A surface treatment method for improving the attachment of PDMS: acoustofluidics as a case study

A method for a permanent surface modification of polydimethylsiloxane (PDMS) is presented. A case study on the attachment of PDMS and the lithium niobate (LiNbO3) wafer for acoustofluidics applications is presented as well. The method includes a protocol for chemically treating the surface of PDMS to strengthen its bond with the LiNbO3 surface. The PDMS surface is modified using the 3-(trimethoxysilyl) propyl methacrylate (TMSPMA) silane reagent. The effect of silane treatment on the hydrophilicity, morphology, adhesion strength to LiNbO3, and surface energy of PDMS is investigated. The results demonstrated that the silane treatment permanently increases the hydrophilicity of PDMS and significantly alters its morphology. The bonding strength between PDMS and LiNbO3increased with the duration of the silane treatment, reaching a maximum of approximately 500 kPa. To illustrate the effectiveness of this method, an acoustofluidic device was tested, and the device demonstrated very promising enhanced bonding and sealing capabilities with particle manipulation at a flow rate of up to 1 L/h by means of traveling surface acoustic waves (TSAW). The device was reused multiple times with no fluid leakage or detachment issues. The utility of the presented PDMS surface modification method is not limited to acoustofluidics applications; it has the potential to be further investigated for applications in various scientific fields in the future.


Micro fabrication and surface treatment protocol
The primary purpose of the surface silanization is to permanently change the surface properties of the PDMS.This change is required to ensure better adhesion between the PDMS and the LiNbO 3 wafer and prevent leakage once the device is in operation mode.The treatment should also enable the microdevice to be used multiple times.In this work, the attachment of PDMS to LiNbO 3 involves five steps: PDMS channel fabrication, TMSPMA silane treatment, washing and drying, plasma treatment, and thermal bonding, as illustrated in Fig. 1.To examine the effect of silanization on PDMS, multiple samples were treated with silane for varying time periods and then tested.Each sample has been labeled according to Table 1.The labels used to represent each sample in Table 1 will be used throughout the work.

PDMS channel fabrication
A PDMS (SYLGARD 184 Silicone Elastomer from DOW chemical company) part with a microchannel is fabricated using standard soft lithography process.A master mold is created on a silicon wafer using SU-8 (negative photoresists from MICRO-CHEM) and then used as a PDMS replica mold.Many published works have already described the steps involved in making a microchannel using PDMS 29,30 .

Silane treatment
To prepare the silane solution, a mixture of ethanol (88 wt.%) (from Fisher Scientific UK), deionized water (6 wt.%), and TMSPMA (6 wt.%) (TMSPMA 98% from Sigma-Aldrich) is prepared based on literature 3 .The solution is prepared by mixing ethanol and deionized water, and then the TMSPMA is added.A beaker is used to immerse the previously prepared PDMS microchannel in the silane solution.PDMS must be completely submerged in the silane solution, and the glass beaker with the sample and silane must be carefully covered and sealed to prevent any evaporation.Six representative samples prepared using the same method but salinized for different time durations were used to assess the silanization effect, as shown in Table 1.

Washing and drying
After the silanization step, ethanol is used to wash the samples.This step ensures that any remaining silane is removed from the PDMS part.As the presence of silane in the microchannel during experiments may affect the results and raise safety concerns, the washing procedure is repeated multiple times to ensure the complete removal of excess silane.Following this washing step, compressed nitrogen is used to dry the treated PDMS microchannel.If the microchannel inlets and outlets were not pierced during fabrication, the holes can also be made after silanization.www.nature.com/scientificreports/Plasma treatment PDMS is hydrophobic by nature, so it must be treated with oxygen/argon plasma before it can be bonded to a LiNbO 3 wafer 31 .Although TMSPMA silanization improved the hydrophilicity of the PDMS surface in a permanent manner, oxygen plasma treatment has been proven in this experiment that it can further improve the hydrophilicity of the PDMS surface temporarily.

Thermal bonding
The PDMS microchannel is thermally bonded to the LiNbO 3 wafer as the final step in the attachment process.Following the plasma treatment of both the PDMS microchannel and the LiNbO 3 wafer, a drop of ethanol is placed on the wafer to facilitate the alignment of the PDMS microchannel at the desired location on the LiNbO 3 wafer.The alignment is performed using a microscope, after which the device is left at room temperature for a few minutes to allow the ethanol to evaporate.The PDMS microchannel and LiNbO 3 wafer are then heated at 90 °C for 4 hours with a weight element of 1-2 kg placed on top of them.This process strengthens the bond between the two parts.The heating process is then terminated, and the device is gradually cooled to room temperature prior to use.

Results and discussion
Several assessments were carried out to study the effect of silanization on the surface of PDMS as well as the attachment of PDMS to LiNbO 3 .Results from these assessments are presented in the following sections.

Microscopic imaging
As depicted in Fig. 2 (a-c, right-side), three of the six representative samples examined under a microscope are shown.The microscopic images clearly show the effect of silanization on the surface morphology of PDMS (extra images are available in the supplementary material).Figure 2a depicts the surface of an untreated sample (S-0).The S-0 surface is devoid of any visible surface characteristics.Tiny, mound-shaped defects are depicted for the S-30 sample, which was treated for only 30 min.It indicates that the silanization process has just begun to affect the surface morphology of the PDMS. Figure 2b depicts microscopic images of the surface of an S-60 sample.
The formation of clear mounds indicates that the silanization process impacts the PDMS surface and changes its morphology.For S-120, which underwent a two-hour silanization process, The mounds are significantly larger than those observed on the surface of S-60.In Fig. 2c, it is evident that the density and size of the mounds increase as the silanization period increases.S-180 PDMS samples have a significantly higher mound density than S-120 samples, and the same is true for S-240 compared to S-180.It is clear from the microscopic images that the silanization process has successfully altered the surface morphology of the PDMS.However, additional testing is needed to get a clear picture of the PDMS surface morphology following the silanization process.

Atomic force microscopy
The atomic force microscopy (AFM) is the second examination that is performed.The AFM is used to examine the PDMS's surface topography after the silanization process.AFM provides valuable information and an indication of changes in the surface topography of the PDMS sample, which directly affect its wettability and bond strength to other materials.Figure 2 illustrates the AFM microscopy results for three samples of the six cases.The results of AFM microscopy are consistent with those of microscopic imaging in terms of the general effect of the silane on the PDMS surface.In Fig. 2a, the AFM results for the untreated PDMS sample, S-0, do not reveal any surface variation or roughness.A minor change in the surface morphology was observed in S-30.This indicates that the silanization process begins to affect the surface roughness of the PDMS after 30 min.Figures 2b and c illustrate the changes in surface morphology, and the results demonstrate the effect of silanization time on the PDMS surface.The root mean square (RMS) value for the PDMS surface morphology is shown in Fig. 3.The RMS, which represents the number of peaks and valleys, or surface height variation created in the PDMS surface after the silanization process, provides a clear indication of the extent to which the silanization process can affect the PDMS surface morphology.As shown in Fig. 3, the RMS value increases significantly after three hours of silanization.It is important to note that the surface of the S-240 sample was clearly curved, as determined by a visual assessment of the sample.The swelling issue has been reported previously in multiple studies 32 .

Wettability test
The surface energy of a microfluidic device is crucial to a number of operations.In this section, the effect of the silanization process on the surface wettability is investigated.Hydrophilic PDMS microchannels have several advantages in biological applications and should enhance the adhesion of PDMS to various surfaces.
To determine the effectiveness of the silanization process on wettability, the contact angle of six representative samples was measured with a goniometer using the sessile drop method.A high-resolution camera-equipped optical subsystem was used to capture the profile of the sessile drop and then calculate the contact angle.
The wettability test results are shown in Fig. 4.After 30 min of silanization, the contact angle has decreased dramatically, as shown in the figure.This demonstrates that the salinization process improves the wettability of the PDMS surface.It is also important to note that the duration of silanization has no significant effect on the contact angle.Contact angles from wettability tests showed high consistency between the five treated samples (excluding S-0). Figure 5 depicts the contact angle for the treated sample (S-60) and the untreated PDMS sample (S-0).The figure shows that the salinization process modifies the wettability of PDMS and improves its ability to support different applications in the microfluidic field.It's important to note that the wettability test was performed directly after the silanization process and again after 3 days, revealing similar results.This indicated that the change in wettability caused by this silanization permanently impacts the PDMS surface's wettability.

Fourier-transform infrared spectroscopy
The Fourier-transform infrared (FTIR) spectra of S-0, S-60, S-60 treated with plasma, S-240, and S-240 treated with plasma are shown in Fig. 6.The samples were studied to compare the chemical groups on the PDMS surface and to analyze the changes of the functional groups on the PDMS surface during silanization with TMSPMA and plasma treatments.The FTIR spectra of the S-0 substrate (Fig. 6) show a peak at 790 cm −1 for the CH3 rocking and Si-C stretching modes in Si-CH3, peaks in the range of 950-1150 cm −1 for the stretching mode of Si-O-Si, a peak at 1258 cm −1 for the symmetric CH3 bending in Si-CH3, and a peak at 2962 cm −1 for the C-H stretching www.nature.com/scientificreports/ in CH3, in agreement with the literature [33][34][35] .The peaks at 1720 and 1640 cm −1 for S-60 and S-240 correspond to the deformation of C=O and C=C groups present in the structure of TMSPMA, respectively, indicating a successful modification process.The absorption peak near 3470 cm −1 corresponds to the presence of -OH groups formed during the hydrolysis of methoxy (OCH3) groups during the silanization process.Oxygen-containing groups such as carbonyl and hydroxyl groups formed during the silanization process are the main reason for the reduction of the contact angle and the improvement of the hydrophilicity of PDMS surfaces.Compared to S-60 and S-240, the intensity of oxygen-containing groups increases with plasma treatment.This can be attributed to the ability of plasma to break C=C bonds in TMSPMA and form new C-O and C=O groups 36 .In addition, the plasma can also transfer unhydrolyzed Si-OCH3 to Si-OH in this step of surface modification 4 .This increase in the concentration of oxygen-containing groups after plasma treatment could be the reason for the improvement in the adhesion between the PDMS microchannel and the LiNbO 3 wafers.Figure 7 illustrates the proposed mechanism of bonding PDMS and LiNbO 3 substrates.

Bonding strength measurement (tensile test)
The silanization effect on the bond strength between PDMS and LiNbO 3 was assessed using a tensile test.The tensile test measures the bonding between the PDMS microchannel and the LiNbO 3 wafer.Tests were conducted

Experiments
The silanization protocol described in this work is used to build acoustofluidic systems and then test their hermetical sealing and applications for microparticle manipulation at very high flow rates.The results of both tests are detailed in the following sections.

Hermeticity test
In the hermeticity test, a simple microchannel with one inlet and two outlets is subjected to varying water flow rates, as depicted in Fig. 10.The microchannel had dimensions of 160 µm in width, 70 µm in height, and 3000 µm in length.The microfluidic device was fabricated using the described silanization technique.The PDMS element was silanized for 60 min, as the S-60 sample demonstrated the highest bonding strength.A syringe pump was used to flow a colored solution of deionized water into the microchannel of the device, thereby testing its hermeticity.The flowrate was increased gradually from 0 to 1 L/h.which is extremely high flowrate for microfluidics.The effect of sudden pressure variation on the hermeticity of the microfluidic device was tested by   www.nature.com/scientificreports/implementing sudden changes in flowrate.The microchannel was able to withstand both the high flowrate and the high flowrate variations.There were no leakage issues or indications of PDMS debonding from the LiNbO 3 wafer.It was also confirmed through repeated testing of the same device that the microdevices can be reused multiple times without degrading in performance.
Compared to other PDMS surface enhancement methods, the proposed method illustrated excellent permanent fluid sealing capabilities and good bonding results.A comparison of different surface enhancement methods is shown in Table 3.In terms of tensile strength capability, the silanization method demonstrated an average value among other enhancement methods using plasma treatment.However, in terms of sealing capabilities, the silanization method demonstrated outstanding performance, handling a flow of up to 1 L/h.This sealing capability can be explored further to be applied in different engineering applications.It is also clearly mentioned that plasma treatment methodologies are of temporary nature, whereas the silanization method demonstrated a permanent nature in terms of PDMS surface enhancement.It's worth mentioning that this PDMS surface enhancement method can be further investigated to be applied in different fields where a high flowrate is needed.

Case study: particle manipulation via TSAW
This case study presents an experiment that demonstrates the use of TSAW to manipulate PS particles on an acoustic manipulation platform that was built in accordance with the prescribed treatment method.The flowrates and micro entity types of TSAW devices available in the literature are presented in Table 4. Clearly, the flowrate observed with the new surface treatment protocol is significantly higher than those reported in the literature.This means that the new method for PDMS surface enhancement is compatible with the majority of current applications using TSAW.Whereas the plasma treatment methodology covers limited applications using TSAW where the flow rate required is below 100 µL/min.The reported bonding method for PDMS and LiNbO 3 paves the way for increased throughput in acoustofluidics applications with high flow rates.In addition, this new method will facilitate the creation of new microsystems and devices that can be utilized to facilitate and accelerate manipulation processes.
Acoustophoresis, the process of controlling particle movement by utilizing sound waves (pressure) to induce particle migration, encompasses three main types of acoustic force application in microfluidics.These methods include bulk acoustic waves, standing SAW, and TSAW.This case examines the latter phenomenon.TSAW manipulation is determined by distinct parameters, including the particle size, particle density, and compressibility of the target particles in relation to the suspending liquid.Before examining the proof-of-concept experiment, a section is dedicated to describing the related fundamentals of the TSAW particle manipulation platform 8 .
Acoustophoresis is a label-free and non-contact particle manipulation technique that enables high-throughput microdevices.Moreover, the use of TSAW has been proven as a secure method to manipulate biological samples because of its biocompatibility, which has highly impacted the development of TSAW-based acoustophoretic www.nature.com/scientificreports/platforms for various types of microparticle migration in a very positive way 30,[37][38][39] .The term "TSAW" refers to travelling or progressive surface waves which dissipate after travelling a certain distance because of the attenuation effect.King 40 studied the effect of acoustic radiation pressure on a rigid sphere.His research resulted in the development of an equation that permits the calculation of the radiation pressure on rigid spheres with a small circumference relative to the applied wavelength, as follows: where ρ 0 and ρ 1 are the medium and the sphere densities respectively.|A| is the coefficient of the incident radiation field and α is a dimensionless parameter defined as α = ka , where a is the radius of the sphere and k is the incident wave number ( k = 2π/ ) .Generally, TSAW can be simply produced by printing a single IDT on a piezoelectric wafer, in this case a LiNbO 3 wafer, and connecting and supplying the IDT to the required power supply to create the required acoustic force.Further investigation into the acoustic radiation pressure by Hasegawa and Yosioka 41 concluded that the sphere's elasticity has an effect on the calculation of the acoustic radiation force.Their studies were focused only on plane progressive TSAW.They derived the following representation of the acoustic radiation force generated by TSAW on an elastic sphere as: where F TSAW is the time averaged acoustic radiation force by TSAW,Y T is a dimensionless parameter called the acoustic radiation force factor defined as the acoustic radiation force per unit acoustic energy density per unit cross sectional area of the microsphere, a is the radius of the sphere, and E is the mean energy density.Similar to the hermeticity test, the PDMS was silanized for sixty minutes as it showed the best results in terms of bonding strength and hermeticity.Figure 11 illustrates schematically the manipulation platform and the working principles.In Fig. 11a, the particles are moving while the IDT is not receiving a signal to generate the necessary acoustic force.In this instance, particles are exiting both outlets.Figure 11b depicts the anticipated result of manipulating PS particles with TSAW.As TSAW pushes the particles away, it is anticipated that they will flow near the far wall.Particles will only exit the channel through outlet B. As depicted in Fig. 12, the TSAW particle manipulation experiment was conducted and visualized under an inverted microscope (Zeiss Axio Observer) equipped with a high-speed camera (Fastcam SA-X2 from Photron) to monitor the manipulation process.PS particles (from micro-Particles GmbH) with a diameter of 6.69 µm were suspended in deionized (1) + terms in α 8 and higher powers (DI) water (suspension medium) and then pumped into the microchannel using a syringe pump at a flow rate of 1 mL/h.This high flow rate was selected to demonstrate the microchannel's ability to withstand high flow rates without leaking.To prevent PS particles from adhering to the channel's walls, a drop of Tween-20 was added to the solution.Once the flow became stable within the microchannel, a waveform generator was used to apply an AC signal with a frequency of 100 MHz (IDT resonant frequency) and a voltage amplitude of 5 V peak-to-peak to the IDT array (RIGOL DG4102).Signal amplification was performed using a power amplifier (ZHL-1-2W-N+).The device was able to manipulate PS particles and direct them toward the opposite channel's wall.The same device has been tested multiple times at various flow rates with no leakage observed.

Conclusion
In conclusion, a novel surface treatment technique has been developed to permanently enhance the bonding and hermeticity of acoustofluidic devices.Using the TMSPMA silane surface modification reagent, a detailed implementation protocol for the surface treatment has been described.The method does not require extremely high temperatures or pressure to be applied.Compared to other temporary plasma-based treatments, the demonstrated method enabled a permanent enhancement of PDMS hydrophilicity.Multiple characterization tests were conducted to examine the effect of the treatment method on the PDMS surface.The results of the tests indicated that a treatment duration of one hour using TMSPMA is optimal.The hydrophilicity of PDMS has been shown to be permanently and significantly improved after one hour of silanization.In addition, the treatment positively affected the morphology of the PDMS surface, while the bonding strength was increased by more than twofold.The maximum sealing pressure of approximately 500 kPa was reached for the sample that was treated for one hour.The PDMS surface energy exhibited a noticeable increase.These improvements demonstrate that the newly introduced method is very promising and can aid in creating acoustofluidic devices.Experiments demonstrating proof-of-concept have illustrated that the fabricated devices can withstand a flow rate of up to 1 L/hr without leaking or debonding, which covers TSAW-based and other microfluidic applications.With particles being manipulated at a flow rate of 1 mL/hr via TSAW, the demonstration experiment of the TSAW-based particle manipulation device showed promising improvements in boning and sealing.It has been demonstrated that the introduced PDMS surface treatment method permanently improves the bonding efficiency and bonding strength of PDMS and LiNbO 3 .Microfluidic devices based on SAW technology can make extensive use of this improved performance.

Figure 3 .Figure 4 .
Figure 3.The RMS values chart that represents the surface height variation for the 6-representative samples.

Figure 5 .
Figure 5.An illustration of the difference in the contact angle of the treated sample (S-60) on the right side compared to the bare PDMS sample (S-0) on the left side.

Figure 8 .
Figure 8.Right side: schematic illustration of the tensile test setup.Left side: tensile testing machine loaded with one of our samples for tensile testing.

Figure 9 .
Figure 9.The applied force vs the elongation curve generated from the tensile test for the 6-representative samples during the bonding strength measurement.

Figure 10 .
Figure 10.A photo of the microchannel leakage testing conducted using colored deionized water and syringe pump.The tested channel consists of one inlet at the left side and two outlets at the right side.

( 2 )Figure 11 .
Figure 11.Schematic illustration of the TSAW based particle manipulation platform.(a) TSAW off: particles are flowing everywhere along the channels width and leaving the channel from both outlets.(b) TSAW on: particles are pushed to the channel's wall away from the IDT and particles are leaving the channel trough outlet B only.

Figure 12 .
Figure 12.PS particles manipulation experiment as observed under an inverted microscope.(a) The location is at the middle of the microfluidic device.The TSAW device is off.The PS particles are flowing everywhere along the width of the channel at flow rate of 1000 µL/h.(b) The location is at the middle of the microfluidic device.The TSAW device is on.The PS particles are pushed toward the opposite channel's wall due to the acoustic radiation force.(c) The location is at the outlet of the microfluidic device.The TSAW device is off.The PS particles are exiting the channel through both outlets at flow rate of 1000 µL/h.(d) The location is at the outlet of the microfluidic device.The TSAW device is on.The PS particles are exiting from the upper outlet only because of the effect of the acoustic radiation force.

Table 1 .
Samples labels used to represent each sample throughout the entire work for accuracy and consistency.

Table 2 .
The results of the tensile strength test for the 6-representative samples showing the average maximum load and the average tensile strength of each sample.Sample's bonded surface area = 15 mm × 15 mm.

Table 3 .
Comparison of different surface treatment methodologies to enhance the bonding between PDMS and LiNbO 3 .Significant values are in bold.

Table 4 .
Literature review of the available manipulation devices (PDMS + LiNbO 3 ) that utilizes TSAW showing the different flowrates (throughput).