Hi yoshi,
This is a rayon or cellulosic “semi-synthetic” material which can be derived from many types of cellulose and is spun into fabric. Their site says it’s “viscous” which means “in between a solid and a liquid” but it’s actually viscose which is a “cellulosic solution”. Bamboo, Beech, Eucalyptus, Aloe Vera, and many others are examples of different sources of cellulose which can be made into different forms of rayon. You can read more about viscose rayon fabrics in post #7 here.
If it comes from Belgium it may be from DesleeClama but of course I don’t know where they buy their ticks from.
I’m looking forward to your pictures and feedback … and congratulations on your new mattress 
Phoenix