Remodeling by fibroblasts alters the rate-dependent mechanical properties of collagen

The ways that fibroblasts remodel their environment are central to wound healing, development of musculoskeletal tissues, and progression of pathologies such as fibrosis. However, the changes that fibroblasts make to the material around them and the mechanical consequences of these changes have proven difficult to quantify, especially in realistic, viscoelastic three-dimensional culture environments, leaving a critical need for quantitative data. Here, we observed the mechanisms and quantified the mechanical effects of fibroblast remodeling in engineered tissue constructs (ETCs) comprised of reconstituted rat tail (type I) collagen and human fibroblast cells. To study the effects of remodeling on tissue mechanics, stress-relaxation tests were performed on ETCs cultured for 24, 48, and 72 h. ETCs were treated with deoxycholate and tested again to assess the ECM response. Viscoelastic relaxation spectra were obtained using the generalized Maxwell model. Cells exhibited viscoelastic damping at two finite time constants over which the ECM showed little damping, approximately 0.2 s and 10-30 s. Different finite time constants in the range of 1-7000 s were attributed to ECM relaxation. Cells remodeled the ECM to produce a relaxation time constant on the order of 7000 s, and to merge relaxation finite time constants in the 0.5-2 s range into a single time content in the 1 s range. Results shed light on hierarchical deformation mechanisms in tissues, and on pathologies related to collagen relaxation such as diastolic dysfunction. Statement of Significance As fibroblasts proliferate within and remodel a tissue, they change the tissue mechanically. Quantifying these changes is critical for understanding wound healing and the development of pathologies such as cardiac fibrosis. Here, we characterize for the first time the spectrum of viscoelastic (rate-dependent) changes arising from the remodeling of reconstituted collagen by fibroblasts. The method also provides estimates of the viscoelastic spectra of fibroblasts within a three-dimensional culture environment. Results are of particular interest because of the ways that fibroblasts alter the mechanical response of collagen at loading frequencies associated with cardiac contraction in humans. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Passive and Active Microrheology for Biomedical Systems

Microrheology encompasses a range of methods to measure the mechanical properties of soft materials. By characterizing the motion of embedded microscopic particles, microrheology extends the probing length scale and frequency range of conventional bulk rheology. Microrheology can be characterized into either passive or active methods based on the driving force exerted on probe particles. Tracer particles are driven by thermal energy in passive methods, applying minimal deformation to the assessed medium. In active techniques, particles are manipulated by an external force, most commonly produced through optical and magnetic fields. Small-scale rheology holds significant advantages over conventional bulk rheology, such as eliminating the need for large sample sizes, the ability to probe fragile materials non-destructively, and a wider probing frequency range. More importantly, some microrheological techniques can obtain spatiotemporal information of local microenvironments and accurately describe the heterogeneity of structurally complex fluids. Recently, there has been significant growth in using these minimally invasive techniques to investigate a wide range of biomedical systems both in vitro and in vivo . Here, we review the latest applications and advancements of microrheology in mammalian cells, tissues, and biofluids and discuss the current challenges and potential future advances on the horizon

Multiscale mechano-morphology of soft tissues : a computational study with applications to cancer diagnosis and treatment

Cooperation of engineering and biomedical sciences has produced significant advances in healthcare technology. In particular, computational modelling has led to a faster development and improvement of diagnostic and treatment techniques since it allows exploring multiple scenarios without additional complexity and cost associated to the traditional trial-and-error methodologies. The goal of this thesis is to propose computational methodologies to analyse how the changes in the microstructure of soft tissues, caused by different pathological conditions, influence the mechanical properties at higher length scales and, more importantly, to detect such changes for the purpose of quantitative diagnosis and treatment of such pathologies in the scenario of drug delivery. To achieve this objective different techniques based on quasi-static and dynamic probing have been established to perform quantitative tissue diagnosis at the microscopic (tissue) and macroscopic (organ) scales. The effects of pathologies not only affect the mechanical properties of tissue (e.g. elasticity and viscoelasticity), but also the transport properties (e.g. diffusivity) in the case of drug delivery. Such transport properties are further considered for a novel multi-scale, patient-specific framework to predict the efficacy of chemotherapy in soft tissues. It is hoped that this work will pave the road towards non-invasive palpation techniques for early diagnosis and optimised drug delivery strategies to improve the life quality of patientsJames-Watt Scholarship, Heriot-Watt Annual Fund and the Institute of Mechanical, Process and Energy Engineering (IMPEE) Grant

Active colloids in complex fluids

We review recent work on active colloids or swimmers, such as self-propelled microorganisms, phoretic colloidal particles, and artificial micro-robotic systems, moving in fluid-like environments. These environments can be water-like and Newtonian but can frequently contain macromolecules, flexible polymers, soft cells, or hard particles, which impart complex, nonlinear rheological features to the fluid. While significant progress has been made on understanding how active colloids move and interact in Newtonian fluids, little is known on how active colloids behave in complex and non-Newtonian fluids. An emerging literature is starting to show how fluid rheology can dramatically change the gaits and speeds of individual swimmers. Simultaneously, a moving swimmer induces time dependent, three dimensional fluid flows, that can modify the medium (fluid) rheological properties. This two-way, non-linear coupling at microscopic scales has profound implications at meso- and macro-scales: steady state suspension properties, emergent collective behavior, and transport of passive tracer particles. Recent exciting theoretical results and current debate on quantifying these complex active fluids highlight the need for conceptually simple experiments to guide our understanding.Comment: 6 figure

Characterising the elastic and viscoelastic interaction between the cell and its matrix in 3D: because it takes two to salsa dance

The extracellular matrix (ECM) is a three-dimensional, acellular component of all organs and tissues. The ECM has elastic and viscoelastic properties, quantified through the elastic modulus (i.e. stiffness) and stress relaxation, respectively, that guide cell fate. Stiffness and stress relaxation drive cellular plasticity in homeostasis and disease. Therefore, to represent the mechanics of the ECM in vitro it is necessary to employ models that recapitulate these properties. Among said models are hydrogels: polymeric networks whose mass mainly consists of water. Importantly, hydrogel viscoelasticity remains an understudied property, notably in cell-loaded materials. Hence, this thesis investigated the elastic and viscoelastic properties of diverse cell-free and cell-loaded hydrogels. The hydrogels evaluated in this thesis include organ-derived ECM, gelatine methacryloyl, agarose, human-derived platelet-poor plasma, alginate and pluronic. These hydrogels have tissue engineering and regenerative medicine (TERM) potential. Particular emphasis was placed on investigating and mathematically modelling the elastic and viscoelastic fate of cell-loaded hydrogels. Our data show that increasing polymer concentration tailored hydrogel elasticity and viscoelasticity. Hydrogel architecture, composition and the bonds forming the polymer network dictated hydrogel elasticity and viscoelasticity. Also, cells altered hydrogel stiffness and stress relaxation in a polymer type, hydrogel concentration and time-dependent manner. A generalised Maxwell model of viscoelasticity further revealed cell-induced changes in hydrogel time-dependent mechanics. Overall, this thesis furthers our understanding of cell-matrix biology in vitro. The data presented here also have implications for the TERM field and areas of hydrogel-based research for cellular applications

Computational model of the human urinary bladder

La propuesta de una vejiga artificial es un obstáculo a trasponer. El cáncer de vejiga está entre los casos más frecuentes de enfermedades oncológicas en Estados Unidos y Europa. Ese cáncer es considerado un problema médico importante una vez que esa enfermedad presenta altas tasas de re-ocurrencia, muchas veces llevando a la remoción del órgano. La solución más sofisticada para remplazar ese órgano es la vejiga ileal, que consiste en una neovejiga hecha de tejido intestinal del enfermo. Desafortunadamente, esa solución presenta no solo problemas mecánicos funcionales, descritos en la literatura como problemas de vaciado y fuga, peo también problemas de orden biológica (como ejemplo pérdida ósea, debido a la absorción por el intestino de substancias que necesitan ser eliminadas del organismo). A través de la solicitación de la comunidad urológica del Hospital Clínico de Barcelona y con su experiencia en modelos numéricos para estructuras biomédicas, el Centro de Métodos Numéricos en Ingeniería (CIMNE) ha tenido la iniciativa de proporcionar actividad investigadora de la mecánica de la vejiga urinaria y de la simulación de interacción fluidoestructura para reproducir el llenado y vaciado de ese órgano con la orina. La simulación de la vejiga humana por el Método de los Elementos Finitos (FEM) y un completo entendimiento de la mecánica de ese órgano y de su interacción con la orina dará la posibilidad de proponer mejora en la geometría y de analizar materiales para la solución artificial en caso de remplazamiento de la vejiga. Para lograr ese objetivo, primeramente procedemos a una revisión bibliográfica de los modelos matemáticos del aparato urinario y un estudio comprehensivo de la fisiología y dinámica de la vejiga. Presentamos una revisión de las principales estructuras urológicas, riñón, uréter y uretra. Las estructuras anexas también son consideradas para entender las condiciones de contorno del problema estudiado. Posteriormente, proponemos el modelo constitutivo para estudiar la vejiga urinaria humana. El comportamiento del musculo detrusor durante llenado y vaciado de la vejiga con orina, su habilidad de retención de orina a baja presión debe ser correctamente representada por medio de la implementación de un modelo constitutivo no-lineal. El modelo matemático necesita representar las variables mecánicas que gobiernan ese órgano, y también las propiedades de la orina. El comportamiento no-lineal de tejidos vivos es implementado y validado con ejemplos de la literatura. La propiedad quasi-incompressible de la orina y las ecuaciones Navier-Stokes son consideradas para análisis del fluido. Para representar la geometría de la vejiga, implementamos un modelo computacional 3D a partir de imágenes de tomografía computadorizada de un cadáver adulto. Los datos son tratados para considerar las condiciones de contorno. Hemos construido dos modelos de malla: un mallado con tetrahedos de cuatro nodos y otro mallado con elementos de membrana de tres nodos. El esquema utilizado para calcular la interacción fluido-estructura debe ser adecuado para materiales de densidad muy parecidas. El análisis numérico de llenado y vaciado de la vejiga humana es validada con tests urodinámicos estandarizados. La parte final de la tesis, presentamos una simulación de una neo-vejiga, siendo el primer paso para representar numéricamente materiales artificiales para remplazamiento de la vejiga.The proposal of an artificial bladder is still a challenge to overcome. Bladder cancer is among the most frequent cases of oncologic diseases in United States and Europe. It is considered a major medical problem once this disease has high rates of reoccurrence, often leading to the extirpation of this organ. The most refined solution to replace this organ is the ileal bladder, which consists of a neobladder made of the patient’s intestinal tissue. Unfortunately this solution presents not only functional mechanical problems, described on the literature as voiding and leaking problems, but also biological ones (i.e. bone loss, given the absorption by the intestine of substances that should be eliminated from the organism). Urged by the urological community of the Hospital Clinic de Barcelona and backgrounded by its experience in the numerical simulation of biomedical structures, the Center of Numerical Methods in Engineering (CIMNE) had the initiative to provide the research of the mechanics of the urinary bladder and the simulation of fluid structure interaction (FSI) to account for the filling and voiding of this organ with urine. The Finite Element Method (FEM) simulation of the real bladder and the comprehensive understanding of the mechanics of this organ and its interaction with urine will give the possibility to propose geometrical improvements and study suitable materials for an artificial solution to address the cases on which the bladder needs to be removed. To reach this goal, first we proceeded to the bibliographic review of mathematical models of the urinary apparatus and to a comprehensive study of the physiology and dynamics of the bladder. A review of the major urological structures, kidney, ureter and urethra, takes place. To consider boundary conditions other surrounding structures to the urinary system are also studied. In the second part of the thesis, we propose the numerical model to study the human urinary bladder. The behavior of the detrusor muscle during filling and voiding of the bladder with urine and its ability to promote the storage of urine under low pressure need to be accurately represented, requiring the implementation of a non-linear constitutive model. The mathematical model needs to be capable to simulate the mechanical variables that govern this organ and the properties of the urine. The nonlinear behavior of living tissues is implemented and validated with examples from the literature. The quasi-incompressibility property of urine and the navierstokes equations for the fluid are taken into account. The geometry of the bladder needs to be taken into account, and the implementation of a 3D computational model obtained from the computerized tomography of a cadaver male adult is considered. The data has been treated to consider boundary conditions. Two models have been conceived: one meshed with four nodes tetrahedral and another meshed with shell elements. FSI must work for the simulation of filling and voiding of the bladder. Due to the close densities of the materials the scheme used to solve fluid-structure needs to be carefully selected. The proposed numerical model and the filling and voiding analysis are finally validated with standardized urodynamic tests. The final part of the thesis, the simulation of a neobladder is presented, being the first step to simulate numerically artificial materials for bladder replacement

Computational model of the human urinary bladder

The proposal of an artificial bladder is still a challenge to overcome. Bladder cancer is among the most frequent cases of oncologic diseases in United States and Europe. It is considered a major medical problem once this disease has high rates of reoccurrence, often leading to the extirpation of this organ. The most refined solution to replace this organ is the ileal bladder, which consists of a neobladder made of the patient’s intestinal tissue. Unfortunately this solution presents not only functional mechanical problems, described on the literature as voiding and leaking problems, but also biological ones (i.e. bone loss, given the absorption by the intestine of substances that should be eliminated from the organism). Urged by the urological community of the Hospital Clinic de Barcelona and backgrounded by its experience in the numerical simulation of biomedical structures, the Center of Numerical Methods in Engineering (CIMNE) had the initiative to provide the research of the mechanics of the urinary bladder and the simulation of fluid structure interaction (FSI) to account for the filling and voiding of this organ with urine. The Finite Element Method (FEM) simulation of the real bladder and the comprehensive understanding of the mechanics of this organ and its interaction with urine will give the possibility to propose geometrical improvements and study suitable materials for an artificial solution to address the cases on which the bladder needs to be removed. To reach this goal, first we proceeded to the bibliographic review of mathematical models of the urinary apparatus and to a comprehensive study of the physiology and dynamics of the bladder. A review of the major urological structures, kidney, ureter and urethra, takes place. To consider boundary conditions other surrounding structures to the urinary system are also studied. In the second part of the thesis, we propose the numerical model to study the human urinary bladder. The behavior of the detrusor muscle during filling and voiding of the bladder with urine and its ability to promote the storage of urine under low pressure need to be accurately represented, requiring the implementation of a non-linear constitutive model. The mathematical model needs to be capable to simulate the mechanical variables that govern this organ and the properties of the urine. The nonlinear behavior of living tissues is implemented and validated with examples from the literature. The quasi-incompressibility property of urine and the navierstokes equations for the fluid are taken into account. The geometry of the bladder needs to be taken into account, and the implementation of a 3D computational model obtained from the computerized tomography of a cadaver male adult is considered. The data has been treated to consider boundary conditions. Two models have been conceived: one meshed with four nodes tetrahedral and another meshed with shell elements. FSI must work for the simulation of filling and voiding of the bladder. Due to the close densities of the materials the scheme used to solve fluid-structure needs to be carefully selected. The proposed numerical model and the filling and voiding analysis are finally validated with standardized urodynamic tests. The final part of the thesis, the simulation of a neobladder is presented, being the first step to simulate numerically artificial materials for bladder replacement

New Tools for Viscoelastic Spectral Analysis, with Application to the Mechanics of Cells and Collagen across Hierarchies

Viscoelastic relaxation spectra are essential for predicting and interpreting the mechanical responses of materials and structures. For biological tissues, these spectra must usually be estimated from viscoelastic relaxation tests. Interpreting viscoelastic relaxation tests is challenging because the inverse problem is expensive computationally. We present here (1) an efficient algorithm and (2) a quasi-linear model that enable rapid identification of the viscoelastic relaxation spectra of both linear and nonlinear materials. We then apply these methods to develop fundamental insight into the mechanics of collagenous and fibrotic tissues. The first algorithm, which we term the discrete spectral approach, is fast enough to yield a discrete spectrum of time constants that is sufficient to fit a measured relaxation spectrum with an accuracy insensitive to further refinement. The algorithm fits a discrete spectral generalized Maxwell (Maxwell-Wiechert) model, which is a linear viscoelastic model, to results from a stress-relaxation test. The discrete spectral approach was tested against trial data to characterize its robustness and identify its limitations and strengths. The algorithm was then applied to identify the viscoelastic response of reconstituted collagen and engineered fibrosis tissues, revealing that cells actively adapted the ECM, and that cells relax at multiple timescales, including one that is fast compared to those of the ECM. The second algorithm, which we term the discrete quasi-linear viscoelastic (DQLV) approach, is a spectral extension of the Fung quasi-linear viscoelastic (QLV) model, a standard tool for characterizing biological materials. The Fung QLV model provides excellent fits to most stress-relaxation data by imposing a simple form upon a material\u27s temporal relaxation spectrum. However, model identification is challenging because the Fung QLV model\u27s “box” shaped relaxation spectrum, predominant in biomechanics applications, because it can provide an excellent fit even when it is not a reasonable representation of a material\u27s relaxation spectrum. The DQLV model is robust, simple, and unbiased. It is able to identify ranges of time constants over which the Fung QLV model\u27s typical box spectrum provides an accurate representation of a particular material\u27s temporal relaxation spectrum, and is effective at providing a fit to this model. The DQLV spectrum also reveals when other forms or discrete time constants are more suitable than a box spectrum. After validating the approach against idealized and noisy data, we applied the methods to analyze medial collateral ligament stress-relaxation and sinusoidal excitation data and identify the strengths and weaknesses of an optimal Fung QLV fit. Taken together, the tools in this dissertation form a comprehensive approach to characterizing the mechanics of viscoelastic biological tissues, and to dissecting the micromechanical mechanisms that underlie a tissue\u27s viscoelastic responses

Research on real-time physics-based deformation for haptic-enabled medical simulation

This study developed a multiple effective visuo-haptic surgical engine to handle a variety of surgical manipulations in real-time. Soft tissue models are based on biomechanical experiment and continuum mechanics for greater accuracy. Such models will increase the realism of future training systems and the VR/AR/MR implementations for the operating room