ICMCTF2016 Session D2-2: Surface Coatings, Micro/Nano Texturing, Nanotubes, Drug Delivery, Biodegradable Implants

The translation of mesenchymal stem cells (MSCs) preclinical results has serious problems to be solved due to the significant variability of MSCs preparations and limited life-span of MSCs. To circumvent such issues, immortalized MSCs have been used. Various studies have demonstrated that such immortalized cells grow fast and are able to produce significant amount of extracellular matrix. However, it remains elusive how good these extracellular matrix can be in terms of mechanical properties and microstructure.

In this work, the nanoindentation technique has been adopted to study the mechanical properties of the matrix produced by immortalized human MSCs at different cell media (basal media and osteogenic media). It has demonstrated that these immortalized cells grow very fast particularly in osteogenic media. For both basal media and osteogenic media, extracellular matrix produced by these cells gets matured quickly and the corresponding elastic moduli decrease from day 7 to day 21. For day 7, the matrix produced in osteogenic media is stiffer compared to its counterpart in basal media. However, such an observation reversed for day 14. In addition, it is found that the extracellular matrix produced in basal media has significant anisotropic property. Statistical analysis has revealed the longitudinal modulus is 2.5-3 times that of transverse direction. This significant anisotropic behaviour is correlated to the fibrous structure of the matrix as observed by atomic force microscopy. The anisotropic behaviour is less profound for the matrix produced in osteogenic media, in which case the longitudinal modulus of extracellular matrix is 1.2-1.5 times that of transverse direction. The change of nanomechanical properties with cell culture media and cell culture duration is due to the change in molecular components and microstructure of extracellular matrix which is related to cell metabolic activities. The findings in this study are useful for developing fundamental understanding of engineered bone tissue as well as cell and materials design for tissue engineering.

Poly-aryl-ether-ether-ketone (PEEK) is a polymeric biomaterial whose high Young’s modulus allows it to be used in load bearing applications in arthroplasty [1]. In particular, PEEK-on-PEEK couples have been of interest for motion preservation spinal implants. Despite the good properties of PEEK, there is currently no definite benefit, in terms of wear rates, in using a PEEK-on-PEEK couple for spinal implants compared with the reference CoCr-UHMWPE couple [2][3]. On the other hand, thin hydrogenated carbon coatings have been studied as coatings aiming at reducing wear and thus improving the lifespan of implants [4].

This study focuses on evaluating and comparing the physico-chemical, mechanical and tribological properties of different thin hydrogenated carbon coatings on PEEK. More precisely, four different coatings were obtained from a hydrocarbon gas by Plasma Enhanced Chemical Vapor Deposition (PECVD). Those four coatings were obtained under different bias voltages and with or without a polarizing titanium layer obtained by cathodic magnetron sputtering.

The physico-chemical properties of surfaces were assessed and compared by wetting, topographical analyses and X-ray photoelectron spectroscopy. Particular attention was paid to the adhesive, scratch and nano-hardness properties of the hydrogenated carbon-PEEK combinations as well as to the residual stress in the coatings: a relationship between residual stress, scratch resistance and nano-hardness was established. The tribological performances of the different hydrogenated carbon-PEEK combinations were investigated by rubbing against alumina balls under physiological conditions and evaluated in terms of coefficient of friction and wear rate and compared to the PEEK surface reference.

[1] S. Kurtz, PEEK Biomaterials Handbook Chapter 13 (2012) 201-220

[2] Grupp et al., Biomaterials 31 (2010) 553-531

[3] Xin et al., Wear 303 (2013) 473-479

[4] R. Hauert et al., Surface & Coatings Technology 223 (2013) 119-130

Diamond-like carbon (DLC) coatings are most successfully used successfully in a broad range of machinery applications due to their ability to improve the mechanical and chemical resistance of surfaces exposed to wear. Quite a few attempts were made to achieve similar effects in load-bearing body implants such as hip, knee and spinal disc prostheses. However, several clinical studies using DLC coatings on articulating joints showed severe problems due to partial coating delamination after several years in vivo [1], which had dramatic effects for the affected patients. Such observations obviously affected the use of DLC in the MedTech field.

Here, we present an overview of the main known failure mechanisms of DLC coated medical alloys and their impact on the lifetime of the coated implant, along with means to predict the in vivo survival time - especially the long-term adhesion stability of the coating. The formation of a few atomic layers of reaction products at the interface, usually metal carbides, will be addressed. Furthermore, any contamination from residual gas or any cross-contamination will result in a different reactively formed interface compound with different properties; several adhesion promoting interlayer materials were tested for their sensitivity towards oxygen contamination.

The most promising interlayer candidate was tested in a spinal disc simulator and maintained stable for a simulated mechanical equivalent of over 100 years of articulation [2] whilst Rockwell-indentation based corrosion tests indicate a corrosion resistance up to 60 years in vivo.

REFERENCES:

[1] R. Hauert, K. Thorwarth, et al., "An overview on diamond-like carbon coatings in medical applications", Surface and Coatings Technology 233, 119, (2013)

[2] K. Thorwarth, G. Thorwarth, et al., "On interlayer stability and high-cycle simulator performance of diamond-like carbon layers for articulating joint replacements", International Journal of Molecular Sciences, 15(6), 10527 (2014)

A novel carrier matrix for antibiotics consisting of phosphatidylcholine (PC) can be directly applied to the surface of an implant material leaving a thin degradable and biocompatible matrix to release antibiotics and prevent bacterial growth. The weakly adherent lipid-based coating must withstand surgical handling and implantation forces for optimal drug release. In this study, we have modeled retention of phosphatidylcholine coatings on common implant metals, titanium and stainless steel while mimicking forces applied to coated implant materials by surgeons upon handling and implantation.

PC coatings fabricated through a process of warming and kneading Phospholipon 90G were directly applied to washed and passivated titanium and stainless steel coupons. Coupons were weighed pre- and post-coating to determine the amount of material applied. Then they were wiped with medium manual pressure (approximately 4 N at 5mm/s) with surgical cloth or low lint tissue . After wiping, the metal coupons were weighed a third time to determine the amount of material removed from the surface. A second PC coating was fabricated by loading 25% Direct Red 80 dye in Phospholipon 90G. This coating was directly applied to sterile titanium and stainless steel wires, before insertion into 2% agar as a tissue mimic. After 24 hours, the agar block was sectioned through the middle of the insertion and elution pattern and distance was observed as a function of insertion distance.

Titanium coupons coated with PC retained an average of 49±10% of material when wiped with low-lint tissue and 18 ± 6 % of material when wiped with cloth. This did not statistically differ from retention values on stainless steel (27±14%). The triangular elution pattern observed in cross-sectioned agar demonstrates that coating is removed and deposited into the tissue phantom material as the implant material is inserted. Pooled coating was observed at the interface of implantation and elution varied from 5 mm radially at the interface to 0 approximately 15 cm in depth.

Point-of-care applied antibiotic-loaded coatings could be advantageous as a preventative method to prevent biofilm formation on implants. In order to release antibiotics at levels that kill bacteria and protect surrounding tissue, enough antibiotic in coating must be retained at the implant surface. These results show that although this coating is weakly adherent and not chemically bound to the implant materials, some coating material remains after implantation procedure simulation. Modifications to implant surfaces or coating formulation may be pursued in future studies to improve retention of coating at implant surfaces.

Thin nanostructured RF magnetron sputter deposited HA-based coating with the thickness in the range 400 - 600 nm was deposited on the surface of AZ31 alloy. Chemical, molecular and phase composition of the biocomposite as well as physico-mechanical properties and corrosion resistance were studied. The analysis of the surface morphology revealed that the coating is homogeneously covering the entire surface of the substrate. No pores or cracks were observed. XRD-spectra revealed that HA coating was prepared. No other phases were present. FTIR results revealed the presence of molecular bonds, which were typical for HA. The mechanical properties of the samples showed that the deformation behavior of the films is elastic-plastic. In case of uncoated AZ31 alloy the deformation was completely plastic, i.e. no recovery of indentations after unloading was observed. Higher values of nanohardness were observed in case of the HA coated AZ31 substrates compared to bare substrate. A higher value of Young’s modulus of the biocomposite AZ31+HA coating compared to the bare substrate, taking into consideration the ratios E/H and H³/E²,enables better biocompoiste behavior being implanted. The corrosion resistance tests were done. The results revealed that an enhanced corrosion resistance was observed in case of the HA coated substrates. Thus, RF magnetron sputtering is a versatile technique to enhance the surface properties of biodegradable AZ31 alloy. The authors acknowledge the support of State order NAUKA #11.1359.2014/K.

We investigated the potential of nano-crystalline chromium nitride thin film implant coatings to reduce the corrosion process and minimise the immune cell response in-vivo faced by patients with osteoarthritis. The films are prepared by reactive magnetron sputtering and characterised for grain growth by scanning electron microscopy. The chemical structure of the prepared films are characterised by X-ray photoelectron spectroscopy and Raman spectroscopy. The nano crystalline structure of the coatings which contributes to their phagocyte activation was probed by x-ray diffraction and radial distribution function analysis. We investigated the presence of surface chemical constituent entities on the coatings with XDLVO surface energy analysis and Kelvin probe contact potential difference/ work function measurements to establish the presence of hydrophobic surface chemical entities on the prepared films. The corrosion susceptibility of the films was investigated in saline solution. Our initial investigation includes open circuit potential measurements (OCP) over several hours, Tafel plots and Potentiodynamic polarization. The coatings show good corrosion resistance against pitting corrosion but could be improved further through a microstructural growth mode switch to eliminate potential pin-holes due to a columnar growth mode. The columnar Volmer-Weber growth mode observed by scanning electron microscopy is suspected to underlie the corrosion behaviour of the coating. The initial in vitro immune cell activation was investigated using peripheral blood mononuclear cells (PBMC) cultured on coated and uncoated control surfaces. Supernatants were collected at various time points and simulation conditions. There was a statistical significance (P<0.01) in the secretion of the inflammatory cytokine, interleukin 6 (IL-6), between the chromium nitride coated and the uncoated control surface. The results of our current in-vitro investigation based on corrosion and cellular response tests confirm the potential promise for the application of chromium nitride coatings prepared by reactive magnetron sputtering in orthopaedic implant applications.