Physiological loading of cartilage replacement materials in a bioreactor environment

Trying to replace injured cartilage by implants is a common practice in biomedical engineering. These implants can be non-seeded or seeded with human cartilage cells. To initiate cell multiplication and oriented cell growth in cell seeded implants, the implants are cultivated and usually stimulated electrically or mechanically in a bioreactor before implanting. In the present study, a knee testing bench combined with a bioreactor environment is developed. Doing so, it is possible to stimulate such implants controlled in a physiologically consistent, multi-dimensional way. The implants are placed in a recreated human knee joint and stimulated with several physiological load cycles of reproduced walking. After some days, the implanted material can be removed and mechanically and biologically evaluated in cooperation with the RWTH Aachen University Hospital. The new experimental set-up enables us for the first time to study the remodelling effect, the efficiency of the preconditioning as well as the influence of the body-conformable load on the material. Furthermore, the need of cell colonisation in the implants shall be investigated. To understand the correlation between tissue remodelling and mechanical load history, the experiment is also numerically investigated, based on a geometrically realistic FE model of the recreated human knee and appropriate material models for the involved structures. Doing so, the strains and stresses, as well as the shear forces in the implant can be evaluated. The results will be compared to experimental data. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Biomechanical modelling of cellular articular cartilage replacement tissues

The aim of the present study is to investigate the strength and damping properties of cellular articular cartilage replacement material. For this purpose, a viscoelastic-diffusion model for the acellular water-saturated condensed collagen gel type I is proposed and validated experimentally. Moreover, a remodelling law for the cell seeded collagen gel is introduced. For an experimental study of the interaction between fibre growth and mechanical stimulation, bioreactors are developed and histological investigations are carried out. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Multi-dimensional,mechanical stimulation of cartilage implants

In biomedical engineering, it is a common practice to replace injured cartilage by implants, which are seeded with human cartilage cells. Before implanting, the implants are cultivated and usually stimulated electrically or mechanically in a bioreactor to initiate cell multiplication and oriented cell growth. A new experimental set-up is developed leading to the possibility of stimulating such implants in a multi-dimensional, physiologically consistent way. In cooperation with the University Medical Centre Aachen, a human knee simulator is developed. Cell-seeded implants are placed in a recreated human environment and stimulated with several load cycles of reproduced walking. After the cultivation period, the implanted material is removed and biologically and mechanically evaluated. The quality of the implanted material as well as the influence of the body-conformable load on the material is studied. To understand the correlation between tissue remodelling and mechanical load history, the load and movement scenario is also numerically investigated. For this reason, the experiment is transferred to a geometrically realistic FE model of a human knee. As a first approach, an elastic material model is used. The aim is to have a predictive FE model with an optimal trade-off between accuracy and efficiency using an appropriate material formulation. The results will be compared to experimental data. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Experimental and numerical investigation of different types of articular cartilage replacement materials

The aim of the presented work is to characterize the mechanical properties of different types of articular cartilage replacement materials. For this propose an elastic-diffusion model is developed to identify the elastic and diffusion properties of the replacement materials. A set of unconfined compression tests were performed with several kinds of implants. By means of finite element simulation integrated with an user-defined material model, the material parameters were identified. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mechanical modelling and experimental validation of meniscus replacement material

In the present study a condensed collagen gel is investigated for its application as a cartilage replacement material. For this reason, the strength and damping properties are examined experimentally and a theoretical model for predicting specimen deformations and stresses is proposed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Theoretical and experimental investigation of cartilage replacement material for medical applications

Soft tissues are commonly applied in surgery to replace the injured articular cartilage. Many biological researches were carried out through mechanical and histological experiments. They focus on the function, degeneration and regeneration of the articular cartilage and fibrocartilage. The aim of the presented work is to develop a method to characterize the mechanical properties of different kinds of soft tissues and to trace the evolution of elastic properties in implants during the remodeling process. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Modeling the migration of cancer cells in the bloodstream

We model the migration of cancer cells that have broken away from a tumor and are circulating in the bloodstream. Using the immersed boundary method and culling from literature the material properties of cancer cells, we simulate how cells deform as a function of the flow properties of the bloodstream as well as the adhesive properties between the cancer cells and the endothelial cells of the bloodstream. We also simulate the migration characteristics as a function of the migrating cell density. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural Changes in the Surface Friction Layer of a Polymeric Endoprosthesis Cup

A concept of creation of a polymeric insert for hip joint endoprostheses with the physiological and biomechanical properties typical of natural cartilage is proposed. The spherical friction surface of the insert is coated with a microporous layer imitating cartilage. This layer carries an electret charge, which improves the lubrication of the endoprosthesis with synovia and serves as a carrier of drugs, thus ensuring their prolonged discharge into the operation wound. Tribotechnical characteristics of an endoprosthesis with such an insert are investigated. It is shown that a drop in the friction coefficient of such a pair is accompanied by a change in the microrelief of the friction surface and in the degree of crystallinity of the material of the porous layer.     

Numerical Simulation of the Application of NiTi Alloys in Medical Technologies

Shape memory alloys are nowadays already established as a material which is able to solve exceptional tasks in practical applications. Particularly, its utilization in the field of medical technologies increases steadily. For example micro tools (staple, catheters) and implants (coronary stents) are made out of Nickel-Titanium well known as a basic shape memory alloy. Apart from the advantages like the avoidance of auxiliary components and joints in the system and to utilize the high volume specific work of shape memory alloys, NiTi alloys exhibit a good biocompatibility. This property is necessary with regard to either permanent or temporary implants. To optimize the use of NiTi alloys in the scope of medical technologies, the support of the development of applicable tools by numerical simulations is highly recommended. However the complex material behaviour containing a profoundly thermomechanical coupling poses indeed a big challenge to the material modeling and its implementation into a finite element code. Particularly, the material model proposed by Helm [1] proves to be a firm model containing the most common properties of shape memory alloys, as the pseudoelasticity, the shape memory effect and the two-way effect. In the present contribution the FE modelling of a medical staple used in foot surgery is presented by considering the model of Helm which was investigated by the authors to improve its performance in the finite element method [2]. The foot staple, produced by a group of members of the SFB 459 which is funded by the DFG, avails the shape memory effect to excite the desired clamping effect [3]. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

A Microscopic model for a cell-seeded material

The aim of the present paper is to account for the growth of fiber which is observed in a cell-seeded material stimulated in a bioreactor. For this purpose, the change of mass is considered in the balance laws, and the deformation energy is assumed to be a function of varying mass and the Helmholtz free-energy. Fiber growth at the microscopic level causes a macroscopic change of the material's mechanical properties. The study is a first approach towards a micromechanical model accounting for remodelling in cartilage replacement materials. In so doing, constitutive equations for renewable soft tissues are proposed. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Anisotropic viscohyperelastic behavior of intervertebral discs: Modeling and experimental validation

Biomechanical investigations of human cartilage, especially intervertebral discs (IVDs), have greatly helped to improve people's health over the last several decades. The study of the underlying biomechanical characteristics of cartilage tissues is a key issue to understand its physiological function and degeneration or damage behavior. The aim of this investigation is to describe the biomechnical behavior of healthy sheep IVDs under various loading conditions. Experimental and cartilage histological data, including fiber orientation, are used to develop a viscohyperelastic material model, which allowed us to numerically study the mechanical behavior of IVDs, consisting of a cartilaginous, fiber-reinforced ring surrounding a highly hydrated, gelatinous core. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nanocrystalline Diamond Coatings

Carbon—in the form of diamond coatings (nanocrystalline diamond—NCD) on suitable substrates—has attractive properties for biomedical applications. The excellent chemical inertness of NCD films makes them a promising material for medical implants, cardiovascular surgery and for coating of certain components of artificial heart valves.

The medical applications of carbon films impose some special requirements on their quality, purity, phase content and the state of the surface. Of particular importance is the smoothness of the surface and good adhesion of the coatings to the substrate. We have investigated carbon films which were synthesized by Radio Frequency Plasma Chemical Vapour Deposition (RF–PCVD).

The specimens obtained have been tested to show their structure and their biological, mechanical and chemical resistance. Additional investigations of the NCD films were carried out by micro-X-ray spectroscopy, Raman spectroscopy, AFM, Auger spectroscopy, corrosion tests, breakdown tests and clinical investigations. Nanocrystalline diamond (NCD) layers obtained by a new method of RF dense plasma CVD onto AISI-316L steel used in surgery were investigated to determine their suitability as biomaterials.     

A DLM/FD/IB Method for Simulating Cell/Cell and Cell/Particle Interaction in Microchannels

A spring model is used to simulate the skeleton structure of the red blood cell （RBC） membrane and to study the red blood cell （RBC） rheology in Poiseuille flow with an immersed boundary method. The lateral migration properties of many cells in Poiseuille flow have been investigated. The authors also combine the above methodology with a distributed Lagrange multiplier/fictitious domain method to simulate the interaction of cells and neutrally buoyant particles in a microchannel for studying the margination of particles.     

A viscoelastic-diffusion model for cartilage replacement material

In order to predict deformations and internal stresses of articular cartilage replacement material, two viscoelastic diffusion models are proposed in the present study. Also, the remodeling effect of the material seeded with human cells is verified experimentally. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mathematical modeling of a temperature-sensitive and tissue-mimicking gel matrix: Solving the Flory–Huggins equation for an elastic ternary mixture system

Programmed to retain active responsivity to environmental stimuli, diverse types of synthetic gels have been attracting interests regarding various applications, such as elastomer biodevices. In a different approach, when the gels are made of tissue-derived biopolymers, they can act as an artificial extracellular matrix (ECM) for use as soft implants in medicine. To explore the physical properties of hydrogels in terms of statistical thermodynamics, the mean-field Flory–Huggins–Rehner theory has long been used with various analytical and numerical modifications. Here, we suggest a novel mathematical model on the phase transition of a biological hybrid gel that is sensitive to ambient temperature. To mimic acellular soft tissues, the ECM-like hydrogel is modeled as a network of biopolymers, such as type I collagen and gelatin, which are covalently crosslinked and swollen in aqueous solvents. Within the network, thermoresponsive synthetic polymer chains are doped by chemical conjugation. Based on the Flory–Huggins–Rehner framework, our analytical model phenomenologically illustrates a well-characterized volume phase behavior of engineered tissue mimics as a function of temperature by formulating the ternary mixing free energy of the polymer–solvent system and by generalizing the elastic free energy term. With this formalism, the decoupling of the Flory–Huggins interaction parameter between the thermoresponsive polymer and ECM biopolymer enables deriving a simple steady-state formula for the volume phase transition as a function of the structural and compositional parameters. We show that the doping ratio of thermoresponsive polymers and the Flory–Huggins interaction parameter between biopolymer and water affect the phase transition temperature of the ECM-like gels.     

A hyperelastic biphasic fiber reinforced model for articular cartilage considering the distribution and orientation of collagen fibers

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. (© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical investigations on the fixation of cemented and uncemented endoprostheses

In this contribution, a constitutive model adopted from the computational plasticity-models of Drucker-Prager and von Mises is presented. This model captures the material behavior of osseointegration and the curing-process of bone cement. With this basic model, both simulations of bone-ingrowth of uncemented implants and simulations of the curing process of bone cement for cemented implants are carried out in a bone-implant interface. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Collapse criteria of foam cells under various loading

Recently porous materials are widely used in civil and mechanical engineering. In particular, such porous materials as metal and polymer foams have applications in lightweight structures. From mechanics point of view foams can demonstrate unusual behavior such as strain localization related to foam cells buckling under certain loads. The aim of this work is the elaboration of the model of foam material taking into account the cell collapse. We consider the cell collapse initiation during the elastic instability and its further evolution under loading. The geometrical structure of foam is generated with the use of the Voronoi algorithm. Based on stochastic distributions of cells we create various geometrical models of foams. The influence of the cell volume, wall thicknesses and material properties of the foam material on critical loads is obtained. The calculations are performed with the use of Abaqus CAD/CAE system. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Modeling and homogenization of textile reinforced composites based on the isogeometric analysis

In this contribution, the isogeometric analysis is used to compute the effective material properties of textile reinforced composites. The isogeometric analysis based on non-uniform rational B-splines (NURBS) provides an efficient approach for numerical modeling because there is no need for a mesh generation. There are further advantages such as the availability of a geometry representation based on NURBS in computer-aided design software and the possibility to apply different refinement methods which do not change the geometry of the numerical model. These properties motivate the combination of the isogeometric analysis with the homogenization method. Therefor, the unit cell model representing the inner architecture of a textile reinforced composite is defined using NURBS. In order to compute the effective mechanical properties of the heterogeneous material, the homogenization method with periodic boundary conditions is applied. Finally, two examples demonstrate the advantages of this approach. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

A homogenization method for heterogeneous materials using a multi-scale technique

For modern microelectronics solders lifetime and stability predictions are important. To perform such an analysis material properties are required. As electronic devices and the corresponding amount of matter used become smaller, the influence of a changing microstructure on mechanical properties must be considered. First some analytical methods were conducted for upper and lower bounds ignoring the exact geometric distribution of the solder phases. Second, analytical equations derived for geometries such as laminate structures were applied to examine the influence of the geometry on homogenized properties. Third, a multi-scale approach for periodic media was presented allowing for a more general analysis of structures. We assume that the solder material is composed of periodic cells, which represent the properties of the whole structure. Composite materials with periodic structures can be investigated by using at least two scales. A global scale is related to the whole piece of material whereas a local scale is related to the periodic cell only. The constitutive equations are stated and a homogenization technique for the elastic properties of arbitrary structures is derived. The resulting equations are solved numerically and results are presented. Again, for layered materials closed-form formulas are derived and compared to the numerical results. The method is also used to obtain effective mechanical properties for materials with linear hardening. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)