Preparation of bovine hydroxyapatite by transferred arc plasma

Current Applied Physics 11 (2011) 702e709

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Preparation of bovine hydroxyapatite by transferred arc plasma C.P. Yoganand a, V. Selvarajan a, *, O.M. Goudouri b, K.M. Paraskevopoulos b, Junshu Wu c, Dongfeng Xue c a

Plasma Laboratory, Department of Physics, Bharathiar University, Coimbatore 641 046, India Department of Physics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece c State Key Laboratory of Fine Chemicals, Department of Materials Science and Chemical Engineering, School of Chemical Engineering, Dalian University of Technology, 158 Zhongshan Road, Dalian 116012, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 March 2010 Received in revised form 11 November 2010 Accepted 12 November 2010 Available online 9 December 2010

Hydroxyapatite (Ca10(PO4)6(OH)2, abbreviated as HA) is a kind of bioactive material that has received considerable attention over the past decades as an implant material due to its excellent biocompatability for clinical applications. In this work, Hydroxyapatite was obtained by plasma processing of the natural bovine bones by Transferred arc plasma (TAP) processing at 5 kW in argon plasma for different processing times (i.e. 30, 45, 60, 90 and 120 s). The TAP synthesized HA bioceramic was characterized by XRD, FTIR, SEM-EDX and TG-DTA analysis. The effect of TAP processing time on the preparation of organic free HA from bovine bone was studied. The study indicated that TAP processing for 30, 45 and 60 s were insufficient for removal of organics from the natural bovine bone. Organic free bovine HA was obtained for 90 s TAP processing with a Ca/P ratio of 1.93 comparable with commercially available natural HA-Endobon powder. Whereas 120 s of processing resulted in trivial thermal decomposition of HA in to its constituent phases. Thus our present investigation suggested that HA production from bovine bone using TAP processing is a time effective advantageous method in comparison to the annealing method. Ó 2010 Elsevier B.V. All rights reserved.

Keywords: Transferred arc plasma (TAP) Bovine bone Hydroxyapatite

1. Introduction The number of treated skeletal deficiencies steadily increases in a global scale. Effective ways for bone replacements and enhancement of bone formation together with research directed to find ideal biomaterials and production techniques, which will feature biocompatibility and production simplicity and economy, are required [1]. Hydroxyapatite (HA, Ca10(PO4)6(OH)2), the main mineral component of bones and teeth, is among the leading biomaterials satisfying these requirements [2,3]. Although HA powder manufacture for implants, prostheses and feed stocks are standard practice, the cost of these materials to countries distant from the HA-manufacturing country or to countries with developing economies remains very high; therefore, requiring a cheaper local HA source [4]. The present methods of preparation of HA range from conventional wet synthesis, solidestate reaction, hydrothermal exchange process and calcination of animal skeletal bone etc. [5,6]. HA can be produced chemically or from natural resources like corals and bovine bones. Recently, annealing method (i.e. heat treatment) has been suggested as an alternative technique to produce HA from bovine bones. At the material level, bovine bone is composed of organic and inorganic components. The organic part * Corresponding author. Tel.: þ91 422 2422222; fax: þ91 422 2422387. E-mail addresses: [email protected] (C.P. Yoganand), vselvrjn47@ rediffmail.com (V. Selvarajan). 1567-1739/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2010.11.035

contains mainly collagen and proteins, whereas the inorganic component is mainly HA with a small percentage of other elements being incorporated in the structure such as carbonate, magnesium and sodium. Moreover the presence of these elements in the HA produced from bovine bones is absent in the case of synthetic HA. The presence of trace elements is very important and plays a vital role in bone metabolism process. The annealing method of HA synthesis usually takes few (5e6 h) or more hours of thermal treatment during which the organic materials in the bovine bones gets removed leaving pure inorganic HA as the residue [7]. It is well known that plasma processing in recent years has emerged as an efficient technique for synthesis of a variety of materials. It is an enabling technology for synthesis of newer and better materials. In this context, the present study is aimed at preparing HA from bovine bone through transferred arc plasma processing method within short duration. Besides that, in this study it is also attempted to determine the effect of different processing times. Our observations based upon the studies of the process are presented and discussed. 2. Methodology 2.1. Preparation of bovine hydroxyapatite by plasma processing HA was obtained by plasma processing of the natural bovine bones. The bones were deproteinized with reagent grade NaOH

C.P. Yoganand et al. / Current Applied Physics 11 (2011) 702e709 Table 1 The typical operating parameters. Torch type:

Transferred arc

Input power: Plasma Gas and flow rate: Cooling water flow rate Processing time Quenching medium

5 kW dc Argon; 10 lpm 12 lpm 30,45,60,90 & 120 s air

treatment (1 h) and were cut into pieces (l  bw2cm  1 cm) were used for plasma processing at 5 kW of plasma power in argon plasma for different processing times. A dc transferred arc plasma torch which can be operated at a maximum power of 10 kW was used for the purpose. The arc is typically struck between the cathode and the anode by applying a high current between the cathode and the anode (at an electrode gap of about 25 mm) and the desired power level was maintained by controlling the flow rate of the plasma gas and the arc current. Plasma was generated and the bovine bone was processed by varying the melting times for 30, 45, 60, 90 and 120 s respectively. The average plasma zone temperature is 2980  C [8]. During the plasma process, the organics from the bovine bone get removed and HA ceramic in the bulk form was produced. The properties of the TAP synthesized HA bioceramic were characterized. The effect of TAP processing time on the formation of organic free HA from bovine bone was studied. The operating parameters are given in Table 1. The Fig. 1a shows the schematic of TAP torch and Fig. 1b shows the photographic image of plasma produced during TAP processing.

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with respect to processing time and this is given in Table 2. The colour of the raw bovine bone was light yellow. Upon TAP processing for 30, 45, 60, 90 and 120 s the colour of the samples changed from yellow to grey and then to white as shown in Table 2 respectively. This series of colour change is believed to be associated with the burn out process of organic matrix (e.g. protein and collagen) in the bovine bone. The darker colours observed for samples at 30 and 45 s of TAP processing indicated incomplete removal of organic compositions [9]. However, for more than 60 s of plasma processing, the samples were white in colour, suggesting complete removal of organic substances. 3.2. X-ray diffraction analysis The XRD patterns of the raw bovine bone and TAP processed samples are presented in Fig. 2. The XRD pattern of raw bovine bone presents the characteristic but rather broad peaks of HA indicating the existence of poor crystallized HA as it is expected for natural, biological apatite. It was observed from the XRD pattern that, as the melting time is increased, the intensity of HA characterized peaks gradually increased. The XRD pattern of the samples processed at 45, 60, 90 and 120 s exhibited a substantial increase in peak height

2.2. Characterization The samples were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy with energy dispersive X-ray spectroscopic analysis (SEM-EDX), thermo gravimetric differential thermal analysis TG-DTA and X-ray diffractometry. In order to characterize the samples by FTIR; a quantity of 3 mg of the sample with 200 mg of KBr was pressed each time in a vacuum press at 7 tonnes in order to produce a pellet with 13 mm diameter and 0.8 mm thickness, while a batch of powder sample was carbon coated for SEM-EDX measurements. The FTIR spectra were collected using a Bruker IFS113v FTIR spectrometer, in transmission mode in mid infrared (MIR) region (400e4000 cm1) and with a resolution of 4 cm1. Surface elemental compositions were performed with a SEM with associated EDX (JEOL J S M.840A). Mass loss measurements of all samples with the temperature were measured in a Thermo gravimetric Differential Thermal Analyzer, Setaram Setsys 1750. The XRD measurements were carried out using a Philips (PW1710) diffractometer with Ni-filtered CuKa radiation. 3. Results and discussion 3.1. General observation A general observation was made upon TAP processing for different processing time, there was colour change of the sample Table 2 Effect of TAP processing on the colour of bovine bone. Sample

Processing time (sec)

Colour

1 2 3 4 5 6

0 30 45 60 90 120

Light yellow Dark grey Light grey White White White

Fig. 1. (a) Schematic of TAP torch (b) Photograph of plasma produced during processing.

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a

1000

Raw bovine bone

HA

60 sec

HA

700

HA

HA

800

d

800

HA

HA

600

Intensity (a.u.)

Intensity (a.u.)

600

HA HA

400

HA HA

HA HA HA

500 400

HA

300

HA HA

HA

200

200

HA

100

0

0

0

10

20

30

40

50

60

70

80

90

0

10

20

30

40

b

700

30 sec

e

800

600

HA

Intensity (a.u.)

Intensity (a.u.)

HA HA

70

80

90

90 sec HA

600

300

60

HA

500 400

50

2 theta scale

2 theta scale

HA

200

HA

HA HA

HA HA HA

400

HA HA

HA HA

200

100 0

HA HA

0

-100 0

10

20

30

40

50

60

70

80

90

0

10

20

30

2 theta scale 800

c

40

50

60

70

80

90

2 theta scale

45 sec

HA

700

F

2000

120 sec

HA

600

HA HA HA

400

1500

Intensity (a.u.)

Intensity (a.u.)

500

HA HA

300

HA

HA HA HA

200

1000

TCP HA 500

100

HA TCP TCP HA TTCP TCP

CaO

0 0

0

10

20

30

40

50

60

70

80

90

0

10

2 theta scale

20

30

40

50

60

70

80

90

2 theta scale Fig. 2. XRD patterns of raw and processed samples.

and a decrease in peak width, thus indicating an increase in crystallinity. All the XRD patterns obtained for the samples were in agreement with the stoichiometric HA characterized pattern. However, the XRD pattern of the 120 s processed sample showed

the diffraction peaks of tricalcium phosphate (TCP), tetracalcium phosphate (TTCP) and CaO, respectively [10]. This may be due to the chemical transformation involved due to the prolonged TAP processing. HA decomposes to form other calcium phosphates at

C.P. Yoganand et al. / Current Applied Physics 11 (2011) 702e709

Table 4 Comparison of the Ca/P ratio of 90 s TAP sample with chemically produced HA and natural HA (Endobon).

Transmittance (AU)

120 sec

Element 90 sec

750

TAP process (90 s)

Ca/P ratio 1.93 Trace element(%) Mg% 0.70 Na% 0.48

Raw bovine bone

500

705

1000 1250 1500 1750 2000 2250 2500 2750 3000 3250 3500 3750 4000

Wave number (cm-1) Fig. 3. FTIR spectra of raw and plasma processed bovine bone at 90 and 120 s.

elevated temperatures [11]. There are two mechanisms for the decomposition as follows:

Ca10 ðPO4 Þ6 ðOHÞ2 52b  Ca3 ðPO4 Þ2 þCa4 P2 O9 þ [H2 O;

HA (chemical)

Endobon (natural)

1.67

1.92

e e

0.576 0.287

reveal the presence of the carbonate group in HA molecule. On the other hand the presence of proteins in the raw bovine bone sample is mainly indicated by the broad peak at the spectral area 1540e1570 cm1 that is attributed to CeN stretching vibration and NeH bending vibration (amide II) and the peak at 1660 cm1 corresponding to C]O stretching vibration (amide I). It can be clearly noticed from the FTIR pattern that the protein peaks i.e. amide II peaks at 1540e1570 cm1 and amide I peak at 1660 cm1 corresponding to C]O stretching are found in the spectra of the raw bovine bone indicating the presence of organic materials in the raw bovine bone sample [14]. These two bands are characteristics of macromolecules of protein in the bovine bone matrix.

Ca10 ðPO4 Þ6 ðOHÞ2 53b  Ca3 ðPO4 Þ2 þCaO þ [H2 O; Thus as the melting time is increased to 120 s, the characteristic peaks for TCP, TTCP and CaO was observed. The present XRD results suggest that the HA stability was not disrupted by TAP processing up to 90 s. 3.3. Fourier transformed infrared spectroscopic (FTIR) analysis The FTIR spectra of raw and plasma processed bovine bone samples for 90 and 120 s is shown in Fig. 3. From the FTIR spectra in general it was noticed that a large number of bands in all the spectra’s corresponded to the standard spectral range (3571e3572 cm1, 3422.3e3425.1 cm1, 2072 cm1, 2002 cm1, 1411e1457 cm1, 1044e1049 cm1, 959e962 cm1, 631 cm1, 601e601 cm1, 568e571 cm1, 472e473 cm1 and 425e426 cm1) and matches with the bands in the HA reference spectrum and are in close agreement with the reported data [12,13]. At a material level, bovine bone is composed of organic and inorganic components. The organic part contains mainly proteins, whereas the inorganic part is mainly biological, namely carbonated hydroxyapatite (HCAp). The participation of poor crystallized HCAp in the raw bovine bone sample is indicated by the broad peak at 560e604 cm1 and the peaks at the spectral area 1020e1110 cm1. Moreover, the peak at 1420e1470 cm1 and the peak at 870 cm1

Table 3 The EDX elemental percentages of raw and TAP processed samples. Percentage of elements with processing time Element

Bovine bone

60 s

90 s

120 s

O Na Mg Al Si P S Cl K Ca

59.21 1.00 0.63 0.06 0.07 13.21 0.02 0.13 0.05 26.52

54.54 2.02 0.72 0.07 0.01 15.17 0.25 0.06 0.04 27.61

48.31 0.48 0.70 0.02 0.02 17.19 0.11 0.01 0.04 33.22

70.01 0.03 0.29 0.12 0.14 5.11 0.07 0.09 0.02 24.21 Fig. 4. TG-DTA of raw and 90 s TAP processed samples.

The FTIR spectra of samples produced for 90 s and 120 s present the characteristic bands of pure HA, namely the peak at 961 cm1 and the double peak at 1050 and 1090 cm1 that are attributed to the stretching vibrational mode of phosphate (PO4) group and the double peak at 570 and 602 cm1 that is attributed to the bending PO4 group. Moreover, the pronounced peaks at 632 and 3570 cm1 are due to the hydroxyl group (OH), while the low intensity double peak at 1410 and 1456 cm1 and the shoulder peak at 870 cm1 correspond to the symmetric and asymmetric vibrational mode of carbonate (CO3) group. The amide I and II peaks found in the case of raw bovine bone sample were absent in the spectra of 90 s and 120 s TAP processed samples. In general, the FTIR spectra indicated the presence of phosphate (PO4), hydroxyl (OH) and carbonate (CO3) ions in the TAP processed samples. With increasing processing time, the PO4 bands of HA located at 961 cm1 and the double peak at 1050 and 1090 cm1 gradually increased. Additionally, the FTIR spectra of samples exhibited a pronounce peak at 632 and 3570 cm1 due to the presence of hydroxyl group. As the processing time is increased, the band of OH at 3570 cm1 gradually increases in its intensity. This can be attributed to the increase in HA crystallinity with increasing the processing time as observed from the XRD patterns of these samples. 3.4. Energy dispersive x-ray spectroscopic (EDX) analysis

Fig. 5. SEM images of raw bovine bone at different magnifications.

The EDX analysis results indicated that the inorganic phases of all the samples were mainly composed of Ca, P and O as the major constituents with some minor components like Na, Mg, Al, Si, S, Cl and K. These are expected results, as these are the chemical elements that which are normally present in the bovine bone. Normally bovine bone composition should contain Ca, P and O including magnesium, sodium, potassium and carbonate salts [15,16]. The Table 3 displays the EDX quantitative analysis of the raw bovine bone and the TAP processed samples. The mean molar Ca/P ratio of the as-received bovine bone was found to be 2.03, while for the samples processed by TAP processing for 120 s is 4.73. In the case of the 120 s processed sample there was a significant variation of the measurements indicating that Ca/P ratio varied from 1.44 to 15.7 which indicates the lack of homogeneity of the sample. The Ca/P ratio of the samples produced for 60 and 90 is fairly higher than the stoichiometric value of 1.67. The Ca/P ratio of the samples produced for 60 s was 1.82, while Ca/P ratio of the samples produced for 90 s was 1.93.The variation in the Ca/P ratio may be attributed to the effect of the varying TAP processing time. The varying Ca/P ratio of the 120 s TAP processed sample suggests that the processing time was too high and which causes deviation from the stoichiometric value of HA. This result is in accordance with the XRD analysis that suggests that 120 s TAP processing causes development of other phases such as TCP, TTCP and CaO due to the thermal decomposition of HA. The obtained EDX results

Fig. 6. SEM images of 30 s TAP processed sample at different magnifications.

C.P. Yoganand et al. / Current Applied Physics 11 (2011) 702e709

Fig. 7. SEM image of (a) 45 s and (b) 60 s TAP processed sample.

Fig. 8. SEM image of (a) 90 s and (b) 120 s TAP processed sample.

Fig. 9. (a,b) SEM images with (c) EDX of 90 s TAP processed sample.

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suggesting that all organic materials are completely removed. The DTA curves of HA processed by TAP method for 90 s (Fig. 4b) confirm its thermal stability, since for heating up to 1200  C, it presents no peaks and a slight change in TG curve. This may be due to the reason that the organics present in the bovine bone gets completely removed on TAP processing and ensure the formation of HA which is thermally stable up to its melting point (1843 K). HA decomposes to form other calcium phosphates at elevated temperatures into tricalcium phosphate, tetracalcium phosphate and calcium oxide [7]. For the above reason the DTA of HA produced by TAP method (90 s) confirm its thermal stability, since for heating up to 1200  C. 3.6. Scanning electron microscopy (SEM) analysis

Fig. 10. Photographic image of 90 s TAP processed sample.

suggests that 90 s of TAP processing produces HA which has a Ca/P value which fairly coincides with the reported values of Endobon HA produced from natural bone sources [17,18]. From the comparative study, it is possible to suggest that 90 s of TAP processing is the optimum processing time to produce HA from bovine bone in comparison with Endobon powder (Table 4).

3.5. Thermal analysis (TG-DTA) The mass loss and the heat flow curves of the raw and TAP processed samples are given in Fig. 4. The TG-DTA analysis of the bovine bone powder (Fig. 4a) shows a strong endothermic peak from room temperature to about 200  C followed by a mass loss of 10% corresponding to the loss of the incorporated water. A continuous weight loss is observed between 300 and about 700  C, which can be associated with the burning of organic substances. The DTA analysis shows a double endothermic peak at about 400e500  C. This may be mainly due to the decomposition of organic traces and also due release of gaseous carbon-di-oxide from carbonates present in the form of carbonated apatite in the bovine bone [1,19e21]. Above 700  C there is no significant weight loss,

Fig. 5 shows SEM images of raw bovine bone powder at different magnifications. It is observed in figure that the microstructure of raw bone is highly dense which may be due to the presence of organic components. From Fig. 5a, b it can be observed that raw bovine bone is much coarser and powder particles of uneven size distribution can be observed. Fig. 5c show much closer magnifications of the raw bovine bone which clearly depicts the highly dense microstructure. Figs. 6e8 show the SEM images of powdered TAP processed samples for 30,45,60,90 and 120 s. It can be observed from the SEM images that all samples commonly exhibited almost much similar micromorphology. From the SEM pictures it was observed that the formation of denser particles is a key feature observed for the 60, 90 and 120 s of plasma processed samples than for 30 and 45 s of processed samples. This may be due to the reason that higher TAP processing time like 60, 90 and 120 s offer stronger thermal treatment so that the sample becomes more denser than for that of 30 and 45 s processed samples. Fig. 9 shows the SEM with EDX image of the as prepared 90 s TAP processed sample. It can be observed that the surface of the sample is quite denser in appearance with a rough and uneven surface morphology. Fig. 10 shows the photograph of the as prepared 90 s TAP processed sample. Generally the TAP prepared HA samples are in bulk form as observed in Fig. 10. The as prepared bulk HA sample can be crushed and powdered using ball mill and the desired particle size range may be obtained using sieves. Fig. 11 shows the photograph of HA powder obtained after powdering the as prepared HA sample bulk. The obtained HA powder can be used for a variety of applications. Annealing method has been suggested as an alternative to obtain HA from bovine bone. Annealing method generally involves a few (5e6 h) or more hours of heat treatment of bovine bone for the production of HA [22,23]. The major drawback of the annealing process is the long time consumption. In our present work the TAP processing method was employed for the production of HA from bovine bone. The bovine bone was plasma processed for 30, 45, 60, 90 and 120 s at a constant power level of 5 kW to check the effect of processing time on HA production. The results of HA production by TAP processing of bovine bone revealed that this route would reduce the long time consumption in comparision to the annealing method. The production of HA by TAP method could be suggested as a very good time effective alternative method for the annealing method. 4. Conclusion

Fig. 11. Photograph of HA powder obtained after powdering the as prepared HA sample bulk.

The results of our study indicated that HA was obtained by TAP method in a quite considerable short time of processing. (1) It was found that both 60 and 90 s of TAP processing were found to produce HA from bovine bone. (2) However the EDX study indicated that 90 s of TAP processing was more beneficial in HA

C.P. Yoganand et al. / Current Applied Physics 11 (2011) 702e709

production with a Ca/P ratio of 1.93 which was well in accordance with the commercially available natural HA-Endobon powder. (3) Incomplete removal of the organic substances was observed for TAP processing at 30 and 45 s; whereas 120 s of TAP processing resulted in formation of tricalcium phosphate (TCP), tetracalcium phosphate (TTCP) and CaO phases. (4) The results of the investigation suggest that the plasma processing is an advantageous method in comparision to the annealing method for the production of HA. Acknowledgements The authors acknowledge Mr. Janarthanan Nair (Ion Arc Technologies Pvt. Ltd., Coimbatore) for providing the torch facilities. Mr. C.P. Yoganand acknowledges the grant of Seniour Research Fellowship (SRF) by the Council of Scientific and Industrial Research (CSIR), Government of India. References [1] D. Tadic, M. Epple, Biomaterials 25 (2004) 987e994. [2] B. Ben-Nissan, Curr. Opin. Solid State Mater. Sci. 7 (2003) 283e288. [3] Limin sun, Christopher C. Berndt, Karlis A. Gross, Ahmet kucuk, J. Biomed. Mater. Res. Appl. Biomater. 58 (2001) 570e592.

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