Tailoring the Orientation and Periodicity of Wrinkles Using Ion-Beam Bombardment

Mechanical Properties of the Carbonized Layer Formed by Ion Flow Orientated at Different Angles to the Polyurethane Surface

Polymer materials are widely used in medicine due to their mechanical properties and biological inertness. When ion-plasma treatment is used on a polymer material, a carbonization process occurs in the surface nanolayer of the polymer sample. As a result, a surface carbonized nanolayer is formed, which has mechanical properties different from those of the substrate. This layer has good biocompatibility. The formation of a carbonized nanolayer on the surface of polymer implants makes it possible to reduce the body's reaction to a foreign body. Typically, to study the properties of a carbonized layer, flat polymer samples are used, which are treated with an ion flow perpendicular to the surface. But medical endoprostheses often have a curved surface, so ion-plasma treatment can occur at different angles to the surface. This paper presents the results of a study of the morphological and mechanical properties of a carbonized layer formed on a polyurethane surface. The dependence of these properties on the directional angle of the ion flow and its fluence has been established. To study the surface morphology and elastic properties, methods of atomic force microscopy and methods of elasticity theory were used. The strength properties of the carbonized layer were studied using a stretching device combined with a digital optical microscope.

Elastic Modulus of a Carbonized Layer on Polyurethane Treated by Ion-Plasma

Nanocoatings formed by various plasma and chemical methods on the surface of polymeric materials have unique properties. However, the applicability of polymeric materials with nanocoatings under specific temperature and mechanical conditions depends on the physical and mechanical properties of the coating. The determination of Young’s modulus is a task of paramount importance since it is widely used in calculations of the stress–strain state of structural elements and structures in general. Small thicknesses of nanocoatings limit the choice of methods for determining the modulus of elasticity. In this paper, we propose a method for determining the Young’s modulus for a carbonized layer formed on a polyurethane substrate. For its implementation, the results of uniaxial tensile tests were used. This approach made it possible to obtain patterns of change in the Young’s modulus of the carbonized layer depending on the intensity of ion-plasma treatment. These regularities were compared with regularities of changes in the molecular structure of the surface layer caused by plasma treatment of different intensity. The comparison was made on the basis of correlation analysis. Changes in the molecular structure of the coating were determined from the results of infrared Fourier spectroscopy (FTIR) and spectral ellipsometry.

Escherichia coli Adhesion and Biofilm Formation on Polydimethylsiloxane are Independent of Substrate Stiffness

Bacterial adhesion and biofilm formation on the surface of biomedical devices are detrimental processes that compromise patient safety and material functionality. Several physicochemical factors are involved in biofilm growth, including the surface properties. Among these, material stiffness has recently been suggested to influence microbial adhesion and biofilm growth in a variety of polymers and hydrogels. However, no clear consensus exists about the role of material stiffness in biofilm initiation and whether very compliant substrates are deleterious to bacterial cell adhesion. Here, by systematically tuning substrate topography and stiffness while keeping the surface free energy of polydimethylsiloxane substrates constant, we show that topographical patterns at the micron and submicron scale impart unique properties to the surface which are independent of the material stiffness. The current work provides a better understanding of the role of material stiffness in bacterial physiology and may constitute a cost-effective and simple strategy to reduce bacterial attachment and biofilm growth even in very compliant and hydrophobic polymers.

Light extraction from quantum dot light emitting diodes by multiscale nanostructures

Improving the light extraction efficiency by introducing optical-functional structures outside of quantum dot light emitting diodes (QLEDs) for further enhancing the external quantum efficiency (EQE) is essential for their application in display and lighting industries. Although the efficiency of QLEDs has been optimized by controlling the synthesis of the quantum dots, the low outcoupling efficiency is indeed unresolved because of total internal reflections, waveguides and metal surface absorptions within the device. Here, we utilize multiscale nanostructures attached to the outer surface of the glass substrate to extract the trapped light from the emitting layers of QLEDs. The result indicates that both the EQE and luminance are improved from 12.29% to 17.94% and 122 400 cd m^-2 to 178 700 cd m^-2, respectively. The maximum EQE and current efficiency improve to 21.3% and 88.3 cd A^-1, respectively, which are the best performances among reported green QLEDs with light outcoupling nanostructures. The improved performance is ascribed to the elimination of total internal reflection by multiscale nanostructures attached to the outer surface of the QLEDs. Additionally, the simulation results of the finite-difference time domain (FDTD) also demonstrate that the light trapping effect is reduced by the multiscale nanostructures. The design of novel light outcoupling nanostructures for further improving the efficiency of QLEDs can promote their application in display and lighting industries.

One-dimensional surface wrinkling for twisted nematic liquid crystal display based on ultraviolet nanoimprint lithography

Surface wrinkling method is used to fabricate a 1-dimensional nanostructure. The structure is transferred to an ultraviolet cured polymer which is used as an alignment layer. The anisotropic geometry serves as a guide for aligning liquid crystal molecules uniformly without defects. The TN-LC cell showed a successful LC switching, with a response time of 20.5 ms, and a threshold voltage of 2.00 V. It also exhibited high thermal stability above 180°C. The proposed UV-cured polymers with 1-D nano wrinkle geometry can be a candidate for alternative alignment techniques, for advanced liquid crystal devices with high thermal budgets.

Controlled Wrinkling Patterns in Periodic Thickness-Gradient Films on Polydimethylsiloxane Substrates

Surface wrinkles in homogeneous and heterogeneous film-substrate systems have received intense attention in both science and engineering. Understanding the wrinkling phenomena of heterogeneous systems with continuously variable features is still a challenge. In this work, we propose an unconventional strategy to prepare periodic thickness-gradient metal films on polydimethylsiloxane (PDMS) substrates by masking of copper grids which are weaved by orthometric copper wires. It is found that a periodic thickness-gradient film spontaneously forms during the sputtering process because of the specific structures of the copper grids. Surface wrinkles are strongly modulated by the copper grid structures and are position-dependent within a period. A phase diagram has been established to correlate the wrinkle morphology with the mesh size and film thickness. The film surfaces at mesh centers are evolved from labyrinth wrinkling to herringbone wrinkling and then to stripe wrinkling and finally to wrinkling-free state when the mesh size decreases and/or the film thickness increases. The morphological characteristics, evolutional behaviors, and underlying mechanisms of such wrinkling are discussed in detail based on the stress theory and numerical simulation.

Facile Fabrication of High-Definition Hierarchical Wrinkle Structures for Investigating the Geometry-Sensitive Fate Commitment of Human Neural Stem Cells

As neural stem cells (NSCs) interact with biophysical cues from their niche during development, it is important to understand the biomolecular mechanism of how the NSCs process these biophysical cues to regulate their behaviors. In particular, anisotropic geometric cues in micro-/nanoscale have been utilized to investigate the biophysical effect of the structure on NSCs behaviors. Here, a series of new nanoscale anisotropic wrinkle structures with the a range of wavelength scales (from 50 nm to 37 μm) was developed to demonstrate the effect of the anisotropic nanostructure on the fate commitment of NSCs. Intriguingly, two distinct characteristic length scales promoted the neurogenesis. Each wavelength scale showed a striking variation in terms of dependency on the directionality of the structures, suggesting the existence of at least two different ways in the processing of anisotropic geometries for neurogenesis. Furthermore, the combined effect of the two distinctive length scales was observed by employing hierarchical multiscale wrinkle structures with two characteristic neurogenesis-promoting wavelengths. Taken together, the wrinkle structure system developed in this study can serve as an effective platform to advance the understanding of how cells sense anisotropic geometries for their specific cellular behaviors. Furthermore, this could provide clues for improving nerve regeneration system of stem cell therapies.

Heterogeneity of hard skin layer in wrinkled PDMS surface fabricated by Ar ion-beam irradiation

The spatial distribution of binding states in the depth direction of a soft polydimethylsiloxane (PDMS) material was investigated in the hard skin layer formed by Ar ion-beam irradiation. The hard skin layer, typically considered silica-like and homogeneous, was heterogeneous, comprising a topmost layer and an intermediate layer. Impinging Ar ions transferred energy to the PDMS medium by collisional energy transfer, which was maximised at the surface and decreased gradually as the ions penetrated the PDMS. The decreasing energy transfer rate from the surface created a heterogeneous hard skin layer. X-ray photoelectron spectroscopic depth profiling showed the existence of the topmost and intermediate layers. In the topmost layer, scission and cross-linking occurred simultaneously; Si-O bonds showed the dissociated state of SiO_x (x = 1.25-1.5). Under the topmost layer, the intermediate layer showed mostly cross-linking, with Si-O bonds showing silica-like binding states of SiO_x (x = 1.75-2). The spatial distribution of carbon-related bonds such as C-Si and sp³ C-C also showed heterogeneity, yielding a gradient of bond distribution. A theoretical analysis of the collisional energy transfer rate and displacement per atom showed consistency with the XPS depth profiling results.