Enzymatic digestion of articular cartilage results in viscoelasticity changes that are consistent with polymer dynamics mechanisms

<p>Abstract</p> <p>Background</p> <p>Cartilage degeneration via osteoarthritis affects millions of elderly people worldwide, yet the specific contributions of matrix biopolymers toward cartilage viscoelastic properties remain unknown despite 30 years of research. Polymer dynamics theory may enable such an understanding, and predicts that cartilage stress-relaxation will proceed faster when the average polymer length is shortened.</p> <p>Methods</p> <p>This study tested whether the predictions of polymer dynamics were consistent with changes in cartilage mechanics caused by enzymatic digestion of specific cartilage extracellular matrix molecules. Bovine calf cartilage explants were cultured overnight before being immersed in type IV collagenase, bacterial hyaluronidase, or control solutions. Stress-relaxation and cyclical loading tests were performed after 0, 1, and 2 days of incubation.</p> <p>Results</p> <p>Stress-relaxation proceeded faster following enzymatic digestion by collagenase and bacterial hyaluronidase after 1 day of incubation (both <it>p </it>≤ 0.01). The storage and loss moduli at frequencies of 1 Hz and above were smaller after 1 day of digestion by collagenase and bacterial hyaluronidase (all <it>p </it>≤ 0.02).</p> <p>Conclusion</p> <p>These results demonstrate that enzymatic digestion alters cartilage viscoelastic properties in a manner consistent with polymer dynamics mechanisms. Future studies may expand the use of polymer dynamics as a microstructural model for understanding the contributions of specific matrix molecules toward tissue-level viscoelastic properties.</p

The behavior of adaptive bone-remodeling simulation models

The process of adaptive bone remodeling can be described mathematically and simulated in a computer model, integrated with the finite element method. In the model discussed here, cortical and trabecular bone are described as continuous materials with variable density. The remodeling rule applied to simulate the remodeling process in each element individually is, in fact, an objective function for an optimization process, relative to the external load. Its purpose is to obtain a constant, preset value for the strain energy per unit bone mass, by adapting the density. If an element in the structure cannot achieve that, it either turns to its maximal density (cortical bone) or resorbs completely.\ud \ud It is found that the solution obtained in generally a discontinuous patchwork. For a two-dimensional proximal femur model this patchwork shows a good resemblance with the density distribution of a real proximal femur.\ud \ud It is shown that the discontinuous end configuration is dictated by the nature of the differential equations describing the remodeling process. This process can be considered as a nonlinear dynamical system with many degrees of freedom, which behaves divergent relative to the objective, leading to many possible solutions. The precise solution is dependent on the parameters in the remodeling rule, the load and the initial conditions. The feedback mechanism in the process is self-enhancing; denser bone attracts more strain energy, whereby the bone becomes even more dense. It is suggested that this positive feedback of the attractor state (the strain energy field) creates order in the end configuration. In addition, the process ensures that the discontinuous end configuration is a structure with a relatively low mass, perhaps a minimal-mass structure, although this is no explicit objective in the optimization process.\ud \ud It is hypothesized that trabecular bone is a chaotically ordered structure which can be considered as a fractal with characteristics of optimal mechanical resistance and minimal mass, of which the actual morphology depends on the local (internal) loading characteristics, the sensor-cell density and the degree of mineralization

Validation of a laboratory method for evaluating dynamic properties of reconstructed equine racetrack surfaces.

BackgroundRacetrack surface is a risk factor for racehorse injuries and fatalities. Current research indicates that race surface mechanical properties may be influenced by material composition, moisture content, temperature, and maintenance. Race surface mechanical testing in a controlled laboratory setting would allow for objective evaluation of dynamic properties of surface and factors that affect surface behavior.ObjectiveTo develop a method for reconstruction of race surfaces in the laboratory and validate the method by comparison with racetrack measurements of dynamic surface properties.MethodsTrack-testing device (TTD) impact tests were conducted to simulate equine hoof impact on dirt and synthetic race surfaces; tests were performed both in situ (racetrack) and using laboratory reconstructions of harvested surface materials. Clegg Hammer in situ measurements were used to guide surface reconstruction in the laboratory. Dynamic surface properties were compared between in situ and laboratory settings. Relationships between racetrack TTD and Clegg Hammer measurements were analyzed using stepwise multiple linear regression.ResultsMost dynamic surface property setting differences (racetrack-laboratory) were small relative to surface material type differences (dirt-synthetic). Clegg Hammer measurements were more strongly correlated with TTD measurements on the synthetic surface than the dirt surface. On the dirt surface, Clegg Hammer decelerations were negatively correlated with TTD forces.ConclusionsLaboratory reconstruction of racetrack surfaces guided by Clegg Hammer measurements yielded TTD impact measurements similar to in situ values. The negative correlation between TTD and Clegg Hammer measurements confirms the importance of instrument mass when drawing conclusions from testing results. Lighter impact devices may be less appropriate for assessing dynamic surface properties compared to testing equipment designed to simulate hoof impact (TTD).Potential relevanceDynamic impact properties of race surfaces can be evaluated in a laboratory setting, allowing for further study of factors affecting surface behavior under controlled conditions

Progress towards ultra sensitive KIDs for future far-infrared missions: a focus on recombination times

Future generations of far-infrared (FIR) telescopes will need detectors with noise-equivalent powers on the order of 5 x 10^(-20) W/Hz^(1/2) in order to be photon background limited by astrophysical sources. One such mission concept in development is the Galaxy Evolution Probe (GEP), which will characterize galaxy formation and evolution from z=0 to beyond z=4. Kinetic inductance detectors (KIDs) have been baselined for the GEP for spectroscopy and imaging science between 10 μm and 400 μm due to their intrinsic frequency multiplexability and simple readout schemes. We focus on quasiparticle recombination times as a strategy for increasing detector responsivities to move towards the NEP requirements of the GEP. We present a new model for quantifying time constants from the responses of detectors to pulses of light, and test this model on a 40 nm thick ¼ λ Al coplanar waveguide KID. We intend to use this measurement scheme to quantify the dependence of the quasiparticle recombination time on Al thickness

Modelling the Interaction Between Racehorse Limb and Race Surface

AbstractMusculoskeletal injuries are the leading cause of racehorse fatalities and attrition. Race surface mechanics affect racehorse limb biomechanics, and therefore can affect musculoskeletal injuries. Installation of experimental race surfaces to determine their effect on racehorse limb kinematics is not financially feasible. Furthermore, field data collection is time consuming, labor intensive, and requires the use of live animals. Computational modelling provides an economical option to survey a wide range of surface mechanics and resulting effects on racehorse limb motions. This research aimed to develop and evaluate an integrated racehorse limb and race surface computational model. The interaction of a virtual galloping racehorse impacting virtual race surfaces was modelled in SIMM using combined forward/inverse dynamics. In vivo kinematic data were averaged to determine proximal forelimb, trunk, and hindlimb kinematic model profiles throughout gallop stance, as well as distal forelimb initial conditions. All distal forelimb joints and hoof translations were free to respond to external forces applied by the race surface model during stance. Race surface model coefficients were determined from previously measured race surface mechanics and forward dynamic simulations of a track-testing device. Simulation results were compared to distal forelimb motions of actual galloping racehorses on mechanically measured race surfaces. Model predicted kinematic profiles (metacarpophalangeal angle and hoof translations) had qualitative shapes and peak magnitudes within ranges of experimental data. Simulated peak metacarpophalangeal angle and hoof translations were within 11 degrees and 4cm respectively. Future model applications include estimation of the effects of variation in race surface parameters on racehorse limb biomechanics

Extending KIDs to the Mid-IR for Future Space and Suborbital Observatories

The galaxy evolution probe (GEP) is a concept for a probe-class space observatory to study the physical processes related to star formation over cosmic time. To do so, the mid- and far-infrared (IR) spectra of galaxies must be studied. These mid- and far-IR observations require large multi-frequency arrays, sensitive detectors. Our goal is to develop low NEP aluminum kinetic inductance detectors (KIDs) for wavelengths of 10–400 μm for the GEP and a pathfinder long-duration balloon (GEP-B) that will perform precursor GEP science. KIDs for the lower wavelength range (10–100 μm) have not been previously implemented. We present an absorber design for KIDs sensitive to wavelengths of 10 μm shown to have around 75–80% absorption efficiency through ANSYS HFSS (high-frequency structure simulator) simulations, challenges that come with optimizing our design to increase the wavelength range, initial tests on our design of fabricated 10 μm KIDs, and theoretical NEP calculations

Responsivity boosting in FIR TiN LEKIDs using phonon recycling: simulations and array design

To characterize further the cosmic star formation history at high redshifts, a large-area survey by a cryogenic 4-6 meter class telescope with a focal plane populated by tens of thousands of far-infrared (FIR, 30-300 μm) detectors with broadband detector noise equivalent powers (NEPs) on the order of 3×10^(-9) W/√ Hz is needed. Ideal detectors for such a surveyor do not yet exist. As a demonstration of one technique for approaching the ultra-low NEPs required by this surveyor, we present the design of an array of 96 350 µm KIDs that utilize phonon recycling to boost responsivity. Our KID array is fabricated with TiN deposited on a silicon-on-insulator (SOI) wafer, which is a 2 μm thick layer of silicon bonded to a thicker slab of silicon by a thin oxide layer. The backside thick slab is etched away underneath the absorbers so that the inductors are suspended on just the 2 μm membrane. The intent is that quasiparticle recombination phonons are trapped in the thin membrane, thereby increasing their likelihood of being re-absorbed by the KID to break additional Cooper pairs and boost responsivity. We also present a Monte-Carlo simulation that predicts the amount of signal boost expected from phonon recycling given different detector geometries and illumination strategies. For our current array geometry, the simulation predicts a measurable 50% boost in responsivity

Identification of material parameters based on Mohr-Coulomb failure criterion for bisphosphonate treated canine vertebral cancellous bone

Nanoindentation has been widely used to study bone tissue mechanical properties. The common method and equations for analyzing nanoindentation, developed by Oliver and Pharr, are based on the assumption that the material is linearly elastic. In the present study, we adjusted the constraint of linearly elastic behavior and use nonlinear finite element analysis to determine the change in cancellous bone material properties caused by bisphosphonate treatment, based on an isotropic form of the Mohr–Coulomb failure model. Thirty-three canine lumbar vertebrae were used in this study. The dogs were treated daily for 1 year with oral doses of alendronate, risedronate, or saline vehicle at doses consistent, on a mg/kg basis, to those used clinically for the treatment of post-menopausal osteoporosis. Two sets of elastic modulus and hardness values were calculated for each specimen using the Continuous Stiffness Measurement (CSM) method (ECSM and HCSM) from the loading segment and the Oliver–Pharr method (EO–P and HO–P) from the unloading segment, respectively. Young's modulus (EFE), cohesion (c), and friction angle (ϕ) were identified using a finite element model for each nanoindentation. The bone material properties were compared among groups and between methods for property identification. Bisphosphonate treatment had a significant effect on several of the material parameters. In particular, Oliver–Pharr hardness was larger for both the risedronate- and alendronate-treated groups compared to vehicle and the Mohr–Coulomb cohesion was larger for the risedronate-treated compared to vehicle. This result suggests that bisphosphonate treatment increases the hardness and shear strength of bone tissue. Shear strength was linearly predicted by modulus and hardness measured by the Oliver–Pharr method (r2 = 0.99). These results show that bisphosphonate-induced changes in Mohr–Coulomb material properties, including tissue shear cohesive strength, can be accurately calculated from Oliver–Pharr measurements of Young's modulus and hardness