· The closed negative pressure wound therapy (NPWT) uses PVA (polyvinyl alcohol) or PU (polyurethane) sponge as an intermediate between the wound and the drainage tube by filling it in the wound formed after the defect, infection, and necrosis of the body's skin or soft tissue. Relying on the biological semi-permeable membrane, it can seal the open wound and make the central or special negative pressure device generate certain negative pressure to change traditional point drainage into omnidirectional drainage.