Very sturdy and durable. I was told the black ones are sturdier than the blue ones, so that is what I got. They are very well made and stay cold for much longer than normal. I after using one for 20 minutes before going to sleep, forgot to put it away, woke up 4 hours later and it was still cold enough to get some relief. Jell is thick enough that it doesn't all flow to one part of the bag, and yet malleable enough to conform to the shape you need when cold.
An excellent ice pack, and it cannot be beat for the price. In fact I was shocked when I saw the price compared to the ice packs we had purchased at Wagreens. It's very durable freezes quickly, stay cold for 30+ minutes- this is the type my physical therapist used and he swears by them, but said he paid much more for his through a medical supplies company. Wrap with thin towel to prevent 'cold burn'.