Synthesis of Layered, Graded Bioscaffolds - Structural Interfaces and Attachments in Biology - page 345

Fig. 16.3 Left, middle : SEM shows the continuity of collagen fibers at the interface between the
CG ( blue line ) and CGCaP ( tan dashed line ) compartments. Scale bars: 500 m m( left ), 200 m m
( middle ). Right : energy dispersive X-ray spectroscopy (EDX) superimposed on SEM image of the
multi-compartment CG-CGCaP scaffold demonstrates the localization of Ca and P mineral content
in the CGCaP (bone) compartment. (Reproduced, with permission, from Harley et al. [ 47 ])

CG microstructure (pore size and morphology) is best quantified via a
stereological approach. Longitudinally and transversely oriented scaffold samples
are embedded in glycolmethacrylate, sectioned, stained, and observed using optical
microscopy. A linear intercept approach can resolve a best-fit ellipse representation
of the pore morphology. Dimensions of the ellipse (major and minor axes) allow
calculation of both an equivalent mean diameter and a mean pore aspect ratio
[ 14 , 19 , 37 , 53 ]. Micro-computed tomography (microCT) has also been used to
analyze pore microstructure [ 14 , 19 , 37 , 54 ], though it often requires significant use
of contrast agents to enable accurate visualization of the non-mineralized CG
content. For subsequent discussion of mechanics and permeability in this chapter,
a model multi-compartment scaffold variant (CG compartment mean pore size:
219 27

m; CGCaP compartment: 200 42 m; Interface region: 208 20

m m

m) will be described.
Cellular solids modeling approaches have proven to be useful tools in the
description and characterization of CG scaffold pore geometry and microstructural
properties (e.g., pore shape, specific surface area) [ 55 , 56 ]. CG scaffolds have been
primarily modeled as low density, open-cell foams using a tetrakaidecahedral
(14-sided polyhedron) unit cell. Application of modeling approaches using the
tetrakaidecahedral unit cell with CG scaffolds has led to estimations of a number
of key microstructural features of the CG scaffolds, notably scaffold-specific surface
area (surface area divided by the volume of the scaffold) [ 19 , 21 ], mechanical
behavior [ 57 ], strut geometry and deformation [ 14 , 15 ], and permeability (SA/V)
[ 58 ]. A key parameter used in these analyses is the relative density ( r */ r s ) of the
scaffold, which is also equivalent to (1 % porosity). Relative density ( r */ r s )is
defined as the ratio of the density of the scaffold ( r *) divided by the density of the
solid material the scaffold was fabricated from ( r s ).
Mechanical testing has been performed to determine both the standalone moduli
of individual scaffold compartments and the behavior of the multi-compartment
scaffolds. Results have demonstrated that these scaffolds possess nonlinear
stress-strain behavior with three distinct regions (linear, collapse plateau, and
densification) as is characteristic of low density open-cell foams (Fig. 16.4 )[ 47 ].

m