Tim Ricken - Academia.edu (original) (raw)
Papers by Tim Ricken
Archive of Applied Mechanics, Nov 28, 2009
ABSTRACT
Proceedings in applied mathematics & mechanics, Dec 1, 2021
Biomechanical modeling enables a better understanding and prediction of various processes in the ... more Biomechanical modeling enables a better understanding and prediction of various processes in the human body. To make this simulation more patient‐specific, realistic geometries of liver lobules are included in an existing knowledge‐based model for the simulation of hepatic function‐perfusion processes in the human liver. This model allows the simulation of liver diseases such as the non‐alcoholic fatty liver disease (NAFLD) or tumor development. The basis of this calculation is a continuum‐biomechanical multiscale and multiphase model based on the Theory of Porous Media. To capture the function‐perfusion and growth processes in the liver, partial differential equations (PDEs) on the lobule scale are coupled to ordinary differential equations (ODEs) describing metabolic processes on the cellular scale. Additionally, we used manifold learning techniques on in silico data for the identification of inverted fat zonation during NAFLD.
Journal of The Mechanical Behavior of Biomedical Materials, Oct 1, 2018
Medical imaging performed in vivo captures geometries under Donnan osmotic loading, even when the... more Medical imaging performed in vivo captures geometries under Donnan osmotic loading, even when the articulating joint is otherwise mechanically unloaded. Hence patient-specific finite element (FE) models constructed from such medical images of cartilage represent osmotically induced prestretched/prestressed states. When applying classical modeling approaches to patient-specific simulations of cartilage a theoretical inconsistency arises: the in-vivo imaged geometry (used to construct the model) is not an unloaded, stress-free reference configuration. Furthermore when fitting nonlinear constitutive models that include osmotic swelling (to obtain material parameters), if one assumes that experimental data-generated from osmotically loaded cartilage-begin from a stress-free reference configuration the fitted stress-stretch relationship (parameters) obtained will actually describe a different behavior. In this study we: (1) establish a practical computational method to include osmotically induced prestretch/prestress in image-driven simulations of cartilage; and (2) investigate the influence of considering the prestretched/prestressed state both when fitting fiber-reinforced, biphasic constitutive models of cartilage that include osmotic swelling and when simulating cartilage responses. Our results highlight the importance of determining the prestretched/prestressed state within cartilage induced by osmotic loading in the imaged configuration prior to solving boundary value problems of interest. With our new constitutive model and modeling methods, we aim to improve the fidelity of FE-based, patient-specific biomechanical simulations of joints and cartilage. Improved simulations can provide medical researchers with new information often unavailable in a clinical set-*
Proceedings in applied mathematics & mechanics, Dec 1, 2018
Proceedings in applied mathematics & mechanics, Dec 1, 2018
With this contribution we would like to communicate the state of the art of TPM 2 application to ... more With this contribution we would like to communicate the state of the art of TPM 2 application to realistic engineering problems. First of all, a conceptional overview of TPM 2 is shortly given, secondly we illustrate the benefit of Computer Tomography (CT) technology to capture geometry and create finite element meshes. Further, the application of the domain decomposition (DD) method for parallel execution will be shown on an example of a fluid saturated porous unit cube and finally we give advise for additional acceleration of computational runtime via model order reduction (MOR) for the TPM 2-Framework.
Proceedings in applied mathematics & mechanics, Nov 1, 2019
Polyelectrolytic gels placed in aqueous solution show effects under various kinds of stimulation.... more Polyelectrolytic gels placed in aqueous solution show effects under various kinds of stimulation. The stimuli could be e.g. of chemical, electrical, mechanical or thermal nature. The hydrogels react via uptake or delivery of mobile ions and solvent, and they show enormous swelling capabilities. This multifunctional behaviour is potentially attractive for chemo-electromechanical energy converters or for the use as actuators or sensors. In the present research, anionic and cationic hydrogels are investigated, which means that the polymer network contains anionic or cationic bound charged groups. The chemical stimulation is applied by a change of boundary conditions in the solution bath for the salt concentrations. The electrical stimulus is realized by incorporating electrodes between which an electric potential difference is applied. The mechanical stimulus is realized by prescribed displacements at a boundary of the hydrogel itself. The thermal stimulus is applied as transient temperature change over the whole domain, incorporating temperature-dependent material parameters and osmotic pressure differences. The reactions of the hydrogel differ depending on the sensitivity of the gel to the applied stimulus. The incorporated chemo-electro-mechanical model enhanced with thermal dependencies is capable of giving local concentrations, electric potential and mechanical displacements.
Computers & Structures, May 1, 2020
Rheumatic heart disease (RHD) is identified as a serious health concern in developing countries, ... more Rheumatic heart disease (RHD) is identified as a serious health concern in developing countries, specifically amongst young individuals, accounting for between 250 000 and 1.4 million deaths annually. As such, attention is initially placed on the importance of the development of a cardiac analysis toolbox with functionality for pathophysiological analysis of the disease. Subsequently, in order to develop a toolbox to further the understanding of the mechanisms of the disease as linked to changes in the cytoskeletal architecture and hypertrophy of cardiac myocytes, a continuum bi-phasic model applicable to cardiac tissue is formulated based on the theory of porous media (TPM). This makes it possible to account for interactions and contributions of multiple phases of constituent materials as well as concentrations of solved components, which in computational cardiac modelling are the solid phase-the cardiac tissue-and the liquid phase-blood and interstitial fluid. Therefore, subsequent attention is paid to the cardiac model development in order to implement a sound base on which to add strain-and nutrient-driven phase transition, in addition to a nutrient phase contained within the liquid phase. To this end, based on thermodynamical restrictions, constitutive relations are proposed for stress, permeability, seepage velocity and interaction forces. The approach is implemented in the in-house computational cardiac mechanics toolbox SESKA which supports finite element as well as Element-free Galerkin-based approximations. This paper considers the passive and active non-linear elastic material behaviour of the myocardium of the left ventricle coupled with porous media theory, along with an additional coupling to the haemodynamics of the circulatory system, facilitating modelling of the full cardiac cycle. In order to illustrate the potential and efficacy of the approach with qualitative results, a human heart affected by RHD is investigated, making use of cardiovascular magnetic resonance scans taken over a period of two years to generate realistic 3D computer models.
Examples and counterexamples, Nov 1, 2022
Springer eBooks, Nov 28, 2013
In many branches of engineering, e.g. material science, soil constructions, and geotechnics, free... more In many branches of engineering, e.g. material science, soil constructions, and geotechnics, freezing and thawing processes of fluid filled porous media play an important role. The coupled fluid-ice-solid behavior is strongly influenced by phase transition, heat and mass transport as well as interactions of fluid-solid/ice pressure depending on the entropy of fusion, and is accompanied by a volume expansion. In this contribution, a macroscopic model based on the Theory of Porous Media (TPM) is presented toward the description of freezing and thawing processes/ freeze-thaw cycles in fluid filled porous media. Therefore, a quadruple model consisting of the constituents solid, ice, liquid and gas is used. Attention is paid to the description of capillary effects, especially, the frost suction, the distribution of fluid and ice pressure as well as solid deformation before, during and after the ice formation in consideration of energetic effects under cycling thermal loading. Numerical examples are presented to demonstrate the usefulness of the model.
International Journal of Solids and Structures, Apr 1, 2022
Gamm-mitteilungen, May 24, 2019
In recent years computational models have become more important for simulating hepatic processes ... more In recent years computational models have become more important for simulating hepatic processes and investigating liver diseases in silico and so various liver models have been published. The complex behavior of biological tissue with its hierarchical structure as well as the blood perfusion through the organ have been described using different approaches and numerical techniques. This paper shows and compares numerical approaches for function and perfusion simulation recently published and compares them with a multiscale function-perfusion model using the extended theory of porous media. We focus on the description of blood perfusion and liver tissue, but also on the simulation of liver diseases or the zonation of processes in the liver. Furthermore, the selected geometry is taken into account.
Proceedings in applied mathematics & mechanics, Dec 1, 2018
Ionic polyelectrolytic gels in an aqueous solution, i.e. hydrogels-also known as smart materials-... more Ionic polyelectrolytic gels in an aqueous solution, i.e. hydrogels-also known as smart materials-react to different kinds of environmental changes, e.g. chemical, electrical, mechanical, and thermal stimulation. As a reaction, they show enormous swelling capabilities resulting from the delivery or uptake of ions and solvent. These properties make them attractive for chemo-electro-mechanical energy converters and for the application as actuators or sensors. The applied multi-field formulation consists of the chemical, electrical, and mechanical field and is capable of giving local concentrations, electric potential distributions and displacements. In this excerpt the reaction of a modelled hydrogel finger gripper under electrical stimulation is shown. The swelling ratio is assumed to be in the regime of small volume changes and corresponding displacements.
CRC Press eBooks, Aug 21, 2019
Articular cartilage is a multiphase material consisting of fluids and electrolytes, which is desc... more Articular cartilage is a multiphase material consisting of fluids and electrolytes, which is described with the Theory of Porous Media. The mechanical characteristics of articular cartilage are porosity, incompressible material behavior combined with transversely isotropic behavior for solid and fluid phases. There are two central points to model articular cartilage: the poro-viscosity of the porous matrix and the visco elasticity, and orientation of the collagen fibers. A numerical example is presented.
Proceedings in applied mathematics & mechanics, Oct 1, 2015
Proceedings in applied mathematics & mechanics, Dec 1, 2017
Thanks to the advancements in the digital era we are able to capture naturally grown and artifici... more Thanks to the advancements in the digital era we are able to capture naturally grown and artificially manufactured microstructures with various scanning devices like CT and MRT and can transfer the digital image data to finite element models. In addition, there has been a permanent improvement in the quality of additive reproduction technology. Looking at the biomedical industry producing organic parts, porous materials saturated with fluids play an important role. For this reason, we also have to develop appropriate simulation technology providing a description for porous materials regarding the underlying microstructure. This contribution presents a numerical experiment for the flow through a porous body with different underlying microstructures applying the TPM 2-Method. The different macroscopic behavior for the displacements, pressure distribution, and volumetric fluid flow for an isotropic and two differently orientated anisotropic microstructures are shown in section 3.
Journal of Applied Mathematics and Mechanics, Jul 2, 2013
In civil engineering, the frost durability of partly liquid saturated porous media under freezing... more In civil engineering, the frost durability of partly liquid saturated porous media under freezing and thawing conditions is a point of great discussion. Ice formation in porous media results from coupled heat and mass transport and is accompanied by ice expansion. The volume increase in space and time corresponds to the moving freezing front inside the porous solid. In this paper, a macroscopic model based on the Theory of Porous Media (TPM) is presented which describes energetic effects of freezing and thawing processes. For simplification a ternary model consisting of the phases solid, ice and liquid is used. Attention is paid to the description of the temperature development, the determination of energy, enthalpy and mass supply as well as volume deformations due to ice formation during a freezing and thawing cycle. For the detection of energetic effects regarding the characterization and control of phase transition of water and ice, a physically motivated evolution equation for the mass exchange between ice and liquid is presented. Comparing experimental data with numerical examples shows that the simplified model is indeed capable of simulating the temperature development and energetic effects during phase change.
Computational Materials Science, May 1, 2009
It is well-known that biological systems have the capacity to change their inner structure and sh... more It is well-known that biological systems have the capacity to change their inner structure and shape for an optimized load transfer. This paper deals with the development of a multiphase model to describe the growth and remodeling phenomenon in biological systems in order to learn more about the biological optimization mechanisms. A continuum triphasic model (i.e., a solid having interstitial space filled with water containing nutrients) based on the multiphase Theory of Porous Media (TPM) is proposed to provide a thermodynamically consistent description of the growth and remodeling phenomenon. The constitutive modeling of stress-strain-or nutrient-driven growth and remodeling phenomena is discussed. Finally, the influence of different driving mechanisms for growth is demonstrated by three illustrative exemplary problems.
Archive of Applied Mechanics, Nov 28, 2009
ABSTRACT
Proceedings in applied mathematics & mechanics, Dec 1, 2021
Biomechanical modeling enables a better understanding and prediction of various processes in the ... more Biomechanical modeling enables a better understanding and prediction of various processes in the human body. To make this simulation more patient‐specific, realistic geometries of liver lobules are included in an existing knowledge‐based model for the simulation of hepatic function‐perfusion processes in the human liver. This model allows the simulation of liver diseases such as the non‐alcoholic fatty liver disease (NAFLD) or tumor development. The basis of this calculation is a continuum‐biomechanical multiscale and multiphase model based on the Theory of Porous Media. To capture the function‐perfusion and growth processes in the liver, partial differential equations (PDEs) on the lobule scale are coupled to ordinary differential equations (ODEs) describing metabolic processes on the cellular scale. Additionally, we used manifold learning techniques on in silico data for the identification of inverted fat zonation during NAFLD.
Journal of The Mechanical Behavior of Biomedical Materials, Oct 1, 2018
Medical imaging performed in vivo captures geometries under Donnan osmotic loading, even when the... more Medical imaging performed in vivo captures geometries under Donnan osmotic loading, even when the articulating joint is otherwise mechanically unloaded. Hence patient-specific finite element (FE) models constructed from such medical images of cartilage represent osmotically induced prestretched/prestressed states. When applying classical modeling approaches to patient-specific simulations of cartilage a theoretical inconsistency arises: the in-vivo imaged geometry (used to construct the model) is not an unloaded, stress-free reference configuration. Furthermore when fitting nonlinear constitutive models that include osmotic swelling (to obtain material parameters), if one assumes that experimental data-generated from osmotically loaded cartilage-begin from a stress-free reference configuration the fitted stress-stretch relationship (parameters) obtained will actually describe a different behavior. In this study we: (1) establish a practical computational method to include osmotically induced prestretch/prestress in image-driven simulations of cartilage; and (2) investigate the influence of considering the prestretched/prestressed state both when fitting fiber-reinforced, biphasic constitutive models of cartilage that include osmotic swelling and when simulating cartilage responses. Our results highlight the importance of determining the prestretched/prestressed state within cartilage induced by osmotic loading in the imaged configuration prior to solving boundary value problems of interest. With our new constitutive model and modeling methods, we aim to improve the fidelity of FE-based, patient-specific biomechanical simulations of joints and cartilage. Improved simulations can provide medical researchers with new information often unavailable in a clinical set-*
Proceedings in applied mathematics & mechanics, Dec 1, 2018
Proceedings in applied mathematics & mechanics, Dec 1, 2018
With this contribution we would like to communicate the state of the art of TPM 2 application to ... more With this contribution we would like to communicate the state of the art of TPM 2 application to realistic engineering problems. First of all, a conceptional overview of TPM 2 is shortly given, secondly we illustrate the benefit of Computer Tomography (CT) technology to capture geometry and create finite element meshes. Further, the application of the domain decomposition (DD) method for parallel execution will be shown on an example of a fluid saturated porous unit cube and finally we give advise for additional acceleration of computational runtime via model order reduction (MOR) for the TPM 2-Framework.
Proceedings in applied mathematics & mechanics, Nov 1, 2019
Polyelectrolytic gels placed in aqueous solution show effects under various kinds of stimulation.... more Polyelectrolytic gels placed in aqueous solution show effects under various kinds of stimulation. The stimuli could be e.g. of chemical, electrical, mechanical or thermal nature. The hydrogels react via uptake or delivery of mobile ions and solvent, and they show enormous swelling capabilities. This multifunctional behaviour is potentially attractive for chemo-electromechanical energy converters or for the use as actuators or sensors. In the present research, anionic and cationic hydrogels are investigated, which means that the polymer network contains anionic or cationic bound charged groups. The chemical stimulation is applied by a change of boundary conditions in the solution bath for the salt concentrations. The electrical stimulus is realized by incorporating electrodes between which an electric potential difference is applied. The mechanical stimulus is realized by prescribed displacements at a boundary of the hydrogel itself. The thermal stimulus is applied as transient temperature change over the whole domain, incorporating temperature-dependent material parameters and osmotic pressure differences. The reactions of the hydrogel differ depending on the sensitivity of the gel to the applied stimulus. The incorporated chemo-electro-mechanical model enhanced with thermal dependencies is capable of giving local concentrations, electric potential and mechanical displacements.
Computers & Structures, May 1, 2020
Rheumatic heart disease (RHD) is identified as a serious health concern in developing countries, ... more Rheumatic heart disease (RHD) is identified as a serious health concern in developing countries, specifically amongst young individuals, accounting for between 250 000 and 1.4 million deaths annually. As such, attention is initially placed on the importance of the development of a cardiac analysis toolbox with functionality for pathophysiological analysis of the disease. Subsequently, in order to develop a toolbox to further the understanding of the mechanisms of the disease as linked to changes in the cytoskeletal architecture and hypertrophy of cardiac myocytes, a continuum bi-phasic model applicable to cardiac tissue is formulated based on the theory of porous media (TPM). This makes it possible to account for interactions and contributions of multiple phases of constituent materials as well as concentrations of solved components, which in computational cardiac modelling are the solid phase-the cardiac tissue-and the liquid phase-blood and interstitial fluid. Therefore, subsequent attention is paid to the cardiac model development in order to implement a sound base on which to add strain-and nutrient-driven phase transition, in addition to a nutrient phase contained within the liquid phase. To this end, based on thermodynamical restrictions, constitutive relations are proposed for stress, permeability, seepage velocity and interaction forces. The approach is implemented in the in-house computational cardiac mechanics toolbox SESKA which supports finite element as well as Element-free Galerkin-based approximations. This paper considers the passive and active non-linear elastic material behaviour of the myocardium of the left ventricle coupled with porous media theory, along with an additional coupling to the haemodynamics of the circulatory system, facilitating modelling of the full cardiac cycle. In order to illustrate the potential and efficacy of the approach with qualitative results, a human heart affected by RHD is investigated, making use of cardiovascular magnetic resonance scans taken over a period of two years to generate realistic 3D computer models.
Examples and counterexamples, Nov 1, 2022
Springer eBooks, Nov 28, 2013
In many branches of engineering, e.g. material science, soil constructions, and geotechnics, free... more In many branches of engineering, e.g. material science, soil constructions, and geotechnics, freezing and thawing processes of fluid filled porous media play an important role. The coupled fluid-ice-solid behavior is strongly influenced by phase transition, heat and mass transport as well as interactions of fluid-solid/ice pressure depending on the entropy of fusion, and is accompanied by a volume expansion. In this contribution, a macroscopic model based on the Theory of Porous Media (TPM) is presented toward the description of freezing and thawing processes/ freeze-thaw cycles in fluid filled porous media. Therefore, a quadruple model consisting of the constituents solid, ice, liquid and gas is used. Attention is paid to the description of capillary effects, especially, the frost suction, the distribution of fluid and ice pressure as well as solid deformation before, during and after the ice formation in consideration of energetic effects under cycling thermal loading. Numerical examples are presented to demonstrate the usefulness of the model.
International Journal of Solids and Structures, Apr 1, 2022
Gamm-mitteilungen, May 24, 2019
In recent years computational models have become more important for simulating hepatic processes ... more In recent years computational models have become more important for simulating hepatic processes and investigating liver diseases in silico and so various liver models have been published. The complex behavior of biological tissue with its hierarchical structure as well as the blood perfusion through the organ have been described using different approaches and numerical techniques. This paper shows and compares numerical approaches for function and perfusion simulation recently published and compares them with a multiscale function-perfusion model using the extended theory of porous media. We focus on the description of blood perfusion and liver tissue, but also on the simulation of liver diseases or the zonation of processes in the liver. Furthermore, the selected geometry is taken into account.
Proceedings in applied mathematics & mechanics, Dec 1, 2018
Ionic polyelectrolytic gels in an aqueous solution, i.e. hydrogels-also known as smart materials-... more Ionic polyelectrolytic gels in an aqueous solution, i.e. hydrogels-also known as smart materials-react to different kinds of environmental changes, e.g. chemical, electrical, mechanical, and thermal stimulation. As a reaction, they show enormous swelling capabilities resulting from the delivery or uptake of ions and solvent. These properties make them attractive for chemo-electro-mechanical energy converters and for the application as actuators or sensors. The applied multi-field formulation consists of the chemical, electrical, and mechanical field and is capable of giving local concentrations, electric potential distributions and displacements. In this excerpt the reaction of a modelled hydrogel finger gripper under electrical stimulation is shown. The swelling ratio is assumed to be in the regime of small volume changes and corresponding displacements.
CRC Press eBooks, Aug 21, 2019
Articular cartilage is a multiphase material consisting of fluids and electrolytes, which is desc... more Articular cartilage is a multiphase material consisting of fluids and electrolytes, which is described with the Theory of Porous Media. The mechanical characteristics of articular cartilage are porosity, incompressible material behavior combined with transversely isotropic behavior for solid and fluid phases. There are two central points to model articular cartilage: the poro-viscosity of the porous matrix and the visco elasticity, and orientation of the collagen fibers. A numerical example is presented.
Proceedings in applied mathematics & mechanics, Oct 1, 2015
Proceedings in applied mathematics & mechanics, Dec 1, 2017
Thanks to the advancements in the digital era we are able to capture naturally grown and artifici... more Thanks to the advancements in the digital era we are able to capture naturally grown and artificially manufactured microstructures with various scanning devices like CT and MRT and can transfer the digital image data to finite element models. In addition, there has been a permanent improvement in the quality of additive reproduction technology. Looking at the biomedical industry producing organic parts, porous materials saturated with fluids play an important role. For this reason, we also have to develop appropriate simulation technology providing a description for porous materials regarding the underlying microstructure. This contribution presents a numerical experiment for the flow through a porous body with different underlying microstructures applying the TPM 2-Method. The different macroscopic behavior for the displacements, pressure distribution, and volumetric fluid flow for an isotropic and two differently orientated anisotropic microstructures are shown in section 3.
Journal of Applied Mathematics and Mechanics, Jul 2, 2013
In civil engineering, the frost durability of partly liquid saturated porous media under freezing... more In civil engineering, the frost durability of partly liquid saturated porous media under freezing and thawing conditions is a point of great discussion. Ice formation in porous media results from coupled heat and mass transport and is accompanied by ice expansion. The volume increase in space and time corresponds to the moving freezing front inside the porous solid. In this paper, a macroscopic model based on the Theory of Porous Media (TPM) is presented which describes energetic effects of freezing and thawing processes. For simplification a ternary model consisting of the phases solid, ice and liquid is used. Attention is paid to the description of the temperature development, the determination of energy, enthalpy and mass supply as well as volume deformations due to ice formation during a freezing and thawing cycle. For the detection of energetic effects regarding the characterization and control of phase transition of water and ice, a physically motivated evolution equation for the mass exchange between ice and liquid is presented. Comparing experimental data with numerical examples shows that the simplified model is indeed capable of simulating the temperature development and energetic effects during phase change.
Computational Materials Science, May 1, 2009
It is well-known that biological systems have the capacity to change their inner structure and sh... more It is well-known that biological systems have the capacity to change their inner structure and shape for an optimized load transfer. This paper deals with the development of a multiphase model to describe the growth and remodeling phenomenon in biological systems in order to learn more about the biological optimization mechanisms. A continuum triphasic model (i.e., a solid having interstitial space filled with water containing nutrients) based on the multiphase Theory of Porous Media (TPM) is proposed to provide a thermodynamically consistent description of the growth and remodeling phenomenon. The constitutive modeling of stress-strain-or nutrient-driven growth and remodeling phenomena is discussed. Finally, the influence of different driving mechanisms for growth is demonstrated by three illustrative exemplary problems.