I. INTRODUCTION

Section:

ChooseTop of pageABSTRACTI. INTRODUCTION <<II. MATERIALS AND METHODSIII. RESULTS AND DISCUSSI...IV. CONCLUSIONSREFERENCESDry reversible adhesives inspired by attachment organs of animals have been first developed 15 years ago.11. L. F. Boesel, C. Greiner, E. Arzt, and A. Del Campo, Adv. Mater. 22, 2125 (2010). https://doi.org/10.1002/adma.200903200 They find several usages, such as in robotics, signaling, or underwater applications due to their specific surface patterning and design.2–72. Y. Li, J. Krahn, and C. Menon, J. Bionic Eng. 13, 181 (2016). https://doi.org/10.1016/S1672-6529(16)60293-73. D. M. Drotlef, M. Amjadi, M. Yunusa, and M. Sitti, Adv. Mater. 29, 1701353 (2017). https://doi.org/10.1002/adma.2017013534. M. K. Kwak, H. E. Jeong, and K. Y. Suh, Adv. Mater. 23, 3949 (2011). https://doi.org/10.1002/adma.2011016945. M. Varenberg and S. Gorb, J. R. Soc. Interface 5, 383 (2008). https://doi.org/10.1098/rsif.2007.11716. E. Kizilkan, J. Strueben, A. Staubitz, and S. N. Gorb, Sci. Robot 2, eaak9454 (2017). https://doi.org/10.1126/scirobotics.aak94547. E. Kizilkan, L. Heepe, and S. Gorb, in Biological and Biomimetic Adhesives Challenges and Opportunities, edited by R. Santos, N. Aldred, S. Gorb, and P. Flammang (RSC, Cambridge, 2013), pp. 65–71. One of the most important abilities of such dry adhesives is their reversibility, which allows them to repeatedly apply in many cycles of attachment and detachment without any permanent (plastic) deformation.8–108. H. Lee, B. P. Lee, and P. B. Messersmith, Nature 448, 338 (2007). https://doi.org/10.1038/nature059689. A. K. Geim, S. Dubonos, I. Grigorieva, K. Novoselov, A. Zhukov, and S. Y. Shapoval, Nat. Mater. 2, 461 (2003). https://doi.org/10.1038/nmat91710. T. Xie and X. Xiao, Chem. Mater. 20, 2866 (2008). https://doi.org/10.1021/cm800173cThe mushroom-shaped microstructure is one of the prominent examples of bioinspired dry adhesives.11–1311. S. Gorb, M. Varenberg, A. Peressadko, and J. Tuma, J. R. Soc. Interface 4, 271 (2007). https://doi.org/10.1098/rsif.2006.016412. L. Heepe, M. Varenberg, Y. Itovich, and S. N. Gorb, J. R. Soc. Interface 8, 585 (2011). https://doi.org/10.1098/rsif.2010.042013. L. Heepe and S. N. Gorb, Annu. Rev. Mater. Res. 44, 173 (2014). https://doi.org/10.1146/annurev-matsci-062910-100458 They are inspired by the attachment organs of a leaf beetle Gastrophysa viridula, whose male demonstrated better adhesion performance when compared to the female.14,1514. D. Voigt, J. Schuppert, S. Dattinger, and S. Gorb, J. Insect Physiol. 54, 765 (2008). https://doi.org/10.1016/j.jinsphys.2008.02.00615. J. M. Bullock and W. Federle, Naturwissenschaften 98, 381 (2011). https://doi.org/10.1007/s00114-011-0781-4 This was attributed to the existence of discoidal hair morphology, which inspired the mushroom-shaped adhesive microstructure.14,16,1714. D. Voigt, J. Schuppert, S. Dattinger, and S. Gorb, J. Insect Physiol. 54, 765 (2008). https://doi.org/10.1016/j.jinsphys.2008.02.00616. E. Gorb, N. Hosoda, C. Miksch, and S. Gorb, J. R. Soc. Interface 7, 1571 (2010). https://doi.org/10.1098/rsif.2010.008117. D. Voigt, M. Varenberg, J. Schuppert, and S. Gorb, J. Insect Physiol. 127, 104158 (2020). https://doi.org/10.1016/j.jinsphys.2020.104158 Previous studies on bioinspired microstructures also showed that among reversible dry adhesives, the mushroom-shaped microstructures possess superior adhesive performance to other bioinspired microstructures that were made of the same silicone elastomer.18,1918. G. Carbone, E. Pierro, and S. N. Gorb, Soft Matter 7, 5545 (2011). https://doi.org/10.1039/c0sm01482f19. A. Del Campo, C. Greiner, and E. Arzt, Langmuir 23, 10235 (2007). https://doi.org/10.1021/la7010502 Their outperforming adhesive abilities were attributed to the van der Waals forces and the specific crack entrapment ability.11,13,2011. S. Gorb, M. Varenberg, A. Peressadko, and J. Tuma, J. R. Soc. Interface 4, 271 (2007). https://doi.org/10.1098/rsif.2006.016413. L. Heepe and S. N. Gorb, Annu. Rev. Mater. Res. 44, 173 (2014). https://doi.org/10.1146/annurev-matsci-062910-10045820. M. Varenberg and S. Gorb, J. R. Soc. Interface 5, 785 (2008). https://doi.org/10.1098/rsif.2007.1201 They can demonstrate similar adhesive performance in millions of cycles.In recent years, several works have been undertaken to enhance the adhesive performance of these microstructures by application of adhesive wax layers onto the microstructures, modifying their geometrical parameters, and by heat or plasma treatment.21–2821. A. E. Kovalev, M. Varenberg, and S. N. Gorb, Soft Matter 8, 7560 (2012). https://doi.org/10.1039/c2sm25431j22. P. Glass, H. Chung, N. R. Washburn, and M. Sitti, Langmuir 26, 17357 (2010). https://doi.org/10.1021/la102924523. S. C. Fischer, K. Groß, O. Torrents Abad, M. M. Becker, E. Park, R. Hensel, and E. Arzt, Adv. Mater. Interfaces 4, 1700292 (2017). https://doi.org/10.1002/admi.20170029224. B. Aksak, K. Sahin, and M. Sitti, Beilstein J. Nanotechnol. 5, 630 (2014). https://doi.org/10.3762/bjnano.5.7425. M. P. Murphy, S. Kim, and M. Sitti, ACS Appl. Mater. Interfaces 1, 849 (2009). https://doi.org/10.1021/am800243926. M. Seong, C. Jeong, H. Yi, H.-H. Park, W.-G. Bae, Y.-B. Park, and H. E. Jeong, Appl. Surf. Sci. 413, 275 (2017). https://doi.org/10.1016/j.apsusc.2017.04.03627. M. Seong, J. Lee, I. Hwang, and H. E. Jeong, Int. J. Precis. Eng. Manuf. 6, 587 (2019). https://doi.org/10.1007/s40684-019-00062-z28. E. Kizilkan and S. N. Gorb, ACS Appl. Mater. Interfaces 10, 26752 (2018). https://doi.org/10.1021/acsami.8b06686 Some of these methods require additional stages of production or higher material consumption that lead to an increase in manufacturing cost and/or contamination of these surfaces exposed to additional surroundings that contain such as dust or an unfavorable amount of adhesive proteins. On the contrary, plasma treatment has several advantages over the other method to develop better reversible adhesives. Atmospheric plasma treatment (APT) is utilized to functionalize the surfaces by modifying the surface chemistry, mechanical properties, or surface topography.29,3029. M. Thomas and K. L. Mittal, Atmospheric Pressure Plasma Treatment of Polymers Relevance to Adhesion (Scrivener, Wiley, Beverly, 2013).30. Y. Kusano, J. Adhes. 90, 755 (2014). https://doi.org/10.1080/00218464.2013.804407 For instance, it can easily be integrated into the production stage, e.g., in an assembly line, which enables the surfaces to be functionalized without a solid contact with other surfaces. On the one hand, even for short treatment times by APT, adhesive performance enhancement can be obtained in comparison to heat treatment, which requires longer treatment times.3131. H. M. Abourayana and D. P. Dowling, “Plasma processing for tailoring the surface properties of polymers,” in Surface Energy, edited by A. Mahmood (InTech, Rijeka, 2015), pp. 123–152. However, too high stickiness might deform the fine contact elements irreversibly in many cycles of use that in turn may induce a consequential deprivation of reversibility and shortage of material lifetime. Besides these, such APTs are used for surface cleaning32,3332. J. M. Koo, J. B. Lee, Y. J. Moon, W. C. Moon, and S. B. Jung, J. Phys. Conf. Ser. 100, 012034 (2008). https://doi.org/10.1088/1742-6596/100/1/01203433. S. M. Hong, S. H. Kim, J. H. Kim, and H. I. Hwang, J. Phys. Conf. Ser. 34, 656 (2006). https://doi.org/10.1088/1742-6596/34/1/108 and surface etching that can be potentially harmful to the microstructured surfaces. Therefore, the surface treatment parameters must be optimized to obtain better reversible adhesive properties.

For the development of the static attachment performances of dry adhesives, we propose APT as an alternative enhancement method that can easily be integrated into an assembly line or production stage because it enables functionalization of microstructured surfaces without any solid or liquid contact. Consequently, in the present study, the time parameter of plasma jet treatment was optimized to obtain maximal adhesive performance of the microstructured samples made of silicone elastomer. After adhesion performance tests, we made effective elastic modulus measurements and optical characterization of sample surfaces to elucidate the influence of the APT on the microstructure, surface, and material that lead to the improvement of adhesive properties.

II. MATERIALS AND METHODS

Section:

ChooseTop of pageABSTRACTI. INTRODUCTIONII. MATERIALS AND METHODS <<III. RESULTS AND DISCUSSI...IV. CONCLUSIONSREFERENCESIn this work, we used mushroom-shaped microstructured surfaces and their unstructured counterpart made of silicone elastomer polyvinylsiloxane (PVS). The individual microstructures consist of a stalk, contact plate on top, and a neck between them (Fig. 1). The mushroom-shaped microstructured surfaces were obtained from Gottlieb Binder GmbH (Holzgerlingen, Germany).

The samples were plasma treated with an atmospheric pressure plasma system (DienerPlasmaBeam, Ebhausen, Germany). As the plasma gas, atmospheric air was used (input pressure, 5.5 bar; power, 300 W; frequency, 20 kHz; gas flow, 2 m3/h).

Similar to our previous work, adhesion performance was characterized by a pull-off test through the measuring forces during separation between a glass sphere of 1 mm radius and the samples made of PVS.66. E. Kizilkan, J. Strueben, A. Staubitz, and S. N. Gorb, Sci. Robot 2, eaak9454 (2017). https://doi.org/10.1126/scirobotics.aak9454 The forces were recorded from a force transducer (10 g capacity; World Precision Instruments, Inc., FL, USA) connected to an amplifier (Biopac Systems Ltd., Goleta, CA, USA). The glass sphere approached the PVS surface with a micromanipulator (DC3314R, World Precision Instruments, Inc., Sarasota, FL, USA). The micromanipulator was driven with a speed of 100 μm/s to preload test samples with a force of 10 mN.

The adhesion experiments were statistically analyzed with one-way ANOVA Shapiro–Wilk normality tests using sigmaplot 12.5 software (t-test; SPSS Inc., San Jose, CA, USA).

Dynamic contact angle measurements of experimental and control surfaces were performed with a high-speed optical contact angle measuring device OCAH 200 (DataPhysics Instruments, Filderstadt, Germany). By using the sessile drop method (drop volume, 1 μl), the test surfaces were tilted until the water drop was moved by gravity. These processes were video-recorded. The advancing and receding contact angles were calculated according to the moment when the drops started to move on the unstructured sample surface.

To monitor the thermal changes on plasma-treated surfaces, the IR images of sample surfaces were recorded with a thermal imaging camera (Trotec IC 080V, Heisenberg, Germany).

The effective elastic modulus of the samples was measured by microindentation using a microforce tester Basalt-02 (TETRA GmbH, Ilmenau, Germany). As a probe, the glass sapphire sphere with a radius of 1 mm attached to the steel cantilever (spring constant = 200 N/m) was brought in contact with the unstructured surface. The indentation depth was 50 μm. From the force–distance curve obtained, the effective elastic modulus was calculated by fitting a JKR model of adhesive contact using MATLAB R2012b (The MathWorks, Inc., Natick, MA, USA).

The surface topographies of mushroom-shaped microstructured surfaces were examined using a scanning electron microscope TM-3000 at 5 kV acceleration voltage (Hitachi Ltd., Tokyo, Japan) before and after particular plasma treatments. The samples were coated with a 10 nm AuPd layer by a BAL-TEC SCD 500 Sputter Coater using a BAL-TEC QSG 100 Quartz Film Thickness Monitor (Bal-tec AG, Balzers, Lichtenstein).

The surface topography of the samples was quantified before and after the particular plasma treatment using a white light interferometer NewView 6k (Zygo, Middlefield, CT, USA) at 50× magnification. The roughness parameter was demonstrated as the root mean square (rms) value.

III. RESULTS AND DISCUSSION

Section:

ChooseTop of pageABSTRACTI. INTRODUCTIONII. MATERIALS AND METHODSIII. RESULTS AND DISCUSSI... <<IV. CONCLUSIONSREFERENCES

The maximal plasma gas beam length was about 10 cm. Therefore, the surfaces of mushroom-shaped microstructured and unstructured samples were kept at 5 cm distance to the plasma nozzle; the distance is half of the maximal plasma beam length. The working distances smaller than 5 cm resulted in no improvement in adhesion performance characterized with the pull-off force measurements. The adhesion performances at 5 and 7 cm working distances did not differ statistically significantly.

The influence of plasma treatment duration on adhesion performance was studied for nine different time intervals (10 s, 30 s, 45 s, 1 min, 2 min, 3 min, 4 min, 5 min, 10 min). The pull-off forces for each time interval were measured before (Fun) and after (Fpl) plasma treatment. The results are depicted as relative adhesion, where pull-off forces after plasma treatment were divided by those before the treatment (Fpl/Fun). The relative adhesion data are shown for unstructured and mushroom-shaped microstructured samples in Figs. 2(a) and 2(b), respectively.For unstructured surface, the adhesion for treatments with a duration from 1 to 5 min was significantly stronger than that for the untreated control sample (one-way ANOVA, for 1 min P = 0.014; for 2, 3, 4, and 5 min, P 3434. K. L. Johnson, K. Kendall, and A. D. Roberts, Proc. R. Soc. London Ser. A 324, 301 (1971). https://doi.org/10.1098/rspa.1971.0141 However, after 10 min of APT, the relative adhesion decreased on average to 80% of that of the untreated sample (P P = 0.208, P = 0.298, and P = 0.085, respectively).

For the mushroom-shaped microstructured surface, however, the influence of APT on adhesion performance was different from that for the unstructured surface. By increasing the time of treatment below 1 min, the relative adhesion increased on average and reached a maximum value at 1 min of treatment (60.5%) (P < 0.001). The treatments of 1  and 2 min showed a highly significant increase in relative adhesion (P < 0.001). After reaching the maximum at the treatment time of 1 min, the relative adhesion decreased stepwise with increasing treatment time to 8.7% at 10 min of treatment time.

The APT might have several influences on a substrate, such as changes in surface chemistry, topological modifications, and heat-induced material changes. Commonly, plasma treatment is employed on the polymer surfaces for their activation purposes through creating more polar units on the surfaces. Consequently, the overall surface-free energy in general and its polar component, in particular, increase, and this, in turn, leads to an increase in adhesion. One way to characterize this change is through the contact angle measurement. Therefore, as the first step to understand the change in the adhesive performance after APT, dynamic contact angle measurements were performed to analyze the influence of APT duration on the surface chemistry of unstructured PVS surfaces. In our previous work, the water contact angles of the microstructured surface (∼130°) were measured, but no changes had been observed or the water droplets had been entrapped between the individual microstructures.2828. E. Kizilkan and S. N. Gorb, ACS Appl. Mater. Interfaces 10, 26752 (2018). https://doi.org/10.1021/acsami.8b06686 For the unstructured surface in this work, the advancing and receding contact angles were measured (Fig. 3). Contact angle hysteresis was calculated by subtracting the advancing and receding contact angles from each other. The measurements indicate that the contact angle and contact angle hysteresis did not demonstrate dramatic changes in the surface-free energy at even relatively long APT durations (also see the supplementary material). This result might be elucidated by the thermal effect that accelerates the hydrophobic recovery, which is mainly due to reorientation and/or diffusion of smaller molecular weight polar units from surface plasma exposed to the bulk polymer.35–3735. Y. I. Yun, K. S. Kim, S.-J. Uhm, B. B. Khatua, K. Cho, J. K. Kim, and C. E. Park, J. Adhes. Sci. Technol. 18, 1279 (2004). https://doi.org/10.1163/156856104158820036. M. Noeske, J. Degenhardt, S. Strudthoff, and U. Lommatzsch, Int. J. Adhes. Adhes. 24, 171 (2004). https://doi.org/10.1016/j.ijadhadh.2003.09.00637. D. Bodas and C. Khan-Malek, Sens. Actuators B 123, 368 (2007). https://doi.org/10.1016/j.snb.2006.08.037To quantify the thermal influence of the APT and to understand the lack of apparent change in the surface hydrophobicity after plasma treatment due to the application of heat to the surface, the temperature of the samples' surfaces were measured using the thermal imaging camera (Fig. 4). It was observed that the plasma treatment time is related to the temperature: longer treatments led to higher temperatures of the surfaces. The surface temperature at the APT durations of up to 10 min was up to 138 °C, whereas at 1 min of APT, at which the maximum relative adhesion increase was observed for the mushroom-shaped microstructured surface, it was 56 °C. The temperature of the plasma device nozzle reached 191 °C in 10 min. Thus, in comparison to several other plasma treatments, such as low-pressure plasma treatment, APT possesses a high density of the active species and relatively higher temperatures, which might lead to a much quicker activation and hydrophobic recovery.3636. M. Noeske, J. Degenhardt, S. Strudthoff, and U. Lommatzsch, Int. J. Adhes. Adhes. 24, 171 (2004). https://doi.org/10.1016/j.ijadhadh.2003.09.006For APT, it is difficult to separate the chemical effect from the morphological effect since both occur at the same time. However, one effect is postulated to be more prevailing than the other since final pull-off forces after APT are significantly altered with the optimum length of treatment time. Such morphological effect or topological modification as the surface roughness is anticipated to be increased by surface etching due to the sputtering of plasma species onto the substrate surface. To investigate this modification, we quantified the surface roughness using the rms parameter by white light interferometry measurements (Fig. 5). The results show that APT led to an increase in nanoroughness on the unstructured surface depending on the treatment time. After 1 min of APT, the rms value was 51.1 nm, and after 10 min of the APT, it was 340 nm. Previous works also demonstrated that the adhesive performances are significantly influenced by the nanoroughness of unstructured and biomimetically structured surfaces.38,3938. N. Cañas, M. Kamperman, B. Völker, E. Kroner, R. M. McMeeking, and E. Arzt, Acta Biomater. 8, 282 (2012). https://doi.org/10.1016/j.actbio.2011.08.02839. H. Kasem and M. Varenberg, J. R. Soc. Interface 10, 20130620 (2013). https://doi.org/10.1098/rsif.2013.0620 At 100 nm and a higher rms, the flexibility of both the material and the microstructure cannot compensate for the surface irregularities during contact formation, and therefore, the pull-off force decays.

The roughness is expected to be obtained through etching, which might lead to the ejection of particular components of the commercial microstructured surfaces made of PVS. The material composition of the surface therefore might be modified, too.

An important feature of the dry adhesive studied is that it provides additional compliance to the substrate surface to adhere. Hence, the enhancement in adhesive performance of the APT-applied microstructured surfaces might be therefore expected due to the altered mechanical properties of the PVS material. To prove this hypothesis, we have tested the effective elastic modulus of PVS after APT. For this test, the unstructured surfaces were employed and the untreated ones were compared with those after 30 s, 1 min, 3 min, 5 min, and 10 min of APT. The results show that a longer APT time leads to the reduction of the effective elastic modulus (Fig. 6). At 1 min of APT, where the maximal relative adhesion was obtained, the average effective elastic modulus was 2.8 MPa. This value was significantly lower than that of the untreated sample (P = 004). The decreased effective elastic modulus can lead to the higher conformity of the surface. According to this explanation, the durations longer than 1 min should lead to higher relative adhesion, but this is not the case.However, for the treatments longer than 1 min, the unstructured and mushroom-shaped microstructured surfaces demonstrate different adhesive performances. For the microstructured surface, the reduction in pull-off forces for APTs longer than 1 min might be influenced by the effective elastic modulus but also by altering some microstructure morphological features. Therefore, the SEM study of the samples was performed (Fig. 7). After 1 min of the treatment, there is no prominent modification of the microstructured surface. However, after 5 min of the treatment, individual microstructures demonstrated condensation (conglutination) to neighboring ones,40,4140. A. Jagota and S. J. Bennison, Integr. Comp. Biol. 42, 1140 (2002). https://doi.org/10.1093/icb/42.6.114041. R. Spolenak, S. Gorb, and E. Arzt, Acta Biomater. 1, 5 (2005). https://doi.org/10.1016/j.actbio.2004.08.004 and contact lips, which are responsible for the conformity with the substrates, were etched on the perimeter and/or were crumbled. As a result, the number of the individual microstructures, contributing to the total adhesion, was reduced.Compared to its unstructured counterpart, the mushroom-shaped microstructure demonstrates stronger adhesive performance, when the relative adhesion data are compared. The superior adhesive performance was previously attributed to the combination of intermolecular van der Waals forces and particular crack entrapment mechanism of the microstructure that allows lower stress concentration on the outer perimeter of individual contact plates.1313. L. Heepe and S. N. Gorb, Annu. Rev. Mater. Res. 44, 173 (2014). https://doi.org/10.1146/annurev-matsci-062910-100458The critical detachment stress, σpull-off, is described by the following expression:where W is the work of adhesion and E is the effective elastic modulus. In our previous work, we observed that the individual mushroom-shaped microstructure shrunk during pull-off.4242. L. Heepe, A. E. Kovalev, A. E. Filippov, and S. N. Gorb, Phys. Rev. Lett. 111, 104301 (2013). https://doi.org/10.1103/PhysRevLett.111.104301 However, the shrinkage of the individual mushroom-shaped microstructure detachment area was 64%, while the treated microstructure by the low-pressure plasma treatment kept its initial area constant.2828. E. Kizilkan and S. N. Gorb, ACS Appl. Mater. Interfaces 10, 26752 (2018). https://doi.org/10.1021/acsami.8b06686 Taken into account the 11% increase of the work of adhesion (Fig. 2) and the 13% decrease of the effective elastic modulus (Fig. 7) after 1 min of APT, the contact area ratio between the plasma-treated and nontreated microstructures at pull-off should be 1.68. This means, that after 1 min of APT, the friction between the PVS microstructure and sapphire sphere is so high and there is no slip at the interface during pull-off. On the contrary, there is almost perfect slip (low friction) at the interface between the untreated PVS microstructure and sapphire sphere, similar to the observation in our previous work.2828. E. Kizilkan and S. N. Gorb, ACS Appl. Mater. Interfaces 10, 26752 (2018). https://doi.org/10.1021/acsami.8b06686 Obviously, the surface energy of PVS increased after the APT. The surface energy has dispersive and polar components. The increase in the polar component should be accompanied by the increase in water contact angle, which was not changed after the APT. Therefore, the surface energy after 1 min of APT increased because of an increase of its dispersive component. No further adhesive performance increase in the microstructured PVS with an increased APT time was observed because the roughness of the surface with an increased APT time increased as well (Fig. 6), and the pull-off force decreases with an increase in size of the pre-existing interface defects.1818. G. Carbone, E. Pierro, and S. N. Gorb, Soft Matter 7, 5545 (2011). https://doi.org/10.1039/c0sm01482fThe recent works demonstrated an enhanced adhesion of the mushroom-shaped adhesive microstructured surfaces by heat treatments.26,2726. M. Seong, C. Jeong, H. Yi, H.-H. Park, W.-G. Bae, Y.-B. Park, and H. E. Jeong, Appl. Surf. Sci. 413, 275 (2017). https://doi.org/10.1016/j.apsusc.2017.04.03627. M. Seong, J. Lee, I. Hwang, and H. E. Jeong, Int. J. Precis. Eng. Manuf. 6, 587 (2019). https://doi.org/10.1007/s40684-019-00062-z An additional production step, such as bringing samples to a heating stage or placing them in an oven, can potentially affect the microstructure contamination, which is known to be one of the biggest obstacles for the effective functioning of the microstructures as reversible adhesives. However, the previous results on heat treatments used relatively long treatment times (30 min and longer).26,2726. M. Seong, C. Jeong, H. Yi, H.-H. Park, W.-G. Bae, Y.-B. Park, and H. E. Jeong, Appl. Surf. Sci. 413, 275 (2017). https://doi.org/10.1016/j.apsusc.2017.04.03627. M. Seong, J. Lee, I. Hwang, and H. E. Jeong, Int. J. Precis. Eng. Manuf. 6, 587 (2019). https://doi.org/10.1007/s40684-019-00062-z The APT method proposed here offers shorter treatment times and easier integration of the treatment into the industrial production line or as a post-treatment process.