Biomedical Engineering Reference
In-Depth Information
The main characteristics of the route by which the mineralized hard tissues are formed are that
the organic matrix is laid down fi rst, and the inorganic reinforcing phase grows within this organic
matrix. Oyster shells, coral, ivory, pearls, sea urchin spines, and cuttlefi sh bone are just a few of the
vast variety of biomineralized materials engineered by living creatures. Many of these biological
structural materials consist of inorganic minerals combined with organic polymers. The study of
these structures has generated a growing awareness that the adaptation of biological processes may
lead to signifi cant advances in the controlled fabrication of superior smart materials. However, to date,
neither the elegance of the biomineral assembly mechanisms nor the intricate composite microarchi-
tectures have been duplicated by nonbiological methods. But tissue engineers are trying very hard to
implement the above-described principles of natural tissues to design and fabricate scaffolds.
Zhou et al.
20
studied so-called second-generation scaffolds made of medical grade polycapro-
lactone and 20% calcium phosphate
in vitro
and
in vivo
. Composite scaffolds were fabricated via
FDM and were studied
in vivo
in conjunction with bone marrow stromal cell (BMSC) sheets for the
engineering of structural and functional bone grafts. The constructs were fabricated and cultured
in vitro
before undergoing an 84-day
in vivo
trial using nude rats as shown in Figure 2.7.
The second-generation FDM scaffolds for bone engineering are now made of polymers and
ceramics. Hutmacher's group at the University of Singapore has designed and built an RP machine
as shown in Figure 2.8. The group has undertaken several studies (both
in vitro
and
in vivo
) utilizing
FDM to produce scaffolds comprising medical grade polycaprolactone and 20% calcium phosphate,
combined with collagen type I, which have been studied for up to 13 months in rat calvarial model.
These scaffolds demonstrated superior bone regeneration compared with control (blank) defects as
seen in Figure 2.9. Larger composite scaffolds have also been produced and placed as a bone graft
in a high-load-bearing application in a pig spinal fusion model as seen in Figure 2.10. Collectively
these methods demonstrate the versatility of FDM techniques in producing tailor-made scaffolds of
different shapes, sizes, and compositions for specifi c anatomical applications.
A traditional FDM machine consists of a head-heated-liquefi er attached to a carriage moving
in the horizontal
x
-
y
plane. The function of the liquefi er is to heat and pump the fi lament material
through a nozzle to fabricate the scaffold following a programmed path, which is based on CAD
model and the slice parameters. Once a layer is built, the platform moves down one step in the
z
-direction to deposit the next layer. Parts are made layer-by-layer with the layer thickness varying
in proportion to the nozzle diameter chosen. FDM is restricted to the use of thermoplastic materi-
als with good melt viscosity properties; cells or other theromosensitive biological agents cannot be
encapsulated into the scaffold matrix during the fabrication process.
A variation of FDM process, the so-called precision extruding deposition (PED) system, was
developed at Drexel University and tested.
31
The major difference between PED and conventional
FDM is that the scaffolding material can be directly deposited without fi lament preparation. Pellet-
formed PCL is fused by a liquefi er temperature provided by two heating bands and respective ther-
mal couples and is then extruded by the pressure created by a turning precision screw.
The pore openings facing the
z
-direction are formed in between the intercrossing of material
struts/bars and are determined by user-defi ned parameter settings. However, the pore openings fac-
ing both the
x
- and
y
-directions are formed from voids created by the stacking of material layers,
and hence, their sizes are restricted to the bar/strut thickness (diameter). As such, systems with a
single extrusion head/liquefi er do not allow variation in pore morphology in all three axes. A design
method exists by extruding one strut/bar directly on top of each other to add design variability in
the
z
-axis.
15
Ya ng e t a fi
32
have prepared fi ne ceramic lattices (hydroxyapatite) using extrusion free forming
utilizing a volatile solvent. The ceramic powder was compounded with binder and solvent, and after
extrusion, the paste was solidifi ed by evaporation, making it possible to form regular quasicrystal
lattices; the fi lament spacing can be varied and the overall structure can be controlled by using a
support structure, and all the organic contents can be removed on sintering at high temperatures,
resulting in a pure ceramic lattice. Moroni et al.
33
have developed a technology to fabricate hollow
