Shape-controlled, high fill-factor microlens arrays fabricated by a 3D diffuser lithography and plastic replication method

Development of Shape Prediction Model of Microlens Fabricated via Diffuser-Assisted Photolithography

The fabrication of microlens arrays (MLAs) using diffuser-assisted photolithography (DPL) has garnered substantial recent interest owing to the exceptional capabilities of DPL in adjusting the size and shape, achieving high fill factors, enhancing productivity, and ensuring excellent reproducibility. The inherent unpredictability of light interactions within the diffuser poses challenges in accurately forecasting the final shape and dimensions of microlenses in the DPL process. Herein, we introduce a comprehensive theoretical model to forecast microlens shapes in response to varying exposure doses within a DPL framework. We establish a robust MLA fabrication method aligned with conventional DPL techniques to enable precise shape modulation. By calibrating the exposure doses meticulously, we generate diverse MLA configurations, each with a distinct shape and size. Subsequently, by utilizing the experimentally acquired data encompassing parameters such as height, radius of curvature, and angles, we develop highly precise theoretical prediction models, achieving R-squared values exceeding 95%. The subsequent validation of our model encompasses the accurate prediction of microlens shapes under specific exposure doses. The verification results exhibit average error rates of approximately 2.328%, 7.45%, and 3.16% for the height, radius of curvature, and contact angle models, respectively, all of which were well below the 10% threshold.

Fabrication of tungsten-based optical diffuser using fiberform nanostructure via efficient plasma route

Optical diffusion is an essential process used to manage photons in a wide range of photoelectric systems. This work proposes an approach to fabricate novel optical diffusers by a plasma-processing technique, using fiberform nanostructures formed by helium plasma irradiation and subsequent annealing. After an annealing procedure in the air for oxidation, the optical properties and the light-diffusing abilities of these nanostructured thin films were studied. In addition to the morphology analysis and total transmittance measurement, the diffusion efficiency of the optical diffusers was analyzed using a transmitted scatter distribution function (TDF). It was revealed that the diffusion efficiency of a device with an irradiation time of 30 minutes could reach 97%. The results demonstrate the potential of these nanostructured optical diffusers for various photoelectric applications.

Parallelization of Microfluidic Droplet Junctions for Ultraviscous Fluids

The parallelization of multiple microfluidic droplet junctions has been successfully achieved so that the production throughput of the uniform microemulsions/particles has witnessed considerable progress. However, these advancements have been observed only in the case of a low viscous fluid (viscosity of 10^-2 -10^-3 Pa s). This study designs and fabricates a microfluidic device, enabling a uniform micro-emulsification of an ultraviscous fluid (viscosity of 3.5 Pa s) with a throughput of ≈330 000 droplets per hour. Multiple T-junctions of a dispersed oil phase, split from a single inlet, are connected into the single post-crossflow channel of a continuous water phase. In the proposed device, the continuous water phase undergoes a series circuit, wherein the resistances are continuously accumulated. The independent corrugations of the dispersed oil phase channel, under the theoretical guidance, compromise such increased resistances; the ratio of water to oil flow rates at each junction becomes consistent across T-junctions. Owing to the design being based on a fully 2D interconnection, single-step soft lithography is sufficient for developing the full device. This easy-to-craft architecture contrasts with the previous approach, wherein complicated 3D interconnections of the multiple junctions are involved, thereby facilitating the rapid uptake of high throughput droplet microfluidics for experts and newcomers alike.

Tunable and Dynamic Optofluidic Microlens Arrays Based on Droplets

Microlens arrays (MLAs) are acquiring a key role in the micro-optical system, which have been widely applied in the fields of imaging processing, light extraction, biochemical sensing, and display technology. Compared with solid MLAs, liquid MLAs have received extensive attention due to their natural smooth interface and adjustability. However, manufacturing tunable liquid MLAs with ideal structures is still a key challenge for current technologies. In this paper, a novel and simple optofluidic method is demonstrated, enabling the tunable focusing and high-quality imaging of liquid MLAs. Tunable droplets are fabricated and self-assembled into arrays as the MLAs, which can be easily adjusted to focus, form images, and display different focal lengths. Tuning of MLAs' focusing properties (range from 550 to 5370 μm) is demonstrated by changing the refractive index (RI) of the droplets with a fixed size of 200 μm, which can be changed by adjusting the flow rates of the two branch streams. Also, the corresponding numerical apertures of the MLAs range from 0.026 to 0.26. Furthermore, the MLAs' functionality for microparticle imaging applications is also illustrated. Combining the MLAs with a 4× objective, microparticle imaging is magnified two times, and the resolution has also been improved on the original basis. Besides, both the size and RI of the MLAs in an optofluidic chip can be further adjusted to detect samples at different positions. These MLAs have the merits of high optical performance, a simple fabrication procedure, easy integration, and good tunability. Thus, it shows promising opportunities for many applications, such as adaptive imaging and sensing.

Cellular shape reinforces niche to stem cell signaling in the small intestine

Niche-derived factors regulate tissue stem cells, but apart from the mechanosensory pathways, the effect of niche geometry is not well understood. We used organoids and bioengineered tissue culture platforms to demonstrate that the conical shape of Lgr5⁺ small intestinal stem cells (ISCs) facilitate their self-renewal and function. Inhibition of non-muscle myosin II (NM II)-driven apical constriction altered ISC shape and reduced niche curvature and stem cell capacity. Niche curvature is decreased in aged mice, suggesting that suboptimal interactions between old ISCs and their niche develop with age. We show that activation of NM IIC or physical restriction to young topology improves in vitro regeneration by old epithelium. We propose that the increase in lateral surface area of ISCs induced by apical constriction promotes interactions between neighboring cells, and the curved topology of the intestinal niche has evolved to maximize signaling between ISCs and neighboring cells.

Self-Assembled Microlens Array with Controllable Focal Length Formed on a Selective Wetting Surface

We report a microlens array with a controllable focal length that is based on a selective wetting surface. A substrate modified with a hydrophobic layer that has an array of microholes exhibits different wettability for inside holes (hydrophilic) versus outside holes (hydrophobic). When liquid flows over the surface, a small amount of liquid is adhered to hydrophilic holes and forms a lens-shaped droplet array that self-assembles because of surface tension. A large-sized plano-convex lens array that has good uniformity is obtained via the blade coating method, and controlling the amount of liquid enables control of the focal length. Our self-assembled microlens array has the merits of high optical performance, a simple fabrication procedure, and good mechanical stability, and thus, it has potential applications in imaging processing, light extraction, protein detection, light-emitting diodes, sensors, and displays.

Using Micromachined Molds, Partial-curing PDMS Bonding Technique, and Multiple Casting to Create Hybrid Microfluidic Chip for Microlens Array

In a previous study, we presented a novel manufacturing process for the creation of 6 × 6 and 8 × 8 microlens arrays (MLAs) comprising lenses with diameters of 1000 μm, 500 μm, and 200 μm within an area that covers 10 mm × 10 mm. In the current study, we revised the manufacturing process to allow for the fabrication of MLAs of far higher density (15 × 15 and 29 × 29 within the same area). In this paper, we detail the revised manufacturing scheme, including the micromachining of molds, the partial-curing polydimethylsiloxane (PDMS) bonding used to fuse the glass substrate and PDMS, and the multi-step casting process. The primary challenges that are involved in creating MLAs of this density were ensuring uniform membrane thickness and preventing leakage between the PDMS and glass substrate. The experiment results demonstrated that the revised fabrication process is capable of producing high density arrays: Design I produced 15 × 15 MLAs with lens diameter of 0.5 mm and fill factor of 47.94%, while Design II produced 29 × 29 MLAs with lens diameter of 0.25 mm and fill factor of 40.87%. The partial-curing PDMS bonding system also proved to be effective in fusing PDMS with glass (maximum bonding strength of approximately six bars). Finally, the redesigned mold was used to create PDMS membranes of high thickness uniformity (coefficient of variance <0.07) and microlenses of high lens height uniformity (coefficient of variance <0.15).

Large-area fabrication of microlens arrays by using self-pinning effects during the thermal reflow process

Generally, the fabrication of curved structures such as microlens arrays has been regarded as an expensive and complicated process. Here, we propose a facile method to form a microlens array with controlled lens curvature by combining residue-free nanoimprint lithography (NIL) with V-shaped molds and the successive thermal reflow procedure of the printed polymeric structures. The V-shaped molds used in this study enable the bottom substrate to be exposed after the NIL process when the initial thickness is controlled. Then, we use the thermal reflow to realize hemi-cylindrical curved lenses by applying heat. The polymers are self-pinned on the exposed substrate, which is strong enough to fix the boundary to not dewet or be flattened in the broad temperature range of the reflow process, which is essential for a large-area fabrication. Furthermore, we demonstrate the modulation of the focal lengths of the lenses by controlling the initial polymer thickness coated on a substrate.

Fabrication of polymer microlens array with controllable focal length by modifying surface wettability

A straightforward technique for fabricating low-cost microlens arrays with controllable focal length is developed. By harnessing and manipulating the interfacial energy between the liquid-state acrylate resin and the solidified polydimethylsiloxane (PDMS), the surface of the acrylate resin in the PDMS microhole presents a spherical shape and the curvature can be flexibly controlled. With the change of the processing time for the surface modification of the PDMS microholes, the focal length of the concave microlenses varies from -296.3 μm to -67.4 μm. The numerical aperture of 0.45 is realized. The focal length and the aperture of the microlenses are also affected by the diameter of the microholes. The fabricated concave microlens array can be employed as a master to further duplicate convex microlens array. A good image quality can be achieved by using the convex microlens arrays.

Anisotropic Electrowetting on Wrinkled Surfaces: Enhanced Wetting and Dependency on Initial Wetting State

Electrowetting on dielectric (EWOD) on unidirectional microstructured surfaces has recently evoked significant interest as they can modulate the effect of electrowetting, and can thus find applications in directional wetting in microfluidic systems. However, the dependency of such EW phenomenon on their initial state of wetting and anisotropy is far from being well understood. The current study addresses the initial wetting states and their implication on the anisotropic electrowetting using a wrinkled EWOD platform. Herein we demonstrate a facile stampless and maskless structure generation technique to fabricate wrinkles of varying topography. Further, we have demonstrated alteration in the interfacial wetting conditions by modulating the wrinkle topography, and its effect on the droplet behavior during electrowetting. The capillary wicking-assisted electrowetting on these wrinkled surfaces is in specific direction dictated by the ordered wrinkles and prompts enhanced spreading of the droplet. We also demonstrate that while the enhancement of unidirectional electrowetting is stronger in conformal wetting state surfaces, composite wetting state surfaces depict a reversal in anisotropy.

Diffraction of partially-coherent light beams by microlens arrays

The synthesis method including wave-optics and ray-tracing for the acceleration of the simulation of micro-optical systems has been developed. The effects of the spatial coherence and randomization of microlens array (MLA) parameters have been considered. The method based on coherent states representation for the calculation of the optical efficiency of microlens arrays taking into account the light source polarization has been developed. Numerical simulations of the intensity distributions and spreading angle of a diffracted beam have been carried out.

High-resolution real-time dual-view imaging with multiple point of view microscopy

Most methods to observe three-dimensional processes in living samples are based on imaging a single plane that is sequentially scanned through the sample. Sequential scanning is inherently slow, which can make it difficult to capture objects moving quickly in three dimensions. Here we present a novel method, multiple point-of-view microscopy (MPoVM), that allows simultaneous capturing of the front and side views of a sample with high resolution. MPoVM can be implemented in most fluorescence microscopes, offering new opportunities in the study of dynamic biological processes in three dimensions.

Fabrication of ellipticity-controlled microlens arrays by controlling the parameters of the multiple-exposure two-beam interference technique

We demonstrate a promising method for fabrication of plastic microlens arrays (MLAs) with a controllable ellipticity and structure, by using the combination of multiple-exposure two-beam interference and plastic replication techniques. Multiple exposures of a two-beam interference pattern with a wavelength of 442 nm into a thick positive photoresist (AZ-4620) were used to form different two-dimensional periodic structures. Thanks to the developing effect of the positive photoresist, fabricated structures consisting of hemielliptical- or hemispherical-shaped concave holes were obtained. By controlling the rotation angle between different exposures, both the shape and structure of the holes varied. By adjusting the dosage ratio between different exposures, the shape of the holes was modified while the structure of the holes was unchanged. The photoresist concave microstructures were then transferred to plastic MLAs by employing replication and embossing techniques. The fabricated MLAs were characterized by a scanning electron microscope and atomic force microscope measurements. We show that the ellipticity of the microlenses can be well controlled from 0 (hemispherical) to 0.96 (hemielliptical) by changing the rotation angle or dosage ratio between the two exposures.

Electrowetting on a polymer microlens array

This paper reports on the electrowetting behavior of a flexible poly(dimethylsiloxane) (PDMS) microlens array. A Cr and Au double-layered electrode was formed on an array of microlenses with diameters of 10 microm and heights of 13 microm. A deposition of parylene and a coating of Teflon were followed for electrical insulation as well as for enhancement of the hydrophobicity. On the nearly superhydrophobic microlens array surface, the electrowetting of a deionized water droplet was observed over the contact angle range of approximately 140 degrees to approximately 58 degrees by applying 0-200 V, respectively. The electrowetting phenomenon was reversible even in air environment with applied voltages of less than 100 V. The electrowetting on the microlens array surface lost its reversibility after the microlens array surface was completely wetted when the water meniscus touched the bottom of the microlens array. Analysis of meniscus shapes and net force direction follows to elucidate the reversibility. The convex curvature of the microlens caused gradual rather than abrupt impalement of water into the gap among the microlenses.

Microdome-gooved Gd(2)O(2)S:Tb scintillator for flexible and high resolution digital radiography

A flexible microdome-grooved Gd(2)O(2)S:Tb scintillator is simulated, fabricated, and characterized for digital radiography applications. According to Monte Carlo simulation results, the dome-grooved structure has a high spatial resolution, which is verified by X-ray image performance of the scintillator. The proposed scintillator has lower X-ray sensitivity than a nonstructured scintillator but almost two times higher spatial resolution at high spatial frequency. Through evaluation of the X-ray performance of the fabricated scintillators, we confirm that the microdome-grooved scintillator can be applied to next-generation flexible digital radiography systems requiring high spatial resolution.

Miniaturized tools and devices for bioanalytical applications: an overview

This article presents an overview of various miniaturized devices and technologies developed by our group. Innovative, fast and cheap procedures for the fabrication of laboratory microsystems based on commercially available materials are reported and compared with well-established microfabrication techniques. The modules fabricated and tested in our laboratory can be used independently or they can be set up in different configurations to form functional measurement systems. We also report further applications of the presented modules e.g. disposable poly(dimethylsiloxane) (PDMS) microcuvettes, fibre optic detectors, potentiometric sensors platforms, microreactors and capillary electrophoresis (CE) microchips as well as integrated microsystems e.g. double detection microanalytical systems, devices for studying enzymatic reactions and a microsystem for cell culture and lysis.

Fabrication of microlens arrays based on the mass transport effect of SU-8 photoresist using a multiexposure two-beam interference technique

Microlens arrays (MLAs) were fabricated based on the mass transport effect of SU-8 photoresist by a multiexposure two-beam interference technique. In particular, a direct single-step fabrication process, i.e., without developing, mask, and pattern transferring processes, is demonstrated. The effects of various parameters such as thicknesses, exposure dosage, and angle between two laser beams on MLAs were investigated. Square and hexagonal lattices of microlenses were obtained by controlling rotation angles between different exposures on SU-8 samples. In addition, microlenses with elliptical shape were fabricated by a double exposure at 0 degrees and 60 degrees. Finally, the surface profiles of microlenses in MLAs were characterized by atomic force microscopy.

Optic diffusers based on photopolymerizable hologram material with an ionic liquid as additive

Symmetric and asymmetric diffusers with a directional diffusion property were both fabricated based on a photopolymerizable hologram material using an ionic liquid as an additive. The diffusion property can be regulated by changing the concentration of the ionic liquid. The fiber structure, the surface-relief structure, and the formation of nanoparticles led to the directional diffusion property of the diffuser.

Two step process for the fabrication of diffraction limited concave microlens arrays

A two step process has been developed for the fabrication of diffraction limited concave microlens arrays. The process is based on the photoresist filling of melted holes obtained by a preliminary photolithography step. The quality of these microlenses has been tested in a Mach-Zehnder interferometer. The method allows the fabrication of concave microlens arrays with diffraction limited optical performance. Concave microlenses with diameters ranging between 30 microm to 230 microm and numerical apertures up to 0.25 have been demonstrated. As an example, we present the realization of diffusers obtained with random sizes and locations of concave shapes.

Microfluidics and photonics for Bio-System-on-a-Chip: a review of advancements in technology towards a microfluidic flow cytometry chip

Microfluidics and photonics come together to form a field commonly referred to as 'optofluidics'. Flow cytometry provides the field with a technology base from which both microfluidic and photonic components be developed and integrated into a useful device. This article reviews some of the more recent developments to familiarize a reader with the current state of the technologies and also highlights the requirements of the device and how researchers are working to meet these needs.

Fabrication of a polymeric vertical microlens with the dip method

We have investigated a process based on the dip method to fabricate a polymeric vertical microlens (PVM). After the primary dip step, the PVM is formed by hanging the liquid SU-8 on a wall in virtue of the strong adhesive force and liquid cohesion. The microlens is then baked and exposed in ultraviolet light to further cross-link the negative photoresist SU-8 to enhance thermal stability and reliability. According to the experimental results, the radius of curvature of the fabricated vertical microlens varies from 120.8 to 34.2 microm, which relates to the dip depth or the thickness of the dipped pool. To characterize the PVM, an edge-emitting laser diode (lambda=1.31 microm) is then bonded onto the optical bench and a detector is utilized to observe the beam divergence with and without the lens insertion. Compared with an angle of 40.8 degrees without the microlens, the beam passing through a suitable PVM shows a vertical far-field angle of 3.32 degrees. Furthermore, the lens efficiency, approximately 83.4%, is also specified by the measurements.

Microlens array diffuser for a light-emitting diode backlight system

Microlens array (MLA) diffusers for light-emitting diode (LED) backlight systems have been developed. A high fill-factor photoresist mold for the MLA was fabricated using three-dimensional diffuser lithography, and the patterns were transferred to a nickel master mold for UV-curable polymer replication. The fabricated microlens had various paraboloidal profiles, and its aspect ratio was controlled from 1.0 to 2.1. The MLA diffuser showed a batwing radiation pattern with a radiation angle of 150 degrees. The fabricated MLA diffuser may greatly enhance the color-mixing characteristics of LED backlight systems and help reduce the number of LEDs required.