Biomineralization: Mechanisms of Hydroxyapatite Crystal Growth - Bioinspiration: From Nano to Micro Scales - page 121

Fig. 3.3 AFM images of HAP c -face growing in pseudo-body fluid. ( a ) Original seed surface.
( b ) Surface after 5 min of growth. Many small islands grew. ( c ) Surface after 2 h of growth.
Number and size of islands increased. Growth proceeded by multiple two-dimensional nucleation
(reproduced with permission from [ 69 ]) (copyright 1998, American Chemical Society)

3.4.6 Analysis of Growth Features and Growth Rate Data

of HAP c-face

The importance of surface observations in the investigation of growth mechanisms
was described above. While surface observation is clearly possible with phase-shift
interferometry, the resolution in the within-plane horizontal direction is limited to
that at the normal optical microscopic level although that in the height direction
is superior. For this reason, detailed surface observations of the HAP c -face using
AFM were performed in combination with the growth rate measurements using the
phase-shift interferometry. While AFM observations are not in situ, they are able to
give a clear picture of the growth mode of the c -face in pseudo-body fluid.
Results of such observations are shown in Fig. 3.3 ,[ 69 ]. The crystal was
grown for 5 min in pseudo body fluid followed by removal and observation with
AFM (Fig. 3.3 b). Innumerable small islands had formed on the surface. After
observation, the crystal was returned to the pseudo body fluid and grown for
an additional 2 h. The number and the size of the islands greatly increased, as
shown in Fig. 3.3 c. The growth of the c -face therefore proceeds in a multiple two-
dimensional nucleation mode. In this study, the lowest supersaturation in which
growth was confirmed by AFM was . However, growth was multiple

two-dimensional nucleation even in this supersaturation, and no spiral growth
was observed. The face growth rate via multiple two-dimensional nucleation is
expressed by the formula below, with h as the height of the formed two-dimensional
nucleus, v as the step velocity of the two-dimensional nucleus, and J as the
frequency of two-dimensional nucleation [ 70 ]:

D 0:85

v 2 J =3/ 1=3 R D h. (3.11)

v D $C e ˇ (3.12)

. ˛ 2 $h=k B T/ D B exp f ˛ 2 $h=.k B T/ 2 ln J D B exp .1 C / g : (3.13)