Nanochannels in Fused Silica through NaOH Etching Assisted by Femtosecond Laser Irradiation.

Sodium hydroxide (NaOH) is increasingly drawing attention as a highly selective etchant for femtosecond laser-modified fused silica. Unprecedented etching contrasts between the irradiated and pristine areas have enabled the fabrication of hollow, high-aspect-ratio structures in the bulk of the material, overcoming the micrometer threshold as the minimum feature size. In this work, we systematically study the effect of NaOH solutions under different etching conditions (etchant concentration, temperature, and etching time) on the tracks created by tightly focused femtosecond laser pulses to assess the best practices for the fabrication of hollow nanostructures in bulk fused silica.

Lab-on-Chip Systems for Cell Sorting: Main Features and Advantages of Inertial Focusing in Spiral Microchannels.

Inertial focusing-based Lab-on-Chip systems represent a promising technology for cell sorting in various applications, thanks to their alignment with the ASSURED criteria recommended by the World Health Organization: Affordable, Sensitive, Specific, User-friendly, Rapid and Robust, Equipment-free, and Delivered. Inertial focusing techniques using spiral microchannels offer a rapid, portable, and easy-to-prototype solution for cell sorting. Various microfluidic devices have been investigated in the literature to understand how hydrodynamic forces influence particle focusing in spiral microchannels. This is crucial for the effective prototyping of devices that allow for high-throughput and efficient filtration of particles of different sizes. However, a clear, comprehensive, and organized overview of current research in this area is lacking. This review aims to fill this gap by offering a thorough summary of the existing literature, thereby guiding future experimentation and facilitating the selection of spiral geometries and materials for cell sorting in microchannels. To this end, we begin with a detailed theoretical introduction to the physical mechanisms underlying particle separation in spiral microfluidic channels. We also dedicate a section to the materials and prototyping techniques most commonly used for spiral microchannels, highlighting and discussing their respective advantages and disadvantages. Subsequently, we provide a critical examination of the key details of inertial focusing across various cross-sections (rectangular, trapezoidal, triangular, hybrid) in spiral devices as reported in the literature.

Methods and Protocol for Single-Cell Motility Assays under Topological Constraints.

The extracellular environment plays a crucial role in many physiological and pathological processes involving cell motility, such as metastatic invasion in cancer development, by heavily impacting the migration strategies adopted by the cells. The study of how mechanical constraints affect the dynamics of cell migration may be relevant to gain more insight into such processes, and it may prove to be a powerful tool in the hands of biologists. In this chapter, we describe the methods used to investigate the ability of neoplastic cells to migrate through narrowing, rigid microstructures upon chemoattractant stimulation.

Anisotropy Parameters for Two-Color Photoionization Phases in Randomly Oriented Molecules: Theory and Experiment in Methane and Deuteromethane.

We present combined theoretical and experimental work investigating the angle-resolved phases of the photoionization process driven by a two-color field consisting of an attosecond pulse train and an infrared pulse in an ensemble of randomly oriented molecules. We derive a general form for the two-color photoelectron (and time-delay) angular distribution valid also in the case of chiral molecules and when relative polarizations of the photons contributing to the attosecond photoelectron interferometer differ. We show a comparison between the experimental data and theoretical predictions in an ensemble of methane and deuteromethane molecules, discussing the effect of nuclear dynamics on the photoionization phases. Finally, we demonstrate that the oscillating component and the phase of the two-color signal can be fitted by using complex asymmetry parameters, in perfect analogy to the atomic case.

In vivo label-free tissue histology through a microstructured imaging window.

Tissue histopathology, based on hematoxylin and eosin (H&E) staining of thin tissue slices, is the gold standard for the evaluation of the immune reaction to the implant of a biomaterial. It is based on lengthy and costly procedures that do not allow longitudinal studies. The use of non-linear excitation microscopy in vivo, largely label-free, has the potential to overcome these limitations. With this purpose, we develop and validate an implantable microstructured device for the non-linear excitation microscopy assessment of the immune reaction to an implanted biomaterial label-free. The microstructured device, shaped as a matrix of regular 3D lattices, is obtained by two-photon laser polymerization. It is subsequently implanted in the chorioallantoic membrane (CAM) of embryonated chicken eggs for 7 days to act as an intrinsic 3D reference frame for cell counting and identification. The histological analysis based on H&E images of the tissue sections sampled around the implanted microstructures is compared to non-linear excitation and confocal images to build a cell atlas that correlates the histological observations to the label-free images. In this way, we can quantify the number of cells recruited in the tissue reconstituted in the microstructures and identify granulocytes on label-free images within and outside the microstructures. Collagen and microvessels are also identified by means of second-harmonic generation and autofluorescence imaging. The analysis indicates that the tissue reaction to implanted microstructures is like the one typical of CAM healing after injury, without a massive foreign body reaction. This opens the path to the use of similar microstructures coupled to a biomaterial, to image in vivo the regenerating interface between a tissue and a biomaterial with label-free non-linear excitation microscopy. This promises to be a transformative approach, alternative to conventional histopathology, for the bioengineering and the validation of biomaterials in in vivo longitudinal studies.

Experimental certification of contextuality, coherence, and dimension in a programmable universal photonic processor.

Quantum superposition of high-dimensional states enables both computational speed-up and security in cryptographic protocols. However, the exponential complexity of tomographic processes makes certification of these properties a challenging task. In this work, we experimentally certify coherence witnesses tailored for quantum systems of increasing dimension using pairwise overlap measurements enabled by a six-mode universal photonic processor fabricated with a femtosecond laser writing technology. In particular, we show the effectiveness of the proposed coherence and dimension witnesses for qudits of dimensions up to 5. We also demonstrate advantage in a quantum interrogation task and show it is fueled by quantum contextuality. Our experimental results testify to the efficiency of this approach for the certification of quantum properties in programmable integrated photonic platforms.

Six-telescope integrated optics beam combiner fabricated using ultrafast laser inscription for J- and H-band astronomy.

We have built and characterized, to our knowledge, the first six-telescope discrete beam combiner (DBC) for stellar interferometry in the astronomical J-band. It is the DBC with the largest number of beam combinations and was manufactured using ultrafast laser inscription in borosilicate glass, with a throughput of ≈56%. For calibration of the visibility-to-pixel matrix, we use a two-input Michelson interferometer and extract the complex visibility. A visibility amplitude of 1.05 and relative precision of 2.9% and 3.8% are extracted for 1328 nm and 1380 nm, respectively. Broadband (≤40n m) characterization is affected by dispersion but shows similar performance.

Structured-light-sheet imaging in an integrated optofluidic platform.

Heterogeneity investigation at the single-cell level reveals morphological and phenotypic characteristics in cell populations. In clinical research, heterogeneity has important implications in the correct detection and interpretation of prognostic markers and in the analysis of patient-derived material. Among single-cell analysis, imaging flow cytometry allows combining information retrieved by single cell images with the throughput of fluidic platforms. Nevertheless, these techniques might fail in a comprehensive heterogeneity evaluation because of limited image resolution and bidimensional analysis. Light sheet fluorescence microscopy opened new ways to study in 3D the complexity of cellular functionality in samples ranging from single-cells to micro-tissues, with remarkably fast acquisition and low photo-toxicity. In addition, structured illumination microscopy has been applied to single-cell studies enhancing the resolution of imaging beyond the conventional diffraction limit. The combination of these techniques in a microfluidic environment, which permits automatic sample delivery and translation, would allow exhaustive investigation of cellular heterogeneity with high throughput image acquisition at high resolution. Here we propose an integrated optofluidic platform capable of performing structured light sheet imaging flow cytometry (SLS-IFC). The system encompasses a multicolor directional coupler equipped with a thermo-optic phase shifter, cylindrical lenses and a microfluidic network to generate and shift a patterned light sheet within a microchannel. The absence of moving parts allows a stable alignment and an automated fluorescence signal acquisition during the sample flow. The platform enables 3D imaging of an entire cell in about 1 s with a resolution enhancement capable of revealing sub-cellular features and sub-diffraction limit details.

3D photopolymerized microstructured scaffolds influence nuclear deformation, nucleo/cytoskeletal protein organization, and gene regulation in mesenchymal stem cells.

Mechanical stimuli from the extracellular environment affect cell morphology and functionality. Recently, we reported that mesenchymal stem cells (MSCs) grown in a custom-made 3D microscaffold, the Nichoid, are able to express higher levels of stemness markers. In fact, the Nichoid is an interesting device for autologous MSC expansion in clinical translation and would appear to regulate gene activity by altering intracellular force transmission. To corroborate this hypothesis, we investigated mechanotransduction-related nuclear mechanisms, and we also treated spread cells with a drug that destroys the actin cytoskeleton. We observed a roundish nuclear shape in MSCs cultured in the Nichoid and correlated the nuclear curvature with the import of transcription factors. We observed a more homogeneous euchromatin distribution in cells cultured in the Nichoid with respect to the Flat sample, corresponding to a standard glass coverslip. These results suggest a different gene regulation, which we confirmed by an RNA-seq analysis that revealed the dysregulation of 1843 genes. We also observed a low structured lamina mesh, which, according to the implemented molecular dynamic simulations, indicates reduced damping activity, thus supporting the hypothesis of low intracellular force transmission. Also, our investigations regarding lamin expression and spatial organization support the hypothesis that the gene dysregulation induced by the Nichoid is mainly related to a reduction in force transmission. In conclusion, our findings revealing the Nichoid's effects on MSC behavior is a step forward in the control of stem cells via mechanical manipulation, thus paving the way to new strategies for MSC translation to clinical applications.

Laser-written vapor cells for chip-scale atomic sensing and spectroscopy.

We report the fabrication of alkali-metal vapor cells using femtosecond laser machining. This laser-written vapor-cell (LWVC) technology allows arbitrarily-shaped 3D interior volumes and has potential for integration with photonic structures and optical components. We use non-evaporable getters both to dispense rubidium and to absorb buffer gas. This enables us to produce cells with sub-atmospheric buffer gas pressures without vacuum apparatus. We demonstrate sub-Doppler saturated absorption spectroscopy and single beam optical magnetometry with a single LWVC. The LWVC technology may find application in miniaturized atomic quantum sensors and frequency references.

Integrated optical device for Structured Illumination Microscopy.

Structured Illumination Microscopy (SIM) is a key technology for high resolution and super-resolution imaging of biological cells and molecules. The spread of portable and easy-to-align SIM systems requires the development of novel methods to generate a light pattern and to shift it across the field of view of the microscope. Here we show a miniaturized chip that incorporates optical waveguides, splitters, and phase shifters, to generate a 2D structured illumination pattern suitable for SIM microscopy. The chip creates three point-sources, coherent and controlled in phase, without the need for further alignment. Placed in the pupil of a microscope's objective, the three sources generate a hexagonal illumination pattern on the sample, which is spatially translated thanks to thermal phase shifters. We validate and use the chip, upgrading a commercial inverted fluorescence microscope to a SIM setup and we image biological sample slides, extending the resolution of the microscope.

On the robustness of machine learning algorithms toward microfluidic distortions for cell classification via on-chip fluorescence microscopy.

Single-cell imaging and sorting are critical technologies in biology and clinical applications. The power of these technologies is increased when combined with microfluidics, fluorescence markers, and machine learning. However, this quest faces several challenges. One of these is the effect of the sample flow velocity on the classification performances. Indeed, cell flow speed affects the quality of image acquisition by increasing motion blur and decreasing the number of acquired frames per sample. We investigate how these visual distortions impact the final classification task in a real-world use-case of cancer cell screening, using a microfluidic platform in combination with light sheet fluorescence microscopy. We demonstrate, by analyzing both simulated and experimental data, that it is possible to achieve high flow speed and high accuracy in single-cell classification. We prove that it is possible to overcome the 3D slice variability of the acquired 3D volumes, by relying on their 2D sum z-projection transformation, to reach an efficient real time classification with an accuracy of 99.4% using a convolutional neural network with transfer learning from simulated data. Beyond this specific use-case, we provide a web platform to generate a synthetic dataset and to investigate the effect of flow speed on cell classification for any biological samples and a large variety of fluorescence microscopes (https://www.creatis.insa-lyon.fr/site7/en/MicroVIP).

Microfluidic Lab-on-a-Chip for Studies of Cell Migration under Spatial Confinement.

Understanding cell migration is a key step in unraveling many physiological phenomena and predicting several pathologies, such as cancer metastasis. In particular, confinement has been proven to be a key factor in the cellular migration strategy choice. As our insight in the field improves, new tools are needed in order to empower biologists' analysis capabilities. In this framework, microfluidic devices have been used to engineer the mechanical and spatial stimuli and to investigate cellular migration response in a more controlled way. In this work, we will review the existing technologies employed in the realization of microfluidic cellular migration assays, namely the soft lithography of PDMS and hydrogels and femtosecond laser micromachining. We will give an overview of the state of the art of these devices, focusing on the different geometrical configurations that have been exploited to study specific aspects of cellular migration. Our scope is to highlight the advantages and possibilities given by each approach and to envisage the future developments in in vitro migration studies under spatial confinement in microfluidic devices.

Effect of 3D Synthetic Microscaffold Nichoid on the Morphology of Cultured Hippocampal Neurons and Astrocytes.

The human brain is the most complex organ in biology. This complexity is due to the number and the intricate connections of brain cells and has so far limited the development of in vitro models for basic and applied brain research. We decided to create a new, reliable, and cost-effective in vitro system based on the Nichoid, a 3D microscaffold microfabricated by two-photon laser polymerization technology. We investigated whether these 3D microscaffold devices can create an environment allowing the manipulation, monitoring, and functional assessment of a mixed population of brain cells in vitro. With this aim, we set up a new model of hippocampal neurons and astrocytes co-cultured in the Nichoid microscaffold to generate brain micro-tissues of 30 µm thickness. After 21 days in culture, we morphologically characterized the 3D spatial organization of the hippocampal astrocytes and neurons within the microscaffold, and we compared our observations to those made using the classical 2D co-culture system. We found that the co-cultured cells colonized the entire volume of the 3D devices. Using confocal microscopy, we observed that within this period the different cell types had become well-differentiated. This was further elaborated with the use of drebrin, PSD-95, and synaptophysin antibodies that labeled the majority of neurons, both in the 2D as well as in the 3D co-cultures. Using scanning electron microscopy, we found that neurons in the 3D co-culture displayed a significantly larger amount of dendritic protrusions compared to neurons in the 2D co-culture. This latter observation indicates that neurons growing in a 3D environment may be more prone to form connections than those co-cultured in a 2D condition. Our results show that the Nichoid can be used as a 3D device to investigate the structure and morphology of neurons and astrocytes in vitro. In the future, this model can be used as a tool to study brain cell interactions in the discovery of important mechanisms governing neuronal plasticity and to determine the factors that form the basis of different human brain diseases. This system may potentially be further used for drug screening in the context of various brain diseases.

Toward Higher Integration Density in Femtosecond-Laser-Written Programmable Photonic Circuits.

Programmability in femtosecond-laser-written integrated circuits is commonly achieved with the implementation of thermal phase shifters. Recent work has shown how such phase shifters display significantly reduced power dissipation and thermal crosstalk with the implementation of thermal isolation structures. However, the aforementioned phase shifter technology is based on a single gold film, which poses severe limitations on integration density and circuit complexity due to intrinsic geometrical constraints. To increase the compactness, we propose two improvements to this technology. Firstly, we fabricated thermal phase shifters with a photolithography process based on two different metal films, namely (1) chromium for microheaters and (2) copper for contact pads and interconnections. Secondly, we developed a novel curved isolation trench design that, along with a state-of-the-art curvature radius, allows for a significant reduction in the optical length of integrated circuits. As a result, curved Cr-Cu phase shifters provide a compact footprint with low parasitic series resistance and no significant increase in power dissipation (∼38 mW) and thermal crosstalk (∼20%). These results pave the way toward the fabrication of femtosecond-laser-written photonic circuits with a steep increase in terms of layout complexity.

Storage and analysis of light-matter entanglement in a fiber-integrated system.

The deployment of a full-fledged quantum internet poses the challenge of finding adequate building blocks for entanglement distribution between remote quantum nodes. A practical system would combine propagation in optical fibers with quantum memories for light, leveraging on the existing communication network while featuring the scalability required to extend to network sizes. Here, we demonstrate a fiber-integrated quantum memory entangled with a photon at telecommunication wavelength. The storage device is based on a fiber-pigtailed laser-written waveguide in a rare earth-doped solid and allows an all-fiber stable addressing of the memory. The analysis of the entanglement is performed using fiber-based interferometers. Our results feature orders-of-magnitude advances in terms of storage time and efficiency for integrated storage of light-matter entanglement and constitute a substantial step forward toward quantum networks using integrated devices.

Strategies for improved temporal response of glass-based optical switches.

We present an optimization of the dynamics of integrated optical switches based on thermal phase shifters. These devices have been fabricated in the volume of glass substrates by femtosecond laser micromachining and are constituted by an integrated Mach-Zehnder interferometer and a superficial heater. Simulations, surface micromachining and innovative layouts allowed us to improve the temporal response of the optical switches down to a few milliseconds. In addition, taking advantage of an electrical pulse shaping approach where an optimized voltage signal is applied to the heater, we proved a switching time as low as 78 µs, about two orders of magnitude shorter with respect to the current state of the art of thermally-actuated optical switches in glass.

Whole transcriptomic analysis of mesenchymal stem cells cultured in Nichoid micro-scaffolds.

Mesenchymal stem cells (MSCs) are known to be ideal candidates for clinical applications where not only regenerative potential but also immunomodulation ability is fundamental. Over the last years, increasing efforts have been put into the design and fabrication of 3D synthetic niches, conceived to emulate the native tissue microenvironment and aiming at efficiently controlling the MSC phenotype in vitro. In this panorama, our group patented an engineered microstructured scaffold, called Nichoid. It is fabricated through two-photon polymerization, a technique enabling the creation of 3D structures with control of scaffold geometry at the cell level and spatial resolution beyond the diffraction limit, down to 100 nm. The Nichoid's capacity to maintain higher levels of stemness as compared to 2D substrates, with no need for adding exogenous soluble factors, has already been demonstrated in MSCs, neural precursors, and murine embryonic stem cells. In this work, we evaluated how three-dimensionality can influence the whole gene expression profile in rat MSCs. Our results show that at only 4 days from cell seeding, gene activation is affected in a significant way, since 654 genes appear to be differentially expressed (392 upregulated and 262 downregulated) between cells cultured in 3D Nichoids and in 2D controls. The functional enrichment analysis shows that differentially expressed genes are mainly enriched in pathways related to the actin cytoskeleton, extracellular matrix (ECM), and, in particular, cell adhesion molecules (CAMs), thus confirming the important role of cell morphology and adhesions in determining the MSC phenotype. In conclusion, our results suggest that the Nichoid, thanks to its exclusive architecture and 3D cell adhesion properties, is not only a useful tool for governing cell stemness but could also be a means for controlling immune-related MSC features specifically involved in cell migration.

Resetting directional couplers for high-fidelity quantum photonic integrated chips.

In this Letter, we propose a fabrication technique based on femtosecond laser secondary direct writing (FsLSDW) that allows us to statically reset the beam-splitting ratio of directional couplers. By modifying the interaction region with a second inscription, the coupling coefficient of the reconstructed devices can be indeed changed continuously within the range of 0.49-2.1 rad/mm, thus enabling a complete tunability of the reconstructed splitting ratio from zero to full power transfer between the waveguides. This powerful reconstruction capability facilitates the arbitrary reset of an imperfect device, from any initial splitting ratio to the correct one. In the future, such static control method could potentially solve the fabrication error problem in the manufacturing of high-fidelity large-scale integrated photonic quantum chips.

First stellar photons for an integrated optics discrete beam combiner at the William Herschel Telescope.

We present the first on-sky results of a four-telescope integrated optics discrete beam combiner (DBC) tested at the 4.2 m William Herschel Telescope. The device consists of a four-input pupil remapper followed by a DBC and a 23-output reformatter. The whole device was written monolithically in a single alumino-borosilicate substrate using ultrafast laser inscription. The device was operated at astronomical H-band (1.6 µm), and a deformable mirror along with a microlens array was used to inject stellar photons into the device. We report the measured visibility amplitudes and closure phases obtained on Vega and Altair that are retrieved using the calibrated transfer matrix of the device. While the coherence function can be reconstructed, the on-sky results show significant dispersion from the expected values. Based on the analysis of comparable simulations, we find that such dispersion is largely caused by the limited signal-to-noise ratio of our observations. This constitutes a first step toward an improved validation of the DBC as a possible beam combination scheme for long-baseline interferometry.