Relating the Frequency Dependent Radial Compliance to the Tensile Modulus of Polyurethane Vascular Grafts
5th World Biomaterials Congress, Transactions Volume 2, 434, (1996)

Bozzi, R1, Conti JC, Rohde D, Soldani G1, Spence L, Strope E, and Withrow D

Dynatek Dalta Scientific Instruments, PO Box 254, Galena, MO 65656
1 Instituto di Fisiologia Clinica, Via Trieste 41, 56126 Pisa, ITALY


Introduction

Since first introduced as a biomaterial in 1967 (I), polyurethanes have been utilized as candidate materials for vascular grafts. They show a high degree of toughness combined with a tendency to be blood compatible. It has been suggested that compliance matching of vascular grafts to recipient vessels is important to long term patency (2).

Polyurethane materials are susceptible to calcification, enzymatic degradation and auto-oxidation (3). There are a large number of polyurethanes that are candidate materials for vascular grafts. Coupled with tunable manufacturing processes, an effective long term implant could be produced. The problem is finding ways to evaluate a large number of materials in a timely and cost effective manner.

It is our intention to develop a three-part series of tests to give biomaterial scientists and engineers a well defined procedure to screen candidate materials for mechanical acceptability.

This report represents the results of our first evaluation towards this goal. In this study we compare the frequency dependent radial compliance of an intact polyurethane vascular graft to the frequency dependent tensile modulus obtained from circumferential samples of the same graft.

Materials and Methods

Two vascular grafts were prepared and supplied according to the technique of Soldani et al (4). The difference in these grafts represented a slight variation in the angle at which the polyurethane was deposited on the receiving mandrel. Both of these grafts were subjected to dynamic internal compliance testing. The radial compliance was evaluated over the pressure excursion from 80/120mm Hg at testing frequencies of 72, 200, 400, 600, 800 and 1000 cycles per minute.

Three 4mm wide hoops were then cut from different regions of the two graft samples. These hoops were split and mounted on a dynamic micromechanical tensile tester. The tensile modulus of these samples was then evaluated at the same cycle rate as the compliance determination.

All samples were moistened with distilled water throughout the testing.

Results

Figure 1 is a summary of the percent radial compliance determined at the various speeds for each sample. Note that there is a general trend of reduced compliance with increased test frequency, which is to be expected. Also note that the largest changes in sample 104 occur when going from 400 to 600 beats per minute and again from 800 to 1000 beats per minute. With sample 106, the largest changes occur at the transition from 72 to 200 beats per minute and from 600 to 800 beats per minute.

Percent Radial Compliance

Test            104        Sample       106
Speed

  72             4.71                        4.86
  200           4.68                        3.93
  400           4.57                        3.62
  600           3.22                        2.93
  800           3.15                        1.49
1000           0.70                        1.24

                           Figure 1


Figure 2 shows the summary of the tensile modulus, here calculated as the simple slope of the stress-strain curve. As you can see, the variation from sample to sample is significant. Some of the frequency dependent transitions in modulus correspond to the same transitions seen in the compliance data. The trend suggests that the dynamic mechanical properties of the graft are influenced by the fact that they have been shaped into a tube and not just by the properties of the polyurethane.

Tensile Modulus

Test  
Speed                         Sample
             104A       1048     104C     106A     106B     106C
   72          47           70         40         59         44         62
   200        60           82         52         68         58         81
   400        62           97         63         78         63         84
   600        76           94         56         91         68         89
   800        64         117         67         93         79         85
  1000       42         103         71        111       126         94

                                  Figure 2


Radial compliance versus frequency information is commonly used to determine the maximum effective test speed when performing accelerated durability testing on vascular graft samples. The modulus versus frequency presented here support the position that changing the structure of the vascular graft while keeping the materials the same can have an influence on the dynamic properties of the resultant device.

We are currently evaluating the hysteresis properties of these samples and will correlate the above compliance and modulus results with hysteresis and eventual accelerated durability of whole grafts.

References

1 Boretos, JW, Pierce, WS, Science 158:1481 (1967)

2 Abbott, WM, Cambria, RB, "Control of Physical Characteristics of Vascular
   Grafts" Biologic and Synthetic Vascular Prosthesis, Ed. JS Stanley, Grune
   and Stratton. 1982. 189-219.

3 Stokes, K, Coury, A, Urbanski, P, "Autooxidative Degradation of Implanted
   Polyether Polyurethane Devices," J Biomat App, 412-448, 1987.

4 Soldani, G, Panol, M, Goddard, M, Sasken, HF, Galletti, PM, J Mat Sci: Materials
   in Medicine, 3:106, 1992.