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Physiology and Basic Investigation of Blood Coagulation
Published in Hau C. Kwaan, Meyer M. Samama, Clinical Thrombosis, 2019
M. M. Samama, C. H. Ts’ao, Hau C. Kwaan
There are two different ways to study the mechanisms of human blood coagulation. The first takes into consideration the numerous reactions acting in vivo. The more important ones involve the vessel wall, platelets, plasma clotting factors, and rheologic factors. The second way is to restrict oneself to simple schemes for the understanding of the mechanism of blood coagulation and the interpretation of blood tests in hypo-, normal, and hypercoagulable states. In vivo, the coagulation system is aimed at inducing, with a very high speed, a localized clot through thrombin formation and preventing any dissemination of the enzyme, thereby limiting the propagation of the clot. Autocatalysis is controlled by feedback reactions.
Diversity, interconnectivity and sustainability
Published in Jan Bogg, Robert Geyer, Complexity, Science and Society, 2017
Peter M. Allen, Pierpaolo Andriani
Among the features of the model used in this paper there is the co-evolution of technologies and institutions. In particular, the rate of entry into any new sector is determined by the joint effect of the expected size of the market and by financial availability, which in the paper represents the fraction of the total financial resources allocated to the new sector. The allocation of financial resources to the new sector often requires new competencies, leading to the creation of new institutions, an example of which would be venture capital firms. This is a specific example of the more general situation of the co-evolution of technologies and institutions. If financial resources increase with the size of the output of the new sector, this provides a synergistic effect which raises the rate of growth of the new sector above the average rate of growth of the economy and allows the new sector to achieve its ‘economic weight’. The co-evolution of technologies and institutions is also an example of autocatalysis [18]: one of the outputs of the process (new firms or a new type of output) accelerates the process itself. Autocatalysis is required in order for a homogeneous system to acquire structure.
The non-linear tradition: historical development of complexity
Published in Keiran Sweeney, Complexity in Primary Care, 2017
production of C, and chemical C catalyses the production of D, and so on. If, somewhere down the line of catalytic reactions, chemical X catalyses the production of chemical A, the loop becomes self-generating and autocatalytic. According to Gribbin (2004), Kauman presents this model, with supportive but not yet definitive evidence, as analogous to the connected-button model, namely as a phase transition in a chemical system involving a sucient number of connections between the chemicals (analogous to the nodes in the button model). This process of chemical autocatalysis is valuable because it illustrates the idea of connectedness – the crucial interaction of individual components within a system, whose iterative patterning forms a self-organising process with the potential to create emergent properties.
Current trends in PLGA based long-acting injectable products: The industry perspective
Published in Expert Opinion on Drug Delivery, 2022
Omkara Swami Muddineti, Abdelwahab Omri
Critical factors that can modulate the hydrolytic tendency of PLGA polymer are hydrophilicity, chemical composition, hydrolysis mechanism, additives, morphology, device dimensions, porosity, Tg, and physicochemical factors (ion exchange, ionic strength, and pH), molecular weight, sterilization, and site of implantation [35]. For instance, the crystallinity of the polymer comprising a microparticle may significantly impact the degradation rate. Usually, an increase in crystallinity results in increased mobility of the partially degraded polymer chains, allowing the polymer chains to be realigned into a more ordered crystalline state [36]. Higher porosity in microspheres aids to permit cellular migration and improves the diffusion of oligomers and low molecular weight degradation polymer whose carboxylic chain ends may ease the autocatalytic degradation. Hence, large molecular weight distribution would specify more carboxylic end groups, enabling the autocatalytic degradation behavior. Similarly, a narrow molecular weight distribution would have less carboxylic acid end groups accessible for autocatalysis [37].
Fluid flow effects on the degradation kinetics of bioresorbable polymers
Published in Computer Methods in Biomechanics and Biomedical Engineering, 2021
Zhitao Liu, Hongbo Zhang, Huanxin Lai
Hydrolysis is known to be the dominant mechanism in hydrolytic degradation. The diffusion and accumulation of water molecules causes hydrolysis of the ester bonds in the polymer matrix. As a result, these chains are split into water soluble shorter chains (oligomers and monomers) characterized by carboxylic ends. Because of the chain scission, the molecular weight of the polymer decreases gradually. The degradation products then diffuse into the surrounding environment, which results in the mass loss of polymers. The diffusion is generally slower in a large-size device, due to the greater diffusion distance (Siepmann et al. 2005; Xu et al. 2017). The slow diffusion may result in accumulation of acidic products inside the polymer matrix, and they accelerate the degradation process. The phenomenon is known as the autocatalysis (Gentile et al. 2014; Laycock et al. 2017).
Poly(lactic-co-glycolic acid) microsphere production based on quality by design: a review
Published in Drug Delivery, 2021
Yabing Hua, Yuhuai Su, Hui Zhang, Nan Liu, Zengming Wang, Xiang Gao, Jing Gao, Aiping Zheng
A PLGA chain with a relatively low averaged Mw of 2 kg/mol consists of approximately 30 repeat units, approximately 7% of which form terminal groups, and 93% are reactive (Machatschek & Lendlein, 2020; Siepmann et al., 2005). The PLGA terminal groups influence the degradation rate. For instance, carboxyl terminal groups can catalyze the hydrolysis of ester bonds, thus producing more acidic groups and establishing an autocatalytic cycle that accelerates polymer degradation. Therefore, the rate of PGLA degradation with carboxyl terminal groups is higher than that with ester terminal groups (Lanao et al., 2011). Furthermore, the end groups significantly affect drug encapsulation efficiency and loading capacity of the polymer. Wang et al. studied the water contact angle of the polymer to find that PLGA with ester terminal groups is more hydrophobic. This allows it to encapsulate increased quantities of drugs, possibly as a result of the delayed hydrolysis of the PLGA microspheres during the curing process. Moreover, they reported that the effect of PLGA end groups on low Mw polymers is more pronounced. At a given mass, PLGA with lower Mw is likely to contain more acidic groups than higher Mw PLGA due to difference in the densities of carboxyl terminal groups for PLGAs of different Mw (Wang et al., 2019). In cases where terminal groups have an autocatalytic effect, the initial Mw distribution can influence PLGA degradation, with polydisperse polymers having more terminal groups than monodisperse polymers.