Don't know why the OP is getting such a hard time here.
I've got a plastic disk which I can heat up in the microwave. It stays warm all night. I would say it's much safer than a hot water bottle and more effective because it stays warm for much longer.
I also have an electoric blanket that goes under my sheet. Both the blanket and the disk are amazing for back pain.
Just because somebody has used hot water bottles without incident for many years, it doesn't prove that they are the safest option. Technology changes.