The following radionuclide solutions in addition to [211At]NaAt solution were used to evaluate the permeability of plastic films and rubber sheets: [225Ac]Ac(NO3)3, [125I]NaI, [111In]InCl3, [201Tl]TlCl, and [99mTc]NaTcO4.
225Ac is also a promising alpha emitter for TAT, and this radionuclide acts as a trivalent cation in a solution. 125I is a halogen that acts as an anion in a solution. 111In is a photon emitter that acts as a trivalent cation in a solution. 201Tl is a photon emitter with a main energy of 70.3 keV, which is similar to that of characteristic X-rays emitted from 211At [3]. 201Tl acts as a cation in a solution. 99mTc is a photon emitter that acts as an anion in the form of [99mTc]TcO4- in a solution. [211At]NaAt was supplied by the RIKEN Institute (Wako, Japan). [225Ac]Ac(NO3)3 was purchased from Global Morpho Pharma (La Chapelle-sur-Erdre, France). [125I]NaI was purchased from PerkinElmer (Waltham, MA, USA). [111In]InCl3 and [201Tl]TlCl were purchased from Nihon Medi-Physics (Tokyo, Japan). [99mTc]NaTcO4 was purchased from FUJIFILM Toyama Chemical (Tokyo, Japan).
Four types of plastic films and two types of rubber gloves were tested. Polyethylene (30 µm), polyvinylidene chloride (11 µm), and polyvinyl chloride (8 µm) and laminated films of polypropylene, ethylene vinyl alcohol copolymer (EVALTM, Kuraray, Tokyo, Japan), and polyethylene (104 µm) were used as plastic films. The numbers in parentheses are the film thicknesses. The first three plastic films are commercially available to wrap perishable food materials and keep them fresh. The laminated film was developed to pack dried bonito flakes so that they are not damaged by oxygen and high humidity. As rubber sheets, pieces of latex and nitrile rubber gloves were used. These gloves are currently used as PPE against COVID-19[4]. The thickness of each glove was more than 130 µm for latex and 70 µm for nitrile.
A three centimeter square piece of filter paper was covered by a sheet of plastic film or a piece of rubber cut out from a rubber glove. Fifty microliters of radionuclide solution whose radioactivity was adjusted to 100 kBq was dropped on a plastic film or rubber (Figure 1). The plastic film or rubber was covered by a plastic Petri dish to minimize the evaporation of the radioactive solution. Each piece of filter paper under the plastic film or rubber was picked up 3.5 h later. This interval is half the half-life of 211At. These pieces of filter paper were placed on imaging plates (FUJIFILM, Tokyo, Japan) for 5 min and approximately 15 h. The imaging plates were scanned with an imaging plate reader (FLA-7000; FUJIFILM, Tokyo, Japan). The acquired images were analyzed using the ImageJ software (U.S. National Institutes of Health, Bethesda, MD, USA). The radioactivity of these pieces of filter paper was also measured using a gamma counter (2480 Wizard2; PerkinElmer, Waltham, MA, USA). These experiments were repeated three times for each radionuclide.