Background The technique of mercury intrusion porosimetry (MIP) is well understood to be of value in the field of bioceramics and rigid high-density polyethylene porous surgical implants. One might be hesitant to use the same pressure-based technique on soft, deformable polymers e.g. polyhydroxyalkanoates (PHAs), chitosan, polylactic acid (PLA), polyglycolic acid (PGA) and their co-polymers (PLGA). However, since the pore sizes involved are typically quite large (usually above 100 microns), the pressures involved are rather small (less than 10 psi). Furthermore, the pressure is applied “isometrically”, not transversely as in bubble-point (permeametry) techniques, so the polymer will not rupture even if in sheet form. MIP is therefore a viable technique.
Thin Sheets Or Regular ShapesNevertheless, if detailed pore size measurements are not required, but just a measurement of overall porosity, the analysis is remarkably straightforward. Pore volume need not be measured directly (by pore filling with mercury) but can be calculated as the difference between bulk volume and “skeletal” volume. Bulk volume can be determined from consideration of geometric dimensions. Many biopolymers are formed as thin sheets or regular shapes. It is a therefore a trivial matter to take micrometer or caliper readings. Alternatively, and certainly for irregularly shaped pieces, bulk volume can be determined by Dry Powder Pycnometry. This technique entails measuring the increase in volume of a packed bed of powder when the sample is immersed in it, Archimedes style. One simply needs a powder, reasonably free flowing, whose particle size precludes it penetrating pores connecting to the polymer surface.
The Only Limit Is Ones ImaginationOver the years a number of different materials have been employed: graphite, aluminum hydroxide, metal powders, rapeseed, glass beads. The list of possible materials is only limited by one’s imagination. To complete the testing a reproducible means of packing the powder, by tapping, is employed. The Autotap is such a device. The powder and sample volumes are simply read from the graduations on the sample container. Both rapid and inexpensive, the Autotap (or its two-station sibling the Dual Autotap) is a valuable addition to any material development lab.
A Good FitThe second half of indirect pore volume determination is the measurement of Skeletal Volume. Nowadays this is normally achieved using a gas expansion pycnometer (dry, non destructive) rather than liquid displacement (solubility problems, solvent disposal etc). The one-sample Ultrapycnometer 1000 and the unique five-station Pentapycnometer (both Quantachrome Instruments) feature simple programming of automatic repeat measurements, balance and PC interfaces, and automatic printed report generation. The Ultrapycnometer is available in a special version, the UltraFoam Pycnometer, which additionally calculates open porosity and closed pore volume (as percentages). Already used for a number of years in non-surgical fields (packaging, insulation, wicking etc), the technology is well established and seems like a good fit to the objectives of the bio-scaffold community.