Skingineering I: engineering porcine dermo-epidermal skin analogues for autologous transplantation in a large animal model

Combining bioengineered human skin with bioprinted cartilage for ear reconstruction

Microtia is a congenital disorder that manifests as a malformation of the external ear leading to psychosocial problems in affected children. Here, we present a tissue-engineered treatment approach based on a bioprinted autologous auricular cartilage construct (EarCartilage) combined with a bioengineered human pigmented and prevascularized dermo-epidermal skin substitute (EarSkin) tested in immunocompromised rats. We confirmed that human-engineered blood capillaries of EarSkin connected to the recipient's vasculature within 1 week, enabling rapid blood perfusion and epidermal maturation. Bioengineered EarSkin displayed a stratified epidermis containing mature keratinocytes and melanocytes. The latter resided within the basal layer of the epidermis and efficiently restored the skin color. Further, in vivo tests demonstrated favorable mechanical stability of EarCartilage along with enhanced extracellular matrix deposition. In conclusion, EarCartilage combined with EarSkin represents a novel approach for the treatment of microtia with the potential to circumvent existing limitations and improve the aesthetic outcome of microtia reconstruction.

Bioprinting and plastic compression of large pigmented and vascularized human dermo-epidermal skin substitutes by means of a new robotic platform

Extensive availability of engineered autologous dermo-epidermal skin substitutes (DESS) with functional and structural properties of normal human skin represents a goal for the treatment of large skin defects such as severe burns. Recently, a clinical phase I trial with this type of DESS was successfully completed, which included patients own keratinocytes and fibroblasts. Yet, two important features of natural skin were missing: pigmentation and vascularization. The first has important physiological and psychological implications for the patient, the second impacts survival and quality of the graft. Additionally, accurate reproduction of large amounts of patient’s skin in an automated way is essential for upscaling DESS production. Therefore, in the present study, we implemented a new robotic unit (called SkinFactory) for 3D bioprinting of pigmented and pre-vascularized DESS using normal human skin derived fibroblasts, blood- and lymphatic endothelial cells, keratinocytes, and melanocytes. We show the feasibility of our approach by demonstrating the viability of all the cells after printing in vitro, the integrity of the reconstituted capillary network in vivo after transplantation to immunodeficient rats and the anastomosis to the vascular plexus of the host. Our work has to be considered as a proof of concept in view of the implementation of an extended platform, which fully automatize the process of skin substitution: this would be a considerable improvement of the treatment of burn victims and patients with severe skin lesions based on patients own skin derived cells.

Biological evaluations of decellularized extracellular matrix collagen microparticles prepared based on plant enzymes and aqueous two-phase method

For patients with extensive full-thickness burns who do not have sufficient autologous split-thickness skin for skin grafts, the application of biological skin substitutes may be considered. The aim of this study was to find an optimal new type method for the production of a biovital skin substitute based on acellular dermal matrix (ADM) and preclinical evaluations. In this work, 25 methods of ADM production were assessed. The proposed methods are based on the use of the following enzymes: papain, Carica papaya lipase (CPL), and purification using a polymer/salt aqueous two-phase system. The obtained ADM samples were characterized via scanning electron microscopy (SEM), porosity measurement and water vapor transmission test. Results showed that the collagen bundles of ADM microparticles were intact and orderly. Through differential scanning calorimetry (DSC), thermo gravimetric analysis (TGA) and biocompatibility tests, the results indicated that the proportion of papain and CPL was the same and 5 h processing time are the optimum conditions for ADM preparation and the material showed good biocompatibility. Our results suggested that the potential of developing this kind of decellularization process to manufacture ADM scaffolds for clinical application.

Bioengineered Skin Substitutes: Advances and Future Trends

As the largest organ in the human body, the skin has the function of maintaining balance and protecting from external factors such as bacteria, chemicals, and temperature. If the wound does not heal in time after skin damage, it may cause infection or life-threatening complications. In particular, medical treatment of large skin defects caused by burns or trauma remains challenging. Therefore, human bioengineered skin substitutes represent an alternative approach to treat such injuries. Based on the chemical composition and scaffold material, skin substitutes can be classified into acellular or cellular grafts, as well as natural-based or synthetic skin substitutes. Further, they can be categorized as epidermal, dermal, and composite grafts, based on the skin component they contain. This review presents the common commercially available skin substitutes and their clinical use. Moreover, the choice of an appropriate hydrogel type to prepare cell-laden skin substitutes is discussed. Additionally, we present recent advances in the field of bioengineered human skin substitutes using three-dimensional (3D) bioprinting techniques. Finally, we discuss different skin substitute developments to meet different criteria for optimal wound healing.

A Cultured Autologous Dermo-epidermal Skin Substitute for Full-Thickness Skin Defects: A Phase I, Open, Prospective Clinical Trial in Children

BACKGROUND

The management of deep partial-thickness and full-thickness skin defects remains a significant challenge. Particularly with massive defects, the current standard treatment, split-thickness skin grafting, is fraught with donor-site limitations and unsatisfactory long-term outcomes. A novel, autologous, bioengineered skin substitute was developed to address this problem.

METHODS

To determine whether this skin substitute could safely provide permanent defect coverage, a phase I clinical trial was performed at the University Children's Hospital Zurich. Ten pediatric patients with acute or elective deep partial- or full-thickness skin defects were included. Skin grafts of 49 cm were bioengineered using autologous keratinocytes and fibroblasts isolated from a patient's small skin biopsy specimen (4 cm), incorporated in a collagen hydrogel.

RESULTS

Graft take, epithelialization, infection, adverse events, skin quality, and histology were analyzed. Median graft take at 21 days postoperatively was 78 percent (range, 0 to 100 percent). Healed skin substitutes were stable and skin quality was nearly normal. There were four cases of hematoma leading to partial graft loss. Histology at 3 months revealed a well-stratified epidermis and a dermal compartment comparable to native skin. Mean follow-up duration was 15 months.

CONCLUSIONS

In the first clinical application of this novel skin substitute, safe coverage of skin defects was achieved. Safety and efficacy phase II trials comparing the novel skin substitute to split-thickness skin grafts are ongoing.

CLINICAL QUESTION/LEVEL OF EVIDENCE

Therapeutic, IV.

Pig Model to Test Tissue-Engineered Skin

Tissue engineering of skin is a field with high research activities and major importance for wound healing, especially following burn injuries. Animal models enable to test tissue-engineered skin as well as different types of cells in a realistic setting. Although there are several challenges in working with pigs, it is a good model because of its similarity to the human skin and a comparable wound regeneration. Here, we explain our routinely used methods for using pig models to test tissue-engineered skin in burn injuries.

Induction of angiogenic and inflammation-associated dermal biomarkers following acute UVB exposure on bio-engineered pigmented dermo-epidermal skin substitutes in vivo

PURPOSE

Ultraviolet (UV) radiation adversely affects skin health at cellular and molecular levels. Hence, UV radiation can directly induce inflammatory responses in the dermis by inducing erythema, edema, inflammation, dermal fibroblasts alterations, and extracellular matrix modifications.

METHODS

Human keratinocytes, melanocytes, and fibroblasts were isolated from skin biopsies, cultured, and expanded in vitro. Fibroblasts were seeded into collagen type I hydrogels that were subsequently covered by keratinocytes and melanocytes. These pigmented dermo-epidermal skin substitutes (pigmDESS) were transplanted for 5 weeks onto full-thickness skin wounds on the back of immuno-incompetent rats, exposed to a single UVB dose of 250 mJ/cm² or unexposed and excised after 1 week. The effects onto the dermis were assessed regarding cell number, cell phenotype, and cell proliferation. Local inflammation by granulocytes (HIS48) or macrophages (CD11b, iNOS) was analyzed by immunohistochemistry staining.

RESULTS

We observed a significantly enhanced ingrowth rate of blood capillaries, but not of lymphatic capillaries at 1 week post-irradiation. Moreover, the enhanced vascularization of pigmDESS after UVB exposure was concomitant with a high infiltration of granulocytes and monocytes/macrophages to the dermal part of grafts. In addition, a heterogeneous expression of HIF-1α and TNFα was detected at this early phase after UVB exposure. In local cellular response examination, results only show a moderate cell proliferation in the dermis.

CONCLUSIONS

We were able to define early markers of UVB-induced effects in the dermis of pigmDESS. Overall, a single UVB dose induces temporary acute angiogenic and immune responses during the early post-irradiation phase in vivo.

Skin-Derived Stem Cells for Wound Treatment Using Cultured Epidermal Autografts: Clinical Applications and Challenges

The human skin fulfills important barrier, sensory, and immune functions-all of which contribute significantly to health and organism integrity. Widespread skin damage requires immediate treatment and coverage because massive skin loss fosters the invasion of pathogens, causes critical fluid loss, and may ultimately lead to death. Since the skin is a highly immunocompetent organ, autologous transplants are the only viable approach to permanently close a widespread skin wound. Despite the development of tissue-saving autologous transplantation techniques such as mesh and Meek grafts, treatment options for extensive skin damage remain severely limited. Yet, the skin is also a rich source of stem and progenitor cells. These cells promote wound healing under physiological conditions and are potential sources for tissue engineering approaches aiming to augment transplantable tissue by generating cultured epidermal autografts (CEAs). Here, we review autologous tissue engineering strategies as well as transplantation products based on skin-derived stem cells. We further provide an overview of clinical trial activities in the field and discuss relevant translational and clinical challenges associated with the use of these products.

Skin Tissue Engineering: Application of Adipose-Derived Stem Cells

Perception of the adipose tissue has changed dramatically over the last few decades. Identification of adipose-derived stem cells (ASCs) ultimately transformed paradigm of this tissue from a passive energy depot into a promising stem cell source with properties of self-renewal and multipotential differentiation. As compared to bone marrow-derived stem cells (BMSCs), ASCs are more easily accessible and their isolation yields higher amount of stem cells. Therefore, the ASCs are of high interest for stem cell-based therapies and skin tissue engineering. Currently, freshly isolated stromal vascular fraction (SVF), which may be used directly without any expansion, was also assessed to be highly effective in treating skin radiation injuries, burns, or nonhealing wounds such as diabetic ulcers. In this paper, we review the characteristics of SVF and ASCs and the efficacy of their treatment for skin injuries and disorders.

Protein nanocoatings on synthetic polymeric nanofibrous membranes designed as carriers for skin cells

Protein-coated resorbable synthetic polymeric nanofibrous membranes are promising for the fabrication of advanced skin substitutes. We fabricated electrospun polylactic acid and poly(lactide-co-glycolic acid) nanofibrous membranes and coated them with fibrin or collagen I. Fibronectin was attached to a fibrin or collagen nanocoating, in order further to enhance the cell adhesion and spreading. Fibrin regularly formed a coating around individual nanofibers in the membranes, and also formed a thin noncontinuous nanofibrous mesh on top of the membranes. Collagen also coated most of the fibers of the membrane and randomly created a soft gel on the membrane surface. Fibronectin predominantly adsorbed onto a thin fibrin mesh or a collagen gel, and formed a thin nanofibrous structure. Fibrin nanocoating greatly improved the attachment, spreading, and proliferation of human dermal fibroblasts, whereas collagen nanocoating had a positive influence on the behavior of human HaCaT keratinocytes. In addition, fibrin stimulated the fibroblasts to synthesize fibronectin and to deposit it as an extracellular matrix. Fibrin coating also showed a tendency to improve the ultimate tensile strength of the nanofibrous membranes. Fibronectin attached to fibrin or to a collagen coating further enhanced the adhesion, spreading, and proliferation of both cell types.

About ATMPs, SOPs and GMP: The Hurdles to Produce Novel Skin Grafts for Clinical Use

BACKGROUND

The treatment of severe full-thickness skin defects represents a significant and common clinical problem worldwide. A bio-engineered autologous skin substitute would significantly reduce the problems observed with today's gold standard.

METHODS

Within 15 years of research, the Tissue Biology Research Unit of the University Children's Hospital Zurich has developed autologous tissue-engineered skin grafts based on collagen type I hydrogels. Those products are considered as advanced therapy medicinal products (ATMPs) and are routinely produced for clinical trials in a clean room facility following the guidelines for good manufacturing practice (GMP). This article focuses on hurdles observed for the translation of ATMPs from research into the GMP environment and clinical application.

RESULTS AND CONCLUSION

Personalized medicine in the field of rare diseases has great potential. However, ATMPs are mainly developed and promoted by academia, hospitals, and small companies, which face many obstacles such as high financial burdens.

Tissue engineering of skin: human tonsil-derived mesenchymal cells can function as dermal fibroblasts

PURPOSE

It is unclear whether dermal fibroblasts are indispensable key players for tissue engineering of dermo-epidermal skin analogs. In this experimental study, we wanted to test the hypothesis that tonsil-derived mesenchymal cells can assume the role of dermal fibroblasts when culturing pigmented skin analogs for transplantation.

METHODS

Mesenchymal cells from excised tonsils and keratinocytes, melanocytes, and fibroblasts from skin biopsies were isolated, cultured, and expanded. Melanocytes and keratinocytes were seeded in a ratio of 1:5 onto collagen gels previously populated either with tonsil-derived mesenchymal cells or with autologous dermal fibroblasts. These laboratory engineered skin analogs were then transplanted onto full-thickness wounds of immuno-incompetent rats and analyzed after 3 weeks with regard to macroscopic and microscopic epidermal characteristics.

RESULTS

The skin analogs containing tonsil-derived mesenchymal cells showed the same macroscopic appearance as the ones containing dermal fibroblasts. Histologically, features of epidermal stratification, pigmentation, and cornification were identical to those of the controls assembled with autologous dermal fibroblasts. Transmission electron microscopy confirmed these findings.

CONCLUSION

These data suggest that human tonsil-derived mesenchymal cells can assume dermal fibroblast functions, indicating that possibly various types of mesenchymal cells can successfully be employed for "skingineering" purposes. This aspect may have clinical implications when sources for dermal fibroblasts are scarce.

Three types of dermal grafts in rats: the importance of mechanical property and structural design

Background

To determine how the mechanical property and micro structure affect tissue regeneration and angiogenesis, three types of scaffolds were studied. Acellular dermal matrices (ADM), produced from human skin by removing the epidermis and cells, has been widely used in wound healing because of its high mechanical strength. Collagen scaffolds (CS) incorporated with poly(glycolide-co-L-lactide) (PLGA) mesh forms a well-supported hybrid dermal equivalent poly(glycolide-co-L-lactide) mesh/collagen scaffolds (PMCS). We designed this scaffold to enhance the CS mechanical property. These three different dermal substitutes—ADM, CS and PMCSs are different in the tensile properties and microstructure.

Methods

Several basic physical characteristics of dermal substitutes were investigated in vitro. To characterize the angiogenesis and tissue regeneration, the materials were embedded subcutaneously in Sprague–Dawley (SD) rats. At weeks 1, 2, 4 and 8 post-surgery, the tissue specimens were harvested for histology, immunohistochemistry and real-time quantitative PCR (RT-qPCR).

Results

In vitro studies demonstrated ADM had a higher Young’s modulus (6.94 MPa) rather than CS (0.19 MPa) and PMCS (3.33 MPa) groups in the wet state. Compared with ADMs and CSs, PMCSs with three-dimensional porous structures resembling skin and moderate mechanical properties can promote tissue ingrowth more quickly after implantation. In addition, the vascularization of the PMCS group is more obvious than that of the other two groups. The incorporation of a PLGA knitted mesh in CSs can improve the mechanical properties with little influence on the three-dimensional porous microstructure. After implantation, PMCSs can resist the contraction and promote cell infiltration, neotissue formation and blood vessel ingrowth, especially from the mesh side. Although ADM has high mechanical strength, its vascularization is poor because the pore size is too small. In conclusion, the mechanical properties of scaffolds are important for maintaining the three-dimensional microarchitecture of constructs used to induce tissue regeneration and vascularization.

Conclusion

The results illustrated that tissue regeneration requires the proper pore size and an appropriate mechanical property like PMCS which could satisfy these conditions to sustain growth.

"Take" of a polymer-based autologous cultured composite "skin" on an integrated temporizing dermal matrix: proof of concept

This study aimed to investigate the ability of an autologous cultured composite skin (CCS) to close similar biodegradable temporizing matrix (BTM)-integrated wounds, and its effectiveness in healing fresh full-thickness wounds after the failure of cultured epithelial autograft in its two forms (sheets and suspensions) to epithelialize over an integrated polymer BTM. Using a porcine model, autologous split-skin grafts were harvested three of four dorsal 8 × 8 cm treatment sites. These three sites were subsequently converted to full-thickness wounds and BTMs were implanted. The grafts were used to produce autologous CCSs for each pig. These consisted of a 1 mm thick biodegradable polymer foam scaffold into which fibroblasts and keratinocytes harvested from the grafts were cocultured. At Day 28, on each animal, the autologous CCSs were applied to two of the integrated BTMs, an autologous split-skin graft was applied to the third integrated BTM, and one CCS was applied immediately into a fresh, "naked" (no BTM applied) wound. The CCSs were capable of generating a bilayer repair over the naked wound's fat base and BTM-integrated wounds, which consisted of dermal elements and a keratinized stratified squamous epidermis anchored with a basement membrane by day 7. The CCSs behaved in different ways: either as a delivery vehicle allowing similar development of a bilayer repair while the polymer foam was shed from the wound, or generating a bilayer repair with the foam scaffold being retained (composite "take"). These results conclude our porcine program and provide proof of concept that the integrated BTM can be closed with an autologous CCS. Once fully optimized, this may provide robust repair without resorting to the split-skin graft, important in those cases where unburned donor site is unavailable.

"Trooping the color": restoring the original donor skin color by addition of melanocytes to bioengineered skin analogs

PURPOSE

Autologous skin substitutes to cover large skin defects are used since several years. Melanocytes, although essential for solar protection and pigmentation of skin, are not yet systematically added to such substitutes. In this experimental study, we reconstructed melanocyte-containing dermo-epidermal skin substitutes from donor skins of different skin pigmentation types and studied them in an animal model. Features pertinent to skin color were analyzed and compared in both skin substitutes and original donor skin.

METHODS

Keratinocytes, melanocytes, and fibroblast were isolated, cultured, and expanded from skin biopsies of light- and dark-pigmented patients. For each donor, melanocytes and keratinocytes were seeded in different ratios (1:1, 1:5, 1:10) onto collagen gels previously populated with autologous fibroblasts. Skin substitutes were then transplanted onto full-thickness wounds of immuno-incompetent rats. After 8 weeks, macroscopic and microscopic analyses were conducted with regard to skin color and architecture.

RESULTS

Chromameter evaluation revealed that skin color of reconstructed light- and dark-pigmented skin was very similar to donor skin, independent of which melanocyte/keratinocyte ratio was added. Histological analyses of the skin analogs confirmed these findings.

CONCLUSION

These data suggest that adding autologous melanocytes to bioengineered dermo-epidermal skin analogs can sustainably restore the patients' native skin color.

Collagen hydrogels strengthened by biodegradable meshes are a basis for dermo-epidermal skin grafts intended to reconstitute human skin in a one-step surgical intervention

Extensive full-thickness skin loss, associated with deep burns or other traumata, represents a major clinical problem that is far from being solved. A promising approach to treat large skin defects is the use of tissue-engineered full-thickness skin analogues with nearly normal anatomy and function. In addition to excellent biological properties, such skin substitutes should exhibit optimal structural and mechanical features. This study aimed to test novel dermo-epidermal skin substitutes based on collagen type I hydrogels, physically strengthened by two types of polymeric net-like meshes. One mesh has already been used in clinical trials for treating inguinal hernia; the second one is new but consists of a FDA-approved polymer. Both meshes were integrated into collagen type I hydrogels and dermo-epidermal skin substitutes were generated. Skin substitutes were transplanted onto immuno-incompetent rats and analyzed after distinct time periods. The skin substitutes homogeneously developed into a well-stratified epidermis over the entire surface of the grafts. The epidermis deposited a continuous basement membrane and dermo-epidermal junction, displayed a well-defined basal cell layer, about 10 suprabasal strata and a stratum corneum. Additionally, the dermal component of the grafts was well vascularized.

Skingineering II: transplantation of large-scale laboratory-grown skin analogues in a new pig model

BACKGROUND

Tissue engineering of skin with near-normal anatomy is an intriguing novel strategy to attack the still unsolved problem of how to ideally cover massive full-thickness skin defects. After successful production of large, pig cell-derived skin analogues, we now aim at developing an appropriate large animal model for transplantation studies.

MATERIALS AND METHODS

In four adult Swiss pigs, full-thickness skin defects, measuring 7.5 × 7.5 cm, were surgically created and then shielded against the surrounding skin by a new, self-designed silicone chamber. In two animals each, Integra dermal regeneration templates or cultured autologous skin analogues, respectively, were applied onto the wound bed. A sophisticated shock-absorbing dressing was applied for the ensuing 3 weeks. Results were documented photographically and histologically.

RESULTS

All animals survived uneventfully. Integra healed in perfectly, while the dermo-epidermal skin analogues showed complete take of the dermal compartment but spots of missing epidermis. The chamber proved effective in precluding ("false positive") healing from the wound edges and the special dressing efficiently kept the operation site intact and clean for the planned 3 weeks.

CONCLUSION

We present a novel and valid pig model permitting both transplantation of large autologous, laboratory-engineered skin analogues and also keeping the site of intervention undisturbed for at least three postoperative weeks. Hence, the model will be used for experiments testing whether such large skin analogues can restore near-normal skin, particularly in the long term. If so, clinical application can be envisioned.