Roman V. Ivanov

Synthesis and characteristics of cryogels of chitosan crosslinked by glutaric aldehyde

The mechanism of cryotropic gelation in moderately frozen solutions of chitosan crosslinked by glutaric aldehyde is studied. Chitosan cryogels with large pores are synthesized at a low content of crosslinking agent in the reaction mixture or under conditions that do not lead to gelation at temperatures above 0°C. The dependences of the yield of gel fractions, the degree of swelling of the polymer phase of cryogels, and the hydrodynamic characteristics of cryogels on the temperature of synthesis are shown to be extremal. This result may be explained by the competition between the cryoconcentration of reagents in the nonfrozen liquid microphase, which assists the development of a crosslinked polymer network, and such factors as an increase in the viscosity of the reaction mixture and a reduced reagent mobility, which prevent gelation. The morphology of chitosan cryogels is studied, and the character of the macroporous structure of the samples prepared at different temperatures is shown to exert a stronger effect on the hydrodynamic characteristics of a cryogel than the degree of swelling of crosslinked polymers in the walls of its macropores.

Redox-Initiated Radical Polymerisation of Acrylamide in Moderately Frozen Water Solutions

The radical polymerisation of acrylamide initiated with a persulfate/tertiary amine redox pair was performed in liquid (+5 to +35°C) and moderately frozen (–10 to –30°C) aqueous solutions. The reaction in frozen systems resulted in the pronounced increase in the molecular weight of the polymer. The temperature dependence of the yield and molecular weight of cryogenically produced polyacrylamide were of extreme character with a maximum for both parameters in the vicinity of –12.5°C.

Selective adsorption of hydrocarbon-oxidizing Rhodococcus cells in a column with hydrophobized poly(acrylamide) cryogel

A method for selective adsorption of Rhodococcus cells in the column with hydrophobized poly(acrylamide) cryogel (cryoPAAG) was developed that allowed rhodococci separation from mixed bacterial populations and their effective concentration within a sponge-like gel matrix. Hydrophobization of cryoPAAG using the n-dodecane graft (C12) was performed to enhance the adhesion of Rhodococcus cells to the cryogel; this was suggested by our finding that alkanotrophic rhodococci possess high adhesive activity (91-98%) towards n-alkanes, whereas other Gram-positive and Gram-negative bacteria tested did not adhere strongly to hydrocarbons. The selective index of the hydrophobic C12-cryoPAAG column for Rhodococcus cells was 72% that ensured their separation from complex bacterial cultures. Respirometry results using the Columbus Micro-Oxymax respirometer showed that the maximal respiratory activity of C12-cryoPAAG-immobilized Rhodococcus cells incubated with petroleum hydrocarbons was 1.6-1.8 times higher than that of freely suspended cells, and this correlated with the largest immobilized cell number. Moreover, high respiration rates were maintained over 3 weeks of incubation, indicating a considerable functional stability of the cryoPAAG-immobilized biocatalyst developed.

Comparison of the methods for seeding human bone marrow mesenchymal stem cells to macroporous alginate cryogel carriers

We performed a comparative study of the localization, distribution, metabolic activity, and surface properties of human bone marrow mesenchymal stromal cells after static and perfusion seeding to macroporous alginate cryogels. A simple perfusion system for mesenchymal stromal cell seeding to macroporous alginate cryogel sponges proposed in this study resulted in rapid and uniform distribution of cells within the whole volume of the scaffold preserving functional and morphological properties of the cells.

Cryopolymerization of acrylamide: Effect of thermal prehistory of a moderately frozen reaction system

The effect of the thermal prehistory (the freezing technique of the starting reagent solution) on the cryopolymerization of acrylamide in a moderately frozen aqueous medium initiated by a ammonium persulfate-tertiary amine redox system has been studied. It has been shown that, for chemically-initiated cryopolymerization processes in moderately frozen solutions, the thermal prehistory of the reaction system, that is, the pathway via which the sample is brought to the desired negative temperature, strongly affects the temperature dependences of the yield and molecular mass of the resulting polymer and its molecular mass distribution.

Effect of polymer-precursor molecular mass on the formation and properties of covalently crosslinked chitosan cryogels

Features of cryotropic gelation in moderately frozen solutions of chitosan samples that have various molecular masses and that are covalently crosslinked with glutaraldehyde are studied. It is shown that the chain length of the macromolecular precursor, chitosan, affects the yield of the gel fraction; the swelling ability of the polymer phase of wide-pore cryogels; the dimensions of the gross capillary pores of cryogels; and, as a consequence, the hydrodynamic parameters of columns packed with a permeable continuous matrix based on the spongy gel material. The characteristics of the latter depend in a sophisticated manner on the molecular mass of the polymer used for formation of cryogels, a circumstance that is associated with competition of differently directed factors, specifically cryoconcentration effects, a strong increase in the viscosity of the reaction medium in the unfrozen liquid microphase, kinetic features of chain crosslinking at a high concentration of reagents, etc. Therefore, in each particular case, individual conditions for specific combination of the above-mentioned parameters of the process arise. As a result, a complex dependence corresponding is observed for the efficiency of cryostructuring of the chitosan-glutaraldehyde system and for the properties of the resulting cryogels.

Cryostructuring of polymer systems. 47. Preparation of wide porous gelatin-based cryostructurates in sterilizing organic media and assessment of the suitability of thus formed matrices as spongy scaffolds for 3D cell culturing

Abstract New gelatin-based cryostructurates have been elaborated and tested as scaffolds for three-dimensional (3D) cell culturing. Scaffold preparation included dissolution of Type A gelatin in dimethylsulfoxide, freezing of such solution, cryoextraction of crystalline phase with cold ethanol, cross-linking of gelatin with carbodiimide in ethanol medium, treatment of the matrix with ethanolic solution of Tris and tanning of the matrix with formaldehyde dissolved in ethanol. The use of organic media during all the preparation stages ensured the sterility of the scaffolds. The matrices thus prepared were seeded with human adipose tissue multipotent mesenchymal stromal cells to confirm the biocompatibility of scaffolds and their possibility to provide necessary environment for the cell growth and differentiation. The cells attached onto the surface of the pore walls, proliferated and differentiated into osteogenic and adipogenic lineages. These results demonstrate that gelatin-based cryostructurates prepared in the sterility ensuring organic media can be used as scaffolds for tissue engineering purposes.

An Open Queueing Network with a Correlated Input Arrival Process for Broadband Wireless Network Performance Evaluation

In the paper characteristics of an open queueing network with correlated input Markovian arrival processes and phase type distribution of service time in the nodes are studied. The developed methodology is used for the performance evaluation of broadband wireless networks with linear topology. A comparison study of numeric results of analytic modelling and simulation is carried out. To study open queueing networks with MAP input a pyQuMo library has been developed using Python 3 language. The library provides a means for model description, model properties computations by taking advantage of an analytical approach, and simulation as well as results visualisation.

Calculating the American options in the default model

For the binomial model of the derivative securities market, consideration was given to calculation of prices and optimal instants of execution for the American instruments in the model with possible default (repudiation of a contract) by the contract holder. The results were obtained for the buyer and seller options with discount.

On predicting the maximum of a semimartingale and the optimal moment to sell a stock

We generalize the results of recent publications on the prediction of an unknown maximum of a process to the case of an exponential semimartingale whose logarithm can be represented as a sum of local martingales and a positive bias. Our results generalize across a wide variety of models, including multifactor models; the results can also be applied to risk reduction problems.