in the readout amplifier. In this work, we connected five
thermocouple junctions in series to yield an overall temperature
responsivity (total Seebeck coefficient) of 110
V/K. In the
completed devices, the electrical components were sealed be-
tween two parylene layers to isolate them from the liquid sample.
We measured the thermoelectric voltage from this thermopile by
amplifying it with a low-noise, dc-coupled preamplifier (CS 3001;
Cirrus Logic). In the output (voltage) domain, the total sensor
noise referred to input was
10 nV in a 1-Hz bandwidth (0.01–1
Hz). Given the aforementioned temperature responsivity, this
indicates the overall temperature resolution for 3:1 signal-to-
noise ratio (SNR) is
T
270
K in a 1-Hz bandwidth.
The individual thermocouple elements are formed from Au–Ni
microjunctions. This choice of materials provides both fabrication
convenience and very low 1/f electrical noise. We anticipate that
substitution of our Au/Ni thermopiles with previously demon-
strated thermopiles providing a higher total Seebeck coefficient
should readily provide more than an order of magnitude sensitivity
improvement beyond that achieved here (19).
We calibrated the calorimeter by measuring the thermoelec-
tric voltage induced in response to an electric power applied to
a gold-resistive heater inside the chamber (Fig. 3
A
). Thermom-
eter signal response to a step function power is predicted by
V
(
t
)
S
(
P/G
)(1
e
t/
), where
G
is thermal conductance,
P
is amplitude of applied power, and
is the thermal time constant.
From the steady-state response, we found that the device has a
heat responsivity, defined as voltage output (
V
) over applied
heating power, of 7.1 V/W. The measured thermal conductance
of the device,
G
S
P
/
V
,is16
W/K under vacuum. This high
vacuum-enabled thermal isolation exceeds the performance of
prior chip-based calorimeters, which generally range from 100
W/K to 10 mW/K (Table 1). When a chemical reaction occurs
in the chamber, the energy of reaction is obtained as
E
t
m
(
t
)/
Sdt
. The time of measurement,
t
m
, is determined by the
longer of either the time of the chemical reaction or the thermal
relaxation time of the calorimeter. The power sensitivity of the
device is
G
T
4.2 nW. During these calibration runs, the
measurement chamber is filled with deionized (DI) water.
Together, the thermal conductance and the thermal time con-
stant allow us to determine the device heat capacity,
C
G
. The
thermal time constant,
1.3 s, is extracted from the measured
rate of exponential growth in response to electrically induced
heat steps. These data indicate the heat capacity of the water-
filled device is
C
21
J/K. By using tabulated values, we
estimate that 3.5 nL of water contributes
15
J/K to the total
heat capacity; hence, we deduce the intrinsic (empty) device heat
capacity to be
6
J/K.
During these calibrations and all other measurements per-
formed, the enclosure box was maintained at 20 °C, with long-term
temperature stability of 0.5 °C (during the
1-h measurement
sessions). Environmental temperature fluctuations have negligible
effect on the accuracy of our measurements because of the close
proximity of the two ends of the thermocouple (
1 mm); the
thermopiles only sense the temperature gradient between their two
ends. Furthermore, the ends of the thermopiles are isolated from
their environment by the vacuum space. To establish a specified
reaction temperature the calorimeter and its fluidic contents are
Table 1. Chip-based and commercial calorimeters
Group (source)
Chamber
Sample handling
Thermal
conductance,
W/K
Resolution*
Type Volume, nL
Power, nW
Theoretical
energy, nJ
†
Practical
energy, nJ
‡
Caltech (this study)
Closed
3.5
Multilevel microfluidics
16
4.2
6
10
Penn State (11)
Closed
15
§
Syringe pump
5,000
300
30
10
5
Eurotronics (12, 13)
Closed
6,000
§
Syringe pump
10,000
30
100
1,000
Columbia (14)
Closed
800
§
Syringe pump
1,500
50
30
5,000
Scripps-Palo Alto Research
Center (4, 5)
Open
500
Electrostatic merging
1,000
50
100
750
Katholieke Universiteit
Leuven (6)
Open
10
5
to 10
7
Micropipette
5,000
5,000
10
5
10
6
University of Glasgow (7)
Open
0.75
Micropipette
100
13
0.16
100
Technische Universität
Bergakademie Freiberg (8)
Open
6,000
Micropipette
30,000
50
500
5,000
Vanderbilt (9)
Open
5–50
Micropipette
170
22
24
132
Vanderbilt (10)
Open
0.05
Inkjet head
90
150
1
500
Commercial
Open
10
6
Micropipette
N/A
10
N/A
100
N/A, not available.
*Power and energy resolution are based on a 3:1 signal-to-noise ratio.
†
Theoretical energy resolution is the product of (noise-limited) power resolution and thermal relaxation time.
‡
Practical energy resolution also includes measurement irreproducibility from fluidic volume uncertainties in injection, mixing, and evaporation
. Evaporation
is particularly deleterious for small, open-chamber calorimeters.
§
Flow calorimeters are typically operated at flow rates of 0.5 mL/h.
Lee et al.
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SCIENCES
maintained within a temperature-regulated enclosure under pro-
portional-integral-derivative control.
Fig. 3
B
shows results from finite element simulation (FEM-
LAB; COMSOL) of the calorimeter’s thermal conductance. The
color scale indicates the temperature change caused by the
uniform power dissipated within the chamber. The device is
designed in such a way that the temperature gradient inside the
measurement chamber (Fig. 3
B
, circle at the center) is relatively
small compared with the gradient over the parylene membrane
(Fig. 3
B
, overall view). The simulation indicates that the maxi-
mum temperature gradient in the measurement chamber is
5%
of the chamber-averaged temperature change caused by heating.
Thus, the temperature inside the measurement chamber can be
considered uniform, and the measurement error arising from the
position of thermopile junctions will not exceed 5% of total
energy. The average temperature change at the thermocouple
junction positions, with 1-
W heating, was 0.15 K. The resulting
thermal conductance of the device was 6.7
W/K.
Fig. 3
C
demonstrates the performance enhancement obtained
from on-chip vacuum isolation. Heat loss through air convection
and conduction contributes
93% of total heat loss at atmo-
spheric pressure. Below 2 mtorr, however, thermal transport
through air becomes negligible, and only the parylene microflu-
idic structure, the electrical leads, and the liquid within the
channel provide appreciable pathways for thermal conduction.
Thus, vacuum encapsulation provides a 14-fold net increase in
signal sensitivity. The solid line in Fig. 3
C
represents the results
of an analytical formula for the thermal conductivity of air
between two infinite planes scaled to represent our device
geometry but without edge corrections (20). This slip-flow
theory provides a reasonably accurate representation for our
purposes here. However, to be precise at low pressures, it should
be replaced with free-molecule theory. The fitting parameter,
d
,
depends on geometric factors of the device, which include the
size of the measurement chamber (200-
m radius) and the
distances between the vacuum walls (50
m, top, and 500
m,
bottom). Fits to the data indicate
d
170
m, which is
comparable with the physical dimensions of the device.
We demonstrated calorimeter performance by observing two
separate exothermic reactions. First, we measured the heat of
reaction of urea hydrolysis catalyzed by urease:
(NH
2
)
2
CO
3H
2
O
O
¡
Urease
2NH
4
HCO
3
OH
,
H
61kJ/mol
To carry out this first measurement, the 3.5-nL reaction chamber
was initially filled with an aqueous solution of urease (0.5
mg/mL; Sigma–Aldrich). Subsequently, an aqueous solution of
urea (50 mM; USB) was injected into the chamber by using the
on-chip PDMS peristaltic pump; this step displaces an equal
amount of the urease solution. Both the urease and urea
solutions were prepared with the same buffer (0.2 M sodium-
phosphate buffer, pH 7.0) to minimize extraneous contributions
caused by the heat of mixing. Fig. 4 shows measurements after
discrete injections of urea solution, with injection volumes
ranging from 700 pL to 2.8 nL, corresponding to 35–140 pmol of
urea. The measurement time scale,
30 s, was limited by enzyme
kinetics and not by the calorimeter’s intrinsic thermal response
time (
1.33 s). The total energy liberated from hydrolysis can
be calculated by integrating the temporal data (Fig. 4
Inset
),
yielding an enthalpy change of 62 kJ/mol. This finding agrees well
with the value of 60.9
2.5 kJ/mol that we measured indepen-
dently on much larger sample volumes (
1.5 mL) with a
commercial calorimeter (VP-ITC; MicroCal).
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
50
100
Thermal Conductance
(
μ
W/K)
Pressure
(Torr)
Fitting
Data
C
14x
G
res
-2
0
2
4
6
8
0
10
20
30
0.0
0.1
0.2
0.3
V
(
t
) =
V
0
(1-e
-
t
/
τ
)
Data
Voltage
(
μ
V
)
Time
(s)
Temperature Rise
,
∆
T
(K)
τ
= 1.33s
A
0.12
0.08
0.04
0.00
∆
T(K)
B
Fig. 3.
Thermal response of device. (
A
) Thermopile response to local electrical heater. A step function of electrical power (4.0
W) is applied beginning at
t
0. A vacuum of 2 mtorr is maintained, and the device reaction chamber is filled with water. (
B
) Simulation of thermal response under heat generation inside the
chamber at 1
W. The measurement chamber is the circle. The six U-turn lines are the metallic sensors, and the four rectangles are fluidic channels. (
C
)
Experimental data of thermal conductance,
G
, of the device under vacuum at different pressures. The fitting formula is
G
(
p
)
G
res
G
air
/(1
171
pd
), where
p
is
the vacuum chamber pressure (in torr). The fitting parameters are residual thermal conductance (caused by parylene, etc.)
G
res
15.5
W/K, the thermal
conductance of air at 1 atm
G
air
214
W/K, and a geometric factor
d
170
m. The geometric factor
d
is related to the thickness of the vacuum jacket and
the size of the measurement chamber.
0 20406080
0.0
0.2
0.4
0.6
0.8
1.0
0
20
40
60
Injection vol.
2.8nl
2.1nl
1.4nl
0.7nl
Exothermic Reaction Power
(
μ
W)
Time
(s)
Temperature Difference,
Δ
T
(mK)
Fig. 4.
Calorimetric signature of urea hydrolysis by urease. Calorimetric
response after the injection of various volumetric aliquots of 50 mM urea
solution into the chamber prefilled with urease solution. Four separate mea-
surements are plotted on the same graph for comparison. (
Inset
) The total
energy of reaction. The red line shows the expected energy of reactions
deduced from the heat of reaction,
H
61 kJ/mol.
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In these measurements, the total noise at the readout amplifier’s
output, referred to input (temperature domain), corresponded to
500
K for the present device (1:1 SNR; bandwidth, 0.01–10 Hz).
The dominant noise contribution arose from the readout amplifier
itself; a secondary contribution arose from the Johnson noise of the
thermopile. The contribution from thermal fluctuations of the
calorimeter was even smaller; we estimated them to be on an order
of 10
K within the measurement bandwidth. In practice, however,
our measurement accuracy does not approach the amplifier-limited
value of sensitivity, because of the variance of injection volumes
from run to run. To attain full performance from these devices, a
more precise sample injection method will be required.
To provide further validation of the capabilities of these
calorimeters, we present a second measurement involving the
heat of mixing of methanol and water. For this experiment,
methanol was prediluted with DI water (14.7% methanol by
mole fraction) to reduce the predicted enthalpy change upon
mixing to
14
J (21). The measurement chamber was initially
filled with DI water, and 1.4 nL of the 14.7% methanol–water
mixture was subsequently injected to attain a final mole fraction
of 5.4% methanol. Fig. 5 shows the enthalpy change ensuing
from the mixing process. The measured total enthalpy change
from the mixing process was on the order of
10
J. With the
current device design, the concentrated sample begins diffusing
into the chamber, and thus mixes before the ‘‘formal’’ injection
step. The measurement error from this uncontrolled diffusion is
rather small for the majority of biochemical reactions because
the associated volume of the fluidic channels near the measure-
ment chamber (0.5 pL/
m) is quite small compared with the
chamber volume (3.5 nL). Hence, during the short interval
between sample loading and injection, only a small amount of
reaction/mixing occurs. However, this effect can potentially
become more problematic for solutes with large diffusion con-
stants, such as methanol. To circumvent such issues, we have
been investigating the separation of samples by immiscible
liquids (22). We also note that the temporal resolution of these
first-generation devices is limited by the rate of mixing within the
chamber (after injection), which occurs primarily by diffusion.
Incorporating microfluidic mixers (23) will greatly enhance the
rate of mixing.
The vacuum-insulated microfluidic reaction chamber devel-
oped in this work is applicable to variety of well-validated
approaches in calorimetry, including isothermal titration calo-
rimetry, differential scanning calorimetry, and flow calorimetry.
In Fig. 6, we demonstrate one possible configuration we have
developed to enable flow calorimetry. Two microfluidic chan-
nels, each isolated on a suspended parylene membrane, pass over
the two ends of the thermopile. This differential configuration
enables suppression of fluidic nonidealities, such as dilution and
mixing. Further improvement of the sensitivity should be attain-
able through use of thermoelectric materials with higher See-
beck coefficients and optimization of parylene suspension ge-
ometry to further suppress residual thermal losses under vacuum
conditions.
Summary
Miniaturization brings significant benefits to calorimetry, in-
cluding the possibility of high-sensitivity, high-throughput anal-
yses with low sample consumption. Past efforts to develop
miniaturized calorimetric sensors have led to faster response
(better temporal resolution) but generally suffer from low power
sensitivity caused by poor thermal isolation. This has proven to
be especially deleterious for closed-chamber miniaturized chip
calorimeters. In this work, we solved this problem by incorpo-
rating vacuum-isolated microfluidics that minimize heat loss and
enhance the calorimeter’s sensitivity. With these first-generation
devices, we demonstrate
4.2-nW sensitivity and
1.3-s re-
sponse time with 3.5 nL of total sample volume. This improved
performance should enable a new class of cost-effective, high-
throughput, automated calorimetric measurements.
Methods
Device fabrication first involves the creation of silicon nitride (SiN) membranes
from a SiN wafer polished on both sides (front and back). These SiN membranes
subsequently serve as sacrificial layers used to support the polymer devices
through the fabrication process. At the end of the fabrication process, they are
removed by reactive ion etching (RIE). Fabrication on these membranes proceeds
as follows: a first parylene layer (1
m thick) is deposited on the SiN membrane
with the PDS 2010 LABCOTER 2 parylene coater (Specialty Coating Systems). An
adhesion promoter, A-174, is applied before the parylene deposition to enhance
its adhesion to the substrate. An 80-nm-thick Ni layer and a 60-nm-thick Au layer
are sequentially e-beam-evaporated on this basal parylene layer. A 4-nm-thick Ti
layer is evaporated as an adhesion layer before both the Ni and Au depositions.
These metal layers are subsequently patterned by optical lithography and wet
chemical etching to form the thermopiles. Thereafter, a second parylene layer
of 1-
m thickness is deposited over the metallic components to isolate them
from the fluidic chamber. The parylene microfluidic chamber and its inlet and
outlet channels are patterned by conventional methods (15, 16). A 15-
m-thick
photoresist is first spun on and patterned to form the inside shapes of the
microfluidic structures. A third parylene layer of 2-
m thickness is deposited on
these photoresist templates to create the parylene microfluidic structures. An
0102030
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0
50
100
150
Temperature Difference,
Δ
T
(mK)
Exothermic Reaction Power
(
μ
W)
Time
(s)
0s
0.1s
1s
5s
A
B
Fig. 5.
Enthalpy of mixing measurement. (
A
) Heat of methanol dilution. A
total of 1.4 nL of 14.7% methanol–water solution was mixed with 2.1 nL of
water. (
B
) Visualization of the diffusive mixing process, enabled by the injec-
tion of red dye into the chamber filled with water.
AB
500
μ
μ
m
Fig. 6.
Microscale flow calorimeter. (
A
) Parylene microfluidics and electronic
sensor fabricated onto a parylene bridge structure. The measurement cham-
ber is located at the middle of the bridge. (
B
) Flow calorimeter chip (3
3 cm)
with two identical measurement chambers (located at the transparent area at
the center) for differential measurement.
Lee et al.
PNAS
September 8, 2009
vol. 106
no. 36
15229
APPLIED PHYSICAL
SCIENCES
SU-8devicetopstructureof
80-
mthicknessispatternedontopoftheparylene
microfluidic components to planarize the surface and to construct the vacuum
encapsulation.
Several etch steps are carried out by using RIE. First, the parylene microflu-
idic channel opening areas are etched by using an O
2
plasma at a pressure of
150 mtorr with a drive power of 140 W. The photoresist inside the channel is
removed by using propylene glycol methyl-ether-acetate. After the microflu-
idic channels are cleared, the parylene covering the electrical components is
etched (conditions as above). Finally, the parylene membrane is suspended by
etching the sacrificial SiN membrane from its backside; for this step, a CF
4
plasma at a pressure of 120 mtorr with a drive power of 140 W is used. For the
flow calorimetry devices, an additional parylene etch step is done to pattern
the suspended parylene bridges. The final step involves sealing the vacuum
encapsulation region with a glass slide by using UV-curable glue.
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cgi
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