biomedical applications such as gene therapies, biomedical implants, controlled drug

delivery, cancer therapies, tissue engineering, etc. is yet underutilized because of

several drawbacks of HA. HA needs to be tailored for a specific application, and

each novel structure of HA has to be treated as a new biomaterial before its

application. The biological response of HA depends on its (a) crystallite size,

shape, and crystallinity, (b) biocompatibility, (c) bioactivity, (d) relatively simple

synthesis protocols, (e) functionalization, and (f) capacity to load therapeutic agents

(Supova 2015).

A better HA product can be obtained by transition to nanodimensional material

because such powders are characterized by a homogeneous structure, small crystal-

lite size, and enhanced performance (Chen et al. 2002). The modification of

hydroxyapatite Ca10(PO4)6(OH)2 through its composition via cationic, anionic, or

their simultaneous substitution can significantly affect its properties due to the

formation of nanodimensional particles in the basic structure, which is of great

interest for medical applications as a component of artificial bones and implants.

The ionic substitution of HA is also essential for promising rate of bone tissue

regeneration and physicochemical parameters close to those of natural bone. Each

substitute ion can affect the features of the lattice, thus impacting its crystal size,

crystallinity degree, stability, and morphology, all promoting its bioactivity and

solubility. The most reported substitute ions for hydroxyapatite for biomedical

applications are Mg²⁺, Mn²⁺, Sr²⁺, and Zn²⁺ for calcium ions and CO3

2
and

SiO4

4
for phosphate ions and F
for hydroxide ions (Norhidayu et al. 2008).

In this chapter, the influence of ionic substitution in HA is examined. The

nanodimensional

hydroxyapatite

powders

substituted

with

various

ions

Ca10
xAx(PO4)6
yBy(OH)2
zCz (where x, y, and z indicate substitution for Ca²⁺,

PO4

3
, and OH
) were precipitated by wet chemical method from solutions. Powder

characteristics like particle size distribution, morphology, phase composition, spe-

cific surface area, etc. have been studied. It was observed that the partial substitution

of calcium ions, phosphate ions, or hydroxide ions or any two of the three leads to a

reduction in the particle size to nanoscale. Furthermore, the amount of substitution

also affects the crystallite size, shape, and crystallinity, biomineralization, bioactiv-

ity, etc. of synthesized powders.

Several methods can be used for the synthesis of hydroxyapatite, but most of

these do not result in good quality hydroxyapatite having high crystallinity, accept-

able biocompatibility, nanoscale particles as compared with natural bone tissue and

enhanced resorption rate, necessary for application in implants, bone reconstruction,

and other applications. Existing synthesis methods lead to the formation of second-

ary phases like α-, β-, and γ-tricalcium phosphate Ca3(PO4)2, affecting biological

properties. The degree of crystallinity required in HA structure can be attained by

heat treatment at temperatures between 400 ^C and 1300 ^C, but it leads to sintering

of powders, thereby increasing the particle size from nanodimension to microscale.

This deteriorates associated characteristics of nanodimensional level initially

obtained during HA synthesis.

It is reported that high crystallinity of nanodimensional particles positively

impacts the growth and development of bone cells. An assessment of particle size

420

S. Kapoor et al.