Influence of the Processes Parameters on the Properties of The ...

Chapter 1.
Polylactide Based Bio-Materials
al. [1988] with mean values <strong>of</strong> approximately 194 and 3.55 MPa as reported by Goulet et al. [1994].
Various parts <strong>of</strong> <strong>the</strong> bones in <strong>the</strong> human body have different mechnical properties. Compact bone is known
to have a compressive strength <strong>of</strong> 150–250 MPa due to variability in bone density [Natali and Meroi, 1989;
Carter, 1976]. Although <strong>the</strong> ideal mechanical strength <strong>of</strong> biomaterial scaffolds has not yet been determined,
previously researched scaffold compressive strengths have fallen within a 2–45 MPa range [Ghosh et al.,
2008; Gomes et al., 2008; Xiong, 2002]. The compressive modulus for bone has been measured to be 5–20
GPa while biomaterial scaffolds vary from 60 MPa to 15 GPa [Xiong, 2002]. Although polymeric scaffolds
have lower compressive strength and modulus than o<strong>the</strong>r biobased scaffolds and natural compact bones, it is
not fully understood to what extent scaffolds must mimic natural bone mechanical properties. They have,
however, demonstrated to be a promising substrate for cell growth and bone regeneration as shown by <strong>the</strong>
cellular studies and sponge-like characteristics <strong>of</strong> <strong>the</strong> scaffolds. The results from <strong>the</strong>ir experiments gave
modulus at a low level (cf. Table 1.1).
Table 1.1: Summary <strong>of</strong> mechanical properties <strong>of</strong> osteoporotic (OP) bone and normal bone.
Material Property OP Bone Normal Bone
E (MPa)
247
310
50 – 410 40 – 460
Yield strength (MPa)
2.5
3.3
0.6 – 5. 8 0.4 – 9.0
Energy absorbed to yield (kJ.m -3 16.3
21.8
)
2 – 52
2 – 90
[Li and Aspden, 1997]
Median values and approximate ranges <strong>of</strong> <strong>the</strong> 5% – 95% confidence limits as given by Li and
Aspden [1997] on <strong>the</strong> mechanical properties <strong>of</strong> human cancellous bone specimen (diameter: 9 mm, mean
cylinder length: 7.7 mm) from OP femoral heads.
1.3 Biodegradable Polymers
During <strong>the</strong> last years, a large number <strong>of</strong> articles and publications have been published which cover
biodegradable polymers <strong>of</strong> <strong>the</strong> different material groups (e.g., polysaccharides, polypeptides, polyesters, and
polyisoprenoides), as well as <strong>the</strong>ir copolymers and blends. PLA (polylactic acid) and PLGA poly (lactideco-glycolide)
are mainly used in medical engineering as biodegradable polymers, because <strong>the</strong>se are naturally
occurring polymers. Degradable/resorbable polymers have been well established in <strong>the</strong> field <strong>of</strong> medicine, for
example, as surgical sutures, implants, and bone plates, since 1960s and 1970s [Schmack, 2009]. It is not
easy to classify biodegradable polymers. They can be sorted according to <strong>the</strong>ir chemical composition,
syn<strong>the</strong>sis method, processing method, economic importance, application, etc. Each <strong>of</strong> <strong>the</strong>se classifications
provides different and useful information. In <strong>the</strong> present overview, we have chosen to classify biodegradable
polymers (hereafter called biopolymers) according to <strong>the</strong>ir origin: natural polymers, polymers coming from
natural resources and syn<strong>the</strong>tic polymers, polymers syn<strong>the</strong>sised from crude oil. Biopolymers from natural
origins include, from a chemical point <strong>of</strong> view, six sub-groups:[Clarinval and Halleux, 2005]
1. polysaccharides (e.g., starch, cellulose, lignin, chitin).
2. proteins (e.g., gelatine, casein, wheat gluten, silk and wool).
3. lipids (e.g., plant oils including castor oil and animal fats).
4. polyesters produced by micro-organism or by plants (e.g., polyhydroxy-alcanoates, poly-3-
hydroxybutyrate).
5. polyesters syn<strong>the</strong>sised from bio-derived monomers (polylactic acid).
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