Mechanics of a plant in fluid flow.

Plants live in constantly moving fluid, whether air or water. In response to the loads associated with fluid motion, plants bend and twist, often with great amplitude. These large deformations are not found in traditional engineering application and thus necessitate new specialized scientific developments. Studying fluid-structure interaction (FSI) in botany, forestry, and agricultural science is crucial to the optimization of biomass production for food, energy, and construction materials. FSIs are also central in the study of the ecological adaptation of plants to their environment. This review paper surveys the mechanics of FSI on individual plants. I present a short refresher on fluid mechanics then dive into the statics and dynamics of plant-fluid interactions. For every phenomenon considered, I examine the appropriate dimensionless numbers to characterize the problem, discuss the implications of these phenomena on biological processes, and propose future research avenues. I cover the concept of reconfiguration while considering poroelasticity, torsion, chirality, buoyancy, and skin friction. I also assess the dynamical phenomena of wave action, flutter, and vortex-induced vibrations.

Failure mechanisms of coiling fibers with sacrificial bonds made by instability-assisted fused deposition modeling.

Instability-assisted 3D printing is a method for producing microstructured fibers with sacrificial bonds and hidden lengths that mimic nature's toughening mechanisms found in spider silk. This hierarchical structure increases the effective toughness of poly(lactic acid) (PLA) fibers by 240-340% in some specimens. Nevertheless, many specimens show worse toughness as low as 25% of that of the benchmark straight fiber due to the incomplete release of hidden lengths caused by premature failures. Here, we report the mechanical tests and simulations of microstructured fibers with coiling loops that identify the material plastic deformation as being crucial to fully release the hidden lengths. Without sufficient material yielding, high local tensile stress results from the bending-torsion-tension coupled deformation of the coiling loop and induces crack initiation at the fiber backbone during the loop unfolding process. On the other hand, the influence of bond-breaking defects is found to be negligible here. Moreover, for a number of broken bonds beyond a critical value, the accumulated elastic energy along the released loops induces a high strain rate (∼1500 mm mm-1 s-1) in a quasi-static tensile test, which fractures the fiber backbone within 0.1 ms after the breaking of a new bond. We also show a size effect in fused deposition modeling (FDM) extruded PLA fibers, which results in a higher effective toughness (∼5 times the performance of the straight fiber benchmark) in small coiling fibers (dia. = 0.37 mm), due to the better ductility in bending and torsion compared to large fibers (dia. = 1.20 mm). The failure mechanisms of single microstructured fibers presented here lay the groundwork for further optimizations of fiber arrays in the next generation of high energy-absorption composites for impact protection and safety-critical applications.

Easy to build, modular and large scale pipe conveying fluid experimental setup.

The pipe conveying fluid is a classic fluid structure interaction experiment. First studied for industrial applications such as liners and pipelines, it became a "paradigm" of non-linear dynamics in the same way as the vertical rotating shaft. Hundreds of papers studying different pipe instabilities and different phenomena with various numerical and analytical methods have been published in the last decades. However, many studies lack the comparison with experimental data to validate the analytical models and numerical simulations. Indeed, designing and building a pipe conveying fluid experimental setup can prove to be a long and a burdensome process. This paper presents an easy to build pipe conveying fluid experimental setup built in the LM2 laboratory at Polytechnique Montréal. Fig. 1 presents the global architecture of this experimental rig. This large scale setup uses relatively high speed cameras to track the pipe in three dimensions. It does not require heavy construction or major plumbing and electrical work. Moreover, it is removable and can be modified easily to observe different phenomena with various large scale pipes or boundary conditions. Lastly, it is relatively inexpensive as it costs less than 20 000 US dollars including all the sensors and acquisition systems.

Cell geometry regulates tissue fracture.

In vascular plants, the epidermal surfaces of leaves and flower petals often display cells with wavy geometries forming intricate jigsaw puzzle patterns. The prevalence and diversity of these complex epidermal patterns, originating from simple polyhedral progenitor cells, suggest adaptive significance. However, despite multiple efforts to explain the evolutionary drivers behind these geometrical features, compelling validation remains elusive. Employing a multidisciplinary approach that integrates microscopic and macroscopic fracture experiments with computational fracture mechanics, we demonstrate that wavy epidermal cells toughen the plants' protective skin. Through a multi-scale framework, we demonstrate that this energy-efficient patterning mechanism is universally applicable for toughening biological and synthetic materials. Our findings reveal a tunable structural-mechanical strategy employed in the microscale design of plants to protect them from deleterious surface fissures while facilitating and strategically directing beneficial ones. These findings hold implications for targeted plant breeding aimed at enhancing resilience in fluctuating environmental conditions. From an engineering perspective, our work highlights the sophisticated design principles the plant kingdom offers to inspire metamaterials.

A low-cost open-source automated shot peen forming system.

The aerospace industry relies on shot peen forming to form sheet metal parts and correct distortion in machined parts. The shot peen forming community is developing simulation and planning methods to meet the industrial need for automation. Researchers need to invest in expensive industrial robots contained in large blasting cabinets to perform the experimental validations of their proposed methods. To bypass this need for expensive industrial-grade equipment, this work presents a low-cost shot peen forming system that enables researchers to validate their methods experimentally on small samples. Conventional installations can cost a few 100,000 USD while the proposed prototype only costs 400 USD to produce. Such a prototype can operate on a tabletop with conventional electrical installations. The use of small samples for validation further reduces the cost of experiments and allows for faster development.

Air leaks: Stapling affects porcine lungs biomechanics.

During thoracic operations, surgical staplers resect cancerous tumors and seal the spared lung. However, post-operative air leaks are undesirable clinical consequences: staple legs wound lung tissue. Subsequent to this trauma, air leaks from lung tissue into the pleural space. This affects the lung's physiology and patients' recovery. The objective is to biomechanically and visually characterize porcine lung tissue with and without staples in order to gain knowledge on air leakage following pulmonary resection. Therefore, a syringe pump filled with air inflates and deflates eleven porcine lungs cyclically without exceeding 10 cmH2O of pressure. Cameras capture stereo-images of the deformed lung surface at regular intervals while a microcontroller simultaneously records the alveolar pressure and the volume of air pumped. The raw images are then used to compute tri-dimensional displacements and strains with the Digital Image Correlation method (DIC). Air bubbles originated at staple holes of inner row from exposed porcine lung tissue due to torn pleural on costal surface. Compared during inflation, left upper or lower lobe resections have similar compliance (slope of the pressure vs volume curve), which are 9% lower than healthy lung compliance. However, lower lobes statistically burst at lower pressures than upper lobes (p-value<0.046) in ex vivo conditions confirming previous clinical in vivo studies. In parallel, the lung deformed mostly in the vicinity of staple holes and presented maximum shear strain near the observed leak location. To conclude, a novel technique DIC provided concrete evidence of the post-operative air leaks biomechanics. Further studies could investigate causal relationships between the mechanical parameters and the development of an air leak.

Characterization of the deflections of a catheter steered using a magnetic resonance imaging system.

PURPOSE: The authors quantify the deflections of a catheter and a guidewire in MR setting with different designs of ferromagnetic tips and a system of high gradient coils which can generate gradients, and thus forces, 20 times larger than a conventional scanner. METHODS: Different designs of catheter tips are experimentally tested in an effort to maximize the deflections. One to two ferromagnetic spheres are attached at the distal tip of the catheter (or guidewire) with different spacing between the spheres. The effect of dipole-dipole interaction on the steering of the catheter is studied through experimentation and theoretical modeling. The effect of using many spheres on the artefact generated in fast imaging sequences is also investigated. RESULTS: A catheter and a guidewire are successfully steered by applying magnetic gradients inside a magnetic resonance scanner. More ferromagnetic material allows for larger magnetic forces, however, the use of two ferromagnetic spheres introduces undesired dipole-dipole interactions. Two ferromagnetic spheres generate a single larger artefact as they are close together. CONCLUSIONS: By varying the distance between the two ferromagnetic spheres, a balance can be struck between the need to minimize the size of the tip and the undesirable dipole-dipole interaction.

Mechanical Stress Initiates and Sustains the Morphogenesis of Wavy Leaf Epidermal Cells.

Pavement cells form wavy interlocking patterns in the leaf epidermis of many plants. We use computational mechanics to simulate the morphogenetic process based on microtubule organization and cell wall chemistry. Based on the in silico simulations and experimental evidence, we suggest that a multistep process underlies the morphogenesis of pavement cells. The in silico model predicts alternatingly located, feedback-augmented mechanical heterogeneity of the periclinal and anticlinal walls. It suggests that the emergence of waves is created by a stiffening of the emerging indented sides, an effect that matches cellulose and de-esterified pectin patterns in the cell wall. Further, conceptual evidence for mechanical buckling of the cell walls is provided, a mechanism that has the potential to initiate wavy patterns de novo and may precede chemical and geometrical symmetry breaking.

One-Step Solvent Evaporation-Assisted 3D Printing of Piezoelectric PVDF Nanocomposite Structures.

Development of a 3D printable material system possessing inherent piezoelectric properties to fabricate integrable sensors in a single-step printing process without poling is of importance to the creation of a wide variety of smart structures. Here, we study the effect of addition of barium titanate nanoparticles in nucleating piezoelectric ß-polymorph in 3D printable polyvinylidene fluoride (PVDF) and fabrication of the layer-by-layer and self-supporting piezoelectric structures on a micro- to millimeter scale by solvent evaporation-assisted 3D printing at room temperature. The nanocomposite formulation obtained after a comprehensive investigation of composition and processing techniques possesses a piezoelectric coefficient, d31, of 18 pC N-1, which is comparable to that of typical poled and stretched commercial PVDF film sensors. A 3D contact sensor that generates up to 4 V upon gentle finger taps demonstrates the efficacy of the fabrication technique. Our one-step 3D printing of piezoelectric nanocomposites can form ready-to-use, complex-shaped, flexible, and lightweight piezoelectric devices. When combined with other 3D printable materials, they could serve as stand-alone or embedded sensors in aerospace, biomedicine, and robotic applications.

Instability-Assisted Direct Writing of Microstructured Fibers Featuring Sacrificial Bonds.

A 30 µm-diameter thread of poly(lactic acid) (PLA) dissolved in dichloromethane is deposited on top of the eye of a sewing needle. The deposition robot traces a straight line; the helical shape of the thread is due to the liquid rope coiling instability. This instability is used to fabricate microstructured fibers with tailored mechanical properties.

In vivo demonstration of magnetic guidewire steerability in a MRI system with additional gradient coils.

PURPOSE: To assess the ability to control the steering of a modified guidewire actuated by the magnetic force of a magnetic resonance imaging system with additional gradient coils for selective arterial catheterization in rabbits. METHODS: Selective catheterizations of the right renal artery, left renal artery, superior mesenteric artery, and iliac artery were performed on two rabbits. A 3D magnetic force was applied onto a magnetic bead placed at the tip of a guidewire. The ability of the guidewire to advance in the aorta without entering the side branches when the magnetic force was not applied was also evaluated. Steering of the guidewire was combined with a dedicated tracking system and its position was registered on the 3D model of a magnetic resonance angiography (MRA). RESULTS: The magnetic catheterization of the renal arteries was successful and showed reproducibility. Superior mesenteric artery and iliac artery showed that the catheterization was feasible. These two arteries were difficult to visualize on MRA, making catheterization and setting the direction of the force more difficult. There was no inadvertent catheterization of side vessels when the guidewire was advanced with magnetic steering despite the hook shape at the tip of the guidewire caused by the alignment of the bead anisotropy with the permanent magnetic field. CONCLUSIONS: This first evaluation of selective catheterization of aortic branches with a magnetic guidewire provided a successful steering in the less angled side branches and this modified guidewire was advanced in the aorta without inadvertent selective catheterization when manipulated without magnetic actuation.

Catheter steering using a Magnetic Resonance Imaging system.

A catheter is successfully bent and steered by applying magnetic gradients inside a Magnetic Resonance Imaging system (MRI). One to three soft ferromagnetic spheres are attached at the distal tip of the catheter with different spacing between the spheres. Depending on the interactions between the spheres, progressive or discontinuous/jumping displacement was observed for increasing magnetic load. This phenomenon is accurately predicted by a simple theoretical dipole interaction model.