Electrochemical and mechanical behavior of implantable materials

Most implantable materials used in medical implants are vulnerable to degradation due to corrosion during their service life. The purpose of this special issue is to review the current understanding of the electrochemical, mechanical and biological processes that are responsible for the degradation of a variety of implantable biomaterials. The specific focus is to establish a forum for the dissemination and discussion of recent advances in understanding degradation mechanisms in most of the metallic implantable materials including stainless steel, titanium and its alloys, cobalt chromium, magnesium alloys, nitinol and other biomaterials used in medical devices (orthopaedic, dental, spinal, and other applications). There is abundance of literature on retrievals and in vitro studies elucidating corrosion-related concerns in metallic medical devices. Despite these findings, corrosion in medical devices is not well understood. This speaks to the need for extending research efforts to gain insights into fundamental understanding of the corrosion processes associated with implantable materials used in the medical devices.

The electrochemical and mechanical aspect of corrosion processes (pitting, crevice, intergranular, stress corrosion cracking, galvanic, fretting, corrosion fatigue and wear) and related issues in different types of metallic biomaterials is comprehensively discussed by Sridhar et al.¹ A complex combination of the above corrosion processes, so-called ‘Mechanically assisted Crevice Corrosion (MACC)’ is described in the review article by Mali.² MACC continues to be a serious concern for highly loaded medical devices especially modular hip prosthesis. MACC is a multifactorial process that includes mechanical, geometric, chemical and electrochemical interactions which can lead to accelerated increase in corrosion rates, thereby decreasing structural and mechanical integrity of the implant.² The review article also provides discussion on retrieval analysis, in vitro testing and recent biological evaluations on orthopaedic implants and materials. The corrosion of biomaterials used for dental implants also has a significant clinical relevance. In a review article, Chaturvedi³ has provided an overview of various aspects of dental implant materials and its interaction with oral environment. Metallic dental implant materials are susceptible to corrosion processes in the presence of hostile electrolytic oral environment (low pH, high fluoride ion concentration).³

Magnesium (Mg) alloys are emerging as important candidates for biodegradable temporary implants applications (e.g. plates, screws, wires, etc.) given their strength-to-weight ratio and biocompatibility. However, in spite of significant research and improvement, problems of fast degradation rates for Mg-based alloys continue to be a challenge for their successful implementation for a variety of applications in the body. Current research now focuses on developing improved Mg alloys with lower biodegradation rates and without any potential toxicity problems of the alloying elements. The biodegradation mechanism of Mg alloy and methods to improve their corrosion resistance by alloying and surface treatment is described by Chen et al.⁴ Similarly, Trivedi et al.⁵ have described the state-of-art knowledge on new class of Mg alloy with rare earth metals for use as biodegradable implant with improved mechanical and biodegradation properties.

Titanium and its alloy are one of the most used metals for biomedical applications owing to their excellent mechanical properties, corrosion resistance and biocompatibility. Despite its excellent properties, titanium and its alloy are susceptible to tribocorrosion processes, stress shielding and possible toxicity issues related to alloying elements (vanadium, aluminium). Significant advancement have been made in the field towards development of new titanium alloys with low modulus,⁶ surface treatment methods and coatings to improve surface-related properties including hardness, wear, corrosion and tribocorrosion.^{6, 7} An interesting surface modification technique, Plasma Electrolytic Oxidation (PEO) for titanium is described by Laurindo et al.⁸ In this study, the porosity, chemical composition, crystalline structure and oxide layer thickness were modified due to increase in PEO voltage and annealing treatment, leading to improved wear resistance of the titanium.

I have been greatly privileged to act as Guest Editor for this Special Issue of Materials Technology: Advance Performance Materials on such an important and interesting subject. I hope that the material presented in this collection of eight original Review and Research papers will provide a glimpse of the various research and development activities already recorded in this field while discussing further topics which needs additional assessments.

I am grateful to the Editor, Professor Devesh Misra, Editor-in-Chief, for constant encouragement and support. I would also like to thank and congratulate the authors for their timely and greatly appreciated contributions to this Special Issue.

Sachin A. Mali