Biomedical Engineering Reference
In-Depth Information
(Popat et al. 2007a). However, the difference between this nanoscaled topography and
dense titania films is not given. Park et al. have systematically investigated the influence of
the pore size of the nanotube on the cell response. Titania nanotube arrays with diameters
ranging from 15 to 100 nm are prepared by controlling the applied potential. The morphol-
ogy of the as-prepared titania nanotube arrays is exhibited in Figure 5.12. Cell adhesion
and spreading are highest on the 15-nm tubes and decline significantly for larger pore
size. Cell growth experiments conducted on the 30-nm and smaller diameter nanotubes
show extensive formation of paxillin-positive focal contact to which actin stress fibers are
anchored and the cells have a highly migratory morphology with long protrusions (Park
et al. 2009; Park et al. 2007). An enhanced cell migration behavior has been reported on
titania nanotube arrays by Brammer et al. (2008). Cell differentiation of mesenchymal stem
cells into osteogenic lineages has been observed and osteocalcin immunofluorescence is
the highest on the 15-nm tubes but severely impaired on the 100-nm nanotubes. The most
remarkable discovery is the strong induction of apoptosis on the 100-nm tubes (Park et
al. 2007). As the predicted surface occupancy size by the head of an integrin heterodimer
consisting of a β-propeller of the R-chain and the domain of β-chain is about 10 nm (Takagi
et al. 2002), it is believed that 15 to 20 nm spacing allows or force clustering of integrins into
the nearly closest spacing possible resulting in optimal integrin activation.
Bioactivity/Bone Conductivity
Bioactivity or named bone conductivity is referred to as the capability that the materials
can induce growth of bone like HA from body fluids. This performance can be evaluated
both in vivo and in vitro. Tsuchiya has reported HA growth behavior on anodic titania
nanotubes. Their study demonstrates that apatite formation on titania nanotubes depends
on the nanotube length and crystallographic phase of titania. The 500-nm-long nanotube
hardly induces precipitation of apatite, while a thick HA layer forms on the 2.5-μm-long
nanotube after soaking in SBF for 2 weeks. It has been reported that a crystalline structure
greatly affects the bioactivity of titania nanotubes. Anatase or anatase and rutile struc-
tures can induce more effective apatite growth on the nanotubes. It is interesting that the
15 nm
20 nm
30 nm
50 nm
70 nm
100 nm
200 nm
FIGURE 5.12
SEM views of a highly ordered titanitube with various pore sizes between 15 and 100 nm fabricated by tailoring
applied potentials ranging from 1 to 20 V. (From Park et al., Nano Lett. , 7, 1686-1691, 2007. With permission.)
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