MNE 14 (2022) 100112 Contents lists available at ScienceDirect Micro and Nano Engineering journal homepage: www.sciencedirect.com/journal/micro-and-nano-engineering Introducing surface functionality on thermoformed polymeric films Carlos Sáez-Comet a, Olga Muntada b, Achille Francone c, Nekane Lozano a, *, Marta Fernandez-Regulez b, Jordi Puiggali d, Nikolaos Kehagias e, Clivia M. Sotomayor Torres c, f, Francesc Perez-Murano b, * a Eurecat. Parc Tecnològic del Vallès, Av. Universitat Autònoma, 23, 08290 Cerdanyola del Vallès, Spain b Instituto de Microelectrónica de Barcelona (IMB-CNM, CSIC). Campus de la Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain c Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain d Universitat Politécnica de Catalunya, Departament d’Enginyeria Química, Av. Eduard Maristany 16, Barcelona 08019, Spain e NCSR Demokritos, Institute of Nanoscience and Nanotechnology, 15341 Agia Paraskevi, Athens, Greece f Institució Catalana de Recerca i Estudis Avancats (ICREA), 08010 Barcelona, Spain A R T I C L E I N F O A B S T R A C T Keywords: We present a fabrication process for the production of 3-dimensional micro-structured polymeric films. The Thermoforming microstructures are fabricated in a single step using thermal nanoimprint lithography as patterning technique. Nanoimprint lithography The micro-structured polymer films are then transformed into a 3D shape by means of a plug-assisted thermo­ Plastic injection molding forming process, while keeping the functionality of the micro-patterned areas. The preserved functionality is Surface functionalization characterized by water contact angle measurements, while the deformation of the micro-structured topographies due to the thermoforming process is analyzed using confocal microscopy and Digital Image Correlation (DIC) techniques. This combined fabrication process represents a promising solution to complement in-mold decora­ tion approaches, enabling the production of new functional surfaces. As the microstructures are fabricated by means of a mechanical modification of the surface, without the need of chemical treatments or coatings, the presented technique represents a promising, simple and green solution, suitable for the industrial fabrication of 3D nonplanar shaped functional surfaces. 1. Introduction optics) [10] as well as home and industrial lighting [11], to mention but a few. In these sectors, a recent manufacturing technology known as in- The industrial interest in micro/nm scale patterning of surfaces is mold electronics is becoming more and more relevant [12]. In-mold broad due to the wide range of applications. Micro / nano-metric labelling (IML) [13] and in-mold decoration (IMD) [14] processes texturing makes it possible to provide surfaces with functional proper­ allow to handle and incorporate decorative 3D formed polymeric films ties, such as, for example, hydrophilicity, hydrophobicity, greater on final injection-molded parts. These manufacturing technologies can adherence, antibacterial, optical or chromatic effects, among others [1]. be further enhanced by using locally micro- and nano- textured films to Several high accuracy methods to pattern surfaces at the micro and obtain local and specific functionalities on the surface of the resulting nanoscale have been developed [2,3]. However, a methodology for high injection molded parts. Furthermore, the micro- and nano- textured volume manufacturing at affordable cost is still missing for many ap­ polymeric films can also be used as replication inserts, adequately plications, especially to define accurate and deterministic patterns on placed on the injection molds [15]. arbitrary and or curved surfaces. In recent years functional micro- and The most widely extended methods to pattern a surface incorporate a nano- textured surfaces on 3D formed polymeric films with potential in lithography process, which defines the lateral dimensions [16]. The various applications have been a subject of investigation [4–7]. Among tremendous progress in lithography made over several decades has them, micro fluidics [8] and tissue culture [9] are some that attract the delivered a robust and reliable technology, covering a wide dimensions most interest. Furthermore, there are other potential applications in range from few nanometers to millimeters. A patterning technique that other industrial sectors such as the automotive industry (interior parts, allows local 3D micro- and nano- structuring of thermoplastic films is * Corresponding authors. E-mail addresses: nekane.lozano@eurecat.org (N. Lozano), Francesc.Perez@csic.es (F. Perez-Murano). https://doi.org/10.1016/j.mne.2022.100112 Received 18 August 2021; Received in revised form 25 January 2022; Accepted 4 February 2022 Available online 7 February 2022 2590-0072/© 2022 Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). C. Sáez-Comet et al. Micro and Nano Engineering 14 (2022) 100112 Fig. 1. (a) Graphical representation of the Young equation and contact angle for a droplet on a surface. (b) Graphical representation of the Wenzel and Cassie-Baxter states for a micro-structured surface. thermal nanoimprint lithography (thermal NIL) [17], a high-resolution and high-throughput lithography technique. Thermal NIL is based on Table 1 Calculated values of Wenzel θW and Cassie-Baxter contact angles θCB for the the mechanical deformation of a thermoplastic material, which can be a designs of the stamps. A nominal pillar height of 6.5 mm is assumed for all the thermoplastic resist coated on a substrate or a bulk thermoplastic sheet/ quadrants. film, with a stamp containing the surface topography to be replicated in Quadrant d (μm) p (μm) a 1 to 1 scale. The achievable resolution is mainly limited by the mini­ θW θCB mum feature size that can be fabricated on the stamp and sub-10 nm Q1 4 8 59◦ 139◦ lateral dimensions have been demonstrated [18]. In order to overcome Q2 6 12 65◦ 139◦ Q3 8 16 68◦ 139◦ possible adhesion issues between the stamp and the surface to be Q4 10 20 70◦ 139◦ patterned, generally an anti-adhesion treatment is applied on the stamp surface [19]. In this work, thermal NIL was used in a step-and-repeat mode, a manufacturing process with potential for high-volume and submitted to a thermoforming process, we have chosen as test patterns low-cost manufacturing [20–22]. After imprinting a die, the stamp is arrays of pillars, because they are relatively simple structures that would released from the substrate, displaced to the next die, put again into allow us to relate the deformation at macroscale due to the thermo­ contact with the substrate, and thermally imprinted [23]. forming process with the deformation at the microscale. As a change in However, most lithography methods require the printed surface to be the morphology implies a change of the functional properties, we have flat. Some examples of patterning on curve surfaces exist, like for quantified the variation of the hydrophobicity by measuring the contact example in nanostencil lithography [24], but they are not a universal angle before and after thermoforming. solution and present limited industrial applicability. When it comes to The hydrophobicity of a surface is commonly defined from the form micro- and nano- textured films into 3D shapes, a well-known and contact angle of a water droplet in contact with the surface as described popular transformation process is thermoforming using vacuum-assisted in the Young’s equation [27]: plugs. In this case, the surface is micro-patterned when it is flat, and later γSG = γSL + γLG cos(θ) it is given the desired final macroscopic shape [25]. Applications of thermoforming in industry are large but so far very few examples of Where γSG is the interfacial tension between solid and gas phases, γSL functional patterned thermoformed pieces have been realized [8,9] . between gas and liquid, γLG between liquid and gas and θ is the angle of a After thermoforming, the patterned areas in the film will suffer droplet on a solid surface. geometrical distortions caused by the induced macroscopic deforma­ To increase the surface contact angle, the interfacial free energy must tion. Those distortions might affect negatively the surface functionality. be increased. This is possible to be achieved by micro-structuring the To understand the achievable maximum 3D- macroscale deformations surface: specific geometries allow air pockets to be trapped between the that can be applied to the films without losing the surface functionality, droplet and the surface. The ideal case where this occurs is known as a study of how the macroscopic deformation translates into geometrical Cassie-Baxter state and is preferable to the Wenzel state in which the distortions at the micro- scale is required. This will then allow to study droplet completely wets the surface (Fig. 1.b) [28]. (See Table 1.) the loss in the initial surface functionality, if any, and how it could be Fig. 2 describes the geometrical parameters of the microstructures identified and prevented. that will be transferred to the polycarbonate films, which will induce an In this article, we present the patterning of polycarbonate (PC) films increment of the water contact angle. Four designs have been selected, by thermal-NIL to obtain hydrophobic surfaces. We investigate how each one consisting of an area of 30 mm × 30 mm covered by pillars their structural and functional properties change when the film is with diameters (d) of 4 μm, 6 μm, 8 μm and 10 μm. In each area, the pitch thermoformed. The local changes in the geometries of the various micro- (p) is defined to be 2d. This design will result on four stamps with arrays structured zones are monitored using confocal scanning microscopy. of holes that, after the thermal-NIL process, will produce pillars in the The result is compared with to macroscopic deformations and draw polycarbonate films. ratios obtained by digital image correlation (DIC) [8,26]. A further The Wenzel and Cassie-Baxter water contact angles can be estimated correlation with surface functionality is carried out by evaluating the according to the following relations [29]: wetting behavior of the micro-structured areas of the film before and cosθW = rcosθ (1) after the thermoforming process. cosθCB = Φs (1 + cosθ) − 1 (2) 2. Pattern and process design where θW and θCB are the apparent contact angle for Wenzel and 2.1. Pattern design Cassie-Baxter states respectively, r is the roughness factor, Φs is the area fraction of the solid surface for micro/nanostructured surfaces and θ is For investigating the change of the functional properties of a surface the water contact angle on flat –non nanostructured & non- 2 C. Sáez-Comet et al. Micro and Nano Engineering 14 (2022) 100112 Fig. 2. Micro-structured texture design. The pattern consists of 4 areas of 30 × 30 mm each that contain arrays of holes pillars diameters d ranging from 4 μm to 10 μm and pitches p = 2d for all of them. Fig. 4. (a) Design of the thermoforming mold geometry showing the placement of the microstructured areas in red. Dimensions are in millimeters. (b) Photo­ graph of the polycarbonate film on top of the mold prior to the thermoforming process. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) thermoformed PC. r and Φs can be calculated as follows [30]: [ 2 ] (p) + πdh r= 2 (3) (p) Fig. 3. Scheme of the overall fabrication process which includes the fabrication of the silicon stamp (steps 1–4) the replication of the stamp in a polycarbonate film by thermal NIL (steps 5–6) and the thermoforming of the film (step 7). πd2 Φs = (4) 4(p)2 Next table show the values for the expected Wenzel and Cassie- Baxter contact angles for each quadrant of the design, assuming a height of 6.5 μm for the pillars, and a contact angle of 77◦ for the flat and non- textured surface: 3 C. Sáez-Comet et al. Micro and Nano Engineering 14 (2022) 100112 Fig. 5. Detail of the strain in the thermoformed film obtained with the Digital Image Correlation technique. (a): Direction of the major (black lines) and minor (white lines) strain directions. A confocal microscopy image corresponding to each of the imprinted thermal NIL areas illustrates how the microstructure deforms. The values of the major and minor strains at each point of the thermoformed film are represented in Figures (b) and (c), respectively. Cassie-Baxter states are expected for the textured surfaces, resulting 3. Materials and methods to be more hydrophobic after the patterning process in comparison to the un-patterned ones. Upon deformation, the change of the geometrical 3.1. Silicon stamps fabrication parameters of the microstructures will induce a variation in the corre­ sponding water contact angle values, which eventually could imply a The first step for the fabrication of the silicon wafers is a 10 min deviation from a pure Cassie-Baxter state. standard cleaning with a piranha solution (H2SO4:H2O2 2:1) and a rinse with 5% hydrofluoric acid (HF) followed by a photolithography process. 2.2. Process design The photolithography starts with by spin coating 2 μm of standard positive photoresist HIPR 6512 (Fujifilm) on the wafer, followed by a The overall process for the fabrication of the thermoformed micro- light exposure through a photomask using a photolithography equip­ structured film is shown in Fig. 3. The first part of the process is the ment (Karl Suss MA6 Mask Aligner). Then, the wafer is immersed in OPD fabrication of the stamp. Four stamps from the same silicon wafer are 4262 developer (Fujifilm) that dissolves the areas that have been fabricated following the designs shown in Fig. 2 by means of optical exposed. The remaining photoresist is used as a mask in a modified lithography and reactive ion etching (steps 1–4). After Step 4, the wafer reactive etching Bosch process (in an Alcatel 601-E equipment) that uses is dived in the four dices, each one containing one of the array designs, combined pulses of SF6 as the etching gas and C4F8 for the passivation which will be used as a master stamp. The master stamp is then pressed step. The alternation of both gases results in “wells” craved in the silicon and heated against a PC sheet by step and repeat thermal NIL (Step 5). with a scalloping contour (Fig. S1 Supplemental). The depth of the wells Five micro- structured dies (see Fig. 4.a) of equal dimensions were is designed to be 7 μm for the smallest well. The depth is different for fabricated in several selected locations of the flat polycarbonate film each array as the silicon etching rate depends on the diameter of the that was subsequently thermoformed in order to obtain the final 3D wells (Table S1-Supplemental). After the etching process, the remaining shape (see Fig. 4.b). After the demolding (step 6) we obtain a planar resist is stripped from the wafer by means of an oxygen plasma (Power micro-structured PC sheet that is thermoformed into a 3D shape using a = 600 W; O2 flow = 600 ml/min; Time = 20 min). thermoforming process (step 7). The conditions of these processes are further detailed in the Materials and Methods section. 3.2. Thermal NIL patterned films The geometrical shape chosen for the thermoforming mold is a half- cylinder with decreasing curvature radius, down to 50 mm, a length of Transparent 500 μm thick polycarbonate sheets were used as ther­ 195 mm and a constant width of 100 mm (Fig. 4.a). The placement of the moplastic substrates. A NPS 300 stepper (SET) was used as imprinting micro-structured zones on the flat film was carefully selected to observe tool for the step-and-repeat thermal NIL process, using the following the effects caused by the film deformation in relation to the increasing process parameters: 190–220 ◦ C as imprinting temperature, 50–80 kg as linear draw ratios and decreasing curvature radii. The mold employed applied weight, 1–2 min as imprinting time, 60 ◦ C as demolding for the thermoforming process, together with the microstructured film temperature. on top of it, is shown in Fig. 4.b. 3.3. Thermoforming process Thermal NIL micro-structured films were thermoformed using a vacuum-assisted aluminum mold with a half-cylinder geometry, as 4 C. Sáez-Comet et al. Micro and Nano Engineering 14 (2022) 100112 Table 2 Summary of the characterization results after thermoforming. Water contact angle (◦ ) Standard Deviation (◦ ) Curvature Radius (mm) Linear Draw Ratio AVG major strain (%) AVG minor strain (%) Non-textured PC 77.0 ±5.3 – – – – Textured PC (flat) 150.6 ±1.6 – – – – Zone 1 147.7 ±4.3 314.5 2.79 2.8 0.3 Zone 2 146 ±2.4 211.3 2.98 7.1 1.2 Zone 3 135.9 ±8.0 59.8 2.57 11.6 4.6 Zone 4 127.1 ±3.1 58.6 2.85 12.1 − 1.8 Zone 5 107.1 ±5.1 50.1 4.63 97.4 − 15.5 shown in Fig. 4 (b), in which the micro-structured zones are pointing outwards. During the thermoforming process, the heating stage pa­ rameters were set at 430 ◦ C for 11.5 s (upper heaters) and 500 ◦ C for 12.5 s (lower heaters), and the deformation stage had a duration of 1 s. The mold was kept at 80 ◦ C during the whole cycle and pre-vacuum and vacuum times were set at 4′′ and 3′′ , respectively, using a vacuum power of 15 W (pressure in the circuit 6 bar). 3.4. Structural characterization The local strains and thickness reductions taking place on the films were assessed using Digital Image Correlation (DIC) technique [26], comparing digital images of the same films (taken from exactly the same place) before and after thermoforming. The DIC technique allows the identification of the position of each object point in the two images (before and after thermoforming) by applying a correlation algorithm Fig. 6. Chart showing the experimental and theoretical (Cassie-Baxter) contact based on the tracking of the grey value pattern G(x,y) in small local angles in relation to the local major/minor strains for each of the micro- neighbourhood facets. Then, the characterization of local in-plane de­ structured zones considered. To calculate the Cassie-Baxter contact angle, the formations can be determined by the position of these points. Prior to procedure described in [28] was followed. performing the DIC analysis, a stochastic dot-based intensity pattern was defined on the thermoplastic film’s surface via serigraphy and a ste­ supplemental information (Figs. S3 and S4). Table 2 summarizes the reoscopic sensor setup was used for the photographic process. results of the characterization of the textured PC films for the zones 1 to In addition, the local linear draw ratios, representative of the degree 5 depicted in Fig. 3. of geometrical deformation and local film stretching of the different In order to determine the wettability behavior for the imprinted micro-structured zones, were determined in order to check the effects on areas (prior to thermoforming), the water contact angle was measured, the microstructures produced by varying deformations. Local linear showing an average value of 150.6◦ . This represents almost a two-fold draw ratios are defined as the ratio of the projected length of a line increase compared to the value of 77◦ obtained for the non- crossing the centre of the thermoformed zone to the length in the un­ microstructured PC film. The micro-structuring therefore modifies the formed sheet and were calculated by DIC for each of the micro- surface wetting behavior of the PC, making it superhydrophobic [31]. structured zones and for the two orthogonal strain directions (major After thermoforming, the hydrophobic character of the surfaces is pre­ and minor) as shown in Fig. 5. served even at curvature radius of 50.1 mm and a linear draw ratio of 4.63. The experimental contact angles and the local deformation (strain) 3.5. Contact angle characterization observed via the DIC technique are represented in Fig. 6 for each of the five selected zones. The zone that suffered the highest stretching is Zone Finally, water contact angle measurements (WCA) of the virgin PC 5, where the water contact angle value descended to 107.1◦ compared to film and the micro-structured zones before and after thermoforming the original value before thermoforming (150.6◦ ). This indicates a loss were taken using a tensiometer (Dataphysics OCA 15 CE) and image of the hydrophobic character, but still representing an improvement processing software (SCA 2.0). Micro-droplets of 150 μL of deionized when compared to that of the unformed flat PC film. Representative water were used. The contact angle values are obtained as the average of optical images of the water droplets for measuring the contact angle are 5 measurements. displayed in the supplemental information (Fig. S5). Initially, a theoretical estimation of the water contact angle for the 4. Results and discussion different deformed microstructures after the thermoforming was con­ ducted applying the analytical method described in [30]. In this model, The study of the effect of the thermoforming process on the micro­ the variation of the contact angle shown by different low aspect ratio structure has been carried out using the stamp with the array Q2 (lateral pillar geometries (almost barrel-shaped) can be estimated as a function dimensions d = 6 μm and p = 12 μm; Fig. 2) for all the zones depicted in of the nanostructure roughness parameters a, b and h; being a the Fig. 4.b except for zone 2, where the array Q3 was used (d = 8 μm and p diameter at the top of the nanopillar, b the distance between the top = 16 μm). As both arrays have the same d/p ratio, the theoretical Cassie- corners of a pillar and h the height of the nanopillar. In our case, these Baxter contact angle remains unaffected, and in consequence, this zone parameters were carefully extracted from confocal microscopy images of is also considered in the analysis. each of the flat and thermoformed nanostructures. However, the images Before the thermoforming, the five zones were imprinted by thermal of the confocal microscope show a more elliptical size for the pillars, as NIL and characterized by confocal microscopy in areas of 250 × 190 well as an asymmetric deformation of the pillar array, in accordance μm2. The height of the pillars ranged between 9.4 and 10.4 μm and the with the difference between major and minor strains. Because of this, we pillar diameter resulted to be 6.6 μm ± 0.3 μm. Representative optical modelled the expected contact angle from the deformed microstructures and SEM images of the microstructured zones are included in the 5 C. Sáez-Comet et al. Micro and Nano Engineering 14 (2022) 100112 the flat micro-structured patches on the films (which represents a two- fold increase with respect to the non-patterned films (77◦ )). The con­ tact angles of the micro-structured patches after the macroscopic de­ formations suffered by the film during thermoforming have been measured and correlated with the local major and minor in-plane strains, using the DIC technique. For extreme deformations during the thermoforming process, the hydrophobic character decreases but a substantial hydrophobicity is still retained compared to that of the initial non micro-structured film. An analytical model developed by Zheng and Lü [28] to predict the water contact angle as a function of micro-pillar geometry based on the Cassie-Baxter equation is applied to the particular case of geometrical variations produced by in-plane deformations resulting from the ther­ moforming process. This prediction seems to work well for most of the thermoforming induced-deformation. However, it deviates for areas were the strain is larger. This deviation might be explained by the fact that, for a very large deformation, either the surface presents a more Wenzel state or the Cassie Baxter modelling does not capture the asymmetric deformation experienced by the pillars. In conclusion, the methodology we have presented here can be used to predict the change in functional properties of a microtextured surface film when subjected to a thermoforming process. Consequently, it could be used, for example, to define geometric parameters of patterns on the NIL stamps to bestow a given texture to the surface prior to thermo­ Fig. 7. (a) Geometrical deformation suffered by the pillars at each zone of the forming, knowing a priori the local deformation each area of the surface thermoformed film. (b) Scheme to refer to each of the geometrical parameters. will experience. It can also provide clues of which maximum strain a thermoformed film can sustain to retain its functional properties. following the procedure described in [28], which accounts for an irregular pillar shape (data is shown in the supplemental information). Declaration of Competing Interest The result is displayed in the red curve of Fig. 6. Even using this second formalism, it is obtained that the Wenzel and Cassie-Baxter contact The authors declare that they have no known competing financial angle theoretical estimation does not follow the trend determined by the interests or personal relationships that could have appeared to influence experimental contact angle. the work reported in this paper. The discrepancy between the experimental contact angle and the contact angle estimation assuming a Cassie-Baxter state can be explain Acknowledgements attending to the specific deformation that the microstructures suffer when submitted to deformation. A detailed characterization by confocal This work has been performed within the PLASTFUN project (Planta microscopy of the pillars after the thermoforming process reveals that Pilot de Peces Plàstiques amb Superficies Funcionals Avançades), within the the base of the pillar increases significantly with the strain at which the Industries of the Future community (IDF) RIS3CAT, supported by the plastic film is submitted to. We have quantified this deformation European Regional Development Fund (ERDF) as part of the operative recording the most significant geometrical parameters, as displayed in frame FEDER of Catalonia 2014-2020 EC [COMRDI 16-1-0018], Fig. 7, for the direction of major strain (Data is shown numerically in the included in the 7th Framework Program. Carlos Sáez also acknowl­ supplemental information). edges the funding received from “Departament d’Economia I Coneixe­ It is remarkable that the slope angle of the pillar decreases signifi­ ment de la Generalitat de Catalunya” in the frame of the “Doctorats cantly for the zones 4 and 5, following the same trend than the experi­ Industrials” program. AF, NK and CMST acknowledge support from the mental water contact angle. According to the above observation, we S. Ochoa program from the Spanish Research Agency (AEI, grant no. hypothesize that the deformation of the pillars increases the actual SEV-2017-0706) and by the CERCA Programme of the Generalitat de surface area that is wet by water. The fact that the pillar slope angle Catalunya. decreases implies that the water wets not only the top of pillar, but it also occupies part of the volume between the pillars. Consequently, the Appendix A. Supplementary data contact angle behavior separates from a pure Cassie-Baxter state and approaches to a Wenzel like state. This observation brings us to conclude Supplementary data to this article can be found online at https://doi. that that the macroscopic determination of the strain caused by the org/10.1016/j.mne.2022.100112. thermoforming process is not enough to predict the change of the sur­ face properties when the functionalization is generated by an array of References microstructures. It implies that in order to predict the final functional of [1] K. Ellinas, A. Tserepi, E. Gogolides, Durable superhydrophobic and a textured surface after being submitted to a thermoforming process, the superamphiphobic polymeric surfaces and their applications: a review, Adv. manner that the microstructures deform must be accurately Colloid Interf. Sci. 250 (2017) 132–157, https://doi.org/10.1016/j. investigated. cis.2017.09.003. [2] E. Gogolides, K. Ellinas, A. 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