11.2 Absorbent Dressings: Foams, Alginates, Hydrofibers
Key Takeaways
- Foam dressings handle moderate exudate and add cushioning; they are withheld on dry eschar that still needs hydration because they wick remaining moisture away.
- Alginates are seaweed-derived calcium/sodium-exchange fibers that gel, support hemostasis, fill dead space, and always need a secondary dressing; they are not for dry wounds.
- Hydrofibers are carboxymethylcellulose fibers that lock high volumes of exudate and tend to wick vertically, which helps tunnels and periwound skin compared with alginate's more lateral spread.
- Change frequency follows saturation, leakage, odor change, or a labeled wear-time ceiling—not a calendar day alone.
- Pack dead space loosely with a gelling filler; overpacking causes pressure ischemia, and a foam sheet alone does not fill a tunnel.
When drainage exceeds what a hydrating dressing can hold, the clinical task flips: capture exudate, protect periwound skin, fill dead space, and still keep the bed moist—not wet, not dry. Foams, alginates, and hydrofibers are the core absorbent family on CWCN-style items. Independent OpenExamPrep material for this section emphasizes chemistry only insofar as it predicts behavior: cushioning, hemostasis, lateral versus vertical wicking, and the rule that change frequency follows saturation, not the calendar.
Foam dressings
Foam dressings (polyurethane, often with a silicone wound-contact layer) absorb moderate exudate, provide thermal insulation, and add cushioning over bony prominences. Many have a waterproof backing that allows showering. Silicone contact layers reduce pain on removal and are preferred on fragile skin. Foam can be a primary dressing on a granulating, moderately draining surface or a secondary dressing over alginate or hydrofiber when the primary filler has gelled.
Use foam when the bed is already moist and drainage is moderate: many venous ulcers after compression is in place, granulating pressure injuries, and donor sites with serous leak. Border foams picture-frame the wound so adhesive sits on intact skin, not on the bed.
Do not put foam on dry eschar that needs hydration. Foam wicks remaining moisture away and can mummify the eschar. Dry stable heel eschar in a poorly perfused limb may be kept dry on purpose, but that is a vascular decision, not a reason to choose foam because it looks modern. Also avoid relying on foam alone in a deep tunnel: foam sheets do not pack dead space. Place a rope or strip filler first, then cover with foam if overflow is expected.
Foam is the wrong step-down when exudate is still heavy enough to strike through in hours; step up to a higher-capacity filler rather than stacking extra foam and hoping. Foam is also the wrong step-up when the bed has dried and needs water donation—go back to hydrogel, not to a thicker foam.
Alginates
Alginates are derived from seaweed (brown algae). Calcium alginate fibers exchange calcium for sodium in wound fluid, forming a moist gel. That ion exchange also supports hemostasis after debridement of a bleeding capillary bed—alginate is a classic post-debridement filler when oozing, not arterial spurting, is the problem. Arterial bleeding needs pressure and surgical control, not a dressing swap.
Alginate fills dead space as a rope or pad. Pack loosely; tight packing creates pressure ischemia in the wall of a tunnel. Alginate requires a secondary dressing (foam, gauze, or film depending on leak). As it gels, it may look yellow-brown; that is expected, not automatically infection. Residual dry fiber can be irrigated away at the next change. Do not pre-moisten alginate on the sterile field and then drop it into a dry wound—the product is designed to use wound fluid. If there is no exudate, you chose the wrong family.
Alginate is not for dry wounds: without exudate, fibers remain dry, adhere, and can leave foreign material. Do not use alginate on dry eschar, dry arterial ulcers, or dry full-thickness burns. If the wound is only slightly moist, the gel will not form.
Alginate holds fluid in a relatively lateral pattern: strikethrough can spread toward the periwound. That lateral spread is the contrast hydrofibers are built to improve.
Hydrofibers (carboxymethylcellulose)
Hydrofiber dressings are carboxymethylcellulose (CMC) soft fibers. They absorb large volumes and lock fluid as a cohesive gel. The exam distinction: hydrofibers tend to wick vertically (into the dressing thickness) with less lateral spread than alginate, which helps protect periwound skin in high-exudate wounds and in tunnels where you want fluid pulled up and out rather than sideways under intact skin.
Use hydrofiber ribbon in undermining and tunnels, and pads on highly exudative venous or lymphatic wounds, including under compression when the cover can handle the volume. Like alginate, hydrofiber needs a secondary dressing. Some products are silver-impregnated; treat those as antimicrobials with a planned step-down, not as forever primaries (see 11.3).
As exudate falls, a hydrofiber that remains dry and clings is a signal to step down to foam or a hydrating product. Leaving a high-capacity gelling fiber on a now-dry bed recreates the alginate-on-dry-wound error.
Frequency of change
Change when the dressing is saturated—strikethrough, loss of gel integrity, leakage onto intact skin, a new odor, or a manufacturer wear-time ceiling—not because the unit always changes on Tuesdays. Premature changes waste product and cool the bed. Delayed changes macerate and allow leakage. Document percent saturation and periwound status so the next clinician can adjust absorbency: step up from foam to hydrofiber, add a more absorbent secondary, or shorten the interval. Wear-time numbers on a box are ceilings, not prescriptions to leave a leaking dressing in place until that hour arrives.
| Dressing | Exudate | Special property | Not for |
|---|---|---|---|
| Foam | Moderate | Cushioning, insulation, optional silicone contact | Dry eschar needing hydration; packing tunnels alone |
| Alginate | Moderate to heavy | Seaweed; calcium/sodium exchange; hemostasis; fills space; secondary needed | Dry wounds; arterial hemorrhage |
| Hydrofiber (CMC) | High | Vertical wicking; cohesive gel; tunnels and undermining | Dry wounds; leftover dry fiber left unassessed |
Scenario. After sharp debridement of a sacral pressure injury, capillary bleeding continues and a 3 cm tunnel produces copious serous drainage. Calcium alginate rope provides hemostasis and fills dead space; a foam secondary captures overflow. Two weeks later the tunnel is cleaner, drainage remains high, and periwound skin is at risk of maceration: switching the filler to hydrofiber ribbon uses vertical wicking. Calendar-based daily changes despite a still-dry outer foam would be wrong; saturation and leakage drive the interval. A hydrogel in this wound would flood the periwound. Overpacking the tunnel until the patient reports pressure pain is also wrong: loosely fill, do not stuff.
Which statement correctly limits foam dressing use?
After conservative sharp debridement, a wound has capillary oozing and a cavity that must be filled. Which dressing property set best matches alginate?
Compared with alginate, hydrofiber (carboxymethylcellulose) is preferred in a high-exudate tunnel primarily because it:
A hydrofiber ribbon in a tunnel is due for change. The outer foam is dry, there is no leakage or new odor, and gel integrity is intact. Which action best matches saturation-based practice?