AU696951B2

AU696951B2 - Electroless deposition of metallic coatings on non-conducting substances

Info

Publication number: AU696951B2
Application number: AU76874/96A
Authority: AU
Inventors: Neil Lawrence Anderson; Robert Driver; Kannai Paulose; Michael George Stevens
Original assignee: Cardiac CRC Nominees Pty Ltd
Current assignee: Amc Technologies Pty Ltd
Priority date: 1995-12-08
Filing date: 1996-12-06
Publication date: 1998-09-24
Anticipated expiration: 2016-12-06
Also published as: AU7687496A

Description

Electroless deposition of metallic coatings on non-conducting substances

Technical Field

This invention relates to method for producing metallic coatings on substrates using electroless deposition and more particularly to the catalytic surface preparation of substrates prior to electroless deposition so as to avoid the inclusion of metals such as tin in the coating. Background Art

It is possible to produce metallic coatings on non-conductive substrates using electroless deposition. In order to produce such coatings, it is essential that the substrate surface to be coated is first treated with a metallic catalyst so as to activate that surface. Activation of the surface then allows for a layer of metal to be deposited onto the surface by electroless deposition. In essence, the catalytic treatment acts to initiate the deposition of the metal onto the surface; the formation of a continuous and adherent layer then proceeds by autocatalysis.

Various catalyst compositions and methods have been used in the past to prepare a substrate prior to electroless deposition. For example, in one technique a substrate is first immersed in a stannous chloride solution followed by immersion in an acidic palladium chloride solution to form an activating deposit of palladium on the substrate.

One of the most widely used catalysts is a colloidal suspension of palladium particles which are embedded in a tin-base matrix. This is descπbed for example in US Patent No 3011920. In this case it is known that the use of this catalyst results in the occlusion of tin in the final coating. In US Patent No 3963841. it is disclosed that the catalyst comprises a complex between a precious metal salt of a metal of the fifth and sixth periods of Group 8 of the periodic table and dimethyl sulfoxide together with a metal salt of Group 4 of the periodic table which is capable of reducing the precious metal salt to its elemental form. Examples of precious metals are palladium, platinum, ruthenium and osmium whilst examples of suitable

Group 4 metals include stannous tin, titanium and germanium.

It is important to note that the complex and the Group 4 metal salt are contained in an organic solvent such as a ketone, alcohol, ether or ester. The present inventors have recognised that in some applications it is desirable that the occlusion of tin or other metals in the final coating be avoided. Such applications include but are not limited to biomedical applications.

It has been further recognised by the present inventors that a particular difficulty in achieving such coatings occurs when one requires an adherent metal coating on a polymer substrate, particularly polyesters.

Through the invention that will now be described, the present inventors seek to provide a process that avoids the inclusion of tin and other contaminating species in the coating.

Disclosure of Invention Accordingly, in a first aspect, the present invention consists in a method for producing a metal coated non-conductive substrate comprising: a) contacting a substrate with a solution of a noble metal compound in a manner such that noble metal ions are adsorbed to the surface of the substrate: b) treating the substrate with a non-contaminating reducing agent so as to reduce the surface-adsorbed noble metal ions to their elemental state; and c) coating the substrate with a metal in an electroless metal-plating bath to foπn a metal coated substrate.

In a second and more general aspect, the present invention further consists in a catalytic process for the preparation of the surface of a substrate prior to the coating of the substrate in an electroless metal-plating bath comprising: a) contacting a substrate with a solution of a noble metal compound in a manner such that noble metal ions are adsorbed to the surface of the substrate; and b) treating the substrate with a non-contaminating reducing agent so as to reduce the surface-adsorbed noble metal ions to their elemental state and thereby provide a substrate suitable for metal coating in an electroless metal- plating bath. Non-aqueous or aqueous solvents can be used in the methods of the present invention.

A solvent is used in this invention to form a solution of a noble metal compound. Such a solvent will desirably be compatible with the substrate to be coated. Amongst the non-aqueous solvents that may be used include dimethyl sulfoxide, dimethyl formamide and tetrachloroethane. The preferred non-aqueous solvent is dimethyl sulfoxide. Particularly suitable compounds for use in an aqueous medium are complexes of ethylenediaminetetraacetic acid (EDTA) and EDTA disodium salt,

When using an aqueous solution, the present inventors have found that the order of steps (a) and (b) is not particularly critical. In this form, the substrate is preferably treated with the reducing agent (step (b)) prior to being contacted with the solution of noble metal compound (step (a)).

A variety of noble metal compounds may be used including salts and complexes of such metals. By noble metals it is meant platinum, palladium, silver, gold, ruthenium, rhodium, osmium and iridium. Suitable salts of these metals include halides, such as palladium chloride and platinum chloride and nitrates such as silver nitrate.

Although the concentration of the noble metal compound is not critical, for guidance solutions in the concentration range of 0.05 to 0.5% w/v may be used. A preferred concentration is 0.2% w/v. When using an aqueous solution, it is preferred that the noble metal is treated with a complexing agent to form a noble metal complex which is soluble in water and stable with time, and which the noble metal complex ion is capable of reduction to the metallic state in the presence of a suitable reducing agent. The complexing agent may be a neutral or ionic species containing one or more amino, carboxylate, or other functional groups capable of acting as metal binding sites. Prior to contact with the aqueous solution containing the noble metal complex, the substrate is preferably immersed in a solution containing a suitable concentration of a non- contaminating reducing agent. The concentration of this reducing agent will depend to some extent on the stability constant of the noble metal complex.

The substrate should be contacted with the solution of the noble metal compound in a manner such that the noble metal ions are adsorbed to the surface of the substrate or undergo reduction in close proximity to the substrate surface. One way of achieving such contact is to immerse the substrate in a suitable volume of the solution.

A variety of non-conductive substrates may be coated using the present invention. These substrates include polymeric materials, particularly polyesters. One preferred polyester is polyethylene terephthalate (PET).

The substrate may be in a variety of physical forms including planar portions or fibres. The preferred substrate is a polyester yarn. Prior to contacting the substrate with the solution of the noble metal compound, it is preferred to wash the substrate with a suitable solvent so as to remove any surface contaminants introduced during manufacture of the substrate. Solvents that may be used to remove contaminants such as lubricating oils include organic solvents such as ethanol, methanol, ethylacetate, acetone or methylethylketone and aqueous surface active agents which may be at elevated temperatures eg. 50-80°C.

Preferably the substrate will be washed with a hot solvent.

It is also desirable, although such a step is optional, to chemically etch the surface of the substrate with an aqueous solution of alkaline metal hydroxide. Suitable alkaline metal hydroxides include sodium hydroxide and potassium hydroxide. Conveniently the alkaline metal hydroxide solution will be hot. up to about 130°C. Alternatively, the alkaline metal hydroxides may be dissolved in ethanol or an ethanol/aqueous mix. This chemical etching step is carried out in order to increase the area of contact between the metal coating and the substrate surface, thereby promoting adhesion.

Following contact of the substrate with the solution of the noble metal compound to form noble metal ion adsorbed to the surface of the substrate, the substrate is treated with a non-contaminating reducing agent so as to reduce the surface adsorbed noble metal ions to their elemental state. Preferably this treatment will follow without rinsing to remove excess noble metal solution by immersing the substrate in a suitable volume of the non- contaminating reducing agent. One reducing agent that has been found to be suitable is hydrazine.

This compound has the advantage that the by products formed during the reduction are nitrogen gas and water. Therefore the likelihood of any substantial retention of a contaminating species in the coating is minimised. The concentration of reducing agent vised will vary according to the actual agent used. Similarly, treatment time will vary although where the treatment comprises immersion in the non-contaminating reducing agent solution, it may as short as one minute.

Following treatment with the non-contaminating reducing agent, it is preferred that the substrate is rinsed thoroughly with water. The substrate may then be coated in an electroless metal-plating bath in a conventional manner. A variety of metal coatings may be applied including titanium, palladium, nickel, copper, cobalt, iron, and gold and platinum.

Using the method of the invention not only circumvents the inclusion of undesirable contaminating species in the coating but also provides a coating which has good adhesion to the surface of a substrate.

Both of these properties are important and desirable to biomedical applications.

Accordingly in a third aspect, the present invention still further consists in a metal-coated substrate formed using the method of the first or second aspect of this invention.

The metal coated substrates of the invention may be used in a variety of biomedical applications such as implantable electrode leads. In such applications the substrate will comprise fibres with a portion coated with metal, such portion serving as an electrode. For this application, biocompatibility is important and as mentioned above this is achieved in the present invention by eliminating the presence of undesirable contaminating species such as tin.

Implantable leads must also be flexible and be highly resistant to fatigue. In addition, the implantable leads must have high electrical conductivity. Metal coating of individual yarns achieve this property. These properties are required owing to the long-life requirements for such leads.

Desirably, the leads should also be capable of being formed into complex shapes such as braided, knitted or woven sheets, threads or tubes.

Other biomedical applications include the formation of coated fibres into textiles for use as patches, sutures, vascular grafts or other applications in place of, for example, Dacron. In this regard it should be noted that platinum is less thrombogenic than Dacron.

In order to modify the metal surface produced by the method of this invention, for specific purposes, pendant groups such as lipids, proteins, peptides and other biologically active species may be added using thiol chemistry.

Another application for textiles produced using fibres coated according to the method of this invention is in iontophoresis. In this application, a drug is attached to the fibres or impregnated in the textile and is able to be released when an electric current is applied. When using metal coated textiles for carrying charge either as conductors or as electrodes, it is important to be able to form reliable electrical connections. Connections that require heat such as welding or crimping are unsuitable due to damage to a polymer substrate such as polyester. Alternative methods include mechanical methods such as crimping or shape memory alloy rings. However, crimping will cause fibre damage due to stress concentration at the crimp points.

Shape memory alloy rings are an excellent alternative. They are placed over the conductive textile e.g. tube or thread, and are then heated causing the ring to contract onto the substrate. A fatigue resistant and highly conductive joint results with high tensile strength. Importantly, the fibres are also undamaged due to the uniform distribution of clamping pressure.

It will of course be evident to those skilled in the art that the metal coated substrates of the invention may, however, be used in a variety of other applications.

In order that the present invention may be more clearly understood, preferred forms will be described with reference to the following examples and drawing.

Brief Description of Drawings Figure 1 is a photomicrograph of a polyester fibre coated according to the present invention.

Figure 2 shows the results of a cell growth inhibition assay.

Figure 3 shows the results of a fatigue test carried out on a fibre coated by the method of the present invention. Modes for Carrying Out the Invention

EXAMPLE 1

A PET fibre was cleaned in boiling ethanol for 20-30 minutes and then dried in air for about one hour at 60°C. The fibre was then chemically etched in a 3% w/v solution of sodium hydroxide in water at a temperature of about 100°C for 5 minutes.

Following etching, the fibre was rinsed thoroughly in de-ionised water.

A non-aqueous solution of 0.2% w/v of palladium chloride in dimethyl sulfoxide was prepared and the fibre immersed for about 5 minutes at room temperature. After the catalyst treatment the PET fibre was transferred to a 4% w/v solution of hydrazine in water. Reduction using the hydrazine solution was carried out at room temperature for about one minute.

Following reduction, the fibre was thoroughly rinsed using de-ionised water before being coated with an approximately 1 micron thick deposit of platinum using a proprietary electroless plating bath.

EXAMPLE 2

A further PET fibre sample was coated with platinum using the method of this invention, to characterise this sample, conductivity was determined vising the 4 point resistance method and adhesion was determined using Australian Standard 2331.4.1-1981 "Qualitative Adhesion Tests". The results for these tests are shown respectively in Tables 1 and 2, with Table 3 being the classification system for the adhesion test. To assess uniformity of the coatings, a fibre sample was examined by

SEM. In Figure 1 there is shown the photomicrograph obtained, which clearly indicates a high degree of uniformity of coating.

TABLE 1

Resistance per 4 cm of yarn (Ω)

6.42 6.77

6.53 7.19

6.74 9.69

8.17 9.76

8.43 7.93

7.66 7.22

6.86 7.19

6.66 7.02

6.63 7.19

Average = 186.2 Ωm Average = 7.45 Ω per 4 cm

TABLE 2 Adhesion

Sample Classification (Result)

1 0 (no coating removed)

2 0 (no coating removed)

3 0 (no coating removed)

(According to classification system developed, see Table 3)

TABLE 3 Classification system for Qualitative Tape Test

Classification Description

0 No coating removed

1 Speckles of coating removed

2 Less than half the coating removed ( <50%)

3^" More than half the coating removed (>50%)

3 ⁺ All of the coating removed (100%) EXAMPLE 3

PET was fibre cleaned and etched as described in the above examples.

The fibre was immersed in a 10% w/v solution of hydrazine in water for 5 minutes. An aqueous palladium catalyst solution containing 0.2% w/v palladium chloride and one mole equivalent of ethylenediaminetetraacetic acid disodium salt (EDTA disodium salt) was prepared by heating a mixture of the constituents in the required volume of water at 80^llC until dissolution of the palladium chloride was complete. After immersion in the hydrazine solution, the fibre was transferred directly to the palladium catalyst solution which was maintained at about 75°C. The immersion time was 10 minutes. Following the palladium catalyst treatment, the fibre was rinsed thoroughly in de-ionised water before being coated with an approximately 1 micron thick deposit of platinum vising a proprietary electroless plating bath. An adherent uniform coating was produced by this method. CYTOTOXICITY TESTING

Platinum coated polyester film (Mylar) was produced by the method described in Example 1 and was tested for cytotoxicity by 1) direct contact and 2) cell growth inhibition assays. Direct cell contact assay

This section describes the testing of biomaterials for cytotoxic potential using an in vitro direct cell contact technique based on ASTM F813. Murine L929 cells were grown to near confluency in plastic petri dishes. The medium was aspirated and replaced with a small volume of fresh medium and materials to be tested were placed directly on the cell monolayer for a 24 hour period. During this time any cytotoxic components from the test materials disrupt the normal functions of the cells. Cultures are stained with a vital stain (Trypan Blue) and cytotoxicity assessed using bright field and phase-contrast microscopy. Cytotoxic responses are graded according to a standard key which quantifies the zonal extent of cell damage (0 to 4 maximum). The results are shown in Table 4. TABLE 4

Plate Sample Description Reactivity Comments Number Index

74 "PT PET "Polymer 1 scattered cell death under sample

75 "PT PET ' Polymer 0

76 "PT PET "Polymer 0

664 "PET "Polymer 0

65 "PET "Polymer 0

66 "PET "Polymer 0

71 Positive Control 4 (Latex)

72 Positive Control 4 (Latex)

73 Positive Control 4 (Latex)

52 Negative Control 0 (Silastic)

53 Negative Control 0 (Silastic)

54 Negative Control 0 (Silastic)

55 Null 0

56 Null 0

57 Null 0 TABLE 5 Reactivity Grades

Grade Reactivity Description of Reactivity Zone

0 None No detectable zone around or under sample

1 Slight Zone limited to area under sample

2 Mild Zone extends less than 0.5 cm beyond sample

3 Moderate Zone extends 0.5 to 1.0 cm beyond sample

4 Severe Zone extends > 1.0 cm beyond sample

A reactivity grade above 1 is indicative of a significant cytotoxic response under the conditions stipulated by this standard operating procedure. A score of 4 indicates severe cytotoxicity.

All controls performed as expected. All of the uncoated polymer samples showed no evidence of cytotoxicity. One metal coated polymer sample had scattered dead cells directly under the sample but not extending beyond the perimeter of the sample. All others showed no evidence of any significant cytotoxic influence. The platinum PET material prepared by the present invention passes this test. Cell growth inhibition assay This section describes the testing of biomaterials for cytotoxic potential vising an in vitro cell growth inhibition techniqvie. The following is a brief overview of the methods used.

Murine L929 cells were established at low density and grown to confluency in plastic petri dishes. Twenty four hours after inoculation, the medium on the test dishes was aspirated and replaced with medium supplemented with extract prepared from the test materials. The extracts were prepared by autoclaving a known amount of test material in saline. The cell monolayer was cultured for a further 72 hour period. During this time cytotoxic components in the extracts, if present, disrupt normal functions of cells in the culture dish. At the end of the test period cells were harvested from the dishes and their numbers counted and compared with untreated cultures. Differences in cell numbers were expressed as a percentage inhibition in comparison to untreated cultures. An inhibition of 30% is considered clear indication of cytotoxic potential in the test material. Negative Controls: i) Saline (NULL) ii) 2 x Saline (extracted). Positive Controls: i) 4% Ethanol ii) 5% Ethanol iii) 7.5% Ethanol Samples: i) "PT PET" ii) "PET"

The results are shown in Figure 2 and Table 6. The controls in this assay performed as expected. The level of inhibition set as indicating significant cytotoxicity is 30 %. The metal coated polymers (PT PET) and uncoated polymers (PET) showed no signs of inhibitory effects. The platinum PET material passes this test.

TABLE 6

Treatment Mean Std Dev % Inhibition

Null 8.99 x 10⁵ 1.11 x lO⁵ 0 %

Saline 1 8.90 x 10^s 6.27 x lO⁴ 1%

Saline 2 9.13 x 10⁵ 1.15 x lO⁵ -1.6%

4% Ethanol 1.34 x 10^s 1.29 x 10⁴ 85.1%

5% Ethanol 8.65 x 10⁴ 1.44 x 10 * 90.4 %

7.5% Ethanol 5.75 x IO⁴ 5.94 x IO³ 93.6 %

PET 9.27 x 10^s 1.44 x IO⁴ -3.1 %

PT PET 8.95 x IO⁴ 6.49 x IO⁴ 0.4 %

Figvire 3 shows some fatigue test results for some PET fibres coated using the process according to the present invention. The results show uniform four (4) point resistance for the duration of the study which was about 6.5 million cycles. The fibres were bent through 180 degrees to a 4 mm radius.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

CLAIMS:

1. A method for producing a metal coated non-conductive substrate comprising: a) contacting a substrate with a solution of a noble metal compound in a manner svich that noble metal ions are adsorbed to the surface of the substrate; b) treating the substrate with a non-contaminating reducing agent so as to reduce the surface-adsorbed noble metal ions to their elemental state: and c) coating the substrate with a metal in an electroless metal-plating bath to form a metal coated substrate.

2. A catalytic process for the preparation of the surface of a substrate prior to the coating of the substrate in an electroless metal-plating bath comprising: a) contacting a substrate with a solution of a noble metal compound in a manner such that noble metal ions are adsorbed to the surface of the substrate; and b) treating the substrate with a non-contaminating reducing agent so as to reduce the surface-adsorbed noble metal ions to their elemental state and thereby provide a substrate suitable for metal coating in an electroless metal- plating bath.

3. The method or process according to claim 1 or 2 wherein the noble metal compound is in a non-aqueous solvent.

4. The method or process according to claim 3 wherein the non-aqueous solvent is selected from the group consisting dimethyl sulfoxide, dimethyl formamide, and tetrachloroethane.

5. The method or process according to claim 4 wherein the non-aqueous solvent is dimethyl sulfoxide.

6. The method or process according to claim 1 or 2 wherein the noble metal compound is in an aqueous solution.

7. The method or process according to claim 6 wherein the noble metal compound is in a complex of ethylenediaminetetraacetic acid disodium salt.

8. The method or process according to any one of claims 1 to 7 wherein the noble metal is selected from the group consisting of platinum, palladivim, silver, gold, ruthenium, rhodium, osmium, iridium, salts and complexes thereof.

9. The method or process according to claim 8 wherein the metal salts are halides or nitrates.

10. The method or process according to claim 9 wherein the metal salts are selected from the group consisting of palladium chloride, platinum chloride and silver nitrate.

11. The method or process according to any one of claims 1 to 10 wherein the concentration of the noble metal compound in (a) is in the range of 0.05 to 0.5% w/v.

12. The method or process according to claim 11 wherein the concentration of the noble metal compound is 0.2% w/v.

13. The method or process according to any one of claims 1 to 12 wherein the non-conductive substrates include polymeric materials including polyesters.

14. The method or process according to claim 13 wherein the non- conductive substrates are in any physical form including planar portions, fibres or yarns.

15. The method or process according to claim 13 or 14 wherein the polyester is polyethylene terephthalate (PET).

16. The method or process according to any one of claims 1 to 15 wherein prior to contacting the substrate with the solution of the noble metal compound, the substrate is washed with a solvent so as to remove any surface contaminants.

17. The method or process according to claim 16 wherein the solvent used to remove contaminants is selected from organic solvents and aqueous surface active agents which may function at elevated temperatures of 50 to

80°C.

18. The method or process according to claim 17 wherein the organic solvents are selected from the group consisting of ethanol, methanol, ethylacetate, acetone, and methylethylketone.

19. The method or process according to any one of claims 1 to 18 wherein the substrate is washed with a hot solvent.

20. The method or process according to any one of claims 1 to 19 wherein prior to contacting the svibstrate with the solution of the noble metal compound, the surface of the svibstrate is chemically etched with an aqvieous solution of alkaline metal hydroxide.

21. The method or process according to claim 20 wherein the alkaline metal hydroxide is selected from sodium hydroxide and potassium hydroxide.

22. The method or process according to claim 21 wherein the alkaline metal hydroxide solution is at a temperature up to about 130°C.

23. The method or process according to claim 22 wherein the alkaline metal hydroxide is dissolved in ethanol or an ethanol/aqueous mix.

24. The method or process according to any one of claims 1 to 23 wherein step b) follows step a) without rinsing to remove excess noble metal solution by immersing the substrate in a suitable volume of the non- contaminating reducing agent.

25. The method or process according to claim 24 wherein the reducing agent is hydrazine.

26. The method or process according to any one of claims 1 to 25 wherein following treatment with the non-contaminating reducing agent, the substrate is rinsed with water.

27. The method or process according to any one of claims 1 to 26 wherein the metal coated by step c) is selected from the group consisting of titanium, palladium, nickel, copper, cobalt, iron, gold, and platinum.

28. The method or process according to any one of claims 1 to 27 wherein the metal surface is further modified vising thiol chemistry to include pendant grovips.

29. The method or process according to claim 28 wherein the pendant group is selected from the group consisting of lipids, proteins, peptides and other biologically active species.

30. A metal-coated substrate formed using the method or process according to any one of claims 1 to 29.

31. The metal-coated substrate according to claim 30 selected from the group consisting of biocompatible implantable electrode leads, braided, knitted or woven sheets, threads or tubes, and coated fibres woven into textiles for vise as patches, sutures, vascular grafts.