Note: Descriptions are shown in the official language in which they were submitted.
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Method for the surface treatment of a biocorrodable implant
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.
The purpose of implants is to support or to replace body functions and the
most varied
embodiments of implants have been used in medical technology. In addition to
implants for
securing tissues, endovascular implants, dental prosthesis implants, joint
replacement
implants, implants are also used for the treatment of bone damage, such as
screws, nails,
plates, or as bone replacements.
Nowadays implants which are applied to the bone are generally manufactured
from titanium.
In spite of the relatively good biocompatibility of titanium implants by
comparison with other
permanent implants, efforts have been made to further improve these titanium
implants.
Coatings are often provided on the surface of the implant in order to improve
the
biocompatibility.
One disadvantage of the application of layers to the surface of the implant is
a change to the
geometry of the implant, even in the event of small layer thicknesses.
Moreover, the
adhesion of the applied layers is generally not optimal.
DE 195 04 386 C2 discloses a method for producing a graduated coating of
calcium
phosphate phases and metal oxide phases on metal implants, preferably made of
titanium.
In the electrochemical method using a substrate electrode formed by the metal
implant and
a counterelectrode, an aqueous solution with calcium and phosphate ions in the
weakly
acidic to neutral range is used as electrolyte and the substrate electrode
formed by the
implant is alternately polarized anodically and cathodically. Good growth of
the bone onto
the implant is achieved by the firm incorporation of calcium phosphate phases
into the
implant surface.
DE 100 29 520 Al describes a coating for a metal implant surface for
improvement of the
osteointegration. In an electrolysis cell the implant is cathodically
polarized in an electrolyte
containing calcium, phosphate and collagen. A mineralized collagen layer is
formed on the
surface of the implant by the method.
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It is often only necessary for the implant to remain temporarily in the body,
in particular in the
case of cardiovascular and orthopedic implants. Implants made of a permanent
material
must then be removed by a further operation. For this reason, biocorrodable
materials are
used for implants. In this case biocorrosion is understood to be the gradual
breakdown of the
material caused by substances in the body. Even with biocorrodable materials
an influence
on the corrosion process is advantageous.
However, in the case of biocorrodable implants complete inhibition of the
corrosion should
not generally be achieved, since it is ultimately desirable for the implant to
dissolve in the
body after a certain time. Instead, it is merely necessary to influence the
rate of corrosion,
which enables a delayed degradation of the implant in the body as required.
One approach in order to improve the protection against corrosion is, as also
in the case of
permanent implants, the application of a corrosion-inhibiting layer.
By way of example reference is made to DE 103 57 281 Al, which discloses a
degradable
stent made of a magnesium material, which is provided with a coating which
delays the
degradation. In this connection the uncoated surface of the implant, which has
a natural
mixed oxide layer, is transformed into a mixed fluoride layer. The coating can
be produced
by dipping into fluoride-containing media with or without electrolytic
assistance.
The object of the present invention is to provide an alternative method for
the surface
treatment of a biocorrodable implant, by which the rate of degradation of the
implant can be
adapted as required.
This object is achieved by the method according to the invention for the
surface treatment of
a biocorrodable implant according to claim 1.
The method according to the invention for the surface treatment of a
biocorrodable implant
by means of electrochemical reactions comprises 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) electrochemically treating the surface of the implant,
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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, the
current density being set to -0.1 to -75 mA/crn2 for the cathodic polarization
and the current
density is set to 0.1 to 25 mA/cm2 for the anodic polarization.
A magnesium hydride layer which grows from the implant surface into the
implant is
produced by the method according to the invention. Hydrogen ions are deposited
cathodically from the electrolyte and are implanted into the surface of the
implant.
A metal hydride layer is formed which, starting from the surface of the
implant, virtually
grows into the implant. Thus the method has the advantage that no change to
the geometry
of the implant takes place, since the metal hydride layer grows into the
implant.
In the context of the present invention an implant should be understood to be
an artificial
material implanted in the body. Due to the use of a biocorrodable alloy for
the implant body,
the material used is gradually broken down by the substances in the body. It
is provided that
the implant consists, completely or in parts, of a biocorrodable alloy. The
implants can fulfill
different purposes and functions as required, such as for example interference
screws,
screws and plates for fixing bones, implants as medication depots, joint
prostheses, stents,
jaw and tooth implants. The list is merely by way of example and should in no
way be
understood as definitive.
The corrosion of the implant is slowed down by the hydride layer. The rate of
corrosion of
the hydride layer is less than that of the actual material without a hydride
layer. So long as a
closed hydride layer is present on the surface of the implant, the rate of
corrosion of the
implant is determined by the corrosion reaction of the magnesium hydride. As
soon as this
hydride layer is broken down by corrosion, the rate of corrosion of the
implant corresponds
to the rate of corrosion of the actual biocorrodable magnesium alloy.
Accordingly, after the
hydride layer is broken down by corrosion the alloy is broken down further, as
would be the
case with an untreated implant. The formation of the hydride layer gives rise
to a two-stage
corrosion behavior.
The method according to the invention is preferably carried out as follows:
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An implant, for example a compression screw, made of a biocorrodable metal
alloy,
preferably a biocorrodable magnesium alloy, is threaded onto a platinum wire.
Then the
surface of the screw is activated by a bath in aqueous citric acid solution,
preferably a 1-10%
solution, for 1 to 10 seconds. Then the testpiece is rinsed in deionized
water, preferably for
approximately 5 to 30 seconds.
For the further treatment the screw is fixed on a non-metallic object carrier.
The platinum
wire is then pulled out. Indentations in the object carrier prevent later
slipping of the screw.
Alternatively, the screw can be inserted through a plate with a hole, wherein
the ends of the
screw are free in order to produce the contact later, for example with
terminals.
The screw is contacted with terminals in order to produce conductive contact.
For this
purpose, the terminals are preferably attached to the external ends of the
screw.
If very small implants such as very small screws or pins are used, no
terminals are used,
since contact cannot be produced by terminals. Instead a fine metal mesh is
used, onto
which the screws or pins are laid. The conductive contact to the implant is
then produced
with the aid of the mesh. The activation in the citric acid solution as well
as the rinsing with
water are likewise preferably carried out with the aid of the mesh.
Then the implant is introduced into the electrolyte. The electrolyte has a
basic pH from pH 9
to pH 13, preferably from pH 9 to pH 10. Furthermore, it is preferable that
the electrolyte
contains 0.01 M NaOH and 0.2 M Na2SO4.
The formation of the magnesium hydride layer is made possible by the basic pH
value. At a
pH value below pH 9 the magnesium material would corrode because of its base
metal
nature.
First of all, cleaning of the surface of the implant takes place by the action
of positive pulses.
In this case the implant forms the working electrode. Furthermore, a
counterelectrode is
present in the arrangement. The counterelectrode preferably consists of a
corrosion-
resistant metal material, for example platinum, chromium-nickel steel, etc.
Glass vessels are
preferably used as electrolysis cells.
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For cleaning of the surface a positive pulse of 15 mA/cm2 to 35 mA/cm2 is
preferable with a
pulse length (pulse duration) of 0.10 seconds to 0.50 s and a total duration
of the pulses of
overall 5 min to 40 min. In this case particular preference is given to a
positive pulse of 25
mA/cm2 at a pulse length of 0.20 s and a total duration of 20 min.
Then the hydrogenation of the implant takes place by negative and positive
pulse changes
alternating multiple times. For the method according to the invention it is
preferable 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.
In a preferred embodiment the current density is set to -35 to -55 mA/cm2 for
the cathodic
polarization and the current density is set to 5 to 25 mA/cm2 for the anodic
polarization.
Furthermore, it is preferable 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.
In the context of the present invention a polarization step should be
understood as a
succession of positive or negative pulses of a specific current density and
pulse length.
Furthermore, it is preferable 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.
In a preferred embodiment of the method according to the invention 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.
In a further preferred embodiment the total duration of all pulses of the
cathodic and anodic
polarization steps is 20 min to 300 min, preferably 120 min to 240 min,
particularly preferably
195 min.
In a preferred embodiment the method according to the invention consists of an
alternating
succession of five polarization steps:
1. Polarization step: cathodic polarization (negative pulse)
Current density: - 0.1 to - 75 mA/cm2
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Pulse length: 0.50 to 2.5 s
Overall for 60 min (3.6 ks)
2. Polarization step: anodic polarization (positive pulse)
Current density: + 0.1 to + 25 mA/cm2
Pulse length: 0.20 to 0.5 s
Overall for 10 min (0.6 ks)
3. Polarization step: cathodic polarization (negative pulse)
Current density: - 0.1 to - 75 mA/cm2
Pulse length: 0.50 to 2.5 s
Overall for 60 min (3.6 ks)
4. Polarization step: anodic polarization (positive pulse)
Current density: + 0.1 to + 15 mA/cm2
Pulse length: 0.20 to 0.5 s
Overall for 5 min (0.3 ks)
5. Polarization step: cathodic polarization (negative pulse)
Current density: - 0.1 to - 75 mA/cm2
Pulse length: 0.50 to 2.5 s
Overall for 60 min (3.6 ks)
A deposition rate of 5 to 8 nm/h is advantageously achieved by the method
according to the
invention.
In the context of the present invention the deposition rate should be
understood to be the
growth of the metal hydride layer from the surface of the implant into the
implant.
After the alternating cathodic and anodic polarization, the implant is removed
from the
electrolyte and rinsed for approximately 30 to 60 seconds with deionized
water. For
passivation the implant is introduced into a hot air stream, preferably at a
temperature of 60
C for 10 to 100 seconds. Until further use, the implant is preferably packaged
in an airtight
manner in order to prevent oxidation.
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With the aid of the method according to the invention a magnesium hydride
layer which
increases the corrosion resistance of the implant is formed on the surface of
the implant.
Higher current intensities than those specified for the method according to
the invention can
actually lead to a faster formation of the hydride layer within a defined time
interval.
However, the faster growth of the hydride layer, which is accompanied by a
different depth
of penetration of the hydride layer, can lead to an inhomogeneous surface and
thus to
inconsistent corrosion. In the event of an inhomogeneous layer thickness of
the formed
magnesium hydride layer, thinner layer zones are completely broken down faster
than
thicker zones. If the magnesium hydride layer is already broken down on some
zones, but
not yet on others, this can lead to a dramatic increase in the rate of
corrosion, since at these
points there is no longer any hydride corrosion, but corrosion of the actual
material takes
place. Thus the implant is broken down inconsistently and can lose its
stability.
The described parameters lead to an optimal surface in a reasonable time.
The growth of the hydride layer takes place during the cathodic polarization
step. Longer or
shorter pulse lengths only have an indirect effect on the growth of the
hydride layer. In
addition to the electrochemical reaction the pulse primarily serves the
purpose that the
formed hydrogen (H2) is released uniformly and in short intervals on the
working electrode.
Accumulations of hydrogen bubbles can lead to slowing down or interruption of
the buildup
of the hydride layer at this point, since in the extreme case contact no
longer takes place
between the material (working electrode) and the electrolyte.
Within a polarization step there is a short resting phase ("break") between
the individual
pulses. In this case this resting phase between 'current is applied' and
'current is not applied'
should preferably be long enough so that the hydrogen bubbles can rise up from
the working
electrode.
In particular square wave pulse currents are advantageous for the method
according to the
invention, since they provide enough time so that the hydrogen can rise up. In
the context of
the present invention a square wave pulse current is a current with a steep
rise and fall and
a constant plateau located between them. The same applies to the pulse length.
With many
short pulses in a time interval, the resting phase is too short and a
substantial accumulation
df hydrogen gas in the form of bubbles occurs on the working electrode.
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In an advantageous configuration of the method according to the invention a
rest break of at
least 0.1 s is provided between two pulses.
In this case the geometry of the implant (working electrode) also has an
effect on the optimal
p.ulse length. A smooth or uniform surface encourages the hydrogen bubble to
drop off. The
pulse length can be truncated here. Samples with an uneven surface or thread,
as in the
case of helical implants, or a supporting mesh as electrode, as used in the
case of small
implants, leads to the hydrogen bubbles requiring more time to drop off.
Thus the pulse length is adjustable according to the geometry of the implant.
If too many
hydrogen bubbles accumulate on the working electrode, the pulse length is made
longer.
In this way the individual method parameters are adaptable to different
implant sizes and
geometries. Moreover, the rate of degradation of the implant can be adapted as
required. If
a fast degradation is desirable, the total duration of the pulses, that is to
say the duration of
the respective cathodic polarization step, is reduced in order to limit the
formation of the
hydride layer to a small depth of penetration and thus to a small layer
thickness. On the
other hand, in the event of a longer total duration of the pulses the depth of
penetration and
thus the layer thickness is increased.
In a further embodiment of the method according to the invention it is
preferable that the
implant provided is made of a biocorrodable magnesium alloy which has a
magnesium
component of at least 50%. The following composition is particularly
preferred: a rare earth
metal component 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,
preferably
less than 0.1 % by weight
and the rest up to 100 % by weight is magnesium.
Furthermore, it is preferable that the implant consists completely or in parts
of a
biocorrodable magnesium alloy.
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Further advantages are provided by an implant with a corrosion-inhibiting
coating, which is
or may be obtained by the method according to the invention.
After the method according to the invention has been carried out, the surface
of the implant
has a hydrogenated outer layer which increases the corrosion resistance. In
this case it is
preferable that the corrosion-inhibiting hydride layer has a layer thickness
of at least 10 nm,
preferably at least 15 nm, particularly preferably 20 nm.
In a further preferred embodiment the implant provided, which is made of a
biocorrodable
magnesium alloy, has a magnesium component of at least 50%. The biocorrodable
magnesium alloy from which the implant is manufactured has the following
composition: a
rare earth metal component 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,
preferably
less than 0.1 % by weight
and the rest up to 100 % by weight is magnesium.
Due to the low, almost negligible content of aluminum the biocorrodable
magnesium alloy is
suitable for the use of implants in human medicine, since aluminum is alleged
to have
properties which are harmful to health, such as the promotion of Alzheimer's
disease or
cancer.
Furthermore, it is preferable that the implant consists completely or in parts
of a
biocorrodable magnesium alloy.
The method according to the invention is explained in greater detail with
reference to an
embodiment.
Embodiment
A round material made of the magnesium alloy ZfW 102 PM F is treated by the
method
according to the invention.
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In this case the magnesium alloy ZfW 102 PM F consists of a rare earth metal
component
(including neodymium) of 4.05 % by weight, the neodymium component
corresponding to
2.35 % by weight, an yttrium component of 1.56 % by weight, a zirconium
component of
0.78 % by weight, a zinc component of 0.4 % by weight, an aluminum component
of 0.0032
% by weight. The rest up to 100 % by weight is magnesium.
The round material is a full cylinder with a diameter of 6 mm and a length of
3 cm. This full
cylinder functions as a working electrode. A platinum electrode with a
titanium core having a
diameter of 6 mm and a length of 7 cm is used as the counterelectrode.
A 500 ml beaker is used as electrolysis cell. The electrolyte consists of 0.01
M NaOH and
0.2 M Na2SO4and has a pH value of 9.4. The method is carried out at 24 C.
For cleaning of the surface a positive pulse of 25 mA/cm2 at a pulse length of
0.20 s and a
total duration of 20 min is used.
Then the hydrogenation of the round piece takes place by alternating negative
and positive
pulse changes. The method according to the invention is carried out in an
alternating
succession of five polarization steps:
1. Polarization step: cathodic polarization (negative pulse)
Current density: -50 mA/cm23
Pulse length: 0.50 s
Overall for 60 min (3.6 ks)
a Polarization step: anodic polarization (positive pulse)
Current density: +20 mA/cm2
Pulse length: 0.20 s
Overall for 10 min (0.6 ks)
3. Polarization step: cathodic polarization (negative pulse)
Current density: -50 mA/cm2
Pulse length: 0.50 s
Overall for 60 min (3.6 ks)
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4. Polarization step: anodic polarization (positive pulse)
Current density: + 10 mA/cm2
Pulse length: 0.20 s
Overall for 5 min (0.3 ks)
5. Polarization step: cathodic polarization (negative pulse)
Current density: -50 mA/cm2
Pulse length: 0.50 s
Overall for 60 min (3.6 ks)
After a total of 195 min a layer thickness of 18 nm is achieved.
The success of the treatment is determined by means of X-ray diffractometry
(RDA),
secondary ion mass spectrometry (SIMS) as well as determination of the free
corrosion
potential. An identical round material made of the magnesium alloy ZfW 102 PM
F which has
not been treated by the method according to the invention serves as a
comparison.
The results are presented in Figures 1 to Figure 4.
Figure 1 shows the hydride detection by means of X-ray diffractometry (XRD).
Figure 2 shows the hydride detection by means of secondary ion mass
spectrometry
(SIMS).
Figure 3 shows the determination of the free corrosion potential.
Figure 4 shows the corrosion rate in a Ringer's lactate solution.
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 Figure 1. The occurrence of
magnesium hydride
phases (MgH2) is evidence of the hydride layer formed by the method according
to the
invention.
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. Figure 2
shows
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=
the hydride detection as a function of the depth of penetration of the
hydrogen ions into the
workpiece.
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. Figure 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.
Figure 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 mmo1/1 Na, 4.0-5.4
mmo1/1 K+,
0.9-2.0 mmo1/1 Ca2, 106-117 mmol/ICI-, 25-31 [mmo1/1] 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
e corrosion rate of 0.224 mm/year after 432 h and a corrosion rate of 0.153
mm/year after
624 h (cf. Figure 4).
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
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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.
