1. Field of the Invention
The present invention relates to surface-modified metallic materials for medical purposes, in particular materials for use in circulatory medical devices. More specifically, the present invention relates to surface-modified metallic materials prepared by coating a thin film of gold or silver on the surface of metallic substrates, attaching to said thin film functional sulfur compounds having high adsorptivity, and chemically bonding sulfonated poly(ethylene oxide) (PEO) (alternatively, referred to as poly(ethylene) glycol, PEG) to functional groups of said sulfur compounds. The materials of the invention have remarkably improved antithrombogenicity and biocompatibility and thus are especially useful for medical devices such as prosthetic cardiac valves, stents and catheters.
2. Description of the Prior Art
Prosthetic cardiac valves have been implanted and used as substitutes for cardiac valves that have been impaired through hereditary or acquired means. Examples of prosthetic cardiac valves include valves made from tissues and mechanical valves made from metallic materials such as titanium. The tissue valves have good biocompatibility but have a drawback in that their internal durability is inferior due to calcification. The mechanical valves have excellent durability but are defective in that they may be accompanied by the formation of thrombus, requiring the patient to take anticoagulants throughout his or her life. Intensive research has been conducted in the art in order to improve the antithrombogenicity of mechanical valves. But because the formation of thrombus is a normal physiological phenomenon, it is impossible to completely prevent it. Moreover, the formation mechanism of thrombus has not yet been completely explained.
In order to treat the stricture of the coronary arteries, a percutaneous transluminal coronary angioplasty in which blood vessels are expanded by inserting intraaortic baloon catheters into the coronary arteries has been commonly used. This angioplasty has produced relatively good results, and its operation method and apparatus have been continuously developed. However, problems such as chronic closure and restenosis are still unsolved.
Stents are metallic implants in the form of a spring which are inserted into blood vessels to keep them expanded after the angioplasty has been carried out in order to prevent restenosis. Recently, the use of stents has increased. Stents are made from stainless steel, tantalum or titanium-nickel alloys and the like, and various types of stents, such as balloons or tubes have been developed and used. However, it has been found that the effort to prevent restenosis meets with failure about 20% to 30% of the time, even in cases when stents are implanted. It has been also ascertained that the main cause of such failure are restenosis, which is caused by an acute and chronic thrombus formation and the proliferation of smooth muscle cells in the internal walls of blood vessels from the wounds inflicted when inserting stents. The inherent properties of metals make it easy for thrombus to easily form on the surface thereof. Metal surfaces generally have a positive charge and, thus, exhibit high interreactivity with blood having a negative charge. Also, it has been ascertained that metals have high critical surface tension and thus are easily susceptible to the formation of thrombus, as noted by M.F.A. Goosen, et al. in Biomaterials, 17, 685-694 (1996).
U.S. Pat. No. 5,824,045, granted to E. Alt, and U.S. Pat. No. 5,976,169, granted to M. A. Imran, disclose attempts to improve the antithrombogenicity and to reduce allergic reactions of stents made from stainless steel and the like, by vapor deposition of a thin film of gold, platinum, silver or an alloy thereof on the surface of stents. However, these attempts have failed to provide a superior antithrombogenicity effect.
Also, U.S. Pat. No. 5,919,126 granted to A. J. Annini, discloses stents for the prevention of the restenosis by beta-ray emission, which are prepared by vapor deposition of a thin film of gold, platinum, titanium, nickel or the like on the surface thereof made from stainless steel, titanium or nickel-titanium alloy, and then by implanting radioisotopes to the film.
Further, many studies have been made in the art wherein polymers are coated on the surface of metallic materials which are to be used in the preparation of mechanical valves and stents in order to improve their antithromgenicity. For example, a method comprising the covering of the metallic surface with nylon mesh (See T. Yoshioka, et al., Am. J. Radiol., 15, 673-676, 1988), or a method comprising the coating of the metallic surface with silicone (See T. Roeren, et al., Radiology 174, 1069, 1990) or polyurethane (See I. K. De Scheerder, et al., J. Am. Coll. Cardiol 23, 186A, 1994) have been proposed, but these methods have not produced satisfactory results.
Further, there are proposed methods comprising the coating of the metallic surface with polymers having grafted heparin as an anticoagulant (See S. Stheth, et al., J. Am. Coll. Cardiol 23, 187A, 1994), with fibrin (See R. S. Schwartz, et al., J. Am. Coll. Cardiol 19, 171A, 1992), or with polymers containing agents such as dexamethasone (See A. M. Lincoff, et al., J. Am. Coll. Cardiol 23, 18A, 1994) such that the agents can be released slowly. However, these methods also failed to obtain the desired results.
In order to enhance the antithromgenicity of metallic materials for medical uses, many studies have been made with respect to anionic surfaces or structures having hydrophilic, hydrophobic and hydrophilic/hydrophobic micro-domains. It has been reported that a PEO-grafted surface prevents the adhesion of blood components such as proteins and blood platelets thereto and thus improves the antithromgenicity thereof (See J. D. Andrade, et al., Biomaterials 11, 455, 1990). It has also been reported that the PEO-grafted surface largely reduces the adhesion and spreading of cells as well as the adhesion and infection of bacteria (See J. A. Hubbell, et al., Biomaterials 13, 417, 1992).
Because metals have no functional groups that are chemically active, unlike polymers, i.e., organic materials, it is impossible to chemically modify them. There are some examples where PEO, poly(vinyl alcohol) or similar hydrophilic polymers have been applied to the surface of metals, especially stents, in order to modify the surface (See U.S. Pat. No. 5,843,172, granted to J. Y. Yan and U.S. Pat. No. 5,897,911, granted to J. P. Loeffler). However, because the polymers were simply coated on the metal surface, adhesive strength is poor and the antithromgenicity is below the desired level.
M. Grunze, et al. studied and reported that in a PEO self-assembled monolayer wherein the PEO is grafted to the surface of gold or silver film by means of sulfur compounds, the adsorption of protein is reduced (See J. Phys. Chem. B, 102, 426-436, 1998). Similar studies are in progress. However, it is impossible to obtain a practically applicable level of antithromgenicity, and there have been no reports on its commercialization.
We, the present inventors, found that the grafting of sulfonated poly(ethylene oxide) to the surface of polymers adds antithrombotic effects of the sulfonate group to the non-adhesion property of the PEO, thereby highly enhancing the antithrombogenicity and biocompatibility of the surface of the polymers (See Korean Patent. No. 62,921 to Y. H. Kim et al.).
The present inventors have now found that excellent antithrombogenicity and biocompatibility of metals can be obtained by chemically binding sulfonated PEO through sulfur compounds to thin film of gold or silver that is then stably coated on the surface of the metals. In other words, the present inventors have solved the problems mentioned above by discovering a thin film of gold or silver that could be stably coated on a metallic surface; sulfur compounds that form charge transfer complexes with said thin films and are strongly adsorbed onto the films; and sulfonated PEO derivatives that chemically bond to said sulfur compounds.
Thus, the object of the present invention is to provide metallic materials for medical purposes with remarkably improved antithrombogenicity and biocompatibility which are prepared by coating a thin film of gold or silver on a metallic surface by electroplating, vacuum vapor deposition, or ion sputtering methods, attaching thereto a sulfur compound having strong adsorptivity to said thin film, and then chemically bonding a sulfonated PEO derivative to functional groups of said adsorbed sulfur compound, and to provide a method for preparing the metallic materials.
The further object of the present invention is to provide circulatory medical devices, in particular, stents, prosthetic cardiac valves and catheters by using metallic materials that reduce the thrombogenicity and the adhesion and interaction of proteins and cells as a result of the synergic effects of the combination of the antithrombogenicity of sulfonic acid and the non-adhesion property of PEO.
The medical metallic materials according to the invention comprise
a metallic substrate;
a thin film of gold or silver coated on the surface of said substrate;
a functional sulfur compound adsorbed on said film; and
a sulfonated poly(ethylene oxide) (PEO) derivative chemically bonded to functional groups of said sulfur compound.
Metals which can be used as metallic substrates include, without limitation, iron, stainless steel, nickel, chromium, copper, titanium, tantalum and alloys thereof. A thin film of gold or silver generally has a thickness of tens of micrometers, but possibly up to several hundred micrometers. The thin film of gold or silver of the inventive metallic materials typically have a thickness of 0.1 to 100 xcexcm; whereas the ultrathin film of chromium, titanium or an alloy thereof having a thickness of about 0.01 to 1 xcexcm can be inserted between the surface of the metallic substrate and the thin film of gold or silver (See U.S. Pat. No. 5,919,126 to A. J. Armini).
A functional sulfur compound is formed as a self-assembled monolayer by chemisorption on the surface of a thin film of gold or silver (analyses have shown that said sulfur compound forms a charge transfer complex). Said functional sulfur compound is prepared by bonding functional groups such as hydroxyl, amino, isocyanate, aldehyde, carboxyl or its acid chloride, acid anhydride or acid amide, succinimidyl ester, succinimidyl carbonate, tresilyl, oxycarbonyl imidazole or nitrophenyl carbonate group to alkanethiol, dialkylsulfide, dialkyldisulfide, alkylxanthate, dialkylthiocarbamate.
More specifically, functional sulfur compounds, which can be used in the present invention, are those represented by the general formulas (1) to (5) that are given below. The compounds of general formula (4) or (5) may be also used in the form of salt or ester thereof:
Yxe2x80x94Rxe2x80x94SHxe2x80x83xe2x80x83(1)
Yxe2x80x94Rxe2x80x94Sxe2x80x94Rxe2x80x2xe2x80x94Yxe2x80x83xe2x80x83(2)
Yxe2x80x94Rxe2x80x94Sxe2x80x94Sxe2x80x94Rxe2x80x2xe2x80x94Yxe2x80x83xe2x80x83(3)
Yxe2x80x94Rxe2x80x94Oxe2x80x94CSSHxe2x80x83xe2x80x83(4)
(Yxe2x80x94R)2xe2x80x94Nxe2x80x94CSSHxe2x80x83xe2x80x83(5)
wherein
Y is hydroxyl, amino, isocyanate, aldehyde, carboxyl or its acid chloride, acid anhydride or acid amide, succinimidyl ester, succinimidyl carbonate, tresilyl, oxycarbonyl imidazole or nitrophenyl carbonate; and
R and Rxe2x80x2 are independently C2-C25 alkyl.
Examples of the alkanethiols-functional sulfur compound represented by the general formula 1 include, without limitation, mercaptoethanol, mercaptopropanol, mercaptobutanol, aminoethanethiol, aminomethylpropanethiol, mercaptoacetic acid, mercaptopropionic acid, mercaptosuccinic acid, thiolactic acid and substituted derivatives thereof.
Examples of the dialkylsulfides-functional sulfur compound represented by the general formula 2 include, without limitation, thiodiethanol, thiodipropanol, methylthioethanol, methylthiopropanol, methylthiobutanol, ethylhydroxyethyl sulfide, glucose dimethyl mercaptal, thioethylethyl amine, thiodiglycolic acid, thiodipropionic acid, methylthioacetic acid and substituted derivatives thereof.
Examples of the dialkyldisulfides-functional sulfur compound represented by the general formula 3 include, without limitation, hydroxyethyl disulfide, cystamine, dithiodipropionic acid, dithiodibutyric acid and substituted derivatives thereof.
Functional sulfur compounds, thus adsorbed on the metallic surface of a thin film of gold or silver, have a very strong bonding force that makes them stable under any kind of friction or various chemical environment such as weak acid or alkali, non-concentrated solvent, etc. Thus, they do not present any risks when applied to the human body or in other medical uses.
The sulfonated PEO derivatives should have a sulfonic acid group on one end and a functional group (X) which can be bonded to a functional group (Y) of sulfur compounds on the other end. Thus, their structure can be represented by the general formula 6 below:
HO3Sxe2x80x94Axe2x80x94PEOxe2x80x94Bxe2x80x94Xxe2x80x83xe2x80x83(6)
wherein
PEO is a poly(ethylene oxide) residue represented by the formula xe2x80x94(CH2xe2x80x94CH2xe2x80x94O)nxe2x80x94, wherein n is a integer from 5 to 250;
A and B are the same or different, and represents a C1-C3 alkylene group; and
X is selected from the group consisting of hydroxyl, amino, carboxyl, epoxy, aldehyde, succinimidyl ester, succinimidyl carbonate, tresilyl, oxycarbonyl imidazole and nitrophenyl carbonate group.
The molecular weight of a PEO unit of sulfonated PEO derivatives is from 200 to 15,000, more preferably from 500 to 10,000. If the molecular weight of PEO is smaller than 500, then the biological function of PEO, i.e., the function to reduce the adhesion of proteins, bloods and cells, is too small to have any effect. Because said function does not increase proportional to the molecular weight, the molecular weight of PEO over 10,000 is unnecessary.
The present invention is also related to a method for the production of a surface-modified medical metallic material, which comprises the steps of:
(a) providing a metallic substrate;
(b) coating a thin film of gold or silver on the surface of said substrate;
(c) applying on said film a functional sulfur compound which can form a charge transfer complex with said film, thereby be adsorbed on said film; and
(d) chemically bonding a sulfonated poly(ethylene oxide) (PEO) derivative to the functional groups of said sulfur compound.
Metals which can be used as metallic substrate include, without limitation, iron, stainless steel, nickel, chromium, copper, titanium, tantalum and alloys thereof.
The coating method in step (b) includes the electroplating, chemical vapor deposition, ion sputtering and thermal vapor deposition method.
The electroplating method is a method wherein a gold or silver coat is plated by using a gold or silver cyanide plating solution and applying a voltage of about 6 volts, and it has the advantage of a uniformly plated coat even though the shape of the substrate is complex. The thermal vapor deposition method is a method wherein thin film materials are deposited under a supervacuum state of 10xe2x88x928 mmHg at high temperature, close to their melting point. The ion sputtering method is also a method wherein thin film materials are ionized deposited by a current energy under a supervacuum state. An example of this method is the vacuum deposition of a thin film of gold, silver, aluminum or the like, which is well known in the art. But such method has disadvantages that it is difficult to uniformly deposit a substrate having a complex shape. The chemical vapor deposition method is characterized by the decomposition of thin film materials on the surface of a substrate to form the thin film. Although such method is carried out at ambient temperature and low pressure, it is not suitable in the present invention.
Generally, a thin film has a thickness of tens of micrometers or lower, but possibly up to several hundred micrometers. According to the present invention, the thin film of gold or silver is formed to a thickness of 0.1 to 100 xcexcm. Generally, stable thin films having abrasion resistance are formed, although they are somewhat different in term of roughness, stability and abrasion resistance, depending on the method used.
According to a preferred embodiment of the present invention, an ultrathin film of chromium, titanium or an alloy thereof can be deposited in a thickness of 0.1 to 0.5 xcexcm prior to step (b) to enhance the adhesion between a metal and a thin film of gold or silver.
In the adsorption of the functional sulfur compound in step (c), a functional sulfur compound is chemically adsorbed on the surface to self-assembled monolayer film (analyses have shown that said sulfur compound forms a charge transfer complex). Functional sulfur compounds useful for the adsorption in step (c) include those prepared by bonding a functional group (Y) to alkanethiol, dialkylsulfide, dialkyldifulfide, alkylxanthate and dialkylthiocarbamate, as described above. The above functional group (Y) is that which can be reacted with the functinal group (X) of the sulfonated PEO derivatives to be bonded in step (d). Group (Y) is selected from the group consisting of hydroxyl, amino, isocyanate, aldehyde, carboxyl or its acid chloride, acid anhydride or acid amide, succinimidyl ester, succinimidyl carbonate, tresilyl, oxycarbonyl imidazole or nitrophenyl carbonate group.
More specifically, functional sulfur compounds useful for the present invention include alkanethiols, such as mercaptoethanol, mercaptopropanol, mercaptobutanol, aminoethanethiol, aminomethylpropanethiol, mercaptoacetic acid, mercaptopropionic acid, mercaptosuccinic acid, thiolactic acid; dialkylsulfides, such as thiodiethanol, thiodipropanol, methylthioethanol, methylthiopropanol, methylthiobutanol, ethylhydroxyethyl sulfide, glucose dimethyl mercaptal, thioethylethyl amine, thiodiglycolic acid, thiodipropionic acid, methylthioacetic acid; dialkyldisulfides, such as hydroxyethyl disulfide, cystamine, dithiodipropionic acid, dithiodibutyric acid, etc. These sulfur compounds can be introduced with said other functional groups by means of a suitable substitution reaction. Alkyxthanates- and dialkylthiocarbamates-sulfur compounds, including suitable functional group (Y), are rarely available commercially as derivatives, but can be prepared by a known method in the art from the corresponding starting materials.
It has been reported that such functional sulfur compounds, in general, more easily and stably adhere to the surface of gold than that of silver (See A. Ulman, Chem. Rev. 96, 1533-1554, 1996).
The adsorption process of functional sulfur compounds is carried out by preparing the above functional sulfur compound as a diluted solution, and immersing a metal sample therein at ambient temperature for 6 to 24 hours. Alcohol is used as a solvent for sulfur compound, and the solution is used at a concentration of from 0.5 mMol to 2 mMol, preferably 1 mMol.
The sulfonated PEO derivatives used in step (d) should have a sulfonic acid group on one end and a functional group (X) which can be bonded with a functional group (Y) of sulfur compounds on the other end as described above.
More specifically, the sulfonated PEO derivatives useful in the present invention can be prepared by reacting usual PEO having hydroxyl group at both ends or by reacting PEO derivatives that are substituted by amino, carboxyl, epoxy, aldehyde, succinimidyl ester or the like at both ends with propane sultone or with a compound containing both sulfonic acid group and amino or other functional groups, for example, taurine (aminoethylsulfonic acid) or the like (See M. Harris, et al., PEO Chemistry and Biological Applications, ACS, 1997), which is shown but not limited to Schemes 1 to 6 below. Said PEO derivatives are commercially available from Shearwater (U.S.A.) or Nippon Oil Industrial Co., Ltd. (Japan):
Scheme 1
HOxe2x80x94PEOxe2x80x94OH+propane sultonexe2x86x92HOxe2x80x94PEOxe2x80x94Oxe2x80x94(CH2)3xe2x80x94SO3H
Scheme 2
H2Nxe2x80x94(CH2)2,3xe2x80x94Oxe2x80x94PEOxe2x80x94Oxe2x80x94(CH2)2,3xe2x80x94NH2+propane sultonexe2x86x92H2Nxe2x80x94(CH2)2,3xe2x80x94Oxe2x80x94PEOxe2x80x94Oxe2x80x94(CH2)2,3xe2x80x94NHxe2x80x94(CH2)3xe2x80x94SO3H
Scheme 3
HOOCxe2x80x94(CH2)1,2xe2x80x94Oxe2x80x94PEOxe2x80x94Oxe2x80x94(CH2)1,2xe2x80x94COOH+taurinexe2x86x92HOOCxe2x80x94(CH2)1,2xe2x80x94Oxe2x80x94PEOxe2x80x94Oxe2x80x94(CH2)1,2xe2x80x94COxe2x80x94NHxe2x80x94(CH2)2xe2x80x94SO3H

Scheme 5
OHCxe2x80x94CH2xe2x80x94CH2xe2x80x94Oxe2x80x94PEOxe2x80x94Oxe2x80x94CH2xe2x80x94CH2xe2x80x94CHO+taurinexe2x86x92OHCxe2x80x94CH2xe2x80x94CH2xe2x80x94Oxe2x80x94PEOxe2x80x94Oxe2x80x94CH2xe2x80x94CH2xe2x80x94CHxe2x95x90Nxe2x80x94(CH2)2xe2x80x94SO3H

wherein,
(CH2)2,3 and (CH2)1,2 represent those having 2 or 3, and 1 or 2 of CH2 groups, respectively.
A functional sulfur compound that can be adsorbed on a thin film of gold or silver, and sulfonated PEO derivatives bonded thereto are in principle indefinite in the selection. Thus, they are commercially available or can be prepared from the available compounds by methods known in the art. An economically feasable method should be found whereby starting materials can be made easily available to reduce costs and enable one to conveniently and simply carry out this process. That is, the reaction scheme should be designed, as in scheme 7 below, such that the sulfonated PEO derivatives have a sulfonic acid on one end and a functional group (X), which can be bonded to a sulfur compound at other end, and that the sulfur compounds contain a functional group (Y) which can be bonded with said functional group (X).
Scheme 7
HO3Sxe2x80x94Axe2x80x94PEOxe2x80x94Bxe2x80x94X+Yxe2x80x94R-Sulfur Compound/Thin film of gold or silver/Metalxe2x86x92HO3Sxe2x80x94Axe2x80x94PEOxe2x80x94Bxe2x80x94Xxe2x80x94Yxe2x80x94R-Sulfur Compound/Thin film of gold or silver/Metal
The functional group (Y) of sulfur compounds that can be bonded with the functional group (X) of sulfonated PEO derivatives is summarized in Table 1 below.
The binding reactions of the sulfur compounds and the sulfonated PEO derivatives according to the above Scheme 7 proceed as in Schemes 8 and 9, and various methods are possible depending on the types of X and Y and the types of sulfur compounds. 
Upon binding the functional group (Y) of the sulfur compounds and the functional group (X) of the sulfonated PEO derivatives, the appropriate catalysts, i.e., the substitution and the addition catalysts such as the esterification catalysts, the amidation catalysts and the like, may be used in order to allow the binding of two compounds proceed more quantitatively.
The binding reactions between the functional group (Y) of the sulfur compound and the functional group (X) of the sulfonated PEO derivatives are carried out in aqueous solutions, more preferably in buffer solutions, except in specific cases. The reactions are performed by adjustments to the acidic or basic pH depending on the nature of each reaction.
In order to estimate the hydrophilicity of the surface as properties of the modified metal, a contact angle was determined (Model CA-DT 11931, Kyowa Interface Sci., Japan). Antithrombogenicity was estimated according to the xe2x80x9cMeasurement of Platelet Adhesionxe2x80x9d as described below. Samples of the surface-modified metal were charged into a disposable syringe and 2 ml of a phosphate buffer solution were added. The phosphate buffer solution was replaced with 2 ml of the human platelet rich plasma (52xc3x97104 of platelet/ul), the syringe was suspended in a shaking incubator controlled at 37xc2x0 C. and was kept at the same temperature for a certain amount of time. The syringe was recovered, and the number of the non-adhesive platelets in the plasma was measured by a Coulter counter or Cytometer, thereby reverse-counting the number of the adhered platelets (See Hee-Jung Lee, et al., Polymer (Korea), 21, 1045-1052, 1997).
The chemical conformation of the surface was analyzed by means of ATR FTIR (Attenuated Total Reflectance Fourier Transform Infrared) and ESCA (Electron Spectroscopy for Chemical Analysis). The ATR FTIR was performed with a Bruker FTIR Apparatus (IFS 66; Bruker, German) using KRS-5 crystals. The ESCA was performed on ESCA 280-S (SSI, USA) with AlK a X-ray, with reference to the binding energy of Cls absorption area of the Cxe2x80x94H group as 285.0 eV, by using 2.50, 1.68, 1.80 and 8.5 as collecting factor of O1s, N1s, S2P3 and Na1s, and counting the ratios of O/C, N/C, S/C and Na/C. The morphology of the metal surface was analyzed by using an AFM (Atomic Force Microscope; Park Scientific Instruments, USA).
The present invention will be further described by the follow examples, but should not be construed as being limited by them. In the examples, the process for forming a thin film of gold or silver on the metallic substrate is performed according to the following thin film process.
Thin Film Process
1. Thermal Vapor Deposition Process
The metal samples (1xc3x971 cm and 1xc3x973 cm) washed with a saturated solution of chromic acid (Aldrich, USA) were mounted on the thermal vapor deposition apparatus (Model RH900, MDC, USA). The distance between the boat-shape thin film material evaporator made of molybdenum and the samples was maintained at 20 cm, the evaporator temperature was adjusted to about 1200xc2x0 C., and the vapor deposition using chromium or titanium was conducted under vacuum of about 1xc3x9710xe2x88x928 torr for 1 hour. The thickness of the resulting chromium or titanium ultrathin films was 200 xc3x85. Gold or silver was then deposited for 3 hours forming a thin film of gold or silver to a thickness of about 0.2 xcexcm.
2. Ion Sputtering Process
The metal samples (1xc3x971 cm and 1xc3x973 cm) washed with a saturated solution of chromic acid (Aldrich, USA) were mounted on the ion sputter (Model IB-3; Eiko Corporation, Japan). Under a vacuum of 1xc3x9710xe2x88x926 torr, the ionic current was adjusted to 7 mA and a chromium or titanium was then deposited for 40 minutes coating a chromium or titanium ultrathin films to a thickness of 200 xc3x85. Consequently, gold or silver was deposited for 1 hour forming a thin film of gold or silver to a thickness of about 0.2 xcexcm.
3. Electroplating Process
The metal samples (1xc3x971 cm and 1xc3x973 cm sizes), washed with a saturated solution of chromic acid (Aldrich, USA), were placed on the cathode of the electrolytic bath containing 500 ml of a solution comprising 15 g/l of gold cyanide or silver cyanide (Aldrich, USA), 100 g/l of potassium cyanide and 0.01 g/l of carbon disulfide. The platinum samples were then mounted on the anode. A current of 6 volts was applied and an ultrathin film of gold or silver was plated for 2 minutes to a thickness of 2 xcexcm.