I have made a fair number of chemo caps and recently (thanks to Jan) found this yarn. It is cotton with a bit of elastic in it and PERFECT for chemo caps. One problem with some caps made of wool is they can be too warm. So alot depends on the time of year and climate the person lives in.
I recently made several for a friend who was in the hospital for a lengthy period and she said the cotton ones were the best because the hospital is usually kept quite warm.
Acrylic is a NONO because it holds the heat worse, unless it is mixed with a natural fiber.
Another consideration is washability and where the person you are making them for is in the disease process. For the early stages of chemo you want to make sure they are washable because of the nausea and vomiting that chemo can cause. You also need to chose a yarn that "grips", some of the softer yarns are just too slippery to stay on a bald head.
I have made two out of the yarn above and they worked great.