Sports midsole (E-TPU and microcellular PU)

The midsole is the intermediate layer of sports footwear soles — between the outsole (which touches the ground) and the insole/upper. It is the piece responsible for the cushioning, energy return and stability properties that differentiate a common sports sneaker from a high-performance one. Historically dominated by expanded EVA, the midsole underwent technological revolution in the past decade with the introduction of E-TPU (Expanded Thermoplastic Polyurethane) and advanced microcellular PU formulations.

E-TPU was launched by BASF under the Infinergy brand and popularized by Adidas with Boost technology in 2013. The material consists of small spheres (approximately 4 to 5 mm) of expanded TPU, sintered in a mold to form the final midsole. Unique properties include: very high energy return (around 85%), property maintenance at varying temperatures (−20°C to +40°C), superior durability over EVA. Adidas licensed the technology for years; today other manufacturers enter with similar or alternative technologies.

High-performance microcellular PU is an alternative — injected systems with specific formulations for high energy return. Technology used by brands like Nike (proprietary systems), New Balance (Fresh Foam X), Saucony (PWRRUN+) and others.