CELL - BASED NANOCOMPOSITES AND BIOMOLECULES FOR BONE TISSUE ENGINEERING - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 549

In a separate study, bioactive glass nanofi bers were electrospun with average
diameters ranging from 85 - to - 400 nm 51 . The presence of polyvinyl pyrrolidone
(PVP) and surfactant pluronic P123 (EO 20 - PO 70 - EO 20 ) resulted in the formation
of smooth nanofi bers and a reduction in diameter respectively. The nanofi bers
were subjected to simulated body fl uid (SBF) whereby its ionic concentration was
similar to human blood plasma and calcium phosphate nanoparticles were depos-
ited on the nanofi ber surfaces after six hours of SBF immersion. With increasing
immersion periods, more apatite was seen and after twenty-four hours of immer-
sion, the bioglass nanofi bers were entirely covered with apatite layers. Unlike
conventional bioglass fi bers, the induction of apatite was accelerated on the bio-
glass nanofi bers and this could be explained by virtue of the fact that the nanofi -
bers had a large surface area that promoted apatite deposition.

16.3.2 Polymers
16.3.2.1 Natural Polymers. Natural polymers such as collagen, gelatin,
chitosan, alginate, hyaluronan, fi brin and silk are frequently used in bone tissue
engineering 52 - 61 . For instance, electrospun silk fi broin fi bers were subjected to an
alternate soaking method for nucleation and growth of apatite 61 . The fi bers were
fi rst immersed in a calcium solution, followed by immersion in phosphate solu-
tion. Mineralization was achieved as apatite preferentially grew along the longi-
tudinal direction of the fi bers. The silk fi broin and acidic peptides allowed the
controlled nucleation and growth of apatite minerals on the fi bers 61 . In a separate
study, porous hyaluronan-based materials coated with fi bronectin that were
implanted in osteochondral defects in rabbits exhibited improved bone repair as
compared to those without the implantation of the hyaluronan-based materials 59 .
One of the drawbacks of natural polymers is the lack of mechanical properties.
Therefore, extensive investigations have been carried out in fi ne - tuning the
material selection and design.

16.3.2.2 Synthetic Polymers. Synthetic materials are gaining popularity
as alternative options because scaling up in production terms is not an issue and
they are mechanically better than natural materials. The commonly used synthetic
polymers encompass poly(lactic - co - glycolic) acid (PLGA), poly - l - lactide acid
(PLLA) and polycaprolactone (PCL) 62 - 66 . Polyglycolic acid (PGA) is often used
in medical applications (such as sutures) because it is a degradable product, since
glycolic acid is a natural metabolite. Glycolic acid can also be excreted out of the
body as urine. Polylactic acid (PLA) is also used and it is generally more hydro-
phobic than PGA. PLA has three isomeric forms, namely: d(

), l(+), and racimic
(d, l). Poly(l)LA and poly(d)LA are semi-crystalline solids and have similar deg-
radation rates as PGA. In general, the (l) isomer of lactic acid (LA) is preferred
because it can be metabolized in the body. The degradation rate of PCL is slower
than that of PLA and is a suitable material for long-term, drug delivery systems.
One of the disadvantages of biodegrabable synthetic polymers is the release of
acidic by-productions during degradation 67 . Typically, a combination of ceramic-

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