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Abstract
We report on a prototype endoscope with tunable-focus liquid microlenses actuated through infrared (IR) light integrated at its distal end. Tunable-focus microlenses scanning the areas of interest with minimal movements of the scopes themselves would greatly advance endoscopy. Multiple IR light-responsive hydrogel posts were used to tune the focal length of the microlens, which were formed by a curved water-oil meniscus. The fabrication and actuation of the hydrogel posts were realized through light transmitted via optical fibers. The focal length varied from 52 mm to 8 mm. Focused images were obtained from an image fiber bundle. A microlens array with larger depth of focus and view of field could be potentially utilized.
... Benefitting from variable focal length of the microlenses, different depth of focus and thus spatial depth perception can be obtained in the serial images acquired. Based on their accomplishment on liquid microlenses actuated through IR-light-responsive hydrogel described in the prior section, Zeng et al. demonstrated prototype endoscopes with tunable-focus liquid microlenses integrated at their distal ends and actuated through IR light [25,75]. Figure 12 shows the schematics of such a prototype endoscope [25]. ...
The recent rapid development in microlens technology has provided many opportunities for miniaturized optical systems, and has found a wide range of applications. Of these microlenses, tunable-focus microlenses are of special interest as their focal lengths can be tuned using micro-scale actuators integrated with the lens structure. Realization of such tunable microlens generally relies on the microelectromechanical system (MEMS) technologies. Here, we review the recent progress in tunable liquid microlenses. The underlying physics relevant to these microlenses are first discussed, followed by description of three main categories of tunable microlenses involving MEMS techniques, mechanically driven, electrically driven, and those integrated within microfluidic systems.
... As shown inFig. 4(a), the focal length of each microlens in the array was initially measured before wrapping on a hemispherical base, as a way of determining the position of the optically minimum focused point of a collimated input light beam along the optical axis [25]. A small thermal resistor heater and the tip of a thermometer were placed under the target microlens in order to control and measure the in situ surrounding temperature, while the focused point of a microlens was being recorded. ...
We present microlens arrays consisting of multiple focus-tunable microlenses omnidirectionally fabricated on spher-ical surfaces, to realize large field of view. Thin flexible polymer bridges connecting adjacent microlenses are designed to reduce the wrapping stress and deformation of the microlens array. Each microlens, formed via water–oil interface, is individually tuned by a thermo-responsive hydrogel actuator. The range of the focal length of each microlens in this array varied from millimeters to infinity. A prototype of optical imaging system based on such a microlens array on a spherical surface, including a charged-coupled device camera, a fiber bundle, and a 3-D rotational stage, is demonstrated as well. [2010-0273] Index Terms—Curvilinear surface, field of view (FOV), focus tunable, microlens array.
We report on the fabrication and characterization of 2x2 arrays of mm-diameter PDMS lenses whose focal length can be electrically tuned. Dielectric elastomer actuators generally rely on carbon powder or carbon grease electrodes, which are not transparent, precluding the polymer actuator from also being a lens. However compliant electrodes fabricated by low-energy ion implantation are over 50% transparent in the visible, enabling the polymer lens to simultaneously be an actuator. We have developed a chip-scale process to microfabricate lens arrays, consisting of a molded socket bonded to a Pyrex chip supporting 4 membrane actuators. The actuators are interconnected via an incompressible fluid. The Pyrex chip has four through-holes, 1 to 3 mm in diameter, on which a 30 μm thick Polydimethysiloxane (PDMS) layer is bonded. The PDMS layer is implanted on both sides with 5 keV gold ions to define the transparent electrodes for EAP actuation. Applying a voltage to one of the lens/actuators leads to an area expansion and hence to a change in radius of curvature, varying the focal length. We report tuning the focal length from 4 mm to 8 mm at 1.7 kV, and present changes in optical transmission and membrane stiffness following gamma and proton irradiation.
Hydrogels have been developed to respond to a wide variety of stimuli, but their use in macroscopic systems has been hindered by slow response times (diffusion being the rate-limiting factor governing the swelling process). However, there are many natural examples of chemically driven actuation that rely on short diffusion paths to produce a rapid response. It is therefore expected that scaling down hydrogel objects to the micrometre scale should greatly improve response times. At these scales, stimuli-responsive hydrogels could enhance the capabilities of microfluidic systems by allowing self-regulated flow control. Here we report the fabrication of active hydrogel components inside microchannels via direct photopatterning of a liquid phase. Our approach greatly simplifies system construction and assembly as the functional components are fabricated in situ, and the stimuli-responsive hydrogel components perform both sensing and actuation functions. We demonstrate significantly improved response times (less than 10 seconds) in hydrogel valves capable of autonomous control of local flow.
Programmable autonomous micromixers and micropumps have been designed and realized via a merger between MEMS and microfluidic tectonics (μFT). Advantages leveraged from both fabrication platforms allow for relatively simple and rapid fabrication of these microfluidic components. Nickel (Ni) microstructures, driven by an external rotating magnetic field, are patterned in situ and serve as the microactuators in the devices. μFT permits in situ patterning through the use of a step-and-repeat fabrication process known as liquid-phase photopolymerization (LP<sup>3</sup>). LP<sup>3</sup> is a polymer-based fabrication process tool that offers additional fabrication materials, including responsive hydrogels that expand and contract under different stimuli. Using pH- and temperature-sensitive hydrogels as clutches, autonomous micromixers and micropumps have been fabricated and tested that perform as closed-loop microsystems. The step-and-repeat fabrication process allows pre-programming of the system, like a programmable read-only memory chip, to be sensitive to a desired stimulus. Different Ni blade designs, and pH-sensitive hydrogel geometries and dimensions have been designed and tested to better ascertain their effects on micromixing efficiency and response times of hydrogels (related to the autonomous functionality), respectively. Temperature-responsive hydrogels have allowed for development of temperature-sensitive micromixers and micropumps with applications in areas demanding temperature control. [1498].
The development of adaptive lenses from liquids or soft polymer materials that can change their focal length in response to external stimuli is discussed. The adaptive optics have the capability to compensate for the distorting effect of the atmosphere by first measuring light wavefront perturbation and then applying an opposite distortion, by making the appropriate changes to a deformable mirror. Adaptive lenses can be made by controlling the refractive index of a layer of nematic liquid crystal, a substance that flows like a liquid but consists of long molecules that line up in the same direction. As the refractive index of the liquid crystals depends on the orientation of the molecules, and the orientation of the molecules changes with the electric field, the varying field sets up a corresponding variation in refractive index. The liquid-crystal layer acts like a lens, with light passing near the edge of the device being bent more than light near the center.
We report on liquid-based tunable microlenses actuated by infrared (IR) light. Multiple micropost structures, made of IR light-responsive hydrogel with entrapped gold nanoparticles, are photopatterned around a lens aperture. The volumetric change in the hydrogel, controlled by IR light, regulates the curvature of a liquid-liquid interface forming the microlens at the aperture and its focal length. The focal length of the microlens can be tuned from −17.4 mm to +8.0±0.4 mm in seconds under IR irradiation. This microlens can be integrated into optical systems, for instance, for fiber endoscopy.
Independent optical control of two valves at a junction within a microfluidic device formed from a optomechanically responsive nanocomposite hydrogels was analyzed. Nanocomposite-hydrogel valves were formed by photopolymerizing the monomer mixture, with the inclusion of the appropriate nanoparticles, within a microfluidic device at a T-junction. It was shown that the lower critical solution temperature (LCST) of the temperature-responsive material could be tailored by altering the copolymer composition. By raising the LCST, one could also use absorbing materials with slightly overlapping absorption curves, since slight heating in non-targeted valve materials would not result in a phase change, allowing the valve to remain close.
A new method for instant oxidation of closed, bonded microchannels is presented and evaluated. By placing the tip-formed electrode of a corona plasma equipment in the reservoir of a PDMS microstructure, the plasma spark can spread into the microchannel and oxidize the inner PDMS channel walls. By applying this process, the non-specific adsorption of hydrophobic affinity analytes is markedly decreased, here evaluated with the fluorescent dye Rhodamine B and standard protein BSA. The results show that the surface adsorption in plasma-treated channels is reduced significantly, e.g. the amount of BSA adsorbed at 35 mm distance from the reservoir is only 35% of the amount of BSA adsorbed in non-treated channels. The surface shows very low adsorption during the first 200 min after oxidation, and has recovered (90%) its hydrophobicity first after 24 h. This method of instant surface oxidation has in our group been widely used to simplify microfluidic studies of microstructure prototypes, since the need of other more complicated surface modifications to lower analyte adsorption is eliminated.
We report on polydimethylsiloxane (PDMS) microlens arrays fabricated through liquid-phase photopolymerization and molding. The gist of this fabrication process is to form liquid menisci of variable radii of curvature at an array of apertures through pneumatic control, followed by photopolymerization under ultraviolet radiance. The resultant polymerized structures are then transferred to PDMS utilizing two molding steps. By adjusting the pneumatic pressure during the process, a single aperture array can be used to fabricate PDMS microlens arrays with variant focal lengths. The liquid menisci are formed by liquid-air interfaces that are pinned at the top edges of the apertures along hydrophobic-hydrophilic boundaries generated through surface chemical treatments. The microlens arrays are optically characterized. Variant focal lengths from 2.35 to 5.54 mm and f -numbers from 1.27 to 5.88, dependent on the diameter of apertures and the applied pressure to form the liquid menisci, are achieved with this relatively simple process and match well with the physical model. Owing to the formation from the liquid-air interfaces, the surface roughness of microlenses is measured to be around 25 nm.
Despite its compactness, the human eye can easily focus on different distances by adjusting the shape of its lens with the help of ciliary muscles. In contrast, traditional man-made optical systems achieve focusing by physical displacement of the lenses used. But in recent years, advances in miniaturization technology have led to optical systems that no longer require complicated mechanical systems to tune and adjust optical performance. These systems have found wide use in photonics, displays and biomedical systems. They are either based on arrays of microlenses with fixed focal lengths, or use external control to adjust the microlens focal length. An intriguing example is the tunable liquid lens, where electrowetting or external pressure manipulates the shape of a liquid droplet and thereby adjusts its optical properties. Here we demonstrate a liquid lens system that allows for autonomous focusing. The central component is a stimuli-responsive hydrogel integrated into a microfluidic system and serving as the container for a liquid droplet, with the hydrogel simultaneously sensing the presence of stimuli and actuating adjustments to the shape--and hence focal length--of the droplet. By working at the micrometre scale where ionic diffusion and surface tension scale favourably, we can use pinned liquid-liquid interfaces to obtain stable devices and realize response times of ten to a few tens of seconds. The microlenses, which can have a focal length ranging from -infinity to +infinity (divergent and convergent), are also readily integrated into arrays that may find use in applications such as sensing, medical diagnostics and lab-on-a-chip technologies.