If you want to avoid harming the environment with your 3D pen, then the plastic we recommend using is PLA, which is short for Poly Lactic Acid. A polyester derived from renewable biomass, typically from fermented plant starch such as corn, cassava, sugarcane or sugar beet pulp, this type of plastic is both biogradable and sustainable. To be specific, it typically takes around six to 12 months to break down, while for most plastics it takes hundreds of years.
The other type of plastic commonly used in 3D pens, ABS, is not so great for the environment. Short for Acrylonitrile butadiene styrene, ABS is petroleum-based and non-biodegradable. It's both bad for the environment and more toxic to humans than PLA.