Biomedical Engineering Reference
In-Depth Information
stress), high strain to failure (>20%), and high strain rate sensitivity. The con-
structs with a porosity of 52% had compressive strengths of 12 MPa and 5
MPa in the directions parallel and perpendicular to the freezing direction,
respectively. The favorable mechanical behavior of the porous constructs,
coupled with the ability to modify their microstructure, indicates the poten-
tial of the present freeze-casting route for the production of porous scaffolds
for bone tissue engineering (Fu et al. 2008b,c). Using the freeze-casting and
drying technique, the biohybrid HAp/gelatine composites could also be pre-
pared by infiltrating HAp lamellar scaffolds (45-55 vol.% of porosity) with
a 10 wt% solution of gelatine. The HAp/gelatin porous lamellar scaffold
showed appropriate compressive mechanical properties improved with the
addition of gelatine: the strength increased up to 5 to 6 times, while the elas-
tic modulus and strain approximately doubled (Landi, Valentini, et al. 2008).
Freeze-casting has quickly gained popularity as a manufacturing route for
bioceramics because it is a comparatively straightforward physical process,
based on biocompatible liquid carriers such as water. Next, the structure of
freeze-cast materials, such as their overall porosity, pore size, and pore geom-
etry, can easily be controlled across several length scales. Of equal impor-
tance is the ability to custom design both during and after freeze-casting the
scaffold's cell-wall properties, such as surface roughness and chemistry, and
with it the interface properties that are of critical importance to tissue-mate-
rial interactions and a scaffold's successful tissue integration. These struc-
tural features, combined with the remarkable mechanical properties that
directionally solidified materials offer despite their high overall porosity, are
the basis of the great promise of freeze-cast biomaterials. Moreover, freeze-
casting is ideally suited for the manufacture of bioceramics with property
gradients. Because the porosity generated by freeze-casting is highly con-
nected, it is possible to create hierarchical microstructures by sequential
freeze-casting, thereby exploring the potential to introduce another level
and another direction of porosity into the material. The high connectivity
also makes it possible to coat the sample once or several times and to infil-
trate it with another phase. Both offer great potential to integrate into the
biomaterial growth factors to stimulate tissue ingrowth (Wegst et al. 2010).
Recently, wood-derived ceramic structures were obtained in a variety
of compositions, through a sequence of chemical and thermal reactions.
Pyrolysis is the starting process aimed at eliminating all the organic frac-
tion of wood, leaving a porous skeleton in carbon, whose structure well
reproduces the cellular organization of the native wood. Rattan wood in
particular has a strong morphology similar to bone (Tampieri et al. 2001);
it is characterized by a total porosity of 85% and large pores with diameter
250±40 µm (Tampieri et al. 2009), evidencing a system of channel-like pores
(simulating the Haversian system in bone) interconnected with a network of
smaller channels (such as the Volkmann system). The criteria for the selec-
tion of rattan wood for bone scaffold development were based on the speci-
fications of spongy bone.
Search WWH ::
Custom Search