MATERIALS FOR ORTHOPEDIC APPLICATIONS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 64

Ning et al. 37 showed that the biocomposite, fabricated from HAp and Ti pow-
ders by powder metallurgy technique, has the ability to induce apatite nucleation
and growth on its surface when immersed in SBF solution. Among all the crystal
phases in the as-fabricated composite, Ti 2 O has the ability to induce apatite for-
mation. Furthermore, the dissolved CaO phase also provides favorable condi-
tions for the apatite nucleation and growth.
In a recent study on HAp-Ti system (sinter at 1200 °C), the composite with
a lower initial Ti content contains major crystalline phases of CaTiO 3 , CaO and
Ti x P y . On increasing the initial Ti content to 50 vol%, Ti 2 O and residual

- Ti
were also observed. Further increase in initial Ti content resulted in a compos-
ite with only

α

α - Ti as main crystalline phase. TCP - TiO 2 biocomposites 38 , with

a controlled α : β -TCP ratio, can be obtained by varying the heat-treatment

conditions.
In order to make a comprehensive study on processing-microstructure-
physical and biological property correlationship, HAp-Ti composites are sintered
at different temperature (1000-1400 °C) in different atmospheres like argon,
hydrogen and air 39 . The sintering reactions of air sintered samples were analyzed
by thermodynamic consideration, which successfully describe the oxidation of
Ti (TiO 2 formation), followed by dissociation of HAp and the formation of
CaTiO 3 . The experimental results clearly reveal that sintering of various HAp-Ti
compositions leads to TCP/HAp-TiO 2 microstructure with little traces of CaTiO 3
and CaO in some of the sintered samples. The microstructural analysis indicated
abnormal grain growth of monolithic HAp, when sintered at a temperature
higher than 1200 ° C.
For various HAp-Ti compositions, the characteristic presence of a larger
TiO 2 lump, surrounded by coarser TCP phase, is also recorded. In case of sam-
ples, sintered in argon atmosphere, HAp-5 wt% Ti exhibits better results in terms
of densifi cation and mechanical properties. The hardness values were in the range
of 4-6 GPa, which is better than air sintered samples (2-4 GPa). Also, in argon
atmosphere, retention of Ti is possible along with undissociated HAp phase (up
to 10 wt% Ti addition to HAp). In contrast, sintering in H 2 atmosphere could not
prevent the oxidation of Ti and HAp dissociation. For example, HAp-40 wt% Ti
sample, sintered at 1400 °C in H 2 atmosphere, contain only single phase CaTiO 3 .
Even the densifi cation process was hindered by high temperature chemical reac-
tions and the formation of gaseous reaction products.
Cell culture experiments on air sintered samples using L929 fi broblast cells
provide clear evidences of cell adhesion and cell proliferation on HAp-Ti com-
posites. The general observation has been that the cells are attached closely with
neighboring cells, and thereby, form a cellular network (characteristic of fi bro-
blast type cells) on the composite surface (Figure 3.7). The observation of cellular
bridges i.e., cell-cell interaction as well as fl attening of the cells indicate good cy-
tocompatibility property of the developed composite (see Figure 3.7). Important-
ly, the addition of bioinert Ti (up to 30 wt%) does not show any degradation in
cell adhesion properties.