Smoothness-based forces for deformable models: A long-range force and a corner fitting force

Deformable models, originally proposed by Terzopoulos et al. (Artif. Intell. 36 (1988) 91) and Kass et al. (Int. J. Comput. Vision 1 (1988) 321) in 1988, have been widely used in medical image segmentation. However, they manifest two well-known limitations: the lack of an appropriate long-range force to drive the model surface towards the object boundary and poor performance at high curvature boundaries (such as corners) due to the models' intrinsic smoothness constraint. In this paper, a new smoothness force with local control is proposed. The local control is used to devise a long-range force, referred to as the self-zoom force, and a corner fitting force. The self-zoom force enables the model surface to expand and shrink without a limit in range. The corner fitting force propels the model surface to fit high-curvature boundaries. Experiments demonstrate that the model surface is driven to the object boundary by the new forces even if the initial estimate is not close and the object is nonconvex or has a high local curvature. © 2002 Elsevier Science Ltd. All rights reserved

The waterbomb actuator: a new origami-based pneumatic soft muscle

This project introduces a new Pneumatic Artificial Muscle (PAM) design based on an origami structure. This artificial muscle is designed to operate at a very low range of pressures while being lightweight and compliant. It is also designed to reduce the pressure threshold and hysteresis problems present on other PAMs like the McKibben actuator. These properties are achieved thanks to a rearranging membrane based on the Waterbomb pattern, which can contract upon inflation while keeping the surface area constant. This concept has been tested using paper prototypes coated with silicone. We created thee different structures (4x8, 6x12 and 8x16 cells waterbomb actuators) from the same paper sheet (14x28cm2) and we actuated them under loads of 2, 4 and 7N. The 4x8 was discarded, but the 6x12 and 8x16 actuators contracted a maximum of 12.5% of the original length (≃10cm) while the operating pressures remained under 5Pa. We also proposed a novel approach to 3D print these actuators using a Stratasys Objet260 Connex3 3D printer. The main idea consists in creating a flat structure that can self-assemble using a technique known as 4D Printing. The pattern is printed as a flat sheet where the hinges are composites composed of an elastomeric material and shape memory polymer (SMP) fibers. These hinges can be activated through a thermomechanical process inducing a self-folding effect. Unfortunately, we were not able to verify this fabrication process due to the lack of material availability

A numerical study of fin and jet propulsions involving fluid-structure interactions

Fish swimming is elegant and efficient, which inspires humans to learn from them to design high-performance artificial underwater vehicles. Research on aquatic locomotion has made extensive progress towards a better understanding of how aquatic animals control their flexible body and fin for propulsion. Although the structural flexibility and deformation of the body and fin are believed to be important features to achieve optimal swimming performance, studies on high-fidelity deformable body and fin with complex material behavior, such as non-uniform stiffness distributions, are rare. In this thesis, a fully coupled three-dimensional high-fidelity fluid-structure interaction (FSI) solver is developed to investigate the flow field evolution and propulsion performance of caudal fin and jet propulsion involving body and/or fin deformation. Within this FSI solver, the fluid is resolved by solving unsteady and viscous Navier-Stokes equations based on the finite volume method with a multi-block grid system. The solid dynamics are solved by a nonlinear finite element method. The coupling between the two solvers is achieved in a partitioned approach in which convergence check and sub-iteration are implemented to ensure numerical stability and accuracy. Validations are conducted by comparing the simulation results of classical benchmarks with previous data in the literature, and good agreements between them are obtained. The developed FSI solver is then applied to study the bio-inspired fin and jet propulsion involving body deformation. Specifically, the effect of non-uniform stiffness distributions of fish body and/or fin, key features of fish swimming which have been excluded in most previous studies, on the propulsive performance is first investigated. Simulation results of a sunfish-like caudal fin model and a tuna-inspired swimmer model both show that larger thrust and propulsion efficiency can be achieved by a non-uniform stiffness distribution (e.g., increased by 11.2% and 9.9%, respectively, for the sunfish-like model) compared with a uniform stiffness profile. Despite the improved propulsive e performance, a bionic variable fish body stiffness does not yield fish-like midline kinematics observed in real fish, suggesting that fish movement involves significant active control that cannot be replicated purely by passive deformations. Subsequent studies focus on the jet propulsion inspired by squid locomotion using the developed numerical solver. Simulation results of a two-dimensional inflation-deflation jet propulsion system, whose inflation is actuated by an added external force that mimics the muscle constriction of the mantle and deflation is caused by the release of elastic energy of the structure, suggest larger mean thrust production and higher efficiency in high Reynolds number scenarios compared with the cases in laminar flow. A unique symmetry-breaking instability in turbulent flow is found to stem from irregular internal body vortices, which cause symmetry breaking in the wake. Besides, a three-dimensional squid-like jet propulsion system in the presence of background flow is studied by prescribing the body deformation and jet velocity profiles. The effect of the background flow on the leading vortex ring formation and jet propulsion is investigated, and the thrust sources of the overall pulsed jet are revealed as well. Finally, FSI analysis on motion control of a self-propelled flexible swimmer in front of a cylinder utilizing proportional-derivative (PD) control is conducted. The amplitude of the actuation force, which is applied to the swimmer to bend it to produce thrust, is dynamically tuned by a feedback PD controller to instruct the swimmer to swim the desired distance from an initial position to a target location and then hold the station there. Despite the same swimming distance, a swimmer whose departure location is closer to the cylinder requires less energy consumption to reach the target and hold the position there.Fish swimming is elegant and efficient, which inspires humans to learn from them to design high-performance artificial underwater vehicles. Research on aquatic locomotion has made extensive progress towards a better understanding of how aquatic animals control their flexible body and fin for propulsion. Although the structural flexibility and deformation of the body and fin are believed to be important features to achieve optimal swimming performance, studies on high-fidelity deformable body and fin with complex material behavior, such as non-uniform stiffness distributions, are rare. In this thesis, a fully coupled three-dimensional high-fidelity fluid-structure interaction (FSI) solver is developed to investigate the flow field evolution and propulsion performance of caudal fin and jet propulsion involving body and/or fin deformation. Within this FSI solver, the fluid is resolved by solving unsteady and viscous Navier-Stokes equations based on the finite volume method with a multi-block grid system. The solid dynamics are solved by a nonlinear finite element method. The coupling between the two solvers is achieved in a partitioned approach in which convergence check and sub-iteration are implemented to ensure numerical stability and accuracy. Validations are conducted by comparing the simulation results of classical benchmarks with previous data in the literature, and good agreements between them are obtained. The developed FSI solver is then applied to study the bio-inspired fin and jet propulsion involving body deformation. Specifically, the effect of non-uniform stiffness distributions of fish body and/or fin, key features of fish swimming which have been excluded in most previous studies, on the propulsive performance is first investigated. Simulation results of a sunfish-like caudal fin model and a tuna-inspired swimmer model both show that larger thrust and propulsion efficiency can be achieved by a non-uniform stiffness distribution (e.g., increased by 11.2% and 9.9%, respectively, for the sunfish-like model) compared with a uniform stiffness profile. Despite the improved propulsive e performance, a bionic variable fish body stiffness does not yield fish-like midline kinematics observed in real fish, suggesting that fish movement involves significant active control that cannot be replicated purely by passive deformations. Subsequent studies focus on the jet propulsion inspired by squid locomotion using the developed numerical solver. Simulation results of a two-dimensional inflation-deflation jet propulsion system, whose inflation is actuated by an added external force that mimics the muscle constriction of the mantle and deflation is caused by the release of elastic energy of the structure, suggest larger mean thrust production and higher efficiency in high Reynolds number scenarios compared with the cases in laminar flow. A unique symmetry-breaking instability in turbulent flow is found to stem from irregular internal body vortices, which cause symmetry breaking in the wake. Besides, a three-dimensional squid-like jet propulsion system in the presence of background flow is studied by prescribing the body deformation and jet velocity profiles. The effect of the background flow on the leading vortex ring formation and jet propulsion is investigated, and the thrust sources of the overall pulsed jet are revealed as well. Finally, FSI analysis on motion control of a self-propelled flexible swimmer in front of a cylinder utilizing proportional-derivative (PD) control is conducted. The amplitude of the actuation force, which is applied to the swimmer to bend it to produce thrust, is dynamically tuned by a feedback PD controller to instruct the swimmer to swim the desired distance from an initial position to a target location and then hold the station there. Despite the same swimming distance, a swimmer whose departure location is closer to the cylinder requires less energy consumption to reach the target and hold the position there

Soft Scalable Self-Reconfigurable Modular Cellbot

Hazardous environments such as disaster affected areas, outer space, and radiation affected areas are dangerous for humans. Autonomous systems which can navigate through these environments would reduce risk of life. The terrains in these applications are diverse and unknown, hence there is a requirement for a robot which can self-adapt its morphology and use suitable control to optimally move in the desired manner. Although there exist monolithic robots for some of these applications, such as the Curiosity rover for Mars exploration, a modular robot containing multiple simple units could increase the fault tolerance. A modular design also enables scaling up or down of the robot based on the current task, for example, scaling up by connecting multiple units to cover a wider area or scaling down to pass through a tight space.Taking bio-inspiration from cells, where – based on environmental conditions – cells come together to form different structures to carry out different tasks, a soft modular robot called Cellbot was developed which was composed of multiple units called ‘cells’. Tests were conducted to understand the cellbot movement over different frictional surfaces for different actuation functions, the number of cells connected in a line (1D), and the shapes formed by connecting cells in 2D. A simulation model was developed to test a large range of frictional values and actuation functions for different friction coefficients. Based on the obtained results, cells could be designed using a material with frictional properties lying in the optimal locomotion range. In other cases, where the application has diverse terrains, the number of connected units can be changed to optimise the robot locomotion. Initial tests were conducted using a ‘ball robot’, where the cellbot was designed using balls which touch ground to exploit friction and actuators to provide force to move the robot. The model was extended to develop, a ‘bellow robot’ which was fabricated using hyper-elastic bellows and employed pneumatic actuation. The amount of inflation of a cell and its neighbouring cells determined if the cell would touch the ground or be lifted up. This was used to change cell behaviour where a cell could be touching ground to provide anchoring friction, or lifted to push or pull the cells and thereby move the robot. The cells were connected by magnets which could be disconnected and reconnected by morphing the robot body. The cellbot can thus reconfigure by changing the number of connected units or its shape. The easy detachment can be used to remove and replace damaged cells. Complex cellbot movements can be achieved by either switching between different robot morphologies or by changing actuation control.Future cellbots will be controlled remotely to change their morphology, control, and number of connected cells, making them suitable for missions which require fault tolerance and autonomous shape adaptation. The proposed cellbot platform has the potential to reduce the energy, time and costs in comparison to traditional robots and has potential for applications such as exploration missions for outer space, search and rescue missions for disaster affected areas, internal medical procedures, and nuclear decommissioning.<br/

Spontaneous formation of a self-healing carbon nanoskin at the liquid-liquid interface

Biological membranes exhibit the ability to self-repair and dynamically change their shape while remaining impermeable. Yet, these defining features are difficult to reconcile with mechanical robustness. Here, we report on the spontaneous formation of a carbon nanoskin at the oil–water interface that uniquely combines self-healing attributes with high stiffness. Upon the diffusion-controlled self-assembly of a reactive molecular surfactant at the interface, a solid elastic membrane forms within seconds and evolves into a continuous carbon monolayer with a thickness of a few nanometers. This nanoskin has a stiffness typical for a 2D carbon material with an elastic modulus in bending of more than 40–100¿GPa; while brittle, it shows the ability to self-heal upon rupture, can be reversibly reshaped, and sustains complex shapes. We anticipate such an unusual 2D carbon nanomaterial to inspire novel approaches towards the formation of synthetic cells with rigid shells, additive manufacturing of composites, and compartmentalization in industrial catalysis.Peer ReviewedPostprint (published version

MotorSkins—a bio-inspired design approach towards an interactive soft-robotic exosuit

The work presents a bio-inspired design approach to a soft-robotic solution for assisting the knee-bending in users with reduced mobility in lower limbs. Exosuits and fluid-driven actuators are fabric-based devices that are gaining increasing relevance as alternatives assistive technologies that can provide simpler, more flexible solutions in comparison with the rigid exoskeletons. These devices, however, commonly require an external energy supply or a pressurized-fluid reservoir, which considerably constrain the autonomy of such solutions. In this work, we introduce an event-based energy cycle (EBEC) design concept, that can harvest, store, and release the required energy for assisting the knee-bending, in a synchronised interaction with the user and the environment, thus eliminating any need for external energy or control input. Ice-plant hydro-actuation system served as the source of inspiration to address the specific requirements of such interactive exosuit through a fluid-driven material system. Based on the EBEC design concepts and the abstracted bio-inspired principles, a series of (material and process driven) design experimentations helped to address the challenges of realising various functionalities of the harvest, storage, actuation and control instances within a closed hydraulic circuit. Sealing and defining various areas of water-tight seam made out of thermoplastic elastomers provided the base material system to program various chambers, channels, flow-check valves etc of such EBEC system. The resulting fluid-driven EBEC-skin served as a proof of concept for such active exosuit, that brings these functionalities into an integrated ‘sense-acting’ material system, realising an auto-synchronised energy and information cycles. The proposed design concept can serve as a model for development of similar fluid-driven EBEC soft-machines for further applications. On the more general scheme, the work presents an interdisciplinary design-science approach to bio-inspiration and showcases how biological material solutions can be looked at from a design/designer perspective to bridge the bottom–up and top–down approach to bio-inspiration.Deutsche Forschungsgemeinschafthttps://doi.org/10.13039/501100001659Peer Reviewe

Finite Element Simulation of Large-Scale Confined Inflatable Structures

The protection of transportation tunnels is one of the top priorities of transportation and government entities. Transportation tunnels have been identified as particularly vulnerable to different threats such as propagation of toxic gasses, or smoke originated by human activities or flooding originated by extreme climatic events such as hurricanes and severe weather. Finding solutions to minimize the consequences of disastrous events has become critical to increase the resiliency of tunnel systems. The implementation of large-scale inflatable structures at specific locations of the tunnel system for containing the propagation of flooding or gases is now possible. When a threat happens, a sensing system detects the threat and triggers the activation of an inflation system which can deploy, inflate and pressurize the inflatable structure in a few minutes. When the inflatable structure is completely inflated, it acts as a barrier that can isolate the compromised region and contain the threat. The feasibility of this concept was demonstrated in 2008, and several experimental evaluations were conducted in the recent years to demonstrate the operational viability of this solution. Despite the successful results seen in the experimental evaluations, the development of simulations that can predict results in advance to reduce the number of experimental iterations is still essential. Finite Element simulation efforts performed in the recent years contributed to the understanding of the dynamics of the deployment and inflation of an inflatable structure for one particular tunnel profile and one folding and deployment configuration. However, if the membrane material of the inflatable changes, or the shape or configuration of the tunnel profile changes, or the position for storage of the folded inflatable changes, the initial behavior of the unstressed membrane during the initial deployment and later inflation, will be different. All this variability increases the need of experimental iterations to determine the appropriate combination of parameters to achieve acceptable results. Considering that the resources for experimental iterations can be very limited, there is a clear need to continue with the development of predictive models that can account for the different factors involved in the implementation of inflatable structures for tunnel protection.;This work presents the development of Finite Element simulations generated for the evaluation of different phases of the operation of a large-scale inflatable structure used for sealing a tunnel segment. The simulations developed in this work focused on reproducing deflation, folding, and placement procedures for deploying an inflatable from the ceiling of a tunnel segment. The models were also used to evaluate the behavior of the inflatable during the initial deployment and the full inflation. Different strategies were analyzed with the ultimate goal of maximizing the global and local conformity, which translate in a better sealing capacity of the inflatable to the tunnel profile. The results of the simulations showed that a very flat shape can be achieved by implementing a controlled deflation of the nominal shape of the inflatable as a starting point of the folding procedures. Moreover, a combination of translational and rotational planes allowed the flattened shape to reach a more compact shape at the end of the folding procedures. Simulation results also showed that the stiffness of the membrane influenced the shape and behavior of the inflatable during the initial deployment. Moreover, results demonstrated that the implementation of passive restrainers to control the movement and release of the membrane during the deflation, folding, deployment and inflation contributed to reach higher levels of local conformity of the inflatable to the tunnel perimeter, as well as an increase of the contact area as the global and local conformity improved. A comparison of simulation results with available experimental data demonstrated a good level of agreement between the finite element simulations and the experimental observations

Passive Hydro-actuated Unfolding of Ice Plant Seed Capsules as a Concept Generator for Autonomously Deforming Devices

In der Natur und ihren biologischen Systemen existieren zahlreiche Beispiele für gerichtete Bewegung durch spezifische Reaktion auf externe Stimuli. Diese potentiellen Quellen der Inspiration dienen oft als Vorbilder für energieeffiziente "Smart" Technologien. Vom Wasser getriebenen schnellen Zuschnappen der Venusfliegenfalle bis zum einfacheren ebenso hydroresponsiven Biegen der Weizengrannen, viele Pflanzen haben im Laufe der Evolution verschiedene Mechanismen entwickelt, um Wasser als Triebkraft ihrer Aktoren-Gewebe zu nutzen, die für spezifische und gerichtete Bewegung sowie die gewünschte Verformung sorgen. Das ist diesen Pflanzen möglich durch die Organisation ihrer Gewebe in ausgereiften, komplexen und hierarchisch organisierten Architekturen auf verschiedensten Skalen. Einige Arten der Familie Aizoaceae, auch bekannt als Mittagsblumen oder Ice plant, zeigen ein geniales Beispiel für solche passiven Betätigungssysteme, da sie einen "intelligenten" Mechanismus entwickelt haben, um ihre Schutzsamenkapseln öffnen zu lassen und die Samen nur in Anwesenheit von flüssigem Wasser (Regen) freizugeben. Schwerpunkt der ersten Phase dieser Arbeit war die Untersuchung der zu Grunde liegenden Mechanismen und der strukturellen und kompositorischen Basis von Wasser-getriebenen Bewegungen der Samenkapseln von Ice plant (Delosperma nakurense) auf ihren verschiedenen hierarchischen Ebenen. Fünf hygroskopische Kiele erwiesen sich als aktive "Muskeln", die zu einer reversiblen origamiartigen Entfaltung der Samenkapsel führen, wenn diese mit Wasser benetzt wird. Jeder Kiel besteht aus zwei wabenartigen Geweben, die aus hochgradig schwellfähigen und elliptisch-sechseckig geformten Zellen zusammengesetzt sind, die entlang einem inerten Träger organisiert sind. Als Hauptmotor der Aktuation wurde die signifikante Schwellung von hochgradig schwellfähigen zellulosereichen Innenschichten (CIL) im Lumen der Zellen identifiziert. Die Morphologie der CIL und deren physikochemische Reaktion auf Wasser wurde unter Verwendung einer Vielzahl von Techniken untersucht und damit gezeigt, dass der Entropiegewinn während der Wasserabsorption die Hauptantriebskraft für die Schwellung der Zellen ist. Die Umsetzung dieser relativ kleinen Energiebeiträge in eine konzertierte und komplexe makroskopische Bewegung, wurde durch ein optimiertes Design auf den verschiedenen Ebenen der hierarchischen Organisation des Systems erläutert. Das kooperative anisotropische Anschwellen der Zellen des hygroskopischen Gewebes führt durch das Timoschenko Doppelschicht-Biegeprinzip zu einer Umsetzung in eine Biegebewegung der Strukturen und letztlich zur Entfaltung der Samenkapseln. Inspiriert von den zugrunde liegenden Mechanismen in Ice plants, wurden zwei unterschiedliche Strategien entwickelt, um durch kleine Dehnungen im mikroskopischen Bereich eine vorprogrammierte Makro-Bewegung einer Wabenstruktur zu ermöglichen. Durch eine geschickte Anwendung dieses einfachen Prinzips, kann eine Mimik des biologischen Vorbilds im weiteren technischen Sinne zu zahlreichen Anwendungsbeispielen führen, wie als passive Schalter und Aktoren in der Biomedizin, Landschaftsgestaltung oder der Architektur.Numerous examples of actuated-movements with specific responses of the structure to external stimuli can be found in biological systems, which can be a potential source of inspiration for the design of energy-efficient "smart" devices. From the hydro-driven rapid snapping of the Venus fly trap leaves to simple hydro-responsive bending of wheat awns, various plants have evolved different mechanisms to utilize water as an actuator to undergo a desired deformation via sophisticated architecture at different hierarchical levels of their systems. Some species of the family Aizoaceae, also known as ice plants, show an ingenious example of such passive actuation systems, as they evolved a smart mechanism to open their protective seed capsules and release their seeds only in the presence of liquid water (rain). The scope of the first phase of the thesis was to investigate the underlying mechanism and the structural and compositional basis of the hydro-actuated movement of the ice plant seed capsules (Delosperma nakurense) at several hierarchical levels. Five hygroscopic keels were found to be the active muscles responsible for the reversible origami-like unfolding of the seed capsule upon wetting. Each keel consists of two honeycomb-like tissues made up of highly swellable hexagonal/elliptical shape cells running along an inert backing tissue. The significant swelling of a highly swellable cellulosic inner layer (CIL) inside the lumen of these cells was found to be the main engine of the actuation. The morphology and physicochemical response of the CIL to water was studied using a variety of techniques and it was shown that the entropic changes during water absorption were the main driving force for swelling of the cells. The translation of such relatively small available energy to the complex movement at a macro scale was explained by an optimized design at different hierarchical levels of the system. The cooperative anisotropic swelling of the cells in the hygroscopic tissue is translated into a flexing movement of the structure via simple Timoshenko’s bilayer bending principle, which then results in an unfolding of the seed capsules. Inspired by the underlying mechanism in ice plant, two different strategies were developed to translate small strains at micro scale into a pre-programmed macro movement of a honeycomb structure. Through a clever application of the same simple concepts, one can "mimic" the biological model system in a broader engineering sense, with potential applications of such passive switches in biomedicine, agricultural engineering or architectural design

Finite Element Analysis to Study Percutaneous Heart Valves

Communications engineering / telecommunication

Modelling of Atherosclerotic Plaque for Use in a Computational Test-Bed for Stent Angioplasty

A thorough understanding of the diseased tissue state is necessary for the successful treatment of a blocked arterial vessel using stent angioplasty. The constitutive representation of atherosclerotic tissue is of great interest to researchers and engineers using computational models to analyse stents, as it is this in silico environment that allows extensive exploration of tissue response to device implantation. This paper presents an in silico evaluation of the effects of variation of atherosclerotic tissue constitutive representation on tissue mechanical response during stent implantation. The motivation behind this work is to investigate the level of detail that is required when modelling atherosclerotic tissue in a stenting simulation, and to give recommendations to the FDA for their guideline document on coronary stent evaluation, and specifically the current requirements for computational stress analyses. This paper explores the effects of variation of the material model for the atherosclerotic tissue matrix, the effects of inclusion of calcifications and a lipid pool, and finally the effects of inclusion of the Mullins effect in the atherosclerotic tissue matrix, on tissue response in stenting simulations. Results indicate that the inclusion of the Mullins effect in a direct stenting simulation does not have a significant effect on the deformed shape of the tissue or the stress state of the tissue. The inclusion of a lipid pool induces a local redistribution of lesion deformation for a soft surrounding matrix and the inclusion of a small volume of calcifications dramatically alters the local results for a soft surrounding matrix. One of the key findings from this work is that the underlying constitutive model (elasticity model) used for the atherosclerotic tissue is the dominant feature of the tissue representation in predicting tissue response in a stenting simulation