I think the term "resistant" in "helium-resistant" is too ambiguous. Helium is inert, meaning it rarely reacts with anything (only a few helium compounds are known). But some things are permeable to helium, including low density solids like polyproplene.

Here's the start of the paper at http://advances.sciencemag.org/content/3/11/eaao2710.full (Thanks to mtgx for the pointer):

"Metals do not absorb helium (He) from the environment. Large quantities of He may nevertheless be introduced into metals by nuclear reactions that produce α particles (He nuclei) (1, 2). Because it is insoluble in metals, any He forced into solution this way rapidly precipitates out into nanometer-scale “bubbles” (3). These bubbles nucleate as small He/vacancy clusters and remain approximately equiaxed as they grow into micrometer-sized voids (4–6). When they impinge on free surfaces, He-filled voids form blisters that release the He trapped inside them as they burst apart (7, 8)."

"We show that confinement within nanoscale layers markedly alters the life cycle of He precipitates. Rather than expanding continuously while remaining equiaxed, He precipitates confined within nanolayers spontaneously coalescence into elongated, He-filled channels."

I had to look up "equiaxed": "Having axes of approximately the same dimensions".