DESIGNING NANOFIBROUS SCAFFOLDS FOR TISSUE ENGINEERING - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 454

solvent system for the synthesis of electrospun PU nanofi bers as wound-dressing
materials [170]. It has been reported that THF as the solvent system leads to the
production of unstable jets moving in all directions. Low fl ow rate of the polymer
solution in THF leads to its drying at the needle tip, while higher fl ow rate pro-
duces larger diameter fi bers. On the other hand, DMF as a solvent system causes
production of droplets with intermediate fi bers. Therefore, Khil et al. optimized
the ratio of DMF :THF and demonstrated the production of PU fi bers in the
range of 250 to 300 nm [170] with DMF :THF in the ratio of 30 : 70 (v/v), thereby
demonstrating that the solvent system can play a major role in the formation of
defect - free fi bers in the nanometer range.

13.4.2.4 Bacterial Polyesters. As the name suggests, these polyesters are
synthesized by bacteria like Bacillus megaterium, Wautersia eutropha , and Pseu-
domonas species and are meant to provide the bacteria with a reserve house for
carbon and energy. Bacterial polyesters mainly include the family of poly-hydroxy
alkanoates (PHA) with the most well studied members being poly-hydroxybutyr-
ate (PHB) and poly-hydroxyvalerate (PHV). Bacterial polyesters have been
explored in tissue engineering due to their biocompatibility and biodegradability.
Further, PHB and a copolymer of PHB and PHV-PHBV are potential candidates
for long-term implants due to their slow degradability.
Synthesis of PHBV nanofi bers by electrospinning has been reported by
Choi et al. [87]. PHBV fi bers were obtained in the range of 1-4 m when

chloroform was used as the solvent system. They further studied the infl uence
of an additive-benzyl trialkylammonium chloride. This salt was soluble in chlo-
roform and its addition caused a signifi cant increase in the conductivity of the
solution, thereby leading to increased elongation effects due to repulsion of
similar charges on the polymer jet. These favorable factors led to a decrease in
average fi ber diameter to 1

μ

m. Further, since PHBV nanofi bers electrospun with
the salts had higher surface area to volume ratio as compared to PHBV fi bers
generated without the use of an additive, they degraded faster. In another study,
Lee et al. reported the synthesis of electrospun PHBV using 2,2,2-trifl uoroethanol
as the solvent system and obtained fi bers having average fi ber diameter of
185 nm [171] .

μ

13.4.3 Polymeric Blends
Current state-of-the-art of design of many tissue engineering scaffolds demands
the combination of properties offered by natural and synthetic polymers. Few
synthetic polymers—for example, poly(

- hydroxyesters) — possess poor process-
ability and release acidic products during their degradation. Further, synthetic
polymers lack cell-recognition moieties, which are present in natural materials.
Natural polymers, on the other hand, form a gel-like phase at low polymer con-
centration and a highly viscous solution at increased polymer concentrations,
making it diffi cult to electrospin. Further natural polymers rapidly go into solu-

α