Pharmaceutical Technology: Controlled Drug Release, Volume 2

IMPLANTS [CH. 17 191
release profiles. Increasing porosity ε or decreasing tortuosity τ may counteract the tendency for
the release rate to decline.
So, constant long-term release from matrix systems may preferably be realized under the
following conditions: (i) hydrocolloids should be used as pore-forming excipients, (ii) excipients
should be leached out very slowly and (iii) the matrix polymer should be elastic and permeable to
water vapour in order to permit swelling and to yield homogeneous water uptake. Finally (iv) the
swelling, drug-saturated excipient should be of higher osmotic activity than the surrounding
medium in order to permit water uptake.
The results show that this concept is capable of long-term ‘anomalous diffusion’ [17]. Further
optimization may lead to matrix-type release systems capable of even longer and more constant
drug release.
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