BIOCOMPATIBLE MATERIAL FOR IMPLANTS

Abstract: The invention relates to a biocompatible ceramic implant material for implants. Said ceramic material comprises NaxKyNbO3, 0</=x</=0.8, 0.2</=y</=1, x+y=1.

Description: Biocompatible material for implants The present invention relates to biocompatible implant materials for implants in accordance with the preamble of claim 1, and particularly to ingrowth-enhancing biocompatible materials. Background to the invention. The most commonly used rigid biocompatible implant materials are ceramic materials that may be used as bulk materials as for instance A1 2 0 3 . They also may be materials normally found as or used as layers on substrates, such as Ti0 2 , TiN (materials that are considered to be ceramic in nature) or hydroxylapatite . These materials normally are not piezoelectric and normally cannot be polarized so as to become piezoelectric. Ceramics having mechanical properties comparable to the above ceramic materials and having good piezoelectric properties, i. e. mainly polarizable ceramic ferroelectrics, normally contain components that can be suspected to be potentially harmful in the implanted state, for instance lead, as for instance in lead titanate, lead metaniobate or lead zirconate. It thus is undesirable that these materials directly or indirectly come into contact with body tissue or fluids. This is particularly important for piezo materials that are to be left in the body for a very long time. Biocompatible phosphate glass ceramics that may contain crystal phases of apatite and A1P0 4 in the tridymite and/or berlinite form are disclosed in US-A-4 , 698 , 318. Berlinite is an isotype to quartz and has inherent piezoelectric properties. It is suggested that the piezoelectric properties of the berlinite can be utilized to promote healing of bone fractures. Berlinite has relatively weak piezoelectric properties. Since berlinite only is a part of 2 the material, the overall piezoelectric properties of this material are weak. The piezoelectric properties are obtained by thermal treatments at relatively high temperatures for long time periods said to cause targeted precipitation of apatite or of apatite and AlP0 4 -crystals . The long-term stability of the material in the implanted state is not discussed, but hydroxylapatite and apatite are at least to some extent biodegradable. There are some other biocompatible materials with piezoelectric properties. These materials mostly are relatively soft piezoelectric polymers with inherent piezoelectric properties. Other materials are biocompatible polymers containing inorganic polarizable, ceramic ferroelectric particles such as lead zirconate titanate (PZT) or barium titanate. Such polymeric materials are for instance disclosed in US-A-5, 68 , 061 where they are used as membranes for enhancing bone regeneration. These materials have relatively weak piezoelectric properties and have a limited use in view of their relative softness, their potential toxicity and their poor mechanical and chemical strength. The inventive concept The object of the invention is to achieve a highly biocompatible material that has a long-term stability in the implanted state and that can be wholly or selectively polarized in order to obtain piezoelectric properties for tissue growth promoting purposes. This object is achieved by means of a biocompatible ceramic material in accordance with claim 1, an implant according to claim 10 and a method of determining the apposition of an implant surface with the adjoining tissue. Preferred embodiments are set forth in the dependent claims. Description of the appended drawings Fig 1 illustrates the biocompatibility of the materials according to the invention by means of the lactate dehydrogenase (LDH) activity as [B-B units/ml] in the culture medium after 23 and 46 h cultivation of lx 10 6 human monocytes, on a ceramic surface (Ceram) , titanium (Ti) and on tissue culture polystyrene (Ps) . Fig 2 shows the LDH activity as μkatal/1, in the culture medium after 23 and 46 h cultivation of lxlO 6 human monocytes on the same surfaces as in Fig 1. Fig 3 shows the LDH activity as [B-B units/ml] , normalized by total amount of DNA in wells after 46 h cultivation of 1x10 s human monocytes on the same surfaces as in Fig 1. Fig 4 illustrates the control of total LDH activity as [B-B units/ml], from dead human monocytes. Detailed description of preferred embodiments of the invention In a preferred embodiment of the invention, the material consists of Na 0 . 5 o. 5 Nb0 3 . This material surprisingly has been found to have excellent properties for the use in implants. In addition to this, it is possible to polarize the material in order to provide it with piezoelectric properties. The material thus combines the following properties : -a very high level of biocompatibility, -mechanical and chemical stability expected to be at least ten years, 4 -a d 33 that can exceed 100 pCN -1 , -resistivity that can exceed 10 12 Ωm, -Curie temperature > 160 °C, Additionally, the material will function as desired at - a working temperature of 36-41 °C, - a band width of 0.3 - 20 Hz. These properties obviously are well suited for use in the human body. It should be noted that the material according to the invention could be provided with piezoelectrical properties (e.g. the dielectric constant) that are better than the corresponding properties of berlinite by a factor of 100. The piezo material preferably should have a relative density of at least 97%, a maximum pore size of 4 μm and a maximum grain size of 4 μm. In some applications, other properties might be more desirable than the best piezoelectric properties. For instance, in an implant the porosity of the material might be more important than the optimal piezoelectric properties . The material may be made as a bulk material by means of the hot isostatic pressing methods using sodium carbonate, potassium carbonate and niobium pentoxide as precursors as defined in the following articles from American Ceramic Society Bulletin: Egerton - Dillon in 42(1959) pp438 - 442, Jaeger - Egerton in 45(1962) pp209 - 213 and Egerton - Bieling in 47(1968) ppll51 - 1156. The material may also be made in the form of films or layers on substrates by means of cathode sputtering methods as for instance described in Margolin et al, "(K,Na)Nb0 3 ferroelectric films synthesized by cathode sputtering", Sov. Phys. Tech. Phys . 33(12), December 1988, or by other suitable thin film techniques. The excellent biocompatible properties of the material are illustrated by the following standard toxicology test. In this test monocytes were cultured on the surface of (Na,K)Nb0 3 (Ceram) , and on two control surfaces titanium(Ti) and Polystyrene (Ps) . The cytoplasmic enzyme lactate dehydrogenase (LDH) is detected extracellularly and is an indicator of the degree of injury inflicted to cells. Measurements of LDH were performed on the cellular medium after 23h and after 46h. Cells can be activated and deteriorated not only by direct material interactions but also by soluble factors, including chemicals released from the material, so therefore the study also examined if there were any bacterial products (endotoxin) present in the culture medium before and after the test. The result show that human monocytes in terms of viability are not negatively affected during culture up to 46 hours by the presence of (Na,K)Nb0 3 . Fig 1 thus illustrates the biocompatibility of the piezomaterials according to the invention by means of the lactate dehydrogenase (LDH) activity as [B-B units/ml] in 6 the culture medium after 23 and 46 h cultivation of lx 10 6 human monocytes, on a ceramic surface (Ceram) , on titanium (Ti) and on tissue culture polystyrene (Ps) . The cells were from two separate blood donors, exp. 1 respectively exp.2. The values in the diagram are mean values, the standard error mean (SEM) also being indicated. Fig 2 shows the LDH activity as μkatal/1, in the culture medium after 23 and 46 h cultivation of lxlO 6 human monocytes, on a ceramic surface, titanium and on tissue culture polystyrene. Again cells from two separate blood donors were used, exp. 1 respectively exp.2. Normal levels in human serum are 8 μkatal/1. The values are mean values ± SEM. Fig 3 shows the LDH activity as [B-B units/ml] , normalized by total amount of DNA in wells after 46 h cultivation of lxlO 6 human monocytes on a ceramic surface, titanium and on tissue culture polystyrene. Cells from two separate blood donors, exp. 1 respectively exp. 2. The values are mean values ± SEM. Fig 4 illustrates the control of total LDH activity as [B-B units/ml], from dead human monocytes by adding 2% Triton X100 to lxlO 5 , lxlO 6 and 2xl0 6 human monocytes. The LDH activity between 500 and 700 corresponds approximately to 0.5-1.5xl0 5 dead cells. As is evident from these diagrams, the ceramic material according to the invention has a biocompatibility that closely follows that of titanium. In view of the fact that titanium has proved to be one of the most biocompatible materials and in view of the doubtful biocompatibility of known polarizable ceramic ferroelectrics, this is very unexpected in a ceramic ferroelectric. 7 Although the test relates to Na 0 . 5 K 0 . 5 NbO 3 , it obviously also is valid for niobates with other proportions of Na and K. potassium-niobates . The material according to the invention can be used in several ways in the medical field. The material as such can be used as a rigid implant material in view of its strength and durability either in bulk form or in the form of films or layers covering other materials . The material may be polarized in bulk form or in film form in order to achieve piezoelectric properties that can be utilized in order to stimulate the regeneration or growth of the tissue in the vicinity of the material by electric stimulation. In these applications the material also can be used in particulate form in a polymer matrix in view of its excellent biocompatibility since it would not matter if the material directly or indirectly would come into contact with body tissue. One important advantage of the material according to the invention is that it can be polarized selectively. This means, for instance, that it is possible to obtain implants that have piezoelectric properties in specified locations only and/or have piezoelectric properties varying from weak to strong in dependence on where and to what degree the tissue growth or generation is to be promoted. As with all ceramic ferroelectrics, the piezoelectric properties can be made directional in dependence of the direction of the polarizing field. 8 Bone implants could for instance be provided with a certain degree of tissue growth promoting capability in one part of the adjoining bone tissue, e. g. the compact bone, and with a different tissue growth promoting capability in another part of the adjoining bone tissue, for instance the cancellous bone. The tissue growth promoting capability alternatively could be matched locally to the local rigidity so as to obtain a constant tissue growth promoting capability along the entire implant surface even if the forces transferred from adjoining tissue varies. Conversely, the implant material can be used in order to give an indication of the size of the forces transferred from an implant to the adjoining tissue when the implant is loaded. The forces exerted on the piezoelectric material will cause an electrical signal that directly corresponds to the totality of the forces actually acting on the implant. A measurement of this signal can be compared with a theoretical value calculated from the nature of the surrounding tissue, from the geometrical shape of the implant and from the way forces ideally would be transferred from a body with this geometrical shape to a surrounding media. This comparison thus would give an indication whether, and to what extent, the implant actually does engage the adjoining tissue so as to transfer forces . One use would be the clinical testing of different bone implant designs, for instance dental implant designs. The above comparison would give a value of the actual bone apposition obtained by a specific design. 9 Since the overall values of the forces transferred are used, the above comparisons only would be mean values, i. e. the comparison would not say anything about the local conditions. A development of this principle thus would be to provide the implant with several separate areas with piezoelectric properties and individually measuring the signal generated from each separate area, thus determining for instance the bone apposition in each separate area. By these means, specific design features of the implant can be evaluated. The above method could also be used for permanent implants for determining for instance the bone apposition after some time and for following up the long-term fixation of the implant. One skilled in the art will appreciate that the present invention can be practiced in other ways than by means of the described embodiments, which are presented for purposes of illustration. The present invention is limited only by the claims that follow.

Claims: 10 CLAIMS 1. Biocompatible ceramic implant material for implants characterized in that said ceramic material comprises Na x K y Nbθ3, 0<x<0.8 , 0.2≤y≤l, x+y=l . 2. Ceramic material according to any of the preceding claims, characterized in that 0.2 < x < 0.8 and 0.2 < y < 0.8. 3. Ceramic material according to claim 2, characterized in that 0.4 < x < 0.6 and 0.4 < y < 0.6. 4. Ceramic material according to claim 5, characterized in that x = y = 0.5. 5. Ceramic material according to any one of the preceding claims, characterized in that the relative density of the piezo material exceeds 97 %. 6. Ceramic material according to any one of the preceding claims, characterized in that the pore size of the piezo material is less than 4 μm. 7. Ceramic material according to any one of the preceding claims, characterized in that the grain size of the piezo material is less than 4 μm. 8. Ceramic material according to any one of the preceding claims, characterized in that the piezoelectric constant d 33 exceeds 100 pCN -1 . 9. Implant or part of implant system including a ceramic material according to any one of the preceding claims . 11 10. Implant or part of implant system according to claim 9, characterized in that the bulk of said implant or part is made of said ceramic material. 11. Implant or part of implant system according to claim 9, characterized in that said ceramic material is provided as a film or layer on an implant body or part comprising a different material than said ceramic material. 12. Implant or part of implant system according to claim 11 - 12, characterized in that said ceramic material has been wholly or partly polarized in order to provide all the material or parts thereof with piezoelectric properties. 13. Implant or part of implant system according to any one of claims 9 - 11, characterized in that said ceramic material has been selectively polarized in order to provide said implant with varying piezoelectric properties. 14. Method of determining the apposition of an implant according to any one of claims 12 - 13 with adjoining tissue, characterized in that the electric signal or signals from the respective piezoelectric regions caused by the interaction of said implant and the adjoining tissue when said implant is loaded are measured.

Inventors: KJELLMAN CHARLOTTE, LIDMAN JOHAN, LJUNGSTROEM KARIN, NILSSON KENTH, KJELLMAN, CHARLOTTE, LIDMAN, JOHAN, LJUNGSTROEM, KARIN, NILSSON, KENTH
Applicants: KJELLMAN CHARLOTTE, LIDMAN JOHAN, LJUNGSTROEM KARIN, NILSSON KENTH, PACESETTER AB, KJELLMAN, CHARLOTTE, LIDMAN, JOHAN, LJUNGSTROEM, KARIN, NILSSON, KENTH
Application year: 1999
Main classification: C04B35
IPC codes: A61L31, A61N1, C04B35