Biomedical Engineering Reference
In-Depth Information
approach to creating a living valve is to implant a valve matrix which attracts the
right cells from the body and induces them to remodel the matrix and create a
living tissue (De Visscher et al., 2007). Preliminary research indicates that the
body does contain the right kinds of cells required for such a task (De Visscher
et al., 2008), and early research is underway to attract those cells to the valve
matrix using the appropriate homing factors (De Visscher et al., 2010).
Undoubtedly, the concepts and technologies being developed for a tissue
engineered heart valve will continue to evolve as we understand how to address
the many challenges posed by such a device. Only time will tell when a tissue
engineered heart valve will become a commercial reality.
5.9
Applicable standards
There are numerous ISO guidelines regulating the design, development and
testing of heart valves, both mechanical and biologic. ISO 5840 describes the
specific test methods that need to be performed to demonstrate the safety of the
valve. Informative Annex K in ISO 5840 describes a number of relevant
standards applicable to testing the materials and components of heart valve
substitutes. While these are too numerous to list here, the Annex includes
specifications for metals, tensile testing of metals, durability crack initiation and
endurance limit testing of metals, fatigue crack growth rate testing, hardness,
microstructure determination, thermal expansion testing, fracture toughness,
and fatigue life. Testing which may be required for polymers include
viscosimetry, melt flow index, determination of breaking strength under a
static load, tensile testing to failure, tensile properties determination, deter-
mination of dynamic mechanical properties, resistance to surface wear,
resistance to scratching, flexural properties and the determination of breaking
strength under dynamic bending load, fatigue crack initiation determination and
measurement of fatigue crack growth rate, determination of compressive
properties, material specifications for various common medical polymers,
density, liquid diffusivity, hardness, wear resistance, creep, fracture toughness,
and hydraulic expansion.
The Annex also identifies relevant standards for ceramics and carbons,
including characterization of physical and chemical properties, measurement of
fatigue crack growth rates, hardness, thermal expansion and fracture toughness.
While there are no specific standards or guidance documents describing the
mechanical testing of biological materials, the Annex identifies some
potentially relevant test methods which might be adapted to the determination
of the tensile properties of biological materials. The Annex also identifies
testing to be completed on the cloth used in the valve. The ISO 10993 series is
applicable to the biocompatibility testing of the device, which is detailed more
fully in Part 1. The FDA has recently distributed its draft guidance on heart
valves (Vaughn, 2010).
