Increasing Heat Transfer from Metal Surfaces through Laser-Interference-Induced Microscopic Heat Sinks.

With the increasing processing power of micro-electronic components and increasing spatial limitations, ensuring sufficient heat dissipation has become a crucial task. This work presents a microscopic approach to increasing the surface area through periodic surface structures. Microstructures with a periodic distance of 8.5 µm are fabricated via Direct Laser Interference Patterning (DLIP) on stainless steel plates with a nanosecond-pulsed infrared laser and are characterized by their developed interfacial area ratio. The optimal structuring parameters for increasing the surface area were investigated, reaching peak-to-valley depths up to 12.8 µm and increasing surface area by up to 394%. Heat dissipation in a natural convection environment was estimated by measuring the output voltage of a Peltier element mounted between a hot plate and a textured sample. The resulting increase in output voltage compared to an unstructured sample was correlated to the structure depth and developed interfacial area ratio, finding a maximum increase of 51.4%. Moreover, it was shown that the output voltage correlated well with the structure depth and surface area.

Tribocorrosion Behavior of NiTi Biomedical Alloy Processed by an Additive Manufacturing Laser Beam Directed Energy Deposition Technique.

The purpose of the present study was to experimentally assess the synergistic effects of wear and corrosion on NiTi alloy in comparison with Ti-6Al-4V alloy, the most extensively used titanium alloy in biomedical applications. Both alloys were processed by an additive manufacturing laser beam directed energy deposition (LB-DED) technique, namely laser engineered net shaping (LENS), and analyzed via tribocorrosion tests by using the ball-on-plate configuration. The tests were carried out in phosphate buffered saline solution at 37 °C under open circuit potential (OCP) to simulate the body environment and temperature. The synergistic effect of wear and corrosion was found to result in an improved wear resistance in both materials. It was also observed that, for the process parameters used, the LB-DED NiTi alloy exhibits a lower tendency to corrosion as compared to the LB-DED Ti-6Al-4V alloy. It is expected that, during the service life as an implant, the NiTi alloy is less susceptible to the metallic ions release when compared with the Ti-6Al-4V alloy.

Conditioning of hiPSC-derived cardiomyocytes using surface topography obtained with high throughput technology.

Surface functionalization of polymers aims to introduce novel properties that favor bioactive responses. We have investigated the possibility of surface functionalization of polyethylene terephthalate (PET) sheets by the combination of laser ablation with hot embossing and the application of such techniques in the field of stem cell research. We investigated the response of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) to topography in the low micrometer range. HiPSC-CMs are expected to offer new therapeutic tools for myocardial replacement or regeneration after an infarct or other causes of cardiac tissue loss. However, hiPSC-CMs are phenotypically immature compared to myocytes in the adult myocardium, hampering their clinical application. We aimed to develop and test a high-throughput technique for surface structuring that would improve hiPSC-CMs structural maturation. We used laser ablation with a ps-laser source in combination with nanoimprint lithography to fabricate large areas of homogeneous micron- to submicron line-like pattern with a spatial period of 3 µm on the PET surface. We evaluated cell morphology, alignment, sarcomeric myofibrils assembly, and calcium transients to evaluate phenotypic changes associated with culturing hiPSC-CMs on functionalized PET. Surface functionalization through hot embossing was able to generate, at low cost, low micrometer features on the PET surface that influenced the hiPSC-CMs phenotype, suggesting improved structural and functional maturation. This technique may be relevant for high-throughput technologies that require conditioning of hiPSC-CMs and may be useful for the production of these cells for drug screening and disease modeling applications with lower costs.

Direct Laser Interference Patterning of Diffraction Gratings in Safrofilcon-A Hydrogel: Fabrication and Hydration Assessment.

Refractive index modification by laser micro-structuration of diffractive optical devices in ophthalmic polymers has recently been applied for refractive correction in the fields of optics and ophthalmology. In this work, Safrofilcon-A hydrogel, used as soft contact lenses, was processed by direct laser interference patterning (DLIP) to fabricate linear periodic patterns on the surface of the samples. Periodic modulation of the surface was attained under two-beam interference by using a Q-switched laser source with emission at 263 nm and 4 ns pulse duration. Features of processed areas were studied as a function of both the interference spatial period and the laser fluence. Optical confocal microscopy used to evaluate the topography of the processed samples showed that both structured height and surface roughness increased with laser fluence. Static water contact angle (WCA) measurements were carried out with deionized water droplets on the structured areas to evaluate the hydration properties of DLIP structures. It was observed that the laser structured areas induced a delay in the hydration process. Finally, microstructural changes induced in the structured areas were assessed by confocal micro-Raman spectroscopy showing that at low laser fluences the polymer structure remained almost unaltered. In addition, Raman spectra of hydrated samples recovered the original shape of areas structured at low laser fluence.

Stable Superhydrophobic Aluminum Surfaces Based on Laser-Fabricated Hierarchical Textures.

Laser-microtextured surfaces have gained an increasing interest due to their enormous spectrum of applications and industrial scalability. Direct laser interference patterning (DLIP) and the well-established direct laser writing (DLW) methods are suitable as a powerful combination for the fabrication of single (DLW or DLIP) and multi-scale (DLW+DLIP) textures. In this work, four-beam DLIP and DLW were used independently and combined to produce functional textures on aluminum. The influence of the laser processing parameters, such as the applied laser fluence and the number of pulses, on the resulting topography was analyzed by confocal microscopy and scanning electron microscopy. The static long-term and dynamic wettability characteristics of the laser-textured surfaces were determined through water contact angle and hysteresis measurements, revealing superhydrophobic properties with static contact angles up to 163° and hysteresis as low as 9°. The classical Cassie-Baxter and Wenzel models were applied, permitting a deeper understanding of the observed wetting behaviors. Finally, mechanical stability tests revealed that the DLW elements in the multi-scale structure protects the smaller DLIP features under tribological conditions.

Fabrication of superhydrophobic and ice-repellent surfaces on pure aluminium using single and multiscaled periodic textures.

Fabricating aluminium surfaces with superhydrophobic and ice-repellent properties present nowadays a challenging task. In this work, multifunctional structures are manufactured by direct laser writing and direct laser interference patterning methods using pulsed infrared laser radiation (1064 nm). Different periodic patterns with feature sizes ranging from 7.0 to 50.0 µm are produced. In addition, hierarchical textures are produced combining both mentioned laser based methods. Water contact angle tests at room temperature showed that all produced patterns reached the superhydrophobic state after 13 to 16 days. In addition, these experiments were repeated at substrate temperatures from -30 °C to 80 °C allowing to determine three wettability behaviours as a function of the temperature. The patterned surfaces also showed ice-repellent properties characterized by a near three-fold increase in the droplets freezing times compared to the untreated samples. Using finite element simulations, it was found that the main reason behind the ice-prevention is the change in the droplet geometrical shape due to the hydrophobic nature of the treated surfaces. Finally, dynamic tests of droplets imping the treated aluminium surfaces cooled down to -20 °C revealed that only on the hierarchically patterned surface, the droplets were able to bounce off the substrate.

The Role of the Surface Nano-Roughness on the Wettability Performance of Microstructured Metallic Surface Using Direct Laser Interference Patterning.

Superhydrophobic natural surfaces usually have multiple levels of structure hierarchy, particularly microstructures covered with nano-roughness. The multi-scale nature of such a surface reduces the wetting of water and oils, and supports self-cleaning properties. In this work, in order to broaden our understanding of the wetting properties of technical surfaces, biomimetic surface patterns were fabricated on stainless steel with single and multi-scale periodic structures using direct laser interference patterning (DLIP). Micropillars with a spatial period of 5.5 µm and a structural depth of 4.2 µm were fabricated and covered by a sub-micro roughness by using ultrashort laser pulses, thus obtaining a hierarchical geometry. In order to distinguish the influence of the different features on the wettability behavior, a nanosecond laser source was used to melt the nano-roughness, and thus to obtain single-scale patterns. Then, a systematic comparison between the single- and multi-scale structures was performed. Although, the treated surfaces showed hydrophilic behavior directly after the laser treatment, over time they reached a steady-state hydrophobic condition. However, the multi-scale structured metal showed a contact angle 31° higher than the single-scale geometry when the steady-state conditions were reached. Furthermore, the impact of the surface chemistry was investigated by energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) analyses. Finally, a hydrophobizing agent was applied to the laser treated samples in order to further enhance the water contact angles and to determine the pure contribution of the surface topography. In the latter case, the multi-scale periodic microstructures reached static contact angles of 152° ± 2° and a contact angle hysteresis of only 4° ± 2°, while the single-scale structures did not show superhydrophobic behavior. These results definitely suggest that multi-scale DLIP structures in conjunction with a surface chemistry modification can promote a superhydrophobic regime.

Fabrication of inclined non-symmetrical periodic micro-structures using Direct Laser Interference Patterning.

The direct fabrication of microstructures, having a non-symmetrical morphology with controllable inclination, presents nowadays a challenging task. Natural examples of surfaces with inclined topographies have shown to provide anisotropic functionalities, which have attracted the interest of several researchers in the last years. This work presents a microfabrication technique for producing microstructures with a determined and controllable inclination angle using two-beam Direct Laser Interference Patterning. Polyimide foils are irradiated with a 4 ns UV (266 nm) laser source producing line-like structures with a period varying from 4.6 µm to 16.5 µm. The inclinations, retrieved by tilting the sample with respect to the optical axis of the setup, are changed from 0° to 75°, introducing a well controllable and defined inclination of the structure walls. The structuring parameters (laser fluence, number of laser pulses and interference period) as well as the inclination of the microstructures are correlated with the global tilting of the sample. As a result, a determined laser fluence and number of pulses are necessary to observe a remarkable non-symmetrical morphology of the structures. In addition, the presence of structural undercuts is reported, which opens the possibility for developing new direction-dependent properties on polymeric materials. As an example, preliminary results on light diffraction are presented, showing a similar behavior as blazed diffraction gratings.

Geometric Control of Cell Alignment and Spreading within the Confinement of Antiadhesive Poly(Ethylene Glycol) Microstructures on Laser-Patterned Surfaces.

In this study, a mask-less laser-assisted patterning method is used to fabricate well-defined cell-adhesive microdomains delimited by protein-repellent poly(ethylene glycol) (PEG) microstructures prepared from multiarm (8-PEG) macromonomers. The response of murine fibroblasts (L-929) toward these microdomains is investigated, revealing effective cell confinement within the cell-adhesive areas surrounded by nonadhesive 8-PEG microstructures. Moreover, the spatial positioning of cells in microdomains of various sizes and geometries is analyzed, indicating control of cell density, size, and elongated cell shape induced by the size of the microdomains and the geometric confinement.

Photopatterning of multifunctional hydrogels to direct adult neural precursor cells.

Matrix-metalloproteinase and photosensitive peptide units are combined with heparin and poly(ethylene glycol) into a light-sensitive multicomponent hydrogel material. Localized degradation of the hydrogel matrix allows the creation of defined spatial constraints and adhesive patterning for cells grown in culture. Using this matrix system, it is demonstrated that the degree of confinement determines the fate of neural precursor cells in vitro.

Large area fabrication of tuned polystyrene/poly(methylmethacrylate) periodic structures using laser interference patterning.

The fabrication of advanced architectures in poly(methylmethacrylate-co-styrene) (PMMA-S) copolymers (ranging from 12 to 66% mol content of methylmethacrylate) using direct laser interference patterning is reported. For all copolymer compositions, two different regimes were observed. At low laser intensities, the irradiated surfaces swell up due to the formation of microbubbles that result from the degradation of the methylmetacrylate (MMA) component. While laser ablation produces concave holes, the swelling process permits fabrication of convex hemispherical dots. At higher intensities the bubbles release from the surface forming a periodic micropored structure. In addition, the laser fluence necessary to swell the polymeric surface (swelling threshold) does not depend on polymer composition, while the ablation threshold, which determines the transition to the periodic micropored structure, strongly depends on the MMA content. This observation can be explained by the mechanical and chemical properties of the copolymer. The method provides a unique way to produce periodical structures protruding from the polymer surface.

Fabrication of highly ordered arrays of platinum nanoparticles using direct laser interference patterning.

Highly ordered electrode arrays composed of lines of platinum nanoparticles deposited onto gold substrates have been made by direct laser interference patterning of polyaniline thin films, followed by electrochemical deposition of platinum nanoparticles. Nanostructured arrays of electrocatalytic platinum particles are built in that way.