silver nanocomposites for biomedical applications

European Polymer Journal 43 (2007) 307–314

EUROPEAN POLYMER JOURNAL www.elsevier.com/locate/europolj

Macromolecular Nanotechnology

Synthesis and characterisation of advanced UHMWPE/silver nanocomposites for biomedical applications c,* ,

a

School of Chemistry, University of Nottingham, University Park, Nottingham, NG7 2RD, UK Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Moscow, Russia Institute of Laser and Information Technologies, Russian Academy of Sciences, Pionerskaya 2, Troitsk, Moscow Region, 142190, Russia d Laboratory of Biophysics and Surface Analysis, School of Pharmacy, University of Nottingham, University Park, Nottingham, NG7 2RD, UK b

c

Received 16 August 2006; received in revised form 12 October 2006; accepted 16 October 2006 Available online 28 November 2006

Abstract A supercritical fluid (SCF) route for facile and homogeneous introduction of silver nanoparticles into polymer hosts is described. Our focus is on ultra-high molecular weight polyethylene (UHMWPE). We demonstrate that the metallic nanoparticles have a substantial effect upon the wear and tribochemical properties of the polymer substrate.  2006 Elsevier Ltd. All rights reserved. Keywords: Ultra high molecular weight polyethylene; Supercritical fluids; Silver; Surface modification

1. Introduction Ultra high molecular weight polyethylene (UHMWPE) has for many years been the material of choice for fabrication of bearing inserts for total joint replacement components [1]. It is a high tensile polymer with high impact strength and is resistant to corrosion and abrasion. However, a major problem with the longevity of prostheses based upon UHMWPE is wear and concomitant debris generation. Once significant wear has occurred, particulate wear debris can be released inside the joint capsule

*

Corresponding author. E-mail address: [email protected] (V.K. Popov).

and this debris can activate macrophages. This often leads to inflammation of the surrounding tissues, and subsequently to necrosis and failure of artificial joints [2,3]. Improving the performance of the loaded surface of a polymer has been previously achieved by reinforcing the polymer with glass, ceramic and metal microparticles and fibers [4]. However, methods of blending two (or more) components usually involve very high temperatures or organic solvents and often lead to phase separated products where the desirable physical properties of the original material is lost. Moreover, melt processing of UHMWPE is not trivial. Extreme shear forces are involved in order to mix such a highly viscous matrix. This often results in the formation of an inhomogeneous spatial distribution of particles

0014-3057/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2006.10.011

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K.S. Morley a, P.B. Webb a, N.V. Tokareva b, A.P. Krasnov b, V.K. Popov J. Zhang d, C.J. Roberts d, S.M. Howdle a

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within the polymer, leading to unacceptable surface properties. Thus, conventional methodologies are not appropriate for this important engineering material and the key hurdle to overcome is how to actually introduce a filler system, i.e. micro-particles, without producing a negative effect on the properties of the polymer host [5]. An alternative methodology is to infuse or impregnate an organometallic precursor complex, which may subsequently be converted into metal or metal oxide nanoparticles. However, conventional liquid solvent impregnation processes require the use of high volumes of volatile organic solvents to deliver a precursor species, and in the case of UHMWPE such solvents are not able to efficiently penetrate into the polymer matrix. Besides, solvents such as chloroform and toluene are considered to be toxic, and can leave residues in the matrix which are difficult to remove completely [6]. In previous work, we have demonstrated the ability of supercritical fluids (SCFs) to impregnate a wide range of host materials, from polyethylene to porous silica, and to allow controlled formation of nanoparticles of metal in situ [7–13]. Additionally, we also reported that SCF impregnation of a copper(II) hexafluoroacetylacetonate precursor could lead to copper nanoparticle formation and a significantly improved polymeric wear resistance for polyarylate hosts [14]. In this paper we focus upon novel methods for processing UHMWPE and the use of supercritical fluid impregnation to incorporate silver nanoparticles into both the polymer surface and internal domains. We report the effects of this modification upon the tribological and mechanical properties. 2. Materials and methods Polymer discs (0.5 mm thickness and 15 mm in diameter) were prepared by punching out from a sheet of UHMWPE (Perplas Medical Ltd., UK, GUR 1050, Mw = 6 · 106) which was used as received. The silver precursor complex – Ag (hfac) (tetraglyme) (Fig. 1) was prepared with a standard literature synthesis [15]. UHMWPE discs and silver precursor were placed in a 10 ml stainless steel autoclave (Thar Designs). The cell was pressurised with scCO2 to 4000 psi at 38 C. After 24 h the pressure was reduced and gaseous CO2 was released from the cell. The autoclave was then pressurised with hydrogen (1000 psi) at 38 C in order to facilitate decomposi-

Ag(hfac)tetraglyme

H3C

O

CF3 O

O Ag

O

O

CF3 O H3C

O

Fig. 1. Precursor complexes used to impregnate the UHMWPE matrix and produce silver nanoparticles.

tion (ligand loss) via reduction of the metal centre. After 24 h, the H2 was released and the autoclave pressurised again with scCO2 for a further 24 h under a continuous flow at 4000 psi and 38 C to ensure complete extraction of the scCO2 soluble organic ligands from the polymer discs and leave behind only metal particles in the UHMWPE matrix. The pressure was then released over a period of 5 min and UHMWPE samples were removed from the autoclave. Physical–chemical characterisation of the nanocomposites was performed by gravimetric analysis, Transmission and Scanning Electron Microscopy (TEM and SEM), Energy Dispersive Analysis of X-rays (EDAX) and tapping mode atomic force microscopy (TMAFM). The tribological properties of the discs both before and after treatment with scCO2 were compared with those containing silver nanoparticles. All UHMWPE samples were studied using a horizontal tribometer [16] (Fig. 2). Dry and wet (in synovial fluid) sliding friction (contact overlapping coefficient = 1) at room temperature under a normal load of 0.5 MPa was applied by leverage against pure titanium (WT-1-0) or Ti6Al4V alloy counterbodies. Each counterbody was micropolished using 1 lm-diamond-powder (Ra < 0.2 lm). A range of different sliding velocities (10–80 mm/s) were applied, and each sample was tested for a continuous period of three hours to study sample wear resistance and changes in friction coefficient (f ). The wear resistance was determined by assessing the mass loss from both the polymer (Qp) and counterbody (Qc). The mass loss of the samples was measured after

309

Fig. 2. Schematic diagram of horizontal tribometer for UHMWPE wear tests.

each hour of testing. The precision of the tribometer measurements was limited by the accuracy of the analytical microbalance (104 g). A tensiometer was also used to analyse f behavior during the experiments. The temperature of the friction contact (T ) was determined by a thermocouple placed inside the counterbody at a depth of 1 mm from the friction surface. Each test was performed at least three times on three sets of the samples in order to prove good reproducibility. 3. Results and discussion After the impregnation process, a distinct colour change was observed throughout the bulk of the UHMWPE material; from white/opaque prior to nanoparticle impregnation to grey with a metallic lustre following treatment, indicating the presence of metallic silver. On removal of the discs from the high-pressure vessel the discs were covered with a residue of metallic silver which was easily removed by wiping. The variation in uptake of metallic silver into the UHMWPE samples was investigated by a systematic gravimetric study. It was found that the impregnation process is capable of producing a consistent and reproducible loading of metallic silver particles trapped within the polymeric matrix. The ratio of UHMWPE: Ag placed in the high pressure cell was varied in order to define the optimum loading conditions. There are a number of factors which control the impregnation process and limit the loading of metal nanoparticles. Impregnation of the precursor complex and subsequent deposition of metal nanoparti-

cles depends strongly upon the solubility of the complex in scCO2 and the partitioning of the complex between scCO2 and polymeric substrate [17]. scCO2 is not a particularly strong solvent and hence only a small quantity of complex will dissolve at any one time. Partitioning of the complex into the polymer then allows for further precursor to be dissolved in scCO2. The polymer itself also has a substantial effect. UHMWPE has approximately 50:50 ratio of amorphous to crystalline regions. The silver can only be deposited within the amorphous regions of the polymer and a maximum loading of ca. 7 wt.% was achieved in our experiments. TEM was utilised to characterize the particles, and their sizes and distribution within the polymer matrix. Nanoparticles of metallic silver were observed and identified from the corresponding diffraction ring patterns [13] (Fig. 3). Additionally, TMAFM performed on fracture surfaces of the treated UHMWPE discs demonstrates the presence of the Ag nanoparticles. Upon impregnation, silver nanoparticles are deposited throughout the matrix as observed in the TMAFM images (Fig. 4). The increasing concentration of metal deposited can also be seen in these images. The additional metal component is deposited almost exclusively in the amorphous regions of the polymer, as demonstrated in the height and phase images. This is consistent with the work published by Shieh et al. concerning processing with scCO2 and the effect on the physical and mechanical properties of the material [18,19]. The results presented substantiate and corroborate the results collected from TEM and unequivocally prove the success of supercritical processing.

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Fig. 3. TEM micrograph of UHMWPE sample impregnated with silver nanoparticles. The diffraction pattern (upper right corner) proves definitively that the particles are metallic silver.

The tribological properties of UHMWPE before and after impregnation were assessed during wear against pure Ti and Ti6Al4V counterbodies in air. For the initial UHMWPE discs there was a significant weight loss from both types of counterbody (Fig. 5 – left hand side). An increase in the sliding velocity of the counterbody from 10 to 80 mm/s lead to a significant rise in the measured tempera-

ture of the friction contact from 20 to 30 C. Additionally it was shown that the friction coefficient f increased from 0.20 to 0.28 and a black deposit appeared on the surface of the polymer. Analysis of this material demonstrated that it was mainly formed of particles of titanium oxides and carbides which had been transferred from the counterbody to the polymer disk. This phenomenon also concealed a significant mass loss from the UHMWPE sample. SEM and EDAX analyses (Fig. 6) confirmed that particles from the metal counterbody had indeed been transferred to UHMWPE surface. Very different behaviour was observed for the silver impregnated samples (Fig. 5 – right hand side). Negligible mass loss was observed for both the counterbodies Qc and the polymer disks Qp. Moreover, no detectable variation of either temperature or friction coefficient was observed over the same range of sliding velocities. Careful analysis of the polymer samples after three hours of testing did reveal a trace level of a black deposit, but the quantities are significantly less than observed in the absence of silver. We have previously investigated the wear process for polyethylene and have demonstrated that there are two competing processes in the early stages of its tribochemical transformation [20]. These are: (1) fracture of the main polymer chain and (2) detachment of side chains and chemical groups followed by free radical and hydrogen formation. As a

Fig. 4.1. 2 lm · 2 lm TMAFM height (a) and phase (b) images of virgin UHMWPE. The images were collected on a cryomicrotomed surface and the lamellar crystalline regions of the polymer are shown in both height and phase images.

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Fig. 4.2. 2 lm · 2 lm TMAFM height (a) and phase (b) images of UHMWPE with silver nanoparticles deposited. The images were collected from a cryomicrotomed surface and the crystalline regions of the polymer and nanoparticles can be clearly identified in both the height and phase images.

3.5

Initial UHMWPE

3 UHMWPE counterbody

Wear (mg/hour)

2.5 2 1.5 1

Impregnated UHMWPE

0.5 0 Ti

Ti6Al4V

Ti

Ti6Al4V

Fig. 5. Alteration of tribological properties of initial and impregnated with silver UHMWPE samples during wear against pure Ti and Ti6Al4V counterbody in air.

result, low molecular weight polymer chains become oriented along the counterbody sliding direction. More importantly, the free radicals and hydrogen can react with the metal and initiate tribooxidation of the metal surface following by microparticle transfer to the polymer surface. This is known as ‘‘hydrogen wear’’ [21,22] and leads to further polymer and metal degradation and debris formation.

Thus, to enhance the wear resistivity of the polymer surface we need to suppress the oxidation process. Following the wear tests, SEM and EDAX analyses were carried out on the samples pre- and postwear treatment. The unmodified UHMWPE sample before wear testing has no debris on the surface and the EDAX spectrum consists of only carbon peaks from the matrix itself (Fig. 6a). However, post-wear analysis of untreated material revealed much debris on the sample surface and this debris results from wear of the counterbody followed by titanium oxides and carbides transfer to the polymer (Fig. 6b). The additional mass gained by the polymer from the counterbody may explain why the UHMWPE samples appear to lose much less mass than the counterbodies are seen to lose (see Fig. 5). Analysis of the impregnated sample, pre-wear, illustrates the presence of only silver on the surface as expected. Finally, analysis of the impregnated sample, postwear testing, also illustrates the presence and deposition of titanium oxides and carbides transferred from the metal counterbody as before, but to a much lesser extent, particularly after wear in synovial fluid which worked as a lubricant [23]. This must be related to the supercritical treatment of the UHMWPE samples and the presence of silver nanoparticles trapped in the matrix. It is important

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Fig. 6. SEM images and EDAX analysis of UHMWPE surfaces. (a) Initial, (b) initial after wear in synovial fluid, (c) UHMWPE impregnated with silver nanoparticles and (d) UHMWPE impregnated with silver nanoparticles after wear in synovial fluid.

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4. Conclusions In this study we have demonstrated that SCF processing of UHMWPE and formation in situ of

silver nanoparticles leads to both physical and chemical stabilization of the polymer surface layer toward friction oxidation and degradation. Preliminary data indicate that this leads to enhancement of the polymer host tribological properties. This simple and inexpensive SCF procedure can significantly decrease the process of polymer/metal tribochemical debris formation and at the same time enhance UHMWPE biocompatibility and antimicrobial activity. Acknowledgements The authors would like to gratefully acknowledge the financial support of Royal Society for a Joint Project, EPSRC and ISTC. SMH is a Royal Society Wolfson Research Merit Award Holder. References [1] Barbour PSM, Stone MH, Fisher J. A study of the wear resistance of three types of clinically applied UHMWPE for total replacement hip prostheses. Biomaterials 1999;20: 2101–2106. [2] Edidin AA, Jewett CW, Kalinowski A, Kwarteng K, Kurtz SM. Degradation of mechanical behavior in UHMWPE after natural and accelerated aging. Biomaterials 2000;21: 1451–1460. [3] Affatato S, Fernandes B, Tucci A, Esposito L, Toni A. Isolation and morphological characterisation of UHMWPE wear debris generated in vitro. Biomaterials 2001;22: 2325–2331. [4] Friedrich K, Lu Z, Hager AM. Recent advances in polymer composites’ tribology. Wear 1995;190:139–44. [5] Tervoort TA, Visjager J, Smith P. On abrasive wear of polyethylene. Macromolecules 2002;35:8467–71. [6] Kurtz SM, Muratoglu OK, Evans M, Edidin AA. Advances in the processing, sterilization, and crosslinking of ultra-high molecular weight polyethylene for total joint arthroplasty. Biomaterials 1999;20:1659–88. [7] Clarke MJ, Cooper AI, Howdle SM, Poliakoff M. Photochemical reactions of organometallic complexes impregnated into polymers: speciation, isomerization and hydrogenation of residual alkene moieties in polyethylene. J Am Chem Soc 2000;122:2523–31. [8] Clarke MJ, Howdle SM, Jobling M, Poliakoff M. Photochemical generation of polymer-bound CpMn(CO)2 (g2-C=C-) complexes (Cp = g5-C5H5) in polyethylene film; a new diagnostic probe for investigating the unsaturation of the polymer. J Am Chem Soc 1994;116:8621–8. [9] Cooper AI, Howdle SM, Hughes C, Johnston KP, Kazarian SG, Poliakoff M, Shepherd LA. Spectroscopic probes for hydrogen-bonding, extraction impregnation and reaction in supercritical fluids. Analyst 1993;118:1111–6. [10] Cooper AI, Howdle SM, Ramsay JM. Spectroscopic analysis and in situ monitoring of impregnation and extraction of polymer films and powders using supercritical fluids. J Polym Sci Polym Phys 1994;32:541–9.

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to note that prior to wear testing, particles of silver are observed and detected on the surface of the impregnated sample. However, post-wear treatment, fewer silver particles are observed. There are two possible explanations for these observations. It is possible that upon wear testing, the particles of silver are pushed deeper into the UHMWPE matrix and therefore not subsequently observed post-wear analysis. Alternatively, UHMWPE fibers may possibly be pushed over and cover the surface particles as the counterbody works on the surface. This might explain why the particles are not observed by EDAX following treatment. It is important to determine how much of an effect the nanoparticles actually have, or if indeed the enhanced tribological properties of the material are due entirely to the processing with scCO2. Summarizing the presented results we can conclude that supercritical fluid treatment of the polymer leads to some wear resistivity enhancement in the initial stage of the wear process due to the morphological surface transformations and the slight (ca. 10%) increase in crystallinity [24]. However, the presence of silver nanoparticles distributed throughout the polymer matrix clearly do stabilize the surface against oxidation which is extremely important for long-term wear stability. The process of SCF treatment, extraction and silver particle impregnation leads to significant improvement of UHMWPE biocompatibility. Additionally, we have shown that a significant antimicrobial activity can also be introduced by silver [25]. Finally, in the general literature there are two main mechanisms of debris formation which are proposed as the sources of potential and actual inflammation in the artificial joint implant site: Firstly, UHMWPE degradation due to oxidation during the gamma-ray sterilization and in body fluids [26]. Secondly, the corrosion of titanium alloys in the highly aggressive living tissue environment [27,28]. In this paper, we also introduce a third possible route to explain the degradation, where tribomechanical modification of UHMWPE leads to formation of free radicals and release of hydrogen. This combination leads to tribochemical destruction (‘‘hydrogen wear’’) [21,22] of titanium and/or titanium alloy surface layers and intensive debris formation.

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