Anisotropic behaviour of human gallbladder walls

Inverse estimation of biomechanical parameters of soft tissues from non-invasive measurements has clinical significance in patient-specific modelling and disease diagnosis. In this paper, we propose a fully nonlinear approach to estimate the mechanical properties of the human gallbladder wall muscles from in vivo ultrasound images. The iteration method consists of a forward approach, in which the constitutive equation is based on a modified Hozapfel–Gasser–Ogden law initially developed for arteries. Five constitutive parameters describing the two orthogonal families of fibres and the matrix material are determined by comparing the computed displacements with medical images. The optimisation process is carried out using the MATLAB toolbox, a Python code, and the ABAQUS solver. The proposed method is validated with published artery data and subsequently applied to ten human gallbladder samples. Results show that the human gallbladder wall is anisotropic during the passive refilling phase, and that the peak stress is 1.6 times greater than that calculated using linear mechanics. This discrepancy arises because the wall thickness reduces by 1.6 times during the deformation, which is not predicted by conventional linear elasticity. If the change of wall thickness is accounted for, then the linear model can used to predict the gallbladder stress and its correlation with pain. This work provides further understanding of the nonlinear characteristics of human gallbladder

Mechanics of bacterial cellulose hydrogel

Natural polymer-based hydrogels, like bacterial cellulose (BC) hydrogels, gained a growing interest in the past decade mainly thanks to their good biological properties and similar fibrous structure as real human tissues that make them good potential candidate materials for various applications in a biomedical field. BC hydrogels are produced in a process of primary metabolism of some microorganisms. They were intensively studied with regard to their biological aspects, revealing many potential applications such as a direct implant replacement of some real tissues and an excellent scaffold for in-vitro tissue regeneration; still, its mechanical behaviour under application-relevant conditions has not been well documented. [Continues.

The Design and Construction of a Bulge Testing Device Platform for Human Skin Tissue Applications

Limited standard mechanical testing practises and stress-strain data are available for anisotropic human skin tissue in biaxial loading configurations to suitably represent skin in vivo. Inconsistencies in mechanical and physical properties in the literature due to numerous physiological factors have restricted development of biaxial testing equipment in laboratories to ad hoc research solutions having limited modifiability and parametric control. This project aims to develop a biaxial tensile testing device and testing platform which can be used in a research laboratory setting to provide a springboard to expediate mechanical skin tissue testing. The device can be easily reconfigured to accommodate a range of bulge pressures, while being driven via a 10bar compressed air supply. Based on simplified modelling of skin as an elastomer, mechanical and pneumatic resistivecapacitive pressure vessel models are developed. These are used respectively to initially specify a modifiable piston-cylinder bulge testing apparatus, and to design a customisable discrete proportional-integral closed-loop feedback pressurisation rate control system and software control environment. Pressure-time histories were successfully collected and stored on a dedicated computer for silicone sheet samples of 50mm diameter, as a surrogate for skin, that were tested using the platform to maximum pressures of about 200 kPa, at rates set between 2 20 kPa/s. The efficacy of the rate control system was affected by resolution of discrete pressurisation components that were used. The described platform is currently suitable for controlled and measured bulge pressurisation of elastomers. It is recommended to extend facility of the current platform by integrating 3D imaging and measurement technologies, to evaluate deformation of bulged anisotropic skin tissue and map inhomogeneous stress-strain fields for complex tensile stress-strain evaluations

Mechanics of Micro- and Nano-Size Materials and Structures

For this reprint, we intend to cover theoretical as well as experimental works performed on small scale to predict the material properties and characteristics of any advanced and metamaterials. New studies on mechanics of small-scale structures such as MEMS/NEMS, carbon and non-carbon nanotubes (e.g., CNTs, Carbon nitride, and Boron nitride nanotubes), micro/nano-sensors, nanocomposites, macrocomposites reinforced by micro-/nano-fillers (e.g., graphene platelets), etc., are included in this reprint

A systematic study of Brain Tissue microstructure: from composition to biomechanics and modelling of White Matter

This thesis aims to shed light on the biomechanical knowledge of the brain, in particular of its white matter (WM). An extensive, multidisciplinary and bottom-up research has been carried out to understand its micromechanical response focusing on three areas: Corpus Callosum, Corona Radiata and Fornix. Axons and the surrounding matrix with its accessory cells, the two main components of the tissue, have been analysed via Focused Ion Beam Scanning Electron Microscopy (FIB-SEM). Tissue volumes have been sampled, stained, embedded and imaged to finally be 3D-reconstructed, appearing as unidirectional composite materials. They have been geometrically characterised, creating a location-specific database of: cross-sectional area, diameter, ellipticity and tortuosity of the axons, together with the volume fraction of the volumes. The AFM-enabled nanoindentations investigated the mechanical response of WM. Stress-relaxation experiments have been performed on samples with axons running either parallel or perpendicular to the testing plane. The tissue showed linear viscoelasticity and slight anisotropy at the investigated level. The perpendicular samples showed a higher initial stiffness than parallel samples while the relative change in stiffness after relaxation was higher for the parallel samples than for the perpendicular ones. Finally, micromechanical modelling of the areas was performed. Location-speci c Representative Volume Elements have been created with the geometrical info obtained via FIB-SEM. Via an inverse-modelling approach, using the AFM data, material parameters of the axons and the matrix, the tissue components, have been obtained. The predicted stress-relaxation curves simulated by the finite element analysis showed good agreement with the experimental curves. The acquired knowledge of the microenvironment is fundamental for a comprehensive microscopical characterisation of the white matter. It provides important information to reduce axonal damage during neurosurgery, by predicting the local mechanical response and planning accordingly, and to improve the efficacy and therapeutic reach of Convection Enhanced Delivery, by exploiting the cytoarchitecture, leading to minimal side effects and maximal efficacy of the treatments.Open Acces

Bridging spatiotemporal scales in biomechanical models for living tissues : from the contracting Esophagus to cardiac growth

Appropriate functioning of our body is determined by the mechanical behavior of our organs. An improved understanding of the biomechanical functioning of the soft tissues making up these organs is therefore crucial for the choice for, and development of, efficient clinical treatment strategies focused on patient-specific pathophysiology. This doctoral dissertation describes the passive and active biomechanical behavior of gastrointestinal and cardiovascular tissue, both in the short and long term, through computer models that bridge the cell, tissue and organ scale. Using histological characterization, mechanical testing and medical imaging techniques, virtual esophagus and heart models are developed that simulate the patient-specific biomechanical organ behavior as accurately as possible. In addition to the diagnostic value of these models, the developed modeling technology also allows us to predict the acute and chronic effect of various treatment techniques, through e.g. drugs, surgery and/or medical equipment. Consequently, this dissertation offers insights that will have an unmistakable impact on the personalized medicine of the future.Het correct functioneren van ons lichaam wordt bepaald door het mechanisch gedrag van onze organen. Een verbeterd inzicht in het biomechanisch functioneren van deze zachte weefsels is daarom van cruciale waarde voor de keuze voor, en ontwikkeling van, efficiënte klinische behandelingsstrategieën gefocust op de patiënt-specifieke pathofysiologie. Deze doctoraatsthesis brengt het passieve en actieve biomechanisch gedrag van gastro-intestinaal en cardiovasculair weefsel, zowel op korte als lange termijn, in kaart via computermodellen die een brug vormen tussen cel-, weefsel- en orgaanniveau. Aan de hand van histologische karakterisering, mechanische testen en medische beeldvormingstechnieken worden virtuele slokdarm- en hartmodellen ontwikkeld die het patiënt-specifieke orgaangedrag zo accuraat mogelijk simuleren. Naast de diagnostische waarde van deze modellen, laat de ontwikkelde modelleringstechnologie ook toe om het effect van verschillende behandelingstechnieken, via medicatie, chirurgie en/of medische apparatuur bijvoorbeeld, acuut en chronisch te voorspellen. Bijgevolg biedt deze doctoraatsthesis inzichten die een onmiskenbare impact zullen hebben op de gepersonaliseerde geneeskunde van de toekomst

Improved human soft tissue thigh surrogates for superior assessment of sports personal protective equipment

Human surrogates are representations of living humans, commonly adopted to better understand human response to impacts. Though surrogates have been widely used in automotive, defence and medical industries with varying levels of biofidelity, their primary application in the sporting goods industry has been through primitive rigid anvils used in assessing personal protective equipment (PPE) effectiveness. In sports, absence from competition is an important severity measure and soft tissue injuries such as contusions and lacerations are serious concerns. Consequently, impact surrogates for the sporting goods industry need a more subtle description of the relevant soft tissues to assess impact severity and mitigation accurately to indicate the likelihood of injury. The fundamental aim for this research study was to establish a method to enable the development of superior, complementary, increasingly complex synthetic and computational impact surrogates for improved assessment of sports personal protective equipment. With a particular focus on the thigh segment, research was conducted to evaluate incremental increases in surrogate complexity. Throughout this study, empirical assessment of synthetic surrogates and computational evaluation using finite element (FE) models were employed to further knowledge on design features influencing soft tissue surrogates in a cost and time efficient manner. To develop a more representative human impact surrogate, the tissue structures considered, geometries and materials were identified as key components influencing the mechanical response of surrogates. As a design tool, FE models were used to evaluate the changes in impact response elicited with different soft tissue layer configurations. The study showed the importance of skin, adipose, muscle and bone tissue structures and indicated up to 15.4% difference in maximum soft tissue displacement caused by failure to represent the skin layer. FE models were further used in this capacity in a shape evaluation study from which it was determined that a full-scale anatomically contoured thigh was necessary to show the full diversity of impact response phenomena exhibited. This was particularly pertinent in PPE evaluations where simple surrogate shapes significantly underestimated the magnitudes of displacements exhibited (up to 155% difference) when rigid shell PPE was simulated under impact conditions. Synthetic PDMS silicone simulants were then fabricated for each of the organic soft tissues to match their dynamic responses. The developed simulants exhibited a superior representation of the tissues when compared to previous single material soft tissue simulant, Silastic 3483, which showed 324%, 11,140% and -15.8% greater differences than the PDMS when compared to previously reported target organic tissue datasets for relaxed muscle, skin and adipose tissues respectively. The impact response of these PDMS surrogates were compared in FE models with previously used single material simulants in representative knee and cricket ball sports impact events. The models were each validated through experimental tests and the PDMS simulants were shown to exhibit significantly closer responses to organic tissue predictions across all impact conditions and evaluation metrics considered. An anatomically contoured synthetic thigh surrogate was fabricated using the PDMS soft tissue simulants through a novel multi-stage moulding process. The surrogate was experimentally tested under representative sports impact conditions and showed a good comparison with FE model predictions with a maximum difference in impactor displacements and peak accelerations of +6.86% and +12.5% respectively at velocities between 2 - 4 m.s-1. The value of increased biofidelity in the anatomical synthetic and virtual surrogate thighs has been proven through the incremental adoption of important surrogate elements (tissue structures, material and geometries). The predictive capabilities of each surrogate have been demonstrated through their parallel developments and staged comparisons with idealised organic tissue responses. This increase in biofidelity is introduced at modestly higher cost compared to Silastic 3483, but, given the benefits of a more representative human impact response for PPE evaluations, this is shown to be worthwhile

Advanced Design Concepts and Efficient Finite Element Modeling for Dielectric Elastomer Devices

Dielectric elastomers (DEs) offer their use in numerous applications, due to their advantages compared to conventional actuators and sensors. They excel in properties such as lightweight, energy efficiency, low-noise and inherent compliance, just to name a few. In particular, actuator and sensor systems based on membrane DEs show their potential in many fields, from the automotive industry to consumer electronics. Defined procedures which permit an efficient design process are required in order to allow the development of novel DE devices. Additionally, numerical methods for the optimization of such processes are of interest. The first part of this dissertation provides advanced design methods for actuator and sensor applications. For DE actuators, systems biased with permanent magnets are investigated and design rules are derived in order to maximize the stroke for a given load case. For DE sensors, the field of high pressure measurements is developed, introducing concepts for intrusive and nonintrusive sensor systems. In the second part of this dissertation, numerical methods for membrane DE actuators based on the Finite Element method are derived. The main focus is fast computation time and numerical efficiency. Two approaches are presented, one based on a two-dimensional continuum formulation and one based on a three-dimensional membrane formulation. The resulting models allow the investigation of local field distributions, such as stresses, thickness and electric field.Dielektrische Elastomere (DE) bieten sich durch ihre Vorteile gegenüber herkömmlichen Aktoren und Sensoren für viele Anwendungen an. Sie zeichnen sich aus durch geringes Gewicht, hohe Energieeffizienz, geräuschlosen Betrieb und inhärente Dehnbarkeit. Um die Entwicklung neuer DE Anwendungen voranzutreiben, werden effiziente Auslegungsprozesse benötigt. Zusätzlich sind numerische Methoden zur Optimierung solcher Prozesse von Interesse. Der erste Teil dieser Dissertation entwickelt fortgeschrittene Entwicklungsmethoden für Aktorund Sensorsysteme. Für DE Aktoren werden Systeme mit Permanentmagneten als Vorspannmechanismus untersucht und eine Prozedur zur Maximierung des Aktorhubs für eine vorgegebene Last hergeleitet. Für DE Sensoren wird das Feld der Hochdruckmessung erschlossen, indem Konzepte für intrusive und nicht-intrusive Druckmessungen entwickelt werden. Der zweite Teil dieser Dissertation leitet numerische Modelle für die Simulation von DE Aktoren basierend auf der Finite Elemente Methode her. Der Hauptfokus liegt hierbei auf schnellen Rechenzeiten und numerischer Effizienz. Der erste diskutierte Ansatz basiert auf einer zweidimensionalen Kontinuumsformulierung, während der zweite Ansatz auf einer dreidimensionalen Membranformulierung basiert. Die resultierenden Modelle erlauben die Untersuchung lokaler Feldverteilungen, beispielsweise der mechanischen Spannung, der Dickenänderung und dem elektrischen Feld