Hui Min Leung

A liquid-filled tunable double-focus microlens

A novel microlens design with tunable double-focus is presented. It is fabricated by adding only one SU-8 photolithography step to the well-developed liquid-filled microlens fabrication process. The thickness of this layer determines the thickness difference between the central and peripheral region of the membrane, the deformation of which is used to define the surface profile of the microlens. The stepped thickness variation is finally manifested as the difference in deformation contour at two different regions of the membrane when subjected to uniform applied pressure, thereby causing two focal lengths to appear. Experimental and simulation results are presented, from which the tunability of the focal lengths of the double-focus microlens is demonstrated to be effective over a wide range through combining the structural design with pressure control. The successful demonstration of this unconventional microlens design concept will potentially extend t application of liquid-filled microlens technology.

Tunable liquid-filled lens integrated with aspherical surface for spherical aberration compensation.

A novel liquid-filled lens design is presented. During fabrication, high precision single point diamond turning (SPDT) is introduced into standard soft lithography process to fabricate an aspherical surface constituting one end of lens. This enables the spherical aberration associated with the operation of the conventional liquid-filled lenses to be compensated for. Through flexibly optimizing this surface contour, it can be designed to work within particular working regions with improved optical quality. At the same time, the deformable elastic membrane is still adopted at the other end of the lens, thus preserving the high focal length tunability. This proof of concept and the performance of the proposed lens have been demonstrated using the lateral shearing interferometry experiment..

Liquid tunable diffractive/refractive hybrid lens

We present a liquid tunable diffractive/refractive hybrid lens fabricated through what we believe to be a novel process that combines single-point diamond turning with soft lithography techniques. The hybrid lens achieves focal length tunability by changing its shape and, at the same time, utilizes the unique dispersion property of diffractive surfaces to enhance its spectral performance within a wide tuning range.

Multi-modality imaging of a murine mammary window chamber for breast cancer research

Window chamber models have been developed and utilized as a means to study the complex microenvironment in which cancers develop, proliferate, and metastasize in small animals. Here we utilize rapid prototyping printer technology to construct a new plastic orthotopic mammary window chamber that is compatible with magnetic resonance imaging, nuclear imaging, and optical imaging. Optical imaging allows for high-resolution cellular and molecular level analysis of tissues; magnetic resonance imaging provides quantitative measures of tumor size, perfusion, diffusion, fat/water content relaxation parameters; and a nuclear imaging technique, called the Beta Imager, supports functional and metabolic imaging. Our demonstration of the multiple imaging capabilities of this model suggests that it can be used as a powerful platform for studying basic cancer biology and developing new cancer therapies.

Diamond turning and soft lithography processes for liquid tunable lenses

Making use of the capability of high precision diamond turning in producing 3-dimensional free form optical surfaces with excellent surface finish, molds for various types of liquid tunable microlenses are fabricated. Subsequently, a rapid prototyping process known as soft lithography is applied to the fabricated mold to replicate multiple lens structures. This method provides an efficient and reliable way of generating rotationally symmetric free form optical surfaces that are otherwise difficult to produce with conventional methods such as lithography and etching methods. Using atomic force microscopy, white light interferometry and a mechanical profiler, it is verified that the surface quality and dimensional accuracy of the replicas are preserved. We demonstrate the practical usefulness of the proposed fabrication methods by developing and experimentally testing three different liquid tunable lenses, namely (1) a diffractive/refractive hybrid lens that reduces chromatic aberration within the visible spectrum, (2) a double focusing lens and (3) an aspherical lens that minimizes spherical aberration.

Flexible, high-resolution micro-optical coherence tomography endobronchial probe toward in vivo imaging of cilia

We report the design and fabrication of a flexible, longitudinally scanning high-resolution micro-optical coherence tomography (μOCT) endobronchial probe, optimized for micro-anatomical imaging in airways. The 2.4 mm diameter and flexibility of the probe allows it to be inserted into the instrument channel of a standard bronchoscope, enabling real-time video guidance of probe placement. To generate a depth-of-focus enhancing annular beam, we utilized a new fabrication method, whereby a hollow glass ferrule was angle-polished and gold-coated to produce an elongated annular reflector. We present validation data that verifies the preservation of linear scanning, despite the use of flexible materials. When utilized on excised, cultured mouse trachea, the probe acquired images of comparable quality to those obtained by a benchtop μOCT system.

Liquid tunable double-focus lens fabricated with diamond cutting and soft lithography

With the use of diamond cutting processes, namely turning and shaping, followed by soft lithography with polydimethylsiloxane, a liquid tunable double-focusing lens is fabricated. Data from a mechanical profiler verified that the dimensions of the features of the lens device adhere well to designed values. In addition, atomic force microscopy results show that this method of fabrication is able to produce multiple replicas of the lens device with a high-quality surface finish that is suitable for optical purposes. Lastly, the tunability of the lens is demonstrated, with experimental results agreeing well with simulation results.

Micro-optical coherence tomography of the mammalian cochlea

The mammalian cochlea has historically resisted attempts at high-resolution, non-invasive imaging due to its small size, complex three-dimensional structure, and embedded location within the temporal bone. As a result, little is known about the relationship between an individual’s cochlear pathology and hearing function, and otologists must rely on physiological testing and imaging methods that offer limited resolution to obtain information about the inner ear prior to performing surgery. Micro-optical coherence tomography (μOCT) is a non-invasive, low-coherence interferometric imaging technique capable of resolving cellular-level anatomic structures. To determine whether μOCT is capable of resolving mammalian intracochlear anatomy, fixed guinea pig inner ears were imaged as whole temporal bones with cochlea in situ. Anatomical structures such as the tunnel of Corti, space of Nuel, modiolus, scalae, and cell groupings were visualized, in addition to individual cell types such as neuronal fibers, hair cells, and supporting cells. Visualization of these structures, via volumetrically-reconstructed image stacks and endoscopic perspective videos, represents an improvement over previous efforts using conventional OCT. These are the first μOCT images of mammalian cochlear anatomy, and they demonstrate μOCT’s potential utility as an imaging tool in otology research.

Development of a Primary Human Co-Culture Model of Inflamed Airway Mucosa

Neutrophil breach of the mucosal surface is a common pathological consequence of infection. We present an advanced co-culture model to explore neutrophil transepithelial migration utilizing airway mucosal barriers differentiated from primary human airway basal cells and examined by advanced imaging. Human airway basal cells were differentiated and cultured at air-liquid interface (ALI) on the underside of 3 µm pore-sized transwells, compatible with the study of transmigrating neutrophils. Inverted ALIs exhibit beating cilia and mucus production, consistent with conventional ALIs, as visualized by micro-optical coherence tomography (µOCT). µOCT is a recently developed imaging modality with the capacity for real time two- and three-dimensional analysis of cellular events in marked detail, including neutrophil transmigratory dynamics. Further, the newly devised and imaged primary co-culture model recapitulates key molecular mechanisms that underlie bacteria-induced neutrophil transepithelial migration previously characterized using cell line-based models. Neutrophils respond to imposed chemotactic gradients, and migrate in response to Pseudomonas aeruginosa infection of primary ALI barriers through a hepoxilin A3-directed mechanism. This primary cell-based co-culture system combined with µOCT imaging offers significant opportunity to probe, in great detail, micro-anatomical and mechanistic features of bacteria-induced neutrophil transepithelial migration and other important immunological and physiological processes at the mucosal surface.

Illuminating dynamic neutrophil trans-epithelial migration with micro-optical coherence tomography

A model of neutrophil migration across epithelia is desirable to interrogate the underlying mechanisms of neutrophilic breach of mucosal barriers. A co-culture system consisting of a polarized mucosal epithelium and human neutrophils can provide a versatile model of trans-epithelial migration in vitro, but observations are typically limited to quantification of migrated neutrophils by myeloperoxidase correlation, a destructive assay that precludes direct longitudinal study. Our laboratory has recently developed a new isotropic 1-μm resolution optical imaging technique termed micro-optical coherence tomography (μOCT) that enables 4D (x,y,z,t) visualization of neutrophils in the co-culture environment. By applying μOCT to the trans-epithelial migration model, we can robustly monitor the spatial distribution as well as the quantity of neutrophils chemotactically crossing the epithelial boundary over time. Here, we demonstrate the imaging and quantitative migration results of our system as applied to neutrophils migrating across intestinal epithelia in response to a chemoattractant. We also demonstrate that perturbation of a key molecular event known to be critical for effective neutrophil trans-epithelial migration (CD18 engagement) substantially impacts this process both qualitatively and quantitatively.