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Objective To study stress relaxation behaviors of cartilage scaffolds under different degradation cycles by using finite element analysis combined with theoretical models. Methods Based on the established degradation theoretical model, the elastic modulus of the scaffold was calculated under different degradation cycles. The finite element model of cartilage scaffolds was established and stress relaxation simulation was performed to analyze the variation of scaffold relaxation stress with time. The stress relaxation constitutive model was established to predict mechanical properties of the scaffold. Results The elastic modulus of cartilage scaffolds at 14 th, 28th, 42nd, 56th day after degradation was 32. 35, 31. 12, 29. 91, 28. 74 kPa, respectively. The upper layer for cartilage scaffolds was the largest. The overall relaxation stress of the scaffold decreased rapidly with time and then tended to be stable. At 8th week after degradation, the stress which the scaffold couldwithstand was still within the physiological load range of the cartilage. The predicted results of the stress relaxation constitutive model were in good agreement with the finite element simulation results. Conclusions The elastic modulus of the scaffold gradually decreases with the increase of degradation time. The longer the degradation period is, the less stress the scaffold can withstand. At the same degradation period, the larger the applied compressive strain, the larger the stress on the scaffold. Both the finite element simulation and stress relaxation constitutive model can effectively predict stress variations of cartilage scaffolds under degradation
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Objective Taking pig kidney as an example, through a series of comparative and analogical experiments, the influencing factors of compressive stress at relaxation stage of biological tissues were analyzed, and a more accurate and widely applicable biomechanical model at relaxation stage was established. Methods The compressive stress relaxation experiments of pig kidney under different conditions were carried out by using the self-built mechanical experiment platform. The collected data were analyzed and mapped, and various factors affecting the relaxation force changes were summarized. Based on the conclusion, the neural network learning algorithm was used to model the force change process at relaxation stage of pig kidney. Results The pre-extrusion pressure and relaxation time were the main influencing factors for compressive stress changes of biological tissues at relaxation stage. The average error of test sample validation experiment was 6.4 mN, and the average prediction error of generalization sample validation experiment was 34.9 mN, so the modeling effect was good. Conclusions Neural network modeling algorithm has the advantages of strong generalization ability and good fault tolerance, which contributes to providing more realistic force tactile feedback prediction for virtual surgery system. It is also a new idea for mechanical modeling of nonlinear biological tissues.
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BACKGROUND: Most of studies on the mechanical properties of modified glass ionomer cement mainly focus on the compression and bending experiments, but there are few reports on the stress relaxation and creep experiments of glass ionomer cement after adding strontium hydroxyapatite. OBJECTIVE: To compare the stress relaxation and creep characteristics of traditional glass ionomer cement, composite resin enamel adhesive and modified glass ionomer cement. METHODS: Strontium doped hydroxyapatite was added into glass ionomer cement according to the mass ratio of 15%, and then the modified glass ionomer cement was prepared. Samples of modified glass ionomer cement, composite resin enamel adhesive and traditional glass ionomer cement were maded. Ten samples from each group were taken for stress relaxation test and another 10 samples for creep test. RESULTS AND CONCLUSION: (1) At 7 200 s in the stress relaxation test, the stress in the traditional group was decreased by 1.18 MPa, decreased by 1.39 MPa in the composite group, and decreased by 1.38 MPa in the modified group. The decreased value in the traditional group was significantly less than that in the composite and modified groups (P 0.05). (2) At 7 200 s in the creep test, the stress in the traditional group was increased by 0.24%, increased by 0.33% in the composite group, and increased by 0.32% in the modified group. The increased value in the traditional group was significantly less than that in the composite and modified groups (P 0.05). (3) The viscoelastic properties of glass ionomer cement are improved by compounding 15% strontium-doped hydroxyapatite with glass ionomer cement. The increase of viscoelasticity is beneficial to the adhesion and bulk bond strength of glass ionomer cement.
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OBJECTIVE: This study was performed to investigate an appropriate degree of prestretch for orthodontic synthetic elastomeric chains focusing on time-dependent viscoelastic properties. METHODS: Orthodontic synthetic elastomeric chains of two brands were prestretched to 50, 100, 150, and 200% of the original length in one and three cycles, and the hysteresis areas of the obtained stress-strain curves were determined. Acrylic plates were employed to maintain constant strain during the experiment. A total of 180 samples were classified into nine groups according to brand, and their stresses and permanent deformations were measured immediately after prestretch (0 hour), after 1 hour and 24 hours, and after 1, 2, 3, 4, 5, 6, 7, and 8 weeks. The relationship between stress relaxation and permanent deformation was investigated for various degrees of prestretch, and the estimated stress resulting from tooth movement was calculated. RESULTS: The degree of prestretch and the stress relaxation ratio exhibited a strong negative correlation, whereas no correlation was found between the degree of prestretch and the average normalized permanent strain. The maximal estimated stress was observed when prestretch was performed in three cycles to 200% of the original length. CONCLUSIONS: Although prestretch benefited residual stress, it did not exhibit negative effects such as permanent deformation. The maximal estimated stress was observed at the maximal prestretch, but the difference between prestretch and control groups decreased with time. In general, higher residual stresses were observed for product B than for product A, but this difference was not clinically significant.
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Elastomers , Relaxation , Tooth Movement TechniquesABSTRACT
Objective To observe the effect of paraformaldehyde fixation on viscoelastic properties of the vertebrae in rats, so as to find the best methods of preserving cancellous bone samples from the perspective of biomechanics. Methods Twenty 8-week-old healthy female Sprague-Dawley rats were selected, and their whole L4 and L5 vertebra were separated by surgery. The total 40 vertebrae were randomly and evenly divided into experimental group and control group. The experimental group was fixed with 4% paraformaldehyde for 72 h, and the control group was transferred to 5 mL EP tube and cryopreserved at -20 ℃. Ten vertebrae were randomly selected from each group for stress relaxation and creep experiments. After 7 200 s, the samples were collected and their micro-structure changes were analyzed by micro-CT. Results The relaxation creep curve of experimental group was smoother than that of control group, the time to reach steady state was shorter, and the total amount of relaxation creep at 500 s and 7 200 s was significantly decreased (P<0.01). Micro-CT results showed that relaxation and creep experiment could cause trabecular rupture, and trabecular damage was more severe in experimental group than that in control group. Conclusions Paraformaldehyde significantly reduce the viscoelasticity of rat vertebrae, and it is more easily to cause microstructure damage under mechanical stimulation, which is detrimental to cancellous bone preservation.
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Objective To construct a variable-parameter nonlinear model for the research on stress relaxation properties of human intervertebral disc under the cyclic strain. Methods The variable-parameter nonlinear model combined with experimental data on stress relaxation and creep response of the intervertebral disc were used to study stress relaxation properties under the cyclic strain and compare the differences of linear and nonlinear model in viscoelastic properties of the intervertebral disc. Results The cycle modulus and relaxation coefficient obtained by the variable-parameter nonlinear model under the of frequency 0.01 Hz was very close to the experimental data, and the cyclic modulus under the frequency of 0.1 and 1 Hz were also close to the experimental data, but the relaxation coefficient obtained in 0.1 and 1 Hz had serious distortion. Conclusions The intervertebral disc experiences a nonlinear stress behavior under the compression strain, so the variable-parameter nonlinear model is more suitable for studying the stress relaxation response of the intervertebral disc under the cyclic strain.
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Objective To evaluate the viscoelastic properties of porcine acellular dermal matrix (PADM) by comparison and analysis on physical parameters of the skin. Methods Full-thickness skin defects were performed on the back of white rabbits as wound models, and randomly divided these wounds into 3 groups according to different methods of skin grafting: PADM group (autogenous skin and PADM were grafted to the surface of the wound), TS group (autogenous skin was grafted in situ), NS group (normal skin as the control). Experiments on the stress-strain relationship of the implanted skin in the three groups were conducted. Results The curves of stress relaxation, creep, and stress-strain relationship showed that under a given stress, PADM group had the lowest strain, NS group had the highest strain, while the strain of TS group was in between the PADM and NS group. Conclusions The skin viscoelasticity mechanical model is a four-parameter solid model. When the skin wounds are grafted by using PADM which may have poor elasticity, its recovery capacity after deformation is also poor, and this is in agreement with the clinical results.
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Objective To compare the stress relaxation and mechanical properties of creep viscoelasticity in the young and old’s spine and to provide the reference for the mechanical mechanism of viscoelasticity of yellow ligament degeneration. Method Twenty specimens of elderly patients with degenerative lumbar yellow ligament degeneration as the older group, and twenty specimens of young patients with lumbar yellow ligament injuries as the youth group were used respectively. All the yellow ligament specimens were from the yellow ligament between the fourth and fifth of lumbar vertebra (L4~L5). Stress relaxation experiment and creep experiment were conducted on the specimens by the strain increasing speed as 1%/s and the stress increasing speed as 0.5 MPa/s respectively. The experimental temperature was (36.5±0.5) ℃ and the experimental time was 7 200 s. Ninety stress relaxation and creep data were collected, and the data were dealt with the statistical analysis and normalization methods. Results The 7 200 s stress of yellow ligament in the youth group decreased by 1.42 MPa, while the 7 200 s stress of yellow ligament in the older group decreased by 1.91 MPa. The 7 200 s strain of yellow ligament in the youth group increased by 3.39%, while the 7 200 s strain of yellow ligament in the older group increased by 2.07%. The 7 200 s stress in two groups all increased and the increased strain data showed statistically significance (P<0.05). The stress relaxation curve changed in the form of logarithm and the creep curve changed in the form of index. Conclusions The 7 200 s stress relaxation and creep in the yellow ligament of the older group all decreased and the mechanical properties of viscoelasticity changed.
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Objective To evaluate the static viscoelastic properties of the porous gradient UHMWPE material by the creep, stress relaxation and creep recovery tests. Method The porous gradient UHMWPE material was prepared by the template leaching (T-L) method. The porous structure was characterized by scanning electron microscopy(SEM) and the porosity and pore distribution of porous layer were measured by a pressure mercury analyzer. The creep, stress relaxation and creep recovery properties were tested by the experimental determination of flat indentation. Results The porous layer of T-L UHMWPE was well bonded with the substrate material. With the increase of NaCl content, the porosity and pore size distribution scope improved clearly. Creep deformation and modulus presented a nonlinear increase with time and the stress and modulus of stress relaxation showed a nonlinear decrease with time. The buffering capacity for impact loads of porous gradient UHMWPE was significantly higher than the compact UHMWPE. Experimental results showed that the creep recovery properties of the reasonable porosity of U50 and U60 samples were almost similar to the compact UHMWPE material. But the higher porosity caused the increase of the plastic deformation. Conclusions The porous structure of UHMWPE effectively increased the elastic property and strain response sensitivity, which may be beneficial to improve the lubrication of frictional contact surfaces and reduce the wear of artificial joints.
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AIM: To explore the characteristic of relaxation of porcine cornea after LASIKMETHODS: Usual LASIK was performed on fresh porcine corneas with stable intraoculer pressure (IOP) maintained through optic nerve irrigation. The ablation depth on stroma is 30%, 50% and 70% respectively. Then the dumbbell-shaped corneal strip specimens were cut and stored in 20% Human Albumin solution for use (4℃).Sip up the Albumin solution on specimens, and fixed them on homemade jig. Stress relaxation tests were erformed on Tytron250 Dynamics Experiment System. the loading speed was 385mm/min,extending ratewas 1.5, and relaxation time was 1 000s. The data werecollected electronically and automatically.RESULTS: In LASIK procedure, though a single flap-cutting can cause a little reduction of corneal stress relaxation (P＜0.05,P=0.49), the cornea may still remain its property of visco-elasticity. When ablation depth was 30% or more, corneal stress relaxation decreased to almost one half (P＜0.01).The change of corneal stress relaxation degree in vertical meridian specimen was lower than that in horizontal specimen, especially when ablation depth was 70%, and it's statistically significant (P＜0.001). In LASIK operation, the more depth the ablation, the more reduce the stress relaxation degree, and it's easy to cause deformation and creep deformation.CONCLUSION: The changes of the stress relaxation in verticai and longitudinal meridian specimens are similar, and slightly obvious in longitudinal specimen, especially in 70%ablation group.