As the name implies, alginate dressings are made of "algae" or seaweed and have been available since the mid-80s in North America. They're biodegradable and most commonly used for wounds with a heavy seepage or exudate.
Aginate dressings are highly absorbent and can absorb 15-20x their own weight. They've available in a variety of formats including ropes, ribbons and flat sheets. Flat sheets are most commonly used for surface/skin wounds and may be self-adhesive or require a secondary support such as a gauze wrap or compression wrap to hold the alginate dressing in place.