Drug release porous matrices

The presence of insoluble ingredients in the formulations helps to maintain the physical dimension of a hydrophobic matrix during drug release. As such, diffusion of the active form from the system is the release mechanism, and the corresponding release characteristic can be described by the Higuchi equation (Eq. 4.11). Very often, pores form within a hydrophobic matrix as a result of the release of the active ingredient. For a porous monolithic system, Eq. (4.11) can be further modified as18... 

In cases where well-defined pores ranging in sizes from a few hundredths to several hundred microns exist throughout the matrix, the kinetics of drug release can still be described by Equations (1)—(3) provided that an effective diffusion coefficient is used. When the drug diffusion only takes place through the solvent filled porous network, the effective diffusion coefficient is further related to the matrix structure by ... 

As in the case of hydrophilic (swelling) matrix systems and reservoir (membrane) systems, drug release profiles from insoluble (porous or non porous) systems are most of the time described on a basis of the diffusion theory. This is not true for every situation and we have for example shown that the release from porous matrix systems is dissolution-controlled above the solubility limit of the drug [150]. A simplified equation for the model proposed and for values of kd t > 4, is ... 

In monolithic devices, the drug is dissolved or dispersed homogeneously throughout the water-insoluble polymer matrix. The polymer matrix can also be non-porous or microporous. Unlike the reservoir devices, the drug release from the monolithic devices rarely provide zero-order release. The zero-order release can be achieved by adjusting the physical shape of the device such as a half sphere (75). 

Generally, in the case of porous silica, a sustained release mechanism could be met. Many controlled-release products are designed on the principle of embedding the drug in a porous matrix. Liquid penetrates the matrix and dissolves the drug. 

The drug release pattern from such porous matrices is strongly influenced by the penetration process of the external liquids inside the microstructure of the matrix. If the liquid can wet the matrix (contact angle below 90 ) the penetration takes place basically via a capillary motion through the pores. In this case the basic physical factors influencing the penetration are the contact angle the pore size and gojy e the surface tension of liquid and its viscosity. If the liquid does not wet the... 

From the data presented above, it is evident that in the design of a porous matrix tablet with specific drug release rate, is strictly necessary to prelyminarly identify the process of penetration taking place in the polymeric system under examination. 

Another important application of boronic acid-containing hydrogel is for drug release. The porous structure of hydrogels allows for loading of drugs into the gel matrix. In addition, their stimuli-responsivity allows for fine control over the drug release rate. 

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