Novel approach for label free super-resolution imaging in far field

Drop-on-Demand Pyro-Electrohydrodynamic Printing of Nematic Liquid Crystal Microlenses

Inkjet printing of liquid crystal (LC) microlens arrays is particularly appealing for the development of switchable 2D/3D organic light-emitting diode (OLED) displays, as the printing process ensures that the lenses can be deposited directly and on-demand onto the pixelated OLED layer without the need for additional steps, thus simplifying fabrication complexity. Even if different fabrication technologies have been employed and good results in LC direct printing have already been achieved, all the systems used require costly equipment and heated nozzles to reduce the LC solution's viscosity. Here, we present the direct printing of a nematic LC (NLC) lens by a Drop-on-Demand (DoD) inkjet printing by a pyro-electrohydrodynamic effect for the first time. The method works at ambient temperature and avoids dispensing nozzles, thus offering a noncontact manipulation approach of liquid with high resolution and good repeatability on different kinds of substrates. NLC microlenses are printed on different substrates and fully characterized. Polarization properties are evaluated for various samples, i.e., NLC lenses on unaligned and indium-tin oxide (ITO) aligned. Moreover, an in-depth characterization of the NLC lenses is reported by polarized optical microscopy and by analyzing the birefringence in digital holographic microscopy.

Skin cancer margin detection using nanosensitive optical coherence tomography and a comparative study with confocal microscopy

Excision biopsy and histology represent the gold standard for morphological investigation of the skin, in particular for cancer diagnostics. Nevertheless, a biopsy may alter the original morphology, usually requires several weeks for results, is non-repeatable on the same site and always requires an iatrogenic trauma. Hence, diagnosis and clinical management of diseases may be substantially improved by new non-invasive imaging techniques. Optical Coherence Tomography (OCT) is a non-invasive depth-resolved optical imaging modality based on low coherence interferometry that enables high-resolution, cross-sectional imaging in biological tissues and it can be used to obtain both structural and functional information. Beyond the resolution limit, it is not possible to detect structural and functional information using conventional OCT. In this paper, we present a recently developed technique, nanosensitive OCT (nsOCT), improved using broadband supercontinuum laser, and demonstrate nanoscale sensitivity to structural changes within ex vivo human skin tissue. The extended spectral bandwidth permitted access to a wider distribution of spatial frequencies and improved the dynamic range of the nsOCT. Firstly, we demonstrate numerical and experimental detection of a few nanometers structural difference using the nsOCT method from single B-scan images of phantoms with sub-micron periodic structures, acting like Bragg gratings, along the depth. Secondly, our study shows that nsOCT can distinguish nanoscale structural changes at the skin cancer margin from the healthy region in en face images at clinically relevant depths. Finally, we compare the nsOCT en face image with a high-resolution confocal microscopy image to confirm the structural differences between the healthy and lesional/cancerous regions, allowing the detection of the skin cancer margin.

Accessing depth-resolved high spatial frequency content from the optical coherence tomography signal

Optical coherence tomography (OCT) is a rapidly evolving technology with a broad range of applications, including biomedical imaging and diagnosis. Conventional intensity-based OCT provides depth-resolved imaging with a typical resolution and sensitivity to structural alterations of about 5–10 microns. It would be desirable for functional biological imaging to detect smaller features in tissues due to the nature of pathological processes. In this article, we perform the analysis of the spatial frequency content of the OCT signal based on scattering theory. We demonstrate that the OCT signal, even at limited spectral bandwidth, contains information about high spatial frequencies present in the object which relates to the small, sub-wavelength size structures. Experimental single frame imaging of phantoms with well-known sub-micron internal structures confirms the theory. Examples of visualization of the nanoscale structural changes within mesenchymal stem cells (MSC), which are invisible using conventional OCT, are also shown. Presented results provide a theoretical and experimental basis for the extraction of high spatial frequency information to substantially improve the sensitivity of OCT to structural alterations at clinically relevant depths.

Nanosensitive optical coherence tomography to assess wound healing within the cornea

Optical coherence tomography (OCT) is a non-invasive depth resolved optical imaging modality, that enables high resolution, cross-sectional imaging in biological tissues and materials at clinically relevant depths. Though OCT offers high resolution imaging, the best ultra-high-resolution OCT systems are limited to imaging structural changes with a resolution of one micron on a single B-scan within very limited depth. Nanosensitive OCT (nsOCT) is a recently developed technique that is capable of providing enhanced sensitivity of OCT to structural changes. Improving the sensitivity of OCT to detect structural changes at the nanoscale level, to a depth typical for conventional OCT, could potentially improve the diagnostic capability of OCT in medical applications. In this paper, we demonstrate the capability of nsOCT to detect structural changes deep in the rat cornea following superficial corneal injury.

Noninvasive detection of nanoscale structural changes in cornea associated with cross-linking treatment

Corneal cross-linking (CXL) using ultraviolet-A (UVA) irradiation with a riboflavin photosensitizer has grown from an interesting concept to a practical clinical treatment for corneal ectatic diseases globally, such as keratoconus. To characterize the corneal structural changes, existing methods such as X-ray microscopy, transmission electron microscopy, histology and optical coherence tomography (OCT) have been used. However, these methods have various drawbacks such as invasive detection, the impossibility for in vivo measurement, or limited resolution and sensitivity to structural alterations. Here, we report the application of oversampling nanosensitive OCT for probing the corneal structural alterations. The results indicate that the spatial period increases slightly after 30 minutes riboflavin instillation but decreases significantly after 30 minutes UVA irradiation following the Dresden protocol. The proposed noninvasive method can be implemented using existing OCT systems, without any additional components, for detecting nanoscale changes with the potential to assist diagnostic assessment during CXL treatment, and possibly to be a real-time monitoring tool in clinics.

Image scanning difference microscopy

Here, we propose a novel imaging method, which is called image scanning difference microscopy (ISDM), for superresolution imaging. In ISDM, we implement a detector array composed of 19 avalanche photodiodes (APD) rather than single-point detector in standard confocal microscopy for reconstructing superresolved images with higher signal-to-noise ratio (SNR). Combining with our former proposed fluorescence emission difference (FED) method, we have achieved a lateral resolution of 111 nm (∼λ/6) without the damage of image quality, the highest FED resolution to the best of our knowledge. With its simple setup and remarkable performance, we believe that ISDM can become a versatile observation tool in biology and other fundamental studies. LAY DESCRIPTION: Fluorescence emission difference (FED) microscopy is a really simple and generalisable superresolved fluorescence microscopy method based on PSF engineering and difference algorithm recently. Compared to stimulated-emission-depletion fluorescence microscopy (STED), FED don't need complicated system or precise alignment and polarisation, available for wide variety of dyes and has low photobleaching and phototoxicity for living cells. However, the distortion caused by negative value is one of the biggest obstacles to the further development of FED. In light of this, we propose a novel superresolution imaging method based on the FED method with parallel detection system, which is called image scanning difference microscopy (ISDM). Our method has achieved a significant breakthrough in FED, increasing the resolution further while reducing artefacts generated by negative values, which cannot be accomplished through combining other methods. In addition, ISDM does not require complex setup and optical alignment, long time imaging and imposing no constraint on dyes. Importantly, we realised a transverse resolution of ∼λ/6 (triple diffraction limit) with single wavelength, single incident path and low light intensity, which has never been achieved in any other far-field superresolution microscopy.

Visualization of ultraviolet absorption distribution beyond the diffraction limit of light by electron-beam excitation-assisted optical microscope

We demonstrated that the high spatial resolution absorption contrast imaging of the crystal of vitamin B9 has absorption at ultraviolet wavelengths. The absorption wavelength matches with the wavelength of the emission of the fluorescent thin film of an electron-beam excitation-assisted (EXA) optical microscope. The fine crystal structure was imaged beyond the optical diffraction limit. The image contrast corresponded with the thickness of the crystal. The illumination light is absorbed with the vitamin B9 crystal and the intensity of the transmitted light depends on the thickness of the vitamin B9 crystal. The EXA optical microscope is useful for analysis of growth of a crystal, bioimaging and so on.

Label-free ultra-sensitive visualization of structure below the diffraction resolution limit

For both fundamental study of biological processes and early diagnosis of diseases, information about nanoscale changes in tissue and cell structure is crucial. Nowadays, almost all currently known nanoscopy methods rely upon the contrast created by fluorescent stains attached to the object or molecule of interest. This causes limitations due to the impact of the label on the object and its environment, as well as its applicability in vivo, particularly in humans. In this paper, a new label-free approach to visualize small structure with nano-sensitivity to structural alterations is introduced. Numerically synthesized profiles of the axial spatial frequencies are used to probe the structure within areas whose size can be beyond the diffraction resolution limit. Thereafter, nanoscale structural alterations within such areas can be visualized and objects, including biological ones, can be investigated with sub-wavelength resolution, in vivo, in their natural environment. Some preliminary results, including numerical simulations and experiments, which demonstrate the nano-sensitivity and super-resolution ability of our approach, are presented.

Super-resolved 3-D imaging of live cells' organelles from bright-field photon transmission micrographs

Current biological and medical research is aimed at obtaining a detailed spatiotemporal map of a live cell's interior to describe and predict cell's physiological state. We present here an algorithm for complete 3-D modelling of cellular structures from a z-stack of images obtained using label-free wide-field bright-field light-transmitted microscopy. The method visualizes 3-D objects with a volume equivalent to the area of a camera pixel multiplied by the z-height. The computation is based on finding pixels of unchanged intensities between two consecutive images of an object spread function. These pixels represent strongly light-diffracting, light-absorbing, or light-emitting objects. To accomplish this, variables derived from Rényi entropy are used to suppress camera noise. Using this algorithm, the detection limit of objects is only limited by the technical specifications of the microscope setup-we achieve the detection of objects of the size of one camera pixel. This method allows us to obtain 3-D reconstructions of cells from bright-field microscopy images that are comparable in quality to those from electron microscopy images.

Review of interferometric spectroscopy of scattered light for the quantification of subdiffractional structure of biomaterials

Optical microscopy is the staple technique in the examination of microscale material structure in basic science and applied research. Of particular importance to biology and medical research is the visualization and analysis of the weakly scattering biological cells and tissues. However, the resolution of optical microscopy is limited to ? 200 ?? nm due to the fundamental diffraction limit of light. We review one distinct form of the spectroscopic microscopy (SM) method, which is founded in the analysis of the second-order spectral statistic of a wavelength-dependent bright-field far-zone reflected-light microscope image. This technique offers clear advantages for biomedical research by alleviating two notorious challenges of the optical evaluation of biomaterials: the diffraction limit of light and the lack of sensitivity to biological, optically transparent structures. Addressing the first issue, it has been shown that the spectroscopic content of a bright-field microscope image quantifies structural composition of samples at arbitrarily small length scales, limited by the signal-to-noise ratio of the detector, without necessarily resolving them. Addressing the second issue, SM utilizes a reference arm, sample arm interference scheme, which allows us to elevate the weak scattering signal from biomaterials above the instrument noise floor.

Multiscale dispersion-state characterization of nanocomposites using optical coherence tomography

Nanocomposite materials represent a success story of nanotechnology. However, development of nanomaterial fabrication still suffers from the lack of adequate analysis tools. In particular, achieving and maintaining well-dispersed particle distributions is a key challenge, both in material development and industrial production. Conventional methods like optical or electron microscopy need laborious, costly sample preparation and do not permit fast extraction of nanoscale structural information from statistically relevant sample volumes. Here we show that optical coherence tomography (OCT) represents a versatile tool for nanomaterial characterization, both in a laboratory and in a production environment. The technique does not require sample preparation and is applicable to a wide range of solid and liquid material systems. Large particle agglomerates can be directly found by OCT imaging, whereas dispersed nanoparticles are detected by model-based analysis of depth-dependent backscattering. Using a model system of polystyrene nanoparticles, we demonstrate nanoparticle sizing with high accuracy. We further prove the viability of the approach by characterizing highly relevant material systems based on nanoclays or carbon nanotubes. The technique is perfectly suited for in-line metrology in a production environment, which is demonstrated using a state-of-the-art compounding extruder. These experiments represent the first demonstration of multiscale nanomaterial characterization using OCT.

Functional imaging for regenerative medicine

In vivo imaging is a platform technology with the power to put function in its natural structural context. With the drive to translate stem cell therapies into pre-clinical and clinical trials, early selection of the right imaging techniques is paramount to success. There are many instances in regenerative medicine where the biological, biochemical, and biomechanical mechanisms behind the proposed function of stem cell therapies can be elucidated by appropriate imaging. Imaging techniques can be divided according to whether labels are used and as to whether the imaging can be done in vivo. In vivo human imaging places additional restrictions on the imaging tools that can be used. Microscopies and nanoscopies, especially those requiring fluorescent markers, have made an extraordinary impact on discovery at the molecular and cellular level, but due to their very limited ability to focus in the scattering tissues encountered for in vivo applications they are largely confined to superficial imaging applications in research laboratories. Nanoscopy, which has tremendous benefits in resolution, is limited to the near-field (e.g. near-field scanning optical microscope (NSNOM)) or to very high light intensity (e.g. stimulated emission depletion (STED)) or to slow stochastic events (photo-activated localization microscopy (PALM) and stochastic optical reconstruction microscopy (STORM)). In all cases, nanoscopy is limited to very superficial applications. Imaging depth may be increased using multiphoton or coherence gating tricks. Scattering dominates the limitation on imaging depth in most tissues and this can be mitigated by the application of optical clearing techniques that can impose mild (e.g. topical application of glycerol) or severe (e.g. CLARITY) changes to the tissue to be imaged. Progression of therapies through to clinical trials requires some thought as to the imaging and sensing modalities that should be used. Smoother progression is facilitated by the use of comparable imaging modalities throughout the discovery and trial phases, giving label-free techniques an advantage wherever they can be used, although this is seldom considered in the early stages. In this paper, we will explore the techniques that have found success in aiding discovery in stem cell therapies and try to predict the likely technologies best suited to translation and future directions.