Skin Substitutes

  • the main requirement in burn wound management is an economical, easy to apply, readily available dressing or method of coverage that will provide good pain relief, protect the wound from infection, promote healing, prevent heat and fluid loss, be elastic and non-antigenic and adhere well to the wound while waiting for spontaneous epithelialization of superficial partial thickness burns or for permanent coverage with autologous epithelium of deeper burn wounds.
  • Methods for handling burn wounds have changed in recent decades. Increasingly, aggressive surgical approach with early tangential excision and wound closure is being applied.
  • It is probably the most significant change in recent years leading to improvement in mortality rates of burn victims at a substantially lower cost
  • By shortening hospital stay, early wound closure reduces pain associated with local burn wound care, number of operative procedures and infective complications.
  • It also decreases the severity of hypertrophic scarring, joint contractures and stiffness, and promotes quicker rehabilitation
  • Irrespective of any other consideration, early healing is paramount for good aesthetic and functional recovery.
  • It has been clearly demonstrated that disruption of epidermal–mesenchymal communication due to a delay in epithelialization, increases the frequency of developing fibrotic conditions such as scar hypertrophy and contractures.
  • successful artificial skin or skinlike material must replace all of the functions of skin and, therefore, consist of a dermal portion and an epidermal portion.
  • True closure is achieved only with living autografts or isografts (identical twins). One exception, though, is human skin allografts in patients taking the usual dosages of immunosuppressants for renal transplantation. In such category of patients, skin allografts seem to survive indefinitely with minimal repopulation of skin allografts by autogenous keratinocytes (KC) and fibroblasts. In case of discontinuation of immunosuppression, the skin allograft does not reject acutely. It persists clinically and the allograft cells are destroyed and replaced slowly with autogenous cells

Classification

  1. Dressing (synthetic)
  2. Skin substitute (biosynthelic or allograft)
  3. Autograft

Skin substitutes may be

  1. permanent or temporary
  2. biological or biosynthetic
  3. dermal or epidermal or combined

Biological

Autologous

  1. SSG
  2. Limited in donor sites with major burns
  3. creation of additional donor site wounds equivalent to second degree burns thus further increasing the total body surface area
  4. CEA
  5. most important advantage of cultured keratinocyte allografts is the large surface area obtained from a relatively small biopsy of healthy skin from the patient.
  6. Deep second degree burns remain an application of choice for the cultured epithelia, as the presence of dermis limits retractions responsible for functional complications usually observed in third degree burns where dermis is absent
  7. at least 3 weeks is needed for growth of cultured epidermal sheets in the laboratory, thus delaying the coverage of wounds
  8. epidermal sheets need to be grafted on a clean wound bed because they are highly sensible to bacterial infection and toxicity of residual antiseptics;
  9. best applied to intact dermis - the regeneration of the dermal compartment underneath the epidermis is a lengthy process, and skin remains fragile for at least 3 years and usually blisters
  10. aesthetic aspect of the healed skin is less acceptable than the one obtained with a split-thickness graft.
  11. Storage and preservation of viable sheets have also been a major handicap
  12. Keratinocytes delivery system
  13. Many researches have been working on the development of alternative systems for the delivery of cultured autologous keratinocytes
  14. fibrin glue suspension
  15. fibrin glue sheets
  16. spray on cells

Allograft

  1. Cadaveric allograft - temporary
  2. Cryopreserved
  3. Glycerolised
  4. cultured allogeneic keratinocytes - temporary
  5. survival appears to be related to presence of dermis (prolongs survival)
  6. regarded as a wound cover (dressing) since they do not achieve wound closure
  7. lyophilized human dermis (Alloderm) - permanent
  8. After processing, the skin is reduced to a basement membrane and properly oriented dermal collagen matrix.
  9. Preserved by freeze-drying
  10. Result is an acellular human dermis that theoretically will not be rejected.
  11. Amnion - temporary
  12. efficiency of amniotic membrane in preserving healthy excised wound bed and maintaining low bacterial count in contaminated wounds parallels that of human skin allograft dressings
  13. Contrary to human skin allograft, it has a fragile structure and is technically more difficult to handle
  14. special appeal in developing countries particularly where religious barriers preclude the acceptance of bovine and porcine skin or cadaveric skin.
  15. greater risk of contamination from pathogenic bacteria on placentas from vaginal deliveries
  16. proven to be impervious to micro-organisms and is usually free from toxic material however, disease transmission remains a possibility as with any biological material.
  17. Unlike skin allografts, amniotic membranes do not get incorporated and vascularised
  18. Though amniotic membranes allow rapid epithelialization and early healing in superficial and intermediate depth dermal burns, in deep dermal burns the membrane is not adequate and usually disintegrates before healing occurs
  19. primarily used to cover debrided second degree burns until complete healing, can provide a useful cover for microskin grafts as well or an overlay of widely meshed autografts promoting early epithelialization and rapid wound healing

Xenograft

  1. Porcine skin is the most common source of xenograft because of its high similarity to human skin.
  2. Sterility is an essential concern with xenogeneic tissues transplanted on wounds. Ionizing radiation appears to be the most suitable method for a guarantee of this sterility and for application in mass production.
  3. irradiation coupled with freeze-drying seems to decrease the antigenic properties of the pigskin graft and to increase its potential to inhibit bacterial growth.
  4. pigskin is a well-suited temporary dressing for the coverage of second-degree burns, especially after early excision. It usually promotes scar-free healing, with an average healing period of about 10 days.
  5. In addition, pigskin provides a suitable overlay to cover widely meshed (1:8 to 1:12) autografts.
  6. Because freeze-drying and irradiation are expensive, a low-cost alternative preservation technique was successfully developed by using 98% glycerin as the antiseptic followed by storage at room temperature for 20-300 days

Biosynthetics

Cell Free matrices

  1. Integra
  2. Dermal regeneration template
  3. Bilaminar structure – cross-liked bovine collagen and shark glycosaminoglycan with a silicone membrane that provides a temporary epidermal function
  4. Pore size 70-200m to allow migration of patient’s own keratinocytes and fibroblasts
  5. Histology shows gradual dermal remodelling resembling papillary and reticular dermis but failing to show rete ridges.
  6. Graft take lower than SSG (80 vs 95%) but the same as allograft take
  7. Advantages
  8. Heals quicker
  9. Less scarring
  10. Better aesthetic result
  11. Reduced donor site morbidity – able to use thinner autografts. Use with CEA reported, and said to be more durable than if CEA used alone.
  12. Disadvantages
  13. High learning curve
  14. Expensive
  15. 2 stage
  1. Biobrane
  2. Bilaminate membrane – nylon mesh fabric bonded to a thin layer of silicone,
  3. Nylon mesh is coated with peptides derived from porcine type 1 collagen in order to aid adherence to the wound bed and fibrovascular ingrowth
  4. As the wound heals, Biobrane separates and can readily be peeled away
  5. Adherence is slightly less than that of allografts before day five, but after a further 72 hours it is greater.
  6. Recommended for use on donor sites and superficial partial-thickness burns within the first 6 hours
  7. Has been used for temporary cover for freshly excised full-thickness wounds
  8. Advantages
  9. The single application reduces pain and costs.
  10. adherence of Biobrane reduces the incidence of infection
  11. allows bathing of the patient
  12. adapts to any body surface
  13. Epithelialization is visible beneath Biobrane.

Cell containing matrices

  1. Transcyte
  2. No longer available
  3. Biobrane seeded with neonatal fibroblasts in order to improve healing properties
  4. Outer layer: synthetic epidermal layer is biocompatible and protects the wound surface from environmental insults. It is semi-permeable to allow fluid and gas exchange.
  5. The Inner layer: bio-engineered human dermal matrix, contains structural proteins (type I, III, and V collagen), provisional matrix proteins (fibronectin, tenascin, SPARC,), glycosaminoglycans (versican, decorin), and growth factors (TGF-B1, KGF, VEGF, IGF-1).
  6. Fibroblasts allowed to grow for 17 days producing fibronectin, type I collagen, proteoglycans and growth factors
  7. Then frozen to -70C which does not maintain metabolic activity of the fibroblasts, however, the tissue matrix and bound growth factors are left intact.
  8. Stored at -20C in specially designed cartridges (2 sheets/cartridge)
  9. recommended for use as a temporary wound covering for surgically excised full thickness and deep partial thickness thermal burn wounds prior to autograft placement.
  10. Autograft adherence as good with transcyte as with allograft
  11. Transcyte easier to remove than allograft and with less bleeding
  12. Heals quicker and less scarring
  13. No trials comparing Transcyte vs Biobrane
  1. Apligraf
  2. Most sophisticated and expensive skin substitute
  3. Outer layer: Neonatal allogeneic keratinocytes
  4. dermal layer: composed of human fibroblasts in a bovine Type I bovine collagen lattice. While matrix proteins and cytokines found in human skin are present in Apligraf, Apligraf does not contain Langerhans cells, melanocytes, macrophages, lymphocytes, blood vessels or hair follicles.
  5. Primary role for treatment of chronic ulcers – three times more effective than compression therapy alone
  6. Donor fibroblasts probably last longer than keratinocytes but will eventually be replaced. The matrix will remain
  1. Dermagraft
  2. Cryopreserved living dermal structure
  3. Cultivated neonatal allogeneic fibroblasts on a polymer scaffold (Dexon or Vicryl)
  4. Cells secrete growth factors and dermal matrix proteins
  5. Remain viable and metabolically active after implantation
  6. Facilitates healing by stimulating the ingrowth of fibrovascular tissue from the wound bed and reeithelialisation from the wound edges
  7. Like Apligraf, is marketed for healing diabetic ulcers and not for burns

The future

Cytokine/ Gene therapy

  • Local application of cytokines as proteins (transforming growth factor-TGFb, heparin binding epidermal growth factor-like growth factor-HB-EGF, etc.) has been shown to be ineffective and of little clinical value due to enzymes and proteases locally present in the wound and because of lack of adequate receptors
  • Large amounts of systemic IGF-I needed for the desired therapeutic effects result, however, in serious side-effects, such as hypoglycemia, mental status changes, edema, fatigue and headache. These adverse side-effects limit the therapeutic utility of IGF-I in the treatment of burns
  • Gene therapy is emerging as an effective therapeutic approach to improve clinical outcomes after thermal injury
  • Particle-mediated gene transfer in burnt skin is feasible and may provide a means of introducing biologic agents into injured tissue capable of enhancing bacterial clearance and improving wound healing.
  • Hurdles include
  • Ensuring therapeutic genes are expressed at the correct level for the right amount of time.
  • Ensuring that delivery vehicle that is taken up by cells