Radiography is a medical technology that transmits X-rays through the human body and it uses the difference in the density of substances in the human body to image anatomical structures1. The penetration of X-rays is limited when the density of the tissue is high, whereas tissues with relatively low density can be easily penetrated2. Therefore, the higher the density of the shield, the more advantageous it can be for radiation protection.
Artificial radiations, such as X-rays, have been developed for medical and industrial technologies. However, owing to the increasing use of medical devices, the general population and medical and industry workers are subjected to increased radiation exposure3. Therefore, active radiation defense technology is required to reduce exposure. Furthermore, the use of mobile X-ray devices has increased because of the recent COVID-19 pandemic4. The International Commission on Radiological Protection (ICRP) specifies that radiation used in the medical field should be used for the benefit of patients and should be optimized5.
Radiation shield technology used in medical institutions is associated with time and distance6. Lead plates or sheets made of lead powder and a polymer, such as rubber, are generally used as X-ray shields7. However, owing to its toxicity, lead poses problems with lead poisoning and disposal. Therefore, shields used in medical institutions are increasingly being manufactured with lead-free materials8. However, most medical devices, supplies, and facilities that use radiation still use lead as a shielding material. Therefore, to overcome this issue, the use of cheap and eco-friendly lead-free materials with shielding performance equivalent to lead should be expanded.
Materials such as tungsten, bismuth oxide, barium sulfate, and boron are typically used as alternatives to lead9. Considering the shielding performance, tungsten is the most useful eco-friendly shielding material. In general, lead replacement shielding materials should be non-toxic and have flexibility and processability. In addition, the materials should be proposed as a material having an excellent affinity with the polymer to be mixed or as a material capable of reducing the weight when manufacturing a shield. The types of shields that can be produced with these shielding materials include plate, fiber, and sheet, and pressing or injection molding into the desired shape is possible depending on the process technology.
A fiber-type shield is woven from a yarn impregnated with the shielding material. However, the shielding performance is limited by the pinholes generated between the yarn during the weaving process. Therefore, fiber-based shields are used primarily to protect against secondary (or scattered) radiation10. A sheet-shaped shield is manufactured by mixing a polymer and shielding material, which is compressed to the required thickness. The most important element of this process is the uniform dispersion of the shielding material. The shielding material dispersion process affects the reproducibility of the shielding performance and is difficult to apply to mass production without standardization of the production process11.
The single component plate-shaped shield comprises 100% of the shielding material and is manufactured through the rolling process. When tungsten is selected as the shielding material during single component plate manufacturing, the production processability is low because of its high melting point12. Therefore, the choice of shielding material for single component plate shields is limited. In recent years, plate flexibility has been obtained by using composite materials and the plate manufactured in this manner has been widely used as a material for applications such as shielding walls13. Materials for other applications of shields, such as blocks, syringe shields, and apertures used in medical institutions, are manufactured by injection molding by mixing a shielding material and polymer14. The miscibility of the metal particles with the polymer is a crucial factor affecting processability and shielding performance of composite materials.
An apron is a representative medical institution X-ray shield, which is manufactured in the form of clothing and worn by medical staff and workers. Therefore, it must be manufactured in a thin and light form to ensure unconstrained mobility of the wearer. The X-ray shielding apron currently available places a physical burden on the wearer because it weighs 2.85–3.15 kg for a product with a lead equivalent of 0.25 mmPb15. The weight reduction of the shielding garment may be limited because it is directly related to the density and mass of the shielding material. Although the mobility of the wearer can be improved by reducing the thickness of the sheet, this would reduce the shielding performance. A method to improve the shielding performance is the controlled dispersion of the shielding material. The attenuation of the incident X-ray energy occurs by its interaction with shielding material particles, which can be increased by allowing the radiation to interact with a greater number of particles16. Therefore, the particle dispersion technology of the shielding material is the most crucial factor in terms of shielding performance, weight reduction, and reproducibility of shielding performance, especially for the thin layer composite structures.
Several types of unusual composite structures are found in nature. The wings of the morpho butterfly are composed of micro-sized multi-layered thin films and show a regular arrangement. Due to the unique surface structure, only blue light is reflected and the butterfly wings appear blue17. The wrinkles are folded on the left and right pillars with an interval of approximately 700 nm and a height of 2 μm, and the interval between the upper and lower wrinkles is approximately 200 nm18. In this study, the structure of butterfly wings was used as a model for the dispersion of particles in an X-ray shield. The pattern was completed by repeated overlapping, similar to the tile used for the roof of a Korean building. In addition, to maintain the reproducibility of this pattern, the electrospinning method was used to apply the same amount of shielding material at the same location. The shielding material was eco-friendly tungsten powder particles. Even though tungsten has an atomic number of 74 and has a density higher than that of lead (19.25 g cm−3), a weight reduction is possible by reducing the thickness of the shield19. Therefore, this study aims to evaluate the shielding performance of a shield with a pattern similar to butterfly wings based on these materials.
In addition, this study aims to improve the shielding performance by making the radiation shielding paper as thin as possible so that it can be used in a multi-layered structure, and inducing interaction with particle radiation. The shielding paper produced can be used in a flexible, laminated structure because of its reduced thickness. Thus, this study reports a novel medical radiation technology for manufacturing thin shielding papers utilizing the morpho butterfly wing structure. Moreover, the proposed method can be used to mass produce the lightweight radiation-shielding paper and improve the safety of medical workers.
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