Biomedical Engineering Reference
In-Depth Information
This is followed by the addition of NH
4
OH to start the crystallization reac-
tion. After 24 h, the final product is sintered at 1100°C for 1 h to obtain nHA.
18
Besides wet chemical precipitation, a second method to create nHA is by
the sol-gel process. This technique is also based on a precipitation reaction,
but the densification of the HA is achieved at lower temperatures. These tem-
peratures are dependent on the calcium and phosphorus precursor solutions
that are used. Pure HA phases can be formed by mixing calcium diethox-
ide (Ca(OEt)
2
) and triethyl phosphate (PO(OEt)
3
) solutions at temperatures
above 600°C, with aging times longer than 24 h.
19, 20
Also, solutions of calcium
acetate and triethyl phosphate can be blended at 775°C, creating a mixture
of HA and CaO. Subsequent purification of the HA is performed by leach-
ing the mixture with hydrochloric acid.
21
A final and more recent sol-gel
technique to produce HA is a two-step procedure. In this procedure, tri-
ethyl phosphate is hydrolyzed with water for 24 h, followed by the addition
of an aqueous nitrate solution. The resulting gel is transformed into a low-
crystallized nanoscale apatitic structure at temperatures between 300°C and
400°C.
22
4.1.2 Nanohydroxyapatite Composite Materials
To obtain optimal composite materials for bone tissue engineering pur-
poses, several natural and synthetic-derived materials have been mixed with
nHA. For instance, a porous scaffold of gelatin-starch enriched with nHA
was fabricated through a microwave vacuum-drying process, followed by
cross-linking with trisodium citrate. Addition of nHA increased the biocom-
patibility and mechanical properties of the material.
23
Another composite
material of chitosan mixed with nHA was prepared by a chemical method
in which nHA formed
in situ
. This preparation method ensured the nHA
to be tightly bound to the scaffold, and the scaffold was shown to be bio-
compatible.
24
Also, collagen matrices with embedded nHA were fabricated
and proved to be biocompatible and osteoconductive; however, the weakness
of these scaffolds was the mechanical strength.
25
Furthermore, silk fibroins
were mixed with nHA into a composite material by a coprecipitation and
freeze-drying method. The nHA provided the material an excellent com-
pressive modulus and strength, especially after adding high amounts of
nHA (up to 70% w/w).
26
Besides natural materials, nHA has also been combined with synthetic
materials. Porous composite materials of the synthetic polymers poly(lactic-
acid) (PLLA) and poly(glycolic-acid) (PLGA) were mixed with nHA by
in situ
polymerization and thermally induced phase separation methods. Both scaf-
folds had a high ability to absorb water and excellent mechanical proper-
ties.
2 7, 2 8
Another composite material, fabricated of polyamide (PA) and nHA,
resulted in a porous scaffold material with good biocompatibility and a com-
pressive strength equal to the upper limit of cancellous bone.
29
Finally, mix-
ing poly(caprolactone) (PCL) with nHA by a layer manufacturing process
