Forced modulation of biological processes: A review

Cell culture or biochemical processes utilizing free or immobilized cells or enzymes are candidates for periodic operation. Even though these processes are isothermal, inhibition frequently arises giving their kinetics a push-pull character of the Turing type so that excited states or multiplicity are observed. Modulation of an input such as a substrate or a nutrient concentration or the cell environment such as the pH can enhance the rates of biochemical reactions that are occurring. This possibility was realized by explorers of reactor periodic operation in the 1980s. However, the use of rotating disc biological filters for aerobic waste treatment precedes these activities by at least a decade. The biologically active slime on the disc surface is exposed alternately to air and to an aqueous media containing dissolved and/or suspended waste matter. Indeed, periodic variation of reactor volume in a CSTR, which is equivalent to periodic variation of space velocity, was explored in the early 1970s. The application of this was to activated sludge waste treatment. Periodic operation has been used to address the problem of plasmid loss in continuous fermenters to produce genetically altered microorganisms. A twofold increase in the concentration of plasmid bearing cells using square wave modulation of the dilution rate has been found in a simulation using a model validated by experimental data. These results are in agreement with other studies attempting to control species populations in chemostats. Improvement in the rate of fermentation of a substrate to form a metabolite has been addressed by modulation. It has been shown that the rate of xylose utilization by a yeast is affected by a periodic variation of pH. These were lowered, but the metabolite yield, ethanol, remained the same. With secondary metabolites associated with the non-growth period in fermentation, modulation of the growth-limiting substrate concentration fed to immobilized live cells resulted in significantly higher yields. Differences in the effect of cycling on primary and secondary metabolites has been observed in several biochemical systems, such as those used for penicillin and monoclonal antibody production or in citric acid fermentation.     

Adsorption of hydrocarbons by ZSM-5 zeolites with different SiO2/Al2O3 ratios: a combined FTIR and gravimetric study

In metal-free ZSM-5, the decrease in the concentration of Bronsted and Lewis acid sites parallels the increase in the SiO₂/Al₂O₃ ratio. In the presence of residual moisture, the adsorption of small alkanes does not markedly depend on the SiO₂/Al₂O₃ ratio in the range from 30 to 150. The acidity is more important for the adsorption of alkenes (propene) which oligomerize following adsorption. The addition of 5 wt.% of La does not significantly affect the strength of the surface acid sites but does decrease their concentration. It causes a transformation, in a part, of the Bronsted acid sites. These new centers could perhaps be assigned to LaO(OH) or La(OH)₂⁺ species which may be formed on the surface in La-containing ZSM-5. La could exchange only the strongest acidic sites without inducing large changes in their total concentration. La addition has a very small effect on the adsorption of small hydrocarbons. The exchange of the protons by Cu²⁺ decreases the concentration of Bronsted acid sites. At a sufficient loading of the ZSM-5 by Cu²⁺, i.e. at an atomic ratio Cu/Al > 1.0, the exchange can almost totally suppress the Bronsted acidity. Another effect of the ion exchange is the suppression of the adsorption capacity of small hydrocarbons due to lowered surface area and internal pore volume. The lowered available internal pore volume also limits the extent of the oligomerization of propene.     

Stabilization of water-in-octane nano-emulsion. Part I: Stabilized by mixed surfactant systems

In this paper, four nonionic binary mixed surfactant systems (Tween 80/Span 80, Tween 80/Span 85, OP 10/Span 80 and OP 10/Span 85) were used to emulsify water/octane mixture. The influence of emulsion composition and preparation method on the stability of water-in-octane emulsion was investigated systematically. The mixture of Tween 80/Span 80 had the best synergistic effect. Those mixed surfactant systems with HLB = 9 stabilized water-in-octane emulsion the best. Under optimal condition, water-in-octane nano-emulsion (the effective diameter of water droplets = 200 nm) could be made. These water-in-octane nano-emulsions could be stored for more than 26 days at 25 °C without phase separation.     

挖掘装载机转斗油缸稳定性分析

对挖掘装载机装载作业中可能发生的转斗油缸失稳弯曲现象的原因进行了分析,给出了不发生失稳的转斗油缸最大长度的限制条件,为装载装置连杆机械的设计打下了基础。     

SBS改性沥青稳定机理的研究

通过加入相容剂和稳定剂来稳定SBS在沥青中的分散,制备贮存稳定性良好的SBS改性沥青,并探讨其稳定定理。研究表明,相容剂能够溶胀SBS的硬嵌段－－聚苯乙烯微区,有利于SBS在沥青中均匀分散,稳定剂能够引发SBS交联形成网络结构和沥青接枝到SBS上,稳定SBS在沥青中的分散,形成宏观均匀稳定的体系。     

钾离子和钙离子协同抑制作用的探讨

为深入探索钾盐和钙盐对泥页岩水化作用的抑制潜能,文中就钾盐和钙盐,特别是混合盐对泥页岩的抑制作用进行了实验研究。结果表明,K~ 和Ca~(2 )抑制页岩分散的能力顺序与离子浓度有关;Ca~(2 )抑制泥页岩分散作用随浓度的变化规律与K~ 的不同。实验还发现,对某些泥页岩而言,当混合盐溶液达到合理的离子配比时,具有离子协同抑制作用。     

增加挡墙稳定的措施

针对挡墙抗滑、抗倾、基底应力及偏心难以满足规范要求的情况，介绍几种增加挡墙稳定的措施。     

Robustness of quantized control systems with mismatch between coder/decoder initializations

This paper analyzes the stability of linear systems with quantized feedback in the presence of mismatch between the initial conditions at the coder and decoder. Under the assumption of the perfect channel, we show that using the scheme proposed in [Liberzon, D., & Neši?, D. (2007). Input-to-state stabilization of linear systems with quantized state measurements. Institute of Electrical and Electronic Engineers Transaction on Automatic Control, 52, 767-781] it is possible to achieve stability with exponential convergence of linear systems with quantized feedback when the coder and decoder are initialized at different initial conditions.     

Synthesis and Characterization of Thrombin Conjugated γ‐Fe2O3 Magnetic Nanoparticles for Hemostasis

Thrombin is the final protease produced in the clotting pathways. Thrombin has been used in the clinic more than six decades for topical hemostasis and wound management. In human plasma the half‐life of thrombin is shorter than 15 seconds due to close control by inhibitors. In order to stabilize thrombin, this enzyme was conjugated covalently and physically to γ‐Fe₂O₃ magnetic nanoparticles. The physical conjugation was accomplished through adsorption of thrombin to BSA coating on the nanoparticles. The coagulant activity of the covalently bound thrombin was significantly lower than that of the physically adsorbed thrombin. Leakage of the physically bound thrombin into PBS containing 4% HSA was negligible. The physical conjugation of thrombin onto the nanoparticles stabilized the thrombin against its major inhibitor antithrombin III and improved its storage stability. At optimal CaCl₂ concentration, the clotting time by the bound thrombin is shorter than that of the free enzyme. This novel conjugated thrombin may be an efficient candidate for topical hemostasis and wound healing.     

Stability tests and stabilization for piecewise linear systems based on poles and zeros of subsystems

This paper provides several stability tests for piecewise linear systems and proposes a method of stabilization for bimodal systems. In particular, we derive an explicit and exact stability test for planar systems, which is given in terms of coefficients of transfer functions of subsystems. Restricting attention to the bimodal and planar case, we show simple stability tests. In addition, we drive a necessary stability condition and a sufficient stability condition for higher-order and bimodal systems. They are given in terms of the eigenvalue loci and the observability of subsystems. All the stability tests provided in this paper are computationally tractable, and our results are applied to the stabilizability problem. We confirm the exactness and effectiveness of our approach by illustrative examples.