materials Article Hierarchical Microtextures Embossed on PET from Laser-Patterned Stamps Felix Bouchard 1, *, Marcos Soldera 1,2 , Robert Baumann 1 and Andrés Fabián Lasagni 1,3   Citation: Bouchard, F.; Soldera, M.; Baumann, R.; Lasagni, A.F. Hierarchical Microtextures Embossed on PET from Laser-Patterned Stamps. Materials 2021, 14, 1756. https:// doi.org/10.3390/ma14071756 Academic Editor: Alenka Vesel Received: 5 February 2021 Accepted: 30 March 2021 Published: 2 April 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Institut für Fertigungstechnik, Technische Universität Dresden, George-Baehr-Str. 3c, 01069 Dresden, Germany; marcos.soldera@mailbox.tu-dresden.de (M.S.); robert.baumann1@tu-dresden.de (R.B.); andres_fabian.lasagni@tu-dresden.de (A.F.L.) 2 PROBIEN-CONICET, Dto. de Electrotecnia, Universidad Nacional del Comahue, Buenos Aires 1400, Neuquén 8300, Argentina 3 Fraunhofer-Institut für Werkstoff- und Strahltechnik (IWS), Winterbergstraße 28, 01277 Dresden, Germany * Correspondence: felix.bouchard1@tu-dresden.de Abstract: Nowadays, the demand for surface functionalized plastics is constantly rising. To address this demand with an industry compatible solution, here a strategy is developed for producing hierarchical microstructures on polyethylene terephthalate (PET) by hot embossing using a stainless steel stamp. The master was structured using three laser-based processing steps. First, a nanosecond- Direct Laser Writing (DLW) system was used to pattern dimples with a depth of up to 8 μm. Next, the surface was smoothed by a remelting process with a high-speed laser scanning at low laser fluence. In the third step, Direct Laser Interference Patterning (DLIP) was utilized using four interfering sub-beams to texture a hole-like substructure with a spatial period of 3.1 μm and a depth up to 2 μm. The produced stamp was used to imprint PET foils under controlled temperature and pressure. Optical confocal microscopy and scanning electron microscopy imaging showed that the hierarchical textures could be accurately transferred to the polymer. Finally, the wettability of the single- and multi-scaled textured PET surfaces was characterized with a drop shape analyzer, revealing that the highest water contact angles were reached for the hierarchical patterns. Particularly, this angle was increased from 77 ◦ on the untreated PET up to 105 ◦ for a hierarchical structure processed with a DLW spot distance of 60 μm and with 10 pulses for the DLIP treatment. Keywords: direct laser writing; direct laser interference patterning; hot embossing; hierarchical structures; stainless steel; polymer; water contact angle; polyethylene terephthalate 1. Introduction Nature provides countless surface structures that influence the macroscopic physical properties. Famous examples are the self-cleaning effect on the lotus leaf, the anti-bacterial behavior of spring-tail skin, and the high adhesion skin on gecko feet [1–3]. All these structures have in common that surface features with different size scales are combined, normally in the micrometer range with nanometer substructures. In the last decades, a lot of effort was spent to mimic these hierarchical structures and their outstanding properties in polymer surfaces, such as for example polyethylene terephthalate (PET). Due to its low weight, chemical stability, optical transparency, and low price, PET is widely used in many fields like packaging, food industry, textiles, electronics, and biomedical devices [3–5]. Especially in the food industry, pharmaceutical packaging, and biomedical applications, the non-wetting and easy-to-clean PET surfaces are of special interest due to the intrinsic antibacterial potential [6,7]. There are several innovative strategies to manipulate the wetting behavior of polymer surfaces, for instance plasma activation, thermal drawing, nanoparticles deposition, or applying functional coatings [8–12]. However, these methods are mainly devised for small areas and are not directly scalable to industrial throughputs. Materials 2021, 14, 1756. https://doi.org/10.3390/ma14071756 https://www.mdpi.com/journal/materials