Mechanical and tribological behaviour of BaTiO3 nanoparticles reinforced UHMWPE nanocomposite for prospective load-bearing applications

Mechanical and tribological behaviour of BaTiO 3 nanoparticles reinforced UHMWPE nanocomposite for prospective load-bearing applications Darshana Havaldar a,* , Jan Walter a , Zdenˇ ek Starý b , Ladislav Cvrˇ cek a,c , Roman Gabor c , Karel Maˇ sek c,d , Elena Filov´ a c,e , Lubica Staˇ nkov´ a e , Zdeˇ nka Jeníkov´ a a , Kiran Pawar f a Department of Materials Engineering, Faculty of Mechanical Engineering, Czech Technical University in Prague, Karlovo n´ amˇ estí 293/13, 120 00 Prague 2, Czech Republic b Department of Polymer Processing, Institute of Macromolecular Chemistry, Czech Academy of Sciences, Heyrovsk´ eho n´ am. 1888, Bˇ revnov, 162 00 Praha 6, Czech Republic c Nanotechnology Centre, CEET, VSB – Technical University of Ostrava, 17. Listopadu 15/2172, 708 00 Ostrava-Poruba, Czech Republic d Department of Surface and Plasma Science, Faculty of Mathematics and Physics, Charles University, V Holeˇ soviˇ ck´ ach 747/2, 180 00 Prague 8, Czech Republic e Laboratory of Biomaterials and Tissue Engineering, Institute of Physiology of the Czech Academy of Sciences, Vídeˇ nsk´ a 1083, 142 00 Prague 4, Czech Republic f School of Nanoscience and Biotechnology, Shivaji University Kolhapur, Vidya Nagar, Kolhapur, Maharashtra 416004, India ARTICLE INFO Keywords: UHMWPE BaTiO 3 Nanocomposite Compression molding Biomaterial Tribology Load-bearing implant ABSTRACT Ultra-high molecular weight polyethylene (UHMWPE), extensively utilised in orthopaedic implants, experiences mechanical degradation and suboptimal wear resistance over time, restraining its long-term reliability. Hence, to modify functionality, barium titanate (BaTiO 3 , 0–10 wt%) reinforced UHMWPE nanocomposites were manufactured employing compression molding. Characterisations, including X-ray diffraction (XRD) and scanning electron microscopy (SEM), validated the efficient filler incorporation within the matrix. The incorporation of 5 wt% BaTiO 3 caused enhancement in tensile strength (31%), flexural strength (28%), and impact resistance (40%), attributed to effective nanoparticle dispersion and improved load transfer. The same composite was the most consistent in tribological performance, reducing specific wear rate by 58% in air and 35% in foetal bovine serum compared to UHMWPE and reducing the average friction coefficient by 0.027 and 0.01 in respective environments. The surface wettability assessments highlighted the relative hydrophilicity of the nanocomposites, signifying enhanced interactions with the biological fluids and thus, biocompatibility. Besides, preliminary in vitro cell viability studies confirmed the non-toxic nature and suitability of the nanocomposites for biological applications. Overall, the nanocomposites demonstrated improved functionalities, with the 5 wt% UHMWPE/ BaTiO 3 nanocomposite outperforming others by a balanced combination of strength, wear resistance, and biocompatibility, making it a promising candidate for load-bearing orthopedic applications. 1. Introduction The necessity for high-performing biomedical implants, particularly orthopaedic implants, is constantly expanding owing to several aspects, notably the incidence of traumatic injuries due to accidents or sports, the ageing population, and escalating rates of degenerative diseases [1–3]. Orthopaedic implants are tailored to endure extensive mechanical loads while fostering bone integration and healing within the body. A bone is a complex biological structure, rendering its absolute restoration and replacement rather challenging. Inevitably, research on orthopaedic implants and their constituents continues to be an area of ongoing research [4–6]. Polymers and bioactive materials have long been employed in the medical sphere to heal and replace tissues, organs, and body parts, primarily due to their non-inflammatory, non-allergic, and non-toxic qualities [7,8]. Ultra-high molecular weight polyethylene (UHMWPE) has arisen as a mainstay among those differing materials leveraged for orthopaedic implants owing to its remarkable properties comprising superior wear resistance, ductility, durability against brittle fractures, chemical inertness, a low friction coefficient and excellent mechanical performance. It has been a preferred material as a bearing surface in joint replacement for decades, providing a vital balance of performance and biocompatibility, which is crucial for long-term utility [9–14]. However, UHMWPE is not without challenges, irrespective of its * Corresponding author. E-mail addresses: [email protected], [email protected] (D. Havaldar). Contents lists available at ScienceDirect Materials & Design journal homepage: www.elsevier.com/locate/matdes https://doi.org/10.1016/j.matdes.2025.114349 Received 7 April 2025; Received in revised form 2 July 2025; Accepted 4 July 2025 Materials & Design 256 (2025) 114349 Available online 5 July 2025 0264-1275/© 2025 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ). myriad advantages. One of the major constraints that researchers have reported since the early studies is the generation and release of wear debris particles. In a comparative study, Shibo et al. inspected the wear behaviour and debris formation by conducting wear simulation tests of UHMWPE acetabular cup articulating against femoral head materials, including Al 2 O 3 ceramic composite, under different lubrication and motion conditions. The outcomes stated the occurrence of debris particles, having distinct morphologies, with noticeably higher wear against metal femoral heads than ceramic heads. Importantly, primary abrasive, adhesive, and fatigue wear mechanisms closely influenced the observed debris morphologies. Release of debris into the joint capsule surrounding the implant material triggers a foreign body reaction to the biomaterial. This inflammatory response further leads to aseptic osteolysis around the implant, and the eventual loosening compromises the implant durability [15–19]. Thereby, the need for substantial research aimed at augmenting the tribological and mechanical behaviour of UHMWPE is required to alleviate this severe issue. In this instance, composite materials have been introduced as a potential approach where fillers reinforce the polymeric matrix and have been shown to boost the mechanical, tribological, and erosive properties of polymer composites [20,21]. Nanofillers, either particle or fibre ones, have come to prominence as an effectual interphase for polymer matrices with the potential of complementing their innate mechanical, physical, and chemical traits. Their characteristic small dimensions and high surface area boost the interfacial crosslinking and seamless blending with polymer matrices over micro-size fillers, resulting in markedly superior properties than just polymers or conventional composites [20–26]. Concomitantly, it is vital to maintain low, but optimum, concentrations of these fillers owing to their tendency to agglomerate and, hence, act as stress concentrators at the filler–matrix interface, compromising the mechanical properties [25,27–29]. Numerous studies have scrutinised the effect of reinforcing nanofillers to UHMWPE, for instance, carbon nanotubes, nanodiamonds, graphene nanoplatelets (GNP), nanoclay, nano-zinc oxide, etc., primarily targeted at the betterment of mechanical and tribological properties, together with biocompatibility [30]. Nayak et al. reported the reinforcement of UHMWPE with multiple additives, namely talc, graphite, and multi-walled carbon nanotubes, to assess the physical, mechanical, and bio-tribological behaviour of composites and recorded the amelioration in terms of the hardness, compressive modulus, friction, and wear for specific composites. The formation of micro-cracks, adhesive wear, surface protrusions, and plastic deformations was found to be dominant surface damage wear mechanisms [31]. The concurrent effect of the GNP and hydroxyapatite (HAp) on UHMWPEbased hip implant liners, to modify biofunctional, mechanical and tribological performance, was explored by Taromsari and the group. At the optimised filler composition, the mechanical and tribological performance was sufficiently amended; also, enhanced biofunctionality while HAp mitigated the adverse effects of GNP [32]. The work by Salari and co-workers inspected the strategy of a dual-reinforcement to transform the conventional UHMWPE using zirconia and hydroxyapatite in search of a more reliable hip implant liner. The composites attained exalted resistance to wear, mechanical integrity, and cellular compatibility at rather reduced additive content [33]. Hussain et al. fabricated hybrid nano-Al 2 O 3 and vitamin C-filled UHMWPE composites to explore the tribological performance for cartilage replacement and noted an attenuation in wear with the addition of fillers [34]. Among a range of organic and inorganic nanomaterials documented to interact with biological systems in many preceding studies, piezoelectric nanomaterials have received great emphasis and are known to undergo electrical polarisation when subjected to mechanical stress [20,35]. Barium titanate (BaTiO 3 ), a piezoelectric ceramic, is naturally biocompatible and deemed to be a promising candidate for biomedical applications. In addition to its antibacterial nature, it has remarkable antibiofilm activity. This is imperative since most orthopedic implant components are vulnerable to biofilm formation, which can result in undesirable implications, specifically delayed healing or implant removal. Consequently, incorporating BaTiO 3 into the UHMWPE matrix can bolster the mechanical strength while simultaneously commencing bioactive features that can encourage cellular responses and osseointegration. Altogether, this positions the use of BaTiO 3 as an achievable strategy for the advancement of UHMWPE composites tailored for orthopedic applications [36–41]. The primary objective for forming a UHMWPE/BaTiO 3 composite is to develop a nanobiocomposite that can transcend the established constraints of UHMWPE in biomedical applications. Remarkably, to the best of current knowledge, UHMWPE/BaTiO 3 composites have not been studied in such a manner. This introduces an innovative approach towards designing a bioactive, mechanically endure, stimulus-responsive implant material that may enhance long-term functionality. Based on the literature, periprosthetic osteolysis triggered by wear debris released during the articulation of UHMWPE against femoral heads is one of the major concerns. It leads to aseptic loosening and compels the necessity for revision surgery of implants [42,43]. Thus, there is a pressing demand to study and strengthen the bio-tribological aspects of commercially available materials. In this regard, this work aims mainly to focus on the mechanical (tensile, flexural, impact strength) and tribological behaviour of a UHMWPE/BaTiO 3 nanocomposite, followed by in vitro evaluation of material biocompatibility. The friction and wear performances were scrutinised with Al 2 O 3 counter-bodies in the air and under lubrication conditions in a fetal bovine serum (FBS) and phosphatebuffered saline (PBS) solution to mimic the physiological environments. Al 2 O 3 was chosen as it represents a material commonly used in joint replacement implants, frequently forming a functional pair with UHMWPE [44]. Additionally, it is often mentioned in literature dealing with tribological performance of implant materials, facilitating comparisons of the performed testing [45–48]. Altogether, this study focuses on enhancing the mechanical robustness and tribological characteristics of materials with the objective of evaluating the feasibility of UHMWPE/ BaTiO 3 nanobiocomposites as effective load-bearing implants. 2. Materials and methods 2.1. Materials For this work, UHMWPE (powder grade GUR 4120) was used as the matrix and purchased from the Celanese Corporation, with an average particle size of 120 µm, a molecular weight of 4.7 ×10 6 g/mol and 0.93 g/cm 3 density. Barium titanate (pure; 99.95 %, particle size 280 nm, tetragonal) was bought from Nanografi Nano Technology Turkey as a nanofiller. Isopropanol was employed as a dispersant to form colloidal material formations. For the tribological testing, Al 2 O 3 ceramic 6 mm balls were preferred as the counter-body with precision class G10, and purchased from the commercial supplier Redhill Precision Speciality Balls. The roughness of the counter-bodies was evaluated using coherence scanning interferometry on a Sensofar S Neox Five Axis optical profiler. The measured Sa value (arithmetical mean height) was Sa =16 ±3 nm. This roughness is well within the roughness limit set for precision class G10. Two fluids were used to simulate the environment of the human body. The solution of PBS was prepared by dissolving one PBS tablet from Sigma-Aldrich in 200 mL of distilled water (DW). The FBS, collected in South America, was purchased from Capricorn Scientific GmbH. Before use, the FBS was diluted in a 1:2 ratio in DW. 2.2. Nanocomposite fabrication Polymer composites of UHMWPE/BaTiO 3 were fabricated at five different filler weight percentages (0-10 wt%); the sample containing 0 wt% filler served as a reference for the rest of the samples. The initial formulation was carried out in three steps using a solvent-assisted D. Havaldar et al. Materials & Design 256 (2025) 114349 2 dispersion methodology. At the outset, the specific amounts of UHMWPE and BaTiO 3 were dispersed in isopropanol separately with an ultrasonication bath at 50 ◦C for 60 min. The exact quantities of the filler and matrix, based on the composites’ composition, were each dispersed in approximately 50 mL and 250 mL of isopropanol, respectively. Whereupon both suspensions were mixed and ultrasonicated for 90 min at the same temperature to obtain a homogenised slurry. The resultant mixture was stirred mechanically in a further step for 60 min at 70 ◦C to avoid any possible precipitation. Lastly, the mixture was dried at 90 ◦C in an oven for 5-6hr and set aside for a day to entirely remove remaining moisture, if any. The collected powder was molded into rectangular sheets using hot compression molding. Following a thermogravimetric study from earlier research, pristine UHMWPE degrades at temperatures exceeding 400 ◦C [20,49]. As a result, the processing temperature for the composites should be below 400 ◦C. Accordingly, the composite blends were introduced into a preheated mold at 220 ◦C and compacted using a Fontijne hydraulic press. No pressure was applied initially for 5 min to let the powder start melting, followed by a gradual pressure increase to 1.5 MPa, 3 MPa, and 6 MPa, with each pressure level being held for 5 min. Then the mold was transferred into a second press heated to 70 ◦C and left to cool for 5 min without external cooling. It was finally cooled with water until it reached room temperature. The resultant sheets (100 ×80 ×2 mm) were labelled as mentioned in the table (Fig. 1, Table 1) and used for further testing and characterisation. Surface roughness of fabricated specimens was evaluated using confocal microscopy on a Sensofar S Neox Five Axis optical profiler and was Sa 160 nm with no differences based on filler concentration. 2.3. Composite characterisations and testing 2.3.1. X-ray diffraction (XRD) The XRD analysis was harnessed to gain insight into the structural properties of the UHMWPE and UHMWPE/BaTiO 3 nanocomposites. The measurements were achieved by deploying an AXS D8 Advances instrument from Bruker Ltd Germany, employing Cu K α radiation (λ = 1.54Å). The XRD patterns were captured at an ambient temperature within the 2θ range of 10◦to 80◦. The operating conditions comprised an accelerating voltage of 40 kV and a current of 40 mA, with a scan step size of 0.02◦and a time per step of 5sec. 2.3.2. Differential scanning calorimetry (DSC) The thermal analysis was carried out with a differential scanning calorimeter (DCS Netzsch STA 409PG LUXX) to explore the melting and crystallisation attributes of the nanocomposites. To eliminate the thermal history of the specimens, each sample was scanned through two heating cycles. Nearly 10 mg of every single sample was sealed in an Fig. 1. Schematic representation of the fabrication of the UHMWPE/BaTiO 3 nanocomposite sheets. Table 1 Nanocomposite designation information. Sample ID Composition (Weight Fraction) Matrix % (UHMWPE) Filler % (BaTiO 3 ) UHMWPE 100 0 PEBT 2.5 97.5 2.5 PEBT 5 95 5 PEBT 7.5 92.5 7.5 PEBT 10 90 10 D. Havaldar et al. Materials & Design 256 (2025) 114349 3 aluminium crucible and held at a heating rate of 10 ◦C/min until reaching a temperature up to 200 ◦C. The temperature was maintained at this level for 3 min ahead of cooling at a rate of 5 ◦C/min, followed by repeating the process in the second heating cycle. The degree of crystallinity (Xc) of the nanocomposites was gauged by: Xc =ΔHm (1− ω )ΔH0 m ⋅100% (1) where ΔHm is the measured heat of fusion, ΔH0 m is the heat of fusion of 100 % crystalline UHMWPE (291Jg −1 ), and w is the weight fraction of BaTiO 3 in the polymer matrix. 2.3.3. Scanning electron microscopy (SEM) with an elemental analysis The composition and morphology of the nanocomposites were comprehensively examined using scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (EDS) (JEOL JSM-7610FPlus) under high vacuum conditions and an accelerating voltage of 10–15 kV. The samples were polished on an EM TIC3X LEICA ion polisher at 3 kV, 1 mA for 25 min and coated with 10 nm of Au on a Q150V Plus coating machine. The SEM imaging was conducted on representative areas, while a simultaneous EDS analysis enabled the elemental mapping to discern the spatial distribution of the elements within the nanocomposite. The SEM images of the wear tracks after the tribology testing were obtained by a JEOL JSM-7600F to analyse the wear mechanism. An accelerating voltage of 10 kV was used, and samples were coated with a thin film of copper on a physical vapour deposition (PVD) coating machine. 2.3.4. Mechanical properties Tensile tests were carried out mechanically at room temperature using a universal testing machine (MTS E42.503 5KN, USA) in accordance with ISO 527–2 guidelines. The samples were cut into dumbbell shapes with the requisite dimensions per the specifications. The crosshead speed was set at 50 mm/min with a load cell of 5kN. The tensile parameters of the composites, namely the tensile strength, yield strength, and elongation at break, were ascertained by measuring the force needed to break a sample and the extent to which the sample elongated to the breaking point [50]. To confirm the reproducibility of the results, five replicates of each composition were tested, and the average values were calculated to describe the tensile characteristics of the composites. The samples were subjected to three-point bending tests using a universal testing machine (MTS E42.503 5KN, USA) at room temperature, following the ISO 178 requirements. It was executed using a machine crosshead speed of 2 mm/min and measures the force needed to bend the samples under three-point loading conditions [50]. To assess the flexural characteristics of each composition, five samples were tested, and the mean results were reported. The Charpy impact test was undertaken at room temperature per the ISO 179 standards by using an impact tester (Instron Ceast, USA) to analyse the impact strength or the tendency of the material to resist breaking on being subjected to sudden shock [51]. For each composition, five notched samples were examined, and the resulting average values were exploited to quantify the composites’ absorbed energy. The quantity of energy absorbed before fracture was taken into account to estimate the toughness. The hardness of the samplings comprising UHMWPE and UHMWPE nanocomposites was gauged by the Vickers hardness tester (Struers Duramin 40AC3) armed with a diamond pyramid indenter at room temperature. The samples were ensured to have uniform thickness and smooth and flat surfaces. A consistent 2 kg load (equivalent to 19.61 N) was applied to dwell each sample for 10sec, creating five indentations at different locations, and then the average values were used. 2.3.5. Surface wettability To quantify the wettability of the polymer surfaces, the composites were subjected to surface contact angle analyses deploying the sessile drop method. A micropipette was used to gently dispense 8 μ L droplets of each test liquid, expressly DW, PBS, and FBS, over the surface of the composites. The droplets were then allowed to rest for 5sec to enable the uniformity of the droplet over the specimen surface, prior to capturing the image by a digital microscope with the objective rotated 90◦ (Olympus DSX1000). Subsequently, the captured images were taken under assessment in the Olympus Stream software to determine the contact angle. The average values of a minimum of five readings from separate areas on the surface for each composite were documented. 2.3.6. Tribological testing The tribological testing was conducted using a CSM Instruments THT pin-on-disc tribometer. Diverse test environments were tested to evaluate the suitability of the tested composites for load-bearing implant applications. Al 2 O 3 in a ball form with a diameter of 6 mm was used as a counter-body. Tests were performed in air, in FBS, and in a solution of PBS. A normal load of 1 N was applied during the test. A linear sliding speed of 50 mm.s −1 , and the number of laps was set to 20,000 for all the tests. In each configuration, the tests were conducted twice on radii of 5 mm and 4 mm. The wear of the counter-body surface and the wear track on the planar sample were analysed using an Olympus DSX1000 digital microscope after the test. For all the tested combinations, a specific wear rate was calculated from the equation: k=V/Fs (2) where k is the specific wear rate, V is the volume of the wear track, F is the normal load, and s is the sliding distance. The volume of the wear track was calculated by multiplying the area of the wear track crosssection, measured by a Zygo NewView 7200 optical profilometer, by the circumference of the wear track. 2.3.7. X-ray photoelectron spectroscopy (XPS) The chemical composition and the chemical state of the elements present in the selected samples were determined by XPS. The XPS spectra were taken using a hemispherical analyser with a multichannel detector at a primary energy of 1486.6 eV of the X-rays (Al K α radiation source). The spectra were corrected for the charging effect by setting up the binding energy (BE) position of C 1 s main emission line to a value of 285 eV, corresponding to the typical BE value for polyethylene [52]. Before the XPS analysis, the samples were cleaned in ethanol using an ultrasonic cleaner for 5 min. The samples were measured untreated after the introduction to the analysis chamber and after the Ar ion bombardment with an energy of 3 keV for 60 min to analyse the composition of the near subsurface region. The relatively high dose of ion bombardment is necessary due to the low sputtering yield in the case of the UHMWPE base material. 2.3.8. In vitro evaluation of material biocompatibility Human osteoblast-like cell line SaOS-2, purchased from CLS Cell Lines Service (Eppelheim, Germany), were cultivated in McCoy ′ s 5A medium (Sigma-Aldrich Co., St. Louis, MO, USA) with 15 % of FBS (Sebak GmbH, Aidenbach, Germany) and 40 µg/ml of gentamicin (Novartis International AG, Basel, Switzerland). UHMWPE/BaTiO 3 nanocomposite sheets were sterilised with 70 % pure ethanol for 30 min and were inserted into the wells of 24-well cell culture polystyrene plates (TPP, Trasadingen, Switzerland). The cells were seeded on the samples at a density of approximately 10,528 cells/cm 2 into 2 mL of the cell culture medium per well and cultivated for 7 days at 37 ◦C in a humidified air atmosphere with 5 % CO 2 . Tissue culture polystyrene (PS) wells were used as a reference material. On days 1, 3 and 7 after seeding, the cells on the samples, in triplicate, were stained with the D. Havaldar et al. Materials & Design 256 (2025) 114349 4 LIVE/DEAD viability/cytotoxicity kit for mammalian cells (Invitrogen, Molecular Probes, Cat. No. L3224) according to the manufacturer’s protocol. Briefly, the cells were incubated in two probes detecting the esterase activity in living cells (calcein AM producing green fluorescence) or detecting the membrane damage in dead cells (ethidium homodimer-1 emitting red fluorescence) for 5 min at 37 ◦C in a humidified air atmosphere containing 5 % CO 2 . Then, live and dead cells were counted, cell viability and live cell densities were calculated on microphotographs taken under an Olympus IX 51 epifluorescence microscope (obj. ×4x or ×10) equipped with a DP 70 digital camera. 3. Results and discussion 3.1. XRD The diffraction patterns of UHMWPE and PEBT 5, as a pictorial representation for nanocomposites, are depicted in Fig. 2, utilised to ascertain the crystallinity as well as the phase composition of materials and are consistent with the literature. Two prominent sharp peaks, characteristic of semicrystalline polyethylene, at 21.54◦and 23.93◦ were located for the UHMWPE allied to the (110) and (200) planes of the orthorhombic crystal structure, respectively (Fig. 2a). The plane (110) corresponds to the primary representation of the crystalline phase, whereas the (200) plane is related to the secondary crystallites. Along with sharp maxima, a broad halo was observed across a wide range of diffraction angles (2θ from 15◦to 25◦), indicating the presence of an amorphous phase [53–56]. The diffraction pattern of the nanocomposite with 5 wt% BaTiO 3 (Fig. 2b) entails peaks peculiar to BaTiO 3 in conjunction with the characteristic UHMWPE peaks. The peaks at 2θ equal to 31.53◦, 38.87◦, 51.07◦, 56.21◦, and 65.99◦are analogous to the respective planes of (110), (111), (210), (211), and (220). The peak splitting of (002) and (200) at 2θ～45◦validates the tetragonal phase of the nanoparticles. The absence of a peak typically found at 22.2◦related to the (100) plane might be due to the overlapping of a broad amorphous region of UHMWPE [57,58]. There were no obvious peak shifts for BaTiO 3 in the nanocomposites when equated with the pure BaTiO 3 data in the literature. Furthermore, the retention of the tetragonal phase supports the structural integrity and phase stability of the filler despite the reinforcement within the matrix and the processing conditions. Interestingly, the intensity of the peaks correlated with UHMWPE enlarged in the case of the composites, proposing enrichment in the crystallinity by BaTiO 3 serving as a nucleating agent. This interaction between the polymeric chains and the filler nanoparticles is plausible in fostering a more ordered crystalline structure within the matrix [59]. The results commensurate with the degree of crystallinity attained by the DSC analysis divulged higher crystallinity with the growing amount of BaTiO 3 . 3.2. DSC The melting temperature, enthalpy of fusion, and the degree of crystallinity of the UHMWPE and UHMWPE/BaTiO 3 nanocomposites were derived from the data obtained through DSC, compiled in Table 2. The corresponding thermograms are shown in Fig. 3. The melting temperatures fluctuated with the filler content. Neat UHMWPE presents a Tm at 144.7 ◦C, which remained almost in the same range at BaTiO 3 loadings of 2.5 wt% and 7.5 wt%; however, noted dropping of around 6 ◦C and 4 ◦C for the 5 wt% and 10 wt% loadings, respectively. There was an overall upsurge in the crystallinity of nanocomposites as BaTiO 3 was introduced. The UHMWPE presented a crystallinity of 36.77 % that substantially elevated to 41.80 %, 42.90 %, and 41.42 % for composites, successively, until PEBT 7.5. Peculiarly, the composite featuring the highest reinforcement amount unveiled salient crystallinity growth by nearly 12 %. It demonstrates that BaTiO 3 particles might serve as proficient heterogeneous nuclei expediting the crystallisation [59]. The effect was observed to be amplified at a higher concentration of particles, reflected by an apparent increase in the crystallinity. 3.3. SEM with elemental analysis The SEM analysis uncovered insights into the dispersion of BaTiO 3 nanoparticles within the UHMWPE/BaTiO3 nanocomposites at various filler contents (2.5, 5, 7.5, and 10 wt%). Fig. 4a-e showcase the distribution of the particles, each with an average diameter of 280 nm, corresponding to certain compositions. The distribution of filler isn’t completely uniform at any observed concentration. At a weight percentage of 2.5 %, a relatively low quantity of nanoparticles exists within the matrix. It is evident that at the 5 % filler content, the particles exhibited efficient distribution through the matrix, indicating an optimal dispersion level compared to the other contents. However, at Fig. 2. XRD patterns of the (a) UHMWPE and (b) PEBT 5 nanocomposite. Table 2 The melting temperature (Tm), enthalpy of fusion (ΔHm), and the degree of crystallinity (Xc) of the UHMWPE and UHMWPE/BaTiO 3 nanocomposites. T m ( o C) ΔH m (J/g) % Crystallinity UHMWPE 144.7 107 36.77 PEBT 2.5 142.4 118.6 41.80 PEBT 5 138.5 118.6 42.90 PEBT 7.5 143.3 111.5 41.42 PEBT 10 140.9 127.2 48.57 D. Havaldar et al. Materials & Design 256 (2025) 114349 5 7.5 % and 10 % filler contents, an increased number of particles led to noticeable aggregations, with a pronounced agglomeration observed at 10 % filler content. The composition and distribution of individual BaTiO 3 elements in the polymer matrix were further analysed using EDS mapping (Fig. 5). Clearly, the composites displayed the simultaneous presence of Ba, Ti, and O elements. 3.4. Mechanical testing The mechanical characteristics of the UHMWPE nanocomposites, especially the tensile properties (tensile strength, yield strength, and elongation at break), flexural strength, impact strength, and microhardness, were ascertained and compared against the pure UHMWPE. The results (Fig. 6) illuminated the pronounced strengthening in the properties of the composites with the weight fraction of the filler. The average values of the results are presented in the table (Supplementary file, Table S1). Fig. 6(a) corresponds to the ultimate tensile strengths of the samples, illustrating that the presence of the filler has positively impacted the material’s ability to withstand the maximum applied stress before rupture. The pure UHMWPE had a tensile strength of 23.88 MPa, which progressively improved and attained its peak of 31.15 MPa at 5 wt% of BaTiO 3 . Beyond this concentration, a slight lessening was observed with strengths of 28.82 MPa and 27.07 MPa for the 7.5 wt% and 10 wt%, respectively. A comparable trend appeared in the yield strengths (Fig. 6b), emphasising an upsurge from 20.98 MPa for UHMWPE to 25.15 MPa for PEBT 5. It disclosed the improved resistance of composites for plastic deformation with the filler inclusion, however, the yield strengths began to decline and dropped to 22.17 MPa for PEBT 10. Regardless, all the values of the tensile strengths and yield strengths of composites remained higher than those of pristine UHMWPE. Measured ultimate tensile strength of PEBT 5 is well comparable with values of tensile strengths obtained by Salari et al. and Taromsari et al for composites with significantly higher amounts of hydroxyapatite, zirconia and GNP, while yield strength of UHMWPE/BaTiO 3 is significantly higher than that of composites prepared in these works [32,33]. Degradation of mechanical properties with high filler amounts has been stated in literature, though at significantly higher amounts of filler [20]. The elongation at break and the tensile stress–strain profiles of the materials are pictured in Fig. 6(c) and 6(d). Pure UHMWPE, renowned for its integral flexibility, exhibits a strain hardening effect following yielding, allowing it to undergo significant deformation before ultimately experiencing failure [60,61]. Irrespective of the blending with the 2.5 wt% of the infill, the capability of the material to endure sizeable deformation ahead of failure has ascended by 20 %. It kept elongating, ultimately reached a peak and reflected an almost 40 % enlargement over the unfilled UHMWPE at PEBT 5. At further filler concentrations, the composites upheld the elongation behaviour, attesting to their adaptability to stretch with the addition of the filler. The stress–strain curves coordinate with the outcomes, revealing distinctive ductile responses with an initial elastic region leading to yielding and prolonged plastic deformation. The curves are indicative that the insertion of filler did not adversely impact the inherent ductility of UHMWPE; rather, it showed enhancement for the nanocomposites. The interrelationship between the flexural strength, maximum stress tolerance when being flexed, and the infill quantities is elucidated in Fig. 6(e). The unfilled polymer exhibited a decent flexural strength of 43.20 MPa, which moderately increased to ～47 MPa for PEBT 2.5, ascribable to the initial reinforcement effect conferred by the filler. The additional filler incorporation to 5 wt% triggered an appreciable upgrade, with the strength jumping to 55 MPa, a notable 28 % increase compared to the original matrix. At the same time, as the weight percentage of BaTiO 3 continued to increase, the resistance to bending gradually diminished, reaching 51 MPa and 48.40 MPa for PEBT 7.5 and PEBT 10, respectively. The composites’ reactions to sudden deformation attributable to the impact loads were quantified in terms of the energy absorbed by the samples (Fig. 6e). The impact strength decreased from 68.12 kJ/m 2 to 56.25 kJ/m 2 at 2.5 wt% of BaTiO 3 than the pristine polymer. The moderated interfacial bonding between the polymer and the nanoparticles at a lower weight fraction may result in an insufficient load transfer between the nanoparticles and the polymer matrix. Subsequently, this inadequacy can lead to depleted toughness and energy absorption during impact testing [62]. A well-discernible surge was detected for the composites PEBT 5 and PEBT 7.5, with impact strengths of 95.65 kJ/m 2 and 92.6 kJ/m 2 , respectively. Nanoparticles, at elevated concentrations, can create a more interconnected network within the polymer matrix, enhancing the energy dissipation mechanisms during impact. This network aids in more evenly distributing the stress throughout the composite, enabling better energy absorption and improving its resistance to impact forces while enhancing its overall toughness. Contrariwise, the strength value radically depleted to less than the reference polymer at PEBT 10 to 54.37 kJ/m 2 . This decrease may be attributed to the excessive insertion of BaTiO 3, which hardened and thereby disturbed the otherwise continuity of the matrix, restraining its competency to absorb the applied impact energy [20]. The graph interprets the change in the Vickers hardness values for UHMWPE and its composites as a function of the BaTiO 3 content Fig. 3. DSC curves of UHMWPE and UHMWPE/BaTiO3 nanocomposites (a) Heating curves (second heating cycle), (b) Cooling curves. D. Havaldar et al. Materials & Design 256 (2025) 114349 6 (Fig. 6f). The hardness of the nanocomposites signifies a slender increase corresponding to the amount of filler added. The initial Vickers hardness of UHMWPE was recorded at 4.17HV, which steadily rose to 4.8HV for PEBT 10 due to the higher hardness of the ceramic filler particles, essentially transferring the load and thus heightening the resistance of the composite to deformation. Additionally, an extended concentration of nanoparticles in the matrix led to reduced inter-particle distances, ultimately enhancing the composite’s resistance to indentation [63–66]. Recorded hardness of neat UHMWPE is analogous to values of hardness recorded for UHMWPE tested under comparable testing parameters [31,67,68]. Observed hardness increase, in our study, of roughly 15 % at 10 % of filler content seems to be comparable with increases of hardness observed when talc or carbon fibres, however, lower than in the case of hydroxyapatite [31,33,68]. Any comparison with reinforced composites in the literature is limited due to the wide range of testing methods and parameters of hardness testing. The mechanical properties were constructively influenced and evidently improved owing to the incorporation of the fillers into UHMWPE. Across all the mechanical tests, except for the impact strength and microhardness, initial advancements in the values were observed, which reached a peak and were followed by a continual decline, although the values consistently exceeded those of the pure UHMWPE. The perceptible amplifications can be supported by the reinforcing effects assigned by the fillers, likely functioning as stress transfer agents. This aided a more consistent stress transfer between the matrix and external forces, stimulating superior mechanical performance [21,69]. Nonetheless, at the maximum filler content of 10 wt%, the strengths dipped. The deterioration might be explained by the agglomeration of filler particles at greater concentrations induced by the enriched filler-filler interactions. It perhaps provides stress concentration areas that degrade the overall structure [70–72]. This is supported by filler agglomerations observed by SEM for this composition. The noteworthy improvement in elongation at break underlines the enhanced ductility of the composites. This feature is particularly appealing for bone implant materials since higher elongation implies that the material is capable of bearing more strain before fracturing, lowering the chance of catastrophic brittle failure under physiological stresses [73–75]. Inclusively, the optimal mechanical performance was Fig. 4. SEM images of (a) UHMWPE, (b) PEBT 2.5, (c) PEBT 5, (d) PEBT 7.5, (e) PEBT 10. D. Havaldar et al. Materials & Design 256 (2025) 114349 7 discerned at a 5 wt% filler concentration, indicating that this composition strikes the most favourable balance between strength and ductility. 3.5. Surface wettability The wetting behaviour of the composite surfaces and their variation with the differing amounts of incorporated BaTiO 3 were studied by conducting contact angle measurements using DW, PBS, and FBS as the testing liquids (Supplementary file, Fig. S1). The outcomes of testing, summarised in Table 3, evince an explicit trend towards a hydrophilic nature with the increasing filler content across all the testing liquids. For pristine UHMWPE, the angles were observed to be close to 90◦, with the DW recorded at 89.4◦, with PBS at 88.79◦and with FBS at 83.91◦, which gradually dropped to 72.96◦, 72.1◦and 70.69◦, respectively, as the concentration of BaTiO 3 reached its highest level. The augmentation in the wettability is likely due to an increase in the surface energy of the UHMWPE matrix due to the addition of BaTiO 3, thereby improving the hydrophilicity. The results align with the previous literature, stating that the inclusion of BaTiO 3 on the titanium scaffold as a coating material greatly advances its wetting ability [76]. The identified sequence of diminishing hydrophilicity (FBS >PBS > water) may be explained by the surface tension characteristics of the liquids. FBS, possessing the lowest surface tension, facilitates enhanced wetting and a reduced contact angle, succeeded by PBS, while water, with a comparatively higher surface tension, establishes larger contact angles [77,78]. The biocompatibility of bioactive materials is prominently influenced by their surface wettability. For the UHMWPE composites approached as an orthopedic implant material, the hydrophilicity facilitates protein adsorption, cell attachment and growth to promote ultimate osseointegration [79–81]. 3.6. Tribology results Figs. 7-9 contain the results of the measurements of the coefficients of friction (COF) and specific wear rates (k), along with the images of the surfaces of the counter-bodies after testing for all the tested combinations. The average values of COF, taken as the average values during the stable phase of friction without considering the run-in phase, and k are presented in the table (Supplementary file, Table S2). During dry sliding with an Al 2 O 3 counter-body, the observed specific wear rate and COF of UHMWPE fall well into the expected range for this material [33,45,46,48,67,82]. The graph of the specific wear rates (Fig. 7a) shows a sizable improvement for composites comprising 5 wt% and higher concentrations of BaTiO 3 . The samples with 5 wt%, 7.5 wt%, and 10 wt% exhibited specific wear rates of roughly 40 % of that observed for neat UHMWPE. The differences between these three samples are relatively small and can be deemed statistically insignificant. The wear of the sample with 2.5 wt% BaTiO 3 is also less than neat UHMWPE, but the difference is slighter than in the other cases and falls on the side of a statistical error. Composite samples displayed a slightly lower COF, though the difference is negligible in the case of the PEBT 2.5 and PEBT 10 samples. The lowest COF was measured for the sample with 5 wt% of BaTiO 3 (Fig. 7b). The polymer transfer film and particles adhered to the ball surface during the test. This film could be easily removed after the test. The performed tests did not leave any observable damage on the ball surfaces (Fig. 7c). The COF measured in the PBS against the Al 2 O 3 ball was, in all cases, consistently lower than during dry sliding. For all the samples except PEBT 2.5, the COF was in the range of 0.06 to 0.08. Whereas for PEBT 2.5, it was even lower, around 0.03 in a stable phase of friction. It only reaches this phase after an extensive run-in phase with almost 10,000 cycles (Fig. 8b). This represents the lowest coefficient of friction observed during testing and a reduction of 61 % compared to the COF of UHMWPE. The wear rates conspicuously declined for UHMWPE and PEBT 2.5 than in air, while it is comparable for PEBT 5. However, for composites with a higher content of BaTiO 3 , the wear is significantly higher (Fig. 8a). This is likely caused by the discernibly thick transfer film formed in the PBS (Fig. 8c). This film is observable on all the counter bodies after the test, but, with a higher amount of BaTiO 3, the film gets detectably thicker. The specific wear rate for all the composites in the FBS is around 5 × 10 -6 mm 3 /Nm regardless of the content of BaTiO 3 (Fig. 9a), representing roughly a 30 % reduction compared to UHMWPE. Wear of UHMWPE is equivalent to that during dry sliding. The COF of the UHMWPE is about as high as during the dry sliding (Fig. 9b). The addition of BaTiO 3 resulted in a slight improvement in the COF. No transfer film or polymer Fig. 5. EDS mapping of the Ba, Ti, and O elements within the UHMWPE/BaTiO 3 nanocomposites. D. Havaldar et al. Materials & Design 256 (2025) 114349 8 particles were present on the surface of the counter-body after the test (Fig. 9c). This can be attributed to the presence of FBS and makes the test more comparable with clinical applications. The influence of FBS on the friction and wear characteristics depends on a large number of factors, such as the roughness of the samples, the roughness of the counterbodies, the specific tribological set-up used, and the protein content of FBS [83,84]. The use of bovine serum as the lubricant is commonly associated with a decrease in friction and wear, particularly in the case of smooth counter bodies [45,83–85]. It should be noted that the literature featuring test parameters most similar to ours shows comparable results for neat UHMWPE, which is attributed to the high viscosity of the bovine serum and adsorbed layer on the polyethylene surface [48]. The effect of the adsorption of proteins is a curious one; the literature records that protein denaturation on hydrophobic surfaces can have an adverse effect on the tribological properties [47,86]. This explains the relatively poor results for UHMWPE as well as the observed decrease in the COF with the addition of BaTiO 3, since it coincides with the increase in the surface energy. Since the study did not include an examination of protein absorption, it is not possible to draw any definitive conclusions on the subject. Overall, there was no distinctive correlation between the specific wear rate and the COF with the amount of BaTiO 3 filler. Unlike hardness, the wear resistance did not increase linearly with filler loading, due to poorer distribution and particle agglomeration at higher filler concentrations. The need for uniform particle distribution and lack of Fig. 6. (a) Tensile strength, (b) Yield strength, (c) Elongation at break, (d) Tensile stress–strain curves, (e) Flexural strength and Impact strength, (f) Vickers Hardness of UHMWPE and UHMWPE/BaTiO 3 nanocomposites. Table 3 Contact angles of the UHMWPE and UHMWPE/BaTiO 3 nanocomposites measured with DW, PBS, and FBS. Contact Angle ( o ) UHMWPE PEBT 2.5 PEBT 5 PEBT 7.5 PEBT 10 DW 89.4 86.6 78.12 75.98 72.96 PBS 88.79 84.33 79.99 74.4 72.1 FBS 83.91 80.25 76.32 71.97 70.69 D. Havaldar et al. Materials & Design 256 (2025) 114349 9 CRediT authorship contribution statement Darshana Havaldar: Writing – original draft, Visualization, Project administration, Methodology, Investigation, Data curation. Jan Walter: Writing – original draft, Visualization, Investigation, Data curation. Zdenˇ ek Starý: Resources. Ladislav Cvrˇ cek: Writing – review & editing, Supervision, Conceptualization. Roman Gabor: Investigation, Funding acquisition. Karel Maˇ sek: Resources, Investigation. Elena Filov´ a: Investigation. Lubica Staˇ nkov´ a: Investigation. Zdeˇ nka Jeníkov´ a: Investigation. Kiran Pawar: Resources. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was supported by the Grant Agency of the Czech Technical University in Prague, project No. SGS23/163/OHK2/3T/12. This work was created as part of the project No. CZ.02.01.01/00/ 22_008/0004631. Materials and technologies for sustainable development within the Jan Amos Komensky Operational Programme, financed by the European Union and from the state budget of the Czech Republic. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.matdes.2025.114349. Data availability Data will be made available on request. References [1] T. Mastnak, U. Maver, M. Finˇ sgar, Addressing the needs of the rapidly aging society through the development of multifunctional bioactive coatings for orthopedic applications, Int. J. Mol. Sci. 23 (2022), https://doi.org/10.3390/ ijms23052786. [2] J. Wawrzynski, J.A. Gil, A.D. Goodman, G.R. 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