Diffusion: Drug transport across a polymeric barrier

Diffusion

Drug transport across a polymeric barrier

Drug transport through a polymeric or biological barrier may occur by simple molecular permeation known as molecular diffusion or by move-ment through pores and channels known as pore diffusion (Figure 4.1).

Figure 4.1 An illustration of passive diffusion processes. (a) Diffusion through the homo-geneous film, (b) diffusion through solvent (usually water)-filled pores, and (c) diffusion through and/or between the fibrous membrane strands.

Molecular diffusion

The transport of a drug molecule through a polymeric membrane that involves dissolution of the drug in the matrix of the membrane, followed by its diffusive transport to the surrounding bulk liquid, is an example of simple molecular diffusion (Figure 4.1a). The release rate of drug by diffusive transport through the polymeric matrix depends on the size and shape of the diffusing molecules, drug solubility in the polymeric matrix, partition coefficient of the drug between the polymeric matrix and the bulk liquid, and the degree of stirring of the bulk liquid at the interface.

Pore diffusion

Pore diffusion involves passage of drug through the solvent-filled pores in the polymeric membrane (Figure 4.1b). In pore diffusion, the release rate of dissolved drug is affected by porosity of the membrane, pore structure, sur-face functional groups (e.g., hydrophobic or hydrophilic), tortuosity, and length of pores.

The molecules may also pass through the tortuous gaps between the over-lapping strands of the polymer (Figure 4.1c). In the cases of both molecu-lar diffusion and pore diffusion, the drug must be available in a dissolved state. This would be the case if the drug product is formulated as a drug solution in the polymer. If a formulation consists of a suspension of drug particles in the polymer, another kinetic step of dissolution of the drug into the polymer or the solvent is involved. The rate of dissolution of a drug would depend on the degree of crystallinity, crystal size, and surface area of the drug; intrinsic dissolution rate of the drug in the polymer and/or the solvent; degree of swelling of the polymer with the solvent; and the extent of mechanical agitation in the system. Drug dissolution from its particles will be discussed in the next section.

Matrix erosion

In addition to molecular diffusion and pore diffusion, erosion of the poly-meric matrix may often be involved in the case of biodegradable polymers. The kinetic contribution of matrix erosion to the drug release rate would depend on the relative rates of drug dissolution, polymer erosion, drug dis-solution in the polymer, drug dissolution in the bulk solvent, molecular diffusion, and pore diffusion.