Rigid Outer Wall Research Articles

Flow rate–pressure drop relations for new configurations of slender compliant tubes arising in microfluidics experiments

We investigate the steady-state fluid–structure interaction between a Newtonian fluid flow and a deformable microtube in two novel geometric configurations arising in recent microfluidics experiments. The first configuration is a cylindrical fluidic channel surrounded by an annulus of soft material with a rigid outer wall, while the second one is a cylindrical fluidic channel extruded from a soft rectangular slab of material. In each configuration, we derive a mathematical theory for the nonlinear flow rate–pressure drop relation by coupling lubrication theory for the flow with linear elasticity for the inner tube wall’s deformation. Using the flow conduit’s axial slenderness and its axisymmetry, we obtain an analytical expression for the radial displacement in each configuration from a plane-strain configuration. The predicted displacement field, and the resulting closed-form flow rate–pressure drop relation, are each validated against three-dimensional direct numerical simulations via SimVascular’s two-way-coupled fluid–structure interaction solver, svFSI, showing good agreement. We also show that weak flow inertia can be easily incorporated in the derivation, further improving the agreement between theory and simulations for larger imposed flow rates.

Structural mechanics of filamentous cyanobacteria.

Filamentous cyanobacteria, forming long strands of connected cells, are one of the earliest and most successful forms of life on Earth. They exhibit self-organized behaviour, forming large-scale patterns in structures like biomats and stromatolites. The mechanical properties of these rigid structures have contributed to their biological success and are important to applications like algae-based biofuel production. For active polymers like these cyanobacteria, one of the most important mechanical properties is the bending modulus, or flexural rigidity. Here, we quantify the bending stiffness of three species of filamentous cyanobacteria, of order Oscillatoriales, using a microfluidic flow device where single filaments are deflected by fluid flow. This is complemented by measurements of Young’s modulus of the cell wall, via nanoindentation, and the cell wall thickness. We find that the stiffness of the cyanobacteria is well-captured by a simple model of a flexible rod, with most stress carried by a rigid outer wall. Finally, we connect these results to the curved shapes that these cyanobacteria naturally take while gliding, and quantify the forces generated internally to maintain this shape. The measurements can be used to model interactions between cyanobacteria, or with their environment, and how their collective behaviour emerges from such interactions.

Rapid auxin‐mediated changes in the proteome of the epidermal cells in rye coleoptiles: implications for the initiation of growth

In axial organs of juvenile plants, the phytohormone auxin (indole-3-acetic acid, IAA) rapidly mediates cell wall loosening and hence promotes turgor-driven elongation. In this study, we used rye (Secale cereale) coleoptile sections to investigate possible effects of IAA on the proteome of the cells. In a first set of experiments, we document that IAA causes organ elongation via promotion of expansion of the rigid outer wall of the outer epidermis. A quantitative comparison of the proteome (membrane-associated proteins), using two-dimensional difference gel electrophoresis (2-D DIGE), revealed that, within 2 h of auxin treatment, at least 16 protein spots were up- or down-regulated by IAA. These proteins were identified using reverse-phase liquid chromatography electrospray tandem mass spectrometry. Four of these proteins were detected in the growth-controlling outer epidermis and were further analysed. One epidermal polypeptide, a small Ras-related GTP-binding protein, was rapidly down-regulated by IAA (after 0.5 h of incubation) by -35% compared to the control. Concomitantly, a subunit of the 26S proteasome was up-regulated by IAA (+30% within 1 h). In addition, this protein displayed IAA-mediated post-translational modification. The implications of these rapid auxin effects with respect to signal transduction and IAA-mediated secretion of glycoproteins (osmiophilic nano-particles) into the growth-controlling outer epidermal wall are discussed.

Case Report: Variably Compliant Transtibial Prosthetic Socket Fabricated Using Solid Freeform Fabrication

Achieving and maintaining a comfortable fit in a lower-limb prosthetic socket is an important goal to help ensure a successful rehabilitation. The purpose of this case study was to assess the performance of sockets with compliant features integrated into the socket wall to relieve in-socket pressure during transtibial amputee gait. Two sockets incorporating variably compliant areas over the fibula head and distal tibia were fabricated for a transtibial amputee. The two sockets were designed with different levels of compliance and were compared with a third more conventional socket without compliant areas. All three sockets were otherwise geometrically identical and were aligned identically to eliminate in-socket pressure differences due to alignment. The three sockets were fabricated using selective laser sintering, a form of solid freeform fabrication. The in-socket pressure measurements were acquired during normal walking. Tekscan F-Socket pressure sensors were attached to the amputee’s residual limb rather than to the socket wall, which made it possible to measure pressures in the same location on the residual limb upon change of socket. A Vicon motion capture system was used to match in-socket pressures to the subject’s gait cycle. The pressure measurements showed that the compliant socket peak pressures over the distal tibia and fibula head were similar for the two compliance levels and nearly 50% and 30% lower than the conventional socket pressures, respectively. This case study showed that selective laser sintering manufactured sockets with variably compliant regions holds great promise for reducing contact pressure in sensitive regions of the residual limb. (J Prosthet Orthot. 2008;20:1‐7.) KEY INDEXING TERMS: prosthetic socket, amputees, CAD, CAM, solid freeform fabrication T he most important aspect of a lower extremity transtibial prosthesis is the socket design. The socket introduces a new interface or a new joint between the human and the mechanical support system. The forces generated through the socket either by body weight or by gait have to be carried by the soft tissue of the residual limb, which when improperly loaded can lead to discomfort or skin breakdown. Ultimately, the design and fit of the socket is what determines patient acceptance, comfort, suspension, and energy expenditure. 1 Achieving and maintaining a comfortable and tissue-tolerant socket fit remains a significant clinical problem. This study explores the use of new manufacturing techniques to fabricate sockets with integrated compliant features specifically designed to improve comfort and fit of the prosthesis.

C1 同心円型膜タンクの縦振動試験

We investigated the dynamic chracteristics of the longitudinal (axi-symmetric) sloshing of liquid in a concentric membrane tank. We conducted an experiment using a circular cylindrical tank having a rigid outer wall and a flexible inner wall. The three materials used for the inner walls, rigid acrylic, Kapton film and polyethylene foam sheeting. We found that in the membrane wall, the outer part resonant phenomenon was difficult to clearly discern, because of the heavy motion coupling of the inner and outer liquids through the flexible membrane wall. Consequently the apparent damping ratio was increased with decreasing rigidity of the membrane wall.

The structure and biochemistry of charophycean cell walls: I. Pectins of Penium margaritaceum

Plant cell walls are essential for proper growth, development, and interaction with the environment. It is generally accepted that land plants arose from aquatic ancestors which are sister groups to the charophycean algae (i.e., Streptophyta), and study of wall evolution during this transition promises insight into structure-function relationships of wall components. In this paper, we explore wall evolutionary history by studying the incorporation of pectin polymers into cell walls of the model organism Penium margaritaceum, a simple single-cell desmid. This organism produces only a primary wall consisting of three fibrillar or fibrous layers, with the outermost stratum terminating in distinct, calcified projections. Extraction of isolated cell walls with trans-1,2-diaminocyclohexane-N,N,N',N'-tetraacetic acid yielded a homogalacturonan (HGA) that was partially methyl esterified and equivalent to that found in land plants. Other pectins common to land plants were not detected, although selected components of some of these polymers were present. Labeling with specific monoclonal antibodies raised against higher-plant HGA epitopes (e.g., JIM5, JIM7, LM7, 2F4, and PAM1) demonstrated that the wall complex and outer layer projections were composed of the HGA which was significantly calcium complexed. JIM5 and JIM7 labeling suggested that highly methyl esterified HGA was secreted into the isthmus zone of dividing cells, the site of active wall secretion. As the HGA was displaced to more polar regions, de-esterification in a non-blockwise fashion occurred. This, in turn, allowed for calcium binding and the formation of the rigid outer wall layer. The patterning of HGA deposition provides interesting insights into the complex process of pectin involvement in the development of the plant cell wall.

A rigid‐bodied Ediacaran Biota from Upper Cambrian strata in Co. Wexford, Eire

AbstractTwo typical representatives of the Ediacaran biota, Ediacaria booleyi n.sp. and Nimbia occlusa have been found in deep‐water turbidites within the Upper Cambrian Booley Bay Formation at Booley Bay in Co. Wexford, Eire. The examples of E. booleyi were transported over short distances by the turbidity currents and then impacted, in some instances at high angles, into the ocean floor muds where they formed moulds but, as their bodies decayed, these were filled by casting sand from the overlying turbidite. They preserved fine detail during transportation and impaction, thereby indicating the existence of a rigid body wall. The three‐dimensional nature of their preservation allows reconstruction, which shows that Ediacaria had a discoid form with prominent relief, particularly on its dorsal side, which was divided into three main concentric zones with thin radial features, more evident at the periphery. The ventral side had lower relief, with alternate areas of coarse and fine concentric markings and numerous fine radial lines, some also thicker at the periphery.Nimbia occlusa are disc‐like bodies of low relief and occur in profusion on one sole. Some show a circular outline indicating that they were preserved flat on the bedding plane, but others occur as semicircular reliefs oriented in the palaeocurrent direction. This indicates that they too were moved by the turbidity current, but their abundance and excellent preservation indicate that transport distances were short. There is no evidence in either genus for the coelenterate characteristics of a mouth and a two‐layered body wall enclosing a single cavity. This, and the evidence for a rigid outer wall, adds to recent doubt on the concept of the world‐wide Ediacaran fauna as dominated by soft‐bodied coelenterates.

Spheroplasts of Methanospirillum hungatii formed upon treatment with dithiothreitol.

Strains of Methanospirillum hungatii, when treated with dithiothreitol at alkaline pH, formed spheroplasts which lysed in the absence of an osmotic stabilizer. Other methanogenic bacteria showed no response to this treatment, including Methanobacterium thermoautotrophicum. Methanobacterium strain M.o.H., Methanobacterium strain G2R, and Methanosarcina barkeri. The reaction with M. hungatii proceeded slowly at pH 8.0, but increased dramatically up to a pH of 10. Spheroplasts did not form in the presence of MgCl2 or if large amounts of FeS precipitate had been deposited on the cells during growth. Ultra-thin sections of M. hungatii GPI confirmed that individual cells, surrounded by a membrane and an inner wall, were contained within an outer wall or sheath. Adhesive material, which stained in the region of the cell spacer, was observed to bind together the inner and outer wall layers. After treatment with dithiothreitol, the spheroplasts, which retained the flexible inner wall and membrane, were released from the rigid outer wall. Examination of the isolated outer cell wall revealed that dithiothreitol (and dodecyl sodium sulfate) dissolved the adhesive material and damaged the cell spacer at the attachment site to the outer wall. It is proposed that dithiothreitol at alkaline pH loosens both the cell spacer and inner wall attachments to the outer wall, thereby allowing the osmotic pressure of the cytoplasm to eject the spheroplast. Resistance of the cells to lysis by Triton X-100 is lost upon spheroplast formation, indicating that protection to the cell is conferred by the outer cell wall.

Vascular modifications induced by flow

1. 1. Flow at critical velocities produced changes in the form of channels lined with a deformable silicone, with the development of structures superficially resembling those seen in blood vessels. These effects are attributed to the operation of mechanical forces. 2. 2. At sites of high velocity, such as occur at confluences or at large pressure gradients, the silicone tended to form bilaterally symmetrical cushions. These increased in size until they virtually occluded the lumen, acting as a stenosis and reducing the flow delivery through the system. In some of these cushions, the downstream end was suddenly hollowed out, producing a valve-like structure. 3. 3. At sites of high stream velocity, as at cushions, the silicone was sometimes pulled away from the rigid outer wall of the channel, producing a “dissection” of the wall. 4. 4. The downstream ends of the cushions, or valves, commonly developed polypoid “streamers” whose peduncles became progressively elongated and attenuated; these masses finally broke free, being swept downstream as “emboli.” 5. 5. Transmural perfusion was normally from stream to wall. At sites of cushion formation, stenosis, or valve formation, the pattern was reversed, with flow taking place from wall to stream. 6. 6. Similitudes of these formations and processes to those occurring in the cardiovascular system are discussed.