Biomedical Engineering Reference
In-Depth Information
internal space within a degradable scaffold will increase with time, allowing increased space for
tissue to develop or remodel.
2.2.2 M
ORPHOLOGY
/A
RCHITECTURE
Gibson and Ashby
18
classifi ed porous solids into two general groups: foams and honeycombs. A
honeycomb consisted of a regular two-dimensional array of polygonal pores each defi ned by a wall
shared between adjacent pores (Figure 2.4). The pores were packed in planar arrays like the hexago-
nal cells of a honeycomb, as seen in Figure 2.4C.
The ASTM terminology
19
for porous materials is similar and classifi ed into three groups: inter-
connecting (open pores), nonconnecting (closed pores), and a combination of both. When the pores
are open, the foam material is usually drawn into struts forming the pore edges. A network of struts
produces a low-density solid with pores connecting to each other through open faces. When the
pores are closed, a network of interconnected plates produces a higher density solid. The closed
pores are sealed off from adjacent neighbors.
Gibson and Ashby
18
describe that the mechanical properties of a porous solid depend mainly
on its relative density, the properties of the material that make up the pore edges or walls, and
the anisotropic nature, if any, of the solid. In general, the stiffness (
E
*) and yield strength (
σ
*), in
compression, of porous solids were each related to the relative density by a power-law relationship.
Given that most constructs require a high degree of porosity to accommodate mass transfer and tis-
sue development, the volume fraction of the scaffold will necessarily be low. In all, especially in the
most biomechanically challenging applications, it is likely that the test for the scaffold engineer is to
A
B
10 mm
C
FIGURE 2.4
Microscopic and macroscopic views of NaOH-degraded PCL scaffolds from 0 to 5 weeks. Top
inset (A) shows SEM micrographs (
×
2000 magnifi cations) of the surface texture of the scaffold fi laments over
time; 0-week, 1-week and 5-week (
left to right
). Middle inset (B) shows the macroscopic view of the degraded
scaffolds. Bottom inset (C) shows SEM micrographs (
×
64) from 0 to 5 weeks. The scaffolds were observed
to degrade via a surface erosion pathway homogenously throughout the scaffold structure, through the thin-
ning of the fi lament diameters. (From Lam, C.X.F., Teoh, S.H., Hutmacher, D.W. Polymer International, 56,
718-728, 2007. With permission from Wiley and Elsevier.)
