Biomedical Engineering Reference
In-Depth Information
TiN phase (the harder phase in the structure).
This technique yields multilayered structures
with sharp interfaces. However, it is a very
slow process, as discussed in Chapter 15 by
MartÃn-Palma and Lakhtakia, and the load-
transfer mechanics are only based on molecular
interaction at interfaces. The restricted choice of
materials is another disadvantage.
Another novel technique that can be catego-
rized as either an LBL technique or as a deposi-
tion technique is sequential inkjet deposition of
different materials on a substrate. This promising
technique demonstrates nanometer control over
the thickness of constituents and well-controlled
mineral concentration. It has two advantages
over traditional LBL techniques: (i) there is no
need to go through several intermediate rinsing
steps, as required for LBL techniques, and
(ii) direct patterning on the substrate is possible,
thereby eliminating the need for subsequent
etching steps. Andres
et al
.
[67]
recently utilized
this technique to develop stable multilayered
nanocomposite films. A combination of this tech-
nique and the traditional LBL technique, has also
been used for several purposes, including selec-
tively activating a multilayered surface for metal
plating or modifying the surface to enhance
adhesion
[69, 70]
.
LBL technique
[48]
. However, the sedimen-
tation technique is much faster than LBL and
could lead to high-throughput nanocomposite
fabrication.
Gel casting can also be used to fabricate pol-
ymer-reinforced platelets. As the first step, the
platelets are well dispersed in a solution that
turns to a gel upon cooling, so that the particles
are locked in their place. The solvent is then
vaporized to make the structure denser. This
polymer-reinforced platelet is then hot-pressed
to further align the platelets. Bonderer
et al
.
[74]
showed that, for platelet concentration up to 40
% v/v, the mechanical behavior of these com-
posites can be improved by increasing the plate-
let content but degrades for higher concentrations.
This composite has strength and stiffness values
of, respectively, 83% and 13 times higher than
the pure polymer matrix.
3.4.5 Template-Assisted Fabrication
Template-assisted fabrication methods are
defined here as the methods in which surface-
modified particles are assembled into a mechan-
ically or chemically modified template. This
template controls the attraction, growth, and
morphology of the resulting structures
[75]
. For
example, use of cavities with the same size as
the colloidal particles would direct the particles
to self-assemble into the cavities in a lock-and-
key manner
[76, 77]
. In many cases, a two-
dimensional patterned template is used to
control the growth and morphology of colloidal
crystals
[78, 79]
. One-dimensional linear tem-
plates like DNA or single-walled carbon nano-
tubes (SWNTs) are also used to direct the
organization of nanoparticles
[80, 81]
or poly-
mers
[82, 83]
.
Template-assisted self-assembly has found
wide applications in bone mimicry and recon-
struction
[84, 85]
. Zhao
et al
. reported the use of
chemically functionalized SWNTs as templates
to grow bone-like materials
[85]
. Hydroxyapatite
3.4.4 Centrifugation, Sedimentation,
Shearing, and Gel Casting
These processes usually start with coating nano-
platelets with polymers. The nanoplatelets can
then be assembled using a variety of techniques,
including centrifugation
[71]
, sedimentation
[49, 72]
, and shearing
[73]
. These techniques are
relatively faster than other bottom-up assembly
techniques such as LBL assembly. Using a sim-
ple sedimentation technique, Walther
et al
.
[49]
recently fabricated a PVA/nanoclay composite
with a tensile strength of 170 MPa and ultimate
tensile strain of 0.014. The results are consis-
tent with the results obtained earlier using the
