Fibroblasts can condense a hydrated collagen lattice to a tissue-like structure 1/28th the area of the starting gel in 24 hr. The rate of the process can be regulated by varying the protein content of the lattice, the cell number, or the con- centration of an inhibitor such as Colcemid. Fibroblasts of high population doubling level propagated in vitro, which have left the cell cycle, can carry out the contraction at least as efficiently as cycling cells. The potential uses of the system as an immu- nologically tolerated "tissue" for wound hea ing and as a model for studying fibroblast function are discussed.

Production of a tissue-like structure by contraction of collagen lattices by human fibroblasts of different proliferative

Quantitative phase imaging (QPI) has emerged as a valuable method for investigating cells and tissues. QPI operates on unlabelled specimens and, as such, is complementary to established fluorescence microscopy, exhibiting lower phototoxicity and no photobleaching. As the images represent quantitative maps of optical path length delays introduced by the specimen, QPI provides an objective measure of morphology and dynamics, free of variability due to contrast agents. Owing to the tremendous progress witnessed especially in the past 10–15 years, a number of technologies have become sufficiently reliable and translated to biomedical laboratories. Commercialization efforts are under way and, as a result, the QPI field is now transitioning from a technology-development-driven to an application-focused field. In this Review, we aim to provide a critical and objective overview of this dynamic research field by presenting the scientific context, main principles of operation and current biomedical applications. Over the past 10–15 years, quantitative phase imaging has moved from a research-driven to an application-focused field. This Review presents the main principles of operation and representative basic and clinical science applications.

Quantitative phase imaging in biomedicine

A cellular-level study of the pathophysiology is crucial for understanding the mechanisms behind human diseases. Recent advances in quantitative phase imaging (QPI) techniques show promises for the cellular-level understanding of the pathophysiology of diseases. To provide important insight on how the QPI techniques potentially improve the study of cell pathophysiology, here we present the principles of QPI and highlight some of the recent applications of QPI ranging from cell homeostasis to infectious diseases and cancer.

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Quantitative phase imaging techniques for the study of cell pathophysiology: from principles to applications.

Boundary Integral and Singularity Methods for Linearized Viscous Flow: Green's functions

Particle tracking is a fundamental technique for investigating a variety of biophysical processes, from intracellular dynamics to the characterization of cell motility and migration. However, observing three-dimensional (3D) trajectories of particles is in general a challenging task in classical microscopy owing to the limited imaging depth of field of commercial optical microscopes, which represents a serious drawback for the analysis of time-lapse microscopy image data. Therefore, numerous automated particle-tracking approaches have been developed by many research groups around the world. Recently, digital holography (DH) in microscopy has rapidly gained credit as one of the elective techniques for these applications, mainly due to the uniqueness of the DH to provide a posteriori quantitative multiple refocusing capability and phase-contrast imaging. Starting from this paradigm, a huge amount of 3D holographic tracking approaches have been conceived and investigated for applications in various branches of science, including optofluids, microfluidics, biomedical microscopy, cell mechano-trasduction, and cell migration. Since a wider community of readers could be interested in such a review, i.e., not only scientists working in the fields of optics and photonics but also users of particle-tracking tools, it should be very beneficial to provide a complete review of state-of-the-art holographic 3D particle-tracking methods and their applications in bio-microfluidics.

Recent advances in holographic 3D particle tracking

Digital holographic microscopy (DHM) is a potent tool to perform three-dimensional imaging and tracking. We present a review of the state-of-the-art of DHM for three-dimensional profiling and tracking with emphasis on DHM techniques, reconstruction criteria for three-dimensional profiling and tracking, and their applications in various branches of science, including biomedical microscopy, particle imaging velocimetry, micrometrology, and holographic tomography, to name but a few. First, several representative DHM configurations are summarized and brief descriptions of DHM processes are given. Then we describe and compare the reconstruction criteria to obtain three-dimensional profiles and four-dimensional trajectories of objects. Details of the simulated and experimental evidences of DHM techniques and related reconstruction algorithms on particles, biological cells, fibers, etc., with different shapes, sizes, and conditions are also provided. The review concludes with a summary of techniques and applications of three-dimensional imaging and four-dimensional tracking by DHM.

https://www.spiedigitallibrary.org/journals/optical-engineering/volume-53/issue-11/112306/Review-of-digital-holographic-microscopy-for-three-dimensional-profiling-and/10.1117/1.OE.53.11.112306.pdf

Review of digital holographic microscopy for three-dimensional profiling and tracking

The traction force produced by biological cells has been visualized as distortions in flexible substrata. We have utilized quantitative phase microscopy by digital holography (DH-QPM) to study the wrinkling of a silicone rubber film by motile fibroblasts. Surface deformation and the cellular traction force have been measured from phase profiles in a direct and straightforward manner. DH-QPM is shown to provide highly efficient and versatile means for quantitatively analyzing cellular motility.

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Measurement of the traction force of biological cells by digital holography.

Three-dimensional profiling and tracking by digital holography microscopy (DHM) provide label-free and quantitative analysis of the characteristics and dynamic processes of objects, since DHM can record real-time data for microscale objects and produce a single hologram containing all the information about their three-dimensional structures. Here, we have utilized DHM to visualize suspended microspheres and microfibers in three dimensions, and record the four-dimensional trajectories of free-swimming cells in the absence of mechanical focus adjustment. The displacement of microfibers due to interactions with cells in three spatial dimensions has been measured as a function of time at subsecond and micrometer levels in a direct and straightforward manner. It has thus been shown that DHM is a highly efficient and versatile means for quantitative tracking and analysis of cell motility.

Four-dimensional motility tracking of biological cells by digital holographic microscopy.

Speckle formation is a limiting factor when using coherent sources for imaging and sensing, but can provide useful information about the motion of an object. Illumination sources with tunable spatial coherence are therefore desirable as they can offer both speckled and speckle-free images. Efficient methods of coherence switching have been achieved with a solid-state degenerate laser, and here we demonstrate a degenerate laser system that can be switched between a large number of mutually incoherent spatial modes and few-mode operation. This technology enables multimodality imaging, where low spatial coherence illumination is used for traditional high-speed videomicroscopy and high spatial coherence illumination is used to extract dynamic information of flow processes. Our implementation is based on a vertical external-cavity surface-emitting laser (VECSEL) architecture. This architecture uses a semiconductor gain module that is electrically pumped, mechanically compact, and supports continuous-wave emission. As an initial example, we perform dynamic multimodality biomedical imaging in Xenopus embryo (tadpole) hearts, an important animal model of human heart disease.

Coherence switching of a degenerate VECSEL for multimodality imaging

Purpose This study is to develop a low-cost, easy-to-use, wide-field smartphone fundus video camera to enable affordable point-of-care examination and telemedicine. Methods The wide-field smartphone fundus camera is based on a unique design of miniaturized indirect ophthalmoscopy. For proof-of-concept prototype, we used a Samsung Galaxy S6 smartphone and all off-the-shelf components. A fiber coupled LED was used to deliver illumination light through a 1 mm micro mirror, which was conjugated to the subject pupil plane, for retinal illumination. A 60D ophthalmic lens was used to image the retina through the eye, and a plano-convex lens with 90 mm focal length was used to relay the retinal image to the smartphone camera. Results A totally wireless smartphone fundus camera was constructed, with a whole weight of 255 g. This device allowed both snapshot fundus photography and continuous video recording. 92° field of view (FOV) was achieved in single-shot images. Optic disc, macula, and retinal blood vasculatures can be clearly observed with image quality comparable to standard fundus camera. Conclusion Miniaturized indirect ophthalmoscopy enabled a low-cost, portable, wide-field smartphone fundus camera, which can foster telemedicine and clinical deployments of wide-field fundus photography for eye disease screening, diagnosis and treatment assessment.

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Changgeng Liu

Papers

Review of digital holographic microscopy for three-dimensional profiling and tracking

Measurement of the traction force of biological cells by digital holography.

Four-dimensional motility tracking of biological cells by digital holographic microscopy.

Coherence switching of a degenerate VECSEL for multimodality imaging

Wide-field smartphone fundus video camera based on miniaturized indirect ophthalmoscopy.