Publications by authors named "Jacques M Huyghe"

25 Publications

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Corrigendum to "Piezoelectricity in the Intervertebral Disc" [J. Biomech. 102 (2020) 109622].

J Biomech 2022 May 18;139:111148. Epub 2022 May 18.

Bernal Institute, University of Limerick, Ireland; Department of Mechanical Engineering, Eindhoven University of Technology, The Netherlands.

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http://dx.doi.org/10.1016/j.jbiomech.2022.111148DOI Listing
May 2022

The strain-generated electrical potential in cartilaginous tissues: a role for piezoelectricity.

Biophys Rev 2021 Feb 19;13(1):91-100. Epub 2021 Feb 19.

Bernal Institute, University of Limerick, Limerick, Ireland.

The strain-generated potential (SGP) is a well-established mechanism in cartilaginous tissues whereby mechanical forces generate electrical potentials. In articular cartilage (AC) and the intervertebral disc (IVD), studies on the SGP have focused on fluid- and ionic-driven effects, namely Donnan, diffusion and streaming potentials. However, recent evidence has indicated a direct coupling between strain and electrical potential. Piezoelectricity is one such mechanism whereby deformation of most biological structures, like collagen, can directly generate an electrical potential. In this review, the SGP in AC and the IVD will be revisited in light of piezoelectricity and mechanotransduction. While the evidence base for physiologically significant piezoelectric responses in tissue is lacking, difficulties in quantifying the physiological response and imperfect measurement techniques may have underestimated the property. Hindering our understanding of the SGP further, numerical models to-date have negated ferroelectric effects in the SGP and have utilised classic Donnan theory that, as evidence argues, may be oversimplified. Moreover, changes in the SGP with degeneration due to an altered extracellular matrix (ECM) indicate that the significance of ionic-driven mechanisms may diminish relative to the piezoelectric response. The SGP, and these mechanisms behind it, are finally discussed in relation to the cell response.
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http://dx.doi.org/10.1007/s12551-021-00779-9DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7930161PMC
February 2021

The Importance of the Mixing Energy in Ionized Superabsorbent Polymer Swelling Models.

Polymers (Basel) 2020 Mar 7;12(3). Epub 2020 Mar 7.

Bernal Institute, University of Limerick, Castletroy, V94 T9PX Limerick, Ireland.

The Flory-Rehner theoretical description of the free energy in a hydrogel swelling model can be broken into two swelling components: the mixing energy and the ionic energy. Conventionally for ionized gels, the ionic energy is characterized as the main contributor to swelling and, therefore, the mixing energy is assumed negligible. However, this assumption is made at the equilibrium state and ignores the dynamics of gel swelling. Here, the influence of the mixing energy on swelling ionized gels is quantified through numerical simulations on sodium polyacrylate using a Mixed Hybrid Finite Element Method. For univalent and divalent solutions, at initial porosities greater than 0.90, the contribution of the mixing energy is negligible. However, at initial porosities less than 0.90, the total swelling pressure is significantly influenced by the mixing energy. Therefore, both ionic and mixing energies are required for the modeling of sodium polyacrylate ionized gel swelling. The numerical model results are in good agreement with the analytical solution as well as experimental swelling tests.
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http://dx.doi.org/10.3390/polym12030609DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7182907PMC
March 2020

Piezoelectricity in the Intervertebral disc.

J Biomech 2020 03 9;102:109622. Epub 2020 Jan 9.

Bernal Institute, University of Limerick, Ireland; Department of Mechanical Engineering, Eindhoven University of Technology, The Netherlands.

Lower back pain is a major global health challenge that can often be caused by degeneration of the Intervertebral Disc (IVD). While IVD biomechanics are a key factor in the degenerative cycle, many mechanotransduction pathways remain unknown, in particular the electro-mechanical coupling in the loaded tissue. However, despite evidence for a role in the mechanically-induced remodelling of similar tissue, piezoelectricity has been overlooked in the IVD. In this study, we investigate the piezoelectric properties of the Annulus Fibrosus (AF) and the Nucleus Pulposus (NP) by measuring the direct piezoelectric effect of mechanically-induced electrical potential change. To verify these findings, we conducted Piezoresponse Force Microscopy (PFM) to measure the inverse effect of electrically-induced deformation. We demonstrate that, for the first time, piezoelectricity is generated throughout the IVD. Piezoelectric effects were greater in the AF than the NP, owing to the organised collagen networks present. However, the piezoresponse found in the NP indicates piezoelectric properties of non-collagenous proteins that have not yet been studied. The voltage generated by longitudinal piezoelectricity in-vivo has been calculated to be ~1 nV locally, indicating that piezoelectric effects may directly affect cell alignment in the AF and may work in conjunction with streaming potentials throughout the IVD. In summary, we have highlighted an intricate electro-mechanical coupling that appears to have distinct physiological roles in the AF and NP. Further study is required to elucidate the cell response and determine the potential role of piezoelectric effects in regeneration and preventative measures from degeneration.
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http://dx.doi.org/10.1016/j.jbiomech.2020.109622DOI Listing
March 2020

Chemically Responsive Hydrogel Deformation Mechanics: A Review.

Molecules 2019 Sep 28;24(19). Epub 2019 Sep 28.

Bernal Institute, University of Limerick, V94 T9PX Limerick, Ireland.

A hydrogel is a polymeric three-dimensional network structure. The applications of this material type are diversified over a broad range of fields. Their soft nature and similarity to natural tissue allows for their use in tissue engineering, medical devices, agriculture, and industrial health products. However, as the demand for such materials increases, the need to understand the material mechanics is paramount across all fields. As a result, many attempts to numerically model the swelling and drying of chemically responsive hydrogels have been published. Material characterization of the mechanical properties of a gel bead under osmotic loading is difficult. As a result, much of the literature has implemented variants of swelling theories. Therefore, this article focuses on reviewing the current literature and outlining the numerical models of swelling hydrogels as a result of exposure to chemical stimuli. Furthermore, the experimental techniques attempting to quantify bulk gel mechanics are summarized. Finally, an overview on the mechanisms governing the formation of geometric surface instabilities during transient swelling of soft materials is provided.
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http://dx.doi.org/10.3390/molecules24193521DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6804226PMC
September 2019

Effects of Intrinsic Properties on Fracture Nucleation and Propagation in Swelling Hydrogels.

Polymers (Basel) 2019 May 27;11(5). Epub 2019 May 27.

Bernal Institute, University of Limerick, Limerick, V94 T9PX, Ireland.

In numerous industrial applications, the microstructure of materials is critical for performance. However, finite element models tend to average out the microstructure. Hence, finite element simulations are often unsuitable for optimisation of the microstructure. The present paper presents a modelling technique that addresses this limitation for superabsorbent polymers with a partially cross-linked surface layer. These are widely used in the industry for a variety of functions. Different designs of the cross-linked layer have different material properties, influencing the performance of the hydrogel. In this work, the effects of intrinsic properties on the fracture nucleation and propagation in cross-linked hydrogels are studied. The numerical implementation for crack propagation and nucleation is based on the framework of the extended finite element method and the enhanced local pressure model to capture the pressure difference and fluid flow between the crack and the hydrogel, and coupled with the cohesive method to achieve crack propagation without re-meshing. Two groups of numerical examples are given: (1) effects on crack propagation, and (2) effects on crack nucleation. Within each example, we studied the effects of the stiffness (shear modulus) and ultimate strength of the material separately. Simulations demonstrate that the crack behaviour is influenced by the intrinsic properties of the hydrogel, which gives numerical support for the structural design of the cross-linked hydrogel.
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http://dx.doi.org/10.3390/polym11050926DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6571733PMC
May 2019

On the Physics Underlying Longitudinal Capillary Recruitment.

Authors:
Jacques M Huyghe

Adv Exp Med Biol 2018;1097:191-200

Bernal Institute, University of Limerick, Limerick, Ireland.

Numerous researchers have found that capillary vessel haematocrit depends on the vasodilatory state of the arterioles. At rest, vessel haematocrit is down to 15 %, suggesting a red blood cell velocity three times higher than the plasma velocity. This finding is analysed in the context of present understanding of propulsion of red blood cells (RBCs) and plasma by means of the arteriovenous pressure gradient. Interfacial forces between the red blood cells and the plasma are proposed as a rational explanation of the observed red blood cell velocities. While the arteriovenous pressure gradient across the capillaries propels the red blood cell and the plasma jointly, interfacial forces along the red blood cell membrane can propel RBCs at the cost of the plasma. Different options are explored for the physical origin of these interfacial forces and oxygen gradients are found to be the most probable source.
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http://dx.doi.org/10.1007/978-3-319-96445-4_10DOI Listing
July 2019

A three-dimensional transient mixed hybrid finite element model for superabsorbent polymers with strain-dependent permeability.

Soft Matter 2018 May;14(19):3834-3848

Mechanical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands.

A hydrogel is a cross-linked polymer network with water as solvent. Industrially widely used superabsorbent polymers (SAP) are partially neutralized sodium polyacrylate hydrogels. The extremely large degree of swelling is one of the most distinctive characteristics of such hydrogels, as the volume increase can be about 30 times its original volume when exposed to physiological solution. The large deformation resulting from the swelling demands careful numerical treatment. In this work, we present a biphasic continuum-level swelling model using the mixed hybrid finite element method (MHFEM) in three dimensions. The hydraulic permeability is highly dependent on the swelling ratio, resulting in values that are orders of magnitude apart from each other. The property of the local mass conservation of MHFEM contributes to a more accurate calculation of the deformation as the permeability across the swelling gel in a transient state is highly non-uniform. We show that the proposed model is able to simulate the free swelling of a random-shaped gel and the squeezing of fluid out of a swollen gel. Finally, we make use of the proposed numerical model to study the onset of surface instability in transient swelling.
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http://dx.doi.org/10.1039/c7sm01587aDOI Listing
May 2018

Convection associated with exclusion zone formation in colloidal suspensions.

Soft Matter 2016 Jan;12(4):1127-32

Department of Mechanical Engineering, Materials Technology, Eindhoven University of Technology, Eindhoven, The Netherlands. and Bernal Institute, University of Limerick, Limerick, Ireland.

The long-range repulsion of colloids from various interfaces has been observed in a wide range of studies from different research disciplines. This so-called exclusion zone (EZ) formation occurs near surfaces such as hydrogels, polymers, or biological tissues. It was recently shown that the underlying physical mechanism leading to this long-range repulsion is a combination of ion-exchange at the interface, diffusion of ions, and diffusiophoresis of colloids in the resulting ion concentration gradients. In this paper, we show that the same ion concentration gradients that lead to exclusion zone formation also imply that diffusioosmosis near the walls of the sample cell must occur. This should lead to convective flow patterns that are directly associated with exclusion zone formation. We use multi-particle tracking to study the dynamics of particles during exclusion zone formation in detail, confirming that indeed two pronounced vortex-like convection rolls occur near the cell walls. These dramatic flow patterns persist for more than 4 hours, with the typical velocity decreasing as a function of time. We find that the flow velocity depends strongly on the surface properties of the sample cell walls, consistent with diffusioosmosis being the main physical mechanism that governs these convective flows.
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http://dx.doi.org/10.1039/c5sm01502bDOI Listing
January 2016

Long-range repulsion of colloids driven by ion exchange and diffusiophoresis.

Proc Natl Acad Sci U S A 2014 May 18;111(18):6554-9. Epub 2014 Apr 18.

Department of Mechanical Engineering and Institute for Complex Molecular Systems , Eindhoven University of Technology, 5612 AZ, Eindhoven, The Netherlands.

Interactions between surfaces and particles in aqueous suspension are usually limited to distances smaller than 1 μm. However, in a range of studies from different disciplines, repulsion of particles has been observed over distances of up to hundreds of micrometers, in the absence of any additional external fields. Although a range of hypotheses have been suggested to account for such behavior, the physical mechanisms responsible for the phenomenon still remain unclear. To identify and isolate these mechanisms, we perform detailed experiments on a well-defined experimental system, using a setup that minimizes the effects of gravity and convection. Our experiments clearly indicate that the observed long-range repulsion is driven by a combination of ion exchange, ion diffusion, and diffusiophoresis. We develop a simple model that accounts for our data; this description is expected to be directly applicable to a wide range of systems exhibiting similar long-range forces.
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http://dx.doi.org/10.1073/pnas.1322857111DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4020040PMC
May 2014

Ageing and degenerative changes of the intervertebral disc and their impact on spinal flexibility.

Eur Spine J 2014 Jun 31;23 Suppl 3:S324-32. Epub 2014 Jan 31.

IRCCS Istituto Ortopedico Galeazzi, via Galeazzi 4, 20161, Milan, Italy,

Purpose: Degeneration of the intervertebral disc is associated with various morphological changes of the disc itself and of the adjacent structures, such as reduction of the water content, collapse of the intervertebral space, disruption and tears, and osteophytes. These morphological changes of the disc are linked to alterations of the spine flexibility. This paper aims to review the literature about the ageing and degenerative changes of the intervertebral disc and their link with alterations in spinal biomechanics, with emphasis on flexibility.

Methods: Narrative literature review.

Results: Clinical instability of the motion segment, usually related to increased flexibility and hypothesized to be connected to early, mild disc degeneration and believed to be responsible for low back pain, was tested in numerous in vitro studies. Despite some disagreement in the findings, a trend toward spinal stiffening with the increasing degeneration was observed in most studies. Tests about tears and fissures showed inconsistent results, as well as for disc collapse and dehydration. Vertebral osteophytes were found to be effective in stabilizing the spine in bending motions.

Conclusions: The literature suggests that the degenerative changes of the intervertebral disc and surrounding structures lead to subtle alteration of the mechanical properties of the functional spinal unit. A trend toward spinal stiffening with the increasing degeneration has been observed in most studies.
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http://dx.doi.org/10.1007/s00586-014-3203-4DOI Listing
June 2014

Confined compression and torsion experiments on a pHEMA gel in various bath concentrations.

Biomech Model Mechanobiol 2013 Jun 25;12(3):617-26. Epub 2012 Aug 25.

Eindhoven, The Netherlands.

The constitutive behaviour of cartilaginous tissue is the result of complex interaction between electrical, chemical and mechanical forces. Electrostatic interactions between fixed charges and mobile ions are usually accounted for by means of Donnan osmotic pressure. Recent experimental data show, however, that the shear modulus of articular cartilage depends on ionic concentration even if the strain is kept constant. Poisson-Boltzmann simulations suggest that this dependence is intrinsic to the double-layer around the proteoglycan chains. In order to verify this premise, this study measures whether--at a given strain--this ionic concentration-dependent shear modulus is present in a polymerized hydroxy-ethyl-methacrylate gel or not. A combined 1D confined compression and torque experiment is performed on a thin cylindrical hydrogel sample, which is brought in equilibrium with, respectively, 1, 0.1 and 0.03 M NaCl. The sample was placed in a chamber that consists of a stainless steel ring placed on a sintered glass filter, and on top a sintered glass piston. Stepwise ionic loading was cascaded by stepwise 1D compression, measuring the total stress after equilibration of the sample. In addition, a torque experiment was interweaved by applying a harmonic angular displacement and measuring the torque, revealing the relation between aggregate shear modulus and salt concentration at a given strain.
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http://dx.doi.org/10.1007/s10237-012-0429-0DOI Listing
June 2013

Design of next generation total disk replacements.

J Biomech 2012 Jan 27;45(1):134-40. Epub 2011 Oct 27.

Department of Biomedical Engineering, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands.

To improve the treatments for low back pain, new designs of total disk replacement have been proposed. The question is how well these designs can act as a functional replacement of the intervertebral disk. Four finite element models were made, for four different design concepts, to determine how well they can mimic the physiological intervertebral disk mechanical function. The four designs were a homogenous elastomer, a multi-stiffness elastomer, an elastomer with fiber jacket, and a hydrogel with fiber jacket. The best material properties of the four models were determined by optimizing the model behavior to match the behavior of the intervertebral disk in flexion-extension, axial rotation, and lateral bending. It was shown that neither a homogeneous elastomer nor a multi-stiffness elastomer could mimic the non-linear behavior within the physiological range of motion. Including a fiber jacket around an elastomer allowed for physiological motion in all degrees of freedom. Replacing the elastomer by a hydrogel yielded similar good behavior. Mimicking the non-linear behavior of the intervertebral disk, in the physiological range of motion is essential in maintaining and restoring spinal motion and in protecting surrounding tissues like the facet joints or adjacent segments. This was accomplished with designs mimicking the function of the annulus fibrosus.
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http://dx.doi.org/10.1016/j.jbiomech.2011.09.017DOI Listing
January 2012

Biomechanical behavior of a biomimetic artificial intervertebral disc.

Spine (Phila Pa 1976) 2012 Mar;37(6):E367-73

Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands.

Study Design: The biomechanical behavior of a biomimetic artificial intervertebral disc (AID) was characterized in vitro in axial compression and compared with natural disc behavior.

Objective: To evaluate the strength and durability of a novel biomimetic AID and to demonstrate whether its axial deformation behavior is similar to that of a natural disc.

Summary Of Background Data: Current clinically used AIDs have reasonable success rates. However, because of their nonphysiological design, spinal mechanics are altered. To avoid long-term complications, a novel biomimetic AID, with a nucleus-annulus structure and osmotic swelling properties has been developed.

Methods: Eighteen AIDs in total were tested in axial compression. Six were loaded monotonically to determine strength. Six were tested in fatigue (600-6000 N). Another 6 were used to characterize the axial creep and dynamic behavior (0.01-10 Hz). Creep and dynamic response were also determined for 4 AIDs after fatigue loading.

Results: The AIDs remained intact up to 15 kN and 10 million cycles. The creep and dynamic behavior were similar to the natural disc behavior, except for hysteresis, which was 20% to 30% higher. After fatigue, creep decreased from 4% to 1%, stiffness increased 2-fold, and hysteresis was reduced to that for a normal disc.

Conclusion: A strong and durable AID design was introduced. Compared with current clinical articulating AIDs, this biomimetic AID introduces the natural disc annulus-nucleus structure, resulting in axial behavior closer to that of the natural disc.
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http://dx.doi.org/10.1097/BRS.0b013e3182326305DOI Listing
March 2012

Biaxial testing of canine annulus fibrosus tissue under changing salt concentrations.

Authors:
Jacques M Huyghe

An Acad Bras Cienc 2010 Mar;82(1):145-51

Department of Biomedical Engineering, Engineering Mechanics Institute, Eindhoven University of Technology, Eindhoven, The Netherlands.

The in vivo mechanics of the annulus fibrosus of the intervertebral disc is one of biaxial rather than uniaxial loading. The material properties of the annulus are intimately linked to the osmolarity in the tissue. This paper presents biaxial relaxation experiments of canine annulus fibrosus tissue under stepwise changes of external salt concentration. The force tracings show that stresses are strongly dependent on time, salt concentration and orientation. The force tracing signature of a response to a change in strain, is one of a jump in stress that relaxes partly as the new strain is maintained. The force tracing signature of a stepwise change in salt concentration is a progressive monotonous change in stress towards a new equilibrium value. Although the number of samples does not allow any definitive quantitative conclusions, the trends may shed light on the complex interaction among the directionality of forces, strains and fiber orientation on one hand, and on the other hand, the osmolarity of the tissue. The dual response to a change in strain is understood as an immediate response before fluid flows in or out of the tissue, followed by a progressive readjustment of the fluid content in time because of the gradient in fluid chemical potential between the tissue and the surrounding solution.
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http://dx.doi.org/10.1590/s0001-37652010000100012DOI Listing
March 2010

Mechanisms that play a role in the maintenance of the calcium gradient in the epidermis.

Skin Res Technol 2007 Nov;13(4):369-76

Laboratory for Biomechanics and Tissue Engineering, Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands.

Background/purpose: Calcium regulates the proliferation and differentiation of keratinocytes and plays a role in restoration of the epidermal barrier function. The factors that maintain the calcium gradient in vivo are still unknown. A numerical model may give more insight into transport processes that maintain the epidermal calcium gradient.

Methods: In this study, transport of free calcium in the epidermis is described with diffusion, convection and electrophoresis. Binding and release of calcium results in equilibrium between free and bound calcium. The physiological epidermal calcium gradient as well as the calcium concentration in a damaged epidermis are modeled.

Results: The typical shape of the calcium gradient in the epidermis, as found in experimental studies, was maintained when separate formulations were used for free and bound calcium. Application of damage results in a decrease of the calcium concentration, especially in the upper living epidermis. Using this model, an estimate could be made about the fraction bound calcium in the epidermis.

Conclusion: The typical shape of the gradient is predominantly determined by the bound calcium concentration. For both a normal and a damaged epidermis, the concentration of free calcium is mainly determined by electrophoresis in the living epidermis, whereas in the largest part of the stratum corneum diffusion is the most important factor. The convection that was determined by the transepidermal water loss did not have an effect on the calcium concentration.
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http://dx.doi.org/10.1111/j.1600-0846.2007.00239.xDOI Listing
November 2007

Are disc pressure, stress, and osmolarity affected by intra- and extrafibrillar fluid exchange?

J Orthop Res 2007 Oct;25(10):1317-24

Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands.

Because extrafibrillar water content dictates extrafibrillar osmolarity, we aimed to determine the influence of intra- and extrafibrillar fluid exchange on intradiscal pressures and stresses. As experimental results showed that extrafibrillar osmolarity affects intervertebral disc cell gene expression and crack propagation, quantification of the effects of changes in intra- and extrafibrillar fluid exchange is physiologically relevant. Therefore, our 3D osmoviscoelastic finite element (FE) model of the intervertebral disc was extended to include the intra- and extrafibrillar water differentiation. Two simulations were performed, one without intrafibrillar fluid and one with intrafibrillar fluid fraction as a function of the extrafibrillar osmotic pressure. The intrafibrillar fluid fraction as a function of the extrafibrillar osmotic pressure was exponentially fitted to human data and implemented into the model. Because of the low collagen content in the nucleus, no noticeable differences in intradiscal pressure estimation were observed. However, values of extrafibrillar osmolarity, hydrostatic pressure, and the total tissue stress calculated for the annulus were clearly different. Stresses, hydrostatic pressure, and osmolarity were underestimated when the intrafibrillar water value was neglected. As the loading increased, the discrepancies increased. In conclusion, the distribution of pressure and osmolarity in the disc is affected by intra- and extrafibrillar water exchange.
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http://dx.doi.org/10.1002/jor.20443DOI Listing
October 2007

Influence of osmotic pressure changes on the opening of existing cracks in 2 intervertebral disc models.

Spine (Phila Pa 1976) 2006 Jul;31(16):1783-8

Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands.

Study Design: An experimental hydrogel model and a numerical mixture model were used to investigate why the disc herniates while osmotic pressure is decreasing.

Objective: To investigate the influence of decreasing osmotic pressure on the opening of cracks in the disc.

Summary Of Background Data: In the degeneration process, the disc changes structure (i.e., cracks occur, and osmotic pressure decreases). Disc herniation typically develops when hydration declines, but, on the other hand, it is said that the anulus of a highly hydrated disc has a high risk of rupture. We hypothesized that disc herniation is preceded by the opening of cracks as a result of decreasing osmotic pressure.

Methods: The osmotic pressure was changed in hydrogel samples with a crack, which was visualized with a confocal laser scanning microscope (Zeiss, Göttingen, Germany). A 2-dimensional finite element mixture model simulated a decrease in osmotic pressure around a crack in a swelling material.

Results: Experiments and simulations show that a decrease in osmotic pressure results in the opening of cracks. The simulations show high effective stress concentrations around the crack tip, while the overall stress level decreases, indicating an increased risk of crack growth.

Conclusions: Decreasing osmotic pressure in a degenerating intervertebral disc enhances the opening of existing cracks, despite the concomitant decrease in anular stresses.
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http://dx.doi.org/10.1097/01.brs.0000227267.42924.bbDOI Listing
July 2006

Osmoviscoelastic finite element model of the intervertebral disc.

Eur Spine J 2006 Aug 25;15 Suppl 3:S361-71. Epub 2006 May 25.

Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands.

Intervertebral discs have a primarily mechanical role in transmitting loads through the spine. The disc is subjected to a combination of elastic, viscous and osmotic forces; previous 3D models of the disc have typically neglected osmotic forces. The fibril-reinforced poroviscoelastic swelling model, which our group has recently developed, is used to compute the interplay of osmotic, viscous and elastic forces in an intervertebral disc under axial compressive load. The unloaded 3D finite element mesh equilibrates in a physiological solution, and exhibits an intradiscal pressure of about 0.2 MPa. Before and after axial loading the numerically simulated hydrostatic pressure compares well with the experimental ranges measured. Loading the disc decreased the height of the disc and results in an outward bulging of the outer annulus. Fiber stresses were highest on the most outward bulging on the posterior-lateral side. The osmotic forces resulted in tensile hoop stresses, which were higher than typical values in a non-osmotic disc. The computed axial stress profiles reproduced the main features of the stress profiles, in particular the characteristic posterior and anterior stress which were observed experimentally.
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http://dx.doi.org/10.1007/s00586-006-0110-3DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2335381PMC
August 2006

Do osmotic forces play a role in the uptake of water by human skin?

Skin Res Technol 2004 May;10(2):109-12

Personal Care Institute, Philips Research, Eindhoven, The Netherlands.

Background/purpose: To describe the water and ion transport through the skin under different conditions, we developed a three-component mixture model. This model has proven to describe the transient change in transepidermal water loss (TEWL) after a change in relative humidity and the result of damage to the skin. Osmotic forces arc present in the model. To assess the influence of osmotic forces on the water uptake of the skin, we investigated transient TEWL values after 1 h application of salt solutions of different molarities (0, 1, and 4 M NaCl).

Methods: Filters saturated with 0, 1, and 4 M NaCl solution were applied for 1 h under occlusion. TEWL was measured 50-90 min after removal of the solution. The transient water loss curves were fit with an exponential function. The area under the fitted curve was calculated and regarded as a measure for the amount of extra water absorbed in the skin.

Results: For all molarities, TEWL is increased immediately after removal of the solution. In time, this increase decays until pre-application values are reached again. The rate of decrease differs significantly for all three molarities. Ninety-five per cent of the increase has been reversed after 30, 19, and 6 min for the 0, 1, and 4 M case, respectively. The amount of water absorbed differs significantly between the three molarities 7.3+/-2.0; 3.9+/-1.0; 2.0+/-0.5 g/m(2), respectively.

Conclusions: In all cases, there was an increase in TEWL immediately after removal of the solution. The significant differences in decay time and amount of water absorbed between the three molarities indicate that osmotic forces do play an important role in the water uptake.
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http://dx.doi.org/10.1111/j.1600-0846.2004.00059.xDOI Listing
May 2004

Finite element model of mechanically induced collagen fiber synthesis and degradation in the aortic valve.

Ann Biomed Eng 2003 Oct;31(9):1040-53

Laboratory for Biomechanics and Tissue-Engineering, Department of Biomedical Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

Tissue-engineered trileaflet aortic valves are a promising alternative to current valve replacements. However, the mechanical properties of these valves are insufficient for implantation at the aortic position. To simulate the effect of collagen remodeling on the mechanical properties of the aortic valve, a finite element model is presented. In this study collagen remodeling is assumed to be the net result of collagen synthesis and degradation. A limited number of fibers with low initial fiber volume fraction is defined, and depending on the loading condition, the fibers are either synthesized or degraded. The synthesis and degradation of collagen fibers are both assumed to be functions of individual fiber stretch and fiber volume fraction. Simulations are performed for closed aortic valve configurations and the open aortic valve configuration. The predicted fiber directions for the closed configurations are close to the fiber directions as measured in the native aortic valve. The model predicts the evolution in collagen fiber content and the effect of remodeling on the mechanical properties.
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http://dx.doi.org/10.1114/1.1603749DOI Listing
October 2003

Computational analyses of mechanically induced collagen fiber remodeling in the aortic heart valve.

J Biomech Eng 2003 Aug;125(4):549-57

Eindhoven University of Technology, Department of Biomedical Engineering, Laboratory for Biomechanics and Tissue Engineering, PO Box 513, 5600 MB Eindhoven, The Netherlands.

To optimize the mechanical properties and integrity of tissue-engineered aortic heart valves, it is necessary to gain insight into the effects of mechanical stimuli on the mechanical behavior of the tissue using mathematical models. In this study, a finite-element (FE) model is presented to relate changes in collagen fiber content and orientation to the mechanical loading condition within the engineered construct. We hypothesized that collagen fibers aligned with principal strain directions and that collagen content increased with the fiber stretch. The results indicate that the computed preferred fiber directions run from commissure to commissure and show a strong resemblance to experimental data from native aortic heart valves.
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http://dx.doi.org/10.1115/1.1590361DOI Listing
August 2003

A case for strain-induced fluid flow as a regulator of BMU-coupling and osteonal alignment.

J Bone Miner Res 2002 Nov;17(11):2021-9

Department of Clinical Physics and Informatics, Vrije Universiteit Medical Center, Amsterdam, The Netherlands.

Throughout life, human bone is renewed continuously in a tightly controlled sequence of resorption and formation. This process of bone remodeling is remarkable because it involves cells from different lineages, collaborating in so-called basic multicellular units (BMUs) within small spatial and temporal boundaries. Moreover, the newly formed (secondary) osteons are aligned to the dominant load direction and have a density related to its magnitude, thus creating a globally optimized mechanical structure. Although the existence of BMUs is amply described, the cellular mechanisms driving bone remodeling-particularly the alignment process-are poorly understood. In this study we present a theory that explains bone remodelling as a self-organizing process of mechanical adaptation. Osteocytes thereby act as sensors of strain-induced fluid flow. Physiological loading produces stasis of extracellular fluid in front of the cutting cone of a tunneling osteon, which will lead to osteocytic disuse and (continued) attraction of osteoclasts. However, around the resting zone and the closing cone, enhanced extracellular fluid flow occurs, which will activate osteocytes to recruit osteoblasts. Thus, cellular activity at a bone remodeling site is well related to local fluid flow patterns, which may explain the coordinated progression of a BMU.
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http://dx.doi.org/10.1359/jbmr.2002.17.11.2021DOI Listing
November 2002

Estimation of the poroelastic parameters of cortical bone.

J Biomech 2002 Jun;35(6):829-35

Department of Clinical Physics and Informatics, Medical Centre Vrije Universiteit, PO Box 7057, 1007 MB Amsterdam, Netherlands.

Cortical bone has two systems of interconnected channels. The largest of these is the vascular porosity consisting of Haversian and Volkmann's canals, with a diameter of about 50 microm, which contains a.o. blood vessels and nerves. The smaller is the system consisting of the canaliculi and lacunae: the canaliculi are at the submicron level and house the protrusions of the osteocytes. When bone is differentially loaded, fluids within the solid matrix sustain a pressure gradient that drives a flow. It is generally assumed that the flow of extracellular fluid around osteocytes plays an important role not only in the nutrition of these cells, but also in the bone's mechanosensory system. The interaction between the deformation of the bone matrix and the flow of fluid can be modelled using Biot's theory of poroelasticity. However, due to the inhomogeneity of the bone matrix and the scale of the porosities, it is not possible to experimentally determine all the parameters that are needed for numerical implementation. The purpose of this paper is to derive these parameters using composite modelling and experimental data from literature. A full set of constants is estimated for a linear isotropic description of cortical bone as a two-level porous medium. Bone, however, has a wide variety of mechanical and structural properties; with the theoretical relationships described in this note, poroelastic parameters can be derived for other bone types using their specific experimental data sets.
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http://dx.doi.org/10.1016/s0021-9290(02)00021-0DOI Listing
June 2002
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