High Frequency Testing of Grafts and Mock Arteries for Stents: Influence of Testing Frequency on Durability
25th Annual Meeting of the Society for Biomaterials, Transaction, 403, (1999)
James C. Conti1, Elaine R. Strope, Karen Price
Dynatek Dalta Scientific Instruments, Fourth and Main St., PO Box 254, Galena, MO 65656 USA
1 Southwest Missouri State University, Dept. of Physics, Springfield, MO USA
There is little precedent in the medical product development world for the recent introduction of new product ideas associated with the developers of stents. Product developers face the competing pressures of the need for verifiable, long term durability testing on these products and the need to minimize time to market. Although there has been some relaxation on the part of regulatory agencies to allow clinical trials to begin without the need for complete durability studies, there still is a need on the part of developers to perform durability tests that are as fast as possible, and yet still reliable.
These same issues are evident in the testing of vascular grafts as well. Since the stents that need to be tested are usually put into mock arteries of known physiologically relevant compliances, the need to evaluate the reliability of high speed testing of vascular grafts and mock arteries for stents is very similar. In our attempts to further determine the ability to test these devices at ever faster frequencies, we have designed an experiment to evaluate the influence of high speed testing on long term durability.
The problem with exceeding the currently accepted frequency of testing is that the engineering properties of these tubes change dramatically at high frequency. To combat this, there has been a tendency to simply overpressurize these devices, so as to overcome the tendency for them to become stiffer or lose compliance at high frequency. This experiment addresses these issues.
Fifty 3.6mm inside diameter by 10cm long natural latex rubber tubes were produced and evaluated using a variety of pressure excursions and frequencies to determine their dynamic internal radial compliance. Pressure excursions and frequencies were adjusted with the intention of carrying out two different long term durability experiments, one at a speed in which it was determined that the tubes still had the same properties as they did at 72bpm, and the other designed to adjust the pressure excursion to allow us to do the same geometric change testing per cycle but instead at 1,600bpm.
Table 1 is a summary of the testing done at various cycles per minute. Above 800bpm the % radial compliance (%C) dropped rapidly, and then above 1,600bpm, the lower compliance remains surprisingly constant.
Table 2 shows an isolated test at 72bpm and 1,000bpm from the same batch of tubing, showing that at 1,000bpm the compliance was still very close to the numbers obtained at 72bpm. As a result, a testing speed of 1,000bpm, representing well-defined engineering properties, was chosen for the further, long term testing. In contrast, 1600bpm is a testing frequency at which the tubes are evidencing lower compliance (see Table 1). This speed, then, was chosen as the testing frequency in a second long term durability experiment.
Table 3 shows a series of pressure excursions, all carried out at 1,600bpm, with corrections showing the actual change in radius per cycle. This table demonstrates that a pressure excursion of 210 to 340 gives the same change in geometry at 1,600bpm that these tubes demonstrate at 72bpm.
Two high speed durability testing instruments were set up, one with a pressure excursion of 180/80mmHg testing at 1,000bpm, and the second at a pressure excursion of 340/210mmHg testing at 1,600bpm. All 24 tubes will be removed each 100 million cycles, and the resulting data will be followed and reported.
Twenty-four of the above mock arteries were mounted on two high speed durability testers. Twelve on system I were tested at a cyclic pressure differential of 180180mm Hg at a frequency of 1000bpm. The other twelve were tested at 340/210nim Hg at a frequency of 1600bpm. Several follow-up compliance determinations were carried out.
Table 4 shows the results of the follow-up compliance evaluations on the low speed testing. Table 5 is similar, but on the high speed testing. Notice a much larger amount of fatigue experienced by the low speed testing.
Table 6 and 7 show the burst history of the samples. These data are complicated by the fact that burst pressures are also the loading pressures and, therefore, the low experiments are bursting at lower pressures. Nonetheless, there is a tendency for the lower tested samples to burst 15-20 million cycles earlier than the high experiments.
These experiments show that overpressurized high speed testing of mock arteries has a tendency to undertest the samples compared to slower, more well-defined testing. This means that high pressurelhigh speed testing could indicate success in products that will fail when subjected to loading that is more physiologically relevant.
1. Pressure Excursion 80-180mmHg
2. Natural latex 3.6mm i.d. x 10cm length
3. Percentage change in radius per 100mmHg normalized
4. Actual % change in radius