Wrinkled Polymer Surfaces

In contrast to conventional patterning methodologies, the use of surface instabilities to pattern polymer surfaces does not require expensive equipment or long and multistep procedures to obtain surface microstructures. In addition, surface instabilities enable the fabrication of intricate and complex multiscale surface features which are difficult to achieve, if not impossible, using conventional patterning techniques. These advantages together with the multiple alternatives to induce surface instabilities have made of these approaches interesting alternatives to pattern polymer surfaces. This chapter briefly summarizes the most common types of surface instabilities and provides a general overview of the main characteristics of each methodology. Finally, a brief summary of the book organization, including the chapter distribution as well as the objectives, will be described.

In this chapter, the different strategies to fabricate wrinkled surface morphologies are described. The methodologies have been classified as a function of the film structure, i.e., layered films, depth-wise gradient, or homogeneous films. Layered films are formed by different substrates and different top layers. The bilayer systems described in the literature have been organized and grouped, and finally, the wrinkle formation is described as a function of the stimulus employed. The same strategy has been employed to describe the wrinkle formation in depth-wise gradient and homogeneous films. As a result, a discussion about the particularities of each stimulus employed is reported.

Frontal polymerization (FP) or vitrification consists in the generation of a reaction front, in a localized sector of the material, that travels in a particular direction. Their remarkable flexibility permits to controllably polymerize materials with different molecular weights and variable chemical nature. One of the advantages of FP is that it is possible to generate thin layers of a polymerized material over an unpolymerized composite, being suitable for creating wrinkled patterns in homogenous polymeric materials. In this chapter, the main types of frontal polymerization are described, as well as several examples and applications which take advantage of this methodology to form wrinkled patterns of variable materials.

In this work, we have reviewed the formation of ion beam-induced self-assembled wrinkle pattern on polymer surfaces, especially PDMS. By exposing the surface with a localized ion beam, it is possible to vary the chemical and/or mechanical properties of the material. Oxidation of the sample is procured, which entails the modification of their Young’s modulus, contact angle, and aspect ratio, among other characteristics. To control the wrinkle distribution and morphology, different parameters of ion beam process could be varied such as temperature, deposition time, different ion beam voltages, and ion implantation. Additionally, the ion fluence and exposure area are factors that also can produce a variety of patterns of different dimensions, from micron to submicron range or from simple one-dimensional wrinkles to complex hierarchical nested wrinkled patterns. Today, different applications have been studied using wrinkled pattern formation from the alignment of liquid crystals until piezoresistive tactile sensor devices.

The development of simple strategies capable of simultaneously producing hydrophilic surfaces and controlled surface topography is rare in spite of their huge potential for a wide myriad of applications. Herein, we first summarize the different strategies to fabricate microstructured hydrogel surfaces and the different biological applications described in the literature. Then, we describe a procedure used to fabricate wrinkled structures in thermoplastics. We present a straightforward approach to form microwrinkled surfaces on polycarbonate (PC) film after a process that involves three different steps: first, the contact between a photosensitive monomer mixture based on vinylpyrrolidone (VP) and the PC substrate; second, a UV-curing step of this solution; and third, the hydrogel detachment as a result of the swelling in ethanol. Several parameters allow us to vary the wrinkle characteristics including the contact time between the PC surface and the photopolymerizable solution prior to the UV-vis irradiation, the type of solvent, as well as the cross-linking degree. By contact angle measurements and by confocal Raman microscopy, we were able to demonstrate that the PC wrinkled surface produced after hydrogel detachment has a thin hydrogel layer. Thus the hydrogel presented an internal rupture close to the PC substrate. Finally, we evaluated biocompatibility analyzing cell proliferation, cell morphology, and cell detachment on the substrates with both variable chemical composition and wrinkle size and period.

Wrinkled surfaces can be obtained through the control of surface instabilities produced by repeated irradiation of polymer surfaces by pulsed lasers. By the combination of the electric field associated with the laser beam and the heating of the polymer surface during a short period of time, which typically is in the range of nanosecond, when the irradiating with nanosecond laser pulses of are used, periodic dissipative structures appear. The periodic rippled topography is directly related to the wavelength of the laser. In this chapter, we discuss the role of actors like the substrate, the absorption of polymer, and the thermal conductivity and diffusivity on tuning the obtaining periodic structures.

In this chapter, a description of the experimental setup required for obtaining LIPSS is presented. Afterward, the necessary conditions to obtain LIPSS in polymer surfaces are discussed, and finally, LIPSS in different polymers are reviewed.

Surfaces with controlled topographic characteristics can provide enhanced properties in comparison to surfaces with a random roughness. Several examples of ordered topographies can be found on the surfaces of different plants and animals, which are the result of several 1000 years of evolution. In this manner, nature has shown to be capable of overcoming survival challenges by using bottom-up approaches of surface texturing.

In this chapter, different aspects of laser-based interferometric methods for the treatment of polymer-based materials are introduced. In the first part of the chapter, the main parameters used to control and obtain interference patterns are introduced. After that, several examples of pattern fabrication are discussed showing the potential of the method. The examples include the fabrication of single-scaled and multiple-scaled patterns as well as the structuring of polymer-polymer foil’s interfaces. Finally, a general model for the simulation of the process is introduced.

In this chapter, several methodologies used to generate surface instabilities on hydrogel films are reviewed. The main advantage of surface instability usage for generating nano- and micro-patterned surfaces is their low cost, ease of fabrication, and the possibility of methodology scalement for industrial processes. Surfaces instabilities are generated by a mismatch of forces or stresses between the different strata of a film. Their inhomogeneous contraction or dilatation could eventually generate out-of-plane deformations; the shape and distribution of the patterns formed on top can be controlled according to the variation of the parameters used for their generation. Particularly, hydrogels have a remarkable importance in biomedical applications due to their high biocompatibility, low toxicity, and facile chemical or physical alteration, being able to be used as a base for shape memory devices, pH- or thermoresponsive materials, or antibacterial/antibiofouling devices. In the final section of this chapter, several applications of nano- or micro-patterned surfaces generated on hydrogels are mentioned and explained.

Covalently anchored hydrogels show spontaneous formation of surface instabilities. Although surface instabilities on polymeric and elastomeric films have been extensively studied, fewer studies focused on the formation of instability patterns in hydrogels. The current and prospective applications have led to a surge in the interest in the study of surface instabilities in hydrogels. These applications include cell culture models, adhesives and optical devices, and stimuli-responsive soft actuators. This chapter gives a detailed review on the recent progress in the formation of surface instabilities in confined hydrogels. A fundamental understanding of surface instabilities in different constructions of hydrogel films is provided. The fabrication approaches to create wrinkles on hydrogel surfaces are classified based on the types of stimuli and the hydrogel systems. Finally, several examples in which surface instabilities in hydrogels have been employed are presented.

Graphene and other low-dimensional materials are a fantastic playground both for fundamental and applied sciences: for the former, to reach beyond the laws of continuum mechanics and expand the realm of bulk materials and, for the latter, to unlock new potential breakthroughs in areas ranging from single-molecule sensors to hydrogen storage and water filtration.

In this review, we explore the physical origins of the unique mechanical properties of mono- and few-layer graphene. For instance, bending resistance builds up in monolayer graphene through pi-orbital misalignment but does not involve any elastic strain, in stark contrast with its bulk counterpart. In addition, thermal fluctuations and physical defects renormalize the effective mechanical behavior of graphene. We then review the various wrinkling processes observed in graphene systems, thermally activated self-tearing, thermal expansion or lattice mismatch, and adsorbate-induced spontaneous curvature, and discuss their relevance in technological applications.

The uniqueness of graphene properties presented here showcases the broad range of disciplines impacted by the (just nucleated) birth of 2D systems.

Static and dynamic periodic patterns (stripes, wrinkles, and dots) are ubiquitous in nature, ranging from small wrinkles in soft materials (such as pumpkins, melons, nuts, and dehydrated fruits or even on animal’s skin) to much larger wavelength buckles (such as in lava flows or in geological structures, as in the desert sand).

In our work, we developed a simple method to fabricate Janus particles (films, spheres, and fibers) from a single urethane/urea elastomeric material, with two different surfaces: one smooth and another wrinkled. Wrinkles were generated by selectively UV-irradiating one-half of the elastomeric particles and permanently imprinted by swelling and drying the particles in an appropriate solvent. More, the particle surface can develop diverse wrinkling wavelengths depending on the swelling conditions.

We are able to fabricate monodisperse Janus particles from a single elastomeric material with two different hemispheres: one “aged” (wrinkled) and another “young” (flat). The hierarchical tuneable surface features produced open new horizons for application of these particles as, for example, components in biosensors.

Wrinkling and buckling of multilayered materials are common methodologies used for generating micro- or nano-patterned topographies on the top of the devices. According to the material used and the size and distribution of those patterns, some of these devices can be used for several interesting purposes like aeronautics, force/pressure sensing, or particle selective transport. Biorelated purposes is also a field of application which has attracted high interest in the last years. To take advantage of micro-pattern characteristics is a clever way to impart some interesting capabilities to the material. Interestingly, these modifications could generate or improve some useful properties like biocompatibility or antibiofouling capacities. In this chapter, some of the most relevant examples of biorelated applications of wrinkled patterns are fully reviewed. As a summary, different topics are mentioned in this section, like scaffolds with cell proliferation improvement, devices which allow cellular alignment or differentiation, surfaces which help in the generation of multicellular-spheroid structures, or antibiofouling surfaces.

Surface wrinkles and many other instability patterns are ubiquitous in nature, such as fruits, vegetables, skins, and biological tissues. Understanding these instability patterns is of paramount importance in physics and mechanics fields, in particular, for their engineering and biology applications. In this chapter, we give an overview of how to tune the surface morphology of polymer thin films through bilayered structures, mechanical forces, and external stimuli through combined theoretical, computational, and experimental studies. First, we demonstrate how the compressibility of the substrate can influence the buckling and post-buckling behaviors of a perfectly bonded hard thin film. We find that Poisson’s ratio of the substrate cannot only shift the critical strain for the onset of buckling but also affect the buckling modes. Second, we explore the surface instability of bilayered hydrogel subjected to both compression and solvent absorption. Our results show that when the thickness of the upper layer is very large, surface wrinkles can exist without transforming into period doublings. The pre-absorption of the water can result in folds or unexpected hierarchical wrinkles, which can be realized in experiments through further efforts. Third, we discuss the transition of surface–interface creasing in bilayered hydrogels. The surface or interface crease of the bilayered hydrogels under swelling is found to be governed by both the modulus ratio and height ratio between the thin film and substrate. Last, we study the surface instability of monolayer graphene supported by a soft (polymer) substrate under equal-biaxial compression. Regardless of the interfacial adhesion strength between the graphene and substrate, herringbone wrinkles have always been observed due to their lowest energy status, compared with the checkerboard, hexagonal, triangular, and one-dimensional sinusoidal modes. These fundamental understandings about the surface morphology of polymer thin films and their bilayered structures will enable their future applications in engineering and biology fields, such as flexible electronics, biofouling, and interfacial adhesion.

In a previous chapter of this book, some biological/biomedical polymeric devices in which wrinkled patterns were formed on the surface were fully revised. Particularly, different materials, with interesting thermal, optical, and mechanical properties, which could be used for several applications like generation of ultrasensitive force/pressure sensors or actuators, smart windows, controllable electronics, conductive deformable devices, shape memory polymers, electrochromic devices, organic light-emitting diode (OLED), and photovoltaic/solar cells, or for the creation of anti-counterfeiting devices, will be reviewed in this chapter. As an example, a polymethylmethacrylate/poly(dimethylsiloxane) (PMMA/PDMS) bilayered system is used to generate selective information storage surfaces where, after repetitive UV and thermal cycles, the patterns printed on the top can be erased or recovered according to the application desired. We expect that several of the usages for wrinkled polymeric surfaces revised in this chapter could be applied in the future in different fields like automotive, aeronautics, information storage, and communications, among others.

Traditional approaches to the patterning process of polymer surfaces (lithography, embossing, imprinting) have been largely explored and covered in this book. Parallel, patterns originated by surface instabilities and, in particular, those forming wrinkled surfaces have been also incorporated. The aim of this book is to cover the above-depicted aspects, describing the different methodologies to fabricate spontaneous wrinkled surfaces by taking advantage of the surface instabilities. These were achieved by separating the book into different sections with a specific thematic. Firstly, generalities about wrinkled pattern formation are reviewed. Secondly, novel approaches to generate pattern formation are mentioned and explained. Then, the third section is related to some of the most common polymeric materials employed to form this patterns. And finally, some of the most recent and innovative applications are mentioned and revised.