Discrete model for laser driven etching and microstructuring of metallic surfaces

We present a unidimensional discrete solid-on-solid model evolving in time using a kinetic Monte Carlo method to simulate micro-structuring of kerfs on metallic surfaces by means of laser-induced jet-chemical etching. The precise control of the passivation layer achieved by this technique is responsible for the high resolution of the structures. However, within a certain range of experimental parameters, the microstructuring of kerfs on stainless steel surfaces with a solution of $\mathrm{H}_3\mathrm{PO}_4$ shows periodic ripples, which are considered to originate from an intrinsic dynamics. The model mimics a few of the various physical and chemical processes involved and within certain parameter ranges reproduces some morphological aspects of the structures, in particular ripple regimes. We analyze the range of values of laser beam power for the appearance of ripples in both experimental and simulated kerfs. The discrete model is an extension of one that has been used previously in the context of ion sputtering and is related to a noisy version of the Kuramoto-Sivashinsky equation used extensively in the field of pattern formation.Comment: Revised version. Etching probability distribution and new simulations adde

Microstructuring of glass by laser irradiation: applications on microoptics and microfluidics

En la presente tesis se propone la fabricación de elementos en vidrio para micro-óptica y microfluídica que exigen microestructuras de alta calidad. Estos elementos son altamente demandados desde sectores industriales, por lo que se precisan técnicas de procesado, rápidas, sencillas y de bajo coste

Fabrication and characterization of microstructured scaffolds for complex 3D cell cultures

In einem natürlichen Gewebe wird das zelluläre Verhalten durch Stimuli der Mikroumgebung reguliert. Verschiedene chemische, mechanische und physikalische Reize befinden sich in einem lokalen Milieu und versorgen die Zellen mit einem biologischen Kontext. Im Vergleich zur in vivo Situation, zeigen Standard 2D in vitro Zellkulturmodelle viele Unterschiede in der zellulären Mikroumgebung und können infolgedessen eine Veränderung der Zellantwort verursachen. Die Schaffung einer physiologisch realistischeren Umgebung auf künstlichem Substrat ist ein Schlüsselfaktor für die Entwicklung zuverlässiger Plattformen, die es den kultivierten Zellen ermöglichen, sich natürlicher zu verhalten. Daher sind neuartige Substrate auf Biomaterialbasis mit maßgeschneiderten Eigenschaften sehr gefragt. Die Mikrotechnik ist ein leistungsstarkes Werkzeug, das bei der Herstellung der Funktionsgerüste hilft, um verschiedene Eigenschaften der in vivo Umgebung zu reproduzieren und auf in vitro Bedingungen zu übertragen. Die Gerüstkonstruktionsparameter können manipuliert werden, um die für das jeweilige Gewebe spezifischen Anforderungen zu erfüllen. Eine der grundlegenden Einschränkungen bei aktuellen Herstellungsverfahren ist jedoch die Unfähigkeit, mehrere Gerüsteigenschaften auf vorgefertigte Weise in eine einzelne Gerüststruktur zu integrieren. Diese Dissertation befasst sich mit Gerüstmikrofabrikations- und Oberflächenmodifikations-techniken, welche die Mikrostrukturierungstechnologie verwenden und die gleichzeitige Kontrolle über verschiedene Gerüsteigenschaften ermöglichen. Diese Ansätze bei der Mikrofabrikation von Polymergerüsten werden verwendet, um physikalische und chemische Eigenschaften bereitzustellen, die für die Leberzellkultur optimiert sind. Die physikochemischen Aspekte, die die zelluläre Mikroumgebung von Lebergewebe in vivo ausmachen, werden diskutiert und anschließend werden relevante Technologien vorgestellt, mit denen einige dieser Aspekte in vitro reguliert werden können. Im ersten Teil dieser Arbeit wird ein neuartiges zweistufiges Verfahren zur Herstellung von Polymergerüsten mit mikroporöser Struktur und definierter Topographie gezeigt. Um 3D-Matrizen mit integrierter Porosität zu erhalten, wurde nach der Herstellung mikroporöser Folien ein Mikrostrukturierungsprozess unter Verwendung der Mehrschicht Polymer-Thermoformtechnologie durchgeführt. Diese Methoden wurden verwendet, um Substrate für die organotypische 3D-Hepatozytenkultivierung herzustellen. Poröse Gerüste mit Mikrokavitäten wurden aus lösungsmittelgegossenen und phasengetrennten Polymilchsäure (PLA) Folien gebildet. Die Proben wurden auf grundlegende mechanische und Oberflächenspezifische Eigenschaften sowie auf die Zellleistung untersucht. Um einen Bezugspunkt für die Bewertung der hergestellten Matrices bereitzustellen, wurden PLA-Gerüste mit zuvor beschriebenen Substraten auf Polycarbonat (PC)-Basis mit ähnlicher Geometrie verglichen. HepG2-Zellen, die in PLA-Gerüsten kultiviert wurden, zeigten eine gewebeartige 3D-Aggregation und eine erhöhte Sekretionsrate von Albumin im Vergleich zu PC-Gerüsten. Anschließend wurde dieses zweistufige Herstellungsverfahren verwendet, um schnell abbaubare Gerüste für die gerüstfreie Zellblatttechnik herzustellen. Gerüste mit kontrollierter Porosität und Topographie, die die Schlüsselmerkmale von Lebersinusoiden nachahmen, wurden aus Poly(milch-co-glykolsäure) (PLGA)-Copolymer hergestellt und für den in vitro Abbau in Zellkultur charakterisiert. Um die Beziehung zwischen dem Abbau des Gerüsts und der Organisation der Zellen in der PLGA-Matrix aufzudecken, wurde die Lebensfähigkeit und Morphologie der kultivierten Zellen zusammen mit der Morphologie des Gerüsts untersucht. Im zweiten Teil dieser Arbeit wurden verschiedene technische Lösungen für die gerichtete Strukturierung mikroporöser Polymergerüste bewertet und ihre Eignung zur Erzeugung einer benutzerdefinierten lebenswichtigen oligozellulären Morphologie auf künstlichem Substrat vorgestellt. Besonderes Augenmerk wurde auf das 3D-Mikrokontaktdruckverfahren (3DµCP) gelegt, das die Vorteile des Mikrothermoformens und des Mikrokontaktdrucks kombiniert und eine räumlich-zeitliche Kontrolle über morphologische und chemische Merkmale in einem einzigen Schritt ermöglicht. Um das Potenzial dieser Technik aufzuzeigen, wurden Gerüste mit bestimmten Mikrostrukturen wie Kanäle mit verschiedenen Tiefen und Breiten sowie komplexere Muster hergestellt und verschiedene ECM-Moleküle gleichzeitig in die vordefinierten Geometrien übertragen. Die Gültigkeit des 3DµCP-Prozesses wurde durch mikroskopische Messungen, Fluoreszenzfärbung und Testen der Substrate auf Zelladhäsionsantwort gezeigt. Schließlich wird in dieser Arbeit die Herstellungsmethode zur Erzeugung komplexer Gerüste für die 3D- und gesteuerte Co-Kultivierung von Leberzellen vorgestellt. Polymermatrizen, die die grundlegende Leberarchitektur replizieren und somit eine gut organisierte Leberzellzusammensetzung ermöglichen, wurden erfolgreich unter Verwendung der 3DµCP-Methode hergestellt. Auf der Polycarbonatoberfläche wurden gleichzeitig chemische und topografische Leitfäden in Form sinusförmiger Strukturen strukturiert. Um die 3D-Gewebemikrostruktur zu replizieren, wurden EA.hy926- und HepG2-Zellen auf beiden Seiten des strukturierten porösen Gerüsts Co-kultiviert und anschließend einander gegenüber gestapelt, wodurch zugehörige Kanäle zur Bildung einer Kapillare führen. Das Potenzial unseres 3DµCP-strukturierten Gerüsts für die gerichtete Co-Kultivierung von Zellen wurde unter statischen Zellkulturbedingungen demonstriert. Am Ende wurden Gerüste für die weiteren Anwendungen im perfundierten Bioreaktorsystem angepasst.In a natural tissue, cellular behavior is regulated by microenvironmental stimuli. Different chemical, mechanical and physical cues reside in a local milieu and provide cells with a biological context. Compare to the in vivo situation, standard 2D in vitro cell culture models show many differences in the cellular microenvironment and as a consequence can cause alteration in cellular response. Creating physiologically more realistic environment on artificial substrate is a key factor for development of reliable platforms that enables the cultured cells to behave in a more natural manner. Therefore, novel biomaterial-based substrates with tailored properties are highly demanded. Microtechnology is a powerful tool that helps in the production of the functional scaffolds for reproducing various characteristics of the in vivo environment and transfer them to in vitro conditions. Scaffold design parameters can be manipulated to meet the needs specific to given tissue. However, one of the fundamental limitations in current fabrication methods is the inability to integrate multiple scaffold characteristics within a single scaffold structure in a pre-designed manner. This dissertation discusses scaffold microfabrication and surface modification techniques that use microstructuring technology and allows simultaneous control over various scaffold properties. These approaches in microfabricating polymeric scaffolds are used to provide physical and chemical characteristic those are more optimal for liver cell culture. The physiochemical aspects that constitute the in vivo cellular microenvironment of liver tissue are discussed and subsequently relevant technologies that can be used to regulate some of those aspects in vitro are presented. In the first part this thesis demonstrates a novel two-step procedure for manufacturing polymeric scaffolds with microporous structure and defined topography. To achieve 3D matrixes with integrated porosity, fabrication of microporous foils was followed by microstructuring process using multilayer polymer thermoforming technology. These methods were used to produce substrates for organotypic 3D hepatocyte cultivation. Porous scaffolds with the structure of microcavities were formed from solvent casted and phase separated polylactic acid (PLA) foils. Samples were investigated for basic mechanical and surface properties as well as cellular performance. Moreover, to provide a reference point for the evaluation of produced matrixes, PLA scaffolds were compared to previously reported polycarbonate (PC) based substrates with similar geometry. HepG2 cells cultured within PLA scaffolds showed 3D tissue-like aggregation and enhanced secretion rate of albumin in comparison to PC scaffolds. Subsequently, this two-step fabrication method was used to produce fast degradable scaffolds for scaffold-free cell sheet engineering. Scaffolds with controlled porosity and topography mimicking the key features of liver sinusoids were produced from poly(lactic-co-glycolic acid) (PLGA) copolymer and characterized for in vitro degradation in cell culture. To reveal the relationship between degradation of the scaffold and organization of the cells in PLGA matrix, viability and morphology of the cultured cells was examined along with scaffolds morphology. In the second part of this work, various technical solutions for directed patterning of microporous polymer scaffolds were evaluated and their suitability for creating a user-defined vital oligocellular morphology on artificial substrate was presented. Special attention was given to the 3D microcontact printing (3DµCP) method that combines the advantages of microthermoforming and microcontact printing and provides spatiotemporal control over morphological and chemical feature in a single step. To show the potential of this technique, scaffolds with determined microstructures like channels with various depths and widths as well as more complex patterns were fabricated and different ECM molecules were simultaneously transferred inside the predesigned geometries. The validity of 3DµCP process has been demonstrated by microscopic measurements, fluorescence staining and testing the substrates to cell adhesion response. Finally, this thesis presents fabrication method for manufacturing complex scaffolds for 3D and guided co-cultivation of liver cells. Polymer matrixes that replicate basic liver architecture and thus facilitate well-organized hepatic cell composition were successfully produced using the 3DµCP method. Chemical and topographical guidance cues in the form of sinusoidal structures were simultaneously patterned on the polycarbonate surface. To replicate 3D tissue microstructure, EA.hy926 and HepG2 cells were co-cultured on both sides of the patterned porous scaffold and subsequently stacked facing each other by virtue of which associated channels results in the formation of a capillary. The potential of our 3DµCP patterned scaffold for directed co-cultivation of cells was demonstrated under static cell culture conditions. At the end, scaffolds were adapted for the further applications in perfused bioreactor system

Serial laser lithography for efficient manufacture of universal microstructures

The technique of microstructuring revolutionises all classical fields of engineering like electronics, optics and mechanics. In order to manufacture a microstructure in large quantities and at a reasonable price, master elements or masks will be formed that can be duplicated in a highly efficient process. Further development in technology leads, on the one hand, to further reduction of possible dimensions of structures down to the range of sub-nano technology and, on the other hand, to the development of more flexible systems in using more reasonably priced technologies for the structuring in the classical micrometre range, which in turn opens a much larger field of use. This study examines the use of serial laser lithography for efficient manufacture of universal microstructures. To facilitate this, a laser beam writer or so-called Laser Pattern Generator (LPG) was developed and described here as well as in a previous work[Samu96a]. The laser beam writer uses a precise positioning system for the movement of a substrate for material processing using a focussed laser beam. This system permits the production of structures with dimensions down to 0.5 μm which can be used in several application fields. This was systematically analysed for optimisation of the production process. Based on the achieved results, a computer-aided simulation system for process parameter determination and optimisation was developed that may be used in order to minimise the experimental effort in LPG manufacturing. The total production process and the individual optimising steps are illustrated by the manufacture of different microstructures. Because of the high reproducibility in manufacturing different structure types and, compared with other manufacturing methods, the low equipment and manufacturing effort, serial laser lithography is an efficient process for the microstructuring of universal microstructures down to the dimensions in the micrometre range

Wear characterization of cemented carbides (WC-CoNi) processed by laser surface texturing under abrasive machining conditions

Cemented carbides are outstanding engineering materials widely used in quite demanding material removal applications. In this study, laser surface texturing is implemented for enhancing, at the surface level, the intrinsic bulk-like tribological performance of these materials. In this regard, hexagonal pyramids patterned on the cutting surface of a tungsten cemented carbide grade (WC-CoNi) have been successfully introduced by means of laser surface texturing. It simulates the surface topography of conventional honing stones for abrasive application. The laser-produced structure has been tested under abrasive machining conditions with full lubrication. Wear of the structure has been characterized and compared, before and after the abrasive machining test, in terms of changes in geometry aspect and surface integrity. It is found that surface roughness of the machined workpiece was improved by the laser-produced structure. Wear characterization shows that laser treatment did not induce any significant damage to the cemented carbide. During the abrasive machining test, the structure exhibited a high wear resistance. Damage features were only discerned at the contacting surface, whereas geometrical shape of pyramids remained unchanged.Peer ReviewedPostprint (author's final draft

Wear Characterization of Cemented Carbides (WC–CoNi) Processed by Laser Surface Texturing under Abrasive Machining Conditions

Cemented carbides are outstanding engineering materials widely used in quite demanding material removal applications. In this study, laser surface texturing is implemented for enhancing, at the surface level, the intrinsic bulk-like tribological performance of these materials. In this regard, hexagonal pyramids patterned on the cutting surface of a tungsten cemented carbide grade (WC–CoNi) have been successfully introduced by means of laser surface texturing. It simulates the surface topography of conventional honing stones for abrasive application. The laser-produced structure has been tested under abrasive machining conditions with full lubrication. Wear of the structure has been characterized and compared, before and after the abrasive machining test, in terms of changes in geometry aspect and surface integrity. It is found that surface roughness of the machined workpiece was improved by the laser-produced structure. Wear characterization shows that laser treatment did not induce any significant damage to the cemented carbide. During the abrasive machining test, the structure exhibited a high wear resistance. Damage features were only discerned at the contacting surface, whereas geometrical shape of pyramids remained unchanged

Material modifications due to nonlinear effects created by multiphoton absorption in single crystalline silicon

Material modification inside its bulk via high powered lasers involves much more than just heat transfer and melting of materials. It entails with it complex nonlinear physical phenomena such as multiphoton absorption, self-phase modulation, and self-focussing, amongst many others. These phenomena occur only with ultrafast lasers at very high intensities. Realising subsurface or bulk modifications in semiconductors such as silicon, opens up new avenues in the fields of optoelectronics and optical computation with the potential of increasing current computational speeds by orders of magnitude. The technology of three dimensional volume modification in materials via ultrafast lasers and nonlinear physics, is however, still in its nascent stages. This work explores the possibility of realising bulk modification in silicon and other polymers, and as well as their integration with optoelectronic devices; thus paving way for the future of optical computation

Laser Surface Structuring of Alumina

Alumina ceramic is an important abrasive material for grinding wheels used for rough grinding/machining of materials in manufacturing industry. Purpose of this work is to explore laser surface structuring of alumina grinding wheels for precision machining/grinding of materials by modifying surface microstructure of wheels. Major objective of this work is to study the evolution of surface microstructure and depth of modification such that microstructures/properties of modified wheels can be efficiently tailored based on fundamental understanding of physical processes taking place during laser surface structuring. Surface structuring of alumina using a continuous wave Nd:YAG laser resulted in significant surface melting and subsequent rapid solidification. The surface modified alumina consisted of microstructure characterized by regular polygonal and faceted surface grains with well defined edges and vertices. Such multifaceted grains act as micro-cutting tools on the surface of grinding wheels facilitating micro-scale material removal during precision machining. The formation of faceted morphology is explained on the basis of evolution of crystallographic texture in laser modified alumina. Furthermore, complete crystallographic description of multifaceted morphology of surface grains is provided based on detailed analysis of surface micro-texture. Due to complexity of microstructure formed during laser surface structuring, a fractal analysisbased approach is suggested to characterize surface microstructures. Detailed analysis of the effects of laser interaction with porous alumina ceramic indicated that melt surface undergoes rapid evaporation resulting in generation of high (\u3e105 Pa) evaporationinduced recoil pressures. These pressures drive the flow of melt through underlying porous alumina during modification extending the depth of modification. An integrative modeling approach combining thermal analysis and fluid flow analysis resulted in better agreements between predicted and experimental values of depths of melting. Finally, improvements in microindentation fracture toughness of alumina ceramic are reported with increasing laser fluence. Such improvements in the fracture toughness seem to be derived from better surface densification and coarsening of grain structure. The understanding of the evolution of faceted morphology, depth of surface modifications and improvements in fracture properties in laser surface microstructured alumina ceramic reached in this work provides the foundation for tailoring of surface microstructures/properties of alumina grinding wheels for precision machining applications

Microstructuring of materials with laser technologies for biomedical applications

This thesis presents the use of laser technologies for structuring different materials for applications in biomedicine. One of the aims of this work is the fabrication of fluidic chips for their employment as preclinical devices. By direct or indirect laser techniques, materials like soda-lime glass, titanium or tantalum are structured. Dimensions from microns to millimetres are achieved, depending on the final application of the chip. In particular, a device that imitates a coronary bifurcation is fabricated by laser technologies and soft-lithography methods. It is validated by culturing endothelial cells in their inner walls that withstand flow conditions. Other structures, like microchannels, a circulating tumour cells capturing chip or patterns over titanium and tantalum are manufactured