Pharmaceutical Manufacturing Handbook: Production and

TABLE 25 Aspect Ratios and Exponent Values ( m ) for
Hydrophilic Matrices Studied
Batch
A
B
C
D
E
Aspect Ratio
3.80
3.59
3.03
2.65
2.16
Exponent (m)
0.45
0.44
0.43
0.42
0.43
TABLE 26 Values of Kinetic constants Derived with Selected Equations in Range
5 – 70% Acyclovir Release for All Batches Studied
Batch
Acyclovir,
A B C D E
% w/w 95 90 80 70 60
Zero - order
equation
k 0
r
1.222 0.122 0.096 0.057 0.042
2
0.984 0.974 0.994 0.995 0.987
Sum of squares
total
3545.9 10,352.6 6,531.4 5,149.2 1,268.0
Sum of squares 53.7 278.3 51.8 24.8 16.4
Higuchi
equation
Korsmeyer –
Peppas
equation
Peppas and
Sahlin
equation
k H
r 2
residual
Sum of squares
total
Sum of squares
residual
k H
n
r 2
Sum of squares
total
Sum of squares
residual
k d
k r
r 2
PHARMACEUTICAL TABLETS USING PERCOLATION THEORY 1039
Sum of squares
total
Sum of squares
residual
12.440
0.998
3,545.9
7.0
3.167
0.782
0.999
13,220.9
16.7
2.239
1.615
0.998
49,567.0
42.9
3.4167
0.993
10,352.6
66.9
0.290
0.843
0.994
26,297.6
159.5
2.056
0.161
0.999
13,220.9
23.3
2.8518
0.956
12,367.8
483.6
0.254
0.856
0.998
33,309.1
65.5
0.357
0.205
0.994
28,716.1
165.0
1.7683
0.932
5,149.2
362.4
0.027
1.114
0.998
13,879.4
10.1
− 0.81
0.202
0.997
13,879.4
43.7
1.4694
0.959
1,268.0
54.6
0.041
1.008
0.997
5,673.5
15.2
− 0.38
0.127
0.997
5,673.5
Notes: k 0 (%/min), zero - order constant; k H (%/min 1/2 ), Higuchi ’ s slope; k (%/min n ), kinetic constant of
Korsmeyer model; n , diffusional exponent; k d (%/min m ), diffusional constant of Peppas and Sahlin model;
k r (%/min 2 m ), relaxational constant of Peppas and Sahlin model; m , diffusional exponent that depends
on geometric shape of releasing device through its aspect ratio (see Table 25 ).
The analysis of the release profi les and the kinetic data indicate two different
behaviors and a sudden change between them. In the fi rst behavior, which corresponds
to the matrices that release the drug at slow rates, the release was controlled
by the fully hydrated gel layer. For these matrices, erosion of the hydrophilic gel
structure has shown an important infl uence on drug release. This is indicated by the
better fi t of the drug release kinetics to the zero - order equation, the n value of
15.2