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Biomedical subjects

P Helander

Publications and source records attributed to P Helander.

At least 19 recordsLinked to original sources
PubMed2006

A histological and immunohistochemical study of tissue reactions to solid poly(ortho ester) in rabbits.

In many cases only the temporary presence of a biomaterial is needed in tissue support, augmentation or replacement. In such cases biodegradable materials are better alternatives than biostable ones. At present, biodegradable polymers are widely used in the field of maxillofacial surgery as sutures, fracture fixation devices and as absorbable membranes. The most often used polymers are aliphatic polyesters, such as polyglycolic acid (PGA) and polylactic acid (PLA). Poly(ortho ester) is a surface eroding polymer, which has been under development since 1970, but is used mostly in drug delivery systems in semisolid form. The aim of this study was to evaluate the tissue reactions of solid poly(ortho ester) (POE), histologically and immunohistochemically. Resorption times and the effect of 2 different sterilization methods (gamma radiation and ethylene oxide) upon resorption were also evaluated. Material was implanted into the tibia and subcutaneously into the mandibular ramus area of 24 rabbits. Follow-up times were 1-10, 14 and 24 weeks. Histological studies showed that POE induces a moderate inflammation in soft tissue and in bone. At 24 week follow-up, inflammation was mild in soft tissue and moderate in bone. In immunohistochemical studies, no highly fluorescent layer of tenascin or fibronectin was found adjacent to the implant. Resorption of gamma-sterilized rods was faster than ethylene oxide-sterilized rods. The total resorption time was more than 24 weeks in both groups. Clinically the healing was uneventful and the implants the well tolerated by the living tissue. This encourages these authors to continue studies with this interesting new material to search for the ideal material for bone filling and fracture fixation.

Absorbable Implants↗
PubMed2006

Model for current-driven edge-localized modes.

Edge-localized modes (ELMs) are cyclic disturbances in the outer region of tokamak plasmas that are influential in determining present and future tokamak performance. In this Letter, we outline an approach to modeling ELMs in which we envisage toroidal peeling modes initiating a Taylor relaxation [Phys. Rev. Lett. 33, 1139 (1974)10.1103/PhysRevLett.33.1139] of a tokamak outer region plasma. Relaxation produces a peeling destabilizing flattened edge current profile and a stabilizing plasma-vacuum current sheet; the balance between the two determines the radial extent of the relaxed region. The model can be used to predict the energy losses due to an ELM and reproduces experimentally observed variations with edge safety factor and plasma collisionality. There is an intrinsic "deterministic scatter" in the model that also accords with observation.

Journal Article↗
PubMed2004

Current dynamics during disruptions in large tokamaks.

Self-consistent modeling of the evolution of the plasma current during disruptions in large tokamaks is presented, taking into account both the generation of runaway electrons and their backreaction on the electric field. It is found that the current profile changes dramatically, so that the postdisruption current carried by runaway electrons is much more peaked than the thermal predisruption current. Although only a fraction of the thermal current is converted into runaway electrons, the central current density increases significantly for typical parameters in JET and ITER. It is also shown that the radial runaway profile can easily become filamented in the radial direction.

Journal Article↗
PubMed2003

Positron creation and annihilation in tokamak plasmas with runaway electrons.

It is shown that electron-positron pair production is expected to occur in post-disruption plasmas in large tokamaks, including JET and JT-60U, where up to about 10(14) positrons may be created in collisions between multi-MeV runaway electrons and thermal particles. If the loop voltage is large enough, they are accelerated and form a beam of long-lived runaway positrons in the direction opposite to that of the electrons; if the loop voltage is smaller, the positrons have a lifetime of a few hundred ms, in which they are slowed down to energies comparable to that of the cool ( less, similar 10 eV) background plasma before being annihilated.

Journal Article↗
PubMed2003

Solar flares as cascades of reconnecting magnetic loops.

A model for the solar coronal magnetic field is proposed where multiple directed loops evolve in space and time. Loops injected at small scales are anchored by footpoints of opposite polarity moving randomly on a surface. Nearby footpoints of the same polarity aggregate, and loops can reconnect when they collide. This may trigger a cascade of further reconnection, representing a solar flare. Numerical simulations show that a power law distribution of flare energies emerges, associated with a scale-free network of loops, indicating self-organized criticality.

Journal Article↗
PubMed2002

Ion acceleration during reconnection in MAST.

Spontaneous acceleration of ions to suprathermal energies is observed during magnetic reconnection in the Mega-Ampere Spherical Tokamak (MAST). A high-energy tail is observed in the ion-distribution function following each internal reconnection event in Ohmic discharges. This phenomenon is explained in terms of runaway ion acceleration in the electric field induced by the reconnection.

Journal Article↗
PubMed2002

Effect of poloidal density variation of neutral atoms on the tokamak edge.

Neutral atoms in the tokamak edge affect the electric field and plasma flow through charge exchange. Here, it is shown that these effects depend sensitively on the poloidal location of the atoms. It is found that the radial electric field and toroidal flow velocity in a collisional edge plasma are largest if the atoms are concentrated on the inboard side. This feature may explain MAST and COMPASS-D observations indicating easier H-mode access for inboard gas puffing. The results suggest an external means of controlling the toroidal flow and radial electric field and their shear, and have motivated further MAST measurements appearing to support the predictions.

Journal Article↗
PubMed2002

Approximate solutions of two-way diffusion equations.

In this paper, a general and systematic scheme is formulated for finding approximate solutions of two-way diffusion equations. This expansion scheme is valid for arbitrary mean-free path and can be carried out to any desired accuracy. Its potential is demonstrated by constructing approximate solutions for two problems concerning the kinetics of an electron beam, and the accuracy is found to be very good even when only a few terms are included in the expansion. The approximate solutions found are compared with numerical calculations and previous analytical work in the literature.

Journal Article↗
PubMed2002

L-H transition in the mega-amp spherical tokamak.

H-mode plasmas have been achieved on the MAST spherical tokamak at input power considerably higher than predicted by conventional threshold scalings. Following L- H transition, a clear improvement in energy confinement is obtained, exceeding recent international scalings even at densities approaching the Greenwald density limit. Transition is accompanied by an order-of-magnitude increase in edge-density gradient, a marked decrease in turbulence, the efficient conversion of internal electron Bernstein waves into free space waves, and the onset and saturation of edge poloidal rotation.

Journal Article↗
PubMed2000

H-Mode operation in the START spherical tokamak

H-mode operation has been achieved in high current (I(p)>200 kA) plasmas in the START spherical tokamak for both neutral-beam-injection-heated and Ohmic discharges. The transition to H mode features the development of well-defined edge pedestals in density and temperature, which signifies the formation of an edge-transport barrier, and associated edge-localized modes. Recent operation at plasma currents exceeding 250 kA shows that these features are accompanied by increases in energy confinement time. This is the first clear demonstration of the H-mode regime in a spherical tokamak.

Journal Article↗
PubMed1999

Exactly solvable sandpile with fractal avalanching.

A simple one-dimensional sandpile model is constructed which possesses exact analytical solvability while displaying both scale-free behavior and fractal properties. The sandpile grows by avalanching on all scales, yet its shape and energy content are described by a simple, continuous (but nowhere differentiable) analytical formula. The avalanche energy distribution and the avalanche time series are both power laws with index -1 ("1/f spectra").

Journal Article↗
PubMed1997

The optothermal approach to a real time monitoring of glucose content during fermentation by brewers' yeast.

During production of both normal and low-alcohol beers, sugar is fermented to ethanol, carbon dioxide and several flavour products. Tight control of fermentation is necessary in order to keep production costs low, and to prevent formation of excessive ethanol in low-alcohol beer. Several types of control devices based on, e.g., determination of carbon dioxide, ethanol, and extract have been developed so far; the main disadvantage of these devices is their unsuitability for on-line applications. Here, the optothermal window was used in a laboratory experiment as a new sensor for real time monitoring fermentation of glucose by Saccharomyces cerevisiae, and the results were compared to those obtained by conventional techniques.

Beer↗
PubMed1996

Compact, open and general purpose cell of variable effective pathlength: direct absorption measurement of SO(4)(2-) in water.

The use of an optothermal window (OW) was proposed for the direct (no need for sample preparation) spectroscopic, non-destructive measurement of SO(4)(2-) in water at 1078 cm(-1). The presently determined limit of detection (LOD) of 1 mmol/L is comparable to that provided by CO(2) laser photoacoustic spectroscopy, but about one order of magnitude superior to that obtainable by the ATR method.

Journal Article↗
PubMed1989

Determination of haemoglobin and prothrombin complex in whole blood using optothermal spectrometry.

Optothermal spectrometry measures the thermal energy produced as a result of absorption by molecules at a given modulation frequency and wavelength. Depending on the modulation frequency used, analysis can be performed in very thin layers (50-150 micrometers). A major advantage of optothermal spectrometry is that it is not very sensitive to light scattering. Haemoglobin in whole blood was measured without any reagent at 16 Hz and 2 Hz frequencies. The precisions (within-series, within-day and between-day) were acceptable, and comparisons with reference methods were excellent. As opposed to the reference methods, optothermal spectrometric determinations were not affected by lipaemia. When measured continuously, the signal for haemoglobin increased due to the packing of erythrocytes towards the light source. By converting soluble fibrinogen into insoluble fibrin, the erythrocyte packing could either be inhibited or stopped, and this process could be monitored by assessing the change in the rate of signal increase for haemoglobin. This principle was utilized to analyse the prothrombin complex; the method was found to have acceptable precision and to be comparable to a routine method.

Blood Coagulation Factors↗
PubMed1988

Hydration of human stratum corneum studied in vivo by optothermal infrared spectrometry, electrical capacitance measurement, and evaporimetry.

Optothermal infrared spectrometry (OTIS) is a recently introduced method for specific measurement, in vivo, of water in the skin. In the present study the method proved well suited to register the increase in water content in stratum corneum following application of emollients. The results were compared with those obtained with a commercial instrument, the Corneometer, and the two methods were found to match very closely. Neither method indicated any difference in hydration of normal skin between young and elderly women. Evaporimetry was used to detect any influence on the water barrier function of the skin following application of emollients; even though the water content of the skin was significantly higher after emollient treatment, the transepidermal water loss remained unchanged.

Adolescent↗