A biomimetic approach to shielding from ionizing radiation: The case of melanized fungi

Abstract

Melanized fungi have been shown to thrive in environments with high radionuclide concentrations, which led to the association of the pigment melanin with the protection against ionizing radiation. Several hypotheses regarding the function of melanin have been proposed. Yet, the exact mechanism behind the protective property of melanin is unclear and poorly explored. A better understanding of the mechanisms that are involved in increasing the tolerance of the organisms to ionizing radiation could lead to technology transfer to human-related applications. Effective protection from radiation is essential for human space flight in general and human missions beyond Low Earth Orbit specifically. In this paper, we follow a biomimetic approach: we test two of current hypotheses and discuss how they could be applied to radiation shield designs. First we focus on the interaction of melanin with high energy electrons, which has been suspected to reduce the kinetic energy of the electrons through a cascade of collisions, thus providing physical shielding. Second, we investigate if the spatial arrangement of melanin, organized as a thin film or a collection of hollow micro-spheres, affects its shielding properties. To this end, we measured experimentally and by numerical simulations the attenuation of β-radiation as pass through solutions and suspensions of melanin and contrasted the values to the ones of cellulose, a substance with similar elemental composition. Further, we investigate the spatial arrangement hypothesis using Monte Carlo simulations. In agreement with the simulations, our experiments indicated that melanin does not provide improved shielding in comparison to cellulose from β-radiation. However, our simulations suggest a substantial effect of the spatial arrangement on the shielding performance of melanin, a pathway that could be transferred to the design of composite radiation shields.

Fig 1. Setup schematic and simulation geometry.

(A) Illustration of the shielding experiments. (B) Simulated geometry for the shielding experiments in Geant4.

Fig 2. Schematic of the simulated spatial arrangements.

(A) Illustrations of the film spatial arrangement, at three relative positions (h_r = 0, 0.5 and 1). The direction of the incoming radiation is indicated by the arrow. (B) Illustration of the lattice spatial arrangement for three configurations: sphere, packed sphere and ghost.

Fig 3. Shielding effectiveness of melanin mixtures.

(A) Shielding effectiveness of melanin mixtures in ammonia (Amm) and water, for synthetic melanin (Mel#1) and melanin of S. officinalis (Mel#2). For comparison, slurry of cellulose nano-crystals (CNC) was also tested. Experimental measurements are shown in circles and numerical simulation results in squares. (B) Comparison between experiment and simulation of the estimated probability density function of the deposited energy spectrum. Results for the S. officinalis and water suspension.

Fig 4. Comparison between different arrangements.

(A) Relative radiant fluence for different spatial arrangements for the melanin-water composite and the ⁹⁰Sr source. The relative film position is marked on the bottom x-axis and the equivalent radius for the lattice configurations at the top logarithmic x-axis. (B) Relative radiant fluence for the W-PE composite and the ⁹⁰Sr source. The CI intervals are smaller than the line width. (C) Relative radiant fluence for the W-PE composite and the 40 kVp X-ray source. The CI intervals are smaller than the line width.