Analysis of the bending behaviour of porcine xenograft leaflets and of neutral aortic valve material: bending stiffness, neutral axis and shear measurements

Flexibility of the materials used in the construction of bioprosthetic heart valves is essential for proper valve operation. We therefore examined the bending behaviour of glutaraldehyde treated porcine aortic valve cusps in comparison with fresh aortic valve tissue. We repeatedly bent a total of 35 strips of fresh and treated tissue to curvatures ranging from 0.2 to 2.2 mm-1. We compared the stiffness of the two materials between circumferential and radial bending, natural and reverse curvatures and constant or variable tensile stress (0.8-40 kPa). Our results showed a weak positive relationship between bending stiffness and applied tensile stress and a strong positive dependance of stiffness on tissue thickness (t). For the fresh tissue, the bending stiffness increased in proportion to t1.14 while for the glutaraldehyde treated tissue it increased with t2.18. Fourteen strips of fresh and treated tissue were also histologically processed, sectioned and examined with polarized light microscopy. Collagen fiber wavelengths and shear deformations were measured utilizing the tissue banding patterns produced by polarized light microscopy. The neutral axis of bending was found to lie very close to the outer surface of the tissue, suggesting that aortic leaflets have a very low compressive elastic modulus. The shear strains measured in fresh tissue were 10 +/- 2.7% vs 3 +/- 4.4% for the treated, indicating a stiffening of the tissue following glutaraldehyde fixation. We conclude that both natural and bioprosthetic valve cusps have a complex flexural behaviour that cannot be modeled using simple bending principles, although the bioprosthetic material more closely approximates the simple beam than does the fresh. The non-linear elastic modulus, high compressibility and shearing between fiber layers are likely responsible for the observed behaviour of the fresh tissue, while the cross-linking and dehydrating effects of glutaraldehyde are believed to be responsible for the alteration in bending properties observed in the treated tissue. Our study suggests that bioprosthetic valve material does not adequately mimic the mechanics of the natural valve tissue, and that the current glutaraldehyde fixation process eliminates many of the beneficial, stress-reducing properties of the aortic leaflet.     

Testing a computer-assisted bending machine for manufacturing orthodontic treatment elements]

The use of suitable orthodontic devices producing desired defined force systems is of importance for successful orthodontic treatment. Bending loops can be difficult and time-consuming. Computerised fabrication would enable very precise reproduction of individual loops. A bending machine has now been developed within the framework of a computer-assisted treatment concept. In this study, a prototype machine was used to fabricate U-, T- and delta loops made of stainless steel, cobalt chromium and titanium molybdenum wire. The various geometric parameters of each loop were measured to determine how precisely they had been produced. Furthermore, the force system of each loop were experimentally investigated during simulated activation in an orthodontic measurement and simulation system. The results indicate that the geometric parameters had an average error of 2.8 degrees for angles and 0.9 mm for lengths. Owing to the fabrication errors, loops of the same type produced different force systems. Overall, the new bending machine can fabricate different types of loop, but the requirements of very precise fabrication are currently not met. This fact, together with further limitations in terms of configuration, means that the machine cannot be used routinely at present. However, the machine can nevertheless be considered a good basis for further development.     

Scheduling algorithms for a two-machine flexible manufacturing system

The flexible manufacturing system (FMS) considered in this paper is composed of two CNC machines working in series—a punching machine and a bending machine connected through rollers acting as a buffer system of finite capacity. The main difference between the present problem and the standard two-machine flow shop problem with finite intermediate capacity is precisely the buffer system, which in our problem consists of two stacks of parts supported by rollers: the first stack contains the output of the punching machine, while the second stack contains the input for the bending machine. When the second stack is empty, the first stack may be moved over. Furthermore, the capacity of each stack depends on the particular part type being processed. The FMS can manufacture a wide range of parts of different types. Processing times on the two machines are usually different so that an unbalance results in their total workload. Furthermore, whenever there is a change of the part type in production, the machines must be properly reset—that is, some tools need to be changed or repositioned. A second important difference between the present problem and the usual two-machine flow shop problem is the objective. Given a list ofp part types to be produced in known quantities, the problem considered here is how to sequence or alternate the production of the required part types so as to achieve various hierarchical targets: minimize the makespan (the total time needed to complete production) and, for instance, compress the idle periods of the machine with less workload into a few long enough intervals that could be utilized for maintenance or other reasons. Although Johnson's rule is optimal in some particular cases, the problem addressed in the paper isNP-hard in general: heuristic procedures are therefore provided.     

The Growth Response of the Stems of Genetically Modified Tobacco Plants (Nicotiana tabacum'Samsun') to Flexural Stimulation

Genetically modified tobacco plants (Nicotiana tabacum‘Samsun’)with antisense cinnamyl alcohol dehydrogenase DNA, produce secondaryxylem of a reduced tensile stiffness. These plants were grownalongside control plants. The stems of the plants were flexedor protected from flexing over a period of several weeks. Thetensile moduli and second moments of areas of the differenttissues inside the stems were measured and used to calculatethe bending stiffness of the plants. In tobacco, the cylinderof xylem was found to be the most important tissue in determiningthe bending stiffness of the plants. The thickness of the xylemtissue cylinder increased when plants were subjected to flexuralstimulation. This increased the bending stiffness of the stems.The response to mechanical stimulation was found to be correlatedwith tissue strain and the genetically modified plants wereable to exactly compensate for the reduced modulus of theirxylem tissue by increasing the thickness of the xylem tissuecylinder more than in control plants.Copyright 1999 Annals ofBotany Company. Tobacco plants, stem bending, xylem tissue, second moment of area, thigmomorphogenesis, mechanical strain.     

Expansion of surface epithelium provides the major extrinsic force for bending of the neural plate.

Neurulation, formation of the neural tube, requires both intrinsic forces (i.e., those generated within the neural plate) and extrinsic forces (i.e., those generated outside the neural plate in adjacent tissues), but the precise origin of these forces is unclear. In this study, we addressed the question of which tissue produces the major extrinsic force driving bending of the neural plate. We have previously shown that 1) extrinsic forces are required for bending and 2) such forces are generated lateral to the neural plate. Three tissues flank the neural plate prior to its bending: surface epithelium, mesoderm, and endoderm. In the present study, we removed two of these layers, namely, the endoderm and mesoderm, underlying and lateral to the neural plate; bending still occurred, often with complete formation of a neural tube, although the latter usually rotated toward the side of tissue depletion. These results suggest that the surface epithelium, the only tissue remaining after microsurgery, provides the major extrinsic force for bending of the neural plate and that the mesoderm (and perhaps endoderm) stabilizes the neuraxis, maintaining its proper orientation and position on the midline.     

Koppel‐, Steck‐ und Einrastmechanismen

Besides basic machine elements as levers and hinge joints, especially more complex ones as connecting devices, plugs and sockets, and other “hard dock” mechanisms show up possibilities of finding biological‐technical analogies. Technical partners often turn out as heurisic valuable approximations. They can direct to elements that are hardly realized by the biologist, who tries to explain more complex biomechanical systems. A kinematic chain for example which is kinematically determined, must contain four parts, combined by four joints. If in a biological system an axis of rotation is missing, it can well be there, manifesting itself in a thin bending area of a bone.     

Intra- and interindividual differences in tensile strength of human bone

In a biomechanical study we evaluated the stability of human femurs. In order to document the inter- and intraindividual differences in bending stability, we tested 200 bone samples, harvested from the femurs of 5 multiorgan donors. The bending tests were performed on a material testing machine (ZWICK) under standardized conditions. Our results revealed significant differences in the stability of the femurs of the 5 multiorgan donors. Furthermore, there was a significant difference in breaking strength at different levels of the same bone. Clinical relevance: In studies aimed at evaluating the primary bony stability of alloimplants after different preservation or sterilization procedures, the above-mentioned results should be taken into consideration.     

Bending strategies of convergently evolved,articulated coralline algae

The evolution of uncalcified genicula in upright calcified corallines has occurred at least three times independently, resulting in articulated corallines within Corallinoideae, Lithophylloideae, and Metagoniolithoideae. Genicula confer flexibility to otherwise rigid thalli, and the localization of bending at discrete intervals amplifies bending stress in genicular tissue. Genicular morphology must, therefore, be balanced between maintaining flexibility while mitigating or resisting stress. Genicula in the three articulated lineages differ in both cellular construction and development, which may result in different constraints on morphology. By studying the interaction between flexibility and morphological variation in multiple species, we investigate whether representatives of convergently evolving clades follow similar strategies to generate mechanically successful articulated fronds. By using computational models to explore different bending strategies, we show that there are multiple ways to generate flexibility in upright corallines but not all morphological strategies are mechanically equivalent. Corallinoids have many joints, lithophylloids have pliant joints, and metagoniolithoids have longer joints—while these strategies can lead to comparable thallus flexibility, they also lead to different levels of stress amplification in bending. Moreover, genicula at greatest risk of stress amplification are typically the strongest, universally mitigating the trade‐off between flexibility and stress reduction.     

Type and orientation of yielded trabeculae during overloading of trabecular bone along orthogonal directions

Trabecular architecture plays a major role in bone mechanics. Osteoporosis leads to a transition from a plate-like to a more rod-like trabecular morphology, which may contribute to fracture risk beyond that predicted by changes in density. In this study, microstructural finite element analysis results were analyzed using individual trabeculae segmentation (ITS) to identify the type and orientation of trabeculae where tissue yielded during compressive overloads in two orthogonal directions. For both apparent loading conditions, most of the yielded tissue was found in longitudinally oriented plates. However, the primary loading mode of yielded trabeculae was axial compression with superposed bending for on-axis loading in contrast to bending for transverse loading. For either loading direction, most plate-like trabeculae yielded in the same loading mode, regardless of their orientation. In contrast, rods oriented parallel to the loading axis yielded in compression, while rods oblique or perpendicular to the loading axis yielded in combined bending and tension. The predominance of tissue yielding in plates during both on-axis and transverse overloading explains why on-axis overloading is detrimental to the off-axis mechanical properties. At the same time, a large fraction of the tissue in rod-like trabeculae parallel to the loading direction yielded in both on-axis and transverse loading. Hence, rods may be more likely to be damaged and potentially resorbed by damage mediated remodeling.     

Role of Mechanical Loading in Growth of Sunflower (Helianthus annuus) Seedlings

Seedlings of the herbaceous annual Helianthus annuus L. weregrown under three regimens of daily bending. Bending wasadjustedduring ontogeny so that the deflexion of the stem tip dividedby the stem height was constant (elastic similarity). After6weeks, mechanically–treated plants exhibited a significantlygreater ratio of stem diameter to stem height, flexural stiffness,stemgrowth rate, and proportion of collenchymatous tissue inthe stem. Treated plants were also significantly smaller inheight,showed a greater proportion of stem tissue as cortex,and greater initial stem growth rate. No significant differenceswereobserved in the elastic modulus of the tissue composingthe stem, above–ground biomass, and stem diameter. Thedegree ofmechanical loading also had a significant effect onmost parameters. The most highly stressed plants were thicker,shorter, and elastically stiffer with more collenchyma and lesscortex in the stem. Since plants were loaded for only 60 s d^–1it is concludedthat mechanical effects early in life can haveprofound effects on the form and hence ecology of seedlings. Key words: Seedling growth, Helianthus annuus L., stem mechanical properties, thigmomorphogenesis, bending stress     

Computational modeling of morphogenesis regulated by mechanical feedback

Mechanical forces cause changes in form during embryogenesis and likely play a role in regulating these changes. This paper explores the idea that changes in homeostatic tissue stress (target stress), possibly modulated by genes, drive some morphogenetic processes. Computational models are presented to illustrate how regional variations in target stress can cause a range of complex behaviors involving the bending of epithelia. These models include growth and cytoskeletal contraction regulated by stress-based mechanical feedback. All simulations were carried out using the commercial finite element code ABAQUS, with growth and contraction included by modifying the zero-stress state in the material constitutive relations. Results presented for bending of bilayered beams and invagination of cylindrical and spherical shells provide insight into some of the mechanical aspects that must be considered in studying morphogenetic mechanisms.     

Impact of spherical applicator diameter on relative biologic effectiveness of low energy IORT X-rays: A hybrid Monte Carlo study

IntroductionLow-kV IORT (Low kilovoltage intraoperative radiotherapy) using INTRABEAM machine and dedicated spherical applicators is a candidate modality for breast cancer treatment. The current study aims to quantify the RBE (relative biologic effectiveness) variations of emitted X-rays from the surface of different spherical applicators and bare probe through a hybrid Monte Carlo (MC) simulation approach.Materials and methodsA validated MC model of INTRABEAM machine and different applicator diameters, based on GEANT4 Toolkit, was employed for RBE evaluation. To doing so, scored X-ray energy spectra at the surface of each applicator diameter/bare probe were used to calculate the corresponding secondary electron energy spectra at various distances inside the water and breast tissue. Then, MCDS (Monte Carlo damage simulation) code was used to calculate the RBE values according to the calculated electron spectra.ResultsPresence of spherical applicators can increase the RBE of emitted X-rays from the bare probe by about 22.3%. In return, changing the applicator diameter has a minimal impact (about 3.2%) on RBE variation of emitted X-rays from each applicator surface. By increasing the distance from applicator surface, the RBE increments too, so that its value enhances by about 10% with moving from 2 to 10 mm distance. Calculated RBE values within the breast tissue were higher than those of water by about 4% maximum value.ConclusionBall section of spherical IORT applicators can affect the RBE value of the emitted X-rays from INTRABEAM machine. Increased RBE of breast tissue can reduce the prescribed dose for breast irradiation if INTRABEAM machine has been calibrated inside the water.     

Spinal biomechanical properties are significantly altered with a novel embalming method

In vitro tests on the biomechanical properties of human spines are often performed using fresh frozen specimens. However, this carries the risk of pathogen transfer from specimen to the worker and the specimens can only be used for a limited amount of time. Human spinal specimens embalmed with formaldehyde carry an almost absent risk of transfer of pathogens and can be stored and used for a long time, but the tissue properties are strongly affected making this method inapplicable for biomechanical testing. In this study, a new embalming technique called Fix for Life (F4L), which claims to preserve the tissue properties, was tested. The range of motion (ROM) and stiffness of six fresh human spinal specimens was measured using a spinal motion simulator before and after F4L embalming. After F4L embalming, spinal stiffness increased in flexion-extension by 230%, in lateral bending by 284% and in axial rotation by 271%. ROM decreased by 46% in flexion-extension, 56% in lateral bending and 54% in axial rotation. In conclusion, based on this study, F4L does not maintain physiological spinal biomechanical properties, and we propose that this method should not be used for biomechanical studies. Nevertheless, the method may be an alternative to formaldehyde fixation in situations such as training and education because the effect on spinal biomechanics is less detrimental than formaldehyde and tissue color is maintained.     

Mechano-regulation of mesenchymal stem cell differentiation and collagen organisation during skeletal tissue repair

A number of mechano-regulation theories have been proposed that relate the differentiation pathway of mesenchymal stem cells (MSCs) to their local biomechanical environment. During spontaneous repair processes in skeletal tissues, the organisation of the extracellular matrix is a key determinant of its mechanical fitness. In this paper, we extend the mechano-regulation theory proposed by Prendergast et al. (J Biomech 30(6):539–548, 1997) to include the role of the mechanical environment on the collagen architecture in regenerating soft tissues. A large strain anisotropic poroelastic material model is used in a simulation of tissue differentiation in a fracture subject to cyclic bending (Cullinane et al. in J Orthop Res 20(3):579–586, 2002). The model predicts non-union with cartilage and fibrous tissue formation in the defect. Predicted collagen fibre angles, as determined by the principal decomposition of strain- and stress-type tensors, are similar to the architecture seen in native articular cartilage and neoarthroses induced by bending of mid-femoral defects in rats. Both stress and strain-based remodelling stimuli successfully predicted the general patterns of collagen fibre organisation observed in vivo. This provides further evidence that collagen organisation during tissue differentiation is determined by the mechanical environment. It is envisioned that such predictive models can play a key role in optimising MSC-based skeletal repair therapies where recapitulation of the normal tissue architecture is critical to successful repair.     

Effect of shoot tip and leaf removal on gravitropism in pea

Intact, light-grown pea (Pisum sativum L. cv. Alaska) seedlings were subjected to continuous horizontal gravistimulation and their growth and bending response compared with seedlings whose shoot tip and youngest leaf had been excised and with seedlings to which a counterweight to replace the mass of the decapitated tissue was added. While all seedlings achieved vertical orientation in 2 to 3 h, seedlings that were counterweighted bent upward at a significantly slower rate than the non-counterweighted, decapitated plants. In addition to this effect of mass on the rate of bending, decapitation also removed a major supply of auxin to cells in the bending zone which resulted in the slower bending of treated plants. Thus when using decapitation both the loss of mass and the time course of the response must be considered to understand its effect on gravitropism.     

Early bending kinetics in response to geostimulation or unilateral IAA application in Lolium nodal segments

Abstract In Lolium multiflorum nodal segments, bending responses both to geostimulation and unilateral indole-3-acetic acid (IAA) application exhibited much variability in their lag times and speeds of early bending. Despite this variability, mean response curves to gravity and auxin stimulus were markedly similar with each having a phase of immediate, negative bending followed by phases of slow, positive bending and eventually more rapid, positive bending within 40 min of initial treatment. Comparison of lag times for response to geostimulation and unilateral IAA application, whether derived from the mean of individual replicates, or from mean curve data, showed that at least 4 min is available in this geotropic system for establishment of asymmetric auxin levels that could lead to differential growth. The hypothesis that variability in georesponse in Lolium nodal segments is linked to variable sensitivity of geosensitive tissue to auxin was tested using matching longitudinally-halved nodal segments and evidence was obtained in support of the hypothesis from lag time but not from early bending speed data. The implications of the findings for an involvement of endogenous IAA in shoot geotropism together with the necessity to understand better the complex behaviour of bending response in individual replicates are discussed.     

Enhanced analytical power of SDS-PAGE using machine learning algorithms

We aim to demonstrate that a complex plant tissue protein mixture can be reliably "fingerprinted" by running conventional 1-D SDS-PAGE in bulk and analyzing gel banding patterns using machine learning methods. An unsupervised approach to filter noise and systemic biases (principal component analysis) was coupled to state-of-the-art supervised methods for classification (support vector machines) and attribute ranking (ReliefF) to improve tissue discrimination, visualization, and recognition of important gel regions.     

Mechanical strength of bone allografts subjected to chemical sterilization and other terminal processing methods

Infectious disease transmission through the use of human donor allografts can be a catastrophic complication in an otherwise straightforward surgical procedure. The use of bone allograft in reconstructive orthopedic surgeries is increasing, yet severe complications, including death, can result if the transplanted tissues transmit a communicable disease to the tissue recipient. The BioCleanse((R)) tissue sterilization process is a fully automated, low-temperature chemical sterilization process that renders allograft tissue sterile. The purpose of this study was to evaluate the effect of a chemical tissue sterilization process on the mechanical strength of cortical bone allografts prior to implantation. Cylindrical cortical bone specimens were harvested from seven human cadaver donors and treated either by: chemical sterilization alone; chemical sterilization and terminal sterilization by gamma irradiation; chemical sterilization, lyophilization, terminal sterilization by STERRAD and rehydration; or untreated. The specimens were tested to failure in axial compression, diametral compression, shear, or bending. There were no significant differences in ultimate stress, strain, or fracture energy between the chemically sterilized and control groups in any of the testing modes.     

On the Strength of Herbaceous Vascular Plant Stems

During the past 120 years researchers have tried to providean understanding of the relationship between the arrangementof stiffening tissue in the cross-section of plant stems andtheir mechanical integrity. The mechanical analysis of verticalstems of self-supporting plants has traditionally been concernedwith issues involving global and local stability of the stem,and with stresses developed due to wind loads. Plant stem tissue,considered as a material, is both heterogeneous and highly anisotropic,and this must be reflected in any characterization of its mechanicalbehaviour. This fact strongly influences the type of failurecriterion which should be applied for compressive failure ofthe stem subjected to loads causing bending. It is shown, here,that applying modern ideas as to the appropriate criteria forcompressive failure of fibre-reinforced composite materialscan influence how we assess the efficacy of various stelar arrangementsconcerning their ability to fulfill their mechanical function.Specifically, it is demonstrated that peripheral arrangementsof supporting tissue are, in some circumstances, less advantageousthan more uniform distributions of this tissue. Plant stems; stem bending; compressive strength; stelar types     

Variable tensility of the ligaments in the stalk of a sea-lily

The stalk of isocrinid sea-lilies consists largely of skeletal plates linked by collagenous ligaments. Although lacking contractile tissue, it can bend in response to external stimuli. The stalk of Cenocrinus asterius was tested mechanically to determine whether the mechanical properties of its ligaments are under physiological control. In bending tests, ligaments at the mobile symplexal junctions showed a limited “slackening” response to high K⁺ concentrations which was blocked reversibly by the anaesthetic propylene phenoxetol. In bending tests and uniaxial loading tests, ligaments at the normally rigid synostosal junctions ruptured in response to high K⁺, confirming that these junctions are specialized for autotomy. It is concluded that the ligaments are mutable collagenous structures whose presence explains the mechanical versatility of the isocrinid stalk.