Biomedical Engineering Reference
In-Depth Information
Bioactive polymers and nanostructured composites
Natural and synthetic polymers have attracted increasing attention over the past
decade for their use as biodegradable scaffold materials [59]. Many have studied
such polymers as biodegradable scaffolds because these polymers allow for the
precise control of chemical composition, crystallinity, molecular weight,
molecular weight distribution, as well as easily controlled microstructure and
macrostructure (including porosity) [60±62]. For example, PLGA (poly-lactide-
co-glycolide) is one of the most popular US Food and Drug Administration
(FDA) approved polymers for orthopedic applications. PLGA was originally
developed for use in resorbable surgical sutures and biodegradable drug delivery
systems. The first commercial suture, Dexon
Õ
(composed of poly-lactide-co-
glycolide), was available in 1970 and the first FDA-cleared drug product was the
Lupron Depot drug-delivery system (TAP Pharmaceutical Products Inc.; Lake
Forest, IL). Since then there has been intensive development of medical devices
composed of PGA (polyglycolide), PLA (polylactide), and their copolymers
[63]. The use of biodegradable polymers in orthopedic devices for fixation of
long bone fractures was first clinically implemented in Finland in 1984 [64, 65].
Since the 1990s, applications of PLA, PGA and PLGAs in tissue engineering
have grown considerably [66].
However, owing to the generally weaker mechanical properties of PLGA and
its acidic degradation by-products compared with bone, numerous attempts have
been made to produce composites of polymers with nanostructured ceramics,
optimizing physical, mechanical and biological properties of scaffolds for bone
regeneration. For example, Liu et al. reported that nanostructured titania/PLGA
composites which had the closest surface roughness to natural bone promoted
osteoblast adhesion and subsequent calcium containing mineral deposition by
osteoblasts the most [67]. For ceramic/metal composites, nanostructured HA has
been coated on titanium and then implanted into canine trabecular bone. Results
showed a significant advantage for the integration of mineralized bone tissue
into HA/titanium composites compared with uncoated titanium [34, 68]. In
addition, it was reported that nano-HA/collagen composites synthesized to have
a porous microstructure similar to bone promoted the deposition of a new bone
matrix more than uncoated collagen. Furthermore, they showed that osteoblasts
within this biologically inspired composite eventually acquired a three-
dimensional polygonal shape that integrated with juxtaposed bone fragments
[69]. A nano-HA/collagen composite product, NanOss
Õ
(Pioneer Surgical
Technology, Inc. http://www.pioneersurgical.com/), has been released, stated to
have superior osteoconduction through a proprietary nanocrystalline HA and
collagen formulation designed to mimic bone tissue.
In conclusion, nanostructured biomaterials (including metals/metal alloys,
ceramics, polymers and composites thereof) can mimic the natural hierarchical
superamolecular organization of bone and, thus, hold great promise for improv-
