Biomedical Engineering Reference
In-Depth Information
have been predicted and compared with the experimental values (Corno et al., 2007). More
recently, these modeled surfaces were also employed to study the adsorption of different
typologies of molecules that span from water (Corno et al., 2009) to organic acids (Canepa et
al., 2011a), amino-acids (Rimola et al., 2008) and small peptides (Corno et al., 2010; Rimola et
al., 2009), providing results comparable with the experimental heats of adsorption, when
available.
Fig. 1. Two views of the unit cell of the HA bulk structure: calcium in light green, oxygen
red, hydrogen light grey and phosphate ions as orange tetrahedra. In the blue cylinders, the
column of OH
-
ions can be observed. Unit cell borders are drawn in light orange.
Nonetheless, the study of the possible terminations along the [010] direction is not yet
complete, because two new kinds of (010) surfaces have been discovered experimentally.
These surfaces are non-stoichiometric, because their composition is different from the bulk.
2.1.1 The (010) non-stoichiometric surfaces
In 2002, a HRTEM study highlighted three possible terminations for the (010) surface. As a
matter of fact, HA shows an alternation of two layers, Ca
3
(PO
4
)
2
(A-type) and
Ca
4
(PO
4
)
2
(OH)
2
(B-type), which can be interrupted in three different ways (Sato et al., 2002).
As the sequence is …-A-A-B-A-A-B-A-A-B-…, the periodicity can be truncated by exposing
as last layers …-A-B-A or …-A-A-B or …-B-A-A, as shown in Fig. 2. As already done in
previous work with other surfaces, we investigated these three possible structures with a
slab approach. From the bulk, we extracted a piece of matter along the desired direction, in
this case the [010], generating two faces which are exposed to vacuum. These faces ought to
be the same to remove any possible dipole moment across the slab, due to geometrical
dissimilarities. This necessity may bring the loss of stoichiometricity, in terms of the
possibility of obtaining the bulk by replicating the slab along the non-periodic direction:
neither B-A-A-B-A-A-B nor A-A-B-A-A-B-A-A can be replicated to regenerate the bulk. Only
the stoichiometric surface, characterized by a Ca/P ratio of 1.67, maintains the bulk
sequence A-B-A-A-B-A. The slab structure terminating in -A-A-B is called
Ca-rich
(010)
