Biomedical Engineering Reference
In-Depth Information
groups of the substrate. Two remaining questions
occur: Are ALD-deposited coatings biocompati-
ble? Are those coatings mechanically and chemi-
cally stable? Stability can be obtained by the
proper choice of thickness and material. Thus,
the biocompatibility of ALD coatings seems to be
the most important question for biological and
medical applications of ALD, and it is astonish-
ing that to date investigations have been sparse.
Most of the published work concentrates either
on the biocompatibility of materials or on the
biocompatibility of the coated structures.
The most common material for ALD deposi-
tion is Al
2
O
3
. Finch
et al
.
[84]
investigated the
proliferation of coronary artery smooth muscle
cells on an Al
2
O
3
-coated glass with 60-nm-
coating thickness and compared it to the prolif-
eration on uncoated glass and a silane-terminated
surface. The Al
2
O
3
surface is terminated by
hydroxyl groups and shows hydrophilic behavior
similar to the glass surface. Consequently, the cell
proliferation on the Al
2
O
3
surface and the glass
surface were similar, whereas the hydrophobic
control surface (i.e., the silane-terminated glass)
showed opposite behavior. This rather simple
test indicated that Al
2
O
3
films deposited by ALD
are not less biocompatible than ordinary glass
surfaces.
Putkonen
et al
. took a more direct approach
to ALD-deposited biocompatible coatings
[85]
.
The idea behind their work was to develop an
ALD process for hydroxyapatite (HA). Being a
ternary compound with many variation possi-
bilities in stoichiometry, HA poses serious prob-
lems for ALD processing, since alternating
pulses of two or three precursors cannot be
applied to synthesize this material.
The approach of Putkonen
et al
. is more com-
plex in comparison to a normal ALD process
and is chemically very sophisticated. Their four-
precursor ALD process consists of the precursor
pairs Ca(thd)
2
/O
3
and (CH
3
O)
3
PO/H
2
O and is
performed at 300 °C. The first precursor pair
produced a film of calcium carbonate. In the
second stage, the carbonate groups were
exchanged with phosphate groups by applying
the second precursor pair. The carbon content
after this process was still high, and thermal
annealing in dry or moist N
2
was necessary to
further reduce it. Eventually, HA films were
obtained once the annealing temperature was
above 500°C. The films were tested for biocom-
patibility by attaching preosteoblast MC 3T3-E1
cells (
Figure 16.8
). Good biocompatibility was
observed, especially with the annealed films;
this was derived from the fact that the cells
FIGURE 16.8
MC 3T3-E1 cells were grown for three hours on (a) as-deposited and (b) annealed hydroxyapatite films
formed on Si(100) by ALD. On the annealed, crystallized film, the cells show clear lamellipodia and filopodia structures.
Cytoskeletal actin was stained by Alexa 488-labeled phalloidin (green) and vinculin with a monoclonal antibody followed
by Alexa 546-labeled anti-mouse antibody (red). Reprinted from Ref.
85
. Copyright © 2009, with permission from Elsevier.
(For interpretation of the references to color in this igure legend, the reader is referred to the web version of this topic.)
