Adela Ben-Yakar

Neurosurgery: Functional regeneration after laser axotomy

Understanding how nerves regenerate is an important step towards developing treatments for human neurological disease, but investigation has so far been limited to complex organisms (mouse and zebrafish) in the absence of precision techniques for severing axons (axotomy). Here we use femtosecond laser surgery for axotomy in the roundworm Caenorhabditis elegans and show that these axons functionally regenerate after the operation. Application of this precise surgical technique should enable nerve regeneration to be studied in vivo in its most evolutionarily simple form.

Cavity Flame-Holders for Ignition and Flame Stabilization in Scramjets: An Overview

This paper describes ongoing research efforts in the scramjet community on cavity e ame holders, a concept for e ame holding and stabilization in supersonic combustors. During the last few years, cavities have gained the attention of the scramjet community as a promising e ame-holding device, owing to results obtained in e ight tests and to feasibility demonstrations in laboratory-scale supersonic combustors. However, comprehensive studies are needed to determine the optimal cone guration that will yield the most effective e ame-holding capability with minimum losses. The e owe eld characteristics of cavities and research efforts related to cavities employed in lowand high-speed e ows are summarized. Open questions impacting the effectiveness of the cavities as e ame holders in supersonic combustors are discussed.

Time evolution and mixing characteristics of hydrogen and ethylene transverse jets in supersonic crossflows

We report an experimental investigation that reveals significant differences in the near-flowfield properties of hydrogen and ethylene jets injected into a supersonic crossflow at a similar jet-to-freestream momentum flux ratio. Previously, the momentum flux ratio was found to be the main controlling parameter of the jet’s penetration. Current experiments, however, demonstrate that the transverse penetration of the ethylene jet was altered, penetrating deeper into the freestream than the hydrogen jet even for similar jet-to-freestream momentum flux ratios. Increased penetration depths of ethylene jets were attributed to the significant differences in the development of large-scale coherent structures present in the jet shear layer. In the hydrogen case, the periodically formed eddies persist long distances downstream, while for ethylene injection, these eddies lose their coherence as the jet bends downstream. The large velocity difference between the ethylene jet and the freestream induces enhanced mixing...

Femtosecond laser ablation properties of borosilicate glass

We study the femtosecond laser ablation properties of borosilicate glass using atomic force microscopy and laser pulses of 200 fs duration, centered at 780 nm wavelength. We show that both single-shot and multishot ablation threshold fluences can be determined by studying the diameter and the depth of single-shot ablated craters. The linear relationship between the square of the crater diameter and the logarithm of the laser fluence in the form of D2=2w02ln(F0∕FthN=1) provides the single-shot ablation threshold, FthN=1, whereas the linear relationship between the ablation depth and the logarithm of laser fluence in the form of ha=αeff−1ln(F0∕FthN>1) provides the multishot ablation threshold, FthN>1. The results depict a multishot ablation threshold of ≈1.7J∕cm2 independent of the atmospheric conditions. The slopes of the linear fits also provide a precise estimate of the beam radius at the surface, w0≈5.9μm, and the “effective optical penetration depth,” αeff−1≈238nm in air. The method is systematic, prov...

Microfluidics for the analysis of behavior, nerve regeneration, and neural cell biology in C. elegans

The nematode Caenorhabditis elegans is a widely adopted model organism for studying various neurobiological processes at the molecular and cellular level in vivo. With a small, flexible, and continuously moving body, the manipulation of C. elegans becomes a challenging task. In this review, we highlight recent advances in microfluidic technologies for the manipulation of C. elegans. These new family of microfluidic chips are capable of handling single or populations of worms in a high-throughput fashion and accurately controlling their microenvironment. So far, they have been successfully used to study neural circuits and behavior, to perform large-scale phetotyping and morphology-based screens as well as to understand axon regeneration after injury. We envision that microfluidic chips can further be used to study different aspects of the C. elegans nervous system, extending from fundamental understanding of behavioral dynamics to more complicated biological processes such as neural aging and learning and memory.

Thermal and fluid processes of a thin melt zone during femtosecond laser ablation of glass: the formation of rims by single laser pulses

We study the formation mechanism of rims created around femtosecond laser ablated craters on glass. Experimental studies of the surface morphology reveal that a thin rim is formed around the smooth craters and is raised above the undamaged surface by about 50–100 nm. To investigate the mechanism of rim formation following a single ultrafast laser pulse, we perform a one-dimensional theoretical analysis of the thermal and fluid processes involved in the ablation process. The results indicate the existence of a very thin melted zone below the surface and suggest that the rim is formed by the high pressure plasma producing a pressure-driven fluid motion of the molten material outwards from the centre of the crater. The numerical solutions of pressure-driven fluid motion of the thin melt demonstrate that the melt can flow to the crater edge and form a rim within the first nanoseconds of the ablation process. The possibility that a tall rim can be formed during the initial stages of the plasma is suggestive that the rim may tilt outwards towards the low pressure region creating a resolidified melt splash as observed in the experiments. The possibility of controlling or suppressing the rim formation is discussed also.

Morphology of femtosecond-laser-ablated borosilicate glass surfaces

We study the morphology of borosilicate glass surface machined by femtosecond laser pulses. Our observations show that a thin rim is formed around ablated craters after a single laser pulse. When multiple laser pulses are overlapped, the crater rims also overlap and produce a surface roughness. The rim appears to be a resolidified splash from a molten layer generated during the ablation process. We estimate that this molten layer is a few micrometers thick and exists for a few microseconds. During this melt lifetime, forces acting on the molten layer move it from the center to the edge of the crater.

Cavity flameholders for ignition and flame stabilization in scramjets - Review and experimental study

This paper describes ongoing research efforts in the scramjet community, in general, as well as specific work at Stanford University on cavity flame-holders, a concept for flame-holding and stabilization in supersonic combustors. During the last few years, cavities have gained the attention of the scramjet community as a promising flameholding device, owing to results obtained in flight test and to feasibility demonstrations in laboratory scale supersonic combustors. However, comprehensive studies are needed to determine the optimal configuration which will yield the most effective flameholding capability with minimum loses. In the first cavity study, reported in this paper, the goals were to demonstrate ultra-high-speed schlieren imaging as applied to study a cavity with length-to-depth-ratio of L/D=3. By combination of simultaneously performed fast response pressure measurements, established cavity oscillations were observed and a sequence of oscillation cycles could be captured during the limited test time (~300us) of the flow facility. The results demonstrate that short duration pulse facilities can be used to study gasdynamics aspects of cavities, though with small dimensions (depth of D=3mm), in hypersonic flows.

Femtosecond laser nanoaxotomy properties and their effect on axonal recovery in C. elegans

We present a study characterizing the properties of femtosecond laser nanosurgery applied to individual axons in live Caenorhabditis elegans (C. elegans) using nano-Joule laser pulses at 1 kHz repetition rate. Emphasis is placed on the characterization of the damage threshold, the extent of damage, and the statistical rates of axonal recovery as a function of laser parameters. The ablation threshold decreases with increasing number of pulses applied during nanoaxotomy. This dependency suggests the existence of an incubation effect. In terms of extent of damage, the energy per pulse is found to be a more critical parameter than the number of pulses. Axonal recovery improves when surgery is performed using a large number of low energy pulses.

Clinical Ultrafast Laser Surgery: Recent Advances and Future Directions

Ultrafast pulsed lasers can be used to achieve remarkable precision during surgical ablation. Through nonlinear interactions with tissue, ultrafast lasers can provide a largely non-thermal mechanism of ablation and a unique ability to create targeted damage within bulk tissue. These advantages have made ultrafast lasers the ideal surgical tool for various novel applications in ophthalmology. Clinical adoption of ultrafast lasers in other surgical applications remains limited in part due to the lack of a means for fiber delivery of ultrafast laser pulses as a flexible, hand-held surgical endoscope. This review provides an overview of the recent advances in bringing this unique surgical tool into the clinic. We discuss fundamental mechanisms and limitations of ultrafast laser ablation, novel techniques for overcoming these limitations, the current state of clinical applications, and conclude with our recent efforts in developing fiber-coupled probes for flexible ultrafast laser surgery and imaging.