4.1 This document describes the basic principles that need to be followed to obtain a mean value of the Darcy permeability coefficient for structures that consist of a series of interconnected voids or pores. The coefficient is a measure of the permeability of the structure to fluid flowing through it that is driven by a pressure gradient created across it.

4.2 The technique is not sensitive to the presence of closed or blind-end pores (Fig. 1

).

4.3 Values of the permeability coefficient can be used to compare the consistency of manufactured samples or to determine what the effect of changing one or more manufacturing settings has on permeability. They can also be used to assess the homogeneity and anisotropy of tissue scaffolds. Variability in the permeability coefficient can be also be indicative of:

4.3.1 Internal damage within the sample e.g., cracking or permanent deformation.

4.3.2 The presence of large voids, including trapped air bubbles, within the structure.

4.3.3 Surface effects such as a skin formed during manufacture.

4.3.4 Variable sample geometry.

4.4 This test method is based on the assumption that the flow rate through a given sample subjected to an applied pressure gradient is constant with time.

Note 1-If a steady state flow condition isn't reached, then this could be due to structural damage (i.e., crack formation or the porous structure deformed as a result of the force being placed upon it by the fluid flowing through it). Sample deformation in the form of stretching (bowing) can also occur for less resilient structures as a result of high fluid flow rates. This topic is discussed in more detail in Section 7.

4.5 Care should be taken to ensure that hydrophobic materials are fully wetted out when using water or other aqueous-based liquids as permeants.

4.6 Conventionally, the pressure differential created across a sample is measured as a function of both increasing and decreasing flow rates. An alternative approach, which may be practically easier to create, is to apply a range of different pressure differentials across the sample and measure the resultant flow of fluid through it. The hysteresis that occurs during a complete cycle of increasing flow rate followed by a progressive decrease in flow rate can provide an excellent measure of the behavioural consistency of the matrix. Significant hysteresis in the measured pressure differential during increasing and decreasing flow rates can indicate the existence of induced damage in the structure, the fact that the material is behaving viscoelastically or suffering from permanent plastic deformation. Some guidance on how to identify which of these factors are responsible for hysteresis is provided in Section 7.

4.7 It is assumed that Darcy's law is valid. This can be established by plotting the volume flow through the specimen against the differential pressure drop across the specimen. This plot should be linear for Darcy's law to apply and a least squares fit to the data should pass through the origin. It is not uncommon for such plots to be non-linear which may indicate that the structure does not obey Darcy's law or that the range of pressures applied is too broad. This topic is further discussed in Section 7.