1,720,982 research outputs found
A population balance approach for the description of water osmosis and intracellular ice formation during cryopreservation
No full textA novel model capable of quantitatively describing and predicting Intracellular Ice Formation (IIF) as a function of temperature in a cell population during the cooling stage of a cryopreservation protocol, without Cryo-Protective Agent (CPA) is proposed. The model accounts for water osmosis and IIF occurrence during freezing of the cell population, whose size distribution dynamics is simulated by means of a suitable population balance approach. It is found that IIF temperature depends upon the cell size, i.e. it is higher for larger cells. Correspondingly, the Probability of IIF (PIIF) results to be dependent on the initial size distribution of the cell population. Model reliability is successfully verified by predicting experimental data available in the literature of PIIF at different, constant cooling rates with better accuracy as compared to previous theoretical approaches
Modelling breakage and reagglomeration during fine dry grinding in ball milling devices
No full textModelling of grinding of fine powders in ball milling devices is addressed. The model quantitatively describes breakage and agglomeration phenomena by considering two populations, i.e. primary particles and porous aggregates. The population balance approach takes into account breakage and aggregation kernels which are considered functions of the size of the two populations. The proposed model is able to properly simulate the inversion from the breakage to the agglomerative regime typical of fragile material powder system undergoing ball milling. A suitable fitting procedure is performed for separately determining the adjustable parameters of the model. Model reliability is tested against experimental data, while the proposed breakage/agglomeration kernels are related to the quantitative description of ball milling apparatus dynamics
The effect of cell size distribution during the cooling stage of cryopreservation without CPA
No full textA novel model capable of quantitatively describing and predicting intracellular ice
formation (IIF) as a function of temperature in a cell population characterized by a
size distribution is proposed. The model overcomes the classical approach which takes
into account a population of identically sized cells. The size distribution dynamics of a
cell population in response to water osmosis and IIF occurrence during the cooling
stage of a standard cryopreservation protocol without using cryoprotective agent
(CPA) is simulated by means of a suitable population balance approach. Specifically,
the model couples the classical water transport equation developed by Mazur1 to the
quantitative description of nucleation and diffusion-limited growth of ice crystals in
the framework of a 1-D population balance equation (PBE). It is found that IIF temperature
depends on the cell size, i.e., it is higher for larger cells. Correspondingly,
the probability of IIF (PIIF) results to be dependent on the initial size distribution of
the cell population. Model’s parameters related to the osmotic behavior of the cell
population and to IIF kinetics are obtained by comparison between theoretical results
and suitable experimental data of isolated rat hepatocytes available in the literature.
Model reliability is successfully verified by predicting the experimental data of PIIF at
different, constant cooling rates with better accuracy as compared to the theoretical
approaches available in the literature
Modelling breakage and reagglomeration phenomena during fine dry grinding in ball milling devices
No full textModelling of grinding of fine powders in ball milling devices is addressed. The model quantitatively describes breakage and agglomeration phenomena by considering two populations, i.e. primary particles and porous aggregates. The population balance approach takes into account breakage and aggregation kernels which are considered functions of the size of the two populations. The proposed model is able to properly simulate the inversion from the breakage to the agglomerative regime typical of a fragile material powder system undergoing ball milling. A suitable fitting procedure is performed for separately determining the adjustable parameters of the model. Model reliability is tested against experimental data, while the proposed breakage/agglomeration kernels are related to the quantitative description of ball milling apparatus dynamics
A simulation model for stem cells differentiation into specialized cells of non-connective tissues
No full textA novel mathematical model to simulate stem cells differentiation into specialized cells of non-connective tissues is proposed. The model is based upon material balances for growth factors coupled with a mass-structured population balance describing cell growth, proliferation and differentiation. The proposed model is written in a general form and it may be used to simulate a generic cell differentiation pathway during in vitro cultivation when specific growth factors are used. Literature experimental data concerning the differentiation of central nervous stem cells into astrocytes are successfully compared with model results, thus demonstrating the validity of the proposed model as well as its predictive capability. Finally, sensitivity analysis of model parameters is also performed in order to clarify what mechanisms most strongly influence differentiation and cell types distribution
Self-propagating High-temperature Synthesis of Barium Titanate and subsequent densification by Spark Plasma Sintering
No full textThis paper describes the self-propagating high-temperature synthesis (SHS) of perovskitic oxides, specifically BaTiO3, and their subsequent densification by spark plasma sintering. With the final goal of obtaining dense nanostructured materials, SHS products were mechanically treated at different milling time conditions, before densification. It was found that the grain size of ball milled powders decreases with increasing milling time, this effect being more evident at early stages of milling. Depending upon the ball milling (BM) conditions adopted, crystallite size in the range 15-70 nm was obtained. After milling for 5 h, the resulting powders (20-30 nm) were sintered by SPS, at 700 A, for different periods of time. By properly varying sintering time in the interval 70-140 s, it is possible to obtain products with relative density in the range 66-99%, respectively. In particular, grain growth during sintering was found to be limited (below 50 nm) if the electric current is applied for time intervals equal to or less than 100 s. The observed dielectric properties are typical of a nanocrystalline BaTiO3 ceramic
Modelling the Cryopreservation Process of a Suspension of Cells: The Effect of a Size-Distributed Cell Population
No full textCryopreservation of biological material is a crucial step of tissue engineering, but biological material can be damaged by the cryopreservation process itself. Depending on some bio-physical properties that change from cell to cell lineages, an optimum cryopreservation protocol needs to be identified for any cell type to maximise post-thaw cell viability. Since a prohibitively large set of operating conditions has to be determined to avoid the principal origins of cell damage (i.e., ice formation and solution injuries), mathematical modelling represents a valuable alternative to experimental optimisation. The theoretical analysis traditionally adopted for the cryopreservation of a cell suspension addresses only a single, average cell size and ascribes the experimental evidence of different ice formation temperatures to statistical variations. In this chapter our efforts to develop a novel mathematical model based on the population balance approach that comprehensively takes into account the size distribution of a cell population are reviewed. According to this novel approach, a sound explanation for the experimental evidence of different ice formation temperatures may now be given by adopting a fully deterministic criterion based on the size distribution of the cell population. In this regard, the proposed model represents a clear novelty for the cryopreservation field and provides an original perspective to interpret system behaviour as experimentally measured so far. First our efforts to successfully validate the proposed model by comparison with suitable experimental data taken from the literature are reported. Then, in absence of suitable experimental data, the model is used to theoretically investigate system behaviour at various operating conditions. This is done both in absence or presence of a cryo-protectant agent, as well as when the extra-cellular ice is assumed to form under thermodynamic equilibrium or its dynamics is taken into account consistently by means of an additional population balance. More specifically, the effect of the cell size distribution on system behaviour when varying cooling rate and cryo-protectant content within practicable values for a standard cryopreservation protocol is investigated. It is demonstrated that, cell survival due to intra-cellular ice formation depends on the initial cell size distribution and its osmotic parameters. At practicable operating conditions in terms of cooling rate and cryo-protectant concentration, intra-cellular ice formation may be lethal for the fraction of larger size classes of the cell population whilst it may not reach a dangerous level for the intermediate size class cells and it will not even take place for the smaller ones
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