Sex- and age-dependent skin mechanics-A detailed look in mice.

Skin aging is of immense societal and, thus, scientific interest. Because mechanics play a critical role in skin's function, a plethora of studies have investigated age-induced changes in skin mechanics. Nonetheless, much remains to be learned about the mechanics of aging skin. This is especially true when considering sex as a biological variable. In our work, we set out to answer some of these questions using mice as a model system. Specifically, we combined mechanical testing, histology, collagen assays, and two-photon microscopy to identify age- and sex-dependent changes in skin mechanics and to relate them to structural, microstructural, and compositional factors. Our work revealed that skin stiffness, thickness, and collagen content all decreased with age and were sex dependent. Interestingly, sex differences in stiffness were age induced. We hope our findings not only further our fundamental understanding of skin aging but also highlight both age and sex as important variables when conducting studies on skin mechanics. STATEMENT OF SIGNIFICANCE: Our work addresses the question, "How do sex and age affect the mechanics of skin?" Answering this question is of both scientific and societal importance. We do so in mice as a model system. Thereby, we hope to add clarity to a body of literature that appears divided on the effect of both factors. Our findings have important implications for those studying age and sex differences, especially in mice as a model system.

Biomechanical characterization of the passive porcine stomach.

The complex mechanics of the gastric wall facilitates the main digestive tasks of the stomach. However, the interplay between the mechanical properties of the stomach, its microstructure, and its vital functions is not yet fully understood. Importantly, the pig animal model is widely used in biomedical research for preliminary or ethically prohibited studies of the human digestion system. Therefore, this study aims to thoroughly characterize the mechanical behavior and microstructure of the porcine stomach. For this purpose, multiple quasi-static mechanical tests were carried out with three different loading modes, i.e., planar biaxial extension, radial compression, and simple shear. Stress-relaxation tests complemented the quasi-static experiments to evaluate the deformation and strain-dependent viscoelastic properties. Each experiment was conducted on specimens of the complete stomach wall and two separate layers, mucosa and muscularis, from each of the three gastric regions, i.e., fundus, body, and antrum. The significant preconditioning effects and the considerable regional and layer-specific differences in the tissue response were analyzed. Furthermore, the mechanical experiments were complemented with histology to examine the influence of the microstructural composition on the macrostructural mechanical response and vice versa. Importantly, the shear tests showed lower stresses in the complete wall compared to the single layers which the loose network of submucosal collagen might explain. Also, the stratum arrangement of the muscularis might explain mechanical anisotropy during tensile tests. This study shows that gastric tissue is characterized by a highly heterogeneous microstructure with regional variations in layer composition reflecting not only functional differences but also diverse mechanical behavior. STATEMENT OF SIGNIFICANCE: Unfortunately, only few experimental data on gastric tissue are available for an adequate material parameter and model estimation. The present study therefore combines layer- and region-specific stomach wall mechanics obtained under multiple loading conditions with histological insights into the heterogeneous microstructure. On the one hand, the extensive data sets of this study expand our understanding of the interplay between gastric mechanics, motility and functionality, which could help to identify and treat associated pathologies. On the other hand, such data sets are of high relevance for the constitutive modeling of stomach tissue, and its application in the field of medical engineering, e.g., in the development of surgical staplers and the improvement of bariatric surgical interventions.

Design and mechanical testing of an adjustable posterior leaf spring ankle-foot orthosis for patients with drop foot.

OBJECTIVES: This study aimed to design an adjustable posterior leaf spring (PLS) ankle-foot orthosis (AFO) with an affordable material in low-income countries and investigate the mechanical properties between an adjustable PLS AFO and a standard PLS AFO. STUDY DESIGN: Static and dynamic mechanical testing. METHODS: This study preliminarily tested a new adjustable PLS AFO against a standard PLS AFO. Each AFO design was tested with mechanical testing using an Instron 8801 universal testing machine. RESULTS: The stiffness value of the adjustable PLS AFO was greater than that of the standard PLS AFO during the static loading test. The energy dissipated ratios were lower in the adjustable PLS AFO than in the standard PLS AFO. After 110,000 cycles of fatigue testing, the distal rivet of the adjustable PLS AFO became loose, although the standard PLS AFO had no problem. CONCLUSIONS: The novel adjustable PLS AFO achieved noninferior mechanical properties except fatigue strength. The connecting area always initiated fatigue failure. It is suggested that this area must be prevented for stress concentration. As a preliminary study, this study is fundamental for future studies.

Bone Plates Runout Prediction Through Tensile Strength and Geometric Properties for Regulatory Mechanical Testing.

Mechanical tests on bone plates are mandatory for regulatory purposes and, typically, the ASTM F382 standard is used, which involves a four-point bending test setup to evaluate the cyclic bending fatigue performance of the bone plate. These test campaigns require a considerable financial outlay and long execution times; therefore, an accurate prediction of experimental outcomes can reduce test runtime with beneficial cost cuts for manufacturers. Hence, an analytical framework is here proposed for the direct estimation of the maximum bending moment of a bone plate under fatigue loading, to guide the identification of the runout load for regulatory testing. Eleven bone plates awaiting certification were subjected to a comprehensive testing campaign following ASTM F382 protocols to evaluate their static and fatigue bending properties. An analytical prediction of the maximum bending moment was subsequently implemented based on ultimate strength and plate geometry. The experimental loads obtained from fatigue testing were then used to verify the prediction accuracy of the analytical approach. Results showed promising predictive ability, with R2 coefficients above 0.95 in the runout condition, with potential impact in reducing the experimental tests needed for the CE marking of bone plates.

Development of a multilayer fetal membrane material model calibrated using bulge inflation mechanical tests.

The fetal membranes are an essential mechanical structure for pregnancy, protecting the developing fetus in an amniotic fluid environment and rupturing before birth. In cooperation with the cervix and the uterus, the fetal membranes support the mechanical loads of pregnancy. Structurally, the fetal membranes comprise two main layers: the amnion and the chorion. The mechanical characterization of each layer is crucial to understanding how each layer contributes to the structural performance of the whole membrane. The in-vivo mechanical loading of the fetal membranes and the amount of tissue stress generated in each layer throughout gestation remains poorly understood, as it is difficult to perform direct measurements on pregnant patients. Finite element analysis of pregnancy offers a computational method to explore how anatomical and tissue remodeling factors influence the load-sharing of the uterus, cervix, and fetal membranes. To aid in the formulation of such computational models of pregnancy, this work develops a fiber-based multilayer fetal membrane model that captures its response to previously published bulge inflation loading data. First, material models for the amnion, chorion, and maternal decidua are formulated, informed, and validated by published data. Then, the behavior of the fetal membrane as a layered structure was analyzed, focusing on the respective stress distribution and thickness variation in each layer. The layered computational model captures the overall behavior of the fetal membranes, with the amnion being the mechanically dominant layer. The inclusion of fibers in the amnion material model is an important factor in obtaining reliable fetal membrane behavior according to the experimental dataset. These results highlight the potential of this layered model to be integrated into larger biomechanical models of the gravid uterus and cervix to study the mechanical mechanisms of preterm birth.

Comparative Mechanical Testing for Digitally Produced Provisional Fixed Partial Dentures vs the Conventional Method: An In Vitro Study

PURPOSE: To examine and compare the fracture strength of implant-cemented fixed partial denture (FPD) prostheses fabricated with digital vs conventional chairside methods. MATERIALS AND METHODS: Three groups of seven specimens each were produced: group A (3D printing); group B (milling); and group C (conventional chairside manufacturing), which served as a control. All groups were cemented to standard implant abutments placed in artificial bone blocks. Fracture strength testing was performed using a universal testing machine. Statistical analysis of the resultant maximum forces was performed using SPSS version 25 software (Mann- Whitney U test, P < .05). RESULTS: The mean fracture load value of the group A FPDs was 260.14 N ± 28.88, for group B was 663.57 N ± 140.55, and for group C was 266.65 N ± 63.66. CONCLUSIONS: Milled provisional FPDs showed a higher fracture resistance compared to 3D-printed and control groups. However, no such difference could be detected between the 3D-printed and control groups.

Recent Advances in Nanomechanical Measurements and Their Application for Pharmaceutical Crystals.

Mechanical behavior of pharmaceutical crystals directly impacts the formulation development and manufacturing of drug products. The understanding of crystal structure-mechanical behavior of pharmaceutical and molecular crystals has recently gained substantial attention among pharmaceutical and materials scientists with the advent of advanced nanomechanical testing instruments like nanoindentation. For the past few decades, instrumented nanoindentation was a popular technique for measuring the mechanical properties of thin films and small-length scale materials. More recently it is being implemented to investigate the mechanical properties of pharmaceutical crystals. Integration of correlative microscopy techniques and environmental control opened the door for advanced structure-property correlation under processing conditions. Preventing the degradation of active pharmaceutical ingredients from external factors such as humidity, temperature, or pressure is important during processing. This review deals with the recent developments in the synchronized nanomechanical measurements of pharmaceutical crystals toward the fast and effective development of high-quality pharmaceutical drug products. This review also summarizes some recent reports to intensify how one can design and control the nanomechanical properties of pharmaceutical solids. Measurement challenges and the scope for studying nanomechanical properties of pharmaceutical crystals using nanoindentation as a function of crystal structure and in turn to develop fundamental knowledge in the structure-property relationship with the implications for drug manufacturing and development are discussed in this review. This review further highlights recently developed capabilities in nanoindentation, for example, variable temperature nanoindentation testing, in situ imaging of the indented volume, and nanoindentation coupled Raman spectroscopy that can offer new quantitative details on nanomechanical behavior of crystals and will play a decisive role in the development of coherent theories for nanomechanical study of pharmaceutical crystal.

Efeito de diferentes protocolos de acabamento/polimento na resistência à fadiga e módulo de Weibull de diferentes tipos de zircônia / Effect of different finishing/polishing protocols on the fatigue strength and Weibull modulus of different types of zirconia

Objetivo: Avaliar o efeito de diferentes protocolos de acabamento/polimento na resistência à fadiga das novas gerações de zircônia. Materiais e Métodos: Foram confeccionados noventa (N=90) discos cerâmicos (Ø:12mm; 1,2 mm-ISO 6872), sendo 45 em zircônia ultratranslúcida (UT- VITA, Vita Zahnfabrik) e 45 de uma cerâmica de zircônia híbrida 3Y-TZP e 5Y-PSZ com gradiente de translucidez (GT- e.max Zircad prime GT, Ivoclar). Após a sinterização dos discos cerâmicos, estes foram divididos em 6 grupos (n=15), de acordo com fatores "cerâmica (zircônia UT e zircônia GT)" e "Protocolo de acabamento e polimento" (Pontas Diamantadas + Borrachas; Borrachas; Controle). Os discos foram submetidos ao ensaio de resistência à fadiga pelo método stepwise stress (5Hz para 10.000 ciclos), com um step de 57 MPa e 80 MPa, começando em 170 e 240 MPa, para a zircônia UT e GT, respectivamente, e prosseguindo até 100.000 ciclos ou a detecção da fratura. Foram realizadas também análises extras de Difração de raios X (DRX), microscopia de força atômica (AFM) e rugosidade superficial. Os dados de resistência à fadiga (MPa) e rugosidade (µm) foram avaliados estatisticamente através de ANOVA 2 fatores e teste de Tukey (5%). Foi realizada a análise de "Kaplan-Meier" seguida pelo teste de Mantel-Cox (Log Rank test) e pela comparação múltipla aos pares, todos com nível de significância de 5%. Além disso, também foi utilizada a análise de Weibull. Resultados: ANOVA (2 fatores) revelou que o fator "Protocolo de Acabamento e Polimento" (p=0,0006), "tipo de zircônia" (p=0,000) e a interação dos dois (p=0,0000) apresentaram significância estatística para a resistência à fadiga. A zircônia GT (761,47 MPa) obteve valores médios de resistência superiores a zircônia UT (385,87 MPa), independentemente do tipo de acabamento e polimento. Para zircônia UT o acabamento com pontas diamantadas reduziu os valores de resistência à fadiga (273,44DMPa) comparados ao grupo controle (503,96CMPa) e estatisticamente semelhante ao grupo polidos apenas com borracha (308,22DMPa). Na zircônia GT o polimento com borrachas melhorou a resistência do material (871,35A MPa) quando comparado ao grupo controle (664,29B MPa) que, por sua vez, foi semelhante ao grupo com acabamento com pontas diamantadas (748,78B MPa). Foi possível observar que o tipo de protocolo de acabamento e polimento influenciou a fratura em fadiga das duas cerâmicas, porém, o número de ciclo de sobrevida só foi significativo para a zircônia UT. Conclusão: A resistência à fadiga das duas zircônias foi influenciada pelos diferentes protocolos de acabamento e polimento. Para zircônia UT o polimento com borrachas e pontas diamantadas reduziu a resistência mecânica, já para a zircônia GT, todos os protocolos de acabamento e polimento melhoraram as propriedades mecânicas do material (AU).

Objective: To evaluate the effect of different finishing/polishing protocols on the fatigue strength of new generations of zirconia. Materials and Methods: Ninety (N=90) ceramic discs (Ø:12mm; 1.5mm-ISO 6872) were made, 45 of ultra-translucent zirconia (UT-VITA, Vita Zahnfabrik) and 45 of a hybrid zirconia ceramic 3Y-TZP and 5Y-PSZ with translucency gradient (GT-e.max Zircad prime GT, Ivoclar). After sintering, the ceramic discs were divided into 6 groups (n=15), according to the factors "ceramic (UT and GT)" and "Finishing and polishing protocol" (Diamond burs + Rubbers; Rubbers and Control). The discs were subjected to the fatigue resistance test by the stepwise stress method (5Hz for 10,000 cycles) with a step increment of 57 MPa and 80 MPa, starting at 170 and 240 MPa, for UT and GT zirconia, respectively, and continuing up to 100,000 cycles or failure detection. Complementary analyzes of X-ray Diffraction (XRD), atomic force microscopy (AFM) and surface roughness were also carried out. Results were statistically evaluated using 2-way ANOVA, Tukey test (5%) and Weibull analysis. The results of fatigue resistance and roughness were statistically evaluated using 2-way ANOVA and Tukey test (5%). The "Kaplan-Meier" analysis was performed followed by the Mantel-Cox test (Log Rank test) and the multiple comparison in pairs, all with a significance level of 5%. In addition, Weibull analysis was also carried out. Results: ANOVA (2-way) revealed that the factor "Finishing and Polishing Protocol" (p=0.0006), "type of zirconia" (p=0.000) and the interaction between them (p=0.0000) were statistically significant for resistance to fatigue. GT zirconia (761.47 MPa) had higher resistance values than UT zirconia (385.87 MPa), regardless of the type of finishing and polishing. For UT zirconia, finishing with diamond burs reduced the values of resistance to fatigue (273.44DMPa) compared to the control group (503.96CMPa) and statistically similar to the group polished only with rubber (308.22DMPa). In GT zirconia, polishing with rubbers improved the resistance of the material (871.35A MPa) when compared to the control group (664.29B MPa), which, in turn, was similar to the group finished with diamond burs (748.78B MPa). The Weibull modulus did not show statistical significance between the groups (p=0.300) but the characteristic strength showed statistically significant differences (p=0.0000). It was possible to observe that the type of finishing and polishing protocol influenced the fatigue fracture of the two ceramics, however, the number of survival cycles was only significant for the UT zirconia. Conclusion: The fatigue strength of the two zirconia was influenced by the different finishing and polishing protocols. For UT zirconia, polishing with rubbers and diamond burs reduced the mechanical resistance, whereas for GT zirconia, all finishing and polishing protocols improved the mechanical properties of the material (AU).

Guidelines for ex vivo mechanical testing of tendon.

Tendons are critical for the biomechanical function of joints. Tendons connect muscles to bones and allow for the transmission of muscle forces to facilitate joint motion. Therefore, characterizing the tensile mechanical properties of tendons is important for the assessment of functional tendon health and efficacy of treatments for acute and chronic injuries. In this guidelines paper, we review methodological considerations, testing protocols, and key outcome measures for mechanical testing of tendons. The goal of the paper is to present a simple set of guidelines to the nonexpert seeking to perform tendon mechanical tests. The suggested approaches provide rigorous and consistent methodologies for standardized biomechanical characterization of tendon and reporting requirements across laboratories.

Subsidence Performance of the Bioactive Glass-Ceramic (CaO-SiO2-P2O5-B2O3) Spacer in Terms of Modulus of Elasticity and Contact Area: Mechanical Test and Finite Element Analysis.

OBJECTIVE: The objective of this study is to evaluate the subsidence performance of a bioactive glass-ceramic (CaO-SiO2-P2O5-B2O3) spacer in terms of its modulus of elasticity and contact area using mechanical tests and finite element analysis. METHODS: Three spacer three-dimensional models (Polyether ether ketone [PEEK]-C: PEEK spacer with a small contact area; PEEK-NF: PEEK spacer with a large contact area; and Bioactive glass [BGS]-NF: bioactive glass-ceramic spacer with a large contact area) are constructed and placed between bone blocks for compression analysis. The stress distribution, peak von Mises stress, and reaction force generated in the bone block are predicted by applying a compressive load. Subsidence tests are conducted for three spacer models in accordance with ASTM F2267. Three types of blocks measuring 8, 10, and 15 pounds per cubic foot are used to account for the various bone qualities of patients. A statistical analysis of the results is conducted using a one-way Analysis of variance and post hoc analysis (Tukey's Honestly Significant Difference) by measuring the stiffness and yield load. RESULTS: The stress distribution, peak von Mises stress, and reaction force predicted via the finite element analysis are the highest for PEEK-C, whereas they are similar for PEEK-NF and BGS-NF. Results of mechanical tests show that the stiffness and yield load of PEEK-C are the lowest, whereas those of PEEK-NF and BGS-NF are similar. CONCLUSIONS: The main factor affecting subsidence performance is the contact area. Therefore, bioactive glass-ceramic spacers exhibit a larger contact area and better subsidence performance than conventional spacers.

Static strength of lower-limb prosthetic sockets: An exploratory study on the influence of stratigraphy, distal adapter and lamination resin.

Knowledge about the mechanical properties of lower-limb prosthetic sockets fabricated with resin infusion lamination and composite materials is limited. Therefore, sockets can be subject to mechanical failure and over-dimensioning, both of which can have severe consequences for patients. For this reason, an exploratory study was conducted to analyze the effect of stratigraphy (layup and fibers), matrix (resin) and mechanical connection (socket distal adapter) on socket static strength, with the objectives of: 1) implementing a mechanical testing system for lower-limb prosthetic sockets based on ISO 10328:2016 and provide the mechanical design of the loading plates, 2) apply the testing system to a series of laminated sockets, and 3) for each type of distal adapter, identify the combinations of stratigraphy and matrix with acceptable strength and minimum weight. Twenty-three laminated sockets were produced and tested. Sixteen met the required strength, with ten exhibiting an excessive weight. Among the remaining six, four combinations of stratigraphy and resin were identified as best option, as they all overcame ISO 10328 P6 loading level and weighted less than 600 g. The selected stratigraphies had limited or absent amount of Perlon stockinettes, which seems to increase weight without enhancing the mechanical strength. Sockets based on Ossur MSS braids and connector show the best compromise between strength and weight when the amount of carbon braids is halved.

Creep mechanical tests and shear rheological model of the anchorage rock mass under water‒rock coupling.

The development of deep geotechnical engineering is restricted by the complex geological conditions of deep rock masses and the unknown creep mechanism of rock in water-rich environments. To study the shear creep deformation law of the anchoring rock mass under different water content conditions, marble was used as the bedrock to make anchoring specimens, and shear creep tests of the anchoring rock mass under different water contents were carried out. The influence of water content on rock rheological characteristics is explored by analysing the related mechanical properties of the anchorage rock mass. The coupling model of the anchorage rock mass can be obtained by connecting the nonlinear rheological element and the coupling model of the anchorage rock mass in series. Related studies show that (1) shear creep curves of anchorage rock masses under different water contents have typical creep characteristics, including decay, stability and acceleration stages. The creep deformation of the specimens can be improved with increasing moisture content. (2) The long-term strength of the anchorage rock mass shows an opposite change law with increasing water content. The creep rate of the curve increases gradually with increasing water content. The creep rate curve shows a U-shaped change under high stress. (3) The nonlinear rheological element can explain the creep deformation law of rock in the acceleration stage. By connecting the nonlinear rheological element with the coupled model of anchoring rock mass in series, the coupled model of water‒rock under water cut conditions can be obtained. The model can be used to study and analyse the whole process of shear creep of an anchored rock mass under different water contents. This study can provide theoretical support for the stability analysis of anchor support tunnel engineering under water cut conditions.

Numerical simulation of mechanical tests on a living skin using anisotropic hyperelastic law.

The skin is a living tissue that behaves in a hyperelastic and anisotropic way. A constitutive law called HGO-Yeoh is proposed to model the skin by improving the classical HGO constitutive law. This model is implemented in a finite element code FER "Finite Element Research" to benefit from its tools, including the bipotential contact method, a very efficient function coupling contact and friction. Identifying the skin-related material parameters is done through an optimisation procedure using analytic and experimental data. A tensile test is simulated using the codes FER and ANSYS. Then, the results are compared with the experimental data. Finally, a simulation of an indentation test using a bipotential contact law is done.

Ex Vivo Analysis of an Association of Mechanical Strength of Dilated Ascending Aorta with Tissue Matrix Metalloproteinases and Cytokines.

We analyzed the associations of the mechanical strength of dilated ascending aorta wall (intraoperative samples from 30 patients with non-syndromic aneurysms) with tissue MMPs and the cytokine system. Some samples were stretched to break on an Instron 3343 testing machine and the tensile strength was calculated; others were homogenized and the concentrations of MMP-1, MMP-2, MMP-7, their inhibitors (TIMP-1 and TIMP-2), and pro- and anti-inflammatory cytokines were determined by ELISA. Direct correlations between aortic tensile strength and concentrations of IL-10 (r=0.46), TNFα (r=0.60), and vessel diameter (r=0.67) and an inverse correlation with patient's age (r=-0.59) were revealed. Compensatory mechanisms supporting the strength of the ascending aortic aneurysm are possible. No associations of MMP-1, MMP-7, TIMP-1, and TIMP-2 with tensile strength and aortic diameter were found.

Mechanical testing of transtibial prosthetic sockets: A discussion paper from the American Orthotic and Prosthetic Association Socket Guidance Workgroup.

BACKGROUND: The advent of novel manufacturing technologies, materials, and socket design concepts could introduce risks to prosthetic limb users, as the existing knowledge base for safe fabrication may not apply. Moreover, although structural test standards exist for mass-produced prosthetic components, they are not applicable to prosthetic sockets. METHODS: The "AOPA Socket Guidance Workgroup" was formed in 2020 to provide the prosthetic community with evidence-based clinical best practices and methods in the field of prosthetic socket structural analysis. This multidisciplinary expert workgroup undertook a critical analysis of the knowledge gaps regarding the requirements for mechanical testing of lower limb prosthetic sockets. RESULTS: The Workgroup identified knowledge gaps in 4 domains. Domain 1 describes the shape and composition of a mock residual limb, required to support and generate in vivo representative loading within the socket. Domain 2 concerns prosthetic socket coordinate systems and alignment. Domain 3 regards the components and requirements of test specimens. Finally, Domain 4 considers test conditions, loading parameters, and acceptance criteria. CONCLUSIONS: This paper describes these knowledge gaps in detail and recommends potential solution approaches based on literature review, group consensus around existing knowledge, or the formation of new study groups to fill each knowledge gap. Our intent is for the recommendations arising from this paper to support the community (e.g., researchers in the clinic, academia, industry, and funders) in addressing these knowledge gaps.

A Bayesian constitutive model selection framework for biaxial mechanical testing of planar soft tissues: Application to porcine aortic valves.

A variety of constitutive models have been developed for soft tissue mechanics. However, there is no established criterion to select a suitable model for a specific application. Although the model that best fits the experimental data can be deemed the most suitable model, this practice often can be insufficient given the inter-sample variability of experimental observations. Herein, we present a Bayesian approach to calculate the relative probabilities of constitutive models based on biaxial mechanical testing of tissue samples. Forty-six samples of porcine aortic valve tissue were tested using a biaxial stretching setup. For each sample, seven ratios of stresses along and perpendicular to the fiber direction were applied. The probabilities of eight invariant-based constitutive models were calculated based on the experimental data using the proposed model selection framework. The calculated probabilities showed that, out of the considered models and based on the information available through the utilized experimental dataset, the May-Newman model was the most probable model for the porcine aortic valve data. When the samples were further grouped into different cusp types, the May-Newman model remained the most probable for the left- and right-coronary cusps, whereas for non-coronary cusps two models were found to be equally probable: the Lee-Sacks model and the May-Newman model. This difference between cusp types was found to be associated with the first principal component analysis (PCA) mode, where this mode's amplitudes of the non-coronary and right-coronary cusps were found to be significantly different. Our results show that a PCA-based statistical model can capture significant variations in the mechanical properties of soft tissues. The presented framework is applicable to other tissue types, and has the potential to provide a structured and rational way of making simulations population-based.

The mechanical testing and performance analysis of three-dimensionally produced lingual retainers.

BACKGROUND: Introducing three-dimensional (3D) printing has opened new visions in the orthodontic field. This research evaluates three-dimensionally produced orthodontic retainers and their future possible uses. For this purpose, in vitro tests were performed for these groups, including bond strength, failure analysis, discoloration, and biodegradation. METHODS: A total of 30 specimens (n = 30), lower incisor human teeth, were randomly divided into three groups for a bond strength failure analysis (for each group n = 10). In the experimental groups, lingual retainers were fabricated using 3D systems (group 1 with 3D dental pen and group 2 with 3D-printed). In the control group (group 3), the retainer system was a combination of a wire and composite, which is being used worldwide. A total of 30 specimens (n = 30) from the 3D dental pen and 3D-printed for discoloration and biodegradation tests were divided into three groups (water, tea, and coffee). Data were analyzed using the Mann-Whitney U test, ANOVA, and chi-square test. RESULTS: For all parameters tested, significant differences were determined among groups. The 3D pen group had the highest score for bond strength, whereas discoloration differed significantly. CONCLUSIONS: According to the limitations of this research, 3D-printed retainers have the potential for clinical use in the near future.

Mechanical and Crack Propagating Behavior of Sierpinski Carpet Composites.

Fractals, mathematically defined as "self-similar subsets at different scales", are ubiquitous in nature despite their complexity in assembly and formulation. Fractal geometry formed by simple components has long been applied to many fields, from physics and chemistry to electronics and architecture. The Sierpinski carpet (SC), a fractal with a Hausdorff dimension of approximately 1.8933, has two-dimensional space-filling abilities and therefore provides many structural applications. However, few studies have investigated its mechanical properties and fracture behaviors. Here, utilizing the lattice spring model (LSM), we constructed SC composites with two base materials and simulated tensile tests to show how fractal iterations affect their mechanical properties and crack propagation. From observing the stress-strain responses, we find that, for either the soft-base or stiff-base SC composites, the second iteration has the optimal mechanical performance in the terms of stiffness, strength, and toughness compared to the composites with higher hierarchies. The reason behind this surprising result is that the largest stress intensities occur at the corners of the smallest squares in the middle zone, which consequently induces crack nucleation. We also find that the main crack tends to deflect locally in SC composites with a soft matrix, but in global main crack behavior, SC composites with a stiff matrix have a large equivalent crack deflection. Furthermore, considering the inherent anisotropy of SC composites, we rotated the samples by 45°. The results show that the tensile strength and toughness of rotated SC composites are inferior and the crack propagating behaviors are distinct from the standard SC composites. This finding infers advanced engineering for crack control and deflection by adjusting the orientation of SC composites. Overall, our study opens the door for future engineering applications in stretchable devices, seismic metamaterials, and structural materials with tunable properties and hierarchies.

Can machine learning accelerate soft material parameter identification from complex mechanical test data?

Identifying the constitutive parameters of soft materials often requires heterogeneous mechanical test modes, such as simple shear. In turn, interpreting the resulting complex deformations necessitates the use of inverse strategies that iteratively call forward finite element solutions. In the past, we have found that the cost of repeatedly solving non-trivial boundary value problems can be prohibitively expensive. In this current work, we leverage our prior experimentally derived mechanical test data to explore an alternative approach. Specifically, we investigate whether a machine learning-based approach can accelerate the process of identifying material parameters based on our mechanical test data. Toward this end, we pursue two different strategies. In the first strategy, we replace the forward finite element simulations within an iterative optimization framework with a machine learning-based metamodel. Here, we explore both Gaussian process regression and neural network metamodels. In the second strategy, we forgo the iterative optimization framework and use a stand alone neural network to predict the entire material parameter set directly from experimental results. We first evaluate both approaches with simple shear experiments on blood clot, an isotropic, homogeneous material. Next, we evaluate both approaches against simple shear and uniaxial loading experiments on right ventricular myocardium, an anisotropic, heterogeneous material. We find that replacing the forward finite element simulations with metamodels significantly accelerates the parameter identification process with excellent results in the case of blood clot, and with satisfying results in the case of right ventricular myocardium. On the other hand, we find that replacing the entire optimization framework with a neural network yielded unsatisfying results, especially for right ventricular myocardium. Overall, the importance of our work stems from providing a baseline example showing how machine learning can accelerate the process of material parameter identification for soft materials from complex mechanical data, and from providing an open access experimental and simulation dataset that may serve as a benchmark dataset for others interested in applying machine learning techniques to soft tissue biomechanics.