Research paper
A combination of interdisciplinary analytical tools for evaluation of multi-layered coatings on medical grade stainless steel for biomedical applications

https://doi.org/10.1016/j.ejpb.2018.05.002Get rights and content

Highlights

  • Combination of interdisciplinary analytical tools for analysis of multi-layered coatings.

  • First use of photothermal beam deflection spectroscopy for coating evaluation.

  • New testing platform for medical grade steel materials for biomedical applications.

Abstract

In this comprehensive study several analytical techniques were used in order to evaluate multi-layered biomedical surface coatings composed of a drug (diclofenac) and a polymer (chitosan). Such a thorough examination is of paramount importance in order to assure safety and prove efficiency of potential biomedical materials already at the in vitro level, hence leading to their potentially faster introduction to clinical trials.

For the first time a novel technique based on thermal diffusivity and conductivity measurements (photothermal beam deflection spectroscopy – BDS) was employed in order to analyse in a non-destructive way the thickness of respective layers, together with their thermal diffusivity and conductivity. In addition to attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR), BDS confirmed successive surface layers of the prepared coatings. Scanning electron microscopy and atomic force microscopy were used to examine structural information on the macro- and micro/nano-scale, respectively. Surface hydrophobicity was measured with the contact angle analysis, which clearly showed differences in hydrophobicity between coated and non-coated samples. Considering the targeted application of the prepared coatings (as implant in orthopaedic treatments), the in vitro drug release was analysed spectrophotometrically to examine the coatings potential for a controlled drug release. Furthermore, the material was also tested by electrochemical impedance spectroscopy and cyclic polarisation techniques, which were able to detect even minor differences between the performance of the coated and non-coated materials. As the final test, the biocompatibility of the coatings with human osteoblasts was determined.

Introduction

For decades steel has been used in regular clinical procedures for various applications [1], [2], ranging from simple screws [3], to more complex structures like artificial hips [4]. Medical grade stainless steel is a specific type of steel with a strictly defined composition and low carbon content [5], and as the name reveals, is primarily used in biomedical applications [6], [7], [8].

Due to the advancement of our knowledge in materials science, steel is nowadays often replaced by other materials like plastics [9], ceramics [10], various metal alloys [11], [12], as well as by different combinations of these materials [13], [14], [15]. Nevertheless, medical grade stainless steel is still commonly used as part of different implants, such as femoral heads, hip stems, and bone screws [16], [17]. Significant developments have been reported in the last two decades regarding the modification of medical grade stainless steel implants [18], [19], [20], including the application of different coatings possessing bioactive properties [21].

These improvements focus either on prolongation of the longevity of these implants, which consequently decrease the probability of repeated surgical procedures [7], or provide additional therapeutic properties (e.g. antimicrobial [22] or anti-inflammatory [23]) to the base material [24].

Regardless of the specific type of biomedical application, each new material to be used in a clinical setting must undergo rigorous testing to prove its safety and efficiency for the desired application [25]. With the aim to improve on one side the patient compliance and on the other the clinical efficiency without sacrificing any profit for the manufacturer, novel and improved testing methods are being developed with a staggering pace [26], [27]. Not only that these are developed with putting an increasing weight on the actual simulation of the targeted environment during the application [28], [29], but also include techniques from a versatile range of scientific fields, which are capable to fully proof the stability of the designed materials also after a prolonged exposure to specific environments, like the harsh and changing conditions in the human body [23], [30].

Unfortunately, not all current research on novel materials for biomedical applications is based on the necessary interdisciplinary foundations that would really cover all potential aspects of the use of such materials. It is still often the case that medical doctors perform clinical studies without considering profound understanding of the materials properties, and vice versa, materials scientists often forget to include medical doctors in the design phase of the necessary experiments to proof a specific materials suitability to be used in a specific application. As a result, very important parameters that are necessary to be evaluated also on the laboratory scale, are in clinical studies often overlooked.

Among the commonly used techniques for evaluation of material properties that are of importance in regard to their biomedical applications, are for sure the evaluation of surface properties by microscopy, like scanning electron microscopy (SEM) [31], [32], transmission electron microscopy (TEM) [33], atomic force microscopy (AFM) [34], [35], through evaluation of the surfaces’ hydrophobicity (contact angle measurements), profilometry [30], and different spectroscopic techniques, e.g. attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) [23]. To get more profound information about the materials structural properties, X-ray diffraction (e.g. XRPD for powders) is commonly used [36], [37]. Recently, the photothermal beam deflection spectroscopy (BDS) was developed for determining the sample structure based on the respective materials thermal properties (e.g. thermal diffusivity and conductivity) [38]. To evaluate the biocompatibility of the material with the target biological environment, several testing approaches using cell cultures originating from the target organ have been reported [39]. These techniques follow either the ISO 10993-5 standard [25] or other proven protocols found either in reference protocols [40] or in separate research studies [41].

The evaluation of the clinical effectiveness, which of course follows the prescribed testing in the lab is also of great importance [42]. These evaluations include strict testing protocols approved by responsible committees of medical ethics, either on the national level or organized inside clinical centres. Either way, these protocols mostly rely on literature sources about the materials to be tested, while it is not necessary to add specific testing of the materials by the group to be conducting the clinical testing. Even though this is a standard practice, and that researchers of course don’t want to conduct testing of not-safe materials, it can still happen that due to a possible limited knowledge of the medical doctors that conduct the clinical tests about the profound properties of the materials to be used, tests go awry and even endanger the patients included in the testing [43].

Based on the above arguments, we believe that our approach is far more superior. Namely, our team is based on researchers from very different fields with profound knowledge about the respective expertise. To prove the efficiency and the potential of such an approach, we conducted a study of different properties of a newly developed polymer-based multi-layered coating on medical grade stainless steel. Our study includes the combination of methods and techniques that were never used before together in a single study and could as such pave the way for future endeavours with the focus on the evaluation of materials safety and efficiency to a much higher extent than that achieved already on the laboratory level.

In this study we report the preparation of a novel drug delivery system on medical grade stainless steel (AISI 316LVM), composed of the polymer chitosan (CHI), which was used for preparation of materials in biomedicine before [30], and a non-steroid anti-inflammatory drug (NSAID), DCF, which we also know quite well from our previous studies [44]. To make our efforts more intriguing, we have chosen a multi-layered nanofilm system for this purpose. The surface properties of the prepared systems were evaluated by AFM, SEM and contact angle measurements. Next, ATR-FTIR and BDS measurements have been employed to evaluate the structural characteristics of respective layers. In addition, electrochemical techniques including electrochemical impedance spectroscopy (EIS) and cyclic polarisation (CP), have been used to evaluate the corrosion susceptibility of these systems in the simulated targeted environment (0.9 wt.% NaCl solution). Finally, the potential interaction of these systems with human bone cells (based on tetrazolium salt MTT and Live/Dead assays), as well as their capability for controlled drug release using in vitro drug release performance testing is reported. Thorough statistical evaluation was conducted in all steps of the study.

We have found that the proposed approach (the combination of methods from very different scientific fields) yields far deeper insights into the material properties and potential performance. Although the list of the used methods could still be extended, we believe that the used combination is the necessary base for the analysis of coatings on medical grade stainless steel to be potentially clinically used in the future.

Section snippets

Sample preparation and denomination

Physiological solution with 0.9 wt.% NaCl was prepared using ultra-pure water (18.2 MΩ cm at 25 °C) produced by the Milli-Q® system (EMD Millipore Corporation, USA). NaCl (pro analysis) was purchased from Carlo Erba (Italy), low molecular weight CHI with high purity (≥93 wt.%), and analytical grade DCF (with ≥98.5 wt.% purity) from Sigma Aldrich, Germany. AISI 316LVM stainless steel (medical grade steel) was purchased from Tiger International, China. The composition of medical grade steel is

Results and discussion

The aim of this study was to develop a novel interdisciplinary approach towards evaluation of medical grade stainless steel materials meant for biomedical applications. Systematic evaluation of materials for biomedical applications is of course not new, since researchers (almost) always try to use a range of tests to the best of their knowledge. The same is true for our previous studies that were conducted on the same premises [23], [30]. Nevertheless, most other research studies focus

Conclusions

The aim of this study is to pave the way for a new approach towards the evaluation of medical grade stainless steel materials meant for biomedical applications (and possibly others) by combining various interdisciplinary characterisation methods, ranging from chemical, physical, biological, as well as functional testing of desired material properties related to the actual use. We believe that this approach could present an appropriate platform for future evaluation of similar materials.

Our

Acknowledgements

The authors would like to acknowledge the financial support for this project received from the Slovenian Research Agency (grant numbers: P1-0034, P3-0036 and P2-0032).

The authors are also grateful to dr. Tina Maver for the execution and the Laboratory for Characterisation and Processing of Polymers, Faculty of Mechanical Engineering, University of Maribor for granting us access to the contact angle measurements.

Conflict of interest

The authors state no conflict of interest.

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