Enhancing Blood Circulation With Epsilon-Near-Zero (ENZ) Materials via the Far-Infrared Window of Human Skin.

Although the near-infrared (NIR) window is widely recognized for its deep tissue applications, the far-infrared (FIR) region, centered near 10 µm, remains largely underexplored. Human skin exhibits a relative absorption minimum in this range, suggesting the potential to deliver thermal energy more deeply while avoiding superficial overheating. Epsilon-near-zero (ENZ) materials provide this capability as their optical resonances confine thermal emission to specific wavelengths. Silicon carbide (SiC) exhibits an ENZ resonance near 10 µm, which aligns directly with the FIR skin window. Here, we report the first systematic investigation of wavelength-matched FIR therapy, comparing SiC with the conventional broadband emitter, graphite. A thermally controlled within-subject study of 25 volunteers (50 hands) simultaneously monitored real-time blood perfusion and cardiovascular parameters. After 20 min of irradiation, SiC increased cutaneous blood flow by an average of 30.2% relative to baseline, significantly exceeding the 18.6% increase observed with graphite. These enhancements occurred without changes in blood pressure or heart rate, and skin surface temperature remained below 36°C. These findings establish wavelength-matched FIR therapy as an effective and thermally safe approach for enhancing peripheral circulation without eliciting systemic effects.