We didn't think of using any other material beside Polypropylene because it is safe, light, readily available, and also used throughout the world in the medical industry. Stainless steel wasn't an option for us because of the weight associated with it. It actually never crossed our mind. Here is why:

Thermos - FooGoo Sippy Cup is a stainless steel sippy cup that holds 7 oz of fluid.

According to the Thermos website the weight of Thermos FooGoo Sippy Cup : .7 lbs

Weight of 7 oz of Fluid : .4 lbs

Total weight of a Stainless Steel Sippy Cup filled : 1.1 lbs.

Weight of a 6 month baby : 18 lbs

That is the equivalent of an 180 lb man or woman drinking out of a 7 lb cup with 4 lbs of fluid in it.

Just to give you a relative comparison. A 42 oz drink with cup weighs under 3 lbs.