Nitrogen release characteristics of polyethylene-coated controlled-release fertilizers and their dependence on membrane pore structure

In this study, controlled-release fertilizers （CRFs） with five different nitrogen release periods were pre- pared by coating large urea particles with polyethylene （PE） membranes under various experimental conditions. The preliminary and differential solubility rates, release periods, and membrane pore sizes of the obtained CRFs were measured using water immersion, scanning electron microscopy, and mercury porosimetry. For all CRF samples, the median pore diameters of the membranes were equal to 4.5-5.3 nm and pores with sizes smaller than 10 nm accounted for 86-96% of the total pore surface area. The obtained pore diameter distributions differed for the five studied types of CRF, having release periods of 1,2, 4, 6, and 8 months. Thus, for the CRFs with a 1-month release period, the maximum pore diameter reached a magnitude of 4000 nm, while this value did not exceed 30 nm for the CRFs with a release period of 8 months. Hence, we have established a relationship between the release period of CRFs and their effective maximum pore sizes.

One discrete dynamical model on the Wolbachia infection frequency in mosquito populations

How to prevent and control the outbreak of mosquito-borne diseases,such as malaria,dengue fever and Zika,is an urgent worldwide public health problem.The most conventional method for the control of these diseases is to directly kill mosquitoes by spraying insecticides or removing their breeding sites.However,the traditional method is not effective enough to keep the mosquito density below the epidemic risk threshold.With promising results international,the World Mosquito Program’s Wolbachia method is helping to reduce the occurrence of diseases transmitted by mosquitoes.In this paper,we introduce a generalized discrete model to study the dynamics of the Wolbachia infection frequency in mosquito populations where infected mosquitoes are impulsively released.This generalized model covers all the relevant existing models since 1959 as some special cases.After summarizing known results of discrete models deduced from the generalized one,we put forward some interesting open questions to be further investigated for the periodic impulsive releases.

Konjac glucomannan and xanthan gum as compression coat for colonic drug delivery:experimental and theoretical evaluations

Compression coated tablets for oral colon specific delivery systems were developed with a mixture polysaccharide of konjac glucomannan(KGM)and xanthan gum(XG)as the compression coat.Diffusion of cimetidine from compression coated tablets was investigated by release experiment in Vitro.0.22U/mLβ-mannanase was applied in the mimic colon solution.The structure of the mixture polysaccharide was studied by an atomic force microscope(AFM).The experimental results indicate that a KGM70 tablet with a 0.4 g coat is of good design,due to a less than 5%drug loss in the mimic upper gastrointestinal solution by the synergistic interaction between XG and KGM,and due to about 50%cumulative release in the mimic colon solution by degradation after 24 hours.The release mechanism and model are discussed based on different periods of drug release including the delay of the drug,the constant release without an enzyme and the delay of degradation.Under hydrolysis byβ-mannanase,drug release from the tablet with KGM coat shows an exponential increase,while that from the dosage with the mixture polysaccharide coat is an approximately zero-order process in which the constant release rate relates to the release velocity of a non-degraded system,the content of KGM within the coat and the average molecular weight ratio of KGM to XG.It was found that XG was the framework of the polysaccharide mixtures by AFM,which is similar to the analysis results from experiments on drug release.