Biomedical Engineering Reference
In-Depth Information
on the strut surfaces upon immersion in SBF [111]. It has also been shown that the foams support the
migration, adhesion, spreading, and viability of MG-63 cells (osteosarcoma cell line) [112].
Poly(ε-caprolactone) (PCL) is also an important member of the aliphatic polyester family. It
has been used to effectively entrap antibiotic drugs and thus a construct made with PCL can be
considered as a drug-delivery system, being used to enhance bone ingrowth and regeneration in
the treatment of bone defects [113]. The degradation of PCL and its copolymers involves similar
mechanisms to PLA, proceeding in two stages: random hydrolytic ester cleavage and weight loss
through the diffusion of oligometric species from the bulk. It has been found that the degradation of
PCL system with a high molecular weight (
__
M
n
of 50,000) is remarkably slow, requiring 3 years for
complete removal from the host body [114].
1.3.5.1.2
Polyhydroxyalkanoates (PHB, PHBV, P4HB, PHBHHx, PHO)
Recently, polyhydroxyalkanoates (PHAs), another type of polyesters, have been suggested for tis-
sue engineering because of their controllable biodegradation and high biocompatibility [115]. They
are aliphatic polyesters as well, but produced by microorganisms under unbalanced growth condi-
tions [116,117]. They are generally biodegradable (via hydrolysis) and thermoprocessable, making
them attractive as biomaterials for application in medical devices and tissue engineering. Over
the past years, PHA, particularly poly-3-hydroxybutyrate (PHB), copolymers of 3-hydroxybutyrate
and 3-hydroxyvalerate (PHBV); poly 4-hydroxybutyrate (P4HB), copolymers of 3-hydroxybutyrate
and 3-hydroxyhexanoate (PHBHHx); and poly 3-hydroxyoctanoate (PHO) were demonstrated to be
suitable for tissue engineering and are reviewed in detail in Refs. 115,116.
Depending on the property requirement of different applications, PHA polymers can be either
blended, surface modifi ed, or composed with other polymers, enzymes, or inorganic materials to
further adjust their mechanical properties or biocompatibility. The blending among the several PHA
themselves can dramatically change their material properties and biocompatibility [115,116].
PHB is of particular interest for bone tissue application as it was demonstrated to produce a
consistent favorable bone tissue adaptation response with no evidence of an undesirable chronic
infl ammatory response after an implantation period of up to 12 months [116]. The bone is formed
close to the material and subsequently becomes highly organized, with up to 80% of the implant
surface lying in direct apposition to the new bone. The materials showed no evidence of extensive
structural breakdown
in vivo
during the implantation period of the study [118].
However, a drawback of some PHA polymers is their limited availability and the time-consuming
extraction procedure from bacterial cultures that is required for obtaining suffi cient processing
amounts as described in the literature [115,119]. Therefore, the extraction process might be a chal-
lenge to a cost-effective industrial upscale production for large amounts of some PHA polymers.
1.3.5.1.3
Polypropylene Fumarate
Poly(propylene fumarate) (PPF) is an unsaturated linear polyester. Similar to PLA and PGA, the
degradation products of PPF through hydrolysis (i.e., propylene glycol and fumaric acid) are bio-
compatible and readily removed from the body. The double bond along the backbone of the polymer
permits cross-linking
in situ
, which causes a moldable composite to harden within 10-15 min.
Mechanical properties and degradation time of the composite may be controlled by varying the
PPF molecular weight. Therefore, preservation of the double bonds and control of molecular weight
during PPF synthesis are critical issues [120].
PPF has been suggested for use as scaffold for guided
tissue regeneration, often as part of an injectable bone replacement composite [121], and has been
used as a substrate for osteoblast culture [122].
1.3.5.2
Surface Bioeroding Polymers
There is a family of hydrophobic polymers that undergo a heterogeneous hydrolysis process, which
is predominantly confi ned to the polymer-water interface. This property is referred to as surface
eroding as opposed to bulk degrading behavior. These surface bioeroding polymers have been
