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Advanced maternal age during pregnancy is associated with increased risk of vaginal tearing during delivery and maladaptive postpartum healing. Although the underlying mechanisms of age-related vaginal injuries are not fully elucidated, changes in vaginal microstructure may contribute. Smooth muscle cells promote the contractile nature of the vagina and contribute to pelvic floor stability. While menopause is associated with decreased vaginal smooth muscle content, whether contractile changes occur before the onset of menopause remains unknown. Therefore, the first objective of this study was to quantify the active mechanical behavior of the murine vagina with age. Further, aging is associated with decreased vaginal elastin content. As such, the second objective was to determine if elastic fiber disruption alters vaginal contractility. Vaginal samples from mice aged 2–14 months were used in maximum contractility experiments and biaxial extension-inflation protocols. To evaluate the role of elastic fibers with age, half of the vaginal samples were randomly allocated to enzymatic elastic fiber disruption. Contractile potential decreased and vaginal material stiffness increased with age. These age-related changes in smooth muscle function may be due, in part, to changes in microstructural composition or contractile gene expression. Furthermore, elastic fiber disruption had a diminished effect on smooth muscle contractility in older mice. This suggests a decreased functional role of elastic fibers with age. Quantifying the age-dependent mechanical contribution of smooth muscle cells and elastic fibers to vaginal properties provides a first step towards better understanding how age-related changes in vaginal structure may contribute to tissue integrity and healing. Advanced maternal age at the time of pregnancy is linked to increased risks of vaginal tearing during delivery, postpartum hemorrhaging, and the development of pelvic floor disorders. While the underlying causes of increased vaginal injuries with age and associated pathologies remain unclear, changes in vaginal microstructure, such as elastic fibers and smooth muscle cells, may contribute. Menopause is associated with fragmented elastic fibers and decreased smooth muscle content; however, how reproductive aging affects changes in the vaginal composition and the mechanical properties remains unknown. Quantifying the mechanical contribution of smooth muscle cells and elastic fibers to vaginal properties with age will advance understanding of the potential structural causes of age-related changes to tissue integrity and healing. [Display omitted] [ABSTRACT FROM AUTHOR]

Constitutive models for biological tissue are typically formulated as a mixture of constituents and the overall response is then assembled by superposition or compatibility. This ensures the stress response of the biological tissue to be in the range of a given constitutive relationship, guaranteeing that at least one parameter combination exists so that an experimental response can be sufficiently well captured. Another, perhaps more challenging, problem is to use constitutive models as a proxy to infer the structure/function of a biological tissue from experiments. In other words, we determine the optimal set of parameters by solving an inverse problem and use these parameters to infer the integrity of the tissue constituents. In previous studies, we focused on the mechanical stress-stretch response of the murine patellar tendon at various age and healing timepoints and solved the inverse problem using three constitutive models, i.e., the Freed-Rajagopal, Gasser-Ogden-Holzapfel and Shearer in order of increasing microstructural detail. Herein, we extend this work by adopting a Bayesian perspective on parameter estimation and implement the constitutive relations in the tulip library for uncertainty analysis, critically analyzing parameter marginals, correlations, identifiability and sensitivity. Our results show the importance of investigating the variability of parameter estimates and that results from optimization may be misleading, particularly for models with many parameters inferred from limited experimental evidence. In our study, we show that different age and healing conditions do not correspond to statistically significant separation among the Gasser-Ogden-Holzapfel and Shearer model parameters, while the phenomenological Freed-Rajagopal model is instead characterized by better indentifiability and parameter learning. Use of the complete experimental observations rather than averaged stress-stretch responses appears to positively constrain inference and results appear to be invariant with respect to the scaling of the experimental uncertainty. [ABSTRACT FROM AUTHOR]

Smooth muscle cells contribute to the mechanical function of various soft tissues, however, their contribution to the viscoelastic response when subjected to multiaxial loading remains unknown. The vagina is a fibromuscular viscoelastic organ that is exposed to prolonged and increased pressures with daily activities and physiologic processes such as vaginal birth. The vagina changes in geometry over time under prolonged pressure, known as creep. Vaginal smooth muscle cells may contribute to creep. This may be critical for the function of vaginal and other soft tissues that experience fluctuations in their biomechanical environment. Therefore, the objective of this study was to develop methods to evaluate the contribution of smooth muscle to vaginal creep under multiaxial loading using extension – inflation tests. The vaginas from wildtype mice (C57BL/6 × 129SvEv; 3–6 months; n = 10) were stimulated with various concentrations of potassium chloride then subjected to the measured in vivo pressure (7 mmHg) for 100 s. In a different cohort of mice (n = 5), the vagina was stimulated with a single concentration of potassium chloride then subjected to 5 and 15 mmHg. A laser micrometer measured vaginal outer diameter in real-time. Immunofluorescence evaluated the expression of alpha-smooth muscle actin and myosin heavy chain in the vaginal muscularis (n = 6). When smooth muscle contraction was activated, vaginal creep behavior increased compared to the relaxed state. However, increased pressure decreased the active creep response. This study demonstrated that extension – inflation protocols can be used to evaluate smooth muscle contribution to the viscoelastic response of tubular soft tissues. [Display omitted] • Experimental methods were developed to evaluate smooth muscle contribution to viscoelasticity under multiaxial loading. • Smooth muscle contraction significantly increased changes in vaginal diameter over time under a sustained pressure. • When maximally contracted an increase in sustained pressure decreased vaginal creep. [ABSTRACT FROM AUTHOR]

Continued advances in the tissue engineering of vascular grafts have enabled a paradigm shift from the desire to design for adequate suture retention, burst pressure and thrombo-resistance to the goal of achieving grafts having near native properties, including growth potential. Achieving this far more ambitious outcome will require the identification of optimal, not just adequate, scaffold structure and material properties. Given the myriad possible combinations of scaffold parameters, there is a need for a new strategy for reducing the experimental search space. Toward this end, we present a new modeling framework for in vivo neovessel development that allows one to begin to assess in silico the potential consequences of different combinations of scaffold structure and material properties. To restrict the number of parameters considered, we also utilize a non-dimensionalization to identify key properties of interest. Using illustrative constitutive relations for both the evolving fibrous scaffold and the neotissue that develops in response to inflammatory and mechanobiological cues, we show that this combined non-dimensionalization computational approach predicts salient aspects of neotissue development that depend directly on two key scaffold parameters, porosity and fiber diameter. We suggest, therefore, that hypothesis-driven computational models should continue to be pursued given their potential to identify preferred combinations of scaffold parameters that have the promise of improving neovessel outcome. In this way, we can begin to move beyond a purely empirical trial-and-error search for optimal combinations of parameters and instead focus our experimental resources on those combinations that are predicted to have the most promise. [ABSTRACT FROM AUTHOR]

Background: The subscapularis is an important mover and stabilizer of the glenohumeral joint. Since the advent of shoulder arthroscopy, partial tears are found in 43% of rotator cuff patients. While partial tears to the upper band occur more commonly, little is known about the structure and mechanical behavior of the individual bands. Therefore, the objective of this study was to measure tensile mechanical properties, corresponding collagen fiber alignment, and histology in the upper and lower bands of the rat subscapularis tendon. Materials and methods: Thirty adult Sprague-Dawley rats were euthanized and subscapularis tendons dissected out for mechanical organization (n = 24) and histologic assessment (n = 6). Collagen organization was measured with a custom device during mechanical testing. Results: Linear-region modulus at the insertion site was significantly lower in the upper band compared to the lower band, while no differences were found at the midsubstance location. The upper band was found to be significantly less aligned and demonstrated a more rounded cell shape than the lower band at the insertion site. Discussion: This study demonstrated that the 2 bands of the subscapularis tendon have differential mechanical, organizational, and histological properties, which suggests a functional deficit exists to the upper band of the subscapularis and may be contributing to the prevalence of partial subscapularis tears. Conclusions: Clinicians should be aware that the upper band of the subscapularis tendon may be at higher risk of developing tears, based on decreased mechanical properties and a more disorganized collagen fiber distribution. [ABSTRACT FROM AUTHOR]

Abstract: Background: Collagen fiber re-alignment and uncrimping are two postulated mechanisms of tendon structural response to load. Recent studies have examined structural changes in response to mechanical testing in a postnatal development mouse supraspinatus tendon model (SST), however, those changes in the mature mouse have not been characterized. The objective of this study was to characterize collagen fiber re-alignment and crimp behavior throughout mechanical testing in a mature mouse SST. Method of approach: A tensile mechanical testing set-up integrated with a polarized light system was utilized for alignment and mechanical analysis. Local collagen fiber crimp frequency was quantified immediately following the designated loading protocol using a traditional tensile set up and a flash-freezing method. The effect of number of preconditioning cycles on collagen fiber re-alignment, crimp frequency and mechanical properties in midsubstance and insertion site locations were examined. Results: Decreases in collagen fiber crimp frequency were identified at the toe-region of the mechanical test at both locations. The insertion site re-aligned throughout the entire test, while the midsubstance re-aligned during preconditioning and the test''s linear-region. The insertion site demonstrated a more disorganized collagen fiber distribution, lower mechanical properties and a higher cross-sectional area compared to the midsubstance location. Conclusions: Local collagen fiber re-alignment, crimp behavior and mechanical properties were characterized in a mature mouse SST model. The insertion site and midsubstance respond differently to mechanical load and have different mechanisms of structural response. Additionally, results support that collagen fiber crimp is a physiologic phenomenon that may explain the mechanical test toe-region. [Copyright &y& Elsevier]

Abstract: A recent study (); reported the properties of human supraspinatus tendon (SST) tested along the predominant fiber direction. The SST was found to have a relatively disperse distribution of collagen fibers, which may represent an adaptation to multiaxial loads imposed by the complex loading environment of the rotator cuff. However, the multiaxial mechanical properties of human SST remain unknown. The objective of this study, therefore, was to evaluate the mechanical properties, fiber alignment, change in alignment with applied load, and structure–function relationships of SST in transverse testing. Samples from six SST locations were tested in uniaxial tension with samples oriented transverse to the tendon long-axis. Polarized light imaging was used to quantify collagen fiber alignment and change in alignment under applied load. The mechanical properties of samples taken near the tendon–bone insertion were much greater on the bursal surface compared to the joint surface (e.g., bursal moduli 15–30 times greater than joint; p<0.001). In fact, the transverse moduli values of the bursal samples were very similar to values obtained from samples tested along the tendon long-axis (). This key and unexpected finding suggests planar mechanical isotropy for bursal surface samples near the insertion, which may be due to complex in vivo loading. Organizationally, fiber distributions became less aligned along the tendon long-axis in the toe-region of the stress–strain response. Alignment changes occurred to a slightly lesser degree in the linear-region, suggesting that movement of collagen fibers may play a role in mechanical nonlinearity. Transverse mechanical properties were significantly correlated with fiber alignment (e.g., for linear-region modulus r s =0.74, p<0.0001), demonstrating strong structure–function relationships. These results greatly enhance current understanding of the properties of human SST and provide clinicians and scientists with vital information in attempting to treat or replace this complex tissue. [Copyright &y& Elsevier]

We used a murine model to assess the evolving biomechanical properties of tissue engineered vascular grafts (TEVGs) implanted in the arterial circulation. The initial polymeric tubular scaffold was fabricated from poly(lactic acid)(PLA) and coated with a 50:50 copolymer of poly(caprolactone) and poly(lactic acid)(P[PC/LA]). Following seeding with syngeneic bone marrow derived mononuclear cells, TEVGs (n=50) were implanted as aortic interposition grafts in wild-type mice and monitored serially using ultrasound. A custom biaxial mechanical testing device was used to quantify the in vitro circumferential and axial mechanical properties of grafts explanted at 3 or 7 months. At both times, TEVGs were much stiffer than native tissue in both directions. Repeated mechanical testing of some TEVGs treated with elastase or collagenase suggested that elastin did not contribute significantly to the overall stiffness whereas collagen did contribute. Traditional histology and immunostaining revealed smooth muscle cell layers, significant collagen deposition, and increasing elastin production in addition to considerable scaffold at both 3 and 7 months, which likely dominated the high stiffness seen in mechanical testing. These results suggest that PLA has inadequate in vivo degradation, which impairs cell-mediated development of vascular neotissue having properties closer to native arteries. Assessing contributions of individual components, such as elastin and collagen, to the developing neovessel is needed to guide computational modeling that may help to optimize the design of the TEVG. [ABSTRACT FROM AUTHOR]

From a biomechanical perspective, female reproductive health is an understudied area of research. There is an incomplete understanding of the complex function and interaction between the cervix and uterus. This, in part, is due to the limited research into multiaxial biomechanical functions and geometry of these organs. Knowledge of the biomechanical function and interaction between these organs may elucidate etiologies of conditions such as preterm birth. Therefore, the objective of this study was to quantify the multiaxial biomechanical properties of the murine cervix and uterus using a biaxial testing set-up. To accomplish this, an inflation-extension testing protocol (n = 15) was leveraged to quantify biaxial biomechanical properties while preserving native matrix interactions and geometry. Ultrasound imaging and histology (n = 10) were performed to evaluate regional geometry and microstructure, respectively. Histological analysis identified a statistically significant greater collagen content and significantly smaller smooth muscle content in the cervix as compared to the uterus. No statistically significant differences in elastic fibers were identified. Analysis of bilinear fits revealed a significantly stiffer response from the circumferentially orientated ECM fibers compared to axially orientated fibers in both organs. Bilinear fits and a two-fiber family constitutive model showed that the cervix was significantly less distensible than the uterus. We submit that the regional biaxial information reported in this study aids in establishing an appropriate reference configuration for mathematical models of the uterine-cervical complex. Thus, may aid future work to elucidate the biomechanical mechanisms leading to cervical or uterine conditions. [ABSTRACT FROM AUTHOR]

Abstract Mounting evidence suggests that cells within soft tissues seek to maintain a preferred biomechanical state. Residual stress is defined as the stress that remains in a tissue when all external loads are removed and contributes to tissue mechanohomeostasis by decreasing the transmural gradient of wall stress. Current computational models of pelvic floor mechanics, however, often do not consider residual stress. Residual strain, a result of residual stress can be quantitatively measured through opening angle experiments. Therefore, the objective of this study is to quantify the regional variations in opening angles along the murine female reproductive system at estrus and diestrus, to quantify residual strain in the maintenance state of sexually mature females. Further, evidence suggests that hydrophilic glycosaminoglycan/proteoglycans are integral to cervical remodeling. Thus, variations in opening angles following hypo-osmotic loading are evaluated. Opening angle experiments were performed along the murine reproductive system in estrus (n = 8) and diestrus (n = 8) and placed in hypo-osmotic solution. Measurements of thickness and volume were also obtained for each group. Differences (p < 0.05) in opening angle were observed with respect to region and loading, however, differences with respect to estrous stage were not significant. Thickness values were significant (p < 0.05) with respect to region only. The effects of both estrous cycle and region resulted in significant differences (p < 0.05) in observed volume. The observed regional differences indicate variation in the stress-free state among the reproductive system which may have implications for future computational models to advance women's reproductive health. [ABSTRACT FROM AUTHOR]

Abstract: The mouse has proven to be an advantageous animal model system in basic science research focused on aiding in development and evaluation of potential treatments; however, the small size of mouse tendons makes consistent and reproducible injury models and subsequent biomechanical evaluation challenging for studying tendon healing. In this study, we investigated the feasibility and reproducibility of multiple mouse tendon injury models. Our hypothesis was that incisional (using a blade) and excisional (using a biopsy punch) injuries would result in consistent differences in tendon material properties. At 16 weeks of age, 17 C57BL/6 mice underwent surgery to create defects in the flexor digitorum longus, Achilles, or patellar tendon. Each animal received 1–2 full-thickness, central-width incisional or excisional injuries per limb; at least one tendon per limb remained uninjured. The injuries were distributed such that each tendon type had comparable numbers of uninjured, incisionally injured, and excisionally injured specimens. Three weeks after injury, all animals were euthanized and tendons were harvested for mechanical testing. As hypothesized, differences were detected for all three different tendon types at three weeks post-injury. While all models created injuries that produced predictable outcomes, the patellar tendon model was the most consistent in terms of number and size of significant differences in injured tendons compared to native properties, as well as in the overall variance in the data. This finding provides support for its use in fundamental tendon healing studies; however, future work may use any of these models, based on their appropriateness for the specific question under study. [Copyright &y& Elsevier]