Selective laser melted titanium alloys for hip implant applications: Surface modification with new method of polymer grafting

https://doi.org/10.1016/j.jmbbm.2018.07.031Get rights and content

Abstract

A significant number of hip replacements (HR) fail permanently despite the success of the medical procedure, due to wear and progressive loss of osseointegration of implants. An ideal model should consist of materials with a high resistance to wear and with good biocompatibility. This study aims to develop a new method of grafting the surface of selective laser melted (SLM) titanium alloy (Ti-6Al-4V) with poly (2-methacryloyloxyethyl phosphorylcholine) (PMPC), to improve the surface properties and biocompatibility of the implant. PMPC was grafted onto the SLM fabricated Ti-6Al-4V, applying the following three techniques; ultraviolet (UV) irradiation, thermal heating both under normal atmosphere and UV irradiation under N2 gas atmosphere. Scanning electron microscopy (SEM), 3D optical profiler, energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) were used to characterise the grafted surface. Results demonstrated that a continuous PMPC layer on the Ti-6Al-4V surface was achieved using the UV irradiation under N2 gas atmosphere technique, due to the elimination of oxygen from the system. As indicated in the results, one of the advantages of this technique is the presence of phosphorylcholine, mostly on the surface, which reveals the existence of a strong chemical bond between the grafted layer (PMPC) and substrate (Ti-6Al-4V). The nano-scratch test revealed that the PMPC grafted surface improves the mechanical strength of the surface and thus, protects the underlying implant substrate from scratching under high loads.

Introduction

Hip replacement (HR) treatments have improved significantly over the last two decades, overcoming the challenge of higher revision surgery. The selection of implant materials is the key issue in minimising the revision rate. Titanium (Ti) and its alloys are extensively used as hip implants as they offer excellent mechanical strength, chemical inertness and high biocompatibility properties (Ramakrishnaiah, 2017, Ozan, 2017, Choudhury, 2016). Ti-6Al-4V alloy has received recent attention due to its relatively low Young's modulus compared to other metals, which helps to avoid the stress shielding effect after implantation (Ghosh and Abanteriba, 2016, Afrin). However, the elastic modulus of the alloy is still much higher than that of cortical bone, leading to aseptic loosening and premature implant failure (Fousová et al., 2017). To overcome this problem, the development of a new titanium β alloy is a noteworthy approach (Mussot-Hoinard et al., 2017). Although several studies have reported new systems with much lower stiffness, they are not identical to bone (Niinomi et al., 2012, Santos, 2016). Selective Laser Melting (SLM), an additive manufacturing technique, makes it possible to produce porous structures to minimise Young's modulus (Bartolomeu et al., 2017), an essential advantage for an implant. SLM provides controlled and interconnected pores which play an important role in creating strong bonds between the powder particles (Tan et al., 2017). This technique offers a rapid production rate with high utilisation of materials, and the macrostructure can be graded in a controlled manner (Bartolomeu et al., 2017). The SLM method facilitates the production of complex shapes over a short period with a uniform distribution of density, a homogeneous structure and minimal post-processing requirements. Hip implants can be produced directly from a CAD model using this technique (Wauthle et al., 2015), resulting in savings in cost and time. However, the widespread application of SLM is constrained due to limitations for long-term use of the product in a biological environment. Manufactured product using this technique has a high surface roughness resulted by few partially-melted particles on the surface. Consequently, the surface becomes more hydrophobic due to high surface roughness, leading to unfavourable protein adsorption (Dylla-Spears et al., 2017). Moreover, unmodified Ti-based materials can gradually stimulate the formation of a fibrous layer even after several years of implantation. Therefore, the fibrous layer interacts with the living tissue and leading to a progressive loss of osseointegration (Roos‐Jansåker et al., 2003, Chouirfa, 2017). Sometimes implant surface itself creates a preferential site for bacterial adhesion and leads to inflammatory disease. The gradual loss of supporting bone due to inflammatory disease is one of the major reasons for implant failure. It is evident that untreated SLM fabricated titanium alloy surfaces are not able to establish strong chemical bonding with the surrounding biological tissues. Proper osseointegration of the joint not only enhance the hip replacement surgeries but also helps enormously to improve the quality of life for patients enlarging the lifetime of artificial hip implants. Hence, surface modifications are needed in order to obtain sufficient integration of the implant. Surface modifications help to achieve the desired properties by tailoring the physical/chemical properties of the surface. Surface modification techniques improve the wettability, biocompatibility and mechanical properties of the surface (Ghosh et al., 2016).

Most importantly, low friction and high wear resistance are desirable for artificial hip implants while the high roughness of 3D printed Ti-6Al-4V surface would have an adverse effect on the useful life of implants. Various surface modification techniques such as plasma treatment, heat treatment and surface coating are usually applied on SLM fabricated sample to improve the mechanical properties of the surface. Controlled surface modification on SLM manufactured sample is very complicated. As a result, commonly used mechanical methods such as sandblasting and grinding or physical methods such as thermal spraying and ion implantation might not be effective in a homogeneous surface treatment (Liu et al., 2004, Zavareh, 2014). However, chemical etching (CHE) and electrochemical polishing (ECP) could be applied to obtain a controlled and homogeneous roughness of the treated surface. Pyka et al. (2012) showed a significant reduction in surface roughness using a combination of CHE and ECP with HF-based solutions. Moreover, the HF-based solution helps to remove weakly attached partially-melted particles from the surfaces by chemical etching. Few studies on biochemical modification aim at in order to obtain faster osseointegration and bone adhesion (Kirmanidou, 2016, Puleo and Nanci, 1999). Surface modification incorporating bioactive biomolecules and/or biocompatible polymers could be the pertinent solution to enhance the osseointegration and wear resistance of SLM fabricated implant. However, polymer coating arises poor adhesion between the substrate and the coated materials. Thus, it could be easily delaminated and decrease the useful life of the implants.

A research group in Japan recently introduced a new surface modification technique known as ‘surface grafting’ (Ishihara et al., 2015) which creates covalent bonding between the grafting material and the substrate. A method of direct grafting of Ti with sulphonate groups has recently been published by a polymer group in France (Chouirfa et al., 2016). Michiardi et al. (2010) also reported that a high number of osteoblasts were cultured on a Ti surface grafted with a phosphonic group polymer. Researchers reported that controlled protein adsorption and cell adhesion could protect hip implants from progressive loss of osseointegration (Karazisis, 2017, Migonney, 2013). The use of biocompatible poly (2-methacryloyloxyethyl phosphorylcholine) (PMPC) polymer on a highly cross-linked polyethylene (CLPE) during the grafting process has revealed a new area of research. The PMPC-grafted layer offers a phospholipid-like layer that mimics the articular cartilage of artificial hip joint (Moro, 2015, Moro, 2014). PMPC grafting offers the following benefits; it forms a hydrated lubricating layer; it possesses excellent biocompatibility and anti-bio fouling ability (Moro et al., 2010); it provides the unique surface properties of high lubricity, low friction, anti-protein adsorption and high cell adhesion resistance (Kyomoto et al., 2010a). However, there are limitations to the use of PMPC on CLPE. Moro et al. (2009) reported that water adsorption of PMPC-grafted CLPE surfaces mainly occurs at the CLPE liner. The use of CLPE may, therefore, have a negative effect on the performance of the PMPC grafting, reducing the productive life of the implant. Due to the significant improvement of 3D printed implants considering the mechanical properties, PMPC polymer grafting on 3D printed Ti-6Al-4V might offer a solution to inhibit biofilm formation and to improve the adhesion between metal implant and peri-implant tissues.

It is well known that a CLPE cup/liner is coupled with metallic or composite head materials in an artificial hip implant. The challenge remains to develop polymer grafting on 3D printed metal parts. However, the development of polymer grafting SLM fabricated Ti-6Al-4V has not been studied. The purpose of this study is to develop a new polymer grafting process to improve chemical bonding between the grafted layer and the substrate. Present study focussed on the characterisation of a polymer grafted layer to analyse the covalent bonding between the substrate and MPC polymer. In this work, a combination of several relevant surface characterisation techniques such as scanning electron microscopy (SEM), 3D optical profiler, energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) were performed to well-understand the production process of different polymer grafting techniques and finally it helped in optimising the grafting techniques.

Section snippets

Materials

Rectangular Ti-6Al-4V implant with the dimensions of 8 × 6 × 3 mm (XYZ) was fabricated using a SLM (SLM 250 GmbH, Germany) machine using Ti-6Al-4V powder (ASTM Grade 23, ELI, TLS Technik GmbH & Co., Bitterfeld-Wolfen, Germany) with an average particle size of 25–45 µm. The composition of Ti-6Al-4V powder is presented in Table 1.

The powder bed was preheated to 200 °C and all processing performed in an argon environment with less than 0.01% Oxygen, to prevent oxidation and degradation of the

Results and discussion

In this study, a 2-methacryloyloxyethyl phosphorylcholine (MPC) monomer was grafted to a SLM fabricated Ti-6Al-4V by radical polymerisation. Covalent bonding formed between the free radicals on the Ti-6Al-4V surface and the polymer chain when the samples were immersed in a heated aqueous solution of MPC monomer (Chouirfa et al., 2016). UV irradiation was used to initiate the polymerisation of the MPC monomers at a normal atmosphere in method 1 (M1). Thermal heating was performed at a closed

Conclusions

In this study, a novel approach for grafting the SLM fabricated Ti-6Al-4V implants with a PMPC polymer was investigated using a variety of grafting techniques. Key findings are as follows:

  • The surface morphology confirmed that the best and most uniform grafted layer was achieved by activating the monomer under an N2 gas atmosphere as the oxygen within the atmosphere inhibits polymerisation.

  • The hydrated PMPC layer minimised the surface roughness significantly on the as-built control surface,

Acknowledgements

The authors acknowledge a RMIT Research Stipend Scholarship (RRSS) support from RMIT University, Australia. The authors would like to acknowledge the facilities within the chemistry lab, MicroNano Research Facility (MNRF) and RMIT Microscopy and Microanalysis Facility (RMMF) in RMIT. Thanks to Zahra Homan for assisting in the experimental set-up for polymer grafting process.

References (57)

  • M. Kyomoto

    Self-initiated surface grafting with poly(2-methacryloyloxyethyl phosphorylcholine) on poly(ether-ether-ketone)

    Biomaterials

    (2010)
  • M. Kyomoto

    A hydrated phospholipid polymer-grafted layer prevents lipid-related oxidative degradation of cross-linked polyethylene

    Biomaterials

    (2017)
  • K. Letchmanan

    Mechanical properties and antibiotic release characteristics of poly (methyl methacrylate)-based bone cement formulated with mesoporous silica nanoparticles

    J. Mech. Behav. Biomed. Mater.

    (2017)
  • X. Liu et al.

    Surface modification of titanium, titanium alloys, and related materials for biomedical applications

    Mater. Sci. Eng.: R: Rep.

    (2004)
  • A. Michiardi

    Bioactive polymer grafting onto titanium alloy surfaces

    Acta Biomater.

    (2010)
  • V. Migonney

    Controlled cell Adhesion and aCtivity onto TAl6V TItanium alloy by grafting of the SURFace: elaboration of orthopaedic implants capable of preventing joint prosthesis infection

    IRBM

    (2013)
  • T. Moro

    Wear resistance of artificial hip joints with poly (2-methacryloyloxyethyl phosphorylcholine) grafted polyethylene: comparisons with the effect of polyethylene cross-linking and ceramic femoral heads

    Biomaterials

    (2009)
  • T. Moro

    Surface grafting of biocompatible phospholipid polymer MPC provides wear resistance of tibial polyethylene insert in artificial knee joints

    Osteoarthr. Cartil.

    (2010)
  • G. Mussot-Hoinard

    Fatigue performance evaluation of a Nickel-free titanium-based alloy for biomedical application-Effect of thermomechanical treatments

    J. Mech. Behav. Biomed. Mater.

    (2017)
  • M. Niinomi et al.

    Development of new metallic alloys for biomedical applications

    Acta Biomater.

    (2012)
  • W. Norde et al.

    Protein adsorption and bacterial adhesion to solid surfaces: a colloid-chemical approach

    Colloids Surf.

    (1989)
  • D.K. Pattanayak

    Bioactive Ti metal analogous to human cancellous bone: fabrication by selective laser melting and chemical treatments

    Acta Biomater.

    (2011)
  • D. Puleo et al.

    Understanding and controlling the bone–implant interface

    Biomaterials

    (1999)
  • R. Ramakrishnaiah

    Preliminary fabrication and characterization of electron beam melted Ti–6Al–4V customized dental implant

    Saudi J. Biol. Sci.

    (2017)
  • P.F. Santos

    Fabrication of low-cost beta-type Ti–Mn alloys for biomedical applications by metal injection molding process and their mechanical properties

    J. Mech. Behav. Biomed. Mater.

    (2016)
  • J. Vaithilingam

    Surface chemistry of Ti6Al4V components fabricated using selective laser melting for biomedical applications

    Mater. Sci. Eng.: C

    (2016)
  • R. Wauthle

    Additively manufactured porous tantalum implants

    Acta Biomater.

    (2015)
  • J. Xu

    Ozone-induced grafting phosphorylcholine polymer onto silicone film grafting 2-methacryloyloxyethyl phosphorylcholine onto silicone film to improve hemocompatibility

    Colloids Surf. B: Biointerfaces

    (2003)
  • Cited by (26)

    • Robust prediction and validation of as-built density of Ti-6Al-4V parts manufactured via selective laser melting using a machine learning approach

      2022, Journal of Manufacturing Processes
      Citation Excerpt :

      Complete parametric and non-parametric analyses were performed in three software packages: MATLAB® 2021b (MathWorks Inc., MA), R 2021 (R Core Team, Austria), and Design-Expert® v13 (Stat-Ease Inc., MN). A total of 446 ‘as-built’ SLM-ed Ti-6Al-4V input arguments were retrieved from papers [1,10,13–15,37–74] throughout literature published over the last decade. To our best knowledge, this forms the most comprehensive dataset for SLM-ed Ti-6Al-4V alloy.

    • Performance analysis of grafted poly (2-methacryloyloxyethyl phosphorylcholine) on additively manufactured titanium substrate for hip implant applications

      2019, Journal of the Mechanical Behavior of Biomedical Materials
      Citation Excerpt :

      The polymerisation time was maintained at 1 h for all samples. The details of polymer grafting process were described in a previous study (Ghosh et al., 2018). Five batches were produced for each monomer concentration and each batch include six samples.

    • Optimisation of grafted phosphorylcholine-based polymer on additively manufactured titanium substrate for hip arthroplasty

      2019, Materials Science and Engineering C
      Citation Excerpt :

      This porous structure has the ability to maintain good mechanical integrity with a satisfactory geometric accuracy of the implants [4,5]. The current challenge with the SLM process is relatively high surface roughness (>Ra 15 μm) of the parts due to the presence of partially-melted particles left on outer surfaces during fabrication [6,7]. These outer surfaces experience different cooling rates due to the heat transfer from the melted part straight to the powder bed, and these areas do not undergo re-melting cycles from subsequent layer laser penetration.

    • Improving the mechanical and tribological properties of amorphous carbon-based films by an a-C/Zr/ZrN multilayered interlayer

      2019, Ceramics International
      Citation Excerpt :

      Medical implants are of great value in that they can maintain the mobility and reduce the pain of osteoarthritis patients [1].

    View all citing articles on Scopus
    View full text