Sports midsole (E-TPU and microcellular PU)
What it is
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.
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Value proposition
Comparison of midsole technologies:
| Technology | Energy return | Relative weight | Price premium |
|---|---|---|---|
| Conventional EVA | ~50% | Low | Baseline |
| High-quality expanded EVA | ~55% | Low | +20–30% |
| HP microcellular PU | ~70% | Medium | +40–60% |
| E-TPU (Boost-type) | ~85% | Medium-high | +80–120% |
| Pebax-based (Nike ZoomX) | ~85% | Low | +100–150% |
Arguments for end consumer (amateur/professional athlete):
- Measurable performance: independent studies show 2 to 4% improvement in running economy with premium midsole technologies
- Reduced muscle fatigue: higher energy return means less muscular work for each stride
- Durability: sneakers with premium technologies maintain performance for 500 to 1,000 km against 300 to 500 km for conventional technologies
- Lower injury risk: better cushioning reduces stress on knees and joints
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