Description:FIELD OF THE INVENTION
[0001] Embodiments of the present disclosure relates to an optimized injection molding process for fabricating medical-grade Ultra-High Molecular Weight Polyethylene (UHMWPE) based polymer blends tailored for implant applications, and more specifically for orthopaedic implants.
BACKGROUND
[0002] Implants, specially orthopaedic implants is a sizable market, which is driven by an aging population, changing lifestyles, and cases of physical trauma, where approaches such as drug-based clinical treatments do not reduce such trauma. Generally, rheumatic diseases, such as arthritis, caused to clinical observations of redness or swelling of hip joint, disturbing bone homeostasis, resulting in inflammation-mediated bone loss and often, extreme pain, and in many situations result in unbearable pain, irreparable deformities, or intolerable functional loss of the hip joint, requiring surgical intervention. Usually, implanted orthopaedic prosthesis is expected to restore the functions of the native hip joint and integrate with the periprosthetic bone.
[0003] From a biomaterials perspective, both hip and knee replacement essentially represent the functional assembly of multiple materials, for example SS 316, CoCr, ZTA, UHMWPE, Ti6Al4V, and their performance is determined by the properties of materials at articulating surfaces. One such biomaterial for articulating surfaces, used in most of the articulating joints is UHMWPE, where machining extruded UHMWPE rods to produce final products results in the loss of material, and post-machining operations are generally needed for obtaining the surface finish as per the ASTM standard, which adds to the final cost and production time. Therefore, there is a need for tuning materials that can be easily molded, where because of its inherent material property like high molecular weight and viscosity, UHMWPE is difficult to be molded, and specifically injection molded, wherein injection molding, is normally suitable for mass production of small-sized polymer products with complicated structures that require precise dimensional tolerance, without the need for any post-processing.

SUMMARY
[0004] Embodiments of the present disclosure relate to a method and system for preparing a polymeric blend for orthopaedic application. An embodiment of the present disclosure to that end includes forming a uniform blend of an ultra-high molecular weight polyethylene (UHMWPE) with maleated polyethylene (mPE) by first selecting a pre-determined quantity of UHMWPE by weight; and then selecting a pre-determined quantity of mPE by weight, wherein the weight of the mPE is a percentage of weight of the UHMWPE. In an embodiment the pre-determined quantity is controlled to obtain a homogeneous blend that can be injection molded to form an object of a desired shape. A further embodiment includes melt mixing the UHMWPE and the mPE forming a homogeneous blend, wherein the homogeneous blend without inducing polymer matrix degradation, or physiochemical properties. Other embodiments are also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS
[0005] The detailed description is described with reference to the accompanying figures. Features, aspects, and advantages of the subject matter of the present disclosure will be better understood with regard to the following description and the accompanying drawings. The figures are intended to be illustrative, not limiting, and are generally described in context of the embodiments, and it should be understood that it is not intended to limit the scope of the disclosure to these particular embodiments. In the figures, the same numbers may be used throughout the drawings to reference features and components. In order that the present disclosure may be readily understood and put into practical effect, reference will now be made to exemplary embodiments as illustrated with reference to the accompanying figures. The figures together with detailed description below, are incorporated in and form part of the specification, and serve to further illustrate the embodiments and explain various principles and advantages.
[0006] Figure 1 illustrates an exemplary process of preparing a homogeneous blend of UHMEPE with mPE in accordance with embodiments of the present disclosure.
[0007] Figure 2A illustrates an exemplary tensile property of the modified UHMWPE (i.e. UHMEPE blend with mPE) with respect to the percentage of mPE in accordance with embodiments of the present disclosure.
[0008] Figure 2B illustrates an exemplary tensile property of the modified UHMWPE with respect to injection molding at room temperature (mold temperature) and different melt temperatures in accordance with embodiments of the present disclosure
[0009] Figure 3 illustrates an exemplary case of a thermophysical analysis of a differential scanning calorimetry (DSC) 300-A and thermogravimetric analysis (TGA) 300B for pure UHMWPE and the modified UHMWPE in accordance with the embodiments of the present disclosure.
[0010] Figure 4 illustrates an exemplary embodiment of the chemical structure of the modified UHMWPE by FTIR analysis
[0011] Figure 5A, which illustrates rheological study of the modified UHMWPE having a temperature sweep at 1% strain and 0.1 rad/sec
[0012] Figure 5B, which illustrates rheological study of the modified UHMWPE having a frequency sweep at 1% strain for a temperature of 260°C.
[0013] Figure 6 is an exemplary illustration of the effect of gamma irradiation on chemical structure and crosslinking mechanisms in accordance with an embodiment of the present disclosure.
[0014] Figure 7 is an exemplary graph of the strain versus stress for the non cross linked and the cross-linked samples in accordance with an embodiment of the present disclosure.
[0015] Figure 8 is an exemplary graph illustration the stress versus strain for vitamin E doped mPE-UHMWPE in accordance with an embodiment of the present disclosure.
[0016] Throughout the drawings, identical reference numbers designate similar, but not necessarily identical elements. The figures as disclosed herein are not necessarily to scale, and the size of some parts may be exaggerated to more clearly illustrate the example shown. Moreover, the drawings are meant to only be provided as examples and/or implementations consistent with the description, and the description may not be limited to the examples and/or implementations provided in the drawings.

DETAILED DESCRIPTION
[0017] The following describes technical solutions in exemplary embodiments of the subject matter of the present disclosure with reference to the accompanying drawings. In this application as disclosed herein, "at least one" means one or more, and "a plurality of" means two or more. The term "and/or" describes an association relationship for describing associated objects and represents that three relationships may exist. For example, A and/or B may represent the following cases: Only A exists, both A and B exist, and only B exists, where A and B may be singular or plural. The character "/" usually indicates an "or" relationship between the associated objects. "At least one item (piece) of the following" or a similar expression thereof means any combination of the items, including any combination of singular items (piece) or plural items (pieces). For example, at least one item (piece) of a, b, or c may represent a, b, c, a and b, a and c, b and c, or a, b, and c, where a, b, and c each may be singular or plural.
[0018] It should be noted that in this application articles “a”, “an” and “the” are used to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article. The terms “comprise” and “comprising” are used in the inclusive, open sense, meaning that additional elements may be included. It is not intended to be construed as “consists of only”. Throughout this specification defined above, unless the context requires otherwise the word “comprise”, and variations such as “comprises” and “comprising”, will be understood to imply the inclusion of a stated element or step or group of elements or steps but not the exclusion of any other element or step or group of elements or steps. The term “including” is used to mean “including but not limited to”. “Including” and “including but not limited to” are used interchangeably. In the structural formulae given herein and throughout the present disclosure, the following terms have been indicated meaning, unless specifically stated otherwise.
[0019] Unless otherwise defined, all terms used in the disclosure, including technical and scientific terms, have meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. By means of further guidance, term definitions are included for better understanding of the present disclosure. The term ‘about’ as used herein when referring to a measurable value such as a parameter, an amount, a temporal duration, and the like, is meant to encompass variations of ±10% or less, preferably ±5% or less, more preferably ±1% or less and still more preferably ±0.1% or less of and from the specified value, insofar such variations are appropriate to perform the present disclosure. It is to be understood that the value to which the modifier ‘about’ refers is itself also specifically, and preferably disclosed.
[0020] It should be noted that in this application, the term such as "example" or "for example" or “exemplary” is used to represent giving an example, an illustration, or descriptions. Any embodiment or design scheme described as an "example" or "for example" in this application should not be explained as being more preferable or having more advantages than another embodiment or design scheme. Exactly, use of the word such as "example" or "for example" is intended to present a related concept in only a specific manner.
[0021] It should be understood that in the embodiments of the present subject matter that "B corresponding to A" indicates that B is associated with A, and B can be determined based on A. However, it should be further understood that determining B based on A does not mean that B is determined based on only A. B may alternatively be determined based on A and/or other information.
[0022] In the embodiments of this application, "a plurality of" means two or more than two. Descriptions such as "first", "second" in the embodiments of this application are merely used for indicating and distinguishing between described objects, do not show a sequence, do not indicate a specific limitation on a quantity of devices in the embodiments of this application, and do not constitute any limitation on the embodiments of this application.
[0023] Exemplary embodiments of the present disclosure relate utilizing deep-polymer process science-driven injection molding protocols for the development of UHMWPE implants. In the exemplary case, as UHMWPE is difficult to be injection molded, a new blend of UHMWPE with mPE or PE-g-MA (polyethylene grafted maleic anhydride) may be used to advantageously injection molded to produce acetabular liner components. In an exemplary case, Polyolefins are a group of thermoplastics that have lower molecular weight and viscosity, suited for injection molding and among those polyolefins, UHMWPE poses significant challenges for injection molding. In an exemplary case, mixing UHMWPE with another member of the same polyolefin family (with short chains or lower molecular weight) may result in modification of rheological and viscoelastic properties, rendering the blend thus prepared to be injection moldable. In the exemplary case, the rationale is to disperse the short chain polyethylene chains in otherwise high molecular weight polyethylene to reduce the viscosity without compromising on the mechanical properties while inducing fewer chemical changes in the macromolecular architecture. In the exemplary case, such changes will also allow in developing blends with favourable rheological properties to make them injection moldable. The implant properties and performance will be compared with the currently used commercial implants Against this background, UHMWPE mixed with Polyethylene graft maleic anhydride (PE-g-MA or mPE) improves the flowability and moldability of thicker parts and the blend thus prepared is injection moldable, using a plunger-based molding facility. By optimizing injection molding parameters and encompassing material properties, defect-free components with clinically desired strength and durability properties may be obtained.
[0024] Exemplary embodiments of the present disclosure relate to a method and system for preparing a polymeric blend for orthopaedic application, particularly to a Laboratory scale production and testing, which may be easily replicated to larger industrial scale production. An exemplary embodiment of the present disclosure to that end includes forming a uniform blend of an ultra-high molecular weight polyethylene (UHMWPE) with maleated polyethylene (mPE) by first selecting a pre-determined quantity of UHMWPE by weight, and then selecting a pre-determined quantity of mPE by weight, wherein the weight of the mPE is a percentage of weight of the UHMWPE. In an exemplary embodiment the pre-determined quantity is controlled to obtain a homogeneous blend that can be injection molded to form an object of a desired shape, wherein the shape of the object can be in the form of a mold. A further exemplary embodiment includes melt mixing the UHMWPE and the mPE forming a homogeneous blend, wherein the homogeneous blend without inducing polymer matrix degradation, or physiochemical properties. If the homogeneous blend with the desired physical, chemical and mechanical properties is not obtained, the parameters of the melt mixing may be changed accordingly to obtain the homogeneous blend of UHMWPE and mPE (hereinafter reference to mPE in the present disclosure related to maleated PE or PE-g-MA)
[0025] In an exemplary embodiment, the pre-determined percentage weight of mPE is in a range of 1% to 5% weight of UHMWPE. In an exemplary case if 100 grams of UHMWPE has been taken then 1% to about 5%, i.e., about 1 gram to about 5 grams, by weight of mPE may be taken to mix with the UHMWPE. The combination of UHMWPE and mPE is melt mixed and injection molded a pre-defined temperature, for example at a temperature of 2600C to obtain a uniform homogeneous blend.
[0026] In an exemplary case, melt mixing is performed by a melt extruder. In an exemplary case, melt extrusion is typically known as a procedure of converting a raw material with plastic properties into a product of uniform shape and density, by forcing it through a die under controlled temperature, product flow, and pressure condition. It should be obvious to a person of ordinary skill in the art that various other techniques may also be used to create a uniform homogeneous blend for injection molding and all such techniques and variation fall within the scope of the present disclosure.
[0027] In an exemplary case, a temperature for melt mixing varies in a range of about 1800C - 2600C. Again it should be obvious to a person of ordinary skill in the art that the temperatures are only indicative and can deviate from this range, and any deviations fall under the scope of the present disclosure. In the exemplary case, the mixing time for performing the melt mixing is proportional/dependent to the quantity of UHMWPE and mPE chosen for melting and obtaining a homogeneous uniform blend for injection molding.
[0028] In an exemplary case, a pre-determined force is maintained over the mixing time, wherein the pre-determined force is dependent on the quantity of UHMWPE and mPE. In a specific case under laboratory conditions, the mixing time varies in the range of 6 – 10 minutes and a pre-determined force varies between 6-7 kN. Again, it should be obvious to a person of ordinary skill in the art that depending on the quantity and amount of blend created, the mixing time and the force may vary accordingly, and all such variation fall within the scope of the present disclosure.
[0029] In an exemplary case, an increase in a percentage of mPE results in an increase in an elongation of the blend. In an exemplary case, an increase in the percentage of mPE results in a decrease in a tensile strength of the blend, which is due to perturbation or degradation of the crystal lattice of UHMWPE upon incorporation of short chain mPE.
[0030] In an exemplary case the method includes creating an object of interest, for example an orthopaedic object, which includes injecting the blend created by the method disclosed above into a specimen of the object with a plunger-based injection molding machine. In an exemplary case, an injection pressure for the blend being injected into the mold is in a pre-determined range. In an exemplary case, under laboratory conditions a pressure in the range of 12-16 bar was maintained. It should be obvious to a person of ordinary skill in the art that under larger scale productions of the object the pressure ranges may vary accordingly and all such pressure ranges fall within the scope of the present disclosure.
[0031] In an exemplary case a melt temperature for injection molding varies in a range of about 1800C - 2600C, wherein the melt is injected into the mold at the said temperature. In an exemplary case, the melt viscosity decreases with an increasing mPE content (0-5%) in a range from 22 kPa·s to 12 kPa·s, under laboratory conditions. It should be obvious to a person of ordinary skill in the art that under larger scale production, such as industrial production the range of the melt viscosity may vary beyond this range and all such variations fall within the scope of the present disclosure. In an exemplary case, a Melt Flow Index (MFI) index increases from 6.2gm/10min to 8.1 gm/10min with addition of the mPE.
[0032] In an exemplary case, irradiating the object created from the blend with gamma irradiation of about 50kGy and 100kGy, results in an increase of a strain to failure of the UHMWPE from 90-450%. In an exemplary case, doping the molded object with Vit-E enhances tensile properties associated with the molded object, which may be attributed to change in surface modification due to diffusion of Vit-E into the molded parts. In an exemplary embodiment, an object of a desired shape and size may be formed by injection molding using the above disclosed methods.
[0033] Reference is now made to Figure 1, which illustrates an exemplary process of preparing a homogeneous blend of UHMEPE with mPE in accordance with embodiments of the present disclosure. As illustrated, at step 110 a pre-determined quantity of UHMWPE by weight is selected. In an exemplary case, for the purpose of laboratory manufacturing and testing the weight of UHMWPE selected was varied in the range of about 6 – 10 grams. It should be obvious to a person or ordinary skill in the art that the above-mentioned weight is only for producing a laboratory scale blend and for an industrial scale production, the weight of UHMWPE may vary accordingly, and such a variation falls within the scope of the present disclosure. At step 115, a pre-determined quantity of mPE be weight is selected. In an exemplary case, for the purpose of laboratory manufacturing and testing the weight of mPE selected was varied in the range of about 0.1 – 0.5 grams, which is about 1% to about 5% the weight of UHMWPE. Again, it should be obvious to a person or ordinary skill in the art that the above mentioned weight is only for producing a laboratory scale blend and for an industrial scale production, the weight of mPE may vary accordingly to be about 1% to about 5% of the weight of UHMWPE and such a variation falls within the scope of the present disclosure.
[0034] At step 120, the mixture of UHMWPE and mPE is put into a melt extruder and mixed under controlled conditions, wherein extrusion is a procedure of converting a raw material with plastic properties into a product of uniform shape and density, by forcing it through a die under controlled temperature, product flow, and pressure condition. In the laboratory setup, the mixing temperature and the rotation speed of the screws of the extruder may be controlled to obtain a homogeneous blend. At step 130, under controlled conditions a homogeneous blend of the mixture of UHMWPE and mPE is obtained.
[0035] As illustrated if the blend is not homogeneous, control is passed back to step 120, until a homogeneous blend is obtained in step 130. Once a homogeneous blend of the mixture of UHMWPE and mPE is obtained, the blend is injection molded under controlled conditions of temperature, pressure and time which may be controlled by a user manually or may be automated by coupling an external computing device to the system.
[0036] Reference is now made to Figure 2A, which illustrates an exemplary tensile property of injection molded (at 2600C) modified UHMWPE with respect to the percentage of mPE in accordance with embodiments of the present disclosure. The graph illustrates measurement of Stress (MPa) versus Strain (%) for different composition of the mPE used. Line 205 represents 0% of mPE added to UHMWPE, line 210 represents 1% of mPE added to UHMWPE, line 215 represents 2% of mPE added to UHMWP, line 220 represents 3% of mPE added to UHMWP, line 225 represents 4% of mPE added to UHMWP and line 230 represents 5% of mPE added to UHMWP. As illustrated mPE content varied to be within a range of 0 to 5%, followed by melt compounding and the injection molding for subsequent fabrication of dumbbell-shaped tensile specimens (object) conforming to ASTM D638 standards, wherein micro injection molding technique was used.
[0037] The results manifest a discernible trend of tensile strength concomitant with an increase in mPE content, alongside a marked increase in elongation from about70 to about 280% as provided in Table 1 below.
Table 1
Samples Modulus (MPa) Strength (MPa) Elongation (%)
0% mPE 747.95 23.23 78.04
1% mPE 655.53 21.16 94.03
2% mPE 674.37 20.36 145.39
3% mPE 651.46 19.98 146.27
4% mPE 626.09 18.86 230.96
5% mPE 645.38 18.84 202.89

As seen, the modulus for the sample decreases from about 750 MPa at a mPE of 0% to about 640 MPa when the mPE mixture is at about 5%. Correspondingly the strength also decreases from about 24 MPa at 0% weight of mPE to about 19 MPa at 5% weight of mPE. This phenomenon emanates mainly due to the perturbation or degradation of the crystalline lattice of UHMWPE upon incorporation of short chain mPE into the UHMWPE, which facilitates enhanced flowability during processing of the object during the injection molding. Consequently, an increased mPE content precipitates a greater degree of crystalline structure disruption within UHMWPE, resulting in diminished tensile strength.
[0038] Reference is now made to Figure 2B, which illustrates an exemplary tensile property of the modified UHMWPE with respect to injection molding at room temperature and different melt temperatures in accordance with embodiments of the present disclosure. Subsequent to optimization of mPE content tailored for orthopedic applications, further enhancements in mechanical properties studied through the optimization of injection molding parameters. This is elucidated in Figure 2B. where a variation in melting temperature spanning 180 to 260°C was explored while maintaining other injection molding parameters such as mold temperature and injection pressure constant. In the exemplary case, the injection pressure was maintained at 14 bar. In accordance with the exemplary embodiment, elevated melt temperatures precipitate a reduction in tensile strength concomitant with an augmentation in elongation due to alterations in the viscoelastic characteristics of polymers at elevated temperatures. Further, augmented temperatures foster an increased prevalence of the amorphous phase, contributing to diminished tensile strength and heightened elongation. Additionally, prolonged exposure to elevated temperatures may instigate chain scission events, leading to the fragmentation of lengthy polymer chains into shorter counterparts, thereby further compromising tensile strength.
[0039] The graph illustrates measurement of Stress (MPa) versus Strain (%) for different composition of the mPE used. Line 250 represents injection molding at room temperature and melt temperature of 180oC of mPE added to UHMWPE, line 255 represents injection molding at room temperature and melt temperature of 200oC of mPE added to UHMWPE, line 260 represents injection molding at room temperature and melt temperature of 220oC of mPE added to UHMWPE, line 265 represents injection molding at room temperature and melt temperature of 260oC of mPE added to UHMWPE, and line 275 represents injection molding at room temperature and melt temperature of 220oC for pure UHMWPE as illustrated in Table 2 below.
Table 2
Temp Modulus (MPa) UTS (MPa) Elongation (%)
RT-180 859.04 39.20 38.31
RT-200 807.61 33.40 41.11
RT-220 771.47 31.38 77.66
RT-260 606.18 20.21 82.61
RT-220 (NC) 709.09 27.48 46.43

RT indicates that the injection molding is performed at room temperature, and the temperature besides RT indicates the melt temperature. RT-220 (NC) indicates non-crosslinked UHMWPE, i.e., pure UHMWPE which is not mixed with any mPE.
[0040] In the exemplary case, the specimens are subjected to conditions denoted as RT-180 (wherein the mold temperature is ambient (Room Temperature) and melt temperature is 180°C), which evince a superior tensile strength of approximately 40 MPa, accompanied by a favourable elongation range of 40-50%, which may be deemed sufficient good for producing objects that may be of interest in orthopaedic applications. An advantageous attribute of mPE-modified UHMWPE is manifested in its amenability to injection molding at relatively lower temperatures (180°C), which in stark contrast to pure UHMWPE, necessitates processing temperatures exceeding 260°C owing to its exceedingly high viscosity and molecular weight.
[0041] Reference is now made to Figure 3A which illustrates an exemplary case of a thermophysical analysis of a differential scanning calorimetry (DSC) 300-A and thermogravimetric analysis (TGA) 300B for pure UHMWPE and the modified UHMWPE in accordance with the embodiments of the present disclosure. For the exemplary case, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were conducted using a TA Instruments Q2000 DSC apparatus in an inert (using Ar) environment. Parameters such as degree of crystallinity (?c), crystallization temperature (Tc), melting temperature (Tm), and specific heat of fusion (?hf) were determined for the samples under consideration. Samples weighing between 4 mg - 7 mg underwent a heating-cooling-heating cycle to eliminate any prior thermal effects. The experimental procedure involved heating the samples to 200 °C, cooling to -30 °C, and reheating to 200 °C, with heating and cooling rates set at 10 °C/min. DSC endotherms were recorded during the second heating cycle. For calculations, ?hf0 represented the heat of fusion for 100% crystalline polyethylene, established at 290 J/g.
[0042] The thermal analysis of modified UHMWPE alongside pure UHMWPE, DSC data depict the melting and crystalline ranges of the modified materials, resembling those of pure UHMWPE. However, a slight reduction in melting temperature (129°C) is evident in the modified materials compared to pure UHMWPE (132°C). This decline may stem from the blending of short-chain mPE into UHMWPE, disturbing its crystalline structure and weakening the intermolecular bonding, thereby facilitating polymer melting at lower temperatures.
[0043] TGA analysis of modified UHMWPE, indicating comparable thermal stability compared to pure UHMWPE. The injection molding processing window spans from 170°C - 260°C, and it may be observed that mPE-modified materials maintain thermal stability within this range, enabling processing within the injection molding parameters. The similarity in thermal stability between pure and modified UHMWPE may arise from blending polyolefins of the same family with shorter chain lengths, requiring similar thermal energy to break chemical bonds at high temperatures, thereby ensuring comparable thermal stability.
[0044] Reference is now made to Figure 4, which illustrates an exemplary embodiment of the chemical structure of the modified UHMWPE by FTIR analysis. The solid line represents the FTIR analysis of pure UHMWPE and the dotted dashed line represents the mPE modified UHMWPE. FTIR analysis was conducted on two distinct samples: pure UHMWPE (solid line) and mPE-modified UHMWPE (dashed-dotted line). For UHMWPE, discernible bands appear at specific wavenumbers, notably at 720 cm?¹ (rocking movement CH2 groups), 1090 cm?¹ (stretching mode of C-O-C groups), 1460 cm?¹ (bending mode of CH2 groups), 2860 cm?¹ (asymmetric stretching mode of CH2 groups), and 2920 cm?¹ (symmetric stretching mode of CH2 groups). Correspondingly, the observed FTIR spectra for both samples exhibited analogous bands at 719 cm?¹, 730 cm?¹, 1462 cm?¹, 1473 cm?¹, 2848 cm?¹, and 2915 cm?¹. These shared bands denote a uniform molecular makeup across all samples, with no discernible alterations in molecular groups observed in either pure UHMWPE or mPE-modified UHMWPE post-melt compounding.
[0045] The FTIR analysis of modified UHMWPE is juxtaposed with pure UHMWPE, revealing analogous FTIR bands in both spectra. Further, the absence of mPE characteristic bands, preferably in the range 1760 cm-1 – 1860 cm-1, such as maleic anhydride peaks, underscores the thorough mixing and blending of mPE within the UHMWPE matrix during melt compounding. As maleic anhydride groups are intricately bound with short-chain polyethylene, their absence in the FTIR spectra signifies the seamless integration of mPE with medical-grade pure UHMWPE, which advantageously provides an efficient way of producing injection moldable blends. The consistency in chemical composition post-blending renders the modified materials conducive for orthopaedic applications, ensuring continuity in material properties and structural integrity akin to medical-grade pure UHMWPE. It should be obvious to a person of ordinary skill in the art that though orthopaedic applications have been stated herein, it should be obvious that various other products and/or object may also be made and all such objects fall within the scope of the present disclosure.
[0046] In the domain of polymer processing, the Melt Flow Index (MFI) is a fundamental parameter influencing material behaviour during manufacturing processes such as injection molding. The ASTM D1238 standard provides a rigorous methodology for determining MFI, ensuring methodological consistency and reliability across testing procedures. The Melt Flow Index (MFI) of mPE-modified UHMWPE was evaluated in accordance with ASTM D1238 standards, yielding values ranging between 7.6-8.2 gm/10min at 190°C and 21.6 kg load. In comparison, the MFI value for pure UHMWPE under identical conditions was measured at 6.2 gm/10min. This augmentation in MFI suggests improved flow characteristics, particularly advantageous for injection molding applications. The elevated MFI values imply enhanced flowability of the polymer blend, enabling lower molding temperatures and more uniform mold cavity filling. Consequently, this enhancement potentially mitigates defects such as voids and shrinkage, thereby elevating the overall quality and efficiency of the manufacturing process.
[0047] Reference is now made to Figure 5A, which illustrates rheological study of the modified UHMWPE having a temperature sweep at 1% strain and 0.1 rad/sec. Rheological analysis is indicative of the intricate flow and deformation behaviours of polymers, offering invaluable insights into properties such as viscosity, elasticity, and viscoelasticity. The viscoelastic attributes of both UHMWPE and its modified variant were comprehensively scrutinized utilizing a stress-controlled discovery hybrid rheometer (DHR-3) equipped with parallel plate geometry (25 mm diameter, 1 mm gap distance). To ensure material uniformity and stability, samples underwent hot pressing (compression molding) at 260 °C. Rheological investigations, including frequency sweep, temperature sweep, and shear rate sweep, were meticulously conducted within the linear viscoelastic region.
[0048] The square dots indicate measurements for pure UHMWPE, the circular dots indicate measurements for 2% mPE, upper triangle indicated measurement for 3% mPE, inverted triangle indicate measurements for 4% mPE and diamond indicated measurements for 5% mPE. For higher percentage of mPE, the viscosity is found to be lower compared to lower percentage of mPE. The temperature sweep, temperatures spanning from 200 to 280°C were traversed at a constant angular frequency of 1 rad/sec and 1% strain rate. Rheological analysis of the blended polymers are illustrated in the graph. A consistent decline in viscosity is measured with frequency or temperature. This phenomenon bodes well for injection molding processes, as lower viscosity facilitates smoother and more uniform filling of the injection mold. Notably, as the mPE content increases, A progressive reduction in viscosity is measured. For instance, as mPE content varies from 1-5%, viscosity diminishes from 18.83 kPa·s to 12.30 kPa·s. The decline in viscosity with temperature or frequency correlates with the viscoelastic behaviour of polymers. As temperature rises, polymer chain mobility increases, leading to reduced alignment and crystalline structure, and an augmentation of the amorphous phase. Consequently, viscosity decreases. This effect is amplified with higher mPE content, exacerbating the reduction in viscosity.
[0049] Reference is now made to Figure 5B, which illustrates rheological study of the modified UHMWOPE having a frequency sweep at 1% strain for a temperature of 260°C. During the frequency sweep, conducted at a temperature of 260°C with a 1% strain rate, incremental variations in angular frequency ranging from 0 to 100 rad/sec were systematically applied. The graph clearly illustrates that beyond a certain angular frequency at a constant temperature of 260°C, the viscosity decreases substantially.
[0050] Reference is now made to Figure 6, which is an exemplary illustration of the effect of gamma irradiation on chemical structure and crosslinking mechanisms in accordance with an embodiment of the present disclosure. The cross-linked ultra-high molecular weight polyethylene (XL-UHMWPE) represented an advancement in orthopedic materials. XL-UHMWPE, identified as a first-generation derivative of UHMWPE, can undergo crosslinking through either irradiation (electron beam or gamma irradiation) or chemical methodologies (peroxide or silane chemistries). While chemical approaches like peroxide crosslinking typically face challenges linked to oxidation and aging, and therefore radiation crosslinking emerged as the favoured method due to its effectiveness in introducing crosslinks throughout UHMWPE's chemical structure.
[0051] The process of crosslinking through irradiation involves the recombination of free radicals generated by radiation penetration, primarily occurring in the amorphous regions of UHMWPE. These free radicals arise from the breakdown of C-C or C-H bonds, forming Y-linkages or H-linkages, depending on temperature. However, the breakage of C-C bonds may lead to chain scissoring and a reduction in molecular weight, which can result in an impairment of mechanical properties while enhancing the wear resistance. Wear resistance correlates directly with cross-linking density, peaking at approximately 100 kGy irradiation dosage. Further irradiation beyond this level may reduce wear rates but could compromise plastic deformation and fatigue resistance.
[0052] The oxidative properties of irradiated UHMWPE are typically inferior to those of conventional UHMWPE due to entrapped free radicals, particularly in the crystalline phase post-irradiation. Post-irradiation thermal treatments, such as remelting or annealing, have been found to be viable solutions. Generally, annealed-irradiated UHMWPE displays superior properties compared to remelted-irradiated UHMWPE, attributed to its increased ability to crystallize. at UHMWPE irradiated with a gamma-ray dosage equivalent to 100 kGy, followed by melting, demonstrates improved wear resistance but compromised fatigue strength. However, fatigue resistance in annealed XL-UHMWPE is less impacted due to its enhanced crystallization ability.
[0053] Reference is now made to Figure 7, which is an exemplary graph of the strain versus stress for the non cross linked and the cross-linked samples in accordance with an embodiment of the present disclosure. Gamma irradiation was applied to UHMWPE samples at doses of 50 kGy and 100 kGy to assess its impact on mechanical and chemical properties as illustrated in the graph. Solid line represents non crosslinked mPE-UHMWPE, the dotted line indicates a dosage of 50kGY and the dashed dotted line indicates a dosage of 100kGY. Upon irradiation, a significant increase in elongation to failure may be observed, ranging from 90% to 450%, while the tensile strength remained relatively unchanged. This substantial enhancement in elongation (90-450%) equates to approximately 4-5 times increment after gamma irradiation. This substantial increase in elongation can be attributed to the formation of crosslinks between polymer chains induced by the irradiation process. When gamma radiation penetrates the material, it breaks C-C or C-H bonds, generating free radicals that subsequently recombine to form new links between chains. This crosslinking enhances the material's flexibility and elongation properties. However, while the elongation increased significantly, the tensile strength remained stable, suggesting that the material retained its overall strength despite becoming more flexible.
[0054] Reference is now made to Figure 8, which is an exemplary graph illustration the stress versus strain for vitamin E doped mPE-UHMWPE in accordance with an embodiment of the present disclosure. The utilization of Vitamin-E reinforced XL-UHMWPE, recognized as Generation II, emerged as a strategic response to address the shortcomings observed in conventional UHMWPE materials. While post-crosslinked UHMWPE exhibited improved wear properties, long-term oxidative stability remained a challenge due to residual free radicals, leading to oxidation mechanisms akin to those observed in polyunsaturated fatty acids.
[0055] Vitamin-E as an antioxidant, stabilizer, and radical scavenger for UHMWPE. Vitamin-E, specifically DL a-tocopherol, proved to be a biocompatible and FDA-approved antioxidant with the capability to recombine with free radicals without necessitating additional thermal treatment. Vitamin-E reinforced XL-UHMWPE composites were therefore built, commonly known as VE-PE or Poly-E, in 2009, exhibiting promising clinical outcomes. The incorporation of Vitamin-E into XL-UHMWPE offer a number of advantage, including enhanced wear resistance, oxidative stability, and fatigue resistance, compared to conventional XL-UHMWPE. Additionally, Vitamin-E demonstrated immunomodulatory properties, augmenting the body's defense against pathogenic bacteria such as S. aureus.
[0056] The antioxidant activity of Vitamin-E stems from its capacity to donate hydrogen to adjacent free radicals, thereby forming tocopherol free radicals capable of further scavenging other radicals. This mechanism effectively quenches alkyl, allyl, and peroxy free radicals generated during irradiation, hindering the oxidation cascade and mitigating oxidative embrittlement of UHMWPE.
[0057] Vitamin-E can be integrated into UHMWPE through two main techniques: blending with UHMWPE followed by consolidation and irradiation, or reinforcement in irradiated/crosslinked UHMWPE. Concentration of Vitamin-E and radiation dosage significantly impact the crosslinking density of the resulting composite, thereby influencing its physicochemical properties. Optimization of these parameters is crucial to achieving the desired balance between wear resistance, oxidative stability, and fatigue resistance.
[0058] To introduce Vitamin E into the UHMWPE, a doping process involving soaking in a Vitamin E solution and subsequent heating at high temperatures under vacuum conditions was employed. This facilitated the diffusion of Vitamin E into the material's surface. After treatment, analysis revealed that approximately 4-5% of the sample's weight consists of Vitamin E, accompanied by a noticeable change in color from white to light yellow because of the presence of Vitiman E, which again is indicative of successful doping. FTIR spectroscopy of the doped samples exhibited new characteristic peaks associated with Vitamin E, confirming its presence within the material. Solid curve represents mPE modified UHMWPE and dashed dotted line represents vitiman E doped mPE-UHMWPE. Tensile testing of the Vitamin E-doped samples revealed an increase in elongation to failure roughly from about 77% to about101%, and a marginal decrease in tensile strength compared to untreated samples. The observed increase in elongation can be attributed to the high-temperature doping process, which most likely facilitated polymer chain relaxation and reorganization. This relaxation reduces internal stresses within the material, thereby enhancing its ductility and allowing for greater elongation before failure. However, despite the increase in flexibility, the slight decrease in tensile strength suggests that the doping process may have induced subtle structural changes in the polymer, resulting in a minor reduction in overall strength as indicated in Table 3 below.
Table 3
Tensile Properties Without Vit-E With Vit-E
UTS (MPa) 31.38 30.77
Elongation (%) 77.38 101.39
Modulus (MPa) 771.47 560.01

[0059] In the exemplary case of UHMWPE injection molding, for enhancing processability and optimizing mechanical properties, it requires a precise calibration of injection molding parameters. In the exemplary case, for the initial task of making UHMWPE injection moldable, adjustments in mold and melt temperatures, cooling and holding times, and injection, back, and holding pressures are inherent and imperative to be precisely controlled. In the exemplary case, these adjustments are critical to overcome UHMWPE's inherent challenges related to high molecular weight and viscosity, ensuring smooth flowability during injection into the mold by injection molding. In an exemplary embodiment, elevated mold and melt temperatures, specifically, contribute to improved rheological behaviour as has been explicitly discussed previously, facilitating efficient mold filling. In the exemplary case of screw-based injection molding there will more shear stress produced by twin screw extruder at high temperature and injection speed. Gamma irradiations influence the chemical structure of polymers and it can lead to crosslinking of polymers chains using various mechanism and discussed previously.
[0060] In an exemplary case, medical grade mPE-blended-UHMWPE polymers were prepared and Injection molded in an microinjection molding Machine at an injection pressure of 12-14 bar with varying melt temperature from 220 to 260oC and mold temperature kept room temperature to get dumbbell-shaped specimens using the ASTM D638-03 Type 5 standard for specific enablement under Laboratory conditions. Various combinations of injection pressure, holding time, melt and mold temperature may optimize the tensile properties such as elongation at break, Young’s modulus and tensile strength (UTS). Optimal blend compositions and processing conditions may be determined to achieve defect-free implants with clinically desired strength and durability.
[0061] Several characterization techniques such as parallel plate rheometer analysis, Differential Scanning Calorimetry (DSC), Fourier Transform Infrared Spectroscopy (FTIR) and rheological analysis were employed to evaluate the chemical composition, thermal behavior, and rheological properties of the modified blend prepared in the laboratory. Mechanical properties, including tensile strength and modulus, were assessed using ASTM D638-standard tensile testing. It should be obvious to a person of ordinary skill in the art that these parameters may need to be tuned for industrial scale production and all such variation are covered under the present disclosure.
[0062] In the experimental setup under laboratory conditions, UHMWPE matrix undergoes modification by blending it with maleated polyethylene to enhance processibility and mechanical properties. In the exemplary case, this is achieved through melt compounding techniques, such as twin-screw extrusion, which ensures uniform dispersion of maleated polyethylene within the UHMWPE matrix at elevated temperatures, which is in the melt state. In the exemplary case, the blending conditions, including temperature and mixing time, are carefully selected to ensure homogeneous distribution of additives without inducing polymer matrix degradation.
[0063] In the exemplary case, melt mixing or conventional melt compounding of maleated polyethylene (mPE) with UHMWPE is performed using a melt extruder. Pellets of UHMWPE and mPE are melt-mixed at a temperature of 220°C and a screw speed of 60 rpm for a duration of 10 minutes under the specific laboratory conditions. The stability of the blend is monitored during mixing, and a stable force of approximately 6 kN is maintained over time for the specific case. The extruded blend is subsequently injection molded into dumbbell-shaped specimens (ASTM D638-03 Type 5 standards) using an plunger based injection molding machine at various injection pressures of 12-16 bar. Various melting and mold temperatures are adjusted and optimized, in order to meet the mechanical strength of commercially available orthopaedic implants.
[0064] Advantages of the current material and process over the previously known material is the enhanced properties where Integration of has been noticed to maleated polyethylene improve mechanical and rheological properties, enhance implant performance and longevity. Other advantages include scalability and reproducibility, facilitating mass production to meet increasing demand with respect to injection molding. Further advantage is the cost associated where materials reduces dependence on costly imported implants is avoided and the embodiments of the present disclosure are economically produced, with optimized injection molding minimizes material wastage compared to conventional machining. A further advantage is that the Injection molding allows for customized implant designs, precise fabrication of complex implant geometries and meeting specific anatomical requirements and enhancing patient outcomes.
[0065] Further, as opposed to conventional manufacturing methods for orthopaedic implants that encounter a number of limitations such as excessive costs, material wastage, and long process cycle time, the injection molding of UHMWPE-based implants is hindered by poor processibility and mechanical strength, exacerbating the financial burden on patients in economically constrained regions. Embodiments of the present disclosure addresses these challenges by optimizing injection molding parameters and integrating maleated polyethylene, aiming to deliver cost-effective orthopaedic implants with superior mechanical and rheological properties. In particular, embodiments of the present disclosure addresses several technical and scientific limitations of existing technologies, for example with respect to poor processibility of UHMWPE for injection molding due to high viscosity, having limited rheological properties of conventional UHMWPE implants and high manufacturing costs and material wastage associated with extrusion and compression molding. By optimizing injection molding parameters and blending UHMWPE with maleated polyethylene, the invention overcomes these challenges, resulting in enhanced processibility, mechanical strength, and cost-effectiveness of orthopedic implants
[0066] Although the present disclosure has been described with reference to several preferred embodiments, it should be understood that the present disclosure is not limited to the preferred embodiments disclosed here. Embodiments of the present disclosure are intended to cover various modifications and equivalent arrangements within the spirit and scope of the appended claims. Although the foregoing disclosure has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practised within the scope of the appended claims. Examples of the present disclosure have been described in language specific to structural features and/or methods. It should be noted that there are many alternative ways of implementing both the process and apparatus of the present invention. Accordingly, embodiments of the present disclosure are to be considered illustrative and not restrictive, and the invention is not to be limited to the details given herein but may be modified within the scope and equivalents of the appended claims. It should be understood that the appended claims are not necessarily limited to the specific features or methods described. Rather, the specific features and methods are disclosed and explained as examples of the present disclosure
, C , Claims:We Claim:
1. A method for preparing a polymeric blend for orthopaedic application, the method comprising:
- forming a blend of an ultra-high molecular weight polyethylene (UHMWPE) with maleated polyethylene (mPE) by:
- selecting a pre-determined quantity of UHMWPE by weight;
- selecting a pre-determined quantity of mPE by weight, wherein the weight of the mPE is a percentage of weight of the UHMWPE; and
- melt mixing the UHMWPE and the mPE forming a homogeneous blend, wherein the homogeneous blend without inducing polymer matrix degradation, or physiochemical properties.

2. The method as claimed in claim 1, wherein the pre-determined percentage weight of mPE is in a range of 1% to 5% weight of UHMWPE.

3. The method as claimed in claim 1, wherein melt mixing is performed by a melt extruder.

4. The method as claimed in claim 3, wherein a temperature for melt mixing varies in a range of about 1800C - 2600C.

5. The method as claimed in claim 1, wherein the mixing time for performing the melt mixing is proportional/dependent to the quantity of UHMWPE and mPE chosen for melting.

6. The method as claimed in claim 1, wherein a pre-determined force is maintained over the mixing time, wherein the pre-determined force is dependent on the quantity of UHMWPE and mPE.

7. The method as claimed in claim 2, wherein an increase in a percentage of mPE results in an increase in an elongation of the blend.

8. The method as claimed in claim 2, wherein an increase in the percentage of mPE results in a decrease in a tensile strength of the blend.

9. A method for creating an object (orthopaedic) comprises injecting the blend created by the method of claims 1 – 6 into a specimen of the object with a plunger-based injection molding machine.

10. The method as claimed in claim 8, wherein an injection pressure for the blend being injected is in a pre-determined range. (12-16 bar).

11. The method as claimed in claim 8, wherein a melt temperature for injection molding varies in a range of about 1800C - 2600C.

12. The method as claimed in claim 8, wherein the melt viscosity decreases with an increasing mPE content (0-5%) in a range from 22 kPa·s to 12 kPa·s.

13. The method as claimed in claim 8, wherein a Melt Flow Index (MFI) index increases from 6.2gm/10min to 8.1 gm/10min with addition of the mPE.

14. The method as in claimed 8, wherein irradiating the object with gamma irradiation of 50kGy and 100kGy, a strain to failure of the UHMWPE increases from 90-450%.

15. The method as claimed in claims 1 – 14, where doping the molded object with Vit-E enhances tensile properties associated with the molded object.

16. An object formed by the method as claimed in any of the preceding claims 1 -14.

17. The object as claimed in claim 16, wherein the object is an orthopaedic implant.

Dated this 18th day of March 2024
Indian Institute of Science
By their Agent & Attorney

Dr. Eric W B Dias/Reg No 1058
of Khaitan & Co