Post-Charging Of Zeolite Doped Plastics With Antimicrobial Metal Ions

Abstract

Methods of post-loading ceramic particles with antimicrobial metal cations are disclosed. In certain embodiments, the post-loaded particles are zeolites, wherein the zeolites have been incorporated into a resin and the combination is used as an implantable device. In certain embodiments, the polymer is a thermoplastic polymer such as polyaryletheretherketone (PEEK). In certain embodiments, the source of antimicrobial activity includes ion-exchangeable cations contained in a zeolite. In certain embodiments, disclosed are methods of imparting antimicrobial activity to devices by controlling the delivery of certain cations through ion-exchange via a zeolite incorporated in the device.

Claims

1. A surgical implant configured for implantation at an implant site of a host, said device having an exposed surface and comprising a thermoplastic resin and zeolite incorporated in said resin, and one or more non-oxidized ion-exchangeable metal ions incorporated in said zeolite, said surgical implant upon implantation and exposure to bodily fluids of said host being capable of releasing from said exposed surface said one or more non-oxidized ion-exchangeable metal ions in an antimicrobially effective amount.

2. The surgical implant of claim 1, wherein said thermoplastic resin comprises PEEK.

3. The surgical implant of claim 1, wherein said one or more non-oxidized ion-exchangeable metal ions are selected from the group consisting of silver, copper, zinc, mercury, tin, lead, gold, bismuth, cadmium, chromium and thallium.

4. The surgical implant of claim 1, wherein said one or more non-oxidized ion-exchangeable metal ions are silver ions.

5. The surgical implant of claim 1, wherein said zeolite comprises an A-type zeolite.

6. The surgical implant of claim 1, wherein said non-oxidized ion-exchangeable metal ions are incorporated in said zeolite only at said exposed surface.

7. The surgical implant of claim 1, wherein said thermoplastic resin has a porosity between 50% and 85% by volume.

8. The surgical implant of claim 2, wherein said one or more non-oxidized ion-exchangeable metal ions are selected from the group consisting of silver, copper, zinc, mercury, tin, lead, gold, bismuth, cadmium, chromium and thallium.

9. The surgical implant of claim 2, wherein said one or more non-oxidized ion-exchangeable metal ions are silver ions.

10. The surgical implant of claim 2, wherein said zeolite comprises an A-type zeolite.

11. The surgical implant of claim 2, wherein said non-oxidized ion-exchangeable metal ions are incorporated in said zeolite only at said exposed surface.

12. The surgical implant of claim 2, wherein said PEEK has a porosity between 50% and 85% by volume.

Description

DETAILED DESCRIPTION

[0035] Embodiments disclosed herein relate to the use of ceramics, preferably zeolites, as a cation cage in combination with medical implants to deliver and dose one or more antimicrobial cations. Suitable cations include silver, copper, zinc, mercury, tin, lead, gold, bismuth, cadmium, chromium and thallium ions, with silver, zinc and/or copper being preferred, and silver being especially preferred.

[0036] Either natural zeolites or synthetic zeolites can be used to make the zeolites used in the embodiments disclosed herein. Zeolite is an aluminosilicate having a three dimensional skeletal structure that is represented by the formula: XM.sub.2/nO.Al.sub.2O.sub.3.YsiO.sub.2.ZH.sub.2O, wherein M represents an ion-exchangeable ion, generally a monovalent or divalent metal ion, n represents the atomic valency of the (metal) ion, X and Y represent coefficients of metal oxide and silica respectively, and Z represents the number of water of crystallization. Examples of such zeolites include A-type zeolites, X-type zeolites, Y-type zeolites, T-type zeolites, high-silica zeolites, sodalite, mordenite, analcite, clinoptilolite, chabazite and erionite.

[0037] Zeolites can be incorporated into masterbatches of a range of polymers. For final incorporation into PEEK, a masterbatch should be produced by incorporating typically about 20% zeolite. When provided in this form, the pellets of masterbatch PEEK containing the zeolite particles can be further reduced by mixing with more virgin PEEK at high temperature and under high shear. If metal were present in the zeolite, this would result in yet a second exposure to conditions which could cause deterioration of the product.

[0038] Other suitable resins include low density polyethylene, polypropylene, ultra high molecular weight polyethylene or polystyrene, polyvinyl chloride, ABS resins, silicones, rubber, and mixtures thereof, and reinforced resins, such as ceramic or carbon fiber-reinforced resins, particularly carbon fiber-reinforced PEEK. The latter can be produced by dispersing the reinforcing material or materials (e.g., carbon fibers) in the polymer matrix, such as by twin screw compounding of implantable PEEK polymer with carbon fibers. The resulting carbon fiber-reinforced product can be used to direct injection mold final devices and near net shapes, or it can be extruded into stock shapes for machining. The incorporation of fibers or other suitable reinforcing material(s) provides high wear resistance, a Young's modulus of 12 GPa (matching the modulus of cortical bone) and providing sufficient strength to permit its use in very thin implant designs which distribute the stress more efficiently to the bone. The amount of reinforcing material such as carbon fiber incorporated into the resin such as PEEK can be varied, such as to modify the Young's modulus and flexural strength. One suitable amount is 30 wt % carbon fiber. The resins also can be made porous, such as porous PEEK, PAEK and PEKK, with suitable porosities including porosities between 50% and 85% by volume. Average pore size is generally greater than 180 microns in diameter, suitably between about 300 and about 700 microns. Porosity can be imparted using a pore forming agent such as sodium chloride, to create a porous polymer comprising a plurality of interconnected pores, by processes known in the art. Each of the foregoing can be formulated to contain suitable amounts of zeolite particles, usually about 20 wt %. An UHMWPE is preferred for the implant devices.

[0039] Typical amounts of zeolite particles incorporated in an implant resin range from 0.01 to 50 wt. %, more preferably 0.01 to 8.0 wt. %, most preferably 0.1 to 5.0 wt. %. If an implant is coated with a coating or resin which is loaded with zeolite, the coating needs to be applied and dried or cured before the infusion is carried out. The method used to coat an implant is not particularly limited, and can include spraying, painting or dipping. When compounded into a PEEK masterbatch, for example, the PEEK should be protected from sources of moisture and contamination prior to reduction with virgin resin. The compounding can be carried out by blending the molten masterbatch and let down resin under conditions of high temperature and high shear.

[0040] The masterbatch is a concentrated mixture of pigments and/or additives (e.g., zeolite powder) encapsulated during a heat process into a carrier resin which is then cooled and cut into a granular shape. Using a masterbatch allows the processor to introduce additives to raw polymer (let down resin) economically and simply during the plastics manufacturing process.

[0041] In accordance with certain embodiments, a purer more stable product can be produced by charging the polymer with pure zeolite (e.g., one that is not yet loaded with antimicrobial metal ions, or one that is only partially loaded), such as type X zeolite, available from W.R. Grace & Co.-Conn., which is capable of carrying a cationic metal ion cargo such as Ag+, Cu++, Cu+, or Zn+, and subsequently charging the cooled (e.g. cooled to a temperature between about 0 and 100 C., preferably about room temperature) zeolite-containing PEEK surface with metal ions from a metal ion source such as an aqueous metal ion solution, such as silver nitrate, copper nitrate and zinc nitrate, alone or in combination. Cooling to lower temperatures gives lower loading rates but higher stability. Loading at even higher temperatures can be carried out at a faster rate by maintaining the system under pressure, such as in a pressure cooker or autoclave. The content of the ions can be controlled by adjusting the concentration of each ion species (or salt) in the solution.

[0042] By incorporating the metal cation into the zeolite after the zeolite has been incorporated into the polymer resin, oxidation of the metal ions is reduced or eliminated. Those skilled in the art will appreciate that other metal ion salt solutions, such as acetates, benzoates, carbonates, oxides, etc., can be used instead of or in addition to nitrates. Addition of nitric acid to the infusion solution also may be advantageous in that it can etch the surface of the implant, providing additional surface area for ion exchange.

[0043] Since PEEK is susceptible to dissolution by strong oxidizing acids, care should be taken to not use too high an acid concentration that may lead to metal zeolite particles being released from the surface. PEEK is very stable and impermeable to water and bodily fluids. As a result, it is expected that metal ions that are incorporated in a zeolite cage dispersed in PEEK will only elute when the cage is exposed at the surface of the polymer. For this reason, it is possible to post incorporate at least as much available metal ions by post treatment from solution as would be available from metal zeolite incorporated into the hot mix. In fact, the availability of metal ions from the post incorporated system is expected to be significantly higher since the metal ions will be pure and will have experienced no thermal oxidation or hot reactions with the polymer.

[0044] The amount of metal ions in the zeolite should be sufficient such that they are present in an antimicrobial effective amount. For example, suitable amounts can range from about 0.1 to about 20 or 30% of the exposed zeolite (w/w %). These levels can be determined by complete extraction and determination of metal ion concentration in the extraction solution by atomic absorption.

[0045] Preferably the ion-exchanged antimicrobial metal cations are present at a level less than the ion-exchange capacity of the ceramic particles. The amount of ammonium ions is preferably limited to from about 0.5 to about 15 wt. %, more preferably 1.5 to 5 wt. %. For applications where strength is not of the utmost importance the loading of zeolite can be taken as high as 50%. At such loadings the permeation of metal ions can permeate well below the surface layer due to interparticle contact, and much greater loadings of metal ions are possible.

[0046] The amount of zeolite incorporated into the resin should also be an amount effective for promoting antimicrobial activity; e.g., a sufficient amount so as to prevent or inhibit the growth of bacterial and/or fungal organisms or preferably to kill the same. Suitable amounts of zeolite in the resin range from about 0.01 to 50.0 wt. %, more preferably from about 0.01 to 8.0 wt. %, most preferably from about 0.1 to about 5.0 wt. %.

[0047] The absorption of metal ions into synthetic zeolites, or natural Zeolites, in an aqueous dispersion, or loaded in a polymer can be carried out from solutions of the metal salts. The rates of absorption will be proportional to the area of zeolite surface available, the concentration of metal ions in solution and the temperature. As the concentration of metal absorbed by the zeolite increases, the rate will be reduced. When the rate of absorption reaches the rate of release, equilibrium is reached at that solution concentration. A higher concentration in solution could drive the loading higher. Loaded zeolite can be rinsed with deionized water to completely remove adherent metal ion solution. The objective is to have only ion exchanged metal cations attached to the cage and these will only be removed by ion exchange, not by deionized water.

[0048] The most useful ions to incorporate, for the purposes of release into orthopedic implants, are silver, copper and zinc ions. All three have antimicrobial properties, silver being the most active. There also may be synergies between the metals, in terms of antimicrobial activity. For instance, if a microorganism is developing resistance to one metal species, it may still be readily killed by one of the others. Copper and zinc ions also exert further functions in healing and wound repair and bone growth.

[0049] For example, the PEEK zeolite composite can be loaded by bringing the material into contact with an aqueous mixed solution containing ammonium ions and antimicrobial metal ions such as silver copper, zinc etc. The most suitable temperatures at which the infusion can be carried out range from 5 C. to 75 C., but higher temperatures may also be used even above 100 C. if the reaction vessel is held under pressure. Higher temperatures will show increased infusion rates but lower temperatures may eventually produce more uniform and higher loadings. The pH of the infusion solution can range from about 2 to about 11 but is preferably from about 4 to about 7.

[0050] Suitable sources of ammonium ions include ammonium nitrate, ammonium sulfate and ammonium acetate. Suitable sources of the antimicrobial metal ions include: a silver ion source such as silver nitrate, silver sulfate, silver perchlorate, silver acetate, diamine silver nitrate and diamine silver nitrate; a copper ion source such as copper(II) nitrate, copper sulfate, copper perchlorate, copper acetate, tetracyan copper potassium; a zinc ion source such as zinc(II) nitrate, zinc sulfate, zinc perchlorate, zinc acetate and zinc thiocyanate.

[0051] The following are illustrative examples of infusion solutions but a wide range of concentrations and ratios are effective.

TABLE-US-00001 Infusion solution A Component Composition (W/W)% Ammonium hydroxide 2.0 Silver Nitrate 1.2 Purified water 96.8 pH can be adjusted with acid such as citric acid or nitric acid Total 100

TABLE-US-00002 Infusion Solution B Component Composition (W/W)% Ammonium hydroxide 2.0 Copper Nitrate 5.0 Purified water 93.0 pH can be adjusted with acid such as citric acid or nitric acid Total 100.0

TABLE-US-00003 Infusion Solution C Component Composition (W/W)% Ammonium hydroxide 2.0 Zinc Nitrate 7.0 Purified water 91.0 pH can be adjusted with acid such as citric acid or nitric acid Total 100.0

TABLE-US-00004 Infusion Solution D Component Composition (W/W)% Ammonium hydroxide 2.0 Silver Nitrate 0.5 Copper Nitrate 2.0 Zinc Nitrate 2.5 Purified water 93.0 pH can be adjusted with acid such as citric acid or nitric acid Total 100

[0052] Since there is a delicate balance between the concentrations of silver, zinc and copper in metabolism for optimum healing, an advantage of the current method is that it will provide and easy method for accurately controlling the relative concentrations of the individual metal ions. The optimum ratios can be achieved by varying the concentrations of the various metal ion salts to load at the appropriate ratios and subsequently release at the appropriate ratios and rates.

[0053] The rates of release of the metal ions into phosphate buffered saline or for example 0.8% sodium nitrate solution, can be quantified by Inductively Coupled Plasma spectroscopy ICP, or Graphite Furnace Atomic Absorption spectroscopy.

[0054] With a ladder study these results can be used to optimize the elution rates. Because the metal ions are never exposed to high temperature, the ions attached and eluting from the zeolite will be pure metal cations.

[0055] Another advantage of the current method is that the amount of metal being incorporated into the implant will be limited to just what is incorporated in the surface layer. In terms of cost and safety it is a superior solution.

[0056] The process will be effective whether the implants are injection molded or machined to achieve the final dimensions of the implant.

[0057] Whereas the process is most applicable to polymers with a high melting point such as PEEK, it could also be used effectively with polymers of lower melting points which are used in a wide range of orthopedic applications. HDPE, for instance, is used in certain elements of hip and knee transplants.

[0058] The post loading process is also appropriate for thermoset resins such as polyesters epoxies and urethanes, etc.

[0059] This approach will avoid contact of the silver ion with the reactants, reactive intermediates and catalysts which form the finished polymer.

[0060] The embodiments disclosed herein are applicable to generating self sterilizing plastic fibers and film. Such materials can be used to produce wound dressings and in a wide array of applications.

[0061] Facemasks which elute silver copper and zinc are used to provide long term control of microorganisms which might be inhaled in a medical setting or increasingly in the case of a possible pandemic. Suitable substrates for such devices include polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), PCT, PETG (PET, type G), Co-PET and co-polyesters generally, Styrene, polytrimethylene terephalate (PTT).sub.m 3GT, Halar, polyamide 6 or 6,6, etc., See U.S. Pat. Nos. 6,946,196 and 6,723,428 to Foss manufacturing, the disclosures of which are incorporated by reference.

[0062] Other applications where self sterilizing fabrics or plastic sheeting find application are within the scope of the embodiments disclosed herein. [0063] Where material is exposed or can be immersed, the depleted zeolite can be recharged with antimicrobial metal ions.

[0064] It is possible to load a polymer with pure zeolite, extrude the polymer into filaments, and post load the material with the antimicrobial metal ions in the manner described for surgical implants.

[0065] Although the focus of the embodiments disclosed herein is on orthopedic implants, those skilled in the art will appreciate that apply to a much wider array of applications, such as toothbrushes, door handles, computer mice and keyboard components, knife handles, and cutting boards, surgical instruments, telephone surface components, water drinking vessels, food storage containers and polymers for producing self sterilizing clothing and self sterilizing face masks.

EXAMPLE 1

[0066] An ion exchange zeolite, natural, or synthetic such as zeolite type A or type X commercially available from W.R. Grace & Co.-Conn., or equivalent, is incorporated into PEEK. Typical amounts of zeolite particles incorporated in an implant resin range from 0.01 to 10 wt. %, more preferably 0.01 to 8.0 wt. %, most preferably 0.1 to 5.0 wt. %. The method used to coat an implant is not particularly limited, and can include spraying, painting or dipping. When compounded into PEEK, for example, the PEEK composite should be protected from sources of moisture and contamination. The compounding can be carried out by blending.

[0067] About 5% by weight of the zeolite powder is mixed thoroughly with the powdered or prilled PEEK. The mixture is brought up to temperature and processed at 400 C. using high shear. The zeolite and PEEK must be dry before processing in order to minimize decomposition and void formation in the product.

[0068] This system containing Zeolite without added silver ions does not show the progressive color development and darkening which is seen with systems containing silver.

[0069] The dark color development in silver zeolite containing systems is thought to be due to silver oxide formation and polymer decomposition.

[0070] The material is processed as before and can be formed into prills for further processing, cast into blocks, extruded into rods or injection molded into the final desired shapes.

[0071] The block and rod materials can be machined into shapes which are suitable for use as orthopedic implants or other designs where antimicrobial PEEK finds application. Implants can be designed to provide enhanced surface area by having grooves cut in the surfaces or by producing products with holes in the body of the pieces. Surface area can be further enhanced by sanding or abrasive blasting of the surfaces.

EXAMPLE 2

Loading the Finished Pieces with Antimicrobial Metal Ions

[0072] Finished pieces produced as described in Example 1 are immersed in an infusion solution to charge the pieces with antimicrobial metal ions.

[0073] A typical solution for infusion is produced by adding 2% silver nitrate, 5% copper nitrate trihydrate and 1% nitric acid to purified water.

TABLE-US-00005 Component Composition (W/W)% Silver Nitrate 2 Copper nitrate trihydrate 5 Nitric acid 1 Purified water 92 Total 100

[0074] The finished pieces are supported or allowed to move freely in the infusion solution. The solution should be agitated to enhance diffusion of ions to and from the surface of the composite. It is advisable to carry out the infusion process in the dark to minimize photo oxidation of the silver in solution. This can be affected on a lab scare by placing an opaque cover such as a tin can over the beaker in which the pieces are being infused.

[0075] The rate of infusion depends on several variables. At normal temperatures, 90 minutes is sufficient time to effectively charge the surfaces with metal ions. The infusion process can be allowed to run for 24 hours or more to maximize the antimicrobial metal loading. [0076] The rate and extent of loading depends on several variables, including solution concentration, solution composition, (metal ion ratios), solution temperature, and agitation rate.

[0077] It should be possible to load the exposed zeolite to as much as 40% by weight with metal ions.

[0078] When infusion is complete or carried out to the desired levels, the pieces are removed from the infusion solution and triply rinsed with purified water. They may then be dried in a stream of hot air or in an oven or desiccator, etc.

[0079] A measure of the antimicrobial activity of an article is the antimicrobial metal (e.g., silver) release from the exterior surface of the article. Metal release can be measured as the amount of antimicrobial metal released from the exterior surface of a 2 inch by 2 inch sample (0.05 meter by 0.05 meter, or 5 cm by 5 cm). The exterior surface of the sample to be tested is contacted in a sodium nitrate solution (40 mL of 0.8% sodium nitrate) for 24 hours at room temperature (i.e., 25 C) to form a test solution. The test solution is then analyzed to measure the amount of antimicrobial metal in the test solution in parts per billion, and thus the exposure of the inorganic antimicrobial agent at the surface of the article. The amount of antimicrobial metal in the test solution may then be measured using a graphite furnace atomic absorption spectrophotometer or ICP. For an article comprising 2.0 percent by weight (wt. %) of an inorganic antimicrobial agent based on the weight of the article or a layer of a multi-layer article, and wherein the inorganic antimicrobial comprises 2.0 wt % of a antimicrobial metal based on the total weight of the inorganic antimicrobial agent, the exterior surface has a antimicrobial metal release of greater than or equal to about 10 parts per billion (ppb), preferably greater than or equal to about 20 ppb, more preferably greater than or equal to about 30 ppb, and most preferably greater than or equal to about 40 ppb.