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Nanocrystalline Cellulose Improves the Biocompatibility and Reduces the Wear Debris of Ultrahigh Molecular Weight Polyethylene via Weak Binding Shiwen Wang, Qiang Feng, Jiashu Sun, Feng Gao, Wei Fan, Zhong Zhang, Xiaohong Li, and Xingyu Jiang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b04393 • Publication Date (Web): 19 Dec 2015 Downloaded from http://pubs.acs.org on December 21, 2015
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Nanocrystalline Cellulose Improves the Biocompatibility and Reduces the Wear Debris of Ultrahigh Molecular Weight Polyethylene via Weak Binding Shiwen Wang,†,‡ Qiang Feng,† Jiashu Sun,†,* Feng Gao,† Wei Fan,† Zhong Zhang,† Xiaohong Li,‡,* and Xingyu Jiang,†,* †
Beijing Engineering Research Center for BioNanotechnology & CAS Key Laboratory for
Biological Effects of Nanomaterials and Nanosafety, National Center for NanoScience and Technology, 11 Beiyitiao, ZhongGuanCun, Beijing, 100190, China ‡
Key Laboratory of Advanced Technologies of Materials, Ministry of Education of China,
School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu, 610031,China KEYWORDS: Artificial joint, Nanocrystalline cellulose, Ultrahigh molecular weight polyethene, Debris, Friction
ABSTRACT: The doping of biocompatible nanomaterials into ultrahigh molecular weight polyethylene (UHMWPE) to improve the biocompatibility and reduce the wear debris is of great significance to prolonging implantation time of UHMWPE as the bearing material for artificial joints. This study shows that UHMWPE can form a composite with nanocrystalline cellulose (NCC, a hydrophilic nanosized material with a high aspect ratio) by ball-milling and hot-pressing.
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Compared to pure UHMWPE, NCC/UHMWPE composite exhibits improved tribological characteristics with reduced generation of wear debris. The underlying mechanism is related to the weak binding between hydrophilic NCC and hydrophobic UHMWPE. The hydrophilic, rigid NCC particles tend to detach from the UHMWPE surface during friction, which could move with the rubbing surface, serve as a thin lubricant layer, and protect UHMWPE substrate from abrasion. The biological safety of NCC/UHMWPE composite, as tested by MC3T3-E1 preosteoblast cells and macrophage RAW 264.7 cells, is high, with significantly lower inflammatory responses/cytotoxicity than pure UHMWPE. NCC/UHMWPE composite therefore could be a promising alternative to the current UHMWPE for bearing applications.
Total joint replacement is a surgical procedure that replaces the dysfunctional joint with prosthesis. Due to the notable self-lubricating and wear-resistant characteristics, ultrahigh molecular weight polyethylene (UHMWPE) has become the primary bearing material in artificial joints.1 However, the failure of the implants is still a problem mainly caused by an osteolytic immune system response initiated by the wear debris released from UHMWPE.2-4 To circumvent the wear-caused failure of total joint replacements, several attempts to modify the UHMWPE, such as cross-linking of UHMWPE, wear-resistant coating, and fabrication of filler/UHMWPE composite, have been made to improve its tribological properties. The crosslinked UHMWPE, which is modified by gamma irradiation or oxidizing agents, exhibits an increased wear resistance, but is more prone to in vivo oxidation than untreated UHMWPE.5 The oxidation of cross-linked UHMWPE results in the degradation of UHMWPE, thus dramatically reducing the implantation time of UHMWPE as artificial joints. The coating of wear-resistant materials onto the surface of UHMWPE faces the risk of the spontaneous peeling off of adhesive layers from UHMWPE substrate.6 The use of fillers could modify the UHMWPE more simply
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than cross-linking, and show a better stability than wear-resistant coating. The filler materials, such as graphene or carbon black, can significantly improve the wear resistance of UHMWPE.7, 8 However, these filler-materials themselves may trigger harmful biological effects, such as inflammation or osteolysis, resulting in the failure of prostheses.9, 10 Nanocrystalline cellulose (NCC), a natural nanomaterial produced mainly by the hydrolysis of cellulosic material, has been recently applied as a promising filler material due to its good mechanical and biocompatible properties.11-14 NCC is typically a rigid, hydrophilic, high aspectratio, and rod-shaped crystal cellulose whisker of 1-20 nm in diameter and tens to hundreds of nm in length.15 NCC also has a large surface area (150-250 m2/g), low coefficient of thermal expansion, and iridescent chiral nematics.16-21 Compared with carbon nanotubes (CNTs) or short asbestos fibers, NCC shows much less cytotoxicity and inflammatory response.22 Due to these unique characteristics, NCC has been incorporated as fillers for development of injectable hydrogel,23 drug delivery system,12, 24 and scaffold for tissue engineering.25 A recent study employs NCC as a filler to improve the hydrophilicity, mechanical and thermal performance of poly(vinylidene fluoride) (PVDF)/poly(methyl methacrylate) (PMMA) blend.26 Despite these interesting applications,27 the use of NCC as a filler for UHMWPE to improve both the tribological properties and biocompatibility has not been studied yet. In this study, we prepare NCC/UHMWPE composites by blending different amounts of NCC with UHMWPE, followed by hot-pressing of dried blends. The tribological property of NCC/UHMWPE composite is investigated and the wear debris from the composite is characterized. The underlying mechanism of NCC to reduce wear debris is discussed. As a potential bearing material, the biocompatibility of NCC/UHMWPE composite is evaluated via in vitro cytotoxicity test and inflammatory response using wear debris from the composite.
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RESULTS AND DISSCUSIONS Preparation of NCC/UHMWPE composite The composite plates of NCC/UHMWPE are fabricated by hot-pressing (Figure 1 A-C, details in Methods Section). The disk-shaped composites of 25 mm in diameter and 1 mm in thickness are fabricated for wear experiments. The dumbbell-shaped composites (details in Figure S1) are used for mechanical tests. The composites with different contents of NCC are marked by S0 (without NCC), S0.1 (with 0.1% w/w of NCC), S0.5 (with 0.5% w/w of NCC), S2 (with 2% w/w of NCC), S4 (with 4% w/w of NCC), and S8 (with 8% w/w of NCC), respectively. The color of the composites containing less than 2% of NCC is almost the same as that of the pure UHMWPE plate (Figure 1E).28 The format of NCC inside the composites is aggregated and dispersed randomly, according to the SEM images (Figure S2) and optical microscope images (Figure S3). Compared with pure UHMWPE, the composite doped with NCC shows a slightly decrease in contact angle, due to the addition of hydrophilic NCC on the surface of composite (Figure S4, Supporting Information).
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Figure 1. The fabrication of NCC/UHMWPE composite. First, NCC (the purple rods) and UHMWPE powder (the white powder) are mixed by stirring and ball-milling (A). After the solvent evaporated completely (B), the mixture is hot pressed to fabricate the composite of NCC/UHMWPE (C). The TEM image of NCC (D) and the photograph of NCC/UHMWPE composites with different contents of NCC (E). S0: pure UHMWPE, S0.1: 0.1% NCC, S0.5: 0.5% NCC, S2: 2% NCC, S4: 4% NCC, S8: 8% NCC.
Tribological and mechanical characteristics of NCC/UHMWPE To investigate the wear resistance of NCC/UHMWPE composite, we use water as the lubricant for the following experiments.29 With the increased concentration of NCC, the coefficient of friction of the composites significantly decreases (Figure 2A). The wear volumes of the composites are also reduced with the addition of NCC (Figure 2B). Moreover, on the worn
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surface, the more NCC fills in the composite, the less and the shallower the furrows (marked by blue arrows) are observed by optical microscopy (Figure S3A-F). After zooming in the furrows by SEM, some micro-cracks perpendicular to the furrows are observed, as a typical phenomenon of fatigue (Figure 2C-H). The number of cracks decreases with the increasing amount of NCC (Figure 2C-H). Previous studies2 show that these cracks are attributed to the submicron debris, and a small number of micro-cracks reveals a reduced amount of micro-debris. These data indicate that the NCC in UHMWPE could efficiently protect the UHMWPE from abrasion. Some holes are observed on the worn surfaces of the composite samples of NCC/UHMWPE (Figure 2F-I). The generation of these holes might be due to the detachment of the aggregates of NCC from UHMWPE during wear based on the following two lines of evidences. The first one is that we can rarely find holes on the worn surface of pure UHMWPE, so that the holes must be related to NCC. Second, we can also observe the aggregation of NCC (pointed by red arrows) on the surface of composite before wearing (Figure S2A). The morphology and size of the aggregated NCC appear similar to those holes after NCC was detached (pointed by yellow arrows) (Figure S2B). The aggregation of NCC inside UHMWPE indicates an incompatible interface between the hydrophilic filler of NCC and the hydrophobic matrix material of UHMWPE.
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Figure 2. The coefficient of friction of the composite samples with different contents of NCC (A), the wear volume of the samples with different contents of NCC (B), and the number of cracks on the worn surfaces of different composites (C). The increase in the NCC content results in the decrease in the number of cracks. The SEM images of the worn surfaces (D: S0, E: S0.1, F: S0.5, G: S2, H: S4, I: S8, the scale bars are 10 µm and 1 µm). The white arrows indicate the direction of motion. (*: P 1 µm from NCC/UHMWPE composites is decreased up to 98 %, and that of wear debris < 1 µm is up to 79 % (Table 1 and Figure 5G). The coefficient of friction of composites is also reduced by 10 % to 56 % with the increased NCC content from 0.1 % to 8 % in comparison with pure UHMWPE (Figure 2A).
Figure 6. The scheme for a possible mechanism of lubrication by NCC. The hydrophilic, rigid NCC particles tend to detach from the UHMWPE surface during friction, which could move
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with the rubbing surface, serve as a thin lubricant layer, and protect UHMWPE substrate from abrasion.
Biocompatibility of NCC NCC used in this study are rod-like particles with 10 nm in diameter and 102 nm in length in average, produced from cotton fibers by acidic hydrolysis (Figure 1D, Figure 5G and Table S1).36 The physical or chemical properties of NCC are summarized in Table S1. The cytotoxicity and inflammatory response of NCC on mouse preosteoblast (MC3T3-E1, an osteoblast precursor cell line that has been widely used in bone biology) and mouse macrophage (RAW 264.7, widely used in studies of inflammation) are evaluated (Figure 7). These in vitro experiments indicate that NCC does not affect the cell viability, nor trigger serious inflammation of cells with the concentrations less than 125 µg/mL. In our experiments, the highest concentration of NCC detached from UHMWPE is less than 1 µg/mL (Figure 5), much lower than 125 µg/mL, suggesting that detached NCC may not cause the bone tissue inflammatory reaction. A previous report demonstrates that NCC shows a much lower cytotoxicity and inflammatory response than multiple-wall carbon nanotubes (MWCNTs) or crocidolite asbestos fibers (CAFs) possibly due to the different surface chemistry, surface charge and stiffness between NCC, MWCNTs and CAFs.22 Therefore NCC has a better biocompatibility than other rod-shape nanomaterials.
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Figure 7. The cytotoxicity of MC3T3-E1 cells (A) and RAW264.7 cells (B) with different concentrations of NCC (per milliliter) by CCK-8. The ALP secreted by MC3T3-E1 (C) and TNF-α and IL-6 secreted by RAW264.7 (D) with different concentrations of NCC (per milliliter). (* or a: P
