Characterization of different materials for corrosion resistance under simulated body fluid conditions

Abstract

A systematic characterization study has been carried out on different materials such as commercial purity titanium, Ti–6Al–4V, 316L stainless steel, and a cobalt-based alloy under simulated body fluid conditions at 37 °C. Breakdown potential, corrosion rates, pitting/crevice corrosion resistance, and the ability to form protective oxide scales were evaluated and compared. The advantages of the titanium alloy over other materials were highlighted. An attempt has also been made to study the suitability of titanium alloy, IMI 834, for biomedical applications. AC impedance measurements were also carried out in order to provide supportive evidence for the above results.

Introduction

Biomedical materials play an important and a critical role in manufacturing a variety of prosthetic devices in a modern world. Prosthetic devices are artificial replacements that are used in a biological system, such as the human body in an effort to provide the function of the original part. These devices are generally made of polymeric, metallic and ceramic materials, or combinations of these materials, depending on the intended use. Metals/alloys are used as surgical implants in the human body primarily for orthopedic purposes. The first requirement for any material to be placed in the human body is that it should be biocompatible and not cause any adverse reaction in the body. The material must withstand the body environment and not degrade to a point that it cannot function in the body as intended.

Corrosion of metal implants is critical because it can adversely affect the biocompatibility and mechanical integrity. The materials used should not cause any adverse biological reaction in the body and simultaneously, it must be stable and retain its functional properties. Corrosion and surface oxide film dissolution are the two mechanisms for introducing additional ions into the body. Extensive release of ions from prosthesis can result in adverse biological reactions and can lead to mechanical failure of the device.

Metals/alloys used in the human body must have a high corrosion resistance and must not be treated or used in a configuration that would degrade the corrosion behavior. Degradation of metals and alloys used as surgical implant orthopedic devices is usually a combination of electrochemical and mechanical effects. Modern-day life has resulted in the increased use of implants at a younger age. The average life span has also been increased. Both these factors necessitated the use of materials with enhanced life and/or innovation of newer implant materials with prolonged life.

Corrosion is the first consideration for a material of any type that is to be used in the body because metal ion release takes place mainly due to corrosion of surgical implants [1]. Therefore, various in vitro and in vivo tests have to be carried out in order to identify appropriate materials for use as surgical implants. It is desirable to keep the metal ion release to a minimum by the use of corrosion-resistant materials. Some effects of incompatible materials include interference with normal tissue growth near the implant, interference with systematic reactions of the body, and transport and deposition of metal ions at selective sites or organs may occur [2], [3], [4], [5]. There is always a concern about the carcinogenic effects of foreign materials in the body both short- and long-term periods exceeding 20 years. Some individuals are sensitive to metals and some develop metal sensitivity later after receiving an implant.

The environment of the body is extremely well buffered so that the pH is maintained at around 7.4 and it is, of course, held at 37 °C. Two features control the severity of this environment. Firstly, the saline solution is an excellent electrolyte and facilitates the electrochemical mechanisms of corrosion and hydrolysis. Secondly, there are many molecular and cellular species in the tissues that have the ability to catalyze certain chemical reactions or rapidly destroy certain components identified as foreign. Corrosion behavior of materials can be studied either in vivo or in vitro using artificial physiological fluids, in which the oxygen content is controlled at a suitably low value at 37 °C. Hank's solution is an example of an artificial solution, which has been used over a period for corrosion testing in the laboratory [6].

In the present investigation, a systematic study was carried out in order to understand the stability of various materials both for short- and long-term biomedical applications and thereby suggest an appropriate material for surgical implants.