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Serum albumins and polylactic acid (PLA) have been used as bioerodable polymers in the preparation of drug-containing microspheres for parenteral drug delivery. The albumin microsphere may be prepared via either chemical cross-linking or heat denaturation of the protein. Heat-denatured albumin microspheres containing mitomycin C (MMC) have been used in pre-clinical and clinical investigations. Due to the high reactivity of MMC as a bifunctional alkylating agent, a study on the stability of MMC in the albumin and PLA microspheres has been carried out using a high-performance liquid chromatographic (HPLC) method. Human serum albumin (HSA) microspheres were prepared using an emulsion method via either heat denaturation at 120 or 170 degrees C or the use of 0.5 M biacetyl as a cross-linking agent. The PLA microspheres were prepared by an emulsion method at 55 degrees C. HPLC analysis of the HSA microspheres showed that about 37% of MMC was converted to 2,7-diaminomitosene derivatives in microspheres prepared by heat denaturation at 120 degrees C. The degradation increased to 82% when the microspheres were prepared with a denaturation temperature of 170 degrees C. The use of biacetyl as a cross-linking agent in the preparation of HSA microspheres resulted in a complete degradation of the incorporated MMC. Biacetyl was found to interact with MMC leading to the formation of 7-aminomitosene derivatives. In contrast to the albumin system, MMC may be incorporated into PLA microspheres without degradation. 相似文献
105.
Relay feedback systems are used to control engineering devices. In practice the switching between different functional forms
of the system is never instantaneous, but takes place after a small delay. In this paper we analyse the dynamics and bifurcations
of a representative example of such systems. In the absence of delay, negative feedback results only in unimodal symmetric
limit cycles, but positive feedback can lead to aperiodic trajectories and chaos. In the presence of delay, the system can
behave as an equivalent system without delay, provided that the delay is small in a sense which we define precisely. For
larger delays, we identify a new bifurcation phenomenon, an event collision, where the delayed switching manifold intersects
the relay hysteretic lines. In this case the dynamics become much more complicated. 相似文献
106.
A three-dimensional trap for Rydberg atoms in selected Stark states has been realized experimentally. H atoms seeded in a supersonic expansion of Ar are excited to the low-field seeking n=30, k=25, |m|=0, 2 Rydberg-Stark states, decelerated from a mean initial velocity of 665 m/s to zero velocity in the laboratory frame and loaded into a three-dimensional electrostatic trap. The motion of the cold Rydberg atom cloud in the trap and the decay of the trapped atoms have been studied by pulsed electric field ionization and imaging techniques. 相似文献
107.
Extensive new high-temperature, high-resolution FTIR emission spectroscopy measurements for the five common isotopomers of GeO are combined with previous diode laser and microwave measurements in combined isotopomer analyses. New Dunham expansion parameters and an accurate analytical potential energy function are determined for the ground X1Sigma+ state. Copyright 1999 Academic Press. 相似文献
108.
Gregory J. Barbante Conor F. Hogan Andrew B. Hughes 《Journal of Solid State Electrochemistry》2009,13(4):599-608
The solid state electrochemistry and solid state spectroelectrochemistry of two ruthenium complexes, ruthenium tris-(4,7-diphenyl-1,10-phenanthroline)
bis-hexafluorophosphate, [Ru(dpp)3](PF6)2, and ruthenium bis-(2,2′-bipyridine)(4,6-diphenyl-2,2′-bipyridine)bis-hexafluorophosphate, [Ru(bpy)2(dpb)](PF6)2, is described. Microparticles of the material are immobilised on ITO electrodes, and stable voltammetric signals are obtained
in contact with aqueous electrolyte solution. Spectral changes monitored during a slow cyclic voltammetric scan confirm the
exhaustive oxidation of the Ru2+ species to the Ru3+ form. The derivative of the absorbance signal monitored at a single wavelength during potential cycling is morphologically
identical to a cyclic voltammogram with no background current. This technique is shown to be useful when peaks of small magnitude
are obscured by capacitive background or when peaks close to the solvent limit are obscured by solvent electrolysis current.
The technique effectively widens the electrochemical window available for voltammetric measurements. After suitable correction
of the signal, the value of the voltammetric peak height (I
p) as well as peak potential (E
p) may be obtained from the derivative absorbance signal. Chronospectrometry is demonstrated to provide the equivalent to a
chronocoulometric response, but is closer to the ideal simulated response. A facile method for simulating time or potential-dependant
spectroelectrochemical responses using commercial electrochemical simulation software is described. Absorbance transients
monitored during the electrolysis of solid particles of [Ru(dpp)3](PF6)2 show best agreement with simulated data at very short and very long timescales. This observation, in conjunction with the
observations from the potential scan experiments, suggests that the absorbance, charge, or current vs. time behaviour of the
system can be adequately described by a semi-infinite diffusional model at short experimental timescales and by a finite diffusional
model at sufficiently long timescales.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
Dedicated to the 80th birthday of Keith B. Oldham. 相似文献
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In the case of low immersion high-speed milling, the ratio of time spent cutting to not cutting can be considered as a small parameter. In this case the classical regenerative vibration model of machine tool vibrations reduces to a simplified discrete mathematical model. The corresponding stability charts contain stability boundaries related to period doubling and Neimark-Sacker bifurcations. The subcriticality of both types of bifurcations is proved in this paper. Further, global period-2 orbits are found and analyzed. In connection with these orbits, the existence of chaotic motion is demonstrated for realistic high-speed milling parameters. 相似文献