Mangirdas Malinauskas

Ultrafast laser processing of materials: from science to industry

Processing of materials by ultrashort laser pulses has evolved significantly over the last decade and is starting to reveal its scientific, technological and industrial potential. In ultrafast laser manufacturing, optical energy of tightly focused femtosecond or picosecond laser pulses can be delivered to precisely defined positions in the bulk of materials via two-/multi-photon excitation on a timescale much faster than thermal energy exchange between photoexcited electrons and lattice ions. Control of photo-ionization and thermal processes with the highest precision, inducing local photomodification in sub-100-nm-sized regions has been achieved. State-of-the-art ultrashort laser processing techniques exploit high 0.1–1 μm spatial resolution and almost unrestricted three-dimensional structuring capability. Adjustable pulse duration, spatiotemporal chirp, phase front tilt and polarization allow control of photomodification via uniquely wide parameter space. Mature opto-electrical/mechanical technologies have enabled laser processing speeds approaching meters-per-second, leading to a fast lab-to-fab transfer. The key aspects and latest achievements are reviewed with an emphasis on the fundamental relation between spatial resolution and total fabrication throughput. Emerging biomedical applications implementing micrometer feature precision over centimeter-scale scaffolds and photonic wire bonding in telecommunications are highlighted.

Three-dimensional laser micro- and nano-structuring of acrylated poly(ethylene glycol) materials and evaluation of their cytoxicity for tissue engineering applications.

The natural cell environment is characterized by complex three-dimensional structures, which contain features at multiple length scales. Many in vitro studies of cell behavior in three dimensions rely on the availability of artificial scaffolds with controlled three-dimensional topologies. In this paper, we demonstrate fabrication of three-dimensional scaffolds for tissue engineering out of poly(ethylene glycol) diacrylate (PEGda) materials by means of two-photon polymerization (2PP). This laser nanostructuring approach offers unique possibilities for rapid manufacturing of three-dimensional structures with arbitrary geometries. The spatial resolution dependence on the applied irradiation parameters is investigated for two PEGda formulations, which are characterized by molecular weights of 302 and 742. We demonstrate that minimum feature sizes of 200nm are obtained in both materials. In addition, an extensive study of the cytotoxicity of the material formulations with respect to photoinitiator type and photoinitiator concentration is undertaken. Aqueous extracts from photopolymerized PEGda samples indicate the presence of water-soluble molecules, which are toxic to fibroblasts. It is shown that sample aging in aqueous medium reduces the cytotoxicity of these extracts; this mechanism provides a route for biomedical applications of structures generated by 2PP microfabrication and photopolymerization technologies in general. Finally, a fully biocompatible combination of PEGda and a photoinitiator is identified. Fabrication of reproducible scaffold structures is very important for systematic investigation of cellular processes in three dimensions and for better understanding of in vitro tissue formation. The results of this work suggest that 2PP may be used to polymerize poly(ethylene glycol)-based materials into three-dimensional structures with well-defined geometries that mimic the physical and biological properties of native cell environments.

Mechanisms of three-dimensional structuring of photo-polymers by tightly focussed femtosecond laser pulses

Three-dimensional (3D) micro/nano-structuring of photo-resists is systematically studied at the close-to-dielectric- breakdown irradiance. It is demonstrated that avalanche absorption is playing a major part in free electron generation and chemical bond breaking at these conditions. The steps of photo-initiation and chemical bond breaking in propagation of polymerization are altered as compared with photo-polymerization at low-irradiance and one-photon stereo-lithography. The avalanche dominates radical generation and promotion of polymerization at tight focusing and a high approximately TW/cm(2) irradiance. The rates of electron generation by two-photon absorption and avalanche are calculated for the experimental conditions. Simulation results are corroborated by 3D polymerization in three resists with different photo-initiators at two different wavelengths and pulse durations. The smallest feature sizes of 3D polymerized logpile structures are consistent with spectral dependencies of the two photon nonlinearities. Implications of these findings for achieving sub-100 nm resolution in 3D structuring of photo-polymers are presented.

Photopolymerized microscopic vortex beam generators: Precise delivery of optical orbital angular momentum

Direct femtosecond laser photopolymerization is used to fabricate high resolution microscopic spiral phase plates. The total phase change all around their center is prepared to be a integer multiple of 2π for the operating wavelength in the visible domain. The optical performances of the spiral plates are measured and we propose a simple single beam interferometric technique to characterize the phase singularity of the generated vortex beams. The experimental results are compared to simulations and a satisfying agreement is obtained. Potential of large scale fabrication, templating, and smart spiral plate architectures are also illustrated.

Three-dimensional micro-/nano-structuring via direct write polymerization with picosecond laser pulses

We demonstrate capability to structure photo-polymers with sub-wavelength resolution, ∼200-500 nm, and retrieve three-dimensional (3D) structures using a picosecond laser exposure. This alternative to commonly used ultra-short femtosecond lasers extends accessibility of 3D direct write. A popular hybrid sol-gel resist SZ2080 was used for quantitative determination of structuring resolution at 1064 nm and 532 nm wavelengths and for pulses of 8-25 ps duration at the repetition rates of 0.2-1 MHz. Systematic study of feature size dependence of 3D suspended nano-rods shows that linear power dependence of photopolymerization on the dose-per-pulse becomes dominant at higher repetition rates (≥0.5 MHz) while the two-photon nonlinear absorption is still distinguishable at rates lower than 0.2 MHz and shorter pulses (≤8 ps). Thermal accumulation defines polymerization when cooling time of the focal volume is larger than separation between pulses. Photopolymerization and its scaling mechanisms, quality, and fidelity at tight focusing of fs-, ps-, and cw-laser radiation are revealed and explained. 3D scaffolds for biomedicine and microlenses for optical applications are fabricated by the ps-laser direct write.

Preclinical study of SZ2080 material 3D microstructured scaffolds for cartilage tissue engineering made by femtosecond direct laser writing lithography

Over the last decade DLW employing ultrafast pulsed lasers has become a well-established technique for the creation of custom-made free-form three-dimensional (3D) microscaffolds out of a variety of materials ranging from proteins to biocompatible glasses. Its potential applications for manufacturing a patients specific scaffold seem unlimited in terms of spatial resolution and geometry complexity. However, despite few exceptions in which live cells or primitive organisms were encapsulated into a polymer matrix, no demonstration of an in vivo study case of scaffolds generated with the use of such a method was performed. Here, we report a preclinical study of 3D artificial microstructured scaffolds out of hybrid organic-inorganic (HOI) material SZ2080 fabricated using the DLW technique. The created 2.1 × 2.1 × 0.21 mm(3) membrane constructs are tested both in vitro by growing isolated allogeneic rabbit chondrocytes (Cho) and in vivo by implanting them into rabbit organisms for one, three and six months. An ex vivo histological examination shows that certain pore geometry and the pre-growing of Cho prior to implantation significantly improves the performance of the created 3D scaffolds. The achieved biocompatibility is comparable to the commercially available collagen membranes. The successful outcome of this study supports the idea that hexagonal-pore-shaped HOI microstructured scaffolds in combination with Cho seeding may be successfully implemented for cartilage tissue engineering.

3D microporous scaffolds manufactured via combination of fused filament fabrication and direct laser writing ablation

A 3D printing fused filament fabrication (FFF) approach has been implemented for the creation of microstructures having an internal 3D microstructure geometry. These objects were produced without any sacrificial structures or additional support materials, just by precisely tuning the nozzle heating, fan cooling and translation velocity parameters. The manufactured microporous structures out of polylactic acid (PLA) had fully controllable porosity (20%–60%) and consisted of desired volume pores (~0.056 μm3). The prepared scaffolds showed biocompatibility and were suitable for the primary stem cell growth. In addition, direct laser writing (DLW) ablation was employed to modify the surfaces of the PLA structures, drill holes, as well as shape the outer geometries of the created objects. The proposed combination of FFF printing with DLW offers successful fabrication of 3D microporous structures with functionalization capabilities, such as the modification of surfaces, the generation of grooves and microholes and cutting out precisely shaped structures (micro-arrows, micro-gears). The produced structures could serve as biomedical templates for cell culturing, as well as biodegradable implants for tissue engineering. The additional micro-architecture is important in connection with the cell types used for the intention of cell growing. Moreover, we show that surface roughness can be modified at the nanoscale by immersion into an acetone bath, thus increasing the hydrophilicity. The approach is not limited to biomedical applications, it could be employed for the manufacturing of bioresorbable 3D microfluidic and micromechanic structures.

Micro-structured polymer scaffolds fabricated by direct laser writing for tissue engineering

This work presents the latest results on direct laser writing of polymeric materials for tissue engineering applications. A femtosecond Yb:KGW laser (300 fs, 200 kHz, 515 nm) was used as a light source for non-linear lithography. Fabrication was implemented in various photosensitive polymeric materials, such as: hybrid organic-inorganic sol-gel based on silicon-zirconium oxides, commercial ORMOCER® class photoresins. These materials were structured via multi-photon polymerization technique with submicron resolution. Porous three-dimensional scaffolds for artificial tissue engineering were fabricated with constructed system and were up to several millimeters in overall size with 10 to 100 μm internal pores. Biocompatibility of the used materials was tested in primary rabbit muscle-derived stem cell culture in vitro and using laboratory rats in vivo. This interdisciplinary study suggests that proposed technique and materials are suitable for tissue engineering applications.

Flat lensing in the visible frequency range by woodpile photonic crystals

We experimentally demonstrate full two-dimensional focalization of light beams at visible frequencies by a three-dimensional woodpile photonic crystal. The focalization (the flat lensing) with focal distances of the order of 50-70 μm is experimentally demonstrated. Experimental results are compared with numerical calculations and interpreted by harmonic expansion studies.

Black silicon: substrate for laser 3D micro/nano-polymerization

We demonstrate that black silicon (b-Si) made by dry plasma etching is a promising substrate for laser three-dimensional (3D) micro/nano-polymerization. High aspect ratio Si-needles, working as sacrificial support structures, have flexibility required to relax interface stresses between substrate and the polymerized micro-/nano- objects. Surface of b-Si can be made electrically conductive by metal deposition and, at the same time, can preserve low optical reflectivity beneficial for polymerization by direct laser writing. 3D laser polymerization usually performed at the irradiation conditions close to the dielectric breakdown is possible on non-reflective and not metallic surfaces. Here we show that low reflectivity and high metallic conductivity are not counter- exclusive properties for laser polymerization. Electrical conductivity of substrate and its permeability in liquids are promising for bio- and electroplating applications.