Biomechanics of Scar Tissue and Uninjured Skin

Enhancing aesthetic outcomes: The role of biomechanics in periorbital and eyelid cosmetic surgery

Cosmetic periorbital and eyelid surgery is a commonly performed procedure in facial plastic surgery. Understanding the biomechanics of periorbital anatomy and its role in aesthetic surgery is essential for achieving optimal outcomes. This review explores the biomechanical processes involved in periorbital age changes and analyzes the impact of cosmetic surgery approaches on these processes. By maintaining the initial mechanical equilibrium of the brows, eyelids, septal fat, and blepharoplasty folds, periorbital and eyelid cosmetic surgery can effectively rejuvenate the appearance. Disruption of this equilibrium can lead to the migration of anatomic components, resulting in signs of aging. Surgeons, by applying biomechanical concepts, can tailor the forces exerted upon the different structures to manifest the patient's aesthetic aspirations. The key to success in periorbital and eyelid cosmetic surgery lies in re-establishing a dynamic mechanical equilibrium within the periorbital framework.

Electrospun Fiber Surface Roughness Modulates Human Monocyte-Derived Macrophage Phenotype

Injuries to fibrous connective tissues have very little capacity for self-renewal and exhibit poor healing after injury. Phenotypic shifts in macrophages play a vital role in mediating the healing response, creating an opportunity to design immunomodulatory biomaterials which control macrophage polarization and promote regeneration. In this study, electrospun poly(-caprolactone) fibers with increasing surface roughness (SR) were produced by increasing relative humidity and inducing vapor-induced phase separation during the electrospinning process. The impact of surface roughness on macrophage phenotype was assessed using human monocyte-derived macrophages in vitro and in vivo using B6.Cg-Tg(Csf1r-EGFP)1Hume/J (MacGreen) mice. In vitro experiments showed that macrophages cultured on mesh with increasing SR exhibited decreased release of both pro- and anti-inflammatory cytokines potentially driven by increased protein adsorption and biophysical impacts on the cells. Further, increasing SR led to an increase in the expression of the pro-regenerative cell surface marker CD206 relative to the pro-inflammatory marker CD80. Mesh with increasing SR were implanted subcutaneously in MacGreen mice, again showing an increase in the ratio of cells expressing CD206 to those expressing CD80 visualized by immunofluorescence. SR on implanted biomaterials is sufficient to drive macrophage polarization, demonstrating a simple feature to include in biomaterial design to control innate immunity.

Mechanical characterization of soft-tissue stiffness during mandibular distraction

The purpose of this study was to determine the stiffness of mandibular soft tissues during mandibular distraction, from the perspective of improving distraction devices such as automated continuous distractors. Uncompleted osteotomy was performed on 11 fresh human hemimandibles via a greenstick fracture, to preserve the uplift of the internal periosteum of the mandibular corpus. In order to replicate continuous distraction, direct measurements were performed through a uniaxial quasi-static tensile test. For all specimens, linear regression was applied to the force-displacement curve for a force region of 10-20 N, and the slope extracted. The mean stiffness was estimated to be 9.12 ± 3.56 N/mm. This study is the first to measure directly the stiffness of the surrounding tissues of the human mandibular corpus, and paves the way to the design of a new generation of distractor devices.

Hierarchical Design of Tissue-Mimetic Fibrillar Hydrogel Scaffolds

Most tissues of the human body present hierarchical fibrillar extracellular matrices (ECMs) that have a strong influence over their physicochemical properties and biological behavior. Of great interest is the introduction of this fibrillar structure to hydrogels, particularly due to the water-rich composition, cytocompatibility, and tunable properties of this class of biomaterials. Here, the main bottom-up fabrication strategies for the design and production of hierarchical biomimetic fibrillar hydrogels and their most representative applications in the fields of tissue engineering and regenerative medicine are reviewed. For example, the controlled assembly/arrangement of peptides, polymeric micelles, cellulose nanoparticles (NPs), and magnetically responsive nanostructures, among others, into fibrillar hydrogels is discussed, as well as their potential use as fibrillar-like hydrogels (e.g., those from cellulose NPs) with key biofunctionalities such as electrical conductivity or remote stimulation. Finally, the major remaining barriers to the clinical translation of fibrillar hydrogels and potential future directions of research in this field are discussed.

Effect of negative pressure therapy on the treatment response to scar thickness and viscoelasticity

Patients with scars face a grave threat to their mental and physical health. Negative pressure has been used for scar therapy in medical care and provides a microenvironment conducive to scar healing while stimulating cell regeneration. Negative pressure may disrupt scar tissue regeneration when the pressure is too high or too low, so finding a suitable negative pressure is important. We hypothesized that different negative pressure magnitudes would affect scar tissue properties differently. This research aimed to provide practical recommendations for scar therapy. This study used three negative pressures (-105 mmHg, -125 mmHg, and -145 mmHg) to compare scar material properties. We measured scar tissue thickness and viscoelasticity with a motor-driven ultrasound indentation system. According to the results of this study, scar thickness is most effectively reduced at a negative pressure of -105 mmHg. In comparison, scar viscoelasticity continuously increases at a negative pressure of -125 mmHg. Negative pressure therapy can be recommended to scar care clinics based on the results of this study.

The scar-reducing effect of a novel chitosan gel: an in vivo study

OBJECTIVE

Scar tissue formation, as a normal part of wound healing, initiates in the proliferation phase, continues after the remodelling phase, and may cause an unpleasant appearance or disruption in normal functioning. This study investigated the effects of a topical gel on acute wound healing and reducing scars in a rat model.

METHOD

ChitoScar (ChitoTech Company, Iran), a commercial scar-reducing gel based on chitosan, was analysed for antibacterial and antiviral activity through a quantitative suspension test. Its cytotoxic effect was investigated, and then irritation and delayed-type hypersensitivity tests were carried out on rabbits through direct application of the gel. Furthermore, the effect of the chitosan-based gel on wound healing and scar tissue formation was studied in rats with an acute wound in two groups: the treatment group (topical application of the chitosan-based gel); and the control group (without treatment). Histopathological examination was carried out based on the inflammatory cells, collagen fibre, keratinocytes and fibroblasts.

RESULTS

Analysis revealed that the chitosan-based gel had no cytotoxicity and caused no erythema, oedema, local or other systemic adverse response. Wound healing occurred earlier in the treatment group, which was a result of a significant increase in re-epithelialisation, angiogenesis, fibroblast population and collagen fibre thickness (p<0.05). In the treatment group, wounds healed completely after 21 days and scars totally disappeared after 28 days, while in the control group, wound healing remained incomplete with distinct scar tissue.

CONCLUSION

The results demonstrated the positive effect of the chitosan-based gel on the duration and quality of the wound healing process, as well as minimising the scar tissue formation in this in vivo study.

Laminin mimetic angiogenic and collagen peptide hydrogel for enhance dermal wound healing

Laminins are essential in basement membrane architecture and critical in re-epithelialization and angiogenesis. These processes and collagen deposition are vital in skin wound healing. The role of angiogenic peptides in accelerating the wound-healing process has been known. The bioactive peptides could be a potential approach due to their similar effects as growth factors and inherent biocompatible and biodegradable nature with lower cost. They can also recognize ligand-receptor interaction and mimic the extracellular matrix. Here, we report novel angiogenic DYVRLAI, CDYVRLAI, angiogenic-collagen PGPIKVAV, and Ac-PGPIKVAV peptides conjugated sodium carboxymethyl cellulose hydrogel, which was designed from laminin. The designed peptide exhibits a better binding with the α3β1, αvβ3, and α5β1 integrins and CXCR2 receptor, indicating their angiogenic and collagen binding efficiency. The peptides were evaluated to stimulate wound healing in full-thickness excision wounds in normal and diabetic mice (type II). They demonstrated their efficacy in terms of angiogenesis (CD31), re-epithelialization through regeneration of the epidermis (H&E), and collagen deposition (MT). The synthesized peptide hydrogel (DYVRLAI and CDYVRLAI) showed enhanced wound contraction up to 10.1 % and 12.3 % on day 7th compared to standard becaplermin gel (49 %) in a normal wound model. The encouraging results were also observed with the diabetic model, where these peptides showed a significant decrease of 5.20 and 5.17 % in wound size on day 10th compared to the commercial gel (9.27 %). These outcomes signify that the modified angiogenic peptide is a cost effective, novel peptide motif to promote dermal wound healing in both models.

A virtual simulation approach to assess the effect of trocar-site placement and scar characteristics on the abdominal wall biomechanics

Analyses of registries and medical imaging suggest that laparoscopic surgery may be penalized with a high incidence of trocar-site hernias (TSH). In addition to trocar diameter, the location of the surgical wound (SW) may affect TSH incidence. The intra-abdominal pressure (IAP) exerted on the abdominal wall (AW) might also influence the appearance of TSH. In the present study, we used finite element (FE) simulations to predict the influence of trocar location and SW characteristics (stiffness) on the mechanical behavior of the AW subject to an IAP. Two models of laparoscopy patterns on the AW, with trocars in the 5-12 mm range, were generated. FE simulations for IAP values within the 4 kPa-20 kPa range were carried out using the Code Aster open-source software. Different stiffness levels of the SW tissue were considered. We found that midline-located surgical wounds barely deformed, even though they moved outwards along with the regular LA tissue. Laterally located SWs hardly changed their location but they experienced significant variations in their volume and shape. The amount of deformation of lateral SWs was found to strongly depend on their stiffness. Trocar incisions placed in a LA with non-diastatic dimensions do not compromise its mechanical integrity. The more lateral the trocars are placed, the greater is their deformation, regardless of their size. Thus, to prevent TSH it might be advisable to close lateral trocars with a suture, or even use a prosthetic reinforcement depending on the patient's risk factors (e.g., obesity).

A pre-post implementation study of a care bundle to reduce perineal trauma in unassisted births conducted by midwives

PROBLEM

The perineal-bundle is a complex intervention widely implemented in Australian maternity care facilities.

BACKGROUND

Most bundle components have limited or conflicting evidence and the implementation required many midwives to change their usual practice for preventing perineal trauma.

AIM

To measure the effect of perineal bundle implementation on perineal injury for women having unassisted births with midwives.

METHODS

A retrospective pre-post implementation study design to determine rates of second degree, severe perineal trauma, and episiotomy. Women who had an unassisted, singleton, cephalic vaginal birth at term between two time periods: January 2011 - November 2017 and August 2018 - August 2020 with a midwife or midwifery student accoucheur. We conducted logistic regression on the primary outcomes to control for confounding variables.

FINDINGS

data from 20,155 births (pre-implementation) and 6273 (post-implementation) were analysed. After implementation, no significant difference in likelihood of severe perineal trauma was demonstrated (aOR 0.86, 95% CI 0.71-1.04, p = 0.124). Nulliparous women were more likely to receive an episiotomy (aOR 1.49 95% CI 1.31-1.70 p < 0.001) and multiparous women to suffer a second degree tear (aOR 1.18 95% CI 1.09-1.27 p < 0.001).

DISCUSSION

This study adds to the growing body of literature which suggests a number of bundle components are ineffective, and some potentially harmful. Why, and how, the bundle was introduced at scale without a research framework to test efficacy and safety is a key concern.

CONCLUSION

Suitably designed trials should be undertaken on all proposed individual or grouped perineal protection strategies prior to broad adoption.

Lower Extremity Pediatric Tissue Expansion: A Single Surgeon's 16-Year Experience

BACKGROUND

Tissue expansion is a well-established approach to soft tissue reconstruction in the pediatric population for lower extremity pathologies. Unfortunately, complication rates range from 19% to 40% in literature, including infection and implant extrusion, leading to delays in reconstruction. These challenges have prompted investigation toward categorizing risk factors for lower extremity tissue expander placement.

METHODS

A retrospective study of pediatric patients who underwent lower extremity tissue expander placement by the senior author (R.J.R.) was performed over a 16-year period. Patient charts were reviewed to categorize baseline and operative characteristics. Primary outcome variables were surgical-site infection, expander extrusion, and expander deflation. Univariate and multivariate logistic regressions were performed ( α < 0.05).

RESULTS

There were 59 tissue expanders in our cohort. The overall complication rate was 27.1% with a 77.2% successful reconstruction rate. Greater number of expanders placed during 1 operation is associated with 2.5 increased odds of having any complication and is associated with 0.4 decreased odds of having a successful reconstruction. Incisions made in scar tissue for expander placement appear to be associated with a greater than 7 times increased odds of readmission.

CONCLUSIONS

Reconstruction of soft tissue pathologies using lower extremity tissue expanders in the pediatric population is an effective yet challenging technique. This study identified that the number of expanders inserted during 1 operation, incisions made over scar tissue, and expanders placed in the anterior thigh were correlated with having a negative impact on reconstructive outcomes. Extra care should be taken with patients who require multiple expanders during 1 operation and with choosing the location and incision of expander placement.

Effective Governing Equations for Viscoelastic Composites

We derive the governing equations for the overall behaviour of linear viscoelastic composites comprising two families of elastic inclusions, subphases and/or fibres, and an incompressible Newtonian fluid interacting with the solid phases at the microscale. We assume that the distance between each of the subphases is very small in comparison to the length of the whole material (the macroscale). We can exploit this sharp scale separation and apply the asymptotic (periodic) homogenization method (AHM) which decouples spatial scales and leads to the derivation of the new homogenised model. It does this via upscaling the fluid-structure interaction problem that arises between the multiple elastic phases and the fluid. As we do not assume that the fluid flow is characterised by a parabolic profile, the new macroscale model, which consists of partial differential equations, is of Kelvin-Voigt viscoelastic type (rather than poroelastic). The novel model has coefficients that encode the properties of the microstructure and are to be computed by solving a single local differential fluid-structure interaction (FSI) problem where the solid and the fluid phases are all present and described by the one problem. The model reduces to the case described by Burridge and Keller (1981) when there is only one elastic phase in contact with the fluid. This model is applicable when the distance between adjacent phases is smaller than the average radius of the fluid flowing in the pores, which can be the case for various highly heterogeneous systems encountered in real-world (e.g., biological, or geological) scenarios of interest.

Anisotropic mechanical characterization of human skin by in vivo multi-axial ring suction test

Human skin is a soft tissue behaving as an anisotropic material. The anisotropy emerges from the alignment of collagen fibers in the dermis, which causes the skin to exhibit greater stiffness in a certain direction, known as Langer's line. The importance of determining this anisotropy axis lies in assisting surgeons in making incisions that do not produce undesirable scars. In this paper, we introduce an open-source numerical framework, MARSAC (Multi-Axial Ring Suction for Anisotropy Characterization: https://github.com/aflahelouneg/MARSAC), adapted to a commercial device CutiScan CS 100® that applies a suction load on an annular section, causing a multi-axial stretch in the central zone, where in-plane displacements are captured by a camera. The presented framework receives inputs from a video file and converts them into displacement fields through Digital Image Correlation (DIC) technique. From the latter and based on an analytical model, the method assesses the anisotropic material parameters of human skin: Langer's line ϕ, and the elastic moduli E₁ and E₂ along the principal axes, providing that the Poisson's ratio is fixed. The pipeline was applied to a public data repository, https://search-data.ubfc.fr/femto/FR-18008901306731-2021-08-25_In-vivo-skin-anisotropy-dataset-for-a-young-man.html, containing 30 test series performed on a forearm of a Caucasian subject. As a result, the identified parameter averages, ϕˆ=40.9±8.2^∘ and the anisotropy ratio E₁ˆ/E₂ˆ=3.14±1.60, were in accordance with the literature. The intra-subject analysis showed a reliable assessment of ϕ and E₂. As skin anisotropy varies from site to site and from subject to subject, the novelty of the method consists in (i) an optimal utilization of CutiScan CS 100® probe to measure the Langer's line accurately and rapidly on small areas with a minimum diameter of 14mm, (ii) validation of an analytical model based on deformation ellipticity.

Nitric oxide-releasing gel accelerates healing in a diabetic murine splinted excisional wound model

Introduction

According to the American Diabetes Association (ADA), 9–12 million patients suffer from chronic ulceration each year, costing the healthcare system over USD $25 billion annually. There is a significant unmet need for new and efficacious therapies to accelerate closure of non-healing wounds. Nitric Oxide (NO) levels typically increase rapidly after skin injury in the inflammatory phase and gradually diminish as wound healing progresses. The effect of increased NO concentration on promoting re-epithelization and wound closure has yet to be described in the context of diabetic wound healing.

Methods

In this study, we investigated the effects of local administration of an NO-releasing gel on excisional wound healing in diabetic mice. The excisional wounds of each mouse received either NO-releasing gel or a control phosphate-buffered saline (PBS)-releasing gel treatment twice daily until complete wound closure.

Results

Topical administration of NO-gel significantly accelerated the rate of wound healing as compared with PBS-gel-treated mice during the later stages of healing. The treatment also promoted a more regenerative ECM architecture resulting in shorter, less dense, and more randomly aligned collagen fibers within the healed scars, similar to that of unwounded skin. Wound healing promoting factors fibronectin, TGF-β1, CD31, and VEGF were significantly elevated in NO vs. PBS-gel-treated wounds.

Discussion

The results of this work may have important clinical implications for the management of patients with non-healing wounds.

Promotion of Lymphangiogenesis by Targeted Delivery of VEGF-C Improves Diabetic Wound Healing

Chronic wounds represent a major therapeutic challenge. Lymphatic vessel function is impaired in chronic ulcers but the role of lymphangiogenesis in wound healing has remained unclear. We found that lymphatic vessels are largely absent from chronic human wounds as evaluated in patient biopsies. Excisional wound healing studies were conducted using transgenic mice with or without an increased number of cutaneous lymphatic vessels, as well as antibody-mediated inhibition of lymphangiogenesis. We found that a lack of lymphatic vessels mediated a proinflammatory wound microenvironment and delayed wound closure, and that the VEGF-C/VEGFR3 signaling axis is required for wound lymphangiogenesis. Treatment of diabetic mice (db/db mice) with the F8-VEGF-C fusion protein that targets the alternatively spliced extra domain A (EDA) of fibronectin, expressed in remodeling tissue, promoted wound healing, and potently induced wound lymphangiogenesis. The treatment also reduced tissue inflammation and exerted beneficial effects on the wound microenvironment, including myofibroblast density and collagen deposition. These findings indicate that activating the lymphatic vasculature might represent a new therapeutic strategy for treating chronic non-healing wounds.

Beyond the Scar: A Basic Science Review of Wound Remodeling

Significance: Increasing development of experimental animal models has allowed for the study of scar formation. However, many pathophysiological unknowns remain in the longest stage of healing, the remodeling stage, which may continue for a year or more. The wound healing process results in different types of scarring classified as normal or pathological depending on failures at each stage. Failures can also occur during wound remodeling, but the molecular mechanisms driving the wound remodeling process have yet to be investigated. Recent Advances: While the current understanding of wound repair is based on investigations of acute healing, these experimental models have informed knowledge of key components of remodeling. This review examines the components that contribute to collagen organization and the final scar, including cell types, their regulation, and signaling pathways. Dysregulation in any one of these components causes pathologic healing. Critical Issues and Future Directions: As wounds continue to remodel months to years after reepithelialization, new models to better understand long-term remodeling will be critical for improving healing outcomes. Further investigation of the contributions of fibroblasts and cell signaling pathways involved during remodeling as well as their potential failures may inform new approaches in promoting regenerative healing beyond reepithelialization.

Bioprinting a skin patch with dual-crosslinked gelatin (GelMA) and silk fibroin (SilMA): An approach to accelerating cutaneous wound healing

Clinical settings often face significant obstacles in treating large acute wounds. The alternative of therapeutic approach is needed urgently. Hydrogels derived from natural or synthetic materials may be designed to perform a variety of functions for promoting wound healing. Herein, a 3D bioprinted hydrogel patch is designed for accelerating acute wound healing, which is fabricated with methacryloyl-substituted gelatin (GelMA) and silk fibroin (SilMA) dual-cross-linked by ultraviolet (UV) light. The GelMA with added silk fibroin (GelSilMA) shows improved biodegradation and mechanical properties. Furthermore, SilMA hydrogel can maintain a moisturized healing environment in wound area persistently with adequate degradation capacity. In vivo, GelSilMA (G-S) hydrogel can help to speed wound closure by the improved microenvironment for epidermal tissue regeneration and endogenous collagen generation accordingly. In summary, the G-S hydrogel patch can accelerate acute wound healing efficiently in a relatively simple and inexpensive manner.

Intrinsic Networks Regulating Tissue Repair: Comparative Studies of Oral and Skin Wound Healing

Wound repair is a systematic biological program characterized by four overlapping phases: hemostasis, inflammation, proliferation, and remodeling. Notwithstanding differences between species and distinct anatomical sites, the fundamental phases in the wound healing process are conserved among mammalian species. Oral wound healing is defined as an ideal wound healing model because it resolves rapidly and without scar formation. Understanding the regulation and contribution of the different molecular events that drive rapid wound healing in oral mucosa compared with those of the skin will help us define how these lesions heal more efficiently and may provide new therapeutic strategies that can be translated to the clinical settings for patients with chronic nonhealing wounds. Although all epithelial tissues have remarkable ability for tissue repair, the efficiency of such repair differs between epithelia (oral mucosa vs. cutaneous). This prompts the long-standing, fundamental biological and clinically relevant questions as to why and how does the oral mucosa achieve its enhanced wound healing capacity. In this review, we focus on (1) distinct innate wound healing capabilities of the oral mucosa, (2) lessons learned from comparative transcriptomic studies of oral mucosa versus skin, and (3) translation of findings to therapeutics for enhanced wound healing.

Advances in non-invasive biosensing measures to monitor wound healing progression

Impaired wound healing is a significant financial and medical burden. The synthesis and deposition of extracellular matrix (ECM) in a new wound is a dynamic process that is constantly changing and adapting to the biochemical and biomechanical signaling from the extracellular microenvironments of the wound. This drives either a regenerative or fibrotic and scar-forming healing outcome. Disruptions in ECM deposition, structure, and composition lead to impaired healing in diseased states, such as in diabetes. Valid measures of the principal determinants of successful ECM deposition and wound healing include lack of bacterial contamination, good tissue perfusion, and reduced mechanical injury and strain. These measures are used by wound-care providers to intervene upon the healing wound to steer healing toward a more functional phenotype with improved structural integrity and healing outcomes and to prevent adverse wound developments. In this review, we discuss bioengineering advances in 1) non-invasive detection of biologic and physiologic factors of the healing wound, 2) visualizing and modeling the ECM, and 3) computational tools that efficiently evaluate the complex data acquired from the wounds based on basic science, preclinical, translational and clinical studies, that would allow us to prognosticate healing outcomes and intervene effectively. We focus on bioelectronics and biologic interfaces of the sensors and actuators for real time biosensing and actuation of the tissues. We also discuss high-resolution, advanced imaging techniques, which go beyond traditional confocal and fluorescence microscopy to visualize microscopic details of the composition of the wound matrix, linearity of collagen, and live tracking of components within the wound microenvironment. Computational modeling of the wound matrix, including partial differential equation datasets as well as machine learning models that can serve as powerful tools for physicians to guide their decision-making process are discussed.

Using different geometries to modulate the cardiac fibroblast phenotype and the biomechanical properties of engineered connective tissues

Tissue engineering with human cardiac fibroblasts (CF) allows identifying novel mechanisms and anti-fibrotic drugs in the context of cardiac fibrosis. However, substantial knowledge on the influences of the used materials and tissue geometries on tissue properties and cell phenotypes is necessary to be able to choose an appropriate model for a specific research question. As there is a clear lack of information on how CF react to the mold architecture in engineered connective tissues (ECT), we first compared the effect of two mold geometries and materials with different hardnesses on the biomechanical properties of ECT. We could show that ECT, which formed around two distant poles (non-uniform model) were less stiff and more strain-resistant than ECT, which formed around a central rod (uniform model), independent of the materials used for poles and rods. Next, we investigated the cell state and could demonstrate that in the uniform versus non-uniform model, the embedded cells have a higher cell cycle activity and display a more pronounced myofibroblast phenotype. Differential gene expression analysis revealed that uniform ECT displayed a fibrosis-associated gene signature similar to the diseased heart. Furthermore, we were able to identify important relationships between cell and tissue characteristics, as well as between biomechanical tissue parameters by implementing cells from normal heart and end-stage heart failure explants from patients with ischemic or dilated cardiomyopathy. Finally, we show that the application of pro- and anti-fibrotic factors in the non-uniform and uniform model, respectively, is not sufficient to mimic the effect of the other geometry. Taken together, we demonstrate that modifying the mold geometry in tissue engineering with CF offers the possibility to compare different cellular phenotypes and biomechanical tissue properties.

Stem Cell-Based Tissue Engineering for the Treatment of Burn Wounds: A Systematic Review of Preclinical Studies

Burn wounds are a devastating type of skin injury leading to severe impacts on both patients and the healthcare system. Current treatment methods are far from ideal, driving the need for tissue engineered solutions. Among various approaches, stem cell-based strategies are promising candidates for improving the treatment of burn wounds. A thorough search of the Embase, Medline, Scopus, and Web of Science databases was conducted to retrieve original research studies on stem cell-based tissue engineering treatments tested in preclinical models of burn wounds, published between January 2009 and June 2021. Of the 347 articles retrieved from the initial database search, 33 were eligible for inclusion in this review. The majority of studies used murine models with a xenogeneic graft, while a few used the porcine model. Thermal burn was the most commonly induced injury type, followed by surgical wound, and less commonly radiation burn. Most studies applied stem cell treatment immediately post-burn, with final endpoints ranging from 7 to 90 days. Mesenchymal stromal cells (MSCs) were the most common stem cell type used in the included studies. Stem cells from a variety of sources were used, most commonly from adipose tissue, bone marrow or umbilical cord, in conjunction with an extensive range of biomaterial scaffolds to treat the skin wounds. Overall, the studies showed favourable results of skin wound repair in animal models when stem cell-based tissue engineering treatments were applied, suggesting that such strategies hold promise as an improved therapy for burn wounds.

Down-Regulating Scar Formation by Microneedles Directly via a Mechanical Communication Pathway

Excessive extracellular matrix deposition drives fibroblasts into a state of high mechanical stress, exacerbating pathological fibrosis and hypertrophic scar formation, leading to tissue dysfunction. This study reports a minimally invasive and convenient approach to obtaining scarless tissue using a silk fibroin microneedle patch (SF MNs). We found that by tuning the MN size and density only, the biocompatible MNs significantly decreased the scar elevation index in the rabbit ear hypertrophic scar model and increased ultimate tensile strength close to regular skin. To advance our understanding of this recent approach, we built a fibroblast-populated collagen lattice system and finite element model to study MN-mediated cellular behavior of fibroblasts. We found that the MNs reduced the fibroblasts generated contraction and mechanical stress, as indicated by decreased expression of the mechanical sensitive gene ANKRD1. Specifically, SF MNs attenuated the integrin-FAK signaling and consequently down-regulated the expression of TGF-β1, α-SMA, collagen I, and fibronectin. It resulted in a low-stress microenvironment that helps to reduce scar formation significantly. Microneedles' physical intervention via the mechanotherapeutic strategy is promising for scar-free wound healing.

Profibrotic Signaling Pathways and Surface Markers Are Upregulated in Fibroblasts of Human Striae Distensae and in a Mouse Model System

INTRODUCTION

Striae distensae (SD) are common disfiguring cutaneous lesions but lack effective treatments due to an incomplete understanding of their pathophysiology. Dermal fibroblasts likely play an important role. We investigate the cellular-molecular features distinguishing fibroblasts from human SD and normal skin (NS). We also develop a mouse model of SD.

METHODS

Human SD and NS samples were compared for tensile strength and histological structure. Fibroblasts from SD and NS were isolated by fluorescence-activated cell sorting (FACS) for gene expression analysis. Immunofluorescence staining and FACS were used to confirm gene expression data at the protein level. A mouse model of SD formation was created by administering corticosteroids and mechanically loading the dorsal skin.

RESULTS

Human SD exhibited reduced tensile strength, more disordered collagen fibers, and epidermal atrophy compared to human NS. There were 296 upregulated genes in SD fibroblasts, including the profibrotic lineage and surface marker CD26. Upregulated genes were involved in profibrotic and mechanoresponsive signaling pathways (TGFβ and FAK-PI3-AKT-signaling). In contrast, 571 genes were downregulated, including CD74 and genes of the AMPK pathway. Increased CD26 and decreased CD74 expression was confirmed by FACS and immunofluorescence. Similar cutaneous histological and gene expression changes were induced in hypercortisolemic mice by mechanically loading the dorsal skin.

CONCLUSIONS

Fibroblasts from human SD exhibit increased profibrotic and decreased antifibrotic signaling. CD26 and CD74 are promising surface markers that may be targeted therapeutically. Our mouse model of SD can be used as a platform to test the efficacy of potential therapeutic agents.

Exploring the Effects of Standardized Soft Tissue Mobilization on the Viscoelastic Properties, Pressure Pain Thresholds, and Tactile Pressure Thresholds of the Cesarean Section Scar

Background:

Objectives of soft tissue mobilization applied to cesarean section (C-section) scars are to decrease stiffness and to reduce pain. Research investigating these effects is lacking.

Materials and methods:

The authors conducted a descriptive, exploratory, proof-of-concept clinical study. Women aged 18 to 40 years who had undergone at least one C-section were recruited. A trained osteopath performed standardized mobilization of the C-section scar once a week for 2 weeks. Scar quality and pain characteristics, viscoelastic properties, pressure pain thresholds, and tactile pressure thresholds were measured before and after each session. Paired Student's t-tests and Friedman's test with Dunn–Bonferroni adjustment were performed to assess the immediate and short-term effects of mobilizations. Kendall's W and Cohen's d were calculated to determine effect sizes over the short term. Simple bootstrapped bias-corrected and accelerated 95% median confidence intervals were computed.

Results:

Thirty-two participants completed the study. The Patient and Observer Scar Assessment Scale questionnaire revealed differences with small and moderate effects for stiffness (p = 0.021, d = 0.43), relief (p < 0.001, d = 0.28), surface area (p = 0.040, d = 0.36), flexibility (p = 0.007, d = 0.52), and participant opinion (p = 0.001, d = 0.62). Mobilizations increased elasticity (p < 0.001, W = 0.11), decreased stiffness (p < 0.001, W = 0.30), and improved pressure pain thresholds (p < 0.001, W = 0.10) of the C-section, with small to moderate effects. The results also showed decreased tone and mechanical stress relaxation time, as well as increased tactile pressure thresholds at the different measurement times (p < 0.05), but trivial effect sizes (W < 0.10). Creep showed trivial effect and no significant difference (p = 0.09).

Conclusion:

This study showed that two sessions of mobilization of C-section scar might have a beneficial effect on some viscoelastic properties of the C-section as well as on pain. Some variables of interest useful for future empirical studies are highlighted.

ClinicalTrial. Gov NCT04320355.

Application of chitosan nanoparticles in skin wound healing

The rising prevalence of impaired wound healing and the consequential healthcare burdens have gained increased attention over recent years. This has prompted research into the development of novel wound dressings with augmented wound healing functions. Nanoparticle (NP)-based delivery systems have become attractive candidates in constructing such wound dressings due to their various favourable attributes. The non-toxicity, biocompatibility and bioactivity of chitosan (CS)-based NPs make them ideal candidates for wound applications. This review focusses on the application of CS-based NP systems for use in wound treatment. An overview of the wound healing process was presented, followed by discussion on the properties and suitability of CS and its NPs in wound healing. The wound healing mechanisms exerted by CS-based NPs were then critically analysed and discussed in sections, namely haemostasis, infection prevention, inflammatory response, oxidative stress, angiogenesis, collagen deposition, and wound closure time. The results of the studies were thoroughly reviewed, and contradicting findings were identified and discussed. Based on the literature, the gap in research and future prospects in this research area were identified and highlighted. Current evidence shows that CS-based NPs possess superior wound healing effects either used on their own, or as drug delivery vehicles to encapsulate wound healing agents. It is concluded that great opportunities and potentials exist surrounding the use of CSNPs in wound healing.

Method development and characterization of chick embryo tendon mechanical properties

Tendons are involved in multiple disorders and injuries, ranging from birth deformities to tendinopathies to acute ruptures. The ability to characterize embryonic tendon mechanical properties will enable elucidation of mechanisms responsible for functional tendon formation. In turn, an understanding of tendon development could inform approaches for adult and embryonic tendon tissue engineering and regenerative medicine. The chick embryo is a scientifically relevant model that we have been using to study Achilles (calcaneal) tendon development. Chick embryo calcaneal tendons are challenging to mechanically test due to small size and delicate nature, and difficulty distinguishing embryonic tendons from muscle and fibrocartilage using the naked eye. Here, we developed and implemented a "marking protocol" to identify and isolate calcaneal tendons at different stages of chick embryonic development. Mechanical testing of tendons isolated using the marking protocol revealed trends in mechanical property development that were not observed with tendons isolated by naked eye (eyeballing). Marked tendons exhibited non-linear increases in tensile modulus and ultimate tensile strength, whereas eyeballed tendons exhibited linear increases in the same properties, reflecting a need for the marking protocol. Furthermore, the tensile mechanical properties characterized for marked tendons are consistent with previously reported trends in cell length-scale mechanical properties measured using atomic force microscopy. This report establishes new methodology to enable tensile testing of chick embryo tendons and provides new information about embryonic tendon mechanical property development.

In vivo printing of growth factor-eluting adhesive scaffolds improves wound healing

Acute and chronic wounds affect millions of people around the world, imposing a growing financial burden on patients and hospitals. Despite the application of current wound management strategies, the physiological healing process is disrupted in many cases, resulting in impaired wound healing. Therefore, more efficient and easy-to-use treatment modalities are needed. In this study, we demonstrate the benefit of in vivo printed, growth factor-eluting adhesive scaffolds for the treatment of full-thickness wounds in a porcine model. A custom-made handheld printer is implemented to finely print gelatin-methacryloyl (GelMA) hydrogel containing vascular endothelial growth factor (VEGF) into the wounds. In vitro and in vivo results show that the in situ GelMA crosslinking induces a strong scaffold adhesion and enables printing on curved surfaces of wet tissues, without the need for any sutures. The scaffold is further shown to offer a sustained release of VEGF, enhancing the migration of endothelial cells in vitro. Histological analyses demonstrate that the administration of the VEGF-eluting GelMA scaffolds that remain adherent to the wound bed significantly improves the quality of healing in porcine wounds. The introduced in vivo printing strategy for wound healing applications is translational and convenient to use in any place, such as an operating room, and does not require expensive bioprinters or imaging modalities.

The interplay of fibroblasts, the extracellular matrix, and inflammation in scar formation

Various forms of fibrosis, comprising tissue thickening and scarring, are involved in 40% of deaths across the world. Since the discovery of scarless functional healing in fetuses prior to a certain stage of development, scientists have attempted to replicate scarless wound healing in adults with little success. While the extracellular matrix (ECM), fibroblasts, and inflammatory mediators have been historically investigated as separate branches of biology, it has become increasingly necessary to consider them as parts of a complex and tightly regulated system that becomes dysregulated in fibrosis. With this new paradigm, revisiting fetal scarless wound healing provides a unique opportunity to better understand how this highly regulated system operates mechanistically. In the following review, we navigate the four stages of wound healing (hemostasis, inflammation, repair, and remodeling) against the backdrop of adult versus fetal wound healing, while also exploring the relationships between the ECM, effector cells, and signaling molecules. We conclude by singling out recent findings that offer promising leads to alter the dynamics between the ECM, fibroblasts, and inflammation to promote scarless healing. One factor that promises to be significant is fibroblast heterogeneity and how certain fibroblast subpopulations might be predisposed to scarless healing. Altogether, reconsidering fetal wound healing by examining the interplay of the various factors contributing to fibrosis provides new research directions that will hopefully help us better understand and address fibroproliferative diseases, such as idiopathic pulmonary fibrosis, liver cirrhosis, systemic sclerosis, progressive kidney disease, and cardiovascular fibrosis.

Elucidating the combinatorial effect of substrate stiffness and surface viscoelasticity on cellular phenotype

Cells maintain tensional homeostasis by monitoring the mechanics of their microenvironment. In order to understand this mechanotransduction phenomenon, hydrogel materials have been developed with either controllable linear elastic or viscoelastic properties. Native biological tissues, and biomaterials used for medical purposes, often have complex mechanical properties. However, due to the difficulty in completely decoupling the elastic and viscous components of hydrogel materials, the effect of complex composite materials on cellular responses has largely gone unreported. Here, we characterize a novel composite hydrogel system capable of decoupling and individually controlling both the bulk stiffness and surface viscoelasticity of the material by combining polyacrylamide (PA) gels with microgel thin films. By taking advantage of the high degree of control over stiffness offered by PA gels and viscoelasticity, in terms of surface loss tangent, of microgel thin films, it is possible to study the influence that bulk substrate stiffness and surface loss tangent have on complex fibroblast responses, including cellular and nuclear morphology and gene expression. This material system provides a facile method for investigating cellular responses to complex material mechanics with great precision and allows for a greater understanding of cellular mechanotransduction mechanisms than previously possible through current model material platforms.

Can the CutiScan CS 100® measure anisotropy and viscoelasticity in scar tissue after mastectomy? A reliability and validity study

BACKGROUND

Scars have different biomechanical characteristics, including anisotropy and viscoelasticity compared to healthy skin. To assess these characteristics, the CutiScan CS 100^® can be used. The aim of the present study is to investigate reliability and validity of this device in breast cancer patients.

MATERIALS AND METHODS

Thirty female patients, with scar adhesions following mastectomy were assessed with the CutiScan CS 100^® . Maximal distensibility (pixels) (V1), after-suction return rate (pixels) (V2), and their ratio (%) (V3) at three points on and around the scar were assessed as measures of viscoelasticity. For intra- and interrater reliabilities, the intra-class correlation coefficient (ICC) and its 95% confidence intervals were calculated. The standard error of measurement (SEM) was calculated to interpret reproducibility of these measurements. To investigate criterion validity of the measurement of anisotropy, measurements in the direction of healthy skin were compared with measurements in the direction of the scar, using a paired t-test.

RESULTS

V1, V2, and V3 show poor to moderate intrarater reliability (ICC 0.00-0.72) and interrater reliability (ICC 0.00-0.53). The maximum displacement (V1) on the measurement point above the scar shows the best reliability (ICC 0.33-0.72). The SEM is about the same for all parameters at all three points. The paired sample t-test showed a significant difference (p < 0.05) between V1 in the direction towards the scar versus the measurement towards healthy tissue, on the point below the scar.

CONCLUSION

These first reliability and validity results of the CutiScan CS 100^® for measuring anisotropy and viscoelasticity in scar tissue adhesions after mastectomy seem promising. Further research is needed addressing the limitations of the present study design.

Evolution of compressive mechanical properties of early hypertrophic scar during laser treatment

Laser therapy has been widely used in the treatment of hypertrophic scars (HPS), but whether the mechanical properties of HPS tissue after laser treatment can be restored to those of normal skin remains unclear. In this paper, the relationship between the evolution of compressive mechanical properties and histological changes of HPS tissues following three successive combined pulsed dye laser (PDL) and fractional CO₂ laser (CO₂) treatments was investigated by compression tests and histological analysis. The early HPS model of rabbit ear was established by CO₂ laser ablation. The loading-unloading tests and strain creep tests under the compression forces of 1 N, 2 N, and 3 N were carried out for normal skin, untreated HPS and HPS after different treatment times, respectively. The results showed that the compression ratio λ of all tissues revealed force dependence and rose with the increasing compression force, which was similar to the trend of most biological soft tissues. The histological changes of HPSs following laser treatment have a significant influence on the compressive mechanical response. Compared with the normal skin, the toughness and anti-deformation ability of HPS reduced due to the proliferation of collagen fibers and the destruction of elastic fibers, resulting in higher energy dissipation, compression ratio λ, and stable creep rate D, and lower elastic modulus. After three successive combined PDL/ CO₂ laser treatments, the compressive mechanical properties and creep properties of HPS gradually approached that of the normal skin owing to the gradual restoration of the amount and distribution of collagen and elastic fibers in HPS. The results provide a new method for evaluating the clinical efficacy of laser therapy for treatment of HPS tissue.

Chronic Sequelae After Muscle Strain Injuries: Influence of Heavy Resistance Training on Functional and Structural Characteristics in a Randomized Controlled Trial

BACKGROUND

Muscle strain injury leads to a high risk of recurrent injury in sports and can cause long-term symptoms such as weakness and pain. Scar tissue formation after strain injuries has been described, yet what ultrastructural changes might occur in the chronic phase of this injury have not. It is also unknown if persistent symptoms and morphological abnormalities of the tissue can be mitigated by strength training.

PURPOSE

To investigate if heavy resistance training improves symptoms and structural abnormalities after strain injuries.

STUDY DESIGN

Randomized controlled trial; Level of evidence, 1.

METHODS

A total of 30 participants with long-term weakness and/or pain after a strain injury of the thigh or calf muscles were randomized to eccentric heavy resistance training of the injured region or control exercises of the back and abdominal muscle. Isokinetic (hamstring) or isometric (calf) muscle strength was determined, muscle cross-sectional area measured, and pain and function evaluated. Scar tissue ultrastructure was determined from biopsy specimens taken from the injured area before and after the training intervention.

RESULTS

Heavy resistance training over 3 months improved pain and function, normalized muscle strength deficits, and increased muscle cross-sectional area in the previously injured region. No systematic effect of training was found upon pathologic infiltration of fat and blood vessels into the previously injured area. Control exercises had no effect on strength, cross-sectional area, or scar tissue but a positive effect on patient-related outcome measures, such as pain and functional scores.

CONCLUSION

Short-term strength training can improve sequelae symptoms and optimize muscle function even many years after a strain injury, but it does not seem to influence the overall structural abnormalities of the area with scar tissue.

REGISTRATION

NCT02152098 (ClinicalTrials.gov identifier).

Histatin 1 enhanced the speed and quality of wound healing through regulating the behaviour of fibroblast

Objectives

Histatin 1(Hst 1) has been proved to promote wound healing. However, there was no specific study on the regulation made by Hst 1 of fibroblasts in the process of wound healing. This research comprehensively studied the regulation of Hst 1 on the function of fibroblasts in the process of wound healing and preliminary mechanism about it.

Materials and methods

The full‐thickness skin wound model was made on the back of C57/BL6 mice. The wound healing, collagen deposition and fibroblast distribution were detected on days 3, 5 and 7 after injury. Fibroblast was cultured in vitro and stimulated with Hst 1, and then, their biological characteristics and functions were detected.

Results

Histatin 1 can effectively promote wound healing, improve collagen deposition during and after healing and increase the number and function of fibroblasts. After healing, the mechanical properties of the skin also improved. In vitro, the migration ability of fibroblasts stimulated by Hst 1 was significantly improved, and the fibroblasts transformed more into myofibroblasts, which improved the function of contraction and collagen secretion. In fibroblasts, mTOR signalling pathway can be activated by Hst 1.

Conclusions

Histatin 1 can accelerate wound healing and improve the mechanical properties of healed skin by promoting the function of fibroblasts. The intermolecular mechanisms need to be further studied, and this study provides a direction about mTOR signalling pathway.

A Polymer-Biologic Hybrid Hernia Construct: Review of Data and Early Experiences

Surgical mesh reinforcement of the human abdominal wall has been found to reduce the chance of recurrence in hernia repairs. While traditionally polymer meshes have been used in hernia repair, alternative mesh options have been engineered to prevent the inflammatory foreign body response invoked by polymers. A reinforced tissue matrix (RTM) mesh has been developed by embedding a polymer within a decellularized extracellular matrix. This combination has been attributed to the recruitment of host cells, a pro-healing response, and attenuation of the foreign body response. This has been observed to lead to the regeneration of functional tissue within the repair site that is reinforced by the polymer to offload abdominal pressures over time. This manuscript presents the review of OviTex, an RTM, in several types of hernia repair. The authors have found that the use of RTM in hernia repair is effective in preventing foreign body response, promoting wound healing, and providing reinforcement to lower the risk of hernia recurrence.

Addressing Full-Thickness Skin Defects: A Review of Clinically Available Autologous Skin Replacements

Autologous keratinocyte culture, and combinations of scaffolds, different cell types, solutions of macromolecules, or growth factors have contributed to the resurfacing of full-thickness skin defects. Ideally, a treatment for full-thickness skin defects should not merely reestablish continuity of the surface of the skin but should restore its structure to allow skin to function as a dynamic biological factory that can participate in protein synthesis, metabolism, and cell signaling, and form an essential part of the body's immune, nervous, and endocrine systems. This paper provides a review of clinically available autologous skin replacements, highlighting the importance of regenerating an organ that will function physiologically.

A method to analyze the influence of mechanical strain on dermal collagen morphologies

Collagen fibers and their orientation play a major role in the mechanical behavior of soft biological tissue such as skin. Here, we present a proof-of-principle study correlating mechanical properties with collagen fiber network morphologies. A dedicated multiphoton stretching device allows for mechanical deformations in combination with a simultaneous analysis of its collagen fiber network by second harmonic generation imaging (SHG). The recently introduced Fiber Image Network Evaluation (FINE) algorithm is used to obtain detailed information about the morphology with regard to fiber families in collagen network images. To demonstrate the potential of our method, we investigate an isotropic and an anisotropic ex-vivo dorsal pig skin sample under quasi-static cyclic stretching and relaxation sequences. Families of collagen fibers are found to form a partially aligned collagen network under strain. We find that the relative force uptake is accomplished in two steps. Firstly, fibers align within their fiber families and, secondly, fiber families orient in the direction of force. The maximum alignment of the collagen fiber network is found to be determined by the largest strain. Isotropic and anisotropic samples reveal a different micro structural behavior under repeated deformation leading to a similar force uptake after two stretching cycles. Our method correlates mechanical properties with morphologies in collagen fiber networks.

Treatment of murine partial thickness scald injuries with multipotent adult progenitor cells decreases inflammation and promotes angiogenesis leading to improved burn injury repair

Stem cells have been shown to have potential as a new therapy for burns and promote wound healing through decreasing inflammation and increasing angiogenesis. Multipotent adult progenitor cells (MAPC® cells) are a subpopulation of bone marrow-derived stem cells with outstanding self-renewal and differentiation capacity. MAPC cells also secrete a wide range of cytokines which can affect cellular activities. This article aimed to examine the effects of MAPC cells treatment on burn injury repair using a mouse model of partial thickness burn injury. The immunomodulatory effect of MAPC cells was investigated in vitro using a simultaneous T-cell proliferation assay. Partial thickness burns were created on the dorsal surface of mice and MAPC cells were administered via intradermal injection to the wound margins 24 h post-burn injury. The burn tissues were analysed macroscopically to determine wound area and histologically assessed to determine wound width and rate of re-epithelialisation. Immunohistochemistry and ELISA were employed to assess cell proliferation, inflammation and angiogenesis and collagen deposition in the burn area. MAPC cells inhibit the proliferation of stimulated T cells in culture. Burns intradermally injected with MAPC cells showed a significant reduction in the macroscopic wound area, histologic wound width and had an increased rate of re-epithelialisation. Immunohistochemistry and ELISA analysis of burn tissues showed dampened inflammation evidenced by a reduction in neutrophilic infiltration and modulation of inflammatory cytokines. Angiogenesis within the burn area was also improved in MAPC cell treated mice. However, no significant effect of MAPC cell treatment was observed on extracellular matrix production. Treatment of burns with MAPC cells improved burn injury repair with reduced time to healing, decreased inflammation and increased angiogenesis. These findings demonstrate the promising effects of MAPC cells on burn injury repair and suggest MAPC cells as a candidate source for clinical cell therapies.

Mechanical stretch promotes hypertrophic scar formation through mechanically activated cation channel Piezo1

Hypertrophic scar (HS) formation is a skin fibroproliferative disease that occurs following a cutaneous injury, leading to functional and cosmetic impairment. To date, few therapeutic treatments exhibit satisfactory outcomes. The mechanical force has been shown to be a key regulator of HS formation, but the underlying mechanism is not completely understood. The Piezo1 channel has been identified as a novel mechanically activated cation channel (MAC) and is reportedly capable of regulating force-mediated cellular biological behaviors. However, the mechanotransduction role of Piezo1 in HS formation has not been investigated. In this work, we found that Piezo1 was overexpressed in myofibroblasts of human and rat HS tissues. In vitro, cyclic mechanical stretch (CMS) increased Piezo1 expression and Piezo1-mediated calcium influx in human dermal fibroblasts (HDFs). In addition, Piezo1 activity promoted HDFs proliferation, motility, and differentiation in response to CMS. More importantly, intradermal injection of GsMTx4, a Piezo1-blocking peptide, protected rats from stretch-induced HS formation. Together, Piezo1 was shown to participate in HS formation and could be a novel target for the development of promising therapies for HS formation.

Unlocking mammalian regeneration through hypoxia inducible factor one alpha signaling

Historically, the field of regenerative medicine has aimed to heal damaged tissue through the use of biomaterials scaffolds or delivery of foreign progenitor cells. Despite 30 years of research, however, translation and commercialization of these techniques has been limited. To enable mammalian regeneration, a more practical approach may instead be to develop therapies that evoke endogenous processes reminiscent of those seen in innate regenerators. Recently, investigations into tadpole tail regrowth, zebrafish limb restoration, and the super-healing Murphy Roths Large (MRL) mouse strain, have identified ancient oxygen-sensing pathways as a possible target to achieve this goal. Specifically, upregulation of the transcription factor, hypoxia-inducible factor one alpha (HIF-1α) has been shown to modulate cell metabolism and plasticity, as well as inflammation and tissue remodeling, possibly priming injuries for regeneration. Since HIF-1α signaling is conserved across species, environmental or pharmacological manipulation of oxygen-dependent pathways may elicit a regenerative response in non-healing mammals. In this review, we will explore the emerging role of HIF-1α in mammalian healing and regeneration, as well as attempts to modulate protein stability through hyperbaric oxygen treatment, intermittent hypoxia therapy, and pharmacological targeting. We believe that these therapies could breathe new life into the field of regenerative medicine.

Magnetic Nanocomposite Hydrogels for Tissue Engineering: Design Concepts and Remote Actuation Strategies to Control Cell Fate

Most tissues of the human body are characterized by highly anisotropic physical properties and biological organization. Hydrogels have been proposed as scaffolding materials to construct artificial tissues due to their water-rich composition, biocompatibility, and tunable properties. However, unmodified hydrogels are typically composed of randomly oriented polymer networks, resulting in homogeneous structures with isotropic properties different from those observed in biological systems. Magnetic materials have been proposed as potential agents to provide hydrogels with the anisotropy required for their use on tissue engineering. Moreover, the intrinsic properties of magnetic nanoparticles enable their use as magnetomechanic remote actuators to control the behavior of the cells encapsulated within the hydrogels under the application of external magnetic fields. In this review, we combine a detailed summary of the main strategies to prepare magnetic nanoparticles showing controlled properties with an analysis of the different approaches available to their incorporation into hydrogels. The application of magnetically responsive nanocomposite hydrogels in the engineering of different tissues is also reviewed.

Biomechanics of Wound Healing in an Equine Limb Model: Effect of Location and Treatment with a Peptide-Modified Collagen-Chitosan Hydrogel

The equine distal limb wound healing model, characterized by delayed re-epithelialization and a fibroproliferative response to wounding similar to that observed in humans, is a valuable tool for the study of biomaterials poised for translation into both the veterinary and human medical markets. In the current study, we developed a novel method of biaxial biomechanical testing to assess the functional outcomes of healed wounds in a modified equine model and discovered significant functional and structural differences in both unwounded and injured skin at different locations on the distal limb that must be considered when using this model in future work. Namely, the medial skin was thicker and displayed earlier collagen engagement, medial wounds experienced a greater proportion of wound contraction during closure, and proximal wounds produced significantly more exuberant granulation tissue. Using this new knowledge of the equine model of aberrant wound healing, we then investigated the effect of a peptide-modified collagen-chitosan hydrogel on wound healing. Here, we found that a single treatment with the QHREDGS (glutamine-histidine-arginine-glutamic acid-aspartic acid-glycine-serine) peptide-modified hydrogel (Q-peptide hydrogel) resulted in a higher rate of wound closure and was able to modulate the biomechanical function toward a more compliant healed tissue without observable negative effects. Thus, we conclude that the use of a Q-peptide hydrogel provides a safe and effective means of improving the rate and quality of wound healing in a large animal model.

Physical characterization of swine and human skin: Correlations between Raman spectroscopy, Tensile testing, Atomic force microscopy (AFM), Scanning electron microscopy (SEM), and Multiphoton microscopy (MPM)

BACKGROUND

Swine dorsum is commonly utilized as a model for studying skin wounds and assessment of dermatological and cosmetic medicaments. The human abdomen is a common location for dermatological intervention.

OBJECTIVE

This study provides a correlation between spectral, mechanical, and structural characterization techniques, utilized for evaluating human abdominal skin and swine dorsum.

METHODS

Raman spectroscopy (RS), tensile testing, ballistometry, AFM, SEM, and MPM were utilized to characterize and compare full-thickness skin properties in swine and human model.

RESULTS

RS of both species' skin types revealed a similar assignment of vibrations in the fingerprint and the high wavenumber spectral regions. Structural imaging and mechanical characterization using ballistometry and tensile testing displayed differences in the inherent functional properties of human and swine skin. These differences correlated with variations in the Raman peak ratios, collagen intensity measured using SEM and MPM and collagen density measured using AFM.

CONCLUSION

A comprehensive evaluation of swine skin as a suitable substitute for human skin for mechanical and structural comparisons was performed. This data should be considered for better understanding the swine skin model for cutaneous drug delivery and wound applications. Additionally, correlation between RS, tensile testing, AFM, SEM, and MPM was performed as skin characterization tools.

Softening of the chronic hemi-section spinal cord injury scar parallels dysregulation of cellular and extracellular matrix content

Regeneration following spinal cord injury (SCI) is challenging in part due to the modified tissue composition and organization of the resulting glial and fibrotic scar regions. Inhibitory cell types and biochemical cues present in the scar have received attention as therapeutic targets to promote regeneration. However, altered Young's modulus of the scar as a readout for potential impeding factors for regeneration are not as well-defined, especially in vivo. Although the decreased Young's modulus of surrounding tissue at acute stages post-injury is known, the causation and outcomes at chronic time points remain largely understudied and controversial, which motivates this work. This study assessed the glial and fibrotic scar tissue's Young's modulus and composition (scar morphometry, cell identity, extracellular matrix (ECM) makeup) that contribute to the tissue's stiffness. The spatial Young's modulus of a chronic (~18-wks, post-injury) hemi-section, including the glial and fibrotic regions, were significantly less than naïve tissue (~200 Pa; p < 0.0001). The chronic scar contained cystic cavities dispersed in areas of dense nuclei packing. Abundant CNS cell types such as astrocytes, oligodendrocytes, and neurons were dysregulated in the scar, while epithelial markers such as vimentin were upregulated. The key ECM components in the CNS, namely sulfated proteoglycans (sPGs), were significantly downregulated following injury with concomitant upregulation of unsulfated glycosaminoglycans (GAGs) and hyaluronic acid (HA), likely altering the foundational ECM network that contributes to tissue stiffness. Our results reveal the Young's modulus of the chronic SCI scar as well as quantification of contributing elastic components that can provide a foundation for future study into their role in tissue repair and regeneration.

Recent innovations in artificial skin

The skin is a "smart", multifunctional organ that is protective, self-healing and capable of sensing and many forms of artificial skins have been developed with properties and functionalities approximating those of natural skin. Starting from specific commercial products for the treatment of burns, progress in two fields of research has since allowed these remarkable materials to be viable skin replacements for a wide range of dermatological conditions. This review maps out the development of bioengineered skin replacements and synthetic skin substitutes, including electronic skins. The specific behaviors of these skins are highlighted, and the performances of both types of artificial skins are evaluated against this. Moving beyond mere replication, highly advanced artificial skin materials are also identified as potential augmented skins that can be used as flexible electronics for health-care monitoring and other applications.

In vivo panoramic human skin shape and deformation measurement using mirror-assisted multi-view digital image correlation

Panoramic shape and deformation measurements of human skin in vivo may provide important information for biomechanical analysis, exercise guidance and medical diagnosis. This work proposes the application of an advanced mirror-assisted multi-view digital image correlation (DIC) method for dynamic measurements of 360-deg shape and deformation of human body parts in vivo. The main advantage of this method consists in its capabilities to perform full-panoramic non-contact measurements with a single pair of synchronized cameras and two planar mirrors thus representing a lean yet effective alternative to conventional multi-camera DIC systems in 'surrounding' configuration. We demonstrate the capabilities of this method by measuring the full-panoramic shape of a plastic human head, the deformation of a woman face and the principal strain distribution over the full-360-deg surface of a forearm during fist clenching. The applications of this method can be the most disparate but, given the possibility to determine the full-field strains and derived information (e.g. skin tension lines), we envisage a great potential for the study of skin biomechanics in vivo.

Current and Emerging Topical Scar Mitigation Therapies for Craniofacial Burn Wound Healing

Burn injury in the craniofacial region causes significant health and psychosocial consequences and presents unique reconstructive challenges. Healing of severely burned skin and underlying soft tissue is a dynamic process involving many pathophysiological factors, often leading to devastating outcomes such as the formation of hypertrophic scars and debilitating contractures. There are limited treatment options currently used for post-burn scar mitigation but recent advances in our knowledge of the cellular and molecular wound and scar pathophysiology have allowed for development of new treatment concepts. Clinical effectiveness of these experimental therapies is currently being evaluated. In this review, we discuss current topical therapies for craniofacial burn injuries and emerging new therapeutic concepts that are highly translational.

The Role of an IL-10/Hyaluronan Axis in Dermal Wound Healing

Scar formation is the typical endpoint of postnatal dermal wound healing, which affects more than 100 million individuals annually. Not only do scars cause a functional burden by reducing the biomechanical strength of skin at the site of injury, but they also significantly increase healthcare costs and impose psychosocial challenges. Though the mechanisms that dictate how dermal wounds heal are still not completely understood, they are regulated by extracellular matrix (ECM) remodeling, neovascularization, and inflammatory responses. The cytokine interleukin (IL)-10 has emerged as a key mediator of the pro- to anti-inflammatory transition that counters collagen deposition in scarring. In parallel, the high molecular weight (HMW) glycosaminoglycan hyaluronan (HA) is present in the ECM and acts in concert with IL-10 to block pro-inflammatory signals and attenuate fibrotic responses. Notably, high concentrations of both IL-10 and HMW HA are produced in early gestational fetal skin, which heals scarlessly. Since fibroblasts are responsible for collagen deposition, it is critical to determine how the concerted actions of IL-10 and HA drive their function to potentially control fibrogenesis. Beyond their independent actions, an auto-regulatory IL-10/HA axis may exist to modulate the magnitude of CD4⁺ effector T lymphocyte activation and enhance T regulatory cell function in order to reduce scarring. This review underscores the pathophysiological impact of the IL-10/HA axis as a multifaceted molecular mechanism to direct primary cell responders and regulators toward either regenerative dermal tissue repair or scarring.

Toward clinical elastography of dermal tissues: A medical device to probe skin's elasticity through suction, with subsurface imaging via optical coherence tomography

The mechanical behavior of dermal tissues is unarguably recognized for its diagnostic ability and in the last decades received a steadily increasing interest in dermatology practices. Among the various methods to investigate the mechanics of skin in clinical environments, suction-based ones are especially noteworthy, thanks to their qualities of minimal invasiveness and relative simplicity of setups and data analysis. In such experiments, structural visualization of the sample is highly desirable, both in its own right and because it enables elastography. The latter is a technique that combines the knowledge of an applied mechanical stimulus and the visualization of the induced deformation to result in a spatially resolved map of the mechanical properties, which is particularly important for an inhomogeneous and layered material such as skin. We present a device, designed for clinical trials in dermatology practices, that uses a handheld probe to (1) deliver a suction-based, controlled mechanical stimulus and (2) visualize the subsurface structure via optical coherence tomography. We also present a device-agnostic data-analysis framework, consisting of a Python library, released in the public domain. We show the working principle of the setup on a polymeric model and on a volunteer's skin.