Mechanism for wound healing of pressure ulcer by hydrogen-dissolved water. We are predicting that ROS can lead to a pressure ulcer, and the causative process is shown by the left illustration. First of all, such diverse factors as bedridden syndrome, mechanical pressure and local ischemia produce ROS which causes necrosis and apoptosis in combination with other pathologic factors, potently resulting in wounds and tissue defects of pressure ulcer. On the other hand, the right illustration shows the healing mechanism. Oral intake of nano-bubble hydrogen water via drinking or tube-feeding passes by the mouth or esophagus, and it is absorbed by the epithelial cells of the small intestines. It is possible that hydrogen gas transpires in past from HW and inhaled by the lung. Then, the absorbed nano-bubble hydrogen migrates to cutis tissue through blood circulation and scavenges ROS abundantly generated in PU. Finally, this process results in collagen reconstruction of fibroblasts in the dermis and the proliferation of keratinocytes in the epidermis, and causes angiogenesis and remodeling for repairs in the defected tissue.