A multiscale computational model of arterial growth and remodeling including Notch signaling

Blood vessels grow and remodel in response to mechanical stimuli. Many computational models capture this process phenomenologically, by assuming stress homeostasis, but this approach cannot unravel the underlying cellular mechanisms. Mechano-sensitive Notch signaling is well-known to be key in vascular development and homeostasis. Here, we present a multiscale framework coupling a constrained mixture model, capturing the mechanics and turnover of arterial constituents, to a cell-cell signaling model, describing Notch signaling dynamics among vascular smooth muscle cells (SMCs) as influenced by mechanical stimuli. Tissue turnover was regulated by both Notch activity, informed by in vitro data, and a phenomenological contribution, accounting for mechanisms other than Notch. This novel framework predicted changes in wall thickness and arterial composition in response to hypertension similar to previous in vivo data. The simulations suggested that Notch contributes to arterial growth in hypertension mainly by promoting SMC proliferation, while other mechanisms are needed to fully capture remodeling. The results also indicated that interventions to Notch, such as external Jagged ligands, can alter both the geometry and composition of hypertensive vessels, especially in the short term. Overall, our model enables a deeper analysis of the role of Notch and Notch interventions in arterial growth and remodeling and could be adopted to investigate therapeutic strategies and optimize vascular regeneration protocols.

Computational analysis of the role of mechanosensitive Notch signaling in arterial adaptation to hypertension

Arteries grow and remodel in response to mechanical stimuli. Hypertension, for example, results in arterial wall thickening. Cell-cell Notch signaling between vascular smooth muscle cells (VSMCs) is known to be involved in this process, but the underlying mechanisms are still unclear. Here, we investigated whether Notch mechanosensitivity to strain may regulate arterial thickening in hypertension. We developed a multiscale computational framework by coupling a finite element model of arterial mechanics, including residual stress, to an agent-based model of mechanosensitive Notch signaling, to predict VSMC phenotypes as an indicator of growth and remodeling. Our simulations revealed that the sensitivity of Notch to strain at mean blood pressure may be a key mediator of arterial thickening in hypertensive arteries. Further simulations showed that loss of residual stress can have synergistic effects with hypertension, and that changes in the expression of Notch receptors, but not Jagged ligands, may be used to control arterial growth and remodeling and to intensify or counteract hypertensive thickening. Overall, we identify Notch mechanosensitivity as a potential mediator of vascular adaptation, and we present a computational framework that can facilitate the testing of new therapeutic and regenerative strategies.

Transcatheter-Delivered Expandable Bioresorbable Polymeric Graft With Stenting Capacity Induces Vascular Regeneration

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We designed a transcatheter balloon-expandable resorbable vascular graft with support capacity.

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After 2 months in vivo, grafts show native-like tissue reconstruction with endoluminal elastin.

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The concept convenes regenerative grafting, minimally invasive delivery, and clinical stenting.

As the next step in the translation of vascular tissue engineering, this study uniquely combines transcatheter delivery and in situ tissue regeneration using a novel bioresorbable electrospun polymer graft that can be implanted minimally invasively. Once delivered inside a small-diameter vessel, the electrospun microstructure supports the vessel wall, facilitates cellular infiltration, and guides organized tissue formation.

Geometry influences inflammatory host cell response and remodeling in tissue-engineered heart valves in-vivo

Regenerative tissue-engineered matrix-based heart valves (TEM-based TEHVs) may become an alternative to currently-used bioprostheses for transcatheter valve replacement. We recently identified TEM-based TEHVs-geometry as one key-factor guiding their remodeling towards successful long-term performance or failure. While our first-generation TEHVs, with a simple, non-physiological valve-geometry, failed over time due to leaflet-wall fusion phenomena, our second-generation TEHVs, with a computational modeling-inspired design, showed native-like remodeling resulting in long-term performance. However, a thorough understanding on how TEHV-geometry impacts the underlying host cell response, which in return determines tissue remodeling, is not yet fully understood. To assess that, we here present a comparative samples evaluation derived from our first- and second-generation TEHVs. We performed an in-depth qualitative and quantitative (immuno-)histological analysis focusing on key-players of the inflammatory and remodeling cascades (M1/M2 macrophages, α-SMA⁺- and endothelial cells). First-generation TEHVs were prone to chronic inflammation, showing a high presence of macrophages and α-SMA⁺-cells, hinge-area thickening, and delayed endothelialization. Second-generation TEHVs presented with negligible amounts of macrophages and α-SMA⁺-cells, absence of hinge-area thickening, and early endothelialization. Our results suggest that TEHV-geometry can significantly influence the host cell response by determining the infiltration and presence of macrophages and α-SMA⁺-cells, which play a crucial role in orchestrating TEHV remodeling.

Computational modeling guides tissue-engineered heart valve design for long-term in vivo performance in a translational sheep model

Valvular heart disease is a major cause of morbidity and mortality worldwide. Current heart valve prostheses have considerable clinical limitations due to their artificial, nonliving nature without regenerative capacity. To overcome these limitations, heart valve tissue engineering (TE) aiming to develop living, native-like heart valves with self-repair, remodeling, and regeneration capacity has been suggested as next-generation technology. A major roadblock to clinically relevant, safe, and robust TE solutions has been the high complexity and variability inherent to bioengineering approaches that rely on cell-driven tissue remodeling. For heart valve TE, this has limited long-term performance in vivo because of uncontrolled tissue remodeling phenomena, such as valve leaflet shortening, which often translates into valve failure regardless of the bioengineering methodology used to develop the implant. We tested the hypothesis that integration of a computationally inspired heart valve design into our TE methodologies could guide tissue remodeling toward long-term functionality in tissue-engineered heart valves (TEHVs). In a clinically and regulatory relevant sheep model, TEHVs implanted as pulmonary valve replacements using minimally invasive techniques were monitored for 1 year via multimodal in vivo imaging and comprehensive tissue remodeling assessments. TEHVs exhibited good preserved long-term in vivo performance and remodeling comparable to native heart valves, as predicted by and consistent with computational modeling. TEHV failure could be predicted for nonphysiological pressure loading. Beyond previous studies, this work suggests the relevance of an integrated in silico, in vitro, and in vivo bioengineering approach as a basis for the safe and efficient clinical translation of TEHVs.

Predicting and understanding collagen remodeling in human native heart valves during early development

The hemodynamic functionality of heart valves strongly depends on the distribution of collagen fibers, which are their main load-bearing constituents. It is known that collagen networks remodel in response to mechanical stimuli. Yet, the complex interplay between external load and collagen remodeling is poorly understood. In this study, we adopted a computational approach to simulate collagen remodeling occurring in native fetal and pediatric heart valves. The computational model accounted for several biological phenomena: cellular (re)orientation in response to both mechanical stimuli and topographical cues provided by collagen fibers; collagen deposition and traction forces along the main cellular direction; collagen degradation decreasing with stretch; and cell-mediated collagen prestretch. Importantly, the computational results were well in agreement with previous experimental data for all simulated heart valves. Simulations performed by varying some of the computational parameters suggest that cellular forces and (re)orientation in response to mechanical stimuli may be fundamental mechanisms for the emergence of the circumferential collagen alignment usually observed in native heart valves. On the other hand, the tendency of cells to coalign with collagen fibers is essential to maintain and reinforce that circumferential alignment during development. STATEMENT OF SIGNIFICANCE: The hemodynamic functionality of heart valves is strongly influenced by the alignment of load-bearing collagen fibers. Currently, the mechanisms that are responsible for the development of the circumferential collagen alignment in native heart valves are not fully understood. In the present study, cell-mediated remodeling of native human heart valves during early development was computationally simulated to understand the impact of individual mechanisms on collagen alignment. Our simulations successfully predicted the degree of collagen alignment observed in native fetal and pediatric semilunar valves. The computational results suggest that the circumferential collagen alignment arises from cell traction and cellular (re)orientation in response to mechanical stimuli, and with increasing age is reinforced by the tendency of cells to co-align with pre-existing collagen fibers.

Dual Electrospun Supramolecular Polymer Systems for Selective Cell Migration

Dual electrospinning can be used to make multifunctional scaffolds for regenerative medicine applications. Here, two supramolecular polymers with different material properties are electrospun simultaneously to create a multifibrous mesh. Bisurea (BU)-based polycaprolactone, an elastomer providing strength to the mesh, and ureido-pyrimidinone (UPy) modified poly(ethylene glycol) (PEG), a hydrogelator, introducing the capacity to deliver compounds upon swelling. The dual spun scaffolds are modularly tuned by mixing UPyPEG hydrogelators with different polymer lengths, to control swelling of the hydrogel fiber, while maintaining the mechanical properties of the scaffold. Stromal cell derived factor 1 alpha (SDF1α) peptides are embedded in the UPyPEG fibers. The swelling and erosion of UPyPEG increase void spaces and released the SDF1α peptide. The functionalized scaffolds demonstrate preferential lymphocyte recruitment proposed to be created by a gradient formed by the released SDF1α peptide. This delivery approach offers the potential to develop multifibrous scaffolds with various functions.

Can We Grow Valves Inside the Heart? Perspective on Material-based In Situ Heart Valve Tissue Engineering

In situ heart valve tissue engineering using cell-free synthetic, biodegradable scaffolds is under development as a clinically attractive approach to create living valves right inside the heart of a patient. In this approach, a valve-shaped porous scaffold "implant" is rapidly populated by endogenous cells that initiate neo-tissue formation in pace with scaffold degradation. While this may constitute a cost-effective procedure, compatible with regulatory and clinical standards worldwide, the new technology heavily relies on the development of advanced biomaterials, the processing thereof into (minimally invasive deliverable) scaffolds, and the interaction of such materials with endogenous cells and neo-tissue under hemodynamic conditions. Despite the first positive preclinical results and the initiation of a small-scale clinical trial by commercial parties, in situ tissue formation is not well understood. In addition, it remains to be determined whether the resulting neo-tissue can grow with the body and preserves functional homeostasis throughout life. More important yet, it is still unknown if and how in situ tissue formation can be controlled under conditions of genetic or acquired disease. Here, we discuss the recent advances of material-based in situ heart valve tissue engineering and highlight the most critical issues that remain before clinical application can be expected. We argue that a combination of basic science - unveiling the mechanisms of the human body to respond to the implanted biomaterial under (patho)physiological conditions - and technological advancements - relating to the development of next generation materials and the prediction of in situ tissue growth and adaptation - is essential to take the next step towards a realistic and rewarding translation of in situ heart valve tissue engineering.

3D Fiber Orientation in Atherosclerotic Carotid Plaques

Atherosclerotic plaque rupture is the primary trigger of fatal cardiovascular events. Fibrillar collagen in atherosclerotic plaques and their directionality are anticipated to play a crucial role in plaque rupture. This study aimed assessing 3D fiber orientations and architecture in atherosclerotic plaques for the first time. Seven carotid plaques were imaged ex-vivo with a state-of-the-art Diffusion Tensor Imaging (DTI) technique, using a high magnetic field (9.4Tesla) MRI scanner. A 3D spin-echo sequence with uni-polar diffusion sensitizing pulsed field gradients was utilized for DTI and fiber directions were assessed from diffusion tensor measurements. The distribution of the 3D fiber orientations in atherosclerotic plaques were quantified and the principal fiber orientations (circumferential, longitudinal or radial) were determined. Overall, 52% of the fiber orientations in the carotid plaque specimens were closest to the circumferential direction, 34% to the longitudinal direction, and 14% to the radial direction. Statistically no significant difference was measured in the amount of the fiber orientations between the concentric and eccentric plaque sites. However, concentric plaque sites showed a distinct structural organization, where the principally longitudinally oriented fibers were closer to the luminal side and the principally circumferentially oriented fibers were located more abluminally. The acquired unique information on 3D plaque fiber direction will help understanding pathobiological mechanisms of atherosclerotic plaque progression and pave the road to more realistic biomechanical plaque modeling for rupture assessment.

Prediction of Cell Alignment on Cyclically Strained Grooved Substrates

Cells respond to both mechanical and topographical stimuli by reorienting and reorganizing their cytoskeleton. Under certain conditions, such as for cells on cyclically stretched grooved substrates, the effects of these stimuli can be antagonistic. The biophysical processes that lead to the cellular reorientation resulting from such a competition are not clear yet. In this study, we hypothesized that mechanical cues and the diffusion of the intracellular signal produced by focal adhesions are determinants of the final cellular alignment. This hypothesis was investigated by means of a computational model, with the aim to simulate the (re)orientation of cells cultured on cyclically stretched grooved substrates. The computational results qualitatively agree with previous experimental studies, thereby supporting our hypothesis. Furthermore, cellular behavior resulting from experimental conditions different from the ones reported in the literature was simulated, which can contribute to the development of new experimental designs.

Percutaneous pulmonary valve replacement using completely tissue-engineered off-the-shelf heart valves: six-month in vivo functionality and matrix remodelling in sheep

AIMS

The objective was to implant a stented decellularised tissue-engineered heart valve (sdTEHV) percutaneously in an animal model, to assess its in vivo functionality and to examine the repopulation and remodelling of the valvular matrix by the recipient's autologous cells.

METHODS AND RESULTS

Prototypes of sdTEHV were cultured in vitro, decellularised and percutaneously implanted into the pulmonary position in 15 sheep. Functionality was assessed monthly by intracardiac echocardiography (ICE). Valves were explanted after eight, 16 or 24 weeks and analysed macroscopically, histologically and by electron microscopy. Implantation was successful in all animals. Valves showed normal pressure gradients throughout the study. Due to a suboptimal design with small coaptation area, stent ovality led to immediate regurgitation which continuously increased during follow-up. Analyses revealed complete endothelialisation and rapid cellular repopulation and remodelling of the entire matrix. Valves were free from endocarditis, calcification and graft rejection.

CONCLUSIONS

sdTEHV can be safely implanted percutaneously. The fast autologous recellularisation and the extensive matrix remodelling demonstrate the valve's potential as a next-generation percutaneous prosthesis with the capacity for tissue self-maintenance and longevity. Regurgitation may be prevented by valve design optimisation.

The Effects of Scaffold Remnants in Decellularized Tissue-Engineered Cardiovascular Constructs on the Recruitment of Blood Cells<sup/>

Decellularized tissue-engineered heart valves (DTEHVs) showed remarkable results in translational animal models, leading to recellularization within hours after implantation. This is crucial to enable tissue remodeling. To investigate if the presence of scaffold remnants before implantation is responsible for the fast recellularization of DTEHVs, an in vitro mesofluidic system was used. Human granulocyte and agranulocyte fractions were isolated, stained, brought back in suspension, and implemented in the system. Three different types of biomaterials were exposed to the circulating blood cells, consisting of decellularized tissue-engineered constructs (DTECs) with or without scaffold remnants or only bare scaffold. After 5 h of testing, the granulocyte fraction depleted faster from the circulation than the agranulocyte fraction. However, only granulocytes infiltrated into the DTEC with scaffold, migrating toward the scaffold remnants. The agranulocyte population, on the other hand, was only observed on the outer surface. Active cell infiltration was associated with increased levels of matrix metalloproteinase-1 secretion in the DTEC, including scaffold remnants. Proinflammatory cytokines such as interleukin (IL)-1α, IL-6, and tumor necrosis factor alpha (TNFα) were significantly upregulated in the DTEC without scaffold remnants. These results indicate that scaffold remnants can influence the immune response in DTEC, being responsible for rapid cell infiltration.

In situ heart valve tissue engineering using a bioresorbable elastomeric implant - From material design to 12 months follow-up in sheep

The creation of a living heart valve is a much-wanted alternative for current valve prostheses that suffer from limited durability and thromboembolic complications. Current strategies to create such valves, however, require the use of cells for in vitro culture, or decellularized human- or animal-derived donor tissue for in situ engineering. Here, we propose and demonstrate proof-of-concept of in situ heart valve tissue engineering using a synthetic approach, in which a cell-free, slow degrading elastomeric valvular implant is populated by endogenous cells to form new valvular tissue inside the heart. We designed a fibrous valvular scaffold, fabricated from a novel supramolecular elastomer, that enables endogenous cells to enter and produce matrix. Orthotopic implantations as pulmonary valve in sheep demonstrated sustained functionality up to 12 months, while the implant was gradually replaced by a layered collagen and elastic matrix in pace with cell-driven polymer resorption. Our results offer new perspectives for endogenous heart valve replacement starting from a readily-available synthetic graft that is compatible with surgical and transcatheter implantation procedures.

Cellular strain avoidance is mediated by a functional actin cap - observations in an Lmna-deficient cell model

In adherent cells, the relevance of a physical mechanotransduction pathway provided by the perinuclear actin cap stress fibers has recently emerged. Here, we investigate the impact of a functional actin cap on the cellular adaptive response to topographical cues and uniaxial cyclic strain. Lmna-deficient fibroblasts are used as a model system because they do not develop an intact actin cap, but predominantly form a basal layer of actin stress fibers underneath the nucleus. We observe that topographical cues induce alignment in both normal and Lmna-deficient fibroblasts, suggesting that the topographical signal transmission occurs independently of the integrity of the actin cap. By contrast, in response to cyclic uniaxial strain, Lmna-deficient cells show a compromised strain avoidance response, which is completely abolished when topographical cues and uniaxial strain are applied along the same direction. These findings point to the importance of an intact and functional actin cap in mediating cellular strain avoidance.

Excessive volume of hydrogel injectates may compromise the efficacy for the treatment of acute myocardial infarction

Biomaterial injectates are promising as a therapy for myocardial infarction to inhibit the adverse ventricular remodeling. The current study explored interrelated effects of injectate volume and infarct size on treatment efficacy. A finite element model of a rat heart was utilized to represent ischemic infarcts of 10%, 20%, and 38% of left ventricular wall volume and polyethylene glycol hydrogel injectates of 25%, 50%, and 75% of the infarct volume. Ejection fraction was 49.7% in the healthy left ventricle and 44.9%, 46.4%, 47.4%, and 47.3% in the untreated 10% infarct and treated with 25%, 50%, and 75% injectate, respectively. Maximum end-systolic infarct fiber stress was 41.6, 53.4, 44.7, 44.0, and 45.3 kPa in the healthy heart, the untreated 10% infarct, and when treated with the three injectate volumes, respectively. Treating the 10% and 38% infarcts with the 25% injectate volume reduced the maximum end-systolic fiber stress by 16.3% and 34.7% and the associated strain by 30.2% and 9.8%, respectively. The results indicate the existence of a threshold for injectate volume above which efficacy does not further increase but may decrease. The efficacy of an injectate in reducing infarct stress and strain changes with infarct size. Copyright © 2016 John Wiley & Sons, Ltd.

First percutaneous implantation of a completely tissue-engineered self-expanding pulmonary heart valve prosthesis using a newly developed delivery system: a feasibility study in sheep

In a European consortium, a decellularized tissue-engineered heart valve (dTEHV) based on vessel-derived cells, a fast-degrading scaffold and a self-expanding stent has been developed. The aim of this study was to demonstrate that percutaneous delivery is feasible. To implant this valve prosthesis transcutaneously into pulmonary position, a catheter delivery system was designed and custom made. Three sheep underwent transjugular prototype implantation. Intracardiac echocardiography (ICE), angiography and computed tomography (CT) were applied to assess the position, morphology, function and dimensions of the stented dTEHV. One animal was killed 3 h after implantation and two animals were followed up for 12 weeks. Explanted valves were analyzed macroscopically and microscopically. In all animals, the percutaneous implantation of the stented dTEHV was successful. The prototype delivery system worked at first attempt in all animals. In the first implantation a 22 F system was used: the valve was slightly damaged during crimping. Loading was difficult due to valve-catheter mismatch in volume. In the second and third implantation a 26 F system was used: the valves fitted adequately and stayed intact. Following implantation, these two valves showed moderate regurgitation due to insufficient coaptation. During follow-up, regurgitation increased due to shortened leaflets. At explantation, macroscopic and microscopic analysis confirmed the second and third valve to be intact. Histology revealed autologous recellularization of the decellularized valve after 12 weeks in vivo. It was demonstrated that completely in vitro tissue-engineered heart valves are thin and stable enough to be crimped and implanted transvenously into pulmonary position.

Modulation of collagen fiber orientation by strain-controlled enzymatic degradation

Collagen fiber anisotropy has a significant influence on the function and mechanical properties of cardiovascular tissues. We investigated if strain-dependent collagen degradation can explain collagen orientation in response to uniaxial and biaxial mechanical loads. First, decellularized pericardial samples were stretched to a fixed uniaxial strain and after adding a collagen degrading enzyme (collagenase), force relaxation was measured to calculate the degradation rate. This data was used to identify the strain-dependent degradation rate. A minimum was observed in the degradation rate curve. It was then demonstrated, for the first time, that biaxial strain in combination with collagenase alters the collagen fiber alignment from an initially isotropic distribution to an anisotropic distribution with a mean alignment corresponding with the strain at the minimum degradation rate, which may be in between the principal strain directions. When both strains were smaller than the minimum degradation point, fibers tended to align in the direction of the larger strain and when both strains were larger than the minimum degradation, fibers mainly aligned in the direction of the smaller strain. However, when one strain was larger and one was smaller than the minimum degradation point, the observed fiber alignment was in between the principal strain directions. In the absence of collagenase, uniaxial and biaxial strains only had a slight effect on the collagen (re)orientation of the decellularized samples.

STATEMENT OF SIGNIFICANCE

Collagen fiber orientation is a significant determinant of the mechanical properties of native tissues. To mimic the native-like collagen alignment in vitro, we need to understand the underlying mechanisms that direct this alignment. In the current study, we aimed to control collagen fiber orientation by applying biaxial strains in the presence of collagenase. We hypothesized that strain-dependent collagen degradation can describe specific collagen orientation when biaxial mechanical strains are applied. Based on this hypothesis, collagen fibers align in the direction where the degradation is minimal. Pericardial tissues, as isotropic collagen matrices, were decellularized and subjected to a fixed uniaxial strain. Then, collagenase was added to initiate the collagen degradation and the relaxation of force was measured to indicate the degradation rate. The V-shaped relationship between degradation rate and strain was obtained to identify the minimum degradation rate point. It was then demonstrated, for the first time, that biaxial strain in combination with collagenase alters the collagen fiber alignment from almost isotropic to a direction corresponding with the strain at the minimum degradation rate.

Age-Dependent Changes in Geometry, Tissue Composition and Mechanical Properties of Fetal to Adult Cryopreserved Human Heart Valves

There is limited information about age-specific structural and functional properties of human heart valves, while this information is key to the development and evaluation of living valve replacements for pediatric and adolescent patients. Here, we present an extended data set of structure-function properties of cryopreserved human pulmonary and aortic heart valves, providing age-specific information for living valve replacements. Tissue composition, morphology, mechanical properties, and maturation of leaflets from 16 pairs of structurally unaffected aortic and pulmonary valves of human donors (fetal-53 years) were analyzed. Interestingly, no major differences were observed between the aortic and pulmonary valves. Valve annulus and leaflet dimensions increase throughout life. The typical three-layered leaflet structure is present before birth, but becomes more distinct with age. After birth, cell numbers decrease rapidly, while remaining cells obtain a quiescent phenotype and reside in the ventricularis and spongiosa. With age and maturation-but more pronounced in aortic valves-the matrix shows an increasing amount of collagen and collagen cross-links and a reduction in glycosaminoglycans. These matrix changes correlate with increasing leaflet stiffness with age. Our data provide a new and comprehensive overview of the changes of structure-function properties of fetal to adult human semilunar heart valves that can be used to evaluate and optimize future therapies, such as tissue engineering of heart valves. Changing hemodynamic conditions with age can explain initial changes in matrix composition and consequent mechanical properties, but cannot explain the ongoing changes in valve dimensions and matrix composition at older age.

Efficient computational simulation of actin stress fiber remodeling

Understanding collagen and stress fiber remodeling is essential for the development of engineered tissues with good functionality. These processes are complex, highly interrelated, and occur over different time scales. As a result, excessive computational costs are required to computationally predict the final organization of these fibers in response to dynamic mechanical conditions. In this study, an analytical approximation of a stress fiber remodeling evolution law was derived. A comparison of the developed technique with the direct numerical integration of the evolution law showed relatively small differences in results, and the proposed method is one to two orders of magnitude faster.

A computational analysis of cell-mediated compaction and collagen remodeling in tissue-engineered heart valves

One of the most critical problems in heart valve tissue engineering is the progressive development of valvular insufficiency due to leaflet retraction. Understanding the underlying mechanisms of this process is crucial for developing tissue-engineered heart valves (TEHVs) that maintain their functionality in the long term. In the present study, we adopted a computational approach to predict the remodeling process in TEHVs subjected to dynamic pulmonary and aortic pressure conditions, and to assess the risk of valvular insufficiency. In addition, we investigated the importance of the intrinsic cell contractility on the final outcome of the remodeling process. For valves implanted in the aortic position, the model predictions suggest that valvular insufficiency is not likely to occur as the blood pressure is high enough to prevent the development of leaflet retraction. In addition, the collagen network was always predicted to remodel towards a circumferentially aligned network, which is corresponding to the native situation. In contrast, for valves implanted in the pulmonary position, our model predicted that there is a high risk for the development of valvular insufficiency, unless the cell contractility is very low. Conversely, the development of a circumferential collagen network was only predicted at these pressure conditions when cell contractility was high. Overall, these results, therefore, suggest that tissue remodeling at aortic pressure conditions is much more stable and favorable compared to tissue remodeling at pulmonary pressure conditions.

Improved Geometry of Decellularized Tissue Engineered Heart Valves to Prevent Leaflet Retraction

Recent studies on decellularized tissue engineered heart valves (DTEHVs) showed rapid host cell repopulation and increased valvular insufficiency developing over time, associated with leaflet shortening. A possible explanation for this result was found using computational simulations, which revealed radial leaflet compression in the original valvular geometry when subjected to physiological pressure conditions. Therefore, an improved geometry was suggested to enable radial leaflet extension to counteract for host cell mediated retraction. In this study, we propose a solution to impose this new geometry by using a constraining bioreactor insert during culture. Human cell based DTEHVs (n = 5) were produced as such, resulting in an enlarged coaptation area and profound belly curvature. Extracellular matrix was homogeneously distributed, with circumferential collagen alignment in the coaptation region and global tissue anisotropy. Based on in vitro functionality experiments, these DTEHVs showed competent hydrodynamic functionality under physiological pulmonary conditions and were fatigue resistant, with stable functionality up to 16 weeks in vivo simulation. Based on implemented mechanical data, our computational models revealed a considerable decrease in radial tissue compression with the obtained geometrical adjustments. Therefore, these improved DTEHV are expected to be less prone to host cell mediated leaflet retraction and will remain competent after implantation.

Diffusion profile of macromolecules within and between human skin layers for (trans)dermal drug delivery

Delivering a drug into and through the skin is of interest as the skin can act as an alternative drug administration route for oral delivery. The development of new delivery methods, such as microneedles, makes it possible to not only deliver small molecules into the skin, which are able to pass the outer layer of the skin in therapeutic amounts, but also macromolecules. To provide insight into the administration of these molecules into the skin, the aim of this study was to assess the transport of macromolecules within and between its various layers. The diffusion coefficients in the epidermis and several locations in the papillary and reticular dermis were determined for fluorescein dextran of 40 and 500 kDa using a combination of fluorescent recovery after photobleaching experiments and finite element analysis. The diffusion coefficient was significantly higher for 40 kDa than 500 kDa dextran, with median values of 23 and 9 µm(2)/s in the dermis, respectively. The values only marginally varied within and between papillary and reticular dermis. For the 40 kDa dextran, the diffusion coefficient in the epidermis was twice as low as in the dermis layers. The adopted method may be used for other macromolecules, which are of interest for dermal and transdermal drug delivery. The knowledge about diffusion in the skin is useful to optimize (trans)dermal drug delivery systems to target specific layers or cells in the human skin.

The Evolution of Collagen Fiber Orientation in Engineered Cardiovascular Tissues Visualized by Diffusion Tensor Imaging

The collagen architecture is the major determinant of the function and mechanical behavior of cardiovascular tissues. In order to engineer a functional and load-bearing cardiovascular tissue with a structure that mimics the native tissue to meet in vivo mechanical demands, a complete understanding of the collagen orientation mechanism is required. Several methods have been used to visualize collagen architecture in tissue-engineered (TE) constructs, but they either have a limited imaging depth or have a complicated set up. In this study, Diffusion Tensor Imaging (DTI) is explored as a fast and reliable method to visualize collagen arrangement, and Confocal Laser Scanning Microscopy (CLSM) was used as a validation technique. Uniaxially constrained TE strips were cultured for 2 days, 10 days, 3 and 6 weeks to investigate the evolution of the collagen orientation with time. Moreover, a comparison of the collagen orientation in high and low aspect ratio (length/width) TE constructs was made with both methods. Both methods showed similar fiber orientation in TE constructs. Collagen fibers in the high aspect ratio samples were mostly aligned in the constrained direction, while the collagen fibers in low aspect ratio strips were mainly oriented in the oblique direction. The orientation changed to the oblique direction by extending culture time and could also be visualized. DTI captured the collagen orientation differences between low and high aspect ratio samples and with time. Therefore, it can be used as a fast, non-destructive and reliable tool to study the evolution of the collagen orientation in TE constructs.

Competition between cap and basal actin fiber orientation in cells subjected to contact guidance and cyclic strain

In vivo, adhesive cells continuously respond to a complex range of physical cues coming from the surrounding microenvironment by remodeling their cytoskeleton. Topographical and mechanical cues applied separately have been shown to affect the orientation of the actin stress fibers. Here we investigated the combined effects of contact guidance by topographical cues and uniaxial cyclic strain on actin cytoskeleton orientation of vascular derived cells. We devised a modular setup of stretchable circular and elliptic elastomeric microposts, capable to expose the cells to both contact guidance and uniaxial cyclic strain. A competition occurs between these cues when both contact guidance and strain are oriented along the same direction. For the first time we show that this competition originates from the distinct response of perinuclear basal and actin cap fibers: While basal fibers follow the contact guidance cue, actin cap fibers respond to the cyclic strain by strain avoidance. We also show that nuclear orientation follows actin cap fiber orientation, suggesting that actin cap fibers are responsible for cellular reorientation. Taken together, these findings may have broad implications in understanding the response of cells to combined topographical and mechanical cues.

Local anisotropic mechanical properties of human carotid atherosclerotic plaques - characterisation by micro-indentation and inverse finite element analysis

Biomechanical models have the potential to predict failure of atherosclerotic plaques and to improve the risk assessment of plaque rupture. The applicability of these models depends strongly on the used material models. Current biomechanical models employ isotropic material models, although it is generally accepted that plaque tissue behaves highly anisotropic. The aim of the present study is to determine the local anisotropic mechanical properties of human atherosclerotic plaque tissue by means of micro-indentation tests. The indentation was performed on top of an inverted confocal microscope allowing the visualisation and quantification of the collagen fibre deformations perpendicular to the indentation direction of the plaque. Based on this, the anisotropic properties of plaque tissue perpendicular to the indentation direction (middle of the fibrous cap, shoulder of the cap, remaining intima tissue) were derived. There were no significant differences between the different indentation locations for the fibre stiffness (total median 80.6kPa, 25th-75th percentile 17.7-157.0kPa), and fibre dispersion.

Synergistic protein secretion by mesenchymal stromal cells seeded in 3D scaffolds and circulating leukocytes in physiological flow

Mesenchymal stromal cells (MSC) play an important role in natural wound healing via paracrine and juxtacrine signaling to immune cells. The aim of this study was to identify the signaling factors secreted by preseeded cells in a biomaterial and their interaction with circulating leukocytes, in the presence of physiological biomechanical stimuli exerted by the hemodynamic environment (i.e. strain and shear flow). Electrospun poly(ε-caprolactone)-based scaffolds were seeded with human peripheral blood mononuclear cells (PBMC) or MSC. Protein secretion was analyzed under static conditions and cyclic strain. Subsequently, the cross-talk between preseeded cells and circulating leukocytes was addressed by exposing the scaffolds to a suspension of PBMC in static transwells and in pulsatile flow. Our results revealed that PBMC exposed to the scaffold consistently secreted a cocktail of immunomodulatory proteins under all conditions tested. Preseeded MSC, on the other hand, secreted the trophic factors MCP-1, VEGF and bFGF. Furthermore, we observed a synergistic upregulation of CXCL12 gene expression and a synergistic increase in bFGF protein production by preseeded MSC exposed to PBMC in pulsatile flow. These findings identify CXCL12 and bFGF as valuable targets for the development of safe and effective acellular instructive grafts for application in in situ cardiovascular regenerative therapies.

Shear flow affects selective monocyte recruitment into MCP-1-loaded scaffolds

Novel cardiovascular replacements are being developed by using degradable synthetic scaffolds, which function as a temporary guide to induce neotissue formation directly in situ. Priming of such scaffolds with fast-releasing monocyte chemoattractant protein-1 (MCP-1) was shown to improve the formation of functional neoarteries in rats. However, the underlying mechanism has not been clarified. Therefore, the goal of this study was to investigate the effect of a burst-release of MCP-1 from a synthetic scaffold on the local recruitment of circulating leucocytes under haemodynamic conditions. Herein, we hypothesized that MCP-1 initiates a desired healing cascade by recruiting favourable monocyte subpopulations into the implanted scaffold. Electrospun poly(ε-caprolactone) scaffolds were loaded with fibrin gel containing various doses of MCP-1 and exposed to a suspension of human peripheral blood mononuclear cells in static or dynamic conditions. In standard migration assay, a dose-dependent migration of specific CD14⁺ monocyte subsets was observed, as measured by flow cytometry. In conditions of pulsatile flow, on the other hand, a marked increase in immediate monocyte recruitment was observed, but without evident selectivity in monocyte subsets. This suggests that the selectivity was dependent on the release kinetics of the MCP-1, as it was overruled by the effect of shear stress after the initial burst-release. Furthermore, these findings demonstrate that local recruitment of specific MCP-1-responsive monocytes is not the fundamental principle behind the improved neotissue formation observed in long-term in vivo studies, and mobilization of MCP-1-responsive cells from the bone marrow into the bloodstream is suggested to play a predominant role in vivo.

Synergy between Rho signaling and matrix density in cyclic stretch-induced stress fiber organization

Cells adapt in response to mechanical stimulation to ensure adequate tissue functioning. F-actin stress fibers provide a key element in the adaptation process. The high sensitivity and fast adaptation of the F-actin cytoskeleton to cyclic strain have been studied extensively in a 2-D environment; however, 3-D data are scarce. Our previous work showed that stress fibers organize perpendicular to cyclic stretching (stretch-avoidance) in three dimensions. However, stretch-avoidance was absent when cells populated a high density matrix. In this study our aim was to obtain more insight into the synergy between matrix density and the signaling pathways that govern stress fiber remodeling. Therefore we studied stress fiber organization in 3-D reconstituted collagen tissues (at low and high matrix density), subjected to cyclic stretch upon interference with molecular signaling pathways. In particular, the influence of the small GTPase Rho and its downstream effectors were studied. Only at low matrix density does stress fiber stretch avoidance show a stretch-magnitude-dependent response. The activity of matrix metalloproteinases (MMPs), Rho-kinase and myosin light chain kinase are essential for stress fiber reorientation. Although high matrix density restricts stress fiber reorientation, Rho activation can overcome this restriction, but only in the presence of active MMPs. Results from this study highlight a synergistic action between matrix remodeling and Rho signaling in cyclic-stretch-induced stress fiber organization in 3-D tissue.

Biomechanics and mechanobiology in functional tissue engineering

The field of tissue engineering continues to expand and mature, and several products are now in clinical use, with numerous other preclinical and clinical studies underway. However, specific challenges still remain in the repair or regeneration of tissues that serve a predominantly biomechanical function. Furthermore, it is now clear that mechanobiological interactions between cells and scaffolds can critically influence cell behavior, even in tissues and organs that do not serve an overt biomechanical role. Over the past decade, the field of "functional tissue engineering" has grown as a subfield of tissue engineering to address the challenges and questions on the role of biomechanics and mechanobiology in tissue engineering. Originally posed as a set of principles and guidelines for engineering of load-bearing tissues, functional tissue engineering has grown to encompass several related areas that have proven to have important implications for tissue repair and regeneration. These topics include measurement and modeling of the in vivo biomechanical environment; quantitative analysis of the mechanical properties of native tissues, scaffolds, and repair tissues; development of rationale criteria for the design and assessment of engineered tissues; investigation of the effects biomechanical factors on native and repair tissues, in vivo and in vitro; and development and application of computational models of tissue growth and remodeling. Here we further expand this paradigm and provide examples of the numerous advances in the field over the past decade. Consideration of these principles in the design process will hopefully improve the safety, efficacy, and overall success of engineered tissue replacements.

Functional tissue engineering: Ten more years of progress

"Functional tissue engineering" is a subset of the field of tissue engineering that was proposed by the United States National Committee on Biomechanics over a decade ago in order to place more emphasis on the roles of biomechanics and mechanobiology in tissue repair and regeneration. Over the past decade, there have been tremendous advances in this area, pointing out the critical role that biomechanical factors can play in the engineered repair of virtually all tissue and organ systems. In this special issue of the Journal of Biomechanics, we present a series of articles that address a broad array of the fundamental topics of functional tissue engineering, including: (1) measurement and modeling of the in vivo biomechanical environment and history in native and repair tissues; (2) further understanding of the biomechanical properties of native tissues across all geometric scales, in the context of repair or regeneration; (3) prioritization of specific biomechanical properties as design criteria; (4) development of biomaterials, scaffolds, and engineered tissues with prescribed biomechanical properties; (5) development of success criteria based on appropriate outcome measures; (6) investigation of the effects of mechanical factors on tissue repair in vivo; (7) investigation of the mechanisms by which physical factors may enhance tissue regeneration in vitro; and (8) development and validation of computational models of tissue growth and remodeling. These articles represent the tremendous expansion of this field in recent years, and emphasize the critical roles that biomechanics and mechanobiology play in controlling tissue repair and regeneration.

The influence of matrix (an)isotropy on cardiomyocyte contraction in engineered cardiac microtissues

In the cardiac microenvironment, cardiomyocytes (CMs) are embedded in an aligned and structured extracellular matrix (ECM) to maintain the coordinated contractile function of the heart. The cardiac fibroblast (cFB) is the main cell type responsible for producing and remodeling this matrix. In cardiac diseases, however, adverse remodeling and CM death may lead to deterioration of the aligned myocardial structure. Here, we present an in vitro cardiac model system with uniaxial and biaxial constraints to induce (an)isotropy in 3D microtissues, thereby mimicking 'healthy' aligned and 'diseased' disorganized cardiac matrices. A mixture of neonatal mouse CMs and cFBs was resuspended in a collagen-matrigel hydrogel and seeded to form microtissues to recapitulate the in vivo cellular composition. Matrix disarray led to a stellate cell shape and a disorganized sarcomere organization, while CMs in aligned matrices were more elongated and had aligned sarcomeres. Although matrix disarray has no detrimental effect on the force generated by the CMs, it did have a negative effect on the homogeneity of contraction force distribution. Furthermore, proliferation of the cFBs affected microtissue contraction as indicated by the negative correlation between the percentage of cFBs in the microtissues and their beating frequency. These results suggest that in regeneration of the diseased heart, reorganization of the disorganized matrix will contribute to recover the coordinated contraction but restoring the ratio in cellular composition (CMs and cFBs) is also a prerequisite to completely regain tissue function.

Degree of scaffold degradation influences collagen (re)orientation in engineered tissues

Tissue engineering provides a promising tool for creating load-bearing cardiovascular tissues. Ideally, the neotissue produced by cells possesses native strength and anisotropy. By providing contact-guiding cues with microfibers, scaffold directionality can guide tissue organization. However, scaffolds transiently degrade, which may induce undesired tissue remodeling in response to applied strain. We hypothesize that in newly formed tissues, the collagen matrix does not yet provide contact guidance to the cells, and collagen orientation is altered via strain-induced remodeling. To test this hypothesis, we studied the influence of lipase-induced scaffold degradation on collagen (re)orientation at static constraint. Myofibroblasts were cultured in electrospun PCL-U4U anisotropic microfiber scaffolds, which were statically constrained perpendicular to the scaffold fibers. During 2 weeks of culture, neotissue formation aligned in the direction of the scaffold fibers, after which scaffolds were degraded to different degrees (12%, 27%, and 79% reduction in scaffold weight) and collagen (re)orientation was studied after one additional week of culturing. High degrees of scaffold degradation (79%) were associated with remodeling of the collagen toward the constraint direction, while collagen organization was maintained at low degrees of scaffold degradation. These results highlight the importance of slow scaffold degradation when aiming at maintaining collagen orientation.

Cell-mediated retraction versus hemodynamic loading - A delicate balance in tissue-engineered heart valves

Preclinical studies of tissue-engineered heart valves (TEHVs) showed retraction of the heart valve leaflets as major failure of function mechanism. This retraction is caused by both passive and active cell stress and passive matrix stress. Cell-mediated retraction induces leaflet shortening that may be counteracted by the hemodynamic loading of the leaflets during diastole. To get insight into this stress balance, the amount and duration of stress generation in engineered heart valve tissue and the stress imposed by physiological hemodynamic loading are quantified via an experimental and a computational approach, respectively. Stress generation by cells was measured using an earlier described in vitro model system, mimicking the culture process of TEHVs. The stress imposed by the blood pressure during diastole on a valve leaflet was determined using finite element modeling. Results show that for both pulmonary and systemic pressure, the stress imposed on the TEHV leaflets is comparable to the stress generated in the leaflets. As the stresses are of similar magnitude, it is likely that the imposed stress cannot counteract the generated stress, in particular when taking into account that hemodynamic loading is only imposed during diastole. This study provides a rational explanation for the retraction found in preclinical studies of TEHVs and represents an important step towards understanding the retraction process seen in TEHVs by a combined experimental and computational approach.

Engineering fibrin-based tissue constructs from myofibroblasts and application of constraints and strain to induce cell and collagen reorganization

Collagen content and organization in developing collagenous tissues can be influenced by local tissue strains and tissue constraint. Tissue engineers aim to use these principles to create tissues with predefined collagen architectures. A full understanding of the exact underlying processes of collagen remodeling to control the final tissue architecture, however, is lacking. In particular, little is known about the (re)orientation of collagen fibers in response to changes in tissue mechanical loading conditions. We developed an in vitro model system, consisting of biaxially-constrained myofibroblast-seeded fibrin constructs, to further elucidate collagen (re)orientation in response to i) reverting biaxial to uniaxial static loading conditions and ii) cyclic uniaxial loading of the biaxially-constrained constructs before and after a change in loading direction, with use of the Flexcell FX4000T loading device. Time-lapse confocal imaging is used to visualize collagen (re)orientation in a nondestructive manner. Cell and collagen organization in the constructs can be visualized in real-time, and an internal reference system allows us to relocate cells and collagen structures for time-lapse analysis. Various aspects of the model system can be adjusted, like cell source or use of healthy and diseased cells. Additives can be used to further elucidate mechanisms underlying collagen remodeling, by for example adding MMPs or blocking integrins. Shape and size of the construct can be easily adapted to specific needs, resulting in a highly tunable model system to study cell and collagen (re)organization.

Matrix production and organization by endothelial colony forming cells in mechanically strained engineered tissue constructs

Aims

Tissue engineering is an innovative method to restore cardiovascular tissue function by implanting either an in vitro cultured tissue or a degradable, mechanically functional scaffold that gradually transforms into a living neo-tissue by recruiting tissue forming cells at the site of implantation. Circulating endothelial colony forming cells (ECFCs) are capable of differentiating into endothelial cells as well as a mesenchymal ECM-producing phenotype, undergoing Endothelial-to-Mesenchymal-transition (EndoMT). We investigated the potential of ECFCs to produce and organize ECM under the influence of static and cyclic mechanical strain, as well as stimulation with transforming growth factor β1 (TGFβ1).

Methods and Results

A fibrin-based 3D tissue model was used to simulate neo-tissue formation. Extracellular matrix organization was monitored using confocal laser-scanning microscopy. ECFCs produced collagen and also elastin, but did not form an organized matrix, except when cultured with TGFβ1 under static strain. Here, collagen was aligned more parallel to the strain direction, similar to Human Vena Saphena Cell-seeded controls. Priming ECFC with TGFβ1 before exposing them to strain led to more homogenous matrix production.

Conclusions

Biochemical and mechanical cues can induce extracellular matrix formation by ECFCs in tissue models that mimic early tissue formation. Our findings suggest that priming with bioactives may be required to optimize neo-tissue development with ECFCs and has important consequences for the timing of stimuli applied to scaffold designs for both in vitro and in situ cardiovascular tissue engineering. The results obtained with ECFCs differ from those obtained with other cell sources, such as vena saphena-derived myofibroblasts, underlining the need for experimental models like ours to test novel cell sources for cardiovascular tissue engineering.

Mechanics of the pulmonary valve in the aortic position

Mathematical models can provide valuable information to assess and evaluate the mechanical behavior and remodeling of native tissue. A relevant example when studying collagen remodeling is the Ross procedure because it involves placing the pulmonary autograft in the more demanding aortic valve mechanical environment. The objective of this study was therefore to assess and evaluate the mechanical differences between the aortic valve and pulmonary valve and the remodeling that may occur in the pulmonary valve when placed in the aortic position. The results from biaxial tensile tests of pairs of human aortic and pulmonary valves were compared and used to determine the parameters of a structurally based constitutive model. Finite element analyzes were then performed to simulate the mechanical response of both valves to the aortic diastolic load. Additionally, remodeling laws were applied to assess the remodeling of the pulmonary valve leaflet to the new environment. The pulmonary valve showed to be more extensible and less anisotropic than the aortic valve. When exposed to aortic pressure, the pulmonary leaflet appeared to remodel by increasing its thickness and reorganizing its collagen fibers, rotating them toward the circumferential direction.

Mechanical analysis of ovine and pediatric pulmonary artery for heart valve stent design

Transcatheter heart valve replacement is an attractive and promising technique for congenital as well as acquired heart valve disease. In this procedure, the replacement valve is mounted in a stent that is expanded at the aimed valve position and fixated by clamping. However, for this technique to be appropriate for pediatric patients, the material properties of the host tissue need to be determined to design stents that can be optimized for this particular application. In this study we performed equibiaxial tensile tests on four adult ovine pulmonary artery walls and compared the outcomes with one pediatric pulmonary artery. Results show that the pediatric pulmonary artery was significantly thinner (1.06 ± 0.36 mm (mean ± SD)) than ovine tissue (2.85 ± 0.40 mm), considerably stiffer for strain values that exceed the physiological conditions (beyond 50% strain in the circumferential and 60% in the longitudinal direction), more anisotropic (with a significant difference in stiffness between the longitudinal and circumferential directions beyond 60% strain) and presented stronger non-linear stress-strain behavior at equivalent strains (beyond 26% strain) compared to ovine tissue. These discrepancies suggest that stents validated and optimized using the ovine pre-clinical model might not perform satisfactorily in pediatric patients. The material parameters derived from this study may be used to develop stent designs for both applications using computational models.

Poly-ε-caprolactone scaffold and reduced in vitro cell culture: beneficial effect on compaction and improved valvular tissue formation

Tissue-engineered heart valves (TEHVs), based on polyglycolic acid (PGA) scaffolds coated with poly-4-hydroxybutyrate (P4HB), have shown promising in vivo results in terms of tissue formation. However, a major drawback of these TEHVs is compaction and retraction of the leaflets, causing regurgitation. To overcome this problem, the aim of this study was to investigate: (a) the use of the slowly degrading poly-ε-caprolactone (PCL) scaffold for prolonged mechanical integrity; and (b) the use of lower passage cells for enhanced tissue formation. Passage 3, 5 and 7 (P3, P5 and P7) human and ovine vascular-derived cells were seeded onto both PGA-P4HB and PCL scaffold strips. After 4 weeks of culture, compaction, tissue formation, mechanical properties and cell phenotypes were compared. TEHVs were cultured to observe retraction of the leaflets in the native-like geometry. After culture, tissues based on PGA-P4HB scaffold showed 50-60% compaction, while PCL-based tissues showed compaction of 0-10%. Tissue formation, stiffness and strength were increased with decreasing passage number; however, this did not influence compaction. Ovine PCL-based tissues did render less strong tissues compared to PGA-P4HB-based tissues. No differences in cell phenotype between the scaffold materials, species or cell passage numbers were observed. This study shows that PCL scaffolds may serve as alternative scaffold materials for human TEHVs with minimal compaction and without compromising tissue composition and properties, while further optimization of ovine TEHVs is needed. Reducing cell expansion time will result in faster generation of TEHVs, providing more rapid treatment for patients.

Engineering skeletal muscle tissues from murine myoblast progenitor cells and application of electrical stimulation

Engineered muscle tissues can be used for several different purposes, which include the production of tissues for use as a disease model in vitro, e.g. to study pressure ulcers, for regenerative medicine and as a meat alternative (1). The first reported 3D muscle constructs have been made many years ago and pioneers in the field are Vandenburgh and colleagues (2,3). Advances made in muscle tissue engineering are not only the result from the vast gain in knowledge of biochemical factors, stem cells and progenitor cells, but are in particular based on insights gained by researchers that physical factors play essential roles in the control of cell behavior and tissue development. State-of-the-art engineered muscle constructs currently consist of cell-populated hydrogel constructs. In our lab these generally consist of murine myoblast progenitor cells, isolated from murine hind limb muscles or a murine myoblast cell line C2C12, mixed with a mixture of collagen/Matrigel and plated between two anchoring points, mimicking the muscle ligaments. Other cells may be considered as well, e.g. alternative cell lines such as L6 rat myoblasts (4), neonatal muscle derived progenitor cells (5), cells derived from adult muscle tissues from other species such as human (6) or even induced pluripotent stem cells (iPS cells) (7). Cell contractility causes alignment of the cells along the long axis of the construct (8,9) and differentiation of the muscle progenitor cells after approximately one week of culture. Moreover, the application of electrical stimulation can enhance the process of differentiation to some extent (8). Because of its limited size (8 x 2 x 0.5 mm) the complete tissue can be analyzed using confocal microscopy to monitor e.g. viability, differentiation and cell alignment. Depending on the specific application the requirements for the engineered muscle tissue will vary; e.g. use for regenerative medicine requires the up scaling of tissue size and vascularization, while to serve as a meat alternative translation to other species is necessary.

Soft substrates normalize nuclear morphology and prevent nuclear rupture in fibroblasts from a laminopathy patient with compound heterozygous LMNA mutations

Laminopathies, mainly caused by mutations in the LMNA gene, are a group of inherited diseases with a highly variable penetrance; i.e., the disease spectrum in persons with identical LMNA mutations range from symptom-free conditions to severe cardiomyopathy and progeria, leading to early death. LMNA mutations cause nuclear abnormalities and cellular fragility in response to cellular mechanical stress, but the genotype/phenotype correlations in these diseases remain unclear. Consequently, tools such as mutation analysis are not adequate for predicting the course of the disease.

 

Here, we employ growth substrate stiffness to probe nuclear fragility in cultured dermal fibroblasts from a laminopathy patient with compound progeroid syndrome. We show that culturing of these cells on substrates with stiffness higher than 10 kPa results in malformations and even rupture of the nuclei, while culture on a soft substrate (3 kPa) protects the nuclei from morphological alterations and ruptures. No malformations were seen in healthy control cells at any substrate stiffness. In addition, analysis of the actin cytoskeleton organization in this laminopathy cells demonstrates that the onset of nuclear abnormalities correlates to an increase in cytoskeletal tension.

Together, these data indicate that culturing of these LMNA mutated cells on substrates with a range of different stiffnesses can be used to probe the degree of nuclear fragility. This assay may be useful in predicting patient-specific phenotypic development and in investigations on the underlying mechanisms of nuclear and cellular fragility in laminopathies.

Multi-scale mechanical characterization of scaffolds for heart valve tissue engineering

Electrospinning is a promising technology to produce scaffolds for cardiovascular tissue engineering. Each electrospun scaffold is characterized by a complex micro-scale structure that is responsible for its macroscopic mechanical behavior. In this study, we focus on the development and the validation of a computational micro-scale model that takes into account the structural features of the electrospun material, and is suitable for studying the multi-scale scaffold mechanics. We show that the computational tool developed is able to describe and predict the mechanical behavior of electrospun scaffolds characterized by different microstructures. Moreover, we explore the global mechanical properties of valve-shaped scaffolds with different microstructural features, and compare the deformation of these scaffolds when submitted to diastolic pressures with a tissue engineered and a native valve. It is shown that a pronounced degree of anisotropy is necessary to reproduce the deformation patterns observed in the native heart valve.

Trans-apical versus surgical implantation of autologous ovine tissue-engineered heart valves

BACKGROUND AND AIM OF THE STUDY

Living tissue-engineered heart valves (TEHVs) based on rapidly degrading scaffolds and autologous cells might overcome the limitations of today's valve substitutes. Following minimally invasive trans-apical implantation into an ovine model, TEHVs showed adequate in-vivo functionality, but a thickening of the leaflets was observed. In order to evaluate the impact of the substantial tissue deformations of TEHVs associated with the crimping procedure during minimally invasive delivery, trans-apical and conventional implantation technologies were compared in an ovine model.

METHODS

Trileaflet heart valves (n=11) based on PGA/P4HB-scaffolds, integrated into self-expandable stents, were engineered from autologous ovine vascular-derived cells. After in-vitro culture, the TEHVs were either implanted surgically (n=5), replacing the native pulmonary valve, or delivered trans-apically (n=6) into the orthotopic pulmonary valve position. In-vivo functionality was assessed by echocardiography and by angiography for up to eight weeks. The tissue compositions of the explanted TEHVs and corresponding control valves were analyzed.

RESULTS

TEHV implantations were successful in all cases. Independent of the implantation method, the explants demonstrated a comparable layered tissue formation with thickening and deposited fibrous layers. Active remodeling of these layers was evident in the explants, as indicated by vascularization of the walls, invasion of the host cells, and the formation of a luminal endothelial layer on the TEHV leaflets.

CONCLUSION

This direct comparison of trans-apical and conventional surgical implantation techniques showed that crimping had no adverse effect on the integrity or functional outcome of TEHVs. This suggests that a thickening of TEHVs in vivo is neither caused by nor enhanced by the crimping procedure, but represents a functional tissue remodeling process.

The influence of matrix integrity on stress-fiber remodeling in 3D

Matrix anisotropy is important for long term in vivo functionality. However, it is not fully understood how to guide matrix anisotropy in vitro. Experiments suggest actin-mediated cell traction contributes. Although F-actin in 2D displays a stretch-avoidance response, 3D data are lacking. We questioned how cyclic stretch influences F-actin and collagen orientation in 3D. Small-scale cell-populated fibrous tissues were statically constrained and/or cyclically stretched with or without biochemical agents. A rectangular array of silicone posts attached to flexible membranes constrained a mixture of cells, collagen I and matrigel. F-actin orientation was quantified using fiber-tracking software, fitted using a bi-model distribution function. F-actin was biaxially distributed with static constraint. Surprisingly, uniaxial cyclic stretch, only induced a strong stretch-avoidance response (alignment perpendicular to stretching) at tissue surfaces and not in the core. Surface alignment was absent when a ROCK-inhibitor was added, but also when tissues were only statically constrained. Stretch-avoidance was also observed in the tissue core upon MMP1-induced matrix perturbation. Further, a strong stretch-avoidance response was obtained for F-actin and collagen, for immediate cyclic stretching, i.e. stretching before polymerization of the collagen. Results suggest that F-actin stress-fibers avoid cyclic stretch in 3D, unless collagen contact guidance dictates otherwise.

A computational model to describe the collagen orientation in statically cultured engineered tissues

Collagen provides cardiovascular tissues with the ability to withstand haemodynamic loads. A similar network is essential to obtain in tissue-engineered (TE) samples of the same nature. Yet, the mechanism of collagen orientation is not fully understood. Typically collagen remodelling is linked to mechanical loading. However, TE constructs also show an oriented collagen network when developed under static culture. Experiments under these conditions also indicate that the tissue gradually compacts due to contractile stresses developed in the α-actin fibres of the cells. Therefore, it is hypothesised that cellular contractile stresses are responsible for collagen orientation. A model describing the cellular α-actin turnover and the stresses developed by them is integrated in a structural constitutive model describing the mechanical behaviour of collagen fibres. Results show that the model can successfully capture the sample compaction, tissue stress generation and its heterogeneous collagen arrangement.

Variation in tissue outcome of ovine and human engineered heart valve constructs: relevance for tissue engineering

AIM

Clinical application of tissue engineered heart valves requires precise control of the tissue culture process to predict tissue composition and mechanical properties prior to implantation, and to understand the variation in tissue outcome. To this end we investigated cellular phenotype and tissue properties of ovine (n = 8) and human (n = 7) tissue engineered heart valve constructs to quantify variations in tissue outcome within species, study the differences between species and determine possible indicators of tissue outcome.

MATERIALS & METHODS

Tissue constructs consisted of polyglycolic acid/poly-4-hydroxybutyrate scaffolds, seeded with myofibroblasts obtained from the jugular vein (sheep) or the saphenous vein (from humans undergoing cardiac surgery) and cultured under static conditions. Prior to seeding, protein expression of α-smooth muscle actin, vimentin, nonmuscle myosin heavy chain and heat shock protein 47 were determined to identify differences at an early stage of the tissue engineering process. After 4 weeks of culture, tissue composition and mechanical properties were quantified as indicators of tissue outcome.

RESULTS

After 4 weeks of tissue culture, tissue properties of all ovine constructs were comparable, while there was a larger variation in the properties of the human constructs, especially the elastic modulus and collagen content. In addition, ovine constructs differed in composition from the human constructs. An increased number of α-smooth muscle actin-positive cells before seeding was correlated with the collagen content in the engineered heart valve constructs. Moreover, tissue stiffness increased with increasing collagen content.

CONCLUSION

The results suggest that the culture process of ovine tissues can be controlled, whereas the mechanical properties, and hence functionality, of tissues originating from human material are more difficult to control. On-line evaluation of tissue properties during culture or more early cellular markers to predict the properties of autologous tissues cultured for individual patients are, therefore, of utmost importance for future clinical application of autologous heart valve tissue engineering. As an example, this study shows that α-smooth muscle actin might be an indicator of tissue mechanical properties.