Instantaneous Modulus Research Articles

Study on microstructure and mechanical properties of steel corrosion products in marine environment

The microstructure and mechanical properties of steel corrosion products in marine environment are key parameters for developing the concrete corrosion-induced model. In this study, steel corrosion products from steel plates, concrete specimens with 10 mm and 20 mm covers, and cracked beams in Zhejiang province were sampled and analyzed. Initially, the microstructure of the steel corrosion products were determined by X-ray diffractometry (XRD), thermogravimetric analysis (TG), scanning electron microscopy (SEM), and energy-dispersive spectrometry (EDS). Subsequently, the mechanical properties of steel corrosion products including nanoindentation elastic modulus, hardness and instantaneous elastic modulus were measured by nanoindentation and consolidation experiments. This study holds potential for establishing the concrete corrosion-induced model and assessment of the concrete structure durability in marine environment.

Relationship between depth-wise refractive index and biomechanical properties of human articular cartilage.

Optical properties of biological tissues, such as refractive index (RI), are fundamental properties, intrinsically linked to the tissue's composition and structure. We hypothesize that, as the RI and the functional properties of articular cartilage (AC) are dependent on the tissue's structure and composition, the RI of AC is related to its biomechanical properties. This study aims to investigate the relationship between RI of human AC and its biomechanical properties. Human cartilage samples ( ) were extracted from the right knee joint of three cadaver donors (one female, aged 47 years, and two males, aged 64 and 68 years) obtained from a commercial biobank (Science Care, Phoenix, Arizona, United States). The samples were initially subjected to mechanical indentation testing to determine elastic [equilibrium modulus (EM) and instantaneous modulus (IM)] and dynamic [dynamic modulus (DM)] viscoelastic properties. An Abbemat 3200 automatic one-wavelength refractometer operating at 600 nm was used to measure the RI of the extracted sections. Similarly, Spearman's and Pearson's correlation coefficients were employed for non-normal and normal datasets, respectively, to determine the correlation between the depth-wise RI and biomechanical properties of the cartilage samples as a function of the collagen fibril orientation. A positive correlation with statistically significant relations ( ) was observed between the RI and the biomechanical properties (EM, IM, and DM) along the tissue depth for each zone, e.g., superficial, middle, and deep zones. Likewise, a lower positive correlation with statistically significant relations ( ) was also observed for collagen fibril orientation of all zones with the biomechanical properties. The results indicate that, although the RI exhibits different levels of correlation with different biomechanical properties, the relationship varies as a function of the tissue depth. This knowledge paves the way for optically monitoring changes in AC biomechanical properties nondestructively via changes in the RI. Thus, the RI could be a potential biomarker for assessing the mechanical competency of AC, particularly in degenerative diseases, such as osteoarthritis.

Об идентификации характеристик неоднородных вязкоупругих тел в рамках модели дробного порядка

Nowadays, the development of models of viscoelastic materials with complex inhomogeneous structure is one of the hottest problems of continuum mechanics. Along with classical models, fractional-differential models of viscoelasticity have become increasingly popular. In this paper, we present a model for describing steady-state oscillations of inhomogeneous viscoelastic bodies in terms of fractional-order differential operators. Taking into account the fractional order of the model operators, the corresponding form of the complex module describing the material properties is constructed. The module includes four characteristics: instantaneous and long-term elastic moduli (in the case of material inhomogeneity, they are the functions of coordinates), relaxation time and fractionality parameter. The properties of the complex module are studied, and the ranges of the model parameters, at which the rheological properties are most pronounced, are identified. A general formulation of the inverse problem (IP) on the identification of function-parameters of the model based on the acoustic sounding data is proposed. In the framework of this formulation, the inverse problems for specific objects , namely, an inhomogeneous rod and a round plate, are considered. In both model problems, the influence of the fractionalization parameter on the amplitude-frequency characteristics is analyzed. It has been found that in the vicinity of viscoelastic resonances the fractionality parameter affects the parameters of the oscillatory process most significantly, which is typical for such problems. A solution to the nonlinear IPs under consideration is constructed based on the linearization method, which also serve as basis for the iterative processes supplemented with the elements of the projection approach. This allows one to determine corrections to the required functions in the specified classes of functions using regularization. For both IPs, a series of computational experiments were carried out. The analysis of the experimental results made it possible to formulate recommendations for the selection of optimal sensing modes.

Auricular Chondrocytes as a Cell Source for Scaffold-Free Elastic Cartilage Tissue Engineering.

Current tissue engineering (TE) methods utilize chondrocytes primarily from costal or articular sources. Despite the robust mechanical properties of neocartilages sourced from these cells, the lack of elasticity and invasiveness of cell collection from these sources negatively impact clinical translation. These limitations invited the exploration of naturally elastic auricular cartilage as an alternative cell source. This study aimed to determine if auricular chondrocytes (AuCs) can be used for TE scaffold-free neocartilage constructs and assess their biomechanical properties. Neocartilages were successfully generated from a small quantity of primary neonatal AuCs of three minipig donors (n = 3). Neocartilage constructs had instantaneous moduli of 200.5 kPa ± 43.34 and 471.9 ± 92.8 kPa at 10% and 20% strain, respectively. TE constructs' relaxation moduli (Er) were 36.99 ± 6.47 kPa Er and 110.3 ± 16.99 kPa at 10% and 20% strain, respectively. The Young's modulus was 2.0 MPa ± 0.63, and the ultimate tensile strength was 0.619 ± 0.177 MPa. AuC-derived neocartilages contained 0.144 ± 0.011 µg collagen, 0.185 µg ± 0.002 glycosaminoglycans per µg dry weight, and 1.7e-3 µg elastin per µg dry weight. In conclusion, this study shows that AuCs can be used as a reliable and easily accessible cell source for TE of biomimetic and mechanically robust elastic neocartilage implants.

A generalized Rivlin–Thomas theory for nucleation of cracks in viscoelastic materials

Recent fracture experiments using the Rivlin–Thomas pure-shear geometry in rubbery viscoelastic materials have suggested nucleation of cracks at a critical stretch, independent on loading rate. For linear material using the rate-independent cohesive model theory, the critical elongation between grips (in a quite general testing geometry) at nucleation is instead monotonically decreasing with rate of up to the square root of the ratio between the instantaneous and relaxed elastic modulus of the material. Shrimali & Lopez-Pamies have made the assumption of a constant critical stretch to develop a theory which contrasts therefore with classical models. However, we further generalize the Rivlin–Thomas theory by assuming an arbitrary relation between nucleation stretch and loading rate, which therefore includes both models as limit cases. We present a simple case of a Double Cantilever Beam (DCB) geometry with linear standard materials, to show an analytical example.

The Biomechanics of Musculoskeletal Tissues during Activities of Daily Living: Dynamic Assessment Using Quantitative Transmission-Mode Ultrasound Techniques.

The measurement of musculoskeletal tissue properties and loading patterns during physical activity is important for understanding the adaptation mechanisms of tissues such as bone, tendon, and muscle tissues, particularly with injury and repair. Although the properties and loading of these connective tissues have been quantified using direct measurement techniques, these methods are highly invasive and often prevent or interfere with normal activity patterns. Indirect biomechanical methods, such as estimates based on electromyography, ultrasound, and inverse dynamics, are used more widely but are known to yield different parameter values than direct measurements. Through a series of literature searches of electronic databases, including Pubmed, Embase, Web of Science, and IEEE Explore, this paper reviews current methods used for the in vivo measurement of human musculoskeletal tissue and describes the operating principals, application, and emerging research findings gained from the use of quantitative transmission-mode ultrasound measurement techniques to non-invasively characterize human bone, tendon, and muscle properties at rest and during activities of daily living. In contrast to standard ultrasound imaging approaches, these techniques assess the interaction between ultrasound compression waves and connective tissues to provide quantifiable parameters associated with the structure, instantaneous elastic modulus, and density of tissues. By taking advantage of the physical relationship between the axial velocity of ultrasound compression waves and the instantaneous modulus of the propagation material, these techniques can also be used to estimate the in vivo loading environment of relatively superficial soft connective tissues during sports and activities of daily living. This paper highlights key findings from clinical studies in which quantitative transmission-mode ultrasound has been used to measure the properties and loading of bone, tendon, and muscle tissue during common physical activities in healthy and pathological populations.

Infrared Spectroscopy Can Differentiate Between Cartilage Injury Models: Implication for Assessment of Cartilage Integrity

In order to improve the ability of clinical diagnosis to differentiate articular cartilage (AC) injury of different origins, this study explores the sensitivity of mid-infrared (MIR) spectroscopy for detecting structural, compositional, and functional changes in AC resulting from two injury types. Three grooves (two in parallel in the palmar-dorsal direction and one in the mediolateral direction) were made via arthrotomy in the AC of the radial facet of the third carpal bone (middle carpal joint) and of the intermediate carpal bone (the radiocarpal joint) of nine healthy adult female Shetland ponies (age = 6.8 ± 2.6 years; range 4–13 years) using blunt and sharp tools. The defects were randomly assigned to each of the two joints. Ponies underwent a 3-week box rest followed by 8 weeks of treadmill training and 26 weeks of free pasture exercise before being euthanized for osteochondral sample collection. The osteochondral samples underwent biomechanical indentation testing, followed by MIR spectroscopic assessment. Digital densitometry was conducted afterward to estimate the tissue's proteoglycan (PG) content. Subsequently, machine learning models were developed to classify the samples to estimate their biomechanical properties and PG content based on the MIR spectra according to injury type. Results show that MIR is able to discriminate healthy from injured AC (91%) and between injury types (88%). The method can also estimate AC properties with relatively low error (thickness = 12.7% mm, equilibrium modulus = 10.7% MPa, instantaneous modulus = 11.8% MPa). These findings demonstrate the potential of MIR spectroscopy as a tool for assessment of AC integrity changes that result from injury.

Brillouin spectroscopy of medically relevant samples of bovine jugular vein and pericardium

There is the rapid growth in application of Brillouin scattering spectroscopy to biomedical objects in order to characterize their mechanoelastic properties in this way. However, the possibilities and limitations of the method when applied to tissues have not yet been clarified. Here, applicability of Brillouin spectroscopy for testing the elastic response of medically relevant tissues of bovine jugular vein and pericardium was considered. Parameters of the Brillouin peak were studied for samples untreated, diepoxide-fixed, and preserved after treatment in alcohol solutions. It was found that diepoxide cross-linking resulted to a slight tendency to increase the Brillouin position for hydrated tissues. The variations in the position and width of the Brillouin peaks, associated with local fluctuations in water concentration, were reduced after diepoxide treatment in the case of the pericardium, but not in the case of the vein wall. To obtain more information about the elastic response of the protein scaffold without the participation of water, dried samples were also studied. Brillouin spectra of the dried pericardium and vein wall revealed a significant increase in the Brillouin peak position (elastic modulus) after conservation in alcohol. In the case of the vein wall, this effect was found for both collagen and elastin-related peaks, which were identified in the Brillouin spectrum. This result corresponds to a denser packing of fibrous proteins after preservation in alcohol solutions. The ability of Brillouin spectroscopy to independently characterize the effect of treatment on the instantaneous elastic modulus of various tissue components is also attractive for its application in the development of new materials for bioimplants. A comparison of the Brillouin longitudinal and Young’s elastic moduli determined for the hydrated samples of the vein and pericardium showed that there is no clear correspondence between these material parameters. The usefulness of using both experimental methods to obtain new information about the elastic response of the material is discussed.

Customizable, biocompatible implants for dorsal nasal augmentation: An in vivo pilot study of eight polylactic acid scaffold designs.

Augmentation of the nasal dorsum often requires implantation of structural material. Existing methods include autologous, cadaveric or alloplastic materials and injectable hydrogels. Each of these options is associated with considerable limitations. There is an ongoing need for precise and versatile implants that produce long-lasting craniofacial augmentation. Four separate polylactic acid (PLA) dorsal nasal implant designs were 3D-printed. Two implants had internal PLA rebar of differing porosities and two were designed as "shells" of differing porosities. Shell designs were implanted without infill or with either minced or zested processed decellularized ovine cartilage infill to serve as a "biologic rebar", yielding eight total treatment groups. Scaffolds were implanted heterotopically on rat dorsa (N = 4 implants per rat) for explant after 3, 6, and 12 months followed by volumetric, histopathologic, and biomechanical analysis. Low porosity implants with either minced cartilage or PLA rebar infill had superior volume retention across all timepoints. Overall, histopathologic and immunohistochemical analysis showed a resolving inflammatory response with an M1/M2 ratio consistently favoring tissue regeneration over the study course. However, xenograft cartilage showed areas of degradation and pro-inflammatory infiltrate contributing to volume and contour loss over time. Biomechanical analysis revealed all constructs had equilibrium and instantaneous moduli higher than human septal cartilage controls. Biocompatible, degradable polymer implants can induce healthy neotissue ingrowth resulting in guided soft tissue augmentation and offer a simple, customizable and clinically-translatable alternative to existing craniofacial soft tissue augmentation materials. PLA-only implants may be superior to combination PLA and xenograft implants due to contour irregularities associated with cartilage degradation.

Removal of the infrapatellar fat pad and associated synovium benefits female guinea pigs in the Dunkin Hartley model of idiopathic osteoarthritis.

Several tissues contribute to the onset and advancement of knee osteoarthritis (OA). One tissue type that is worthy of closer evaluation, particularly in the context of sex, is the infrapatellar fat pad (IFP). We previously demonstrated that removal of the IFP had short-term beneficial effects for a cohort of male Dunkin-Hartley guinea pigs. The present project was designed to elucidate the influence of IFP removal in females of this OA-prone strain. It was hypothesized that resection of the IFP would reduce the development of OA in knees of a rodent model predisposed to the disease. Female guinea pigs (n=16) were acquired at an age of 2.5 months. Surgical removal of the IFP and associated synovium complex (IFP/SC) was executed at 3 months of age. One knee had the IFP/SC resected; a comparable sham surgery was performed on the contralateral knee. All animals were subjected to voluntary enclosure monitoring and dynamic weight-bearing, as well as compulsory treadmill-based gait analysis monthly; baseline data was collected prior to surgery. Guinea pigs were euthanized at 7 months. Knees from eight animals were evaluated via histology, mRNA expression, and immunohistochemistry (IHC); knees from the remaining eight animals were allocated to microcomputed tomography (microCT), biomechanical analyses (whole joint testing and indentation relaxation testing), and atomic absorption spectroscopy (AAS). Fibrous connective tissue (FCT) replaced the IFP/SC. Mobility/gait data indicated that unilateral IFP/SC removal did not affect bilateral hindlimb movement. MicroCT demonstrated that osteophytes were not a significant feature of OA in this sex; however, trabecular thickness (TbTh) in medial femorae decreased in knees containing the FCT. Histopathology scores were predominantly influenced by changes in the lateral tibia, which demonstrated that histologic signs of OA were increased in knees containing the native IFP/SC versus those with the FCT. Similarly, indentation testing demonstrated higher instantaneous and equilibrium moduli in the lateral tibial articular cartilage of control knees with native IFPs. AAS of multiple tissue types associated with the knee revealed that zinc was the major trace element influenced by removal of the IFP/SC. Our data suggest that the IFP/SC is a significant component driving knee OA in female guinea pigs and that resection of this tissue prior to disease has short-term benefits. Specifically, the formation of the FCT in place of the native tissue resulted in decreased cartilage-related OA changes, as demonstrated by reduced Osteoarthritis Research Society International (OARSI) histology scores, as well as changes in transcript, protein, and cartilage indentation analyses. Importantly, this model provides evidence that sex needs to be considered when investigating responses and associated mechanisms seen with this intervention.

Viscoelastic Hertzian Impact

The problem of normal impact of a rigid sphere on a Maxwell viscoelastic solid half-space is considered. The first-order asymptotic solution is constructed in the framework of Hunter’s model of viscoelastic impact. In particular, simple analytical approximations have been derived for the maximum contact force and the time to achieve it. A linear regression method is suggested for evaluating the instantaneous elastic modulus and the mean relaxation time from a set of experimental data collected for different spherical impactors and impact velocities.

Quantitative study on fatigue characteristics of warm mix recycled asphalt

The warm mix recycling technology of asphalt pavement can improve the utilization rate of solid waste in highway construction industry on a large scale, and reduce the emission of harmful gases ( NOX, CO2, benzopyrene, etc.) in the process of road construction. It is one of the effective ways to realize carbon neutrality in highway construction industry. The main purpose of this study is to systematically analyze the fatigue characteristics of warm mix recycled asphalt (WMRA) from the perspective of quantitative research, and to provide scientific basis for the popularization and application of this technology. Therefore, this study mainly evaluated the anti-fatigue characteristics of WMRA under different conditions based on the linear amplitude sweeping test ( LAS test). Based on the calculation and analysis of the commonly used fatigue judgment indexes ( stress-strain curve, phase angle-strain curve, S×N curve ( S is the ratio of the instantaneous modulus to the initial modulus of the material, which is often replaced by the virtual modulus in the LAS test, and N is the number of cyclic loadings.), virtual strain storage energy-strain curve), the peak strain of virtual strain storage energy is recommended as the fatigue judgment standard from the perspective of reliability and accuracy, and the peak strain of damage factor is proposed as one of the new fatigue judgment standards. In this paper, the nucleation of microcracks and the coalescence of microcracks are distinguished in the process of fatigue damage evolution, and the peak strain of virtual strain energy storage and the peak strain of damage factor are used as the judging indexes respectively. Finally, based on the theory of continuum damage mechanics, the loss modulus of WMRA is selected as the representative value of material properties to calculate the virtual modulus and damage factor, and the fatigue damage evolution model of WMRA based on the viscoelastic continuum damage model ( VECD model) was constructed. Based on this model, the anti-fatigue characteristics of WMRA with the reclaimed asphalt pavement ( RAP) content were analyzed.

Site-specific elastic and viscoelastic biomechanical properties of healthy and osteoarthritic human knee joint articular cartilage

Articular cartilage exhibits site-specific biomechanical properties. However, no study has comprehensively characterized site-specific cartilage properties from the same knee joints at different stages of osteoarthritis (OA).Cylindrical osteochondral explants (n = 381) were harvested from donor-matched lateral and medial tibia, lateral and medial femur, patella, and trochlea of cadaveric knees (N = 17). Indentation test was used to measure the elastic and viscoelastic mechanical properties of the samples, and Osteoarthritis Research Society International (OARSI) grading system was used to categorize the samples into normal (OARSI 0–1), early OA (OARSI 2–3), and advanced OA (OARSI 4–5) groups.OA-related changes in cartilage mechanical properties were site-specific. In the lateral and medial tibia and trochlea sites, equilibrium, instantaneous and dynamic moduli were higher (p < 0.001) in normal tissue than in early and advanced OA tissue. In lateral and medial femur, equilibrium, instantaneous and dynamic moduli were smaller in advanced OA, but not in early OA, than in normal tissue. The phase difference (0.1–0.25 Hz) between stress and strain was significantly smaller (p < 0.05) in advanced OA than in normal tissue across all sites except medial tibia.Our results indicated that in contrast to femoral and patellar cartilage, equilibrium, instantaneous and dynamic moduli of the tibia and trochlear cartilage decreased in early OA. These may suggest that the tibia and trochlear cartilage degrades faster than the femoral and patellar cartilage. The information is relevant for developing site-specific computational models and engineered cartilage constructs.

Anisotropy and viscoelasticity of different corneal regions in rabbit corneal ectasia model

The mechanical properties of the cornea in corneal ectasia disease undergo a significant reduction, yet the alterations in mechanical properties within distinct corneal regions remain unclear. In this study, we established a rabbit corneal ectasia model by employing collagenase II to degrade the corneal matrix within a central diameter of 6 mm. Optical coherence tomography was employed for the in vivo assessment of corneal morphology (corneal thickness and corneal curvature) one month after operation. Anisotropy and viscoelastic characteristics of corneal tissue were evaluated through biaxial and uniaxial testing, respectively. The results demonstrated a marked decrease in central corneal thickness, with no significant changes observed in corneal curvature. Under different strains, the elastic modulus of the cornea exhibited no significant differences in the up-down and naso-temporal directions between the control and model groups. However, the cornea in the model group displayed a significantly lower elastic modulus compared to the control group. Specifically, the elastic modulus of the central region cornea in the model group was significantly lower than that of the entire cornea within the same group. Moreover, in comparison to the control group, the cornea in the model group exhibited a significant increase in both creep rate and overall deformation rate. The instantaneous modulus and equilibrium modulus were significantly reduced in the model cornea. No significant differences were observed between the entire cornea and the central cornea concerning these parameters. The results indicate that corneal anisotropy remains unchanged in collagenase-induced ectatic cornea. However, a significant reduction in viscoelastic properties is noticed. This study provides valuable insights for investigating changes in corneal mechanical properties within different regions of ectatic corneal disease.

Creep behavior of dry and saturated medium-grain sandstone and its relationship with conventional mechanical properties

With the large scale mining of coal and the increase of abandoned goafs under weakly cemented aquifer strata in Western China, it is urgent to study the time dependent mechanical properties of water sensitive aquifer strata. In this paper, creep behavior of dry and saturated medium-grain sandstone, which represent two limit states affected by water, were studied and compared. The results showed that water greatly weakens the compressive strength of medium-grained sandstone, but the difference in axial strain between dry and saturated rock samples decreases with the increase of confining pressure. The creep compression volume of rocks decreases with the increase of deviatoric stress, and only under low confining pressure does the creep volume of rocks exhibit expansion. There is an order of magnitude difference in creep strain between medium grained sandstone and other common rocks. The instantaneous elastic modulus has a negative exponential relationship with deviating stress. The creep rate has a positive exponential relationship with deviating stress. Based on Burges model with exponential damage variables, the law of the influence of confining pressure on creep model parameters has been discussed. The linear relationship between elastic modulus of medium-grained sandstone and parameters of Burgers model with damage was found. The research results are conducive to the convenient prediction of creep behavior of medium grained sandstone engineering and the long-term stability control of the surrounding rock.

Effect of Cyclic Loading Treatment on the Compression Energy and Creep Properties of Machine-Harvested Seed Cotton

Machine-harvested seed cotton was taken as the research object to further clarify its creep performance, minimize its power consumption during the loading process, and obtain a better loading method. The uniaxial compression creep test was carried out using the Instron universal material test bench to apply cyclic loading treatment. The test factors included cyclic loading times, cyclic stress peak, and cyclic loading frequency. The energy consumption curve of the machine-harvested seed cotton during cyclic loading was obtained through OriginPro 2019b software, and its energy change law was analyzed. Creep strain was divided into two parts, namely, initial creep strain and creep strain increment, to elucidate the creep mechanism. The Burgers model was chosen to describe the creep strain increment. Results show that machine-harvested seed cotton exhibits energy consumption hysteresis during cyclic loading. The compression energy rapidly decreases with increasing cyclic loading times and then stabilizes. Meanwhile, the compression energy increases with increasing cyclic stress peak and cyclic loading frequency. The creep strain mechanism is also the same, which first rapidly increases and then levels off. Cyclic loading times, cyclic stress peak, and cyclic loading frequency have different effects on creep strain increment, instantaneous elastic modulus, hysteresis elastic modulus, viscosity coefficient, delay time, and relative deformation index. Finally, disregarding power consumption and interaction, extending the cyclic loading time, and increasing the cyclic stress peak while simultaneously minimizing the cyclic loading frequency can reduce the relative deformation index in the creeping stage. Accordingly, the deformation retention ability in the creep is improved, but the compression energy in the cyclic loading increases. The results can provide theoretical and data support for studying the theoretical basis of the rheological properties of machine-harvested seed cotton, the design of seed cotton baling devices, and the study of bale (mold) forming quality.

Mechanical and histological properties of native medial menisci compared to allograph medial menisci in the osteoarthritic knee.

This study was designed to provide information on how the menisci change over the course of osteoarthritis, particularly with regard to their mechanical properties. The aim was to determine the difference between healthy menisci (fresh frozen meniscal transplants) and menisci harvested during total knee arthroplasty. The latter allows the grading of age-related and osteoarthritic changes in the menisci on macroscopic and microscopic levels. A total of 10 menisci from arthritic knee joints (medial) harvested during total knee arthroplasty were used and compared with 10 medial fresh frozen meniscal transplants. The mechanical measurements were carried out on a Mach-1 testing machine using indentation testing to determine the instantaneous modulus and the thickness of the menisci. The specimens were then embedded in paraffin, sectioned on a microtome, and stained with hematoxylin-eosin and safranin-O. All measurements were divided into the anterior horn, pars intermedia, and posterior horn. There was no significant difference in the instantaneous modulus for the posterior horn in the fresh frozen menisci with 0.27 ± 0.1MPa compared to the arthritic menisci with 0.18 ± 0.03MPa. No significant difference could be determined for the meniscus thicknesses. There was a significant difference in the safranin-O staining. There were also significant differences in the Pauli score: the arthrosis menisci showed a sum score that was, on average, four times higher than the sum score of the fresh frozen menisci. In the present study, it could be shown very well that there are significant differences in the mechanical properties as well as in the macroscopic and histopathological scores, such as the Pauli score, between the fresh frozen meniscus allografts considered healthy and osteoarthritic menisci resulting from total knee arthroplasty. With a degradation score of 3 (Pauli), the instantaneous modulus was reduced by more than 50% compared to healthy controls. More importantly, however, the fresh frozen menisci only show a grade 2 when converting the sum values into grades, where a grade 2 indicates slight degeneration. This is interesting because fresh frozen meniscus transplants were always considered healthy in previous publications and should, therefore, actually have a grade 1.

In situ forming, mechanically resilient hydrogels prepared from 4a-[PEG-b-PTMC-Ac] and thiolated chondroitin sulfate for nucleus pulposus cell delivery.

Intervertebral disk degeneration (IVDD) is a common condition that causes severe back pain and affects patients' mobility and life quality considerably. IVDD originates within the central region of the disk called the nucleus pulposus (NP). Removing the damaged tissue and replacing it with NP cells (NPCs) delivered within an in situ forming hydrogel is a promising treatment approach. Herein we describe a hydrogel formulation based on 4-arm [poly(ethylene glycol)-b-poly(trimethylene carbonate)-acrylate] (4a[PEG-b-PTMC-Ac]) crosslinked with thiolated chondroitin sulfate via Michael-type reaction for this purpose. A library of hydrogels based on 15 kDa 4a-[PEG] with PTMC blocks of varying molecular weight were prepared and characterized. The instantaneous moduli of the hydrogels were adjustable from 24 to 150 kPa depending on the length of the PTMC block and the polymer volume fraction. The influence of each of these parameters was effectively explained using both scaling or mean field theories of polyelectrolyte hydrogels. The hydrogels were resistant to cyclic compressive loading and degraded gradually over 70 days in vitro. A hydrogel formulation with an instantaneous modulus at the high end of the range of values reported for human NP tissue was chosen to assess the ability of these hydrogels for delivering NPCs. The prepolymer solution was injectable and formed a hydrogel within 30 minutes at 37 °C. Bovine NPCs were encapsulated within this hydrogel with high viability and proliferated throughout a 28 day, hypoxic culture period. The encapsulated NPCs formed clusters and deposited collagen type II but no collagen type I within the hydrogels. Despite an initial gradual decrease, a steady-state modulus was reached at the end of the 28 day culture period that was within the range reported for healthy human NP tissue. This in situ forming hydrogel formulation is a promising approach and with further development could be a viable clinical treatment for IVDD.

Characterization of the Age-Related Differences in Porcine Acetabulum and Femoral Head Articular Cartilage.

The use of porcine animal models for cartilage injury has increased recently due to their similarity with humans with regard to cartilage thickness, limited intrinsic healing of chondral defects, and joint loading biomechanics. However, variations in the mechanical and biochemical properties of porcine hip articular cartilage among various tissue ages and weightbearing (WB) regions are still unknown. This study's aim was to characterize the mechanical and biochemical properties of porcine hip articular cartilage across various ages and WB regions. Articular cartilage explants were harvested from WB and non-weightbearing (NWB) surfaces of the femoral head and acetabulum of domesticated pigs (Sus scrofa domesticus) at fetal (gestational age: 80 days), juvenile (6 months), and adult (2 years) ages. Explants underwent compressive stress-relaxation mechanical testing, biochemical analysis for total collagen and glycosaminoglycan (GAG) content, and histological staining. Juvenile animals consistently had the highest mechanical properties, with 2.2- to 7.6-time increases in relaxation modulus, 1.3- to 2.3-time increases in instantaneous modulus, and 4.1- to 14.2-time increases in viscosity compared with fetal cartilage. Mechanical properties did not significantly differ between the WB and NWB regions. Collagen content was highest in the NWB regions of the juvenile acetabulum (65.3%/dry weight [DW]) and femoral head (75.4%/DW) cartilages. GAG content was highest in the WB region of the juvenile acetabulum (23.7%/DW) and the WB region of the fetal femoral head (27.5%/DW) cartilages. Histological staining for GAG and total collagen content followed the trends from the quantitative biochemical assays. This study provides a benchmark for the development and validation of preclinical porcine models for hip cartilage pathologies.

The 2-layer elasto-visco-plastic rheological model for the material parameter identification of bone tissue extended by a damage law

The response of bone tissue to mechanical load is complex and includes plastic hardening, viscosity and damage. The quantification of these effects plays a mayor role in bone research and in biomechanical clinical trials as to better understand related diseases.In this study, the damage growth in individual wet human trabeculae subjected to cyclic overloading is quantified by inverse rheological modeling.Therefore, an already published rheological material model, that includes linear elasticity, plasticity and viscosity is extended by a damage law. The model is utilized in an optimization process to identify the corresponding material parameters and damage growth in single human trabeculae under tensile load.Results show that the damage model is leading to a better fit of the test data with an average root-mean-square-error (RMSE) of 2.52 MPa compared to the non-damage model with a RMSE of 3.03 MPa. Although this improvement is not significant, the damage model qualitatively better represents the data as it accounts for the visible stiffness reduction along the load history. It returns realistic stiffness values of 11.92 GPa for the instantaneous modulus and 5.73 GPa for the long term modulus of wet trabecular human bone. Further, the growth of damage in the tissue along the load history is substantial, with values above 0.8 close to failure. The relative loss of stiffness per cycle is in good agreement with comparable literature.Inverse rheological modeling proves to be a valuable tool for quantifying complex constitutive behavior from a single mechanical measurement. The evolution of damage in the tissue can be identified continuously over the load history and separated from other effects.