Copper-containing intrauterine contraceptive devices are one of the most commonly used forms of contraceptive in the world. Copper IUDs have grown in popularity because they offer many benefits over hormonal IUDs and oral contraceptives including a longer half-life (12 years), high corrosion resistance, low failure rate, and high cost-effectiveness. Still, relatively little is known about the exact mechanism of action that prevents pregnancy with the device, but there are two leading hypotheses. First, the dissolution of copper ions may interact with the sperm, preventing their movement. Alternatively, the copper causes inflammation, which prevents the movement of the sperm. After Cu-IUDs are inserted, a layer of cuprous oxide covers the surface preventing further corrosion. Continued corrosion, or the dissolution of the material into the surrounding solution, is a property of metals that is allows the release of copper from IUDs. Different metals corrode at different rates, so understanding how copper reacts to the intrauterine environment is necessary to create an optimal device.
The paper “Corrosion Behavior and Mechanical Properties of Ultrafine-Grained Pure Copper with Potential as a Biomaterial” discusses the effects that grain size of copper has on mechanical properties of the material and release of copper ions. The research uses swaging to study how grain size can affect strength and ductility as well as the corrosive behavior of the material in physiological conditions.
Originally, course-grained copper was used, but this resulted in a high release of copper ions and corrosion failure. Modifications of the shape and size of course grained Cu-IUDs resulted in little improvement. Copper tubes instead of wire resulted in an unnecessarily high release of copper ions, reducing the life span of the device. The authors believe that an ultrafine-grained Cu-IUDs offers better corrosion properties than previous materials, and therefore is suited for future contraceptive devices.
In order to generate ultrafine-grained Cu, swaging, a severe plastic deformation technique, was utilized to create non-uniform ultrafine-grains and further swaging resulted in homogenous ultrafine-grains throughout the copper bar. These techniques create a high density of dislocations with high-angle grain boundaries. As we learned in class, the application of stress created dislocations allowing the metal to deform. In class we discussed how cold working could cause plastic deformation and alter the properties of the metal. In this paper, swaging was utilized to generate more grains and dislocations. The finer grain improves ductility, increases strength, increases workability, and increases failure resistance compared to course-grained Cu.
The researchers used commercial copper, which was rotary swaged to decrease the cross-sectional area of the bar. The image below shows optical micrographs of the copper at increasing small areas and finer grain sizes.
Next the ultimate tensile strength and yield strength were tested. As we learned in class, smaller grain size resulted in higher tensile strength and higher yield strength, as shown in the graph below. After a point, the tensile strength begins to decline which suggests there’s an optimum grain size to achieve maximum strength. Ductility – measured by elongation – decreased with decreased grain size.
Finer grains resulted in lower corrosion resistance and a higher corrosion rate because high dislocation density occurred due to the plastic deformation. The high density of grain boundaries in finer grained copper were associated with greater number of electrons and resulted in more immediate formation of a surface layer of Cu2O. They further investigated the corrosion resistance using electrochemical impedance spectroscopy. They found that the fine grain had a more homogenous nanocrystalline microstructure, which resulted in a more uniform distribution of grain boundaries, which created a more compact protective layer and reduced polarization. Larger grains were found to be less resistant to corrosion.
The thick oxide layer found in fine grain copper was found to facilitate the release of soluble copper ions necessary for infertility. Finer grains are seen to generate more uniform corrosion. In conclusion, the authors believe that fine-grain copper led to improved mechanical properties and more uniform corrosion, making it a promising material for Cu-IUDs. The paper is significant because optimizing Cu-IUDs can provide more effective and longer lasting contraceptive devices. Creating a more even dissolution of copper ions could minimize negative side effects by providing only the necessary amount of ions necessary to prevent sperms movement. Providing better contraceptive devices has the potential to improve quality of life by minimizing unplanned pregnancies and decreasing physician visits.
Source: Gholami M, Mhaede M, Pastorek F, Altenberger I, Hadzima B, et al. 2015. Corrosion Behavior and Mechanical Properties of Ultrafine-Grained Pure Copper with Potential as a Biomaterial. Advanced Engineering Materials. 18(4):615–23