Aspen is fine. Heres a trick we use if we find some of ours have shedding problems. we either offer them a humidity hide(a container thats big enough for them to crawl into with either spaghnum moss or moist paper towel in it) OR spray the whole cage completely and cover it with a towel once he clears from the blue. Do you feed in the cage or out? All of yours are fed in the cage but we dont use aspen. A trick someone i know uses is she puts a piece of paper towel over the aspen and puts the mouse or rat there, wich reduces the chance of ingesting apen.