permeability

Background

Historically, porosity and permeability have been interchanged when describing textile fabrics. Apparel permeability is most frequently measured by controlled air flow through fabric. Values recorded for this test are cubic feet of air per square foot of fabric for 1 minute of time or volumetric flow rate. Vascular grafts are not tested using air flow, but rather liquid flow measurements. Fluid volumetric flow is recorded as milliliter of fluid per square centimeter of fabric per minute. To measure meaningful values of vascular graft permeability, a controlled pressure must be maintained across the fabric surface.

Porosity of a fabric is defined by the ratio of free space to fiber in a given volume of fabric. A fabric with a porosity of 0.25 has less free space than a fabric with a porosity of 0.75. A porosity of 1 is a totally open fabric, while a porosity of 0 is a film with no open spaces.

Fig. 1 As described above, porosity and permeability have been interchanged; however, one skilled in the art will discern a difference. Permeability and porosity are related, but not the same. Two extreme cases are described below. If a fabric has very high porosity, it can be assumed that it is permeable. A fabric with zero porosity can be assumed to have zero permeability. Now consider a fabric with porosity between 0.1 and 0.9. Permeability is more difficult to predict. The position and size of the free space in a fabric effect the permeability while porosity can stay constant. Figure 1 illustrates an example of this phenomenon for two materials of equal thickness.

Details of the project

Model fluid flow through permeable fabric

Design and produce samples

Develop a Permeability Tester

Compare experimental and theoretical predictions Improve model

Porosity

Pores are void spaces which must be distributed more or less frequently through the material if the latter is called "porous". Extremely small voids in a solid is called "molecular interstices", very large ones are called " caverns." Pores are void spaces intermediate between caverns and molecular interstices. The pores in a porous systems may be interconnected or non-interconnected. The interconnected part of the pore system is called the effective pore space of the porous medium.

A textile material is regarded as an assembly of yarns. This assembly will be permeable to fluid flow because of the existence of spaces or pores between the yarns (inter-yarn pores). The yarns themselves will also be permeable to fluid flow because of the spaces or pores that separate the individual filaments in the yarn (intra-yarn pores). The total flow of liquid through a textile will therefore depend on the part played by both types of pore.

Permeability

The units for measuring graft water permeability are milliliters of water per square centimeter of fabric per minute (ml/cm²/min.). Knits, braids, and weaves fall into ranges of permeability as controlled by the variables listed above. Tightly woven grafts have permeability below 800, tight compacted knits are from 1200 to 2000, loose knits from 2000 to 5000, and braids from 350 to 2500 . This list does not include ePTFE vascular grafts. ePTFE permeability is measured using a different technique called water entry pressure, also described in the AAMI literature.

Many researchers have shown that healing response is influenced by and permeability and porosity . Higher permeability and porosity have been shown to induce faster healing. However, permeability over 800 ml/cm²/min requires preclotting before implantation. If preclotting is not performed, the graft will leak and hemostasis will never take place. Permeability below 600 ml/cm²/min does not require preclotting, but grafts with this level of permeability have been shown to heal slowly.

Arterial Grafts

Arterial graft fabrics are required to have minimal thickness yet maintain specified levels of strength and blood permeability. The models of thickness and strength are fairly well understood. But, although there is significant work in the literature addressing the flow of fluid through fabric structures (see Mc Gregor, Pierce, Baker, etc) this work has focused on Newtonian fluid flow through regular structures. It is well known that blood behaves in a non-Newtonian manner, and can even be considered as Einstenian particulate reinforced fluid.

The use of small yarns (1-7 tex) and small fibers (1-5 mili-micron diameter) in the production of the fabrics requires improved modelling capabilities. Since blood cells are similar in dimension to the fibers, the presence of these cells directly influences the flow through fabric.

Vascular Graft placed inside the Aortic Abdominal Aneurysm

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This page was last updated on 07.02.1999