Advances in tissue-engineered skin substitutes
01/02/11 | Complex wounds, Skin integrity | Damanhuri M, Boyle J, Enoch,S
THE DEVELOPMENT OF SKIN SUBSTITUTES
Xenografts in the form of frog skin were fi rst used to provide wound coverage as early as 1,500 BC  and a product made from the skin of the bullfrog is still used in certain parts of the world such as Vietnam and South America . Water lizard skin was also used in Western culture in the 1600s . More recently, in the 20th century rabbit, dog and pig skin gained acceptance.
As the understanding of immunology and principles of critical care management has increased, patients with extensive burns are now surviving well beyond the acute phase . This has paved the way for the development of homografts in the form of cadaveric skin and autografts. Subsequently, evolving technologies led to the production of tissue- engineered skin substitutes (TESS).
The first transplantation of cultured epidermal autograft (CEA) took place in the 1980s , followed by the transplantation of cultured composite skin substitutes in the 1990s . Research and development in the field have since progressed to include synthetic materials and genetic modifications. The fi rst genetically modifi ed CEA for epidermolysis bullosa treatment was put to use in 2005 .
TYPES OF SKIN INJURY The skin is the largest organ of the body and performs a number of vital functions [Box 2]. The timely restoration of the protective and homeostatic functions of the skin is essential for successful clinical outcomes.
- Burn injuries, depending on depth, can be divided into:
- Superficial (also called epidermal)
- Superficial dermal (also called superficial partial thickness)
- Deep dermal (also called deep partial thickness)
- Full thickness.
Although this classification is mainly used in burns, the principle nevertheless applies to any tissue defect, eg an abrasion or split skin graft donor site is equivalent to a superficial burn and a grade IV pressure ulcer is equivalent to a full thickness burn. Ulcers of varying aetiologies range from superficial to full thickness.
Superficial (epidermal) injuries heal by re-epithelialisation from existing keratinocytes or keratinocyte stem cells and scarring in such injuries is minimal. If the injury extends to the superficial layer of the dermis it is possible that regeneration of the epidermis will occur without surgical intervention, provided there are a sufficient number of keratinocyte stem cells. If epidermal keratinocytes are lost, redevelopment may be performed by epithelial stem cells derived from hair follicles and/or sweat glands that are present in the deep layers of the skin (dermis).
However, if the injury extends to the deeper dermis (including the hypodermis, fat, muscle or bone), the injured surface is depleted of its keratinocytes, fibroblasts and any stem cells). Thus surgical excision of the involved tissue is frequently required along with reconstruction using STSG, which contains all the epidermis and superficial parts of the dermis, thereby transferring self-renewing keratinocyte stem cells to the recipient area.
Deep dermal and full-thickness skin injuries usually require surgical excision and will inevitably result in scarring due to the action of myofibroblasts, even after skin grafting .