Skin Substitutes and ‘the next level’

Introduction

Structure and Function of the Skin

Skin, the body's largest organ, is incredibly complex. Functionally, there are two layers with a highly specialized and effective bonding mechanism. Numerous appendages traverse the skin, and a rich and reactive capillary network provides nutrient flow while controlling temperature. The epidermis, consisting of the strata basale, spinosum, granulosum, and corneum, provides a vapor and bacterial barrier. The dermis provides strength, mechanical resistance, and elasticity. The thin epidermal layer is constantly refreshing itself from its basal layer, with new keratinocytes undergoing terminal differentiation over approximately 4 weeks to anuclear keratin-filled cells that make up the stratum corneum, which provides much of the barrier function of the epidermis. The basal layer of the epidermis is firmly attached to the dermis by a complex bonding mechanism containing among others collagen types IV and VII. When this bond fails, serious morbidity results, as demonstrated by the disease processes of toxic epidermal necrolysis (TEN) ( Fig. 15.1 ) and epidermolysis bullosa.

Fig. 15.1, Patient with toxic epidermal necrolysis.

Consequences of Loss of Barrier Function

Loss of the epidermal barrier has serious adverse physiologic effects. Direct and evaporative fluid losses are immediately seen. If wounds are large, this quickly leads to dehydration and shock. Protein losses are also substantial, leading to loss of colloid oncotic pressure and secondary edema. Microorganisms have unimpeded access to the microcirculation with resulting systemic infection. Deep tissues become desiccated with secondary cell death and progression of wound depth.

Open wounds present a high risk of fluid loss, high inflammation, bacterial colonization, infection, and sepsis, which can lead to a poor prognosis in terms of prolonged healing times and scarring. Timely wound closure is therefore one of the key objectives in modern burn care.

An increasingly effective group of temporary and permanent wound coverings is available.

The objective of this chapter is to review the currently available strategies for temporary and (semi-) permanent skin substitutes in terms of origin of the material (tissue, biological, or synthetic material) and wound indication (partial-thickness vs full-thickness wounds).

Temporary Skin Substitutes and Dressings

Partial-thickness wounds remain confined to the dermal part of the skin. Usually, these wounds have a good healing potential because epidermal cells present in (remnants of) sebaceous glands, sweat glands, and hair follicles are available to close the wound.

In these wounds, the main demands to reach pain reduction and high-quality wound healing without scarring is to warrant undisturbed wound healing. A moist environment and protection against bacterial invasion are the most important qualities that need to be provided.

Wound dressings for partial-thickness wounds should:

▪
Provide a moist environment (migration of cells is more difficult if when there is a dry scab on the wound).
▪
Protect the wound from excessive fluid loss and bacterial invasion.
▪
Require a limited number of dressing changes (pain reduction).

Modern wound dressing are able to fulfil these requirements. Generally, membranous dressings such as hydrocolloids and hydrofibers fulfill these requirements better than topical antimicrobial creams such as silver sulfadiazine (SSD) cream. The main indication for temporary skin substitutes is partial-thickness wounds, of which donor sites are a special category, but essentially are similar wounds.

We can classify temporary skin substitute materials by their tissue origin into:

1.
Biological tissues, such as allograft, xenograft, and amnion (keratinocyte sheets and cells)
2.
Synthetic materials such as hydrocolloids and hydrofibers

Biological Tissues

Allograft

The first type of membranous wound coverage used was human allograft skin. Human allograft is generally used as a split-thickness graft after being procured from organ donors. When used in a viable fresh or cryopreserved state, it vascularizes and remains the “gold standard” of temporary wound closures. It can be refrigerated for up to 7 days, but it can be stored for extended periods when cryopreserved. It is also used in a nonviable state after preservation in glycerol or after lyophilization. Viable split-thickness allograft provides durable biologic cover until it is rejected by the host, usually within 3 or 4 weeks. Prolongation of allograft survival, through the use of antirejection drugs, has been advocated but is not generally practiced for fear that antirejection drugs will increase the risk of infection.

A frequently used application of glycerolized allograft skin is as a membranous dressing on partial-thickness burns, especially scald burns in children ( Fig. 15.2 ). Modern banking techniques and regulations warrant the safety and quality of banked skin. Allograft is also effectively used in combination with meshed autograft in patients with large burns, the interstices of the meshed graft being immediately closed by the overlying unexpanded allograft, possibly reducing metabolic stress and local wound inflammation.

Fig. 15.2, Glycerolized allograft on a partial-thickness scald burn.

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