In the field of in vitro fertilisation (IVF), there is a need to understand how cell properties of the cumulus oocyte complex may be used to predict successful pregnancy and live birth rates post-IVF. Here we used optical tweezers for the first time to measure the viscosity of the cumulus cell matrix surrounding the oocyte (egg). This study aimed to determine whether the viscosity of the cumulus cell matrix – prior to fertilisation – is reflective of subsequent embryo developmental potential and indicative of pregnancy success.
Measurements were performed using a 1µm diameter silica probe particle trapped by a focused 1064nm laser. We benchmarked the accuracy of the system by measuring the viscosity of glycerol with varying mass fractions. Viscosity measurements of the cumulus cell matrix were performed in isolation from both the cumulus cells and the oocyte. This showed that the viscosity of cumulus matrix was significantly higher when sampled from oocytes with a higher developmental potential (in vivo matured) compared to those of lower quality (in vitro matured).
We report on the fabrication of a self-written microfiber waveguide with a first-order Bessel beam. The fabricated microfiber waveguide exhibits a diameter of ~7 μm and a millimeter-scale length. We also address the optical waveguide properties of the fabricated fiber.
Optical binding in liquid and in vacuum continues to unveil and enable new physics. We present two results. Firstly, we show how, for particles beyond the Rayleigh limit, surprising asymmetric forces can arise in counter-propagating beams. These can cause particle transport or instability. Secondly, we show how optical binding between two microspheres levitated in vacuum can be used for sympathetic cooling of the centre of mass motions of the spheres to sub-Kelvin temperatures.
In optical tweezers, reductions in the symmetry of the particle or trapping field introduce nonconservative optical forces, producing a variety of nonequilibrium effects. Here we show nonconservative optical traps in a vacuum using birefringent microspheres in linearly polarised (LP) and circularly polarised (CP) Gaussian beams. Coherent and self-sustained oscillations emerge in LP due to nonsymmetric coupling between rotational and translational degrees of freedom, while stochastic orbital rotation and coherent limit cycles arise in CP about the beam axis. These nonconservative effects play a critical role in the rotational dynamics of a levitated birefringent microsphere.
We demonstrate parametric feedback cooling of a levitated birefringent microsphere held in a circularly polarised Gaussian trap. We show efficient cooling of the birefringent microsphere to the temperature less than a millikelvin, which is orders of magnitude lower than that of a silica counterpart. We reveal that the transverse spin momentum present in circularly polarised light plays a role in the trapped particle dynamics, which exhibits a range of nonequilibrium phenomena, namely, (i) biased stochastic motion, (ii) coupling of mechanical modes, (iii) nonconservative rotation-translation coupling. We further discuss the temperature of the trapped particle in a nonconservative vacuum optical trap.
Optical trapping describes the interaction between light and matter to manipulate micro-objects through momentum transfer. In the case of 3D trapping with a single beam, this is termed optical tweezers. Optical tweezers are a powerful and noninvasive tool for manipulating small objects, and have become indispensable in many fields, including physics, biology, soft condensed matter, among others. In the early days, optical trapping was typically accomplished with a single Gaussian beam. In recent years, we have witnessed rapid progress in the use of structured light beams with customized phase, amplitude, and polarization in optical trapping. Unusual beam properties, such as phase singularities on-axis and propagation invariant nature, have opened up novel capabilities to the study of micromanipulation in liquid, air, and vacuum. We summarize the recent advances in the field of optical trapping using structured light beams.
When particles are held in optical tweezers, we assume that they are in thermal equilibrium. Here we show that this condition only holds for high symmetry cases, e.g. perfectly isotropic particles in unaberrated, linearly polarized Gaussian traps. We show both experimentally and theoretically that when a birefringent microsphere is held in a linearly polarised Gaussian optical trap in vacuum, spontaneous oscillations emerge that grow rapidly in amplitude and become increasingly coherent as the air pressure is reduced. Furthermore, when parametrically driven, these self-sustained oscillators exhibit an ultrahigh quality factor exceeding 200 million, which can be highly sensitive to external perturbations.
When particles are held in optical tweezers, we assume that they are in thermal equilibrium, in which the equipartition of energy determines the position variance of optically trapped spheres. Here we show that this condition only holds for very high symmetry cases, e.g. perfectly isotropic particles in unaberrated, linearly polarized Gaussian traps. We report an experimental observation that when a birefringent microsphere is held in a linearly polarised Gaussian optical trap in vacuum, spontaneous oscillations emerge that grow rapidly in amplitude and become increasingly coherent as the air pressure is reduced. Furthermore, when parametrically driven, these self-sustained oscillators exhibit an ultrahigh mechanical quality factor > 2 x 108, which can be highly sensitive to external purtabations.
Light may carry both orbital angular momentum (AM) and spin AM. The former is a consequence of its helical wavefront, and the latter is a result of its rotating transverse electric field. Intriguingly, the light–matter interaction with such fields shows that the orbital AM of light causes a physical “twist” in a range of materials, including metal, silicon, azopolymer, and even liquid-phase resin. This process may be aided by the light’s spin AM, resulting in the formation of various helical structures. The exchange between the AM of light and matter offers not only unique helical structures at the nanoscale but also entirely novel fundamental phenomena with regard to the light–matter interaction. This will lead to the future development of advanced photonics devices, including metamaterials for highly sensitive detectors as well as reactions for chiral chemical composites. Here, we focus on interactions between the AM of light and azopolymers, which exhibit some of the most diverse structures and phenomena observed. These studies result in helical surface relief structures in azopolymers and will leverage next-generation applications with light fields carrying optical AM.
Particles held in optical tweezers are commonly thought to be at thermodynamic equilibrium with their environment. Under this assumption the elastic energy of the trap is equal to the thermal energy. As a result the variance of the particle position is completely independent of viscosity and inversely proportional to the optical power in the trap. Here we show that these conditions only hold for very high symmetry cases e.g. perfectly spherical particles in unaberrated, linearly polarized Gaussian traps. Here we show that any reduction in symmetry leads to asymmetrically coupled degrees of freedom. The associated force field is linearly non-conservative and the tweezer is no longer at equilibrium. In overdamped systems the effect is a underlying systematic bias to the Brownian motion. In underdamped systems, this systematic component can accumulate momentum, eventually destabilizing the trap. We illustrate this latter effect with reference to two systems, (i) an isotropic sphere in a circularly polarized trap, and (ii) a birefringent sphere in a linearly polarized trap. In both cases the instability can be approached either by decreasing air pressure or by increasing optical power. Close to instability, the trapped particle executes increasingly coherent motion that is highly sensitive to external perturbations. Potential applications to weak force sensing are discussed.
Optical vortex possesses an on-axial phase singularity and an orbital angular momentum (OAM) due to its spiral wavefront characterized by a topological charge ℓ. OAM of the optical vortex can twist or spin the target materials, such as silicon, metal, and polymer, to form chiral structures.
In this paper, we report on the creation of helical microfibers by irradiating picosecond optical vortex pulses with a wavelength of 532 nm to ultraviolet curing resin via a two-photon-absorption photopolymerization process.
Self-focusing effect of incident vortex pulses, arising from the photo-polymerization, confines efficiently optical vortex field to form a self-written helical fiber waveguide with the help of OAM transfer effect.
The resulting helical microfibers exhibited a length of ~300 μm Also, we could control the twisted direction of fibers merely by changing the sign of the topological charge of optical vortex. These experiments will open up a new way to the practical application of helical microfiber to optical communications.
Photopolymerization, the process of using ultraviolet light to activate polymerization within resins, is a powerful approach to create arbitrary, transparent micro-objects with a resolution below the diffraction limit. Importantly, to date all photopolymerization studies have been performed with incident light fields with planar wavefronts and have solely exploited the intensity profile of the incident beam. We investigate photopolymerization with light fields possessing orbital angular momentum (OAM), characterized by the topological charge “l”. We show that, as a consequence of nonlinear self-focusing of the optical field, photopolymerization creates an annular-shaped vortex-soliton and an associated optical fibre, which exhibits a helical trajectory, with a chirality determined by the sign of “l”. In particular, due to a transverse modulation instability in the nonlinear self-focusing photopolymer, the vortex beam breaks up into the “l” solitons or microfibers, each of which exhibit helical trajectories and together form a bundle of helical microfibers. Our numerical simulations, based on the nonlinear paraxial wave equation for the photopolymer, captures all the experimental observations for a variety of optical vortices characterized by “l”. This therefore represents a new physical manifestation of the use of OAM light fields. This research opens up a new application for light fields with OAM, and our generated microfibers may have applications in optical communications and micromanipulation. In a broader context, our work adds a new facet to the emergent field of helical fibres that have themselves recently come to the fore in the photonic crystal community as a route to generating fields with OAM.
Temperature at the mesoscale is important in many fields due to its key role in, e.g., cell mechanics, quantum ground state studies and hydrodynamics. In levitated optomechanics, measuring temperature is challenging at the same time as necessary to understand the dynamics of the optically trapped particle. Generally, the particle’s temperature has been directly correlated to its centre-of-mass (CoM) motion (i.e. translational dynamics). This, together with the rotational dynamics, encompasses the particle’s external degree of freedom which is affected by the external temperature. However, the particle presents an internal structure that is at another internal temperature. Generally, the CoM temperature is experimentally measured and compared to a theoretically calculated internal temperature. The rotation rate (i.e. rotational dynamics) has also been correlated to an experimentally measured internal temperature for thermometric studies. Despite its importance, the temperature of these three degrees of freedom had never been simultaneously measured and correlated. We developed a tripartite method able to independently measure both the internal temperature of the particle (through temperature-dependent luminescence) and the external temperature (through the rotational rate and trap stiffness). We found that, even though they are strongly coupled, the external and internal degrees of freedom present distinct temperatures. This study gives new insight into thermometry at the mesoscale where the appropriate parameter should be carefully chosen for an accurate characterisation of temperature. Moreover, experiments attempting to cool levitated particles to the quantum ground state, in which all degrees of freedom must be independently controlled and characterised, will also benefit from this advance.
Photopolymerization, the process of using ultraviolet light to activate polymerization within resins, is a powerful approach to create arbitrary, transparent micro-objects with a resolution below the diffraction limit. Such microstructures have been optimized for optical manipulation and are finding application elsewhere, including micro-optics, mechanical microstructures and polymer crystallography. Furthermore, due to self-focusing, photopolymerization can form a waveguide, which develops into an optical fibre as long as submillimeters. Importantly, to date virtually all photopolymerization studies have been performed with incident light fields possessing planar wavefronts and simply exploit the beam intensity profile. Here we investigate photopolymerization of ultraviolet curing resins with a light field possessing orbital angular momentum (OAM). We show that the annular vortex beam breaks up via modulation instability into the m-microfibers, depending on the azimuthal index m of an incident optical vortex. These microfibers exhibit helical structures with chirality determined by the sign of m and mirror the helical nature of the incident vortex beam wavefront. We have developed a numerical model based on the Beam Propagation Method that captures the key experimental observations for a variety of optical vortices characterized by their azimuthal index m. This research opens up a range of new vistas and has broad consequences for the fields of structured light, new approaches to writing novel mesoscopic structures and applications such as in detecting or sorting the OAM mode (e.g. photonic lanterns) in areas including optical communications and manipulation.
We demonstrate a self-written sub-millimeter (>300 μm) helical fiber in a photo-cure resin by irradiation of non-diffractive 1st-order Bessel beam with an orbital angular momentum. The twisted direction of the helical fiber can be controlled by only reversing the sign of the topological charge of Bessel beam.
We demonstrate the transfer of orbital angular momentum (OAM) to optically levitated microparticles in vacuum. We create two-dimensional (2D) and three-dimensional (3D) optical potentials possessing OAM. In the former case the microparticle is placed within a Laguerre-Gaussian (LG) beam and orbits the annular beam profile with increasing angular velocity as the air drag coefficient is reduced. Our results reveal that there is a fundamental limit to the OAM that may be transferred to a trapped particle, dependent upon the beam parameters and inertial forces present. Whilst a LG beam scales in size with azimuthal index, recently we have created a “perfect vortex” beam whose radial intensity profile and radius are both independent of topological charge. As the Fourier transform of a “perfect vortex” yields a Bessel beam, imaging a “perfect vortex”, with its subsequent propagation thus realises a complex three-dimensional optical field. In this scenario we load individual silica microparticles into this field where the optical gradient and scattering forces interplay with the inertial and gravitational forces acting on the trapped particle. As a result the trapped microparticle exhibits a complex three-dimensional motion that includes a periodic orbital motion between the Bessel and the “perfect vortex” beam. We are able to determine the three dimensional optical potential in situ by tracking the particle. This first demonstration of trapping microparticles within a complex 3D optical potential in vacuum opens up new possibilities for fundamental studies of many-body dynamics, mesoscopic entanglement, and optical binding.
Photopolymerization, the process of using ultraviolet light to activate polymerization within resins, is a powerful approach to create arbitrary, transparent micro-objects with a resolution below the diffraction limit. We investigate photopolymerization of ultraviolet curing resins with a light field possessing orbital angular momentum (OAM). Due to total internal reflection of the beam within the cured polymer, the material acts as a waveguide, which develops into an optical fibre as long as submillimeters. Importantly, the cured polymer mirrors the energy flow and helical nature of the incident OAM field, resulting in an equivalent spiralling of the cured polymer waveguide itself. Moreover, the chirality of the helix, which is the direction of the spiral, can be controlled by inverting the handedness (reversing the sign of the azimuthal index or topological charge) of the incident field. As the fibre diameter becomes narrower due to self-focusing effects, incident light with higher topological charge cannot be maintained within the single-mode structure of the fibre. We show that the fibre repeatedly branches into a bundle of intertwined fibre guides, each of which carries a fundamental mode of the OAM. Such fibre structures represent a novel form of photonic lanterns. This research opens up a range of new vistas and has broad consequences for the fields of structured light, new approaches to writing novel mesoscopic structures and applications such as in detecting or sorting the OAM mode in areas including optical communications and manipulation.
Optical manipulation of gold nanoparticles has facilitated a wide range of innovative science and applications such as nanothermometry, cell poration, optical binding and optomechanics, with many exploiting the surface plasmon resonance. However, conventional gold nanoparticles usually depart from spherical shape and typically exhibit icosahedron, decahedron, triangular or hexagonal prism, thus making such studies less controlled and leading to potential artefacts in trapping behaviour. We successfully synthesise ultrasmooth gold nanoparticles of 50nm and 100nm in diameter with an improved monodispersity both in shape and size compared to conventional gold nanoparticles. We demonstrate the first optical manipulation of such ultrasmooth gold nanoparticles in a near infrared optical trap, and show trap stiffness with up to a three-fold reduction in standard deviation both in liquid and air, compared to conventional gold nanoparticles. Our trapping study highlights the exceptional sensitivity of the trapping parameters of gold nanoparticles on their morphology. Furthermore, we for the first time quantify the particle temperature of airborne gold nanoparticles for a range of optical powers based on the trap stiffness measurements and reveal the effects of particle morphology. We show our ultrasmooth gold nanoparticles exhibit lower temperature than the conventional counterparts due to the lack of additional morphological features that induce plasmonic heating. The use of ultrasmooth gold nanoparticles can pave the way for more controlled studies of optical binding and plasmon mediated light-matter interactions and novel applications in optomechanics such as optically controllable nanoprobes of weak forces and torques.
Optical binding refers to an optically mediated inter-particle interaction
that creates new equilibrium positions for closely spaced particles [1–5]. Optical binding
of mesoscopic particles levitated in vacuum can pave the way towards the realisation
of a large scale quantum bound array in cavity-optomechanics [6–9]. Recently we have
demonstrated trapping and rotation of two mesoscopic particles in vacuum using a
spatial-light-modulator-based approach to trap more than one particle, induce controlled
rotation of individual particles, and mediate interparticle separation [10]. By trapping
and rotating two vaterite particles, we observe intensity modulation of the scattered light
at the sum and difference frequencies with respect to the individual rotation rates. This
first demonstration of optical interference between two microparticles in vacuum has
lead to a platform to explore optical binding. Here we demonstrate for the first time
optically bound two microparticles mediated by light scattering in vacuum. We investigate
autocorrelations between the two normal modes of oscillation, which are determined by the
centre-of-mass and the relative positions of the two-particle system. In situ determination
of the optical restoring force acting on the bound particles are based on measurement
of the oscillation frequencies of the autocorrelation functions of the two normal modes,
thereby providing a powerful and original platform to explore multiparticle entanglement
in cavity-optomechanics.
We demonstrate the transfer of orbital angular momentum to optically levitated
microparticles in vacuum [1]. We prepare two-dimensional and three-dimensional optical
potentials. In the former case the microparticle is placed within a Laguerre-Gaussian beam
and orbits the annular beam profile with increasing angular velocity as the air drag coefficient
is reduced. We explore the particle dynamics as a function of the topological charge
of the levitating beam. Our results reveal that there is a fundamental limit to the orbital angular
momentum that may be transferred to a trapped particle, dependent upon the beam
parameters and inertial forces present. This effect was predicted theoretically [2] and can be
understood considering the underlying dynamics arising from the link between the magnitude
of the azimuthal index and the beam radius [3].
Whilst a Laguerre-Gaussian beam scales in size with azimuthal index `, recently we
have created a “perfect” vortex beam whose radial intensity profile and radius are both
independent of topological charge [4, 5]. As the Fourier transform of a perfect vortex yields
a Bessel beam. Imaging a perfect vortex, with its subsequent propagation thus realises a
complex three dimensional optical field. In this scenario we load individual silica microparticles
into this field and observe their trajectories. The optical gradient and scattering forces
interplay with the inertial and gravitational forces acting on the trapped particle, including
the rotational degrees of freedom. As a result the trapped microparticle exhibits a complex
three dimensional motion that includes a periodic orbital motion between the Bessel and
the perfect vortex beam. We are able to determine the three dimensional optical potential
in situ by tracking the particle. This first demonstration of trapping microparticles within
a complex three dimensional optical potential in vacuum opens up new possibilities for
fundamental studies of many-body dynamics, mesoscopic entanglement [6, 7], and optical
binding [8, 9].
We synthesize, optically trap, and rotate individual nanovaterite crystals with a
mean particle radius of 423 nm. Rotation rates of up to 4.9 kHz in heavy water are recorded [1].
Laser-induced heating due to residual absorption of the nanovaterite particle results in the
superlinear behavior of the rotation rate as a function of trap power. A finite element method
based on the Navier-Stokes model for the system allows us to determine the residual optical
absorption coefficient for a trapped nanovaterite particle. This is further confirmed by the
theoretical model. Our data reveal that the nanoparticle experiences a different Stokes drag
torque or force depending on whether we consider rotational or translational motion, which is
in a good agreement with the theoretical prediction of the rotational hot Brownian motion [2].
The data allow us to determine the correction factors for the local viscosity for both the
rotational and translational motion of the nanoparticle. The use of nanovaterite particles opens
up new studies for levitated optomechanics in vacuum [3–6] as well as microrheological
properties of cells or biological media [7]. For these latter studies, nanovaterite offers prospects
of microviscosity measurements in ultrasmall volumes and, due to its size, potentially simpler
uptake by cellular media [8].
We presented the irradiation of optical vortex to ultraviolet (UV) curing resin structures a twisted polymer fiber. A
continuous-wave ultraviolet optical vortex, focused at a glass cell containing the cure resin, allowed us to shape a twisted
polymeric fiber with a diameter of a few micrometer and a length of ~160 μm with an exposure time of < 1 second. Twisted
direction of the fiber was also controlled by inverting the handedness of the optical vortex.
Rotational control over optically trapped particles has gained significant prominence in recent years. The marriage between light fields possessing optical angular momentum and the material properties of microparticles has been useful to controllably spin particles in liquid, air and vacuum. The rotational degree of freedom adds new functionality to optical traps: in addition to allowing fundamental tests of optical angular momentum, the transfer of spin angular momentum in particular can allow measurements of local viscosity and exert local stresses on cellular systems.
We demonstrate optical trapping and controlled rotation of nanovaterite crystals. These particles represent the smallest birefringent crystals ever trapped and set into rotation. Rotation rates of up to 5kHz in water are recorded, representing the fastest rotation to date for dielectric particles in liquid. Laser-induced heating results in the superlinear behaviour of the rotation rate as a function of trap power. We study both the rotational and translational modes of trapped nanovaterite crystals. The particle temperatures derived from those two optomechanical modes are in good agreement, which is supported by a numerical model revealing that the observed heating is dominated by absorption of light by the particles rather than by the surrounding liquid. A comparison is performed with trapped silica particles of similar size.
The use of nanovaterite particles open up new studies for levitated optomechanics in vacuum as well as microrheological properties of cells or biological media. Their size and low heating offers prospects of viscosity measurements in ultra-small volumes and potentially simpler uptake by cellular media.
Vortex beams with different topological charge usually have different profiles and radii of peak intensity. This introduces a degree of complexity the fair study of the nature of optical OAM (orbital angular momentum). To avoid this, we introduced a new approach by creating a perfect vortex beam using an annular illuminating beam with a fixed intensity profile on an SLM that imposes a chosen topological charge. The radial intensity profile of such an experimentally created perfect vortex beam is independent to any given integer value of its topological charge. The well-defined OAM density in such a perfect vortex beam is probed by trapping microscope particles. The rotation rate of a trapped necklace of particles is measured for both integer and non-integer topological charge. Experimental results agree with the theoretical prediction. With the flexibility of our approach, local OAM density can be corrected in situ to overcome the problem of trapping the particle in the intensity hotspots. The correction of local OAM density in the perfect vortex beam therefore enables a single trapped particle to move along the vortex ring at a constant angular velocity that is independent of the azimuthal position. Due to its particular nature, the perfect vortex beam may be applied to other studies in optical trapping of particles, atoms or quantum gases.
Cell selective introduction of therapeutic agents remains a challenging problem. Cavitation-based therapies including ultrasound-induced sonoporation and laser-induced optoporation have led the way for novel approaches to provide the potential of sterility and cell selectivity compared with viral or biochemical counterparts. Acoustic streaming, shockwaves and liquid microjets associated with the cavitation dynamics are implicated in gene and drug delivery. These approaches, however, often lead to non-uniform and sporadic molecular uptake that lacks refined spatial control and suffers from a significant loss of cell viability. Here we demonstrate spatially controlled cavitation instigated by laser-induced breakdown of an optically trapped single gold nanoparticle. Our unique approach employs optical tweezers to trap a single nanoparticle, which when irradiated by a nanosecond laser pulse is subject to laser-induced breakdown followed by cavitation. Using this method for laser-induced cavitation, we can gain additional degrees of freedom for the cavitation process - the particle material, its size, and its position relative to cells or tissues. We show the energy breakdown threshold of gold nanoparticles of l00nm with a single nanosecond laser pulse at 532 nm is three orders of magnitude lower than that for water, which leads to gentle nanocavitation enabling single cell transfection. We optimize the shear stress to the cells from the expanding bubble to be in the range of 1-10 kPa for transfection by precisely positioning a trapped gold nanoparticle, and thus nanobubble, relative to a cell of interest. The method shows transfection of plasmid-DNA into individual mammalian cells with an efficiency of 75%.
Shear stress has been recognized as one of the biophysical methods by which to permeabilize plasma membranes of cells. In particular, high pressure transient hydrodynamic flows created by laser-induced cavitation have been shown to lead to the uptake of fluorophores and plasmid DNA. While the mechanism and dynamics of cavitation have been extensively studied using a variety of time-resolved imaging techniques, the cellular response to the cavitation bubble and cavitation induced transient hydrodynamic flows has never been shown in detail. We use time-resolved quantitative phase microscopy to study cellular response to laser-induced cavitation bubbles. Laser-induced breakdown of an optically trapped polystyrene nanoparticle (500nm in diameter) irradiated with a single nanosecond laser pulse at 532nm creates transient shear stress to surrounding cells without causing cell lysis. A bi-directional transient displacement of cytoplasm is observed during expansion and collapse of the cavitation bubble. In some cases, cell deformation is only observable at the microsecond time scale without any permanent change in cell shape or optical thickness. On a time scale of seconds, the cellular response to shear stress and cytoplasm deformation typically leads to retraction of the cellular edge most exposed to the flow, rounding of the cell body and, in some cases, loss of cellular dry mass. These results give a new insight into the cellular response to laser-induced shear stress and related plasma membrane permeabilization. This study also demonstrates that laser-induced breakdown of an optically trapped nanoparticle offers localized cavitation (70 μm in diameter), which interacts with a single cell.
Femtosecond photoporation is an optical, non-invasive method of injecting membrane impermeable substances
contained within the surrounding medium into cells. The technique typically addresses individual cells in a static
monolayer. While this gives excellent selectivity, it can be time consuming or impractical to treat larger samples. We
build on previous work using a microfluidic platform, which allows for a suspension of cells to be dosed with
femtosecond light as they flow through a microfluidic channel. A reusuable quartz chip is designed with an 's'-bend with facilitates the delivery of a 'non-diffracting' femtosecond Bessel beam along the centre of the channel. By implementing off-chip hydrodynamic focusing, cells are confined to the central region of the channel and pass along the Bessel beam core where they are photoporated. This new parallel approach allows for higher flow rates to be used compared to the previous, orthogonal, design whilst maintaining the necessary dwell time in the Bessel beam core. Optical injection of the cell membrane impermeable stain propidium iodide has been successful with two cell lines. These have yielded viable injection efficiencies of 31.0±9.5% Chinese hamster ovary cells (CHO-K1) and 20.4±4.2% human promyelocytic cells (HL60) with a cell throughput of up to 10 cells/second. This marks an order of magnitude increase compared to the previous microfluidic design.
We use stroboscopic quantitative phase microscopy to study cell deformation and the response to cavitation bubbles and transient shear stress resulting from laser-induced breakdown of an optically trapped nanoparticle. A bi-directional transient displacement of cytoplasm is observed during expansion and collapse of the cavitation bubble. In some cases, cell deformation is only observable at the microsecond time scale without any permanent change in cell shape or optical thickness. On a time scale of seconds, the cellular response to shear stress and cytoplasm deformation typically leads to retraction of the cellular edge most exposed to the flow, rounding of the cell body and, in some cases, loss of cellular dry mass. These results give a new insight into the cellular response to cavitation induced shear stress and related plasma membrane permeabilization. This study also demonstrates that laser-induced breakdown of a nanoparticle offers localized cavitation, which interacts with a single cell but without causing cell lysis.
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