ArticlePDF Available

Acoustically Triggered Disassembly of Multilayered Polyelectrolyte Thin Films through Gigahertz Resonators for Controlled Drug Release Applications

Article

Acoustically Triggered Disassembly of Multilayered Polyelectrolyte Thin Films through Gigahertz Resonators for Controlled Drug Release Applications

Abstract

Controlled drug release has a high priority for the development of modern medicine and biochemistry. To develop a versatile method for controlled release, a miniaturized acoustic gigahertz (GHz) resonator is designed and fabricated which can transfer electric supply to mechanical vibrations. By contacting with liquid, the GHz resonator directly excites streaming flows and induces physical shear stress to tear the multilayered polyelectrolyte (PET) thin films. Due to the ultra-high working frequency, the shear stress is greatly intensified, which results in a controlled disassembling of the PET thin films. This technique is demonstrated as an effective method to trigger and control the drug release. Both theory analysis and controlled release experiments prove the thin film destruction and the drug release..
micromachines
Article
Acoustically Triggered Disassembly of Multilayered
Polyelectrolyte Thin Films through Gigahertz
Resonators for Controlled Drug Release Applications
Zhixin Zhang , Zifan Tang , Wenpeng Liu, Hongxiang Zhang, Yao Lu, Yanyan Wang, Wei Pang,
Hao Zhang and Xuexin Duan *
State Key Laboratory of Precision Measuring Technology & Instruments, Tianjin University, Tianjin 300072,
China; zxzhang10@tju.edu.cn (Z.Z.); zifantang@tju.edu.cn (Z.T.); liuwenpeng@tju.edu.cn (W.L.);
tjzhx@tju.edu.cn (H.Z.); luyao@tju.edu.cn (Y.L.); yanyanwang@tju.edu.cn (Y.W.); weipang@tju.edu.cn (W.P.);
haozhang@tju.edu.cn (H.Z.)
*Correspondence: xduan@tju.edu.cn; Tel.: +86-22-2740-1002
The authors contributed equally to this work.
Academic Editors: Renshi Sawada and Nam-Trung Nguyen
Received: 15 September 2016; Accepted: 19 October 2016; Published: 1 November 2016
Abstract:
Controlled drug release has a high priority for the development of modern medicine
and biochemistry. To develop a versatile method for controlled release, a miniaturized acoustic
gigahertz (GHz) resonator is designed and fabricated which can transfer electric supply to mechanical
vibrations. By contacting with liquid, the GHz resonator directly excites streaming flows and induces
physical shear stress to tear the multilayered polyelectrolyte (PET) thin films. Due to the ultra-high
working frequency, the shear stress is greatly intensified, which results in a controlled disassembling
of the PET thin films. This technique is demonstrated as an effective method to trigger and control the
drug release. Both theory analysis and controlled release experiments prove the thin film destruction
and the drug release.
Keywords: controlled drug release; layer-by-layer (LbL); bulk acoustic resonator; MEMS
1. Introduction
Every drug molecule has a therapeutic window in terms of the concentration above which the drug
is toxic and below which it is ineffective [
1
]. Developing controlled drug release techniques to precisely
tune the dosage of drugs is a critical issue in modern medical and biotechnology research [
2
4
]. Many
research interests are focused on developing a system based on molecular self-assembly approaches as
novel drug carriers to realize controlled drug release [
5
,
6
], where the drug molecule is slowly released
over time. Among these dedicated chemical approaches, drug carriers based on the self-assembly
of polyelectrolytes (PETs) through layer-by-layer (LbL) techniques are one of the most promising
methods, in which multilayered thin film is established by alternating the assembly of oppositely
charged PETs on surfaces or in solutions [
7
9
]. Drug molecules can be incorporated directly during
the LbL assembly process or absorbed into the nanoporous PET films after the LBL approach [
10
].
It is a very simple and reliable technique as the architectures of the thin films can be well controlled
at nanometer precision; thus, the amount of the drug molecules loaded can be precisely determined.
The drug release can be controlled by an external stimulus to trigger the disassembly of the PETs,
such as pH [
11
13
], electrochemistry [
14
16
], temperature [
17
19
], or light [
20
22
]. However, these
methods are limited by either chemical properties of the PET systems or specifications on the operating
environment. For practical applications, it is desired to develop methods which are not limited by the
chemical properties of the system or the environment, and with low power consumption.
Micromachines 2016,7, 194; doi:10.3390/mi7110194 www.mdpi.com/journal/micromachines
Micromachines 2016,7, 194 2 of 9
Owing to their small size and high precision, micro-electromechanical systems (MEMS)
technologies are thought to be a potential candidate to realize controlled release. Micro-devices
such as the microneedle [
23
25
] and micropump [
26
28
] have been developed for this purpose. In this
work, we developed a novel versatile controlled drug release method which utilizes a miniaturized
MEMS acoustic resonator to excite streaming flows and physical shear stress which acts as a tearing
force to trigger the disassembly of the multilayered PET thin films. The resonator transfers an electric
supply to the mechanical vibrations of the gigahertz (GHz), which greatly strengthens the shear stress
due to the ultra-high frequency of the vibration. The input power required is around several hundreds
of milliwatts (mW), which is a low power consumption. Additionally, owing to the miniature size of the
device, the resonator opens the possibilities of being implanted
in vivo
to realize site-targeted delivery.
2. Materials and Methods
2.1. Materials
Sodium hyaluronate (HA), Poly-L-lysine (PLL), Poly-L-lysine-FITC (PLL-FITC),
N-hydroxysulfosuccinimide sodium salt (sulfoNHS), and 1-ethyl-3-(3-dimethylaminopropyl)
carbodiimide hydrochloride (EDC) are purchased from Sigma-Aldrich (San Antonio, TX, USA).
Doxorubicin hydrochloride (DOX·HCl) is obtained from Aladdin (Shanghai, China).
2.2. Device Fabrication
The device is fabricated through standard MEMS process (shown in the Supplementary Figure S1).
The process starts from deposition of brag reflection layers on silicon substrate, and the reflectors are
composed of 1.2
µ
m AlN, 0.7
µ
m SiO
2
, 1.0
µ
m AlN, 1.3
µ
m SiO
2
, 1.0
µ
m AlN, 0.65
µ
m SiO
2
, from
bottom to up. Thin film of molybdenum (Mo) was deposited on stacked reflector substrates using RF
sputtering. After that, Mo was patterned as the bottom electrode by plasma etching. Then, AlN and
Mo were deposited in turn onto the bottom electrode. Wet etching was then used to pattern the top
electrode using the mixed etchant of nitric acid, acetic acid, and phosphoric acid. Subsequently, AlN
was etched by a combination of plasma etching and potassium hydroxide (KOH) wet etching. After
AlN etching, the bottom electrode can be accessed. Finally, Gold (Au) was evaporated and patterned
by lift-off process, serving as electrical connections and testing pads.
2.3. Polyelectrolyte Multilayers Preparation and DOX Loading
Polyelectrolyte multilayers were prepared by alternating adsorption of PLL and HA on the surface
of resonators or glass substrates from 2 mg/mL PLL to 5 mg/mL HA aqueous solutions in the presence
of 0.15 M NaCl (pH = 6.5) and at 15 min of incubation time for each layer. The total number of polymer
layers was 12 for each sample. The polymer films were coated on the whole resonator, both the
active area and the non-active area of the device. The thin films were immersed in the DOX solution
(5 mg/mL, in the presence of 0.15 M NaCl) for 12 h at 37
C. Chemical cross-linking of the multilayers
was performed by incubating the thin films in the EDC/sulfo-NHS solution (pH 6.5) overnight. The
concentration of the EDC and the sulfo-NHS was 400 mM and 50 mM, respectively.
2.4. Release System
A chamber is sealed on the top of the resonator, which was coated with multilayered
polyelectrolyte thin films. The chamber was filled with buffer solution (0.15 M NaCl aqueous solution,
typical 100
µ
L). Sinusoidal signal of 1.5 GHz (intrinsic resonant frequency of the resonator) was
generated by a signal generator (N5181A, Agilent, Santa Clara, CA, USA), amplified by a power
amplifier (ZHL-5W-422, Mini-Circuits, New York, NY, USA), and sent to the resonator, which then
transduced the electrical supply to mechanical vibrations, and further induced fluidic flow in the liquid.
Micromachines 2016,7, 194 3 of 9
2.5. Assessment of the DOX Release
Fluorescence microscope (BX53, Olympus, Center Valley, PA, USA) was used to examine
the fluorescence intensities of DOX left in the multilayers. To quantify the DOX release, the
fluorescence intensities of the buffer which dissolved the released DOX was measured by a fluorescence
spectrophotometer (Varioksan LUX, Thermal Scientific, Beaumont, TX, USA), at Ex 485 nm and Em
550 nm.
2.6. SEM Characterization of the Thin Films
To investigate the thin films disassembly, the scanning electron microscope (SEM, FEI F50,
Houston, TX, USA) was utilized to characterize the morphology of the multilayers. The films on the
surface of the device (or glass) was washed by the buffer, and then dried under nitrogen. Finally, the
carrier sample was examined with the SEM.
3. Results
3.1. Theory Consideration
It is known that when the acoustic resonator works in liquid, the vibration of the resonator will
trigger the micro-streaming in the liquid [
29
], providing a local flow condition. To investigate the fluid
behaviors triggered by the gigahertz resonator, we applied a two-dimensional finite element model
(2D FEM) to analyze the patterns and the velocities of the flow. As the flow trigger, the resonator was
modeled at the bottom of the liquid. The simulation results are shown in Figure 1. Two symmetric
micro-vortices are clearly observed under the vibration stimulus. The liquid near the edge of the
device ejects upward and returns from the periphery, forming the flow circulation. The simulated
color patterns indicate the velocity distributions in the liquid. The device located in the streaming
hinders the flow and decreases the velocity. Thus, in the micro-sized area above the device, the velocity
expresses the descended distribution from top to bottom, and approaches zero at the surface of the
device. The vertical gradient of the velocity produces shear stress, which acts as a tearing force at
the solid-liquid interface. Following Newton’s viscosity law, the shear stress is proportional to the
amplitude of the gradient.
Micromachines 2016, 7, 194 3 of 9
2.5. Assessment of the DOX Release
Fluorescence microscope (BX53, Olympus, Center Valley, PA, USA) was used to examine the
fluorescence intensities of DOX left in the multilayers. To quantify the DOX release, the fluorescence
intensities of the buffer which dissolved the released DOX was measured by a fluorescence
spectrophotometer (Varioksan LUX, Thermal Scientific, Beaumont, TX, USA), at Ex 485 nm and Em
550 nm.
2.6. SEM Characterization of the Thin Films
To investigate the thin films disassembly, the scanning electron microscope (SEM, FEI F50,
Houston, TX, USA) was utilized to characterize the morphology of the multilayers. The films on the
surface of the device (or glass) was washed by the buffer, and then dried under nitrogen. Finally, the
carrier sample was examined with the SEM.
3. Results
3.1. Theory Consideration
It is known that when the acoustic resonator works in liquid, the vibration of the resonator will
trigger the micro-streaming in the liquid [29], providing a local flow condition. To investigate the
fluid behaviors triggered by the gigahertz resonator, we applied a two-dimensional finite element
model (2D FEM) to analyze the patterns and the velocities of the flow. As the flow trigger, the
resonator was modeled at the bottom of the liquid. The simulation results are shown in Figure 1.
Two symmetric micro-vortices are clearly observed under the vibration stimulus. The liquid near the
edge of the device ejects upward and returns from the periphery, forming the flow circulation. The
simulated color patterns indicate the velocity distributions in the liquid. The device located in the
streaming hinders the flow and decreases the velocity. Thus, in the micro-sized area above the
device, the velocity expresses the descended distribution from top to bottom, and approaches zero at
the surface of the device. The vertical gradient of the velocity produces shear stress, which acts as a
tearing force at the solid-liquid interface. Following Newton’s viscosity law, the shear stress is
proportional to the amplitude of the gradient.
Figure 1. Simulated results of the 2D FEM analysis. Micro-vortices are formed under the resonator
stimulation of (a) 1.5 GHz and (b) 750 MHz. The simulated shear stress induced by the flow with the
excitation frequency of (c) 1.5 GHz and (d) 750 MHz.
To further understand the relationship between the shear stress and the resonant frequencies,
we simulated the flow conditions under the resonant frequencies of 1.5 GHz (Figure 1a) and 750
MHz (Figure 1b). It clearly shows that by levitating the frequency of the excitation, though the
velocities remain the same, the vortex contracts in the vertical direction, and intensifies the vertical
gradient of the velocity, thus leading to an enhancement of the shear stress. The simulated shear
Figure 1.
Simulated results of the 2D FEM analysis. Micro-vortices are formed under the resonator
stimulation of (
a
) 1.5 GHz and (
b
) 750 MHz. The simulated shear stress induced by the flow with the
excitation frequency of (c) 1.5 GHz and (d) 750 MHz.
To further understand the relationship between the shear stress and the resonant frequencies, we
simulated the flow conditions under the resonant frequencies of 1.5 GHz (Figure 1a) and 750 MHz
(Figure 1b). It clearly shows that by levitating the frequency of the excitation, though the velocities
remain the same, the vortex contracts in the vertical direction, and intensifies the vertical gradient of
Micromachines 2016,7, 194 4 of 9
the velocity, thus leading to an enhancement of the shear stress. The simulated shear stress induced
by streaming flows is shown in Figure 1c,d. The results show that the shear stress increases with
the exciting frequency, which confirms the above theoretical predictions. It also indicates that the
high-resonant-frequency device will induce a much stronger fluid flow and further increase the
shear stress, which can be used as the driving force to manipulate the thin films assembled at the
liquid-solid interface.
3.2. Triggered PET Surface Disassembly
According to the FEM analysis, the stress induced by the high-frequency resonator can directly
interact with the thin films at the liquid-solid interface and contribute to their disassemblies.
To demonstrate the concept, we designed and fabricated a bulk acoustic resonator composed of
a thin piezoelectric layer sandwiched by two metal electrodes which vibrates at 1.5 GHz. The device is
fabricated through a Complementary Metal Oxide Semiconductor-compatible (CMOS-compatible)
process. Figure 2c shows the scanning electron microscope (SEM) image of the fabricated device.
To maximize the utilization efficiency of energy, a polygon-shaped resonator is designed which can
focus more energy in the main working mode.
A reaction chamber–based drug release setup was built up for the convenience of drug release
and detection (Figure 2a). A chamber was sealed on a multilayered polyelectrolyte-coated device
which was filled with buffer solution. The amplified sinusoidal signal of 1.5 GHz was sent to the
device, transducing the electrical supply to mechanical vibrations and exciting a swift flow in the
liquid. The electric input is conveniently switched on and off using a program control, and thus the
treatment time can be well controlled. Since the released drug molecules can dissolve in the buffer, the
amount of the released drug can be quantified by the fluorescence technique either from the buffer or
on the surface.
Micromachines 2016, 7, 194 4 of 9
stress induced by streaming flows is shown in Figure 1c,d. The results show that the shear stress
increases with the exciting frequency, which confirms the above theoretical predictions. It also
indicates that the high-resonant-frequency device will induce a much stronger fluid flow and further
increase the shear stress, which can be used as the driving force to manipulate the thin films
assembled at the liquid-solid interface.
3.2. Triggered PET Surface Disassembly
According to the FEM analysis, the stress induced by the high-frequency resonator can directly
interact with the thin films at the liquid-solid interface and contribute to their disassemblies. To
demonstrate the concept, we designed and fabricated a bulk acoustic resonator composed of a thin
piezoelectric layer sandwiched by two metal electrodes which vibrates at 1.5 GHz. The device is
fabricated through a Complementary Metal Oxide Semiconductor-compatible (CMOS-compatible)
process. Figure 2c shows the scanning electron microscope (SEM) image of the fabricated device. To
maximize the utilization efficiency of energy, a polygon-shaped resonator is designed which can
focus more energy in the main working mode.
A reaction chamber–based drug release setup was built up for the convenience of drug release
and detection (Figure 2a). A chamber was sealed on a multilayered polyelectrolyte-coated device
which was filled with buffer solution. The amplified sinusoidal signal of 1.5 GHz was sent to the
device, transducing the electrical supply to mechanical vibrations and exciting a swift flow in the
liquid. The electric input is conveniently switched on and off using a program control, and thus the
treatment time can be well controlled. Since the released drug molecules can dissolve in the buffer,
the amount of the released drug can be quantified by the fluorescence technique either from the
buffer or on the surface.
Figure 2. (a) Schematic of the release system. The streaming flow was triggered by the stimulation of
the resonator, inducing shear stress on the solid-liquid interface, and further leading to the
disassembly of PET thin films. (b) Multilayered PET-coated resonator composed of HA and
PLL-FITC through LbL approach. (c) SEM image of the fabricated resonator, which is comprised by a
thin-film piezoelectric layer sandwiched by two metal electrodes and vibrates at 1.5 GHz. (d)
Fluorescence images of the PET thin films, which were exposed to the buffer with (experiment
group) or without (control group) the stimulation of the resonator. (e) Schematic process of the
multilayers’ disassembly under the stimulation of the resonator. (f) SEM images of the PET thin films
before and after the treatment.
Figure 2.
(
a
) Schematic of the release system. The streaming flow was triggered by the stimulation of
the resonator, inducing shear stress on the solid-liquid interface, and further leading to the disassembly
of PET thin films; (
b
) Multilayered PET-coated resonator composed of HA and PLL-FITC through LbL
approach; (
c
) SEM image of the fabricated resonator, which is comprised by a thin-film piezoelectric
layer sandwiched by two metal electrodes and vibrates at 1.5 GHz; (
d
) Fluorescence images of the
PET thin films, which were exposed to the buffer with (experiment group) or without (control group)
the stimulation of the resonator; (
e
) Schematic process of the multilayers’ disassembly under the
stimulation of the resonator; (f) SEM images of the PET thin films before and after the treatment.
Micromachines 2016,7, 194 5 of 9
To demonstrate the controlled molecular surface disassembly, thin films of sodium hyaluronate
(HA) and Poly-L-lysine (PLL) were used as a template polyelectrolyte (PET) assembly system. HA is a
negatively charged PET while PLL is positively charged; thus, they could assemble on the substrate in
a layer-by-layer (LbL) fashion and form stable PET thin films. In order to facilitate the visualization
and quantification of the PET disassembly, the PLL was labeled with FITC; thus, the disassembly
process can be directly evaluated through fluorescence detections. Six bilayers of HA and PLL-FITC
were first coated on the resonator surface. After thoroughly cleaning with the buffer, the device was
fixed in the reaction chamber filled with pure buffer solutions and 640 mW power was applied to the
resonator to trigger the PET disassembly, while another device coated with the same HA and PLL-FITC
was immersed in the same buffer system without stimulation of the resonator as a control group. The
fluorescence intensities of the thin films were checked at 0, 30, 60, and 120 min, and the corresponding
images are shown in Figure 2d. No appreciable deduction of the florescence intensity was detected
in the control group which indicates the stability of the PET thin films in the buffer system, while in
the experiment group, the fluorescence intensity clearly decreases with time. The result demonstrates
that the disassembly of the PET thin films only occurs in the presence of the resonator stimulations,
which further indicates that the resonator can effectively trigger the destruction of the PET thin films,
as schematically shown in Figure 2e. In addition, there was little fluorescence left on the device after
the treatment of 2 h, suggesting the resonator stimulus has successfully disassembled most of the PETs.
To further ensure the destruction of the PETs, the scanning electron microscope (SEM) was utilized
to characterize the morphologies of the thin films. As shown in Figure 2f, no obvious changes take
place in the control group. In contrast, most of the PETs have been removed from the surface after the
device simulation, which indicates the success of the disassembly. All the results agree well with the
above fluorescent characterizations.
3.3. Controlled DOX Release
As demonstrated above, the GHz resonator can be used as an effective source to trigger the PETs’
surface disassembly. Since the PET assembly system can be used to entrap the drug molecules [
3
,
30
],
the demonstrated acoustic triggered disassembly can be utilized as an effective technique to activate
the drug release. As shown in Figure 2d, the fluorescence intensity decreased gradually under the
acoustical stimulus, suggesting that the PET thin films disassemble little by little. This can be used
for sustained drug release applications, which is one of the most important issues in drug delivery.
Further experiments were carried out to study the kinetics of the drug release under the stimulations
of different input powers.
Doxorubicin (DOX) was chosen as the model drug and was trapped in PET thin films by
immersion of the films in the DOX solution (5 mg/mL, in the presence of 0.15 M NaCl) for 12 h
at 37
C. Since the released DOX is dissolved in the buffer, the amount of the released drug can be
quantitatively monitored by detecting the fluorescence intensities of the buffer (Figure 3a). According
to the results, it clearly showed that the amount of the released DOX increases with treatment time for
all power supplies, which coincides with the results from the HA-PLL surface disassembly. However,
different input power leads to different percentages of DOX release. Higher power corresponds to
a larger amount of release, showing that the release progress is input power–dependent. It further
indicates that the amount of the release can be well controlled by tuning the input power. Similarly, to
reach the same target amount of release, the required stimulation time can also be controlled through
the adjustment of the input power. Taking 60% DOX release as an example, the required stimulation
time with an input power of 10 mW is 120 min. Increasing the power amplitude to 40, 160, and 640 mW,
the required treatment time decreases to 90, 60, and 40 min, respectively.
To further prove the DOX release, fluorescence images of the PET thin films were taken before and
after the experiments (Figure 3b). Compared with the control group, the experiment groups exhibit
an obvious fluorescence deduction after the acoustic stimulation, which demonstrates the successful
release of DOX from the carrier. It also shows a clear difference between the two experiment groups
Micromachines 2016,7, 194 6 of 9
with the input power of 40 mW and 640 mW. The latter group emits much less fluorescence than
the former, confirming that much more DOX has been released under the stimulation of the higher
input power.
Micromachines 2016, 7, 194 6 of 9
experiment groups with the input power of 40 mW and 640 mW. The latter group emits much less
fluorescence than the former, confirming that much more DOX has been released under the
stimulation of the higher input power.
All these results indicate that the drug release triggered by the resonator is a power-dependent
process, and the stronger power will lead to a higher release rate. This also can be explained in view
of the flow velocity. Increasing the input power accelerates the velocity of the flow, which enhances
the vertical gradient of the velocity, thus strengthening the shear stress, and further speeds up the
drug release.
Figure 3. (a) DOX release under the stimulus of the resonator with different power inputs. (b)
Fluorescence images of the thin films taken before and after the experiments.
4. Discussion
As shown above, the main driving force to trigger the PET surface disassembly/DOX release is
the tear force from the high-speed micro-vortex at the solid-liquid interface induced by the GHz
resonator. In principle, such force can not only work on the films assembled on the device surface,
but also on other substrates which are able to contact the micro-vortex. To demonstrate this, the
same multilayered PET thin films (HA-PLL) incorporated with DOX were coated on a piece of glass
and fixed above the resonator at a distance of 2 mm, by leaving the PET films toward the resonator.
Then, 640 mW power was continuously applied to the device for 2 h. The input power of 640 mW
can heat the liquid from room temperature to around 37 °C. To exclude the impact of the
temperature on the film disassembly, the control experiment was also conducted using a 37 °C
solvent. The fluorescent intensities of the released DOX were measured every 20 min and plotted in
Figure 4b. It clearly shows that up to 90% of DOX has been released, showing the successful
controlled release which is triggered by the acoustic resonator. In contrast, only a little DOX is
released in the control group, suggesting the heat effect on liquid has little impact on film stability.
The results indicate that the acoustically triggered disassembly of multilayered polyelectrolyte thin
films through gigahertz resonators is a versatile method for controlled drug release applications not
only for carriers on the device surface, but also on other substrates close to the device, thus
increasing the throughput of this technique. In addition, owing to the miniature size of the
resonator, it opens possibilities to be implanted in vivo to realize site-targeted delivery.
Figure 3.
(
a
) DOX release under the stimulus of the resonator with different power inputs;
(b) Fluorescence images of the thin films taken before and after the experiments.
All these results indicate that the drug release triggered by the resonator is a power-dependent
process, and the stronger power will lead to a higher release rate. This also can be explained in view
of the flow velocity. Increasing the input power accelerates the velocity of the flow, which enhances
the vertical gradient of the velocity, thus strengthening the shear stress, and further speeds up the
drug release.
4. Discussion
As shown above, the main driving force to trigger the PET surface disassembly/DOX release
is the tear force from the high-speed micro-vortex at the solid-liquid interface induced by the GHz
resonator. In principle, such force can not only work on the films assembled on the device surface,
but also on other substrates which are able to contact the micro-vortex. To demonstrate this, the same
multilayered PET thin films (HA-PLL) incorporated with DOX were coated on a piece of glass and
fixed above the resonator at a distance of 2 mm, by leaving the PET films toward the resonator. Then,
640 mW power was continuously applied to the device for 2 h. The input power of 640 mW can heat
the liquid from room temperature to around 37
C. To exclude the impact of the temperature on the
film disassembly, the control experiment was also conducted using a 37
C solvent. The fluorescent
intensities of the released DOX were measured every 20 min and plotted in Figure 4b. It clearly shows
that up to 90% of DOX has been released, showing the successful controlled release which is triggered
by the acoustic resonator. In contrast, only a little DOX is released in the control group, suggesting
the heat effect on liquid has little impact on film stability. The results indicate that the acoustically
triggered disassembly of multilayered polyelectrolyte thin films through gigahertz resonators is a
versatile method for controlled drug release applications not only for carriers on the device surface,
but also on other substrates close to the device, thus increasing the throughput of this technique.
In addition, owing to the miniature size of the resonator, it opens possibilities to be implanted
in vivo
to realize site-targeted delivery.
Micromachines 2016,7, 194 7 of 9
Figure 4.
(
a
) Schematic of the release system where the PET thin films were coated on a piece of glass
and fixed above the device 2 mm away; (
b
) DOX release with time under the stimulus of the resonator.
The input power is 640 mW.
5. Conclusions
In summary, we have developed a novel versatile physical technique to trigger disassembly of
multilayered polyelectrolyte thin films for controlled drug release applications which utilizes the
GHz bulk acoustic resonator to excite the local micro-vortex and further induce shear stress at the
solid-liquid interface. The ultra-high frequency of the resonator enhances the vertical gradient of
the flow velocities, and therefore intensifies the shear stress, making it a powerful source to tear and
disassemble the PET thin films. Due to the moderate physical interactions between the flow and the
films, the multilayers disassemble step by step; thus, sustained drug release can be achieved in this
manner. The input power required is around several hundreds of milliwatts (mW), indicating the
method requires low power consumption. Owing to the intrinsic miniature size of the resonator, it
opens possibilities for implantation
in vivo
to realize site-targeted delivery. In addition, the shear
wave mode can be considered as a future development of the proposed application due to the direct
generation of shear stress during wave propagation.
Supplementary Materials:
The following are available online at www.mdpi.com/2072-666X/7/11/194/s1,
Figure S1: Schematic of the fabrication process of hypersonic resonator. (
a
) A silicon substrate is prepared; (
b
) Brag
reflector layers are deposited in turn on the substrate; (
c
) Molybdenum (Mo) is deposited and patterned as the
bottom electrode; (
d
) Aluminum nitride (AlN) is deposited as a piezoelectric layer; (
e
) Mo is deposited and
patterned as the top electrode; (
f
) AlN is etched, and gold (Au) is deposited and patterned as the connecting and
testing pad.
Acknowledgments:
The authors gratefully acknowledge financial support in part from the National High
Technology Research and Development Program of China (863 Program No. 2015AA042603), the Natural Science
Foundation of China (NSFC No. 61674114, NSFC No. 61501320) and the 111 Project (B07014). Xuexin Duan
acknowledges support from Tianjin Applied Basic Research and Advanced Technology (14JCYBJC41500).
Author Contributions:
Xuexin Duan conceived and supervised the project. Zifan Tang and Zhixin Zhang
designed the experiments and wrote the manuscript. Hongxiang Zhang and Zifan Tang conducted the simulations.
Zifan Tang and Zhixin Zhang performed the experiments. Wenpeng Liu, Hao Zhang, and Wei Pang fabricated the
device. Yanyan Wang and Yao Lu provided technical support. All authors discussed the results and commented
on the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
Anderson, C.; Bates, I.; Brock, T. Seven billion humans and 98 trillion medicine doses. Am. J. Pharm. Educ.
2011,75, 194. [CrossRef] [PubMed]
2. Langer, R. Drug delivery and targeting. Nature 1998,392, 5–10. [PubMed]
3.
Hua, A.; Jones, S.A.; Villiers, M.M.D.; Lvov, Y.M. Nano-encapsulation of furosemide microcrystals for
controlled drug release. J. Control. Release 2003,86, 59–68.
Micromachines 2016,7, 194 8 of 9
4.
Loira-Pastoriza, C.; Todoroff, J.; Vanbever, R. Delivery strategies for sustained drug release in the lungs.
Adv. Drug Deliv. Rev. 2014,75, 81–91. [CrossRef] [PubMed]
5.
Donath, E.; Sukhorukov, G.B.; Caruso, F.; Davis, S.A.; Möhwald, H. Novel hollow polymer shells by
colloid-templated assembly of polyelectrolytes. Angew. Chem. Int. Ed. 1998,37, 2201–2205. [CrossRef]
6.
Zhang, S. Fabrication of novel materials through molecular self-assembly. Nat. Biotechnol.
2003
,21, 1171–1178.
[CrossRef] [PubMed]
7.
Lvov, Y.; Decher, G.; Moehwald, H. Assembly, structural characterization, and thermal behavior of
layer-by-layer deposited ultrathin films of poly(vinyl sulfate) and poly(allylamine). Langmuir
1993
,9,
481–486. [CrossRef]
8.
Kotov, N.A.; Dekany, I.; Fendler, J.H. Layer-by-layer self-assembly of polyelectrolyte-semiconductor
nanoparticle composite films. J. Phys. Chem. 1995,99, 13065–13069. [CrossRef]
9.
Tang, Z.; Wang, Y.; Podsiadlo, P.; Kotov, N.A. Biomedical applications of layer-by-layer assembly: From
biomimetics to tissue engineering. Adv. Mater. 2007,18, 3203–3224. [CrossRef]
10.
Liu, X.Q.; Picart, C. Layer-by-layer assemblies for cancer treatment and diagnosis. Adv. Mater.
2016
,28,
1295–1301. [CrossRef] [PubMed]
11.
Burke, S.E.; Barrett, C.J. Ph-responsive properties of multilayered poly(l-lysine)/hyaluronic acid surfaces.
Biomacromolecules 2003,4, 1773–1783. [CrossRef] [PubMed]
12.
Sui, Z.; Schlenoff, J.B. Phase separations in ph-responsive polyelectrolyte multilayers: Charge extrusion
versus charge expulsion. Langmuir 2004,20, 6026–6031. [CrossRef] [PubMed]
13.
Chaturbedy, P.; Jagadeesan, D.; Eswaramoorthy, M. Ph-sensitive breathing of clay within the polyelectrolyte
matrix. ACS Nano 2010,4, 5921–5929. [CrossRef] [PubMed]
14.
Malinova, V.; Wandrey, C. Loading polyelectrolytes onto porous microspheres: Impact of molecular and
electrochemical parameters. J. Phys. Chem. B 2007,111, 8494–8501. [CrossRef] [PubMed]
15.
Mendes, P.M. Stimuli-responsive surfaces for bio-applications. Chem. Soc. Rev.
2008
,37, 2512–2529.
[CrossRef] [PubMed]
16.
Rydzek, G.; Thomann, J.S.; Ben, A.N.; Jierry, L.; Mésini, P.; Ponche, A.; Contal, C.; El Haitami, A.E.; Voegel, J.C.;
Senger, B. Polymer multilayer films obtained by electrochemically catalyzed click chemistry. Langmuir
2010
,
26, 2816–2824. [CrossRef] [PubMed]
17.
Büscher, K.; Graf, K.; Ahrens, H.; Helm, C.A. Influence of adsorption conditions on the structure of
polyelectrolyte multilayers. Langmuir 2002,18, 3585–3591. [CrossRef]
18.
Steitz, R.; Leiner, V.; Tauer, K.; Khrenov, V.; Klitzing, R.V. Temperature-induced changes in polyelectrolyte
films at the solid–liquid interface. Appl. Phys. A 2002,74, s519–s521. [CrossRef]
19.
Köhler, K.; Shchukin, D.G.; Möhwald, H.; Sukhorukov, G.B. Thermal behavior of polyelectrolyte multilayer
microcapsules. 1. The effect of odd and even layer number. J. Phys. Chem. B
2005
,109, 18250–18259.
[CrossRef] [PubMed]
20.
Skirtach, A.G.; Antipov, A.A.; Shchukin, D.G.; Sukhorukov, G.B. Remote activation of capsules containing
Ag nanoparticles and IR dye by laser light. Langmuir Acs J.Surf. Colloids
2004
,20, 6988–6992. [CrossRef]
[PubMed]
21.
Xia, T.; Li, J.; Möhwald, H. Self-assembly, optical behavior, and permeability of a novel capsule based on an
azo dye and polyelectrolytes. Chemistry 2004,10, 3397–3403.
22.
Koo, H.Y.; Lee, H.J.; Kim, J.K.; Choi, W.S. UV-triggered encapsulation and release from polyelectrolyte
microcapsules decorated with photoacid generators. J. Mater. Chem. 2010,20, 3932–3937. [CrossRef]
23.
Henry, S.; Mcallister, D.V.; Allen, M.G.; Prausnitz, M.R. Microfabricated microneedles: A novel approach to
transdermal drug delivery. J. Pharm. Sci. 1998,87, 922–925. [CrossRef] [PubMed]
24.
Mcallister, D.V.; Allen, M.G.; Prausnitz, M.R. Microfabricated microneedles for gene and drug delivery.
Annu. Rev. Biomed. Eng. 2000,2, 289–313. [CrossRef] [PubMed]
25.
Rajeswari, N.R.; Malliga, P. Design of mems based microneedle for drug delivery system. Procedia Eng.
2014
,
97, 2001–2010. [CrossRef]
26.
Van Lintel, H.T.G.; Van De Pol, F.C.M.; Bouwstra, S. A piezoelectric micropump based on micromachining of
silicon. Sens. Actuators 1988,15, 153–167. [CrossRef]
27.
Van De Pol, F.C.M.; Van Lintel, H.T.G.; Elwenspoek, M.; Fluitman, J.H.J. A thermopneumatic micropump
based on micro-engineering techniques. Sens. Actuators A Phys. 1990,21, 198–202. [CrossRef]
Micromachines 2016,7, 194 9 of 9
28.
Bourouina, T.; Grandchamp, J.P. Design and simulation of an electrostatic micropump for drug-delivery
applications. J. Micromech. Microeng. 1997,7, 186–188. [CrossRef]
29.
Tam, C.K.W.; Ju, H.; Walker, B.E. Numerical simulation of a slit resonator in a grazing flow under acoustic
excitation. J. Sound Vib. 2008,313, 449–471. [CrossRef]
30.
Sukhorukov, G.B.; Donath, E.; Davis, S.; Lichtenfeld, H.; Caruso, F.; Popov, V.I.; Möhwald, H. Stepwise
polyelectrolyte assembly on particle surfaces: A novel approach to colloid design. Polym. Adv. Technol.
1998
,
9, 759–767. [CrossRef]
©
2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC-BY) license (http://creativecommons.org/licenses/by/4.0/).
Article
Microcentrifugation constitutes an important part of the microfluidic toolkit in a similar way that centrifugation is crucial to many macroscopic procedures, given that micromixing, sample preconcentration, particle separation, component fractionation, and cell agglomeration are essential operations in small scale processes. Yet, the dominance of capillary and viscous effects, which typically tend to retard flow, over inertial and gravitational forces, which are often useful for actuating flows and hence centrifugation, at microscopic scales makes it difficult to generate rotational flows at these dimensions, let alone with sufficient vorticity to support efficient mixing, separation, concentration, or aggregation. Herein, the various technologies—both passive and active—that have been developed to date for vortex generation in microfluidic devices are reviewed. Various advantages or limitations associated with each are outlined, in addition to highlighting the challenges that need to be overcome for their incorporation into integrated microfluidic devices. An overview is presented of the current state‐of‐the‐art in microfluidic vortex technology for driving on‐chip rotational flows for a range of microcentrifugation processes such as micromixing, sample preconcentration, particle separation, component fractionation, and cell agglomeration, among others.
Article
Hypersound (ultrasound of gigahertz (GHz) frequency) has been recently introduced as a new type of membrane-disruption method for cells, vesicles and supported lipid bilayers (SLBs), with the potential to improve the efficiency of drug and gene delivery for biomedical applications. Here, we fabricated an integrated microchip, composed of a nano-electromechanical system (NEMS) resonator and a gold electrode as the extended gate of a field effect transistor (EGFET), to study the effects of hypersonic poration on an SLB in real time. The current recordings revealed that hypersound enabled ion conduction through the SLB by inducing transient nanopores in the membrane, which act as the equivalent of ion channels and show gating behavior. The mechanism of pore formation was studied by cyclic voltammetry (CV), atomic force microscopy (AFM) and laser scanning microscopy (LSM), which support the causality between hypersound-triggered deformation and the reversible membrane disruption of the SLB. This finding contributes to the development of an approach to reversibly control membrane permeability by hypersound.
Article
Nonspecific binding (NSB) is a general issue for surface based biosensors. Various approaches have been developed to prevent or remove the NSBs. However, these approaches either increased the background signals of the sensors or limited to specific transducers interface. In this work, we developed a hydrodynamic approach to selectively remove the NSBs using a microfabricated hypersonic resonator with 2.5 gigahertz (GHz) resonant frequency. The high frequency device facilitates to generate multiple controlled micro-vortices which then create cleaning forces at the solid-liquid interfaces. The competitive adhesive and cleaning forces have been investigated using the finite element method (FEM) simulation, identifying the feasibility of the vortices induced NSB removal. NSB proteins have been selectively removed experimentally both on the surface of the resonator and on other substrates which contact the vortices. Thus, the developed hydrodynamic approach is believed to be a simple and versatile tool for NSB removal and compatible to many sensor systems. The unique feature of the hypersonic resonator is that it can be used as a gravimetric sensor as well, thus a combined NSB removal and protein detection dual functional biosensor system is developed.
Article
Full-text available
The layer-by-layer (LbL) technique was introduced in the early 1990s. Since then, it has undergone a series of technological developments, making it possible to engineer various theranostic platforms, such as films and capsules, with precise control at the nanometer and micrometer scales. Recent progress in the applications of LbL assemblies in the field of cancer therapy, diagnosis, and fundamental biological study are highlighted here. The potential of LbL-based systems as drug carriers is discussed, especially with regard to the engineering of innovative stimuli-responsive systems, and their advantageous multifunctionality in the development of new therapeutic tools. Then, the diagnostic functions of LbL assemblies are illustrated for detection and capture of rare cancer cells. Finally, LbL-mimicking extracellular environments demonstrate the emerging potential for the study of cancer cell behavior in vitro. The advantages of LbL systems, important challenges that need to be overcome, and future perspectives in clinical practice are then highlighted.
Article
Full-text available
Microneedles have been recently given much attention as a novel way for transdermal drug delivery. In this paper, the mechanics of microneedle insertion into soft tissue during drug delivery and also the changes in the forces and moments of the microneedle is to be presented. A non-linear displacement of the microneedle insertion into the soft tissue is expressed as a quadratic polynomial function and solved using Galerkin technique. The geometrical behavior of the microneedle tip is to be considered in the analytical model. The graphical analysis of the analytical model are presented using MATLAB. The analytical model helps to improve the microneedle design by minimizing the pain of insertion and increasing the mechanical strength.
Article
Full-text available
The design of a micropump employing only one valve is presented. This device was simulated by means of a model based on electrical analogies and equivalent networks. With a total size of , and a working voltage of 10 V, the device sizing gives flow rates in the 10 - 100 range, which are very suitable for drug-delivery applications.
Article
In the present study, a versatile layer-by-layer technique using polyelectrolyte multilayers (PEMs) was used to prepare microcapsules with walls containing photoacid generators (PAGs), which made them optically addressable. By exposure to UV light with a wavelength of 254 nm, the PAGs within the capsule walls were activated, which caused the release of protons from the capsules. This release of protons caused a decrease in the pH of the capsule solution, which then triggered a swelling of the microcapsules. The resulting microcapsules could be opened and closed via alternate repetition of exposure to UV light and washing with neutral water. Prolonged exposure led to a breakage of the capsules, which promoted a rapid release of the entrapped substances.
Article
Polyelectrolyte multilayers were deposited onto polystyrene and melamine formaldehyde latex particles by means of consecutive adsorption. Two different methods of multilayer growth were employed. First, adsorption of polyelectrolytes at a concentration exceeding saturation amounts was combined with the removal of the nonbound polyelectrolyte by means of centrifugation. Second, adsorption of polyelectrolyte was performed at a concentration just sufficient for saturation coverage. Both methods yielded continuous layer growth. The process of film formation was followed by electrophoresis, dynamic light scattering, single particle light scattering and fluorescence intensity measurements. Layer deposition onto partially crosslinked melamine resin latex particles, which were soluble at pH values of less than 1.6, resulted in the production of three-dimensional thin polyelectrolyte shells upon dissolving the core. The ultrathin shells were observed by means of scanning and transmission electron microscopy. © 1998 John Wiley & Sons, Ltd.
Article
We have recently introduced a new method of creating homogeneous ultrathin films on solid supports, which is based on the electrostatic attraction between opposite charges. Consecutively alternating adsorption of anionic and cationic polyelectrolytes from their aqueous solution leads to the formation of multilayer assemblies. The multilayer buildup is easily monitored by small angle X-ray scattering (SAXS) and a linear increase of the film thickness with the number of adsorbed layers is observed. In the present study we concentrate on the polyelectrolytes poly(vinyl sulfate) (PVS) and poly(allylamine) (PAH), discussing especially the influence of the adsorption conditions of PVS on the growth and the properties of the resulting films. A stable film growth was found to depend on two prerequisites for the PVS deposition: an adsorption time of at least 15 min and a maximum concentration of added sodium chloride of 0.2 mol/L. In the regime of stable growth the multilayer film is able to smooth the surface roughness of the underlying glass substrate from 18 A to a value of 4.5 A. The addition of small amounts of electrolyte to the PVS solution leads to an increase of the thickness of one layer pair from 13 A (at 0.0 mol of NaCl/L) to 34 angstrom (at 0.9 mol of NaCl/L). This behavior is similar to the well-known ''rod-to-coil'' transition of polyelectrolytes in solution and due to screening of electrostatic charges on the polymer chain. The films are stable up to temperatures of at least 160-degrees-C but seem to lose trapped water above 60-degrees-C.
Article
Spectroscopic (absorption and emission), microscopic (transmission electron and atomic force), X-ray diffraction and photocurrent measurements have provided evidence for the formation of stable ultrathin films with regular periodicities, by the layer-by-layer self-assembly of polycations and cadmium sulfide, lead sulfide and titanium dioxide nanoparticles.