HK1180979A - Implant with antimicrobial coating - Google Patents

Abstract

The invention relates to an implant with a coating (23) which releases silver ions in the human body and as a result has antimicrobial action. According to the invention, a first surface component of the coating (23) is formed by an anode material (25). A second surface component of the coating (23) is formed by a cathode material (26). The cathode material is higher in the electrochemical voltage series than the anode material (25). The cathode material (26) and the anode material (25) are connected to one another in an electrically conductive manner. Together with the body electrolyte in the environment of the implant, the anode material (25) and the cathode material (26) form a multitude of local galvanic elements. The antimicrobial action of the coating (23) is improved as a result.

Description

The invention relates to an implant with a coating that releases silver ions in the human body, thereby having an antimicrobial effect, wherein the implant is an endoprosthesis for bones and joints.

When implants are placed into the human body, there is a risk of infections. The infections can be caused by microorganisms that are introduced into the human body along with the implant or that reside on the surface of the implant. It is known that the risk of infection can be reduced by coating the implant with a material that releases silver ions into its surroundings. Silver ions are known to have an antimicrobial effect. Furthermore, they have the advantage that, if they do not encounter and act against a microorganism, they can bind with the chloride from the body's electrolytes to form AgCl, which can then be excreted from the body. Unlike other antimicrobially active substances, silver ions therefore do not accumulate in the body.

The known silver coatings release silver ions only to a limited extent. Moreover, the released silver ions move randomly in the vicinity of the implant. Therefore, there is a high probability that the silver ions will combine with chloride in the body's electrolyte to form AgCl, thereby losing their antimicrobial effectiveness before they can reach a microorganism.

The application GB2073024 describes antimicrobial surgical implants, particularly orthopedic endoprostheses, with biodegradable metallic silver.

The invention is based on the object of presenting an implant whose coating has improved antimicrobial effectiveness. Starting from the aforementioned state of the art, the object is solved by the features of claim 1. Advantageous embodiments are found in the dependent claims.

According to the invention, a first surface portion of the coating is formed from a silver-containing anode material intended for releasing silver ions. A cathode material is provided for a second surface portion. The cathode material has a higher position in the electrochemical series than the anode material. The cathode material and the anode material are electrically connected to each other.

First, some terms are explained. The term "implant" is limited to endoprostheses for bones or joints.

The terms "first" and "second surface portion" indicate that the cathode material in the coating is spatially separated from the anode material. It is not intended to refer to a coating in which multiple materials are uniformly mixed together. It is possible, but not mandatory, that the second surface portion is completely covered with the cathode material.

In the electrochemical series, substances are arranged according to their standard electrode potential. The higher a substance is positioned in the electrochemical series, the lower its solubility pressure, meaning its tendency to release ions into surrounding water. A metal that is higher in the electrochemical series is referred to as noble; a metal that is lower in the electrochemical series is called non-noble. For most substances, their position in the electrochemical series is known and can be found in relevant tables. If the position of a substance in the electrochemical series is not known, it can be determined by setting up a galvanic cell with a known substance and measuring the resulting potential difference. Based on the potential difference, the position in the series can be determined. The terms anode material and cathode material are used to represent the relative positions of the materials used in the electrochemical series. Anode material and cathode material are electrically conductive materials.

When the implant is placed into the body, the anode material and cathode material of the coating form a local galvanic cell with the body electrolyte present in the surrounding area of the implant. The tendency of the anode material to release silver ions into the surroundings is thereby enhanced. The electrons remaining in the anode material after the release of silver ions can move into the cathode material due to the electrical connection. Due to the potential difference, the silver ions are attracted towards the cathode material.

The effect of the inventive coating is therefore twofold. Firstly, due to the local galvanic cell, the anode material has a stronger tendency to release silver ions into the surrounding body electrolyte. Compared to a coating consisting only of the respective anode material, a larger number of silver ions are thus released, thereby increasing the antimicrobial effectiveness. Moreover, the movement of the released silver ions no longer occurs in arbitrary directions, but the silver ions are moved towards the potential difference between the two materials, that is, towards the cathode material. This increases the likelihood that the silver ions will actually be effective against microorganisms located on the surface of the implant, rather than combining with chloride in the body electrolyte to form AgCl and thereby losing their antimicrobial effectiveness. Therefore, the effect of the inventive coating is focused on the surface of the implant. The coating is particularly suitable for combating the dangerous biofilm that can form on the surface of implants.

The coating can cover the entire surface of the implant. This is particularly suitable for the inventive implants (endoprostheses) that are entirely implanted into the body. In particular, for joint endoprostheses, it may also be provided that only a part of the surface is coated. The coating can be applied to the part of the surface that comes into contact with body tissue when the prosthesis is implanted, while another part of the surface, which is intended, for example, to interact with another prosthetic component, remains free of the coating.

The anode material can be pure silver. With a standard electrode potential of approximately +0.8 V, silver is a relatively noble metal that belongs to the upper range of the electrochemical series. The reference for the voltage values of the standard electrode potential is the standard hydrogen electrode.

The cathode material, which interacts with pure silver, must have a standard electrode potential of more than +0.8 V. Therefore, if the cathode material is a metal, it is nobler than silver. For example, gold is a suitable cathode material for interaction with pure silver, as it has a standard electrode potential on the order of +1.5 V. Even if not pure silver but an alloy of silver and another substance is used as the anode material, the standard electrode potential of the cathode material should still be higher than +0.8 V. Preferably, the standard electrode potential of the cathode material is at least 0.3 V, more preferably at least 0.5 V, even more preferably at least 0.7 V higher than the standard electrode potential of the anode material.

The effect of the local galvanic cell is stronger the greater the difference between the standard electrode potential of the anode material and the standard electrode potential of the cathode material. In an advantageous embodiment, therefore, an silver-containing material is used as the anode material, whose standard electrode potential is less than +0.8 V. The anode material then contains, in addition to the silver components that are intended to dissolve from the anode, further components. The standard electrode potential specified for the anode material refers to the solubility pressure for silver ions. Preferably, a material is selected for the anode from which no other substances besides silver ions are released into the body electrolyte. If other substances besides silver ions are released, there is a risk that these other substances may have unwanted effects in the body. Moreover, for both the anode and the cathode, a material should be selected that is biocompatible.

The antimicrobial effect of the coating according to the invention depends on the silver ions released from the anode material. The number of released silver ions increases as the surface area of the coating occupied by the anode material increases. Therefore, the surface portion of the coating occupied by the anode material is preferably greater than 50%, more preferably greater than 70%, even more preferably greater than 80%. The area portion of the coating occupied by the cathode material is comparatively less important. However, in order to achieve good effectiveness of the galvanic elements, the proportion of the cathode material must not be too small. Preferably, the proportion of the cathode material on the surface of the coating is greater than 0.1%, more preferably greater than 1%, even more preferably greater than 5%.

It is desired that the silver ions, after having left the anode material, can travel a certain distance before reaching the cathode material. During this movement, the silver ions can exert an antimicrobial effect. Therefore, the surface areas occupied by the anode material and the cathode material should be separated sufficiently so that the silver ions do not necessarily hit the cathode material directly. The coating therefore preferably comprises a plurality of circular surface areas with a diameter greater than 1 µm, more preferably greater than 5 µm, even more preferably greater than 15 µm, even more preferably greater than 50 µm, which are exclusively formed by the anode material and are free from cathode material. On the other hand, it is also not advantageous for the effectiveness of the coating if the free path length of the silver ions is too long. Therefore, the diameter of the circular surface areas should be smaller than 5 mm, preferably smaller than 1 mm, even more preferably smaller than 0.5 mm. Preferably, more than 30%, more preferably more than 50% of the surface area of the coating is covered by such surface areas.

Silver ions emerging from the center of such a region must travel a certain distance before reaching the cathode material. During this journey, they can exert an antimicrobial effect. The free path that silver ions need to cover can be oriented according to the diameter of bacteria, which is also in the micrometer range. It can be assumed that the silver ions move along a curved path, and the maximum distance from the surface that the silver ions have on their way is in a similar order of magnitude as the distance traveled parallel to the surface. Therefore, when the free path to be covered is approximately equal to the diameter of the bacteria, it is achieved that the silver ions can act against bacteria situated on the surface throughout their entire path.

The coating can be designed such that the cathode material is embedded in the anode material in a self-shaped manner or applied in a self-shaped manner onto the anode material. The cathode material can be applied in the form of continuous area regions with a diameter, for example, of several micrometers. It is not excluded that the cathode material is applied on the second surface region in the form of individual particles without the anode material being uniformly coated in this area.

In many cases, the surface of the implant should be smooth. This can be achieved by ensuring that the anode material and cathode material are flush with each other. In an alternative embodiment, the cathode material may protrude relative to the anode material. The silver ions then move at a small distance from the surface of the coating, thereby achieving a good effect against microorganisms in the immediate vicinity of the coating. It is suitable to first apply the anode material in a uniform layer thickness, and then to apply the cathode material to the coating in selected areas. The layer thickness of the anode material can range between 100 nm and 10,000 nm, preferably between 200 nm and 400 nm.This area is particularly relevant when the anode material is pure silver. The thickness of the cathode material applied to the anode material can also range between 100 nm and 10,000 nm, preferably between 200 nm and 400 nm. It is also possible first to apply a uniform layer of the cathode material. A layer of anode material can then be placed on the cathode material, which contains breaks or openings, thus making the cathode material accessible from the outside through the anode material. If the anode material is applied using a plasma coating process, these openings can be created by directing larger fragments with a diameter, for example, of 20 pm onto the surface during the coating process.removing a piece from the forming layer, see WO 2009/036846. In this approach, the thickness of the layers is preferably between 100 nm and 10,000 nm, more preferably between 200 nm and 400 nm. The invention will be described below with reference to the accompanying drawings by way of example, based on advantageous embodiments. They show: Fig. 1: A first embodiment of an implant according to the invention; Fig. 2: A component of the implant from Fig. 1; Fig. 3: A second embodiment of a non-inventive implant; Fig. 4: An excerpt from the body of an implant according to the invention with coating; Fig. 5: The coating from Fig. 4 in plan view; Fig. 6: The view from Fig.4 in another embodiment of the invention; Fig. 7: the view from Fig. 5 in the embodiment according to Fig. 6; Fig. 8: the view from Fig. 4 in a further embodiment of the invention; and Fig. 9: the view from Fig. 5 in a further embodiment of the invention.

An implant shown in Fig. 1 is intended to replace a part of the human skeleton extending from the hip down below the knee. A spherical joint head 10 forms a joint surface designed to work together with an acetabulum. The joint head 10 is connected to a head piece 11 of the implant via a screw connection. The part of the implant that replaces the shaft of the femur comprises three implant components 12, 13, 14. The implant components 12, 13, 14 are connected to each other and also to the head piece 11 via screw connections. A knee piece 15 forms a jointed connection to a shaft 16, which is intended to connect the implant to a lower leg bone.

The implant components 12, 13, 14 are available in different lengths, so that the implant can be adapted to femurs of varying lengths. Figure 2 shows an implant component 17 corresponding to the implant components 12, 13, 14, depicted in an enlarged view. The implant component 17 includes a screw bolt 18 and a dashed-lined screw hole 19. Via the screw bolt 18 and the screw hole 19, the implant component 17 can be connected at both ends to further implant components. In the implanted state of the implant component 17, the screw bolt 18, the screw hole 19, as well as the adjacent end faces 20 and 21 do not contact the patient's body tissue, but rather other implant components. On the other hand, the outer surface 22 of the implant component 17 is designed to come into contact with human tissue in the implanted state. The outer surface 22 is provided with an antimicrobial coating 23 indicated by means of punctures. The remaining surface of the implant component is free from the coating 23.

Coating 23 is shown enlarged in Figures 4 and 5. Coating 23 mainly consists of pure silver, which covers the entire outer surface. As shown in Fig. 5, gold material is incorporated into the silver layer in the form of several rectangular islands. The gold material is embedded into the silver layer so that the two materials meet flush with each other and a smooth surface is formed. A smooth surface is desired because the surrounding body tissue should be irritated as little as possible by friction. Coating 23 has a first surface portion 28, which is formed by the silver material, and a second surface portion 29, which is formed by the gold material. The surface portion 28 formed by the silver material occupies more than 80% of the surface of coating 23. Between the islands, circular surface areas 27 remain, as indicated by dashed lines in Fig. 5, in which the surface of coating 23 is entirely made of silver material and not interrupted by gold material. The surface area 27 has a diameter of more than 0.1 mm.

Silver and gold are electrically connected to each other within the coating 23. Silver is a less noble metal than gold and is positioned lower in the electrochemical series than gold. In terms of the inventive function of the coating, silver is therefore an anode material 25 and gold is a cathode material 26.

After implantation, the coating 23 is surrounded by body electrolyte. The silver material has a tendency to release positively charged silver ions into the body electrolyte. This tendency is called the dissolution pressure. When silver ions are released from the coating, excess electrons remain in the coating, and an excess of negative charge carriers forms within the coating. Since the silver material and the gold material are electrically connected, the excess electrons can freely move toward the gold material. The gold material also experiences some dissolution pressure, releasing ions into the body electrolyte. However, since gold is a more noble metal than silver and is positioned higher in the electrochemical series, its dissolution pressure is lower than that of silver. Silver ions, which are released in greater concentration, move towards the gold material. In this way, the body electrolyte together with silver as the anode material 25 and gold as the cathode material 26 form local galvanic cells. Silver ions exit from the anode material 25 and move parallel to the coating 23 toward the cathode material 26. Along this path, the silver ions can exert an antimicrobial effect against microorganisms located on the surface of the coating 23.

The dental implant shown in Fig. 3 is an alternative embodiment (not according to the invention). An implant body 30 is screwed into the jawbone 31 at its lower end. The upper end of the implant body 30 protrudes from the jawbone 31 and the surrounding gum tissue 32. A post 34, which is covered with an artificial crown 33, is screwed into the free end of the implant body 30. In this way, the dental implant replaces a natural tooth. The implant body 30 is again provided with a coating 23 indicated by dots.

Coating 23 is shown in an enlarged representation in figures 6 and 7. Initially, a silver coating is applied to the surface of the implant 30, having a uniform thickness of approximately 400 nm. On the surface of the silver coating, gold material is applied in a grid-like arrangement with a layer thickness of also approximately 400 nm. The areas enclosed within the grid, where the surface of the coating 23 is formed by the silver material, collectively constitute the first surface portion 28 of the coating 23. The grid-like arrangement of the gold material forms the second surface portion 29 of the coating. The grid shape of the gold material is dimensioned such that circular surface areas 27 with a diameter of more than 50 µm remain free of the gold material.

In the coating shown in Fig. 8, the implant component 17 is first completely covered with a layer of gold as cathode material 26. A silver layer applied on top as anode material 25 has a large number of breaks. These breaks collectively form the second surface portion 29, in which the cathode material 26 is accessible from the outside through the anode material 25.

In the embodiment of Fig. 9, the cathode material 26 is not applied over the entire area on the second surface portion 29, but rather as a plurality of individual particles. This does not affect the inventive effect of the coating.

As explained above, the silver, in the sense of the invention, is an anode material 25 and the gold is a cathode material 26. Together with the body electrolyte in the environment of the implant body 30, the coating 23 forms a plurality of local galvanic cells. Since the gold as cathode material 26 protrudes relative to the anode material 25, silver ions can also move in a small distance from the silver layer toward the cathode material 26.

Regarding the (non-inventive) dental implant, the antimicrobial coating 23 is particularly designed to act against microorganisms at the interface between the implant body 30 and the gum 32 or the jawbone 31. It is well known that there are numerous microorganisms in the oral environment, and the risk of infection around the implant body 30 is high. If the penetration of microorganisms between the implant body 30 and the gum 32 is prevented by the antimicrobial coating 23, unpleasant infections for the patient can be avoided.

Claims (11)

1. An implant for bones or a joint having a coating (23), that releases silver ions in the human body and thereby provides an antimicrobial effect, characterized in that a first surface component (28) of the coating (23) is formed by an anode material (25) comprising silver, that a cathode material (26) is provided on the second surface component (29), which is spatially separated from the first surface component (28), wherein the cathode material (26) in the electrochemical voltage sequence is higher than the anode material (25), and that the cathode material (26) and the anode material (25) are coupled in an electrically conducting manner, wherein the implant is an endoprosthesis.
2. The implant according to claim 1, characterized in that the anode material (25) is pure silver.
3. The implant according to claim 1, characterized in that the standard electrode potential for release of silver ions of the anode material (25) is less than +0.8 V.
4. The implant according to any of claims 1 to 3, characterized in that the standard electrode potential of the cathode material is greater than +0.8 V.
5. The implant according to claim 4, characterized in that the cathode material (26) is gold.
6. The implant according to any of claims 1 to 5, characterized in that the standard electrode potential of the cathode material (26) is greater than the standard electrode potential of the anode material (25) by at least 0.3 V, preferably by 0.5 V, further preferably by 0.7 V.
7. The implant according to any of claims 1 to 6, characterized in that the cathode material (26) is embedded in the anode material (25) in an island-shaped manner.
8. The implant according to any of claims 1 to 7, characterized in that the first surface component (28) that is formed by the anode material (25) occupies greater than 50%, preferably greater than 70%, or further preferably greater than 80% of the surface area of the coating (23).
9. The implant according to any of claims 1 to 8, characterized in that the coating (23) has circular surface regions (27) with a diameter of more than 0.1 mm, preferably more than 0.5 mm, further preferably more than 1 mm, said surface regions being free from cathode material (26).
10. The implant according to any of claims 1 to 9, characterized in that the anode material (25) and the cathode material (26) abut against one another in flush manner.
11. The implant according to any of claims 1 to 10, characterized in that the cathode material (26) protrudes relative to the anode material (25).