WO1996039479A1

WO1996039479A1 - Cleaning process for implantable medical device

Info

Publication number: WO1996039479A1
Application number: PCT/US1996/008192
Authority: WO
Inventors: Stephen M. Ramsey
Original assignee: Intermedics Inc
Current assignee: Intermedics Inc
Priority date: 1995-06-06
Filing date: 1996-05-31
Publication date: 1996-12-12
Anticipated expiration: 1997-12-06

Abstract

An improved process for cleaning critical parts and subassemblies comprised of metal, plastics, and other materials, and/or for enhancing the physical properties of such materials. The process involves the contacting of such parts and/or subassemblies with a blend of an alcohol and a non-polar hydrocarbon and may be conducted at room temperature or at temperatures other than room temperature. The blend of the present invention includes a non-polar hydrocarbon such as butane, pentane, hexane, heptane, or octane and an alcohol such as methanol, propanol, isopropanol, butanol, isobutanol, pentanol, or hexanol, and in a particularly preferred embodiment is an azeotropic blend.

Description

CLEANING PROCESS FOR IMPLANTABLE MEDICAL DEVICE Technical Field

The present invention relates to an improved process for cleaning metal and plastic. In more detail, the present invention relates to a cleaning or degreasing process utilizing a blend of polar and non-polar organic molecules, and to the blend of polar and non-polar organic molecules itself, for cleaning and/or degreasing critical metal and plastic component parts of, for instance, implantable medical devices. Background Art The cleaning and degreasing of the metal and plastic parts of implantable medical devices, and subassemblies comprised of such parts from which such devices are assembled, is an essential step in their manufacture. With respect to subassemblies for use in constructing such devices, and as a general rule in the manufacturing business, it is extremely important to insure intimate contact between an adhesive and the parts to be adhered to each other to increase the strength of the glue joint such that cleaning and degreasing such parts and subassemblies is critical to their performance.

It is known to clean such parts and subassemblies in FREON and other chlorofluorocarbons and in blends of such chlorofluorocarbons with alcohols such as are available under the trademark FREON TMS (E.I. du Pont de Nemours Co., Wilmington, DE). Apparatus for use in cleaning and de-greasing processes are available from a number of vendors, including Baron Blakeslee/ Allied Signal (Melrose Park, IL) and S & K, Inc. (Chestnut Ridge, NY), the former intended specifically for use in processes utilizing chlorofluorocarbons. However, with governmental regulations requiring that the use of chlorofluorocarbons be eliminated, there is a need for cleaning processes which do not use these harmful organic chemicals.

It is known to use various liquid hydrocarbons to remove greases and oils from such parts and alcohols are often utilized as solvents. Acetone and methyl ethyl ketone are routinely used for cleaning glass and painted or porcelained metal parts, respectively. It is even known to use alkanes and alcohols sequentially in a process for removing hardened photoresist material from printed circuit conductors (see U.S. Patent No. 3,081,203). Similarly, on information and belief, printed circuit boards are cleaned in boiling isopropyl alcohol. A problem with the use of such solvents arises because of the need for cleaning parts made of diverse materials. Many types of plastics are used in making critical parts, and a variety of adhesives, including epoxies, silicone polymers and cements, elastomer-solvent cements, polysulfϊde sealants, thermoplastic resins, and natural adhesives such as cellulosics and mineral-based adhesives, are used for assembling such parts to each other. Likewise, some parts are coated or treated such that their surface properties are different than those of the underlying material. For example, the electrodes of an implantable medical device such as a pacemaker may be coated with an iridium oxide coating such as the IROX coating utilized on the electrode tip of pacemaker leads manufactured by Intermedics, Inc. (Angleton, TX). Many solvents degrade (e.g., dissolve, adversely affect the electrical, chemical, and/or physical properties of, weaken a glue joint, or otherwise damage) such materials, coatings, and/or adhesives such that there is a need for an effective replacement for chlorofluorocarbon cleaning which can be used on any such parts, and it is an object of the present invention to provide such a cleaning process.

Those skilled in the art will recognize from this disclosure that it is also an object of the present invention to provide a process and composition for use in cleaning the metal, plastic, and polymeric parts of any device or article of manufacture in which avoidance of contamination is desirable. For instance, it is well known in the automobile repair business that carburetor parts and fuel injection jets must be scrupulously clean to function properly, and for years such parts have been cleaned in FREON, gasoline, and a variety of other solvents. The present invention is intended to provide an effective, environmentally sound alternative to such practices for cleaning and/or degreasing any part or article of manufacture which is referred to as a "critical" part in the sense that its cleanliness is important to its function and/or its ability to be effectively assembled to another part. Such parts include the parts of implantable and other medical devices, electrical circuitry, carburetor parts, fuel injectors and other parts of engines and vehicles, metal parts for painting, coating, or adhering, plastic parts which need to be cleaned prior to assembly, painting, or coating, control components, valves, semiconductor devices, and many other parts of types known in the art but too many in number and too diverse in type to list here, it being intended to exemplify the utility of the present invention with the preceding list. Disclosure of Invention

These objects are met by providing an improved method for cleaning critical parts comprised of metal, plastic, and naturally occurring materials such as the parts of an implantable medical device in which the improvement comprises the contacting the parts with a blend of an alcohol and a non-polar hydrocarbon selected from the group consisting of alkanes, alkenes, and alkynes. The method of the present invention also contemplates increasing the temperature of either the parts or the blend and the use of ultrasonic energy during the cleaning process. In another aspect, the present invention provides a composition comprising a blend of alcohol and non-polar hydrocarbon for use in cleaning or degreasing critical parts. Best Mode of Carrying Out the Invention

The preferred embodiments of the present invention are described herein with reference to the cleaning of the parts of an implantable medical device such as a heart pacemaker, it being understood by those skilled in the art who have the benefit of this disclosure that such parts are referenced herein for purposes of exemplification and not limitation, it being intended to provide a composition and process for cleaning a wide variety of parts used for many different purposes. Another reason for exemplifying the present invention with reference to the parts of such a device is that pacemakers are themselves comprised of a variety of materials such that the use of the parts of a pacemaker for exemplification of the present invention provides a description of a relatively widely-based spectrum of the types of parts which are contemplated herein as being critical parts and with which the present invention is used to advantage.

Specifically, pacemaker leads are comprised of metal parts (stainless steel, titanium), coated metal parts (the aforementioned iridium oxide coating, for instance), polyurethane tubing, and silicone molded parts and tubing. These various parts are assembled to each other by adhesives such as epoxies, silicones, and polyurethanes such that the testing of the present invention on these parts and subassemblies provides broad-based verification of its efficacy. Further, it was during the testing of the present invention on such parts that a surprising result was discovered. Specifically, it was discovered that cleaning a subassembly including a glue joint in accordance with the method of the present invention resulted in an increase in the strength of the component plastic materials and the glue joints.

As set out above, the process of the present invention comprises contacting the parts to be cleaned with a blend of an alcohol and a non-polar hydrocarbon. The non-polar hydrocarbon is any straight, branched, or cyclical hydrocarbon, saturated or unsaturated, which is relatively non-polar. By use of the phrase "relatively non-polar," reference is being made to hydrocarbons and/or substituted hydrocarbons having a dipole moment (μ) of less than about zero to about five Debye. Preferred non¬ polar hydrocarbons are hydrocarbons which are liquids at room temperature, non-mutagenic and non- carcinogenic for safety, and which have relatively low vapor pressures so as to minimize loss to the atmosphere. The non-polar hydrocarbons contemplated include those alkanes, alkenes, and alkynes comprised of carbon chains of between about four and about ten carbons in length. The word "about" is used to describe the length, including branches, of the carbon chain because of the interaction of temperature with the non-polar hydrocarbon. Specifically, although the process of the present invention is advantageously conducted at about room temperature, temperatures higher and lower than room temperature may also be used to advantage and shorter chain hydrocarbons which are gases at room temperature are liquids at lower temperatures. Similarly, longer chain hydrocarbons which are viscous enough at room temperature to decrease their utility as the non-polar component of the blend of the present invention at that temperature experience a decrease in viscosity at increased temperature which renders them quite efficacious when used in the process of the present invention.

Having broadly described the non-polar hydrocarbons appropriate for use with the present invention in this fashion, particularly preferred hydrocarbons are alkanes and their straight chain and cyclic isomers having carbon chains of between five and eight carbons in length such as pentanes, cyclopentanes, pentenes, pentynes, and their isomers, hexanes, cyclohexanes, hexenes, hexynes, and their isomers, heptanes, heptenes, heptynes, and their isomers, octanes, octenes, octynes, and their isomers, toluene, benzene, xylene, and methyl-, ethyl-, propyl- and butyl- substituted derivatives of these alkanes, alkenes, and alkynes. Suitable substituted non-polar hydrocarbons and their derivatives include, for instance, methyl-, ethyl-, propyl-, and butyl- ketone, methyl-, ethyl-, propyl-, methylchloro-, ethylchloro- and butyl- acetate, halide substituted derivatives of alkanes such as methyl-, ethyl-, and propylchloride, methylene chloride, chloroform, carbon tetrachloride, methyl-, ethyl-, propyl-, and butylchloroform, and alkanes, alkenes, and alkynes having aldehyde, ketone, ester, carboxylic acid, amide, and/or amine functional groups.

The alcohol contemplated for use in connection with the present invention is any alcohol which acts as a solvent for polar contaminants found on the critical parts to be cleaned and/or degreased which is also miscible with the non-polar hydrocarbon described above. Alcohols comprised of a carbon chain of between one and about ten carbons are specifically contemplated for use in connection with the present invention and may be diols and even polyols. Particularly preferred are alcohols comprised of a saturated or unsaturated carbon chain of one to six carbons in length, the hydoxyl group being located at any position on the carbon chain comprising the alcohol, for instance, propanol, butanol, and isopropyl and isobutyl alcohols. It will also be understood by those skilled in the art who have the benefit of this disclosure, however, that carbon chains substituted with aldehyde and carboxyl groups are likewise included within the scope of the present invention if able to dissolve polar contaminants on the parts to be cleaned and miscible with the non-polar hydrocarbon.

The relative proportions of alcohol and non-polar hydrocarbon utilized depend in part on the specific components of the blend of the present invention. Another factor in selecting the specific components and their relative proportions is whether the blend of alcohol and non-polar hydrocarbon is itself to be cleaned for re-use. Such cleaning can be carried out in a number of ways known in the art, but if one commonly used process, e.g. distillation, is used to clean the blend, it may be advantageous to utilize a blend of alcohol and non-polar hydrocarbon which is azeotropic. As set out in Example 1, infra, when isopropyl alcohol (IPA) and heptane are utilized as alcohol and non-polar hydrocarbon, respectively in the composition of the present invention, they may be utilized in a 50.5/49.5 (percentages by weight) ratio so as to give an azeotropic blend. If it is desired to utilize isobutyl alcohol in the blend to maintain the azeotropic property of the blend, the different molecular weight of isobutyl alcohol changes the ratio accordingly. Other factors to be considered in preparing the alcohol-non-polar hydrocarbon blend are the viscosities of the particular alcohol or non-polar hydrocarbon utilized, the nature of the contaminant(s) to be removed from the critical parts, and the composition of the critical parts . It will be recognized from this disclosure by those skilled in the art that the ratio of alcohol and non-polar hydrocarbon may vary continuously in any proportion in which the components are miscible with each other. In a preferred embodiment, the proportions may vary from 1 :99 to 99: 1 alcohohnon- polar hydrocarbon such that many formulations other than an azeotropic blend may be utilized to advantage in connection with the present invention. The blend may also utilize ratios beyond this range if, for instance, the blend is heated to make the components miscible.

As noted above, one advantage of the cleaning process of the present invention is that it is effective in cleaning critical parts at room temperature, but the invention is not so limited. The cleaning process may also be conducted at elevated temperature or even at a temperature lower than room temperature. As noted in connection with the description of those non-polar hydrocarbons which are appropriate for use in connection with the present invention, there may be situations in which the process must be conducted at a temperature other than room temperature. In one embodiment, the process is conducted at a temperature which is increased to the boiling point of the blend of the present invention. In this latter instance it may also be advantageous to use an azeotropic blend of alcohol and non-polar hydrocarbon.

The process of the present invention may also include subjecting the critical parts to ultrasonic energy either during the contacting of the parts with the blend of the present invention or sequentially, i.e. , either before or after contacting the parts with the blend. Similarly, the use of ultrasonic energy may be combined with the use of increased temperature in the process of the present invention. The present invention may be better understood by reference to the following examples, set out for purposes of exemplification of the invention and not to define the limits of its scope.

Example 1

In a first example demonstrating the effectiveness of the cleaning process of the present invention, an azeotropic blend of isopropyl alcohol (IPA) and heptane was prepared by mixing 50.5% IPA and 49.5% heptane (percentages by weight). A number of 316-type stainless steel washers were

"contaminated" with synthetic skin oil (D.T. Downing, et al., "Variability in the Chemical Composition of Human Skin Surface Lipids", 53 J. Investigative Dermatology 322-327 (1969)) and two each of the

"contaminated" washers were cleaned with FREON TMS in a Baron Blakeslee Model MSR-120LE degreaser, with a CFM Model Ultraflow cleaner, in the sump of the S & K Model IG-200 degreaser in boiling IPA and ultrasonically agitated, and with ultrasonic agitation in the blend of IPA and heptane at room temperature. The effectiveness of each cleaning process was assessed by electron disperse X- ray (EDX) using the following method. EDX is a semi-quantitative method in which carbon counts are compared to a standard, the ratio providing an indication of the relative effectiveness of the cleaning process. A Jeol JSM 35CF scanning electron microscope was used to zero in on the area to be analyzed and a Model TN5000 Tracor Northern, Inc. EDX analyzer used for analysis of that area. The results indicated that the IP A/heptane blend of the present invention was approximately as effective in removing carbon-containing contaminants from the surface of the washers as degreasing in FREON TMS and that those two cleaning processes might both have been somewhat more effective at removing such contaminants than the use of room temperature or boiling IPA. Example 2

In a second experiment, the IP A/heptane azeotropic blend of Example 1 was used to clean 30 subassemblies of parts for use in an implantable cardiac pacemaker. The particular subassembly cleaned was comprised of the inner coil and bipolar pin of an Intermedics, Inc. (Angleton, TX) bipolar pacemaker lead, which are assembled with a commercially available epoxy and allowed to cure overnight. 30 additional such sub-assemblies were made for cleaning with FREON TMS vapor in a

Baron Blakeslee FREON degreaser. All 60 subassemblies were allowed to cure overnight after assembly, then cleaned, and dried overnight. Each was then subjected to a pull test using a Model_1130 electromechanical tensile strength pull tester (Instron) in accordance with ASTM D412. The results were as follows: FREON TMS (lbs) IPA/Heptane (lbs) mean 3.0 3.3 standard deviation 0.6 0.4

CpK 0.8 1.5

The CpK, or process capability index, is calculated by dividing the difference between the mean and the lower specification limit (1.5_lbs) by three times the standard deviation, it being desirable to exceed a CpK of 1.3 for efficient and reliable manufacturing process. Increase in both pull strength and CpK clearly indicates that, as noted above, the cleaning of the subassembly with the azeotropic IP A/heptane blend had a beneficial effect on performance.

Example 3 A third example tested the effect of the azeotropic IP A/heptane blend of Example 1 on silicone rubber. Thirty each silicone rubber coupons were soaked in FREON TMS, the azeotropic blend, or boiling azeotropic IP A/heptane blend, allowed to dry and tested as follows:

Durometer

(Shore A) Tensile (psi) Control (not soaked) 79 1098

FREON TMS 79 1130

IPA/Heptane 80 1126

Boiling IPA/Heptane 79 1111 These data indicate that silicone rubber is not degraded by cleaning by the process of the present invention. Other similar tests indicate that the physical properties of the plastic (such as tensile strength) may actually be improved by contact with or cleaning in the blend of the present invention.

Example 4 In another test, it was determined that silicone rubber glue joints are not degraded by cleaning in accordance with the process of the present invention. In this test, silicone rubber coupons were adhered to stainless steel coupons using a commercially available silicone adhesive, cured overnight, and subjected to the pull test described in Example 2. The results were as follows (n = 30):

Lap shear (psi) Stnd. Dev. Control (not soaked) 103 12

FREON TMS 107 18

IPA/Heptane 106 10

Boiling IPA/Heptane 112 12

These data indicate that silicone rubber glue joints are not degraded, and may be enhanced, by cleaning in accordance with the process of the present invention.

Example 5 Glue joints between polyurethane and stainless steel were tested by cleaning the tubing by the process of the present invention followed by adhering 80 A polyurethane tubing to stainless steel rods. The results of the pull test described in Example 2 were as follows (n_= 17, lower specification limit equals 3.0 lbs):

FREON TMS (lbs) IPA/Heptane (lbs) mean 5.8 6.1

Stnd. Dev. 0.51 0.26

CpK 1.8 4.0 As was the case with silicone rubber glue joints, these data indicate significant improvement in the cleaning of polyurethane tubing as evidenced by the improved performance and reliability of polyurethane glue joints.

Claims

What is claimed is:

1. In a method of cleaning the metal and plastic parts of implantable medical devices, the improvement comprising contacting the parts with a blend of an alcohol and a non-polar hydrocarbon selected from the group consisting of alkanes, alkenes, and alkynes.

2. The method of claim 1 wherein the alcohol is comprised of a carbon chain of between one and about eight carbons in length.

3. The method of claim 1 additionally comprising increasing the temperature of the blend.

4. The method of claim 3 wherein the blend is an azeotropic blend.

5. The method of claim 4 wherein the temperature of the azeotropic blend is increased to the boiling point thereof.

6. The method of claim 1 additionally comprising agitating the parts with ultrasonic energy.

7. The method of claim 6 additionally comprising increasing the temperature of the azeotropic blend.

8. The method of claim 1 wherein the non-polar hydrocarbon is comprised of a carbon chain of between four and about ten carbons in length.

9. The method of claim 1 wherein the non-polar hydrocarbon includes functional groups selected from the group consisting of aldehydes, ketones, esters, carboxylic acids, amides and amines.