Recurrent and hard-to-heal diabetic foot ulcers (DFUs) represent a clinically challenging subgroup with delayed closure and frequent non-response to standard of care (SOC). Impaired local perfusion and superficial tissue hypoxia, commonly attributed to microcirculatory dysfunction, are proposed contributors to impaired wound-bed progression and prolonged healing trajectories in diabetes. Transcutaneous gaseous carbon dioxide (CO₂) therapy is a non-invasive adjunct intervention with a mechanistic rationale to modulate local microcirculation and tissue oxygenation; however, controlled clinical evidence in recurrent, hard-to-heal DFUs remains limited. This prospective, randomized, controlled, open-label, parallel-group clinical investigation compares transcutaneous CO₂ therapy plus SOC versus SOC alone over 4 weeks. The primary objective is to determine whether the proportion of completely healed DFUs at Week 4 differs between groups under a predefined healing confirmation procedure. Key secondary objectives include quantifying changes in superficial tissue oxygenation (StO₂) using hyperspectral imaging and assessing pain intensity (NPRS). Supportive outcomes include ulcer area reduction metrics and wound-bed appearance in unhealed.

Inclusion Criteria:

• diabetes mellitus type 1 or 2
• recurrent hard to heal DFU

Exclusion Criteria:

• KOPD stage IV
• CKD stage IV
• malignant diseases
• pregnancy
• infected ulcer at baseline
• inability to comply with visits