Shutterstock | Denis Simonov
Biocompatible polymers and metals are increasingly being used to make medical devices including heart valves and medical implants such as joint replacements.
Cardiac devices and dental and orthopedic implants have become more common than ever as people are living longer.
Biocompatibility in the exotic environment of the human body is an important requirement in the ever-developing field of biomaterials. Implanted devices, should be constructed of materials that don't cause any adverse effects such as allergy, toxicity, and inflammation.
In order to not fracture or break and require replacement, the biomaterials should have enough mechanical strength to tolerate the forces that they are subjected to.
The environment of the human body is known to be highly corrosive, and with an expected working life of 15 to 20 years1, high wear and corrosion resistance are important requirements of a bio-implant. Three factors decide a successful biomaterial:
- Chemical resistance (biocompatibility and electrochemistry)
- Mechanical strength (stress and fractures)
- Tribological strength (friction and wear)
Electrochemistry and Corrosion
The gradual degradation of materials by electrochemical effect is the definition of corrosion, which is of particular concern for metal implants in the hostile 'electrolytic' environment of the human body.2
Dental and orthopedic implants function in an extremely corrosive environment, which includes many body fluids such as blood, water, amino acids, plasma, glycopolysaccharide mucin, and proteins2. This liquid medium in the human body comprises of a complex mixture of ions including bicarbonate, phosphate, chloride, calcium, potassium, sodium, and magnesium as well as dissolved oxygen and polymers.3
The implants’ electrochemical activity (cathodic or anodic) and its interaction with bacteria, biological molecules, and cells significantly increases the difficulty of any corrosive processes with potential unexpected results. An example of such a circumstance in the highly oxygenated saline include:
Hydrogen formed by a cathodic reaction often acts as a corrosion inhibitor, however corrosion can be enhanced by bacteria absorbing hydrogen near implants.3
Corrosion is also influenced by pH changes. The human body has a pH of about 7.4, but this can go up to 9 or come down to 3 with infection, disease and following surgery. 4
Preferential corrosion is promoted by the reduction of oxygen diffusion in certain areas, and is caused by adsorption of proteins on the surface of the implants.
This means that it is necessary that the implant materials are tested under biological conditions. 4,5
The electrochemical corrosion of metal that occurs in vivo occurs by the same process, oxidation, that causes iron to rust. Shutterstock | Olegusk
Electrochemical testing involves evaluating a material’s resistance to corrosion and the resulting mechanical failure, e.g., orthopedic devices made of titanium undergoing corrosion, pitting, and fracture failure. The testing can also determine the possibility of harmful metal ions leaching into the body (e.g., nickel ion leaching).
The question of a metal implant’s corrosion causing clinical problems is therefore relevant to a device manufacturer.
Various laboratory tests can be performed to determine the corrosion resistance of bio-device materials in the laboratory. These usually consist of electrochemical measurements taken in a imitation body fluid environment (in vitro experiments).6
Imitation body fluids consisting of ions, amino acids, and proteins at physiological pH and temperature conditions are used for in vitro experiments. Due to the complexity of biological systems, the tests are difficult to perform, which means they are difficult to reproduce in the laboratory.7
Testing implants in a live animal involves in vivo measurements but these should be avoided as they raise ethical issues. Laboratory experiments are preferred as similar results have been obtained for both in vitro and in vivo studies.8
Complex implants such as heart valves are designed to be highly reliable over a long lifespan. Shutterstock | pirke
The F2129 test method is recommended by ASTM as the method to test small implant devices.9 The method involves immersing the device in a deaerated (nitrogen purged) simulated physiological solution, and measuring the open circuit (rest) potential [Er] for one hour.
This method can be used to evaluate the corrosion mechanism and recommendations on the electrochemical properties of implant alloys can be made. Several factors need to be considered for corrosion of alloys such as passivation of surfaces, processes which involve deactivation of alloy surfaces, surface oxides, and the effect of bacteria, cells, and changing pH.
One of the major issues for the biomedical implant device failure is corrosion. A comprehensive F2129 test methodology to examine the activity and corrosion processes of implant alloy materials is provided by Fort Wayne Metals. The methodology can help customers to develop better biocompatible alloys by suggesting developments for future materials.
- G. Manivasagam and D. Dhinasekaran et al., Biomedical Implants: Corrosion and its Prevention - A Review, Recent Patents on Corrosion Science, 2010, 2, 40-54
- Williams DF. Review-Tissue-biomaterial interactions. J Mater Sci, 1987; 22: 3421-45.
- Mohanty M, Baby S, Menon KV. Spinal fixation device: a 6-year post implantation study. J Biomater Appl 2003; 18: 109-21.
- Joshua JJ, Gilbert JL, Urban RM. Current concepts review: corrosion of metal orthopaedic implants. J Bone Joint Surg 1998; 80: 268 -82.
- Atkinson JR, Jobbins B. Properties of engineering materials for use in body, In: Dowson D, Wright V, Eds. Introduction to biomechanics of joint and joint replacement. London: Mechanical Engineering Publications 1981; pp. 141-5
- Jones DA. Principles and prevention of corrosion. USA: Macmillan Publishing Company 1992; 74-115.
- Dearnley PA. A brief review of test methodologies for surface engineered biomedical implant alloys. Surf Coat Technol 2005; 98: 483- 90.
- D. F. Williams, Annu. Rev. Mater. Sci., 6, 237 (1976).
- ASTM F 2129-01. Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implant Devices. In: Annual book of ASTM Standards, vol. 13.01, Conshohocken, PA: ASTM International (2003).
This information has been sourced, reviewed and adapted from materials provided by Fort Wayne Metals.
For more information on this source, please visit Fort Wayne Metals.