Physiological monitoring of optically trapped cells: assessing the effects of confinement by 1064-nm laser tweezers using microfluorometry

The bacterial flagellum as an object for optical trapping

This letter considers the possibility of using the optical trap to study the structure and function of the microbial flagellum. The structure of the flagellum of a typical gram-negative bacterium is described in brief. A standard mathematical model based on the principle of superposition is used to describe the movement of an ellipsoidal microbial cell in a liquid medium. The basic principles of optical trapping based on the combined action of the light pressure and the gradient force are briefly clarified. Several problems related to thermal damage of living microscopic objects when the latter gets to the focus of a laser beam are shortly discussed. It is shown that the probability of cell damage depends nonlinearly on the wavelength of laser radiation. Finally, the model systems that would make it possible to study flagella of the free bacteria and the ones anchored or tethered on the surface of a solid material are discussed in detail.

Time and power dependence of laser-induced photodamage on human sperm revealed by longitudinal rolling measurement using optical tweezers

Lasers are widely applied in assisted reproductive technologies, including sperm fixation, sperm selection and intracytoplasmic sperm injections, to reduce procedure time and improve consistency and reproducibility. However, quantitative studies on laser-induced photodamage of sperm are lacking. In this study, we demonstrated that, by using optical tweezers, the kinematic parameters of freely swimming sperm are correlated with the frequency as well as the percentage of pausing duration of longitudinal rolling of the same sperm head in the optical trap. Furthermore, by trapping individual sperm cells using 1064-nm optical tweezers, we quantitatively characterized the time-dependence of longitudinal rolling frequency and percentage of pausing duration of sperm under different laser powers. Our study revealed that, as trapping time and the laser power time increase, the longitudinal rolling frequency of the optically trapped sperm decreases with an increasing percentage of pausing duration, which characterizes the effect of laser power and duration on the photodamage of individual sperm cells. Our study provides experimental basis for the optimization of laser application in assisted reproductive technology, which may reduce the photodamage-induced biosafety risk in the future.

Applications of laser technology in the manipulation of human spermatozoa

The application of laser technology in the field of assisted reproductive technology (ART) has experienced rapid growth over the past decades owing to revolutionary techniques such as intracytoplasmic sperm injection (ICSI), preimplantation genetic testing (PGT), and in vitro manipulation of gametes and embryos. For male gametes, in vitro manipulation techniques include spermatozoa selection, sorting, immobilization, and quality assessment. A number of studies have been conducted to investigate the application of different laser technologies in the manipulation of human spermatozoa. However, there is a lack of a unified understanding of laser application in the in vitro manipulation of sperm and safety considerations in ART and, subsequently, the inability to make clear and accurate decisions on the clinical value of these laser technologies. This review summarizes the advancements and improvements of laser technologies in the manipulation of human spermatozoa, such as photobiomodulation therapy, laser trap systems for sperm analysis and sorting, laser-assisted selection of immotile sperm and laser-assisted immobilization of sperm prior to ICSI. The safety of those technologies used in ART is also discussed. This review will provide helpful and comprehensive insight into the applications of laser technology in the manipulation of human spermatozoa.

Gentle label-free nonlinear optical imaging relaxes linear-absorption-mediated triplet

Sample health is critical for live-cell fluorescence microscopy and has promoted light-sheet microscopy that restricts its ultraviolet-visible excitation to one plane inside a three-dimensional sample. It is thus intriguing that laser-scanning nonlinear optical microscopy, which similarly restricts its near-infrared excitation, has not broadly enabled gentle label-free molecular imaging. We hypothesize that intense near-infrared excitation induces phototoxicity via linear absorption of intrinsic biomolecules with subsequent triplet buildup, rather than the commonly assumed mechanism of nonlinear absorption. Using a reproducible phototoxicity assay based on the time-lapse elevation of auto-fluorescence (hyper-fluorescence) from a homogeneous tissue model (chicken breast), we provide strong evidence supporting this hypothesis. Our study justifies a simple imaging technique, e.g., rapidly scanned sub-80-fs excitation with full triplet-relaxation, to mitigate this ubiquitous linear-absorption-mediated phototoxicity independent of sample types. The corresponding label-free imaging can track freely moving C. elegans in real-time at an irradiance up to one-half of water optical breakdown.

Deep learning-based method for analyzing the optically trapped sperm rotation

Optical tweezers exert a strong trapping force on cells, making it crucial to analyze the movement of trapped cells. The rotation of cells plays a significant role in their swimming patterns, such as in sperm cells. We proposed a fast deep-learning-based method that can automatically determine the projection orientation of ellipsoidal-like cells without additional optical design. This method was utilized for analyzing the planar rotation of trapped sperm cells using an optical tweezer, demonstrating its feasibility in extracting the rotation of the cell head. Furthermore, we employed this method to investigate sperm cell activity by examining variations in sperm rotation rates under different conditions, including temperature and laser output power. Our findings provide evidence for the effectiveness of this method and the rotation analysis method developed may have clinical potential for sperm quality evaluation.

Non-Invasive Dynamic Reperfusion of Microvessels In Vivo Controlled by Optical Tweezers

Distributive shock is considered to be a condition of microvascular hypoperfusion, which can be fatal in severe cases. However, traditional therapeutic methods to restore the macro blood flow are difficult to accurately control the blood perfusion of microvessels, and the currently developed manipulation techniques are inevitably incompatible with biological systems. In our approach, infrared optical tweezers are used to dynamically control the microvascular reperfusion within subdermal capillaries in the pinna of mice. Furthermore, we estimate the effect of different optical trap positions on reperfusion at branch and investigate the effect of the laser power on reperfusion. The results demonstrate the ability of optical tweezers to control microvascular reperfusion. This strategy allows near-noninvasive reperfusion of the microvascular hypoperfusion in vivo. Hence, our work is expected to provide unprecedented insights into the treatment of distributive shock.

Simulation of thermal field distribution in biological tissue and cell culture media irradiated with infrared wavelengths

In recent years, there has been a growing interest in the singlet form of oxygen as a regulator of the physiological functions of cells. One of the ways to generate singlet oxygen is direct optical excitation of the triplet oxygen form. Since molecular oxygen weakly absorbs light, high power is required to obtain sufficient concentrations of singlet oxygen. However, the increase in the radiation power of laser can induce a local temperature increase around the laser spot. This may be critical considering the temperature governs every biological reaction within living cells, in particular. Here, the interaction of laser radiation of infrared wavelengths, generating singlet oxygen, with biological tissues and cell culture media was simulated. Using the COMSOL Multiphysics software, the thermal field distribution in the volume of skin, brain tissue and cell culture media was obtained depending on the wavelength, power and exposure time. The results demonstrate the importance of taking temperature into account when conducting experimental studies at the cellular and organismal levels.

A Critical Review on the Sensing, Control, and Manipulation of Single Molecules on Optofluidic Devices

Single-molecule techniques have shifted the paradigm of biological measurements from ensemble measurements to probing individual molecules and propelled a rapid revolution in related fields. Compared to ensemble measurements of biomolecules, single-molecule techniques provide a breadth of information with a high spatial and temporal resolution at the molecular level. Usually, optical and electrical methods are two commonly employed methods for probing single molecules, and some platforms even offer the integration of these two methods such as optofluidics. The recent spark in technological advancement and the tremendous leap in fabrication techniques, microfluidics, and integrated optofluidics are paving the way toward low cost, chip-scale, portable, and point-of-care diagnostic and single-molecule analysis tools. This review provides the fundamentals and overview of commonly employed single-molecule methods including optical methods, electrical methods, force-based methods, combinatorial integrated methods, etc. In most single-molecule experiments, the ability to manipulate and exercise precise control over individual molecules plays a vital role, which sometimes defines the capabilities and limits of the operation. This review discusses different manipulation techniques including sorting and trapping individual particles. An insight into the control of single molecules is provided that mainly discusses the recent development of electrical control over single molecules. Overall, this review is designed to provide the fundamentals and recent advancements in different single-molecule techniques and their applications, with a special focus on the detection, manipulation, and control of single molecules on chip-scale devices.

Temperature Effects on Optical Trapping Stability

In recent years, optically trapped luminescent particles have emerged as a reliable probe for contactless thermal sensing because of the dependence of their luminescence on environmental conditions. Although the temperature effect in the optical trapping stability has not always been the object of study, the optical trapping of micro/nanoparticles above room temperature is hindered by disturbances caused by temperature increments of even a few degrees in the Brownian motion that may lead to the release of the particle from the trap. In this report, we summarize recent experimental results on thermal sensing experiments in which micro/nanoparticles are used as probes with the aim of providing the contemporary state of the art about temperature effects in the stability of potential trapping processes.

Infrared Excitation Induces Heating and Calcium Microdomain Hyperactivity in Cortical Astrocytes

Unraveling how neural networks process and represent sensory information and how these cellular signals instruct behavioral output is a main goal in neuroscience. Two-photon activation of optogenetic actuators and calcium (Ca²⁺) imaging with genetically encoded indicators allow, respectively, the all-optical stimulation and readout of activity from genetically identified cell populations. However, these techniques locally expose the brain to high near-infrared light doses, raising the concern of light-induced adverse effects on the biology under study. Combining 2P imaging of Ca²⁺ transients in GCaMP6f-expressing cortical astrocytes and unbiased machine-based event detection, we demonstrate the subtle build-up of aberrant microdomain Ca²⁺ transients in the fine astroglial processes that depended on the average rather than peak laser power. Illumination conditions routinely being used in biological 2P microscopy (920-nm excitation, ∼100-fs, and ∼10 mW average power) increased the frequency of microdomain Ca²⁺ events but left their amplitude, area, and duration largely unchanged. Ca²⁺ transients in the otherwise silent soma were secondary to this peripheral hyperactivity that occurred without overt morphological damage. Continuous-wave (nonpulsed) 920-nm illumination at the same average power was as damaging as femtosecond pulses, unraveling the dominance of a heating-mediated damage mechanism. In an astrocyte-specific inositol 3-phosphate receptor type-2 knockout mouse, near-infrared light-induced Ca²⁺ microdomains persisted in the small processes, underpinning their resemblance to physiological inositol 3-phosphate receptor type-2-independent Ca²⁺ signals, whereas somatic hyperactivity was abolished. We conclude that, contrary to what has generally been believed in the field, shorter pulses and lower average power can help to alleviate damage and allow for longer recording windows at 920 nm.

Spatially selective manipulation of cells with single-beam acoustical tweezers

Acoustical tweezers open major prospects in microbiology for cells and microorganisms contactless manipulation, organization and mechanical properties testing since they are biocompatible, label-free and have the potential to exert forces several orders of magnitude larger than their optical counterpart at equivalent power. Yet, these perspectives have so far been hindered by the absence of spatial selectivity of existing acoustical tweezers - i.e., the ability to select and move objects individually - and/or their limited resolution restricting their use to large particle manipulation only and/or finally the limited forces that they could apply. Here, we report precise selective manipulation and positioning of individual human cells in a standard microscopy environment with trapping forces up to ~200 pN without altering their viability. These results are obtained with miniaturized acoustical tweezers combining holography with active materials to synthesize specific wavefields called focused acoustical vortices designed to produce stiff localized traps with reduced acoustic power.

Optical Tweezers in Studies of Red Blood Cells

Optical tweezers (OTs) are innovative instruments utilized for the manipulation of microscopic biological objects of interest. Rapid improvements in precision and degree of freedom of multichannel and multifunctional OTs have ushered in a new era of studies in basic physical and chemical properties of living tissues and unknown biomechanics in biological processes. Nowadays, OTs are used extensively for studying living cells and have initiated far-reaching influence in various fundamental studies in life sciences. There is also a high potential for using OTs in haemorheology, investigations of blood microcirculation and the mutual interplay of blood cells. In fact, in spite of their great promise in the application of OTs-based approaches for the study of blood, cell formation and maturation in erythropoiesis have not been fully explored. In this review, the background of OTs, their state-of-the-art applications in exploring single-cell level characteristics and bio-rheological properties of mature red blood cells (RBCs) as well as the OTs-assisted studies on erythropoiesis are summarized and presented. The advance developments and future perspectives of the OTs' application in haemorheology both for fundamental and practical in-depth studies of RBCs formation, functional diagnostics and therapeutic needs are highlighted.

Super-low-power optical trapping of a single nanoparticle

We propose and demonstrate a simple approach for noncontact, three-dimensional, and stable trapping of a single nanoparticle with a super-low incident laser power (0.7 mW) via the single-fiber optical tweezers. We splice a section of single-mode fiber and a section of multimode fiber to construct a Bessel-like beam, which produces narrow output laser beams. We integrate a high-refractive-index glass microsphere on the tip of the multimode fiber to focus the narrow output laser beams. The focused beams provide a nanoscale optical trap for a single nanoparticle (polystyrene sphere, diameter of 200 nm). This optical fiber probe has the advantages of high laser transmission efficiency, high spatial resolution, and minimum joule heating. The proposed approach extends the application potential of fiber-based optical manipulations, such as nanoparticle sorting, single-cell organelle analysis, and bio-sensing.

Precise and non-invasive circulating tumor cell isolation based on optical force using homologous erythrocyte binding

Precise isolation of circulating tumor cells (CTCs) is proved to be significant for early cancer diagnosis and downstream analysis. Most of the existing strategies yield low purity or cause unexpected damage to cells because of foreign material introduction. To avoid foreign material caused damage and achieve high efficiency simultaneously, this work presents an innovative strategy using tumor cell targeting molecules to bind homologous red blood cells (RBCs) with tumor cells, which results in obvious optical constant differences (both size and mean refractive index) between CC-RBCs (RBC conjugated CTCs) and other blood cells. Then the modified CTCs can be precisely separated under laser illumination in an optofluidic system. Experiments show that CTCs are efficiently modified with erythrocytes and finally isolated from blood at high purity (more than 92%) and a high recovery rate (over 90%). In the whole process, CTCs are proved to keep membrane and function integrity. The combination of homologous RBC binding and an optofluidic system will provide a convenient tool for cancer early diagnosis and treatment monitoring, which exhibits good performance in CTC non-invasive and precise isolation, thus showing great potential.

Optical Tweezers: Phototoxicity and Thermal Stress in Cells and Biomolecules

For several decades optical tweezers have proven to be an invaluable tool in the study and analysis of myriad biological responses and applications. However, as with every tool, they can have undesirable or damaging effects upon the very sample they are helping to study. In this review the main negative effects of optical tweezers upon biostructures and living systems will be presented. There are three main areas on which the review will focus: linear optical excitation within the tweezers, non-linear photonic effects, and thermal load upon the sampled volume. Additional information is provided on negative mechanical effects of optical traps on biological structures. Strategies to avoid or, at least, minimize these negative effects will be introduced. Finally, all these effects, undesirable for the most, can have positive applications under the right conditions. Some hints in this direction will also be discussed.

Translational and rotational manipulation of filamentous cells using optically driven microrobots

Optical cell manipulation has become increasingly valuable in cell-based assays. In this paper, we demonstrate the translational and rotational manipulation of filamentous cells using multiple cooperative microrobots automatically driven by holographic optical tweezers. The photodamage of the cells due to direct irradiation of the laser beam can be effectively avoided. The proposed method will enable fruitful biomedical applications where precise cell manipulation and less photodamage are required.

Soft trapping lasts longer: Dwell time of a Brownian particle varied by potential shape

It is often regarded that the dwell time (or residence time, escape time, trapping duration) of trapped Brownian particles is described by the multiplication of two separate factors, i.e., the diffusive traveling time of the trapping domain size without taking into account the trapping force, and the stochastic event of overcoming the trapping energy by thermal one instantaneously. However, we show that the ratio of dwell time to the typical traveling time for the trapping domain size depends on the shape of the force field. The shape of the trapping potential affects this ratio even if the trapping energy gap is the same and the smooth potential has a single minimum. Our finding suggests the possible application of the potential shape to realize the desired trapping characteristics.

Caring for cells in microsystems: principles and practices of cell-safe device design and operation

Microfluidic device designers and users continually question whether cells are 'happy' in a given microsystem or whether they are perturbed by micro-scale technologies. This issue is normally brought up by engineers building platforms, or by external reviewers (academic or commercial) comparing multiple technological approaches to a problem. Microsystems can apply combinations of biophysical and biochemical stimuli that, although essential to device operation, may damage cells in complex ways. However, assays to assess the impact of microsystems upon cells have been challenging to conduct and have led to subjective interpretation and evaluation of cell stressors, hampering development and adoption of microsystems. To this end, we introduce a framework that defines cell health, describes how device stimuli may stress cells, and contrasts approaches to measure cell stress. Importantly, we provide practical guidelines regarding device design and operation to minimize cell stress, and recommend a minimal set of quantitative assays that will enable standardization in the assessment of cell health in diverse devices. We anticipate that as microsystem designers, reviewers, and end-users enforce such guidelines, we as a community can create a set of essential principles that will further the adoption of such technologies in clinical, translational and commercial applications.

Precise label-free leukocyte subpopulation separation using hybrid acoustic-optical chip

Leukocyte subpopulations contain crucial physiological information; hence, precise and specific leukocyte separation is very important for leukemia diagnosis and analysis. However, conventional centrifugation and immunofluorescence-based separation methods are inaccurate and inconvenient due to the overlapping cell size and density or complex marking processes. Herein, we report a new label-free technology for precise leukocyte subpopulation separation by synergy of acoustic and optical technologies. Standing surface acoustic wave (SSAW) solved the problem of gentle and precise focusing of cells in optical systems. In addition, SSAW was used for the separation of granulocytes, which have evident size distinction from other components. In case of lymphocytes and monocytes, which have overlap in size/density, optical force could distinguish them accurately based on the RI difference, with the convenience of acoustic pre-focusing. In this experiment, separation of three types of leukocyte subtypes with considerable throughput and purity was conducted, through which we obtained 99% pure lymphocytes, 98% pure monocytes, and 95% pure granulocytes. Experimental results prove that the device has robust ability in separating leukocyte phenotypes and have the advantages of being non-invasive, label-free and precise. In the future, this convenient hybrid method will be a potential powerful tool for auxiliary clinical diagnosis and analysis.

On-the-Fly Calibrated Measure and Remote Control of Temperature and Viscosity at Nanoscale

A novel on-the-fly calibration method of optical tweezers is presented, which enables in situ control and measure of absolute temperature and viscosity at nanoscale dimensions. Such noncontact measurement and control at the nanoscale are challenging as the present techniques only provide off-line measurements that do not provide absolute values. Additionally, some of the present methods have a low spatial resolution. We simultaneously apply the high temporal sensitivity of position autocorrelation and equipartition theorem to precisely measure and control in situ temperature and the corresponding microrheological property around the focal volume of the trap at high spatial resolution. The femtosecond optical tweezers (FOTs) use a single-beam high repetition rate laser for optical trapping to result in finer temperature gradients in comparison to the continuous-wave laser tweezers. Such finer temperature gradients are due to the additional nonlinear optical (NLO) phenomena occurring only at the nanoscale focal plane of the FOTs. Because NLO processes are laser peak power-dependent, they promote an effective study of physical properties occurring only at the focal plane. Using FOTs at optically benign near-infrared wavelengths, we demonstrate microrheological control and measurement in water by adding a highly absorbing yet low fluorescent dye (IR780).

Single Cell Isolation Using Optical Tweezers

Optical tweezers offer a non-contact method for selecting single cells and translocating them from one microenvironment to another. We have characterized the optical tweezing of yeast S. cerevisiae and can manipulate single cells at 0.41 ± 0.06 mm/s using a 26.8 ± 0.1 mW from a 785 nm diode laser. We have fabricated and tested three cell isolation devices; a micropipette, a PDMS chip and a laser machined fused silica chip and we have isolated yeast, single bacteria and cyanobacteria cells. The most effective isolation was achieved in PDMS chips, where single yeast cells were grown and observed for 18 h without contamination. The duration of budding in S. cerevisiae was not affected by the laser parameters used, but the time from tweezing until the first budding event began increased with increasing laser energy (laser power × time). Yeast cells tweezed using 25.0 ± 0.1 mW for 1 min were viable after isolation. We have constructed a micro-consortium of yeast cells, and a co-culture of yeast and bacteria, using optical tweezers in combination with the PDMS network of channels and isolation chambers, which may impact on both industrial biotechnology and understanding pathogen dynamics.

Influence of experimental parameters on the laser heating of an optical trap

In optical tweezers, heating of the sample due to absorption of the laser light is a major concern as temperature plays an important role at microscopic scale. A popular rule of thumb is to consider that, at the typical wavelength of 1064 nm, the focused laser induces a heating rate of B = 1 °C/100 mW. We analysed this effect under different routine experimental conditions and found a remarkable variability in the temperature increase. Importantly, we determined that temperature can easily rise by as much as 4 °C at a relatively low power of 100 mW, for dielectric, non-absorbing particles with certain sets of specific, but common, parameters. Heating was determined from measurements of light momentum changes under drag forces at different powers, which proved to provide precise and robust results in watery buffers. We contrasted the experiments with computer simulations and obtained good agreement. These results suggest that this remarkable heating could be responsible for changes in the sample under study and could lead to serious damage of live specimens. It is therefore advisable to determine the temperature increase in each specific experiment and avoid the use of a universal rule that could inadvertently lead to critical changes in the sample.

Label-free analysis of the characteristics of a single cell trapped by acoustic tweezers

Single-cell analysis is essential to understand the physical and functional characteristics of cells. The basic knowledge of these characteristics is important to elucidate the unique features of various cells and causative factors of diseases and determine the most effective treatments for diseases. Recently, acoustic tweezers based on tightly focused ultrasound microbeam have attracted considerable attention owing to their capability to grab and separate a single cell from a heterogeneous cell sample and to measure its physical cell properties. However, the measurement cannot be performed while trapping the target cell, because the current method uses long ultrasound pulses for grabbing one cell and short pulses for interrogating the target cell. In this paper, we demonstrate that short ultrasound pulses can be used for generating acoustic trapping force comparable to that with long pulses by adjusting the pulse repetition frequency (PRF). This enables us to capture a single cell and measure its physical properties simultaneously. Furthermore, it is shown that short ultrasound pulses at a PRF of 167 kHz can trap and separate either one red blood cell or one prostate cancer cell and facilitate the simultaneous measurement of its integrated backscattering coefficient related to the cell size and mechanical properties.

Direct 1O2 optical excitation: A tool for redox biology

Molecular oxygen (O2) displays very interesting properties. Its first excited state, commonly known as singlet oxygen (1O2), is one of the so-called Reactive Oxygen Species (ROS). It has been implicated in many redox processes in biological systems. For many decades its role has been that of a deleterious chemical species, although very positive clinical applications in the Photodynamic Therapy of cancer (PDT) have been reported. More recently, many ROS, and also 1O2, are in the spotlight because of their role in physiological signaling, like cell proliferation or tissue regeneration. However, there are methodological shortcomings to properly assess the role of 1O2 in redox biology with classical generation procedures. In this review the direct optical excitation of O2 to produce 1O2 will be introduced, in order to present its main advantages and drawbacks for biological studies. This photonic approach can provide with many interesting possibilities to understand and put to use ROS in redox signaling and in the biomedical field.

Tweezing of Magnetic and Non-Magnetic Objects with Magnetic Fields

Although strong magnetic fields cannot be conveniently "focused" like light, modern microfabrication techniques enable preparation of microstructures with which the field gradients - and resulting magnetic forces - can be localized to very small dimensions. This ability provides the foundation for magnetic tweezers which in their classical variant can address magnetic targets. More recently, the so-called negative magnetophoretic tweezers have also been developed which enable trapping and manipulations of completely nonmagnetic particles provided that they are suspended in a high-magnetic-susceptibility liquid. These two modes of magnetic tweezing are complimentary techniques tailorable for different types of applications. This Progress Report provides the theoretical basis for both modalities and illustrates their specific uses ranging from the manipulation of colloids in 2D and 3D, to trapping of living cells, control of cell function, experiments with single molecules, and more.

Radiation forces on a Rayleigh dielectric sphere produced by highly focused parabolic scaling Bessel beams

The radiation forces on a Rayleigh dielectric particle induced by a highly focused parabolic scaling Bessel beam (PSBB) are investigated. Numerical results show that the zero-order PSBB can be used to trap a high-index particle at the focus and near the focus by the first-order PSBB. For the low-index particle, it can be guided or confined in the dark core of the nonzero-order PSBB but cannot be stably trapped in this single-beam trap. Further, we analyze the condition of trapping stability. It is found that the lower limit in the particle radius for stable trapping is different for different orders.

Energy-Looping Nanoparticles: Harnessing Excited-State Absorption for Deep-Tissue Imaging

Near infrared (NIR) microscopy enables noninvasive imaging in tissue, particularly in the NIR-II spectral range (1000-1400 nm) where attenuation due to tissue scattering and absorption is minimized. Lanthanide-doped upconverting nanocrystals are promising deep-tissue imaging probes due to their photostable emission in the visible and NIR, but these materials are not efficiently excited at NIR-II wavelengths due to the dearth of lanthanide ground-state absorption transitions in this window. Here, we develop a class of lanthanide-doped imaging probes that harness an energy-looping mechanism that facilitates excitation at NIR-II wavelengths, such as 1064 nm, that are resonant with excited-state absorption transitions but not ground-state absorption. Using computational methods and combinatorial screening, we have identified Tm(3+)-doped NaYF4 nanoparticles as efficient looping systems that emit at 800 nm under continuous-wave excitation at 1064 nm. Using this benign excitation with standard confocal microscopy, energy-looping nanoparticles (ELNPs) are imaged in cultured mammalian cells and through brain tissue without autofluorescence. The 1 mm imaging depths and 2 μm feature sizes are comparable to those demonstrated by state-of-the-art multiphoton techniques, illustrating that ELNPs are a promising class of NIR probes for high-fidelity visualization in cells and tissue.

Highly tunable plasmonic nanoring arrays for nanoparticle manipulation and detection

The advancement of trapping and detection of nano-objects at very low laser powers in the near-infra-red region (NIR) is crucial for many applications. Singular visible-light nano-optics based on abrupt phase changes have recently demonstrated a significant improvement in molecule detection. Here, we propose and demonstrate tunable plasmonic nanodevices, which can improve both the trapping field enhancement and detection of nano-objects using singular phase drops in the NIR range. The plasmonic nanostructures, which consist of gaps with dimensions 50 nm × 50 nm connecting nanorings in arrays is discussed. These gaps act as individual detection and trapping sites. The tunability of the system is evident from extinction and reflection spectra while increasing the aperture size in the arrays. Additionally, in the region where the plasmonic nano-array exhibits topologically-protected, near-zero reflection behaviour, the phase displays a rapid change. Our experimental data predict that, using this abrupt phase changes, one can improve the detection sensitivity by 10 times compared to the extinction spectra method. We finally report experimental evidence of 100 nm polystyrene beads trapping using low incident power on these devices. The overall design demonstrates strong capability as an optical, label-free, non-destructive tool for single molecule manipulation where low trapping intensity, minimal photo bleaching and high sensitivity is required.

Damage induced in red blood cells by infrared optical trapping: an evaluation based on elasticity measurements

We evaluated the damage caused to optically trapped red blood cells (RBCs) after 1 or 2 min of exposure to near-infrared (NIR) laser beams at 785 or 1064 nm. Damage was quantified by measuring cell elasticity using an automatic, real-time, homemade, optical tweezer system. The measurements, performed on a significant number (hundreds) of cells, revealed an overall deformability decrease up to ∼104% after 2 min of light exposure, under 10 mW optical trapping for the 785-nm wavelength. Wavelength dependence of the optical damage is attributed to the light absorption by hemoglobin. The results provided evidence that RBCs have their biomechanical properties affected by NIR radiation. Our findings establish limits for laser applications with RBCs.

Plasmon enhanced optical tweezers with gold-coated black silicon

Plasmonic optical tweezers are a ubiquitous tool for the precise manipulation of nanoparticles and biomolecules at low photon flux, while femtosecond-laser optical tweezers can probe the nonlinear optical properties of the trapped species with applications in biological diagnostics. In order to adopt plasmonic optical tweezers in real-world applications, it is essential to develop large-scale fabrication processes without compromising the trapping efficiency. Here, we develop a novel platform for continuous wave (CW) and femtosecond plasmonic optical tweezers, based on gold-coated black silicon. In contrast with traditional lithographic methods, the fabrication method relies on simple, single-step, maskless tabletop laser processing of silicon in water that facilitates scalability. Gold-coated black silicon supports repeatable trapping efficiencies comparable to the highest ones reported to date. From a more fundamental aspect, a plasmon-mediated efficiency enhancement is a resonant effect, and therefore, dependent on the wavelength of the trapping beam. Surprisingly, a wavelength characterization of plasmon-enhanced trapping efficiencies has evaded the literature. Here, we exploit the repeatability of the recorded trapping efficiency, offered by the gold-coated black silicon platform, and perform a wavelength-dependent characterization of the trapping process, revealing the resonant character of the trapping efficiency maxima. Gold-coated black silicon is a promising platform for large-scale parallel trapping applications that will broaden the range of optical manipulation in nanoengineering, biology, and the study of collective biophotonic effects.

Trapping and viability of swimming bacteria in an optoelectric trap

Non-contact manipulation methods capable of trapping and transporting swimming bacteria can significantly aid in chemotaxis studies. However, high swimming speed makes the trapping of these organisms an inherently challenging task. We demonstrate that an optoelectric technique, rapid electrokinetic patterning (REP), can effectively trap and manipulate Enterobacter aerogenes bacteria swimming at velocities greater than 20 μm s(-1). REP uses electro-orientation, laser-induced AC electrothermal flow, and particle-electrode interactions for capturing these cells. In contrast to trapping non-swimming bacteria and inert microspheres, we observe that electro-orientation is critical to the trapping of the swimming cells, since unaligned bacteria can swim faster than the radially inward electrothermal flow and escape the trap. By assessing the cell membrane integrity, we study the effect of REP trapping conditions, including optical radiation, laser-induced heating, and the electric field on cell viability. When applied individually, the optical radiation and laser-induced heating have negligible effect on cells. At the standard REP trapping conditions fewer than 2% of cells have a compromised membrane after four minutes. To our knowledge this is the first study detailing the effect of REP trapping on cell viability. The presented results provide a clear guideline on selecting suitable REP parameters for trapping living bacteria.

Changes in radiation forces acting on a Rayleigh dielectric sphere by use of a wavefront-folding interferometer

We consider a class of fields generated by passing an isotropic Gaussian Schell-model beam through a wavefront-folding interferometer. The output field has various intensity profiles for different phase differences, including the central peak and doughnut shapes. The radiation force on a Rayleigh dielectric particle produced by the highly focused fields is investigated. Numerical results demonstrate that the new fields can be used to trap high-index particles at the focus for the specular case and nearby the focus for the anti-specular case. It is further revealed that the position, the range of particle sizes and the low limit of correlation length for stable trapping could be modulated by adjusting the phase difference.

Background-free Brillouin spectroscopy in scattering media at 780 nm via stimulated Brillouin scattering

We demonstrate the effectiveness of stimulated Brillouin scattering for background-free Brillouin spectroscopy in scattering media within the biological spectral window. Using two nearly counter-propagating continuous-wave diode laser beams at 780 nm, we acquired transmission stimulated Brillouin point spectra in 10 mm and 500 μm thick Intralipid tissue phantoms with ∼100  μm and ∼16  μm diameter focal points, respectively. Stimulated gain spectra with high signal-to-noise ratio (8.7-30.7 dB) and frequency accuracy (6-72 MHz) were obtained at 20  MHz/10  ms and 20  MHz/100  ms through 0.24-3.36 mean-free paths of tissue phantoms. Our results suggest that stimulated Brillouin gain can be useful for imaging of Brillouin resonances in submillimeter-thick scattering samples.

Precise control and measurement of solid–liquid interfacial temperature and viscosity using dual-beam femtosecond optical tweezers in the condensed phase

We present a novel method of microrheology based on femtosecond optical tweezers, which in turn enables us to directly measure and controlin situtemperature at microscale volumes at the solid–liquid interface.

Up-converted fluorescence from photosynthetic light-harvesting complexes linearly dependent on excitation intensity

Weak up-converted fluorescence related to bacteriochlorophyll a was recorded from various detergent-isolated and membrane-embedded light-harvesting pigment-protein complexes as well as from the functional membranes of photosynthetic purple bacteria under continuous-wave infrared laser excitation at 1064 nm, far outside the optically allowed singlet absorption bands of the chromophore. The fluorescence increases linearly with the excitation power, distinguishing it from the previously observed two-photon excited fluorescence upon femtosecond pulse excitation. Possible mechanisms of this excitation are discussed.

Controlling local temperature in water using femtosecond optical tweezer

A novel method of directly observing the effect of temperature rise in water at the vicinity of optical trap center is presented. Our approach relies on changed values of corner frequency of the optical trap that, in turn, is realized from its power spectra. Our two color experiment is a unique combination of a non-heating femtosecond trapping laser at 780 nm, coupled to a femtosecond infrared heating laser at 1560 nm, which precisely controls temperature at focal volume of the trap center using low powers (100-800 µW) at high repetition rate. The geometric ray optics model quantitatively supports our experimental data.

Generation of Autologous Platelet-Rich Plasma by the Ultrasonic Standing Waves

Platelet-rich plasma (PRP) is a volume of autologous plasma that has a higher platelet concentration above baseline. It has already been approved as a new therapeutic modality and investigated in clinics, such as bone repair and regeneration, and oral surgery, with low cost-effectiveness ratio. At present, PRP is mostly prepared using a centrifuge. However, this method has several shortcomings, such as long preparation time (30 min), complexity in operation, and contamination of red blood cells (RBCs). In this paper, a new PRP preparation approach was proposed and tested. Ultrasound waves (4.5 MHz) generated from piezoelectric ceramics can establish standing waves inside a syringe filled with the whole blood. Subsequently, RBCs would accumulate at the locations of pressure nodes in response to acoustic radiation force, and the formed clusters would have a high speed of sedimentation. It is found that the PRP prepared by the proposed device can achieve higher platelet concentration and less RBCs contamination than a commercial centrifugal device, but similar growth factor (i.e., PDGF-ββ). In addition, the sedimentation process under centrifugation and sonication was simulated using the Mason-Weaver equation and compared with each other to illustrate the differences between these two technologies and to optimize the design in the future. Altogether, ultrasound method is an effective method of PRP preparation with comparable outcomes as the commercially available centrifugal products.

Determination of motility forces on isolated chromosomes with laser tweezers

Quantitative determination of the motility forces of chromosomes during cell division is fundamental to understanding a process that is universal among eukaryotic organisms. Using an optical tweezers system, isolated mammalian chromosomes were held in a 1064 nm laser trap. The minimum force required to move a single chromosome was determined to be ≈ 0.8-5 pN. The maximum transverse trapping efficiency of the isolated chromosomes was calculated as ≈ 0.01-0.02. These results confirm theoretical force calculations of ≈ 0.1-12 pN to move a chromosome on the mitotic or meiotic spindle. The verification of these results was carried out by calibration of the optical tweezers when trapping microspheres with a diameter of 4.5-15 µm in media with 1-7 cP viscosity. The results of the chromosome and microsphere trapping experiments agree with optical models developed to simulate trapping of cylindrical and spherical specimens.

Mitigating phototoxicity during multiphoton microscopy of live Drosophila embryos in the 1.0-1.2 µm wavelength range

Light-induced toxicity is a fundamental bottleneck in microscopic imaging of live embryos. In this article, after a review of photodamage mechanisms in cells and tissues, we assess photo-perturbation under illumination conditions relevant for point-scanning multiphoton imaging of live Drosophila embryos. We use third-harmonic generation (THG) imaging of developmental processes in embryos excited by pulsed near-infrared light in the 1.0-1.2 µm range. We study the influence of imaging rate, wavelength, and pulse duration on the short-term and long-term perturbation of development and define criteria for safe imaging. We show that under illumination conditions typical for multiphoton imaging, photodamage in this system arises through 2- and/or 3-photon absorption processes and in a cumulative manner. Based on this analysis, we derive general guidelines for improving the signal-to-damage ratio in two-photon (2PEF/SHG) or THG imaging by adjusting the pulse duration and/or the imaging rate. Finally, we report label-free time-lapse 3D THG imaging of gastrulating Drosophila embryos with sampling appropriate for the visualisation of morphogenetic movements in wild-type and mutant embryos, and long-term multiharmonic (THG-SHG) imaging of development until hatching.

Indirect pushing based automated micromanipulation of biological cells using optical tweezers

In this paper, we introduce an indirect pushing based technique for automated micromanipulation of biological cells. In indirect pushing, an optically trapped glass bead pushes a freely diffusing intermediate bead that in turn pushes a freely diffusing target cell towards a desired goal. Some cells can undergo significant changes in their behaviors as a result of direct exposure to a laser beam. Indirect pushing eliminates this problem by minimizing the exposure of the cell to the laser beam. We report an automated feedback planning algorithm that combines three motion maneuvers, namely, push, align, and backup for micromanipulation of cells. We have developed a dynamics based simulation model of indirect pushing dynamics and also identified parameters of measurement noise using physical experiments. We present an optimization-based approach for automated tuning of planner parameters to enhance its robustness. Finally, we have tested the developed planner using our optical tweezers physical setup and carried out a detailed analysis of the experimental results. The developed approach can be utilized in biological experiments for studying collective cell migration by accurately arranging the cells in arrays without exposing them to a laser beam.

Plasmonic optical trapping in biologically relevant media

We present plasmonic optical trapping of micron-sized particles in biologically relevant buffer media with varying ionic strength. The media consist of 3 cell-growth solutions and 2 buffers and are specifically chosen due to their widespread use and applicability to breast-cancer and angiogenesis studies. High-precision rheological measurements on the buffer media reveal that, in all cases excluding the 8.0 pH Stain medium, the fluids exhibit Newtonian behavior, thereby enabling straightforward measurements of optical trap stiffness from power-spectral particle displacement data. Using stiffness as a trapping performance metric, we find that for all media under consideration the plasmonic nanotweezers generate optical forces 3–4x a conventional optical trap. Further, plasmonic trap stiffness values are comparable to those of an identical water-only system, indicating that the performance of a plasmonic nanotweezer is not degraded by the biological media. These results pave the way for future biological applications utilizing plasmonic optical traps.

Trapping of a mie sphere by acoustic pulses: effects of pulse length

The acoustic counterpart of optical tweezers shows great promise as a single-particle manipulator using a highly focused acoustic beam. Understanding the dependence of the trapping performance of the acoustic beam on the acoustic pulse length may facilitate its development and extend the applications. Herein, we propose a ray-based model for the time-course simulation of instantaneous forces exerted on single Mie spheres by highly focused acoustic pulses of arbitrary lengths. The simulations considered single fat/lipid spheres with a density of 950 kg/m3 and speed of sound of 1450 m/s, suspended in water and located on the beam axis. Simulation was used to establish the spatial and temporal pressure data of pulsed acoustic fields transmitted from a 100-MHz transducer with a half-power bandwidth of 50% and an f-number of 1. The instantaneous intensity vectors were calculated to represent rays for estimating forces exerted by consecutive wave-particle interactions. The results suggest that short acoustic pulses can exert negative forces pulling spheres beyond the focus in the direction opposite to that of wave propagation. Varying the excitation pulse duration has no effect on the region where the exerted forces are averagely negative. Lengthening the excitation pulse duration rapidly increases the amplitude of the average force. A smaller sphere experiences a greater average force when the spatial length of a transmitted acoustic pulse is comparable to the sphere diameter. The amplitude of the instantaneous force can be maximized as long as the acoustic pulse length is longer than the sphere diameter. Regulating the relation between acoustic pulse length and sphere size may be advantageous in particle sorting applications.

Bio-Rapid-Prototyping of Tissue Engineering Scaffolds and the Process-Induced Cell Damage

Tissue scaffolds play a vital role in tissue engineering by providing a native tissue-mimicking environment for cell proliferation and differentiation as well as tissue regeneration. Fabrication of tissue scaffolds has been drawing increasing research attention and a number of fabrication techniques have been developed. To better mimic the microenvironment of native tissues, novel techniques have emerged in recent years to encapsulate cells into the engineered scaffolds during the scaffold fabrication process. Among them, bio-Rapid-Prototyping (bioRP) techniques, by which scaffolds with encapsulated cells can be fabricated with controlled internal microstructure and external shape, shows significant promise. It is noted in the bioRP processes, cells may be continuously subjected to environmental stresses such as mechanical, electrical forces and laser exposure. If the stress is greater than a certain level, the cell membrane may be ruptured, leading to the so-called process-induced cell damage. This paper reviews various cell encapsulation techniques for tissue scaffold fabrication, with emphasis on the bioRP technologies and their technical features. To understand the process-induced cell damage in the bioRP processes, this paper also surveys the cell damage mechanisms under different stresses. The process-induced cell damage models are also examined to provide a cue to the cell viability preservation in the fabrication process. Discussions on further improvements of bioRP technologies are given and ongoing research into mechanical cell damage mechanism are also suggested in this review.

Continuous live cell imaging of cellulose attachment by microbes under anaerobic and thermophilic conditions using confocal microscopy

Live cell imaging methods provide important insights into the dynamics of cellular processes that cannot be derived easily from population-averaged datasets. In the bioenergy field, much research is focused on fermentation of cellulosic biomass by thermophilic microbes to produce biofuels; however, little effort is dedicated to the development of imaging tools to monitor this dynamic biological process. This is, in part, due to the experimental challenges of imaging cells under both anaerobic and thermophilic conditions. Here an imaging system is described that integrates confocal microscopy, a flow cell device, and a lipophilic dye to visualize cells. Solutions to technical obstacles regarding suitable fluorescent markers, photodamage during imaging, and maintenance of environmental conditions during imaging are presented. This system was utilized to observe cellulose colonization by Clostridium thermocellum under anaerobic conditions at 60 degrees C. This method enables live cell imaging of bacterial growth under anaerobic and thermophilic conditions and should be widely applicable to visualizing different cell types or processes in real time.

Stimulated Raman scattering imaging by continuous-wave laser excitation

We demonstrate a low-cost-stimulated Raman scattering (SRS) microscope using continuous-wave (cw) lasers as excitation sources. A dual modulation scheme is used to remove the electronic background. The cw-SRS imaging of lipids in fatty liver is demonstrated by excitation of C─H stretch vibration.

Measurements of forces produced by the mitotic spindle using optical tweezers

An optical trap is used to stop chromosome movement in spermatocytes from an insect and a flatworm and to stop pole movement in PtK cells. The forces required are much smaller than previously believed.

We used a trapping laser to stop chromosome movements in Mesostoma and crane-fly spermatocytes and inward movements of spindle poles after laser cuts across Potorous tridactylus (rat kangaroo) kidney (PtK2) cell half-spindles. Mesostoma spermatocyte kinetochores execute oscillatory movements to and away from the spindle pole for 1–2 h, so we could trap kinetochores multiple times in the same spermatocyte. The trap was focused to a single point using a 63× oil immersion objective. Trap powers of 15–23 mW caused kinetochore oscillations to stop or decrease. Kinetochore oscillations resumed when the trap was released. In crane-fly spermatocytes trap powers of 56–85 mW stopped or slowed poleward chromosome movement. In PtK2 cells 8-mW trap power stopped the spindle pole from moving toward the equator. Forces in the traps were calculated using the equation F = Q′P/c, where P is the laser power and c is the speed of light. Use of appropriate Q′ coefficients gave the forces for stopping pole movements as 0.3–2.3 pN and for stopping chromosome movements in Mesostoma spermatocytes and crane-fly spermatocytes as 2–3 and 6–10 pN, respectively. These forces are close to theoretical calculations of forces causing chromosome movements but 100 times lower than the 700 pN measured previously in grasshopper spermatocytes.

Optical trapping of microalgae at 735-1064 nm: photodamage assessment

Living microalgal cells differ from other cells that are used as objects for optical micromanipulation, in that they have strong light absorption in the visible range, and by the fact that their reaction centers are susceptible to photodamage. We trapped cells of the microalga Trachydiscus minutus using optical tweezers with laser wavelengths in the range from 735 nm to 1064 nm. The exposure to high photon flux density caused photodamage that was strongly wavelength dependent. The photochemical activity before and after exposure was assessed using a pulse amplitude modulation (PAM) technique. The photochemical activity was significantly and irreversibly suppressed by a 30s exposure to incident radiation at 735, 785, and 835 nm at a power of 25 mW. Irradiance at 885, 935 and 1064 nm had negligible effect at the same power. At a wavelength 1064 nm, a trapping power up to 218 mW caused no observable photodamage.

Optical trapping of nanoparticles

Optical trapping is a technique for immobilizing and manipulating small objects in a gentle way using light, and it has been widely applied in trapping and manipulating small biological particles. Ashkin and co-workers first demonstrated optical tweezers using a single focused beam. The single beam trap can be described accurately using the perturbative gradient force formulation in the case of small Rayleigh regime particles. In the perturbative regime, the optical power required for trapping a particle scales as the inverse fourth power of the particle size. High optical powers can damage dielectric particles and cause heating. For instance, trapped latex spheres of 109 nm in diameter were destroyed by a 15 mW beam in 25 sec, which has serious implications for biological matter. A self-induced back-action (SIBA) optical trapping was proposed to trap 50 nm polystyrene spheres in the non-perturbative regime. In a non-perturbative regime, even a small particle with little permittivity contrast to the background can influence significantly the ambient electromagnetic field and induce a large optical force. As a particle enters an illuminated aperture, light transmission increases dramatically because of dielectric loading. If the particle attempts to leave the aperture, decreased transmission causes a change in momentum outwards from the hole and, by Newton's Third Law, results in a force on the particle inwards into the hole, trapping the particle. The light transmission can be monitored; hence, the trap can become a sensor. The SIBA trapping technique can be further improved by using a double-nanohole structure. The double-nanohole structure has been shown to give a strong local field enhancement. Between the two sharp tips of the double-nanohole, a small particle can cause a large change in optical transmission, thereby inducing a large optical force. As a result, smaller nanoparticles can be trapped, such as 12 nm silicate spheres and 3.4 nm hydrodynamic radius bovine serum albumin proteins. In this work, the experimental configuration used for nanoparticle trapping is outlined. First, we detail the assembly of the trapping setup which is based on a Thorlabs Optical Tweezer Kit. Next, we explain the nanofabrication procedure of the double-nanohole in a metal film, the fabrication of the microfluidic chamber and the sample preparation. Finally, we detail the data acquisition procedure and provide typical results for trapping 20 nm polystyrene nanospheres.