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On-demand droplet dispensing systems are indispensable tools in bioanalytical fields, such as microarray fabrication. Biomaterial solutions can be very limited and expensive, so minimizing the use of solution per spot produced is highly desirable. Here, we proposed a novel droplet dispensing method which utilizes a gigahertz (GHz) acoustic resonator to deposit well-defined droplets on-demand. This ultra-high frequency acoustic resonator induces a highly localized and strong body force at the solid–liquid interface, which pushes the liquid to generate a stable and sharp “liquid needle” and further delivers droplets to the target substrate surface by transient contact. This approach is between contact and non-contact methods, thus avoiding some issues of traditional methods (such as nozzle clogging or satellite spots). We demonstrated the feasibility of this approach by fabricating high quality DNA and protein microarrays on glass and flexible substrates. Notably, the spot size can be delicately controlled down to a few microns (femtoliter in volume). Because of the CMOS compatibility, we expect this technique to be readily applied to advanced biofabrication processes.
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Lab on a Chip
Cite this: DOI: 10.1039/c8lc00540k
Received 29th May 2018,
Accepted 17th July 2018
DOI: 10.1039/c8lc00540k
An on-demand femtoliter droplet dispensing
system based on a gigahertz acoustic resonator
Meihang He,
Yangchao Zhou,
Weiwei Cui,
Yang Yang,
Hongxiang Zhang,
Xuejiao Chen,
Wei Pang*
and Xuexin Duan *
On-demand droplet dispensing systems are indispensable tools in bioanalytical fields, such as microarray
fabrication. Biomaterial solutions can be very limited and expensive, so minimizing the use of solution per
spot produced is highly desirable. Here, we proposed a novel droplet dispensing method which utilizes a
gigahertz (GHz) acoustic resonator to deposit well-defined droplets on-demand. This ultra-high frequency
acoustic resonator induces a highly localized and strong body force at the solidliquid interface, which
pushes the liquid to generate a stable and sharp liquid needleand further delivers droplets to the target
substrate surface by transient contact. This approach is between contact and non-contact methods, thus
avoiding some issues of traditional methods (such as nozzle clogging or satellite spots). We demonstrated
the feasibility of this approach by fabricating high quality DNA and protein microarrays on glass and flexible
substrates. Notably, the spot size can be delicately controlled down to a few microns (femtoliter in vol-
ume). Because of the CMOS compatibility, we expect this technique to be readily applied to advanced bio-
fabrication processes.
Dispensing and manipulating micro- or nanoscale droplets
are essential in developing a highly integrated lab-on-a chip
system for bioanalytical applications.
For example, chemi-
cal regents,
and cells
are deposited on target
substrates at precise locations to form a microarray, which be-
comes an indispensable tool in drug discoveries, genomics,
proteomics, and cell analysis.
Different droplet or liquid
dispensing methods have been developed in recent years.
Generally, they can be categorized as contactand non-
contactapproaches according to whether there is a physical
contact between the device and substrate.
At present, pin
and inkjet printing
are the two major tech-
niques which represent the contact and non-contact methods,
respectively. Biomaterial solutions can be very limited and ex-
pensive, so minimizing the use of solution per droplet pro-
duced is highly desirable. The printed spot resolution of the
current system is 50 μm in diameter.
On average, about 1
nL of solution is deposited per spot. Thus, developing a
higher resolution liquid dispensing system with consumption
of fewer samples is one of the driving forces for the develop-
ment of advanced microarray fabrication techniques. In the
past decade, acoustics-based techniques have been developed
as alternative approaches for liquid handling and dispens-
In general, acoustic methods utilize acoustic radia-
tion forces induced by vibration of piezoelectric materials to
atomize liquid
or eject liquid droplets out of the airliquid
thus microarraying in a non-contact manner. A
variety of biomaterials have been successfully patterned on
different substrates with high biocompatibility.
acoustic techniques do not require any pins or nozzles, which
avoid the risks of damaging the substrate
or clogging noz-
However, they still suffer from spot uniformity issues,
such as forming satellite spots because the ejected droplets
tend to settle randomly in any given direction before landing
on the substrate.
Such a settling effect also limits the spot
resolution and may induce misalignment of the spots.
Here, in this work, we reported a new concept of an
acoustic-based on-demand droplet dispensing technique
which is between the contact and non-contact methods. It
utilises a microfabricated gigahertz (GHz) acoustic resonator
to induce a highly localized and strong body force at the
solidliquid interface, which then pushes the liquid upwards
to overcome its surface tension and generates a stable and
sharp liquid needle.
By coming into contact with the tar-
get substrate, controlled liquid droplets can be fabricated on
the target substrate through transient contact between the
liquid needle and the substrate. 3D simulation is applied to
Lab ChipThis journal is © The Royal Society of Chemistry 2018
State Key Laboratory of Precision Measuring Technology & Instruments, Tianjin
University, Tianjin 300072, China. E-mail:
College of Precision Instrument and Optoelectronics Engineering, Tianjin
University, Tianjin 300072, China. E-mail:
Electronic supplementary information (ESI) available: Device fabrication pro-
cess and microarray printing process. See DOI: 10.1039/c8lc00540k
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theoretically understand the formation of the liquid needle.
The performance of the spotting system was demonstrated by
printing DNA and protein microarrays on glass and flexible
substrates. Notably, the spot size of the printed droplets can
be well modulated from several microns to hundreds of
microns, corresponding to liquid volumes from femtoliters to
nanoliters, which shows one order of magnitude smaller than
those of current commercial methods. The liquid needle is
formed in an open well and is always vertical to the device,
thus, there is no concern about clogging or droplet misdirec-
tion issues. Besides, since resonators are fabricated with a
CMOS compatible process, they can be easily integrated as a
multiplexed spotting system for high throughput large-scale
fabrication processes.
Materials and methods
Cy3-labelled goat anti-human IgG was obtained from Biosyn-
thesis Biotechnology Co., Ltd. (Beijing, China). The fluores-
cein isothiocyanate (FITC) modified oligonucleotide: 5FITC-
CAG TCG CGA GTA-3was obtained from Sangon Biotech Co.,
Ltd. (Shanghai, China). TrichloroIJ1H,1H,2H,2H-perfluorooctyl)-
silane was obtained from Energy Chemical (Shanghai, China).
The polydimethylsiloxane (PDMS) precursor and curing agent
(Sylgard 184) were obtained from Dow Corning. Poly-L-lysine
(PLL) was obtained from Sigma-Aldrich (USA).
Setup construction
Fig. 1A illustrates the setup of the automatic droplet dispens-
ing system. The acoustic resonator is excited using a radio
frequency signal (RFS) source (Keysight, N5171B) combined
with a power amplifier (Mini-Circuits, ZHL-5W-422+). A reser-
voir connected with the microchannel made from PDMS was
fixed on top of the resonator to store and supply liquid sam-
ples. The target substrate is located above the sample reser-
voir which is precisely controlled with an XYZ precision posi-
tioning stage. To observe and characterize the liquid
dispensing dynamics, a camera with a speed of 240 fps was
utilized to record the dispensing process in real time. A
custom-developed LABVIEW program was used to control the
system including the XYZ stage, camera and RFS source.
The acoustic resonator is fabricated via standard MEMS
technology. The fabrication process has been reported in pre-
vious literature studies
and briefly described in the ESI
(Fig. S1 and Table S1). As shown in Fig. 1C, the resonator is
composed of a 1.1 μm aluminum nitride piezoelectric (PZ)
layer sandwiched between 0.15 μm and 0.17 μm molybdenum
as the top and bottom electrodes, respectively, a 0.2 μm sili-
con dioxide passivation layer on the surface, and multilayers
of alternately deposited molybdenum (0.64 μm) and silicon
dioxide (0.65 μm) serving as Bragg reflectors at the bottom of
the sandwich structure. Fig. 1B is the top view of the resona-
tor, where the square region is the Bragg reflectors, the mid-
dle pink electrode is the top electrode and the electrodes on
both sides are the bottom electrodes. The area where the top
and bottom electrodes overlap, i.e. the pentagon, is the reso-
nant region. Fig. 1D presents an assembled printing head.
Pretreatment of glass slides and the PDMS substrate
Microscope slides were firstly activated with O
plasma under
120 W for 5 minutes, and then placed in a desiccator together
with an open vial containing several droplets of
trichloroIJ1H,1H,2H,2H-perfluorooctyl)silane. The desiccator
was evacuated and left overnight. Finally, the glass slides were
taken out and baked at 85 °C for 0.5 h. The PDMS substrate
was soaked for 0.5 h in HEPES buffer with 3% PLL, rinsed in
ultra-pure water, and dried with a stream of nitrogen.
Results and discussion
Principle and theory
The resonator is activated via an inverse piezoelectric effect,
which can generate vertically propagated acoustic waves in
thickness-extension mode. The resonator is designed to be
pentagonal in shape so that the transverse spurious modes
can be further eliminated. To investigate the acoustic field in-
duced by the resonator, we carried out simulations using
multi-physics finite element analysis software (COMSOL 5.0).
The displacement magnitude of the resonator vibration U=
(x,y,z,t) is as follows:
where c
represents the speed of the thickness-extensional
acoustic waves in the piezoelectric layer of the resonator. As
the displacement magnitude of the vibration at the boundary
of the resonant region is zero, the boundary condition can be
expressed as:
At Boundary
= 0 (2)
Fig. 1 (A) Schematic illustration of the automatic droplet dispensing
system. (B) Top view of a fabricated acoustic resonator. (C) Schematic
sectional view of section AA'. (D) The assembled dispenser head.
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At the resonatorair interface (z= 0), the displacement
magnitude of the vibration is as follows:
where uIJx,y) is the vibration displacement magnitude at the
resonator surface. Substituting eqn (3) into eqn (1) and eqn
(2), the numerical solution of uIJx,y) can be obtained, as
shown in Fig. 2A. It is shown that the intensity of the acous-
tic field reaches the maximum at the center and sharply de-
creases around the center, which indicates that the acoustic
energy is highly concentrated at the center of the resonant
area. It is notable that the acoustic energy can be precisely
tuned by adjusting the amplitude of the RFS without chang-
ing its distribution.
When the acoustic field interacts with liquid, the gener-
ated acoustic waves will propagate into the liquid and attenu-
ate within a short distance. The coefficient of the attenuation
is given by
where ωis the angular frequency of the acoustic waves, ρis
the liquid density, c
is the sound speed in the liquid, and
B, where μand μ
are the viscosity and bulk viscos-
ity of the fluid, respectively. As the attenuation coefficient
scales with the square of acoustic frequency, the GHz acous-
tic beam generated by the resonator attenuates within a very
short distance.
Due to the nonlinear attenuation of the propagating
acoustic beam, a finite time-averaged momentum flux is gen-
erated, exerting a body force (F
) on the liquid to drive a di-
rected fluid motion along the propagation path of the acous-
tic waves.
The body force induced by the acoustic waves is
expressed as:
The induced body force decreases exponentially along the
zdirection. As for an acoustic frequency of 2.45 GHz, Fig. 2B
presents the maximum body force as a function of the dis-
tance from the resonatorliquid interface, revealing an enor-
mous body force on the scale of 10
generated within
a very short distance. The decay length, which is defined as
the distance where the body force magnitude attenuates to
1/eof its initial value, is 1.36 μm. In the (x,y) plane, since the
body force also scales with the square of uIJx,y), the magni-
tude of the body force has a similar distribution to the vibra-
tion field of the resonator, i.e., the maximum value is located
at the center of the resonator and decreases sharply towards
the edges. The inset in Fig. 2B shows the calculated body
force magnitude distribution of the plane 1 μm above the res-
onatorliquid interface. A locally concentrated body force is
formed without the aid of any additional acoustic focus strat-
egy. Fig. 2B also reveals that the size of the body force distri-
bution region is determined by the area of the resonator.
Moreover, as the body force scales with ω
, a higher acoustic
wave frequency provides a more efficient approach to gener-
ate great body force instead of applying high acoustic power.
Under the effect of the highly center-concentrated body
force, the liquid is pushed upward, producing similarly
Fig. 2 (A) 3-D simulation result of the acoustic field generated by the GHz acoustic resonator, wherein the pentagon indicates the resonant region
of the resonator. (B) Calculated maximum body force magnitude along the zdirection. Inset: The planar distribution of the body force magnitude
of the plane 1 μm above the resonatorliquid interface. (C) Liquid needles generated by resonators with different sizes, where the area of the reso-
nant region of the device is 0.4k μm
and 5k μm
, respectively. Scale bar is 100 μm.
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center-concentrated momentum. From the perspective of en-
ergy, the coupled acoustic energy converts into kinetic energy
of the fluid and thermal energy. The locally confined body
force drives the liquid towards the liquidair interface,
whereas the liquid surface tension and gravity are directed to
the opposite direction. Consequently, a sharp liquid needle
is stably formed. Fig. 2C presents the liquid needles gener-
ated by resonators with different sizes (i.e., the area of the
resonant region). The resonator with a larger size can gener-
ate a larger liquid needle, primarily because the body force
distribution region is determined by the area of the resona-
tor. Therefore, in addition to RFS power, the size of the reso-
nator is another alternative variable for effective control of
the liquid needle size.
Droplet dispensing process
When the tip of the liquid needle comes into contact with
the target substrate, the latter will be wetted due to the inter-
molecular attraction forces between these two phases. When
the RFS is turned off, the liquid needle will retract to the res-
ervoir immediately under the effect of surface tension and
gravity. Consequently, the needle will be cut off and a droplet
will be deposited on the substrate due to the surface tension
between the liquid needle and the substrate. Hence, in our
system, the acoustic resonator works intermittently. The dis-
pensing process is presented by a sequence of images cap-
tured using a high speed camera (Fig. 3). The images show
that once being actuated by the acoustic resonator, the liquid
surface deforms into a needle shape and grows high enough
to come into contact with the substrate within a very short
time (17 ms). It is notable that the shape of the liquid nee-
dle fits well with the distribution of the acoustics-induced
body force presented in Fig. 2B. The liquid needle finally re-
mains very stable during its contact with the substrate which
is very important to fabricate uniform droplets. After turning
off the RFS, the liquid needle disappeared immediately, and
the residual liquid that was left on the substrate formed a
droplet. The whole process of liquid dispensing is completed
in a very short time (33 ms), enabling a fast and high effi-
ciency droplet dispensing system.
During the deposition process, the device has no physical
contact with the substrate, while the liquid needle is directly in
contact with the substrate, thus this method is between contact
and non-contact printing. Compared to conventional microar-
ray fabrication methods which utilize pins or nozzles to transfer
liquid samples, this acoustic method utilizes a liquid needle as
a virtual pin to deposit droplets from an open source well.
Meanwhile, the virtual pin is flexible to installor dismantle,
thus eliminating concerns such as nozzle clogging, being
hard to rinse, and being prone to damage due to pins.
Spot size control
For microarray fabrication, it is of great importance to gener-
ate uniform and size-controllable spots. In our system, the
size of the printed spot can be well controlled by different
factors, including the hydrophobicity of the substrate, device
size, RFS power and RFS duration. Here, we focused on the
physical effects that determine the spot size. For a given dis-
tance between the substrate and the liquid samples, the
acoustic power is optimized and set at a fixed value (200 to
500 mw). We investigated the influence of the device size and
the RFS duration on the spot size. For this purpose, Cy3 con-
jugated goat anti-human IgG solution was deposited onto the
glass slide surface as an example by using three different de-
vices (resonator sizes ranging from 0.4k μm
to 10k μm
Meanwhile, the RFS duration time was varied from 30 ms to
150 ms with an interval of 20 ms. The printing results are
presented and summarized in Fig. 4. It clearly shows that
both the device size and RFS working time have a profound
effect on the printed spot size. The spot size is proportional
to the device size. And for a fixed device, the spot size shows
a linear relationship to the RFS duration (Fig. 4D). These re-
sults are consistent with the theoretical analysis. In sum-
mary, the spot size can be delicately modulated by adjusting
the device size and RFS duration.
Viscosity effect
In addition, we also investigated the dispensing of viscous
liquid using the acoustic device. In principle, viscous liquid
can be deposited with a higher RFS power, which provides a
larger body force. However, the maximum power that the cur-
rent device can bear is around 1.5 W, thus it has a limitation
on handling highly viscous liquid. Glycerol was mixed with
ultra-pure water at different ratios to achieve liquids with dif-
ferent viscosities. The printing results are shown in Fig. S2.
It clearly shows that the acoustic device can handle liquids
with a wide range of viscosity and liquid droplets with a vis-
cosity as high as 15.2 mPa s were successfully printed.
Biomaterial printing and patterning
Before printing, the glass slides were treated to become hydro-
phobic for better adsorption of DNA and protein.
FITC la-
belled DNA and cy3 conjugated goat anti-human IgG solution
Fig. 3 Droplet dispensing process captured using a CMOS camera,
where the time interval between two adjacent images is 4.1 ms.
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were respectively diluted in HEPES buffer at concentrations of
10 μM and 0.1 μM. 5% (v/v) glycerol was added to improve the
spot morphology.
We fabricated 4 ×4 DNA and protein
microarrays with a spacing of 250 μm×250 μm on the glass
slide in a series of spot diameters (Fig. 5, Video S1). Since
the system works in an intermittent manner, the temperature
of the liquid sample is below 26 °C during the printing pro-
cess, which is acceptable for most biomaterials. To prevent
evaporation of the dispensed droplets, the humidity was kept
at 50%. The spot diameter of the four 4 ×4 microarrays in
Fig. 5 is 8 μm, 60 μm, 120 μm and 180 μm, respectively. It is
notable that the smallest spot we have achieved so far is only
8μm in diameter, corresponding to a droplet volume on the
femtoliter scale. Therefore, the system has great potential in
making microarrays with very high spot density, which is very
important for high-throughput applications.
Accurate quantitative analysis of the printed microarrays
is only possible if spot uniformity (i.e., spot-to-spot size and
shape repeatability) and positional accuracy are achieved.
As the step motor of the precision positioning stage has a
precision of 5 μminXYZ directions, the droplets can be
spaced out and arrayed accurately. In the pin printing pro-
cess, since a pin can only carry a limited amount of liquid at
a time, repetitive reloading of samples is required during the
printing process, making the pin-generated spots grow
smaller in size after each transfer. This is not an issue for the
method we present here since the liquid sample is supplied
in real time. For reported acoustic jet printing, satellite drop-
lets are usually produced due to the difficulty in precisely con-
trolling the settling position of the ejected droplets.
misalignment due to the droplet settling effect is not an issue
in our system since the liquid needle is well aligned vertically
to the device and is directly in contact with the targeted sub-
strate. To accurately characterize the spot uniformity, we
conducted fluorescence intensity (FI) analysis through a
MATLAB program, and the FI analysis results confirm the
high uniformity of the fabricated microarrays (Fig. 5).
In addition to uniformity, morphology is another impor-
tant issue for the array technology. Since most microarray
fabrication methods rely on the dispensing liquid, the perfect
spot shape is round. In fact, some reported results suffer
from spot morphology issues, e.g. the coffee ringeffect,
where the sample concentration is greater at the outer circle
than in the middle,
or bullseyespots, where the sample
concentration is highest in the center.
These defects greatly
affect the analysis accuracy. In our experiment, none of the
coffee ringor bullseyespots appear, which is due to the
hydrophobic treatment of the substrate surface.
Besides DNA and protein arrays, we also tested the dis-
pensing of living cells in a droplet array with the acoustic de-
vice. It shows that cells can be successfully printed on the tar-
get substrate with retained bioactivity (Fig. S3). These
results indicate that the system is capable of dispensing a va-
riety of different biomaterials with high biocompatibility.
Fig. 4 IJA)(C) Variable-size spots dispensed by using resonators of different sizes and RFS duration periods. The spots grouped in A, B, and C were
dispensed by resonators with areas of 400 μm
, 2000 μm
, and 10 000 μm
, respectively. For every group, the RFS duration of each spot varies
from 30 ms to 150 ms with an interval of 20 ms. (D) Summarized relationship of spot diameter versus RFS duration and resonator size.
Fig. 5 Fluorescence images of 4 ×4 DNA (A) and (B) and protein (goat
anti-human IgG) microarrays (C) and (D). The fluorescence intensity anal-
ysis was acquired along the white dashed line, which gave quantification
of the printed spot size and uniformity. Scale bar in the inset is 5 μm.
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Printing on a flexible substrate
A recent trend in micro/nanotechnology is developing flexible
electronics and analytical devices, which requires technolo-
gies to pattern materials on flexible substrates.
methylsiloxane (PDMS) is a type of mouldable silicone rub-
ber, which has attracted growing attention over the past
decade as a flexible material for microarray applications or in
situ synthesis of biomaterials (e.g. through soft lithogra-
because of its intrinsic hydrophobicity, transpar-
ency, flexibility and biocompatibility. Here, we demonstrated
the direct spotting of large-scale protein arrays on a flexible
PDMS substrate. The PDMS substrate was first coated with
PLL by electrostatic adsorption to improve the affinity with
the protein which facilitates the spotting.
A protein micro-
array comprising 200 spots (20 ×10) was then successfully
deposited on the PDMS substrate with the acoustic system
(Fig. 6A). The spot size is 110 μm and separated by 40 μm.
The coefficient of variation (CV) of the 10 ×20 microarray is
2.93% which is likely due to the flexible nature of the sub-
strate. In addition, we printed TJUcharacters on this flexi-
ble substrate using writing ink as another sample (Fig. 6B).
The result indicates that our system is robust for large-scale
fabrication and proves the ability of the system to print dif-
ferent patterns on different substrates.
In this work, we developed a novel droplet dispensing system
based on a MEMS acoustic resonator with a resonant fre-
quency as high as 2.45 GHz. A controlled liquid needle is gen-
erated at the liquidair interface through strong body force in-
duced by the acoustic device. By controlling the time duration
of the liquid needle which is in contact with the target sub-
strate, different size high-quality DNA and protein arrays are
successfully patterned on different substrates. Due to its dis-
tinct working manner, the system avoids some drawbacks of
conventional methods (e.g. clogging or satellite spots). The
size of the printed spot as well as the volume dispensed can
be well controlled down to the micrometer scale (femtoliter
in volume). Considering that the acoustic devices are CMOS
compatible, they can be further minimized and integrated,
enabling dispensing liquid samples with ultra-small volume,
as well as fabricating multiplexed and large-scale patterns.
Since this acoustic-based approach is non-invasive, such a
technique can be applied to a variety of different materials in-
cluding inorganic, organic, and biological inks for generating
analytical bio-chips and has great potential in developing
other interesting droplet-based applications.
Conflicts of interest
There are no conflicts of interest to declare.
The authors gratefully acknowledge financial support from the
National Natural Science Foundation of China (NSFC No.
61674114, 91743110, 21861132001), the National Key R&D Pro-
gram of China (2017YFF0204600), and the 111 Project (B07014).
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Lab on a Chip Paper
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Carbon nanotube (CNT)-based chemiresistors are promising gas detectors for gas chromatography (GC) due to their intrinsic nanoscale porosity and excellent electrical conductivity. However, fabrication reproducibility, long desorption time, limited sensitivity, and low dynamic range limit their usage in real applications. This paper reports a novel on-chip monolithic integrated multimode CNT sensor, where a micro-electro-mechanical system-based bulk acoustic wave (BAW) resonator is embedded underneath a CNT chemiresistor. The device fabrication repeatability was improved by on-site monitoring of CNT deposition using BAW. We found that the acoustic stimulation can accelerate the gas desorption rate from the CNT surface, which solves the slow desorption issue. Due to the different sensing mechanisms, the multimode CNT sensor provides complementary responses to targets with improved sensitivity and dynamic range compared to a single mode detector. A prototype of a chromatographic system using the multimode CNT sensor was prepared by dedicated design of the connection between the device and the separation column. Such a GC system is used for the quantitative identification of a gas mixture at different GC conditions, which proves the feasibility of the multimode CNT detector for chromatographic analysis. The as-developed CMOS compatible multimode CNT sensor offers high sensing performance, miniaturized size, and low power consumption, which are critical for developing portable GC.
Conference Paper
In this study, an acoustofluidic printing system for generation of single-cell droplets based on a gigahertz acoustic resonator was proposed and verified. The working area of the resonator has a typical dimension of 10×10 micrometer which is very suitable for single cell printing. Single cells were encapsulated in picoliter droplets and printed directly to a flat substrate without any significant influence on their viability. By combining an optic feed-back loop, a 100% single-cell encapsulation rate is achieved.Clinical Relevance- This acoustic-based system has good biocompatibility and high encapsulation rate, which expands the mechanism of medical and biology studies.
Conference Paper
In this study, an acoustofluidic based wireless micropump for drug delivery was proposed and fabricated. The key actuator of this micropump is a small gigahertz piezoelectric resonator, which could induce strong fluidic streaming at low applied power. This acoustofluidic micropump has stable and accurate dosage resolution (7.0 μL), and sufficient flow rate (1.34 mL/min). The miniaturized size and wireless controlled operation prove it as a portable drug delivery system.Clinical Relevance- The acoustofluidic based micropump could apply for the drug administration in a safe, effective and stable form. It has potential to integrate with miniaturized sensors and electronic circuit to form portable drug delivery systems, realizing smart on-demand drug delivery.
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Recent advances in nano/microfluidics have led to the miniaturization of surface-based chemical and biochemical sensors, with applications ranging from environmental monitoring to disease diagnostics. These systems rely on the detection of analytes flowing in a liquid sample, by exploiting their innate nature to react with specific receptors immobilized on the microchannel walls. The efficiency of these systems is defined by the cumulative effect of analyte detection speed, sensitivity, and specificity. In this perspective, we provide a fresh outlook on the use of important parameters obtained from well-characterized analytical models, by connecting the mass transport and reaction limits with the experimentally attainable limits of analyte detection efficiency. Specifically, we breakdown when and how the operational (e.g., flow rates, channel geometries, mode of detection, etc.) and molecular (e.g., receptor affinity and functionality) variables can be tailored to enhance the analyte detection time, analytical specificity, and sensitivity of the system (i.e., limit of detection). Finally, we present a simple yet cohesive blueprint for the development of high-efficiency surface-based microfluidic sensors for rapid, sensitive, and specific detection of chemical and biochemical analytes, pertinent to a variety of applications.
This paper investigates the mechanism of a new acoustic micro-ejector using a Lamb wave transducer array, which can stably generate picoliter (pL) droplet jetting without nozzles. With eight transducers arranged as an octagon array, droplets are ejected based on the mechanism of combined acoustic pressure waves and acoustic streaming. The acoustic focusing area is designed as a line at the liquid center, which is the key factor for a large working range of liquid height. The experimental results show that the ejector can produce uniform water droplets of 22 μm diameter (5.6 pL in volume) continuously at a rate of 0.33 kHz with high ejection stability, owing to a large liquid height window and high acoustic wave frequency. By delivering precise ∼pL droplets without clogging issues, the acoustic ejector has great potential for demanding biochemical applications.
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Controllable manipulation and effective mixing of fluids and colloids at the nanoscale is made exceptionally difficult by the dominance of surface and viscous forces. The use of megahertz (MHz)‐order vibration has dramatically expanded in microfluidics, enabling fluid manipulation, atomization, and microscale particle and cell separation. Even more powerful results are found at the nanoscale, with the key discovery of new regimes of acoustic wave interaction with 200 fL droplets of deionized water. It is shown that 40 MHz‐order surface acoustic waves can manipulate such droplets within fully transparent, high‐aspect ratio, 100 nm tall, 20–130 micron wide, 5‐mm long nanoslit channels. By forming traps as locally widened regions along such a channel, individual fluid droplets may be propelled from one trap to the next, split between them, mixed, and merged. A simple theory is provided to describe the mechanisms of droplet transport and splitting.
Microdroplets and their dispersion, with a large specific surface area and a short diffusion distance, have been applied in various unit operations and reaction processes. However, it is still a challenge to control the size and size distribution of microdroplets, especially for high-throughput generation. In this work, a novel ultra-high speed rotating packed bed (UHS-RPB) was invented, in which rotating foam packing with a speed of 4000–12000 r·min–1 provides microfluidic channels to disperse liquid into microdroplets with high throughput. Then generated microdroplets can be directly dispersed into a continuous falling film for obtaining a mixture of microdroplet dispersion. In this UHS-RPB, the effects of rotational speed, liquid initial velocity, liquid viscosity, liquid surface tension and packing pore size on the average size (d32) and size distribution of microdroplets were systematically investigated. Results showed that the UHS-RPB could produce microdroplets with a d32 of 25–63 μm at a liquid flow rate of 1025 L·h–1, and the size distribution of the microdroplets accords well with Rosin–Rammler distribution model. In addition, a correlation was established for the prediction of d32, and the predicted d32 was in good agreement with the experimental data with a deviation within ±15%. These results demonstrated that UHS-RPB could be a promising candidate for controllable preparation of uniform microdroplets.
The development of rapid and efficient tools to modulate neurons is vital for the treatment of nervous system diseases. Here, a novel non-invasive neurite outgrowth modulation method based on a controllable acoustic streaming effect induced by an electromechanical gigahertz resonator microchip is reported. The results demonstrate that the gigahertz acoustic streaming can induce cell structure changes within a 10 min period of stimulation, which promotes a high proportion of neurite bearing cells and encourages longer neurite outgrowth. Specifically, the resonator stimulation not only promotes outgrowth of neurites, but also can be combined with chemical mediated methods to accelerate the direct entry of nerve growth factor (NGF) into cells, resulting in higher modulation efficacy. Owing to shear stress caused by the acoustic streaming effect, the resonator microchip mediates stress fiber formation and induces the neuron-like phenotype of PC12 cells. We suggest that this method may potentially be applied to precise single-cell modulation, as well as in the development of non-invasive and rapid disease treatment strategies.
Valve-controlled pneumatic jetting technology, which is characterized by high accuracy, fast dispensing speed, and strong adaptability, has been widely applied in the field of life sciences. This paper examines the formation and separation of droplets by reporting the comprehensive influence of system parameters on the morphological changes of droplets, which was simulated by using several dimensionless numbers, including the Weber number, Reynolds number, and Ohnesorge number. In the next step, the simulation results are verified through experimental results, and the rules of the regular ejection of droplets are summarized. The simulation and experimental results indicate that if We is higher than the upper critical value or lower than the lower critical value, jetting failure situations, such as adhesion, satellite droplets, sputtering, and flow-stream will occur. Similarly, jetting failures such as adhesion or flow-stream will appear with oversize or undersize Reynolds number. Through Oh, only when the system parameters should be adjusted to keep the values of We and Re within the proposed range can normal droplets be jetted. The research also provides the recommended ranges of system parameters for ensuring successful droplet ejection, which has important reference significance for guiding the design and control of valve-controlled pneumatic jetting systems.
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Microarrays of proteins and peptides make it possible the screening of thousands of binding events in a parallel and high throughput fashion; therefore they are emerging as a powerful tool for proteomics and clinical assays. The complex nature of Proteome, the wide dynamic range of protein concentration in real samples and the critical role of immobilized protein orientation must be taken into account to maximize the utility of protein microarrays. Immobilization strategy and designing of an ideal local chemical environment on the solid surface are both essential for the success of a protein microarray experiment. This review article will focus on protein and peptide arrays highlighting their technical challenges and presenting new directions by means of a set of selected recent applications.
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Microarrays with biomolecules (e.g., DNA and proteins), cells, and tissues immobilized on solid substrates are important tools for biological research, including genomics, proteomics, and cell analysis. In this paper, the current state of microarray fabrication is reviewed. According to spot formation techniques, methods are categorized as "contact printing" and "non-contact printing." Contact printing is a widely used technology, comprising methods such as contact pin printing and microstamping. These methods have many advantages, including reproducibility of printed spots and facile maintenance, as well as drawbacks, including low-throughput fabrication of arrays. Non-contact printing techniques are newer and more varied, comprising photochemistry-based methods, laser writing, electrospray deposition, and inkjet technologies. These technologies emerged from other applications and have the potential to increase microarray fabrication throughput; however, there are several challenges in applying them to microarray fabrication, including interference from satellite drops and biomolecule denaturization.
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Fluorescent detection of calcium mobilization has been used successfully to identify modulators of G-protein-coupled receptors (GPCRs); however, inherent issues with fluorescence may limit its potential for high-throughput screening miniaturization. The data presented here demonstrate that the calcium-sensitive photoprotein aequorin (AequoScreen), when compared with FLUO-4 in the same cellular background, allows for miniaturization of functional kinetic calcium flux assays, in which the rank order of potency and efficacy was maintained for a series of diverse small-molecule modulators. Small-volume (<10 microL) 384- and 1536-well aequorin assays were implemented by integration of acoustic dispensing (Echo 550) and kinetic flash luminometry (CyBi Lumax). The enhanced high signal-to-background ratios observed relative to fluorescence were readily manipulated by altering per-well cell densities and yielded acceptable screening statistics in miniaturized format for both agonist and antagonist screening scenarios. In addition, the authors demonstrate the feasibility of using agonist concentrations less than EC(50) in a miniaturized antagonist assay. These features, coupled with improved sample handling, should enhance sensitivity and provide the benefits of miniaturization including cost reduction and throughput gains.
Despite advances in breast cancer prevention and treatment, variability in patient-response has revealed the need for a more "personalized" approach to medicine, in which treatments are tailored to each patient's biology. Motivated by this idea, we introduce a technique that allows for quantification of small-molecule analytes directly from core needle biopsy (CNB) tissue samples on a miniaturized platform. The new technique, powered by digital microfluidics, integrates tissue-liquid extraction and magnetic bead-based competitive immunoassay for quantification of estradiol in milligram-sized CNB samples. Each measurement (from start to finish) requires ∼40 minutes, a duration consistent with a visit to a doctor's office. The performance of the new technique was validated by the gold-standard analysis method (high performance liquid chromatography coupled to tandem mass spectrometry), and was applied to evaluate human patient samples before and after a course of treatment with aromatase inhibitor therapy. We propose that the new technique has great potential for eventual use for fast, automated, and quantitative analysis of biomarkers in tissue samples, towards a personalized medicine approach.
The physical origin of the radiation force exerted by an ultrasonic beam on an absorbing target is explained. It is shown that the force may have two sources-a nonzero time-averaged sound pressure in the ultrasonic beam, and the momentum transported by the beam-but that there is a movement of material out of the ultrasonic beam to prevent a nonzero time-averaged sound pressure from being established. Consequently, the transfer of wave momentum is the sole cause of the force. The assumptions made in evolving this explanation are discussed, and the resulting change in mean density within the ultrasonic beam is considered. It is pointed out that the acoustic radiation force is but one example of a universal phenomenon associated with all forms of wave motion.
We report the use of focused acoustic beams to eject discrete droplets of controlled diameter and velocity from a free‐liquid surface. No nozzles are involved. Droplet formation has been experimentally demonstrated over the frequency range of 5–300 MHz, with corresponding droplet diameters from 300 to 5 μm. The physics of droplet formation is essentially unchanged over this frequency range. For acoustic focusing elements having similar geometries, droplet diameter has been found to scale inversely with the acoustic frequency. A simple model is used to obtain analytical expressions for the key parameters of droplet formation and their scaling with acoustic frequency. Also reported is a more detailed theory which includes the linear propagation of the focused acoustic wave, the coupling of the acoustic fields to the initial surface velocity potential, and the subsequent dynamics of droplet formation. This latter phase is modeled numerically as an incompressible, irrotational process using a boundary integral vortex method. For simulations at 5 MHz, this numerical model is very successful in predicting the key features of droplet formation.
Manipulation of microscale particles and fluid liquid droplets is an important task for lab-on-a-chip devices for numerous biological researches and applications, such as cell detection and tissue engineering. Particle manipulation techniques based on surface acoustic waves (SAWs) appear effective for lab-on-a-chip devices because they are non-invasive, compatible with soft lithography micromachining, have high energy density, and work for nearly any type of microscale particles. Here we review the most recent research and development of the past two years in SAW based particle and liquid droplet manipulation for lab-on-a-chip devices including particle focusing and separation, particle alignment and patterning, particle directing, and liquid droplet delivery.
Arrays of circular spots of glucose oxidase have been obtained on functionalized silicon oxide by piezoelectric inkjet printing and the enzymatic activity toward glucose recognition has been monitored. The addition of glycerol to the molecular ink allows to obtain high spot definition and resolution (tens of micrometers wide; one molecule tall), but in spite of its well-known structural stabilizing properties, in dynamic conditions it may lead to increased protein stresses. The jetting voltage and pulse length have been found to be critical factors for both activity retention and pattern definition. High voltages and pulse lengths results in stress effects along with the loss of activity, which, at least in our experimental conditions, has been found to be recovered in time.
Recent developments in the rapid sequencing, mapping, and analysis of DNA rely on the specific binding of DNA to specially treated surfaces. We show here that specific binding of DNA via its unmodified extremities can be achieved on a great variety of surfaces by a judicious choice of the pH. On hydrophobic surfaces the best binding efficiency is reached at a pH of approximately 5.5. At that pH a approximately 40-kbp DNA is 10 times more likely to bind by an extremity than by a midsegment. A model is proposed to account for the differential adsorption of the molecule extremities and midsection as a function of pH. The pH-dependent specific binding can be used to align anchored DNA molecules by a receding meniscus, a process called molecular combing. The resulting properties of the combed molecules will be discussed.
This paper presents the theory of operation, fabrication, and experimental results obtained with a new acoustically actuated two-dimensional (2-D) micromachined microdroplet ejector array. Direct droplet based deposition of chemicals used in IC manufacturing such as photoresist and other spin-on materials, low-k and high-k dielectrics by ejector arrays is demonstrated to reduce waste contributing to environmentally benign fabrication and lower production cost. These ejectors are chemically compatible with the materials used in IC manufacturing and do not harm fluids that are heat or pressure sensitive. A focused acoustic beam overcomes the surface tension and releases droplets in air in every actuation cycle. The ejectors were operated most efficiently at 34.7 MHz and generated 28mum diameter droplets in drop-on-demand and continuous modes of operation as predicted by the finite element analysis (FEA). Photoresist, water, isopropanol, ethyl alcohol, and acetone were ejected from a 4times4 2-D micromachined ejector array. Single photoresist droplets were printed onto a silicon wafer by drop-on-demand and continuous modes of operation. Parallel photoresist lines were drawn and a 4-in wafer was coated by Shipley 3612 photoresist by using acoustically actuated 2-D micromachined microdroplet ejector arrays