Biomedical Engineering Reference
In-Depth Information
intensively investigated as drug-delivery vehicles. The surface-eroding characteristic offers three
key advantages over bulk degradation when used as scaffold materials: (1) retention of mechani-
cal integrity over the degradative lifetime of the device, owing to the maintenance of mass to vol-
ume ratio; (2) minimal toxic effects (i.e., local acidity), owing to lower solubility and concentration
of degradation products; and (3) signifi cantly enhanced bone ingrowth into the porous scaffolds,
owing to the increment in pore size as the erosion proceeds [123].
1.3.5.2.1
Poly(anhydrides)
Poly(1,3-
bis
-
p
-carboxyphenoxypropane anhydride) [124] and poly(erucic acid dimer anhydride)
[125] are biodegradable polymers for controlled drug delivery in a form of implant or injectable
microspheres. Studies in rabbits have shown that the osteocompatibility of poly(anhydrides) that
undergo photocuring are comparable to PLA and that the implants of poly(anhydrides) show
enhanced integration with the surrounding bones in comparison to PLA controls [126].
1.3.5.2.2
Poly(ortho-esters)
Poly(ortho-esters) (POE) scaffolds were coated with cross-linked acidic gelatine to improve surface
properties for cell attachment. Preliminary
in vitro
and
in vivo
results revealed that POE did not
show any infl ammation and had little or no effect on bone formation while PLA provoked a chronic
infl ammatory response and inhibited bone formation [127,128].
1.3.5.2.3
Polyphosphazenes
These polymers seem to be potential bioerodible materials capable of controlled degradation and
sustained drug delivery for therapeutic use [101,129] and bone regeneration [130]. Their tailored
side groups enable a wide variety of hydrolytic properties to be designed into selected polymers
for application in biological environments without the release of harmful degradation products at
physiological concentration.
1.3.6 B
IOCOMPOSITES
From a biological perspective, it is a natural strategy to combine polymers and ceramics to fabricate
scaffolds for bone tissue engineering because native bone is the combination of a naturally occurring
polymer and a biological apatite. From the point of view of materials science, a single material type
does not always provide the necessary mechanical and chemical properties desired for a particular
application. In these instances, composite materials designed to combine the advantages of both
components may be most appropriate. Polymers and ceramics that degrade
in vivo
should be cho-
sen for designing biocomposites for tissue-engineering scaffolds. While massive release of acidic
degradation from polymers can cause infl ammatory reactions [4,92,131], the basic degradation of
calcium phosphate or bioactive glasses would buffer the acidic by-products of polymers and may
thereby help to avoid the formation of an unfavorable environment for cells due to a decreased pH
level. Mechanically, bioceramics are much stronger than polymers and play a critical role in provid-
ing mechanical stability to constructs prior to the synthesis of a new bone matrix by cells. However,
ceramics and glasses are very fragile because of their intrinsic brittleness and fl aw sensitivity. To
capitalize on their advantages and minimize their shortcomings, ceramic and glass materials have
been combined with various biopolymers to form composite biomaterials for osseous regenera-
tion. Table 1.7 lists selected ceramic/glass-polymer composites, which were designed as biomedical
devices or scaffold materials for bone tissue engineering, and their mechanical properties.
In general, all these synthetic composites have good biocompatibility. Kikuchi et al. [132], for
instance, combined TCP with PLA to form a polymer-ceramic composite, which was found to pos-
sess the osteoconductivity of β-TCP and the degradability of PLA [132].
The research team led by Laurencin [147] synthesized porous scaffolds containing PLGA and
hydroxyapatite, which were reported to combine the degradability of PLGA with the bioactiv-
ity of hydroxyapatite, fostering cell proliferation and differentiation as well as mineral formation
