Polycaprolactone-based Scaffolds Fabricated Using Fused Deposition Modelling or Melt Extrusion Techniques for Bone Tissue Engineering - Biointerfaces: Where Material Meets Biology - page 229

relationship between scaffold strength and porosity was reported by
Hoque et al. 15 Increased deposition speeds (240-360 mm min 1 ) resulted
in reduced RW and consequently higher porosity (45 1.80 to 75 3.0%)
for PCL scaffolds. 15 This lead to a proportional decrease in yield strength
(4.58 0.18 to 2.12 0.08 MPa). 15
It should be noted that Zein et al., 13 carried out mechanical testing fol-
lowing the ASTM F451 standard for testing acrylic bone cement. This
standard has no requirement for mimicking in vivo conditions such as
moisture during testing so it could be argued that the mechanical strength
values obtained are potentially not physiologically relevant. 16 It has in fact
been shown that similarly fabricated PCL scaffolds behave very differently
when mechanically tested in saline at 37 1C. 17 PCL scaffolds, fabricated with
two different raster angles (0/60/1201 and 0/72/144/36/1081), each with 61%
porosity, were all shown to be weaker when tested in saline at 37 1C for both
stiffness and yield strength. 17 Stiffness dropped from 41.9 3.5 to 29.4
4.0 MPa and yield strength dropped from 3.1 0.1 to 2.3 0.2 MPa. 17
Another factor that few consider is the sterilisation method used prior to cell
culture/implantation. There is some evidence to suggest that ethanol ster-
ilisation has a dramatic effect on the mechanical properties of long-chain
polyesters. 18,19 If PCL constructs are to be applied to load-bearing situations
based solely on mechanical testing data then it is very important that this
data applies to the actual in vivo situation and it is recommended to always
test samples as close to the physiologically relevant scenario as possible.
By nature, PCL will never have the same mechanical properties as bone
but it is important to consider this during the initial fabrication/character-
isation stages. The exact requirements of scaffolds for bone tissue engi-
neering remain largely undefined; there are no target design tolerances for
these materials but for load bearing applications the scaffold must behave as
predicted in vivo. It appears that mechanical testing under physiological
conditions is a necessity for analysis of additive manufactured PCL based
scaffolds. PCL based scaffolds for load bearing applications are often used
with some type of internal or external fixator. 20,21 Although these scaffolds
are designed to provide structural support, they are not load-bearing.

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9.3.2 Polycaprolactone Scaffold In Vitro Analysis
We know that the raster pattern affects mechanical properties, but how do
these differences on the macro-scale influence cells? Hoque et al. 15 dem-
onstrated that different raster patterns (0/301, 0/601 and 0/901) had no sig-
nificant influence on the attachment or proliferation of smooth muscle cells.
This is unsurprising considering the macro-scale of the scaffold struts, al-
though it should be noted that a traditional static seeding method was used.
The differences in attachment between the scaffolds may have varied in vivo
or in a perfusion culture due to different flow characteristics imposed by the
different scaffold architectures. This type of analysis was beyond the scope of
this research paper and is an area of current scientific interest.