Biomedical Engineering Reference
In-Depth Information
Figure 21.7.
Accelerated durability test systems for the evaluation of artifi cial heart valves.
The set up simulates the loading conditions in the heart and is capable of cycling at up to 12
times faster than the heart. Five valves can be tested simultaneously using each system.
TABLE 21.10. Accelerated Durability Test Conditions
Parameter
Unit
Value
Cycling rate
Cycles/min
720 ± 80
Valve closing pressure
mm Hg
120 ± 10
Average fl ow rate
lpm
3.5 ± 0.5
Test fl uid
—
Distilled water + Fungal growth inhibitors
Test temperature
° C
37 ± 5
Four tilting disc type mechanical valves with different material combinations
were tested (see Table 21.11). TTK Chitra heart valve, which is already in clinical
use, was chosen as the control device in these studies.
The valves were removed from the test system at intervals of 40 million cycles
and are examined for evidence of structural deterioration. The valves are weighed
accurately in an analytical balance and their weights recorded. The end points in
this accelerated durability test were when: there is a failure of any of its compo-
nents; excessive wear is noticed and there is a likelihood of failure or; when the
valve crosses 400 million cycles of operation (Figure 21.8).
At the end of the test, the valves were examined for signs of wear and other
structural deterioration. The volume of wear is calculated from the weight loss
and the density of the material that is wearing out. The wear rates of all the valves
models were comparable with average rates less than 0.5 microlitres/year of run
(corresponding to 40 million cycles). The DLC-coated valve showed marginally
lower wear rates. Especially in the case of valve with DLC-coated UHMWPE, the
disc edge was very uniform and edge surface fi nish remained extremely good,
which indicates smooth rotation of the disc in the valve assembly and is a very
important factor. This smooth rotation of the disc ensures uniform distribution of
wear in the longer run and can enhance the service life of the valve considerably.
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