Non-Fickian diffusion mechanism

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]... 

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. 

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... 

Although Fickian diffusion has been used to describe the diffusion mechanism of liquid through the rubbery material because of its simplicity and mathematical tractability, most of the rubber-solvent systems do not fit such a simplified explanation. Then, non-Fickian diffusion was developed. The case of non-Fickian diffusion was investigated using the following equation ... 

Whereas the value of n equals 0.5, the mechanism of diffusion described by Case I of Fickian model, when n values range between 0.5 and 1 that exhibits anomalous transport model and when n value equals 1, Case II of non-Fickian model is used to determine. Fickian model describing the rate of diffusion of penetrant molecule is much less than the relaxation rate of polymer chains while Case II of non-Fickian diffusion representing rate of diffusion is rapid than relaxation process. For the anomalous transport model, both solvent diffusion rate and polymer relaxation rate are comparable. Figure 27.2 shows various types of non-Fickian model anomalous transport and Case II of non-Fickian are included in this group. ... 

For diffusion of liquid through rubbery polymer composites, Fickian and non-Fickian diffusion theories are frequently used to describe the mechanism of transport, but for gas or vapour, other models have been developed to fit experimental data of diffusion profiles. The models of gas transport include Maxwell s model," free volume increase mechanism," solubility increase mechanism," nanogap hypothesis," Nielsen model, " " Bharadwaj model, ° Cussler model " " and Gusev and Lusti model, " etc. 

Hydrophilicity-to-hydrophobicity ratio It affects dmg solubihsation and release rate and mechanism (the increase of hydrophihcity enhances sweUing, causing a shift from Fickian to non-Fickian diffusion) Ikeda et al. (1990) Hafeman et al. (2010) Wang et al. (2013) Mattu et al. (2013) Ferreira et al. (2014) Gentile et al. (2015a)... 

Evidently, this approach is favorable in order to understand the relationship between diffusivity and concentration. Nevertheless, deviations occur at the region of lower concentration. This deviation is attributed to an error of the presumed diffusivity, as well as the unknown mechanisms governing the desorption process such as the transition from the Fickian diffusion to the Non-Fickian diffusion in PS. 

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. 

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. 

A drug release mechanism from compressed cellulose ether matrices using dimensionless analysis has been discussed (17). A non-Fickian mechanism could be dismissed when calculating the Deborah and the Swelling interface numbers from relaxation, penetrant diffusion, swelling and drug diffusivity data (17). 

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