Peel Test to Assess the Adhesion Strength of the Dermal-Epidermal Junction in Tissue-Engineered Skin

The role of interface geometry and appendages on the mesoscale mechanics of the skin

The skin is the largest organ in the human body and serves various functions, including mechanical protection and mechanosensation. Yet, even though skin's biomechanics are attributed to two main layers-epidermis and dermis-computational models have often treated this tissue as a thin homogeneous material or, when considering multiple layers, have ignored the most prominent heterogeneities of skin seen at the mesoscale. Here, we create finite element models of representative volume elements (RVEs) of skin, including the three-dimensional variation of the interface between the epidermis and dermis as well as considering the presence of hair follicles. The sinusoidal interface, which approximates the anatomical features known as Rete ridges, does not affect the homogenized mechanical response of the RVE but contributes to stress concentration, particularly at the valleys of the Rete ridges. The stress profile is three-dimensional due to the skin's anisotropy, leading to high-stress bands connecting the valleys of the Rete ridges through one type of saddle point. The peaks of the Rete ridges and the other class of saddle points of the sinusoidal surface form a second set of low-stress bands under equi-biaxial loading. Another prominent feature of the heterogeneous stress pattern is a switch in the stress jump across the interface, which becomes lower with respect to the flat interface at increasing deformations. These features are seen in both tension and shear loading. The RVE with the hair follicle showed strains concentrating at the epidermis adjacent to the hair follicle, the epithelial tissue surrounding the hair right below the epidermis, and the bulb or base region of the hair follicle. The regions of strain concentration near the hair follicle in equi-biaxial and shear loading align with the presence of distinct mechanoreceptors in the skin, except for the bulb or base region. This study highlights the importance of skin heterogeneities, particularly its potential mechanophysiological role in the sense of touch and the prevention of skin delamination.

Characterization of the skin-to-bone mechanical interaction on porcine scalp: A combined experimental and computational approach

Fasciae are soft tissues permitting a large but finite sliding between organs, but also between skin and its underlying elements. The contribution of fasciae has been seldomly reported in the literature, and is usually neglected or overly simplified within simulations. In the present contribution, we propose to use peeling tests in order to quantify the skin-to-bone interaction associated with a simple computational approach based on a geometrical modeling of the skin-to-bone interface. To this aim, a new experimental set up combined with a computational model to characterize the skin-to-bone interaction were proposed. The current work is devoted to the porcine scalp complex since it constitutes a common mechanical surrogate for the human scalp complex. The ad hoc computational approach and peeling set up were firstly evaluated on a validation material, before being used to characterize the skin-to-bone interaction within 6 porcine specimens harvested from the scalp. Our experimental setup allowed to measure the peeling response of porcine scalp, showing a three-regimes response including a plateau force. The computational approach satisfyingly reproduced the peeling response based uniquely on experimental-based parameters and on a discrete modeling of skin-to-bone interface. The presented methodology is a first attempt to propose a computationally efficient geometrically based model able to take into account the skin-to-bone interaction up to failure and corroborated by experimental data, and may be largely extended to the modeling of soft interactions between biological human tissues in the future.

In vitro comparison of human plasma-based and self-assembled tissue-engineered skin substitutes: two different manufacturing processes for the treatment of deep and difficult to heal injuries

Background

The aim of this in vitro study was to compare side-by-side two models of human bilayered tissue-engineered skin substitutes (hbTESSs) designed for the treatment of severely burned patients. These are the scaffold-free self-assembled skin substitute (SASS) and the human plasma-based skin substitute (HPSS).

Methods

Fibroblasts and keratinocytes from three humans were extracted from skin biopsies (N = 3) and cells from the same donor were used to produce both hbTESS models. For SASS manufacture, keratinocytes were seeded over three self-assembled dermal sheets comprising fibroblasts and the extracellular matrix they produced (n = 12), while for HPSS production, keratinocytes were cultured over hydrogels composed of fibroblasts embedded in either plasma as unique biomaterial (Fibrin), plasma combined with hyaluronic acid (Fibrin-HA) or plasma combined with collagen (Fibrin-Col) (n/biomaterial = 9). The production time was 46-55 days for SASSs and 32-39 days for HPSSs. Substitutes were characterized by histology, mechanical testing, PrestoBlue™-assay, immunofluorescence (Ki67, Keratin (K) 10, K15, K19, Loricrin, type IV collagen) and Western blot (type I and IV collagens).

Results

The SASSs were more resistant to tensile forces (p-value < 0.01) but less elastic (p-value < 0.001) compared to HPSSs. A higher number of proliferative Ki67+ cells were found in SASSs although their metabolic activity was lower. After epidermal differentiation, no significant difference was observed in the expression of K10, K15, K19 and Loricrin. Overall, the production of type I and type IV collagens and the adhesive strength of the dermal-epidermal junction was higher in SASSs.

Conclusions

This study demonstrates, for the first time, that both hbTESS models present similar in vitro biological characteristics. However, mechanical properties differ and future in vivo experiments will aim to compare their wound healing potential.

Speeding up the Production of Clinical-Grade Skin Substitutes Using Off-the-shelf Decellularized Self-Assembled Dermal Matrices

Patients with deep and extensive wounds need urgent skin coverage to re-establish the cutaneous barrier that prevents life-threatening infections and dehydration. However, the current clinically-available skin substitutes intended for permanent coverage are limited in number, and a trade-off between production time and quality must be made. Here, we report the use of decellularized self-assembled dermal matrices to reduce by half the manufacturing process time of clinical-grade skin substitutes. These decellularized matrices can be stored for over 18 months and recellularized with patients' cells in order to generate skin substitutes that show outstanding histological and mechanical properties in vitro. Once grafted in mice, these substitutes persist over weeks with high graft take, few contraction events, and high stem cell content. These next-generation skin substitutes constitute a substantial advancement in the treatment of major burn patients, combining, for the first time, high functionality, rapid manufacturability and easy handling for surgeons and healthcare practitioners. Future clinical trials will be conducted to assess the advantages of these substitutes over existing treatments. STATEMENT OF SIGNIFICANCE: The number of patients in need for organ transplantation is ever-growing and there is a shortage in tissue and organ donors. In this study, we show for the first time that we can preserve decellularized self-assembled tissues and keep them in storage. Then, in only three weeks we can use them to produce bilayered skin substitutes that have properties very close to those of the native human skin. These findings therefore represent a major step forward in the field of tissue engineering and organ transplantation, paving the way toward a universal off-the-shelf biomaterial for tissue reconstruction and surgery that will be beneficial for many clinicians and patients.

α-Linolenic acid and linoleic acid modulate the lipidome and the skin barrier of a tissue-engineered skin model

Polyunsaturated fatty acids (PUFAs) play an important role in the establishment and the maintenance of the skin barrier function. However, the impact of their derived lipid mediators remains unclear. Skin substitutes were engineered according to the self-assembly method with a culture medium supplemented with 10 μM of both α-linolenic acid (ALA) and linoleic acid (LA). The supplementation with ALA and LA decreased testosterone absorption through a tissue-engineered reconstructed skin model, thus indicating an improved skin barrier function following supplementation. The exogenously provided fatty acids were incorporated into the phospholipid and triglyceride fractions of the skin substitutes. Indeed, the dual supplementation increased the levels of eicosapentaenoic acid (EPA) (15-fold), docosapentaenoic acid (DPA) (3-fold), and LA (1.5-fold) in the epidermal phospholipids while it increased the levels of ALA (>20-fold), DPA (3-fold) and LA (1.5-fold) in the epidermal triglycerides. The bioactive lipid mediator profile of the skin substitutes, including prostaglandins, hydroxy-fatty acids, N-acylethanolamines and monoacylglycerols, was next analyzed using liquid chromatography-tandem mass spectrometry. The lipid supplementation further modulated bioactive lipid mediator levels of the reconstructed skin substitutes, leading to a lipid mediator profile more representative of the one found in normal human skin. These findings show that an optimized supply of PUFAs via culture media is essential for the establishment of improved barrier function in vitro. STATEMENT OF SIGNIFICANCE: Supplementation of the culture medium with 10 μM of both α-linolenic acid (ALA) and linoleic acid (LA) improved the skin barrier function of a tissue-engineered skin model. The exogenously provided fatty acids were incorporated into the phospholipid and triglyceride fractions of the skin substitutes and further modulated bioactive lipid mediator levels, including prostaglandins, hydroxy-fatty acids, N-acylethanolamines and monoacylglycerols. These findings highlight the important role of ALA and LA in skin homeostasis and show that an optimized supply of polyunsaturated fatty acids via culture media is essential for the establishment of improved barrier function in vitro.

The Human Epidermal Basement Membrane: A Shaped and Cell Instructive Platform That Aging Slowly Alters

One of the most important functions of skin is to act as a protective barrier. To fulfill this role, the structural integrity of the skin depends on the dermal-epidermal junction-a complex network of extracellular matrix macromolecules that connect the outer epidermal layer to the underlying dermis. This junction provides both a structural support to keratinocytes and a specific niche that mediates signals influencing their behavior. It displays a distinctive microarchitecture characterized by an undulating pattern, strengthening dermal-epidermal connectivity and crosstalk. The optimal stiffness arising from the overall molecular organization, together with characteristic anchoring complexes, keeps the dermis and epidermis layers extremely well connected and capable of proper epidermal renewal and regeneration. Due to intrinsic and extrinsic factors, a large number of structural and biological changes accompany skin aging. These changes progressively weaken the dermal-epidermal junction substructure and affect its functions, contributing to the gradual decline in overall skin physiology. Most changes involve reduced turnover or altered enzymatic or non-enzymatic post-translational modifications, compromising the mechanical properties of matrix components and cells. This review combines recent and older data on organization of the dermal-epidermal junction, its mechanical properties and role in mechanotransduction, its involvement in regeneration, and its fate during the aging process.