Andrés Fabián Lasagni

Direct Fabrication of Hierarchical Microstructures on Metals by Means of Direct Laser Interference Patterning

We have studied the fabrication of hierarchical periodic microstructures on metals by means of direct laser interference patterning. A nanosecond pulsed Nd: YAG laser at 355 nm wavelength was used to produce the microstructures with grating periods ranging from 1 μm to 10 μm on stainless steel, titanium, and aluminum. The results indicate that the geometrical characteristics of the interference patterns as well as the thermal properties of the substrates determine the quality of the fabricated structures. In particular, the best structures were obtained when the material at the interference minima position remained in the solid state and the temperature at the interference maxima is below the vaporization temperature. Thermal simulations by finite element method were carried out modeling photothermal interactions of the interference pattern with the metallic substrates to evaluate laser induced thermal effects, such as temperature distribution and temperature gradients and, thus, enabling us to explain the obtained results.

Quantitative allocation of Bragg scattering effects in highly efficient OLEDs fabricated on periodically corrugated substrates.

Bragg scattering effects in bottom-emitting organic light-emitting diodes (OLEDs) grown on corrugated aluminum-doped zinc oxide electrodes are analyzed. Periodic corrugation is introduced by structuring the oxide electrode via UV laser ablation, a process that enables flexible adjustment of the period and height of corrugation. We demonstrate that fabrication of stable and electrically efficient OLEDs on these rough substrates is feasible. Sharp spectral features are superimposed onto the broad emission spectra of the OLEDs, providing clear evidence for Bragg scattering of light from guided modes into the air cone. Theoretical analysis based on an emissive dipole model and conservation of momentum considerations allows a quantitative description of scattering and the associated dispersion relations.

Evaluation of surface microtopography engineered by direct laser interference for bacterial anti-biofouling

Modification of the biomaterial surface topography is a promising strategy to prevent bacterial adhesion and biofilm formation. In this study, we use direct laser interference patterning (DLIP) to modify polystyrene surface topography at sub-micrometer scale. The results revealed that three-dimensional micrometer structures have a profound impact on bacterial adhesion. Thus, line- and pillar-like patterns enhanced S. aureus adhesion, whereas complex lamella microtopography reduced S. aureus adhesion in static and continuous flow culture conditions. Interestingly, lamella-like textured surfaces retained the capacity to inhibit S. aureus adhesion both when the surface is coated with human serum proteins and when the material is implanted subcutaneously in a foreign-body associated infection model.

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.

Laser interference induced nano-crystallized surface swellings of amorphous carbon for advanced micro tribology

We have investigated the direct laser interference patterning (DLIP) of tetrahedral amorphous carbon (ta-C) thin films in the nanometer region. In this paper we achieved texturing of ta-C from a surface swelling either by a phase change from sp 3 to sp 2 or thin film delamination. In contrast to surface ablation, the thin film is not removed from the silicon surface. With this technique a surface nanoswelling of ta-C with an area-speed of 1m 2 min �1 and nanometric resolution can be realized. The phase changes in the ta-C, due to DLIP material heating are undermined by thermal simulations of the patterning process. Additionally we investigated for the first time the influence of periodic nano-swelling on the coefficient of friction (COF). The COF of periodic graphitized surfaces was found to be lower (∼50%) compared to as grown ta-C. This reduction of the COF can be attributed to a reduced area of contact.

To use or not to use (direct laser interference patterning), that is the question

Direct Laser Interference Patterning (DLIP) has shown to be a fabrication technology capable of producing large area periodic surface patterns on almost any kind of material. The produced structures have been used in the past to provide surfaces with new enhanced properties. On the other hand, the industrial use of this technology is still at the beginning due to the lack of appropriate and affordable systems, especially for small and medium enterprises. In this paper, the use of DLIP for the fabrication of periodic structures using different structuring strategies and optical concepts is discussed. Different technological challenges are addressed.

High speed surface functionalization using direct laser interference patterning, towards 1 m2/min fabrication speed with sub-μm resolution

Periodic patterned surfaces can be used to provide unique surface properties in applications, such as biomaterials, surface engineering, photonics and sensor systems. Such periodic patterns can be produced using laser processing tools, showing significant advantages due to a precise modification of the surfaces without contamination, remote and contactless operation, flexibility, and precise energy deposition. On the other hand, the resolution of such laser based surface structuring methods, like direct laser writing, is generally inversely proportional to the fabrication speed. Therefore, the development of new laser structuring technologies as well as strategies offering both high speed and resolution is necessary. In this study, the fabrication of spatially ordered structures with micrometer and submicrometer lengthscales at high surface processing fabrication speed is demonstrated. The procedures shown here are applied to process both planar surfaces and also three dimensional components. Different application examples of structured surfaces on different materials are also described. The applications include the development of thin film structured electrodes to improve the efficiency of organic light emitting diodes (OLEDs) as well as the direct fabrication of decorative elements on technological steels. Finally, an example of fabrication at high fabrication speed is shown.

Patterning of periodic nano-cavities on PEDOT–PSS using nanosphere-assisted near-field optical enhancement and laser interference lithography

A simple approach for creating periodic nano-cavities and periodic stripes of nano-cavity arrays on poly (3,4-ethylene dioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) thin films using a combination of optical near-field enhancement through self-assembled silica nanospheres and laser interference lithography is presented. Monolayers of close-packed silica nanospheres (800, 600, and 430 nm in diameter) are self-assembled on 2 µm thick PEDOT-PSS electropolymerized films and are subsequently irradiated with 10 ns pulses of 355 nm wavelength laser light. Over areas spanning 2 cm(2), circular nano-cavities with central holes of size 50-200 nm and surrounding craters of size 100-400 nm are formed in the PEDOT-PSS films directly underneath the nanospheres due to strong enhancement (11-18 fold) of the incident light in the near-field, which is confirmed through Mie scattering theory. Predictions from theoretical simulations examining the combined effects of near-field enhancement and interference are in good agreement with the experimental results. The results illustrate the versatility of the described technique for creating nano-cavity arrays or nano-pores in PEDOT-PSS over large areas with designed periodicity and hole size.

Development of a general model for direct laser interference patterning of polymers

This study investigates the general mechanism of Direct Laser Interference Patterning (DLIP) involved in the structuring process of polymer materials. An empirical model is developed taking into account experimental observations of DLIP-treated pigmented and transparent polycarbonate substrates with UV (263 nm) and IR (1053 nm) laser radiation. Depending on the used laser processing conditions, the type of material as well as the spatial period of the interference pattern, four different structuring mechanisms can be identified. The treated surfaces are investigated using confocal microscopy, scanning electron microscopy and focus ion beam and as a result from the experimental data analysis, the developed model predicts the material surface topography after the patterning process, by means of a set of material-dependent coefficients.

Laser Surface Pattering of Titanium for Improving the Biological Performance of Dental Implants.

Direct laser interference patterning (DLIP) is used to produce periodic line-like patterns on titanium surfaces. An Nd:YAG laser operating at 532 nm wavelength with a pulse duration of 8 ns is used for the laser patterning process. The generated periodic patterns with spatial periods of 5, 10, and 20 µm are produced with energy densities between 0.44 and 2.6 J cm-2 with a single laser pulse. With variation of energy density, different shapes of the arising topography are observed due to the development of the solidification front of the molten material at the maxima positions. Characterization of the surface chemistry shows that the DLIP treatment enhances the content of nitrogen of the titanium reactive layer from 3.9% up to 23.4%. The structural analysis near the titanium surface shows no changes in microstructure after the laser treatment. Contact angles between 65° and 79° are measured on both structured and turned reference surfaces. Cell viability of human osteoblasts on line-like patterned surfaces after 7 d in cultivation medium is 16% higher compared to the grit-blasted and acid-etched references. Finally, the possibility of patterning complex 3D dental implants is shown.