| dc.description.abstract | Light allows to non-invasively study the complex and dynamic biological phenomenon undergoing within cells and tissues in their native state. The development of super-resolution microscopes in recent years has helped to overcome the fundamental limitation imposed by Abbe’s diffraction limit, thereby revolutionizing the field of molecular and cellular biology. With the advancement of various super-resolution techniques (like STED, PALM, and 4Pi) it is now possible to visualize the nanometeric cellular structures and their dynamics in real time. The limitations of existing fluorescence microscopy techniques are: poor axial resolution when compared to their lateral counterpart, and their inability to produce high resolution images of dynamic samples. This thesis covers two broadly connected areas of fluorescence imaging techniques while addressing these limitations. First, the PSF engineering and spatial filtering technique for axial super-resolution microscopy and second, the integration of light sheet illumination PSF with microfluidic cytometry for imaging cells on-the-go.
The first chapter gives an explicit description on the fundamentals of fluorescence imaging. This introductory chapter includes a variety of optical microscopes, PSF engineering, the resolution limit imposed by the wave nature of light, the photochemistry of the fluorescent dyes, and their proper selection for fluorescence experiments. In addition to the state-of-art imaging techniques, namely Laser Scanning Confocal Microscopy and Light Sheet Microscopy, this chapter also gives a brief explanation on the evolution of imaging cytometry techniques. Their high speed analytic capability (i.e sorting and counting) makes this technique an important tool in health care diagnosis and other various biomedical applications. The chapter ends with a discussion on the operating principle of the flow cytometers and their limitations.
The second chapter in this thesis describes the spatial filtering technique for engineering the PSF to eliminate the side-lobes in the system PSF of the 4Pi Confocal Microscopes. Employing an amplitude mask with binary light transmission windows (also called binary filters), the incident light is structured to minimize the secondary lobes. These lobes are responsible for exciting the off-focal planes in the specimen, hence provide incorrect map of the fluorophore distribution in the object. The elimination of the side-lobes is essential for the artifact-free axial super-resolution microscopy. This second chapter describes the spatial filtering technique in details (its mathematical formulation, application in fluorescence microscopy for generation of desired PSF including Bessellike beam). Specifically, spatial filtering technique is employed in 4Pi type-C Confocal Microscope. The spatial mask used results in the reduction of the side-lobes in 1PE case while they are nearly eliminated in 2PE variant of the proposed technique. The side-lobes are reduced by 46% and 76% for 1PE and 2PE when compared to the existing 4Pi type-C Confocal Microscope system. Moreover, OTF of the proposed system confirms the presence of higher frequencies in the Fourier domain indicating high resolution imaging capability.
Apart from the resolution in lateral and axial dimension, achieving high resolution while imaging dynamic samples is another challenge that is limiting the field of fluorescence microscopy to flourish. The third and fourth chapters are entirely dedicated towards the work that was carried out to develop imaging techniques on a microfluidic platform for imaging dynamic samples. The fusion of microscopy and flow cytometry has given rise to the celebrated field of imaging flow cytometry. In recent years, the focus has shifted towards miniaturized cytometry devices. Apart from the reduced cost of the sample reagents and the assays, portability and easy handling make the microfluidic devices more relevant to developing countries. The commercially available cytometers are bulky and quite costly. In addition to these practical concerns, they are complex in operation and limited in performance. Most of the existing cytometers use different inlets for sheath and sample flow to achieve the hydrodynamic focusing of the sample assays in a narrow and confined region. The laser beam in the illumination arm interrogates with the flowing samples at this region and the response is captured by the detection optics. The same principle is extensively used in most of the microfluidic based flow cytometers reported till date. Apart from the hydrodynamic force other effects like electro-osmotic, acoustic, and dielectrophoresis have also been exploited to achieve flow focusing in the microfluidic channel. Despite omitting the necessity of external syringe pump as required in pressure driven based cytometers, they all rely upon point-source based excitation scheme and thereby can not interrogate the cells flowing through the entire microfluidic channel.
The third chapter describes the integration of light sheet illumination PSF with microfluidic flow cytometry for simultaneous counting and imaging cells on-the-go. The chapter starts with the description on photolithography procedure for preparing SU8 master and PDMS casting procedure adopted to prepare dedicated microfluidic chips for the developed imaging system. The research work reported here demonstrates the proof-ofprinciple of light sheet based imaging flow cytometer. A light sheet fills the entire microfluidic channel and thus omits the necessity of flow focusing and point-scanning based technology. Another advantage lies in the orthogonal detection geometry that totally cuts-off the incident light, thereby substantially reducing the background in the acquired images. Compared to the existing state-of-the-art techniques, the proposed technique shows marked improvement. Using fluorescently coated Saccharomyces cerevisiae cells, cell counting with throughput as high as 2090 cells/min was recorded. Overall the proposed system is cost-effective and simple in channel geometry. Apart from achieving efficient counting in operational regime of low flow rate, high contrast images of the dynamic samples are also acquired using the proposed cytometry technique.
Further, visualization of intra-cellular organelles is achieved during flow in light sheet based high-throughput cytometry system. The fourth chapter demonstrates the proof of concept of light-sheet-based microfluidic cytometer in conjugation with 2π/3 detection system for high-throughput interrogation and high resolution imaging. This system interrogates the flow channel using a sheet of light rather than the existing point-scanning based techniques. This ensures single-shot scanning of specimens flowing through the microfluidic flow channel at variable flow rates. In addition to high throughput counting at low flow rate, visualization of the intra-cellular organelle (mitochondrial network in human cancerous cells) during flow is achieved with sub-cellular resolution. Using mitochondrial network tagged HeLa cells, a maximum count of 2400 cells/min at the optimized flow rate of 700 nl/min was recorded. The 2π/3 detection system ensures efficient photon collection and minimal background caused by scattered illumination light. The other advantage of this kind of detection system which includes 8f detection optics, is the capability to produce variable magnification using the same high NA objective.
This thesis opens up in vivo imaging of sub-cellular structures and simultaneous cell counting in a miniaturized flow cytometry system. The developed imaging cytometry technique may find immediate applications in the diverse field of healthcare diagnostics, lab-on-chip technology, and fluorescence microscopy. The concluding chapter summarizes the results with a brief discussion on the future aspects of this field (e.g., live-cell imaging of infectious RBC in microfluidic device and 3D optical sectioning of flowing cells). The field of imaging flow cytometry has immense applications in the overlapping areas of physics and biology. The hydrodynamic forces which are used to achieve flow focusing of the sample assays can have an adverse effect in the cell morphology, thereby altering the cellular functions. Light sheet based cytometry system lifts off the requirement of flow focusing and ensures a single shot scanning of entire samples flowing through the microfluidic channel. The similar concept can be used to study the developmental biology of an entire organism, such as C. elegans. This enables the direct observation of developmental and physiological changes in the entire body. Such an organism can be kept alive for a longer duration in microfluidic chambers, and the neural development and mating behaviors can be extensively studied. | en_US |
