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The selection of biomaterials as biomedical implants is a significant challenge. Ultra-high molecular weight polyethylene (UHMWPE) and composites of such kind have been extensively used in medical implants, notably in the bearings of the hip, knee, and other joint prostheses, owing to its biocompatibility and high wear resistance. For the Anterior Cruciate Ligament (ACL) graft, synthetic UHMWPE is an ideal candidate due to its biocompatibility and extremely high tensile strength. However, significant problems are observed in UHMWPE based implants, such as wear debris and oxidative degradation. To resolve the issue of wear and to enhance the life of UHMWPE as an implant, in recent years, this field has witnessed numerous innovative methodologies such as biofunctionalization or high temperature melting of UHMWPE to enhance its toughness and strength. The surface functionalization/modification/treatment of UHMWPE is very challenging as it requires optimizing many variables, such as surface tension and wettability, active functional groups on the surface, irradiation, and protein immobilization to successfully improve the mechanical properties of UHMWPE and reduce or eliminate the wear or osteolysis of the UHMWPE implant. Despite these difficulties, several surface roughening, functionalization, and irradiation processing technologies have been developed and applied in the recent past. The basic research and direct industrial applications of such material improvement technology are very significant, as evidenced by the significant number of published papers and patents. However, the available literature on research methodology and techniques related to material property enhancement and protection from wear of UHMWPE is disseminated, and there is a lack of a comprehensive source for the research community to access information on the subject matter. Here we provide an overview of recent developments and core challenges in the surface modification/functionalization/irradiation of UHMWPE and apply these findings to the case study of UHMWPE for ACL repair.
Keywords:
ultra-high molecular weight polyethylene, ligament, tendon, surface modification, biofunctionalization, synthetic graft
The biomaterials used as biomedical implants are expected to be biocompatible such as they need to be non-toxic, non-inflammatory, and should not cause any allergic reactions in the human body [1]. Moreover, the material must have an excellent combination of high strength and low Young’s modulus closer to the implant to ensure longer service life and avoid implant loosening and revision surgery [2]. Ultra-high molecular weight polyethylene (UHMWPE) is distinguished by its high ultimate tensile strength, good biocompatibility, corrosion resistance, low water uptake, low coefficient of friction, and high abrasion resistance [3]. Such properties define UHMWPE’s use in many development areas and in medicine and biology, including the manufacture of artificial joints and implants for orthopedic surgery. All knee replacements and 85% of hip replacements today use UHMWPE on their bearing surfaces, which represents over two million orthopedic implants per year [4,5]. Two key factors decide the quantity and consistency of cell adherence to the implants: implant wettability (surface chemistry) and surface topography (surface roughness) [6,7]. Currently, UHMWPE is commercially fabricated under several brand names: Polymin SK (BASF, Ludwigshafen, Germany), Polystone M (Roechling, Mannheim, Germany), Tivar (Quadrant, Tielt, Belgium), Tecafine PE10 (Ensinger, Nufringen, Germany), Okulen 2000 (SP-Plast, Helsinki, Finland), GUR (Tina, Solidurraz, Württemberg, Germany), and by various companies, such as Goodfellow (Huntingdon, United Kingdom) and Braskem (São Paulo, Brazil, Brazilian Chemicals) [8].
In ACL and other ligament and tendon reconstructions, UHMWPE fiber is selected because it is one of the most durable materials known in the biomedical field [9,10]. In addition, it possesses excellent tensile strength, enough to support human load-bearing demands [11]. Despite these features, particular drawbacks have been noted, such as UHMWPE fibers being problematic to bond to most materials due to their chemical inertness and poor wear resistance. Wear debris generated during joint motions could cause osteolysis and implant displacement, contributing to the primary reason for joint revision [12].
UHMWPE fibers’ appealing physical and mechanical qualities are related to their highly aligned crystalline microstructure polythene chains [13]. Currently, gel-spinning processes are usually used to manufacture UHMWPE filaments. In this technique, an oxygen-rich slim limit layer is created during the turning of UHMWPE filaments, which is responsible for decreasing the bond properties of strands [14]. As a result, eliminating oxygen-rich boundaries is required to maximize fiber adhesion to other materials through surface modification of UHMWPE. Multiple methods have been utilized to modify the surface biocompatibility and wear resistance of UHMWPE [14,15]. These modifications can be divided into two types: chemical and dry techniques. Chemical surface modifications were conducted with oxidative acid etching [14], coating treatment [16,17], and chemical grafting of UHMWPE [18,19,20]. Dry surface modification techniques include different types of plasma treatments, grafting, and UV and gamma irradiation treatments [21,22,23]. Typically, molecular modification processes involve the insertion of oxygen-rich functional groups on the surface of UHMWPE fibers, which provide excellent chemical bonding sites. Additionally, the surface treatment would introduce imperfections or roughening, such as micro-pits, which act as mechanical anchor points, facilitating mechanical interlocking of the polymer matrix to fibers. Sometimes, combined methods are applied to improve interfacial adhesion of the materials [24]. Nano-reinforcement, such as carbon nanotubes (CNTs), nano clay, graphene, boron carbide, nano alumina (Al2O3), and vitamin C, has recently been employed to change the polymer matrix to be used with fiber in order to create the best potential interfacial connection through resonance [25,26,27,28].
The improvement of its surface can modify its biological and tribological properties [29,30]. The use of these materials can improve the surface hardness and abrasion resistance of the UHMWPE. Traditional ways of upgrading the wear performance of the UHMWPE include techniques such as gamma or electron beam radiation followed by thermal stabilization [31]. These techniques are accompanied by an increase in bulk mechanical properties, such as toughness, tensile strength, fatigue performance, and wear resistance [32].
Plasma treatment is currently another technologically successful and safe method (which does not need any corrosive reagents/solvents) for the surface modification of polymeric material. Properties can be improved by surface treatment of UHMWPE with argon plasma, cold atmospheric plasma (CAP), dielectric barrier discharge (DBD) plasma, plasma-assisted chemical vapor deposition (PACVD), and plasma immersion ion implantation (PIII) methods followed by protein immobilization. To resolve the issue of wear and to enhance the life of UHMWPE as an implant, in recent years, this field has witnessed numerous innovative methodologies such as biofunctionalization or high temperature melting of UHMWPE to enhance its toughness and strength. The modifications to the surface of the material through plasma can improve its hydrophilicity, surface energy, and wear resistance by introducing functional groups to the material which have been characterized by water contact angle, Fourier Transform Infrared (FTIR) and Scanning Electron Microscope (SEM) [33,34].
The surface functionalization/modification/treatment of UHMWPE is very challenging in orthopedic applications such as ligament regeneration. In spite of these difficulties, several surface roughening, functionalization, and irradiation processing technologies have been developed and applied in the recent past [35,36,37,38]. The basic research and direct industrial applications of such material improvement technology are very significant, as evidenced by the significant amount of open literature [39]. However, the available literature on the research methodology and techniques related to material property enhancement and protection from wear of UHMWPE is disseminated and there is a lack of a comprehensive source for the research community to access information on the subject matter. Therefore, the objective of this review is to provide an overview of recent developments and core challenges in the surface modification/functionalization/irradiation of UHMWPE and apply these findings to the case study of UHMWPE for ligament, e.g., anterior cruciate ligament, reconstruction. illustrates the overview of the surface treatments of UHMWPE and summarizes the influence of surface properties on UHMWPE after surface treatments.
Open in a separate windowUHMWPE belongs to a subgroup of thermoplastic polyethylene (PE) that is obtained from monomers of ethylene via a polymerization reaction. It is composed of extremely long polyethylene chains which effectively transfer load and provide a polymer backbone by reinforcing intermolecular interactions [57]. The desired degree of polymerization of UHMWPE is dependent on its end applications, the degree of polymerization is observed in orthopedic applications within a range of 71,000–214,000 with a molecular weight ranging from 2 to 6 million g/mole [58,59]. UHMWPE is a semicrystalline polymer, and its properties are strongly dependent on its microstructure [60]. The semicrystalline structure of UHMWPE consists of two phases known as crystalline and amorphous phases. Its properties are determined by the relations between amorphous and crystalline phases, such as binding molecules, crystallinity, degree of crosslinks and entanglements, and the crystallite positions [61]. The crystalline phase comprises lamellae consisting of strongly directed folded chains [62]. UHMWPE is also known as high modulus PE or high-performance PE because of its toughness and good impact strength. High density polyethylene (HDPE) has also been used for biomedical skeletal and orthopedic applications [63]. It also has extraordinary properties such as nontoxicity, high resistance to corrosive chemicals, and wear strength that makes it reliable for orthopedic applications, but UHMWPE is more abrasion and wear resistant than HDPE. represents the physical properties of HDPE and UHMWPE. Several monomer units attach during polymerization based on metallocene catalysts to make UHMWPE stronger compared to HDPE.
In 1962, Sir John Charnley introduced UHMWPE (−[CH2−CH2]n−) for biomedical use, and it was then applied as a joint surface load bearing material for hip and knee replacements. Hip and knee replacements are prosthetic joints that replace human joints affected by arthritis. The oxidation resistance of UHMWPE was improved by cross-linking, high-pressure crystallization, and introducing antioxidants.
UHMWPE can also be used as woven, knitted, or nonwoven sheets to provide three-dimensional structures for cell ingrowth. UHMWPE fabrics can be produced by a gel spinning technique that allows for the parallel orientation of the fibers resulting in a high modulus of elasticity and strength. The market demand for medical-grade UHMWPE has risen tremendously from 60.9 kilotons (2015) to a projected 204.8 kilotons in (2024), according to a survey conducted by Grand view research [64]. Extensive use of UHMWPE in the medical field is due to its superior biocompatibility, chemical resistance, low wear volume, ultimate tensile strength, and low coefficient of friction.
Ligaments are connective tissues with strong mechanical properties that can stretch a joint and become hooked at either end [155]. They attach two bones together, prevent dislocation, and restrain movement of the joints. They differ in location, size, shape, and orientation. There are four different types of ligaments in the knee, namely: medial collateral ligament (MCL), lateral collateral ligament (LCL), posterior cruciate ligament (PCL) and anterior cruciate ligament (ACL) [156,157].
The knee joint is complex and is composed of three separate joints: the tibiofemoral, patellofemoral, and the proximal tibiofibular joints [158]. The knee joint most referred to is the tibiofemoral joint. These knee joints are stabilized by several ligaments, including the anterior cruciate ligament (ACL), the posterior cruciate ligament (PCL), the medial collateral ligament (MCL), and the lateral collateral ligament (LCL). Ligaments are made of bands of collagenous connective tissue [159]. These paralleled collagen bundles are linked to each other by cross-linking [160]. Ligaments contain two-thirds water and one-third solid. Collagen is the major component of the ligament with five prominent collagen types which are I, III, VI, XI, and XIV [161,162]. The majority (90%) of the collagen in ligaments is type I, which is responsible for its tensile strength. To maintain the mechanical and biological stability of ligaments, related organ systems, as well as the bone, play a vital role [163].
ACL injuries are common in sports such as football, soccer, or with uneven surfaces. ACL injuries more commonly cause knee instability that further causes injury to other knee ligaments. Injuries of the ACL range from mild, such as small tears, to severe, such as when the ligament is completely torn [164,165]. Allograft, autograft, and synthetic grafts have been used for ACL reconstruction [166]. Due to the drawbacks of allografts and autografts, synthetic grafts may be a good choice for ACL reconstruction. The synthetic ligament graft is an artificial ligament device for joining the ends of two bones. Laflamme reported that the type of material and its particle size are important factors regarding synthetic ligament graft selection [167]. UHMWPE fibers are commonly used in synthetic ligament implants due to their excellent tensile strength and elastic modulus. Mechanical properties of several materials used for ACL regeneration have been presented in .
Overen et al. [169] reported on thrombogenicity testing of UHMWPE compared to expanded polytetrafluoroethylene (ePTFE) and polyethylene terephthalate (PET) fibers for vascular applications. Hemobile is a method used to detect the damage of blood components and activation of platelets throughout the material/device. It is also used for testing vascularity of UHMWPE, ePTFE and PET fiber. Due to lower hemolysis and low activation of inflammatory responses, UHMWPE showed better hemocompatibility than ePTFE and PET fibers [169].
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Hunter et al. [170] reported on attachment and proliferation of a variety of cell types on UHMWPE for orthopedic implantation. Cell multiplication was measured with a tritiated thymidine assay. Radioactively labelled thymidine (tritium) was used to measure lymphocyte proliferation by incorporation of 3H thymidine into the dividing cell’s DNA. The component vinculin, a cell focal adhesion plaque, was labeled by indirect immunofluorescence to assess the attachment of cells [171]. Fibroblasts and osteoblasts cultured directly on UHMWPE were tested by determining the mean number of adhesion plaques using an image analysis system. Fibroblasts attached well on UHMWPE fabric. High tensile strength, bio-inertness, and fibroblast adhesion makes it appropriate for ACL reconstruction [172]. Several materials have been developed that can be used for the reinforcement of the matrix to modify the properties of UHMWPE, utilized for its application in ACL reconstruction.
Surface modification plays a major role in helping osteogenesis and bone anchorage of synthetic grafts [173]. Chitosan is a naturally derived polysaccharide that has been used for the modification of synthetic grafts. Chitosan-hyaluronic is a composite that promotes new bone formation at the graft bone interface because of its porosity, biodegradability, biocompatibility, anti-infective activity, and ability to accelerate wound healing [174]. Vaquette et al. [175] reported that polystyrene sodium sulfonate as a surface modifier could improve the osteointegration of a synthetic graft [176].
Bioactive glass has been used for ligament graft modification due to its stimulation of angiogenic growth factors [177] A composite of UHMWPE-PCL-bioglass was developed as a synthetic graft using an electrospinning method. Bioglass was coated on UHMWPE via slurry dipping technique; melt derived glass particles were suspended in demineralized water to make a slurry with 5% w/v concentration, followed by 30 min agitation in a magnetic stirrer. Fibroblast cells were seeded on a composite graft to examine cell adhesion. Cells adhering to UHMWPE composite were well flattened and more spread out compared to cells on pure UHMWPE. Excellent fibroblastic cell growth on a composite UHMWPE-PCL-bioglass synthetic graft is shown in b [154].
Bioactive glass has unique compositional ranges of dense amorphous calcium sodium phosphosilicate (CSPS) that develop strong chemical bonds with the collagen of living tissues [178,179]. The composition of 45S5 bioglass is 45% SiO2, 24.5% CaO, 24.5% Na2O, and 6% P2O5 [180]. Bioactive glass dissolves slowly in a simulated body fluid (SBF) with some reactions taking place on the surface of the glass [174,181,182,183,184]. These reactions include: (i) ions releasing due to the ion exchange between the solution and surface of the glass, but other components of the glass remain intact [185,186]; (ii) H+ ions attacking the silica network and as a result Si-O-Si bonds breaking down, and new Si-OH and Si (OH)4 groups forming at the surface of the glass; (iii) a soluble porous silica-rich layer forming on the surface of the glass due to condensation and re-polymerization; (iv) a calcium phosphate-rich layer forming on the Si-rich layer due to the migration of Ca2+ and (PO4)3− ions; and (v) a polycrystalline apatite layer forming on the surface of the bioglass. Collagen fibers can attach to the surface of the bioactive glass. The transparent silica-rich layer induces precipitation of the hydroxyapatite-like (HCA) layer. Interactions between bioglass and collagen fibers occur and become stronger when HCA precipitation increases [187].
Guidoin et al. [188] reported that a thick collagenous tissue partly penetrated the outer layers of the braided structure of a UHMWPE prosthesis. This collagen penetration caused the expansion and separation of the multifilament yarns into individual fibers. However, while the knit fabric was encapsulated by thin collagenous tissue, there was no significant infiltration into the structure. Thus, a hollow braided structure was designed with a core of parallel poly (vinyl alcohol) (PVA) cord wrapped by the braided diamond structure of UHMWPE threads for better mechanical performance [189]. Bach et al. [35] have invented a hydrogel fiber for ACL reconstruction, made from PVA hydrogel. Tensile strength was enhanced by incorporating UHMWPE fibers around the PVA cord.
Zhang et al. [190] reported that UHMWPE filament could be modified with polycaprolactone for ligament and tendon regeneration [191]. Absorbable polycaprolactone PCL has attracted mainstream attention in recent years for the development of tendon/ligament repair materials due to its excellent performance attributes of low degradation, high stability, non-toxicity, and bioresorbability [192]. Fibrous PCL has also been reported to be able to help cell growth.
Schmidt et al. [193] reported that growth factors play an essential role in the stimulation of fibroblast division and ligament healing. Growth factors such as platelet-derived growth factor AA, platelet-derived growth factor-BB, basic fibroblast growth factor, insulin-like growth factor 1, and interleukin 1- alpha can enhance the proliferation of fibroblastic cells. Growth factors can elicit specific biological responses such as proliferation, chemotaxis, matrix synthesis, and secretion of other growth factors during wound healing. Molloy et al. [194] investigated some of the recent research into the functions of five growth factors whose actions were better defined during tendon healing. Those five growth factors are: Insulin-like growth factor I (IGF-I), Transforming growth factor β (TGF -β), Vascular endothelial growth factor (VEGF), Platelet-derived growth factor (PDGF), and Basic fibroblast growth factor (BFGF). summarizes the role of the growth factors in tendon or ligament healing process.
Based on previous research studies, the biofunctionalization of UHMWPE was conducted by loading of VEGF (vascular endothelial growth factor) into UHMWPE followed by SF (Silk fibroin) coating for ACL reconstruction [37]. Firstly, UHMWPE fibers were treated with ethanol and chromic acid to remove impurities. Chromic acid introduced additional functional groups to the surface of the fibers and etched the amorphous regions of threads. The chromic acid-treated UHMWPE was then immersed in either SF or VEGF/SF solution at 4 °C for 12 h. SF loading growth factor VEGF was used to achieve the sustained release and to improve the neovascularization. b presents the whole procedure of the SF/VEGF coating and reconstruction model. Cell morphology of bone marrow mesenchymal stem cells (BMSCs) is shown in c. Filopodia of BMSCs attached to the surface of bare UHMWPE were not visible until after 14 days of cultivation, but it was noticed on the surface of UHMWPE–SF and the UHMWPE–SF/VEGF group after 7 days of cultivation [202].
Open in a separate windowSeveral animal models of ACL reconstruction have been reported in different articles [203,204]. According to these methods, an inhalation mask was used to administer two percent isoflurane in O2 gas (1.5 L/min) to the animals. Procedures were carried out on a heating blanket in a sterile atmosphere. The animal was put in the supine position on the surgical table. The selected knee section was sanitized before skin cuts were made. The lateral parapatellar arthrotomy was utilized to uncover the knee joint of the animal. A notchplasty was performed to remove remnants of ligaments [205]. The bone tunnels were made using a 3.0 mm drill in the anatomic sites of the natural ACL in the femur and tibia. The UHMWPE grafts were threaded through the tunnels and knotted out of the femoral and tibial bone tunnels on both ends. The wound was irrigated with sterile saline solution after the graft was permanently attached. Lastly, sutures were used to seal the capsular layers and skins [206].
While many polymers, metals, ceramics, and composite materials are in use as biomaterials, UHMWPE is one of the most important of the bioinert polymers used in the manufacture of medical implants. Problems associated with the use of UHMWPE as implants include wear debris and oxidative degradation due to the generation of free radicals when exposed to irradiation with gamma rays for grafting or sterilization.
To resolve the issue of wear and to enhance the life of UHMWPE as an implant, in recent years this field has witnessed numerous innovative methodologies, such as biofunctionalization or high temperature melting of UHMWPE to enhance its toughness and strength. Sometimes one surface modification strategy is taken to solve a particular wear problem but may lead to a new problem and further strategy is required to eliminate that new problem. For example, the bioreaction of soft tissues is triggered by UHMWPE wear particles that can ultimately lead to aseptic loosening of hip implants. Therefore, high dose irradiation is used to highly cross-link UHMWPE which decreases the wear rate but initiates free radical formation that causes oxidative degradation in UHMWPE. To reduce or eliminate the free radical formation, annealing or post-irradiation techniques are used. Despite this, there is a chance of increased incidences of rim fracture under impingement and adverse loading conditions due to the lowered fatigue strength of this material. Thus, an alternative method of vitamin E stabilization of UHMWPE is carried out to provide oxidation resistance without sacrificing fatigue strength. However, vitamin E has a capacity to act as a free radical scavenger during irradiation which can lower the cross-linking efficiency of UHMWPE and limits the vitamin E concentration in the blend to less than 0.3 wt%.
Surface modification can improve functional properties such as mechanical properties, resistance to wear, biocompatibility, cytocompatibility, wettability, and biomaterial surface properties. Chromic acid and hydrogen peroxide can be used to reduce the smoothness of the surface, and polydopamine can be used to add functional amine groups on the surface along with protein immobilization. Recently, much attention has been focused on plasma treatment for surface modification of UHMWPE. DBD plasma was introduced to modify the surface properties of materials and then later PACVD, ECR, CAP, and PIII introduced a new era in surface modification compared to other surface functionalization methods. Plasma treatments can improve the hydrophilicity of materials, reduce the smoothness of the surface, and increase protein attachments through cross-linking and covalent bonding. It has been stated that different cellular functions such as adhesion, proliferation, and differentiation are influenced by surface energy, surface functionalization, and surface morphology. Among different plasma methods, plasma immersion ion implantation (PIII) is receiving attention due to the biofunctionalization of materials with complex shapes. In addition, it allows the use of protein immobilization. The surface functionalization of UHMWPE is quite straightforward, and surface treatments can be used to change only the surface properties without affecting the bulk properties of the material.
UV irradiation is the most common method used for cross-linking of free radicals with the substrate. UV irradiation and grafting can modify the wear and mechanical properties of material. Adverse effects of oxidation can be improved by blending vitamin E with the substrate during the treatment.
UHMWPE is a unique material due to its high capacity for vascularization throughout the whole structure, which is considered a primary requirement of grafts and other biomedical prostheses. Ligament/tendon reconstruction is considered a promising research application area for UHMWPE. Owing to the extreme hydrophobicity of UHMWPE and its surface chemistry, which is very different from that of natural ligaments and tendons, modern ACL reconstructions do not enjoy the low friction and wear of the original ligaments. Chitosan-hyaluronic acid composite has been used for modification of UHMWPE grafts. Bioglass and PCL coatings on UHMWPE show excellent results in fibroblastic cell adhesion assays and ligament regeneration. Several proteins and growth factors on the surface of UHMWPE showed significantly improved outcomes on ligament/tendon regeneration. The results described in the literature were related to the surface improvement of UHMWPE with protein adsorption making the surface bioactive for cell adhesion by displaying the signaling motifs of biological molecules. However, successful biofunctionalization depends on selection of the proper type and concentration of protein molecules to minimize inflammation, friction, and wear related issues of UHMWPE implants. Surface functionalization of biomaterials for ligaments/tendons is in need of being further developed, with the potential for improved outcomes for patients.
Many of these procedures have been proven to be successful in the laboratory by competent chemists, but that is not the case in manufacturing. The essential chemicals are potentially dangerous, costly, or currently unavailable in the large quantities required for manufacturing. Plasma treatment for polymer surface alteration has acquired an amazing consideration, attributable to its potential benefits in improvement of surface properties without influencing mass properties. Yet, non-uniformity, instability, inhomogeneity, and transfer into hot plasma over long treatment periods remain challenges to some plasma treatment. The integration of two or more modification methods revealed fascinating multi-functional characteristics; however, due to complex methodology and high cost, this strategy does not appear to be scalable.
Significant progress has been made in the field of functionalization of UHMWPE implants for ligament/tendon regeneration. Biological functionalization of orthopedic surfaces is a well-studied field with a lot of opportunities for advancement. The adaptation of current biomolecule immobilization techniques is the challenge for the next generation of research in this subject. Among different plasma treatments, plasma immersion ion implantation showed promising results due to covalently binding biomolecules to the surface of UHMWPE. A better understanding of the surface chemistry of UHMWPE with proteins is needed for more innovative biomolecule selection and design, potentially resulting in more effective multifunctional interfaces. If this is achieved, it will be a major step forwards in the development of this fiber for a variety of high-performance applications.
The authors gratefully acknowledge the technical and intellectual support from The University of Queensland (Australia).
This study was undertaken as a part of a PhD program funded by Australia Awards.
Conceptualization, S.B.W. and C.R.D.; Writing–original draft preparation, S.B.W.; Data curation, S.B.W.; Resources, S.A., Writing–review and editing, S.B.W., C.R.D., P.A.B., A.J.R., S.N.F., T.B.W., B.S., X.C., Y.Z., C.H.W., M.S.I. and S.A. All authors have read and agreed to the published version of the manuscript.
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The authors declare no conflict of interest.
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