Method and device of enhancing diffusibility of metallic surfaces and applications thereof

Abstract

A method for increasing diffusibility of metallic surfaces and its applications in surface treatment of metallic implants, which combines a mechanic means and a chemical means, whereby the mechanic means increases the diffusibility of the surface while the chemical means forms a chemically modified surface layer for improved properties suitable for particular application of the metallic materials, especially in the bio-medical areas. This surface treatment can increase the hardness and corrosion resistance of stainless steel and reduces Ni release from implants made from NiTi wires.

Description

FIELD OF THE INVENTION

The present invention relates to a method and device for increasing diffusibility of metallic surfaces and its applications. Particularly, the present invention relates to a surface treatment method combining a mechanic means and a chemical means, whereby the mechanic means increases the diffusibility of the surface while the chemical means forms a chemically modified surface layer for improved properties suitable for particular application of the metallic materials, especially in the bio-medical areas.

BACKGROUND OF THE INVENTION

Stainless steel is widely used in daily life and in industries. For example, it is widely used in the bio-medial industry to manufacture orthopedic, dental and other implants, such as bone plates, artificial hip joints, intramedullary nails, etc, see U.S. Pat. Nos. 4,964,925, 4,718,908, and 4,775,426. However, it is known that stainless steel has its shortcomings. Since the surface hardness of stainless steel is low (<300 Hv), wear resistance of the implants is weak in vivo. The weak wear resistance may damage the implant fixation and the wear debris is harmful to the host tissue. In addition, stainless steel has low pit corrosion resistance, especially in the warm salty body fluid containing Cl⁻ ions. To overcome some of the weaknesses of stainless steel, U.S. Pat. Nos. 5,205,921 and 5,482,731 describe two processes to coat bioactive calcium phosphate coatings on metallic implant devices for their bioactive fixation. U.S. Pat. No. 5,057,108 discloses a surface treatment process for stainless steel orthopedic implant devices, in which a heavily cold-worked outer layer is formed that enhances fatigue properties of the devices after two shot blasting steps, electropolishing and passivation.

In another bio-medical areas, with the fast development of interventional therapeutic procedures, NiTi shape memory alloy wires are being widely used to produce cardiovascular, tracheal, oesophageal and other implants due to their excellent mechanical performance, strong shape memory effect, super-elastic properties, reasonably good chemical resistance and biocompatibility, see U.S. Pat. Nos. 6,375,458, 6,224,625, 5,882,444, C.N. Pat. Nos. 02124291.7, 02240165.2, etc. The wide application of these implants implies two concerns in terms of biocompatibility. First, there is the possibility of adverse tissue reactions after implantation, suggesting that the biocompatibility of NiTi wires is not perfect and needs further improvement. Second, the out-diffusion of harmful Ni+ ions from NiTi implants, which have been found to be carcinogenic during prolonged use inside the human body, poses a potential health problem. U.S. Pat. No. 7,000,305 describes a process to coat NiTi or other wires with the expanded fluoropolymer after heat setting to reduce platelet adhesion to the stent. No other inorganic coatings, e.g. titania or carbon coatings that have good hemocompatibility, on NiTi wires are found in the prior art disclosures. By adding a separate layer of coating material which is different from the underlying material, stainless steel or NiTi, for example, is not an ideal method to modify metallic surfaces and there is a need for a thinking in a different direction for different solutions.

SUMMARY OF THE INVENTION

Accordingly, one object of the present invention is to provide a new surface modification method which does not relying on a separate coating layer on the metallic surfaces to increase hardness and corrosion resistance of stainless steel and to reduce Ni release from NiTi wires. Another object of the present invention is to provide an automatic device for mechanically retreating metallic surfaces

This and other objects of the present invention are realized by providing a method that combines mechanic and chemical means to enhance physical and chemical properties of the metallic surfaces to suit particular purposes, for example, for application as implants. In essence, the present invention uses a mechanic process to increase diffusibility of the metallic surfaces to be treated then follows up with chemical process to physically and chemically changes the surfaces. The mechanical process is to facilitates the subsequent chemical process to achieve effects either the mechanical process or the chemically process alone could not achieve. It is believed that the mechanical process create certain nanocrystalline surface structure, characterized by ultrafine grains, typically with at least one dimension of less than 100 nm. Although not be bound by the theory, the nanocryatlline surfaces structure may serve as the basis of the observed enhancement of surface diffusibility. Several mechanical methods and mechanical devices have been invented to generate a layer of nanometric microstructures, or nanostructures on metallic pieces, as disclosed the present inventor's previous U.S. Pat. Nos. (7,147,726 and 7,300,622). The surface modification produced according to the present invention have the following advantages: (1) the layer with nanostructures are directly formed on the surface of metallic pieces and it has the same material as the substrate, and thus the nanostructured layer is dense and highly pure; (2) the grain size has a smooth transition from the nanostructured surface layer to the underlying substrate, and the absence of a clear interface can avoid bonding problem that is often encountered for coating materials; and (3) the process is carried out by mechanical means in air or in low level vacuum conditions, and therefore the complex electrical devices e.g. for sputtering or vaporizing and the high level vacuum pumps are avoided. The low requirement for operational atmosphere is beneficial for treating large pieces in industrial applications.

As a particular example, due to the increase in hardness of the treated stainless steel, 316 stainless steel which is commonly used in implants can now be replaced 304 stainless steel with a lower nickel content.

Furthermore, a new surface treatment device is designed so as to accommodate metallic medical implants with complex shapes and large sizes.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be made to the drawings and the following description in which there are illustrated and described preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A represents a diagram of a device for generating nanostructures on a stainless steel intramedullary nail by bombardment;

FIG. 1B represents top view of the device in FIG. 1A without cover of the chamber and the balls;

FIG. 2A represents a diagram of a device for generating nanostructures on a long stainless steel bone plate by bombardment;

FIG. 2B represents a diagram of a device for generating nanostructures on two short stainless steel bone plates by bombardment;

FIG. 3A represents top view of the cover of the device in FIG. 3B;

FIG. 3B represents a diagram of a device for generating nanostructures, with the roof height of one room being adjustable;

FIG. 3C represents a diagram of the device in FIG. 3B, in which two artificial joints are mounted by the fixtures and treated by bombardment respectively in two rooms separated by a fence;

FIG. 4 represents the device and the fixtures to generate nanostructures on NiTi wires by bombardment;

FIG. 5A represents microhardness of the as received stainless steel bone plate and the plates pre-treated by generating nanostructures and plasma nitrided at 450° C. for different time according to this invention;

FIG. 5B represents nitride layer thickness of the stainless steel bone plates in FIG. 5A;

FIG. 5C represents potentiodynamic scanning plots of the as received stainless steel bone plate and the plates pre-treated by generating nanostructures and plasma nitrided at 450° C. for 120 min according to this invention;

FIG. 6A represents titanium oxide thickness of the as received 200 mm NiTi wire and the wire pre-treated by generating nanostructures for 3 min and oxidized in 10% H2O2 solution at 80° C. for 24 hours according to this invention;

FIG. 6B represents Ni release to 50 ml 0.9% NaCl solution after soaked with 20 cm long NiTi wires of FIG. 6A for 6 months at ambient temperature (20-25° C.).

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS OF THE INVENTION

FIG. 1A represents a diagram of a device for generating nanostructures on a stainless steel intramedullary nail (10) by bombardment and the top view of the device without cover (20) of the chamber (21) and the balls (22). The cover (20) is fixed on the chamber wall by screws (201). The principle of generating nanostructures on a rotating stainless steel implant (10) with large length by bombardment according to FIG. 1 is to set the balls (22) in motion using two ultrasonic generators (23) operating at a given frequency, which are positioned at the bottom of the two rooms for projecting perfectly spherical balls (22). The amplitude of the movement of the sonotrode (23) could be chosen so as to be from a few microns to a few hundred microns. The balls (22) draw their energy from the movement of the sonotrode and hit the surface of the rotating implant a large number of times, at variable and multiple incident angles, creating with each impact a plastic deformation of the grains of the alloy in any direction. A ball that loses its energy in contact with the implant bounces off the sonotrode so as to acquire a new speed in a direction which, seen from the implant, seems random but is determined by physical laws. If the balls lose their energy, they will slide down along the sloping floor (24) of the rooms, and are collected and reused. Each end of the implant is mounted by a clamp (25), and the clamp is fixed by an insertion (26) that is fixed in a tube (27) by screws (28). The tube is supported by two bearings (29). The rotation of a motor (30) can be communicated to the gears (31) at the middle of the tube through a gear group (32). The implant is rotated at a given speed during the treatment so that the whole surface can be treated. The distance between the sonotrode (23) and the treated implant (10) can be controlled through adjusting the vertical position of the sonotrode. In FIG. 1 only two rooms are drawn for treating the slender implant. In the case of even longer and wider implants, the number of rooms and the rows of rooms can be further increased.

According to a variant of embodiment represented in FIG. 2A, the implant to be treated can be a long bone plate (11). Each end of the implant is mounted by a clamp (41), and the clamp is fixed by an insertion (42) that is fixed in a tube (27), which is rotated during treatment. In FIG. 2B two short bone plates (12) are connected as a long piece for treatment. Other treatment conditions are similar to those in FIG. 1.

According to another variant of embodiment represented in FIG. 3, the implant to be treated can be an artificial hip joint (13). Due to its complex shape, the head and the stem are treated respectively in two rooms. In such case the two rooms are separated by a fence (51). The untreated part of the implant can be held by a negative mold (52) or only clamped to a block during treatment. The vertical position of the mold or the block can be controlled by the screws (53). For large amplitude of height adjustment, a cap (54) is mounted on the cover (20) of the working chamber by screws (55). The pad (56) filled in the hole of the cover can be replaced with the mold (52) or the block attached with the implant if the vertical dimension of the implant is too large during treatment. In FIG. 3C, the artificial hip joint on the left can be rotated at a given speed during the treatment. Other treatment conditions are similar to those in FIG. 1.

FIG. 4 represents the fixtures and the device to generate nanostructures on a roll of NiTi wire by bombardment. The left fixture (14) can release and collect the NiTi wire (15), and the right fixture (16) is to reverse the moving direction of the wire. The original wire is released from the upper roll (61) on the left fixture, and the down roll (62) is driven by a motor (63) to collect the treated wire. The power of the motor (63) is supplied by the two metallic rings (64) on the left fixture (14), which are contacted with the two elastic electrodes (65) fixed on the table (33). A slide (66) is pressed on the rim of the upper roll (61) by a spring to exert an adjustable tensile stress in the wire. The two fixtures and the fillers (67) are fixed in the tube (27), which is rotated at a given speed during treatment. In FIG. 4 a fragment of the wire passes the working chamber twice with the help of the roller group (68), and thus the treating time is doubled. The treating time can be increased by further folding the wire, e.g., three or more times. The total treating time for a fragment of the wire is calculated by (width of an individual room ‘number of the rooms/linear moving speed of the wire)’ times of the wire passing through the chamber. Other treatment conditions are similar to those in FIG. 1.

The device is not limited to the embodiments described above, but encompasses any embodiment that makes it possible to generate nanostructured layers on metallic implants by the mechanical means.

The general principle for choosing the parameters to generate nanostructures on the implants according to the invention is that the greater the kinetic energy of the balls, the greater the level of stress generated in the underlying layer. The upper limit of the kinetic energy is defined particularly by the heating caused by the release of this kinetic energy during the impact on the surface being treated, and by the mechanical strength of the balls and of the material constituting the implants. The hardness of the balls plays a role, particularly in the transfer of kinetic energy from the ball to the surface of the implants. Experience has shown that the larger the diameter of the balls used, within a range of dimensions on the order of a few hundred microns to a few millimeters, the larger the nanostructured layer obtained.

Likewise, the treatment time is involved in determining the thickness of the nanostructures. It has been noted that up to a given time value, which is different and depends on the size of the balls, the more the time increases, the more the thickness of the nanostructured layer increases up to a time that corresponds to saturation and allows no further modification of the thickness of the layer. This given value is obtained either through experience, or from a mathematical model for a given material. However, when the time becomes greater than the given value, the thickness of the nanostructured layer decreases. This phenomenon is due to the fact that the impact of the balls on the treated surface generates an emission of heat, which heats up the material. Beginning at a certain threshold, the result of the heat is to increase the size of the metal grains.

The rotation speed of the implant being treated, e.g. from 0.5 rpm to 5 rpm, is another parameter. The rotation will help decrease local temperature rising of the treated implant, because the side opposite to the sonotrode has less bombardment and there is an interval for it to release heat. The higher the rotation speed is, the less heat is accumulated in a local area of the treated surface in a unit time. Thus, for thin wires or thin sheets, the rotation speed should be larger so as to avoid local overheating by bombardment. For the implants with large dimension, the rotation speed can be lower, but grain growth caused by the rapid local temperature rising should be also avoided.

Other parameters may also act on the nanostructure formation during treatment. In the device where an ultrasonic generator is used to set the balls in motion, the acoustic pressure generated by the sound waves may influence the nanostructure generating process. The temperature rising of the implant can be reduced with a cooling system so as to avoid the overheating effect on grain growth of the implant. The device can also be placed in an inert gas atmosphere or in vacuum to avoid oxidization of the surface of the metallic implants being treated.

The generation of nanostructures on the treated surfaces of the implants causes a modification of the law of diffusion in the treated area. In essence, the multiplication of the metal grains also multiplies the number of boundaries between the grains. These boundaries constitute nanometric channels that allow the diffusion of chemical compounds. Thus, these compounds can penetrate more deeply and more completely into the treated surface of the implants, making it possible to obtain a compound layer with advantageous mechanical and chemical properties.

As an example, FIGS. 5A and 5B represent microhardness and nitride layer thickness of the as received stainless steel bone plate and the plates pre-treated by generating nanostructures and plasma nitrided at 450° C. The stainless steel bone plates are pre-treated for 0.5, 1, 2, 4, 8, 15 and 30 min. The as received (or non pre-treated) and the pre-treated plates were plasma nitrided for 30, 60 and 120 min, respectively. Hardness of the as received and the nitrided plates was tested on a Vicker's microhardness tester with an indentation force of 100 gf. The nitride thickness was measured in cross-section under a microscope. It can be seen from FIG. 5A that hardness of the plasma nitrided plates is higher than that of the as received one, which is only 253 Hv. The hardness values increase with pre-treatment time before 4 min, and are near or above 1000 Hv after 1 min SMAT (surface mechanical attrition treatment). At the same pre-treatment time, the hardness increases with nitriding time. Pre-treatment has markedly increased thickness of the nitride layer. For example, the nitride layer thickness is only 8.5 mm for the non pre-treated plate, and is increased to 16.3 mm for the plate pre-treated for 30 min after 120 min plasma nitriding. Since the nitride layer has higher hardness than the stainless steel substrate, it is reasonable that the pre-treated plates have higher hardness than the non pre-treated one after plasma nitriding. The nanostructured layer with high hardness thus produced is beneficial for increasing wear resistance and fatigue life of the implants.

In fact, the presence of nanostructures, and in particular nanometric diffusion channels, allows faster diffusion of compounds into the superficial layer of metal pieces. The above results have evidenced that the growth of nitride layer on stainless steel plates pre-treated by generating nanostructures is quicker than that on the plate without pre-treatment, that is, the nitriding kinetics is accelerated. In addition, optical microscopic observation shows that the nitride layer is more even and dense for the pre-treated plates than the plates without pre-treatment.

FIG. 5C represents potentiodynamic scanning plots of the as received stainless steel bone plate and the plates pre-treated by generating nanostructures and plasma nitrided at 450° C. for 120 min according to this invention. The corrosion test was carried out in 50 ml 0.9% NaCl solution at ambient conditions with the test area 0.5 cm2. The as received plate has a breakdown potential of 0.31 V and there are corrosion pits after test, indicating low pit corrosion resistance. By comparing the potential-current plots and microscopic observation, it can be seen that the plates treated by 0.5-4 min pre-treatment and 120 min plasma nitriding show good resistance to pit corrosion; but the samples treated by 8-30 min pre-treatment and 120 min plasma nitriding show overall corrosion. Therefore, stainless steel implants by 0.5-4 min pre-treatment and plasma nitriding have high surface hardness as well as good pit corrosion resistance.

FIG. 6A represents oxide layer thickness of the as received 200 mm NiTi wire and the wire pre-treated by generating nanostructures for 3 min and oxidized in 10% H2O2 solution at 80° C. for 24 hours. It can be seen that the natural titanium oxide layer on NiTi wire is really thin, only about 50 nm. This thickness is determined from the profiles of DSIMS (dynamic second ion mass spectrum). The titanium oxide layer is formed because Ti has much higher affinity to O than Ni. In H2O2 solution, O atoms are rich at the NiTi wire surface, and Ti atoms in the alloy substrate can diffuse out to the surface to form titania compound under the high chemical driving force, leaving a Ni rich sub-layer. Since the titania layer and the substrate are different in nature, thick titania layer can cause crack or bonding problem of the system. The titania thickness can be adjusted by changing the oxidization parameters. The titanium oxide thickness shown in FIG. 6A is the optimized thickness that does not lead to crack or peeling of the layer by our experience. The titanium oxide layer may act as a shield to block the out-diffusion of Ni atoms from the wire implants to the surrounding body fluid. This effect can be exemplified by FIG. 6B, which represents Ni release to 50 ml 0.9% NaCl solution after soaked with 20 cm long NiTi wires of FIG. 6A for 6 months at ambient temperature (20-25° C.). The Ni content in solution is measured by atomic absorption spectroscopy. The Ni release is decreased from 26 ppb for the as received wire to only 5 ppb for the treated wire. The reduced release of harmful Ni ions helps to improve biocompatibility of the stent implants made of NiTi wires. At the same time, the increased corrosion resistance of NiTi wires is beneficial for the stability of stent implants in human body, which is a complex environment where multiple fields are coupled.

Device Examples

The device for generating nanostructures in a given thickness on metallic implants comprises a working chamber that can be divided into multiple rooms, means for setting in motion a given quantity of perfectly spherical balls of given dimension at a given speed in each room for a given duration, means for reusing the balls continuously in each room, and means for mounting and rotating the implants at a given speed to obtain variable angles of incidence at the same impact point, so that the impact points as a group cover the entire surface of the implant. The method for surface modifying metallic implants comprises means for forming protective compound layers in a sealed chamber or in a chemical solution at given conditions by using the high diffusion properties of the nanostructured layers.

In another embodiment, the working chamber has two rooms with a fence to separate them, and at least one room has adjustable roof height.

In another embodiment, the means for setting the balls in motion includes an ultrasonic generator at the bottom of each room, causing the balls to move in random directions, and the means for reusing the balls is the sloping floor of each room.

In another embodiment, the implant to be treated is a stainless steel intramedullary nail with diameter 10 mm and length up to 240 mm.

In another embodiment, the implant to be treated is a stainless steel bone plate with width 40 mm and length 220 mm.

In another embodiment, the implant to be treated is a stainless steel bone plate with width 50 mm and length 120 mm.

In another embodiment, the implant to be treated is a stainless steel artificial hip joint with stem length 90 mm and head diameter 28 mm.

In another embodiment, the implant to be treated is NiTi wire with diameter from 0.1 mm to 0.5 mm.

In another embodiment, the implant is fixed by clamps or other fixtures, which are mounted in the tubes driven by a motor through gears at a rotation speed between 0.5 rpm and 5 rpm.

In another embodiment, the linear moving speed of the NiTi wire is between 10 cm/min and 40 cm/min.

In another embodiment, the perfectly spherical balls are made of stainless steel.

In another embodiment, the perfectly spherical balls are made of zirconia.

In another embodiment, the diameter of the balls is between 0.3 mm and 3 mm, depending on the thickness of the nanostructures desired by the user.

In another embodiment, the balls are of a quantity such that, when the means for setting them in motion using ultrasound are inactive, they occupy a surface area greater than 30% of the surface of the sonotrode.

In another embodiment, the ball speed is between 5 mps and 100 mps.

In another embodiment, for a given ball size and a given material constituting the implant with given size, the projection time is determined based on the nanostructured layer thickness desired by the user.

In another embodiment, the projection time of the balls is between 30 s and 1800 s.

In another embodiment, the device for generating nanostructures includes means for adjusting emission time of the balls and their speed.

In another embodiment, the device includes means for adjusting the distance between the emission source of the balls and the implant to be treated.

In another embodiment, the distance is between 20 mm and 80 mm.

In another embodiment, the device for generating nanostructures includes means for performing a local cooling of the treated area of the implant.

In another embodiment, the projection step is performed after the chamber has been filled with inert gas.

In another embodiment, the device is enclosed in a vacuum cabinet.

In another embodiment, the device is enclosed in an acoustic isolation chamber.

In another embodiment, the step to form a protective nitride layer on stainless steel implants is plasma nitriding comprising the placement of implants in a nitrogen atmosphere at a given temperature around 450° C. for a given amount of time between 30 min to 120 min.

In another embodiment, the step to form a protective oxide layer on NiTi wires includes the placement of the wires in H2O2 solution with concentration from 10% to 30% at 60-100° C. for a given time between 12 and 24 hr.

While there have been described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes, in the form and details of the embodiments illustrated, may be made by those skilled in the art without departing from the spirit of the invention. The invention is not limited by the embodiments described above which are presented as examples only but can be modified in various ways within the scope of protection defined by the appended patent claims.

Claims

1. A method of enhancing diffusibility of a metallic surface, comprising a step of generating crystalline surface structure by surface mechanical attrition treatment.

2. A device for generating nanostructures in a given thickness on metallic implants comprises a working chamber that can be divided into multiple rooms, means for setting in motion a given quantity of perfectly spherical balls of given dimension at a given speed in each room for a given duration, means for reusing the balls continuously in each room, and means for mounting and rotating the implants at a given speed to obtain variable angles of incidence at the same impact point, so that the impact points as a group cover the entire surface of the implant.

3. The device for generating nanostructures in a given thickness on metallic implants according to claim 2, further comprises fences to separate the rooms of the working chamber.

4. The device for generating nanostructures in a given thickness on metallic implants according to claim 3, at least one room has adjustable roof height.

5. The device for generating nanostructures in a given thickness on metallic implants according to claim 4, further comprises an ultrasonic generator at the bottom of each room to set the balls in motion, causing the balls to move in random directions.

6. The device for generating nanostructures in a given thickness on metallic implants according to claim 4, further comprises the sloping floor of each room to reuse the balls.

7. The device for generating nanostructures in a given thickness on metallic implants according to claim 6, in which the implants are intramedullary nails, bone plates, artificial hip joints, etc made of stainless steel, and NiTi wires.

8. The device for generating nanostructures in a given thickness on metallic implants according to claim 6, further comprises clamps or other fixtures to fix the implants, which are mounted in the tubes driven by a motor through gears at a rotation speed between 0.5 rpm and 5 rpm.

9. The device for generating nanostructures in a given thickness on metallic implants according to claim 6, further comprises fixtures to move NiTi wires in a linear speed between 10 cm/min and 40 cm/min.

10. The device for generating nanostructures in a given thickness on metallic implants according to claim 6, in which the perfectly spherical balls are made of stainless steel or zirconia with diameter, depending on the thickness of the nanostructures desired by the user, being 0.3 mm to 3 mm.

11. The device for generating nanostructures in a given thickness on metallic implants according to claim 6, in which the balls are of a quantity such that, when the means for setting them in motion using ultrasound are inactive, they occupy a surface area greater than 30% of the surface of the sonotrode.

12. The device for generating nanostructures in a given thickness on metallic implants according to claim 6, in which the ball speed is between 5 mps and 100 mps.

13. The device for generating nanostructures in a given thickness on metallic implants according to claim 6, in which the projection time, determined based on the nanostructured layer thickness desired by the user for a given ball size and a given material constituting the implant with given size, is between 30 s and 1800 s.

14. The device for generating nanostructures in a given thickness on metallic implants according to claim 13, further comprises means for adjusting the emission time of the balls.

15. The device for generating nanostructures in a given thickness on metallic implants according to claim 6, in which the distance between the emission source of the balls and the implant being treated is between 20 mm and 80 mm.

16. The device for generating nanostructures in a given thickness on metallic implants according to claim 15, further comprises means for adjusting the distance.

17. The device for generating nanostructures in a given thickness on metallic implants according to claim 6, further comprises means for performing a local cooling of the treated area of the implant.

18. The device for generating nanostructures in a given thickness on metallic implants according to claim 6, in which the projection step is performed after the chamber has been filled with inert gas.

19. The device for generating nanostructures in a given thickness on metallic implants according to claim 5, which is enclosed in a vacuum cabinet.