Variations In Biomechanical Properties Research Articles

Seasonal variations in biomechanical properties of Elymus species: Implications for assessing coastal ecosystem services

AbstractHigher wave heights and currents can greatly increase erosion rates. If the coastal region is exposed to long‐term erosion without protective measures, the coastline will irreversibly retreat landwards and endanger coastal assets. One of the solutions is to make use of the natural coastal vegetation, which can both stabilise the soil and contribute to wave attenuation. One of the most important plants for ecosystem services is the Elymus due to its ability to stabilise upper salt marshes and prevent erosion. In order to better predict the fluid‐plant interaction and wave attenuation by vegetation, an experimental characterization has been carried out on Elymus species in this study to determine the bending flexibility and material stiffness of the plants. For this purpose, three‐point and two‐point bending tests were performed on different sections of Elymus collected in March, Jun and September to investigate the seasonal changes in the tissue‐related mechanical behaviour. Based on the structural properties obtained from histological studies of the cross‐sections, the non‐linear material behaviour of Elymus was unravelled from the experimentally measured bending behaviour using the inverse finite element method. The results of these experiments revealed some surprising differences in the flexibility of the stems at different seasons. Furthermore, by comparing the data from the tissue samples and the simulations, valuable insight is gained into how the morphology and mechanical behaviour of plants change throughout the year. The experimental and numerical results obtained will help to predict how plants interact with the fluid and how wave attenuation relates to different vegetation conditions, for example, plant density, stiffness and dimensions.

Regional variation in biomechanical properties of ascending thoracic aortic aneurysms.

This study aims to characterize the material properties of ascending thoracic aortic aneurysmal tissue, using regional biomechanical assessment of both tensile and dissection propagation peel strength. Thirty-four aneurysm specimens (proximal thoracic aorta) were harvested en-bloc from patients undergoing surgery for aneurysm replacement. Specimens were processed into regional samples of similar shapes covering the whole aneurysm isosurface, according to a structured protocol, in both orientations (longitudinal and circumferential). Thickness mapping, uniaxial tensile and peel tests were conducted, enabling calculation of the following parameters: true stress/strain, tangential modulus, tensile strength, peeling force and dissection energy. Two constitutive material models were used (hyperelastic models of Delfino and Ogden) to fit the data. A circumferential strip of tissue was also obtained for computational histology [regional quantification of (i) elastin, (ii) collagen and (iii) smooth muscle cells]. The aortic wall was thinner on the outer curve (2.21, standard deviation (SD) 0.4 mm vs inner curve 2.50, SD 0.12 mm). Advanced patient age and higher pulse wave velocity (externally measured) were predictors of increased aortic wall thickness. Tensile strength was higher in the circumferential versus longitudinal direction when analysed according to anatomical regions. Both peel force (35.5, 22 N/m) and dissection energy (88.5, 69 J/m2) were on average lowest at the outer curve of the aneurysm in the longitudinal orientation. Delfino and Ogden model constants varied throughout anatomical regions, with the outer curve being associated a higher ɑ constant (Delfino) and lower µ1 constant (Ogden) (P < 0.05) indicating increased stiffness. Histologically, collagen abundance was significantly related to circumferential and longitudinal strength (P= 0.010), whilst smooth muscle cell count had no relation with any mechanical property (P > 0.05). Our results suggest that the outer aortic curve is more prone to dissection propagation and perhaps less prone to rupture than the inner aortic curve. This strengthens the notion of disease heterogeneity in ascending thoracic aortic aneurysms and has implications for the pathogenesis of aortic dissection.

Covariation of Pluripotency Markers and Biomechanical Properties in Mouse Embryonic Stem Cells.

Pluripotent cells are subject to much interest as a source of differentiated cellular material for research models, regenerative medical therapies and novel applications such as lab-cultured meat. Greater understanding of the pluripotent state and control over its differentiation is therefore desirable. The role of biomechanical properties in directing cell fate and cell behavior has been increasingly well described in recent years. However, many of the mechanisms which control cell morphology and mechanical properties in somatic cells are absent from pluripotent cells. We leveraged naturally occurring variation in biomechanical properties and expression of pluripotency genes in murine ESCs to investigate the relationship between these parameters. We observed considerable variation in a Rex1-GFP expression reporter line and found that this variation showed no apparent correlation to cell spreading morphology as determined by circularity, Feret ratio, phase contrast brightness or cell spread area, either on a parameter-by-parameter basis, or when evaluated using a combined metric derived by principal component analysis from the four individual criteria. We further confirmed that cell volume does not co-vary with Rex1-GFP expression. Interestingly, we did find that a subpopulation of cells that were readily detached by gentle agitation collectively exhibited higher expression of Nanog, and reduced LmnA expression, suggesting that elevated pluripotency gene expression may correlate with reduced adhesion to the substrate. Furthermore, atomic force microscopy and quantitative fluorescent imaging revealed a connection between cell stiffness and Rex1-GFP reporter expression. Cells expressing high levels of Rex1-GFP are consistently of a relatively low stiffness, while cells with low levels of Rex1-GFP tend toward higher stiffness values. These observations indicate some interaction between pluripotency gene expression and biomechanical properties, but also support a strong role for other interactions between the cell culture regime and cellular biomechanical properties, occurring independently of the core transcriptional network that supports pluripotency.

Extracellular Matrix Composition Is Altered Alongside Dysmorphology in the Developing Palate

Normal morphogenesis of the primary palate involves directed outgrowth of the facial primordia until they meet and fuse. Gradients of cellular proliferation induced by growth factors are the current paradigm for directed outgrowth, but active cellular migration and physical force propagated through the extracellular matrix (ECM) likely also direct growth. Previously we showed that two of the main components of the ECM, collagen IV and laminin, are thinner in regions of the frontonasal process (FNP) undergoing growth, suggesting tissue stiffness may passively constrain growth in certain directions. We have also shown that increasing Fibroblast growth factor (FGF) signaling in the FNP of chick embryos prior to palate fusion caused overgrowth and disrupted cellular organization. Since cells form and modify the ECM, and since the ECM is required for cellular organization and movement, the effects of FGF signaling on morphogenesis may be due in part to active feedback between ECM and cells. Our goal was to examine the changes to the ECM in the FNP following alteration of FGF signaling. Blocking FGF signaling with SU5402 caused hypoplasia of the FNP and increased the concentration of collagen IV and laminin in regions of expected outgrowth. Conversely, increasing FGF signaling with exogenous FGF2 caused overgrowth of the FNP and decreased concentration of the ECM components. Analysis using transmission electron microscopy of the FGF2 treated samples suggests increasing FGF signaling also decreases the thickness of the epithelium and basement membrane. These results provide preliminary evidence that ECM may play an important role in facial morphogenesis, and that FGF signaling may affect an active biomechanical feedback system between cells and ECM. In the future I will further characterize ECM composition and compliance across the FNP using Fourier transform infrared spectroscopy and atomic force microscopy to generate 3-dimensional maps of biomechanical properties in the face. I will also use confocal microscopy to map cellular organization across the face. I will overlay these maps onto representative reconstructions of the face to correlate regional variation in biomechanical properties and cellular organization with dysmorphologies that occur following exogenous modulation of FGF signaling.

Experimental study and biomechanical characterization for the passive small intestine: Identification of regional differences

This article reports regional differences in the inflation/extension properties and the no-load and zero-stress geometry, serving as the reference state for defining the multiaxial response, within and across the individual duodenum, jejunum, and ileum of the small intestine of middle-aged rats in the normal condition. The descriptive/predictive capacity of three phenomenological models, regularly appearing in the biomechanics literature, to characterize the anisotropic response of intestinal tissue was investigated in terms of best-fit parameters. Our inflation/extension results showed that the pressure-radius relationship was nonlinear, dissimilar to the near-constant, linear force-pressure relationship at all axial stretches, suggesting an energetically-favorable response in the entire loading range. The geometrical results showed that small intestinal dimensions were greatest in the proximal duodenum and smallest in the proximal jejunum, unlike the opening angle and circumferential residual strains that were less spatially variable; contrasting previous findings on relatively immature animals. The quadratic and exponential model was the most suitable descriptor of the passive pseudo-elastic response for the entire loading range from all regions, while the four- and seven-parameter exponential models afforded good data representations at the physiologic loading range primarily. The stress-strain and material characterization results showed progressive softening in the axial direction within the duodenum and jejunum, unlike the ileum that stiffened axially towards the caecum. Much remains to be learned as regards regional variations in biomechanical properties and the adaptive response during aging, but the reported data provide input for a quantitative analysis of physiological functions, mechanobiology, and clinical interventions, e.g. small intestine-repair device interaction.

Corneal Biomechanical Parameters during Pregnancy

To evaluate the variation in biomechanical properties and central corneal thickness (CCT) for each trimester during pregnancy and to compare the values with those in nonpregnant women. We prospectively studied the eyes of 32 pregnant and 34 age-matched non-pregnant women. The parameters included corneal hysteresis (CH), corneal resistance factor (CRF), Goldmann-correlated intraocular pressure (IOP), and corneal-compensated IOP measured by the Ocular Response Analyzer (ORA). The CCT was also measured with an ultrasonic pachymeter attached to the ORA. The mean age was 27.0 ± 3.8 years in the study group and 28.0 ± 4.1 years in the control group. The mean CH measurement was 10.6 ± 1.4 mmHg in the study group and 10.1 ± 1.3 mmHg in the control group. The mean CRF value was 9.6 ± 1.7 mmHg in the study group and 10.0 ± 1.4 mmHg in the control group. The mean CCT value was 541.1 ± 22.4 µm in the study group and 536.5 ± 27.1 µm in the control group. No statistically significant differences were found regarding CH, CRF, or CCT values between the 2 groups (independent t test, p = 0.160, p = 0.355, p = 0.450, respectively). Hormonal changes during pregnancy may not affect corneal biomechanics. This may be due to the balanced effect of the various hormones on the cornea during pregnancy.

Regional variation in the biomechanical properties of the human sclera

The study aimed to determine the variation in thickness and biomechanical properties between the different regions of the human sclera. Thickness measurements were carried out along eight meridian lines extending from the posterior pole to limbus in 36 human donor scleras aged 52-96 years. Strip specimens were extracted from areas close to the limbus, equator and posterior pole, and tested under cycles of uniaxial tension. Two strain rates were considered to assess the viscoelasticity effects on the regional variation in material behaviour. The results were used to derive the stress-strain behaviour of each specimen and to calculate the tangent modulus at each stress level. The scleras had a variable thickness from maximum at the posterior pole to minimum close to the equator, and increasing again towards the limbus. All scleral specimens demonstrated nonlinear behaviour with an initially low tangent modulus (a measure of stiffness) increasing gradually under higher stresses. With reference to specific stress levels, the behaviour comparisons between regions showed a gradual growth in material tangent stiffness with progression from the posterior region towards the limbus. The viscoelasticity of the tissue, which was evident with significant increases in stress (157-203%) and tangent modulus (30.3-38.8%) with strain rate rise (from 8% to 200% per min), was associated with reductions in the regional variation in stiffness. The considerable variation in biomechanical behaviour found in this study should be useful in improving the accuracy of representing the sclera's real-life conditions in numerical simulations.

Effect of dermal thickness, tissue composition, and body site on skin biomechanical properties

Quantitative measurement of skin biomechanical properties has been used effectively in the investigation of physiological changes in tissue structure and function and to determine treatment efficacy. As the methods are applied to new questions, tissue characteristics that may influence the resultant biomechanical properties are important considerations in the research design. For certain applications, variables such as dermal thickness and subdermal tissue composition, as well as age and/or solar exposure, may influence the skin biomechanics. We determined the influence of dermal thickness, tissue composition, and age on the skin biomechanical properties at the shoulder, thigh, and calf among 30 healthy females. We compared two devices, the Biomechanical Tissue Characterization System and the Cutometer SEM 575 Skin Elasticity Meter , to determine the effect of tissue sampling size. Dermal thickness was measured with 20 MHz ultrasound (Dermascan C) and tissue composition was inferred from anthropomorphic data. Skin thickness was significantly correlated with stiffness, energy absorption, and U(r)/U(f) for the shoulder. Body mass index (BMI) was significantly correlated with stiffness (negative correlation), energy absorption (positive), and skin thickness (negative) for the shoulder. Significant differences across body sites were observed. The calf was significantly different from the thigh and shoulders for all parameters (P<0.05, one-way anova). The calf had significantly lower laxity, laxity%, elastic deformation, energy absorption, elasticity, elasticity %, U(r), U(f), and U(r)/U(f) and significantly higher stiffness compared with the thighs and shoulders. sites. The thigh and shoulder sites were significantly different for all parameters except U(r)/U(f), elasticity %, laxity%, and stiffness. The dominant and non-dominant sides were significantly different. The dominant side (right for 90% of the subjects) had increased stiffness and decreased energy absorption (tissue softness, compliance) compared with the left side. A significant (P< or =0.02) negative relationship with age was seen for all biomechanical measures except stiffness at the shoulder. For the thigh and calf sites, significant negative correlations with age were found for elasticity %, U(r), and U(r)/U(f). Age and skin thickness were not correlated in this population. Skin thickness and age influenced the energy absorption at the shoulder site. The biological elasticity at the calf site could be predicted by age and BMI. The biological activity at the thigh site could be predicted by skin thickness and BMI. Significant regional variations in biomechanical properties and dominant side effects were observed. The biomechanical properties were significantly influenced by age. Certain properties varied with dermal thickness and tissue composition. The parameters were well correlated between the two instruments. The Cutometer, with its smaller aperture, was found to be more sensitive to age relationships.

Biomechanics of Corneal Refractive Surgery

From London Vision Clinic, London, United Kingdom (Reinstein); and the Departments of Ophthalmology and Biomedical Engineering, The Ohio State University, Columbus, Ohio (Roberts). I n this issue of the Journal of Refractive Surgery, we introduce a Biomechanics Section, which will feature one to two original articles per issue, highlighting current work devoted to investigating the biomechanics of the cornea. A new era is dawning in biomechanical research that comes with the ability to not only measure corneal biomechanical properties in vivo,1,2 but also determine the internal structure of the cornea layer by layer.3,4 This is analogous to the early days of topographers or wavefront sensors, when the devices and their capabilities were not well understood. Optical aberrations initially provided an explanation for the patient with visual complaints, despite excellent visual acuity. Ultimately, wavefront customization grew from the ability to objectively document image quality on an individual basis. Similarly, individual variations in biomechanical properties may offer the ability to initially explain, and ultimately predict, corneal response to a surgical procedure, leading to the next generation of biomechanical customization. Some of the evidence pointing to the impact of corneal biomechanical properties on surgical outcomes lies in the measurement of intraocular pressure (IOP), both before and after refractive surgery. It is well known that measured IOP is reduced, on average, following a refractive procedure. It has been assumed that this is the result of reduced curvature and thickness in myopic procedures. However, Chang and Stulting5 performed a retrospective review of over 8000 myopic LASIK patients, and determined that although measured pressure was reduced on average by approximately 2 mmHg, the range of change was approximately 10 to 15 mmHg. Every patient in this population had reduced thickness and curvature, and yet almost half of them had an increase in measured IOP. Clearly, the artifact in IOP measurement cannot be explained by thickness alone, and “correction” of measured IOP postoperatively using a linear correction factor based on thickness is problematic.6 This leads to the conclusion that refractive surgery likely alters the fundamental biomechanical properties of the cornea. Reinstein et al have demonstrated that after LASIK, peripheral thickening of the stromal layer occurs outside of the zone of tissue ablation,4 and this has been corroborated by topography studies demonstrating peripheral steepening after LASIK.7 Therefore, as our understanding of corneal biomechanics improves, surgical outcomes may also improve.8-11 The Biomechanics Section is initiated in this issue with two original articles that present fi nite element based models of corneal behavior. Deenadayalu et al offer a model that predicts central fl attening and a subsequent hyperopic shift after lamellar fl ap creation that supports clinical data in the literature. Fernandez et al offer a model of corneal response to transplantation, including lamellar and penetrating procedures. Both articles advance our knowledge of corneal behavior.

TISSUE REACTIONS OF 5 SLING MATERIALS AND TISSUE MATERIAL DETACHMENT STRENGTH OF 4 SYNTHETIC MESH MATERIALS IN A RABBIT MODEL

We evaluated tissue reactions to 5 sling materials used in tension-free vaginal tape (TVT), intravaginal slingplasty (IVS), polypropylene mesh hernia repair, the suprapubic approach to suburethral polypropylene tape (SPARC) and cadaveric fascia lata procedures. We also compared the mesh-to-tissue attachment strength of 4 sling mesh materials (TVT, IVS, surgical polypropylene mesh and SPARC) at on days 2, 7, 15 and 30 after implantation. A total of 20 female New Zealand White rabbits were randomized to group 1-2 days, group 2-7 days, group 3-15 days and group 4-30 days. After the rabbits were anesthetized an 8 cm midline incision was made for rectus muscle access, and 0.5 x 1 cm pieces of TVT, IVS, SPARC, surgical polypropylene mesh and cadaveric fascia lata were sewn to the rectus muscle with direct contact. At the same time 4 subfascial tunnels in the medial surface of the upper extremities were prepared, and 1.0 x 0.5 cm strips of TVT, IVS, SPARC and polypropylene mesh were implanted in each tunnel. On days 2, 7, 15 and 30 after implantation mechanical testing was performed to define tissue detachment strength. The strips of 5 sling materials were then harvested with the surrounding tissue. Specimens were studied by light microscopy. Mean detachment strength, that is the minimum weight needed to move the mesh, of the synthetic meshes from days 2 to 30 were 291.6 to 2,390.0 gm for TVT, 178.4 to 2,160.0 gm for SPARC, 188.4 to 1,850.0 gm for hernia mesh and 92.8 to 1,510.0 gm for IVS (at all data points TVT vs IVS p < 0.05). Light microscopy revealed a quite uniform tissue reaction with a sign of marked acute inflammation in and around the mesh fibers on days 2 and 7 after implantation. All meshes showed stable fibrosis and muscle infiltration on day 30. All 5 synthetic sling materials produce similar tissue reactions beginning soon after implantation. Cadaveric fascia lata persisted in tissue with remarkable perifascial fibrosis at day 30. When comparing the 4 polypropylene mesh materials; the attachment capacity of TVT was superior and that of IVS was the least of the 4. TVT was statistically better than IVS at all data points. SPARC and hernia mesh provided results similar to those of TVT.

Experimental Evaluation of Free Versus Pedicled Fascial Flaps for Sling Surgery of Urinary Stress Incontinence

Bladder neck suspension using an autologous fascial sling is an established surgical technique for treatment of urinary stress incontinence. However, the biological fate and physical properties of autologous fascial sling yet remain to be determined. Our study was designed to evaluate in an animal model of twenty rabbits fascial slings from free and pedicled fascial flaps of two different widths (7 mm and 15 mm.) and uniform length (60 mm.) in respect to changes of length, width, tensile strength and histological criteria of vitality and integrity 3 months after surgery. The results 3 months after surgery revealed shrinkage in length by 37% (31%-46%) of the original length, shrinkage in width by 63% (60%-69%) of the original width and reduction of tensile strength of 53% (51%-55%) without consistent differences between free and pedicled flaps or 7 mm. and 15 mm. wide flaps. In the groups with 7 mm. wide flaps a total of two dystrophic slings (1 pedicled, 1 free) were seen; however, no dystrophies were seen in 15 mm. wide flaps. On microscopy, all slings were vital regardless of their surgical configuration. In summary, on macroscopic, microscopic and physical examination no gross differences between free and pedicled slings could be detected, so that preferences for use of free or pedicled fascial flaps could not be established for clinical applications in sling surgery. However, 15 mm. wide flaps had a lower incidence of dystrophy and retained greater absolute tensile strength than 7 mm. wide flaps.

In vivo skin elasticity of 22 anatomical sites: The vertical gradient of skin extensibility and implications in gravitational aging.

Regional anatomical variations in bio-mechanical properties of skin were not mapped systematically in the past, despite the obvious clinical variation of this function. The object was to describe variations in skin distensibility and elasticity by the use of a suction chamber device. 22 different anatomical sites were measured in 25 healthy volunteers aged 5-72 years. Anatomical sites were numbered systematically from the head toward the foot according to a vertical principle as a part of the study design. A systematic and vertically oriented decrease of distensibility and hysteresis from the head toward the foot was observed with a gradual increase of skin stiffness toward the ground. Skin elasticity decreased significantly with age. Acral female skin was more easily distended than male skin. The vertical gradient of skin distensibility was described for the first time. This appears a prerequisite to resist the hydrostatic pressure and influences of gravity. The findings may have important implications in leg ulcer and gravitational syndrome.

Variations in the Biomechanical Properties of Articular Cartilage of the Midcarpal Joint of Normal Horses

SummaryCarpi from six clinically normal adult horses were used to determine biomechanical properties of equine midcarpal articular cartilage, and compare sites of high and low incidence of osteochondral injury. Test sites were central and dorsal C3, both facets, central and dorsal distal CR and QI central C4 and QU. An automated indentation apparatus was used to obtain creep behaviour, aggregate modulus (HA ), Poisson’s ratio (vS), permeability (k), and cartilage thickness (h) at each site. Results were analyzed using ANOVA and multiple comparisons of means (p&lt;0.05). Results revealed significant variations in HA and h between test sites, and in HA between dorsal and central sites (p = 0.0001). Right cartilage overall was stiffer (p = 0.0006), less permeable (p = 0.0001), and more compressible (p = 0.0001) than left. Topographical variations in biomechanical properties existed, with significant differences between sites of high and low incidence of lesions, and between right and left sides.An automated indentation apparatus was used to determine and topographically map the biomechanical properties of equine midcarpal articular cartilage, and to compare them at sites predisposed to osteochondral injury with those at rare lesion sites. Results showed topographical variations exist, with significant differences between sites of high and low incidence of lesions, and between right and left sides.