The diverse vision systems found in nature can provide interesting design inspiration for imaging devices, ranging from optical subcomponents to digital cameras and visual prostheses, with more desirable optical characteristics compared to conventional imagers. The advantages of natural vision systems include high visual acuity, wide field of view, wavelength-free imaging, improved aberration correction and depth of field, and high motion sensitivity. Recent advances in soft materials, ultrathin electronics, and deformable optoelectronics have facilitated the realization of novel processes and device designs that mimic biological vision systems. This review highlights recent progress and continued efforts in the research and development of bioinspired artificial eyes. At first, the configuration of two representative eyes found in nature: a single-chambered eye and a compound eye, is explained. Then, advances in bioinspired optic components and image sensors are discussed in terms of materials, optical/mechanical designs, and integration schemes. Subsequently, novel visual prostheses as representative application examples of bioinspired artificial eyes are described.

Bioinspired Artificial Eyes: Optic Components, Digital Cameras, and Visual Prostheses

Fluorescence microscopy plays a vital role in modern biological research and clinical diagnosis. Here, we report an imaging approach, termed pattern-illuminated Fourier ptychography (FP), for fluorescence imaging beyond the diffraction limit of the employed optics. This approach iteratively recovers a high-resolution fluorescence image from many pattern-illuminated low-resolution intensity measurements. The recovery process starts with one low-resolution measurement as the initial guess. This initial guess is then sequentially updated by other measurements, both in the spatial and Fourier domains. In the spatial domain, we use the pattern-illuminated low-resolution images as intensity constraints for the sample estimate. In the Fourier domain, we use the incoherent optical-transfer-function of the objective lens as the object support constraint for the solution. The sequential updating process is then repeated until the sample estimate converges, typically for 5-20 times. Different from the conventional structured illumination microscopy, any unknown pattern can be used for sample illumination in the reported framework. In particular, we are able to recover both the high-resolution sample image and the unknown illumination pattern at the same time. As a demonstration, we improved the resolution of a conventional fluorescence microscope beyond the diffraction limit of the employed optics. The reported approach may provide an alternative solution for structure illumination microscopy and find applications in wide-field, high-resolution fluorescence imaging.

High-resolution fluorescence imaging via pattern-illuminated Fourier ptychography

The extraordinary properties of biological materials often result from their sophisticated hierarchical structures. Through multilevel and cross-scale structural designs, biological materials offset the weakness of their individual building blocks and enhance performance at multiple length scales to match the multifunctional needs of organisms. One essential merit of hierarchical structure is that it can optimize the interfacial features of the "building blocks" at different length scales, from the molecular level to the macroscale. Understanding the roles of biological material interfaces (BMIs) on the determination of properties and functions of biological materials has become a growing interdisciplinary research area in recent years. A pivotal aim of these studies is to use BMIs as inspiration for developing bioinspired and biomimetic materials and devices with advanced structures and functions. Given these considerations, this review aims to comprehensively discuss the structure-property-function relationships of BMIs in nature. We particularly focus on the discussion of BMIs and their inspired materials from mechanical and optical perspectives because these two directions are the most well-investigated and closely related. The challenges and directions of design and fabrication of BMI-inspired mechanical and optical materials are also discussed. This review is expected to garner interest from advanced material communities as well as environmental, nanotechnology, food processing, and engineering fields.

Biological Material Interfaces as Inspiration for Mechanical and Optical Material Designs

The natural compound eye is a striking imaging device with a wealth of fascinating optical features such as a wide field of view (FOV), low aberration, and high sensitivity. Dragonflies in particular possess large, sophisticated compound eyes that exhibit high resolving power and information-processing capacity. Here, a large-scale artificial compound eye inspired by the unique designs of natural counterparts is presented. The artificial compound eye is created by a high-efficiency strategy that combines single-pulse femtosecond laser wet etching with thermal embossing. These eyes have a macrobase diameter of 5 mm and ≈30 000 close-packed ommatidia with an average diameter of 24.5 μm. Moreover, the optical properties of the artificial compound eyes are investigated; the results confirm that the eye demonstrates advanced imaging quality, an exceptionally wide FOV of up to 140°, and low aberration.

Dragonfly-Eye-Inspired Artificial Compound Eyes with Sophisticated Imaging

The increasing interest and recent developments in nanotechnology pose previously unparalleled challenges in understanding the effects of nanoparticles on living tissues. Despite significant progress in in vitro cell and tissue culture technologies, observations on particle distribution and tissue responses in whole organisms are still indispensable. In addition to a thorough understanding of complex tissue responses which is the domain of expert pathologists, the localization of particles at their sites of interaction with living structures is essential to complete the picture. In this review we will describe and compare different imaging techniques for localizing inorganic as well as organic nanoparticles in tissues, cells and subcellular compartments. The visualization techniques include well-established methods, such as standard light, fluorescence, transmission electron and scanning electron microscopy as well as more recent developments, such as light and electron microscopic autoradiography, fluorescence lifetime imaging, spectral imaging and linear unmixing, superresolution structured illumination, Raman microspectroscopy and X-ray microscopy. Importantly, all methodologies described allow for the simultaneous visualization of nanoparticles and evaluation of cell and tissue changes that are of prime interest for toxicopathologic studies. However, the different approaches vary in terms of applicability for specific particles, sensitivity, optical resolution, technical requirements and thus availability, and effects of labeling on particle properties. Specific bottle necks of each technology are discussed in detail. Interpretation of particle localization data from any of these techniques should therefore respect their specific merits and limitations as no single approach combines all desired properties.

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Overview about the localization of nanoparticles in tissue and cellular context by different imaging techniques.

Structured illumination microscopy (SIM) breaks the optical diffraction limit by illuminating a sample with a series of line-patterned light. Recently, in order to alleviate the requirement of precise knowledge of illumination patterns, structured illumination microscopy techniques using speckle patterns have been proposed. However, these methods require stringent assumptions of the speckle statistics: for example, speckle patterns should be nearly incoherent or their temporal average should be roughly homogeneous. Here, we present a novel speckle illumination microscopy technique that overcomes the diffraction limit by exploiting the minimal requirement that is common for all the existing super-resolution microscopy, i.e. that the fluorophore locations do not vary during the acquisition time. Using numerical and real experiments, we demonstrate that the proposed method can improve the resolution up to threefold. Because our proposed method succeeds for standard fluorescence probes and experimental protocols, it can be applied in routine biological experiments.

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Fluorescent microscopy beyond diffraction limits using speckle illumination and joint support recovery

Increased demand for compact devices leads to rapid development of miniaturized digital cameras. However, conventional camera modules contain multiple lenses along the optical axis to compensate for optical aberrations that introduce technical challenges in reducing the total thickness of the camera module. Here, we report an ultrathin digital camera inspired by the vision principle of Xenos peckii, an endoparasite of paper wasps. The male Xenos peckii has an unusual visual system that exhibits distinct benefits for high resolution and high sensitivity, unlike the compound eyes found in most insects and some crustaceans. The biologically inspired camera features a sandwiched configuration of concave microprisms, microlenses, and pinhole arrays on a flat image sensor. The camera shows a field-of-view (FOV) of 68 degrees with a diameter of 3.4 mm and a total track length of 1.4 mm. The biologically inspired camera offers a new opportunity for developing ultrathin cameras in medical, industrial, and military fields.

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Xenos peckii vision inspires an ultrathin digital camera

Natural compound eyes comprise an array of integrated optical units called ommatidia whose individual component includes a facet lens, a crystalline cone, a light-guiding rhabdom, and photoreceptor cells. The unique schemes exhibit distinguished benefi ts in wide fi eld-of-view, fast motion, or polarization detection. [ 1–3 ] The compound eyes have attracted extensive research interest in photonics because the physiological features and the principles of visual information processing can provide technical solutions for cutting-edge optical systems in medical, industrial, and military fi elds. [ 4–11 ] The vision has been emulated with multiapertures on an image sensor arrays by stitching multiple sets of angular images. [ 4–8 ] The physiological structures were mimicked by spherically arranging artifi cial ommatidia on a hemispherical dome. [ 9 ] The previous works, however, still have some technical limitations in mining smartness from diverse natural compound eyes. Nature provides ten different optical schemes of compound eyes. The distinctive features include seven apposition and three superposition types. The apposition types cover simple apposition, open rhabdom apposition, neural superposition, afocal apposition, transparent apposition lightguide, transparent apposition axial gradient, and transparent apposition radial gradient types. [ 12–16 ] The superposition types can also be classifi ed into refracting, refl ecting, and parabolic types. [ 12 , 17–19 ] The individual scheme was not well researched for engineering applications even though it has some attractive fi gures-of-merit for sustainable life style in visual acuity, photon collection effi ciency, and spectral or polarization sensitivity. In particular, the cross-sectional anatomical structures of ten different types provide a clear indication to understand their functions and even to utilize the optical schemes for advanced photonic sensors. The diverse optical schemes can be predetermined on a planar substrate as illustrated in Figure 1 . A natural ommatidium can be emulated by a cylindrical microlens, a conical structure, a waveguide, and a photodetector. Like the natural one, the ommatidia of the apposition type are optically isolated so that each waveguide

Planar emulation of natural compound eyes.

We report a new method, termed geometry-guided resist reflow, for the batch fabrication of asymmetric optical microstructures. Thermoplastic microstructures reflow along the geometric boundaries of the adjacent thermoset microstructures above the glass transition temperature of thermoplastic resin. The shape profiles can be freely formed as a concave, convex, or linear shape and the slope angle can also be tuned from 7 to 68 degrees, depending on the geometric parameters. This new method provides a new route for developing functional optical elements.

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Asymmetric optical microstructures driven by geometry-guided resist reflow.

This work reports an artificial compound eye inspired by the imaging principle of Xenos peckii, which is an endoparasite of paper wasps The unique eye design exhibits higher spatial resolution and better sensitivity than conventional compound eyes The biomimetic compound eye comprises three layers, a microprism arrays, a microlens arrays and an aperture arrays All of the layers were formed on a planar substrate, the device can be directly integrated with a commercial image sensor Each channel detects a different part of the whole scene, and the partial images are stitched in the following image processing step The proposed artificial compound eye can create great opportunities for applications in medical, industrial and military fields

Dongmin Keum

Papers

Fluorescent microscopy beyond diffraction limits using speckle illumination and joint support recovery

Xenos peckii vision inspires an ultrathin digital camera

Planar emulation of natural compound eyes.

Asymmetric optical microstructures driven by geometry-guided resist reflow.

Artificial compound eye inspired by imaging principle of Xenos peckii