Higuchi’s equation

Several useful approximate analytical solutions to Eq. (4.8) were developed. A well-known example is Higuchi s equation, based on a pseudosteady-state approach7 ... 

However, because of the pseudosteady state assumptions involved, Higuchi s equation is only valid when the drug loading is in excess of the drug solubility (A>>C ). At the limit of A+C, Higuchi s equation gives a result 11.31 smaller than the exact solution. Lee (4)recently presented a simple analytical solution for this problem which is uniformly valid over all A/Cg values ... 

When Equation (3) is applied to drug release, the deviations from the exact results are consistently one order of magnitude smaller than those of Higuchi s equation. As A/C >1.04, Equation (3) has an accuracy within 1% of the exact solution. Therefore, Equation (3) is much more accurate than Equation (2), particularly at low A/C values. The latter case occurs quite often in delivery systems involving hydrophilic polymers and drugs of high water solubility. 

The value of D determined by Higuchi s equation was observed to be about 30% larger than that calculated by Equation (6.80). 

Equation (6.89) is identical with Equation (6.72) derived under the pseudosteady-state approximation. The error caused by Higuchi s equation increases with decreasing values of A/Cs. 

The first step necessary to release the drug is water penetration with formation of a gelified outer layer. For water-soluble cellulose derivatives, both water penetration and drug release are generally controlled by Fickian diffusion and are described by the so-called Higuchi s equations (see e.g. [14]). Recently, we have developed a model that can predict the solute (drug) and penetrant (water) concentration profiles with simultaneous swelling [76] ... 

Fick s laws apply for the release kinetics whereas the release of dispersed drugs can be described by Higuchi s equation (1). Release kinetics for both conditions and covering different device geometries were recently reviewed by Baker and Lonsdale (2). 

Figure 8.19 Schematic representation of the variation of the defect concentrations in BaPrj- YbjOs-s as a function of dopant concentration, assuming fixed temperature, water, and oxygen pressure. The electroneutrality equation used is [h ] = [YbPr]. [Adapted from data in S. Mimuro, S. Shibako, Y. Oyama, K. Kobayashi, T. Higuchi, S. Shin, and S. Yamaguchi, Solid State Ionics, 178, 641-647 (2007).]...

Release kinetic expression for a tablet (radius rG and thickness /0) can be developed by using Higuchi s pseudo-steady-state approximation and mass balance. With macroscopic observation of the moving boundary of a dispersed drug tablet, Equation (6.76) can be formulated by ... 

Sims, S. M., Srinivasan, V., Higuchi, W. I. and Peck, K. D. (1993). Ionic partition coefficients and electroosmotic flow in cylindrical pores Comparison of the predictions of the Poisson-Boltzmarm equation with experiment. J. Colloid Interface Sci. 755 210-220. 

The data for the copolymer systems and the 25 75 [FUPC] samples is plotted according to the Higuchi equation in Figure 8. Although the [FUPC] sample approximates a straight line, neither the powdered nor the pellet form of the EMCF MMA 50 50 copolymer shows such behavior. On the contrary, both copolymer samples exhibit an S-shaped curve. If a zero-order system is plotted against the square root of time, the resultant... 

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