Freeze casting of hydroxyapatite scaffolds for bone tissue engineering
Introduction
The requirements for a synthetic bone substitute appear deceptively simple, that is, to supply a porous matrix with interconnecting porosity that promotes rapid bone ingrowth, yet possesses sufficient strength to prevent crushing under physiological loads during integration and healing. The ideal bone substitute is not a material that interacts as little as possible with the surrounding tissues, but one that will form a secure bond with the tissues by allowing, and even encouraging new cells to grow and penetrate. One way to achieve this is to use a material that is osteophilic and porous, so that new tissue, and ultimately new bone, can be induced to grow into the pores and help prevent loosening and movement of the implant. Resorbable bone replacements have been developed from inorganic materials that are very similar to the apatite composition of natural bone [1].
In recent years, considerable attention has been given to the development of fabrication methods to prepare porous ceramic scaffolds for osseous tissue regeneration [2], [3], [4], [5], [6], [7], [8], [9]. The ideal fabrication technique should produce complex-shaped scaffolds with controlled pore size, shape and orientation in a reliable and economical way. However, all porous materials have a common limitation: the inherent lack of strength associated with porosity. Hence, their application tends to be limited to low-stress locations, such as broken jaws or fractured skulls. Therefore, the unresolved dilemma is how to design and create a scaffold that is both porous and strong.
Freeze casting is a simple technique to produce porous complex-shaped ceramic or polymeric parts [10]. In freeze casting, a ceramic slurry is poured into a mold and then frozen. The frozen solvent acts temporarily as a binder to hold the part together for demolding. Subsequently, the part is subject to freeze drying to sublimate the solvent under vacuum, avoiding the drying stresses and shrinkage that may lead to cracks and warping during normal drying. After drying, the compacts are sintered in order to fabricate a porous material with improved strength, stiffness and desired porosity. The result is a scaffold with a complex and often anisotropic porous microstructure generated during freezing. By controlling the growth direction of the ice crystals, it is possible to impose a preferential orientation for the porosity in the final material [11].
The technique was applied more specifically to polymeric materials, for tissue engineering. A wide variety of materials have already been investigated, including chitin [12], gelatin [11], [13], collagen [14], PLA [15], [16], PDLLA [15], [17], PLGA [15], [17], poly(HEMA) [18], agarose [19], sericin [20] and alginate [16], [21], [22], [23], [24]. Although not stiff and strong enough for load-bearing applications, all these materials have in common an homogeneous structure with open porosity, favorable for rapid cell proliferation. In particular, pore size and its structure can be controlled by heat transfer rate.
We show here how freeze casting can be applied to hydroxyapatite (HAP), an osteophilic ceramic related to the inorganic component of bone, to process bone substitute materials with suitable physical and mechanical properties. In particular, we describe here how the processing conditions (concentration, freezing rate, sintering) affects the scaffold characteristics (size and amount of porosity, compressive strength) and discuss the limits of the technique.
Section snippets
Experimental techniques
The porous inorganic scaffolds were produced by controlled freezing of HAP slurries. Slurries were prepared by mixing distilled water with a small amount (typically 1 wt%) of ammonium polymethacrylate anionic dispersant (Darvan 811, R. T. Vanderbilt Co., Norwalk, CT), an organic binder (1 wt%, polyvinyl alcohol) and the HAP powder in various content (Hydroxyapatite#30, Trans-Tech, Adamstown, MD), depending on the desired total porosity. Slurries were ball-milled for 20 h with alumina balls and
Results
By directional freezing of the slurry, we force the particles in suspension to be rejected from the moving ice front and piled up between the growing columnar ice crystals [2]. Afterwards, the ice is sublimated by freeze drying, such that a ceramic scaffold whose microstructure is a negative replica of the ice is produced. The porosity of the sintered materials is a replica of the ice structure before drying.
Control of the microstructure and the physics of ice formation
The final microstructure is a replica of the ice. Modifying the amount of water in the slurry as well as the shape of the ice crystals will modify the final amount, shape and size of porosity of the porous ceramic scaffold. In particular, during the steady freezing regime, the ice crystals exhibit a homogeneous morphology throughout the whole sample; this explains why the lamellae thickness is very homogeneous throughout the whole sample. This also means that the ratio porosity thickness/layer
Conclusions
Based on an experimental study of freeze drying of hydroxyapatite powders with various slurry concentrations and sintering conditions, the following conclusions can be made:
- (1)
Porous scaffolds with total porosity ranging from at least 40% to 65% can be obtained by freezing of hydroxyapatite aqueous suspensions and subsequent ice sublimation and sintering. The resultant porosity is open and unidirectional, exhibiting a lamellar morphology. Size of the porosity can be controlled by modifying the
Acknowledgments
This work was supported by the National Institute of Health (NIH/NIDCR) under Grant no. 5R01 DE015633 (Novel Scaffolds for Tissue Engineering and Bone-Like Composites).
References (61)
- et al.
Fabrication of low temperature macroporous hydroxyapatite scaffolds by foaming and hydrolysis of an alpha-TCP paste
Biomaterials
(2004) - et al.
Hydroxyapatite cement scaffolds with controlled macroporosity: fabrication protocol and mechanical properties
Biomaterials
(2003) - et al.
A new method to produce macropores in calcium phosphate cements
Biomaterials
(2002) - et al.
Preparation and properties of dense and porous calcium phosphate
Ceram Int
(1999) - et al.
Preparation of porous hydroxyapatite scaffolds by combination of the gel-casting and polymer sponge methods
Biomaterials
(2003) - et al.
Fabrication of porous gelatin scaffolds for tissue engineering
Biomaterials
(1999) - et al.
Porous chitosan scaffolds for tissue engineering
Biomaterials
(1999) - et al.
In vitro characterization of chitosan–gelatin scaffolds for tissue engineering
Biomaterials
(2005) - et al.
Dendritic ice morphology in unidirectionally solidified collagen suspensions
J Cryst Growth
(2000) - et al.
Preparation of porous scaffolds by using freeze-extraction and freeze-gelation methods
Biomaterials
(2004)
Porous poly([alpha]-hydroxyacid)/Bioglass(R) composite scaffolds for bone tissue engineering. I: preparation and in vitro characterisation
Biomaterials
Macroporous hydrogels for biomedical applications: methodology and morphology
Biomaterials
The fabrication and characterization of linearly oriented nerve guidance scaffolds for spinal cord injury
Biomaterials
Novel alginate sponges for cell culture and transplantation
Biomaterials
Tailoring the pore architecture in 3-D alginate scaffolds by controlling the freezing regime during fabrication
Biomaterials
Preparation of alginate/galactosylated chitosan scaffold for hepatocyte attachment
Biomaterials
HA/alginate hybrid composites prepared through bio-inspired nucleation
Acta Biomater
Carbonated hydroxyapatite as bone substitute
J Eur Ceram Soc
Influence of porosity and pore size on the compressive strength of porous hydroxyapatite ceramic
Ceram Int
Biphasic calcium phosphate nanocomposite porous scaffolds for load-bearing bone tissue engineering
Biomaterials
Cellular biocompatibility and resistance to compression of macroporous beta-tricalcium phosphate ceramics
Biomaterials
Porous calcium polyphosphate scaffolds for bone substitute applications—in vitro characterization
Biomaterials
Preparation of macroporous calcium phosphate cement tissue engineering scaffold
Biomaterials
A synthetic bone implant macroscopically identical to cancellous bone
Biomaterials
Mechanical and in vivo performance of hydroxyapatite implants with controlled architectures
Biomaterials
Slow crack growth behaviour of hydroxyapatite ceramics
Biomaterials
Interaction of particles and a moving ice-liquid interface
J Cryst Growth
Osteoconduction at porous hydroxyapatite with various pore configurations
Biomaterials
Porous calcium polyphosphate scaffolds for bone substitute applications—in vitro characterization
Biomaterials
Nanotopographical guidance of C6 glioma cell alignment and oriented growth
Biomaterials
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