Biomedical Engineering Reference
In-Depth Information
Low water concentration
Low hydrolysis rate
High water concentration
High hydrolysis rate
Figure 4.10
Schematic of proposed erosion mechanism.
convincingly for a process confi ned predominantly to the surface layers of the
polymer matrix.
The erosion process is shown schematically in Figure 4.10 [20]. Surface erosion
demands a much higher rate of hydrolysis in the surface layers of a solid device
as compared to the interior of the device, and pure surface erosion can only take
place if no hydrolysis occurs in the interior of the device. This can only take place
if no water penetrates the polymer and since no polymer is so hydrophobic that
no water can penetrate the matrix, some hydrolysis will always take place in the
interior of the matrix. However, there is a signifi cant difference in hydrolysis rates
between surface and interior due to differences in water concentration. In the
surface layers, the concentration of water is fairly high, and the rate of hydrolysis
is also high. But there is a progressively lower concentration of water with deeper
layers, so the rate of hydrolysis will also progressively decrease with end- result
being that erosion is confi ned predominantly to the surface layers.
Implicit in the use of latent acid is an expectation that an increased amount of
latent acid in the polymer backbone would translate into increased rate of polymer
erosion. That this is actually the case can be seen in Figure 4.11 where the latent
acid content was varied from 5 to 0.1mol% [23]. Clearly, there is a correlation
between latent acid content and polymer erosion rate.
An erosion process that is confi ned predominantly to the surface layers has a
number of important benefi ts. First, if the drug is well immobilized in the matrix,
its release is controlled by polymer erosion so that an ability to control polymer
erosion translates into an ability to control rate of drug release. Second, because
drug release is controlled by polymer erosion, drug release and polymer erosion
take place concomitantly and when drug release has been completed, no polymer
remains. Third, because most of the hydrolysis occurs in the outer layers of the
device, acidic hydrolysis products can diffuse away from the device and do not
accumulate in the bulk material. Thus, the interior of the matrix does not become
