Helical Ridges Research Articles

Experimental and numerical study of a novel inbuilt helical ridge small-caliber graft for artery bypass surgery

Small-caliber (≤ 6 mm) vascular grafts are commonly used to treat vascular diseases, yet intimal hyperplasia at the distal end-to-side anastomosis remains a significant complication with long-term use. Helical flow has been shown to reduce the risk of cardiovascular complications effectively. Integrating helical flow through an inbuilt helical ridge in vascular grafts is intended to improve hemodynamic performance and mitigate issues such as flow-induced thrombosis and intimal hyperplasia. However, inadequate helical flow strength may limit its effectiveness. This study proposes a novel design for an inbuilt helical ridge in small-caliber grafts to achieve enhanced helical flow. Two novel small-caliber vascular grafts with variable-pitch helical ridges, incorporating both decreasing and increasing pitches, were designed. Their effectiveness was assessed through numerical simulations and in vitro experiments to evaluate their performance in arterial graft applications. The small-caliber vascular grafts incorporating variable-pitch helical ridges significantly improved helical flow structure and distribution, optimized wall shear stress distribution, and reduced platelet adhesion compared to conventional and constant-pitch helical ridge grafts. Among the various new designs, the small-caliber vascular graft featuring decreasing variable-pitch helical ridges has demonstrated better performance. This design is well-suited for small-caliber bypass surgeries, as it is effective in improving hemodynamic performance by improving helical flow into asymmetric structures and increasing helical flow density distribution.

Shape-Setting of Self-Expanding Nickel–Titanium Laser-Cut and Wire-Braided Stents to Introduce a Helical Ridge

PurposeAltered hemodynamics caused by the presence of an endovascular device may undermine the success of peripheral stenting procedures. Flow-enhanced stent designs are under investigation to recover physiological blood flow patterns in the treated artery and reduce long-term complications. However, flow-enhanced designs require the development of customised manufacturing processes that consider the complex behaviour of Nickel-Titanium (Ni-Ti). While the manufacturing routes of traditional self-expanding Ni–Ti stents are well-established, the process to introduce alternative stent designs is rarely reported in the literature, with much of this information (especially related to shape-setting step) being commercially sensitive and not reaching the public domain, as yet.MethodsA reliable manufacturing method was developed and improved to induce a helical ridge onto laser-cut and wire-braided Nickel–Titanium self-expanding stents. The process consisted of fastening the stent into a custom-built fixture that provided the helical shape, which was followed by a shape-setting in air furnace and rapid quenching in cold water. The parameters employed for the shape-setting in air furnace were thoroughly explored, and their effects assessed in terms of the mechanical performance of the device, material transformation temperatures and surface finishing.ResultsBoth stents were successfully imparted with a helical ridge and the optimal heat treatment parameters combination was found. The settings of 500 °C/30 min provided mechanical properties comparable with the original design, and transformation temperatures suitable for stenting applications (Af = 23.5 °C). Microscopy analysis confirmed that the manufacturing process did not alter the surface finishing. Deliverability testing showed the helical device could be loaded onto a catheter delivery system and deployed with full recovery of the expanded helical configuration.ConclusionThis demonstrates the feasibility of an additional heat treatment regime to allow for helical shape-setting of laser-cut and wire-braided devices that may be applied to further designs.

Helical-Ridge-Membranes from PVDF for enhanced gas–liquid mass transfer

New membrane geometries have the potential to increase mixing at the feed and permeate side to counteract concentration polarization and fouling. Such membrane geometries can be of very different architecture. Here, we address a new class of hollow fiber membranes having helical ridges. We focus on gas–liquid mass transfer, which is significantly slowed down by a liquid-side diffusion resistance. We present hydrophobic polyvinylidene fluoride (PVDF) helical ridge hollow fiber membranes produced by a rotating needle spinneret. Conceptually, the wet spinning methodology builds upon our Rotation-in-a-Spinneret platform technology, featuring customized microstructured rotating needles. The microstructured needle orifice includes two grooves to initiate ridge formation on the lumen side of the hollow fiber, while the ridges twist helically upon needle rotation. The new generation spinneret device produces hollow fiber membranes with reduced fiber diameter as compared to previous versions. It is specifically designed such that rotating spinning parameters enable an adjustable helical ridge pitch. Ridge formation and ridge shape strongly depend on rotational speed. The latter affects characteristic membrane properties such as membrane permeability and molecular selectivity. The helical ridges induce secondary flow in the lumen of the hollow fiber membranes, proven by pressure drop manipulation and experimental flow streamline visualization. In-depth analysis by flow simulation identifies rotational flow patterns as the governing flow phenomena. Ultimately, application in gas–liquid membrane contactors for oxygenation and CO2 capture revealed up to 10-fold improved transmembrane gas fluxes. Hence, the helical ridges cause turbulence promotion to introduce significant mass transfer enhancement.

‘Heart-like’ cross-sectional shape can better improve the hemodynamics in spiral laminar flow graft for small-caliber bypass application: a numerical study

Small-caliber grafts remain disappointed in the long-term bypass surgeries of coronary and peripheral arterial diseases. In order to improve the hemodynamics in small-caliber artery bypass grafts (ABGs), an improved spiral laminar flow (improved-SLF) graft with a ‘heart-like’ cross-sectional shape was proposed and verified by computational fluid dynamics simulation in this study. The results show that such graft can indeed induce a spiral flow and enhance the WSS distribution on the graft section. Furthermore, the helically distributed ribbon of unfavorable WSS observed in the original SLF graft was eliminated in the improved-SLF graft due to its smoothed and gentle helical ridge. On the other hand, improved-SLF ABG improved the WSS distribution in the distal anastomosis as well, because it maintained the strength of spiral flow when entering the anastomosis region. Finally, the improved-SLF ABG slightly increased the pressure drop along the bypass due to its small change of the general graft structure. As a proof-of-concept study, it can be concluded that improved-SLF graft can not only evenly enhance the WSS distribution in the graft section, but also improve the hemodynamic environment in the distal anastomosis without significantly increasing the pressure drop along the bypass, indicating such new helical-type graft may be more suitable to be used in the small-caliber graft bypass surgeries.

Rotating microstructured spinnerets produce helical ridge membranes to overcome mass transfer limitations

Membrane geometry evolution boosts membrane applications to become even more sustainable, resource- and energy-efficient. This evolution is crucial as increasingly permeable membrane materials introduce the major drawback of promoting fluid resistance due to boundary layer formation. We present how to break these boundary layers with Helical Ridge Membranes produced by rotating microstructured spinnerets. 3D printing enables us to manufacture polymeric, microstructured spinnerets featuring grooved orifices. When integrating these spinnerets into a wet spinning process, microstructured hollow fiber membrane surfaces evolve. Our home-engineered spinning technology sets the spinneret in motion. Rotation twists the nascent microstructure and creates a helical ridge on the lumen side. A robust spinning process especially establishes for our novel spinneret device to rotate the needle inside the spinneret. The interplay of spinning conditions and spinneret rotation uncovers a range of producible helical ridge shapes, sizes and pitches. In addition, spinneret rotation speed affects intrinsic membrane properties, about which we derive general correlations. The helical ridges prove the manipulation of hydrodynamics inside hollow fiber membranes by inducing secondary flow. The latter enhances mass transfer to diminish boundary layers. Ultimately, a cross-flow ultrafiltration showcase reveals TMP gradients reduced by 350% and demonstrates the disruptive impact of Helical Ridge Membranes on membrane filtration.

Mechanical and hydrodynamic analyses of helical strake-like ridges in a glass sponge.

From the discovery of functionally graded laminated composites, to near-structurally optimized diagonally reinforced square lattice structures, the skeletal system of the predominantly deep-sea sponge Euplectella aspergillum has continued to inspire biologists, materials scientists and mechanical engineers. Building on these previous efforts, in the present study, we develop an integrated finite element and fluid dynamics approach for investigating structure-function relationships in the complex maze-like organization of helical ridges that surround the main skeletal tube of this species. From these investigations, we discover that not only do these ridges provide additional mechanical reinforcement, but perhaps more significantly, provide a critical hydrodynamic benefit by effectively suppressing von Kármán vortex shedding and reducing lift forcing fluctuations over a wide range of biologically relevant flow regimes. By comparing the disordered sponge ridge geometry to other more symmetrical strake-based vortex suppression systems commonly employed in infrastructure applications ranging from antennas to underwater gas and oil pipelines, we find that the unique maze-like ridge organization of E. aspergillum can completely suppress vortex shedding rather than delaying their shedding to a more downstream location, thus highlighting their potential benefit in these engineering contexts.

Effect of geometrical irregularities on fatigue of lead sheathing for submarine high voltage power cable applications

High voltage subsea power cables are typically composed of several layers, including: a conductive core, a dielectric layer, a ductile metallic watertight barrier and a series of radial and axial armours and polymer layers. The watertight barrier serves the purpose of preventing water permeating to the dielectric due to the outer layers' polymer hygroscopicity. Water impregnation may cause dielectric breakdown, with fatal consequences on the integrity of the power cable. The watertight barrier is made of extruded lead alloy and is subjected, during the installation and operational life of the component, to axial and radial displacement-controlled fatigue which might lead to cracking, thus to a loss of its isolating properties. Lead, due to the low melting temperature, is characterized by creep deformation and damage even at room temperature. A steel tape is winded around the polyethylene sheathing surrounding the lead barrier, which is deformed, and a helical ridge is generated. This irregularity causes a local concentration of strain which might potentially lead to early failure of the lead sheathing. In this work the results of displacement-controlled fatigue testing at different strain rates of sheathing material including the irregularity is presented and compared to the results in equivalent conditions, but in the absence of the irregularity. The strain concentration caused by the defect is analysed with the implementation of a creep material model calibrated on the cyclic properties of the alloy and the notch sensitivity of the material for the class of defects, temperature and frequency in exam is obtained. The results are applied to the modelling of the stress-strain field in the component with and without irregularity and conclusions are drawn on the danger that such irregularities pose on the structural integrity on the power cable.

Compressed sensing MRI to characterize sodium alginate deposits during cross-flow filtration in membranes with a helical ridge

Cross-flow membrane ultrafiltration of the polysaccharide sodium alginate in aqueous solution was investigated in polymeric Helix membranes by in-situ magnetic resonance imaging (MRI) non-invasively and non-destructively. MRI allows velocity and NMR intensity measurements during filtration. In the present case and under the conditions used in the experiments, signal intensity is mainly generated by 1H of water. Lower intensities can therefore be understood as a measure for material accumulation. Compressed sensing allows faster data acquisition, enabling a reliable, time resolved measurement of the ultrafiltration progress. Even under cross-flow filtration conditions, the membrane filtration is impaired by the increasing filtration resistance due to gel layer formation during the filtration of aqueous sodium alginate with CaCl2. The Helix membrane has an additional helical ridge along the membrane wall to provide fouling inhibition, resulting in a complex three-dimensional flow field that was measured via compressed sensing MRI. Furthermore, the behavior of the deposit was classified more precisely by backwashing in the Helix membrane. Summarizing, the flow field, the deposit formation during cross-flow filtration in a Helix membrane and its backwashing was monitored and investigated by fast MRI methods.

Haemodynamic optimisation of a dialysis graft design using a global optimisation approach.

Disturbed flow and the resulting non‐physiological wall shear stress (WSS) at the graft‐vein anastomosis play an important role in arteriovenous graft (AVG) patency loss. Modifying graft geometry with helical features is a popular approach to minimise the occurrence of detrimental haemodynamics and to potentially increase graft longevity. Haemodynamic optimisation of AVGs typically requires many computationally expensive computational fluid dynamics (CFD) simulations to evaluate haemodynamic performance of different graft designs. In this study, we aimed to develop a haemodynamically optimised AVG by using an efficient meta‐modelling approach. A training dataset containing CFD evaluations of 103 graft designs with helical features was used to develop computationally low‐cost meta‐models for haemodynamic metrics related to graft dysfunction. During optimisation, the meta‐models replaced CFD simulations that were otherwise needed to evaluate the haemodynamic performance of possible graft designs. After optimisation, haemodynamic performance of the optimised graft design was verified using a CFD simulation. The obtained optimised graft design contained both a helical graft centreline and helical ridge. Using the optimised design, the magnitude of flow disturbances and the size of the anastomotic areas exposed to non‐physiological WSS was successfully reduced compared to a regular straight graft. Our meta‐modelling approach allowed to reduce the total number of CFD model evaluations required for our design optimisation by approximately a factor 2000. The applied efficient meta‐modelling technique was successful in identifying an optimal, helical graft design at relatively low computational costs. Future studies should evaluate the in vivo benefits of the developed graft design.

Interaction between particulate fouling and precipitation fouling: Sticking probability and deposit bond strength

Sticking probability (P) and deposit bond strength (ξ) are the two most important factors that determine the fouling process, but no calculation correlations were found for P and ξ in current models. This study investigated the interaction between particulate fouling and precipitation fouling based on the values of P and ξ, taking into account the growth characteristics of the combination of particulate fouling and precipitation fouling on the micro-surface of the heat transfer tube. Three kinds of fouling were measured continuously over 720 h in each of three individual tests: particulate fouling, precipitation fouling, and the combined fouling. In order to recognize the growth mechanism of the combined fouling on the heat transfer surface and to solve the engineering problems, this study provides a reliable method to calculated P and ξ. The results indicated that the solution of CaCO3 mixed with the particles of SiO2 at the mass ratio of 1:1 (CaCO3:SiO2) improved sticking probability. And the enhanced tubes, the helical ridge tube with start-number of 45 and start-number of 10, compared with the plain tube, would not easily stick, but hardly denude by flow which reduced the sticking probability and increased the deposit bond strength. For the helical ridge tube with start-number of 45, when compared with the helical ridge tube with start-number of 10, the particles were more likely to stick to the surface of, but the precipitation fouling was harder to occur. It was observed that deposits were difficult to remove once they were deposited by the particulate fouling or precipitation fouling on the surface of the helical ridge tube with start-number of 45.

Impact of a Helical Ridge within a Tubular Membrane Channel on Fluid Flow and Particle Behavior: A Model-Based Analysis

Helical inserts or baffles in tubular membranes can provide flux increases without additional energy usage. However, the optimal shape of these turbulence-enhancing structures remains debated, with many experimental studies being performed, though few full-scale applications reported. A model-based approach can assist experimental work in getting detailed insights into the hydrodynamic phenomena near the membrane surface. This creates the opportunity for model-based design of such membranes or an optimization of operating conditions. Both the fluid flow and particle behavior were simulated in a single Pentair X-Flow Compact Helix membrane with helical ridge, and results were compared to those of a plain tubular membrane. Higher mean wall shear stresses were found for the membrane with helical ridge as well as a wider distribution of wall shear stresses, which promote fouling mitigation. Within a shadow zone behind the helical ridge, wall shear stress was, however, slightly lower compared to the plain memb...

On the effects of surface corrugation on the hydrodynamic performance of cylindrical rigid structures.

In this work, we perform fully three-dimensional numerical simulations of the flow field surrounding cylindrical structures characterized by different types of corrugated surface. The simulations are carried out using the Lattice Boltzmann Method (LBM), considering a flow regime with a Reynolds number [Formula: see text]. The fluid-dynamic wake structure and stability are investigated by means of PSD analyses of the velocity components and by visual inspection of the vortical coherent structure evolution. Moreover, the energy dissipation of the flow is assessed by considering an equivalent discharge coefficient [Formula: see text], which measures the total pressure losses of the flow moving around the various layout under investigation. Outcomes from our study demonstrate that the helical ridges augment energy dissipation, but might also have a role in the passive control of the characteristic frequencies of the unsteady wake flow.

Enhancement in heat transfer during condensation of an HFO refrigerant on a horizontal tube with 3D fins

In this work, heat transfer coefficients during condensation of an environment-friendly hydro-fluoro-olefin (HFO) refrigerant R1233zd(e) on the outside surface of two horizontal tubes are separately measured. The cooling water flows inside the tubes and provides cooling to the vapor refrigerant. One tube is a plain tube (commercially smooth both inside and outside) while the other tube is an enhanced tube, with the inside surface having 2D helical ridges and the outside surface having 3D extruded fins. The tests were conducted at the saturation temperature 36.1 °C. The condensation heat transfer degrades with an increase in the condensation temperature difference; the degree of degradation is the nearly the same for the two tubes. Compared with the plain surface, the condensation heat transfer from the enhanced surface is approximately 10.8 times as high. In addition to the area increase due to the extruded fins, the enhancement in the condensation heat transfer is partly attributed to more effective condensate draining mechanism of the 3D fins where surface tension plays an important role. Further analysis reveals that heat transfer during the condensation process on the tube with 3D fins follows the Nusselt correlation with a multiplier that accounts for the enhancement in heat transfer, which is desirably simple approach to modeling condensation heat transfer on the complex 3D enhanced surfaces. This work provides more insights into the physical mechanisms underlying the complex condensation process on tubes with 3D fins.

The X-Ray Crystal Structure of the Keratin 1-Keratin 10 Helix 2B Heterodimer Reveals Molecular Surface Properties and Biochemical Insights into Human Skin Disease

Keratins 1 (K1) and 10 (K10) are the primary keratins expressed in differentiated epidermis. Mutations in K1/K10 are associated with human skin diseases. We determined the crystal structure of the complex between the distal (2B) helices of K1 and K10 to better understand how human keratin structure correlates with function. The 3.3 Å resolution structure confirms many features inferred by previous biochemical analyses, but adds unexpected insights. It demonstrates a parallel, coiled-coil heterodimer with a predominantly hydrophobic intermolecular interface; this heterodimer formed a higher order complex with a second K1-K10-2B heterodimer via a Cys401K10 disulfide link, although the bond angle is unanticipated. The molecular surface analysis of K1-K10-2B identified several pockets, one adjacent to the disulfide linkage and conserved in K5-K14. The solvent accessible surface area of the K1-K10 structure is 20-25% hydrophobic. The 2B region contains mixed acidic and basic patches proximally (N-terminal), whereas it is largely acidic distally (C-terminal). Mapping of conserved and nonconserved residues between K1-K10 and K5-K14 onto the structure demonstrated the majority of unique residues align along the outer helical ridge. Finally, the structure permitted a fresh analysis of the deleterious effects caused by K1/K10 missense mutations found in patients with phenotypic skin disease.

Theory of the development of curved barbs and their effects on feather morphology.

Feathers exhibit an extraordinary diversity of shapes, which are used by birds to accomplish a diverse set of functions. Pennaceous feathers have a double branched morphology that develops from a tube of epidermis, and variation in branch geometry determines feather shape. Feather development is both complex (i.e., a simple developmental modification can have multiple effects on mature feather shape), and redundant (i.e., different developmental modifications can create the same shape). Due to this, it is not readily apparent how different feather shapes develop. In many feathers, barbs are not straight, but instead curve in toward, or away, from the feather tip. Barb curvature can affect the shape of mature feathers but the development of curved barbs is unknown. Previous research has hypothesized that barb curvature could develop either during the helical growth of barb ridges in the tubular feather germ, or during barb angle expansion as the feather unfurls from the sheath. To better understand the development of curved barbs and their effects on mature feathers we present a theoretical model of curved barb development and test the model with empirical investigations of feathers. We find that curved barbs affect many aspects of feather morphology including vane width, barb length, and barb spacing. In real feathers, curved barbs can develop both during helical barb ridge growth and during barb angle expansion, with most of the observed curvature due to barb angle expansion. Our results demonstrate that barb angle expansion as a feather unfurls from the sheath is a complex and dynamic process that plays an important role in determining the shape and structure of mature feathers. Curved barbs create heterogeneity in barb geometry within the feather vane, which could have important implications for aerodynamic function and the development of within feather pigmentation patterns. J. Morphol. 277:995-1013, 2016. © 2016 Wiley Periodicals, Inc.

Generation of Subwavelength Plasmonic Nanovortices via Helically Corrugated Metallic Nanowires.

We demonstrate that plasmonic helical gratings consisting of metallic nanowires imprinted with helical grooves or ridges can be used efficiently to generate plasmonic vortices with radius much smaller than the operating wavelength. In our proposed approach, these helical surface gratings are designed so that plasmon modes with different azimuthal quantum numbers (topological charge) are phase-matched, thus allowing one to generate optical plasmonic vortices with arbitrary topological charge. The general principles for designing plasmonic helical gratings that facilitate efficient generation of such plasmonic vortices are derived and their applicability to the conversion of plasmonic vortices with zero angular momentum into plasmonic vortices with arbitrary angular momentum is illustrated in several particular cases. Our analysis, based both on the exact solutions for the electromagnetic field propagating in the helical plasmonic grating and a coupled-mode theory, suggests that even in the presence of metal losses the fundamental mode with topological charge m = 0 can be converted to plasmon vortex modes with topological charge m = 1 and m = 2 with a conversion efficiency as large as 60%. The plasmonic nanovortices introduced in this study open new avenues for exciting applications of orbital angular momentum in the nanoworld.

모바일 기기용 초소형 나사의 냉간 단조 공정 설계

Abstract A screw is a type of fastener characterized by a helical ridge known as thread. The demands for screws with the miniaturization and weight reduction are increasing for the trend of small size of mobile devices. The successful designs of mold and process are very important to obtain screws with good mechanical properties and highprecision. In this study, the design of cold forging process of micro screw was carried out by using finite elementmethod. In particular, in order to investigate the effects of die geometries and friction, design of experiment methodwas adopted and it was revealed that the friction was the dominant factor of folding defects. From these results, the design of die was modified and experiments were carried out with the modified die. From the experimental results,it was found that the folding defects disappeared. Key Words : Cold Forging, Die Design, Finite Element Method, Micro Screw 본 논문은 2014년도 정부의 재원으로 한국산업단지공단의 산업집적지경쟁력강화사업의 지원을 받아 수행되었음. * Corresponding Author : Du-Soon Choi(Korea National University of Transportation) Tel: +82-43-841-5128 email: dschoi@ut.ac.krReceived April 30 2015Accepted June 11, 2015Revised June 2, 2015Published June 30, 2015

Wnt3a gradient converts radial to bilateral feather symmetry via topological arrangement of epithelia

The evolution of bilaterally symmetric feathers is a fundamental process leading toward flight. One major unsolved mystery is how the feathers of a single bird can form radially symmetric downy feathers and bilaterally symmetric flight feathers. In developing downy feather follicles, barb ridges are organized parallel to the long axis of the feather follicle. In developing flight-feather follicles, the barb ridges are organized helically toward the anterior region, leading to the fusion and creation of a rachis. Here we discover an anterior-posterior molecular gradient of wingless int (Wnt3)a in flight but not downy feathers. Global inhibition of the Wnt gradient transforms bilaterally symmetric feathers into radially symmetric feathers. Production of an ectopic local Wnt3a gradient reoriented barb ridges toward the source and created an ectopic rachis. We further show that the orientation of the Wnt3a gradient is dictated by the dermal papilla (DP). Swapping DPs between wing covert and breast downy feathers demonstrates that both feather symmetry and molecular gradients are in accord with the origin of the DP. Thus the fates of feather epidermal cells are not predetermined through some molecular codes but can be modulated. Together, our data suggest feathers are shaped by a DP--> Wnt gradient-->helical barb ridge organization-->creation of rachis-->bilateral symmetry sequence. We speculate diverse feather forms can be achieved by adjusting the orientation and slope of molecular gradients, which then shape the topological arrangements of feather epithelia, thus linking molecular activities to organ forms and novel functions.

Fouling in enhanced tubes using cooling tower water: Part I: long-term fouling data

This paper describes the results of long-term fouling tests for cooling tower water flowing inside enhanced tubes. The data were taken using 800 ppm calcium hardness water supplied to an operating chiller/cooling tower system. The fouling mechanism is a combination of precipitation fouling and particulate fouling. Fouling data were measured for seven different enhanced tube geometries over a 2500 h operating period. The tubes tested had internal helical ridges, and the number of ridge starts varied from 10 to 45. Little fouling occurred until 1300 h. After 1300 h, moderate fouling began to occur in the tubes having a large number of starts. These tubes also provide the highest water-side heat transfer coefficients. Significant fouling existed in the tubes having 30–45 starts at the end of the test period. The salient finding of this work is that the potential for fouling increases as the number of starts and helix angle increases.

Left- and right-handed helical tubule intermediates from a pure chiral phospholipid

Differential phase contrast microscopy under conditions of very slow cooling reveals the dominant self-assembly mechanism of a chiral diacetylenic phospholipid into multilamellar tubules to be the rapid growth of helical ribbons from spherical multilamellar vesicles. Surprisingly, in the first seconds of tubule formation, a pure enantiomer of 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine [DC(8,9)PC] yields roughly equal numbers of left- and right-handed helices, inconsistent with the heretofore observed relationship between tubule helix handedness on phospholipid chirality. A much slower process follows by which tubules become multilamellar with a prominent helical ridge upon the exterior. Interestingly, these exterior helical ridges are of only a single handedness.