Biomedical Engineering Reference
In-Depth Information
structure and shape during the fabrication process. Once a polymer has been
heated and deposited, it remains soft until it cools and the generation of
complex overhanging shapes can lead to sagging and ultimate deformation
of the final structure. This is where a support material is crucial. The support
material is often water soluble facilitating easy removal. Driven by rollers,
the polymer filaments are fed through the extrusion head and into the li-
quefier. The liquefier consists of heating elements which can be accurately
controlled. This temperature is very important as it affects polymer flow
characteristics; specific layer parameters are tailored to certain flow char-
acteristics. As the filament passes through the liquefier, it is melted and
extruded through a nozzle onto the collecting platform (Figure 9.1a).
Another variation of AM process, known as ME, uses a melting chamber
(rather than spools) (Figure 9.1b) allows the use of different and exotic
polymers that are unavailable in spools.
12
Medical grade polymers ordered
in small custom batches can be fabricated into scaffolds using this method.
The temperature of melting and extrusion chambers can be accurately
controlled. Air pressure is applied to the inlet of the melting chamber to
push molten polymer against the inlet of the extrusion chamber. This
positive pressure gradient then allows a rotating screw within the extrusion
chamber to collect molten polymer and exert a constant pressure along the
length of the screw. Once the nozzle tip is reached the polymer is deposited
onto the collecting platform. This rotating screw facilitates the mixing of
molten polymer thereby producing a more homogeneous melt. Additive
manufacturing processes involve complex interactions between the hard-
ware, software and material properties. When generating 3D models with
ME, processing parameters must be carefully optimised to achieve the de-
sired result. The processing parameters of ME and FDM along with the re-
spective effects are summarised in Table 9.2.
13,14
The different mechanical
set-ups between FDM (spool) and ME (chamber) machines lead to different
terminology in some cases as shown in Table 9.2.
When fabricating a scaffold for bone tissue engineering purposes, it is of
the utmost importance to optimise the parameters listed in Table 9.2. to
generate a scaffold with favourable porosity and mechanical strength.
Changes to these processing parameters can affect the scaffold structure by
influencing the characteristics below (Figure 9.1c):
d
n
3
r
4
n
g
|
1
.
Road width:
the diameter of
the circular cross section of
laid
microfilament
Fill gap:
the edge-to-edge horizontal distance between adjacent
filaments
Fill diameter: the centre-to-centre horizontal distance between two
consecutive filaments in the same layer
Slice thickness: the vertical distance between the filament centre of ad-
jacent layers
Layer gap: the edge-to-edge vertical distance between layers of the same
microfilament alignment
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