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-particle-emitting radionuclide 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 [4]. 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.

The details of these radioactive solutions are shown in Additional file 1.

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 (EVAL™, 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 and used for wrapping perishable food materials to maintain freshness. The laminated film was developed to pack dried fish flakes so that they are not damaged by oxygen and high humidity. As rubber sheets, pieces of latex rubber gloves (Diamond Grip PLUS, 63-754, Ansell, Brussels, Belgium) and nitrile rubber gloves (STERLING 5070, HALYARD, Mechanicsville, VA, USA) were used. These gloves are currently used as PPE against COVID-19 [5]. 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 the plastic film or rubber (Fig. 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.

Schema of the experiments to evaluate the permeability of films and rubber. a Photograph indicating the configuration of a radioactive drop, sheet of film or rubber, piece of filter paper, and so on. b The schema of the configuration. 1: Film or rubber, 2: filtering paper, 3: radioactive drop, 4: aluminum ring (to keep the drop at the same position), 5: plastic plate (to avoid contamination by volatilized radionuclides), 6: water (to avoid vaporization of drop). c The schema of the section of the configuration