Light‐Powered Microrobots: Challenges and Opportunities for Hard and Soft Responsive Microswimmers

Worldwide research in microrobotics has exploded in the past two decades, leading to the development of microrobots propelled in various manners. Despite significant advances in the field and successful demonstration of a wide range of applications, microrobots have yet to become the preferred choice outside a laboratory environment. After introducing available microrobotic propulsion and control mechanisms, microrobots that are manufactured and powered by light are focused herein. Referring to pioneering works and recent interesting examples, light is presented not only as a fabrication tool, by means of twophoton polymerization direct laser writing, but also as an actuator for microrobots in both hard and soft stimuli–responsive polymers. In this scenario, a number of challenges that yet prevent polymeric light-powered microrobots from reaching their full potential are identified, whereas potential solutions to overcome said challenges are suggested. As an outlook, a number of real-world applications that light-powered microrobots should be particularly suited for are mentioned, together with the advances needed for them to achieve such purposes. An interdisciplinary approach combining materials science, microfabrication, photonics, and data science should be conducive to the next generation of microrobots and will ultimately foster the translation of microrobotic applications into the real world

Fast Objective Coupled Planar Illumination Microscopy

Among optical imaging techniques light sheet fluorescence microscopy stands out as one of the most attractive for capturing high-speed biological dynamics unfolding in three dimensions. The technique is potentially millions of times faster than point-scanning techniques such as two-photon microscopy. This potential is especially poignant for neuroscience applications due to the fact that interactions between neurons transpire over mere milliseconds within tissue volumes spanning hundreds of cubic microns. However current-generation light sheet microscopes are limited by volume scanning rate and/or camera frame rate. We begin by reviewing the optical principles underlying light sheet fluorescence microscopy and the origin of these rate bottlenecks. We present an analysis leading us to the conclusion that Objective Coupled Planar Illumination (OCPI) microscopy is a particularly promising technique for recording the activity of large populations of neurons at high sampling rate. We then present speed-optimized OCPI microscopy, the first fast light sheet technique to avoid compromising image quality or photon efficiency. We enact two strategies to develop the fast OCPI microscope. First, we devise a set of optimizations that increase the rate of the volume scanning system to 40 Hz for volumes up to 700 microns thick. Second, we introduce Multi-Camera Image Sharing (MCIS), a technique to scale imaging rate by incorporating additional cameras. MCIS can be applied not only to OCPI but to any widefield imaging technique, circumventing the limitations imposed by the camera. Detailed design drawings are included to aid in dissemination to other research groups. We also demonstrate fast calcium imaging of the larval zebrafish brain and find a heartbeat-induced motion artifact. We recommend a new preprocessing step to remove the artifact through filtering. This step requires a minimal sampling rate of 15 Hz, and we expect it to become a standard procedure in zebrafish imaging pipelines. In the last chapter we describe essential computational considerations for controlling a fast OCPI microscope and processing the data that it generates. We introduce a new image processing pipeline developed to maximize computational efficiency when analyzing these multi-terabyte datasets, including a novel calcium imaging deconvolution algorithm. Finally we provide a demonstration of how combined innovations in microscope hardware and software enable inference of predictive relationships between neurons, a promising complement to more conventional correlation-based analyses

Micro-Resonators: The Quest for Superior Performance

Microelectromechanical resonators are no longer solely a subject of research in university and government labs; they have found a variety of applications at industrial scale, where their market is predicted to grow steadily. Nevertheless, many barriers to enhance their performance and further spread their application remain to be overcome. In this Special Issue, we will focus our attention to some of the persistent challenges of micro-/nano-resonators such as nonlinearity, temperature stability, acceleration sensitivity, limits of quality factor, and failure modes that require a more in-depth understanding of the physics of vibration at small scale. The goal is to seek innovative solutions that take advantage of unique material properties and original designs to push the performance of micro-resonators beyond what is conventionally achievable. Contributions from academia discussing less-known characteristics of micro-resonators and from industry depicting the challenges of large-scale implementation of resonators are encouraged with the hopes of further stimulating the growth of this field, which is rich with fascinating physics and challenging problems

Development of a light-sheet fluorescence microscope employing an ALPAO deformable mirror to achieve video-rate remote refocusing and volumetric imaging.

There are numerous situations in microscopy where it is desirable to remotely refocus a microscope employing a high numerical aperture (NA) objective lens. This thesis describes the characterisation, development and implementation of an Alpao membrane deformable mirror-based system to achieve this goal for a light-sheet fluorescence microscope (LSFM). The Alpao deformable mirror (DM) DM97-15 used is this work has 97 actuators and was sufficiently fast to perform refocus sweeps at 25 Hz and faster. However, a known issue with using Alpao deformable mirrors in open-loop mode is that they exhibit viscoelastic creep and temperature- dependent variations in the mirror response. The effect of visco-elastic creep was reduced by ensuring that the mirror profile was on average constant on timescales shorter than the characteristic time of the visco-elastic creep. The thermal effect was managed by ensuring that the electrical power delivered to the actuators was constant prior to optimisation and use. This was achieved by ensuring that the frequency and amplitude of oscillation of the mirror was constant prior to optimisation, so that it reached a thermal steady state, was approximately constant during optimisation and constant during use. The image-based optimisation procedure employed used an estimate of the Strehl ratio of the optical system calculated from an image of an array of 1 μm diameter holes. The optimisation procedure included optimising the amount of high-NA defocus and the Zernike modes from Noll indices 4 to 24. The system was tested at 26.3 refocus sweeps per second over a refocus range of -50 to 50 μm with a 40x/0.85 air objective and a 40x/0.80 water immersion objective. The air objective enabled a mean Strehl metric of more than 0.6 over a lateral field of view of 200x200 microns and for a refocus range of 45 microns. The water objective achieved a mean Strehl metric of more than 0.6 over a lateral field of view of 200x200 microns over a larger refocus range of 77 microns. The DM-based refocusing system was then incorporated into a LSFM setup. The spatial resolution of the system was characterised using fluorescent beads imaged volumetrically at 26.3 volumes per second. The performance of the system was also demonstrated for imaging fluorescence pollen grain samples.Open Acces

Experimental study of the behaviour of the synthetic jets for effective flow separation control

The increase in fuel price and the enforcement of more restrictive regulations have presented significant challenges to the aircraft industry and forced the development and implementation of revolutionary technologies so as to reduce operational costs and environmental impact of aircraft transportation. The development of active flow control is believed to be one of the key measures to meet these challenges.Synthetic jet actuators, as a promising method for flow separation control, are capable of injecting momentum to the external flow with zero-net-mass-flux, which negate the need for air supplies and complex piping systems. So far, the capability of synthetic jets in controlling flow separation has been demonstrated in many laboratory-based investigations. However, our understanding of the fluid mechanics of synthetic jets is still limited. An improved understanding is essential to the optimisation of the synthetic jet actuator operating conditions for a given flow so as to maximise their flow control effectiveness for a practical setting. The present research is aimed at achieving this goal through a series of carefully designed experiments.Firstly, PIV measurements and smoke-wire flow visualisation of circular synthetic jets in quiescent air are undertaken to study the effect of dimensionless parameters on the formation and the extent of vortex roll-up. The results show the dimensionless stroke length (L) affects the onset of suction cycle on the vortex ring and the Stokes number (S) determines the strength of vortex roll-up. A criterion for vortex roll-up is also established. Based on PIV (Particle Image Velocimetry) measurements of both micro-scale and macro-scale synthetic jets, it is confirmed that the characteristic of synthetic jets of different scales will be identical when the L, S and ReL are the same.Dye visualisation of round synthetic jets issued into a zero-pressure gradient laminar boundary layer is carried out in a water flume. Three different types of vortical structures are found to be produced as a result of the interaction of a synthetic jet with a laminar boundary layer as the operating condition of synthetic jets changes, i.e., hairpin vortices, stretched vortex rings and tilted vortex rings. Hairpin vortices and stretched vortex rings are formed as the result of the upstream branches of the vortex rings being weakened by the resident vorticity in the boundary layer. In the case of tilted vortex rings, the vortex rings remain intact as they emerge from the boundary layer due to an increased jet strength.Finally, laser induced fluorescent flow visualisation and PIV measurement are undertaken to study the flow separation control effectiveness of a synthetic jet array issued upstream of a separated laminar flow over a flap in water. Based on the results from the experiment, parameter maps showing the effectiveness of separation control at different actuator operating conditions are produced. It is found that a good control effect is associated with the presence of hairpin type of structures and a velocity ratio (VR) in the range of 0.3 and 0.7. For the experimental conditions tested in this thesis, operating the synthetic jets at a dimensionless stroke length (L) around 2 and velocity ratio (VR) around 0.6 would deliver the best flow control effect with the least energy consumption.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Nondestructive testing of metals and composite materials using ultrasound thermography : comparison with pulse-echo ultrasonics

La thermographie stimulée par ultrasons (TU) est une méthode de contrôle non destructif qui a été inventée en 1979 mais qui a s'est répandue à la fin des années 90. L'idée de cette méthode est d'exciter le matériau à inspecter avec des ondes mécaniques à des fréquences allant de 20kHz à 40kHz et d'observer ensuite leur température de surface avec une caméra infrarouge. TU est une méthode de thermographie active; les autres méthodes les plus connues sont la thermographie optique et celle stimulée par courants de Foucault. Son habilité à révéler des défauts dans des cas où les autres techniques échouent, fait d'elle une méthode pertinente ou complémentaire. L'inconvénient de la TU est que beaucoup de conditions expérimentales doivent être respectées pour obtenir des résultats adéquats incluant quelques paramètres qui doivent être bien choisis. Le but de ce projet est d'explorer les capacités, les avantages et les limites de la TU. Pour comparer la performance de la TU à celle des ultrasons conventionnels, des tests ultrasons de type C-Scan ont été réalisés pour quelques échantillons. Quatre matériaux différents avec quatre types de défauts ont été investigués afin de mieux définir les conditions optimales pour améliorer la détection des défauts. Les résultats bruts obtenus étaient traités dans chaque cas afin de mieux visualiser les contrastes thermiques causés par les discontinuités cachées

Concept definition for space station technology development experiments. Experiment definition, task 2

The second task of a study with the overall objective of providing a conceptual definition of the Technology Development Mission Experiments proposed by LaRC on space station is discussed. During this task, the information (goals, objectives, and experiment functional description) assembled on a previous task was translated into the actual experiment definition. Although still of a preliminary nature, aspects such as: environment, sensors, data acquisition, communications, handling, control telemetry requirements, crew activities, etc., were addressed. Sketches, diagrams, block diagrams, and timeline analyses of crew activities are included where appropriate

NASA Tech Briefs, December 1990

Topics: New Product Ideas; NASA TU Services; Electronic Components and Circuits; Electronic Systems; Physical Sciences; Materials; Computer Programs; Mechanics; Machinery; Fabrication Technology; Mathematics and Information Sciences; Life Sciences