In vivo and ex vivo approaches to studying the biomechanical properties of healing wounds in rat skin

The biomechanics of wounds at physiologically relevant levels: Understanding skin's stress-shielding effect for the quantitative assessment of healing

Wounds are responsible for the decrease in quality of life of billions of people around the world. Their assessment relies on subjective parameters which often delays optimal treatments and results in increased healthcare costs. In this work, we sought to understand and quantify how wounds at different healing stages (days 1, 3, 7 and 14 post wounding) change the mechanical properties of the tissues that contain them, and how these could be measured at clinically relevant strain levels, as a step towards quantitative wound tracking technologies. To achieve this, we used digital image correlation and mechanical testing on a mouse model of wound healing to map the global and local tissue strains. We found no significant differences in the elastic and viscoelastic properties of wounded vs unwounded skin when samples were measured in bulk, presumably as these were masked by the protective mechanisms of skin, which redistributes the applied loads to mitigate high stresses and reduce tissue damage. By measuring local strain values and observing the distinct patterns they formed, it was possible to establish a connection between the healing phase of the tissue (determined by the time post-injury and the observed histological features) and the overall mechanical behaviour. Importantly, these parameters were measured from the surface of the tissue, using physiologically relevant strains without increasing the tissue's damage. Adaptations of these approaches for clinical use have the potential to aid in the identification of skin healing problems, such as excessive inflammation or lack of mechanical progression over time. An increase, decrease, or lack of change in the elasticity and viscoelasticity parameters, can be indicative of wound state, thus ultimately leading to improved diagnostic outcomes.

Basic fibroblast growth factor gel preparation induces angiogenesis during wound healing

PURPOSE

This study aimed to observe the effect of basic fibroblast growth factor (bFGF) gel preparation on wound repair in a full-thickness skin defect rat model and to further explore its mechanism.

METHODS

The full-thickness skin defect model of Wistar rats was created with circular wounds of 20 mm or 10 mm in diameter on both sides of the spine. The animals were divided into the normal, model, control gel, and bFGF gel groups (300 IU/cm²). The effects of the bFGF gel on wound healing were evaluated and compared. Optical coherence tomography (OCT)-based angiography (OCTA) was used to investigate the effects of bFGF on angiogenesis during wound healing. Western blotting, polymerase chain reaction (PCR), and enzyme-linked immunosorbent assay (ELISA) kits were used to detect the effect of the gel preparation on the levels of vascular endothelial growth factor (VEGF) and matrix metalloproteinases (MMP2 and MMP9) on the wound surface to explore the mechanism.

RESULTS

The bFGF gel significantly reduced wound area, promoted the formation of wound granulation tissue, and accelerated wound healing in the bFGF gel group on days 7 and 14, compared with the control gel group. OCTA results showed that bFGF significantly improved wound vascular density, diameter, and circumference. Western blot, PCR, and ELISA results showed that the gel preparation could promote the expression levels of MMP2, MMP9, and VEGF on the wound surface 7 and 14 days after injury.

CONCLUSION

bFGF promotes angiogenesis in wound areas. Topical gel preparations of bFGF can be developed for use in wound repair.

A review of the characterizations of soft tissues used in human body modeling: Scope, limitations, and the path forward

Soft tissue material properties are vital to human body models that evaluate interactions between the human body and its environment. Such models evaluate internal stress/strain responses in soft tissues to investigate issues like pressure injuries. Numerous constitutive models and parameters have been used to represent mechanical behavior of soft tissues in biomechanical models under quasi-static loading. However, researchers reported that generic material properties cannot accurately represent specific target populations due to large inter-individual variability. Two challenges that exist are experimental mechanical characterization and constitutive modeling of biological soft tissues and personalization of constitutive parameters using non-invasive, non-destructive bedside testing methods. It is imperative to understand the scope and appropriate applications for reported material properties. Thus, the goal of this paper was to compile studies from which soft tissue material properties were obtained and categorize them by source of tissue samples, methods used to quantify deformation, and material models used to describe tissues. The collected studies displayed wide ranges of material properties, and factors that affected the properties included whether tissue samples were in vivo or ex vivo, from humans or animals, the body region tested, body position during in vivo studies, deformation measurements, and material models used to describe tissues. Because of the factors that affected reported material properties, it is clear that much progress has been made in understanding soft tissue responses to loading, yet there is a need to broaden the scope of reported soft tissue material properties and better match reported properties to appropriate human body models.

Activin-mediated alterations of the fibroblast transcriptome and matrisome control the biomechanical properties of skin wounds

Matrix deposition is essential for wound repair, but when excessive, leads to hypertrophic scars and fibrosis. The factors that control matrix deposition in skin wounds have only partially been identified and the consequences of matrix alterations for the mechanical properties of wounds are largely unknown. Here, we report how a single diffusible factor, activin A, affects the healing process across scales. Bioinformatics analysis of wound fibroblast transcriptome data combined with biochemical and histopathological analyses of wounds and functional in vitro studies identify that activin promotes pro-fibrotic gene expression signatures and processes, including glycoprotein and proteoglycan biosynthesis, collagen deposition, and altered collagen cross-linking. As a consequence, activin strongly reduces the wound and scar deformability, as identified by a non-invasive in vivo method for biomechanical analysis. These results provide mechanistic insight into the roles of activin in wound repair and fibrosis and identify the functional consequences of alterations in the wound matrisome at the biomechanical level.

The relationship between histopathology, gene expression, and biochemical and mechanical properties of wounds is largely unknown. Here, the authors show that activin A alters wound healing at multiple levels by promoting pro-fibrotic gene expression and matrix deposition, thereby affecting biomechanical properties of skin wounds.

Spatial-Temporal Changes of Mechanical Microenvironment in Skin Wounds During Negative Pressure Wound Therapy

Cell migration, proliferation, and differentiation are regulated by mechanical cues during skin wound healing. Negative pressure wound therapy (NPWT) reduces the healing period by optimizing the mechanical microenvironment of the wound bed. Under NPWT, it remains elusive how the mechanical microenvironment (e.g., stiffness, strain gradients) changes both in time and space during wound healing. To illustrate this, the healing time of full-thickness skin wounds under NPWT, with pressure settings ranging from -50 to -150 mm Hg, were evaluated and compared with gauze dressing treatments (control group), and three-dimensional finite element models of full-thickness skin wounds on days 1 and 5 after treatment were developed on the basis of MR 3D imaging data. Shear wave elastography (SWE) was applied to detect the stiffness of wound soft tissue on days 1 and 5, and nonlinear finite element analysis (FEA) was used to represent the spatial-temporal environment of the 3D strain field of the wound under NPWT vs the control group. Compared with the control group, NPWT with -50, -80, and -125 mm Hg promoted wound healing. SWE showed that the elastic modulus of wounded skin increased during healing. Meanwhile, the elastic modulus in wounded skin under NPWT was significantly smaller than in the control group. Strain and its gradient decreased under NPWT during wound healing, while no significant change was observed in the control group. This study, which is based on MR 3D imaging, shear wave elastography, and nonlinear FEA, provides an in-depth understanding of changes of the skin mechanical microenvironment under NPWT in the time-space dimension and the associated wound healing.

High myopia induced by form deprivation is associated with altered corneal biomechanical properties in chicks

The cornea is a soft, transparent, composite organic tissue, which forms the anterior outer coat of the eyeball. Although high myopia is increasing in prevalence worldwide and is known to alter the structure and biomechanical properties of the sclera, remarkably little is known about its impact on the biomechanics of the cornea. We developed and validated a novel optical-coherence-tomography-indentation probe–to measure corneal biomechanical properties in situ, in chicks having experimentally-induced high myopia, while maintaining intraocular pressure at levels covering the physiological range. We found that the cornea of highly myopic chicks was more steeply curved and softer, at all tested intraocular pressures, than that in contralateral, non-myopic eyes, or in age-matched normal, untreated eyes. These results indicate that the biomechanical properties of the cornea are altered in chicks developing experimentally-induced myopia.

Assessment of Corneal Biomechanical Properties with Inflation Test Using Optical Coherence Tomography

Biomechanical properties are important for the cornea to maintain its normal shape and function. There is still a need to develop better methods for accurate measurement of corneal mechanical properties. In this study, we propose to introduce the optical coherence tomography (OCT) in inflation test for the imaging of corneal deformation in order to measure its biomechanical properties. Ten cornea-mimicking silicone phantoms with different stiffness and five fresh porcine corneas were tested using the proposed method. Intra-ocular pressure was changed from 10 to 90 mmHg using two different loading rates to observe the pressure-apex displacement relationship and calculate the apparent stiffness of the corneas. Stiffness of the corneal phantoms obtained by the inflation test ranged from 0.2 to 1 MPa, which was highly consistent with the results from the mechanical tensile test (y = 0.70x, p < 0.001). The porcine corneas showed highly viscoelastic behavior with obvious hysteresis in inflation. The apparent stiffness of the porcine corneas was 0.63 ± 0.07 and 1.05 ± 0.08 MPa with loading rates of 3.3 and 33 mmHg/min, respectively. Mapping of corneal surface displacement was also generated for both the phantom and porcine corneas. This study showed that it is feasible to incorporate the high resolution OCT imaging in inflation test to measure the biomechanical properties of the cornea.

The mechanical fingerprint of murine excisional wounds

A multiscale mechanics approach to the characterization of murine excisional wounds subjected to uniaxial tensile loading is presented. Local strain analysis at a physiological level of tension uncovers the presence of two distinct regions within the wound: i) a very compliant peripheral cushion and ii) a core area undergoing modest deformation. Microstructural visualizations of stretched wound specimens show negligible engagement of the collagen located in the center of a 7-day old wound; fibers remain coiled despite the applied tension, confirming the existence of a mechanically isolated wound core. The compliant cushion located at the wound periphery appears to protect the newly-formed tissue from excessive deformation during the phase of new tissue formation. The early remodeling phase (day 14) is characterized by a restored mechanical connection between far field and wound center. The latter remains less deformable, a characteristic possibly required for cell activities during tissue remodeling. The distribution of fibrillary collagens at these two time points corresponds well to the identified heterogeneity of mechanical properties of the wound region. This novel approach provides new insight into the mechanical properties of wounded skin and will be applicable to the analysis of compound-treated wounds or wounds in genetically modified tissue.

STATEMENT OF SIGNIFICANCE

Biophysical characterization of healing wounds is crucial to assess the recovery of the skin barrier function and the associated mechanobiological processes. For the first time, we performed highly resolved local deformation analysis to identify mechanical characteristics of the wound and its periphery. Our results reveal the presence of a compliant cushion surrounding a stiffer wound core; we refer to this heterogeneous mechanical behavior as "mechanical fingerprint" of the wound. The mechanical response is shown to progress towards that of the intact skin as healing takes place. Histology and multiphoton microscopy suggest that wounded skin recovers its mechanical function via progressive reconnection of the newly-deposited collagen fibers with the surrounding intact matrix.

TGF-β1 Pretreatment Improves the Function of Mesenchymal Stem Cells in the Wound Bed

The wound healing process initiates after injury to a tissue and involves a series of orchestrated events to minimize the invasion of foreign matters such as bacteria and efficiently regenerate the damaged tissue. A variety of cells must be recruited to the tissue during wound healing. However, this process is severely disrupted in patients suffering from chronic illness, including diabetes, leading to impaired healing or non-healing wounds. Current avenues of treatment include negative-pressure therapy, wound debridement, growth factor replacement, and cell-based therapies. Among these therapies, mesenchymal stem cells (MSCs) delivery to the wound holds a very high promise due to the innate abilities of MSCs that include immunogenicity, plasticity, and self-renewal. Bone marrow derived MSCs have been shown to promote more rapid wound healing by increased cytokine production in diabetic mice. However, the lack of understanding of the mechanical and chemical interaction of the transplanted MSCs with the factors present in the regenerative niches limits their efficacy in the wound bed. In this study, we sought to understand how the changes in MSC biochemical and biophysical properties can affect their function in vitro and in vivo. We demonstrate that pretreatment of MSCs with the mechano-stimulatory soluble factor transforming growth factor (TGF-β1), which is highly expressed in injury sites, improves wound closure in a syngeneic murine wound model. This improved wound closure correlated with increased invasion into the wound bed. In vitro studies demonstrated that TGF-β1 pretreatment expedited wound closure by increasing adhesion, traction force, and migration even after removal of the stimulus. Furthermore, this response was mediated by the cytoskeletal protein focal adhesion kinase. Taken together, this study suggests that defined chemical stimuli can benefit site specific adaptability of MSCs to improve their function and therapeutic usefulness.

Semianalytical Solution for the Deformation of an Elastic Layer under an Axisymmetrically Distributed Power-Form Load: Application to Fluid-Jet-Induced Indentation of Biological Soft Tissues

Fluid-jet-based indentation is used as a noncontact excitation technique by systems measuring the mechanical properties of soft tissues. However, the application of these devices has been hindered by the lack of theoretical solutions. This study developed a mathematical model for testing the indentation induced by a fluid jet and determined a semianalytical solution. The soft tissue was modeled as an elastic layer bonded to a rigid base. The pressure of the fluid jet impinging on the soft tissue was assumed to have a power-form function. The semianalytical solution was verified in detail using finite-element modeling, with excellent agreement being achieved. The effects of several parameters on the solution behaviors are reported, and a method for applying the solution to determine the mechanical properties of soft tissues is suggested.

Cutaneous tissue damage induces long-lasting nociceptive sensitization and regulation of cellular stress- and nerve injury-associated genes in sensory neurons

Tissue damage is one of the major etiological factors in the emergence of chronic/persistent pain, although mechanisms remain enigmatic. Using incision of the back skin of adult rats as a model for tissue damage, we observed sensitization in a nociceptive reflex enduring to 28days post-incision (DPI). To determine if the enduring behavioral changes corresponded with a long-term impact of tissue damage on sensory neurons, we examined the temporal expression profile of injury-regulated genes and the electrophysiological properties of traced dorsal root ganglion (DRG) sensory neurons. The mRNA for the injury/stress-hub gene Activating Transcription Factor 3 (ATF3) was upregulated and peaked within 4 DPI, after which levels declined but remained significantly elevated out to 28 DPI, a time when the initial incision appears healed and tissue-inflammation largely resolved. Accordingly, stereological image analysis indicated that some neurons expressed ATF3 only transiently (mostly medium-large neurons), while in others it was sustained (mostly small neurons), suggesting cell-type-specific responses. In retrogradely-traced ATF3-expressing neurons, Calcium/calmodulin-dependent protein kinase type IV (CAMK4) protein levels and isolectin-B4 (IB4)-binding were suppressed whereas Growth Associated Protein-43 (GAP-43) and Neuropeptide Y (NPY) protein levels were enhanced. Electrophysiological recordings from DiI-traced sensory neurons 28 DPI showed a significant sensitization limited to ATF3-expressing neurons. Thus, ATF3 expression is revealed as a strong predictor of single cells displaying enduring pain-related electrophysiological properties. The cellular injury/stress response induced in sensory neurons by tissue damage and indicated by ATF3 expression is positioned to contribute to pain which can occur after tissue damage.

Automatic Segmentation of Mechanically Inhomogeneous Tissues Based on Deformation Gradient Jump

Variations in properties, active behavior, injury, scarring, and/or disease can all cause a tissue's mechanical behavior to be heterogeneous. Advances in imaging technology allow for accurate full-field displacement tracking of both in vitro and in vivo deformation from an applied load. While detailed strain fields provide some insight into tissue behavior, material properties are usually determined by fitting stress-strain behavior with a constitutive equation. However, the determination of the mechanical behavior of heterogeneous soft tissue requires a spatially varying constitutive equation (i.e., one in which the material parameters vary with position). We present an approach that computationally dissects the sample domain into many homogeneous subdomains, wherein subdomain boundaries are formed by applying a betweenness based graphical analysis to the deformation gradient field to identify locations with large discontinuities. This novel partitioning technique successfully determined the shape, size and location of regions with locally similar material properties for: (1) a series of simulated soft tissue samples prescribed with both abrupt and gradual changes in anisotropy strength, prescribed fiber alignment, stiffness, and nonlinearity, (2) tissue analogs (PDMS and collagen gels) which were tested biaxially and speckle tracked (3) and soft tissues which exhibited a natural variation in properties (cadaveric supraspinatus tendon), a pathologic variation in properties (thoracic aorta containing transmural plaque), and active behavior (contracting cardiac sheet). The routine enables the dissection of samples computationally rather than physically, allowing for the study of small tissues specimens with unknown and irregular inhomogeneity.

Marker-Free Tracking of Facet Capsule Motion Using Polarization-Sensitive Optical Coherence Tomography

We proposed and tested a method by which surface strains of biological tissues can be captured without the use of fiducial markers by instead, utilizing the inherent structure of the tissue. We used polarization-sensitive optical coherence tomography (PS OCT) to obtain volumetric data through the thickness and across a partial surface of the lumbar facet capsular ligament during three cases of static bending. Reflectivity and phase retardance were calculated from two polarization channels, and a power spectrum analysis was performed on each a-line to extract the dominant banding frequency (a measure of degree of fiber alignment) through the maximum value of the power spectrum (maximum power). Maximum powers of all a-lines for each case were used to create 2D visualizations, which were subsequently tracked via digital image correlation. In-plane strains were calculated from measured 2D deformations and converted to 3D surface strains by including out-of-plane motion obtained from the PS OCT image. In-plane strains correlated with 3D strains (R(2) ≥ 0.95). Using PS OCT for marker-free motion tracking of biological tissues is a promising new technique because it relies on the structural characteristics of the tissue to monitor displacement instead of external fiducial markers.