Biomedical Engineering Reference
In-Depth Information
3.3.2.
Carbon-based implants
Carbon-based materials can be obtained in various allotropic
forms; but only the low-temperature isotropic (LTI) and the
ultralow-temperature isotropic (ULTI) pyrolytic carbons are widely
used as biomaterials, mainly as surface coating onto articulating joint
surfaces for ULTI carbon, and hand joints and heart valves for LTI
carbon. Pyrolytic carbons are man-made pure elemental carbon
materials obtained from the pyrolysis of hydrocarbon precursors. They
are partially crystalline materials since they have a turbostratic
structure which consists of graphene sheets held by Van der Waals
forces and stacked in a disordered manner through random rotations or
displacements of the layers relative to each other. Pyrolytic carbons
exhibit small crystallites (2.5-4.0 nm for LTI and 0.8-1.5 nm for
ULTI) randomly oriented conferring to the material its isotropic
behavior [BOE 11].
High-purity pyrolytic carbons (Table 3.3) are bioinert materials
that possess low modulus, quite high strength in comparison with
glassy carbon and graphite (compressive strength = 172 and 138 MPa,
respectively), high fatigue and wear resistance, good compatibility
with blood and soft tissue. The mechanical properties are related to the
density, that is to say to the material aggregate structure. High density
LTI carbons are strong materials which can be designed as coating or
as monoliths, while ULTI carbons can also be obtained with high
density (1.5-2.2 g.cm
−3
) and strength but only as a thin coating
(0.1 to 1 µm). The combination of low modulus and high flexural
strength (275-550 MPa for LTI and 345 to >690 MPa for ULTI) leads
to large strain to failure (2% for LTI and >5% for ULTI). As a
consequence, it is possible to coat flexible polymeric biomaterials
since the coating does not fracture under flexion of the substrate
[DAV 03]. Another unique property of pyrolytic carbons is their
durability, and they do not fail in fatigue because of the absence of
mobile defects in their crystalline structures. Furthermore, up to 20%
of silicon can be added to LTI resulting in a structure composed of
sub-micron β-SiC particles randomly dispersed in a matrix of roughly
spherical micro-size subgrains of pyrolytic carbon. These silicon-
alloyed LTI carbons are developed to improve stiffness, hardness,
