Elsevier

Materials Letters

Volume 62, Issues 6–7, 15 March 2008, Pages 1055-1057
Materials Letters

Hydroxyapatite foams produced by gelcasting using agarose

https://doi.org/10.1016/j.matlet.2007.07.043Get rights and content

Abstract

There is a technological need for porous hydroxyapatite (HA) to be produced in an environmentally friendly manner. Gelcasting of hydroxyapatite foams using agarose as gelling agent was evaluated and the rheological behavior of the suspensions was studied. The viscosity of the slurries could be adjusted by agarose concentration and HA solid loading. These parameters were essential in tailoring the porosity as well as the cell and window sizes of the resulted HA foams. Depending of HA solid loading (24–29 vol.%) and agarose concentration (1.1–1.5 wt.% with regard to water) in the starting slurry, the mean cell size ranged from 130 to 380 μm, while the mean window size varied from 37 to 104 μm. Depending on the porosity range (73–92%) and related with this parameter the mean cell and window size, the compressive strength of HA foams was found to be in the range 0.8–5.9 MPa.

Introduction

Porous hydroxyapatite (HA) and related calcium phosphate ceramic materials with open pore structures are of particular interest, since they have excellent permeability and a large surface area as well as excellent biocompatibility [1], [2]. These properties allow them to be used in biomedical engineering applications, such as bone scaffolds and drug carriers [2].

Several technologies exist today for manufacturing porous bioceramics. Important advantages have been found in the production of macroporous ceramics by the gelcasting of foams. This method was developed as a result of the association of the gelcasting process and the aeration of the ceramic suspensions containing foaming agents and gelling agents [3], [4]. In the early gelcasting systems developed, acrylamide monomers were used as gelling agents in manufacturing of HA foams [4]. In order to avoid the health hazard problems with monomers, other gelling systems were evaluated for the gelcasting of HA foams. Polymers such as polyethyleneimine crosslinked by polyfunctional epoxy compound [5], modified or naturally occurred biopolymers such as methyl cellulose [6], [7], albumin [8] and starch [9] have been used as gelling agents in manufacturing of HA foams. Methyl cellulose, albumin and starch form gels on heating. Other gelling agents such as agaroids create gels on cooling. These polysaccharides were evaluated for the gelcasting of alumina [10] and silicon carbide [11] foams.

In this paper agarose was studied for the gelcasting process of HA foams. Particular attention was paid to the viscosity of starting slurry, which resulted in changes in the cell and window sizes of the hydroxyapatite foams.

Section snippets

Experimental procedure

Commercial hydroxyapatite (HA) powder (Ca10(PO4)6(OH)2, Merck, Darmstadt, Germany) was selected as the bioactive material. The as-received HA powder was calcined at 900 °C for 1 h in order to decrease the specific surface area from 65.5 m2/g to 14.4 m2/g (BET, Quantachrome Instruments). Ceramic suspensions were prepared to a solid loading of 39 vol.% of HA by dispersing them with 0.64 wt.% of dispersant (Darvan 811, R.T. Vanderbilt, U.S.A.). Homogenisation was carried out by ball milling.

Results and discussion

Fig. 1 shows the influence of hydroxyapatite solid loading and agarose concentration on the viscosity of slurries at 60 °C. Both agarose concentration and hydroxyapatite solid loading affected the viscosity. For foaming process, the slurry constituents should not increase the suspensions viscosity to a level that would limit the production of low-density foams. On the other hand, the amount of agarose in the liquid phase of the slurry should be sufficient to create strong gel that would

Conclusions

Gelcasting with agarose as a gelling agent was developed for manufacturing of HA foams. The microstructure of HA foams was typically composed of approximately spherical cells interconnected by circular windows. The total volume fraction of porosity as well as the cell and window size could be adjusted by agarose concentration and HA solid loading in the starting slurry. Depending of HA solid loading (24–29 vol.%) and agarose concentration (1.1–1.5 wt.% with regard to water) in the starting

Acknowledgements

R.T. Vanderbilt Company Inc. for providing free samples of Darvan 811 is gratefully acknowledged. This work was supported by Grant No. 3 T09B 048 28 of Polish Ministry of Higher Education and Science.

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    Moreover, it is found that environmentally friendly biopolymers can be applied to fabricate ceramics with gelcasting technique [7]. For example, Potoczek et al. [8] prepared hydroxyapatite foams using gelcasting method with agarose as gel former. Fey et al. [9] also successfully fabricated alumina ceramics with agarose as gelling agent.

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