RU2508130C1 - Fabrication of cardio implant from titanium nickelid-based alloy with surface layer modified by ion-plasma processing - Google Patents

Abstract

FIELD: process engineering.SUBSTANCE: invention relates to production of cardio implants from titanium nickelid-based alloy with shape memory and superelasticity effects modified surface layer. Said implants are intended for long-term operation in cardiovascular system and feature corrosion resistance, biocompatibility and nontoxicity in biological media. Proposed method comprises making said cardio implant, chemical and electrochemical surface cleaning, surface cleaning by silicon ion flows produced by spraying silicon cathode in vacuum under conditions of high-dose ion implantation with fluence of (0.5÷6.0)×10cmto obtain modified 80-95 nm-thick surface layer. The latter consists of at least two sublayers. Outer 20-25 nm-thick sublayer contains oxygen, carbon, silicon and titanium at the following ratio of components, at.%: oxygen - 25-65, carbon - 1-5, silicon - 1-10, titanium making the rest. Mid 60-70 nm-thick sublayer contains oxygen, carbon, silicon, titanium and nickel at the following ratio of components, at.%: oxygen - 5-30, carbon - 1-5, silicon - 10-30, nickel - 1-50, titanium making the rest. Note here that maximum concentration of silicon is reached at 30-35 nm from the surface.EFFECT: modified surface layer features no apparent interface between sublayers.9 cl, 1 dwg

Description

The invention relates to the manufacture of cardio implants from an alloy based on titanium nickelide with shape memory effect (EPF) and superelasticity with a modified ion-plasma surface layer for long-term use in the cardiovascular system of the body and possessing corrosion resistance, biocompatibility and non-toxicity in biological environments .

Known biocompatible multicomponent nanostructured coatings (MNP) for implants (RU 2281122, A61L 27/02, A61K 6/02, publ. 10.08. 2006) [1] operating under load, are based on titanium carbonitride with the introduction of additional elements that improve mechanical and tribological properties of the coating, as well as ensuring its bioactivity, biocompatibility and non-toxicity. The total concentration of the main and additional elements in the coating have the following ratio:

$$1.2<\frac{{\displaystyle \sum X_i}}{{\displaystyle \sum X_j}}<\mathrm{twenty}$$

where X _i is the total concentration of the main elements Ti, C, N in the coating, Y _j is the total concentration of additional elements Ca, Zr, Si, K, Mn, O, P in the coating, while the concentration of elements in the coating is chosen in the following ratio of components , at.%: Ti 30-50, C 15-40, N 0.5-30, O 5-25, Ca 0-7, Zr 0-20, Si 0-30, P 0-1.5, Mn 0-1.0, K 0-1.0. To measure the physicomechanical and tribological properties, MNPs were deposited on substrates of titanium alloy grade VT 1-0, nickel alloy Celite-N, cobalt alloy Celite-K, and titanium nickelide.

The disadvantage of this method is that the materials on which MNP was deposited, in addition to titanium nickelide, cannot be used for the manufacture of cardioimplants, in particular for clogging the left atrium of an umbrella device, since they do not have thermal shape memory or superelasticity in the human temperature range body.

The disadvantage of this method is the difficulty of achieving the specified component composition of the elements in the MNE coating.

A method for forming a nanostructured biocompatible coating on implants is described (RU 2448741, A61L 27/30, A61L 27/56, A61F 2/28, publ. 04/27/2012) [2], which consists in the deposition of a polysilicon film on the surface of the implant in a reactor. The resulting polysilicon film is subjected to chemical etching to form a nanostructured surface layer of porous polysilicon. The polysilicon film is etched when the implant is immersed in a mixture containing a 50-55% aqueous solution of tetrafluoroboric acid (HBF ₄ ), a 70-90% aqueous solution of nitric acid (HNO ₃ ) and an anionic surfactant based on the ammonium salt of perfluorosulfonic acid R _f PSC ₃ NH ₄ in an amount of 5 · 10 ^-3 -10 ^-2 (wt.%), Where R _f -C ₈ F ₁₇ , or C ₂ F ₅ OC ₃ F ₆ OC ₂ F ₄ , or C ₆ F ₁₃ CH ₂ CH ₂ . Aqueous solutions of acids are used at a ratio of their volume parts: HBF ₄ : HNO ₃ , as (100-800) :( 1: 1,1), followed by washing of the implant with deionized water and drying. The technological capabilities of the method are expanding, regardless of the materials used and the design features of the implants.

The disadvantage of this method is that the polysilicon film on the surface of materials selected from the series: metals (titanium, titanium alloys, steel), cermets and glass ceramics, as a result of chemical etching acquires a porous structure. Such a near-surface layer based on polysilicon with a porous structure cannot play the role of a barrier layer upon contact with a biological medium (blood, soft and hard tissues of the human body), which prevents the release of nickel ions into it. Another disadvantage of the known near-surface layer is its fragility and relatively low adhesive strength of adhesion to the surface of the implant, as a result of which this coating can break when the implant is deformed, which will disrupt the continuity of the barrier layer.

A known method of surface treatment of materials with shape memory, such as alloys based on titanium nickelide (US 2006157159, A61L 27/06, A61L 27/50, C23C 14/48, C23C 8/36, publ. 2006.07.20) [3], using methods of ion-beam or plasma-immersion implantation or ion deposition in order to change the surface properties of these materials for use, first of all, as medical materials. The surface treated with nitrogen, oxygen and carbon becomes bioinert, which can also be achieved after ion implantation of other chemical elements, such as silicon.

The disadvantage of this method is that it describes the methods of plasma-immersion ion implantation and deposition of coatings on medical structures made of titanium nickelide alloys based on ion-beam and plasma surface treatment methods, which allow to obtain thin coatings of a relatively small thickness - not more than 100 nm, which have the same disadvantages of a known source [2].

Also in the known method, it is noted as a recommendation that silicon can be used to create a bioinert surface of the implant. However, no data are presented on the chemical composition in the surface layer, including the content of nickel atoms and its change after ion-beam or ion-plasma treatment of an alloy based on titanium nickelide using fluxes of silicon ions.

The closest in technical essence is a method for producing a material based on titanium nickelide with EPF (RU 2191842, C22C 19/03, C23C 14/06, publ. 10/27/2002) [4] with a surface layer modified by ion implantation with alloying elements, in the quality of which oxygen, carbon, titanium and / or zirconium are selected, has a depth of the modified layer of 50-300 nm, and the composition of the modified layer has the following ratio of elements, at.%: oxygen 25-75, carbon 5-10, titanium and / or zirconium 20-50, nickel 0-20. In addition, titanium nickelide of the following composition was selected as the basis, at.%: Nickel 49-51, titanium - the rest. A material with such elements and at their given concentration in the modified layer has high corrosion resistance both in the initial state and after repeated deformation cycles in the load-unloading mode, as well as low solubility of nickel ions in aggressive environments.

The disadvantage of this method is that the material based on titanium nickelide with EPF with a surface layer modified by ion implantation with the alloying elements mentioned in the invention, although it has low solubility of nickel in biochemical solutions, but this factor is not enough to use this material based on nickelide titanium with EPF for the manufacture of cardio implants, as this does not provide the rapid integration of the implant with the body of an animal or person by creating on its surface and an organic film that insulates the inorganic (metallic) material from direct contact with blood.

The objective of the invention is to develop a method for the manufacture of a cardio implant from an alloy based on titanium nickelide with a modified ion-plasma surface layer treatment. Obtained by the proposed method, a cardio implant with a modified surface layer has biocompatibility, corrosion resistance and lack of toxicity.

The specified technical result is achieved by the fact that the method of manufacturing a cardio implant from an alloy based on titanium nickelide with a modified ion-plasma surface treatment involves ion implantation of alloying elements into the surface layer of a cardio implant with a fluence (0.5 ÷ 6.0) × 10 ¹⁷ cm ^-2 first, a cardio implant is made, then chemical and electrochemical cleaning of its surface is carried out, then the surface of the cardio implant is treated with flows of silicon ions obtained by spraying a silicon cathode in vacuum, in the high-dose ion implantation mode to obtain a surface modified layer with a thickness of 80-95 nm, consisting of at least two sublayers:

the outer sublayer with a thickness of 20-25 nm contains oxygen, carbon, silicon and titanium in the following ratio of elements, at.%:

oxygen 25-65 carbon 1-5 silicon 1-10 titanium rest;

the intermediate sublayer 60-70 nm thick contains oxygen, carbon, silicon, titanium and nickel in the following ratio of elements, at.%:

oxygen 5-30 carbon 1-5 silicon 10-30 nickel 1-50 titanium rest,

and the maximum concentration of silicon reaches a depth of 30-35 nm from the surface.

The initial alloy based on titanium nickelide has the following composition of chemical elements, at.%:

Titanium 49.00-49.50 Nickel 50.50-51.00.

The initial alloy based on titanium nickelide has additional impurities of O, N, C incorporation and Fe, Co substitution up to 0.2 at.%.

An alloy based on titanium nickelide has a temperature of completion of the reverse martensitic transformation (Ak) when it is heated no more than 23 ° C.

An alloy based on titanium nickelide has a reversible inelastic strain of at least 6%.

The cardio implant for clogging the ear of the left atrium is made in the form of an umbrella device (hereinafter referred to as an umbrella device).

Chemical cleaning of a cardio implant is carried out in a mixture of nitric and hydrofluoric acids at a temperature of 50 ° C.

The electrochemical purification of a cardio implant is carried out in a mixture of acids CH ₃ COOH (97%): HCIO ₄ (70%), taken in a ratio of 3: 1 vol.h.

The surface treatment of the cardioimplant with silicon ion flows is carried out at an accelerating voltage of 40-80 kV, with a pulse repetition rate of 50-100 Hz for 5-60 minutes.

The use of an alloy based on titanium nickelide, in particular, in the range of the following compositions, at.%:

titanium 49.00-49.50 nickel 50.50-51.00

in cardiovascular surgery, as a material for cardio implants, it is associated with the presence of valuable mechanical properties in this alloy, such as thermal shape memory, superelasticity. In particular, the aforementioned titanium nickelide double alloy may additionally contain impurities of O, N, C incorporation and substitution of Fe, Co up to 0.2 at.%. The content of such a small amount of these impurities does not significantly affect its functional properties. The titanium nickelide-based alloy used has a temperature of completion of the reverse martensitic transformation (Ak) when it is heated no more than 23 ° C and a reversible inelastic strain of at least 6%.

The indicated properties of an alloy based on titanium nickelide are necessary in the manufacture of cardioimplants of certain designs, requiring restoration of their shape at the temperature of the human body, as well as for their long-term work in the organs of the cardiovascular system under load (muscle pressure on the structure from the side of the vessel walls).

The presence of a significant fraction of nickel atoms in the alloy (50.50-51.00 at.%) Necessitates the creation of a barrier layer that prevents the penetration of nickel ions into the biological medium (tissues and liquids - blood, blood lymph) as a result of corrosion processes occurring on the surface of a cardio implant made from an alloy based on titanium nickelide in contact with biological tissues and fluids and accompanied by the release of nickel ions. An increase in the concentration of nickel atoms in tissues above the permissible level contributes to the deterioration of the biological compatibility of the implant, has a toxic and allergic effect on the body, and can lead to inflammatory processes.

It is known that surface modification of cardio implants from titanium nickelide-based alloys can lead to a change in the mechanical characteristics of the surface layers of the alloys.

Despite a significant amount of published experimental data [1-4], the problem of finding optimal methods for surface treatment of cardio implants from an alloy based on titanium nickelide with EPF has not yet been solved.

The present invention provides a method for manufacturing a cardio implant from an alloy based on titanium nickelide with a modified ion-plasma surface treatment.

A private cardio implant is an umbrella device and can be made of an alloy tube based on titanium nickelide using laser cutting and forming templates; in this way, a frame blank of the umbrella device is obtained.

Then, chemical and electrochemical cleaning of the frame blank of the umbrella device is carried out, which consists in removing laser cutting defects such as sagging, splashing and reducing the thickness of the oxide film.

The choice of silicon as a chemical element for alloying the surface layers of an alloy based on titanium nickelide will solve some of these problems due to its chemical (electronic carbon analog) and physical (partially soluble in the titanium lattice with the formation of a limited substitutional solid solution and forms bioinert, insoluble phases based on titanium, nickel and silicon, which can be the basis for the synthesis of composite cermets based on titanium and silicon) and biological (high compatibility bridge with living cells) properties.

Ion-plasma treatment of the surface of the cardio implant with flows of silicon ions is accompanied by sputtering of the surface layer, its texturing and lowering the roughness class, which increases the ability of cells to attach to the surface of the cardio implant, and the developed surface landscape becomes a natural reservoir of the extracellular matrix, that is, a nutrient medium that always surrounds a living cell .

The experiments performed on the modification of the surface layer of a cardio implant from an alloy based on titanium nickelide by treating the surface of a cardio implant with flows of silicon ions obtained by sputtering a silicon cathode in vacuum in the high-dose ion implantation regime with fluence (D) equal to (0.5 -6.0) x10 ¹⁷ cm ^-2 show that in the surface layer with a thickness of 80-95 nm a silicon-containing layer is formed, the maximum concentration of silicon atoms in which reaches 30 at.% at a depth of 30-35 nm (see Fig. 1).

The claimed fluence range provides the required, predetermined concentration of atoms of added (alloying) chemical elements - silicon, oxygen and carbon, their distribution over the depth of the modified surface layer during ion processing, as well as the thickness of the modified layer (see Fig. 1).

The fluence values necessary for achieving the technical result within the stated limits are obtained by varying the parameters of the ion-plasma treatment, namely, an accelerating voltage of 40 - 80 kV, a pulse repetition rate of 50-100 Hz and a processing time of 5-60 minutes. In the present invention, in the section of the invention, one example of achieving the desired declared fluence value is given. Based on professional knowledge in the field of ion-plasma treatment, a specialist is able to obtain other fluence values within the stated limits, varying the above processing parameters.

Figure 1 shows the Auger spectrum corresponding to the composition of the initial sample in the layer at a depth of ~ 10 nm (a), the distribution profiles of the concentration of the main elements in the surface layers of the TiNi alloy before (b) and after ion modification with silicon (c).

Under conditions of ion implantation, the concentration of elements in the surface layer of the cardio implant is redistributed from an alloy based on titanium nickelide, which leads to a significant decrease in the nickel content in the outer sublayer with a thickness of 20 to 35 nm until they are completely absent to a depth of 20 nm from the surface of the cardio implant.

As a result of ion-plasma exposure, the surface layer of a cardio implant from an alloy based on titanium nickelide is fragmented to a depth of more than 15 μm with the formation of a grain-subgrain structure with subgrain sizes in the range from 100 nm to 1000 nm. Grinding the grain structure in the surface region of a cardio implant from an alloy based on titanium nickel leads to an increase in the corrosion resistance of the surface layer in the biological medium due to a significant increase in the concentration of grain boundaries and activation of oxidative reactions in relation to titanium in their vicinity, and the rapid formation of an oxide film based on titanium and, as a result, a significant increase in the potential for the passivation of EPP.

Thus, when silicon ions act on the surface of a cardio implant from an alloy based on titanium nickelide, which was carried out in an ion-plasma installation, the surface region of the cardio implant is modified in the form of a surface layer consisting of at least two sublayers, which differ significantly in the ratio of oxygen concentrations , carbon, nickel and silicon. The modified surface layer with a modified composition does not have a pronounced interface between the sublayers characteristic of the deposited layer (see figure 1).

Figure 1 shows the concentration distribution profiles of the main chemical elements in the surface layers of a cardioimplant of titanium nickelide alloy. It is seen that the maximum oxygen and carbon content on the surface, 50 at.% And 20 at.%, Respectively, decreases exponentially in a rather narrow outer layer and already at a depth of 10-20 nm each does not exceed 5 at. The distribution of embedded silicon ions is described by a curve with a maximum silicon concentration of 30 at.%, Located in the near-surface region, as already noted above, at a depth of 30-35 nm from the surface.

The presence of carbon in the modified layer is due to the existence of adsorbed carbon on the initial surface of the cardioimplant, on the surface of the sprayed material (silicon cathode), as well as the presence of carbon in the residual atmosphere of the technical vacuum, which (carbon) penetrates into the deeper layers of the processed by atomic mixing and diffusion mechanisms titanium nickelide alloy.

After ion modification, the concentration of oxygen in the surface layer increased, and the thickness of this layer was 2 times the thickness of the oxide layer of the cardioimplant before ion treatment. This is due to the fact that, in the process of ion implantation, in addition to the atoms of the main implantable chemical element (silicon), as a rule, in the ion beam there are elements such as oxygen, carbon, present both in the residual atmosphere of the working chamber and adsorbed on the surface of the irradiated cardioimplant . In the process of ion-beam exposure, due to atomic mixing of the surface and inner layers, and also as a result of radiation-stimulated diffusion, the ions of these elements fall into the surface layer and are distributed in it, participating in the formation of the modified surface layer.

Particularly noteworthy are the features of the redistribution of nickel atoms in the surface modified layer. Figure 1 shows that in the outer (outer) sublayer with a thickness of 20 nm, nickel atoms are practically absent, then there is an increase in their concentration to a value corresponding to its initial concentration in the alloy at a depth of 80-95 nm from the irradiated surface.

From the results of the corrosion tests it follows that the potential for the passivation of E _{pp of an} alloy based on titanium nickelide in biochemical aqueous solutions of 0.9% NaCl and blood plasma after modification of the surface of a cardioimplant is on average 0.9 V, in contrast to its value E _pp ≈0.1 V obtained during testing of a cardioimplant with unmodified surface. The increase in the corrosion resistance of the surface of a cardio implant from an alloy based on titanium nickelide with a modified silicon ion surface layer is due to a change in the chemical composition in the surface layer of a cardio implant after treatment with an silicon ion beam, which resulted in the formation of at least two highly corrosion-resistant sublayer on the cardio implant located one under another: the first - the outer - based on titanium oxy-carbides (a sublayer based on titanium, oxygen and carbon); and the second - intermediate - based on the silicides of titanium and nickel; as well as the fact that, firstly, silicon in the composition of the surface layer does not participate in redox processes and does not form soluble forms at pH 7.3, and secondly, a change in the structural state, namely, fragmentation and nanostructuring of the surface layer, accelerates the passivation process during corrosion tests with the formation of an additional protective passivating layer on the surface of the cardioimplant. As a result of potentiostatic exposure in biochemical solutions at a value of the anode potential E exceeding the value of the stationary potential E _article for an alloy based on titanium nickelide, the formation of a poorly soluble compound Ni ₂ SiO ₄ is possible, which may be one of the reasons for the stabilization of the passive state of the surface layer of a cardioimplant in the region of positive potentials increasing its corrosion resistance.

From in vitro biocompatibility tests with living cells (rat bone marrow mesenchymal stem cells (MSCs) were used) of a cardiac implant from an alloy based on titanium nickelide with an unmodified and modified (silicon) surface layer, it was found that the proliferation efficiency of MSC cells on the surface of a cardioimplant after ion modification turned out to be ~ 1.3 times higher than on the surface of a cardio implant, which was not subjected to ion-beam treatment with silicon. This means that after modification with beams of silicon ions, the biocompatibility of the cardio implant has increased. The increase in the proliferation efficiency of MSC cells is due to the physicochemical properties of the outer layer formed by ion beam exposure, which led to an increase in passivability, high corrosion resistance, and bioinertness of a cardioimplant with a modified surface layer.

The invention is as follows.

Holes and cuts of the desired geometry are cut out from the tube of an alloy based on titanium nickelide 3 mm in diameter using a laser cutting apparatus (disk laser with diode pumping) according to a specially defined program to obtain a frame blank.

Carry out a chemical cleaning of the frame blank, which is necessary to remove defects in laser cutting such as sagging, splashing and reducing the thickness of the oxide film. For this, the frame billet is treated with a mixture of nitric and hydrofluoric acids at a temperature of 50 ° C for 5-7 seconds. Carry out the cleaning of the frame billet in an ultrasonic bath in distilled water.

Next, the frame blank with the help of forming templates give the necessary three-dimensional shape of the umbrella device. For this, the pre-chilled frame blank is pulled onto a chilled inner forming template and clamped by a chilled external forming template. The operation is performed at a temperature of minus 15 to plus 5 ° C. The pressed workpiece, together with the forming templates, is placed in the melt of a mixture of inorganic salts at a temperature of 450-475 ° C for 15 minutes. The blank is quenched in room temperature water, and then the bulk blank of the umbrella device (hereinafter in the example of a cardioimplant) is freed from the external template and removed from the internal template.

Next, the cardio implant is subjected to electrochemical purification in a mixture of CH ₃ COOH (97%): HCIO ₄ (70%), taken in a ratio of 3: 1 part by volume, at an electrolyte temperature of 0 ° C, processing time 10-15 seconds at a voltage of 30 V After electrochemical cleaning, the cardio implant is washed in distilled water in an ultrasonic bath. The washed cardio implant is dried.

Next, the surface of the cardio implant is treated with accelerated silicon ions in a vacuum of an ion-plasma installation (hereinafter, the installation).

For this, similar cardio implants to be processed are fixed in the holders of the positioning system, the installation desktop. The positioning system provides uniform processing of the external and internal surfaces of several (up to 10 pieces) cardio implants simultaneously with a stream of silicon ions. The desktop is moved to the working chamber of the installation. The chamber is pumped to a pressure of at least 3 · 10 ^-3 Pa (2 · 10 ^-5 torr) to reduce the content of impurity atoms (oxygen, carbon) in the residual atmosphere to their minimum concentration.

Turn on the heating-cooling system of the working chamber. The camera should warm up to a temperature of 30-35 ° C. Install an accelerating voltage of 60 kV, a bias voltage of 1000 V, a pulse repetition rate of 50 Hz, include a pulse accelerator of silicon ions and a positioning system. Ionic modification of the surfaces of cardio implants is carried out using pulsed single-component beams of silicon ions in oil-free pumping conditions and high vacuum, in the high-dose ion implantation regime with a fluence of 2 × 10 ¹⁷ cm ^-2 . Processing is carried out for from 15 minutes During processing, each cardio implant is rotated along a predetermined path to achieve uniform distribution of the radiation dose over the surface of the treated cardio implant. To unload the treated cardio implants, air is introduced into the chamber, the table is rolled out and the processed cardio implants are removed from the holders. The proposed ion-plasma treatment provides the claimed properties of the modified surface layer of the cardioimplant.

Claims (9)

1. A method of manufacturing a cardio implant from an alloy based on titanium nickelide with a modified ion-plasma surface treatment, including the ion implantation of alloying elements into the surface layer of a cardio implant with a fluence (0.5 ÷ 6.0) × 10 ¹⁷ cm ^-2 , characterized in that a cardio implant is first made, then chemical and electrochemical cleaning of its surface is carried out, then the surface of the cardio implant is treated with flows of silicon ions obtained by sputtering a silicon cathode in vacuum, high-dose ion implantation mode to obtain a surface modified layer with a thickness of 80-95 nm, consisting of at least two sublayers:
the outer sublayer with a thickness of 20-25 nm contains oxygen, carbon, silicon and titanium in the following ratio of elements, at.%:
oxygen 25-65 carbon 1-5 silicon 1-10 titanium rest;
the intermediate sublayer 60-70 nm thick contains oxygen, carbon, silicon, titanium and nickel in the following ratio of elements, at.%:
oxygen 5-30 carbon 1-5 silicon 10-30 nickel 1-50 titanium rest,
and the maximum concentration of silicon reaches a depth of 30-35 nm from the surface. 2. The method according to claim 1, characterized in that the initial double alloy based on titanium nickelide has the following composition of chemical elements, at.%:
Titanium 49.00-49.50 Nickel 50.50-51.00. 3. The method according to claim 2, characterized in that the initial double alloy based on titanium nickelide has additional impurities of incorporation of O, N, C and substitution of Fe, Co up to 0.2 at.%. 4. The method according to any one of claims 2 or 3, characterized in that the titanium nickelide double alloy has a temperature of completion of the reverse martensitic transformation when it is heated no more than 23 ° C 5. The method according to any one of claims 2 or 3, characterized in that the titanium nickelide double alloy has a reversible inelastic strain of at least 6%. 6. The method according to claim 1, characterized in that the cardiac implant for clogging the ear of the left atrium is made in the form of an umbrella device. 7. The method according to any one of claims 1 or 6, characterized in that the chemical purification of the cardiac implant is carried out in a mixture of nitric and hydrofluoric acids at a temperature of 50 ° C. 8. The method according to any one of claims 1 or 6, characterized in that the electrochemical purification of the cardiac implant is carried out in a mixture of acids CH ₃ COOH (97%): HCIO ₄ (70%), taken in a ratio of 3: 1 vol.h. 9. The method according to any one of claims 1 or 6, characterized in that the surface treatment of the cardioimplant with silicon ion flows is carried out at an accelerating voltage of 40-80 kV, with a pulse repetition rate of 50-100 Hz for 5-60 minutes.

Images