In the last two decades quartz has become a relevant material for sensing technology since it has been used for realization of various devices, such as Quartz Crystal Microbalance (QCM) or Quartz-Tuning-Fork (QTF). Micromachining of quartz can be realized through various techniques, such as diamond cutting, lithography, wet and dry etching, ion beam etching and Ultra-Short-Pulsed-Laser (USPL) processing. At the state-of-the-art USPL has been efficiently applied to quartz micromachining, e.g., for drilling and stealth dicing. In this study, the influence of the incubation effect and the repetition rate on USPL ablation threshold of quartz was systematically investigated. The multi-pulse ablation threshold of quartz was evaluated using 200 fs laser pulses at a wavelength of 1030 nm, at three different repetition rates, i.e., 0.06, 6, 60 and 200 kHz. Results show a strong decrease in the multi-pulse ablation threshold with the number of pulses N, as a consequence of the effect of incubation during the fs-laser ablation. Conversely, the influence of the repetition rate on incubation is negligible in the investigated frequency range. A saturation of the threshold fluence value occurs at number of pulses N > 100 and this trend is well fitted by an exponential incubation model. Using such a model, the single-pulse ablation threshold value and the incubation coefficient for quartz have been estimated. This investigation represents a first step towards the micro- and nano-texturing of quartz crystal for tailoring its mechanical, electrical, and optical properties.
Development of new lab-on-a-chip (LoC) devices requires an optimization phase in which it could be necessary to continuously modify the architecture and geometry. However, this is only possible if easy, controllable fabrication methods and low-cost materials are available. For this reason, rapid prototyping approaches for the fabrication of polymeric LoC are on the rise, as they allow high degrees of precision and flexibility. Here, we describe the fabrication platform of polymeric microfluidic devices, from the design (CAD) to the proof-ofconcept application as LoC for biological applications. The fabrication procedure is mainly based on fs-laser micromachining techniques. The ability of femtosecond (fs)-laser pulses to produce localized modification of the materials, thereby avoiding either debris, recast layers or unsought thermal affected zones, without restriction of the substrate materials, makes this technology particularly suitable for microfluidic device fabrication. In our work, fs-laser has been also possibly combined with other techniques, without the need for the expensive masks and facilities required by the lithographic process. The LoC devices have been realized in polymethyl methacrylate (PMMA), a low cost and biocompatible material. The fs-based smart fabrication platform has been exploited in the fabrication of disposable LoC devices for particles manipulation. In particular, a serpentine microchannel able to distinguish cancer from non-cancer cells without labeling and a fully inertial sorting 3D device have been fabricated and tested.
We report on an experimental and theoretical investigation on the laser ablation of silicon with THz bursts of fs pulses. Craters were generated by varying the burst features, i.e., the number of pulses and the intra-burst repetition rate, and compared to those obtained in Normal Pulse Mode (NPM). A general reduction of the thermal load was observed using bursts, though with a lower ablation rate. In fact, shallower craters were obtained when increasing the number of pulses and reducing the intra-burst repetition rates at fixed processing time and burst energy. However, for bursts at 2 THz, some combinations of process parameters allowed a higher specific ablation rate compared to NPM. Simulations based on the numerical solution of the density-dependent two temperature model showed that bursts with more pulses or with lower intra-burst repetition rates lead to a lower final temperature, thus supporting the experimental findings. This is ascribed to changes of the reflectivity dependent on the number of pulses. Accordingly, different amounts of energy are transferred from the laser pulse to the sample, which also leads to changes in specific ablation rates. The origin of such a behavior was found to be the non-linear absorption processes, especially the two-photon absorption.
Utilization of parts made by combining dissimilar materials, such as different polymers, metals, or semiconductor to polymers, are nowadays highly demanded for the fabrication of electronic, electromechanical, medical micro-devices, and analytical systems (e.g., lab-on-chip). Techniques for joining such hybrid micro-devices, generally based on gluing or thermal processes, remain a challenging task presenting some drawbacks, such as deterioration and contamination of the substrates. Ultrashort laser welding is a non-contact and flexible technique to precisely weld similar and dissimilar materials. In this case, the only constrain is that the upper substrate is transparent to the laser wavelength. This technique has been demonstrated both for welding polymers and polymers to metallic substrates, but never for joining polymers to silicon. In this work, we report on direct femtosecond laser welding of Poly(methyl methacrylate) (PMMA) and silicon. The laser welding was performed in ambient air by focusing ultrashort laser pulses at high repetition rate at the interface between the two, being PMMA transparent to the laser wavelength. A mechanical homogenous pressure was applied on the sandwiched substrates during all the laser process. The Si-PMMA weld strength was evaluated as a function of the laser and processing parameters, e.g., repetition rate, scan speed, and the overlap between adjacent scan lines.
In this work, we report on a single-pass method for cutting 250-μm thick Z-cut quartz plates using 200 fs laser pulses at the wavelength of 1030 nm. In particular, we delve into the influence of the process parameters, i.e. laser repetition rate, scan speed and pulse energy, on the generation of a controlled stress-induced fracture which ultimately leads to the final cut. Processing above a certain threshold pulse energy caused significant damage, resulting in poor quality cuts. Whereas, a correct combination of these parameters led to a flat and almost defect-free cut edges, in a single pass.
Many surfaces in nature, e.g. lotus leaf, exhibit superhydrophobicity. Some of the most attractive applications of these surfaces are based on their self-cleaning properties and anti-icing capability. Many strategies are used by researchers to replicate these natural phenomena on metallic substrates. Among them, short/ultrashort pulsed laser technologies can functionalize surfaces with micro/nano-textures enabling strong water-repellent properties and low adhesiveness, which represent a promising solution to anti-icing properties. In this work, several patterns of micro-structures were textured by femtosecond laser on metallic materials of aeronautic and aerospace interest. The wettability properties of the surfaces were investigated in terms of water contact angle (CA) under different ambient conditions. The reversibility of the sample superhydrophobicity after exposure to a highly humid environment was studied. Water-dripping tests were carried out at subzero temperature finding that, while the untreated samples were covered with ice, no frozen spot was observed on the superhydrophobic textured surfaces.
Inertial microfluidic particles sorting represents a critical task in many areas of biology, biotechnology, and medicine, including the isolation from blood of rare target cell populations, like e.g. circulating tumor cells (CTCs) and circulating fetal cells (CFCs). Usually, cell sorter microfluidic devices are fabricated by PDMS soft lithography, which is the most widespread micromanufacturing platform enabling to cost-effectively produce Lab-on-a-Chip with resolution in the nanometer scale. However, this technology presents some drawbacks: (i) due to PDMS softness, especially for high-pressure flows, the microfluidic behavior may change along the devices, leading to ambiguous results; (ii) soft-lithography allows to pattern structures on just one side of the chip thus limiting the affordable geometries to enhance the throughput of target particles. In this work, we develop a PMMA continuous size-based inertial microfluidic sorter by femtosecond laser microfabrication (FLM). The device design includes contracting and expanding channels (microchambers) provided with siphoning outlets on the backside of the chip. Since FLM technology is in principle applicable to any type of polymer, we chose PMMA, which is a biocompatible and transparent thermoplastic polymer much stiffer than PDMS. FLM allows machining the microfluidic network on both sides of the chip, making it possible the parallelization of the sorting process. In addition, thanks to the FLM flexibility, we easily varied the chambers number and the collecting strategy (at different flow rates) in order to define a device layout maximizing the trapping efficiency and throughput.
We present a cost-effective and highly-portable plastic prototype that can be interfaced with a cell phone to implement an optofluidic imaging cytometry platform. It is based on a PMMA microfluidic chip that fits inside an opto-mechanical platform fabricated by a 3D printer. The fluorescence excitation and imaging is performed using the LED and the CMOS from the cell phone increasing the compactness of the system. A custom developed application is used to analyze the images and provide a value of particle concentration.
Microfluidic optical stretchers are valuable optofluidic devices for studying single cell mechanical properties. These usually consist of a single microfluidic channel where cells, with dimensions ranging from 5 to 20 μm are trapped and manipulated through optical forces induced by two counter-propagating laser beams. Recently, monolithic optical stretchers have been directly fabricated in fused silica by femtosecond laser micromachining (FLM). Such a technology allows writing in a single step in the substrate volume both the microfluidic channel and the optical waveguides with a high degree of precision and flexibility. However, this method is very slow and cannot be applied to cheaper materials like polymers. Therefore, novel technological platforms are needed to boost the production of such devices on a mass scale.
In this work, we propose integration of FLM with micro-injection moulding (μIM) as a novel route towards the cost-effective and flexible manufacturing of polymeric Lab-on-a-Chip (LOC) devices. In particular, we have fabricated and assembled a polymethylmethacrylate (PMMA) microfluidic optical stretcher by exploiting firstly FLM to manufacture a metallic mould prototype with reconfigurable inserts. Afterwards, such mould was employed for the production, through μIM, of the two PMMA thin plates composing the device. The microchannel with reservoirs and lodgings for the optical fibers delivering the laser radiation for cell trapping were reproduced on one plate, while the other included access holes to the channel. The device was assembled by direct fs-laser welding, ensuring sealing of the channel and avoiding thermal deformation and/or contamination.
Femtosecond-pulsed laser welding of transparent materials on a micrometer scale is a versatile tool for the fabrication and assembly of electronic, electromechanical, and especially biomedical micro-devices. In this paper, we report on microwelding of two transparent layers of polymethyl methacrylate (PMMA) with femtosecond laser pulses at 1030 nm in the MHz regime. We aim at exploiting localized heat accumulation to weld the two layers without any preprocessing of the sample and any intermediate absorbing media, by focusing fs-laser pulses at the interface.
The modifications produced by the focused laser beam into the bulk material have been firstly investigated depending on the laser process parameters aiming to produce continuous melting. Results have been evaluated based on heat accumulation models. Finally, fs-laser welding of PMMA samples have been successfully demonstrated and tested by leakage tests for application in direct laser assembly of microfluidic devices.
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