Fabric porosity is an important factor in vascular graft effectiveness and should be related to permittivity. [E.S. Greenhalge & M.W. Dunn; Ann. Rpt: Nat. Textile Ctr; Nov 1998, p352] The initial formation of a cohesive fibrin lining is closely related to blood flow through the graft and hence permittivity of the fabric. The fibrin lining is subsequently replaced by fibrous tissue growth through the pores of the graft. [J. H. Harrison, Am. J. Surgery, 95(1958)3]

A possible device that can utilize the Advanced Permittivity Testing Method for biotextile purposes is shown in Figure 5.1a. The tubular specimen, mounted in the specimen holder, is first inserted into the device. By means of the thrust shaft, the drive motor moves the pistons at a constant speed, forcing the fluid through the specimen fabric at a constant flow rate.  A motor controller will hold a variety of constant motor speeds to determine biotextile permittivity. The pressure difference can be set to typical peristaltic or diastolic values.

The use of the device is quite flexible.  For example, a series of vascular grafts removed from animals during fibrin formation can be tested by the Advanced Permittivity Testing Method to document changes in permittivity against measured thickness of the fibrin layer.

With the quick release mechanism, the pistons can be readily removed from the device, as shown in Figure 5.1b, permitting thorough cleaning. Figure 5.2 illustrates possible specimen holder configurations. A tubular specimen can be either cemented or clamped to the insert. Assuming circular tubes — although not necessary — then of course the effective area of the specimen A=pdL.

The permittivity testing procedure is fully automated, as shown in Figure 5.3, using the electric valve for fluid control. Automatic motor sequencing permits a full series of tests to be run by a specially programmed central processing unit. In this manner, reproducible permittivity determinations can be made for biotextiles.

Figure 5.4a represents an example of the initiation stage of a fully automated testing procedure using the Advanced Permittivity Testing Method. After the specimen holder has been inserted, the fluid chambers are evacuated to remove air bubbles from the fabric. The electric valve then connects the test chamber to the fluid source, as shown in Figure 5.4b, and a previously de-aerated fluid is then automatically injected into the test chamber.

Piston motion at a fixed specified speed is initiated, as represented in Figure 5.5a, forcing the fluid into the mouth of the tubular specimen shown in Figure 5.1a, with the pressure difference across the fabric recorded.

To prevent collapse of the tube on piston motion reversal, the chambers are connected across the electric valve, as represented in Figure 5.5b. This process is repeated at different motor speeds to obtain the information required to calculate the permittivity, as shown in Figure 2.2. If any deviation from linearity appears, this can be readily discerned.

Test completion is represented by Figure 5.6a. The fluid is evacuated from the chambers, as represented by Figure 5.6b, and air is then bled into the chambers.  The specimen holder can then be removed.

Figure 5.7 shows a possible design of a commercial device using the Advanced Permittivity Testing Method. The allowable flow rate and pressure difference range for this apparatus would cover essentially all biomedical requirements for organic and inorganic fluids.  Modification for different temperature operations would be optional. Operation is fully automated.