Vascular tissue engineering - Biomaterials and Devices for the Circulatory System - page 312

Newer biopolymeric scaffolds have been developed to avoid some of the
concerns pertaining to PLA and PGA. Certain groups have employed
HYAFF-11 TM , a commercially available biopolymer produced by the esteri-
fi cation of the glycosaminoglycan, hyaluronan, an ECM component
(Campoccia et al. , 1998). Esterifi cation induces a hydrophobic polymer
which degrades through hydrolysis without prompting an infl ammatory
response or fragmentation, instead becoming increasingly hydrophilic
forming a gel resembling ECM hyaluronan. In vitro , ECs have demon-
strated satisfactory adherence to HYAFF-11 TM , with proliferation and
manufacture of an organised ECM (Turner et al. , 2004). The HYAFF-11 TM
biopolymer was fashioned into conduits which were then grafted into
animal models (Zavan et al. , 2008). Grafts with 2 and 5 mm diameters were
examined. Once grafted into the host animal's circulatory system, the
HYAFF-11 TM conduits maintained their integrity. Whenever the conduit
was examined, using Duplex scan imaging, there was no evidence of aneu-
rysmal dilatation or collapse of the conduit. Grafts examined with the
Duplex scanner at 4 months demonstrated macroscopic resorption of the
scaffold with excellent blood fl ow at the site where the graft was implanted
(Zavan et al. , 2008). Following explantation and more detailed examina-
tion, the prosthesis was found to contain ECs, fi broblasts and VSMCs.
These cells reportedly increased in number in a time-dependent manner
and were arranged in the prosthesis in a way that closely resembled a natu-
rally occurring artery. Analysis of genes involved with ECM synthesis and
remodelling demonstrated increased expression of type I collagen in a time-
dependent manner. Diffuse layers of well-defi ned elastic layers were clearly
visible in the regenerated vessels. Indeed, tropoelastin expression decreased
by the fi fth month, which paralleled the process of elastic fi bre reorganisa-
tion within the conduits (Zavan et al. , 2008). These very exciting reports
may demonstrate a new and unique method of engineering arterial conduits
in animals but its effi cacy is yet to be proven in humans.
The manufacture of scaffold materials is not a precise science as charac-
teristics such as pore dimensions cannot be consistently reproduced
(Hutmacher et al. , 2004; Sachlos and Czernuszka, 2003). Hence the devel-
opment of the solid free-form technique in which scaffolds with a precise
architecture can be designed using a computer program, regardless of their
complexity. The scaffolds are then manufactured using 'bioprinting', which
avoids the use of moulds and is reproducible, and cell seeding may even be
integrated into the process (Hutmacher et al. , 2004; Salusbury, 2005).

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12.4.3 Natural biomaterials or biopolymers

Naturally occurring blood vessels possess an extensive and well-developed
ECM. Since most of the constituents of the ECM and the blood have been