Standing surface acoustic wave assisted fabrication of region

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Standing surface acoustic wave assisted fabrication of regionselective microstructures via user-defined waveguides Yancheng Wang, Chenyang Han, and Deqing Mei Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b01565 • Publication Date (Web): 07 Aug 2019 Downloaded from pubs.acs.org on August 14, 2019

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Standing surface acoustic wave-assisted fabrication of region-selective microstructures via user-defined waveguides Yancheng Wang1*, Chenyang Han2, Deqing Mei1 1State

Key Laboratory of Fluid Power and Mechatronic Systems, School of Mechanical Engineering, Zhejiang University, Hangzhou, 310027, China 2Key Laboratory of Advanced Manufacturing Technology of Zhejiang Province, School of Mechanical Engineering, Zhejiang University, Hangzhou, 310027, China

Abstract Polymer-based substrate with region-selective microstructures are crucial for many biomedical applications. Here, we explored a novel method based on standing surface acoustic waves (SSAWs) for the fabrication of localized polymer-based microstructures via a predefined waveguide. By exciting the SSAWs, the generated acoustic pressure field can be controlled in a pre-determined region of the fluid surface through controlling the size and shape of the waveguide geometry. Based on the capillary wave motion, the generated acoustic pressure field can excite micro-wavy patterned structures on the surface. Then, using ultraviolet (UV) solidification, the polymer-based substrates with region-selective patterned microstructures can be successfully fabricated. Both finite element modelling and experimental studies demonstrated that the polymer substrate with different region-selective microstructures can be achieved by selecting the pairs of interdigital transducers (IDTs) and shapes of the predefined waveguides. The results showed that the proposed method is effective for fabricating polymer-based substrate with region-selective microstructures and may have potential in cell-laden chips for tissue engineering, cell-cell interactions, and other biomedical applications. Keywords: Surface Acoustic Wave; Microstructures; Capillary Wave Motion; Waveguide

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1. Introduction Polymer-based substrate with patterned microstructures have been widely utilized in tissue engineering,1-2 biomimetic smart surfaces,3-5 and organs-on-chips.6-7 For instance, the substrate with aligned micro-channels can guide the self-assembly of nerve2 or myocardium3 cells and provides a biomimetic environment to study the cell’s alignment behavior and drug responses. The bio-functional surfaces with dot-like patterns, which were inspired from leaves’ or animals’ skin, have been validated with several interesting properties such as hydrophobicity,5 optics,6 and/or high-adhesives.7 A three-dimensional (3D) liver chip can be designed on the concave-shaped substrate to study the liver cell’s migration behavior without cell-cell contact.8 Besides, the patterned microstructures with a high aspect ratio, e.g., micro-towers9 or pyramid arrays,10 on the dielectric layer can enhance the sensitivity and response behavior of artificial-skin sensors. Several approaches and technologies have been developed for the fabrication of polymer-based substrate with patterned microstructures. These fabrication approaches can be divided into two groups. The first uses a physical mould (e.g., polydimethylsiloxane (PDMS),11 silicon,12 or metal alloys13) with inverse-shaped microstructures to transfer the patterns into the substrate. The second is additive manufacturing, such as extraction-based printing14 and stereolithography (SLA),15 that can be used to fabricate either metal-based or polymer-based microstructures. As for extrusion-based printing, the dispensing ink is usually modified to dispense the protein and/or cell solution, and various polymer-based microstructures or cell arrays can be printed through nozzle extrusion.14 With the assistance of digital micro-mirror device (DMD) and ultraviolet light exposure, the PEGDA film with microvasculature structure can be printed with good biocompatibility.15 Among these fabrication methods, physical or chemical moulds and templates are usually required and additional fabrication processes and procedures are involved, which lead to a long fabrication period and high cost. Recently, we developed a novel method using SSAWs to excite the wavy- and lattice-shaped patterns on the liquid film while also implementing UV polymerization to solidify the patterned surfaces.16-17 This fabrication method has several advantages, such as a rapid fabrication period (approximately 1~3 s), a relatively low cost, and no required physical or chemical templates or moulds (which is particularly important). For the developed SSAWs-assisted process, the fabricated patterned microstructures are


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relatively simple (e.g., micro-wavy or latticed patterns), which corresponding to the acoustic pressure distribution of acoustofluidics. In addition, the area of the patterned microstructures is determined by the boundary of the photosensitive fluid film in an open-air environment. Therefore, this method lacks the ability to fabricate region-selective or localized microstructures with more complex-shaped surfaces on the substrate. To overcome these shortcomings, acoustic energy transmitted to the fluid film needs to be locally controlled instead of evenly distributed on the liquid film surface. In acoustofluidics, several strategies have been reported, including the waveguide with the fluid chamber20 and several specifically designed acoustic enclosures21-23 with the purpose of precise control or specified acoustic energy in a localized region. However, region-selective or localized patterned surface fabrication in a small region still remains a challenge. Here, we report a novel method for the fabrication of region-selective microstructures through SSAWs and user-defined acoustic waveguides with the assistance of UV polymerization. Our acoustic waveguides with predesigned structures used to bond on a SSAW device. In addition, they enable a specific and localized acoustic pressure field within the prescribed regions in an open-air environment, which makes them promising candidates for the fabrication of localized microstructures. In the following study, the working mechanism of the user-defined waveguide for a specific region of the acoustic pressure field based on Rayleigh’s radiation is presented. We show in detail by turning the shape and size of the waveguide structure for region-selective patterned microstructure fabrication and compare it to that of numerical simulation to verify the theoretical predictions. This research will pave the way for the fabrication of region-selective or localized patterned microstructures for many biomedical applications such as tissue engineering and cell-cell interactions.

2. Materials and Methods 2.1 Working Mechanism Figure 1a shows the schematic view of the SSAW device with acoustic waveguide for fabrication of a polymer-based substrate with localized microstructures. The specifically designed acoustic waveguide made by polydimethylsiloxane (PDMS) is sandwiched between the SSAW device and the top glass wafer at the central region. For the SSAW device, we chose a double polished 128°Y-cut


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LiNbO3 piezoelectric wafer as the substrate due to its optical transparency and high electromechanical coupling.20 Two pairs of identical IDTs with straight electrodes at a spacing of a quarter of the wavelength of the surface acoustic wave (SAW) are symmetrically deposited onto the LiNbO3 substrate. To simplify, we choose local coordinates so the SSAWs will transmit in the x-axis and y-axis as shown in Figure 1a. A thin layer of liquid photosensitive PEGDA polymer film is evenly placed on the top surface of the glass wafer, which provides a hydrophilic environment for the fluid film.

Figure 1. (a) Schematic view of the SAW device with specific designed PDMS acoustic waveguide; as the excitation of two pairs of IDTs, the patterned microstructures will be formed on the top PEGDA fluid film (green dots) with the area confined in the cross-section shape of the acoustic waveguide; (b) The working principles of the utilized acoustic waveguide for localized microstructures’ fabrication. Red lines indicate the propagation of the leaky acoustic wave transmission.

Figure 1b illustrates the 2D schematic diagram of the wave propagation and localized capillary wave on the fluid surface. A sinusoidal signal is split and applied to each pair of IDTs and generates SSAWs. In the region where the waveguide makes contact with the LiNbO3 substrate, a leak wave is coupled to the PDMS waveguide and only the bulk wave is considered because the acoustic


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characteristic of PDMS is similar to fluid (like water).23,33 Therefore, the leak wave will propagate at the Rayleigh’s radiation angle (θR)23 as shown in Figure 1b. As the leaky SAW propagate upward, the reflection and refraction will also occur at the interfaces between the glass wafer and PDMS waveguide due to the differences in acoustic impedance.22 The transmitted wave at the central region of the thin glass wafer is usually called the Lamb wave.26,32 This region has nearly the same area as the cross-sectional shape of the acoustic waveguide according to the refraction angle. Furthermore, structure-acoustic coupling from the glass wafer to the PEGDA fluid will form and result in periodic longitudinal pressure waves in a defined region inside the PEGDA fluid medium. Finally, through UV exposure, the photosensitive fluid with a localized patterned surface will solidify and can be peeled off from the platform for further applications. 2.2 Material Preparation Figure 2a illustrates the photographs of the SSAW device. It has two pairs of IDTs orthogonally arranged on a LiNbO3 piezoelectric substrate. The fabrication process of the SSAW device was adopted according to Mei’s study.16 In consideration of the size of device and the acoustic energy, the wavelength of surface acoustic wave is usually between 20 to 500 μm. Here we choose the wavelength of two pairs of IDTs is equal to 200 μm, which means the period of microstructures is 100 μm. At the central of the SSAW device is a PDMS acoustic waveguide, which has a thickness of 0.2 mm. The waveguide is fabricated by the mould replica method through an aluminum mould; the utilized PDMS prepolymers (Dow Corning 184A and B) are mixed at a weight ratio of 10:1. A square-shaped glass wafer 8 mm in width and 0.1 mm in thickness is prepared and bonded onto the top surface of the PDMS waveguide. The fabrication result is shown in Figure 2b. The micro-pipettor is used to add the liquid PEGDA to the glass wafer. The volume of the liquid PEGDA is controlled, so that the final thickness of the liquid film is about 30-50 μm.37 The thickness of the PEGDA will affect the final height of the fabricated region-selective microstructures. Under the same strength of the SSAW, the height of the fabricated patterns will be decreased with the increasing of the thickness of the fluid film due to the acoustic energy propagation loss. Typically, if the thickness of the fluid film is larger than 100 μm, the acoustic flow will be too strong to generate the wavy patterns on the liquid surface. On the other hand, if the fluid film has the thickness less than 20 μm, it is hard to place the liquid film on the glass wafer and will be easily tear up by the acoustic


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force. So the suitable thickness of the fluid film for microstructure’s fabrication needs to be controlled as about 30-50 μm. The PEGDA is a mixture of 99.5% (w/v) PEGDA (Mn 250, Sigma, USA) and 0.5% (w/v) Irgacure 2959 (Sigma, USA). Three types of waveguides, including a circular-shaped waveguide with a diameter of 2.0 mm, a square-shaped PDMS waveguide with a side length of 2.0 mm, and a triangle-shaped waveguide with side length of 2.0 mm are fabricated as shown in Figure 2c.

Figure 2. (a) and (b) Photographs of the designed and fabricated SSAW device with the PDMS acoustic waveguide and glass wafer placed at the central area of the LiNbO3 substrate; (c) three different shaped PDMS acoustic waveguides with a thickness of 0.2 mm: circular-shaped, triangular-shaped, and square-shaped waveguides.

2.3 Numerical Modeling The numerical modelling of the SSAWs and the waveguide for the generation of localized microstructure patterns was established. According to Tan’s study, the anisotropy piezoelectric substrate is governed by25


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Di S Ek  eikl kl   ikS t t t

Tij t

E  cijkl

S kl E  ekij k t t

(1a) (1b)

where eikl are the piezoelectric stress coefficients, εikS are the dielectric coefficients at constant strain S, Tij are the stress components, and cijklE are the elastic stiffness coefficients at a constant electric displacement D or field E25 and they are calculated by the Maxwell equation.23,29 In addition, the strain-stress equation is calculated for the mechanical motion inside the glass wafer.29 The damping effect is considered to reveal an accurate solution. Here, we use the Rayleigh damping in the glass wafer with the mass damping parameter αK = 2.22106 s-1 and the stiffness damping parameter βK = 1.4510-10 s calculated by the damping ratio ξ from the previous report.32,35 So the equation can be written as



2u u S )  K     (S   K 2 t t t

(2)

where u is the displacement and S is the strain. As we neglect the bulk wave in the waveguide, the governing equation in the waveguide can be simplified as the linear wave equation26,30

2 p 

1 2 p 0 c f 2 t 2

where p and cf indicate the acoustic pressure and sound speed of PDMS.


(3)

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Figure 3. (a) z-axis displacement and acoustic pressure distribution in the x-z cross section of the SSAW device with the acoustic waveguide, which is calculated by the numerical model at t = 60 T; (b) acoustic pressure fluctuation at the B-B’ line of the PEGDA fluid film surface when t = 60 T and t = 60.5 T, respectively; (c) the height of the PEGDA calculated by the time-average acoustic potential.

The perturbation theory is applied to calculate acoustic field inside the acousticfluidics.36 Since it is much time consuming to calculate the acoustic flow in time domain for the 3D model, thus first-order term in perturbation theory is utilized. That means the governing equation in the photosensitive fluid also can be written as Eq. (3). We applied the following boundary conditions. The mechanical motion in the LiNbO3 substrate, which is driven by electric potential at the IDTs, can be expressed as

V (t )  V0 eit

(4)


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where V0 and ω are the voltage and angle frequency of the input signal, respectively. The lossy-wall condition is used to calculate the acoustic losses when the leak wave propagates from the calculated domain to the air through the side of the waveguide and the top of the fluid domain. This condition is often given as

n p  i

 f p  g cg

(5)

where ρg (1.21 kg/m3) and cg (343 m/s) are the density and sound speed of air, respectively.26 The generation of the wavy patterns on the liquid film can be calculated by the force balance between the gravity, capillary pressures, and acoustic radiation pressure. Several research have showed the force balance after the deformation of the fluid interface.38,39 although the fluid deformation are not induced by the SSAW.

According to our previous

study, the height of the 1D and 2D capillary wave motion on the PEGDA surface can be predicted by the acoustic pressure at the fluid film under the excitation of the SSAWs in the x- and y-axes (px and py) in a simply form.16

h1D  px 2

(6a)

h2D  px 2  p y 2

(6b)

The cross term px × py is eliminated in Eq. (6b) because of the different frequency in the xand y-axis.34 Therefore, Eq. (6b) can be used to calculate the height of the microarrays based on superposition after we calculate the piezoelectric effect and the wave propagation in the xand y-axes, respectively.

Note that Eq. (6a) and Eq. (6b) are good approximated from our

previous experiments, they can be used to calculate the distribution of the patterns. As for the accurate relationship between the acoustic pressure p and height of capillary motion h, it is much more complicated. The numerical modelling of the SSAWs and waveguide for the generation of localized microstructure patterns is established in COMSOL v5.3 Multiphysics software. In numerical


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modelling, the thicknesses of these four layers are set as 300, 200, 100, and 40 μm, which correspond to the experiments. The material properties of the LiNbO3 substrate,23 PDMS waveguide,23 glass wafer,26 and PEGDA fluid film26,27 were adopted from previous studies. The densities for the PDMS waveguide and PEGDA fluid are 1030 kg/m3 and 1100 kg/m3, whereas the sound speeds in these layers are 1076.5 m/s and 1650 m/s, respectively. The elasticity modulus and Poisson’s ratio of the glass wafer are 62 GPa and 0.24, respectively. During simulation, we investigated the resonant frequencies of the LiNbO3 substrate in the numerical model, which were 19.57 MHz in the x-axis and 18.69 MHz in the y-axis and correspond with the experimental settings (19.8 MHz in the x-axis and 18.4 MHz in the y-axis). To ensure the same strength of the SSAWs in the x- and y-axis on the LiNbO3 substrate, the input voltage in the y-axis is approximately 1.6 times greater than that of the x-axis due to the anisotropy of the 128° Y-cut piezoelectric substrate. The convergence tests were also conducted to ensure that meshing did not interfere with the simulation results. The displacement and acoustic pressure distribution at the cross-section of the x-z plane are illustrated in Figure 3a. We can see the generated and transmitted acoustic waves on the whole device at t = 60 T (T is the period of excitation sinusoidal signal). Most acoustic energies are concentrated into an area that corresponds to the size of the utilized acoustic waveguide. Only a few acoustic energies will propagate at the side region of the device and they dissipate quickly since energy decays with propagation. The acoustic pressure distribution between the half period at the top surface of the PEGDA fluid film is shown in Figure 3b. The sinusoidal-shaped acoustic pressure fluctuation with a period of λSAW can be observed and most acoustic pressure was focused in the region area of the acoustic waveguide boundaries. The contrary distribution of acoustic pressure (p) between the half period corresponds to the hypothesis of the harmonic time dependence of acoustic pressure (p(r,t) = p(r)exp(iωt)).28,30 Furthermore, the time-averaged acoustic potential based on acoustic pressure in time domain is calculated by Eq. (6) and the height of the PEGDA fluid film is simulated as shown in Figure 3c. We can also see that micro-wavy arrays are neatly distributed inside the region of the waveguide on the top of the fluid film, and the period of each peak is confirmed as the half value of the wavelength of the excited SAWs.


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2.4 Experimental Setup To validate the proposed method and numerical predictions, experiments were performed to examine the user-defined acoustic waveguide for the fabrication of localized patterned microstructures. Two sets of experiments are conducted: Firstly, to verify our hypothesis that the waveguide will determine the final area of the region-selective patterned microstructures, the circular-shaped waveguide with diameter of 2.0 mm is utilized. And using one pair and two pairs of IDTs to fabricate the linear and latticed patterned microstructures, respectively. Secondly, the triangular-shaped and square-shaped acoustic waveguides are utilized for patterned microstructures fabrication, and compared to those of using circular-shaped waveguide to find out whether different shaped waveguides will affect the final shaped of the region-selective patterned microstructures. The experimental process is adopted from previous report.16 During the experiments, a signal generator (Tektronix AFG, 3052C) is used to generate the sinusoidal waves, which amplified by a coaxial amplifier (ZHL-1-2W+). The input sinusoidal waves are set as 19.7 MHz and 300 mVpp in the x-axis and 18.4 MHz and 480 mVpp in the y-axis. For the solidification of the prepared fluid film, a 365 nm UV light source is used to cure the fluid with the protection of nitrogen.16 After the localized microstructures fabrication, a laser scanning confocal microscope (OLS4100, Olympus, Japan) is utilized for the measurement of the patterned microstructures, both the surface topography and height of the microstructures are measured.

3. Results and Discussion 3.1 Fabrication of Localized Microstructures With Circular Waveguides We first considered the circular-shaped acoustic waveguide for the fabrication of localized patterned microstructures. When using one pair and two pair of IDTs to excite the SSAWs, the numerical simulated acoustic pressure fields on the top of the fluid surfaces are shown in Figure 4a and 4b. We can see that the excited undulated patterns are distributed inside the region formed by the boundary of the utilized acoustic waveguide. Typically, when using one pair of IDTs, the micro-wavy patterned surface in the predetermined region can be generated, while the localized dotted patterns can be generated when using two pair of IDTs. As in Figure 4a and 4b, the nodes (green and other light colours) and antinodes (blue) are clearly distributed and they have the same


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period values of 100 μm. Figure 4c and 4d show the measured surface topography of the patterned surfaces when using one pair and two pair of IDTs; the localized micro-wavy and dotted patterned surfaces are successfully fabricated. Generally, the distribution and period of the fabricated patterned surfaces matched well with the numerical simulation results. However, the amplitude of the fabricated microstructures is more evenly than that of numerical results. That is due to only linear acoustic wave is considered in the acoustofluidics and the second-term in perturbation theory are neglect during numerical modeling.36 These neglected issues would make the induced patterns more evenly distributed without sharp or salient fluctuate at the central region in the fabricated patterns. Furthermore, the approximate relationship between acoustic pressure p and height of capillary wave motion h in Eqs. (6a)-(6b) may also induced some differences between the numerical modeling and experimental measurements. Some flaws happened in the fabrication results (Figure 4c and 4d). That may mainly attribute to the material defect and impurity in the piezoelectric substrate, the waveguide, the glass wafer and the fluid. Because they will also affect the acoustic wave propagation, thus affect the final fabrication resolution. Figure 4e and 4f shows the 3D laser scanning results of the fabricated microstructures in the region marked in Fig. 4(c) and (d). Results confirm that the microstructures with sinusoidal micro-wavy and dotted-like patterned surfaces can be fabricated, and both types of undulated periodical patterned surfaces have nearly the same period value of 100 μm, which equals half the value of the wavelength utilized for the SSAW device. The region for 3D laser scanning is selected near the central area of the fabricated microstructures, those regions usually have much more smooth and evenly distributed wavy patterns. The height of the micro-wavy patterned and dotted-like patterned surfaces are approximately 4.5 μm and 5.0 μm, respectively. This also demonstrates that the acoustic energy excited by the IDTs in the x- and y-axis are almost the same according to our experimental design.

From our previous study, the final height of the fabricated microstructures

can be controlled by the voltage input to the SAW device.37 According to our utilized amplifier, the input voltage can be adjusted from 100 mVpp to 700 mVpp. If the voltage is too low, the induced wavy patterns will be too small to be fabricated. Also, if the input voltage greater than 700 mVpp, the output sinusoidal-shaped signals will become distortion, thus the SAW device will not be operated on its resonating mode.


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Figure 4. Fabrication of localized patterned microstructures using the circular-shaped waveguide: the numerical simulation predicted results when using (a) one pair of IDTs and (b) two pair of IDTs; the measured surface topography of the localized patterned surfaces when using (c) one pair of IDTs and (d) two pair of IDTs; and 3D scanning results of (e) the micro-wavy patterned surface and (f) the dotted patterned surface.

Compared to the fabricated patterned microstructures when not using the waveguide, we can observed that the waveguide not only determine the region of the patterned microstructure, but also have some effects on the shape of microstructures at the boundary area. We can see that the


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region-selective patterns will be curved and dissipated at the boundary area of the waveguide, the width is about 2λ. That is mainly due to the boundary of the waveguide is contacted with air, making the acoustic waves propagate from the device to the air. The acoustic wave reflection happens on the interfaces between two different materials when it propagation across the PDMS waveguide, glass wafer and PEGDA film. Thus, the acoustic energy in this area would be weaker than the central region. So the amplitude of the fabricated region-selective patterned microstructures at the boundary area of the waveguide will be lower. However, the waveguide will not affect the period distribution of the patterned microstructures. Because the standing surface acoustic wave propagation in different materials will not affect its energy distribution, such as the nodes and antinodes distribution, these results can be shown in Fig. 3(a).

3.2 Fabrication of Localized Microstructures With Different Types of Waveguides We further examined the effects of using different shapes of the acoustic waveguide for the fabrication of localized microstructures. The square-shaped and triangle-shaped PDMS acoustic waveguides are utilized, and the side length of these two waveguides are the same as 2.0 mm. The numerical simulation predicted results and the fabrication results of the region pre-determined patterned microstructures are shown in Figure 5. The square-shaped and triangle-shaped patterned surfaces with inner sine-shaped undulate microstructures were successfully fabricated. Furthermore, the regions of these two patterns are determined by the boundary area of the waveguides. As for the sine-shaped undulate microstructures (Figure 5c and 5d) inside the square-shaped and triangle-shaped patterns, the periodical period and surface topography are similar with results obtained when using the circular-shaped waveguide. Similarly, the localized microstructures in various regions can be fabricated by using more complex waveguide. The size of the waveguide should be larger than 3λ (about 0.6 mm), or the boundary of the waveguide will have great effects, the random distributed instead of aligned or dotted microstructures will be generated inside the waveguide.20 Also, the smaller size of the waveguide will weaken the acoustic energy propagation upwards and leads to higher fabrication cost.


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Figure 5. Fabrication of region predetermined patterned microstructures using different shaped acoustic waveguides: numerical simulation predicted results using (a) a square-shaped waveguide and (b) a triangular-shaped waveguide; The measured surface topography of the fabricated results are presented using (c) a square-shaped waveguide and (d) a triangular-shaped waveguide.

4. Conclusions In summary, we proposed and demonstrated a novel method to fabricate polymer-based localized microstructures via SSAWs and user-defined acoustic waveguides. The working principle and mechanism of this method for the fabrication of localized microstructures are presented. Through numerical modelling and experimental demonstrations, the region pre-determined micro-wavy and dotted-like patterned microstructures can be successfully fabricated when using one pair or two pair of IDTs. The results highlight the feasibility for rapid and controllable fabrication of polymer-based region-selective microstructures for future biomedical applications. Further, the fabrication of region-selective microstructures can be controlled and adjusted by the shapes of the acoustic


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waveguide and the parameters (working frequency, pair of IDTs, and input voltage) of the standing SAWs. For future work, the accurate relationship between the capillary wave motion, acoustic pressure, and material properties, such as surface tension coefficient, acoustic speed, et al., needs to be developed. In addition, the application of this fabrication method for the region-selective and localized microstructures will be conducted.

Author Information Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest.

Acknowledgements The research is funded by the National Natural Science Foundation of China (Grant No. 51575485), Zhejiang Provincial Funds for Distinguished Young Scientists of China (Grant No. LR19E050001), Zhejiang Province Key Research and Development Plan Projects (Grant No. 2018C01053), and the Creative Research Groups of the National Natural Science Foundation of China (Grant No. 51521064).

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For Table of Contents Use Only Title: Standing surface acoustic wave-assisted fabrication of region-selective microstructures via user-defined waveguides Authors: Yancheng Wang*; Chenyang Han; Deqing Mei


Standing surface acoustic wave assisted fabrication of region

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