Matrices, bulk erosion, drug

In their study of branched PSA, Maniar et al. (1990) found that the molecular architecture of branched polymers affects the release kinetics in a variety of ways. They found that the branched polymers degraded faster than linear PSA of comparable molecular weight (Maniar et al., 1990). They also noted that drug (morphine) release profiles were more characteristic of bulk erosion than surface erosion An initial lag time during which very little drug was released was associated with the time required for water to swell the polymer. This was followed by a period of relatively fast release, which tapered off as the device disintegrated. The polymer matrix lost its mechanical integrity before the release experiment was complete (Maniar et al., 1990). Despite the increase... 

Matrix degradation for controlled drug release (A) bulk erosion of a matrix, (B) surface erosion of a matrix. Source. From Ref. 34. 

A much more desirable erosion mechanism is surface erosion, where hydrolysis is confined to a narrow zone at the periphery of the device. Then, if the drug is weU-immobihzed in the matrix so that drug release due to diffusion is minimal, the release rate is completely controlled by polymer erosion, and an ability to control erosion rate would translate into an ability to control dmg delivery rate. For a polymer matrix that is very hydrophobic so that water penetration is limited to the surface (thus Hmiting bulk erosion), and at the same time, allowing polymer hydrolysis to proceed rapidly, it should be possible to achieve a drug release rate that is controlled by the rate of surface erosion. Two classes of biodegradable polymers successfully developed based on this rationale are the polyanhydrides [31] and poly (ortho esters) [32], the latter of which is the subject of this chapter. 

Bulk erosion takes place when water molecules infuse into the bulk of the matrix at a faster velocity than erosion, consequently displaying complex degradation/ erosion kinetics. Majority of the biodegradable polymers used in drug delivery undergo bulk erosion. Nevertheless employment of nanoparticle or microparticle formulations acquiring massive surface areas results in bulk- and surface-eroding materials that display similar erosion kinetics. 

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