Many types of optical tweezers arrays have been proposed and developed for use in conjunction with microfluidics for
bio-chemical essays. Trap arrays rely on different methods allowing various degrees of flexibility and relative trapping
efficiencies. Among the different techniques currently employed, it is not simple to distinguish which ones produce
adequate performances for a given task in bio-chemistry. Experimental results for trapping efficiently diverse biological
specimens allow distinguishing between the properties of optical trap arrays based on techniques as different as
interferometry, holography, Fresnel or Fraunhoffer diffraction of diffractive structures, generalized phase contrast,
microlens assemblies, micro-mirrors matrices, and also clusters of individual tweezers. The bulkiness of those systems is
another important factor in the design of labs-on-a-chip; in particular the use of cumbersome microscope objectives can
be detrimental to chip optimization. Arrangements of tweezers produced with different concepts should be compared in
terms of efficiency, ease of use, and number of traps simultaneously exploitable
Micro-optical components offer several possibilities for creating
large matrices of optical traps, either when working on inverted
microscopes, or by directly integrating miniaturized optical
components at the level of a micro-fluidic chip. In this article we
focus on two particular configurations, both allowing to generate
large arrays of 3D optical traps. The first configuration takes
advantage of an array of refractive microlenses to generate multiple
optical tweezers within the focal plane of a high-NA microscope
objective. The second configuration relies on an array of focusing
high-NA micromirrors which are directly integrated at the level of a
micro-fluidic chip. We also present measurements of the maximal
optical trapping forces that can be reached with several types of
cells commonly employed in biology and biotechnology, and
demonstrate that these forces are essentially related to the bulk
refractive index of the cells.
We present a multiple laser tweezers system based on refractive optics. The system produces an array of 100
optical traps thanks to a refractive microlens array, whose focal plane is imaged into the focal plane of a high-NA
microscope objective. This refractive multi-tweezers system is combined to micro-fluidics, aiming at performing
simultaneous biochemical reactions on ensembles of free floating objects. Micro-fluidics allows both transporting
the particles to the trapping area, and conveying biochemical reagents to the trapped particles. Parallel trapping
in micro-fluidics is achieved with polystyrene beads as well as with native vesicles produced from mammalian
cells. The traps can hold objects against fluid flows exceeding 100 micrometers per second. Parallel fluorescence
excitation and detection on the ensemble of trapped particles is also demonstrated. Additionally, the system is
capable of selectively and individually releasing particles from the tweezers array using a complementary steerable
laser beam. Strategies for high-yield particle capture and individual particle release in a micro-fluidic environment
are discussed. A comparison with diffractive optical tweezers enhances the pros and cons of refractive systems.
In biological investigations, many protocols using optical trapping call for the possibility to trap a large number
of particles simultaneously. Interference fringes provide a solution for massively parallel micro-manipulation of
mesoscopic objects. Concurrently, the strength of traps can be improved by raising the slope of fringe profiles,
such as to create intensity gradients much higher than the ones formed by sinusoidal fringes (Young's fringes). We
use a multiple-beam interference system, derived from the classical Fizeau configuration, with semitransparent
interfaces to generate walls of light with a very high intensity gradient (Tolansky fringes). These fringes are
formed into a trapping set-up to produce new types of trapping templates. The possibility to build multiple trap
arrays of various geometries is examined; a high number of particles can be trapped in those potential wells. The
period of the fringes can easily be changed in order to fit traps sizes to the dimensions of the confined objects.
This is achieved by modifying several parameters of the interferometer, such as the angle and/or the distance
between the beam-splitter and the mirror. It is well known that optical trapping presents a great potential when
used in conjunction with microfluidics for lab-on-a-chip applications. We present an original solution for multiple
trapping integrated in a microfluidic device. This solution does not require high numerical aperture objectives.
Shaping optical fields is the key issue in the control of optical forces that pilot the manipulation of mesoscopic polarizable dielectric particles. The latter can be positioned according to endless configurations. The scope of this paper is to review and discuss several unusual designs which produce what we think are among some of the most interesting arrangements. The simplest schemes result from interference between two or several coherent light beams, leading to periodic as well as pseudo-periodic arrays of optical traps. Complex assemblages of traps can be created with holographic-type set-ups; this case is widely used by the trapping community. Clusters of traps can also be configured through interferometric-type set-ups or by generating external standing waves by diffractive elements. The particularly remarkable possibilities of the Talbot effect to generate three-dimensional optical lattices and several schemes of self-organization represent further very interesting means for trapping. They will also be described and discussed. in this paper. The mechanisms involved in those trapping schemes do not require the use of high numerical aperture optics; by avoiding the need for bulky microscope objectives, they allow for more physical space around the trapping area to perform experiments. Moreover, very large regular arrays of traps can be manufactured, opening numerous possibilities for new applications.
Optical low coherence reflectometry (OLCR) was used for the measurement of the local fiber Bragg grating (FBG) birefringence. The birefringence with resolution of 4μ10-6 was determined directly from oscillations in the OLCR amplitude with a spatial resolution on the millimeter scale. Using OLCR phase in combination with inverse scattering algorithm the birefringence was retrieved with a high spatial resolution of 25 microns. The technique using OLCR amplitude was applied for measuring birefringence of a FBG under diametric loading.
An optical fiber sensor to simultaneously measure strain and temperature was designed and embedded into an adaptive composite laminate which exhibits a shape change upon thermal activation. The sensor is formed by two fiber Bragg gratings, which are written in optical fibers with different core dopants. The two gratings were spliced close to each other and a sensing element resulted with Bragg gratings of similar strain sensitivity but different response to temperature. This is due to the dependence of the fiber thermo-optic coefficient on core dopants and relative concentrations. The sensor was tested on an adaptive composite laminate made of unidirectional Kevlar-epoxy pre-preg plies. Several 150μm diameter pre-strained NiTiCu shape memory alloy wires were embedded in the composite laminate together with one fiber sensor. Simultaneous monitoring of strain and temperature during the curing process and activation in an oven was demonstrated.
We present a new light source for parallel Optical Coherence Tomography (OCT) based on multiple waveguides written in Ti:sapphire. Each channel can generate a spectrum of 174 nm bandwidth centered at 772 nm, with an optical power on sample of 30 uW. A system depth resolution of 1.9 um is obtained, which correspond to 1.5 um in tissue.
Very high frequency oscillations of intense light fields interact with micron-size dielectric objects to exert dc optical forces that allow polarizable particles to levitate, to be trapped and to be bound. Such optical forces are also suitable to arrange cold atoms in optical lattices. Various assemblages of optical traps, including periodic arrays, can be constructed either with independent lasers, or with a single laser beam split into different parts later recombined by interference, as well as through the use of diffractive elements. These optical-well arrays serve as templates for writing and erasing dynamic two-dimensional and three-dimensional "optical crystals", composed of mono-dispersed polystyrene spheres in water. Subsequently, the crystals become diffractive structures themselves.
The association of micro-fluidics and optical trapping allows for the formation of optical traps into micro-channels. This leads to perform microchemistry experiments, such as fluorescence detection, on individual bodies attached to trapped particles. Self-trapping due to the optical binding force relates to the interaction between different dielectric objects located in an electromagnetic field; each one reacts not only to the field of the incident beam, but also to the induced fields radiated coherently by all other particles. Optical binding strongly influences the equilibrium state and the behavior of optical crystals. It must have the potential for creating collective effects.
Nd:YAG laser welding of high reflectivity metals is difficult because of the highly non-linear light-material interaction yielding a narrow process window and poor reliability. However, achieving high
reliability is mandatory for applying this technique in industrial production lines. The welding control can be improved by real-time monitoring of the process evolution with sensors. Such sensor signals are particularly useful for weld classification and for laser power control in off-line or in closed-loop feedback configurations. The latter possibility is difficult to implement in pulsed lasers and requires a careful sensor choice. Here, we report on laser lap micro-spot welding of thin copper sheets using a pulsed Nd:YAG laser. The welding was performed under atmospheric conditions on pure, 50 μm thick, slightly oxidized copper sheets with pulse durations and energies of less than 8 ms and 8 J, respectively. The process was experimentally analyzed by detecting normal laser reflection, heat emission, and instantaneous laser power with high time resolution. The meaningful signal parameters have then been selected for a closed loop feedback control. The variance of top and bottom weld spot diameters could be reduced by more than a factor of 8 in the case of closed loop control.
We report on the fabrication of Ti:sapphire channel waveguides. Such channel waveguides are of interest, e.g., as low-threshold tunable lasers. We investigated several structuring methods including ion beam implantation followed by wet chemical etching strip loading by polyimide spin coating and subsequent laser micro-machining, direct laser ablation or reactive ion etching through laser-structured polyimide contact masks. The later two methods result in ribs having different widths and heights up to ~5 μm. By reactive ion etching we have obtained channel waveguides with strong confinement of the Ti:sapphire fluorescence emission.
First results and analysis of second harmonic generation with a Nd:YAG laser working in the long pulse, free running mode are presented in this paper. Second harmonic generation conversion efficiencies of up to 17.5% and pulse powers of 162W have been generated with a free running Nd:YAG laser and a KTP non-linear crystal. The conversion efficiency is limited by a saturation effect and optical damage occurring at ~50 times lower peak intensities than in the Q-switch mode. The saturation and damage mechanism involves creation of temporary 'color centers' induced by the second harmonic radiation and subsequent increased fundeamental wave absorption.
Experimental, analytical and finite-element-simulation approaches are presented for the characterization of fibre Bragg grating sensors written in conventional monomode and polarization-maintaining fibers subjected to transverse loading. Firstly, a diametrical-load configuration is considered. Numerical simulations show the behavior to be nonlinear as a function of the applied load when an appropriate analytical model for the opto-mechanical response is employed. Secondly, experiments are carried out with the sensors embedded in epoxy specimens, when the latter are subjected to transversal biaxial loading. The response is monitored as a function of the vertical/horizontal load ratio. A finite-element model of the specimen with the embedded fibre and the previous analytical procedure are used to calculate the strain distributions in the fibre core resulting from loading, and predict the corresponding Bragg wavelength shifts. Experimental results are then compared to numerical predictions.
A complete set-up for local annealing of Shape Memory Alloys (SMA) is proposed. Such alloys, when plastically deformed at a given low temperature, have the ability to recover a previously memorized shape simply by heating up to a higher temperature. They find more and more applications in the fields of robotics and micro engineering. There is a tremendous advantage in using local annealing because this process can produce monolithic parts, which have different mechanical behavior at different location of the same body. Using this approach, it is possible to integrate all the functionality of a device within one piece of material. The set-up is based on a 2W-laser diode emitting at 805nm and a scanner head. The laser beam is coupled into an optical fiber of 60(mu) in diameter. The fiber output is focused on the SMA work-piece using a relay lens system with a 1:1 magnification, resulting in a spot diameter of 60(mu) . An imaging system is used to control the position of the laser spot on the sample. In order to displace the spot on the surface a tip/tilt laser scanner is used. The scanner is positioned in a pre-objective configuration and allows a scan field size of more than 10 x 10 mm2. A graphical user interface of the scan field allows the user to quickly set up marks and alter their placement and power density. This is achieved by computer controlling X and Y positions of the scanner as well as the laser diode power. A SMA micro-gripper with a surface area less than 1 mm2 and an opening of the jaws of 200(mu) has been realized using this set-up. It is electrically actuated and a controlled force of 16mN can be applied to hold and release small objects such as graded index micro-lenses at a cycle time of typically 1s.
In order to improve the reliability of micro-spot welding of metal parts in production such as e.g. in electron guns for TV picture tubes, real-time information about the evolution of the welding process should be available to allow on-line modification of the laser parameters. Such information can be derived from a set of sensors that are mounted on a laser-scanning head. Different sensors are used to monitor the optical fiber output power to determine the evolution of temperature during the spot welding process, to measure plasma emission and back-reflected laser light. A vision channel and a CCD camera are used to control the position of the laser spot on the parts to be processed. The compact scanning head is composed of a tip/tilt laser scanner, a collimating lens and a focusing lens. The scanner is fast steering, with a bandwidth of 700Hz, and can tilt by +/- 3.5 degree(s) with a repeatability better than 50(mu) rad. The settling time for maximum deflection is less that 10ms. The scanning lens is a newly developed focusing lens designed to replace commercial cumbersome scanning lenses such as F-(theta) lenses, which have large volume, weight and price. This lens is based on the well-known Cooke triplet design and guarantees a constant shape of the spot all over the scan surface and is specially well suited for high power beam delivery. The scan field achieved by the system is limited to 25mm x 25mm. The laser used for this application is a pulsed Nd:YAG laser delivered by an optical fiber to the optical head. However, the system can be adapted to different types of lasers. Laser micro-spot welding on copper substrate has been performed in the frame of the Brite-Euram project MAIL. Smaller tolerances (a factor of 2 less) on the spot diameters were obtained in the case of a sensor controlled operation compared to the case where sensor control is not used.
Frequency doubled Nd:YAG lasers represent an attractive alternative to other laser tools for many material processing applications, but frequency doubling with pulsed Nd:YAG lasers has been performed until now only with pulses of tens of nanoseconds. In material processing with longer pulses (10-1000 microsecond(s) ), such as encountered in typical 1.06 micrometers industrial Nd:YAG applications, the laser-material interaction is different and, in particular, higher material ablation rates are performed. Furthermore, the green light material processing permits a better focusability and a higher absorption in most materials. However, frequency doubling with long pulse lasers is much more difficult and less efficient up to now. The main problems are the generation of a fundamental 1.064 micrometers beam of high quality necessary for the non-linear process, and the low damage threshold of the non linear materials in the long pulse regime. Therefore, a zigzag slab laser, which has a high beam quality and an inherently linear polarization of the beam, is an ideal candidate for non-linear processes. The optical damage threshold in the non-linear materials is the main limiting parameter. The 140 W instantaneous power obtained for a 200 microsecond(s) pulse duration in extra-cavity configuration allows us to finely process sheets up to 200 micrometers thick.
This paper investigates the use of embedded optical fiber Bragg gratings to measure strain near a stress concentration within a solid structure. Due to the nature of a stress concentration (i.e. the strong non-uniformity of the strain field) the assumption that the grating spectrum in reflection remains a single peak with a constant bandwidth may not be valid. Compact tension specimens including a controlled notch shape are fabricated with embedded optical fiber Bragg gratings at identical locations but with different gauge lengths. The spectra in transmission varies between such specimens for given loading conditions. This variation is shown to be due to the difference in gauge length. By using the strain field measured on the specimen surface with electronic speckle pattern interferometry and a discretized model of the grating, the spectra in transmission are then verified analytically. Thus, by considering the non-uniformity of the strain field, the optical fiber Bragg gauge functions well as an embedded strain gauge near the stress concentration. Due to the distributed nature of the measurements within a specific gauge length, the optical fiber Bragg gauge has a large potential to measure debonding in fiber reinforced composites.
Wolfgang Andreasch, Christian de Graffenried, Robert Kohler, Reymond Clavel, Thomas Sidler, Rene-Paul Salathe, Ronald Mueller, Hannes Bleuler, Bernhard Gaechter, Hartmut Ehbets
An automated assembly technique for small optical components has been developed. It concerns components such as, e.g., laser diodes and LEDs, fibers, lenses beamsplitters, polarizers, mirrors, crystals, prisms, diffractive elements or photodiodes. It is based on the flexible 2-dimensional arrangement of a universal tripod holder (10 by 10 by 4 mm) on a planar mounting plate. Its particular mechanical structure allows to align the optical elements on-line and to attach them to the mounting plate in a one step procedure. The different elements are aligned with an accuracy of plus or minus 1 micrometer and attached one after the other. Very good position stability (plus or minus 0.7 micrometer, plus or minus 0.2 mrad) during the attachment procedure has been achieved by laser point welding. They are optically interconnected by free-space propagation of a light beam with diameter of up to 8 millimeters. Mass production has been shown with a collimator as test vehicle. The collimator is composed of two elements (laser diode and collimating lens) and is mounted entirely automatically by two co-working robots. Easy prototyping has been shown with the realization of the optical position sensing system featuring a high precision linear magnetic bearing. Flexibility, simple handling, high packaging density and low cost make this new assembly technique satiable to both mass production and prototyping of small opto electronical devices.
The mechanical resistance of UV irradiated optical fibers has been investigated by dynamic fatigue tests. The intrinsic Weibull distribution of the pristine and chemically stripped fiber were almost identical. The high germanium doped fiber was homogeneously irradiate using a KrF excimer with different pulse energy densities and different total doses, both corresponding to typical Bragg grating fabrication conditions. The Weibull distribution of the irradiated fibers depends on the pulse energy density and the total irradiation dose. Both a strong broadening and a reduction of the fiber strength were observed. With increasing total irradiation dose, the median breaking stress of the fiber decreases from 4.8 GPa for the pristine fiber to 2.5, 2.2, and 1.8 GPa for pulse energy densities of 50, 100 and 200 mJ/cm2, respectively, for a total dose of 1 kJ/cm2. At higher total irradiation levels the median breaking stress and hence the strain were found to be independent of laser fluence with a value of 2.7 GPa for 200 mJ/cm2.
A series of carotid end-to-end diode laser assisted microvascular anastomosis (LAMA) versus control ateral conventional suture microanastomosis (CMA) were performed in 120 Wistar rats (in the same animal, LAMA performed in the left side and CMA in the right). The optic and scanning electron microscopic examinations were assessed from day 0 to day 210. The results revealed that on day 0 LAMA gave rise to proteins deneturation and collagens fusion of the media and adventitia in the arterial wall. Re-endothelialization of anastomotic line began at day 3, as well as a large number of inflammatory aggregated in the adventitia. On day 10 the endothelial cells were restored on the anastomotic site and collagenous network developed in the media. On day 90 proliferation and disorientation of the elastic fibers appeared. A part of elastic laminae had been reconstructed on day 210. In the group CMA, the re- endothelialization developed later than LAMA, and the reconstruction of the elastic laminae failed to happen until day 210. These data suggest that the results of long term healing process after diode LAMA is better than that of CMA in normal artery repair.
In a series of 70 Wistar rats submitted to a noncontact diode laser-assisted carotid end-to-end anastomosis (LAMA) versus controlateral manual suture microanastomosis (CMA), contrast angiography and scanning electron microscopy were performed in a group of 18 animals at a long-term follow up of 7 months after operation. The investigation of angiography realized by means of a CGR General Electric DG 200 with a 512 digital matrix gave precise images of vessel wall repair and vessel patency. It permitted a simultaneous comparison between laser and manual anastomosis. Patency rate was 83% in LAMA and 78% in CMA. Nonlethal complications such as 1 stenosis, 2 thromboses in LAMA and 1 stenosis, 2 thromboses, and 1 pseudoaneurysm in CMA were observed. The results could be correlated to scanning electron microscopy
findings and confirmed the morphological superiority of the laser technique.
Two different methods are presented and discussed for the evaluation of the point spread function (PSF) in multicore fibers (MCFs) used as an image guide in microendoscopy. In the first method, the coupling intensities between the fiber cores in a MCF are measured at three different wavelengths (632, 520, and 488 nm) by scanning the
input face of the MCF with an illuminated pinhole. In the second method, the intensity coupled into a fiber core from all the neighboring fiber cores is measured by obturating the fiber core and measuring the output light intensity. The nonnegligible contribution of leaky modes in the large intercore distance coupling is demonstrated. The two methods are fairly well correlated. The investigations have been carried out for two MCFs presenting well-differentiated characteristics. The major coupling mechanisms between fiber cores have been identified and their roles quantified as functions of the coupling distance. At short distance, crosstalk dominates, with a maximum coupling intensity of 15% at 632 nm for the
nearest neighbor fiber core. At larger distances (ranging from 3.8 to 5 intercore distances), leaky modes play a predominant role in the coupling, which remains nonnegligible.
For a few years promising techniques have been worked out for microsurgery of cells and organelles by laser micro-cutting and micro-drilling. In this paper the specific requirements for controlled and safe laser surgery at this scale are addressed. Laser micro-drilling is investigated for various interaction parameters. For this a Nd:YAG laser ((lambda) equals 1064 nm) in Q-switched (7 ns) mode and a cw diode laser ((lambda) equals 1.48 micrometers ) in pulsed mode are used. The lasers are coupled to an inverted microscope and focused through the microscope objective (40X and 45X) in a spot of 1 to 3 micrometers in diameter. Ablation of biological targets (mouse zygotes zona pellucida) is compared with ablation of model targets (ink films), all in liquid environment. Possible generation of acoustical transients and cavitation bubbles is monitored with a PVDF pressure transducer and by time resolved video flash photography. The results show that, for the pulse durations investigated, a large absorption coefficient is a necessary condition for controlled micro-drilling. In liquid environment, short pulse lasers in combination with a low light absorption coefficient of the target induce unacceptable mechanical effects related to the generation of a cavitation bubble. These effects are due to a too high energy threshold for ablation in these processes. Video images of the drilled holes reveal walls of regular shape with small surrounding damage immediately after laser irradiation, which are completely destroyed at the bubble collapse. By contrast, with the 1.48 micrometers laser diode, which is strongly absorbed by the target, reproducible micro-drillings without mechanical effects are performed.
Erbium and Holmium lasers are ideally suited for cutting and drilling biological tissue. This is due to the fact that their wavelengths (Er:YSGG at 2.79 micrometers and Ho:YAG at 2.12 micrometers ) are strongly absorbed in water which is present in all tissues. Combined with an optical fiber these lasers seem to be optimal instruments for endoscopic and/or minimal invasive applications in surgery. In this study we focused our interest on cutting of human meniscus in the knee where, besides a very limited operation field, the standard arthroscopic treatment is performed in a liquid, highly absorbing environment. The bubble formation process, therefore, has to be well understood because it mainly determines relevant aspects of tissue ablation. The influence of the laser parameters in general and the influence of pulse duration in particular are determined in this paper for two different laser wavelengths. The goal was to determine the optimum laser parameters in view of a high ablation efficiency, a high precision and a minimal destruction of the adjacent tissue. To determine the optimum pulse duration for ablating tissue under water and to obtain a better understanding of the channel formation process, transmission and pressure measurements together with video flash photography were performed. Additionally, we determined experimentally the ratio between initial laser pulse energy and energy available for tissue treatment under water. To prove the results obtained, cuts in human meniscus were performed, sectioned and evaluated. The comparison between the results obtained with the Erbium and Holmium laser revealed a strong influence of the absorption coefficients on the tissue effects, especially on the ablation efficiency and on the zone of thermally and mechanically damaged tissue.
Diode laser assisted microvascular anastomosis (LAMA) versus control ateral conventional suture microanastomosis (CMA) were performed in 50 rats. The laser source (830 nm) was included into a micromanipulator without optical fiber connection. The beam was directly focused on a deviating mirror giving a 200 micrometers spot size. The LAMA was performed on the left common carotid by means of 3 laser shots of 360 mW power and 8 s exposure time regulated by an auto-stop system, the irradiance being 1,146 W/cm2. CMA was performed on the right carotid by means of size 10.0 Ethilon stitches (BV 70, Ethnor, Neuilly, France). The anastomoses were evaluated by macroscopic controls and Doppler spectral analysis (D0 to D60). After LAMA, light and scanning electron microscopy visualized the rapid re-endothelialization of the anastomotic line with longitudinal migration of endothelial cells, while the media was repaired by collagen network due to tissular fusion. The procedure demonstrated the usefulness and the adaptability of our new laser system. The prototype, characterized by low weight (2,500 g), small size 30 X 10 X 6 cm), good laser transmission, equipped with a feed-back system is well adapted to microsurgery.
Laser tissue welding is used to close wounds or to anastomose conducts such as arteries. The welding effect is due to tissue denaturation which depends on the irradiation parameters. An excessive laser irradiation leads to irreversible thermal damage. In conventional welding procedures, laser irradiation is controlled by visual inspection of the welded zone. A new device based on an 800 nm diode laser allowing an automated welding procedure is described here. The device measures the variation of the diffuse reflectance on the treatment area and stops laser irradiation when a given optimal value is reached. This allows the user to increase the reproducibility of the treatment by avoiding the subjective aspect of welding completion assessment by visual criteria. The system has been tested by in vitro welding an egg yolk membrane on mesenteric bovine arteries. The results have shown that for a given laser power, there is a specific reflectance decrease threshold value leading to an optimal welding. Reproducible welds with minimal thermal damage could be obtained.
An experimental procedure of antitumoral interstitial diode laser hyperthermia was achieved in a series of 30 Swiss nu/nu mice (10 weeks old, 25 g weight). The tumor model was a subcutaneous HT29 colonic carcinoma. The diode laser (830 nm) was applied through a 300 micrometers optic fiber implanted in the tumor and delivered at different powers as follow: group 1 -- 0.2 W (n equals 5); group 2 -- 0.5 W (n equals 4); group 3 -- 0.8 W (n equals 4); group 4 -- 1.0 W (n equals 4); group 5 -- 1.2 W (n equals 4); group 6 -- 1.5 W (n equals 4); group 7 -- 1.8 W (n equals 5). The exposure time was 900 sec (360 to 1 620 J). According to the different laser applications the temperature range was 48.2 degree(s)C to 73.7 degree(s)C in the center of the tumor and 42.0 degree(s)C to 63.5 degree(s)C in its peripheral part. The tumor was removed 24 h after laser treatment. The largest necrosis diameter evaluated by macroscopic and histological sections were 8, 11, 13.25, 17.65, 18, 18.25 and 18.5 mm in the group 1, 2, 3, 4, 5, 6, 7, respectively. In the groups 6 and 7 (1.5 and 1.8 W) all animals died after treatment, the death being explained by extensive hyperthermia. Our results suggest that 1.0 W/900 sec might be the most effective condition for treating this tumor model in Swiss nu/nu mice.
A cw 1.48 micrometers diode-laser microsurgical procedure is investigated allowing drilling of mouse zona pellucida without micromanipulators or handling of the egg outside the culture dish. The laser beam (60 - 70 mW at the focal point) and a coaxial red light aiming laser are directed through the objective (45 X) of an inverted microscope and focused in spots of 2 - 3 micrometers diameter. Mice zygotes are suspended in groups of 15 - 20 (Nunc culture dishes) in 1 ml culture medium. Egg zona is positioned with the microscope stage on the control spot and exposed to laser light (10 - 20 ms; 0.65 - 1.3 mJ). One laser pulse is sufficient to drill openings ranging from 5 - 7 micrometers diameter depending on laser power and exposure time. Drilled zygotes (N equals 150) develop to the blastocysts stage at a rate (70%) comparable to the control. There is no evidence of thermal damage under optical microscopic observation. In conclusion, the 1.48 micrometers laser radiation allows us to drill holes in mouse zona pellucida in a rapid, simple and non touch procedure. Its high absorption by water and non-mutagenicity makes it a useful tool for assisted fertilization procedures.
The intense interest in the investigation of erbium laser radiation in medicine is due to the fact that radiation at 3 micrometers is very strongly absorbed by water, which is present in all biological tissue. As a consequence of this high absorption the interaction of pulsed radiation is characterized by an explosive process with a low ablation threshold and a thin coagulation zone along the laser incisions. Erbium lasers, therefore, have a wide field of potential medical applications which become even more attractive with the availability of reliable delivery systems. An interesting situation arises in orthopaedics and angioplasty, where a precise cutting instrument is needed in a liquid environment. For this reason, we experimentally investigated the interaction mechanism of fiber transmitted, pulsed, free-running and Q- switched Erbium:YSGG ((lambda) equals 2.79 micrometers ) and Erbium:YAG ((lambda) equals 2.94 micrometers ) laser radiation with liquid water. The dynamics of the bubble formation and the propagation of shockwaves in water was studied and visualized by flash photography. Acoustic transients of a few hundreds of bars accompanying the ablation process were measured with a needle hydrophone. A clear correlation between the spikes of the laser pulse and those of the pressure signal was observed. Additionally, strong pressure transients were measured after the end of the laser pulse, which could be associated with the collapse of the vapor bubble and further collapses after multiple rebounds. The influence of pulse energy, fiber size and pulse duration on the formation and the amplitude of the pressure waves is demonstrated.
In this study the role of acoustical transients during pulsed holmium laser ablation is addressed. For this the collapse of cavitation bubbles generated by 2.12 micrometers Cr:Tm:Ho:YAG laser pulses delivered via a fiber in water is investigated. Multiple consecutive collapses of a single bubble generating acoustic transients are documented. Pulse durations are varied from 130 - 230 microsecond(s) and pulse energies from 20 - 800 mJ. Fiber diameters of 400 and 600 micrometers are used. The bubble collapse behavior is observed by time resolved fast flash photography with 1 microsecond(s) strobe lamp or 5 ns 1064 nm Nd:YAG laser illumination. A PVDF needle probe transducer is used to observe acoustic transients and measure their pressure amplitudes. Under certain conditions, at the end of the collapse phase the bubbles emit spherical acoustic transients of up to several hundred bars amplitude. After the first collapse up to two rebounds leading to further acoustic transient emissions are observed. Bubbles generated near a solid surface under water are attracted towards the surface during their development. The final phase of the collapse generating the acoustic transients takes place directly on the surface, exposing it to maximum pressure amplitudes. Our results indicate a possible mechanism of unwanted tissue damage during holmium laser application in a liquid environment as in arthroscopy or angioplasty that may set limits to the choice of laser pulse duration and energies.
A randomized study of interstitial hyperthermia with diode laser was performed on 40 Swiss nu/nu mice. The series was divided as follows: group 1 a control group; and group 2 treated by diode laser. The tumor model was a subcutaneous HT29 colonic carcinoma treated at the same size. The diode laser (830 nm) was applied through a 300 micrometers optic fiber implanted in the tumor and delivered at a power output of 200 mW and 1800 sec exposure time (360 J). The temperature range was 46 degree(s)C in the center of tumor and 42 degree(s)C in its peripheral part. In both groups, the tumor was removed 3 days after laser treatment. The tumor volume (TV) was evaluated and compared using nonparametric tests (Kruskal Wallis). Microscopic examination of tumors showed extensive and complete necrosis in group 1. By day 30 after removal the tumoral recurrence rate was 20% in group 2 versus 55% in group 1. The technical advantages of diode laser are pointed out.
Direct diode laser assisted carotid end-to-end microanastomosis (LAMA) versus contralateral manual sutures microanastomosis (CMA) were performed in 70 Wistar rats. The laser beam- wavelength 830 nm, maximum power 3 W and continuous wave was transmitted through a micromanipulator and provided a focused spot of 300 micrometers in diameter. After placement of three 10.0 stitches for edge coaptation, LAMA was achieved on left common carotid (0.8 - 1.2 mm) using laser shots (average 3) of 500 mW power, 4.5 s duration and 700 W/cm2 irradiance each. CMA was performed on right carotid by means of six 10.0 stitches. Light and scanning electron microscopy (n equals 82) showed in LAMA that re-endothelialization was complete on day 10 while collagenous fusion of media and adventitia was evident. The patency rate was 93% after LAMA versus 93% after CMA. The intra operative advantages of LAMA versus CMA: shorter operating time minimizing organ ischemia (13 min versus 22 min) and reduced endothelial trauma were evidenced. The technical advantages of diode laser were pointed out: small size of device, absence of water cooling system, no maintenance and decreasing price of diodes.
An Optical Low Coherence Reflectometer with a spatial resolution of 1.5 micrometers (FWHM) in biological tissue has been developed. This high resolution is obtained using the fluorescence generated by an Ar-laser pumped Ti-Sapphire crystal. Measurements have been performed on freshly excised pig eyes and bovine mesenteric arterial walls, as examples of transparent and diffusive biological tissues, respectively. The improved spatial resolution is used to determine more precisely optical distances and to resolve details of the intima structure.
Mechanical effects induced by laser pulses with nanosecond to microsecond duration are studied on an optical fiber as a model target. The strength of the mechanical effects is documented by monitoring the stresses induced on the target fiber and the pressure transients detected in its direct environment. Pressure transients and stresses are detected during the laser pulse and some hundreds of microseconds later at the collapse of the resulting cavitation bubble. For microsecond pulse duration, the largest induced stress is observed at the cavitation bubble collapse. For nanosecond duration already a very large stress is observed during the laser pulse, followed by a second large stress. A continuous transition between the two regimes is observed for intermediate pulse duration, as also confirmed by the pressure measurements.
Optical low coherence reflectometry (OLCR) is applied to the precise characterization of Bragg gratings in optical fibers. Grating with lengths below two millimeters have been fabricated in standard telecommunication fibers. Their OLCR spectra can be used to determine the induced index changes, which range from 0.64*10-3 to 1.16*10-3, with an accuracy of a few percent. These changes are obtained by adjusting the position of the oscillation minima in the OLCR spectra.
We present in this paper the effect of dehydration on the behavior of reflectance (R) and transmittance (T) on the arterial wall. The behavior of R and T at 633 nm as a function of tissue dehydration, measured on fresh arteries exposed to air, shows two domains of variation. For water loss up to a value of about 60% of the initial tissue weight, the tissue shrinks, but its reflectance and transmittance remain almost constant. In this region, the absorption and scattering coefficients, deduced from a diffusion model, increase with dehydration. For greater water loss, a significant increase of T and correspondent decrease of R are observed. This behavior is related to a tissue denaturation. This study shows that the dehydration is an important parameter in determining the correct dose of laser light that needs to be applied to obtain the desired effect, since it leads to variations of the tissue optical properties.
Optical coherence domain reflectometry is applied here to the study of diffusive biological tissue. This non-contact technique allows us to probe the light reflected at a tissue interface, and the back scattered light through the tissue thickness, with very fine spatial resolution and large dynamics. We present such measurements performed on an arterial wall, with a spatial resolution of 15 micrometers and a dynamic of 110 dB, and deduce from these measurements an attenuation coefficient (Sigma) t equals 19 cm-1 through the arterial wal
The variations of reflection and transmission coefficients, as well as of the fluorescence response of the arterial wall are studied as a function of temperature. In a first experiment, reflection coefficient, transmission coefficient, and temperature, are measured during laser irradiation of the tissue. The fluorescence spectrum is recorded in a second experiment, together with temperature, during heating of the arterial wall in a vapor bath. The temperature dependence of the optical parameters and of the fluorescence response are then deduced. For the reflection and transmission coefficients, a direct correlation of their variations with temperature is measured. Extrema are observed in the temperature region at about 50 degree(s)C. A global decrease of the fluorescence intensity is characteristic of a temperature increase. These variations can be used as thermal markers, which in turn could give a criterion to control heat deposition in the tissue during laser anastomose.
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