https://orcid.org/0000-0003-0870-1664
Abstract
Burn injuries are a severe form of skin damage with a significant risk of scarring and systemic sequelae. Approximately 11 million individuals worldwide suffer burn injuries annually, with 180,000 people dying due to their injuries. Wound healing is considered the main determinant for the survival of severe burns and remains a challenge. The surgical treatment of burn wounds entails debridement of necrotic tissue, and the wound is covered with autologous skin substitutes taken from healthy donor areas. Autologous skin transplantation is still considered to be the gold standard for wound repair. However, autologous skin grafts are not always possible, especially in cases with extensive burns and limited donor sites. Allografts from human cadaver skin and xenografts from pig skin may be used in these situations to cover the wounds temporarily. Alternatively, dermal analogs are used until permanent coverage with autologous skin grafts or artificial skins can be achieved, requiring staged procedures to prolong the healing times with the associated risks of local and systemic infection. Over the last few decades, the wound healing process through tissue-engineered skin substitutes has significantly enhanced as the advances in intensive care ensuring early survival have led to the need to repair large skin defects. The focus has shifted from survival to the quality of survival, necessitating accelerated wound repair. This special volume of JBCR is dedicated to the discoveries, developments, and applications leading the reader into the past, present, and future perspectives of skin tissue engineering in burn injuries.
THE PAST: BIRTH OF SKIN TISSUE ENGINEERING AND CONVENTIONAL TREATMENTS FOR BURNS
The year 1975 appears to be a milestone period in the history of skin tissue engineering.1,2 This took place before the Washington National Science Foundation bioengineering panel adopted the “tissue engineering” terminology, and other academics expanded upon it from 1987 to 1993.3–5
The pioneering research on the development of skin tissue engineering commenced 40 years ago (1975) through the efforts of two groups of American researchers who worked concurrently. The effective in vitro cultivation of human epidermal keratinocytes was stated by Rheinwald and Green in a study of so-called “tissue engineering of the skin epidermis.” Their research highlighted the aforementioned cell’s potential to be expanded for grafting through a minor skin biopsy.6,7 Yannas et al concurrently presented a study in 1975 that forested the ground for the emergence of an engineered dermal skin substitute, culminating in “tissue engineering of the dermis”.2,8,9 It took 6 years for both research teams to publicly demonstrate utilizing their tissue-engineered replacements in the clinic to treat severe and extensive burns, however, in distinct ways. The first conquest in grafting employing sheets of cultured epithelium on two patients with severe burns was revealed in research conducted by O’Connor et al.10,11 Following O’Connor’s study, the effective utilization of an artificial dermis in treating 10 patients with severe burn injuries was also revealed by Burke et al.12
THE PRESENT: COMMERCIALLY AVAILABLE TISSUE-ENGINEERED SKIN SUBSTITUTES
Several methods and technologies have been reported, including acellular skin substitutes, cellular allogeneic/autologous skin substitutes, and synthetic and xenogeneic dermal skin substitutes. Figure 1 depicts milestones in the evolution of tissue-engineered skin substitutes for burn treatments.2,13,14
History of skin tissue engineering in burn treatment—“Burn engineering.” CEA, Cultured Epidermal Autograft; MSCs, mesenchymal stem cells.
Acellular skin substitutes have been employed for superficially wounds and burns since the 1980s. These acellular skin substitutes are typically made up of a “dermis” of nylon mesh (Biobrane®) or collagen and an “epidermis” of silicon membrane (Integra™). Another significant progress was the development of a bilayered “artificial skin.” This acellular-based skin substitute is commercially available as Integra™, designed in the 1980s and introduced commercially in 1996 in the United States. Integra™ is the most extensively used artificial biological dermis substitute, with documented good cosmetic and functional results. Infection, on the other hand, is still the most commonly reported Integra™ complication. Other than Integra™, several other commercially available acellular skin substitutes, including Alloderm®, MatriDerm®, and BTM® (Biodegradable Temporizing Matrix), have also been introduced into the market.3,13–15 NovoSorb™ BTM, a synthetic wound dressing, is used to stimulate neodermis growth during the wound closure. BTM is approved for the management of infected wounds, burns, and traumas.16
Synthetic dermal replacements appear to be less extensively used since their emergence in the 1990s for burn therapy. Transcyte®, Dermagraft®, and Biobrane® are some commercial examples of such products. Transcyte® and Dermagraft® are currently off the market. Biobrane®, however, is still applied as a synthetic skin substitute because of its effectiveness in partial-thickness burns.1–3,13–17
Cellular allogeneic skin substitutes are mostly obtained by culturing foreskin fibroblast cells on a mesh or matrix that has been clinically tested. Dermagraft®, Apligraf®, and Graftskin® are a few commercially available examples. Dermagraft® is an allogeneic dermal replacement made from foreskin fibroblast cells combined with biodegradable mesh. Once implemented in the wounds, the cryopreserved fibroblasts at −80°C acquire viability and begin to create growth factors and extracellular matrix (ECM) components.3,13–17
Allogeneic skin replacements, both cellular and acellular, can cover wounds temporarily. In the case of severe burn wounds, the dermal scaffold affords an environment of tissue-guided regeneration; the epidermal replacement some weeks later is achieved with a traditional split skin graft cultured autologous cells or noncultured cell populations. Cellular autologous skin replacements come in two classifications: Cultured Epidermal Autograft (CEA) and Cultured Skin Substitutes (CSS).2,13–16
CEAs have been used in burn therapy since 1981. A small biopsy of healthy skin tissue can be used to grow keratinocytes, which are then expanded into sheets over a few weeks. Cuono reported satisfactory graft take in a full-thickness wound bed by combining cultured epithelial autografts with split-thickness allografts in the mid-1980s. However, they need a delivery vehicle or a supportive dressing because CEA is costly, difficult to handle, and its uptake is unpredictable. Since the dermal–epidermal interface of CEA has not yet fully developed, friction-induced blister development is another issue. In addition, this method has also reported several possible side effects, including scarring, contracture, and hyperkeratosis.2,3,14–16
CSS is a type of autologous graft that includes both epidermal and dermal layers. It offers a permanent covering, has a well-formed dermal–epidermal junction, and is simple to apply. CSS demonstrates clinical outcomes that are close to autograft skin tissue and decrease the necessity for donor skin autograft for wound treatments.18 CSS enables decreased morbidity and mortality in treating burns, chronic wounds, and dermal reconstruction.19 However, CSS costs more and takes a longer time to produce. The most promising autologous dermo-epidermal skin substitute known as PermaDerm™ was established in Cincinnati, Ohio, in the United States.2,3,13–17
Hydrogels are deemed to be safe and biocompatible for burn wound treatment. Hydrogels could be used in irregularly shaped and deep burn wounds. Hydrogels have recently been used as carriers for delivering therapeutics to wounded areas. Growth factors, antimicrobials, and antibiotics are some therapeutics that can be loaded into hydrogel carriers to improve burn wound healing.17,20
ReCell, a cell suspension spray obtained from the dermal–epidermal interface at the point of care, has gained popularity for various indications, including superficial second-degree burns, widely meshed donor sites, and donor sites. The data of the larger multicenter trials are emerging, indicating the efficacy and potential of this cell suspension spray technique. In 2019, this technology became commercially available and shortened patients’ recovery, minimizing pain and scarring while also allowing for faster healing.1,21
Self-Assembled Skin Substitute, a biological skin substitute from fibroblasts and keratinocytes, has entered the clinical arena with promising data. Currently, a large multicenter trial is underway to determine its efficacy.21 All the aforementioned skin substitutes have disadvantages, and no perfect or ideal skin substitute has yet to be developed.
THE FUTURE: ADVANCED APPROACHES IN TISSUE ENGINEERING
Developing skin substitutes and tissue engineering products for human patients is already a reality. Even while tissue-engineered skin substitutes have become a reality in effectively treating severe burns, the fact is that current skin replacements have therapeutic limitations. They all have the same issues: high costs, subpar skin microstructure, and erratic engraftment, particularly in full-thickness burns. The optimistic vision is to develop a dermo-epidermal substitute that vascularizes quickly, accommodates stratifying epidermal grafts optimally on a biodegradable matrix, and is easy to handle by the surgeon.14 The following are some of the most recent advances in tissue engineering, along with a brief description of each:
Stem Cell Therapy
Available skin substitutes are mostly made up of fibroblasts and keratinocytes and cannot thus induce differentiated structures, including hair and sweat glands. Consequently, utilizing the new cell types, eg, endothelial cells, mesenchymal stem cells, and Induced Pluripotent Stem Cells (iPSCs) in engineered skin substitutes, could have promising results.1 The capability of stem cells to self-renew and differentiate into various cell types makes them unique. These characteristics provide stem cells with the ability to regenerate and uniquely replace tissue. Employing co-cultured cells in conjunction with complex three-dimensional (3D) models has recently been introduced to create artificial tissues similar to their native counterparts.1
New Skin Substitutes
Full-thickness skin substitutes such as Tiscover™, DenovoDerm™, and DenovoSkin™ are currently being tested. DenovoSkin™ (CUTISS) is a bioengineered tailored autologous human skin that may be provided from a small patch of healthy skin substitute recognized as an Advanced Therapy Medicinal Product that can be produced from a small skin biopsy. It reduces the number of corrective procedures required in the future, especially in children. DenovoSkin™ is undergoing Phase IIb clinical trials in Switzerland.3,22–24 Currently, there are a variety of skin substitutes on the market. However, a promising new approach is emerging that utilizes biomaterials and stem cells/iPSCs to develop skin substitutes.
Full-Thickness Skin Bioprinting
Skin bioprinting is a relatively new technology with unique properties in skin tissue engineering. 3D bioprinting provides distinct benefits for developing clinically relevant skin constructs. Additionally, the skin has been rebuilt utilizing 3D bioprinting, which entails layer-by-layer deposition of cells on top of supporting materials over damaged tissues. In recent years, stem cell therapies combined with 3D bioprinting and biofabrication techniques have proven to be valuable research platforms in regenerative medicine and burn wound healing.25–27
Skin Spray
Other wound coverage options include in situ cell sprays and skin guns transporting autologous cells to the wounded areas to stimulate reepithelialization. They can be applied for any burn on any part of the body. ReCell, a cell suspension spray obtained from the dermal–epidermal interface at the point of care, has gained popularity for various indications, including superficial second-degree burns, widely meshed donor sites, and donor sites. The data of the larger multicenter trials are emerging, indicating the efficacy and potential of this cell suspension spray technique. In 2019, this technology became commercially available and shortened patients’ recovery, minimizing pain and scarring while also allowing for faster healing.1,28
In Situ Tissue-Engineered Products: Skin Biopens, and Handheld Bioprinters
Handheld bioprinters are advanced coaxial extrusion devices that could be used as 3D bioprinters. Handheld devices deliver cells within a bioink similarly to 3D bioprinters. O’Connell et al presented the prototype of the biopen to fabricate 3D tissue scaffolds in situ and deliver human stem cells for tissue regeneration.29 In 2018, the first handheld 3D bioprinting equipment prototype was unveiled. Hakimi et al successfully demonstrated printing mesenchymal stem cells to improve reepithelialization and neovascularization in porcine models of full-thickness burn.30
CONCLUSION
Recent biomedical advancements have enabled the application of innovative methodologies and new biomaterials to mimic the biological, structural, and functional complexity of native skin. These techniques range from cellular-scale therapies like the delivery of mesenchymal stem cells or growth factors to large-scale biofabrication techniques such as 3D bioprinting, both in the lab and at the point of care. In developing new stem cell-based therapies, concerns like availability, biocompatibility, immunological rejection, vascularization, reinnervation, and adnexal skin structures will have to be addressed. In addition, the pathways employed by new 3D bioprinted skin substitutes to deliver stem cells in the wound are being finessed. The field of stem cell and 3D bioprinting in burn wound healing is promising, and it has the potential to become the next standard of treatment for burn wounds in the future.
Funding: We received funding from the following sources: Canadian Institutes of Health Research (# 123336), CFI Leader’s Opportunity Fund: Project (#25407), National Institutes of Health (#2R01GM087285-05A1), Ontario Institute of Regenerative Medicine, and a generous donation from Toronto Hydro.
Supplement sponsorship: This article appears as part of the supplement “Skin Regeneration and Wound Healing in Burn Patients: Are We There Yet?,” sponsored by Mallinckrodt Pharmaceuticals.
Conflict of interest statement: All authors have no conflicts of interest to declare.
References
