Abstract
The biomechanical properties are crucial indicators for the functional characterization of cartilaginous tissue. In this contribution native articular cartilage and three-dimensional scaffold-free cartilage constructs (SFCCs) are characterized by hyperelastic material models (Yeoh, Ogden and Demiray). SFCCs were developed for the therapy of damaged articular cartilage. The normalized error (NE) of fit and experiment is in the range of 0.04 and 0.13. The material model Yeoh with two parameters yields the best fit. The stress-like parameterc20 is 0.489 MPa for native cartilage, 0.120 MPa and 0.041 MPa for SFCCs produced from mesenchymal stem cells and chondrocytes, respectively. The significance of the fits and the derived parameters are presented and evaluated.
1 Introduction
Articular cartilage and SFCCs are investigated and characterized by biomechanical, biochemical and histological analysis. The biomechanical properties contain a variety of relevant information for the functional characterization of cartilaginous tissue [1]. For this purpose the modulus of elasticityE is commonly evaluated at a defined pressure applied to the cartilaginous tissue. However, the modulus of elasticityE cannot describe the entire strain behavior, because of the non-linear, non-homogeneous and anisotropic structure of cartilaginous tissue. On this account, the hyperelastic material models Yeoh, Ogden and Demiray are utilized for the characterization of the non-linear stress–strain–behavior of cartilage and SFCCs. Hence, within the scope of this article isotropic hyperelastic material models are fitted to the experimentally determined stress–strain–curves.
2 Methods
In this section the experimental approach and the hyper-elastic material models Yeoh, Ogden and Demiray are explained. Furthermore, the method to evaluate model quality is described.
2.1 Experimental approach
The stress–strain–curves of native cartilage and SFCCs were determined experimentally using a material testing machine INSTRON® 4466 (100 N). The uniaxial compression of the specimens was effected at a testing velocity of v = 1 mm/s, with an initial load of F = 0.1 N and a strain of 30 %.
The experiment was carried out on four native cartilage specimens, on five SFCCs-MSC produced from mesenchymal stem cells and five SFCCs-Chondro produced from chondrocytes. The native cartilaginous tissue was obtained from the equine knee joint (Trochlea ossis femoris). The SFCCs were produced according to patented technology [2] and implanted in the equine knee joint (Trochlea ossis femoris). After one-year-long implantation time the SFCCs were extracted for biomechanical characterization. In Figure 1 native cartilaginous tissue and SFCC are depicted. For the modelling averaged values were used.
2.2 Hyperelastic material models
For the characterization of the stress–strain–behavior the following isotropic incompressible hyperelastic material models were utilized: Yeoh [3], Ogden [4] and Demiray [5].

SFCC (left) and native cartilage (right).
The material model Yeoh was calculated with equation 1
where Ψ is the strain energy density, I1 is the first invariant of the right Cauchy-Green tensor C and ci0 are stress-like material parameters (defining the stiffness of material). For a unique solution c30 had to be fixed on 0.01 MPa.
The parameters of the model Ogden were calculated with equation
where λ1, λ2 and λ3 are the principle stretch ratios, μr are stress-like material parameters (defining the stiffness of material) and αr are dimensionless material parameters. Applying the one-term formulation, the material parameters μ2 and μ3 were fixed on 1 MPa and α2 and α3 on zero, respectively.
The material model Demiray was calculated with equation
where a is the stress-like material parameter (defining the stiffness of material) and b is the dimensionless material parameter (defining progression of stress–strain–curve). The material models shown here are commonly used to describe the stress–strain–behavior of rubber-like solids. However, the model Demiray was also applied to characterize the stress–strain–behavior of soft tissue [5]. The calculations were carried out with the software Hyperfit® [6].
For the evaluation of model quality the normalized error (NE) of fit and experiment was calculated with equation
where ye is the experimental (observed) value, ym is the model (theoretical) value,i is the data-point index, n is the number of data-points and
3 Results
In Figures 2 – 4 the material model fits of Yeoh, Ogden and Demiray at the averaged experimental stress–strain– curves of native cartilage and SFCCs-MSC and SFCCsChondro are depicted.
It is evident that stress–strain–behavior is non-linear. The stress behavior of native cartilage and SFCCs-MSC and SFCCs-Chondro are nearly equal up to 10 % strain. From 10 % strain the stress values of the native cartilage increase stronger than those of the SFCCs. At 30 % strain the stress value of native cartilage is six times and three times higher than those of SFCCs-Chondro and SFCCs-MSC, respectively.

Stress–strain–curve experimental data (red, green and black) and material model fit Yeoh (blue).

Stress–strain–curve experimental data (red, green and black) and material model fit Ogden (blue).

Stress–strain–curve experimental data (red, green and black) and material model fit Demiray (blue).
In Table 1 the hyperelastic material models are summarized with the respective normalized errors (NE) of experiment and fit as well as the number of parameters necessary for the calculation. The model Yeoh yielded the best fit for the characterization of the non-linear stress–strain– behavior. The normalized error (NE) of these models lies below 0.05. The models Ogden and Demiray are not suited for native cartilage.
The calculated parameters for native cartilage and SFCCs-MSC and SFCCs-Chondro, respectively, are listed in Tables 2 – 4. The results show, that the native cartilage has a higher stiffness than the SFCCs. This is illustrated particularly by the parametersc20 (Yeoh),µ1 (Ogden) und a (Demiray). Furthermore, the parameters calculated from the model Yeoh and Demiray show that the SFCCs-MSC have a higher stiffness than SFCCs-Chondro. The parameters calculated from the model Ogden show contrarious behavior and no significant differences in the calculated stiffness between native cartilage and SFCCs.
Number of parameters and the quality (NE) of the models for the characterization of native cartilage and SFCCs.
Models | Number of Parameters | NE (native cartilage) | NE (SFCCs) |
---|---|---|---|
Yeoh | 2 | 0.04 | 0.04 |
Ogden | 2 | 0.09 | 0.06 |
Demiray | 2 | 0.13 | 0.05 |
Calculated material parameters Yeoh.
Parameter | native cartilage | SFCCs- Chondro | SFCCs- MSC |
---|---|---|---|
c10 [MPa] | 0.001 | 0.011 | 0.018 |
c20 [MPa] | 0.489 | 0.041 | 0.120 |
Calculated material parameters Ogden.
Parameter | native cartilage | SFCCs- Chondro | SFCCs- MSC |
---|---|---|---|
μ1 [MPa] | 0.008 | 0.007 | 0.004 |
α1 [] | 15.453 | 11.520 | 11.356 |
4 Conclusion
In this contribution isotropic incompressible hyperelastic material models (Yeoh, Ogden and Demiray) were utilized for the characterization of non-linear stress–strain– behavior of native cartilage and SFCCs. Particularly the model Yeoh (two parameters) with a normalized error of 0.04 is suitable for this purpose. The number of parameters can be increased to three for the models Yeoh and Ogden. Thereby the quality of the fit would be improved [7]. However, negative fit parameters may occur, which are not allowed physically [8]. SFCCs may be described with the model Demiray, but for native cartilage the model is insufficient. The stiffness mismatch between native cartilage and SFCCs resulted from the incomplete maturing process of the SFCCs. Thus the SFCCs can be used for the mathematical modelling of cartilaginous tissue as intermediate stages or for the development of a mathematical arthrosis model. Furthermore, the parameters obtained from the model may be used for FEM simulations. The models used so far regarded the monophasic state, i.e. the time-depended behavior (relaxation and creep) was not considered.
Further research will be extended to material models of the biphasic or triphasic theory [9]. Additionally, the material behavior of the native cartilage and SFCCs will be examined under cyclic load (stress softening, hysteresis, viscoelastic analysis) [10, 11].
Calculated material parameters Demiray.
Parameter | native cartilage | SFCCs- Chondro | SFCCs- MSC |
---|---|---|---|
a [MPa] | 0.095 | 0.026 | 0.053 |
b [] | 14.122 | 7.990 | 9.339 |
Acknowledgment
The authors would like to thank Petra Prokop for translation and editing.
Funding: This study is part of a research project supported by the German Ministry of Economy and Technology: INNO-WATT Reg. Nr.: IW091034.
Author’s Statement
Conflict of interest: Authors state no conflict of interest. Material and Methods: Informed consent: Informed consent has been obtained from all individuals included in this study. Ethical approval: The research related to human use has been complied with all the relevant national regulations, institutional policies and in accordance the tenets of the Helsinki Declaration, and has been approved by the authors’ institutional review board or equivalent committee.
References
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This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
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- Mathematical, numerical and in-vitro investigation of cooling performance of an intra-carotid catheter for selective brain hypothermia
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- An ideally parameterized unscented Kalman filter for the inverse problem of electrocardiography
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- Interactive visualization of cardiac anatomy and atrial excitation for medical diagnosis and research
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- Virtualizing clinical cases of atrial flutter in a fast marching simulation including conduction velocity and ablation scars
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- Mesh structure-independent modeling of patient-specific atrial fiber orientation
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- Accelerating mono-domain cardiac electrophysiology simulations using OpenCL
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- Understanding the cellular mode of action of vernakalant using a computational model: answers and new questions
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- A java based simulator with user interface to simulate ventilated patients
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- Evaluation of an algorithm to choose between competing models of respiratory mechanics
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- Numerical simulation of low-pulsation gerotor pumps for use in the pharmaceutical industry and in biomedicine
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- Numerical and experimental flow analysis in centifluidic systems for rapid allergy screening tests
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- Biomechanical parameter determination of scaffold-free cartilage constructs (SFCCs) with the hyperelastic material models Yeoh, Ogden and Demiray
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- FPGA controlled artificial vascular system
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- Simulation based investigation of source-detector configurations for non-invasive fetal pulse oximetry
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- Test setup for characterizing the efficacy of embolic protection devices
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- Impact of electrode geometry on force generation during functional electrical stimulation
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- 3D-based visual physical activity assessment of children
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- Realtime assessment of foot orientation by Accelerometers and Gyroscopes
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- Image based reconstruction for cystoscopy
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- Image guided surgery innovation with graduate students - a new lecture format
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- Multichannel FES parameterization for controlling foot motion in paretic gait
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- Smartphone supported upper limb prosthesis
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- Use of quantitative tremor evaluation to enhance target selection during deep brain stimulation surgery for essential tremor
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- Evaluation of adhesion promoters for Parylene C on gold metallization
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- The influence of metallic ions from CoCr28Mo6 on the osteogenic differentiation and cytokine release of human osteoblasts
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- Increasing the visibility of thin NITINOL vascular implants
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- Possible reasons for early artificial bone failure in biomechanical tests of ankle arthrodesis systems
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- Development of a bending test procedure for the characterization of flexible ECoG electrode arrays
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- Tubular manipulators: a new concept for intracochlear positioning of an auditory prosthesis
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- Investigation of the dynamic diameter deformation of vascular stents during fatigue testing with radial loading
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- Electrospun vascular grafts with anti-kinking properties
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- Integration of temperature sensors in polyimide-based thin-film electrode arrays
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- Use cases and usability challenges for head-mounted displays in healthcare
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- Device- and service profiles for integrated or systems based on open standards
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- Risk management for medical devices in research projects
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- Simulation of varying femoral attachment sites of medial patellofemoral ligament using a musculoskeletal multi-body model
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- Does enhancing consciousness for strategic planning processes support the effectiveness of problem-based learning concepts in biomedical education?
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- SPIO processing in macrophages for MPI: The breast cancer MPI-SNLB-concept
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- Numerical simulations of airflow in the human pharynx of OSAHS patients