Auxiliary Optomechanical Tools for 3D Cell Manipulation

Optically Actuated Soft Microrobot Family for Single-Cell Manipulation

Precisely controlled manipulation of nonadherent single cells is often a pre-requisite for their detailed investigation. Optical trapping provides a versatile means for positioning cells with submicrometer precision or measuring forces with femto-Newton resolution. A variant of the technique, called indirect optical trapping, enables single-cell manipulation with no photodamage and superior spatial control and stability by relying on optically trapped microtools biochemically bound to the cell. High-resolution 3D lithography enables to prepare such cell manipulators with any predefined shape, greatly extending the number of achievable manipulation tasks. Here, it is presented for the first time a novel family of cell manipulators that are deformable by optical tweezers and rely on their elasticity to hold cells. This provides a more straightforward approach to indirect optical trapping by avoiding biochemical functionalization for cell attachment, and consequently by enabling the manipulated cells to be released at any time. Using the photoresist Ormocomp, the deformations achievable with optical forces in the tens of pN range and present three modes of single-cell manipulation as examples to showcase the possible applications such soft microrobotic tools can offer are characterized. The applications describe here include cell collection, 3D cell imaging, and spatially and temporally controlled cell-cell interaction.

Optically Controlled Dissolution Kinetics of Vaterite Microcapsules: Toward Novel Crystal Growth Strategies

Controllable continuous release of functional materials from capsules is one of the unmet functions of theragnosis particles; on this way, understanding cargo-fluid interactions in vitro is an essential milestone. We develop a flexible platform to investigate single particle-fluid interactions utilizing a glass micropipette as a highly localized flow source around an optically trapped particle. In proof-of-concept experiments, this microparticle is sensitive to local microflow distribution, thus serving as a probe. The very same flows are capable of the particle rotating (i.e., vaterite drug cargo) at frequencies dependent on the mutual particle-pipette position. Platform flexibility comes from different interactions of a tweezer (optical forces) and a pipette (mechanical/hydrodynamical) with a microparticle, which makes this arrangement an ideal microtool. We studied the vaterite dissolution kinetics and demonstrated that it can be controlled on demand, providing a wide cargo release dynamic rate. Our results promote the use of inorganic mesoporous nanoparticles as a nanomedicine platform.

Universal optothermal micro/nanoscale rotors

Rotation of micro/nano-objects is important for micro/nanorobotics, three-dimensional imaging, and lab-on-a-chip systems. Optical rotation techniques are especially attractive because of their fuel-free and remote operation. However, current techniques require laser beams with designed intensity profile and polarization or objects with sophisticated shapes or optical birefringence. These requirements make it challenging to use simple optical setups for light-driven rotation of many highly symmetric or isotropic objects, including biological cells. Here, we report a universal approach to the out-of-plane rotation of various objects, including spherically symmetric and isotropic particles, using an arbitrary low-power laser beam. Moreover, the laser beam is positioned away from the objects to reduce optical damage from direct illumination. The rotation mechanism based on opto-thermoelectrical coupling is elucidated by rigorous experiments combined with multiscale simulations. With its general applicability and excellent biocompatibility, our universal light-driven rotation platform is instrumental for various scientific research and engineering applications.

Optical microlever assisted DNA stretching

Optical microrobotics is an emerging field that has the potential to improve upon current optical tweezer studies through avenues such as limiting the exposure of biological molecules of interest to laser radiation and overcoming the current limitations of low forces and unwanted interactions between nearby optical traps. However, optical microrobotics has been historically limited to rigid, single-body end-effectors rather than even simple machines, limiting the tasks that can be performed. Additionally, while multi-body machines such as microlevers exist in the literature, they have not yet been successfully demonstrated as tools for biological studies, such as molecule stretching. In this work we have taken a step towards moving the field forward by developing two types of microlever, produced using two-photon absorption polymerisation, to perform the first lever-assisted stretches of double-stranded DNA. The aim of the work is to provide a proof of concept for using optical micromachines for single molecule studies. Both styles of microlevers were successfully used to stretch single duplexes of DNA, and the results were analysed with the worm-like chain model to show that they were in good agreement.

Nanometer optical trap based on stimulated emission in evanescence of a totally reflected Arago spot : Nanometer optical trap for fluorescent nanoparticles

Optical tweezers have paved the way towards the manipulation of particles and living cells at the micrometer range. Its extension towards the nanometer world may create unprecedented potentialities in many areas of science. Following a letter (O. Emile, J. Emile, H. Tabuteau, EPL 129, 58001 (2020)) that reported the observation of the trapping of a single 200nm diameter fluorescent particle in a nanometric volume, we detail here our experimental findings. In particular, the trapping mechanism is shown to be based on the radiation pressure of light in one direction and on the stimulated emission of the particle in the evanescent wave of a nanometer Arago spot on a glass/liquid interface on the other directions. The trapping volume is a 200nm height cylinder whose radius varies with the spreading of the evanescent wave near the spot and can reach 50nm. The calculation of the force and the parameters limiting the lifetime are detailed. Applications to laser trapping of atoms and molecules are also discussed.

Single-Cell Elasticity Measurement with an Optically Actuated Microrobot

A cell elasticity measurement method is introduced that uses polymer microtools actuated by holographic optical tweezers. The microtools were prepared with two-photon polymerization. Their shape enables the approach of the cells in any lateral direction. In the presented case, endothelial cells grown on vertical polymer walls were probed by the tools in a lateral direction. The use of specially shaped microtools prevents the target cells from photodamage that may arise during optical trapping. The position of the tools was recorded simply with video microscopy and analyzed with image processing methods. We critically compare the resulting Young’s modulus values to those in the literature obtained by other methods. The application of optical tweezers extends the force range available for cell indentations measurements down to the fN regime. Our approach demonstrates a feasible alternative to the usual vertical indentation experiments.

Second-harmonic computer-generated holographic imaging through monolithic lithium niobate crystal by femtosecond laser micromachining

The computer-generated holography technique is a powerful tool for three-dimensional display, beam shaping, optical tweezers, ultrashort pulse laser parallel processing, and optical encryption. We have realized nonlinear holography in ferroelectric crystals by utilizing spatial light modulators in our previous works. Here, we demonstrate an improved method to realize second-harmonic (SH) holographic imaging through a monolithic lithium niobate crystal based on binary computer-generated holograms (CGHs). The CGH patterns were encoded with the detour phase method and fabricated by femtosecond laser micromachining. By the use of the birefringence phase-matching process in the longitudinal direction, bright nonlinear holograms can be obtained in the far-field. The realization of SH holography through monolithic crystal opens wide possibilities in the field of high power laser nonlinear holographic imaging.

Editorial for the Special Issue on Optical Trapping and Manipulation: From Fundamentals to Applications

This Special Issue of Micromachines is devoted to optical trapping, and the enormous range of uses the method has found in the decades since its first demonstration [...].