Optofluidic Lenses for 2D and 3D Imaging

Optofluidic Lenses for 2D and 3D imaging

Hanyang Huang, and Yi Zhao

Department of Biomedical Engineering

The Ohio State University

Abstract

Optofluidic lenses that modulate the light using fluidic materials have been recognized as a powerful alternative to solid lenses. These lenses can achieve many advanced imaging capabilities that are difficult, if not impossible, to obtain otherwise. In particular, such adaptive optical elements have shown great potentials in a broad array of applications in two- dimensional (2D) and three-dimensional (3D) imaging, including macro/microscopic imaging, optical coherence tomography, wide-angle imaging, spectral imaging, optical zooming, and switchable monocular/binocular vision. This article reviews the current research efforts on optofluidic lenses for 2D/3D imaging. The configurations and actuation mechanisms of different types of optofluidic lenses and their representative imaging applications are introduced.

The opportunities and challenges in this field are also discussed. This review is expected to provide a snapshot of the landscape of current optofluidic lenses research to attract more interests in this exciting field and foster the development of next-generation optofluidic lenses.

Keywords: optofluidics; lenses; adaptive optics; actuators; stereoscopic; optical aberrations.

1. Introduction

Advanced imaging capabilities, such as wide-angle imaging, stereoscopic vision, and depth perception, are often preferable in optical imaging systems. In conventional optical imaging systems, these functions are typically achieved through replacement and/or displacement of multiple solid optical elements. This may complicate the optical system configuration and increase the overall device dimension and cost. In the past two decades, miniature imaging systems are increasingly employed in consumer electronics (e.g. mobile devices and digital cameras), biomedical imaging (e.g. microscopes, endoscopes, dermatoscopes, and optical coherence tomography), industrial (e.g. robotics, and machine vision), and military applications

(e.g. unmanned aerial vehicles, head mount display, remote surveillance systems) [1-5]. New adaptive optics approaches that can incorporate advanced imaging capabilities in miniature imaging systems are thus of imperative needs.

Microfluidics provides a viable solution of adaptive optics. Different from the systems using solid optical elements with a fixed shape and a fixed refractive index (RI), microfluidic-based adaptive optical systems rely on the use of fluidic optical elements with a tunable shape and/or a tunable RI. For example, the RI of a liquid crystal optical element can be modulated by an applied electric field [6]; and a tunable refractive power of a fluid/fluid lens can be obtained by deforming the interface between the two immiscible fluids [7]. This burgeoning research discipline that utilizes the adaptive optical properties of fluidic materials is known as optofluidics [8-12], which has widespread applications in optical lenses [13], displays [14], switches [15], prisms [16], apertures [17], and mirrors [18]. The advent of optofluidics provides

a new route for designing and improving new imaging systems.

Optofluidic lenses are at the epicenter of this technological innovation and have attracted considerable attention in the recent past. They serve as the basis of miniature and adaptive imaging devices by courtesy of their compact size and tunable optical powers. Many pioneer studies have been dedicated to the development of different types of optofluidic lenses for rendering two-dimensional (2D) and/or three-dimensional (3D) images. Extensive literature about the configurations and working principles of optofluidic lenses is available [13, 19-21].

There, however, lacks a comprehensive review of optofluidic lenses in the context of 2D/3D imaging, one essential application of such lenses.

In this review, we introduce the recent advances of optofluidic lenses and their applications in

2D and 3D imaging. Here, we group optofluidic lenses into three major types based on their different configurations (Figure 1). The first type (Type I) refers to the lenses with two fluid components, which have different RIs and are contacting with each other via a tunable interface.

Droplet lenses and electrowetting lenses fall into this category. The second type (Type II) refers to the lenses with one compliant solid component and one fluid component that contact with each other. Elastomer-liquid lenses fall into this category. In the third type (Type III), the RI of the lens material changes upon particular external stimulations. Liquid crystal (LC) lenses fall into this category. Due to different configurations and different lens materials, the actuation mechanisms of these three types of lenses differ from each other. In section 2 of this review, we will introduce representative configurations of each type of optofluidic lenses and the typical

actuation approaches. Section 3 reviews recent 2D/3D imaging applications of the optofluidic lenses. The opportunities in this field and the potential challenges are discussed in section 4.

2. Configuration and Actuation

2.1. Type I: lenses with tunable fluid/fluid interface(s)

When two fluid components with different RIs are put into contact, a refractive interface forms.

If the curvature of the refractive interface reaches an optimal profile, the two fluids with the interface can serve as a refractive lens. The two fluids can either be air and a liquid material or two immiscible liquid materials. These two subtypes are elaborated below.

2.1.1. Lenses with air/liquid interface(s)

If the refractive interface is formed by air and a liquid material, the lens often refers to a droplet lens. It can be formed by dispensing a single liquid droplet on a solid substrate and exposing it to air [22, 23]. Such lenses have the simplest configuration and are very affordable. For example, one can turn a smartphone camera into a macroscope in a few seconds by dispensing a water droplet on the camera lens surface, for examining the fine details of tiny objects [24].

The refractive power of droplet lenses can be tuned by ferrofluidic [25-27], acoustic [28-31], thermal [32, 33] and electrical [34-36] means. Xiao et al. developed a ferrofluidic actuator to drive a 2mm droplet lens [26]. The droplet sits in a cylindrical well that connects to a microchannel. Once the ferrofluid piston in the channel moves by an external magnetic field, the focal length of the droplet lens changes.

Acoustic actuation can cause droplet deformations at resonance. Feng et al. developed a 5 mm droplet lens by dispensing a droplet in a round well with a ring piezoelectric transducer sidewall and a flexible bottom [31]. The droplet develops a dome shape when the transducer sidewall vibrates at resonance. The droplet curvature changes with the vibration amplitude to realize different refractive powers (Figure 2a). In a similar study, Lopez et al. developed a droplet lens by dispensing a water droplet in a through hole of a Teflon plate [29]. The droplet oscillates upon acoustic activation (Figure 2b). This small droplet lens can operate at as high as 100 kHz, showing good potential for fast adaptive imaging.

Thermal actuation utilizes the thermal expansion of liquid or air to change the refractive power of droplet lenses. In a recent study, a 2 mm thermopneumatic droplet lens was reported [32].

The lens has a liquid cavity and an adjacent air cavity. The two cavities are separated by an elastomer membrane. The thermal expansion of air deflects the elastomer membrane, causing fluid redistribution and curvature change at the distal end of the liquid cavity (Figure 2c). The response time is on the order of tens of seconds. It needs even longer time to cool down due to the thermal mass. To address this, Alireza et al. devised a 2 mm droplet lens driven by a bi- directional thermoelectric actuator (Figure 2d), where the thermal expansion of optical fluid directly changes the lens power [33]. The response time is 0.8 sec.

Electrochemical actuation has been reported recently. López et al. proposed a 0.89 mm droplet lens that uses a water-soluble ferrocenyl surfactant as the optical fluid [34]. The fluid is

overfilled into a through hole of a Teflon plate and forms two air/liquid interfaces on the two sides of the plate (Figure 2e). When a low voltage bias of less than 5VDC is applied between the opposing sidewall electrodes, the surfactant solution near the electrodes undergoes the reduction-oxidation process, where oxidation increases the surface tension and reduction decreases the surface tension. Accordingly, the droplet curvature on the reduction side increases and that on the oxidation side decreases, changing the focal length of the droplet lens.

Reversible focal lengths between 0.5 mm and infinity were demonstrated with the response time over 1 min. Dielectrophoretic force can also drive droplet lenses [37-39]. In such lenses, a dielectric droplet is placed on a rigid substrate that is patterned with electrodes and coated with a hydrophobic layer. The contact angle of the droplet changes once a non-uniform electric field is applied. Since the droplet size is fairly small, the contact angle change can cause the curvature change in the entire droplet, thus changing the focusing power.

The limitations of droplet lenses are apparent. First, the droplet volume keeps reducing due to evaporation. For a wettable droplet, the curvature decreases during evaporation. The focal length of the droplet increases accordingly. For a non-wettable droplet, the lens aperture decreases during evaporation. Since the ambient environment of a droplet lens is often not predictable, it is challenging to control the focal length and the lens aperture change. Long-term imaging is not possible. Besides, droplet lenses are vulnerable to asymmetric deformation caused by external inertial disturbance and the gravity effect. Moreover, many droplet lenses have fairly long response time, making it difficult for practical and commercial uses.

2.1.2. Lenses with tunable liquid/liquid interface(s)

The two liquid materials in this subtype must be immiscible with each other and have different

RIs. The two liquid components are often sealed within a rigid chamber. Such a configuration makes the lenses less vulnerable to external inertial disturbance and the gravity effect.

Evaporation is also no longer a critical concern. The actuation mechanisms including electrowetting [40-44], mechanical-wetting [45-48], stimuli-responsive [49-51] and dielectric actuation [52-54] can be used.

An electrowetting liquid/liquid lens often comprises of a conductive liquid component and an insulating liquid component, both are hosted in a sealed chamber. The sidewall is patterned with a conductive electrode and coated with a dielectric layer and a hydrophobic layer. A bottom electrode directly contacts the conductive fluid (Figure 3a). Once a voltage bias is applied between the sidewall electrode and the conductive fluid, the curvature of the interface between the conductive liquid and the insulating liquid changes, which in turn changes the focal length.

Since abundant papers have been published to report the electrowetting effect and various electrowetting lenses, we choose not to repeat but to only introduce the recent progress on switchable electrowetting lenses/prisms [55-57]. Different from previous studies that are only able to tune the refractive power of lenses, these studies can switch an electrowetting device between a lens and a prism. For example, Terrab et al. proposed an oil-water electrowetting element consisting of an electrode on the bottom that is grounded and two electrodes on the sidewalls. The sidewall electrodes are coated with a hydrophobic layer and a dielectric layer

[56] (Figure 3b). When the same DC voltage is applied between the bottom electrodes and the

two sidewall electrodes, the contact angles at the two sidewalls yield the same value, and the element serves as a lens, whose refractive power changes with the applied voltage. If the voltage reaches a critical value, the oil-water interface becomes planar to allow the light to pass through without refraction. When the voltages applied between the bottom electrode and the two sidewall electrodes differ, the different contact angles at the two sidewalls cause asymmetric interface profile that tilts towards the lower voltage side, exemplifying a prism shape. The prism angle changes with the applied voltage difference. The device provides up to 4.3° steering range at 30VDC. The focal length changes from 78 mm to −192 mm with the voltages from 0VDC to

21VDC.

Mechanical forces can also change the interface curvature [45-47]. Unlike electrowetting lenses where the interface often displaces along the chamber wall, mechanical-wetting lenses need a pinned contact line. This reduces the risk of lens asymmetry during operation and reduces focal length hysteresis [36]. In a typical mechanical wetting configuration, two immiscible liquids are filled into the top and bottom chambers, respectively. A long screw is inserted in the bottom chamber through a threaded hole on the wall. The pressure in the bottom chamber changes as the screw is further inserted or withdrew, changing the curvature of the liquid/liquid interface

[47] (Figure 3c). Although a low hysteresis can be achieved, the response time is fairly long

(>1 sec) and the optical power is small (<10 diopters).

Hydrogels that expand or shrink upon an external stimulus (such as pH, temperature, light, antigen, etc.) can also tune the liquid/liquid surface. In a well-known study by Dong et al.,

stimuli-responsive tunable lenses responsive to temperature change or pH value change were demonstrated [49]. This is done by controlling the oil-water interface using a hydrogel ring

(Figure 3d). The temperature-sensitive hydrogel expands at low temperatures and shrinks at high temperatures. The pH-sensitive hydrogel expands in basic solutions and shrinks in acid solutions. The water volume changes when the hydrogel ring is exposed to the temperature or the pH value changes, resulting in a tunable focal length. Although the lenses using stimuli- responsive hydrogels can be made very small, this lens suffers from slow response that is typically on the order of tens of seconds. An improved design was reported by the same group using a different infrared light-responsive hydrogel [58]. The response time is reduced to 7 sec.

The dielectrophoretic effect can also change the liquid/liquid interface(s). These lenses have similar configurations as electrowetting lenses, except that the two fluids are both nonconductive and must have different dielectric constants. Various electrodes layouts, such as a planar electrode with an array of holes [53] and a non-planar electrode patterned on a surface with an array of microwells [59] are used to generate inhomogeneous electric fields. This approach requires a relatively high voltage on the order of hundreds of volts. The response time is typically on the order of hundreds of milli-sec. Such lenses often have good temperature stability and low power consumption.

In most studies, the interface of liquid/liquid lenses approximates a spherical shape as governed by the surface tension as well as the gravitational force. The collimated light passing through the peripheral region of the lenses cannot converge at the same focal point as the light passing

through the center region. This is known as spherical aberration. Since it is difficult to control the interface curvature, spherical aberration at high diopters, along with other optical aberrations, is not negligible and deteriorates the optical performance. The tuning range of liquid/liquid lenses is also limited by the contact-angle saturation [60]. The liquid-liquid lenses based on stimuli-responsive materials often have fairly slow response. Also, because the capillary effect dominates only within the close vicinity of the chamber sidewall, the liquid/liquid lenses usually have limited aperture size.

2.2. Type II: lenses with tunable solid/fluid interface(s)

By encapsulating an optical fluid within a deformable cavity made of solid materials, an optofluidic lens with tunable solid/fluid interface(s) is constructed. Elastomer-liquid lenses represent the common configuration of this lens type, where the deformable cavity consists of one or more elastomer membranes and a rigid framework. Compared to fluid/fluid optofluidic lenses, elastomer-liquid lenses are more resistant to external inertial disturbance and can be fabricated into different lens types. One notable feature of this type of lenses is that the curvature between solid and fluid components no longer solely depends on surface tension and gravity, but also depends on the local thickness and stiffness of the elastomer. It is thus possible to finely tune the curvature profile in specific areas to reduce the optical aberrations.

Hydraulic and pneumatic actuations are the simplest approaches to drive elastomer-liquid lenses. By pumping or withdrawing fluid into/from the lens chamber, the lens shape can be changed [61-65]. These actuation approaches allow for a large refractive power change. The

system, however, can be bulky and prone to fluid leakage.

Lens chambers with movable parts can also be used for actuation [66-68]. Ren et al. designed a tunable liquid lens where the optical fluid is encircled by an annular sealing ring with an adjustable diameter [66]. As the diameter of the annular ring decreases, the lens membrane deflects outwards (Figure 4a), increasing the refractive power. A similar approach is to wrap an elastic string around the lens with one end fixed and the other end mounted to a servo motor

[67]. The lens membrane deforms as the motor arm pulls the string (Figure 4b).

Electrostatic, piezoelectric and electromagnetic approaches can also deform the lens membrane.

These have been extensively investigated and achieved remarkable commercial success [69-

74]. In a representative configuration, an actuation chamber connecting to the lens chamber is used, which is filled with the same optical fluid and sealed by an actuation elastomer membrane

(Figure 4c). Once the actuation membrane is deformed by electrostatic, piezoelectric, or electromagnetic forces, fluid in the lens chamber redistributes, changing the refractive power of the lens. The major challenge is that the actuation force is relatively small. As a result, the tunable range of the refractive power is limited.

Dielectric electroactive polymer (DEAP) emerges as another promising actuation solution with

good commercialization potential. A DEAP actuator has a dielectric elastomer membrane

sandwiched between two compliant electrodes. When a high electric field is applied across the

deflected DEAP membrane with fixed boundaries, the membrane bulges and causes fluid

redistribution in the chamber, which in turn causes the deflection of the lens membrane [75- 11

78]. In an alternative design, the in-plane extension of the DEAP membrane squeezes the lens

aperture, causing the out-of-plane deflection of the lens membranes (Figure 4d). The response

time as fast as 175 s for a 20% focal length change is reported [79]. To save space, the DEAP

membrane itself can also serve as the lens membrane by using transparent compliant electrodes

[80, 81]. Despite its compact configuration, low cost and fast response, DEAP actuation often

requires fairly high voltages (typically on the order of a few kV).

Thermal expansion of air can also drive elastomer-liquid lenses [82-85]. For example, platinum heaters can be activated to expand the air sealed in an annular air chamber and deform a 2 mm lens membrane [84] (Figure 4e). Thermopneumatic lenses often undergo considerable hysteresis due to the low thermal expansion coefficient, and thus a lag in response. The mechanical structures in the lenses also risk to damages due to repeated heating and cooling.

2.3. Type III: lenses made of materials with tunable RI

Instead of changing the shape of the optical fluids or the interfaces, tunable refractive power can also be achieved by the RI modulation of the optical fluid materials. Two common optofluidic lenses with adaptive RIs are LC lenses [6, 86, 87] and gradient refractive index

(GRIN) lenses [88-90]. GRIN lenses are not covered in this review since they are not primarily used for imaging.

A typical LC lens consists of two glass substrates with transparent indium tin oxide (ITO) electrodes on the opposing surfaces. The space between the two glass substrates is known as

the LC cell gap. An alignment layer is coated on each opposing surface and rubbed along one direction to ensure the director field through the cell gap be always in the plane containing the cell normal direction and the rub direction. Once a sufficient voltage bias is applied to the ITO electrodes, the director orientation changes from being parallel to the surfaces to being perpendicular to the surfaces, which causes the effective RI change.

If the opposing surfaces are parallel to each other, an array of patterned electrodes, such as hole- patterned electrodes and ring-patterned electrodes, are often used [91-93], which can generate an inhomogeneous electric field by applying different voltages to different electrodes. The light rays passing through different electrodes exhibit different propagating speeds, which makes the light bend after passing through the LC cell. The inhomogeneous electric field can also be generated using two parallel electrodes and embedding a polymeric layer with spatially various dielectric constant into the LC cell gap [94] (Figure 5a). The director field is initially aligned homogeneously by the polymeric layer and the alignment layer, and reoriented by the inhomogeneous electric field upon the applied voltage.

Alternatively, the inhomogeneous electric field can also be achieved by using non-planar opposing surfaces [95-97] (Figure 5b). If one glass substrate has a concave surface, and the opposing surface of the other glass substrate is flat, an inhomogeneous electric field can be generated due to the spatially varied electrode gap, leading to a centrosymmetric RI profile change in the LC cell, and convergence/divergence of the light.

LC lenses often require lower driving voltages (typically on the order of several volts) than electrowetting and DEAP lenses. In general, the response time (typically on the order of milli- sec) is also smaller than other types of optofluidic lenses. However, the refractive power of LC lenses is relatively small (typically a few diopters).

In addition to LC lenses and GRIN lenses, the deformation of live cells can also be used to modulate the light. Miccio et al. demonstrated in vivo imaging using a red blood cell (RBC) as the optical element, where the cell deforms from a biconcave disk to a sphere due to the osmolarity change of the ambient buffer [98]. The response time of the RBC lens is about ~10 sec.

3. 2D/3D Imaging

The dynamically tunable refractive power of optofluidic lenses allows for rapid acquisition of a planar object at a certain distance, acquiring the depth information of a 3D object by scanning the entire view field, obtaining a wide viewing angle with low image distortion, or rendering stereoscopic vision from the image disparity at different viewing directions. None of these advanced imaging capabilities requires the displacement or the replacement of optical elements, significantly reducing the configuration and operation complexities. A non-exclusive review of

2D/3D imaging enabled by optofluidic lenses is elaborated below.

3.1. Optofluidic macro/microscopy

3.1.1. Variable focusing

The minimal feature size that can be resolved by an optical macro/microscope is governed by the diffraction limit, which reversely relates to the numerical aperture of the objective lens. In order to reveal the fine details of the object, an objective lens with a sufficiently large numerical aperture is needed. This can be done by using a high magnification objective lens but is often accompanied by a reduced field of view. A low magnification objective lens, on the other end, offers a relatively large field of view with low resolution. In practice, the objective lenses with different magnifications need to be frequently switched to meet varied imaging needs, complicating and elongating the imaging process. Optofluidic lenses allow for dynamic tuning of the numerical aperture and the magnification on demand without physically replacing any lens elements. A typical configuration can be seen in the paper by Chowdbury et al. that reports a variable focusing microscope using a water droplet objective lens [99]. The droplet is coated with an oil layer to reduce the evaporation rate. The droplet shape deforms upon water addition or mechanical compression, yielding a refractive power change (Figure 6a). A reflective microscope using the droplet lens is developed which can achieve tunable magnifications from

37× to 47× (Figure 6b). This microscope has a very simple configuration, yet poor stability and slow response time due to the utilization of the droplet lens. By elastomer-liquid lenses, the tuning range of the magnification and the stability can be largely improved [100, 101]. A magnification range from 20× to 50× is achieved [101].

3.1.2. Scanning

In conventional macro/microscopes, the depth of field (DOF) decreases with the increasing magnification and the increasing numerical aperture. As a result, under a high magnification,

the distance between the object and the objective lens needs to be very carefully adjusted to bring the projected image focused on the imager/retina surface. Such tuning can be challenging in portable microscopes that are recently developed to work with smartphones [102, 103] due to the lack of appropriate structural supports and fine tuning mechanisms in a portable setting.

This limits wide applications of such portable imaging devices in global healthcare, agriculture and field studies where the infrastructures are not available.

Optofluidic lenses with variable focusing capability provide a promising solution to address this [104, 105]. Qu et al. placed an electrowetting lens to the rear plane of a 10×/0.25 objective lens in a microscope [105]. As the optical power of the lens changes from -5 diopters to 15 diopters, different object planes are brought into focus. The overall axial imaging range can be increased from 8.5 m to 20 m by axially scanning the field. In this design, however, the magnification and the numerical aperture keep changing during the scanning, making it difficult to compare and integrate the images at different object planes. Such an issue can be addressed by inserting a 4f-system composed of two achromatic relay lenses and placing the liquid lens at the conjugated pupil plane of the 4f-system (Figure 6c). Due to the telecentricity of the configuration, the magnification of the microscope does not depend on the optical power of the liquid lens (Figure

6d), which allows for axial scanning without a change in magnification and numerical aperture

[106].

3.1.3. Optical coherence tomography (OCT)

Optical coherence tomography (OCT) is an optical imaging modality that can perform cross-

sectional imaging of internal structures in materials and biological tissues with the resolution on the order of m. Since OCT allows for non-invasive examination of live tissues with the spatial resolution close to that of optical microscopy, it has been widely employed in ophthalmology for examining the morphology of various diseased tissues in the eyes, e.g. optical nerve disorders on the retina surface, as well as widely scattered applications in other clinical domains. In OCT, the image forms from the interference of the light backscattered by the reference mirror and the light backscattered by the sample. Elastomer-liquid lenses [107],

LC lenses [108], and electrowetting lenses [109] have all been used in different types of OCT systems (time-domain, spectral-domain, and sweep-source) to allow dynamic focusing as well as to extend the axial scanning range while maintaining the highest possible lateral resolution.

A low-cost spectral-domain OCT was also recently reported that uses two commercial electrowetting lenses in combination with a MEMS mirror to scan the entire field of view [110].

3.2. Spectral imaging

A digital color micrograph or a photograph visible to human eyes is a combination of three colors: red, green and blue. Spectral imaging refers to the technology that can obtain the information in each of the three colors, which has found important uses in photo repair [111], antique furniture restoration [112], and color enhancement for endoscopy [113, 114].

Optofluidic hyperchromatic lenses can be used to separate light in different colors. Such a lens consists of a diffractive lens and a tunable refractive lens [115]. Two different configurations have been reported (Figure 7a). The high chromatic aberration of the lens allows only a narrow wavelength band to be focused on the image plane at each time. The wavelength collected at

the image plane varies as the lens is tuned (Figure 7b).

3.3. Optical zooming

Optical zooming is a highly desired function in imaging systems, which magnifies images without changing the working distance. Conventional optical zooming systems change the zooming factor through the physical movements of multiple solid optical elements. This necessitates an extended axial dimension as well as precise actuation mechanisms. Multiple types of optofluidic lenses, including elastomer-liquid lenses [116-118], electrowetting lenses

[119, 120], and LC lenses [121, 122], can be used for building optofluidic zooming systems.

For example, Savidis et al. developed a telescopic zooming system based on elastomer-liquid lenses [118], where two elastomer-liquid lenses are separated by the distance equals to the sum of their focal lengths to form an afocal zooming group. The first liquid lens converges collimated light and focuses it on the front focal plane of the second liquid lens. The light rays become collimated again after passing through the second liquid lens, then form images onto the image sensor. The magnification from 0.12× to 10.5× is achieved while the image plane is kept stationary.

Zooming lenses based on LC lenses are also reported [122], which consists of an LC objective lens, an LC eyepiece lens, and a camera module (Figure 8a). The focal lengths of the objective lens and the eyepiece lens need to be tuned synchronously to make the objective lens converge the light from the object to focus on the front focal plane of the eyepiece lens. The light becomes collimated after passing through the eyepiece and then forms an image on the image sensor.

The device can be zoomed in/out continuously with the working distance from 10 cm to infinity and the magnification from 0.29× to 2.3×. A similar configuration using three electrowetting lenses was also reported [120], where the magnification ranges from 7.8× to 13.2× (Figure 8b).

In optofluidic zooming systems, at least two optofluidic lenses are used. The refractive powers of the lenses must be tuned synchronously to achieve variable optical magnification. Each lens requires an independently controlled actuation module, which inevitably increases the complexity of the system configuration and size. Synchronized actuation of multiple lenses is also challenging.

3.4. Wide-angle imaging

Imaging with a wide field of view (FOV) is another advanced capability that is critical for surveillance, robotic vision, laparoendoscopic imaging, etc.. Current wide-angle imaging devices based on solid lenses, such as compound lenses and fisheye lenses, either lack sufficient spatial resolution or do not allow for depth perception. Optofluidic devices with reconfigurable lens structures can offer both features [123, 124]. Kang et al. developed a device that incorporates the architecture merits of the wide-FOV of insect compound eyes and the accommodation capability of human eyes [124, 125]. The device comprises an array of elastomer-liquid lenses sit on a big elastomer membrane (Figure 9a). Both the elastomer-liquid lenses and the big membrane can be actuated independently using embedded microfluidic channels. When the big membrane is deformed to a dome shape, the optical axes of the peripheral elastomer-liquid lenses orient outwards, increasing the overall viewing angle. In the

meantime, the optical power of each small elastomer-liquid lens can be adjusted, allowing for the imaging of the objects at different depths (Figure 9b). The overall FOV up to 120° can be reached, with the focal length of each elastomer-liquid lens ranges from 3.6 mm to infinity.

3.5. Stereoscopic imaging

Stereoscopic imaging acquires not only the images on a planar object plane normal to the optical axis but also the images on a curvilinear surface as well as the depth information of the object.

Stereoscopic vision has been increasingly employed in many emerging applications enabled by portable electronic devices, such as virtual reality (VR) and augmented reality (AR). Due to the limited space of these portable devices, liquid lenses that offer smaller footprints, low power consumptions, and light weight are expected to substitute conventional stereoscopic instruments in these emerging applications.

Lenticular lenses are widely used in 3D displays. In a typical lenticular display design, multiple images are pixelated and arranged interdigitated beneath a lenticular lens array, where each lens covers multiple pixels. The left and right eyes of the observer positioned at two different viewing directions can respectively perceive pixels from two different images due to refraction, rendering a 3D image. Lenticular displays allow the observer to view the stereoscopic images without using anaglyph glasses or polarized glasses, however, at the cost of reduced spatial resolution. In order to achieve both 3D vision and high-resolution 2D images, electrowetting lenses [126-129], elastomer-liquid lenses [130] and LC lenses [131, 132] can be used. In an electrowetting lenticular display design, an electrowetting lens array is formed by placing oil

solution in an array of micro-wells and encapsulated by water. 2D images can be achieved when the electric voltage is tuned to obtain a planar oil/water interface; while 3D vision can be achieved by forming a convex oil shape by increasing the voltage [129] (Figure 10a&b).

Similarly, lenticular display using elastomer-liquid lenses has been demonstrated by changing the hydrostatic pressures across the elastomer membranes of an array of elastomer-liquid lenses covered on pixelated images [130].

LC lenticular displays consist of a polymeric LC lenticular lens array, a polarizer, and a twisted- nematic (TN) cell [131, 132] (Figure 10c). When no voltage is applied to the TN cell, the optical axis of the polarizer is parallel to the rubbing direction of the TN cell. The polarization of the polarized light is rotated 90° by the TN cell and orthogonal to the LC directors in the lenses. As a result, the ordinary light is not refracted by the LC lenses, allowing for 2D display.

When a voltage is applied to the TN cell, the polarization of the polarized light is no longer changed by the TN cell. The light rays become extraordinary and are focused by the LC lenses, allowing for 3D vision.

3.6. Binocular vision for laparoendoscopy

Laparoendoscopic imaging is an important clinical field that liquid lenses are found useful.

Different from open surgeries where stereoscopic vision acquired by the two eyes of the doctor can be achieved, the limited caliber of laparoendoscopic tubes used in minimally invasive surgeries and examinations, with the diameter on the order of a few mm, does not allow stereoscopic imaging with high resolution. A reconfigurable optofluidic device was developed

to switch between monocular vision and binocular vision [133]. The device combines an elastomer-liquid singlet lens and an elastomer-liquid binocular lens within a multi-layered microfluidic chip (Figure 11a). The two lenses are arranged on the two sides of a glass substrate and share the same optical axis and the image sensor. Under the 2D mode, the singlet lens is actuated while the binocular lens has no power. The entire image sensor is used for acquiring the 2D image with high resolution. Under the 3D mode, the singlet lens has no power while the binocular lens is actuated. Each lenslet in the binocular lens projects the object on a half area of the image sensor. The image disparity indicates the depth of the object. Such a configuration can also be adopted by LC lenses [134-137]. Hassanfiroozi et al. reported an LC lens with 1.42 mm lens diameter for 2D/3D imaging [137] (Figure 11b). The lens consists of two LC cell layers: there are three small LC cells in the top layer and a big LC cell in the bottom layer. The

LC cells can be controlled independently, and switch between monocular and binocular imaging modes.

4. Opportunities and Challenges

This paper reviews the recent development of optofluidic lenses for 2D/3D imaging. The actuation mechanisms of optofluidic lenses with different configurations are introduced.

Typical advanced 2D/3D imaging capabilities enabled by optofluidic lenses that are difficult, if not impossible, to be achieved using conventional solid lenses are discussed. It is apparent that optofluidic lenses provide a powerful alternative to current solid lenses and are expected to foster new imaging capabilities, especially when they are used in combination with the increasingly powerful portable electronics devices. It should be noted that the wide adoption of

optofluidic lenses in the broad spectrum of imaging devices is hindered by their limited optical performance, reliability, and manufacturability. Fortunately, rapid technology development in optofluidics and adjacent research domains make it promising to overcome these challenges.

The low image resolution due to optical aberrations remains as a primary limitation of optofluidic lenses, regardless of the lens type. It is difficult to compensate for the optical aberrations without compromising the miniaturization and the range of adaptive refractive power. To date, only a few pioneering studies have been performed to reduce the optical aberrations in optofluidic lenses [138-142]. For elastomer-liquid lenses, Wei et al. designed an aspherical lens membrane with inhomogeneous thickness profile to reduce the edge clamping effect [143] (Figure 12a&b). The imaging resolutions in both the central and the peripheral regions of the lens aperture gain significant improvements as compared to conventional elastomer-liquid lenses whose lens membrane has a constant thickness profile. Mishra et al. combined both the electric actuation and hydraulic actuation to yield a hyperbolic liquid-liquid profile in an electrowetting lens for reducing the spherical aberration and the image distortion

[144] (Figure 12c). Although these efforts have not yet yielded image resolutions comparable to or exceeding those with high-quality solid lenses, they show promising potential to overcome the bottleneck of optical aberration limitations.

One key area that optofluidic lenses is expected to over-perform their solid counterparts is clinical imaging with space constraints, such as laparoscopic imaging and OCT scanning.

Before breakthroughs are made, biocompatibility issues must be addressed. Here,

biocompatibility refers not only to the use of biocompatible lens materials and optical fluids, but also to appropriate packaging of fluidic materials, and the uses of low hydraulic pressures

(for mechanically driven lenses) and low electrical voltages/currents (for electrically driven lenses). More biocompatible materials with excellent optoelectromechanical properties are expected to be designed for the next-generation optofluidic lenses. The research outcomes of smart materials responsive to various electromechanical stimuli will also provide a solid technical base for the development of miniaturized actuators that can drive optofluidic lenses without disturbing natural tissues and organs in the close vicinity. New materials will also help to improve the lens reliability at low costs, which is another critical concern of current optofluidic lenses. For example, many optical fluids are reported to degrade over time after the exposure to light. The short shelf time prevents the integration of optofluidic lenses in many commercial electronics devices. Once the degradation issue is resolved, more portable electronic devices, such as smartphones, VR, and AR devices, are expected to embrace optofluidic lenses with adaptive optical performance to enhance their imaging capabilities.

Last but not the least, poor manufacturability of optofluidic lenses is a critical hurdle for broad commercial uses. Different from most optoelectromechanical devices, an optofluidic lens includes not only rigid and flexible solid materials, but also transparent liquid materials, and in many cases conductive electrodes. Although these materials are generally of low costs, the manufacturing cost of miniature hybrid devices using these materials is not trivial. Since there are no well-established manufacturing protocols for optofluidic lenses, the few lens manufacturers on the market developed their own fabrication protocols based on their

respective lens designs. Since most protocols are lens specific and are not modular, they often associate with high fabrication cost and low throughput. This is expected to be addressed by inputs from the rapidly developing precision additive manufacturing technologies, which have already shown the capabilities of creating surfaces with optical grade roughness, and of incorporating hybrid structures made of different materials with varied stiffness and optical transparency. Although hybrid additive manufacturing that can create structures with both liquid and solid materials have yet to be reported, there would be a strong call of such technologies due to the huge market potentials of optofluidic lenses in the board range of clinical, civilian, and military applications.

Acknowledgments

This work is funded by NSF grants under the award numbers 1509727 and 1701038. The authors also acknowledge the Pelotonia program for graduate fellowship support.

Disclosure

The authors declare no conflict of interest.

References: [1] Zhang Y S, Trujillo-de Santiago G, Alvarez M M, Schiff S J, Boyden E S and Khademhosseini A 2017 Expansion mini-microscopy: An enabling alternative in point-of-care diagnostics Current opinion in biomedical engineering 1 45-53 [2] Meyer T, Baumgartl M, Gottschall T, Pascher T, Wuttig A, Matthäus C, Romeike B F, Brehm B R, Limpert J and Tünnermann A 2013 A compact microscope setup for multimodal nonlinear imaging in clinics and its application to disease diagnostics Analyst 138 4048-57 [3] Kim S B, Koo K-i, Bae H, Dokmeci M R, Hamilton G A, Bahinski A, Kim S M, Ingber D E and Khademhosseini A 2012 A mini-microscope for in situ monitoring of cells Lab on a Chip 12 3976-82 [4] Ostergard T 2005 Microminiature zoom system for digital camera. Google Patents) [5] Gao Q K, Liu J, Duan X H, Zhao T, Li X and Liu P L 2017 Compact see-through 3D head- mounted display based on wavefront modulation with holographic grating filter Optics Express 25 8412-24 [6] Ren H and Wu S T Liquid Crystal Lens Introduction to Adaptive Lenses 189-269 [7] Li L, Liu C, Ren H W, Deng H and Wang Q H 2015 Annular folded electrowetting liquid lens Optics Letters 40 1968-71 [8] Monat C, Domachuk P and Eggleton B J 2007 Integrated optofluidics: A new river of light Nature Photonics 1 106-14 [9] Psaltis D, Quake S R and Yang C H 2006 Developing optofluidic technology through the fusion of microfluidics and optics Nature 442 381-6 [10] Levy U and Shamai R 2008 Tunable optofluidic devices Microfluid Nanofluid 4 97-105 [11] Pang L, Chen H M, Freeman L M and Fainman Y 2012 Optofluidic devices and applications in photonics, sensing and imaging Lab on a Chip 12 3543-51 [12] Zhao Y H, Stratton Z S, Guo F, Lapsley M I, Chan C Y, Lin S C S and Huang T J 2013 Optofluidic imaging: now and beyond Lab on a Chip 13 17-24 [13] Nguyen N T 2010 Micro-optofluidic Lenses: A review Biomicrofluidics 4 [14] Heikenfeld J, Zhou K, Kreit E, Raj B, Yang S, Sun B, Milarcik A, Clapp L and Schwartz R 2009 Electrofluidic displays using Young-Laplace transposition of brilliant pigment dispersions Nature Photonics 3 292-6 [15] Murade C U, Oh J M, van den Ende D and Mugele F 2011 Electrowetting driven optical switch and tunable aperture Optics Express 19 15525-31 [16] Smith N R, Abeysinghe D C, Haus J W and Heikenfeld J 2006 Agile wide-angle beam steering with electrowetting microprisms Optics Express 14 6557-63 [17] Schuhladen S, Banerjee K, Sturmer M, Muller P, Wallrabe U and Zappe H 2016 Variable optofluidic slit aperture Light-Sci Appl 5 [18] Kilaru M K, Yang J and Heikenfeld J 2009 Advanced characterization of electrowetting retroreflectors Optics Express 17 17563-9 [19] Mishra K, van den Ende D and Mugele F 2016 Recent Developments in Optofluidic Lens Technology Micromachines-Basel 7 [20] Song W Z, Vasdekis A E and Psaltis D 2012 Elastomer based tunable optofluidic devices Lab on a Chip 12 3590-7 [21] Chiu C P, Chiang T J, Chen J K, Chang F C, Ko F H, Chu C W, Kuo S W and Fan S K 2012 Liquid Lenses and Driving Mechanisms: A Review J Adhes Sci Technol 26 1773-88

[22] Duocastella M, Florian C, Serra P and Diaspro A 2015 Sub-wavelength Laser Nanopatterning using Droplet Lenses Sci Rep-Uk 5 [23] Hu H and Larson R G 2002 Evaporation of a sessile droplet on a substrate J Phys Chem B 106 1334-44 [24] Huang H and Zhao Y 2018 Smartphone based focus-free macroscopy using an adaptive droplet lens. In: Solid-State Sensors, Actuators and Microsystems Workshop Hilton Head Island, (SC, USA: Transducer Research Foundation) p 97050P [25] Cheng H C, Xu S, Liu Y F, Levi S and Wu S T 2011 Adaptive mechanical-wetting lens actuated by ferrofluids Optics Communications 284 2118-21 [26] Xiao W J and Hardt S 2010 An adaptive liquid microlens driven by a ferrofluidic transducer J Micromech Microeng 20 [27] Schultheis T, Hoheisel D, Xiao W J, Molella L S, Reithmeier E, Rissing L and Hardt S 2012 Performance of an adaptive liquid microlens controlled by a microcoil actuator Microfluid Nanofluid 13 299-308 [28] Koyama D, Isago R and Nakamura K 2010 Compact, high-speed variable-focus liquid lens using acoustic radiation force Optics Express 18 25158-69 [29] López C A and Hirsa A H 2008 Fast focusing using a pinned-contact oscillating liquid lens Nature Photonics 2 610 [30] Knollman G C, Bellin J L S and Weaver J L 1971 Variable-Focus Liquid-Filled Hydroacoustic Lens J Acoust Soc Am 49 253-& [31] Feng G H and Liu J H 2013 Simple-structured capillary-force-dominated tunable-focus liquid lens based on the higher-order-harmonic resonance of a piezoelectric ring transducer Applied Optics 52 829-37 [32] Lee J K, Park K W, Lim G, Kim H R and Kong S H 2012 Variable-focus Liquid Lens Based on a Laterally-integrated Thermopneumatic Actuator J Opt Soc Korea 16 22-8 [33] Ashtiani A O and Jiang H R 2013 Thermally actuated tunable liquid microlens with sub-second response time Applied Physics Letters 103 [34] Lopez C A, Lee C C and Hirsa A H 2005 Electrochemically activated adaptive liquid lens Applied Physics Letters 87 [35] Liu C, Wang Q H, Yao L X and Wang M H 2014 Adaptive Liquid Lens Actuated by Droplet Movement Micromachines-Basel 5 496-504 [36] Krupenkin T, Yang S and Mach P 2003 Tunable liquid microlens Applied Physics Letters 82 316- 8 [37] Cheng C C, Chang C A and Yeh J A 2006 Variable focus dielectric liquid droplet lens Optics Express 14 4101-6 [38] Xu M, Xu D M, Ren H, Yoo I S and Wang Q H 2014 An adaptive liquid lens with radial interdigitated electrode J Optics-Uk 16 [39] Jin B Y, Xu M, Ren H W and Wang Q H 2014 An adaptive liquid lens with a reciprocating movement in a cylindrical hole Optics Express 22 31041-9 [40] Hendriks B, Kuiper S, As M V, Renders C and Tukker T 2005 Electrowetting-based variable- focus lens for miniature systems Optical review 12 255-9 [41] Li L, Wang J H, Wang Q H and Wu S T 2018 Displaceable and focus-tunable electrowetting optofluidic lens Optics Express 26 25839-48 [42] Strauch M, Shao Y F, Bociort F and Urbach H P 2017 Study of surface modes on a vibrating

electrowetting liquid lens Applied Physics Letters 111 [43] Hendriks B H W, Kuiper S, Van As M A J, Renders C A and Tukker T W 2005 Electrowetting- based variable-focus lens for miniature systems Optical Review 12 255-9 [44] Liu C X, Park J and Choi J W 2008 A planar lens based on the electrowetting of two immiscible liquids J Micromech Microeng 18 [45] Xu S, Liu Y, Ren H and Wu S-T 2010 A novel adaptive mechanical-wetting lens for visible and near infrared imaging Optics express 18 12430-5 [46] Ren L, Park S, Ren H and Yoo I S 2012 Adaptive Liquid Lens by Changing Aperture J Microelectromech S 21 953-8 [47] Patra R, Agarwal S, Kondaraju S and Bahga S S 2017 Membrane-less variable focus liquid lens with manual actuation Optics Communications 389 74-8 [48] Malouin B A, Vogel M J, Olles J D, Cheng L L and Hirsa A H 2011 Electromagnetic liquid pistons for capillarity-based pumping Lab on a Chip 11 393-7 [49] Dong L, Agarwal A K, Beebe D J and Jiang H R 2006 Adaptive liquid microlenses activated by stimuli-responsive hydrogels Nature 442 551-4 [50] Xu S, Ren H W, Lin Y J, Moharam M G J, Wu S T and Tabiryan N 2009 Adaptive liquid lens actuated by photo-polymer Optics Express 17 17590-5 [51] Nagelberg S, Zarzar L D, Nicolas N, Subramanian K, Kalow J A, Sresht V, Blankschtein D, Barbastathis G, Kreysing M, Swager T M and Kolle M 2017 Reconfigurable and responsive droplet-based compound micro-lenses Nat Commun 8 [52] Cheng C C and Yeh J A 2007 Dielectrically actuated liquid lens Optics Express 15 7140-5 [53] Ren H and Wu S T 2008 Tunable-focus liquid microlens array using dielectrophoretic effect Optics Express 16 2646-52 [54] Ren H W, Xianyu H Q, Xu S and Wu S T 2008 Adaptive dielectric liquid lens Optics Express 16 14954-60 [55] Lee D G, Park J, Bae J and Kim H Y 2013 Dynamics of a microliquid prism actuated by electrowetting Lab on a Chip 13 274-9 [56] Terrab S, Watson A M, Roath C, Gopinath J T and Bright V M 2015 Adaptive electrowetting lens-prism element Optics Express 23 25838-45 [57] Cheng J T and Chen C L 2011 Adaptive beam tracking and steering via electrowetting- controlled liquid prism Applied Physics Letters 99 [58] Zeng X F, Smith C T, Gould J C, Heise C P and Jiang H R 2011 Fiber Endoscopes Utilizing Liquid Tunable-Focus Microlenses Actuated Through Infrared Light J Microelectromech S 20 583-93 [59] Xu S, Lin Y J and Wu S T 2009 Dielectric liquid microlens with well-shaped electrode Optics Express 17 10499-505 [60] Mugele F and Baret J C 2005 Electrowetting: From basics to applications J Phys-Condens Mat 17 R705-R74 [61] Jeong K H, Liu G L, Chronis N and Lee L P 2004 Tunable microdoublet lens array Optics Express 12 2494-500 [62] Marks R, Mathine D L, Schwiegerling J, Peyman G and Peyghambarian N 2009 Astigmatism and defocus wavefront correction via Zernike modes produced with fluidic lenses Applied Optics 48 3580-7 [63] Xiong G R, Han Y H, Sun C, Sun L G, Han G Z and Gu Z Z 2008 Liquid microlens with tunable focal length and light transmission Applied Physics Letters 92

[64] Chronis N, Liu G L, Jeong K H and Lee L P 2003 Tunable liquid-filled microlens array integrated with microfluidic network Optics Express 11 2370-8 [65] Huang H, Wei K and Zhao Y 2016 Adaptive optofluidic lens (es) for switchable 2D and 3D imaging. In: Microfluidics, BioMEMS, and Medical Microsystems XIV: International Society for Optics and Photonics) p 97050P [66] Ren H and Wu S T 2005 Variable-focus liquid lens by changing aperture Applied Physics Letters 86 [67] Ren H, Fox D, Anderson P A, Wu B and Wu S-T 2006 Tunable-focus liquid lens controlled using a servo motor Optics express 14 8031-6 [68] Ren H W and Wu S T 2007 Variable-focus liquid lens Optics Express 15 5931-6 [69] Lee S W and Lee S S 2007 Focal tunable liquid lens integrated with an electromagnetic actuator Applied Physics Letters 90 [70] Batchko R G, Mansell J and Szilagyi A 2010 Fluidic lens with electrostatic actuation. Google Patents) [71] Aschwanden M, Niederer D, Schmidhausler T, Romer C, Kern T, Yang S-H, King C, Kirchhoefer D R and Warren D 2014 Lens assembly apparatus and method. Google Patents) [72] Oh S H, Rhee K and Chung S K 2016 Electromagnetically driven liquid lens Sensor Actuat a- Phys 240 153-9 [73] https://www.optotune.com/ [74] https://www.holochip.com/ [75] Wei K, Domicone N W and Zhao Y 2014 Electroactive liquid lens driven by an annular membrane Optics Letters 39 1318-21 [76] Carpi F, Frediani G, Turco S and De Rossi D 2011 Bioinspired Tunable Lens with Muscle-Like Electroactive Elastomers Adv Funct Mater 21 4152-8 [77] Choi S T, Lee J Y, Kwon J O, Lee S and Kim W 2011 Varifocal liquid-filled microlens operated by an electroactive polymer actuator Optics Letters 36 1920-2 [78] Wei K, Domicone N W and Zhao Y 2014 A tunable liquid lens driven by a concentric annular electroactive actuator. In: Micro Electro Mechanical systems (MEMS), 2014 IEEE 27th International Conference on: IEEE) pp 909-12 [79] Maffli L, Rosset S, Ghilardi M, Carpi F and Shea H 2015 Ultrafast All-Polymer Electrically Tunable Silicone Lenses Adv Funct Mater 25 1656-65 [80] Hwang T, Kwon H Y, Oh J S, Hong J P, Hong S C, Lee Y, Choi H R, Kim K J, Bhuiya M H and Nam J D 2013 Transparent actuator made with few layer graphene electrode and dielectric elastomer, for variable focus lens Applied Physics Letters 103 [81] Shian S, Diebold R M and Clarke D R 2013 Tunable lenses using transparent dielectric elastomer actuators Optics Express 21 8669-76 [82] Wang W S and Fang J 2006 Design, fabrication and testing of a micromachined integrated tunable microlens J Micromech Microeng 16 1221-6 [83] Werber A and Zappe H 2008 Tunable Pneumatic Microoptics J Microelectromech S 17 1218- 27 [84] Zhang W, Aljasem K, Zappe H and Seifert A 2011 Completely integrated, thermo- pneumatically tunable microlens Optics Express 19 2347-62 [85] Zhang W, Zappe H and Seifert A 2014 Wafer-scale fabricated thermo-pneumatically tunable microlenses Light-Sci Appl 3

[86] Ren H W, Fan Y H, Gauza S and Wu S T 2004 Tunable-focus flat liquid crystal spherical lens Applied Physics Letters 84 4789-91 [87] Ren H and Wu S-T 2006 Adaptive liquid crystal lens with large focal length tunability Optics Express 14 11292-8 [88] Mao X L, Lin S C S, Lapsley M I, Shi J J, Juluri B K and Huang T J 2009 Tunable Liquid Gradient Refractive Index (L-GRIN) lens with two degrees of freedom Lab on a Chip 9 2050-8 [89] Barretto R P J, Messerschmidt B and Schnitzer M J 2009 In vivo fluorescence imaging with high-resolution microlenses Nature Methods 6 511-U61 [90] Ye C F and McLeod R R 2008 GRIN lens and lens array fabrication with diffusion-driven photopolymer Optics Letters 33 2575-7 [91] Chiu C W, Lin Y C, Chao P C P and Fuh A Y G 2008 Achieving high focusing power for a large- aperture liquid crystal lens with novel hole-and-ring electrodes Optics Express 16 19277-84 [92] Ye M, Wang B and Sato S 2008 Realization of liquid crystal lens of large aperture and low driving voltages using thin layer of weakly conductive material Optics Express 16 4302-8 [93] Valley P, Mathine D L, Dodge M R, Schwiegerling J, Peyman G and Peyghambarian N 2010 Tunable-focus flat liquid-crystal diffractive lens Optics Letters 35 336-8 [94] Lin H C and Lin Y H 2012 An electrically tunable-focusing liquid crystal lens with a low voltage and simple electrodes Optics Express 20 2045-52 [95] Ren H W, Fox D W, Wu B and Wu S T 2007 Liquid crystal lens with large focal length tunability and low operating voltage Optics Express 15 11328-35 [96] Ren H W and Wu S T 2006 Adaptive liquid crystal lens with large focal length tunability Optics Express 14 11292-8 [97] Lin H-C, Chen M-S and Lin Y-H 2011 A review of electrically tunable focusing liquid crystal lenses Transactions on Electrical and Electronic Materials 12 234-40 [98] Miccio L, Memmolo P, Merola F, Netti P A and Ferraro P 2015 Red blood cell as an adaptive optofluidic microlens Nat Commun 6 [99] Chowdhury F A and Chau K J 2012 Variable focus microscopy using a suspended water droplet J Optics-Uk 14 [100] Huang H, Wei K and Zhao Y 2016 Variable focus smartphone based microscope using an elastomer liquid lens. In: Micro Electro Mechanical Systems (MEMS), 2016 IEEE 29th International Conference on: IEEE) pp 808-11 [101] Fuh Y K, Lai Z H, Kau L H and Huang H J 2017 A lab-on-phone instrument with varifocal microscope via a liquid-actuated aspheric lens (LAL) Plos One 12 [102] Breslauer D N, Maamari R N, Switz N A, Lam W A and Fletcher D A 2009 Mobile phone based clinical microscopy for global health applications PloS one 4 e6320 [103] Zhu H, Isikman S O, Mudanyali O, Greenbaum A and Ozcan A 2013 Optical imaging techniques for point-of-care diagnostics Lab on a Chip 13 51-67 [104] Liu S and Hua H 2011 Extended depth-of-field microscopic imaging with a variable focus microscope objective Optics Express 19 353-62 [105] Qu Y F and Yang H J 2015 Optical microscopy with flexible axial capabilities using a vari-focus liquid lens J Microsc-Oxford 258 212-22 [106] Fahrbach F O, Voigt F F, Schmid B, Helmchen F and Huisken J 2013 Rapid 3D light-sheet microscopy with a tunable lens Optics Express 21 21010-26 [107] Divetia A, Hsieh T H, Zhang J, Chen Z P, Bachman M and Li G P 2005 Dynamically focused

optical coherence tomography for endoscopic applications Appl Phys Lett 86 [108] Murali S, Lee K S and Rolland J P 2007 Invariant resolution dynamic focus OCM based on liquid crystal lens Opt Express 15 15854-62 [109] Jian Y F, Lee S, Ju M J, Heisler M, Ding W G, Zawadzki R J, Bonora S and Sarunic M V 2016 Lens-based wavefront sensorless adaptive optics swept source OCT Sci Rep-Uk 6 [110] Kim S, Crose M, Eldridge W J, Cox B, Brown W J and Wax A 2018 Design and implementation of a low-cost, portable OCT system Biomed Opt Express 9 1232-43 [111] Lin X, Liu Y B, Wu J M and Dai Q H 2014 Spatial-spectral Encoded Compressive Hyperspectral Imaging Acm T Graphic 33 [112] Bufler T D, Narayanan R M and Dogaru T 2015 Radar Signatures of Furniture Elements Ieee T Aero Elec Sys 51 521-35 [113] Miyake Y, Kouzu T, Takeuchi S, Yamataka S, Nakaguchi T and Tsumura N 2005 Development of new electronic endoscopes using the spectral images of an internal organ. In: Color and Imaging Conference: Society for Imaging Science and Technology) pp 261-3 [114] Song L M W K, Adler D G, Conway J D, Diehl D L, Farraye F A, Kantsevoy S V, Kwon R, Mamula P, Rodriguez B and Shah R J 2008 Narrow band imaging and multiband imaging Gastrointestinal Endoscopy 67 581-9 [115] Cu-Nguyen P H, Grewe A, Fesser P, Seifert A, Sinzinger S and Zappe H 2016 An imaging spectrometer employing tunable hyperchromatic microlenses Light-Sci Appl 5 [116] Zhang D Y, Justis N and Lo Y H 2005 Fluidic adaptive zoom lens with high zoom ratio and widely tunable field of view Optics Communications 249 175-82 [117] Wippermann F, Schreiber P, Bräuer A and Berge B 2006 Mechanically assisted liquid lens zoom system for mobile phone cameras. In: Novel Optical Systems Design and Optimization IX: International Society for Optics and Photonics) p 62890T [118] Savidis N, Peyman G, Peyghambarian N and Schwiegerling J 2013 Nonmechanical zoom system through pressure-controlled tunable fluidic lenses Applied optics 52 2858-65 [119] Duparre J, Wippermann F, Dannberg P and Brauer A 2008 Artificial compound eye zoom camera Bioinspir Biomim 3 [120] Li L, Wang D, Liu C and Wang Q H 2016 Zoom microscope objective using electrowetting lenses Optics Express 24 2931-40 [121] Valley P, Dodge M R, Schwiegerling J, Peyman G and Peyghambarian N 2010 Nonmechanical bifocal zoom telescope Optics Letters 35 2582-4 [122] Lin Y H, Chen M S and Lin H C 2011 An electrically tunable optical zoom system using two composite liquid crystal lenses with a large zoom ratio Optics Express 19 4714-21 [123] Huang H and Zhao Y 2018 A reconfigurable optofluidic device for adaptive imaging and position estimation with a wide field of view. In: Solid-State Sensors, Actuators and Microsystems Workshop Hilton Head Island, (SC, USA: Transducer Research Foundation) p 97050P [124] Wei K, Zeng H and Zhao Y 2014 Insect–Human Hybrid Eye (IHHE): an adaptive optofluidic lens combining the structural characteristics of insect and human eyes Lab on a Chip 14 3594-602 [125] Wei K and Zhao Y 2013 A three-dimensional deformable liquid lens array for directional and wide angle laparoscopic imaging. In: Micro Electro Mechanical Systems (MEMS), 2013 IEEE 26th International Conference on: IEEE) pp 133-6 [126] Won Y H 2014 Electrowetting-based adaptive vari-focal liquid lens array for 3D display. In:

Optoelectronic Devices and Integration V: International Society for Optics and Photonics) p 92700W [127] Kim J, Shin D, Lee J, Koo G, Kim C, Sim J H, Jung G and Won Y H 2018 Electro-wetting lenticular lens with improved diopter for 2D and 3D conversion using lens-shaped ETPTA chamber Optics Express 26 19614-26 [128] Sim J H, Kim J, Kim C, Shin D, Lee J, Koo G, Jung G S and Won Y H 2018 Novel biconvex structure electrowetting liquid lenticular lens for 2D/3D convertible display Sci Rep-Uk 8 [129] Lee J, Kim J, Kim C, Shin D, Koo G, Sim J H and Won Y H 2016 Improving the performance of an electrowetting lenticular lens array by using a thin polycarbonate chamber Optics Express 24 29972-83 [130] Iimura Y, Teshima T, Heo Y, Morimoto Y, Yoshida S, Onoe H and Takeuchi S 2013 Microfluidically tunable lenticular lens. In: Solid-State Sensors, Actuators and Microsystems (TRANSDUCERS & EUROSENSORS XXVII), 2013 Transducers & Eurosensors XXVII: The 17th International Conference on: IEEE) pp 1787-90 [131] Zhu R D, Xu S, Hong Q, Wu S T, Lee C, Yang C M, Lo C C and Lien A 2014 Polymeric-lens- embedded 2D/3D switchable display with dramatically reduced crosstalk Applied Optics 53 1388-95 [132] Ren H, Xu S, Liu Y F and Wu S T 2013 Switchable focus using a polymeric lenticular microlens array and a polarization rotator Optics Express 21 7916-25 [133] Huang H, Wei K and Zhao Y 2016 A compound optofluidic lens for switchable 2D/3D imaging. In: Micro Electro Mechanical Systems (MEMS), 2016 IEEE 29th International Conference on: IEEE) pp 2-5 [134] Hassanfiroozi A, Huang Y P, Javidi B and Shieh H P D 2015 Hexagonal liquid crystal lens array for 3D endoscopy Optics Express 23 971-81 [135] Chen H S and Lin Y H 2013 An endoscopic system adopting a liquid crystal lens with an electrically tunable depth-of-field Optics Express 21 18079-88 [136] Wang Y J, Shen X, Lin Y H and Javidi B 2015 Extended depth-of-field 3D endoscopy with synthetic aperture integral imaging using an electrically tunable focal-length liquid-crystal lens Optics Letters 40 3564-7 [137] Hassanfiroozi A, Huang Y P, Javidi B and Shieh H P D 2016 Dual layer electrode liquid crystal lens for 2D/3D tunable endoscopy imaging system Optics Express 24 8527-38 [138] Flores-Bustamante M C, Rosete-Aguilar M and Calixto S 2016 Mechanical and optical behavior of a tunable liquid lens using a variable cross section membrane: modeling results. In: Optics and Biophotonics in Low-Resource Settings II: International Society for Optics and Photonics) p 969908 [139] Fuh Y-K, Lin M-X and Lee S 2012 Characterizing aberration of a pressure-actuated tunable biconvex microlens with a simple spherically-corrected design Optics and Lasers in Engineering 50 1677-82 [140] Huang H, Wei K, Wang Q and Zhao Y 2016 Improved optical resolution for elastomer-liquid lens at high diopter using varied thickness membrane. In: Microfluidics, BioMEMS, and Medical Microsystems XIV: International Society for Optics and Photonics) p 970504 [141] Lima N, Mishra K and Mugele F 2017 Aberration control in adaptive optics: a numerical study of arbitrarily deformable liquid lenses Optics express 25 6700-11 [142] Reichelt S and Zappe H 2007 Design of spherically corrected, achromatic variable-focus liquid

lenses Optics express 15 14146-54 [143] Wei K, Huang H, Wang Q and Zhao Y 2016 Focus-tunable liquid lens with an aspherical membrane for improved central and peripheral resolutions at high diopters Optics Express 24 3929-39 [144] Mishra K, Murade C, Carreel B, Roghair I, Oh J M, Manukyan G, van den Ende D and Mugele F 2014 Optofluidic lens with tunable focal length and asphericity Sci Rep-Uk 4

Figures

Figure 1. Three types of optofluidic lenses based on different configurations. (a&b)

Type I: lenses with fluid/fluid interface(s), where the two fluids with different RIs contact with each other. The tunable refractive power is achieved by deforming the fluid/fluid interface(s).

(a) refers to lenses with air/liquid interface(s), and (b) refers to lenses with liquid/liquid interface(s). (c) Type II: lenses with fluid/solid interface(s), where the solid material is transparent and deformable. The tunable refractive power is achieved by deforming the fluid/solid interface(s). (d) Type III: lenses made of the liquid materials with a tunable refractive index (RI).

Figure 2.Droplet lenses. (a) The schematic of an acoustic droplet lens where the optical liquid resides in a round well defined by a ring piezoelectric transducer sidewall and a flexible bottom surface. The droplet forms a dome shape when the transducer sidewall vibrates at resonance.

(b) The time-lapse micrographs of a pinned-contact oscillating droplet lens. Reproduced from

[29]. (c) The schematic of a thermopneumatic droplet lens. The thermal expansion of the air deflects the elastomer membrane, causing fluid redistribution in the lens cavity and driving the droplet lens. (d) The schematic of a droplet lens driven by a bi-directional thermoelectric (TE) heater which directly contacts with the optical fluid. The thermal expansion of optical fluid directly deforms the droplet. (e) The schematic of an electrochemical lens where the droplet is overfilled in a through hole of a Teflon plate. The optical power is tuned by modulating the surface tension through the reduction-oxidation process of the surfactant fluid near the electrodes.

Figure 3. Optofluidic lenses with a liquid/liquid interface. (a) The schematic of an electrowetting lens. Both a conductive liquid component and an insulating liquid component are sealed in a closed chamber. The chamber sidewalls patterned with an electrode, which is covered by a dielectric layer and a hydrophobic layer. The liquid/liquid interface can be deformed by applying an applied voltage bias between the sidewall electrode and the bottom electrode. (b) The schematic of a switchable electrowetting element. Electric voltages are applied between the bottom electrode and the two sidewall electrodes to drive the device. If the voltages biases of the two sidewall electrodes equal to each other, the device serves as a lens

(left panel). If the voltages biases of the two sidewall electrodes differ from each other, the device serves as a prism (center panel). If both voltage biases equal to a critical voltage, a planar interface can be obtained to allow the light to pass through without refraction (right panel).

Reproduced from [56] with permission of The Optical Society of America. (c) The schematic of a mechanical-wetting lens where the liquid-liquid interface is deformed by manually moving a screw. (d) The schematic of a stimuli-responsive liquid/liquid lens. The curvature of the water- oil interface is tuned by swelling or shrinking stimuli-responsive hydrogel through temperature or pH value changes. Reproduced from [49].

Figure 4. Elastomer-liquid lenses. (a) The schematic of an elastomer-liquid lens with a tunable iris diaphragm. The impeller is pushed towards the lens center and deflects the lens membrane. (b) The schematic of an elastomer-liquid lens driven by a deformable rubber membrane. (c) The schematic of an elastomer-liquid lens with an actuation chamber, where the deformation of the actuation membrane in the actuation chamber causes liquid redistribution and deflects the lens membrane. (d) The schematic of an elastomer-liquid lens connecting to an annular dielectric electroactive polymer (DEAP) actuator, where the in-plane expansion of the

DEAP membrane upon an elevated electric field reduces the lens aperture and decreases the focal length. Reproduced from [79]. (e) The schematic of an elastomer-liquid lens driven by the thermal expansion of air, where the actuation membrane deformation due to the temperature change pushes the liquid to deflect the lens membrane.

Figure 5. Liquid crystal lenses. (a) The schematic of a LC lens with a homogeneous cell gap and a polymer network layer. (b) The schematic of a LC lens with a planar electrode and a curved opposing electrode (inhomogeneous cell gap).

Figure 6. Optofluidic macro/microscopy. (a) The configuration of a variable focusing macro/microscope using a water-in-oil droplet objective lens. Adapted from [99] with the permission of IOP Publishing. (b) Microscopic images of a calibration slide captured at different magnifications by (a). The adjacent lines on the slide are separated by 10 µm . Adapted from [99] with the permission of IOP Publishing. (c) The schematic of an infinity-corrected scanning microscope with a 4f-system composed of two achromatic relay lenses and a liquid lens placed on the conjugated pupil plane of the 4f-system. (d) Time-lapse microscopic images of a beating zebrafish heart captured at different object planes by (c). Scale bar: 50 μm.

Reproduced from [106] with permission of Optical Society of America.

Figure 7. Spectral imaging. (a) The schematic of two hyperchromatic lens designs. The lens on the left consists of a rigid diffractive plate and a polymer membrane, and optical fluid sandwiched in between, where the polymer membrane can be deformed to tune the refractive power. The lens on the right includes a polymer membrane that is patterned with diffractive structures to allow simultaneous tuning of the refractive power and the diffractive pattern. (b)

The schematic of spectral imaging with a hyperchromatic lens. Reproduced from [115].

Figure 8.Optical zooming. (a) The optical configuration of a zoom system using two LC lenses.

By synchronizing the focal lengths of the objective lens and the eyepiece lens, light rays keep being collimated after passing through the eyepiece to form an image on the image sensor.

Reproduced from [122] with permission of the Optical Society of America. (b) Micrographs of a resolution target at different magnifications using an electrowetting zoom system. Reproduced from [120] with permission of the Optical Society of America.

Figure 9.Wide-angle imaging. (a) The configuration of a compound lens that hosts an array of small elastomer-liquid lenses on a big elastomer membrane. During operation, the big membrane deforms to a dome shape to enlarge the overall viewing angle, while each elastomer- liquid lens can be tuned to focus on objects at different depths. (b) Wide-angle imaging with depth perception demonstration by (a). Reproduced from [124] with permission of the Royal

Chemistry Society.

Figure 10. Lenticular display using optofluidic lenses. (a) The display using electrowetting lenses. (b) Multi-view images through the electrowetting lenticular lenses.

Overlapping images of the two objects are projected to the two eyes of the observer when no voltage is applied. Each object is projected only to one eye under the applied voltage of 24 V.

Reproduced from [129] with permission of the Optical Society of America. (c) The display using polymeric LC lenses with a TN cell. The left panels in (a)&(c) illustrate the non-focusing state (2D mode); and the right panels in (a)&(c) illustrate the focusing state (3D mode).

Reproduced from [131] with permission of the Optical Society of America.

Figure 11. Optofluidic laparoendoscopic imaging. (a) An optofluidic device that can image with switchable monocular vision and binocular vision. 3D anaglyph images can be rendered for stereoscopic vision. (b) An LC endoscope with two LC cell layers. Under the 2D mode, the LC lens in the bottom layer is activated for monocular vision. Under the 3D mode, the small LC lenslets in the top layer are activated for stereoscopic vision. Reproduced from

[137] with permission of the Optical Society of America.

Figure 12. Optical aberrations correction in optofluidic lenses. (a) Optomechanical design of an elastomer-liquid lens with a 6 mm aspherical elastomer membrane that has inhomogeneous thickness profile for spherical aberration correction. (b) shows the resolution comparison of this lens with an elastomer-liquid lens of the same lens aperture and with a constant thickness elastomer membrane. Reproduced from [143] with the permission of Optical

Society of America. (c) An electrowetting lens with an aspherical liquid/liquid interface for reducing the spherical aberration and the image distortion. Reproduced from [144].