METHOD FOR THE SURFACE TREATMENT OF A BIOCORRODABLE IMPLANT

Abstract

The present invention relates to a method for the surface treatment of a biocorrodable implant by means of alternating cathodic and anodic polarization, and also to a corresponding implant.

Claims

1. A method for the surface treatment of a biocorrodable implant by means of electrochemical reactions comprising the steps of: a) providing an implant made of a bicorrodable magnesium alloy; b) introducing the implant into an electrolyte with a pH value of pH 9 to pH 13; c) hydrogenation of the implant electrochemically treating the surface of the implant, wherein the implant serves as the working electrode and there is also a counterelectrode, and wherein the working electrode is alternately polarized cathodically and anodically with a pulsed voltage, the current density being set to 0.1 to 75 mA/cm.sup.2 for the cathodic polarization and the current density is set to 0.1 to 25 mA/cm.sup.2 for the anodic polarization, and wherein the total duration of the pulses in a cathodic polarization step is 5 min to 90 min and the total duration of the pulses in an anodic polarization step is 1 min to 20 min.

2. The method according to claim 1, characterized in that the working electrode is alternately polarized cathodically and anodically multiple times, starting with a cathodic polarization and ending the deposition with a cathodic polarization.

3. The method according to claim 1 or 2, characterized in that the current density and the total duration of the pulses are lower in an anodic polarization step than in a preceding anodic polarization step.

4. The method according to one of the preceding claims, characterized in that the pulse length in the cathodic polarization is 0.40 s to 2.5 s and in the anodic polarization is 0.10 s to 0.50 s.

5. The method according to one of the preceding claims, characterized in that the total duration of all pulses is 20 min to 300 min.

6. The method according to one of the preceding claims, characterized in that a hydride layer having a hydride layer thickness of at least 10 nm, preferably at least 15 nm is achieved on the surface of the implant.

7. An implant obtained by the method according to one of claims 1 to 6 consisting of a biocorrodable magnesium alloy and having a corrosion-inhibiting coating, wherein the corrosion-inhibiting coating consists of a hydride layer having layer thickness of at least 10 nm, preferably at least 15 nm, and the biocorrodable magnesium alloy contains a rare earth metal component without yttrium of 2.5 to 5% by weight, an yttrium component of 1.5 to 5% by weight, a zirconium component of 0.1 to 2.5% by weight, a zinc component of 0.01 to 0.8% by weight, as well as unavoidable impurities, wherein the total content of possible contaminants is below 1% by weight and the aluminum component is less than 0.5% by weight, and the rest up to 100% by weight is magnesium.

Description

[0118] The results are presented in FIG. 1 to FIG. 4.

[0119] FIG. 1 shows the hydride detection by means of X-ray diffractometry (XRD).

[0120] FIG. 2 shows the hydride detection by means of secondary ion mass spectrometry (SIMS),

[0121] FIG. 3 shows the determination of the free corrosion potential.

[0122] FIG. 4 shows the corrosion rate in a Ringer's lactate solution.

[0123] A round piece which had been treated by the method according to the invention according to exemplary embodiment 1 was examined by means of X-ray diffractometry. The phases present in the material are illustrated in FIG. 1. The occurrence of magnesium hydride phases (MgH.sub.2) is evidence of the hydride layer formed by the method according to the invention.

[0124] Moreover, a round piece which had been treated by the method according to the invention according to exemplary embodiment 1 was examined by means of SIMS. FIG. 2 shows the hydride detection as a function of the depth of penetration of the hydrogen ions into the workpiece.

[0125] Moreover, the free corrosion potential of a round piece which had been treated by the method according to the invention according to exemplary embodiment 1 as well as an untreated round piece is determined. FIG. 3 shows that at 1680 mV the treated round piece (H-EIR, H electrochemical induced reaction) has a more positive corrosion potential than the untreated round piece.

[0126] FIG. 4 shows the corrosion rate of an untreated round piece and a round piece which had been treated by the method according to the invention according to exemplary embodiment 1. The corrosion rate was determined under conditions similar to those in the human body in each case at 37 C. in a Ringer's lactate solution (125-134 mmol/l Na.sup.+, 4.0-5.4 mmol/l K.sup.+, 0.9-2.0 mmol/l Ca.sup.2, 106-117 mmol/l Cl.sup., 25-31 [mmol/l] lactate). A Ringer's solution has a composition comparable to that of the blood plasma and the extracellular liquid. It can be seen that the treated round piece has a lower corrosion rate than the untreated round piece. Thus for example the untreated round piece has a corrosion rate of 0.415 mm/year after 432 h and a corrosion rate of 0.339 mm/year after 624 h, and on the other hand the round piece treated by the method according to the invention according to exemplary embodiment 1 has a corrosion rate of 0.224 mm/year after 432 h and a corrosion rate of 0.153 mm/year after 624 h (cf. FIG. 4).

[0127] Thus because of the slower rate of degradation, after implantation into the human body a biocorrodable implant which is treated by the method according to the invention has a longer service life than an untreated implant of the same structural design. Thus with the aid of the method according to the invention the rate of degradation can be adapted to the particular purpose and to the necessary residence time of the implant in the body. If it is necessary to have a longer residence time in the body than the actual material permits, the corrosion resistance can be increased by the treatment of an implant by the method according to the invention. Moreover, the increased corrosion resistance gives the implant an increased stability, since corrosion is accompanied by a loss of mass of the implant. If the implant breaks down too quickly in the body, in some circumstances the bone does not have sufficient time to grow into the implant and to replace the material by bone material. Thus the choice of the corrosion resistance is dependent upon the position of the implant in the body or also dependent upon the patient. Thus in the case of older people, who exhibit slower bone growth, a biocorrodable implant with a substantially slower rate of degradation can be used. On the other hand, if only a small implant into the bone is used, which is not subject to substantial mechanical stresses, an implant with a smaller hydride layer thickness can be used.