What is the influence of using generic material properties on the estimation of the pelvis sagging when sitting from a Finite Element model of the buttock region?

Ischial pressure sores are painful, slow healing wounds that develop during prolonged sitting. Its formation is associated with the high internal strains induced by the compression of the soft tissues under the ischium [1]. 3D Finite Element (FE) models have been developed to estimate internal strains in the subdermal soft tissues. Some authors have also investigated the influence of the material properties of the soft tissues [2]. However, the interval of variation of the parameters in these sensitivity studies are not necessarily representative of the variability of subgroups of population. In this contribution, we investigate the influence of using the material properties of one given individual (generic material properties) as representative of a population. The generic material properties were obtained by Finite Element Updating to fit the experimental sagging of the pelvis of one subject when sitting. The 3D subject-specific FE model was generated from the combination of bi-planar Radiography, ultrasound imaging and optical scanner and is composed of the pelvis (rigid body) and 3 homogeneous layers representing the muscle tissue, fat and skin. The adipose tissue and the muscle layer were modelled as an Ogden quasi-incompressible hyperelastic material. The same material parameters were used to estimate the pelvis sagging of 7 healthy subjects. The estimated sagging was compared to the experimental one measured by computing the vertical displacements of both ischial tuberosities visible on the radiographs before and after sitting (Figure 1). For 5 subjects, the differences between both were below 1mm. For the two other subjects, the differences were 4 and 6 mm. These findings suggest that using generic material properties allow to reproduce the biomechanical response of the buttock when sitting for healthy subjects. The same approach could be applied to spinal cord injury population, which will allow to clarify the necessity of personalizing the material properties in models developed for this population

Development and validation of a new methodology for the fast generation of patient-specific FE models of the buttock for pressure ulcer prevention.

Ischial pressure sores are painful, slow healing wounds that develop during prolonged sitting. Its formation is associated with the high internal strains induced by the compression of the soft tissues under the ischium [1]. Although, many 3D Finite Element (FE) models have been developed to predict the mechanical response of the subdermal soft tissues, they are always constructed from segmentation of MRI or CT-Scan acquisitions limiting the studies to only one individual and overlooking the inter-individual variability. In this contribution, we present a new methodology for a fast 3D FE model generation of the buttock for PU prevention. The 3D subject-specific FE model was generated from the combination of bi-planar Radiography, ultrasound imaging and optical scanner and is composed of the pelvis (rigid body) and 3 homogeneous layers representing the muscle tissue, fat and skin. The adipose tissue and the muscle layer were modelled as an Ogden quasi-incompressible hyperelastic material and the material properties were calibrated to fit the experimental data. The validation of the model was performed from external pressure measurement on a population of 6 healthy subjects. The mean difference of the median pressure was 0.32kPa (std 0.8kPa), showing good agreement between the experiments and FE models and representing 2% of the mean value. The low generation time of this model compared to existing methodologies will allow to investigate the influence of pelvis and buttock geometry on the biomechanical response of the subdermal soft tissues under the ischium during sitting

Feasibility of sub-dermal soft tissue deformation assessment using B-mode ultrasound for pressure ulcer prevention

Pressure Ulcer (PU) prevention remains a main public health issue. The physio-pathology of this injury is not fully understood, and a satisfactory therapy is currently not available. Recently, several works suggested that mechanical strains are responsible of deformation-induced damage involved in the initiation of Deep Tissue Injury (DTI). A better assessment of the internal behavior could allow to enhance the modeling of the transmission of loads into the different structures composing the buttock. A few studies focused on the experimental in vivo buttock deformation quantification using Magnetic Resonance Imaging (MRI), but its use has important drawbacks. In clinical practice, ultrasound imaging is an accessible, low cost, and real-time technic to study the soft tissue. The objective of the present work was to show the feasibility of using B-mode ultrasound imaging for the quantification of localised soft-tissue strains of buttock tissues during sitting. An original protocol was designed, and the intra-operator reliability of the method was assessed. Digital Image Correlation was used to compute the displacement field of the soft tissue of the buttock during a full realistic loading while sitting. Reference data of the strains in the frontal and sagittal planes under the ischium were reported for a population of 7 healthy subjects. The average of shear strains over the region of interest in the fat layer reached levels up to 117% higher than the damage thresholds previously quantified for the muscular tissue in rats. In addition, the observation of the muscles displacements seems to confirm previous results which already reported the absence of muscular tissue under the ischium in the seated position, questioning the assumption commonly made in Finite Element modeling that deep tissue injury initiates in the muscle underlying the bone.This work was supported by the Fondation de l'Avenir (grant number AP-RM-2016-030), by la Fondation des Arts et Métiers and the Fond de dotation Clinatec. The authors are also grateful to the ParisTech BiomecAM chair program on subject-specific musculoskeletal modeling

Is a simplified Finite Element model of the gluteus region able to capture the mechanical response of the internal soft tissues under compression?

Internal soft tissue strains have been shown to be one of the main factors responsible for the onset of Pressure Ulcers and to be representative of its risk of development. However, the estimation of this parameter using Finite Element (FE) analysis in clinical setups is currently hindered by costly acquisition, reconstruction and computation times. Ultrasound (US) imaging is a promising candidate for the clinical assessment of both morphological and material parameters. Method: The aim of this study was to investigate the ability of a local FE model of the region beneath the ischium with a limited number of parameters to capture the internal response of the gluteus region predicted by a complete 3D FE model. 26 local FE models were developed, and their predictions were compared to those of the patient-specific reference FE models in sitting position. Findings: A high correlation was observed (R = 0.90, p-value < 0.01). A sensitivity analysis showed that the most influent parameters were the mechanical behaviour of the muscle tissues, the ischium morphology and the external mechanical loading. Interpretation: Given the progress of US for capturing both morphological and material parameters, these results are promising because they open up the possibility to use personalised simplified FE models for risk estimation in daily clinical routine.This work was supported by the Fondation de l'Avenir (grant number AP-RM-2016-030), by la Fondation des Arts et Métiers and the Fonds de dotation Clinatec. The authors are also grateful to the ParisTech BiomecAM chair program on subject-specific musculoskeletal modelling

Hydrogels in aqueous media : from macroscopic adhesion to molecular mechanisms

L'adhésion d'hydrogels sur surfaces minces de polymères a été étudiée de manière systématique au moyen d'un test de contact plan-plan réalisé en milieu immergé. A l'échelle moléculaire, l'adhésion macroscopique se traduit à l'interface [gel/surface mince] par la formation d'interactions spécifiques réversibles (liaisons hydrogène, interactions électrostatiques). Nous nous sommes interrogés sur les paramètres clés qui pilotent la formation de ces interactions en solution aqueuse. Nous avons ainsi établi l'importance de la composition de l'hydrogel (concentration initiale en polymère et taux de réticulation), du type d'interactions physiques mises en jeu à l'interface et de la distance d'interpénétration des chaînes de polymères sur la probabilité de créer des interactions au niveau de l'interphase volumique. Par ailleurs, les résultats des suivis cinétiques d'adhésion in situ au cours du gonflement des gels ont permis de quantifier la perte d'adhésion entre leur état de préparation et leur équilibre de gonflement, survenant même dans le cas de dilutions relativement faibles. En cause, la cinétique de formation d'interactions multiples à l'interface [gel/surface mince] plus lente à l'équilibre de gonflement qu'à l'état de préparation. Toutefois en combinant des énergies de liaisons physiques élevées (interactions électrostatiques) à une distance d'interpénétration plus grande et à des effets de dissipation élastique importants (gel mince de polymère comme surface mince), nous avons montré qu'il est possible d'améliorer considérablement l'adhésion du système immergé tout en maintenant l'énergie d'adhésion constante, même à l'équilibre de gonflement.Adhesion of hydrogels on thin polymer surfaces has been studied systematically via an underwater flat-flat contact test. Macroscopic adhesion at the [gel/thin surface] interface is due to reversible and specific interactions (hydrogen bonds, electrostatic interactions) created at molecular scale. We wondered about the key parameters that control the formation of these interactions in aqueous solution. Thus, we have established the importance of the composition of the hydrogel (initial concentration of polymer and cross-linking ratio), of the nature of the physical interactions involved in the system and of the interpenetrating distance of polymer chains. Furthermore, the results of the kinetics studies of the evolution of adhesion properties during the swelling of the networks were helpful to quantify the loss of adhesion between state preparation and swelling equilibrium of hydrogels, occurring even in the case of relatively low dilution factors. The kinetics slowdown of the formation of multiple interactions at the [gel/thin surface] interface is involved in the decrease of the energy of adhesion measured at swelling equilibrium compared to state preparation.However by mixing physical bonds with higher energy (electrostatic interactions) at greater interpenetrating distance of chains and elastic dissipation effects (thin polymer gel as thin surface), we have significantly improved the underwater adhesion of the system, while retaining the energy of adhesion constant, even at swelling equilibrium

Hydrogels in aqueous media : from macroscopic adhesion to molecular mechanisms

L'adhésion d'hydrogels sur surfaces minces de polymères a été étudiée de manière systématique au moyen d'un test de contact plan-plan réalisé en milieu immergé. A l'échelle moléculaire, l'adhésion macroscopique se traduit à l'interface [gel/surface mince] par la formation d'interactions spécifiques réversibles (liaisons hydrogène, interactions électrostatiques). Nous nous sommes interrogés sur les paramètres clés qui pilotent la formation de ces interactions en solution aqueuse. Nous avons ainsi établi l'importance de la composition de l'hydrogel (concentration initiale en polymère et taux de réticulation), du type d'interactions physiques mises en jeu à l'interface et de la distance d'interpénétration des chaînes de polymères sur la probabilité de créer des interactions au niveau de l'interphase volumique. Par ailleurs, les résultats des suivis cinétiques d'adhésion in situ au cours du gonflement des gels ont permis de quantifier la perte d'adhésion entre leur état de préparation et leur équilibre de gonflement, survenant même dans le cas de dilutions relativement faibles. En cause, la cinétique de formation d'interactions multiples à l'interface [gel/surface mince] plus lente à l'équilibre de gonflement qu'à l'état de préparation. Toutefois en combinant des énergies de liaisons physiques élevées (interactions électrostatiques) à une distance d'interpénétration plus grande et à des effets de dissipation élastique importants (gel mince de polymère comme surface mince), nous avons montré qu'il est possible d'améliorer considérablement l'adhésion du système immergé tout en maintenant l'énergie d'adhésion constante, même à l'équilibre de gonflement.Adhesion of hydrogels on thin polymer surfaces has been studied systematically via an underwater flat-flat contact test. Macroscopic adhesion at the [gel/thin surface] interface is due to reversible and specific interactions (hydrogen bonds, electrostatic interactions) created at molecular scale. We wondered about the key parameters that control the formation of these interactions in aqueous solution. Thus, we have established the importance of the composition of the hydrogel (initial concentration of polymer and cross-linking ratio), of the nature of the physical interactions involved in the system and of the interpenetrating distance of polymer chains. Furthermore, the results of the kinetics studies of the evolution of adhesion properties during the swelling of the networks were helpful to quantify the loss of adhesion between state preparation and swelling equilibrium of hydrogels, occurring even in the case of relatively low dilution factors. The kinetics slowdown of the formation of multiple interactions at the [gel/thin surface] interface is involved in the decrease of the energy of adhesion measured at swelling equilibrium compared to state preparation.However by mixing physical bonds with higher energy (electrostatic interactions) at greater interpenetrating distance of chains and elastic dissipation effects (thin polymer gel as thin surface), we have significantly improved the underwater adhesion of the system, while retaining the energy of adhesion constant, even at swelling equilibrium

Hydrogels en milieux immergés : de l'adhésion macroscopique aux mécanismes moléculaires

Adhesion of hydrogels on thin polymer surfaces has been studied systematically via an underwater flat-flat contact test. Macroscopic adhesion at the [gel/thin surface] interface is due to reversible and specific interactions (hydrogen bonds, electrostatic interactions) created at molecular scale. We wondered about the key parameters that control the formation of these interactions in aqueous solution. Thus, we have established the importance of the composition of the hydrogel (initial concentration of polymer and cross-linking ratio), of the nature of the physical interactions involved in the system and of the interpenetrating distance of polymer chains. Furthermore, the results of the kinetics studies of the evolution of adhesion properties during the swelling of the networks were helpful to quantify the loss of adhesion between state preparation and swelling equilibrium of hydrogels, occurring even in the case of relatively low dilution factors. The kinetics slowdown of the formation of multiple interactions at the [gel/thin surface] interface is involved in the decrease of the energy of adhesion measured at swelling equilibrium compared to state preparation.However by mixing physical bonds with higher energy (electrostatic interactions) at greater interpenetrating distance of chains and elastic dissipation effects (thin polymer gel as thin surface), we have significantly improved the underwater adhesion of the system, while retaining the energy of adhesion constant, even at swelling equilibrium.L'adhésion d'hydrogels sur surfaces minces de polymères a été étudiée de manière systématique au moyen d'un test de contact plan-plan réalisé en milieu immergé. A l'échelle moléculaire, l'adhésion macroscopique se traduit à l'interface [gel/surface mince] par la formation d'interactions spécifiques réversibles (liaisons hydrogène, interactions électrostatiques). Nous nous sommes interrogés sur les paramètres clés qui pilotent la formation de ces interactions en solution aqueuse. Nous avons ainsi établi l'importance de la composition de l'hydrogel (concentration initiale en polymère et taux de réticulation), du type d'interactions physiques mises en jeu à l'interface et de la distance d'interpénétration des chaînes de polymères sur la probabilité de créer des interactions au niveau de l'interphase volumique. Par ailleurs, les résultats des suivis cinétiques d'adhésion in situ au cours du gonflement des gels ont permis de quantifier la perte d'adhésion entre leur état de préparation et leur équilibre de gonflement, survenant même dans le cas de dilutions relativement faibles. En cause, la cinétique de formation d'interactions multiples à l'interface [gel/surface mince] plus lente à l'équilibre de gonflement qu'à l'état de préparation. Toutefois en combinant des énergies de liaisons physiques élevées (interactions électrostatiques) à une distance d'interpénétration plus grande et à des effets de dissipation élastique importants (gel mince de polymère comme surface mince), nous avons montré qu'il est possible d'améliorer considérablement l'adhésion du système immergé tout en maintenant l'énergie d'adhésion constante, même à l'équilibre de gonflement

Thin Hydrogel-Elastomer Multilayer Encapsulation for Soft Electronics

In the exciting race to design and engineer biointegrated and body-like electronic systems, many efforts concentrate on the integration of hydrogels in electronic assemblies. The versatility of hydrogels chemistry combined with their tissue-mimicking properties inspires numerous demonstrations of hydrogel-based touch panels, robots, and sensors over the years. However, their long-term integration in a thin and functional electronic assembly remains a challenge: their sensitivity to both air-drying and water swelling leads to important volume change of the network that is incompatible with the cohesion of a multilayer system, and has irreversible impact on the electronic properties of the assembly. To tackle this issue, proposed is a method to fabricate a hydrogel-elastomer micrometric bilayer with a stable interface, using of a low-swelling type of hydrogel, i.e., poly(2-hydroxyethyl methacrylate) and silicone rubber. The bilayer can sustain multiple hydration/dehydration cycles without delamination and can be kept for several months in its dry configuration. Combined with soft metallization technology, the bilayer can be readily integrated into a soft electronic circuit thereby opening a technological route for microfabricated, on-demand morphing systems

Development and evaluation of a new methodology for the fast generation of patient-specific Finite Element models of the buttock for sitting-acquired deep tissue injury prevention

International audienceThe occurrence and management of Pressure Ulcers remain a major issue for patients with reduced mobility and neurosensory loss despite significant improvement in the prevention methods. These injuries are caused by biological cascades leading from a given mechanical loading state in tissues to irreversible tissue damage. Estimating the internal mechanical conditions within loaded soft tissues has the potential of improving the management and prevention of PU. Several Finite Element models of the buttock have therefore been proposed based on either MRI or CT-Scan data. However, because of the limited availability of MRI or CT-Scan systems and of the long segmentation time, all studies in the literature include the data of only one individual. Yet the inter-individual variability can’t be overlooked when dealing with patient specific estimation of internal tissue loading. As an alternative, this contribution focuses on the combined use of low-dose biplanar X-ray images, B-mode ultrasound images and optical scanner acquisitions in a non-weight-bearing sitting posture for the fast generation of patient-specific FE models of the buttock. Model calibration was performed based on Ischial Tuberosity sagging. Model evaluation was performed by comparing the simulated contact pressure with experimental observations on a population of 6 healthy subjects. Analysis of the models confirmed the high inter-individual variability of soft tissue response (maximum Green Lagrange shear strains of 213 ± 101% in the muscle). This methodology opens the way for investigating inter-individual factors influencing the soft tissue response during sitting and for providing tools to assess PU risk

Equilibrium and Out-of-Equilibrium Adherence of Hydrogels against Polymer Brushes

At low pH, underwater adherence between poly(N,N-dimethylacrylamide) hydrogels and poly(acrylic acid) brushes is due to the formation of multiple hydrogen bonds. The effect of the key parameters controlling the formation of these interactions (contact time and composition of the hydrogel) was investigated with a contact mechanics test using a flat probe. We specifically quantified the difference in adherence observed between the gel and the brush as gels were in their preparation state or in their equilibrated state. The progressive swelling to equilibrium of the gels results (for a fixed contact time) in a significant decrease in adherence even in the case of relatively low dilution factors. This adherence loss was attributed to the slowdown of the kinetics of formation of multiple H-bond interactions as the gel approached its equilibrated state. In both equilibrated and nonequilibrated conditions the energy of adherence scaled with the polymer concentration, independent of the cross-linking density of the hydrogel, suggesting that the Lake and Thomas amplification factor is not relevant for these weak bonds