Difference in metallic wear distribution released from commercially pure titanium compared with stainless steel plates.

INTRODUCTION: Stainless steel and commercially pure titanium are widely used materials in orthopedic implants. However, it is still being controversially discussed whether there are significant differences in tissue reaction and metallic release, which should result in a recommendation for preferred use in clinical practice. MATERIALS AND METHODS: A comparative study was performed using 14 stainless steel and 8 commercially pure titanium plates retrieved after a 12-month implantation period. To avoid contamination of the tissue with the elements under investigation, surgical instruments made of zirconium dioxide were used. The tissue samples were analyzed histologically and by inductively coupled plasma atomic emission spectrometry (ICP-AES) for accumulation of the metals Fe, Cr, Mo, Ni, and Ti in the local tissues. Implant corrosion was determined by the use of scanning electron microscopy (SEM). RESULTS: With grades 2 or higher in 9 implants, steel plates revealed a higher extent of corrosion in the SEM compared with titanium, where only one implant showed corrosion grade 2. Metal uptake of all measured ions (Fe, Cr, Mo, Ni) was significantly increased after stainless steel implantation, whereas titanium revealed only high concentrations for Ti. For the two implant materials, a different distribution of the accumulated metals was found by histological examination. Whereas specimens after steel implantation revealed a diffuse siderosis of connective tissue cells, those after titanium exhibited occasionally a focal siderosis due to implantation-associated bleeding. Neither titanium- nor stainless steel-loaded tissues revealed any signs of foreign-body reaction. CONCLUSION: We conclude from the increased release of toxic, allergic, and potentially carcinogenic ions adjacent to stainless steel that commercially pure Ti should be treated as the preferred material for osteosyntheses if a removal of the implant is not intended. However, neither material provoked a foreign-body reaction in the local tissues, thus cpTi cannot be recommend as the 'golden standard' for osteosynthesis material in general.

Ligand-receptor interaction under periodic stimulation: a modeling study of concentration chemoreceptors.

The first step of chemosensory transduction consists in the association of ligand molecules with receptor proteins borne by the cell membrane. In this article, the time evolution of ligand-receptor complexes is studied in the presence of a periodically changing ligand concentration. This type of stimulation is a close approximation to some natural situations, for example in olfaction. The transient and steady-state periodic levels of the complexes, resulting from a single-step (binding) or double-step (binding and activation) reaction, are determined. When possible, analytical solutions are given, if not for the complete model, at least for its simplified version at low ligand concentration. Otherwise, solutions are found numerically and both the complete and simplified versions of the model are compared. The results obtained are discussed with respect to actual experimental data based on the moth sex-pheromone receptor. Periodic steady states are achieved very quickly and their amplitude decreases when the stimulation frequency increases. We show that the simplified description is adequate if only a fraction of activated receptors is sufficient to produce the maximum response, as is actually the case in the example treated. The role of the frequency of stimulation is investigated and it is shown to possess an optimal range between 2 and 5 Hz.

Alternative food, switching predators, and the persistence of predator-prey systems.

Sigmoid functional responses may arise from a variety of mechanisms, one of which is switching to alternative food sources. It has long been known that sigmoid (Holling's Type III) functional responses may stabilize an otherwise unstable equilibrium of prey and predators in Lotka-Volterra models. This poses the question of under what conditions such switching-mediated stability is likely to occur. A more complete understanding of the effect of predator switching would therefore require the analysis of one-predator/two-prey models, but these are difficult to analyze. We studied a model based on the simplifying assumption that the alternative food source has a fixed density. A well-known result from optimal foraging theory is that when prey density drops below a threshold density, optimally foraging predators will switch to alternative food, either by including the alternative food in their diet (in a fine-grained environment) or by moving to the alternative food source (in a coarse-grained environment). Analyzing the population dynamical consequences of such stepwise switches, we found that equilibria will not be stable at all. For suboptimal predators, a more gradual change will occur, resulting in stable equilibria for a limited range of alternative food types. This range is notably narrow in a fine-grained environment. Yet, even if switching to alternative food does not stabilize the equilibrium, it may prevent unbounded oscillations and thus promote persistence. These dynamics can well be understood from the occurrence of an abrupt (or at least steep) change in the prey isocline. Whereas local stability is favored only by specific types of alternative food, persistence of prey and predators is promoted by a much wider range of food types.

Analysis of high purity graphite and silicon carbide by direct solid sampling electrothermal atomic absorption spectrometry.

Solid sampling electrothermal atomic absorption spectrometry using the boat technique and a transversely heated graphite tube was applied to direct analysis of graphite and silicon carbide powders for 14 and 12 impurity elements, respectively. With graphite, for all analytes under investigation, a very effective in-situ analyte/matrix separation was achieved. That was the case also for analytes in silicon carbide requiring atomization temperatures below 2400 degrees C. At higher atomization temperatures, the decomposition products of silicon carbide give rise to significant background, which can still be corrected. Sample amounts of up to 4 mg graphite and 8 mg silicon carbide per analysis cycle were applied. For all analytes in both materials, limits of detection at the lower ng g(-1) and sub-ng g(-1) level were achieved, excluding arsenic for which they were 50 ng g(-1) and 23 ng g(-1) for graphite and silicon carbide, respectively. Quantification was performed using calibration curves measured with aqueous standard solutions. The accuracy was checked by comparison of the results with those obtained by instrumental neutron activation analysis and by other independent methods.

Optimal intraguild foraging and population stability.

This article explores effects of adaptive intraguild predation on species coexistence and community structure in three species' food webs. Two Lotka-Volterra models that assume a trade-off between competition and predation strength are considered in detail. The first model does not explicitly model resource dynamics and is considered with both nonadaptive and adaptive intraguild predation; in the latter case predators choose their diet in order to maximize their instantaneous population growth rate. The second model includes resource population dynamics. Effects of adaptive intraguild predation on the community structure along a gradient in environment productivity are analyzed and compared with some experimental results of protist food webs. Conditions under which intraguild predation is adaptive are discussed for both models. It is proved that if intraguild predators are perfect optimizers then intraguild predation should decrease with increasing environmental productivity and adaptive intraguild predation is a stabilizing factor provided environmental productivity is high enough.

Perireceptor and receptor events in olfaction. Comparison of concentration and flux detectors: a modeling study.

Transduction in chemosensory cells begins with the association of ligand molecules to receptor proteins borne by the cell membrane. The receptor-ligand complexes formed act as signaling compounds that trigger a G-protein cascade. This receptor-ligand interaction, described here by a single-step or double-step reaction, depends on factors controlling the access of the ligand molecules to the cell membrane. Two basic mechanisms can be distinguished: concentration detectors (CD), in which the ligand can freely diffuse to the membrane, and flux detectors (FD), in which it accumulates irreversibly in a distinct perireceptor space where it is chemically deactivated. These two systems, plus their generalization, are investigated and compared. The transient and steady-state numbers of complexes are studied as a function of the external ligand concentration. The biological significance of the results is shown in a well-studied example of FD, the insect sex-pheromone olfactory receptor neuron. How the number of complexes can code for the intensity of stimulation is analyzed using the size, dynamic range and sensitivity of the steady-state responses, and the time needed to reach a predefined level of the transient responses. It is shown that the FD design affords a large increase in sensitivity (a shift of the threshold response towards low concentration) with respect to the CD design, which is paid for by a lesser ability to follow fast changes in stimulus intensity.

A mechanistic model for partial preferences.

Classic prey optimal foraging model assumes that individual predators are globally omniscient; that is, they have exact knowledge of prey population densities in the environment. This study examines a spatially explicit individual-based model of a one-predator two-prey system where individual predators are assumed to be omniscient only locally, i.e., to know prey population densities only in the range of their perception. Due to local variations in prey numbers, the probability of acceptance of less profitable prey shifts from the zero-one rule to a gradually decreasing function, for which an explicit formula is derived, giving way to partial preferences. A corresponding predator functional response to more profitable prey is shown to have a sigmoid-like form.

Patch choice under predation hazard.

In this paper we study optimal animal movement in heterogeneous environments consisting of several food patches in which animals trade-off energy gain versus predation risk. We derive a myopic optimization rule describing optimal animal movements by fitness maximization assuming an animal state is described by a single quantity (such as weight, size, or energy reserves). This rule predicts a critical state at which an animal should switch from a more dangerous and more profitable patch to a less dangerous and less profitable patch. Qualitatively, there are two types of behavior: either the animal switches from one patch to another and stays in the new patch for some time before it switches again, or the animal switches between two patches instantaneously. The former case happens if animal state growth is positive in all patches, while the latter case happens if animal state growth is negative in one patch. In particular, this happens if one patch is a refuge. We consider in detail two special cases. The first one assumes a linear animal state growth while the second assumes a saturating animal state growth described by the von Bertalanffy curve. For the first model the proportion of time spent in the more profitable and more risky patch increases with profitability of this patch when state growth is positive in both patches. On contrary, if state growth is negative in the less profitable and safer patch, animals spend proportionally less time in the more profitable and more risky patch as its profitability increases. As a function of the predation risk in the more profitable patch the time spent there proportionally decreases. When animal state growth is described by the saturating curve, time spent in the more risky patch is a hump-shaped curve if state growth is positive in both patches. Our results extend the mu/f rule, which predicts that animals should behave in such a way as to minimize mortality risk to resource intake ratio.

Direct solid sampling electrothermal atomic absorption spectrometry for the analysis of high-purity niobium pentaoxide.

A direct solid sampling electrothermal atomic absorption spectrometry (SoS-ETAAS) method for ultratrace analysis of powdered niobium pentaoxide for Al, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni and Zn has been developed. The elements K, Mg, Na and Zn could be determined without any chemical modification. However, in the determination of the elements Al, Co, Cr, Cu, Fe, Mn and Ni, serious matrix-caused non-spectral interferences and background occurred which made their determination impossible. This problem was remedied by conversion of the niobium pentaoxide matrix into the thermally stable niobium carbide by using methane atmosphere during the pyrolysis stage. The development resulted in establishing an extraordinary powerful method for the analysis of niobium pentaoxide in term of limits of detection, accuracy, simplicity and analysis time. Quantification was performed using calibration curves measured with aqueous standard solutions. The accuracy was checked by comparing the results with those obtained by ETAAS in analysis of slurries and digests of the sample. Due to almost complete freedom of blank and high applicable sample amounts (up to 15 mg), extremely low limits of detection (0.5-2 ng/g) were achieved.

Optimal foraging and predator-prey dynamics, II.

In this paper we consider one-predator-two-prey population dynamics described by a control system. We study and compare conditions for permanence of the system for three types of predator feeding behaviors: (i) specialized feeding on the more profitable prey type, (ii) generalized feeding on both prey types, and (iii) optimal foraging behavior. We show that the region of parameter space leading to permanence for optimal foraging behavior is smaller than that for specialized behavior, but larger than that for generalized behavior. This suggests that optimal foraging behavior of predators may promote coexistence in predator-prey systems. We also study the effects of the above three feeding behaviors on apparent competition between the two prey types.

Effects of Optimal Antipredator Behavior of Prey on Predator-Prey Dynamics: The Role of Refuges.

The influence of optimal antipredator behavior of prey on predator-prey dynamics in a two-patch environment is studied. One patch represents an open habitat while the other is a refuge for prey. It is assumed that prey maximize their fitness measured by the instantaneous per capita growth rate. In each patch population dynamics is described by the Lotka-Volterra time continuous model. The refuge is characterized by its protectiveness which is inversely related to the predation risk for prey, and the dependence of population dynamics on protectiveness is studied. It is shown that adaptive behavior of prey changes qualitative properties of the underlying Lotka-Volterra model due to the appearance of a bounded attractor. Adaptive prey behavior does not lead to a stable equilibrium but to the reduction of population fluctuations. Dynamic consequences of a limited carrying capacity of the refuge are also considered. Low refuge carrying capacity leads to stability of predator-prey dynamics while stability is lost when the carrying capacity of the refuge is high. Lastly, it is shown that optimal antipredator behavior of prey leads to persistence and reduction of oscillations in population densities. Copyright 1998 Academic Press.

An electrothermal atomic absorption spectrometer using semiconductor diode lasers and a tungsten coil atomizer: design and first applications.

A new type of atomic absorption spectrometer using a laser diode as light source and a tungsten coil as atomizer is described. Compared to established atomic absorption spectrometers, it is much simpler in construction, smaller in size, and less expensive and it provides inherent background correction and high detection power. The performance of this concept is demonstrated by the determination of aluminum and chromium in water, blood serum and, using the slurry sampling technique, in powdered high-purity graphite and titanium dioxide samples. For calibration, the standard addition method was used. Possible interferences by impurities originating from the tungsten coils are discussed. Applying aqueous solutions of Al and Cr, detection limits of 0.9 and 0.03 ng/mL, respectively, were obtained, and for serum, they were 2.5 and 0.3 ng/mL, respectively. For these elements in graphite and titanium dioxide applied as slurry, the detection limits are between 0.02 (Cr in TiO2) and 0.6 micrograms/g (Al in graphite). The accuracy was checked by comparison of the results with those of other methods. The described system is especially suitable for on-site and on-line analysis.

A graphite furnace electrothermal vaporization system for inductively coupled plasma atomic emission spectrometry.

An improved graphite furnace electrothermal vaporization device equipped with an autosampler for inductively coupled plasma atomic emission spectrometry is presented. The transport losses of eight selected analytes in the individual segments of the device were determined by means of the radiotracer technique by applying amounts traced comparable to those to be determined in real samples. The results obtained from the radiotracer study were the basis for further improvement of the interface design, leading to considerable increase of the total transport efficiency, which finally was found to be between 26 (for Cr) and 57% (for Ga). The whole system consists of a graphite furnace vaporizer, a power supply, a gas flow box, and an autosampler with incorporated microbalance. The temperature program, gas flows, and autosampler functions are controlled by a data station which also provides the data acquisition and processing of the transient signals. The performance parameters of the developed system were evaluated using aqueous standard solutions. Absolute limits of detection for most analytes were between 0.1 and 1 ng, and for As, K, Ni and Pb, they were between 2 and 3.2 ng.

Searching for Food or Hosts: The Influence of Parasitoids Behavior on Host-Parasitoid Dynamics

A host-parasitoid system with overlapping generations is considered. The dynamics of the system is described by differential equations with a control parameter describing the behavior of the parasitoids. The control parameter models how the parasitoids split their time between searching for hosts and searching for non-host food. The choice of the control parameter is based on the assumption that each parasitoid maximizes the instantaneous growth rate of the number of copies of its genotype. It is shown that optimal individual behavior of parasitoids, with respect to time sharing between hosts and food searching, may have a stabilizing effect on the host-parasitoid dynamics.

Optimal Foraging and Predator-Prey Dynamics

A system consisting of a population of predators and two types of prey is considered. The dynamics of the system is described by differential equations with controls. The controls model how predators forage on each of the two types of prey. The choice of these controls is based on the standard assumption in the theory of optimal foraging which requires that each predator maximizes the net rate of energy intake during foraging. Since this choice depends on the densities of populations involved, this allows us to link the optimal behavior of an individual with the dynamics of the whole system. Simple qualitative analysis and some simulations show the qualitative behavior of such a system. The effect of the optimal diet choice on the stability of the system is discussed.

Radiotracers for the determination of the accuracy of trace element analyses.

Many analytical techniques for the determination of trace elements in environmental samples are plagued by ill-defined systematic errors. Investigations of analytical techniques with radioactively labelled analytes can identify systematic errors, lead to their elimination, and can improve the accuracy of the results. Radiotracers are directly detected with high sensitivity and great rapidity in most cases by gamma-ray counters. Examples of the use of radiotracer techniques for the investigation of the following steps of analytical procedures are presented: presampling events, sampling and sample handling, mineralization of samples, separation and preconcentration, and determination. Most of the examples discussed in the text were taken from the analytical chemistry of mercury, sulfur, lead, sodium, zinc and selenium. Other examples are summarized in tables. The results of these investigations clearly show the superiority of the radiotracer technique over other techniques. A more frequent use of the radiotracer technique in investigations aimed at improving the accuracy of analytical procedures is certainly desirable.

Extraction of Mo, W and Tc with polyurethane foam and with cyclic polyether from SCN-/HCL medium.

The extraction of molybdenum, tungsten and technetium by polyether-based polyurethane foam and by a cyclic polyether from aqueous thiocyanate solutions is described. The influence of the reductants stannous chloride and ascorbic acid has also been studied. The possibilities of the polyurethane foam for preconcentration and determination of molybdenum are discussed.

The distribution of elements between polyether-type polyurethane foam, cyclic polyethers and hydrofluoric acid solution.

The extraction of fifteen elements in the systems consisting of polyurethane foam (polyether type)/HF-alkali-metal fluoride and cyclic polyether/HF-alkali-metal fluoride has been investigated. Little or no extraction was found for Co, Zn, Fe, Zr, Hf, Sn(IV), Nb, Pa, As(V), Mo, and W. Only Ta, Sb, Re, and Tc are well extracted with either polyurethane foam or dicyclohexano-18-crown-6 in dichloro-ethane under the conditions investigated. The mechanism of the extraction is discussed. The most interesting separation possibilities have been tested.

Open wet ashing of some types of biological materials for the determination of mercury and other toxic elements.

An open wet decomposition procedure for trace element analysis of biological material has been worked out. It is based on the utilization of nitric acid, hydrochloric acid and hydrogen peroxide and is well suited for the determination of As, Bi, Cd, Cr, Hg, Pb, Sb, Se and Tl. For these elements the recoveries were examined by the radiotracer technique, with lichens, pine needles, grass and human hair as representative biological samples. In addition to the conventional labelling technique, use was also made of in situ labelling. In all cases, recoveries between 93 and 100% were achieved, which are adequate for quantitative trace element analysis.

A group separation based on anion- and cation-exchange from hydrofluoric acid medium Application to multielement neutron-activation analysis of niobium.

A group separation procedure for the determination of 33 elements has been developed and applied to the neutron-activation analysis of niobium. It is based on cation- and anion-exchange chromatography in HF media and separates the elements into five groups suitable for gamma spectrometry. The recoveries and their reproducibilities have been studied by the radiotracer technique. Recovery 95% (mean deviation 3%) was obtained for all the elements tested. For a 12-hr irradiation at a thermal neutron flux of 8 x 10(13) n.cm(-2).sec(-1), the limits of detection for a niobium matrix vary between 0.6 pg (for Mn) and 0.6 mug (for Fe).