Artificial Skin: A Literary Review Wayne R. Fischer ME 597 Introduction to Solid Biomechanics Boise State University May 8th, 2003
Introduction • Our skin is a major organ of the body that acts as a barrier to pathogens and trauma. • Skin defects caused by burns, venous and diabetic ulcers, or acute injury occasionally induce life-threatening situations. • Thus, the need for a functional and cost-effective permanent skin substitute for burn victims has always been garnered.
U.S. Burn Statistics(May/June 1992 issue of the Journal of Burn Care & Rehabilitation) • Approximately 2.4 million burn injuries are reported per year in the United States. • Medical professionals treat approximately 650,000 of the injuries; 75,000 are hospitalized. Of those hospitalized, 20,000 have major burns involving at least 25% of their total body surface. • Between 8,000 and 12,000 of patients with burns die, and approximately one million will sustain substantial or permanent disabilities resulting from their burn injury. • Patients with major burns exceeding 60% of their total body surface area often do not survive since too much of the organ has been destroyed and cannot be permanentlyreplaced.
Hospital Costs • Burns are one of the most expensive • catastrophic injuries to treat. For example, a burn of 30% of total body area can cost as much as $200,000 in initial hospitalization costs and physicians fees. • The cost of waiting for your own skin • to grow can be more painful than the burn itself!
Background Information • Although attempts to cover wounds and treat severe • burns is cited as far back as 1500 B.C., it has only been in the past few centuries that a significant number of solutions have emerged. • The bulk of these solutions involve using skin grafts • from humans (allografts) or animals (xenografts), or using membranes fabricated from natural or synthetic polymers.
The best material for wound closure is the patient’s own skin; however autografting has several disadvantages(Schulz, 2000): • The donor site is a new wound. • Scarring and pigmentation changes occur. • Dermis is not replaced. • Donor site is a potential site for infection. • Donor site is not unlimited. • Extensive burns makes it impossible.
Cadaver Skin: Allograft as a Temporary Skin Substitute • The annual national requirement for cadaver skin is estimated to be only 3000 m2. • Yet only 14% to 19% of human skin needed is being recovered.
Xenografts • Xenografts, particularly porcine skin grafts, are • commercially available and are an effective means of short-term wound closure (Yannas, 1980). • A Xenograft is normally removed on the third or • fourth day of use before extensive adhesion onto the wound bed sets in, thereby necessitating its traumatic excision prior to drying and sloughing off (Yannas, 1980).
Synthetic Polymers (Yannas, 1980) • The use of synthetic polymers has not so far led to the solution of the problem of a skin substitute. • A high incidence of infection and a relatively low capacity for inducing vascularisation and epithelialisation are frequently reported. • However, useful insights into the requirements for a satisfactory skin replacement have been discovered through the use of synthetic polymers.
Purpose of this Review In the past thirty years the ability to apply engineering principles of materials science and biomechanics to designing tissues has emerged as a thriving and productive field; yet, the goal of making a cost-effective, viable, and permanent skin substitute remains elusive. The purpose of this literary review is to focus upon the development of artificial skin substitutes and propose further research.
Presentation Outlined • What is skin and its functions? • What are some of the design requirements for an artificial skin substitute? • Literary Review? • What should be researched and studied?
The Anatomy of Human Skin • Epidermis (5 layers) Keratinocytes provide protective properties. Melanocytes provide pigmentation. Langerhans’ cells help immune system. Merkel cells provide sensory receptors. • Dermis (2 layers) Collagen, glycoaminoglycans, elastine, ect. Fibroblasts are principal cellular constituent. Vascular structures, nerves, skin appendages. • Hypodermis (fatty layer) Adipose tissue plus connective tissue. Anchors skin to underlying tissues. Shook absorber and insulator.
Eight Functions of Human Skin • Protect underlying tissues from injury: mechanical, heat, cold, biological. • Prevent excess water loss. • Act as a temperature regulator. • Serve as a reservoir for food and water: adipose tissue • Assist in the process of excretion: H20, Salt, Urea, Lactic Acid. • Serve as a sense organ for cutaneous senses: pain, heat, cold, pressure, touch. • Prevent entrance of foreign bodies: microorganisms. • Serve as a seat of origin for Vitamin D.
Phases of Wound Healing • Vascular Response • Blood coagulation • Inflammation • Formation of new tissue • Epithelialisation • Contraction & Remodeling
Given the structural, functional, and wound healing constraints, what are the minimum design requirements for a viable artificial skin substitute?
General Design Properties • Essential Design Properties • "The dermal replacement should provide both the information necessary to control the inflammatory and contractile processes and also the information necessary to evoke ordered recreation of autologous tissue in the form of a neodermis" (Schulz, 2000). • "The initial replacement material should provide immediate physiologic wound closure and be eliminated once it has provided sufficient information for reconstitution of neodermis" (Schulz). • It should protect the wound by providing a barrier to the outside (Beele, 2002) • It should control water evaporation and protein and electrolyte loss (Beele) • It should limit excessive heat loss (Beele) • It should decrease pain and allow early mobilization (Beele) • It should provide an environment for accelerated wound healing (Beele) • The risk of infection must be taken into account (Beele)
More General Design Properties • Physical Characteristics • It should be easy to manipulate the product, i.e. easy to place and dress the skin substitute effectively (Beele) • It should improve the cosmetic appearance of the scar (Beele) • Availability • It should be readily available off the shelf and custom made. • Cost • Cost should not preclude the use of the device.
Schematic Representation of Specific Mechanical Problems that Should Not Arise (Yannas, 1985).
Specific Physiochemical and Mechanical Problems to Overcome (Yannas, 1985). • Skin graft does not displace air pockets efficiently from graft-woundbed interface. • c) Shear stress causes buckling of graft, rupture of graft woundbed bond and formation of air pockets. • e) Excessively high moisture flux rate through graft causes dehydration and development of shrinkage stresses at edges and peeling.
Specific Physiochemical and Mechanical Problems to Overcome (Yannas, 1985). • Flexural rigidity of graft is excessive; graft does not deform sufficiently under its own weight to make contact with depressions in woundbed surface, thus air pockets form. • Peeling force lifts graft away from woundbed. • f) Very low moisture flux causes fluid accumulation at graft-woundbed interface and peeling.
Literary Review Up to 1990’s (Beele, 2002) Antiquity: Indian description of using autologous soft tissue flaps. Greeks used dressings for skin wounds. Renaissance: Amboise-Pare provide wound healing foundation. 1850’s: Reverdin and Thiersch use autologous skin grafts. 1914: Kreibich was the first person to cultivate keratinocytes in vitro. 1948: Medawar autotransplanted keratinocytes. 1960’s: Yannas and Burke begin their work using materials science and mechanics. 1975: Rheinwald & Green describe a technique to cultivate human keratinocytes. 1980’s: Yannas and Burke describe a bilaminate collagen-glycosaminoglycan matrix with a silicon surface. After take of the matix. The silicon surface is removed and can be replaced with autologous cultured epidermal cells. 1981: Bell constructs the first living skin equivalent with collagen fibroblast gel with keratinocytes cultured on top of contracted gel. 1983: Helton used cultured allografts in burn patients 1985: Boyce and Ham introduce an alternative culturing method. 1989: Possible to cryo-preserve keratinocyte sheets.
Literary Review: 1990’s to Present (Chart in British 2002 Journal of Plastic Surgery)
Observations from designing dermal replacements (Schulz, 2000) • The thicker the dermal layer of a split-thickness skin graft, the less the graft contracts. • Partial-thickness wounds with superficial dermal loss heal with less hypertrophic scarring. • Full-thickness skin grafts contract minimally. • The length of illness in burn cases is essentially restricted to the length of time the burn wound is open. • Full-thickness dermal injuries heal by contraction and hypertonic scarring, producing subepithelial scar tissue that is nothing like the original dermis.
Further Research (Buras, 1989) • The actual biological elements and events being critically tested in mechanical studies are only guessed at, and analysis can rarely go beyond the science of mechanics. • There are promising possibilities: • Pulsed ultrasound techniques may soon provide accurate imaging of skin structures as well as measurements of blood flow in the skin. • The multifrequency shear wave method may be able to resolve mechanical properties of the epidermal tissues discretely.
Research topics of Dr. Yannas at Dept. of Engineering, MIT • Study the mechanical behavior of artificial skin as a function of processing variables. • Study the surface tension of artificial skin. • Study the stress relaxation rate of artificial skin in standardized solutions of tissue enzymes. • Study the design of novel processes for the inexpensive and reproducible fabrication of artificial skin. • Study the pore structure of artificial skin by scanning electron microscopy. • Study the moisture permeability of artificial skin.
My Questions for Future Research How does the skin transform and grow naturally on a biochemical and physiologic level? How can these natural transformations be combined with concepts from materials science and biomechanics in order to develop and design a cost effective and viable skin substitute? Which designs already incorporate natural growth components with concepts from materials science and biomechanics? How can these designs be enhanced or re-deigned using the concepts within the domain of materials science and biomechanics?
Some References And we’re up and walking again! Beele, H. Artificial skin: Past, present and future. The International Journal of Artificial Organs. 25(3): 163-173, 2002. Jones, I., Currie, L., Martin, R. A guide to biological skin substitutes. British Journal of Plastic Surgery. 55: 185-193, 2002. Schulz III, J.T., Tompkins, R.G., Burke, J.F. Artificial Skin. Annu. Rev. Med. 51: 231-244, 2000. Yannas, I.V. Artificial Skin and Dermal Equivalents. In The Biomedical Engineering Handbook, ed. J. D. Bronzino, pp. 2025-2038. Boca Raton: CRC Press, 1995.