Biomedical Engineering Reference
In-Depth Information
This decrease in the strength of porous titanium can be prevented by combining with a
biocompatible polymer. Penetration of the polymer into the porous titanium can be
achieved by pressing. In the pressing method, HMDP (high molecular density polyethylene)
is pressed into porous titanium.
Another proposed method for penetration of a polymer into the titanium pores (Nakai, 2010)
involves firstly using the monomer of PMMA. The porous titanium (pTi) is first immersed
into a monomer solution of PMMA leading to penetration of the monomer into the pores of
titanium. The PMMA monomer in the porous titanium is then subjected to polymerization by
heating. By combination with PMMA, the strength of porous titanium increases as shown in
Fig. 5 (Nakai, 2010). The tensile strength of PMMA infiltrated porous titanium is greater than
that of porous titanium, whereas the Young's modulus of PMMA infiltrated porous titanium
is nearly equal to that of porous titanium as shown in Fig. 6 (Nakai, 2010). The tensile
strength of PMMA infiltrated porous titanium increases by silane coupling treatment while
the Young's modulus remains unchanged as shown in Figs. 5 and 6.
400
pTi
pTi/PMMA
Si-treated pTi/PMMA
350
300
250
200
150
PMMA
PMMA
= 50 - 80 MPa
100
50
0
pTi45-22
pTi45-35
pTi150-27 pTi150-38 pTi150-45
pTi250-45 pTi250-50
Fig. 5. Tensile strengths of pTi, pTi/PMMA, and Si-treated pTi/PMMA.
Another advantage offered by porous titanium and polymer composites is the ease with
which bio-functionalities may be added given that the surface of porous titanium can be
covered with polymers. Instead of PMMA, biodegradable PLLA can also be filtrated into the
pores of porous titanium by modifying the process for PMMA filtration. Fig. 7 (Nakai,
2011b) shows the compressive 0.2% proof stress of porous titanium and PLLA infiltrated
porous titanium as a function of porosity in the range of 5%-45%. In this figure, the
compressive 0.2% proof stress of PLLA obtained experimentally is also shown for
comparison. The compressive 0.2% proof stress of PLLA infiltrated porous titanium is
higher than those of porous titanium independent to porosity. This result indicates that the
PLLA filling can improve the compressive 0.2% proof stress of porous titanium at any
degree of porosity. In particular, the increase in compressive 0.2% proof stress due to PLLA
filling is relatively large for porosities higher than or equal to 35%. The compressive 0.2%
proof stress of PLLA obtained is around 80-120 MPa, which is higher than that of PMMA
(around 50-80MPa) (Honda, 1961; Imai and Brown, 1976).
