Flexible Bio-Scaffold Analysis Using Mercury Porosimetry and Helium Pycnometry

The techniques of mercury intrusion porosimetry (MIP), and Helium pycnometery have long been recognised as valuable tools in understanding the 3 dimensional structure of rigid bio scaffolds such as those used as supports for the growth of bone. Recent work performed by scientists at Quantachrome UK Ltd, and a leading medical research institute in London has revealed that the two techniques can also provide valuable information, both quantitative and qualitative about the pore structure of a new generation of flexible bio scaffolds being investigated as candidates for arterial replacement. Key attributes of these flexible bio scaffolds are that they must be elastic, as this contributes to the mechanism of pumping blood around the body, and they must be porous to allow growth of endothelial cells inside the artery, and diffusion into the vessel wall.

Mercury intrusion was carried out using Quantachrome PoreMaster 33GT. Three samples were tested to investigate whether minor differences in pore sizes could be detected. The samples are POSS modified polyurethane with different percentages of sodium bicarbonate. The NaHCO3 acts as a porogen which is leached out in water leaving behind the porous structured membrane. The three samples were tested to see the effect of different concentrations of NaHCO3 on the porosity. SEM analysis had previously been performed which showed a very open pore structure and a broad pore size range, but due to the shape and highly porous nature of the material neither the pore size, nor the pore volume could be determined from the images.

The analysis was a blind test, with the operator unaware of the composition of the three samples. A good correlation was shown between the concentration of NaHCO3, and both the pore volume and modal pore size. MIP was able to quantify both the increase in pore volume and the change in pore size which results from increasing concentrations of NaHCO3.

NaHCO3 | Pore volume | Pore size

concentration | (cc/g) | (mode)

55% NaHCO3 | 3.37 | 30.68um

45% NaHCO3 | 2.56 | 19.14um

35% NaHCO3 | 1.33 | 15.06um

The intrusion and extrusion cycle produced some interesting phenomena. Some small intrusion steps were seen at high pressure, which are probably due to compression of the flexible structure. On some of the samples an unusual step could be seen in the extrusion cycle, which was both reproducible between samples, and repeatable on the same sample. Some further investigation is required to fully understand this unusual peak, but we can speculate that this is due to the elastic deformation of the sample causing a sudden emptying of pores as the pressure drops.

While mercury porosimetry is a useful technique for probing pores which are accessible from the outside of the sample, it cannot be used to determine the presence of any completely sealed pores which are not accessible from the sample surface. To determine the presence of sealed pores the technique of Helium Pycnometry can be utilised. Density can be described as sample mass/sample volume, therefore if part of the sample volume is composed of the sealed pores, the density will decrease (assuming that air has a lower density than the matrix of the sample). Helium Pycnometry analysis was performed using a Quantachrome Ultrapycnometer 1200; the following table shows a correlation between the amount of porogen, and decrease in density and hence increase in sealed pores.

NaHCO3 | Density

concentration | (g/cc)

55% NaHCO3 | 1.080

45% NaHCO3 | 1.202

35% NaHCO3 | 1.638

In summary, the techniques of Helium Pycnometry and Mercury Intrusion Porosimetry have proved to be valuable tools in the quantitative analysis of flexible bio scaffolds; some caution must be used as these materials may deform under high pressure, but nonetheless the techniques allow a rapid and sensitive determination of these materials’ porous characteristics.