Biomedical Engineering Reference
In-Depth Information
6
P
T
2
4
7
5
1
3
Fig. 3
Schematic diagram of the experimental supercritical impregnation apparatus: (
1
)CO
2
reservoir; (
2
) high pressure CO
2
pump; (
3
) water bath; (
4
) high pressure stainless steel impreg-
nation cell; (
5
) magnetic stirrer; (
6
) pressure transducer and (
7
) glass trap
6
Effect of the Process Conditions on the Polymeric Matrices
As previously mentioned, one of the advantages of using SCF technology is the
possibility to change the matrix porosity by changing process conditions like
pressure, residence time and depressurization rate. This is an important factor when
developing wound dressing materials as it can influence the permeability of the
dressing to gases and moisture and also the kinetic release of a bioactive compound
to the wound in the case of impregnated materials. The porosity of the CBC films
and foams and AGA foams was measured by mercury porosimetry (Micromeritics,
Pore Sizer 9320). Table
3
presents the relative variation induced by the process
on the porosity, pore size and apparent density of the three samples studied. As
expected the pore diameter of the foam like structure is higher (10-500
m) than
that of the film (0-10
m). This difference is illustrated in Fig.
4
which compares
the pore size distribution of the samples before and after processing at 323 K
and 20 MPa for 3 h. The processing of the materials followed an experimental
procedure similar to the one previously described except that in this case any
bioactive compound was included in the high pressure cell. The vertical axis in
Fig.
4
,(
dV/dlogD) is a differential of the volume of mercury intruded at each
pore diameter (D) and is therefore related to the volume of macropores of each
diameter. As can be seen, scCO
2
processing increased sample porosity, acting
as a morphological and porogenic agent for all the samples being this effect most
pronounced for CBC films with a porosity increase of over 40% and only slightly for
CBC foam. When comparing CBC and AGA foams it can be seen that processing
has a higher effect on the number and size of the pores of AGA foams.
