Porous ceramic body

Details Porous ceramic body Porous ceramic body

2000-12-15

2002-11-12

Koslow, C. Melissa (Department: 1755)

Compositions: ceramic

Ceramic compositions

Pore-forming

C501S082000, C501S083000, C106S035000, C623S023560

Reexamination Certificate

active

06479418

ABSTRACT:

The invention relates to a method of preparing a porous ceramic body and to a body obtainable by said method. The invention further relates to the use of said body as a scaffold in tissue engineering.
Regeneration of skeletal tissues has been recognized as a new means for reconstruction of skeletal defects arising from abnormal development, trauma, tumors and other conditions requiring surgical intervention. Autologous bone grafting is considered the gold standard of bone transplantation with superior biological outcomes. However, autologous bone stocks are limited and often insufficient, particularly when large skeletal defects are encountered. Allografts are used as alternative materials but are associated with immunologically mediated complications and risks of disease transmission. Additional disadvantages of autograft and allograft materials include their limited potential for molding or shaping to achieve an optimum fit with bone voids.
As surgical techniques and medical knowledge continue to advance, there is an increasing demand for synthetic bone replacement materials, resulting from the limited supply of autograft materials and the health risks associated with the use of allografts. Hydroxyapatite has been investigated for use in the osseous environment for over 20 years, and the biocompatibility of the ceramic and its osteoconductive behavior is well established. Since porous HA is more resorbable and more osteoconductive than dense HA, there is an increasing interest in the development of synthetic porous hydroxyapatite (HA) bone replacement materials for the filling of both load-bearing and non-load-bearing osseous defects. Such technology could have the potential for restoration of vascularization and complete penetration of osseous tissue throughout the repair site.
Variation of the scaffold design as three-dimensional superstructures has been demonstrated as an approach to optimize the functionality of bone regeneration materials so that these materials may be custom designed for specific orthopedic applications in the form of void fillers, implants, or implant coatings. In attempt to develop a skeletal cell and tissue carrier, which could provide optimal spatial conditions for cell migration and maintenance by the arrangement of structural elements such as pores and fibers, the feasibility of using “live” material is under investigation. Such live material could take the form of an open-porous implant system together with living tissue. In other words, this is so-called hard tissue engineering.
The most traditional way of preparing a porous HA ceramic is to use a foaming agent like hydrogen peroxide (H
2
O
2
). In detail, an HA slurry is made by mixing HA powder with water and a H
2
O
2
solution. Then, samples of the slurry are put in oven, under elevated temperature. H
2
O
2
decomposes and O
2
is released from the bulk material, leaving a porous structure. Until today, this technique is still widely used in both clinical applications and research areas. However, porous ceramics made by this H
2
O
2
method has an intrinsic shortcoming: it possesses only “laminar porosity”. In other words, the pores are interconnected mostly in a laminar way, so there is no truly three-dimensional interconnected structure.
Slip-casting is another way of synthesizing porous ceramics. The manufacturing route comprises preparing an HA slurry (slip) by mixing HA powder under stirring with water, a deflocculant and binder agents. In this slurry, a kind of foam (sponge) is immersed and pressed. As a result, the slurry will be sucked into the foam. A layer of ceramic will be coated on all the struts of the sponge after removing the extra slurry by squeezing the samples. Then the samples will be dried in microwave oven and finally sintered in a furnace. This method is often referred to as a positive replica method.
Slip-casted materials are highly porous; they have a reticulate structure. However, due to the inner flaws in the ceramic, which are left after the sponge is burnt off, the strength of the material can not be increased to meet the requirement of tissue engineering application.
Meanwhile, coral HA has gained a wide interest in biomaterial spheres. An example of such a material is Interpore®, which has a high porosity and excellent microporous surface structure. However, it is an expensive material and, more importantly, its mechanical strength is insufficient for tissue engineering applications.
In summary, there are several known methods of preparing porous ceramics. However, there are specific requirements for porous ceramic which are to be used for skeletal regeneration, hard tissue repairing, and even for hard tissue engineering purpose. For bony tissue ingrowth, it is accepted that the pore size should be in the range of 100 to 300 microns, and the pores should be fully interconnected. This specification gives rise to a desire for a more suitable porous ceramic (tissue carrier or 3-D scaffold).
The present invention provides an improved method for preparing a porous ceramic body. The method leads to a ceramic body having interconnected pores of a controllable pore size. Furthermore, the mechanical properties of the ceramic body are superior to those of ceramic bodies prepared by the above discussed, known methods. Particularly its compressive strength is much higher.
A process for preparing a porous ceramic body according to the invention is based on a negative replica method. More in detail, the present method comprises the steps of:
1) preparing an aqueous slurry of a ceramic material;
2) mixing the slurry with a liquid, viscous organic phase to obtain a dough, wherein the organic phase is substantially insoluble in water, and is thermally decomposable into gaseous residues;
3) drying the dough; and
4) removing the organic phase by thermal decomposition.
The features of various preferred embodiments are discussed below with reference to the accompanying figures in which:
FIG. 1
shows the heating profile used in one embodiment to heat and sinter a body produced in accordance with a preferred embodiment.
FIG. 2
shows a microscopic photograph (magnification of 26.4) of a body produced in accordance with the embodiment of FIG.
1
.
FIG. 3
shows a microscopic photograph (magnification of 1250) of a body produced in accordance with the embodiment of FIG.
1
.
FIG. 4
shows a microscopic photograph (magnification of 2500) of a body produced in accordance with the embodiment of FIG.
1
.
As has been mentioned, the present method has the advantage that a ceramic body is obtained which has a porous structure consisting of interconnected pores. Moreover, a particularly high porosity can be achieved while maintaining superior mechanical properties.
In addition, it has been found that a ceramic body obtainable by the present method possesses a specific microporous surface inside the macropores. In other words, the surface of the ceramic body, including the surface within the pores, has a certain advantageous rugosity. By virtue of this feature, a good attachment of cells is obtained when cells are seeded onto the body, e.g. in tissue engineering applications. Also, by virtue of this feature the ceramic body may give rise to osteoinduction. The above described ceramic materials prepared by a slip-casting method have been found not to have this feature.
The ceramic material of which the slurry is prepared can in principle be any material of which it is desired to prepare a porous body of. In other words, the choice for a particular ceramic material will depend on the objective application of the final product. In view of the envisaged application of the porous body as a scaffold in tissue engineering, in accordance with the invention it is preferred that the ceramic material is a calcium phosphate. Highly preferred calcium phosphates may be chosen from the group of octacalcium phosphate, apatites, such as hydroxyapatite and carbonate apatite, whitlockites, &agr;-tricalcium phosphate, &bgr;-tricalcium phosphate, sodium calcium phosphate, and combinations thereof.
Under certain

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