Processing and mechanical properties of hydroxyapatite pieces obtained by the gelcasting method

https://doi.org/10.1016/j.jeurceramsoc.2004.02.017Get rights and content

Abstract

The aim of this work was to prepare hydroxyapatite (HA) pieces, by the gelcasting method, with appropriate mechanical properties for using as bone implants. The mechanical properties of the obtained pieces were evaluated. The influence of the solid content on the rheological behaviour of the slurries and, in turn, on the mechanical properties, was investigated. Pieces were prepared by using slurries containing 55 and 60 vol.% of an HA calcined at 1200 °C and 47 vol.% of HA calcined at 1100 °C. HA powders calcined at lower temperature than 1100 °C were not suitable for producing concentrated slurries. The raw and calcined HA were characterised by XRD, FTIR, X-ray fluorescence and elemental analysis. The pieces sintered at different temperatures and times were characterised by XRD, and the sintering treatment applied to the specimens for mechanical testing (1300 °C for 24 h) was selected taking into account the phases present after sintering.

The mechanical properties of the green bodies were higher than obtained by other methods and the ones of the final pieces were comparable to those obtained by other techniques. The results indicated that slurries with a solid content higher than a limit value (for which limited contraction occurs) must be used and that the rheological properties of the slurries play an important role in the mechanical properties of the resulting pieces.

Introduction

Hydroxyapatite (HA) is the main inorganic component of the hard tissues (bone and teeth) of vertebrate animals and humans. For this reason HA has received considerable attention over the past three decades as an implant material showing excellent biocompatibility.1., 2. Many efforts have been made in order to improve the mechanical properties of HA and several forming techniques have been used. In addition to common pressing method (uniaxial,3 hot pressing,4 hot isostatic pressing5), other methods have been proposed, such as slip-casting,6., 7. tape-casting,8., 9. injection moulding10 and viscous plastic processing.11

Colloidal processing offers the potential to produce reliably ceramic films and bulk forms through careful control of the initial suspension “structure” and its evolution during fabrication.12 Among these methods, the gelcasting has significant advantages over the other processes, in terms of dimensional accuracy and complex shaping capabilities, the uniform structure and high strength of the green bodies, simplicity, as well reduced manufacturing cost. Therefore, the gelcasting method would be very useful to obtain pieces of HA with complex shapes, as it is required for clinical applications.

The gelcasting process is based on the gelling, by in situ polymerisation, of a concentrated ceramic slurry suspended in a monomeric solution. The as-formed organic network encapsulates the ceramic particles, producing very uniform green bodies with high strength. This method differs from slip-casting, which has been used for complex forming of HA, in the solidification step. In the former, the solidification occurs via gelification and in the latter it is induced by fluid removal: the fluid flows into a porous gypsum mould via capillary-driven transport. Therefore, slip-casting requires rather long process times and the strength of the green bodies is weak. In addition, the density gradient in thick green bodies become significant. The notable merit of gelcasting, compared to slip-casting, is that the green bodies are uniform and strong.

Gelcasting has rapidly developed in the past decade as a promising colloidal in situ forming technique. It has been utilised in the forming of many sorts of ceramic material systems.13., 14., 15., 16., 17., 18., 19., 20., 21., 22., 23., 24. However, few reports about the use of this method for HA shaping are found in the literature.25 In that work25 a 38 vol.% suspension was prepared and by using a foaming agent porous ceramics were obtained. One of the reasons why this method has not been used to prepare HA ceramics seems to be the difficulty of preparing HA slurries with a high solid content (>50 vol.%), which is one of the requirements. In most of the works dealing with the preparation of HA suspensions, only solid contents equal29 or lower7., 26., 27., 28. than 50 vol.% have been achieved.

In order to prepare HA suspensions with a solid content higher than 50 vol.%, different factors affecting the colloidal stability of HA slurries were studied in a previous work.30 The parameters studied were the calcination temperature of the HA powder, its particle size and distribution, the type and concentration of dispersant, the pH of the dispersing medium, the mixing time and solid concentration. Slurries with a concentration of solids as high as 60 vol.% were obtained by using an HA calcined at 1200 °C, dry milled for 20 h and using Darvan 811 as dispersant. The decrease of the surface area and porosity of the HA powders with the calcination temperature reduced significantly the viscosity of the slurries and, therefore, suspensions with a solid content higher than 50 vol.% could only be obtained with HA calcined at 1200 °C.

The aim of this work was to evaluate the mechanical properties of the pieces obtained by the gelcasting method and to study the influence of the solid content on the rheological behaviour of the slurries and, in turn, on the mechanical properties. Pieces were obtained from slurries containing 55 and 60 vol.% of an HA calcined at 1200 °C. On the other hand, considering that the sintering process could not be effective enough when this HA was used, pieces from a slurry containing HA calcined at 1100 °C were also prepared, but in this case the maximum solid content that could be reached was lower.

Section snippets

HA preparation and characterisation

The synthesis of HA was carried out by the precipitation method by reaction of an aqueous slurry of Ca(OH)2 (Riedel-DeHäen) with a solution of H3PO4 (Merck) as previously described.30 The obtained powder was ground in a vibrating mill and then calcined at 1100 °C (HA-1100) or at 1200 °C (HA-1200) for 1 h. The calcined powders were dry milled for 20 h.

The initial and calcined HA were characterised by XRD, FTIR, elemental analysis and X-ray fluorescence. Phase composition was determined by XRD in a

HA characterisation

X-ray fluorescence was carried out on the raw powder finding the following elemental composition: 40.15 wt.% Ca, 18.47 wt.% P, which leads to a molar ratio Ca/P=1.68±0.04. The XRD patterns of the initial and calcined HA at 1100 and 1200 °C are shown in Fig. 2a. Raw HA and calcined powders at 1100 °C only showed maxima corresponding to HA33 while the pattern of HA calcined at 1200 °C showed in addition two maxima corresponding to CaO.34 The content of CaO as determined by the Rietveld method was 1.3 

Discussion

The HA used as raw material is a carbonatehydroxyapatite type B, where the CO3 groups are substituting the PO4 groups. The carbonate incorporation seems to be due to the carbonatation of raw Ca(OH)2 or during the ageing of the HA slurry. By thermal treatment the decomposition takes place, with the formation of CaO and CO2 (Eq. (1)), being the CaO observed (by DRX) at 1200 °C.35Ca10−x+y(PO4)6−x(CO3)x(OH)2−x+2y(s)→(1−x/6)Ca10(PO4)6(OH)2(s)+(y+2/3x)CaO(s)+xCO2(g),0≤x≤2and2y≤x

At higher temperature

Conclusions

  • 1.

    The gelcasting resulting a simple, reproducible and non-expensive method to prepare HA pieces with the complex shapes that are required in clinical applications.

  • 2.

    It was possible to prepare slurries with a high solid content (60 vol.% using HA-1200 and 47 vol.% using HA-1100).

  • 3.

    The mechanical properties of the green bodies were higher than those obtained by other methods and the ones of the ceramic pieces were comparable to those obtained by other techniques.

  • 4.

    Pieces containing 47 vol.% of HA-1100

Acknowledgements

Financial support of CICYT, Spain, through research projects MAT02-0025 and ICI-MEC/PR264/97 are acknowledged. Authors also thank C.A.I. electron microscopy and C.A.I. X-ray diffraction, UCM, for valuable technical and professional assistance, and to Ricardo Molina S.A. for kindly supply the dispersant employed in this work.

References (40)

  • M. Jarcho

    Calcium phosphate ceramics as hard tissue prosthetics

    Clin. Orthop. Rel. Res

    (1981)
  • Aoki, H., Science and Medical Applications of Hydroxyapatite. Takayama Press, Tokyo, 1991, p....
  • S.R. Levitt et al.

    Forming methods for apatite prosthesis

    J. Biomed. Mater Res

    (1969)
  • K. Uematsu et al.

    Transparent HA prepared by HIP of filter cake

    J. Am. Ceram. Soc

    (1989)
  • R. Rao et al.

    Dispersion and slip-casting of hydroxyapatite

    J. Am. Ceram. Soc

    (2001)
  • I.H. Arita et al.

    Synthesis and processing of hydroxyapatite ceramic tape casting with controlled porosity

    J. Mater. Sci.: Mater. Med

    (1995)
  • R.L. Reis et al.

    Relation between processing and mechanical properties of injection molded high molecular mass polyetilene + hydroxyapatite composites

    Mat. Res. Innovat

    (2001)
  • J.H. Shaw et al.

    Study of the application of viscous plastic processing to hydroxyapatite

    J. Mater. Sci. Lett

    (1995)
  • J.A. Lewis

    Colloidal processing of ceramics

    J. Am. Ceram. Soc

    (2000)
  • Janney, M. A., Method for Forming Ceramic Powders into Complex Shapes. U.S. Patent 4894194,...
  • Cited by (50)

    • Calcium orthophosphate (CaPO<inf>4</inf>)-based bone-graft substitutes and the special roles of octacalcium phosphate materials

      2019, Octacalcium Phosphate Biomaterials: Understanding of Bioactive Properties and Application
    • Calcium-orthophosphate-based bioactive ceramics

      2018, Fundamental Biomaterials: Ceramics
    • Calcium phosphates for biomedical applications

      2017, Boletin de la Sociedad Espanola de Ceramica y Vidrio
      Citation Excerpt :

      Processing normally consist in two steps, conformation and following sintering. Most common conformation methods include: uniaxial compaction [45], isostatic pressing [46], granulation [47], slip casting [48], gel casting [49], freeze casting [50], atomization [51], polymer replication [52], extrusion [53], low pressure injection [20], slurry, dipping or thermal spraying [54]. Also, shaping can be realized during cooling and solidification of previously melted raw materials [12].

    View all citing articles on Scopus
    View full text