Fickian-diffusion mechanism

SANS. The polymers were crosslinked with hi- and trifunctional cross-linking units that reacted via Michael addition. These nanoparticles were then studied as a potential nanocarrier for vitamin B9. The particles were loaded with vitamin Bg and the release was monitored by UV-Vis spectroscopy. Complete release of Bg was achieved in 5-6 hours in neutral pH water. The release exponent was calculated to be approximately 0.5, which suggests a Fickian diffusion mechanism for this SCNP based system. 

Psyllium gum modified with polyacrylamide Salicylic acid and tetracycline Hydrogel Drug released from the polymeric matrix follows non-Fickian diffusion mechanism. [156,157]... 

Psyllium gum and methacrylamide polymeric complex using N,N-methylene-bisacrylamide as crosshnker Insulin hydrogel Non-Fickian diffusion mechanism has been observed for insuhn release profile in pH 2.2 buffer and pH 7.4 buffer. [158]... 

Carboxymethyaltion of Gum kondagogu with monochloro-acetic acid Metformin ionotropically gelled beads Ex-vivo bio adhesion studies of CMGK beads revealed bioadhesion greater than 80% over a period of 24 h and drug release rate from the beads followed Fickian diffusion mechanism with zero-order release kinetics. [6]... 

Ionic gelation between Arabic gum and chitosan Insulin Nanoparticles Enhanced solubility of chitosan in acidic medium and better swelling of Arabic gum chains at pH >6.5 resulted in lower insulin release at pH 6.5 in comparison with that of the other pH mediums. Drug release profile followed non-Fickian diffusion mechanism. [160]... 

Here, Mt is the amount of water absorbed at time t, M is the amount of water absorbed at equilibrium, k is a constant and n is a diffusion exponent. When the value of n lies between 0.5 and 1.00, it indicates non-Fickian diffusion mechanism. For P(HEMA-co-AA) (60 40 molar basis), the value of n increased from 0.65 to 0.85 (approximately) at pH 4.9, showing the pH dependence of the diffusion mechanism (Table 1 and Figure 2). As the pH of the swelling medium increased, the diffusion mechanism became more anomalous. 

Fickian diffusion mechanism may take place. Furthermore, all the other affinity events, as discussed for the pore model, could occur at interactive sites establishing a more complex transport process. 

The linear trend of water absorption vs. the square root of time is in accordance with a Fickian diffusion mechanism. The diffusion coefficient D was evaluated by the following equation, where Mt is the mass of the water absorbed at time t, Mq is the mass of the water at equilibrium, t is time in seconds, L is the sample thickness in centimeters and D is the diffusion coefficient incm sec". ... 

Release of tetracycUne hydrochloride from PCL fibers was evaluated as a means of controlled administration to periodontal pockets (69). Only small amounts of the drug were released rapidly in vitro or in vivo, and poly(ethylene-co-vinyl acetate) gave superior results. Because Fickian diffusion of an ionic hydrochloride salt in a UpophiUc polymer is unlikely, and because PCL and EVA have essentially identical Fickian permeabilities, we attribute this result to leaching of the charged salt by a mechanism similar to release of proteins from EVA (73). Poly-e-caprolactone pellets have been found unsuitable for the release of methylene blue, another ionic species (74,75). In this case, blending PCL with polyvinyl alcohol (75% hydrolyzed) increased the release rate. 

Criteria 1-3 are the cardinal characteristics of Fickian diffusion and disregard the functional form of D(ci). Violation of any of these is indicative of non-Fickian mechanisms. Criterion 4 can serve as a check if the D(ci) dependence is known. As mentioned, it is crucial that the sorption curve fully adhere to Fickian characteristics for a valid determination of D from the experimental data. At temperatures well above the glass transition temperature, 7 , Fickian behavior is normally observed. However, caution should be exercised when the experimental temperature is either below or slightly above 7 , where anomalous diffusion behavior often occurs. 

Drug release from soluble polymers is accompanied by the gradual erosion-type dissolution of the polymer. Therefore, polymer dissolution and drug diffusion may be the overall hybrid mechanism of release. Drug release from nonsoluble hydrogels generally follows Fickian or non-Fickian diffusion kinetics [51]. The mechanism of... 

Dynamic swelling parameters of bioactive substances into hydrogel matrices are summarized in Table 19.1. The values of n>0.5 indicate that the loading mechanism of bioactive substances deviates from Fickian diffusion. For NIPA-APSA and NIPA-AA/Richlocaine systems the diffusion mechanism is relaxation-controlled because their n values are close to 1. Temperature-dependent release of richlocaine from the NIPA-AA hydrogels is shown in Figs. 19.3 and 19.4. 

In early works, the reactivity associated widi TiO2 photocatalysis was assigned to free OH radicals, but more recently die oxidative species has been identified as bound HO radicals and photogenerated holes. Reaction of the substrate with these species would require either die direct adsorption of organic compounds to the semiconductor interface or Fickian diffusion of the substrate to the semiconductor surface under depletion conditions. However, these two pathways imply different mechanisms, and in some cases different reaction products or intermediates. 

The analysis of transfer mechanisms of drugs across the intestinal epithelial layer has passed a long way since the theory of lipid pore membrane [118] in which the total pore area of the intestinal membranes was calculated (and found to be low compared with the total surface of the mucosal aspect of the gut), through the Fickian diffusion calculations of the transport of unionized moieties of drug molecules (the Henderson-Hasselbach equation), which led to the conclusion that acidic drugs are absorbed in the stomach [119,120]. 

The importance of adsorbent non-isothermality during the measurement of sorption kinetics has been recognized in recent years. Several mathematical models to describe the non-isothermal sorption kinetics have been formulated [1-9]. Of particular interest are the models describing the uptake during a differential sorption test because they provide relatively simple analytical solutions for data analysis [6-9]. These models assume that mass transfer can be described by the Fickian diffusion model and heat transfer from the solid is controlled by a film resistance outside the adsorbent particle. Diffusion of adsorbed molecules inside the adsorbent and gas diffusion in the interparticle voids have been considered as the controlling mechanism for mass transfer. 

Equation (6.94) illustrates that zero-order release kinetics are obtained if drug dissolution controls the release kinetics. However, as soon as the last particle in the matrix dissolves, the controlling mechanism of drug release shifts to Fickian diffusion. Figure 6.19 shows the dissolution-controlled release of KC1 at the early stage of release and the diffusion-controlled release at the later stage of release from an ethyl cellulose tablet. 

When Fickian diffusion in normal Euclidean space is justified, further verification can be obtained from the analysis of 60% of the release data using the power law in accord with the values of the exponent quoted in Table 4.1. Special attention is given below for the values of b in the range 0.75-1.0, which indicate a combined release mechanism. Simulated pseudodata were used to substantiate this argument assuming that the release obeys exclusively Fickian diffusion up to time t = 90 (arbitrary units), while for I, > 90 a Case II transport starts to operate too this scenario can be modeled using... 

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