Optical trapping and propulsion of red blood cells on waveguide surfaces

Optical tweezing of microparticles and cells using silicon-photonics-based optical phased arrays

Integrated optical tweezers have the potential to enable highly-compact, low-cost, mass-manufactured, and broadly-accessible optical manipulation when compared to standard bulk-optical tweezers. However, integrated demonstrations to date have been fundamentally limited to micron-scale standoff distances and, often, passive trapping functionality, making them incompatible with many existing applications and significantly limiting their utility, especially for biological studies. In this work, we demonstrate optical trapping and tweezing using an integrated OPA for the first time, increasing the standoff distance of integrated optical tweezers by over two orders of magnitude compared to prior demonstrations. First, we demonstrate trapping of polystyrene microspheres 5 mm above the surface of the chip and calibrate the trap force. Next, we show tweezing of polystyrene microspheres in one dimension by non-mechanically steering the trap by varying the input laser wavelength. Finally, we use the OPA tweezers to demonstrate, to the best of our knowledge, the first cell experiments using single-beam integrated optical tweezers, showing controlled deformation of mouse lymphoblast cells. This work introduces a new modality for integrated optical tweezers, significantly expanding their utility and compatibility with existing applications, especially for biological experiments.

Advances in Microfluidics for Single Red Blood Cell Analysis

The utilizations of microfluidic chips for single RBC (red blood cell) studies have attracted great interests in recent years to filter, trap, analyze, and release single erythrocytes for various applications. Researchers in this field have highlighted the vast potential in developing micro devices for industrial and academia usages, including lab-on-a-chip and organ-on-a-chip systems. This article critically reviews the current state-of-the-art and recent advances of microfluidics for single RBC analyses, including integrated sensors and microfluidic platforms for microscopic/tomographic/spectroscopic single RBC analyses, trapping arrays (including bifurcating channels), dielectrophoretic and agglutination/aggregation studies, as well as clinical implications covering cancer, sepsis, prenatal, and Sickle Cell diseases. Microfluidics based RBC microarrays, sorting/counting and trapping techniques (including acoustic, dielectrophoretic, hydrodynamic, magnetic, and optical techniques) are also reviewed. Lastly, organs on chips, multi-organ chips, and drug discovery involving single RBC are described. The limitations and drawbacks of each technology are addressed and future prospects are discussed.

The Rise of the OM-LoC: Opto-Microfluidic Enabled Lab-on-Chip

The integration of optical circuits with microfluidic lab-on-chip (LoC) devices has resulted in a new era of potential in terms of both sample manipulation and detection at the micro-scale. On-chip optical components increase both control and analytical capabilities while reducing reliance on expensive laboratory photonic equipment that has limited microfluidic development. Notably, in-situ LoC devices for bio-chemical applications such as diagnostics and environmental monitoring could provide great value as low-cost, portable and highly sensitive systems. Multiple challenges remain however due to the complexity involved with combining photonics with micro-fabricated systems. Here, we aim to highlight the progress that optical on-chip systems have made in recent years regarding the main LoC applications: (1) sample manipulation and (2) detection. At the same time, we aim to address the constraints that limit industrial scaling of this technology. Through evaluating various fabrication methods, material choices and novel approaches of optic and fluidic integration, we aim to illustrate how optic-enabled LoC approaches are providing new possibilities for both sample analysis and manipulation.

Multimodal on-chip nanoscopy and quantitative phase imaging reveals the nanoscale morphology of liver sinusoidal endothelial cells

Visualization of three-dimensional (3D) morphological changes in the subcellular structures of a biological specimen is a major challenge in life science. Here, we present an integrated chip-based optical nanoscopy combined with quantitative phase microscopy (QPM) to obtain 3D morphology of liver sinusoidal endothelial cells (LSEC). LSEC have unique morphology with small nanopores (50-300 nm in diameter) in the plasma membrane, called fenestrations. The fenestrations are grouped in discrete clusters, which are around 100 to 200 nm thick. Thus, imaging and quantification of fenestrations and sieve plate thickness require resolution and sensitivity of sub-100 nm along both the lateral and the axial directions, respectively. In chip-based nanoscopy, the optical waveguides are used both for hosting and illuminating the sample. The fluorescence signal is captured by an upright microscope, which is converted into a Linnik-type interferometer to sequentially acquire both superresolved images and phase information of the sample. The multimodal microscope provided an estimate of the fenestration diameter of 119 ± 53 nm and average thickness of the sieve plates of 136.6 ± 42.4 nm, assuming the constant refractive index of cell membrane to be 1.38. Further, LSEC were treated with cytochalasin B to demonstrate the possibility of precise detection in the cell height. The mean phase value of the fenestrated area in normal and treated cells was found to be 161 ± 50 mrad and 109 ± 49 mrad, respectively. The proposed multimodal technique offers nanoscale visualization of both the lateral size and the thickness map, which would be of broader interest in the fields of cell biology and bioimaging.

Study of waveguide background at visible wavelengths for on-chip nanoscopy

On-chip super-resolution optical microscopy is an emerging field relying on waveguide excitation with visible light. Here, we investigate two commonly used high-refractive index waveguide platforms, tantalum pentoxide (Ta₂O₅) and silicon nitride (Si₃N₄), with respect to their background with excitation in the range 488-640 nm. The background strength from these waveguides were estimated by imaging fluorescent beads. The spectral dependence of the background from these waveguide platforms was also measured. For 640 nm wavelength excitation both the materials had a weak background, but the background increases progressively for shorter wavelengths for Si₃N₄. We further explored the effect of the waveguide background on localization precision of single molecule localization for direct stochastic optical reconstruction microscopy (dSTORM). An increase in background for Si₃N₄ at 488 nm is shown to reduce the localization precision and thus the resolution of the reconstructed images. The localization precision at 640nm was very similar for both the materials. Thus, for shorter wavelength applications Ta₂O₅ is preferable. Reducing the background from Si₃N₄ at shorter wavelengths via improved fabrication will be worth pursuing.

Optical transport of sub-micron lipid vesicles along a nanofiber

Enhanced manipulation and analysis of bio-particles using light confined in nano-scale dielectric structures has proceeded apace in the last several years. Small mode volumes, along with the lack of a need for bulky optical elements give advantages in sensitivity and scalability relative to conventional optical manipulation. However, manipulation of lipid vesicles (liposomes) remains difficult, particularly in the sub-micron diameter regime. Here we demonstrate the optical trapping and transport of sub-micron diameter liposomes along an optical nanofiber using the nanofiber mode's evanescent field. We find that nanofiber diameters below a nominal diffraction limit give optimal results. Our results pave the way for integrated optical transport and analysis of liposome-like bio-particles, as well as their coupling to nano-optical resonators.

Highly temporal stable, wavelength-independent, and scalable field-of-view common-path quantitative phase microscope

SIGNIFICANCE

High temporal stability, wavelength independency, and scalable field of view (FOV) are the primary requirements of a quantitative phase microscopy (QPM) system. The high temporal stability of the system provides accurate measurement of minute membrane fluctuations of the biological cells that can be an indicator of disease diagnosis.

AIM

The main aim of this work is to develop a high temporal stable technique that can accurately quantify the cell's dynamics such as membrane fluctuations of human erythrocytes. Further, the technique should be capable of acquiring scalable FOV and resolution at multiple wavelengths to make it viable for various biological applications.

APPROACH

We developed a single-element nearly common path, wavelength-independent, and scalable resolution/FOV QPM system to obtain temporally stable holograms/interferograms of the biological specimens.

RESULTS

With the proposed system, the temporal stability is obtained ∼15  mrad without using any vibration isolation table. The capability of the proposed system is first demonstrated on USAF resolution chart and polystyrene spheres (4.5-μm diameter). Further, the system is implemented for single shot, wavelength-independent quantitative phase imaging of human red blood cells (RBCs) with scalable resolution using color CCD camera. The membrane fluctuation of healthy human RBCs is also measured and was found to be around 47 nm.

CONCLUSIONS

Contrary to its optical counterparts, the present system offers an energy efficient, cost effective, and simple way of generating object and reference beam for the development of common-path QPM. The present system provides the flexibility to the user to acquire multi-wavelength quantitative phase images at scalable FOV and resolution.

Optimisation and scaling effect of dual-waveguide optical trapping in the SOI platform

Optical trapping has potential applications in biological manipulation, particle trapping, Raman spectroscopy, and quantum optomechanics. Among the various optical trapping schemes, on-chip dual-waveguide traps combine benefits of stable trapping and mass production. However, no systematic research has been conducted to optimise on-chip dual-waveguide traps so that the trapping capability is maximised. Here, a numerical simulation of an on-chip silicon on insulator (SOI) dual-waveguide optical trap based on Lumerical FDTD Solutions is carried out to optimise the on-chip dual-waveguide trap. It was found that the waveguide thickness is a crucial parameter when designing a dual-waveguide trap, and its optical trapping capability largely depends on the distance between the two waveguides. We show that the optimal waveguide thickness to achieve the maximum trapping capability generally increases with the gap distance, accompanied by a periodic feature due to the interference and the resonant effects within the gap. This optimal waveguide thickness and gap distance are analysed to have clear scaling effects over the input optical wavelength, which paves the way for the design and optimisation of dual-waveguide traps for various applications.

High Trap Stiffness Microcylinders for Nanophotonic Trapping

Nanophotonic waveguides have enabled on-chip optical trap arrays for high-throughput manipulation and measurements. However, the realization of the full potential of these devices requires trapping enhancement for applications that need large trapping force. Here, we demonstrate a solution via fabrication of high refractive index cylindrical trapping particles. Using two different fabrication processes, a cleaving method and a novel lift-off method, we produced cylindrical silicon nitride (Si₃N₄) particles and characterized their trapping properties using the recently developed nanophotonic standing-wave array trap (nSWAT) platform. Relative to conventionally used polystyrene microspheres, the fabricated Si₃N₄ microcylinders attain an approximately 3- to 6-fold trap stiffness enhancement. Furthermore, both fabrication processes permit tunable microcylinder geometry, and the lift-off method also results in ultrasmooth surface termination of the ends of the microcylinders. These combined features make the Si₃N₄ microcylinders uniquely suited for a broad range of high-throughput, high-force, nanophotonic waveguide-based optical trapping applications.

Tunable optical lattices in the near-field of a few-mode nanophotonic waveguide

Due to the action of the scattering force, particles that are optically trapped at the surface of a waveguide are propelled in the direction of the light propagation. In this work, we demonstrate an original approach for creating tunable periodic arrays of optical traps along a few-mode silicon nanophotonic waveguide. We show how the near-field optical forces at the surface of the waveguide are periodically modulated when two guided modes with different propagation constants are simultaneously excited. The phenomenon is used to achieve stable trapping of a large number of dielectric particles or bacteria along a single waveguide. By controlling the light coupling conditions and the laser wavelength, we investigate several techniques for manipulating the trapped particles. Especially, we demonstrate that the period of the optical lattice can be finely tuned by adjusting the laser wavelength. This effect can be used to control the trap positions, and thus transport the trapped particles in both directions along the waveguide.

Quantitative phase microscopy of red blood cells during planar trapping and propulsion

Red blood cells (RBCs) have the ability to undergo morphological deformations during microcirculation, such as changes in surface area, volume and sphericity. Optical waveguide trapping is suitable for trapping, propelling and deforming large cell populations along the length of the waveguide. Bright field microscopy employed with waveguide trapping does not provide quantitative information about structural changes. Here, we have combined quantitative phase microscopy and waveguide trapping techniques to study changes in RBC morphology during planar trapping and transportation. By using interference microscopy, time-lapsed interferometric images of trapped RBCs were recorded in real-time and subsequently utilized to reconstruct optical phase maps. Quantification of the phase differences before and after trapping enabled study of the mechanical effects during planar trapping. During planar trapping, a decrease in the maximum phase values, an increase in the surface area and a decrease in the volume and sphericity of RBCs were observed. QPM was used to analyze the phase values for two specific regions within RBCs: the annular rim and the central donut. The phase value of the annular rim decreases whereas it increases for the central donut during planar trapping. These changes correspond to a redistribution of cytosol inside the RBC during planar trapping and transportation.

Photonic and Plasmonic Nanotweezing of Nano- and Microscale Particles

The ability to manipulate and sense biological molecules is important in many life science domains, such as single-molecule biophysics, the development of new drugs and cancer detection. Although the manipulation of biological matter at the nanoscale continues to be a challenge, several types of nanotweezers based on different technologies have recently been demonstrated to address this challenge. In particular, photonic and plasmonic nanotweezers are attracting a strong research effort especially because they are efficient and stable, they offer fast response time, and avoid any direct physical contact with the target object to be trapped, thus preventing its disruption or damage. In this paper, we critically review photonic and plasmonic resonant technologies for biomolecule trapping, manipulation, and sensing at the nanoscale, with a special emphasis on hybrid photonic/plasmonic nanodevices allowing a very strong light-matter interaction. The state-of-the-art of competing technologies, e.g., electronic, magnetic, acoustic and carbon nanotube-based nanotweezers, and a description of their applications are also included.

Resonant optical propulsion of a particle inside a hollow-core photonic crystal fiber

Resonant propulsion of small nonresonant particles inside metal waveguides due to the formation of resonant states by the guided modes below their cutoffs has been predicted in the past. Here it is shown that stable resonant propulsion exists in hollow-core photonic crystal fibers, which are all-dielectric structures and are a major platform for various photonic applications. Specific features of the resonant propulsion are discussed together with the fiber design issues. The results may enable power-efficient transport of particles over long distances, particle sorting, and sensitive detection.

Metal-insulator-metal waveguides for particle trapping and separation

Optical particle trapping and separation are essential techniques in the fields of biology and chemistry. In many applications, it is important to identify passive separation techniques that only rely on intrinsic forces in a system with a fixed device geometry. We present a dual-waveguide sorter that utilizes the loss of metal-insulator-metal (MIM) waveguides for completely passive particle trapping and separation and is created using a unique angle sidewall deposition process. Our experiments show that an inner Au-Si3N4-Au waveguide is able to trap particles within the propagation distance of its dominant modes and release the particles into an outer Au-H2O-Au waveguide. The outer waveguide then propels the particles and separates them by size. The separation results are accurately modeled by a first-principles, analytical model.

Rib waveguides for trapping and transport of particles

Rib waveguides are investigated as an alternative to strip waveguides for planar trapping and transport of microparticles. Microparticles are successfully propelled along the surface of rib waveguides and trapped in the gap between opposing rib waveguides. The trapping capabilities of waveguide end facets formed by a single and opposing waveguide geometries are investigated. The slab beneath a rib waveguide continues to guide light after the end facet of a rib waveguide. Thus particles can be trapped in wider gaps formed by opposing rib waveguides than with strip waveguides. Rib waveguides were found more efficient in trapping a collection of particles in the gap and particles could be moved to different locations in the gap by changing the relative power in the two opposing rib waveguides. Numerical simulations are used to show that the trapping efficiency on the surface of rib and strip waveguides is comparable. The simulations also confirm the advantage of opposing rib waveguides for trapping particles in wide gaps. The low sidewalls of rib waveguides give low propagation losses and make it easy to integrate rib waveguides with other functions in a lab-on-a-chip where particle trapping and transport is required.

Recent advances in optical fiber devices for microfluidics integration

This paper examines the recent emergence of miniaturized optical fiber based sensing and actuating devices that have been successfully integrated into fluidic microchannels that are part of microfluidic and lab-on-chip systems. Fluidic microsystems possess the advantages of reduced sample volumes, faster and more sensitive biological assays, multi-sample and parallel analysis, and are seen as the de facto bioanalytical platform of the future. This paper considers the cases where the optical fiber is not merely used as a simple light guide delivering light across a microchannel, but where the fiber itself is engineered to create a new sensor or tool for use within the environment of the fluidic microchannel.

Squeezing red blood cells on an optical waveguide to monitor cell deformability during blood storage

Red blood cells squeeze through micro-capillaries as part of blood circulation in the body. The deformability of red blood cells is thus critical for blood circulation. In this work, we report a method to optically squeeze red blood cells using the evanescent field present on top of a planar waveguide chip. The optical forces from a narrow waveguide are used to squeeze red blood cells to a size comparable to the waveguide width. Optical forces and pressure distributions on the cells are numerically computed to explain the squeezing process. The proposed technique is used to quantify the loss of blood deformability that occurs during blood storage lesion. Squeezing red blood cells using waveguides is a sensitive technique and works simultaneously on several cells, making the method suitable for monitoring stored blood.

Effect of multiple scattering to optical forces on a sphere near an optical waveguide

We have investigated the effect of multiple scattering to optical forces on a particle in the evanescent field produced by an optical waveguide. Considering the multiple scattering between the sphere and the waveguide, we extend the formalism based on transition matrix and reflection matrix to calculate the optical forces on a sphere near an optical waveguide. Numerical results show that the influence that multiple scattering has on the optical forces can't be ignored, especially when the structure resonance of the particle arises. Moreover, the effect of multiple scattering to optical forces is also studied in detail on the condition that the distance between the sphere and the waveguide is within the effective operating distance.

Review of methods to probe single cell metabolism and bioenergetics

Single cell investigations have enabled unexpected discoveries, such as the existence of biological noise and phenotypic switching in infection, metabolism and treatment. Herein, we review methods that enable such single cell investigations specific to metabolism and bioenergetics. Firstly, we discuss how to isolate and immobilize individuals from a cell suspension, including both permanent and reversible approaches. We also highlight specific advances in microbiology for its implications in metabolic engineering. Methods for probing single cell physiology and metabolism are subsequently reviewed. The primary focus therein is on dynamic and high-content profiling strategies based on label-free and fluorescence microspectroscopy and microscopy. Non-dynamic approaches, such as mass spectrometry and nuclear magnetic resonance, are also briefly discussed.

Long-distance laser propulsion and deformation- monitoring of cells in optofluidic photonic crystal fiber

We introduce a unique method for laser-propelling individual cells over distances of 10s of cm through stationary liquid in a microfluidic channel. This is achieved by using liquid-filled hollow-core photonic crystal fiber (HC-PCF). HC-PCF provides low-loss light guidance in a well-defined single mode, resulting in highly uniform optical trapping and propulsive forces in the core which at the same time acts as a microfluidic channel. Cells are trapped laterally at the center of the core, typically several microns away from the glass interface, which eliminates adherence effects and external perturbations. During propagation, the velocity of the cells is conveniently monitored using a non-imaging Doppler velocimetry technique. Dynamic changes in velocity at constant optical powers up to 350 mW indicate stress-induced changes in the shape of the cells, which is confirmed by bright-field microscopy. Our results suggest that HC-PCF will be useful as a new tool for the study of single-cell biomechanics.

Trapping and delivery of Escherichia coli in a microfluidic channel using an optical nanofiber

The stable trapping and direct delivery of bacteria are very important in studying their nanoscopic biochemical processes. Here, we report the stable optical trapping and direct delivery of Escherichia coli in a microfluidic channel. By placing an optical nanofiber (NF, diameter: 600 nm) in the channel and directing a laser beam with a wavelength of 980 nm and an optical power over 25 mW into the NF, E. coli were stably trapped by the NF. The trapping stability was tested in flowing suspensions and the delivery efficiency of the trapped E. coli was demonstrated. Experimental results were interpreted by numerical simulations and theoretical calculations.

Influence of electrostatic forces on particle propulsion in the evanescent field of silver ion-exchanged waveguides

The effect of electrostatic interaction between carboxylate- and amino-functionalized polystyrene particles and a charged waveguide surface on the propulsion speed in optical tweezers is considered to be a function of the pH and ionic strength. It was shown that with the variation of the pH of the aqueous solution in which the particles were immersed, a systematic change in propulsion speed with a maximum speed could be achieved. The appearance of a maximum speed was ascribed to changes in the particle-waveguide separation as a result of the combination of two forces: Coulomb repulsion/attraction and induced dipole forces. The highest maximum speed at low ionic strength was around 12 μm/s. Changes in the ionic strength of the solution influenced the gradient of the dielectric constant near the involved surfaces and also led to a slightly reduced hydrodynamic radius of the particles. The combination of these effects subsequently increased the maximum speed to about 23 μm/s.

Serial Raman spectroscopy of particles trapped on a waveguide

We demonstrate that Raman spectroscopy can be used to characterize and identify particles that are trapped and propelled along optical waveguides. To accomplish this, microscopic particles on a waveguide are moved along the waveguide and then individually addressed by a focused laser beam to obtain their characteristic Raman signature within 1 second acquisition time. The spectrum is used to distinguish between glass and polystyrene particles. After the characterization, the particles continue to be propelled along the straight waveguide. Alternatively, a waveguide loop with a gap is also investigated, and in this case particles are held in the gap for characterization before they are released.

Surface transport and stable trapping of particles and cells by an optical waveguide loop

Waveguide trapping has emerged as a useful technique for parallel and planar transport of particles and biological cells and can be integrated with lab-on-a-chip applications. However, particles trapped on waveguides are continuously propelled forward along the surface of the waveguide. This limits the practical usability of the waveguide trapping technique with other functions (e.g. analysis, imaging) that require particles to be stationary during diagnosis. In this paper, an optical waveguide loop with an intentional gap at the centre is proposed to hold propelled particles and cells. The waveguide acts as a conveyor belt to transport and deliver the particles/cells towards the gap. At the gap, the diverging light fields hold the particles at a fixed position. The proposed waveguide design is numerically studied and experimentally implemented. The optical forces on the particle at the gap are calculated using the finite element method. Experimentally, the method is used to transport and trap micro-particles and red blood cells at the gap with varying separations. The waveguides are only 180 nm thick and thus could be integrated with other functions on the chip, e.g. microfluidics or optical detection, to make an on-chip system for single cell analysis and to study the interaction between cells.

Efficient transportation of nano-sized particles along slotted photonic crystal waveguide

We design a slotted photonic crystal waveguide (S-PhCW) and numerically propose that it can efficiently transport polystyrene particle with diameter as small as 50 nm in a 100 nm slot. Excellent optical confinement and slow light effect provided by the photonic crystal structure greatly enhance the optical force exerted on the particle. The S-PhCW can thus transport the particle with optical propulsion force as strong as 5.3 pN/W, which is over 10 times stronger than that generated by the slotted strip waveguide (S-SW). In addition, the vertical optical attraction force induced in the S-PhCW is over 2 times stronger than that of the S-SW. Therefore, the S-PhCW transports particles not only efficiently but also stably. We anticipate this waveguide structure will be beneficial for the future lab-on-chip development.

Determination of surface protein coverage by composite waveguide based polarimetric interferometry

Deposition of a tapered thin film of Ta(2)O(5) onto a single-mode, polarization-insensitive slab glass waveguide enables the resultant structure to serve as a simple, inexpensive yet highly sensitive polarimetric interferometer for trace, even ultra-trace, detection of chemical and biochemical analytes. By comparing the measured refractive-index sensitivity with that simulated based on a four-layer homogeneous waveguide, the equivalent thickness for the tapered layer of Ta(2)O(5) and the sensitivity of the sensor to adlayer thickness were determined. Responses of the sensor to unspecific adsorption of bovine serum albumin (BSA) and to surface antibody-antigen interaction were investigated in situ and the corresponding surface coverages were obtained with the adlayer-thickness sensitivity. The interferometer sensor shows good long-term stability and its phase drift is lower than π over 10 h.

Waveguide trapping of hollow glass spheres

Microparticles can be trapped and propelled by the evanescent field of optical waveguides. As the evanescent field only stretches 100-200 nm from the surface of the waveguide, only the lower caps of the microparticles interact directly with the field. This is taken advantage of by trapping hollow glass spheres on waveguides in the same way as solid glass spheres. For the chosen waveguide, numerical simulations show that hollow microspheres with a shell thickness above 60 nm can be stably trapped, while spheres with thinner shells are repelled. The average refractive index of the sphere-field intersection volume is used to explain the result in a qualitative way.