According to Wikipedia, a hydrocolloid is a substance, typically a gel, in which hydrophilic polymers are suspended in water. Hydrocolloids are often used medically in hydrocolloid dressings, which are impervious to bacteria and biodegradable. They also adhere to the skin without any adhesive.
As Wound Care Advisor points out, hydrocolloid dressings are best used with non-infected, necrotic or newly healed wounds. They are not effective on infected or weeping wounds. Hydrocolloid dressings stay in place for up to seven days without changing and are easy to apply and to remove. They may be used under venous compression. According to Wikipedia, hydrocolloid dressings are also used to treat acne and eczema.