Biomedical Engineering Reference
In-Depth Information
break down into the original particles. It has been shown that those particles can
induce inflammatory responses depending on their size [
56
,
57
]. On the other hand,
controlled surface reactivity can be used to provide the surrounding cells with the
building blocks, such as calcium and phosphate, for the new tissue or stimulate the
cells with leaching bioactive components [
1
]. Osteoclasts and macrophages are
involved in phagocytosis and in resorption of bioactive ceramics and glasses [
54
].
The non-degraded materials tend to accumulate to form extracellular deposits [
58
].
2.3 Polymers
The development of artificial polymers has changed almost all fields of modern
science and engineering. Biodegradable implants are no exception to this. Bio-
degradable polymers typically contain linkages susceptible to hydrolysis. If such a
material is hydrophilic and capable of absorbing water, the physiological envi-
ronment will lead to its degradation [
59
]. Tissue enzymes can greatly influence
this process. Enzymes are catalysts for specific biochemical reactions. Certain
enzymes are able to induce or accelerate hydrolytic degradation of polymers,
which are normally not degraded at body temperature [
1
,
60
-
62
]. In addition,
certain cells of the immune system are involved in the degradation and resorption
process, either by their attachment to polymer surfaces and the release of
destructive enzymes on to the surface [
63
] or by the ingestion of fragments of the
polymer. Degradable polymers include: polyglycolide acid (PGA) [
64
,
65
],
poly(L-lactide) (PLL), PLGA [
66
,
67
], PLLA [
68
,
69
], poly(D-lactide) (PDL),
poly(c-caprolactone) (PCL) [
70
], polyphosphazenes [
71
,
72
], poly(orthoester)
(POE) [
73
], poly(beta-hydroxybutyrate) (PHB) [
64
], polyhydroxyalkanoates
(PHA) [
74
,
75
], polyesters based on fumaric acid (PPF) [
76
-
79
], tyrosine-based
polycarbonates [
80
,
81
], rosin-based polymers [
82
-
84
], and the naturally occur-
ring polymer collagen [
85
].
3 Measurements of Bioresorption
To be able to develop new biomaterials with tailored degradation characteristics, it
is necessary to have methods available to quantify the extent of the degradation
under different conditions (Table
1
).
3.1 ISO Medical Biomaterials Degradation Assays
The International Organization for Standardization (ISO) is a worldwide federa-
tion which mainly focuses on establishing standards worldwide (
www.iso.org
).
The main advantage of this approach for testing of biodegradation is that even if
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