Growth of hydroxyapatite whisker by hydrolysis of -tricalcium - - PowerPoint PPT Presentation

Growth of hydroxyapatite whisker by hydrolysis of α-tricalcium phosphate studied by transmission electron microscopy

• T. Isshiki, Y. Takino, M. Tamai, K. Nishio, A. Nakahira and H. Endoh

Kyoto Institute of Technology, Matsugasaki, Kyoto 606-8585, Japan (mailto:isshiki@dj.kit.ac.jp)

Introduction

Hydroxyapatite

Hydroxyapatite (Ca10(PO4)6(OH)2: HAp)

• Crystal structure: Similar to a bone

⇓ widely noticed and expected

• As an alternative material for bone
• Application to a biosensor making

good use of biocompatibility

Synthesis of Hydroxyapatite

• Dry process

⇒ Stoichiometric HAp 6CaHPO4 + 4CaCO3 → HAp + 2H2O + 4CO2

• Wet process

⇒ Non-stoichiometric HAp – Sol-gel method – Hydrolysis method [1]

Ca-deficient Hydroxyapatite [1]

α-tricalcium phosphate (α-Ca3(PO4)2: α-TCP) ⇓ Hydrolysis in a mixture of water and alcohol Ca-deficient HAp (Ca10−Z(HPO4)Z(PO4)6−Z(OH)2−Z ⋅ nH2O (Z = 0-1)) The merits

• Easy to systhesis

(under mild environment within several hours)

• Control of crystal morphology
• Whisker shape favorable as a source to produce

porous biomaterials

Investigation

It is discussed that the crystal growth of HAp whiskers produced by hydrolysis of α-TCP studied by transmission electron microscopy (TEM).

Experimental Procedure

Sintered and shaped into a disk α-TCP powder for mass production Thin α-TCP for HRTEM

• bservation

α-TCP powder Thinned with a dimple grinder

Specimen I Specimen II Dripped on a TEM grid TEM observation : JEOL JEM-2010/SP & JEM-2000EX Hydrolysis in a water solvent suspending 1-octanol α-TCP : water : 1-octanol = 0.01 mol : 100 ml : 60 ml Initial pH = 11 (adjusted with NH4OH) Stirred at 70 C for 1, 2, 3, 4, 5, 24, and 48 hours

Results and Discussion

Initial stage of hydrolysis

TEM images of the thinned sample (Specimen I) are shown in Figs. 1-3. The surface of the α-TCP was clean before hydroly- sis as shown in Fig. 1.

• Fig. 1 Surface of α-TCP

before hydrolysis.

The surface of the α-TCP hydrolyzed for a few hours was covered with an amorphous layer (Fig. 2). Nuclei appeared not on the α-TCP crystal but on the surface of the amorphous layer (Fig. 3a, b), and they grew into the dendritic structure com- posed of twigs with a few nm in width and trunks with several tens nm in width (Fig. 3c). The den- drites are regarded as an embryo of HAp crystal.

• Fig. 3 (right) TEM images of the surface of α-TCP

hydrolyzed for 2 (a) and 4 (b, c) hours, respectively.

• Fig. 2 (a) TEM image of α-TCP hydrolyzed for 2 hours,

(b, c) enlarged images of the areas indicated by squares in a, respectively, and (d) SAED pattern of the area c.

Growth of hydroxyapatite whiskers

• Figs. 4, 5 show that the progress of hydrolysis of

the α-TCP powder (Specimen II) and conversion ratio of HAp to α-TCP calculated from the X-ray diffraction intensities obtained from the samples.

• Fig. 4 (a-f) TEM images of the samples hydrolyzed for 0,

1, 2, 3, 5, and 48 hours, respectively.

Needle-like HAp crystals with a few tens nm in width and a few hundreds nm in length grew around the α-TCP particle after hydrolysis for 1 hour (Fig. 4b). After 3 hours of hydrolysis, most of growing HAp crystals had whisker shape (Fig. 4d). Growth of the needle-like HAp crystal oc- curs at first, and then deposition of HAp on the needle-like crystals and aggregation of them form HAp whiskers. Hydrolysis of the α-TCP powder (Specimen II) was completed in 3 hours (Figs. 4d-f, 5).

20 40 60 80 100 1 2 3 4 5 Conversion rate [%] Hydrolyzed time [hour] 20 30 40 50 Intensity [a.u.] Diffraction angle 2θ [degree] 0 h 1 h 2 h 3 h 24 h 48 h HAp

a b

• Fig. 5 Dependence of conversion rate to HAp from α-TCP
• n hydrolyzed time (a) calculated from X-ray diffraction

intensities obtained from the samples at various hydrolyzed time (b).

Summary

Growth process of HAp whiskers by hydrolysis of α-TCP is schematically illustrated as follows. α-TCP Surface is dissolved Amorphous layer on the surface of α-TCP

Nucleation of HAp on the amorphous layer Deposition from the solution Dendrite structures

• n the nuclei

Successive deposition Needle-like HAp crystals Successive deposition and aggre- gation HAp whiskers

References [1] A. Nakahira, K. Sakamoto, S. Yamaguchi, M. Ka- neno, S. Takeda and M. Okazaki, J. Am. Ceram.

• Soc. 82 (1999) 2029-2032