best yarn for chemo cap

[tarrentella]

i know there have been threads about this before but i can't seem to find them.

What is the best types of yarn to use for a chemo cap? or should i just find a pattern and stick to the yarn it says use? i know it needs to be soft but im worried about alergies and things too.

I want to make a cap for a lovely woman who is the mother of one of my students. she has recently found out that her breast cancer is back and is shortly going to be undergoing a course of chemo (her hair has only just grown back after last time) she is a supermum and i want to do somehting nice for her.