Constitutive Effects of Hydrolytic Degradation in Electro-Spun Polyester-Urethane Scaffolds for Soft Tissue Regeneration - Tissue Engineering: Computer Modeling, Biofabrication and Cell Behavior - page 50

thesis. Implants need to be designed such that their behaviour matches, in the ideal
case both biologically and mechanically, that of the organ or tissue to be replaced
in its healthy state [ 1 ]. In the treatment of cardiovascular diseases, examples of re-
generative medicine include tissue regenerative small diameter vascular grafts. In
such grafts, porosity allowing the ingrowth of cells and tissue is a key factor for the
long-term success [ 3 ]. Porous scaffolds have been manufactured in different ways
including phase inversion and porogen extraction [ 4 , 5 ] salt leaching [ 6 ], gas foam-
ing [ 7 ], extrusion-phase-inversion [ 8 ], thermally induced phase separation [ 9 ] and
electro-spinning [ 10 - 12 ].
While imperative for healing and tissue regeneration, porosity may adversely
affect the structural properties of the vascular scaffold, in particular when viscoelas-
tic polymeric materials are used. This increases the complexity of the structural
design of tissue regenerative vascular scaffolds which should, ideally, mimic non-
linear elastic mechanics of the vascular soft tissue [ 13 , 14 ]. The desired structural
properties may not be attainable with prostheses consisting of a single material but
demand a more complex structure comprising multiple components and/or mate-
rials. Due to the intricacy of the structural design process, computational methods
have been employed for the development and optimisation of cardiovascular im-
plants [ 15 - 17 ]. The numerical prediction of structural properties of grafts requires
the knowledge of the mechanical properties of the scaffold materials. Based on ex-
perimentally determined material properties, constitutive models can be used, or
developed if required, for a realistic computational representation of the material's
mechanics. Constitutive models of porous structures, such as foams, can be based at
microscopic and macroscopic scale, respectively. At microscopic level, a constitu-
tive model utilises the bulk properties of the porous material combined with repre-
sentation of the microscopic geometrical features. This approach requires typically
extensive computational resources and may not be feasible for large ranges between
microscopic and macroscopic dimensions of the physical problem. Alternatively, a
constitutive model may utilise the mechanical properties of the porous material at
macroscopic level, e.g. of a foam. In this 'smearing' approach, the porous struc-
ture is numerically treated as a homogeneous material without the need to represent
microscopic geometrical features [ 16 , 18 ].
The optimal design of tissue regenerative vascular prostheses, thus, needs to
consider the mechanical properties of the initial scaffold as well as effects of bio-
degradation, and healing, on the structural mechanics of the implant. Consequently,
detailed knowledge is required of the mechanical effects of scaffold degradation.
The mechanical characterisation of biodegradable polymeric materials used for
tissue regenerative medical implants such as vascular grafts, has received attention
from various research groups. Lendlein et al. [ 19 , 20 ] studied the mechanical bulk
properties of a degradable polyester-urethane (DegraPol ® ) prior to degradation and
during degradation, whereas electro-spun polyester-urethane membranes were me-
chanically characterised by Riboldi et al. [ 12 ]. Kwon et al. [ 21 ] determined struc-
tural and mechanical properties of electro-spun biodegradable co-polyesters. Me-
chanical properties prior to degradation have also been reported for electro-spun
scaffolds using poly( ε -caprolactone) [ 22 ], poly( ε -caprolactone)/collagen [ 23 ] and
poly( ε -caprolactone)/poly-lactic acid [ 24 ].