An integrated optofluidic device for single-cell sorting driven by mechanical properties

Efficient Surfactant-Mediated Photovoltaic Manipulation of fL-Scale Aqueous Microdroplets for Diverse Optofluidic Applications on LiNbO3 Platform

The electrodeless biocompatible manipulation of femtoliter-scale aqueous microdroplets remains challenging. The appropriate isolation of electrostatic charges from femtoliter-scale aqueous microdroplets is crucial for electrodeless optoelectronic manipulation based on space-charge-density modulation. Here, surfactant-mediated photovoltaic manipulation is proposed, where the surfactant layers self-assembled at the water-oil and oil-Lithium niobate interfaces are employed to isolate photovoltaic charges. The reduced electrostatic attenuation, remarkable hydrophobicity, and strong electrical breakdown suppression of the surfactant layers enable the stable and swift manipulation of femtoliter-scale aqueous microdroplets using µW-level laser in oil media. By virtue of the surfactant-mediated photovoltaic manipulation, a controllable merging/touching/detaching switch of aqueous microdroplets by adjusting the laser illumination intensity and position is realized and the cascading biochemical operations and microreactions of aqueous microdroplets and microdroplet strings are demonstrated. To demonstrate its potential in photonic Micro-Electro-Mechanical-System assemblies, the end coupling of a focused-laser-beam into a ZnO microrod leveraging the refraction effect occurring at the water/oil interface is demonstrated. Moreover, because of the selective permeability of the droplet-interface-bilayer developed between the touching microdroplets, in situ adjustment of the size of the microdroplets and the fluorescent solute contained in the microdroplets are achieved, aiming at constructing multicomponent fluorescent microdroplets with tunable whispering-gallery-mode characteristics.

Dielectrophoresis-electrophoresis transition during the photovoltaic manipulation of water microdroplets on LiNbO3:Fe platform

The abrupt behaviors of microdroplets during the LN-based photovoltaic manipulation may cause the transient instability and even failure of the microfluidic manipulation. In this paper, we perform a systematical analysis on the responses of water microdroplets to laser illumination on both naked and PTFE-coated LN:Fe surface, and find that the abrupt repulsive behaviors of the microdroplets are due to the electrostatic transition from the dielectrophoresis (DEP) to electrophoresis (EP) mechanism. Charging of the water microdroplets through the Rayleigh jetting from electrified water/oil interface is suggested as the cause of the DEP-EP transition. Fitting the kinetic data of the microdroplets to the models describing the motion of the microdroplets under the photovoltaic field yields the charging amount depending on the substrate configuration (∼1.7 × 10^-11 and 3.9 × 10^-12 C on the naked and PTFE-coated LN:Fe substrates), and also reveals the dominance of the EP mechanism in the co-existence of the DEP and EP mechanisms. The outcome of this paper will be quite important to the practicalization of the photovoltaic manipulation in LN-based optofluidic chips.

Optical Deformation of Biological Cells using Dual-Beam Laser Tweezer

Optical tweezer is a non-contact tool to trap and manipulate microparticles such as biological cells using coherent light beams. In this study, we utilized a dual-beam optical tweezer, created using two counterpropagating and slightly divergent laser beams to trap and deform biological cells. Human embryonic kidney 293 (HEK-293) and breast cancer (SKBR3) cells were used to characterize their membrane elasticity by optically stretching in the dual-beam optical tweezer. It was observed that the extent of deformation in both cell types increases with increasing optical trapping power. The SKBR3 cells exhibited greater percentage deformation than that of HEK-293 cells for a given trapping power. Our results demonstrate that the dual-beam optical tweezer provides measures of cell elasticity that can distinguish between various cell types. The non-contact optical cell stretching can be effectively utilized in disease diagnosis such as cancer based on the cell elasticity measures.

3D Photovoltaic Router of Water Microdroplets Aiming at Free-Space Microfluidic Transportation

So far, microfluidic navigation based on space-charge modulation is limited in a two-dimensional (2D) substrate plane. In this paper, a three-dimensional (3D) photovoltaic water-microdroplet router based on a superhydrophobic LiNbO₃:Fe crystal is reported. This router employs the repulsive electrostatic force induced by the positive photovoltaic charges generated under focused laser illumination and permits traveling microdroplets to be routed in both in-plane and out-of-plane ways. By analyzing the dynamic process of microdroplet routing, it is found that the microdroplets can gain positive charges through traveling on a superhydrophobic surface and that the positive photovoltaic charges exert an electrophoretic (EP) force on the microdroplets being charged and make them either routed inside the 2D substrate plane or jump out of the 2D plane through electrostatic ballistic ejection. The laser-illumination and microdroplet-size dependence of the deflecting parameters of the in-plane microdroplet routing as well as the jumping trajectory of the out-of-plane routing are investigated. An electrostatic kinetic model is established for both routing ways, and the simulation based on this model predicts well the experimental dependence. A few examples of cascaded free-space microfluidic transportation using the 3D photovoltaic router are demonstrated, showing the potential of this technique in future biological applications.

Cancer-Nano-Interaction: From Cellular Uptake to Mechanobiological Responses

With the advancement of nanotechnology, the nano-bio-interaction field has emerged. It is essential to enhance our understanding of nano-bio-interaction in different aspects to design nanomedicines and improve their efficacy for therapeutic and diagnostic applications. Many researchers have extensively studied the toxicological responses of cancer cells to nano-bio-interaction, while their mechanobiological responses have been less investigated. The mechanobiological properties of cells such as elasticity and adhesion play vital roles in cellular functions and cancer progression. Many studies have noticed the impacts of cellular uptake on the structural organization of cells and, in return, the mechanobiology of human cells. Mechanobiological changes induced by the interactions of nanomaterials and cells could alter cellular functions and influence cancer progression. Hence, in addition to biological responses, the possible mechanobiological responses of treated cells should be monitored as a standard methodology to evaluate the efficiency of nanomedicines. Studying the cancer-nano-interaction in the context of cell mechanics takes our knowledge one step closer to designing safe and intelligent nanomedicines. In this review, we briefly discuss how the characteristic properties of nanoparticles influence cellular uptake. Then, we provide insight into the mechanobiological responses that may occur during the nano-bio-interactions, and finally, the important measurement techniques for the mechanobiological characterizations of cells are summarized and compared. Understanding the unknown mechanobiological responses to nano-bio-interaction will help with developing the application of nanoparticles to modulate cell mechanics for controlling cancer progression.

3D printed microfluidic lab-on-a-chip device for fiber-based dual beam optical manipulation

3D printing of microfluidic lab-on-a-chip devices enables rapid prototyping of robust and complex structures. In this work, we designed and fabricated a 3D printed lab-on-a-chip device for fiber-based dual beam optical manipulation. The final 3D printed chip offers three key features, such as (1) an optimized fiber channel design for precise alignment of optical fibers, (2) an optically clear window to visualize the trapping region, and (3) a sample channel which facilitates hydrodynamic focusing of samples. A square zig–zag structure incorporated in the sample channel increases the number of particles at the trapping site and focuses the cells and particles during experiments when operating the chip at low Reynolds number. To evaluate the performance of the device for optical manipulation, we implemented on-chip, fiber-based optical trapping of different-sized microscopic particles and performed trap stiffness measurements. In addition, optical stretching of MCF-7 cells was successfully accomplished for the purpose of studying the effects of a cytochalasin metabolite, pyrichalasin H, on cell elasticity. We observed distinct changes in the deformability of single cells treated with pyrichalasin H compared to untreated cells. These results demonstrate that 3D printed microfluidic lab-on-a-chip devices offer a cost-effective and customizable platform for applications in optical manipulation.

Hydrophobic-substrate based water-microdroplet manipulation through the long-range photovoltaic interaction from a distant LiNbO3:Fe crystal

Development of photovoltaic water-microdroplet manipulation using LN:Fe crystals has to meet the requirement of the hybrid and heating-avoided design of biological lab-on-chips. To fulfill this, we demonstrate a successful manipulation of a water microdroplet on a hydrophobic substrate by utilizing the long-range photovoltaic interaction from a distant LN:Fe crystal (see Visualization 1). The maximal manipulation distance (MMD) is found to be dependent on the laser-illumination intensity at the LN:Fe crystal and it can be tuned up to a sub-centimeter level (∼4 mm). Basing on the two-center model of light-induced charge transport in the LN:Fe crystal, we establish an analytic model to describe the force balance during the microdroplet manipulation under a long-range photovoltaic interaction. Either shortening the manipulation distance or increasing the illumination intensity can enhance the photovoltaic interaction and increase the velocity of the microdroplet being manipulated. An abrupt shape change followed by a fast repelling movement of the water microdroplet is observed under a strong photovoltaic interaction (see Visualization 2).

Characterization of optofluidic devices for the sorting of sub-micrometer particles

In this work, we investigate methods of fabricating a device for the optical actuation of nanoparticles. To create the microfluidic channel, we pursued three fabrication methods: SU-8 to molded polydimethylsiloxane soft lithography, laser etching of glass, and deep reactive ion etching of fused silica. We measured the surface roughness of the etched sidewalls, and the laser power transmission through each device. We then measured the radiation pressure on 0.5-µm particles in the best-performing fabricated device (etched fused silica) and in a square glass capillary.

Visible-light-assisted condensation of ultrasonically atomized water vapor on LiNbO3:Fe crystals

Optically massive trapping of the moisture in the air into an adjacent surface is a potential technique in the fields of bacterial adhesion and microfluidic generation, which is quite important to the development of LN-based biological lab-on-chips. Here we demonstrate on a LiNbO₃:Fe substrate the visible-light-assisted condensation of the water vapor in a flowing stream created by an ultrasonic atomizer. Through analyzing the dynamic processes of the visible-light-assisted water condensation at different illumination intensities, it is found that the extent of the water condensation, the bending angle of water vapor trails and the interaction range of the condensation effect are highly dependent on the illumination intensity. According to these findings and the simulated trajectories of the water vapor stream at different illumination intensities, we propose that this visible-light-assisted water condensation is an aggregation process of tiny water droplets driven by the dielectrophoretic interaction of inhomogeneous photovoltaic field and also an electrostatic screening course of photovoltaic charges through the charged evaporation of condensed water. The prolonged condensation of water vapor after a high-intensity illumination and that of oil vapor at a super-low evaporation rate are also studied, and the agreement between the simulation and experimental results reinforces the above mechanism. The reported technique, employing the inexpensive, safe-for-cell visible laser beam, is quite convenient for the controllable generation of various biological microdroplets, and thus it is promising for the microfluidic functionality integration of LN-based biological lab-on-chips.

Sheath-less high throughput inertial separation of small microparticles in spiral microchannels with trapezoidal cross-section

Various mechanisms of different designs have emerged for the purpose of microparticle separation and cell sorting. The main goals behind such designs are to create high throughput and high purity sample isolation. In this study, high efficiency, high throughput and precise separation of microparticles under inertial lift and drag forces induced by trapezoidal curvilinear channels are reported. This work is the first to focus and recover 2 from 5 μm and 2 from 10 μm particles in spiral channels in a sheath-less flow device, which reduces the overall complexity of the system and allows for higher throughput. The new microfluidic chip design is fabricated in glass using femtosecond laser ablation. In addition, mathematical force calculations were conducted during the design phase of the microfluidic channels and compared with experiments. The results show a close prediction of the equilibrium position of the tested microparticles.

All-optical splitting of dielectric microdroplets by using a y-cut-LN-based anti-symmetrical sandwich structure

We demonstrate an all-optical active mode of dielectric microdroplet splitting in a sandwich structure consisting of two anti-symmetrical y-cut LN:Fe substrates. The dynamic process of the microdroplet splitting and the simulation of the electrostatic interaction inside the sandwich gap show that the combination of two anti-symmetrical substrates are capable to provide a sufficient dielectrophoretic force and to reduce the unbalance of the drag forces for a stable and efficient splitting of the microdroplet. The dependences of the splitting time on the illumination intensity and the initial microdroplet size are also studied, and the results show that the microdroplet splitting process is fully governed by the establishment of the superposed photovoltaic field inside the sandwich gap. A key ratio E_r/E₀, representing the microdroplet splitting difficulty for a given sandwich structure, is found linearly dependent on the initial microdroplet size. These points are quite important to the integration of splitting functionality on the LN-based microfluidic chip.

Enhanced inertial focusing of microparticles and cells by integrating trapezoidal microchambers in spiral microfluidic channels

In this work, manipulating width and equilibrium position of fluorescent microparticles in spiral microchannel fractionation devices by embedding microchambers along the last turn of a spiral is reported. Microchambers with different shapes and sizes were tested at Reynolds numbers between 15.7 and 156.6 (100-1000 μL min-1) to observe focusing of 2, 5 and 10 μm fluorescent microparticles. This paper also discusses the fabrication process of the microfluidic chips with femtosecond laser ablation on glass wafers, as well as a particle imaging velocimetry (μPIV) study of microparticle trajectories inside a microchamber. It could be demonstrated with an improved final design with inclined microchamber side walls, that the 2 μm particle equilibrium position is shifted towards the inner wall by ∼27 μm and the focusing line's width is reduced by ∼18 μm. Finally, Saccharomyces cerevisiae yeast cells were tested in the final chip and a cell focusing efficiency of 99.1% is achieved.

Direct writing of optical microresonators in a lab-on-a-chip for label-free biosensing

Whispering gallery mode (WGM) resonators are promising optical structures for microfluidic label-free biosensors mainly due to their high sensitivity, but from a practical point of view they present numerous constraints that make their use in real laboratory diagnosis application difficult. Herein we report on a monolithic lab on a chip fabricated by a hybrid femtosecond laser micromachining approach, for label-free biosensing. It consists of a polymer WGM microresonator sensor integrated inside a glass microfluidic chip, presenting a refractive index change sensitivity of 61 nm per RIU. The biosensing capabilities of the device have been demonstrated by exploiting the biotin-streptavidin binding affinity, obtaining a measurable minimum surface density increase of 67 × 10³ molecules per μm².

Microfluidic technologies in cell isolation and analysis for biomedical applications

Efficient platforms for cell isolation and analysis play an important role in applied and fundamental biomedical studies. As cells commonly have a size of around 10 microns, conventional handling approaches at a large scale are still challenged in precise control and efficient recognition of cells for further performance of isolation and analysis. Microfluidic technologies have become more prominent in highly efficient cell isolation for circulating tumor cells (CTCs) detection, single-cell analysis and stem cell separation, since microfabricated devices allow for the spatial and temporal control of complex biochemistries and geometries by matching cell morphology and hydrodynamic traps in a fluidic network, as well as enabling specific recognition with functional biomolecules in the microchannels. In addition, the fabrication of nano-interfaces in the microchannels has been increasingly emerging as a very powerful strategy for enhancing the capability of cell capture by improving cell-interface interactions. In this review, we focus on highlighting recent advances in microfluidic technologies for cell isolation and analysis. We also describe the general biomedical applications of microfluidic cell isolation and analysis, and finally make a prospective for future studies.

Continuous detection of micro-particles by fiber Bragg grating Fabry-Pérot flow cytometer

A novel method to detect different sizes of micro-particles using a fiber Bragg grating Fabry-Pérot (FBG-FP) flow cytometer is presented. The chip is composed of a FBG-FP cavity integrated in a microfluidic channel. Solution with three different sizes of polystyrene particles flowing through the channel induces variations in the transmission spectrum of the FBG-FP cavity. Theoretical and experimental data show that different sizes of particles reveal different resonant wavelengths with a good resonance shift sensitivity of 10^-5. Additionally, the chip is easy to fabricate and features with non-contact and label-free operation. This study demonstrates a promising potential of the FBG-FP flow cytometer in medical and biological sensing.

Particle Manipulation by Optical Forces in Microfluidic Devices

Since the pioneering work of Ashkin and coworkers, back in 1970, optical manipulation gained an increasing interest among the scientific community. Indeed, the advantages and the possibilities of this technique are unsubtle, allowing for the manipulation of small particles with a broad spectrum of dimensions (nanometers to micrometers size), with no physical contact and without affecting the sample viability. Thus, optical manipulation rapidly found a large set of applications in different fields, such as cell biology, biophysics, and genetics. Moreover, large benefits followed the combination of optical manipulation and microfluidic channels, adding to optical manipulation the advantages of microfluidics, such as a continuous sample replacement and therefore high throughput and automatic sample processing. In this work, we will discuss the state of the art of these optofluidic devices, where optical manipulation is used in combination with microfluidic devices. We will distinguish on the optical method implemented and three main categories will be presented and explored: (i) a single highly focused beam used to manipulate the sample, (ii) one or more diverging beams imping on the sample, or (iii) evanescent wave based manipulation.

Biophysical isolation and identification of circulating tumor cells

Isolation and enumeration of circulating tumor cells (CTCs) from blood is important for determining patient prognosis and monitoring treatment. Methods based on affinity to cell surface markers have been applied to both purify (via immunoseparation) and identify (via immunofluorescence) CTCs. However, variability of cell biomarker expression associated with tumor heterogeneity and evolution and cross-reactivity of antibody probes have long complicated CTC enrichment and immunostaining. Here, we report a truly label-free high-throughput microfluidic approach to isolate, enumerate, and characterize the biophysical properties of CTCs using an integrated microfluidic device. Vortex-mediated deformability cytometry (VDC) consists of an initial vortex region which enriches large CTCs, followed by release into a downstream hydrodynamic stretching region which deforms the cells. Visualization and quantification of cell deformation with a high-speed camera revealed populations of large (>15 μm diameter) and deformable (aspect ratio >1.2) CTCs from 16 stage IV lung cancer samples, that are clearly distinguished by increased deformability compared to contaminating blood cells and rare large cells isolated from healthy patients. The VDC technology demonstrated a comparable positive detection rate of putative CTCs above healthy baseline (93.8%) with respect to standard immunofluorescence (71.4%). Automation allows full enumeration of CTCs from a 10 mL vial of blood within <1 h after sample acquisition, compared with 4+ hours with standard approaches. Moreover, cells are released into any collection vessel for further downstream analysis. VDC shows potential for accurate CTC enumeration without labels and confirms the unique highly deformable biophysical properties of large CTCs circulating in blood.

Optofluidic light modulator integrated in lab-on-a-chip

Microfluidic lenses are relevant optical components for sensing application in lab-on-a-chip devices, guaranteeing a robust alignment of the elements, a high level of compactness and tunable optical properties. In this work we describe an innovative integrated in-plane microfluidic lens. The device shows both an optimized shape capable of reducing spherical aberrations and periodically tunable optical properties. Indeed through the combination of the lens with a droplet generator module, we have been able to obtain an integrated optofluidic modulator capable of both on-demand on/off switching and periodic modulation of light. The device possesses a simple 3D geometry, which has been realized by exploiting the 3D capability of the femtosecond laser micromachining fabrication technique.

Integrated Optofluidic Chip for Low-Volume Fluid Viscosity Measurement

In the present work, an integrated optofluidic chip for fluid viscosity measurements in the range from 1 mPa·s to 100 mPa·s is proposed. The device allows the use of small sample volumes (<1 µL) and the measurement of viscosity as a function of temperature. Thanks to the precise control of the force exerted on dielectric spheres by optical beams, the viscosity of fluids is assessed by comparing the experimentally observed movement of dielectric beads produced by the optical forces with that expected by numerical calculations. The chip and the developed technique are validated by analyzing several fluids, such as Milli-Q water, ethanol and water–glycerol mixtures. The results show a good agreement between the experimental values and those reported in the literature. The extremely reduced volume of the sample required and the high flexibility of this technique make it a good candidate for measuring a wide range of viscosity values as well as for the analysis of nonlinear viscosity in complex fluids.

Ultrafast Laser Fabrication of Functional Biochips: New Avenues for Exploring 3D Micro- and Nano-Environments

Lab-on-a-chip biological platforms have been intensively developed during the last decade since emerging technologies have offered possibilities to manufacture reliable devices with increased spatial resolution and 3D configurations. These biochips permit testing chemical reactions with nanoliter volumes, enhanced sensitivity in analysis and reduced consumption of reagents. Due to the high peak intensity that allows multiphoton absorption, ultrafast lasers can induce local modifications inside transparent materials with high precision at micro- and nanoscale. Subtractive manufacturing based on laser internal modification followed by wet chemical etching can directly fabricate 3D micro-channels in glass materials. On the other hand, additive laser manufacturing by two-photon polymerization of photoresists can grow 3D polymeric micro- and nanostructures with specific properties for biomedical use. Both transparent materials are ideal candidates for biochips that allow exploring phenomena at cellular levels while their processing with a nanoscale resolution represents an excellent opportunity to get more insights on biological aspects. We will review herein the laser fabrication of transparent microfluidic and optofluidic devices for biochip applications and will address challenges associated with their potential. In particular, integrated micro- and optofluidic systems will be presented with emphasis on the functionality for biological applications. It will be shown that ultrafast laser processing is not only an instrument that can tailor appropriate 3D environments to study living microorganisms and to improve cell detection or sorting but also a tool to fabricate appropriate biomimetic structures for complex cellular analyses. New advances open now the avenue to construct miniaturized organs of desired shapes and configurations with the goal to reproduce life processes and bypass in vivo animal or human testing.

Photo-assisted splitting of dielectric microdroplets in a LN-based sandwich structure

We demonstrate an active and controllable photo-assisted splitting of dielectric microdroplets inside a LiNbO₃-based sandwich structure by utilizing the pyroelectric and photovoltaic effects of LiNbO₃ crystals. Basing on electrostatic simulation results, the mechanism of the photo-assisted splitting is explained; meanwhile, the microdoplet pre-polarizing and the inclined electrostatic force induced by the photovoltaic charges inside the sandwich structure are emphasized. We also study the dependence of the splitting part size on the substrate temperature variation, the irradiation intensity, and the duration. Featured dependence is found to follow the theoretical curve predicted by the compensation between the photo-excited and bound surface charges.

The physical origins of transit time measurements for rapid, single cell mechanotyping

The mechanical phenotype or 'mechanotype' of cells is emerging as a potential biomarker for cell types ranging from pluripotent stem cells to cancer cells. Using a microfluidic device, cell mechanotype can be rapidly analyzed by measuring the time required for cells to deform as they flow through constricted channels. While cells typically exhibit deformation timescales, or transit times, on the order of milliseconds to tens of seconds, transit times can span several orders of magnitude and vary from day to day within a population of single cells; this makes it challenging to characterize different cell samples based on transit time data. Here we investigate how variability in transit time measurements depends on both experimental factors and heterogeneity in physical properties across a population of single cells. We find that simultaneous transit events that occur across neighboring constrictions can alter transit time, but only significantly when more than 65% of channels in the parallel array are occluded. Variability in transit time measurements is also affected by the age of the device following plasma treatment, which could be attributed to changes in channel surface properties. We additionally investigate the role of variability in cell physical properties. Transit time depends on cell size; by binning transit time data for cells of similar diameters, we reduce measurement variability by 20%. To gain further insight into the effects of cell-to-cell differences in physical properties, we fabricate a panel of gel particles and oil droplets with tunable mechanical properties. We demonstrate that particles with homogeneous composition exhibit a marked reduction in transit time variability, suggesting that the width of transit time distributions reflects the degree of heterogeneity in subcellular structure and mechanical properties within a cell population. Our results also provide fundamental insight into the physical underpinnings of transit measurements: transit time depends strongly on particle elastic modulus, and weakly on viscosity and surface tension. Based on our findings, we present a comprehensive methodology for designing, analyzing, and reducing variability in transit time measurements; this should facilitate broader implementation of transit experiments for rapid mechanical phenotyping in basic research and clinical settings.

Dielectrophoretic behaviours of microdroplet sandwiched between LN substrates

We demonstrate a sandwich configuration for microfluidic manipulation in LiNbO₃ platform based on photovoltaic effect, and the behaviours of dielectric microdroplet under this sandwich configuration are investigated. It is found that the microdroplet can generate in the form of liquid bridge inside the LiNbO₃-based sandwich structure under the governing dielectrophoretic force, and the dynamic process of microdroplet generation highly depends on the substrate combinations. Dynamic features found for different combinations are explained by the different electrostatic field distribution basing on the finite-element simulation results. Moreover, the electrostatic field required by the microdroplet generation is estimated through meniscus evolution and it is found in good agreement with the simulated electrostatic field inside the sandwich gap. Several kinds of microdroplet manipulations are attempted in this work. We suggest that the local dielectrophoretic force acting on the microdroplet depends on the distribution of the accumulated irradiation dosage. Without using any additional pumping or jetting actuator, the microdroplet can be step-moved, deformed or patterned by the inconsecutive dot-irradiation scheme, as well as elastically stretched out and back or smoothly guided in a designed pass by the consecutive line-irradiation scheme.

A Comprehensive Review of Optical Stretcher for Cell Mechanical Characterization at Single-Cell Level

This paper presents a comprehensive review of the development of the optical stretcher, a powerful optofluidic device for single cell mechanical study by using optical force induced cell stretching. The different techniques and the different materials for the fabrication of the optical stretcher are first summarized. A short description of the optical-stretching mechanism is then given, highlighting the optical force calculation and the cell optical deformability characterization. Subsequently, the implementations of the optical stretcher in various cell-mechanics studies are shown on different types of cells. Afterwards, two new advancements on optical stretcher applications are also introduced: the active cell sorting based on cell mechanical characterization and the temperature effect on cell stretching measurement from laser-induced heating. Two examples of new functionalities developed with the optical stretcher are also included. Finally, the current major limitation and the future development possibilities are discussed.

Selective plane illumination microscopy on a chip

Selective plane illumination microscopy can image biological samples at a high spatiotemporal resolution. Complex sample preparation and system alignment normally limit the throughput of the method. Using femtosecond laser micromachining, we created an integrated optofluidic device that allows obtaining continuous flow imaging, three-dimensional reconstruction and high-throughput analysis of large multicellular spheroids at a subcellular resolution.

Nanomechanical probing of soft matter through hydrophobic AFM tips fabricated by two-photon polymerization

Atomic force microscopy (AFM) nanoindentation of soft materials is a powerful tool for probing mechanical properties of biomaterials. Though many results have been reported in this field over the last decade, adhesion forces between the tip and the sample hinder the elastic modulus measurement when hydrophilic soft samples are investigated. Here, two-photon polymerization (2PP) technology was used to fabricate hydrophobic perfluoropolyether-based AFM tips. The hydrophobic 2PP tips allowed us to overcome the limitations of commercial and functionalized tips as well as to successfully measure the elastic modulus of medically relevant soft materials in air. Our results obtained in the characterization of poly(dimethyl siloxane) and polyethylene glycol hydrogels showed lower adhesion forces over a larger measurement range when compared to measurements performed with commercial tips. The elastic moduli measured by means of hydrophobic 2PP AFM tips were also found to be comparable to those obtained using conventional techniques for macroscopic samples. We successfully showed that the hydrophobic AFM tips developed by this highly versatile technology enable the study of mechanical properties of soft matter, benefiting from reduced sample-tip interactions, and a custom-made shape and dimension of the tips.

A comprehensive strategy for the analysis of acoustic compressibility and optical deformability on single cells

We realized an integrated microfluidic chip that allows measuring both optical deformability and acoustic compressibility on single cells, by optical stretching and acoustophoresis experiments respectively. Additionally, we propose a measurement protocol that allows evaluating the experimental apparatus parameters before performing the cell-characterization experiments, including a non-destructive method to characterize the optical force distribution inside the microchannel. The chip was used to study important cell-mechanics parameters in two human breast cancer cell lines, MCF7 and MDA-MB231. Results indicate that MDA-MB231 has both higher acoustic compressibility and higher optical deformability than MCF7, but statistical analysis shows that optical deformability and acoustic compressibility are not correlated parameters. This result suggests the possibility to use them to analyze the response of different cellular structures. We also demonstrate that it is possible to perform both measurements on a single cell, and that the order of the two experiments does not affect the retrieved values.

An optofluidic constriction chip for monitoring metastatic potential and drug response of cancer cells

Cellular mechanical properties constitute good markers to characterize tumor cells, to study cell population heterogeneity and to highlight the effect of drug treatments. In this work, we describe the fabrication and validation of an integrated optofluidic chip capable of analyzing cellular deformability on the basis of the pressure gradient needed to push a cell through a narrow constriction. We demonstrate the ability of the chip to discriminate between tumorigenic and metastatic breast cancer cells (MCF7 and MDA-MB231) and between human melanoma cells with different metastatic potential (A375P and A375MC2). Moreover, we show that this chip allows highlighting the effect of drugs interfering with microtubule organization (paclitaxel, combretastatin A-4 and nocodazole) on cancer cells, which leads to changes in the pressure-gradient required to push cells through the constriction. Our single-cell microfluidic device for mechanical evaluation is compact and easy to use, allowing for an extensive use in different laboratory environments.

Digital Microfluidics for Manipulation and Analysis of a Single Cell

The basic structural and functional unit of a living organism is a single cell. To understand the variability and to improve the biomedical requirement of a single cell, its analysis has become a key technique in biological and biomedical research. With a physical boundary of microchannels and microstructures, single cells are efficiently captured and analyzed, whereas electric forces sort and position single cells. Various microfluidic techniques have been exploited to manipulate single cells through hydrodynamic and electric forces. Digital microfluidics (DMF), the manipulation of individual droplets holding minute reagents and cells of interest by electric forces, has received more attention recently. Because of ease of fabrication, compactness and prospective automation, DMF has become a powerful approach for biological application. We review recent developments of various microfluidic chips for analysis of a single cell and for efficient genetic screening. In addition, perspectives to develop analysis of single cells based on DMF and emerging functionality with high throughput are discussed.

Investigation of temperature effect on cell mechanics by optofluidic microchips

Here we present the results of a study concerning the effect of temperature on cell mechanical properties. Two different optofluidic microchips with external temperature control are used to investigate the temperature-induced changes of highly metastatic human melanoma cells (A375MC2) in the range of ~0 - 35 °C. By means of an integrated optical stretcher, we observe that cells' optical deformability is strongly enhanced by increasing cell and buffer-fluid temperature. This finding is supported by the results obtained from a second device, which probes the cells' ability to be squeezed through a constriction. Measured data demonstrate a marked dependence of cell mechanical properties on temperature, thus highlighting the importance of including a proper temperature-control system in the experimental apparatus.