Femtosecond laser processing of bone tissue has shown great potential for osteotomy procedures where high precision cutting and the preservation of bone tissue functions are of primary importance. Nonetheless, the ablation rates related to this kind of process still remain too low for this technology to be exploitable in a real surgical procedure. Moreover, the strong dependence of the process outcomes from factors such as the processing environment, the type of pre-processing and post-processing treatment of the bone tissue, the species of the processed animal and the bone part itself, stall the full development and advancement of this technique. This study highlights the key role of the anatomical region (femur, tibia, etc.) and species (pig, chicken, etc.) of the investigated bone tissue samples to provide a solid reference on the impact of the choice of types of samples on laser ablation studies of bone tissue. Results show that it is essential to choose the best animal model for a specific case study, which depends to a large extent on the objectives of the research subject. There are no perfect animal models: the selection of one animal model is often associated with its similarity to the human model because the goal is the validation of experiments in clinical setting. However, most studies do not take these variabilities into account in their conclusions.
Use of kW class Ultra-Short Pulse lasers jointly with a suitable beam engineering strategy makes possible to achieve highthroughput production of laser functionalised surfaces. Nonetheless, the production of complex parts is still limited by several issues like the difficulty to reach certain regions where the geometry presents high aspect ratio shapes or tortuous profiles, and the need to adapt the laser processing workstation to a specific geometry. To overcome this issue, metallic moulds can be efficiently laser treated and employed to reproduce the surface morphology, and its specific properties, on a final polymeric replica. Forming process is a well-known method to produce any-shape part from metallic foils by applying specific constraints. In this work, forming of laser-treated metallic foils and the mechanical properties of the final formed functionalised parts are investigated in order to evaluate the possibility to produce laser-functionalised 3D complex products. By using a Rollto- Roll pilot line we textured stainless-steel 200 μm-thick foils by Laser Induced Periodic Surface Structures (LIPSS). The LIPSS morphology has been firstly optimised. Then, three types of mechanical tests were carried on laser-treated and untreated foils: standard tensile, fatigue and cruciform specimen tests. We measured and compared ultimate tensile strength, breaking strength, maximum elongation, and area reduction on specimens with and without LIPSS obtained from the same foil. By SEM and AFM analysis we compared the LIPSS morphology on samples subjected to mechanical tests and those just textured. For both, we evaluated the surface wettability through a measurement of the contact angle.
The nature of transparent materials makes their laser-based functionalisation more complex, highly material dependant and often characterised by process throughputs too low to be considered competitive for an industrial uptake. In this frame, direct laser interference patterning (DLIP) appears to be the right compromise to achieve 100s-nm surface features while running at competitive throughputs. A state-of-the-art DLIP processing setup is employed to shape the 100s-nm nanostructure features to obtain highly homogeneous morphologies in different regimes of interaction on polycarbonate, fused silica and sapphire, including a DLIP-tailored f-theta lens connected to a 30-mm aperture high-speed galvanometric scanning head.
Selective laser etching (SLE) has demonstrated to be an effective technique to push further the exploitation of laser-based surface functionalisation techniques allowing to reach always smaller surface structures at higher resolution and with controlled aspect-ratios. In this work, SLE is investigated on sapphire as a method to induce different surface functionalities such as antireflective effect and wettability modification. The effect of laser process parameters on the etching rate is evaluated in Gaussian and Bessel laser-beam configurations for KOH etching solution by means of scanning electron microscope (SEM) and confocal microscope analyses.
Direct Laser Interference Patterning (DLIP) and Laser Induced Period Surface Structures (LIPSS) are two distinct technologies for periodic surface texturing. Based on different physics phenomena, they enable structures often showing dissimilar morphology and pitch. It has been reported the possibility of superposing LIPSS over DLIP with a final multiscale, hierarchical morphology where the two structures coexist. Here, we report a novel approach in DLIP structuring based on the use of a galvo scanner with large aperture (30 mm) and an F-theta lens combining large entrance pupil (< 20 mm) and relatively small focal length (30 mm). We show that by using a 10 ps laser source emitting at λ=1064 nm, this set-up makes possible a DLIP pitch value Λ as low as 1.4 μm which become comparable with the LIPSS period ≈ 800 – 900 nm. Interestingly, in these experimental conditions, we identified a process window (fluence, number of passes, polarization) where LIPSS formation on stainless-steel surface is strongly affected by the presence of DLIP. Two highly homogeneous, uninterrupted, regular LIPSS arise between two successive DLIP crests with a period reduced to 470 nm, which is sensibly lower than expected. As a result, highly regular ripples with a narrow angular distribution and having a period < λ/2 are observed. Finally, all the generated structures have been characterised by SEM and FFT. We believe that our results represent a promising approach for the high throughput generation over large surface of highly regular structures in the range of few hundreds of nm.
Femtosecond laser processing is well known because of the achievable unparalleled precision with minimized thermal effects on most materials, making this technology competitive for its clinical use on living tissue. Nevertheless, femtosecond laser processing often requires an optimization of the parameters in order to increase the ablation rate which remains too limited compared to CW or QCW lasers. This study identifies an optimized set of process parameters (wavelength, temperature, bone hydration) for efficiently ablating bone tissue. Three different absorption regimes are studied using fs laser sources centered at wavelengths of 1030 nm, 515 nm and 343 nm. The thermal effects generated by the interaction of the fs laser and the bone were analyzed using a thermal camera and thermocouples with different cooling methods (water and air). Our results highlight (i) the significant capacity of the visible wavelength (515 nm) to ablate bone tissue with a maximum value of 0.66 mm3/s on pork femurs and that (ii) the use of water for cooling the sample is the most effective method of cooling and reduce thermal propagation without impacting the ablation rate. This study also raises awareness about the role of the anatomical region (femur, tibia, etc.) and species (pig, chicken, etc.) of the investigated bone tissue samples which may induce significant differences in ablation efficiency.
Nowadays Er:YAG lasers are employed for bone surgery because of their emission wavelengths of 2.94µm which allows for optimization of material removal rate. Nevertheless, high degrees of tissue carbonization are unavoidable and prevent the process of tissue regeneration. In this work, a femtosecond source with a pulse duration of 350fs was employed to carry a comparative study on ablation efficiency and quality at wavelengths of 1030nm and 515nm. Laser-treated bones were analyzed by optical profilometry, SEM and EDX. Results show that on optimization of the process parameters is necessary to achieve an optimal quality of ablation without tissue carbonization.
In the present work, we investigate the benefits and the drawbacks in using on dual-wavelength double fs-pulse laser irradiation for fused silica processing. Our purpose of this pump-pump experiment is to tune the electron dynamics in order to optimize energy deposition and then to improve ablated volume. We use green wavelength (515 nm) for the first pulse to enhance photo-ionization and near-infrared (1030 nm) for the second pulse to maximize electron heating and impact ionization. The investigated parameters are pulse-to-pulse delay (up to 20 ps), second pulse duration (1 and 10 ps) and total fluence (up to 20 J/cm²). The results will be discussed in terms of ablated volume and optical transmission. We demonstrate that (i) there is an optimum delay and (ii) the ablation behavior is intermediate between green and near-infrared single pulse irradiation. Our results are supported by a numerical model taking into account electron dynamics and absorbed energy density.
Over the last ten years, the need for organ donors for transplants became critical due to the increased incidence of organ failure [1]. Today, the development of tissue engineering (TE) appears as the best opportunity to overcome this shortage. TE is an interdisciplinary emerging field that aims to restore and maintain human tissue functions by applying engineering and live science principles [2]. However, one of its greatest challenges is the vascularization of tissue for the transport of oxygen and nutrients to prevent cell death. Here an innovative method is proposed to answer vascularization issues and the difficulty to create blood microcapillaries constructs, with a special interest to renal microcapillaries, which allow blood filtration. A cell-bilayer covering a tubular collagen I matrix with a diameter of about 150μm was developed and treated by ultra-short pulse (USP) laser processing in order to selectively remove the collagen core to create a capillary. The precise laser treatment allows indeed for the creation of voids in the fibre-shaped construct which results in the final formation of the capillary. Firstly, experiments were carried on a 2D model of gelatine hydrogel. The hydrogel-laser interaction was parametrically investigated in order to define a window of laser process parameters allowing the creation of voids within the hydrogel. The best window of laser process parameters was then applied to the 3D cell bilayer microfibres. Confocal microscopy examination demonstrated the presence of a lumen through the collagen I matrix without extended damage to surrounding cells. Live/Dead assays were also carried to assess cell viability.
Ultra-short pulse laser texturing is a well-known one-step technique used to transform the surface properties of different materials in order to functionalize them for specific applications. According to the laser and process parameters, several features can be achieved, as surface coloring, blackening and super-hydrophobicity. In this work, an upscaling approach is considered for generation of surface structures and thermal effects, connected to the use of high-average power lasers are considered in relation to the influence of the laser pulse duration and repetition rate on the final surface morphology. Mirror-polished 316L steel samples were textured by an UPS laser source with pulse duration of about 450fs and running at 1030nm, at two different repetition rates, 250kHz and 1000kHz. Results show that two main sources of thermal effects are identified: (i) heat accumulation due to the use of high repetition rates and (ii) thermal diffusion effects linked to the intrinsic nature of the material. When employing high repetition rates, a lower cumulative energy is necessary to highlight the influence of the pulse duration on the surface morphology. Finally, the influence of pulse duration and wavelength on the wetting properties of the material surface are also investigated.
Super-hydrophobic surfaces are nowadays of primary interest in several application fields, as for de-icing devices in the automotive and aerospace industries. In this context, laser surface texturing has widely demonstrated to be an easy one-step method to produce super-hydrophobic surfaces on several materials. In this work, a high average power (up to 40W), high repetition-rate (up to 1MHz), femtosecond infrared laser was employed to produce super-hydrophobic surfaces on 316L steel. The set of process and laser parameters for which the super-hydrophobic behavior is optimized, was obtained by varying the laser energy and repetition rate. The morphology of the textured surfaces was firstly analyzed by SEM and confocal microscope analyses. The contact angle was measured over time in order to investigate the effect of air environment on the hydrophobic properties and define the period of time necessary for the super-hydrophobic properties to stabilize. An investigation on the effect of after-processing cleaning solvents on the CA evolution was carried to assess the influence of the after-processing sample handling on the CA evaluation. Results show that the highest values of contact angle, that is the best hydrophobic behavior, are obtained at high repetition rate and low energy, this way opening up a promising scenario in terms of upscaling for reducing the overall process takt-time.
Pulsed laser ablation in liquid (PLAL) is nowadays gaining popularity as innovative, reliable and efficient technique to produce high-purity nanoparticles (NPs) of many inorganic and organic materials. In this context, attention has been recently focused on luminescent up-conversion NPs (UCNPs) which, being characterized by sharp emission bands in ultraviolet (UV)-to-near-infrared (NIR) range upon NIR irradiation, are in fact of great interest in many biological and biomedical applications. Moreover, with respect to organic dyes NPs and quantum dots, UCNPs show less toxicity, increased chemical stability, long-lifetime decays and lack of photo-bleaching. Our research focuses on generation of UCNPs of rare earth lanthanide-doped crystalline material, namely 18%Yb:1%Er:NAYF4, by PLAL in water. It is well known that optical properties of NPs strongly depend on their features, as for instance size and shape, which in turn may be controlled by laser ablation parameters. Therefore, two different laser sources are used for the ablation processes in order to find the set of laser parameter, i.e. pulse duration, laser fluence and repetition rate, for which the luminescence of UPNPs is optimized: (i) Amplitude Satsuma HP3 system: 330 fs pulse duration, 1030 wavelength and (ii) Eolite Hegoa system: 50 ps pulse duration, 1030 nm wavelength. UCNPs are finally characterized by spectrophotometer analyses to define emission range and intensity under NIR light and by transmission electron microscopy (TEM) to determine their size and shape.
Recently a parametric decay model was proposed in order to foresee LIPSS interspaces, and experimental results are in reasonable agreement. To confirm the possibility assumed by the model of pre-formed plasma generation, Ti surface was irradiated by a femtosecond (fs) laser beam composed by double fs pulses, with a fixed delay of 160 fs. The fluence of the first pulse (FPP), responsible for surface plasma formation, was varied in the range 10-50 mJ cm-2 and always kept below the LIPSS formation threshold fluence (FLIPSS) of Ti for 50-single-shots exposure. The fluence of the delayed pulse (FLP), responsible for LIPSS formation, was varied in the range 60-150 mJ cm-2 and always kept above FLIPSS. Regardless the specific fluence FLP of the delayed pulse, the interspace of the grating structures increases with the increase of FPP, that is the increase of the surface plasma density. This tendency suggests that a variation of the surface plasma density, due to a variation of FPP, actually leads to a modification of the grating features, highlighting the driving role of the first pulse in LIPSS formation. Moreover, we observed that the LIPSS periodicities after double pulse exposures are in quite good agreement with data on LIPSS periodicities after single 160 fs pulse irradiations on Ti surface and with the curve predicted by the parametric decay model. This experimental result suggests that the preformed plasma might be produced in the rising edge of the temporal profile of the laser pulse.
An overview of Czech national R&D project HiLASE (High average power pulsed LASEr) is presented. The HiLASE project aims at development of pulsed DPSSL for hi-tech industrial applications. HiLASE will be a user oriented facility with several laser systems with output parameters ranging from a few picosecond pulses with energy of 5 mJ to 0.5 J and repetition rate of 1-100 kHz (based on thin disk technology) to systems with 100 J output energy in nanosecond pulses with repetition rate of 10 Hz (based on multi-slab technology).
Nowadays, more powerful and challenging laser systems are built to meet the need of evolving technology. In this
context, the aim of the HiLASE project [1] is to develop a multi-joule picosecond laser system working in kHz repetition
rate regime. The outputs of the project will provide not only unique source for both scientific and industrial applications,
but also great challenge for supporting technologies. The key parameter of all optical components in laser and beam
delivery structure is the laser induced damage threshold, which limits intensities manageable by the system. The
following paper presents results of LIDT test of mirrors intended to use in laser system built within the HiLASE project
as well as advanced LIDT test station design, which will use HiLASE laser as source.
We have developed a novel method for efficient structuring of the surface of materials by applying femtosecond near
infrared laser pulses simultaneously with a weak extreme ultraviolet beam, which leads to a very strong radiation-matter
interaction and brings a dramatic increase of the surface processing speed. We present our recent experimental results on
surface nanostructuring of thin films of amorphous carbon and polymethyl methacrylate deposited on bulk substrates and
discuss the underlying physical mechanisms. In the case of amorphous carbon, large areas of laser-induced periodic
surface structures with a spatial period of 550 nm were created, having their origin in laser-induced convective currents.
Our method provides a powerful tool for fast modification of tribological properties of the irradiated sample.
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