Hmmm...I keep some in my working EMT kit which is normally stored in my car and they've held up just fine. Some of them will seem to harden but if you just break them up and release the liquid portion, they still get nice and cold.

I buy mine at the, don't tell anyone, 99 cent store and thru McMaster-Carr, depending on how low my stock is and how fast I need them.

John E