Used shoes. Tuned for Running Bubble Tech Energy Return SwellFoam N2™ midsole SwellFoam N2™ is our supercritical foam midsole technology, ideal for running footwear. It’s partly made from sugar cane, which captures carbon from the atmosphere as it grows and reduces our reliance on traditional, fossil-based materials in production. Swell Foam N2™ uses a 50/50 cane/fossil composition, a 20% bio increase from the Swell Foam in our training shoe. The supercritical foaming process allows us to utilize a higher bio content whilst still maintaining the performance attributes needed for running.