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The use of a closed pump provides a holding force that prevents
uncontrollable flooding of the chip, even when the liquid
contacts the chip, while also protecting sensitive reagents from
environmental contamination until they are needed. Though
syringe pumps are capable of dispensing accurate volumes, there
is significant evaporation of volatile liquids from the open end of
the dispensing interface, and compensation is required to ensure
high accuracy of delivered volumes after arbitrary delays. This
compensation is provided by using on-chip impedance sensing to
create a very sensitive liquid detector using only minimal
hardware beyond what is normally needed for EWOD actua-
tion.17,18 With our particular configuration, evaporative losses
are compensated with a resolution of about 2 nL.
electrodes to transport droplets from the loading interface into the
EWOD chip. Individual control of electrodes was achieved using
manual switches that connected each electrode to the 100 Vrms AC
signal or to ground. The ground electrode of the cover plate was
connected to the power supply ground.
Design of reagent dispensing system
A schematic of the reagent dispensing system is depicted in
Fig. 1A. The volatile reagent is first loaded into a syringe pump
(PSD4, Hamilton Company, USA) that is connected via a PTFE
delivery line (1/32’’ OD) to the EWOD chip. The delivery line is
terminated in the blunt end of a 30 G needle (Becton, Dickinson
and Co., USA) with the tip positioned adjacent to the gap
between the two substrates of the EWOD chip at the loading site.
The syringe valve is switched to the dispense position, and the
delivery tube is primed by pumping the reagent toward the
EWOD chip until all air is eliminated from the tip of the needle.
The syringe pump is controlled by a computer to deliver the
desired volume of reagent on demand to the EWOD chip.
To prime the delivery line, and to compensate for evaporation
loss from the needle interface between dispensing operations, an
on-chip liquid detector was implemented based on impedance
sensing between the loading electrode (EWOD electrode adjacent
the loading site) in the bottom substrate and the ground
electrode in the cover plate (Fig. 1B, C). To prime the system,
the pump is advanced in small increments of 2.1 nL, achieved
using a 50 mL syringe with 24 000 steps per stroke, until the
leading meniscus of the liquid is detected at the chip. Prior to
delivery of a droplet, the same action is performed, but is
immediately followed by rapid dispensing of the desired reagent
volume from the syringe pump. The EWOD chip is then
actuated to cut the droplet from the fluid in the needle and
transport it to the desired location for processing or analysis.
We demonstrate the use of this interface to load the volatile
solvent acetonitrile (MeCN) for several steps of the radiochemical
synthesis of 2-[18F]fluoro-2-deoxy-D-glucose ([18F]FDG), a mole-
cular probe used in positron emission tomography19 for diagnosis
of cancer,20 searching for metastases,21 monitoring response to
therapy22 and drug development.23 Automation of reagent
dispensing is particularly important in this application due to
the need to shield the operator from radiation emitted by the
starting materials and synthesized probe as well as the require-
ment for multiple precision loadings of the solvent at various
stages of the reaction.
Experimental
Fabrication of EWOD chip
The bottom substrate of the parallel-plate EWOD chip was
fabricated from a 700 mm thick glass wafer coated with 140 nm
indium tin oxide (ITO) (TechGophers Inc., USA). The wafer was
first covered with 20 nm of chromium and 200 nm of gold using
an e-beam evaporator. Metal and ITO layers were etched to
form EWOD electrodes (2 6 2 mm), heater electrodes (circular
electrode with 6 mm radius for application-specific radiosynth-
esis chip described later), connection lines, and contact pads.
1 mm silicon nitride was coated as a dielectric layer by plasma-
enhanced chemical-vapor-deposition (PECVD), and 1 mm of
Cytop1 was spin-coated and annealed at 200 uC to make the
surface hydrophobic. A top substrate (cover plate) was prepared
from 700 mm thick glass coated with 150 nm ITO (Delta
Technologies Inc., USA) to serve as a ground electrode for
electrowetting. The cover plate was coated with PECVD silicon
nitride (100 nm) and Cytop1 (100 nm). One edge of the cover
plate was also coated with Cytop1 film and served as the
reagent-loading edge. The cover plate was affixed to the bottom
EWOD substrate with double-sided adhesive (3 M Inc., USA)
such that the reagent-loading edge was aligned with the edge of
the loading electrode on the bottom EWOD substrate. The
nominal adhesive thickness (and thus gap height) was 100 mm;
however, significant variation was observed and the gap for each
chip was measured.
Liquid detection subsystem
The basic elements of the liquid detection circuit are shown in
Fig. 1A. A resistor, R#1 kV, is placed between the ground
electrode in the cover plate and the power supply ground. The
activation of an EWOD electrode, as discussed above, generates
a sinusoidal current and a corresponding voltage drop, VR,
across this resistor, which is sensitive to the impedance between
the ground electrode in the cover plate and the (activated)
loading electrode in the bottom substrate. Approximated as a
parallel-plate capacitor, the impedance is affected by the amount
and type of liquid located in the gap. This technique affords real-
time pico-liter liquid volume sensing resolution18 on the EWOD
chips used in this paper, effectively eliminating priming error due
to liquid detection. A comprehensive theoretical model of the
relationship between liquid volume (and other parameters) and
VR is also described in ref. 18.
To sense the liquid, the loading electrode is actuated normally
and VR is measured by the DAQ module continuously. Using the
phase information from Vref
, Im{VR} is determined and
Actuation of droplets
compared to a threshold value to detect the presence or absence
of liquid at the electrode site. Threshold detection and control of
the syringe pump were integrated into a custom LabView
program. The details on the selection of threshold value are
discussed in the results. It should be noted that most of the
A 10 kHz, 0.7 Vrms signal, Vref, was generated with a data
acquisition (DAQ) module (NI USB-6211, National Instruments,
USA) and amplified with a custom-built amplifier to y100 Vrms
.
The 10 kHz, 100 Vrms signal was used to activate EWOD
3332 | Lab Chip, 2012, 12, 3331–3340
This journal is ß The Royal Society of Chemistry 2012