The NRG² was designed with a special purpose in mind: to be the engine of the racket! This is the primary meaning of NRG², the most powerful string in the range. Key Features: Specific Construction: More than 1200 microfilaments made of Elastyl (12% more elastic than standard polyamide). Impregnated with 400 polyurethane to generate power and comfort. Exclusive S.P.L Surface Treatment: Provides slip and effective snap back, so you don't have to re-string during play. Welcome to NRG²! String made in France Technologies: PU 400: 400% elasticity for great comfort and dynamic performance. Elastyl Fibres (100%): +12% elasticity compared to standard PA, providing enormous elasticity for maximum power. S.P.L Final Coating: Anti-abrasion coating for more resistance.