Pneumatically tunable optofluidic 2 × 2 switch for reconfigurable optical circuit

Adaptive liquid lens with controllable light intensity

An adaptive liquid lens with controllable light intensity is demonstrated, which can modulate both light intensity and beam spot size. The proposed lens consists of a dyed water solution, a transparent oil, and a transparent water solution. The dyed water solution is used to adjust light intensity distribution by varying the liquid-liquid (L-L) interface. The other two liquids are transparent and designed to control the spot size. In this way, two problems can be solved: the inhomogeneous attenuation of light can be achieved through the dyed layer, and a larger optical power tuning range can be achieved through the two L-L interfaces. Our proposed lens can be used for homogenization effects in laser illumination. In the experiment, an optical power tuning range from - 44.03 m^-1 ∼ + 39.42 m^-1 and an ∼ 89.84% homogenization level are achieved. Our proposed lens may also ease the vignetting problem in imaging systems.

Optofluidic ptychography on a chip

We report the implementation of a fully on-chip, lensless microscopy technique termed optofluidic ptychography. This imaging modality complements the miniaturization provided by microfluidics and allows the integration of ptychographic microscopy into various lab-on-a-chip devices. In our prototype, we place a microfluidic channel on the top surface of a coverslip and coat the bottom surface with a scattering layer. The channel and the coated coverslip substrate are then placed on top of an image sensor for diffraction data acquisition. Similar to the operation of a flow cytometer, the device utilizes microfluidic flow to deliver specimens across the channel. The diffracted light from the flowing objects is modulated by the scattering layer and recorded by the image sensor for ptychographic reconstruction, where high-resolution quantitative complex images are recovered from the diffraction measurements. By using an image sensor with a 1.85 μm pixel size, our device can resolve the 550 nm linewidth on the resolution target. We validate the device by imaging different types of biospecimens, including C. elegans, yeast cells, paramecium, and closterium sp. We also demonstrate a high-resolution ptychographic reconstruction at a video framerate of 30 frames per second. The reported technique can address a wide range of biomedical needs and engenders new ptychographic imaging innovations in a flow cytometer configuration.

M × N electrically controlled optofluidic matrix switch

A completely non-blocking M×N electrically controlled optofluidic matrix switch that uses a 1×3 optical switch with a V-shaped microchannel as the switching unit is proposed. Its light paths and output ports are selected by a micro-actuator matrix and a control circuit. There are few reports of optofluidic matrix switches. Here the given electrostatic micro-actuator and the basic switch structure provide an effective feasible manner for the matrix switch due to the simple and compact structure as well as the operation style. The impacts of microchannels and intersecting waveguides on the switch performance are discussed, and multiple optimization schemes are proposed to reduce the insertion loss efficiently and significantly. The research results indicate that the M×N matrix switch has the advantages of good matrix controllability, simple structure, wide waveband (400-1700 nm), negligible polarization-dependent loss, small insertion loss, and low cross talk. For 1550 nm wavelength, the insertion loss of a 2×6 matrix switch is about 0.17-0.55 dB, and the maximum cross talk is less than -26.8dB. In addition, the performance parameters of a 4×8 matrix switch are given and compared with other reported matrix switches. The proposed M×N matrix switch solves the problem of large insertion loss of general optical matrix switches and can be expanded to a large-scale matrix switch. Moreover, the design of multiple output ports has more flexible applications in systems with multiple branch optical paths and network nodes.

Ultracompact fiber all-optical router using a photo-controlled microbubble

An ultracompact fiber router based on a photo-controlled microbubble was proposed in this Letter. Because the microbubble can be repositioned precisely in the fiber microcavity by adjusting the drive laser power, the target light beam that was incident on the gas-liquid interface of the microbubble was routed toward different directions by the light refraction inside the photo-controlled microbubble. Experimental results showed that the device had a low insertion loss of 0.64 dB, response time of (1.2-1.8s), and can achieve the continuous beam redirections within an angle range of 56° by exploiting a drive laser power of only 1.8 mW. With the characteristics of excellent controllability, low consumption, and no electromechanical parts, such a fiber all-optical router has potential to be used for the multiplex treatments and analysis applications of the photonic laboratory on a chip (PLOC).

In Vivo Optofluidic Switch for Controlling Blood Microflow

Control of blood microflow is crucial for the prevention and therapy of blood disorders, such as cardiovascular diseases and their complications. Conventional control strategies generally implant exogenous synthetic materials into blood vessels as labeling markers or actuating sources, which are invasive and incompatible with biological systems. Here, a label-free, noninvasive, and biocompatible device constructed from natural red blood cells (RBCs) for controlling blood microflow in vivo is reported. The RBCs, optically manipulated, arranged, and rotated using scanning optical tweezers, can function as an optofluidic switch for targeted switching, directional enrichment, dynamic redirecting, and rotary actuation of blood microflow inside zebrafish. The regulation precision of the switch is determined to be at the single-cell level, and the response time is measured as ≈200 ms using a streamline tracking method. This in vivo optofluidic switch may provide a biofriendly device for exploring blood microenvironments in a noncontact and noninvasive manner.

1550 nm infrared/visible light switchable liquid optical switch

In this paper, a liquid optical switch is proposed, and the 1550 nm infrared/visible switching function based on hydraulic control can be realized. An infrared light switch cavity, a visible light cavity and a liquid control cavity are stacked to form the main framework of the device. The glycerol, dyed liquid, and transparent liquid are filled in the cavities, respectively. Two elastic films are fabricated between the cavities for controlling the liquid volume of the cavities. With such a structure, in the initial state, the 1550 nm infrared light and visible light are absorbed by the glycerol and dyed liquid, respectively. The device shows infrared light-off and visible light-off states. When the elastic film is actuated by the liquid pressure, the shape of the elastic film can be changed. Once the elastic film touches to the substrate, a light channel can be formed so that the infrared light or visible light can pass through it. It shows infrared light-on or visible light-on states. In this way, the device can be worked as an infrared light and visible light switchable optical switch. The experiments show that the device can obtain the optical attenuation from ∼1.02 dB to ∼18.24 dB for 1550 nm infrared light optical switch and ∼0.66 dB to ∼8.70 dB @ λ=450 nm; ∼0.62 dB to ∼8.74 dB @ λ=532 nm; ∼0.77 dB to ∼9.00 dB @ λ=633 nm for visible light optical switch. The device has potential applications in the fields of optical fiber communications, variable optical attenuators, and light shutters.

Tunable-Focus Liquid Lens through Charge Injection

Liquid lenses are the simplest and cheapest optical lenses, and various studies have been conducted to develop tunable-focus liquid lenses. In this study, a simple and easily implemented method for achieving tunable-focus liquid lenses was proposed and experimentally validated. In this method, charges induced by a corona discharge in the air were injected into dielectric liquid, resulting in “electropressure” at the interface between the air and the liquid. Through a 3D-printed U-tube structure, a tunable-focus liquid lens was fabricated and tested. Depending on the voltage, the focus of the liquid lens can be adjusted in large ranges (−∞ to −9 mm and 13.11 mm to ∞). The results will inspire various new liquid-lens applications.

Microfluidic-controlled optical router for lab on a chip

In multiplexed analysis, lab on a chip (LoC) devices are advantageous due to the low sample and reagent volumes required. Although optical detection is preferred for providing high sensitivity in a contactless configuration, multiplexed optical LoCs are limited by the technological complexity for integrating multiple light sources and detectors in a single device. To address this issue, we present a microfluidic-controlled optical router that enables measurement in four individual optical channels using a single light source and detector, and without movable parts. The optofluidic device is entirely fabricated in polydimethylsiloxane (PDMS) by soft-lithography, compatible with standard microfabrication technologies, enabling monolithic integration in LoCs. In the device, in-coupled light from an optical fiber is collimated by a polymeric micro-lens and guided through a set of four sequentially connected micro-chambers. When a micro-chamber is filled with water, light is transmitted to the next one. If it is empty of liquid, however, total internal reflection (TIR) occurs at the PDMS-air interface, re-directing the light to the output optical fiber. The router presents high performance, with low cross-talk (<2%) and high switching frequencies (up to 0.343 ± 0.006 Hz), and provides a stable signal for up to 91% of the switching time. With this miniaturized, low-cost, simple and robust design, we expect the current technology to be integrated in the new generation of multiplexed photonic LoCs for biomarker analysis, even at the point of care.

Variable aperture with graded attenuation combined with adjustable focal length lens

In this paper, we demonstrate a variable aperture with graded attenuation combined with adjustable focal length lens actuated by hydraulic control. Two cylindrical chambers and a middle substrate are stacked to form the device body. An elastic film is fabricated in the middle substrate like a sandwich. In the initial state, the dyed liquid is fully covered on the elastic film. The variable aperture shows the state of the maximum optical attenuation. When the bottom chamber is injected with liquid, the elastic film can form a convex surface. The dyed liquid will be pushed to the side wall of the chamber by the raised elastic film and the optical attenuation can be varied by changing the volume of the injected liquid. The proposed device can achieve both the variable attenuator function and the variable-focus lens function. The experiments show that the variable aperture can obtain dynamic attenuation ranges from 33.01 dB to 0.71 dB, and the zoom liquid lens can reach 2.9☓magnifying power. The device can be applied in imaging systems and fiber-optic communications.

Dielectrophoresis-actuated liquid lenses with dual air/liquid interfaces tuned from biconcave to biconvex

This paper reports an electrically reconfigurable optofluidic lens with two air-liquid (silicone oil) interfaces actuated by dielectrophoretic (DEP) force. Initially, a symmetric biconcave air-liquid lens is formed by the surface tension in a microfluidic chip. Then, the DEP force deforms the air-liquid interfaces from biconcave to biconvex, tuning the focal length from -0.5 mm to infinite to +0.5 mm. The wide tunability of the focal length results from the large refractive index difference (∼0.4 at the air-liquid interface), which is only 0.1 in previous liquid-liquid lenses. In the experiment, the lens achieves an ƒ number of 0.91 while consuming only 6.7 nJ per circle. Some asymmetric working states, such as concave-convex and plano-convex lenses, have also been demonstrated. Compared with continuous liquid flow-sustained lenses, this stationary liquid lens holds promise of better compatibility and higher scalability. Its wide tunability, low power consumption and easy operation make it suitable for light manipulation in microfluidic networks.

Light Manipulation in Inhomogeneous Liquid Flow and Its Application in Biochemical Sensing

Light manipulation has always been the fundamental subject in the field of optics since centuries ago. Traditional optical devices are usually designed using glasses and other materials, such as semiconductors and metals. Optofluidics is the combination of microfluidics and optics, which brings a host of new advantages to conventional solid systems. The capabilities of light manipulation and biochemical sensing are inherent alongside the emergence of optofluidics. This new research area promotes advancements in optics, biology, and chemistry. The development of fast, accurate, low-cost, and small-sized biochemical micro-sensors is an urgent demand for real-time monitoring. However, the fluid flow in the on-chip sensor is usually non-uniformed, which is a new and emerging challenge for the accuracy of optical detection. It is significant to reveal the principle of light propagation in an inhomogeneous liquid flow and the interaction between biochemical samples and light in flowing liquids. In this review, we summarize the current state of optofluidic lab-on-a-chip techniques from the perspective of light modulation by the unique dynamic properties of fluid in heterogeneous media, such as diffusion, heat transfer, and centrifugation etc. Furthermore, this review introduces several novel photonic phenomena in an inhomogeneous liquid flow and demonstrates their application in biochemical sensing.

Dielectrophoresis-actuated in-plane optofluidic lens with tunability of focal length from negative to positive

This paper reports a tunable in-plane optofluidic lens by continuously tuning a silicone oil-air interface from concave to convex using the dielectrophoresis (DEP) force. Two parallel glasses are bonded firmly on two sides by NOA 81(Norland Optical Adhesive 81) spacers, forming an open microfluidic channel. An ITO (indium tin oxide) strip and another unpatterned ITO layer are deposited on two glasses as the top and bottom electrodes. Initially, a capillary concave liquid-air interface is formed at the end of the open channel. Then the DEP force is enabled to continuously deform the interface (lens) from concave to convex. In the experiment, the focal length gradually decreases from about -1 mm to infinite and then from infinite to around + 1 mm when the driving voltage is increased from 0 V to 260 V. Particularly, the longitudinal spherical aberration (LSA) is effectively suppressed to have LSA < 0.04 when the lens is operated in the focusing state. This work is the first study of in-plane tunable lenses using the DEP force and possesses special merits as compared to the other reported tunable lenses that are formed by pumping different liquids or by temperature gradient, such as wide tunability, no need for continuous supply of liquids, low power consumption (~81 nJ per switching) due to the capacitor-type driving, and the use of only one type of liquid. Besides, its low aberration makes it favorable for light manipulation in microfluidic networks.

A liquid thermal gradient refractive index lens and using it to trap single living cell in flowing environments

A gradient refractive index (GRIN) lens has a great potential for on-chip imaging and detection systems because of its flat surface with reduced defects. This paper reports a liquid thermal GRIN lens prepared using heat conduction between only one liquid, and uses it as a tunable optical tweezer for single living cell trapping in a flowing environment. This liquid GRIN lens consists of a trapezoidal region in the upper layer which is used to establish a GRIN profile by the heat conduction between three streams of benzyl alcohol with different temperatures, and subsequently a rhombus region in the lower layer with compensation liquids to form a steady square-law parabolic refractive index profile only in transverse direction. Simulations and experiments successfully show the real-time tunability of the focusing properties. The focal length can be modulated in the range of 500 μm with the minimum focal length of 430 μm. A considerable high enhancement factor achieves 5.4 whereas the full width at half maximum is 4 μm. The response time of the GRIN lens is about 20 ms. Based on this enhancement, tunable optical trapping for single human embryonic kidney 293 cell in the range of 280 μm is demonstrated by varying the focal length and working distance which is difficult for solid optical tweezers. The considerable quality of this liquid GRIN lens indicates on-chip applications especially in high quality optical imaging, detection and cells' handling.

Portable microsystem integrates multifunctional dielectrophoresis manipulations and a surface stress biosensor to detect red blood cells for hemolytic anemia

Hemolytic anemia intensity has been suggested as a vital factor for the growth of certain clinical complications of sickle cell disease. However, there is no effective and rapid diagnostic method. As a powerful platform for bio-particles testing, biosensors integrated with microfluidics offer great potential for a new generation of portable point of care systems. In this paper, we describe a novel portable microsystem consisting of a multifunctional dielectrophoresis manipulations (MDM) device and a surface stress biosensor to separate and detect red blood cells (RBCs) for diagnosis of hemolytic anemia. The peripheral circuit to power the interdigitated electrode array of the MDM device and the surface stress biosensor test platform were integrated into a portable signal system. The MDM includes a preparing region, a focusing region, and a sorting region. Simulation and experimental results show the RBCs trajectories when they are subjected to the positive DEP force, allowing the successful sorting of living/dead RBCs. Separated RBCs are then transported to the biosensor and the capacitance values resulting from the variation of surface stress were measured. The diagnosis of hemolytic anemia can be realized by detecting RBCs and the portable microsystem provides the assessment to the hemolytic anemia patient.

Flexible optofluidic waveguide platform with multi-dimensional reconfigurability

Dynamic reconfiguration of photonic function is one of the hallmarks of optofluidics. A number of approaches have been taken to implement optical tunability in microfluidic devices. However, a device architecture that allows for simultaneous high-performance microfluidic fluid handling as well as dynamic optical tuning has not been demonstrated. Here, we introduce such a platform based on a combination of solid- and liquid-core polydimethylsiloxane (PDMS) waveguides that also provides fully functioning microvalve-based sample handling. A combination of these waveguides forms a liquid-core multimode interference waveguide that allows for multi-modal tuning of waveguide properties through core liquids and pressure/deformation. We also introduce a novel lifting-gate lightvalve that simultaneously acts as a fluidic microvalve and optical waveguide, enabling mechanically reconfigurable light and fluid paths and seamless incorporation of controlled particle analysis. These new functionalities are demonstrated by an optical switch with >45 dB extinction ratio and an actuatable particle trap for analysis of biological micro- and nanoparticles.

Microfluidic fabrication of cholesteric liquid crystal core-shell structures toward magnetically transportable microlasers

We report a magnetically transportable microlaser with cholesteric liquid crystal (CLC) core-shell structure, operating in band-edge mode. The dye doped CLC shells as a water-in-oil-in-water (W/O/W) double emulsion were fabricated by microfluidics. Water-dispersible Fe3O4 magnetic nanoparticles were incorporated in the inner aqueous phase by taking advantage of the immiscibility with the middle CLC oil phase. The influence of temperature and shell thickness on laser properties was discussed in detail. The non-invasive manipulation of microlasers was realized under a magnetic field. The dependence of velocity on the viscosity of the carrying fluid and size of the core-shell structure was theoretically analyzed and experimentally investigated using a prototype electromagnetic platform. We also discussed the design principles for this type of DDCLC core-shell structure. Such magnetically transportable microlasers offer promise in in-channel illumination applications requiring active control inside micro-channels.

Dynamic manipulation of particles via transformative optofluidic waveguides

Optofluidics is one of the most remarkable areas in the field of microfluidic research. Particle manipulation with optofluidic platforms has become central to optical chromatography, biotechnology, and μ-total analysis systems. Optical manipulation of particles depends on their sizes and refractive indices (n), which occasionally leads to undesirable separation consequences when their optical mobilities are identical. Here, we demonstrate rapid and dynamic particle manipulation according to n, regardless of size. Integrated liquid-core/solid-cladding (LS) and liquid-core/liquid-cladding (L²) waveguides were fabricated and their characteristics were experimentally and theoretically determined. The high and low n particles showed the opposite behaviors by controlling the contrast of their n values to those of the working fluids. The LS waveguide was found to successfully manipulate particles according to n, and the L² waveguide was found to provide additional system stability and flexibility, compared to the LS system.

Reconfigurable optofluidic switch for generation of optical pulse width modulation based on tunable reflective interface

We present a numerical method for generation of optical pulse width modulation (PWM) based on tunable reflective interface by using a microfluidic droplet. We demonstrate a single layer, planar, optofluidic PWM switch that is driven by excited alternating microbubbles. The main parameters of generation of this PWM such as frequency and speed of switching can be controlled by the mass flow rates of input fluids, and the shape of plug or droplet. Advantages of this design are the reconfigurability in design and the easy control of the switching parameters. The validation of the proposed design is carried out by employing the finite element method (FEM) for the mechanical simulation and the finite-difference time-domain (FDTD) for the optical simulation.

Simulation, fabrication, and characterization of a tunable electrowetting-based lens with a wedge-shaped PDMS dielectric layer

Microlenses with tunable focal length have wide applications in optofluidic devices. This work presents a numerical and experimental investigation on a tunable electrowetting-based concave lens. Optical properties such as focal length of the lens and visibility of images were investigated numerically and experimentally. A finite element analysis and a ZEMAX simulation were used for determination of surface profile and focal length of the lens. The results show that the theoretical surface profile and focal length of the lens are in good agreement with the experimental ones. The lens has a wide tuning focal length equal to 6.5 (cm). Because the polydimethylsiloxane (PDMS) layer is wedge shaped (as both the dielectric and hydrophobic layers), lower applied voltage is needed. A commercial program was used to find the focal length of the lens from maximum visibility value by tuning the applied voltage.

Electrowetting-actuated optical switch based on total internal reflection

In this paper we demonstrate a liquid optical switch based on total internal reflection. Two indium tin oxide electrodes are fabricated on the bottom substrate. A conductive liquid (Liquid 1) is placed on one side of the chamber and surrounded by a density-matched silicone oil (Liquid 2). In initial state, when the light beam illuminates the interface of the two liquids, it just meets the conditions of total internal reflection. The light is totally reflected by Liquid 2, and the device shows light-off state. When we apply a voltage to the other side of the indium tin oxide electrode, Liquid 1 stretched towards this side of the substrate and the curvature of the liquid-liquid interface changes. The light beam is refracted by Liquid 1 and the device shows light-on state. So the device can achieve the functions of an optical switch. Because the light beam can be totally reflected by the liquid, the device can attain 100% light intensity attenuation. Our experiments show that the response time from light-on (off) to light-off (on) are 130 and 132 ms, respectively. The proposed optical switch has potential applications in variable optical attenuators, information displays, and light shutters.

Elastomer-based opto-thermo-mechanical actuation for autonomous, self-powered light level control

We present an autonomous, self-powered solar light controller based on functional integration of a flexible cantilever light guide and a paraffin wax-based optothermal actuator. The controller utilizes the optothermally induced volume increase in the elastomer-encapsulated paraffin wax to produce pneumatic force, which subsequently actuates the cantilever light guide to control the level of frustrated total internal reflection. In its linear response regime, it demonstrated 33% reduction in light intensity fluctuation in terms of the root-mean-square value.

Optofluidic router based on tunable liquid-liquid mirrors

We present an electrically tunable 1 × 5 optofluidic router for on-chip light routing. The device can redirect light from an optical input channel into five output channels by exploiting total internal reflection (TIR) at a liquid-liquid interface. The liquid-liquid mirrors, demonstrated for the first time, are tuned using integrated electrowetting-on-dielectrics (EWOD) actuators. The router is assembled from two chips fabricated by standard MEMS techniques. Through a combination of microfluidic with micro-optical components on chip, reliable light routing is achieved with switching times of [1.5-3.3] s, efficiencies of coupling into channels of up to 12%, optical cross-talk as low as -24 dB, a required drive voltage of 50 V, and a low power consumption of <5 mW, using a device 12 × 13 × 2 mm(3) in size. The optofluidic approach enables addressing of multiple channels over a broad wavelength range. Such optical routing capabilities are important for lab-on-chip devices focusing on optical spectroscopy, optical detection, or even optical manipulation. When integrated with external light sources and a low-cost disposable photonic lab-on-a-chip, the router could thus lead to novel laboratory measurement systems.

In-line microfluidic integration of photonic crystal fibres as a highly sensitive refractometer

Photonic crystal fibres appear to be an ideal platform for the realisation of novel optofluidic devices and sensors due to their waveguide nature and microstructured architecture.

Electrically tunable optofluidic light switch for reconfigurable solar lighting

We describe a reconfigurable lighting system for indoor solar illumination which provides a new way for solar energy conservation. We have experimentally demonstrated an electrically tunable optofluidic light switch which takes the key role for the reconfigurability. The working principle of the switch is based on applying a dielectrophoretic force on a thin oil film hence inducing a surface deformation and consequent leakage of the guided light propagating along the waveguide. A maximum modulation frequency of 2 Hz was achieved. The switch has the advantages of simple fabrication, compact size and low power consumption. The potential applications of such an optofluidic switch are also discussed.

Optofluidic imaging: now and beyond

More than a decade of research work in optofluidics has yielded a large catalogue of optofluidic elements that can manipulate light at the micro-scale (e.g., lenses, prisms). Although these elements have proven useful for many on-chip processes (e.g., miniaturized flow cytometry, interferometry and sample spectroscopy), certain deficiencies have precluded their use in micro-scale imaging. However, recent work in optofluidic imaging has avoided optofluidic elements entirely and focused instead on image capture and composition techniques, demonstrating impressive resolution in both 2D imagery and 3D tomography. In this Focus article, we will discuss some of the recent successes in optofluidic imaging and will expound our expectations for the near future of the optofluidic imaging discipline.

Optofluidic devices and applications in photonics, sensing and imaging

Optofluidics integrates the fields of photonics and microfluidics, providing new freedom to both fields and permitting the realization of optical and fluidic property manipulations at the chip scale. Optofluidics was formed only after many breakthroughs in microfluidics, as understanding of fluid behaviour at the micron level enabled researchers to combine the advantages of optics and fluids. This review describes the progress of optofluidics from a photonics perspective, highlighting various optofluidic aspects ranging from the device's property manipulation to an interactive integration between optics and fluids. First, we describe photonic elements based on the functionalities that enable fluid manipulation. We then discuss the applications of optofluidic biodetection with an emphasis on nanosensing. Next, we discuss the progress of optofluidic lenses with an emphasis on its various architectures, and finally we conceptualize on where the field may lead.

Optofluidic variable-focus lenses for light manipulation

This paper presents a planar optofluidic lens for light manipulation utilizing a combination of optofluidic biconvex lens with micromixer. Three light manipulation techniques including tunable optical diverging, collimating and focusing are realized by altering the refractive index of the optofluidic variable-focus lenses formed by solid polydimethylsiloxane (PDMS) walls and tunable liquid lens body. The optical power from the laser input can be increased or decreased with the tuning of the variable-focus lenses' refractive indexes. The optical power adjustment capabilities are demonstrated and characterized. The combinations of benefits of all lens' optical manipulation capabilities, greater mechanical stability, significant increase of optofluidic device's life time and seamless integration with other lab-on-a-chip functionalities provide a promising and versatile optofluidic compartment to integrate with lab-on-a-chip excitation and sensing applications. Optofluidic lens-including system for tunable fluorescence sensing is demonstrated showing 186% increase in detected fluorescence intensity.

A biomimetic mass-flow transducer utilizing all-optofluidic generation of self-digitized, pulse code-modulated optical pulse trains

We present a new mass-flow transducer producing responses in the form of optical pulse trains that are encoded with information on the strength and position of the stimulus. We implemented the self-digitization and encoding capabilities all-optofluidically, without involving external electronics, by integrating one optical fiber cantilever with multiple polymer optical waveguides on a microfluidic platform. The transducer can also be configured to respond only to transitional stimuli. These features closely mimic the rate-coding, action potential labeling, and rapid adaptation processes observed in biological mechanoreceptors and allow multiple transducers to transmit signals over a single, shared channel. We fabricated the transducer using polymer-based soft-lithography techniques. Its characterization confirmed the stimulus strength-dependent generation of optical pulses and the feasibility of multiplexing 2(n-1) to 2(n) transducers using n waveguides.

Elastomer based tunable optofluidic devices

The synergetic integration of photonics and microfluidics has enabled a wide range of optofluidic devices that can be tuned based on various physical mechanisms. One such tuning mechanism can be realized based on the elasticity of polydimethylsiloxane (PDMS). The mechanical tuning of these optofluidic devices was achieved by modifying the geometry of the device upon applying internal or external forces. External or internal forces can deform the elastomeric components that in turn can alter the optical properties of the device or directly induce flow. In this review, we discuss recent progress in tunable optofluidic devices, where tunability is enabled by the elasticity of the construction material. Different subtypes of such tuning methods will be summarized, namely tuning based on bulk or membrane deformations, and pneumatic actuation.

Characterization of microdroplets using optofluidic signals

We develop a simple method to determine the microdroplet features in a microfluidic chip fabricated by conventional soft lithography. Different sizes of microdroplets are generated through a typical microfluidic T-junction by adjusting the flow rates of the two immiscible liquids. Droplet size and content can be determined by monitoring the optofluidic signals reflected at the fluid-polydimethylsiloxane (PDMS) interface. The demonstrated droplet characterization system can be readily integrated with other microfluidic networks, making it promising for biochemical and biosensing applications.

A single-layer, planar, optofluidic switch powered by acoustically driven, oscillating microbubbles

Merging acoustofluidic mixing with optofluidic integration, we have demonstrated a single-layer, planar, optofluidic switch that is driven by acoustically excited oscillating microbubbles. The device was found to have a switching speed of 5 Hz, an insertion loss of 6.02 dB, and an extinction ratio of 28.48 dB. With its simplicity, low fluid consumption, and compatibility with other microfluidic devices, our design could lead to a line of inexpensive, yet effective optical switches for many lab-on-a-chip applications.

Optofluidic differential spectroscopy for absorbance detection of sub-nanolitre liquid samples

We present a novel optofluidic differential method for carrying out absorbance spectroscopy of sub-nanolitre volumes of liquid samples on a microfluidic chip. Due to the reduction of liquid volume, the absorbance detection in microfluidics is often hindered by either low sensitivity or complex fabrication. To address this issue, we introduced an optofluidic modulator which can be easily integrated into a PDMS (polydimethylsiloxane) based microfluidic chip. The modulator was controlled by the fluid pressure and the absorbance spectrum of the analyte was obtained by taking differential measurements between the analyte and reference medium. An advantage is that this method doesn't need a complicated fabrication step. It is compatible with conventional microfluidic chips and measurements can be carried out on a normal transmission microscope. The performance of the device was tested by measuring solutions containing methylene blue, with concentrations as low as 13 μM.

Microfluidic Wheatstone bridge for rapid sample analysis

We developed a microfluidic analogue of the classic Wheatstone bridge circuit for automated, real-time sampling of solutions in a flow-through device format. We demonstrate precise control of flow rate and flow direction in the "bridge" microchannel using an on-chip membrane valve, which functions as an integrated "variable resistor". We implement an automated feedback control mechanism in order to dynamically adjust valve opening, thereby manipulating the pressure drop across the bridge and precisely controlling fluid flow in the bridge channel. At a critical valve opening, the flow in the bridge channel can be completely stopped by balancing the flow resistances in the Wheatstone bridge device, which facilitates rapid, on-demand fluid sampling in the bridge channel. In this article, we present the underlying mechanism for device operation and report key design parameters that determine device performance. Overall, the microfluidic Wheatstone bridge represents a new and versatile method for on-chip flow control and sample manipulation.

Optofluidic membrane interferometer: An imaging method for measuring microfluidic pressure and flow rate simultaneously on a chip

We present a novel image-based method to measure the on-chip microfluidic pressure and flow rate simultaneously by using the integrated optofluidic membrane interferometers (OMIs). The device was constructed with two layers of structured polydimethylsiloxane (PDMS) on a glass substrate by multilayer soft lithography. The OMI consists of a flexible air-gap optical cavity which upon illumination by monochromatic light generates interference patterns that depends on the pressure. These interference patterns were captured with a microscope and analyzed by computer based on a pattern recognition algorithm. Compared with the previous techniques for pressure sensing, this method offers several advantages including low cost, simple fabrication, large dynamic range, and high sensitivity. For pressure sensing, we demonstrate a dynamic range of 0-10 psi with an accuracy of ±2% of full scale. Since multiple OMIs can be integrated into a single chip for detecting pressures at multiple locations simultaneously, we also demonstrated a microfluidic flow sensing by measuring the differential pressure along a channel. Thanks to the simple fabrication that is compatible with normal microfluidics, such OMIs can be easily integrated into other microfluidic systems for in situ fluid monitoring.