The effects of ovariectomy on the mechanical properties of skin in rats

Tailoring the multiscale mechanics of tunable decellularized extracellular matrix (dECM) for wound healing through immunomodulation

With the discovery of the pivotal role of macrophages in tissue regeneration through shaping the tissue immune microenvironment, various immunomodulatory strategies have been proposed to modify traditional biomaterials. Decellularized extracellular matrix (dECM) has been extensively used in the clinical treatment of tissue injury due to its favorable biocompatibility and similarity to the native tissue environment. However, most reported decellularization protocols may cause damage to the native structure of dECM, which undermines its inherent advantages and potential clinical applications. Here, we introduce a mechanically tunable dECM prepared by optimizing the freeze-thaw cycles. We demonstrated that the alteration in micromechanical properties of dECM resulting from the cyclic freeze-thaw process contributes to distinct macrophage-mediated host immune responses to the materials, which are recently recognized to play a pivotal role in determining the outcome of tissue regeneration. Our sequencing data further revealed that the immunomodulatory effect of dECM was induced via the mechnotrasduction pathways in macrophages. Next, we tested the dECM in a rat skin injury model and found an enhanced micromechanical property of dECM achieved with three freeze-thaw cycles significantly promoted the M2 polarization of macrophages, leading to superior wound healing. These findings suggest that the immunomodulatory property of dECM can be efficiently manipulated by tailoring its inherent micromechanical properties during the decellularization process. Therefore, our mechanics-immunomodulation-based strategy provides new insights into the development of advanced biomaterials for wound healing.

Reversible adhesives with controlled wrinkling patterns for programmable integration and discharging

Switchable and minimally invasive tissue adhesives have great potential for medical applications. However, on-demand adherence to and detachment from tissue surfaces remain difficult. We fabricated a switchable hydrogel film adhesive by designing pattern-tunable wrinkles to control adhesion. When adhered to a substrate, the compressive stress generated from the bilayer system leads to self-similar wrinkling patterns at short and long wavelengths, regulating the interfacial adhesion. To verify the concept and explore its application, we established a random skin flap model, which is a crucial strategy for repairing severe or large-scale wounds. Our hydrogel adhesive provides sufficient adhesion for tissue sealing and promotes neovascularization at the first stage, and then gradually detaches from the tissue while a dynamic wrinkling pattern transition happens. The gel film can be progressively ejected out from the side margins after host-guest integration. Our findings provide insights into tunable bioadhesion by manipulating the wrinkling pattern transition.

Mechanical Properties and Functions of Elastin: An Overview

Human tissues must be elastic, much like other materials that work under continuous loads without losing functionality. The elasticity of tissues is provided by elastin, a unique protein of the extracellular matrix (ECM) of mammals. Its function is to endow soft tissues with low stiffness, high and fully reversible extensibility, and efficient elastic-energy storage. Depending on the mechanical functions, the amount and distribution of elastin-rich elastic fibers vary between and within tissues and organs. The article presents a concise overview of the mechanical properties of elastin and its role in the elasticity of soft tissues. Both the occurrence of elastin and the relationship between its spatial arrangement and mechanical functions in a given tissue or organ are overviewed. As elastin in tissues occurs only in the form of elastic fibers, the current state of knowledge about their mechanical characteristics, as well as certain aspects of degradation of these fibers and their mechanical performance, is presented. The overview also outlines the latest understanding of the molecular basis of unique physical characteristics of elastin and, in particular, the origin of the driving force of elastic recoil after stretching.

Mechanic-Driven Biodegradable Polyglycolic Acid/Silk Fibroin Nanofibrous Scaffolds Containing Deferoxamine Accelerate Diabetic Wound Healing

The extracellular matrix (ECM), comprising of hundreds of proteins, mainly collagen, provides physical, mechanical support for various cells and guides cell behavior as an interactive scaffold. However, deposition of ECM, especially collagen content, is seriously impaired in diabetic wounds, which cause inferior mechanical properties of the wound and further delay chronic wound healing. Thus, it is critical to develop ECM/collagen alternatives to remodel the mechanical properties of diabetic wounds and thus accelerate diabetic wound healing. Here, we firstly prepared mechanic-driven biodegradable PGA/SF nanofibrous scaffolds containing DFO for diabetic wound healing. In our study, the results in vitro showed that the PGA/SF-DFO scaffolds had porous three-dimensional nanofibrous structures, excellent mechanical properties, biodegradability, and biocompatibility, which would provide beneficial microenvironments for cell adhesion, growth, and migration as an ECM/collagen alternative. Furthermore, the data in vivo showed PGA/SF-DFO scaffolds can adhere well to the wound and have excellent biodegradability, which is helpful to avoid secondary damage by omitting the removal process of scaffolds. The finite element analysis results showed that the application of silk fibroin-based scaffolds could significantly reduce the maximum stress around the wound. Besides, PGA/SF-DFO scaffolds induced collagen deposition, re-vascularization, recovered impaired mechanical properties up to about 70%, and ultimately accelerated diabetic wound healing within 14 days. Thus, our work provides a promising therapeutic strategy for clinically chronic wound healing.

Improvement of Biophysical Skin Parameters of Topically Applied Fermented Soybean Extract-Loaded Niosomes with No Systemic Toxicity in Ovariectomized Rats

Despite the known beneficial impacts of estrogen used as hormone replacement therapy to ameliorate signs of skin aging in postmenopausal women, its compliance rates are low. A significant amount of estrogen may be absorbed into the blood circulation and can lead to systemic actions. Soy isoflavone exhibits biological activities similar to synthetic estrogen because it is a heterocyclic phenolic compound. The disadvantage of most topical ingredients based on isoflavone is that they contain biologically inactive glycoside forms, which must be converted to a readily absorbed aglycone for the topical application. The purposes of this study were to develop niosomes-loaded Aspergillus oryzae-fermented soybean extract (FSE) to enhance skin absorption with proven systemic side effect compared to estrogen application. Skin hydration and viscoelasticity of 75 days post-ovariectomized (OVX) Wistar rats following 84-day topical treatment with various tested gel formulations containing fermented soybean extract (FSE) were evaluated. The tested formulations were gel + FSE nanoniosomes, gel + FSE microniosomes, gel + FSE (200 µg FSE/9 cm²/rat), gel + blank nanoniosomes (a negative control), and gel + 17β-estradiol (E2) nanoniosomes (a positive control, 20 µg E2/9 cm²/rat). Changes in vaginal cornifications and weights of uteri, livers, and kidneys in the OVX rats and signs of primary skin irritation in the rabbits were evaluated for their toxicities. Results showed that FSE-loaded nanoniosomes improved the skin hydration and viscoelasticity better than gel + FSE microniosomes and gel + FSE, respectively, but lower than those of gel + E2 nanoniosomes (p < 0.05). Unlike all gel + E2 nanoniosomes, the FSE formulations showed no changes in vaginal cells and weights of uteri, livers, and kidneys and no signs of skin irritation. In conclusion, The FSE niosome-based gels should be promising candidates for delivering phytoestrogens against signs of skin aging with no systemic toxicities.

Physicochemical Properties and Cell Viability of Shrimp Chitosan Films as Affected by Film Casting Solvents. I-Potential Use as Wound Dressing

Chitosan solubility in aqueous organic acids has been widely investigated. However, most of the previous works have been done with plasticized chitosan films and using acetic acid as the film casting solvent. In addition, the properties of these films varied among studies, since they are influenced by different factors such as the chitin source used to produce chitosan, the processing variables involved in the conversion of chitin into chitosan, chitosan properties, types of acids used to dissolve chitosan, types and amounts of plasticizers and the film preparation method. Therefore, this work aimed to prepare chitosan films by the solvent casting method, using chitosan derived from Litopenaeus vannamei shrimp shell waste, and five different organic acids (acetic, lactic, maleic, tartaric, and citric acids) without plasticizer, in order to evaluate the effect of organic acid type and chitosan source on physicochemical properties, degradation and cytotoxicity of these chitosan films. The goal was to select the best suited casting solvent to develop wound dressing from shrimp chitosan films. Shrimp chitosan films were analyzed in terms of their qualitative assessment, thickness, water vapor permeability (WVP), water vapor transmission rate (WVTR), wettability, tensile properties, degradation in phosphate buffered saline (PBS) and cytotoxicity towards human fibroblasts using the resazurin reduction method. Regardless of the acid type employed in film preparation, all films were transparent and slightly yellowish, presented homogeneous surfaces, and the thickness was compatible with the epidermis thickness. However, only the ones prepared with maleic acid presented adequate characteristics of WVP, WVTR, wettability, degradability, cytotoxicity and good tensile properties for future application as a wound dressing material. The findings of this study contributed not only to select the best suited casting solvent to develop chitosan films for wound dressing but also to normalize a solubilization protocol for chitosan, derived from Litopenaeus vannamei shrimp shell waste, which can be used in the pharmaceutical industry.

Morphological and Functional Changes in Skin of Adult Male Rats Chronically Treated with Letrozole, a Nonsteroidal Inhibitor of Cytochrome P450 Aromatase

Skin is a target for hormones and a site of hormone production. Aromatase inhibitors such as letrozole reduce circulating estrogen. The aim of the study was to investigate the morphology of the dermis and immunoexpression of androgen receptor (AR), estrogen receptor α and β (ERα, ERβ), luteinizing hormone receptor (LHR), follicle-stimulating hormone receptor (FSHR), and cytochrome P450 aromatase (P450arom) in male rats with a deficit of estradiol. Experiments were performed on skin of 12 male rats. Rats in the experimental group received per os letrozole for 6 months. For morphological analysis, van Gieson, Sirius Red and orcein staining of sections was performed. In immunohistochemistry, reactions with specific antibodies (anti-P450arom, LHR, FSHR, ERα, ERβ) were used. In morphometric analysis, sections were stained with hematoxylin and eosin. Differences between groups were assessed by Mann-Whitney U-test. There were no differences in the diameter of collagen fibers. The dermis of letrozole-treated animals showed areas without collagen fibers, and expression of P450arom, ERα and ERβ was diminished in the skin of these animals. This study indicates that estrogens exert an effect via ERs that has a role in maintaining proper skin morphology in males, together with androgen. This is also the first documented expression of FSHR in the skin of male rats.

Silk Sponges Ornamented with a Placenta-Derived Extracellular Matrix Augment Full-Thickness Cutaneous Wound Healing by Stimulating Neovascularization and Cellular Migration

Regeneration of full-thickness wounds without scar formation is a multifaceted process, which depends on in situ dynamic interactions between the tissue-engineered skin substitutes and a newly formed reparative tissue. However, the majority of the tissue-engineered skin substitutes used so far in full-thickness wound healing cannot mimic the natural extracellular matrix (ECM) complexity and thus are incapable of providing a suitable niche for endogenous tissue repair. Herein, we demonstrated a simple approach to fabricate porous hybrid ECM sponges (HEMS) using a placental ECM and silk fibroin for full-thickness wound healing. HEMS with retained cytokines/growth factors provided a noncytotoxic environment in vitro for human foreskin fibroblasts (HFFs), human epidermal keratinocytes (HEKs), and human amniotic membrane-derived stem cells to adhere, infiltrate, and proliferate. Interestingly, HEMS-conditioned media accelerated the migration of HFFs and HEKs owing to the presence of cytokines/growth factors. Also, the ex vivo chick chorioallantoic membrane assay of HEMS demonstrated its excellent vascularization potential by inducing and supporting blood vessels. Additionally, HEMS when subcutaneously implanted demonstrated no severe immune response to the host. Furthermore, HEMS implanted in full-thickness wounds in a rat model showed augmented healing progression with well-organized epidermal-dermal junctions via pronounced angiogenesis, accelerated migration of HFFs/HEKs, enhanced granulation tissue formation, and early re-epithelialization. Taken together, these findings show that porous HEMS ornamented with cytokines/growth factors having superior physicomechanical properties may be an appropriate skin substitute for full-thickness cutaneous wounds.

Polycaprolactone nanofibers functionalized with placental derived extracellular matrix for stimulating wound healing activity

Impaired wound healing is primarily associated with inadequate angiogenesis, repressed cell migration, deficient synthesis of extracellular matrix (ECM) component/growth factors, and altered inflammatory responses in the wound bed environment.

Fabrication of a three dimensional spongy scaffold using human Wharton's jelly derived extra cellular matrix for wound healing

The Wharton's jelly (WJ) contains significant amounts of extracellular matrix (ECM) components and rich source of endogenous growth factors. In this study, we designed a new biomimetic spongy scaffold from decellularized WJ-derived ECM and used it as a skin substitute. Histological analysis and biochemical assays showed that bio-active molecules preserved in the fabricated scaffolds and that the scaffolds have highly interconnected porous structure. Cytotoxicity and mechanical evaluation of the scaffold indicated that it is non-toxic and has appropriate mechanical properties. MTT assay, SEM and histological analysis of human fibroblast, seeded on the scaffolds, confirmed cellular viability, attachment, penetration and proliferation. The effectiveness of WJ-derived scaffolds in the regeneration of full-thickness wound was assessed through an in vivo experiment. Our results demonstrated that the scaffolds were well integrated into the mouse tissue and absorbed the exudates after one week. Unlike the controls, in WJ group there were not only complete wound closing and disappearance of the scab, but also complete reepithelialization, newly generated epidermal layers and appendages after 12days of implantation. Taken together, our results indicate that WJ-derived scaffolds are able to improve attachment, penetration and growth of the fibroblast cells and speed up the healing processes, which would offer a proper skin graft for wound healing.

A NONLINEAR HYPERELASTIC BEHAVIOR TO IDENTIFY THE MECHANICAL PROPERTIES OF RAT SKIN UNDER UNIAXIAL LOADING

Skin is a thin membrane which provides many biological functions, such as thermoregulation and protection from mechanical, bacterial, and viral insults. The mechanical properties of skin tissue are extremely hard to measure and may vary according to the anatomical locations of a body. However, the mechanical properties of skin at different anatomical regions have not been satisfactorily simulated by conventional engineering models. In this study, the linear elastic and nonlinear hyperelastic mechanical properties of rat skin at different anatomical locations, including back and abdomen, are investigated using a series of tensile tests. The Young's modulus and maximum stress of skin tissue are measured before the incidence of failure. The nonlinear mechanical behavior of skin tissue is also experimentally and computationally investigated through constitutive equations. Hyperelastic strain energy density functions are adjusted using the experimental results. A hyperelastic constitutive model is selected to suitably represent the axial behavior of the skin. The results reveal that the maximum stress (20%) and Young's modulus (35%) of back skin are significantly higher than that of abdomen skin. The Ogden model is selected to closely address the nonlinear mechanical behavior of the skin which can be used in further biomechanical simulations of the skin tissue. The results might have implications not only for understanding of the mechanical behavior of skin tissue at different anatomical locations, but also to give an engineering insight for a diversity of disciplines, such as dermatology, cosmetics industry, clinical decision making, and clinical intervention.

An anisotropic, hyperelastic model for skin: experimental measurements, finite element modelling and identification of parameters for human and murine skin

The mechanical characteristics of skin are extremely complex and have not been satisfactorily simulated by conventional engineering models. The ability to predict human skin behaviour and to evaluate changes in the mechanical properties of the tissue would inform engineering design and would prove valuable in a diversity of disciplines, for example the pharmaceutical and cosmetic industries, which currently rely upon experiments performed in animal models. The aim of this study was to develop a predictive anisotropic, hyperelastic constitutive model of human skin and to validate this model using laboratory data. As a corollary, the mechanical characteristics of human and murine skin have been compared. A novel experimental design, using tensile tests on circular skin specimens, and an optimisation procedure were adopted for laboratory experiments to identify the material parameters of the tissue. Uniaxial tensile tests were performed along three load axes on excised murine and human skin samples, using a single set of material parameters for each skin sample. A finite element model was developed using the transversely isotropic, hyperelastic constitutive model of Weiss et al. (1996) and was embedded within a Veronda-Westmann isotropic material matrix, using three fibre families to create anisotropic behaviour. The model was able to represent the nonlinear, anisotropic behaviour of the skin well. Additionally, examination of the optimal material coefficients and the experimental data permitted quantification of the mechanical differences between human and murine skin. Differences between the skin types, most notably the extension of the skin at low load, have highlighted some of the limitations of murine skin as a biomechanical model of the human tissue. The development of accurate, predictive computational models of human tissue, such as skin, to reduce, refine or replace animal models and to inform developments in the medical, engineering and cosmetic fields, is a significant challenge but is highly desirable. Concurrent advances in computer technology and our understanding of human physiology must be utilised to produce more accurate and accessible predictive models, such as the finite element model described in this study.

Full-thickness skin wound healing using human placenta-derived extracellular matrix containing bioactive molecules

The human placenta, a complex organ, which facilitates exchange between the fetus and the mother, contains abundant extracellular matrix (ECM) components and well-preserved endogenous growth factors. In this study, we designed a new dermal substitute from human placentas for full-thickness wound healing. Highly porous, decellularized ECM sheets were fabricated from human placentas via homogenization, centrifugation, chemical and enzymatic treatments, molding, and freeze-drying. The physical structure and biological composition of human placenta-derived ECM sheets dramatically supported the regeneration of full-thickness wound in vivo. At the early stage, the ECM sheet efficiently absorbed wound exudates and tightly attached to the wound surface. Four weeks after implantation, the wound was completely closed, epidermic cells were well arranged and the bilayer structure of the epidermis and dermis was restored. Moreover, hair follicles and microvessels were newly formed in the ECM sheet-implanted wounds. Overall, the ECM sheet produced a dermal substitute with similar cellular organization to that of normal skin. These results suggest that human placenta-derived ECM sheets provide a microenvironment favorable to the growth and differentiation of cells, and positive modulate the healing of full-thickness wounds.

Estrogen depletion results in nanoscale morphology changes in dermal collagen

Tissue cryo-sectioning combined with atomic force microscopy imaging reveals that the nanoscale morphology of dermal collagen fibrils, quantified using the metric of D-periodic spacing, changes under the condition of estrogen depletion. Specifically, a new subpopulation of fibrils with D-spacings in the region between 56 and 59 nm is present 2 years following ovariectomy in ovine dermal samples. In addition, the overall width of the distribution, both values above and below the mean, was found to be increased. The change in width due to an increase in lower values of D-spacings was previously reported for ovine bone; however, this report demonstrates that the effect is also present in non-mineralized collagen fibrils. A nonparametric Kolmogorov-Smirnov test of the cumulative density function indicates a statistical difference in the sham and OVX D-spacing distributions (P<0.01).

Indentation versus tensile measurements of Young's modulus for soft biological tissues

In this review, we compare the reported values of Young's modulus (YM) obtained from indentation and tensile deformations of soft biological tissues. When the method of deformation is ignored, YM values for any given tissue typically span several orders of magnitude. If the method of deformation is considered, then a consistent and less ambiguous result emerges. On average, YM values for soft tissues are consistently lower when obtained by indentation deformations. We discuss the implications and potential impact of this finding.

The mechanical properties of the skin epidermis in relation to targeted gene and drug delivery

A challenge in combating many major diseases is breaching the skin's tough outer layer (the stratum corneum (SC)) and delivering drugs and genes into the underlying abundant immunologically sensitive viable epidermal cells with safe, practical physical technologies. To achieve this effectively and accurately, design information is needed on key skin mechanical properties when pushing into and through epidermal skin cells. We measure these important mechanical properties by penetrating through the intact SC and viable epidermis (VE) of freshly excised murine skin with a NANO-indenter, using custom tungsten probes fabricated with nominally 5 and 2 microm diameters (with nanoscale tips). We show the skin Young's modulus, storage modulus and stress all dramatically decreased through the SC. Also, for a given penetration depth, decreasing the probe size significantly increases the storage modulus. Biological variation in penetrating the skin was shown. These collective findings advance the rational design of physical approaches for delivering genes and drugs within key cells of the VE.

In vivo measurements of the elastic mechanical properties of human skin by indentation tests

Knowledge about the human skin mechanical properties is essential in several domains, particularly for dermatology, cosmetic or to detect some cutaneous pathology. This study proposes a new method to determine the human skin mechanical properties in vivo using the indentation test. Usually, the skin mechanical parameters obtained with this method are influenced by the mechanical properties of the subcutaneous layers, like muscles. In this study, different mechanical models were used to evaluate the effect of the subcutaneous layers on the measurements and to extract the skin elastic properties from the global mechanical response. The obtained results demonstrate that it is necessary to take into account the effect of the subcutaneous layers to correctly estimate the skin Young's modulus. Moreover, the results illustrate that the variation of the measured Young's modulus at low penetration depth cannot be correctly described with usual one-layer mechanical models. Thus a two-layer elastic model was proposed, which highly improved the measurement of the skin mechanical properties.

Empirically inspired simulated electro-mechanical model of the rat mystacial follicle-sinus complex

In whiskered animals, activity is evoked in the primary sensory afferent cells (trigeminal nerve) by mechanical stimulation of the whiskers. In some cell populations this activity is correlated well with continuous stimulus parameters such as whisker deflection magnitude, but in others it is observed to represent events such as whisker-stimulator contact or detachment. The transduction process is mediated by the mechanics of the whisker shaft and follicle-sinus complex (FSC), and the mechanics and electro-chemistry of mechanoreceptors within the FSC. An understanding of this transduction process and the nature of the primary neural codes generated is crucial for understanding more central sensory processing in the thalamus and cortex. However, the details of the peripheral processing are currently poorly understood. To overcome this deficiency in our knowledge, we constructed a simulated electro-mechanical model of the whisker-FSC-mechanoreceptor system in the rat and tested it against a variety of data drawn from the literature. The agreement was good enough to suggest that the model captures many of the key features of the peripheral whisker system in the rat.