Alkali and Heat Treatment of Titanium

8/10/2019 Alkali and Heat Treatment of Titanium

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776Volume 27, Number 4, 2012

In recent years, titanium (Ti) has become a materialof great interest in dentistry and orthopedics. Ti hasmany advantages, such as excellent biocompatibility,corrosion resistance, and desirable physical and me-

chanical properties.1,2 The biocompatibility of Ti may

be attributed to its surface oxide film. This oxide film,

formed naturally in air, is a dense and stable anatase

(TiO2) with a thickness of few nanometers.3,4 Com-

mercially pure Ti is available in four grades, which vary

according to the oxygen (0.18% to 0.40% by weight)and iron (0.2% to 0.5% by weight) content. These ap-

parently slight differences in concentration have a

substantial effect on the physical and mechanical

properties.5

Among the most important applications of Ti and

its alloys is for dental implants, in addition to their use

for implant surface coatings, crowns, partial and com-

plete dentures, and orthodontic wires. For success-

ful implantation, the surface character of the implant

material becomes an important factor.5,6In spite of its

excellent properties, Ti is usually bioinert, and integra-

tion between Ti and tissues is only a morphologic con-nection, although direct bone-implant contact, called

osseointegration, could occur.4

Considerable efforts have been directed toward

improving the strength of the bond between Ti im-

plants and bone. Among these techniques is roughen-

ing of the Ti surface by coating, blasting with various

substances, acid etching, or combinations of these

treatments. As reported in many studies,79strong in-

terfacial bonding and active new bone formation have

been confirmed in the peripheral area around rough-

ened implant surfaces. Other attempts to increase the

1

Professor and Chairman, Department of Dental Biomaterialsand Restorative Dentistry, Faculty of Dentistry, Mansoura and

Umm Al Qura Universities, Mansoura, Egypt.2Demonstrator, Department of Dental Biomaterials, Faculty of

Dentistry, Mansoura University, Mansoura, Egypt.3Professor, Department of Dental Biomaterials, Faculty of

Dentistry, Mansoura University, Mansoura, Egypt.4Professor, Department of Metal Physics, Faculty of Science,

Mansoura University, Mansoura, Egypt.

Correspondence to:Dr Ibrahim M. Hamouda, Departmentof Dental Biomaterials and Restorative Dentistry, Faculty of

Dentistry, Mansoura and Umm Al Qura Universities, Mansoura,

Egypt. Fax: +20-50-2260173. Email: [email protected]

Alkali and Heat Treatment of

Titanium Implant Material for Bioactivity

Ibrahim M. Hamouda, MSc, PhD1/Enas T. Enan, MSc2/

Essam E. Al-Wakeel, MSc, PhD3/Mostafa K. M. Yousef, MSc, PhD4

Purpose:This study was conducted to evaluate alkali- and heat-treated titanium implant material. Materials

and Methods:Ninety-eight square plates of commercially pure titanium were divided into three groups. Group

1 plates were left untreated, and groups 2 and 3 were subjected to anodization and alkali treatment for 24

and 48 hours, respectively. Treated specimens were then subdivided into three equal subgroups (a, b, and

c), which were heat treated for 1 hour at temperatures of 500C, 700C, and 800C, respectively. Changes

in the crystalline structure were analyzed using x-ray diffractometry. Surface roughness was measured

using a surface roughness tester. Selected specimens were immersed in a specially prepared simulated

body fluid for 10 days. Calcium and phosphorous deposition on the specimens was detected using energy

dispersive x-ray analysis. Results: Increasing the alkali treatment period and heat treatment temperature

positively affected surface roughness and formation of a bioactive sodium titanium oxide (sodium titanate)

layer on the titanium surface, especially after heat treatment at 800C. There was a significantly higher

calcium deposition on specimens of group 3 in comparison with those of groups 1 and 2. The results of pH

and ion concentration changes of the used simulated body fluid confirmed the results of energy dispersive

x-ray analysis. Conclusion:Alkali and heat treatment of titanium implant materials created better treatment

conditions for obtaining a bioactive implant material.INTJ ORALMAXILLOFACIMPLANTS2012;27:776784.

Key words:alkali treatment, bioactivity, heat treatment, implant materials, titanium

2 01 2 BY QUINTESS ENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RES TRICTED TO PERSONAL US E ONLY.

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Hamouda et al

The International Journal of Oral & Maxillofacial Implants 777

strength of the bone-implant bond have made use of

a Ti substrate that is either plasma sprayed or coated

with a thin layer of calcium phosphate ceramic as tri-

calcium phosphate or hydroxyapatite. The rationale for

coating an implant with calcium phosphate ceramic is

to produce a bioactive surface that promotes bone

growth and induces direct bonding between the im-

plant and hard tissue, whereas the rationale behindplasma spraying is to provide a roughened but biologi-

cally acceptable surface for bone ingrowth to ensure

anchorage in bone.10

It was recently claimed that Ti and its alloys can bond

to living bone by the formation of a bonelike apatite

layer on the surface of the metal without being coated

by hydroxyapatite but through chemical treatment

with sodium hydroxide (NaOH) solution, followed by

heat treatment.11,12The hypothesis of this study was

to develop a new method that would increase bond

strength between Ti implants and surrounding bone.

Consequently, this study was designed to assess theeffects of alkali and heat treatments on Ti.

MATERIALS AND METHODS

Specimen PreparationNinety-eight square plates of commercially pure Ti

(10 10 1 mm) were prepared by machining by

the manufacturer. They were polished with 400-grit

diamond paste and washed with pure acetone and

distilled water. They were classified into three main

groups: group 1 (14 specimens), as received (control);group 2 (42 specimens), alkali treatment for 24 hours;

and group 3 (42 specimens), alkali treatment for 48

hours. Before alkali treatment, the specimens were

subjected to anodization to increase reactivity.13The

specimens were chemically cleaned for 5 minutes in

5.5 mol/L of nitric acid with three drops of hydroflu-

oric acid, rinsed with distilled water, and dried at 40C.

They were immersed in an electrolyte solution (0.5%

by weight of hydrofluoric acid in water). A platinum

electrode (0.1 mm thick) was used as the cathode. A

direct current (20 V) was employed for 2 minutes at

room temperature.14

Alkali and Heat TreatmentsA 5-mol/L concentration of NaOH aqueous solution

was prepared by dissolving 200 g of NaOH powder

in 1 L of distilled water. Each anodized specimen was

immersed in 15 mL of the prepared NaOH solution at

60C.15Group 2 specimens were immersed in the pre-

pared solution for 24 hours, while group 3 specimens

were immersed for 48 hours.11To guarantee a uniform

and steady temperature of 60C during alkali treat-

ment, a heating furnace (Stuart Scientific Furnace) was

used. After immersion, the specimens were washed

with distilled water and dried at room temperature.4

The alkali-treated specimens were divided into three

equal subgroups (14 specimens each): subgroup A was

heated at 500C, subgroup B was heated at 700C, and

subgroup C was heated at 800C, all for 1 hour. After

heat treatment, the specimens were allowed to cool

gradually to room temperature in the heat-treatment

furnace.15

Assessment of Crystal Structure

An x-ray diffractometer (XRD) (Heraeus T 5025) with acopper target and nickel filter was used. The test was

conducted at an excitation voltage of 40 Kv and tube

current of 20 mA. Two specimens from each group

were subjected to irradiation, and the refracted and

transient rays were detected with the XRD. The indi-

vidual peak positions (2), relative intensities (I/I),

and the corresponding interplanar spacing (d) were

obtained from a computer program printout. The crys-

talline structures of the alkali- and heat-treated speci-

mens were identified and compared with those of the

control specimens.

Determination of Surface Roughness andMorphologyA surface roughness tester (Surftest SJ-201P, Mitutoyo)

with a diamond stylus was used. Surface roughness

(Ra) was determined for each specimen at three dif-

ferent sites and the average was calculated. The cutoff

sampling length was set at 0.25 mm. The Ra value was

measured and calculated for 10 specimens from each

subgroup. Surface morphology was observed using a

scanning electron microscope (SEM) (JEOL JXA-840A)

at a magnification of 3,500.

Table 1 Ion Concentrations and pH of SBF andHuman Blood Plasma16

Ion

Concentration (mmol/L)

SBF Blood plasma

Na+ 142.0 142.0

K+ 5.0 5.0

Mg2+ 1.5 1.5

Ca2+ 2.5 2.5

Cl 147.8 103.8

HCO3 4.2 27.0

HPO42 1.0 1.0

SO42 0.5 0.5

pH 7.40 7.207.40




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Hamouda et al


Assessment of Surface CompositionFive Ti specimens from each subgroup were soaked for

10 days in acellular simulated body fluid (SBF) with a

pH of 7.4. The composition of SBF and concentration of

the ions were nearly identical to those of human blood

plasma (Table 1).16The SBF was prepared by dissolv-

ing 7.987 g of reagent-grade sodium chloride (NaCl),

0.352 g of NaHCO3, 0.622 g of potassium chloride

(KCl), 0.262 g of K2HPO43H2O, 0.17 g of MgCl26H2O,0.278 g of calcium chloride (CaCl2), and 0.071 g of so-

dium sulfate (Na2SO4) into 1 L of distilled water and

buffered at a pH of 7.4 with tris(hydroxymethyl) ami-

nomethane ([CH2OH]3CNH3) and hydrochloric acid at

36.5C.11Each specimen was soaked in 25 mL of SBF

at 36.5C for 10 days in a Heraeus furnace. After soak-

ing, the specimen was removed from the fluid, washed

with deionized water, and dried at room temperature.

The surface composition of the incubated specimens

was analyzed and compared with that of the control

specimens using energy dispersive x-ray (EDX) analysis

(INCA X-sight) in conjunction with the SEM.2,15Calcium

and phosphorous (Ca-P) deposition on the treated Ti

specimens was assessed.

Evaluation of pH and Ion Concentration ChangesFive specimens from each subgroup were compared

with five specimens from the control group. Each spec-

imen was immersed in 25 mL of SBF at 36.5C for 10

days and then removed from the fluid, which was then

collected and subjected to evaluation of the pH and

ion concentrations.15 Changes in pH were measured

using a pH meter (Consort p901). Calcium and sodium

ion concentrations in SBF were measured using an

atomic absorption spectrometer (Perkin-Elmer 2380).

One milliliter of each liquid sample of SBF was fed,

without dilution, into the apparatus, which combined

it with a stream of acetyleneair fuel and oxidant. The

mixture then passed into a burner in which the com-

pounds making up the sample were broken into free

atoms. The absorbance values for calcium were 0.036,

0.072, and 0.278, while those for sodium were 0.217,0.442, and 0.669 for 1, 2, and 3 ppm, respectively. To

measure the calcium ion concentration, a current of 10

mA and a wavelength of 422.7 nm were used. To mea-

sure the sodium ion concentration, a current of 8 mA

and a wavelength of 589 nm were used.

Statistical AnalysisData of Ra, pH, and calcium and sodium ion concentra-

tions were collected. Means and standard deviations

were calculated for each group and compared by one-

way analysis of variance (ANOVA) and least significant

difference (LSD) tests. Significance for all statistical

tests was set at = .05. Statistical analysis was per-

formed with SPSS 14.0 (IBM) for Windows.

RESULTS

XRD FindingsThe XRD spectra obtained from as-received and from

alkali- and heat-treated Ti specimens are shown in Figs

1 and 2, respectively. These figures are plots of the rela-

tive intensity (counts per second) versus diffraction

T

T

T T

T

20

100

150

50

010

200

250

350

300

400

450

550

500

650

600

44 48 52 56 60 64 68 72 76 8028 32 36 40

2(stop)

Coun

ts/s

24

T

T

T

T

R

R

R

R

R R

R

RN

NNN

N

N

20

100

150

50

010

200

250

350

300

400

450

550

500

650

600

44 48 52 56 60 64 68 72 76 8028 32 36 40

2(stop)

Coun

ts/s

24

Fig 1 Representative x-ray spectrum for as-received Ti (T ). Fig 2 Representative x-ray spectrum for Ti specimen (T) sub-jected to alkali treatment for 48 hours followed by heat treatmentat 800C. T = titanium; R = rutile; N = sodium titanium oxide.




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angle (2). Miller indices (hkl) of planes in the phases

present, which have been correlated with various dif-

fraction peaks, are shown on the plots. The peak angles

show some variations in intensity of the diffracted

peaks because of preferred orientation. The XRD spec-

tra for all specimens over the 2 interval from 20 to

80 showed the typical {100}, {002}, {101}, {102}, and

{110} peaks for the hexagonal Ti phase. Other peaks

{110}, {101}, {200}, {111}, {210}, {211}, {311}, {202}, and{321}were attributed to the titanium oxide (rutile)

tetragonal phase. But other peaks, present at 2 of 34,

35, 50, 60, 67, 73, and 77 degrees, were attributed to

the sodium titanium oxide phase, as shown by d val-

ues, without indicating the Miller indices (hkl), because

no information was available about them, as indicated

by the ASTM card #11-0239. Tables 2 and 3 show the

calculated average d spacing values for each line corre-

sponding to the lines reported on ASTM card #44-1294

for Ti, corresponding to rutile (titanium oxide) lines re-

ported on ASTM card #87-0710, and corresponding to

sodium titanium oxide (sodium titanate) lines reported

on ASTM card #11-0239.

Surface Roughness MorphologyMean surface Ra values and standard deviations for all

groups are shown in Table 4. The results indicated that

group 3C (alkali-treated for 48 hours and heat-treated

at 800C) had the highest mean Ra value, while group

1 (control) showed the lowest value. ANOVA showed

that there were significant differences (P < .0001)

among the surface roughness values of the studied

groups (Table 4). The LSD test showed that there were

no significant differences between groups 1 (control),

2A (alkali-treated for 24 hours and heat-treated at

500C), and 2B (alkali-treated for 24 hours and heat-

treated at 700C) (P > .05). There were significant dif-

ferences between the control group and groups 2C

(alkali-treated for 24 hours and heat-treated at 800C),

3A (alkali treated for 48 hours and heat-treated at

500C), 3B (alkali-treated for 48 hours and heat-treated

at 700C), and 3C (alkali-treated for 48 hours and heat-

treated at 800C) (P .05). There were significant differ-

ences between group 2A and groups 2C, 3A, 3B, and 3C

(P .05). Significant differences were found between

group 2B and groups 2C, 3A, 3B, and 3C (P .05). Sig-

nificant differences were also detected between group

2C and groups 3A, 3B, and 3C (P .05). There were also

Table 2 Average d Spacing Values for TitaniumOxide (Rutile) Lines Shown on the DiffractionPatterns and the Corresponding ASTM Cards

Card/line 2 d () hklASTM card

d ()

#44-1294

1 41 2.553 100 2.5552 45 2.339 002 2.341

3 47 2.247 101 2.243

4 63 1.727 102 1.726

5 75 1.474 110 1.475

#87-0710

1 32 3.226 110 3.241

2 42 2.479 101 2.482

3 46 2.281 200 2.292

4 48 2.179 111 2.183

5 52 2.047 210 2.050

6 64 1.683 211 1.684

Table 3 Average d Spacing Values for SodiumTitanium Oxide Lines Shown on the Dif fractionPatterns and the Corresponding ASTM Card(#11-0239)

Line 2 d ()ASTM card

d ()

1 34 3.039 3.012 35 2.967 2.95

3 50 2.105 2.10

4 60 1.785 1.73

5 67 1.649 1.64

6 73 1.512 1.51

7 77 1.445 1.44

Table 4 Surface Roughness (Ra, Means SDs,in m) of the Studied Groups

Group Ra F P LSD

1 0.174E 0.006 855.38 < .0001 0.013

2A 0.180E 0.010

2B 0.184E 0.011

2C 0.240D 0.007

3A 0.318C 0.008

3B 0.422B 0.015

3C 0.492A 0.008

Means with the same superscripts are not significantly different.

1 = control (as received); 2 = alkali treatment for 24 hours then heat

treatment at (A) 500C, (B) 700C, or (C) 800C; 3 = alkali treatment

for 48 hours then heat treatment at (A) 500C, (B) 700C, or (C) 800C.




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significant differences between group 3A and groups

3B and 3C (P .05). There was a significant difference

between groups 3B and 3C (P .05).

Surface MorphologyFigure 3 shows scanning electron micrographs of the

studied specimens. These images showed a difference

in surface texture between the control specimen and

the others. The control specimen (1) showed longitu-

dinal elevations and depressions corresponding to the

direction of cutting, while images of specimens subject-

ed to alkali treatment for 24 hours (2A, 2B, 2C) showed

some increase in surface irregularities and porosity. Ti

specimens that were subjected to alkali treatment for

48 hours (3A, 3B, 3C) showed a more granular texture.

Surface CompositionFigures 4 to 6 are examples of EDX data showing peaks

corresponding to the different elements present in the

analyzed specimens after immersion in SBF. Weight

percentages for the detected elements are shown in

Table 5. Figure 4 shows that the control specimen was

composed mainly of Ti. Figure 5 shows EDX data for Ti

specimens subjected to alkali treatment for 24 hours

followed by heat treatment at 800C (group 2C). EDX

data for specimens of group 2C showed the highest

percentage of oxygen among the group 2 subgroups. In

addition, small percentages of calcium were detected

in subgroups 2B and 2C. Figure 6 shows EDX data for

specimens of group 3C (alkali-treated for 48 hours and

heat-treated at 800C). There were peaks correspond-

Fig 3 SEM (3,500) of the studied specimens. 1 = control specimen; 2 = specimens subjected to alkali treatment for 24 hours fol-lowed by heat treatment at 500C (A), 700C (B), and 800C (C); 3 = specimens subjected to alkali treatment for 48 hours followed

by heat treatment at 500C (A), 700C (B), and 800C (C).

1 2A

3A

2B

3B

2C

3C

6420 12 14 16 18 2010

Energy (keV)

8

Ti

T

6420 12 14 16 18 2010

Energy (keV)

8

Ca

Ca

Ti

O

T

Fig 4 EDX spectrum for as-received Ti specimen (group 1). Fig 5 EDX spectrum for Ti specimens subjected to alkali treat-ment for 24 hours followed by heat treatment at 800C.




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Hamouda et al


ing to calcium, phosphorous, and sodium, in addition

to titanium and oxygen. EDX data for specimens of

group 3C showed the highest percentage of oxygen

among the group 3 subgroups.

Changes in pHMean pH values and standard deviations for the SBF

for all groups are shown in Table 6. A comparison of

mean pH values of the tested specimens showed that

the SBF used with specimens of group 3C had the high-

est mean pH, while that used with specimens of group

1 (control) showed the lowest value. ANOVA showed

that there was a significant difference (P < .0001) be-

tween pH values of SBF used with the studied groups

(Table 6). The LSD test showed that there were no sig-

nificant differences between group 2A and groups 2B

and 2C (P > .05). No significant difference was detected

between groups 2B and 2C (P > .05). There were

significant differences between group 1 and groups

2A, 2B, and 2C (P .05). There were significant differ-

ences between group 1 and groups 3A, 3B, and 3C

(P .05). There were significant differences between

group 2A and groups 3A, 3B, and 3C (P .05). There

were significant differences between group 2B and

groups 3A, 3B, and 3C (P .05). In addition, significant

differences were found between group 2C and groups

3A, 3B, and 3C (P .05). Significant differences were de-

tected between group 3A and groups 3B and 3C. There

was a significant difference between groups 3B and 3C

(P .05).

6420 12 14 16 18 2010

Energy (keV)

8

Na P

Ca

Ca

Ti

O

T

Fig 6 EDX spectrum for Ti specimens subjected to alkali treat-ment for 48 hours followed by heat treatment at 800C (group

3C).

Table 5 Average Composition of Ti Specimens

Group

Elements (% by weight)

Titanium Oxygen Calcium Phosphorous Sodium

1 98.54

2A 65.04 34.96

2B 65.73 33.98 0.20

2C 86.90 41.93 0.25

3A 83.67 39.36 0.30

3B 42.08 32.52 1.72 1.26 1.72

3C 58.75 89.13 5.56 0.4 1.27




Table 6 pH (Means SDs) of SBF Used with

the Studied GroupsGroup pH F P LSD

1 7.462E 0.008 1,812.41 < .0001 0.019

2A 7.954D 0.006

2B 7.952D 0.005

2C 7.958D 0.008

3A 7.980C 0.01

3B 8.256B 0.03

3C 8.326A 0.02



treatment at (A) 500C, (B) 700C, or (C) 800C; 3 = alkali treatmentfor 48 hours then heat treatment at (A) 500C, (B) 700C, or (C) 800C.

Table 7 Ca2+ and Na+ Concentrations (Means

SDs, in ppm) of SBF Used with the StudiedGroups

Group Calcium ions (Ca2+) Sodium ions (Na+)

1 5.226A 0.1 10.078D 0.001

2A 5.188A 0.2 10.080D 0.1

2B 5.1A,B 0.1 10.080D 0.002

2C 5.026B 0.129 10.084D 0.1

3A 3.260C 0.2 14.340C 0.9

3B 2.620D 0.1 16.200B 0.3

3C 2.596D 0.1 17.814A 0.4

F 517.96 376.1

P < .0001 < .0001

LSD 0.159 0.505








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Calcium Ion ConcentrationsMeans and standard deviations of the calcium ion con-

centration in SBF used with all studied groups are shown

in Table 7. A comparison of mean calcium ion concen-

trations of the tested specimens showed that SBF used

with specimens of group 3C had the lowest mean con-

centration, while that used with specimens of group 1

showed the highest concentration. ANOVA showed thatthere was a significant difference (P < .0001) between

calcium ion concentration values of SBF used with the

studied groups (Table 7). The LSD test showed that there

were no significant differences between groups 1, 2A,

and 2B (P > .05). No significant differences were found

between groups 2B and 2C (P > .05). No significant dif-

ferences were seen between groups 3B and 3C (P > .05).

There were significant differences between group 1 and

groups 2C, 3A, 3B, and 3C (P .05). There were signifi-

cant differences between group 2A and groups 2C, 3A,

3B, and 3C (P .05). There were significant differences

between group 2B and groups 3A, 3B, and 3C (P .05).In addition, significant differences were found between

group 2C and groups 3A, 3B, and 3C (P .05). Significant

differences were found between group 3A and groups

3B and 3C (P .05).

Sodium Ion ConcentrationMeans and standard deviations of sodium ion concen-

trations in SBF used with all studied groups are shown

in Table 7. A comparison of mean sodium ion concen-

tration values of the tested specimens showed that SBF

used with group 3C had the highest mean sodium ion

concentration, while that used with group 1 had thelowest value. ANOVA showed that there was a signifi-

cant difference between sodium ion concentrations of

SBF used with the studied groups (P < .0001). The LSD

test showed that there were no significant differences

between groups 1, 2A, 2B, and 2C (P > .05). There were

significant differences between group 1 and groups 3A,

3B, and 3C (P .05). There were significant differences

between group 2A and groups 3A, 3B, and 3C (P .05).

Significant differences were found between group 2B

and groups 3A, 3B, and 3C (P .05). There were signifi-

cant differences between group 2C and groups 3A, 3B,

and 3C (P .05). In addition, there were significant dif-

ferences between group 3A and groups 3B and 3C and

also between groups 3B and 3C (P .05).

DISCUSSION

Biomedical and materials researchers have tried to

design the ideal surface to ensure long-lasting anchor-

age of implants. All bioactive materials developed up

to 1990 were based on calcium phosphate ceramics.16

It was later revealed that materials that form a calcium

phosphate layer, usually called a bonelike apatite, on

their surfaces in the living body bond to living bone

through this apatite layer, as it seems to activate bone

morphogenetic proteins and osteogenic cells to start

the cascade of events that result in bone formation.17

Apatite formation on a material can be induced by for-

mation of functional groups such as TiOH, SiOH, TaOH,

and ZrOH on its surface. Based on these findings, bio-active Ti was prepared by forming sodium titanate,

which induces TiOH formation, on its surface via alkali

(NaOH) and heat treatments.16,18

XRD patterns for specimens of group 2 showed

sharp peaks with low intensity of the sodium titanate

phase, in addition to peaks of Ti and rutile, only after

heat treatment at 700C and 800C. Specimens that

were heat-treated at 500C did not show peaks of so-

dium titanate. On the other hand, the definition and

intensity of sodium titanate peaks were stronger in

specimens of group 3, which was treated with NaOH

for 48 hours. The broad peak of sodium titanate thatwas shown after heat treatment at 500C may indicate

that this layer forms first in an amorphous form. This

form precipitated crystalline sodium titanate at 700C

and had fully crystallized at 800C, as indicated by the

sharper and more intense peaks present in the XRD

patterns, especially after 800C heat treatment. This in-

dicated that 48 hours of NaOH immersion followed by

800C heat treatment produced the highest intensity

for the sodium titanate layer.

Leaching of Ti in NaOH results in the formation of a

hydrated titanium oxide gel layer containing alkali ions

on its surface (sodium titanate hydrogel layer).15,1921

This layer is dehydrated and condensed to form an

amorphous sodium titanate layer by heat treatment

below 600C.20,22 Regarding the time required for ef-

fective alkali treatment, one study11showed disagree-

ment with the present study. The authors concluded

that 24 hours of alkali treatment was sufficient for the

formation of a sufficient sodium titanate layer to start a

bioactive reaction on the Ti surface, while the results of

the present study revealed that 48 hours of alkali treat-

ment was more effective for sodium titanate forma-

tion, as shown by the sharper and more intense sodium

titanate peaks in XRD patterns. This difference could be

attributed to variations among the experimental con-

ditions, such as reactivity of the used solutions.

The hydrogel layer formed by alkali treatment is

mechanically unstable and requires further heat treat-

ment to convert the gel layer into a more stable form.

The amorphous sodium titanate layer is converted

into crystalline sodium titanate and is rutile above

700C.15,1921Sodium titanate results from the reaction

between titania (TiO2), which forms during anodiza-

tion, and NaOH. When soaked in NaOH solution, tita-

nia react with OH, thus forming HTiO3;then titanate




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Hamouda et al


hydroxide (HTiO3nH2O) is formed by the hydration of

HTiO3. These hydroxides are joined with sodium ions

in NaOH solution, and a porous network sodium tita-

nate hydrogel layer is formed. After heat treatment,

a stable sodium titanate (Na2Ti5O11) layer is formed

eventually by the removal of nH2O from the sodium

titanate hydrogel layer.20,21

Surface roughness measurements and SEM in thepresent study showed increased roughness, especially

in the case of specimens subjected to alkali treatment

for 48 hours. These results proved the presence of a

direct proportionality between the surface roughness

of Ti and the duration of alkali treatment, which can

be explained by the occurrence of a more prominent

reaction between Ti and NaOH with longer treatment

time.13It was found that this increase in surface rough-

ness favors implant fixation by inducing both bone-

anchoring and biomechanical stability.18,23

After soaking of the alkali- and heat-treated Ti

specimens in the prepared SBF, EDX performed in thecurrent study revealed the presence of calcium, phos-

phorous, and sodium on specimens that were sub-

jected to alkali treatment for 48 hours, especially after

heat treatment at 700C and 800C. These results may

indicate the formation of a calcium phosphate layer

on the analyzed specimens. The presence of sodium

means that the reaction between sodium titanate and

the surrounding SBF was not yet completed, as more

sodium ions were still available for ion exchange and

formation of more calcium phosphate (apatite). On

the other hand, specimens that were alkali-treated for

24 hours showed only calcium deposition after 700Cand 800C heat treatment, while they did not show any

phosphorous peaks. These findings may indicate that

a longer alkali treatment period and subsequent heat

treatment above 700C led to a faster rate of apatite

formation on the treated specimens upon immersion

in SBF, indicating stronger bioactive behavior. In con-

trast to the treated groups, untreated specimens did

not show any calcium or phosphorous peaks, which

signals the absence of any bioactive reaction.

The formation of a calcium phosphate layer (bone-

like apatite) on a materials surface is an essential re-

quirement for bone growth on a synthetic material.

The role of this layer lies in the fact that it has a sig-

nificant effect on cell adhesion and differentiation of

osteoblastlike cells, resulting in bone formation and a

tighter bone-implant bond.17,24 Regarding the effect of

the alkali treatment period on apatite formation on Ti,

the results of the present study were in disagreement

with a previous study,15which detected apatite forma-

tion on Ti after alkali treatment for only 24 hours. On

the other hand, the present study revealed more obvi-

ous apatite deposition after 48 hours of alkali immer-

sion. The different ability of alkali-treated Ti to induce

apatite nucleation could be explained by differences in

the Ti surface that depend on its treatment conditions.

The mechanism of apatite formation on alkali and

heat-treated Ti in SBF was interpreted in terms of an

electrostatic interaction between the Ti surface layer

and the ions in SBF. The previously formed sodium ti-

tanate layer releases sodium ions via exchange with

H3O+ ions in the SBF to form many Ti-OH groups onthe surface. As a result, the surface becomes nega-

tively charged and reacts with the positively charged

calcium ions in the SBF to form calcium titanate. As cal-

cium ions accumulate, the surface becomes positively

charged and reacts with the negatively charged phos-

phate ions to form amorphous calcium phosphate.

Because amorphous calcium phosphate is metastable

in SBF, it eventually transforms into stable crystalline

bonelike apatite.11,16

The pH and ion concentration analysis performed

for SBF used in the present research confirmed the

aforementioned mechanism of apatite formation. Re-sults of pH analysis showed that immersion of group 2

specimens (alkali-treated for 24 hours) in the prepared

SBF caused an increase in its pH in comparison with

the results of the control group. SBF used with group 3

(alkali-treated for 48 hours) showed a greater increase

in pH, which showed its highest value on heat-treated

specimens at 800C. Elevated pH values of SBF after im-

mersion of the treated specimens can be surely linked

to the ionic movements that took place between the

Ti surface and the surrounding solution, especially

the release of sodium ions, leading to changes in ionic

concentrations and therefore changes in pH.The current data indicated that the ionic movement

in the soaking solution (SBF) leads to increase in its pH.

The alkali release and ion exchange in SBF resulted in

an increase in the pH of the surrounding fluid.15,23,24

This pH increase accelerates apatite nucleation by in-

creasing the ionic activity product of apatite according

to the following equilibrium in SBF: 10Ca2++ 6PO43++

2OHCa10(PO4)6(OH)2.9

Measurement of calcium and sodium ion concen-

trations in the used SBF showed that specimens of

group 2 (alkali treated for 24 hours) did not change sig-

nificantly in this regard. This may be attributed to the

presence of a weak ionic reaction among this group,

leading to weak bioactive behavior. Analysis of the re-

sults of group 3 (alkali-treated for 48 hours) showed

a notable decrease in calcium ion concentration si-

multaneous with a significant increase in sodium ion

concentration. A comparison of results of the different

heat treatment temperatures revealed that the highest

changes were related to Ti specimens that were heat

treated at 700C and 800C. Changes in calcium and

sodium ions concentrations may confirm the previ-

ously illustrated ion exchange cascade that ends with




9/9

Hamouda et al


apatite deposition on treated Ti specimens by consum-

ing calcium and phosphate ions from the SBF in which

they were soaked. Dependence of ionic changes on

the heat treatment temperature may be interpreted in

terms of more crystallization of the bioactive sodium

titanate layer at these temperatures (700C and 800C).

Based on the results of the present work, it can

be reported that ion exchanges between an implantsurface and SBF may strongly support the biochemi-

cal bonding theory of alkali- and heat-treated Ti with

the surrounding bonelike environment. In an in vivo

study,25both treated and untreated porous Ti cylinders

were implanted in rabbit femoral condyles. Unexpect-

edly, there was no significant difference in bone in-

growth at the early postimplantation times of 2 and 4

weeks. Over time, however, the alkali- and heat-treated

implants showed increased osseointegration, whereas

the untreated implants did not. The authors attrib-

uted the delayed bone ingrowth, even with treated Ti,

to the type of bone in which implants were placed. Itwas mentioned that the cancellous bone model used

in that study does not enhance bone ingrowth at ear-

ly postimplantation periods, as it has low osteogenic

capacity, while the opposite occurs in cortical bone.

It was reported in the same study that osseointegra-

tion of untreated implants tends to be lost over time,

whereas the treated implants maintained osseointe-

gration throughout the experiment.25

CONCLUSIONS

Based on the results and within the limitations of this

study, the following conclusions can be made.

1. Untreated titanium specimens showed the lowest

surface roughness values and no signs of any bio-

active reaction.

2. Specimens treated in alkali for 48 hours showed

greater surface roughness and the formation of

a bioactive sodium titanate layer on the titanium

surface.

3. During heat treatment, the role of temperature

was obvious, particularly at 800C.

4. An increase in the length of the alkali treatment

period and an increase in the temperature of heat

treatment above 700C are recommended to in-

crease the formation of a bioactive sodium titanate

layer.

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Alkali and Heat Treatment of Titanium