Patent Application
20100022677
1. Field of the Invention
The present invention relates to crosslinked ultra-high molecular weight polyethylene and, particularly, to annealed, crosslinked ultra-high molecular weight polyethylene.
2. Description of the Related Art
Ultra-high molecular weight polyethylene (UHMWPE) is commonly utilized in medical device applications. In order to beneficially alter the material properties of UHMWPE and decrease its wear rate, UHMWPE may be crosslinked. For example, UHMWPE may be subjected to electron beam or gamma radiation, causing chain scission of the individual polyethylene molecules as well as the breaking of C—H bonds to form free radicals on the polymer chains. While free radicals on adjacent polymer chains may bond to one another to form crosslinked UHMWPE, some free radicals may remain in the UHMWPE following irradiation, which could potentially combine with oxygen and result in oxidation of the UHMWPE. Oxidation may detrimentally affect the wear properties of the UHMWPE and may also increase its wear rate. As a result, the oxidized layer of the UHMWPE, which may be a significant depth of the outer portion of the UHMWPE, may need to be removed prior to utilizing the UHMWPE in medical device applications.
To help eliminate the free radicals that are formed during irradiation that fail to cross-link and therefore may cause oxidation, the UHMWPE may be melt annealed by heating the crosslinked UHMWPE to a temperature in excess of its melting point. By increasing the temperature of the UHMWPE above its melting point, the mobility of the individual polyethylene molecules significantly increases, facilitating additional crosslinking of the polyethylene molecules and the quenching of free radicals. To heat the UHMWPE above its melting point, the UHMWPE may be placed in a convection oven in ambient air. A convection oven operates by activating a heating element or burner that comes in contact with the ambient air in the oven. By contacting the heating element or burner, the internal energy of the air is increased, causing a corresponding increase in its temperature. The air, in turn, then contacts the UHMWPE and increases the internal energy of the UHMWPE, causing a corresponding increase in the temperature of the UHMWPE.
The present invention relates to reducing the concentration of free radicals in crosslinked UHMWPE. In one exemplary embodiment, UHMWPE is exposed to crosslinking radiation and is then heated by thermal radiation. For example, the use of thermal radiation may replace the use of convection to heat the UHMWPE. In convection heating, a heat source, such as a heating element or open flame, is used to increase the temperature of an intermediate medium, such as air or water, that then contacts the object to be heated and transfers thermal energy thereto. As a result, convection heating cannot work in a vacuum. In contrast, thermal radiation does not require an intermediate medium as it utilizes electromagnetic waves that are absorbed by the object to be heated. The absorption of the electromagnetic waves by the object to be heated results in an increase in the thermal energy of the object and, correspondingly, an increase in the temperature of the object.
In one exemplary embodiment, the UHMWPE is exposed to infrared radiation. In this embodiment, the infrared radiation generated may be in the near infrared spectrum, the mid infrared spectrum, or the far infrared spectrum. In particular, the infrared radiation generated may have a wavelength from approximately one micron to fifteen microns. In one exemplary embodiment, the infrared radiation is provided by an infrared heater having a tungsten heating element with a quartz tube. In this embodiment, the infrared radiation may have the wavelength from about 0.50 micron to about 5.0 microns. In another exemplary embodiment, the UHMWPE is compression molded into bars prior to exposure to the crosslinking radiation.
In one exemplary embodiment, once the UHMWPE bars are crosslinked, the UHMWPE bars are hung from a rotating conveyor for exposure to the infrared radiation. In another exemplary embodiment, the UHMWPE bars are placed on a rack for exposure to the infrared radiation. In yet another embodiment, the UHMWPE bars may be placed on a conveyor that travels through an oven having a plurality of infrared heating elements. As the bars travel through the oven, the UHMWPE bars are exposed to the infrared radiation. In order to decrease the heating of the air between the infrared heating element and the UHMWPE bar, a fan may be used to move warm air away from the UHMWPE bar and draw cooler air toward the UHMWPE bar. By keeping the air surrounding the UHMWPE bar at a lower temperature during irradiation, the surrounding air is less reactive, lessening the likelihood of the UHMWPE bar experiencing surface oxidation while annealing. In one exemplary embodiment, a plurality of infrared heating elements and a plurality of fans are arranged to facilitate the desired heating of the UHMWPE bar and also to achieve the desired movement of air surrounding the UHMWPE bar.
Advantageously, by utilizing infrared radiation to melt anneal UHMWPE, oxidation of the exterior surface of the UHMWPE bar is substantially lessened. For example, during traditional melt annealing in a convection oven in ambient air, up to eight millimeters of the UHMWPE bar may be oxidized and, thus, rendered unsuitable for use in medical device applications. In contrast, by infrared melt annealing the UHMWPE bars, two millimeters or less of the UHMWPE bar is oxidized. Advantageously, this results in a substantial cost savings as less of the UHMWPE bar is rendered unsuitable for use in its intended application.
Additionally, infrared melt annealing of the UHMWPE bar results in the UHMWPE bar experiencing homogeneous heating and cooling, at a substantially faster rate than in a convection oven. For example, a conventional melt annealing cycle in a convection oven, which begins with a temperature ramp up, extends through a temperature hold, and ends with a temperature cool down, may be approximately 48 hours. In contrast, the use of infrared radiation to melt anneal a UHMWPE bar may be performed in approximately eight hours or less. This results in a substantial reduction in cycle time, which also provides significant cost savings. Further, the need to utilize a convection oven, which may be large, bulky, and expensive, is obviated.
In one form thereof, the present invention provides a method of processing UHMWPE for medical device applications, the method comprising the steps of: providing a quantity of UHMWPE; crosslinking the UHMWPE; and heating the UHMWPE by exposing the UHMWPE to thermal radiation at a watt density of at least 1 watt per square centimeter.
In another form thereof, the present invention provides a crosslinked UHMWPE for use in medical implants prepared by a process comprising the steps of: providing a quantity of UHMWPE; crosslinking the UHMWPE; and heating the UHMWPE by exposing the UHMWPE to thermal radiation with a watt density of at least 1 watt per square centimeter.
The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:
Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate preferred embodiments of the invention and such exemplifications are not to be construed as limiting the scope of the invention in any manner.
In one exemplary embodiment of the present invention, UHMWPE is exposed to crosslinking radiation and then melt annealed by exposure to thermal radiation. In one exemplary embodiment, the UHMWPE is melt annealed by exposure to infrared radiation. While described herein with specific reference to infrared radiation, the present invention may be used in conjunction with any type of thermal radiation, such as microwave radiation or ultraviolet radiation, for example.
Once annealed by exposure to infrared radiation, the UHMWPE may be subjected to additional processing steps, such as packaging and/or sterilization. Any medical grade UHMWPE powder may be utilized in conjunction with the present invention to form a UHMWPE bar or another form of stock material suitable for exposure to crosslinking radiation. For example, GUR1050 and GUR1020 powders, both commercially available from Ticona, having North American headquarters located in Florence, Ky., may be used. In one exemplary embodiment, the UHMWPE powder is blended with an antioxidant. Exemplary methods for creating a UHMWPE/antioxidant blend are disclosed in copending U.S. patent application Ser. No. 12/100,894, entitled AN ANTIOXIDANT STABILIZED CROSSLINKED ULTRA-HIGH MOLECULAR WEIGHT POLYETHYLENE FOR MEDICAL DEVICE APPLICATIONS, filed on Apr. 10, 2008, the entire disclosure of which is expressly incorporated by reference herein. The UHMWPE powder may then be processed by compression molding, net shape molding, injection molding, ram extrusion, or monoblock formation, for example.
In one exemplary embodiment, the UHMWPE is compression molded into the form of a bar. In this embodiment, the UHMWPE bar may be molded to a length of substantially between four feet and five feet. Additionally, the UHMWPE bar may be molded into any desired geometric shape, such that the UHMWPE bar has a substantially round cross-section or a substantially square cross-section, for example. Alternatively, the UHMWPE may be net shape molded so that the UHMWPE has a shape substantially similar to the shape of a final orthopedic component.
In another exemplary embodiment, the UHMWPE may be compression molded into a substrate. In one exemplary embodiment, the substrate may be a highly porous biomaterial useful as a bone substitute and/or cell and tissue receptive material. A highly porous biomaterial may have a porosity as low as 55, 65, or 75 percent or as high as 80, 85, or 90 percent. An example of such a material is produced using Trabecular Metal™ technology generally available from Zimmer, Inc., of Warsaw, Ind. Trabecular Metal™ is a trademark of Zimmer Technology, Inc. Such a material may be formed from a reticulated vitreous carbon foam substrate which is infiltrated and coated with a biocompatible metal, such as tantalum, etc., by a chemical vapor deposition (“CVD”) process in the manner disclosed in detail in U.S. Pat. No. 5,282,861, the entire disclosure of which is expressly incorporated herein by reference. In addition to tantalum, other metals such as niobium, or alloys of tantalum and niobium with one another or with other metals may also be used.
After processing, the UHMWPE may be heated to a temperature below the melting point of the UHMWPE blend to relieve any residual stresses that may have been formed during processing and to provide additional dimensional stability. As used herein, the melting point of the UHMWPE is the melting point as determined by ASTM International F2625-07, Standard Test Method for Measurement of Enthalpy of Fusion, Percent Crystallinity, and Melting Point of Ultra-High-Molecular Weight Polyethylene by Means of Differential Scanning Calorimetry. Heating the UHMWPE to a temperature below the melting point of the UHMWPE creates a more homogenous mixture and increases the final crystallinity. For example, the UHMWPE may be preheated using a convection oven or by exposing the UHMWPE to infrared radiation.
Irrespective of whether or not the UHMWPE is heated to a temperature below the melting point of the UHMWPE to relieve any residual stress, the processed UHMWPE may then by preheated in preparation for receiving crosslinking irradiation. As used herein, “crosslinking irradiation” refers to exposing the consolidated UHMWPE blend to ionizing irradiation to form free radicals which may later combine to form crosslinks. For example, the UHMWPE may be preheated using a convection oven or by exposing the UHMWPE to infrared radiation. In one exemplary embodiment, the processed UHMWPE may be preheated to any temperature between room temperature, approximately 23° C., up to the melting point of the UHMWPE, approximately 140° C. In another exemplary embodiment, the UHMWPE is preheated to a temperature between 60° C. and 130° C. In other exemplary embodiments, the UHMWPE may be heated to a temperature as low as 60° C., 70° C., 80° C., 90° C., or 100° C. or as high as 110° C., 120° C., 130° C., 135° C., 140° C. By preheating the processed UHMWPE before irradiation, the material properties of the resulting irradiated UHMWPE are affected. Exemplary methods of preheating UHMWPE prior to irradiation are disclosed in copending U.S. patent application Ser. No. 12/100,894, which is expressly incorporated by reference herein above. Thus, the material properties for a UHMWPE irradiated at a relatively cold, e.g., approximately 40° C., temperature are substantially different than the material properties for a UHMWPE irradiated at a relatively warm, e.g., approximately 120° C. to approximately 140° C., temperature.
Once the UHMWPE is prepared as desired for crosslinking, the UHMWPE may be exposed to crosslinking irradiation to induce crosslinking of the UHMWPE. The irradiation may be performed in air at atmospheric pressure, in a vacuum chamber at a pressure substantially less then atmospheric pressure, or in an inert environment, i.e., in an argon environment, for example. In one exemplary embodiment, crosslinking is induced by exposing the UHMWPE blend to a total radiation dose between about 25 kGy and 1,000 kGy. The irradiation is, in one exemplary embodiment, electron beam irradiation. In another exemplary embodiment, the irradiation is gamma irradiation. In yet another exemplary embodiment, the crosslinking does not utilize radiation, but instead utilizes silane or other forms of chemical crosslinking.
Once the UHMWPE is irradiated, the UHMWPE may be heated above its melting point, i.e., melt annealed, to decrease the free radical concentration in the UHMWPE. In one exemplary embodiment, the UHMWPE is heated above its melting point by exposing the UHMWPE to infrared radiation. For example, the infrared radiation may be provided to the UHMWPE at a watt density as low as 1.0, 1.5, 2.0, 2.5, 5.0, 10, or 20 watts per square centimeter and as high as 30, 40, 50, 60, 70, 80, or 100 watts per square centimeter. Exemplary calculations of the watt density of an emitter are set forth in Example 1 below.
Referring to
Advantageously, infrared melt annealing of the UHMWPE bar results in the UHMWPE bar experiencing homogeneous heating and cooling at a substantially faster rate than in a convection oven. For example, a conventional melt annealing cycle in a convection oven may be approximately 48 hours. In contrast, the use of infrared radiation to melt anneal a UHMWPE bar may be performed in approximately three hours or less. Further, by utilizing infrared radiation, the air or other medium that is surrounding the UHMWPE bar remains at a lower temperature during annealing. As a result, the surrounding air is less reactive, lessening the likelihood of the UHMWPE bar experiencing surface oxidation during annealing.
Referring to
Operation of infrared heater 14 and fan 24 may be controlled by temperature controller 28 having thermocouples 30, 32 electronically connected thereto. Thermocouples 30, 32 monitor the temperature of the air surrounding UHMWPE bar 12 and the internal temperature of UHMWPE bar 12, respectively. Based on the readings from thermocouples 30, 32, controller 28 may turn infrared heater 14 on and off in order to reach and maintain a bar temperature in excess of the melting point of the UHMWPE. Similarly, controller 28 may turn fan 24 on and off as needed to ensure that the air temperature around UHMWPE bar 12 remains below a predetermined temperature threshold. For example, in one exemplary embodiment, controller 28 controls the operation of infrared heater 14 so that the temperature of UHMWPE bar 12 is raised to and maintained at substantially 150° Celsius. However, controller 28 may be configured to raise and maintain the temperature of UHMWPE bar 12 at any temperature in excess of the melting point of the UHMWPE. Similarly, in one exemplary embodiment, controller 28 may activate fan 24 when the temperature of the air around UHMWPE bar 12 exceeds 25° Celsius. In another exemplary embodiment, controller 28 activates fan 24 to maintain the air temperature surrounding UHMWPE bar 12 at substantially room temperature, e.g., 23° Celsius.
Referring to
As central spindle 52 continues to rotate counter-clockwise about centerpoint C, incoming UHMWPE bars 42 are exposed to infrared radiation from infrared heaters 14, positioned at various points throughout the path of central spindle 52. For example, in one exemplary embodiment, infrared heaters 14 are positioned at each corner 58, 60 defined by sections of outer reflective shielding 62 and inner reflective shielding 64, respectively. In another exemplary embodiment, panel heaters may be positioned substantially entirely along portions of outer and inner reflective shielding 62, 64. In one exemplary embodiment, fans 24, shown in
As central spindle 52 continues to travel about centerpoint C in a counter-clockwise direction, UHMWPE bars 42 may also be rotated. For example, connection portions 54 of spindles 50 may also be connected to a motor, such as motor 20 described above, by a combination of shafts and/or gears to cause corresponding rotation of UHMWPE bars 42. In one exemplary embodiment, connection portions 54 are open on one side and rotate 180 degrees for every 45 degrees that center spindle 52 rotates. As a result, the open end of connection portions 54 are aligned with attachment members 56 of UHMWPE bars 42 to allow connection portions 54 to engage attachment members 56 of incoming UHMWPE bars 42 and disengage attachment members 56 of outgoing UHMWPE bars 44. In this manner, UHMWPE bars 42 eventually return to spindle 48 as outgoing UHMWPE bars 44, i.e., UHMWPE bars that have been exposed to infrared radiation, and are received between opposing arms 46 of spindle 48. Specifically, outgoing UHMWPE bars 44 are retained between arms 46 of spindle 48 in a substantially similar manner as incoming UHMWPE bars 42, describe in detail above. As spindle 48 continues to rotate, UHMWPE bars 44 are received by the track where they may be transported to another location and/or apparatus for further processing.
Referring to
Referring to
As shown in
As a result of the rotation of connection portion 54, attachment member 56 and UHMWPE bar 84 are correspondingly rotated. Thus, operation of rotation device 96 in conjunction with the operation of conveyor 82 results in rotation device 96 providing rotational movement of UHMWPE bar 84 while substantially linear movement of UHMWPE bar 84 is provided by conveyor 82. While rotation device 96 is described and depicted herein with specific reference to conveyor 82, rotation device 96 may also be used in connection with other embodiments of the present invention, such as center spindle 52 of apparatus 40, shown in
Advantageously, by utilizing attachment member 56 to secure UHMWPE bars to corresponding apparatuses for exposing the UHMWPE bars to infrared radiation, the UHMWPE bars are allowed to expand as they are subjected to infrared radiation. Specifically, when exposed to infrared radiation and correspondingly heated, the UHMWPE bars undergo thermal expansion. If the thermal expansion of the UHMWPE bars is restricted, the UHMWPE bars may deform from their intended shape, potentially rendering the resulting melt annealed bars unusable. By utilizing attachment member 56, UHMWPE bars 42, 44, 84 are allowed to expand and, when cooled, contract back to their original shape, without any substantial, permanent deformation.
An alternative embodiment of an attachment mechanism for securing UHMWPE bars to the apparatuses described herein is shown in
Thus, threaded ends 114 of crossbars 104 are inserted through upper and lower apertures in opposing angle braces 102 until heads 106 of crossbars 104 contact a first one of angle braces 102. Springs 108 are then advanced over opposing, threaded ends 114 of crossbars 104 and positioned thereon. Springs 108 are secured in position on crossbars 104 by washers 110 and nuts 112. Specifically, washers 110 are first positioned on crossbars 104 and then nuts 112 are threadingly engaged with threaded ends 114 of crossbars 104. Nuts 112 may be tightened until springs 108 are slightly compressed between nuts 112 and angle braces 102. Additionally, in one exemplary embodiment, one of crossbars 104 includes an eyelet (not shown) substantially similar to eyelet 92 of attachment member 56. By utilizing an eyelet, attachment mechanism 100 may be connected to connecting portion 54, shown in
Advantageously, by utilizing attachment mechanism 100 to secure UHMWPE bars to corresponding apparatuses for exposing the UHMWPE bars to infrared radiation, the UHMWPE bars are allowed to expand as they are subjected to the infrared radiation. As described in detail above with respect to attachment member 56, UHMWPE bars undergo thermal expansion when heated. Thus, when a UHMWPE bar secured by attachment mechanism 100 begins to expand, the UHMWPE bar forces angle braces 102 against the bias of springs 108, thereby compressing springs 108 against washers 110, which is held in position by nuts 112. By compressing springs 108, the UHMWPE bars may expend while remaining in substantially the same position.
Referring to
Referring to
Additionally, as shown in
Referring to
Referring to
The following non-limiting Examples illustrate various features and characteristics of the present invention, which is not to be construed as limited thereto. The following abbreviations are used throughout the Examples unless otherwise indicated.
Calculations were performed to identify the increased watt density that is generated by using thermal radiation. Specifically, as set forth in TABLE 2 below, the watt density emitted from both the stainless steel walls of a convection oven and a tungsten aged filament from an infrared heater were calculated. The watt density, P, expressed as watts per square centimeter, was calculated using the Stefan-Boltzmann Law, i.e., P=eσA(T4−TC4), where σ is equal to 5.6703×10−8 watt/m2K4. In performing each of the calculations, a radiating area, A, of one square centimeter was used.
Additionally, the watt density received, i.e., the amount of radiation that reaches a material to be heated, was calculated for different distances between the radiation source and the material to be heated. Specifically, the watt density received at 1 inch, i.e., when 1 inch separates the source of the radiation and the material to be heated, and at 6 inches, i.e., when 6 inches separate the source of the radiation and the material to be heated, were determined by calculating the heat flow at a given distance from the radiation source and multiplying the same by the emissivity coefficient of the material, which is set forth in TABLE 2 below. The heat flow may be calculated using the following equation:
q=56.69×10−9×VF(1-2)×A×(T4−TC4),
where q is the heat flow, A is the area of the opposing flat surfaces, T is the temperature of the radiation emitter, and TC is the temperature of the material or part to be heated. Additionally, VF(1-2) is the view factor, which at 1 inch is equal to 0.0448244 and at 6 inches is equal to 0.0013666. An area, A, of 1 square centimeter was used in performing each of the calculations.
The feasibility of utilizing infrared radiation to melt anneal crosslinked UHMWPE and the mechanical properties of the resulting infrared melt annealed UHMWPE were investigated. To perform this investigation, Design Expert 6.0.10 software, obtained from Stat-Ease, Inc. Minneapolis, Minn., was utilized to create a Design of Experiment (DOE) to evaluate the mechanical properties of the infrared melt annealed crosslinked UHMWPE. The DOE evaluated three different variables: the maximum temperature of the UHMWPE, cooling of the external surface of the UHMWPE, and the amount of time elapsed during temperature ramp up.
Medical grade UHMWPE powder, GUR 1050, was obtained from Ticona, having North American headquarters in Florence, Ky. The UHMWPE was compression molded into approximately 3.5″ square bars. The bar was then cut into sections measuring 6 in length. Each 6″ section was then subjected to electron beam irradiation in air and received a 100 kGy dose. Once irradiated, each 6″ section of UHMWPE bar was immediately packaged in nitrogen where it remained until the time of testing.
In order to subject the sections of UHMWPE bar to infrared radiation, two Chromalox® T-3 quartz heaters were obtained from Thermtech Systems, Inc. of Chesterfield, Ind. The heaters were positioned 6″ from where a face of a section of the UHMWPE bar is positioned during exposure to the infrared radiation, as shown in
A portion of each bar was then microtomed into 2000 micron thick films. These films were then subjected to FTIR analysis on a Bruker Optics FTIR spectrometer, available from Bruker Optics of Billerica, Mass. The FTIR results were analyzed to determine the OI and the TVI. The OI was determined by calculating the ratio of the area under the carbonyl peak on the FTIR chart at 1765-1680 cm−1 to the area of the polyethylene peak at 1392-1330 cm−1. The TVI was determined by calculating the ratio of the area on the FTIR chart under the vinyl peak at 980-947 cm−1 to the area under the polyethylene peak at 1392-1330 cm−1.
Additional testing was then performed to determine the izod impact strength, elongation, UTS, YS, storage modulus, percentage crystallinity, and free radical concentration. The mechanical properties were tested according to corresponding available ASTM standards for UHMWPE. Specifically, Type V tensile specimens, as defined by the American Society for Testing and Materials (ASTM) Standard D638, Standard Test Method for Tensile Properties of Plastics, were machined and subjected to elongation, UTS, and YS testing in accordance with ASTM Standard D638. Izod specimens were also machined and subjected to testing according to ASTM Standard F-648, Standard Test Methods for Ultra-High-Molecular-Weight Polyethylene Power and Fabricated Form for Surgical Implants. The % crystallinity was determined using DSC and measuring the crystallinity between 40° C. and 160° C. Additionally, the storage modulus was measured using a DMA. This method begins by ramping the temperature from room temperature to 150° C. at a rate of 10° C./min and then ramping the temperature from 150° C. to 210° C. at a rate of 2° C./min at 1 Hz. The DMA measurement corresponds to the Storage Modulus of the UHMWPE at 200° C. Additionally, the free radical concentration of the UHMWPE was analyzed using a Bruker EMX/EPR (electron paramagnetic resonance) spectrometer, which has a detection limit of 0.01×1015 spins/gram and is available from Bruker Optics of Billerica, Mass.
Based on the results of the analysis, the infrared melt annealed crosslinked UHMWPE had OI values lower than the control, which in this case is a convection oven melt annealed crosslinked UHMWPE, and had substantially similar or improved mechanical properties under all of the varying test conditions.
The optimal distance between an infrared heating element and crosslinked UHMWPE for infrared melt annealing was investigated. To perform this investigation, Design Expert 6.0.10 software, obtained from Stat-Ease, Inc. Minneapolis, Minn., was utilized to create a Design of Experiment (DOE) to evaluate the mechanical properties of the infrared melt annealed crosslinked UHMWPE. The DOE evaluated two different variables: the distance from an infrared heating element to a face of the UHMWPE and the percentage of total heating time that a fan was activated to cool the surface of the bar.
Medical grade UHMWPE powder, GUR 1050, was obtained from Ticona, having North American headquarters in Florence, Ky. The UHMWPE was compression molded into approximately 3.5″ square bars. The bar was then cut into sections measuring 6″ in length. Each 6″ section was then subjected to electron beam irradiation and received a 100 kGy dose. Once irradiated, each 6″ section of UHMWPE bar was immediately packaged in nitrogen where it remained until the time of testing.
In order to subject the sections of UHMWPE bar to infrared radiation, four Chromalox® T-3 quartz heaters were obtained from Thermtech Systems, Inc. of Chesterfield, Ind. The heaters were positioned in the four corners of a square-shaped steel frame. The sections of UHMWPE bar were then removed from their packaging and positioned in a holder that held the sections stationary between the four heaters so that the flat sides of the sections were each directly facing one of the infrared heaters, as shown in
Testing was then performed to determine the izod impact strength, elongation, UTS, YS, and free radical concentration. The mechanical properties were tested according to ASTM standards corresponding to UHMWPE. Specifically, Type V tensile specimens, as defined by the American Society for Testing and Materials (ASTM) Standard D638, Standard Test Method for Tensile Properties of Plastics, were machined and subjected to elongation, UTS, and YS testing in accordance with ASTM Standard D638. Izod specimens were also machined and subjected to testing according to ASTM Standard F-648, Ultra-High-Molecular-Weight Polyethylene Powder and Fabricated Form for Surgical Implants. Additionally, the free radical concentration was analyzed using a Bruker EMX/EPR (electron paramagnetic resonance) spectrometer, which has a detection limit of 0.01×1015 spins/gram and is available from Bruker Optics of Billerica, Mass.
Overall, the experiment showed that the UHMWPE had substantially equivalent or better mechanical properties irrespective of heater distance up to 10 inches.
The effects of using different wavelengths of infrared radiation to infrared melt anneal crosslinked UHMWPE was investigated. Medical grade UHMWPE powder, GUR 1050, was obtained from Ticona, having North American headquarters in Florence, Ky. The UHMWPE was compression molded into approximately 3.5″ square bars. The bar was then cut into sections measuring 6″ in length. Each 6″ section was then subjected to electron beam irradiation and received a 100 kGy dose. Once irradiated, each 6″ section of UHMWPE bar was immediately packaged in nitrogen where it remained until the time of testing.
Three different types of infrared heaters were acquired, as set forth in TABLE 5 below. In order to expose the UHMWPE sections to infrared radiation, two heaters of the same type were positioned approximately 24 inches apart from and facing one another. The heaters were attached to a steel frame and held stationary. The sections of the UHMWPE bar were mounted between the heaters so that the flat sides of the bar were facing the heaters. A thermocouple was mounted inside the sections of the UHMWPE bar to monitor the temperature within the UHMWPE bar. The bar was heated to 150° C. and the time that elapsed until the bar was substantially entirely melted, i.e., the time that elapsed from the initiation of the heating until the opaque crystalline regions of the bar became amorphous and, thus, were optically transparent as determined by visual observation, was recorded. This process was repeated for each type of heater set forth in TABLE 5 below.
Once all of the sections of the UHMWPE bar had been tested, the material properties of the resulting sections were analyzed. A portion of each section was microtomed into 2000 micron thick films. These films were then subjected to FTIR analysis on a Bruker Optics FTIR spectrometer, available from Bruker Optics of Billerica, Mass. The FTIR results were analyzed to determine the OI. The OI was determined by calculating the ratio of the area under the carbonyl peak on the FTIR chart at 1765-1680 cm−1 to the area of the polyethylene peak at 1392-1330 cm−1.
Additional testing was then performed to determine the elongation, YS, UTS, and free radical concentration. The mechanical properties were tested according to ASTM standards corresponding to UHMWPE. Specifically, Type V tensile specimens, as defined by the American Society for Testing and Materials (ASTM) Standard D638, Standard Test Method for Tensile Properties of Plastics, were machined and subjected to elongation, UTS, and YS testing in accordance with ASTM Standard D638. Additionally, the free radical concentration was analyzed using a Bruker EMX/EPR (electron paramagnetic resonance) spectrometer, which has a detection limit of 0.01×1015 spins/gram and is available from Bruker Optics of Billerica, Mass.
The analysis, the results of which are set forth below in TABLE 6, showed that the material properties did not vary substantially between wavelengths. However, the polyethylene appeared most quickly to absorb the short wavelengths, i.e., those generated by the heaters with tungsten filaments.
While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.
