Negative pressure wound therapy (NPWT) works by placing a sealed dressing over a wound and applying controlled suction, typically around -120 to -125 mmHg, to physically draw the wound edges together, pull out excess fluid, and trigger cellular responses that accelerate healing. The system consists of a porous filler material (foam or gauze) placed directly in the wound, an airtight adhesive film over the top, and tubing connected to a vacuum pump. What looks like a simple suction device actually sets off a cascade of mechanical and biological effects at every level, from the visible shrinking of the wound down to changes in how individual cells behave.
Macrodeformation: Shrinking the Wound
The most immediate and visible effect is macrodeformation. When the vacuum turns on, the porous filler compresses by roughly 80%. Because the filler is firmly attached to the wound edges, this compression physically pulls those edges inward, reducing the overall wound surface area. Think of it like a sponge being squeezed inside a bowl: as the sponge shrinks, anything connected to it gets drawn toward the center.
How quickly this works depends on the tissue surrounding the wound. Wounds in areas with loose, flexible skin (like the abdomen or thigh) close faster because the tissue can move more freely toward the center. Wounds on tighter surfaces like the scalp respond more slowly because the surrounding tissue resists being pulled inward.
Microdeformation: Stimulating Cells to Rebuild
Below the surface, the suction creates tiny mechanical forces that tug and stretch individual cells at the wound bed. This microdeformation is where much of NPWT’s healing power comes from. When cells experience these pulling forces, their internal scaffolding (the cytoskeleton) physically deforms, which triggers a chain of chemical signals inside the cell. Those signals alter gene activity and push the cell to divide and produce new tissue.
In animal studies, wounds treated with NPWT produced more than 60% more granulation tissue, the pink, grainy tissue that fills in a healing wound, compared to wounds covered with standard gauze dressings. Standard dressings also resulted in more cell death and less growth of fibroblasts, the cells responsible for building new connective tissue. The mechanical stretching from NPWT essentially tells cells to survive longer and multiply faster, while suppressing programmed cell death.
Fluid Removal and Edema Reduction
Chronic and acute wounds often sit in a pool of excess fluid, a mix of inflammatory chemicals, dead cells, and bacteria-laden exudate. This fluid creates swelling (edema) that compresses tiny blood vessels and starves the wound of oxygen and nutrients. NPWT continuously suctions this fluid away from the wound bed.
The mechanism works on two levels. The vacuum physically pulls interstitial fluid (fluid trapped between cells) out of the tissue and into the collection canister. At the same time, the compressive force of the sealed dressing pushes edema fluid away from the wound, functioning similarly to a compression garment. As swelling decreases, blood flow through the small vessels around the wound improves, delivering more oxygen and immune cells to the area. This is especially useful in patients with conditions that cause significant swelling, such as lymphedema or abdominal compartment syndrome.
How Blood Flow and New Vessels Respond
NPWT doesn’t just passively improve circulation by reducing swelling. It actively promotes the growth and maturation of new blood vessels, a process called angiogenesis. The mechanical deformation of tissue stimulates the release of growth factors that signal blood vessel formation.
Research on wound models shows that NPWT significantly increases blood flow perfusion and the maturity of new microvessels, particularly between days 7 and 10 of treatment. The therapy boosts production of a protein called angiopoietin-1, which acts as a maturation signal for new blood vessels. This protein binds to a receptor on the vessel walls that recruits stabilizing cells (pericytes) to wrap around the new vessels, reinforcing their walls and preventing them from leaking. At the same time, collagen is deposited around these new vessels to form a sturdy basement membrane. The result is not just more blood vessels, but sturdier, more functional ones that deliver blood effectively rather than collapsing or leaking.
Foam vs. Gauze Filler
The two main filler materials used in NPWT systems are open-cell foam and woven gauze, and they behave differently in practice. Foam is typically used at higher pressures (around -125 mmHg), while gauze works at lower pressures (-40 to -80 mmHg). Both achieve similar wound closure rates, but the tissue they produce differs.
Foam has tiny pores that allow granulation tissue to grow into the material itself. This creates a slightly irregular wound bed and, after skin grafting, tends to produce thicker, less pliable scar tissue. Ultrasound measurements have shown scar depth averaging 18 mm in foam-treated wounds compared to 7 mm in gauze-treated wounds. Gauze, with its denser weave, prevents tissue ingrowth and produces a smoother wound bed and more flexible final result.
In practice, foam tends to be the better choice for wounds producing large amounts of fluid or those with dead tissue that needs to separate. Gauze works well for wounds with poor blood supply, irregular shapes, tunneling, or undermined edges where the material needs to conform to complex geometry. One practical difference: gauze can safely stay in place for up to 72 hours, while foam should be changed more frequently to prevent tissue from growing too deeply into the pores.
Pressure Settings and Modes
The standard clinical pressure for NPWT is -120 to -125 mmHg, a level that maximizes wound edge contraction without causing excessive discomfort. Research measuring wound edge forces at pressures from -20 to -160 mmHg found that the pulling force on wound edges peaks at -120 mmHg, with no additional benefit at higher pressures.
That said, -125 mmHg causes pain for many patients. Lower pressures in the -75 to -80 mmHg range are sometimes used, particularly with gauze filler, and still produce effective results with better comfort. The vacuum can run continuously or cycle on and off (intermittent mode). Continuous mode is standard for most wounds, while intermittent cycling can further stimulate granulation tissue but is often less tolerable.
What the Treatment Feels Like
When the vacuum first engages, you feel a firm tightening sensation as the dressing compresses and pulls the wound edges inward. Most patients describe this as uncomfortable pressure rather than sharp pain, though sensitivity varies depending on wound location and depth. The sensation usually decreases within 15 to 30 minutes as the tissue adjusts.
Dressing changes are the most uncomfortable part of the process. Traditionally, dressings were changed every 2 to 3 days, but studies comparing 3-day and 7-day change intervals in open fracture wounds found no difference in infection rates or healing complications. Extending the interval to 7 days cut the average number of dressing changes from about 4.5 to 2, reducing both cost and discomfort. Treatment duration before a wound is ready for final closure (such as skin grafting) typically runs 3 to 4 weeks. In clinical studies, foam-treated wounds averaged about 26 days and gauze-treated wounds about 25 days before grafting.
The device itself is portable. Modern units are small enough to carry in a bag or strap to your body, which allows you to move around and, in many cases, continue treatment at home rather than remaining in the hospital.