A Structural Model for the Flexural Mechanics of Nonwoven Tissue Engineering Scaffolds

Mechano-regulated cell-cell signaling in the context of cardiovascular tissue engineering

Cardiovascular tissue engineering (CVTE) aims to create living tissues, with the ability to grow and remodel, as replacements for diseased blood vessels and heart valves. Despite promising results, the (long-term) functionality of these engineered tissues still needs improvement to reach broad clinical application. The functionality of native tissues is ensured by their specific mechanical properties directly arising from tissue organization. We therefore hypothesize that establishing a native-like tissue organization is vital to overcome the limitations of current CVTE approaches. To achieve this aim, a better understanding of the growth and remodeling (G&R) mechanisms of cardiovascular tissues is necessary. Cells are the main mediators of tissue G&R, and their behavior is strongly influenced by both mechanical stimuli and cell-cell signaling. An increasing number of signaling pathways has also been identified as mechanosensitive. As such, they may have a key underlying role in regulating the G&R of tissues in response to mechanical stimuli. A more detailed understanding of mechano-regulated cell-cell signaling may thus be crucial to advance CVTE, as it could inspire new methods to control tissue G&R and improve the organization and functionality of engineered tissues, thereby accelerating clinical translation. In this review, we discuss the organization and biomechanics of native cardiovascular tissues; recent CVTE studies emphasizing the obtained engineered tissue organization; and the interplay between mechanical stimuli, cell behavior, and cell-cell signaling. In addition, we review past contributions of computational models in understanding and predicting mechano-regulated tissue G&R and cell-cell signaling to highlight their potential role in future CVTE strategies.

Anisotropic Fiber-Reinforced Glycosaminoglycan Hydrogels for Heart Valve Tissue Engineering

This study investigates polymer fiber-reinforced protein-polysaccharide-based hydrogels for heart valve tissue engineering applications. Polycaprolactone and gelatin (3:1) blends were jet-spun to fabricate aligned fibers that possessed fiber diameters in the range found in the native heart valve. These fibers were embedded in methacrylated hydrogels made from gelatin, sodium hyaluronate, and chondroitin sulfate to create fiber-reinforced hydrogel composites (HCs). The fiber-reinforced gelatin glycosaminoglycan (GAG)-based HC possessed interconnected porous structures and porosity higher than fiber-only conditions. These fiber-reinforced HCs exhibited compressive modulus and biaxial mechanical behavior comparable to that of native porcine aortic valves. The fiber-reinforced HCs were able to swell higher and degraded less than the hydrogels. Elution studies revealed that less than 20% of incorporated gelatin methacrylate and GAGs were released over 2 weeks, with a steady-state release after the first day. When cultured with porcine valve interstitial cells (VICs), the fiber-reinforced composites were able to maintain higher cell viability compared with fiber-only samples. Quiescent VICs expressed alpha smooth muscle actin and calponin showing an activated phenotype, along with a few cells expressing the proliferation marker Ki67 and negative expression for RUNX2, an osteogenic marker. Our study demonstrated that compared with the hydrogels and fibers alone, combining both components can yield durable, reinforced composites that mimic heart valve mechanical behavior, while maintaining high cell viability and expressing positive activation as well as proliferation markers.

A Tensile Constitutive Relationship and a Finite Element Model of Electrospun Nanofibrous Mats

It is difficult to establish a numerical model for a certain structure of electrospun nanofibrous mats, due to their high porosity and non-linear characteristics, that can fully consider these characteristics and describe their mechanical behaviors. In this paper, an analytical method of meso-mechanics was adopted to establish the tensile constitutive relationship between a single fiber and mats from fiber-web microstructures. Meanwhile, a macroscopic finite element model was developed and verified through uniaxial tensile stress-strain experimental data of silk fibroin (SF)/polycaprolactone (PCL) nanofibrous mats. The compared results show that the constitutive relation and finite element model could satisfactorily express elastic-plastic tensile mechanical behaviors of the polymer. This model helps regulate the microstructure of nanofibrous mats to meet the mechanical requirements in engineering applications.

A mathematical model for the determination of forming tissue moduli in needled-nonwoven scaffolds

Formation of engineering tissues (ET) remains an important scientific area of investigation for both clinical translational and mechanobiological studies. Needled-nonwoven (NNW) scaffolds represent one of the most ubiquitous biomaterials based on their well-documented capacity to sustain tissue formation and the unique property of substantial construct stiffness amplification, the latter allowing for very sensitive determination of forming tissue modulus. Yet, their use in more fundamental studies is hampered by the lack of: (1) substantial understanding of the mechanics of the NNW scaffold itself under finite deformations and means to model the complex mechanical interactions between scaffold fibers, cells, and de novo tissue; and (2) rational models with reliable predictive capabilities describing their evolving mechanical properties and their response to mechanical stimulation. Our objective is to quantify the mechanical properties of the forming ET phase in constructs that utilize NNW scaffolds. We present herein a novel mathematical model to quantify their stiffness based on explicit considerations of the modulation of NNW scaffold fiber-fiber interactions and effective fiber stiffness by surrounding de novo ECM. Specifically, fibers in NNW scaffolds are effectively stiffer than if acting alone due to extensive fiber-fiber cross-over points that impart changes in fiber geometry, particularly crimp wavelength and amplitude. Fiber-fiber interactions in NNW scaffolds also play significant role in the bulk anisotropy of the material, mainly due to fiber buckling and large translational out-of-plane displacements occurring to fibers undergoing contraction. To calibrate the model parameters, we mechanically tested impregnated NNW scaffolds with polyacrylamide (PAM) gels with a wide range of moduli with values chosen to mimic the effects of surrounding tissues on the scaffold fiber network. Results indicated a high degree of model fidelity over a wide range of planar strains. Lastly, we illustrated the impact of our modeling approach quantifying the stiffness of engineered ECM after in vitro incubation and early stages of in vivo implantation obtained in a concurrent study of engineered tissue pulmonary valves in an ovine model.

STATEMENT OF SIGNIFICANCE

Regenerative medicine has the potential to fully restore diseased tissues or entire organs with engineered tissues. Needled-nonwoven scaffolds can be employed to serve as the support for their growth. However, there is a lack of understanding of the mechanics of these materials and their interactions with the forming tissues. We developed a mathematical model for these scaffold-tissue composites to quantify the mechanical properties of the forming tissues. Firstly, these measurements are pivotal to achieve functional requirements for tissue engineering implants; however, the theoretical development yielded critical insight into particular mechanisms and behaviors of these scaffolds that were not possible to conjecture without the insight given by modeling, let alone describe or foresee a priori.

Biomechanical Behavior of Bioprosthetic Heart Valve Heterograft Tissues: Characterization, Simulation, and Performance

The use of replacement heart valves continues to grow due to the increased prevalence of valvular heart disease resulting from an ageing population. Since bioprosthetic heart valves (BHVs) continue to be the preferred replacement valve, there continues to be a strong need to develop better and more reliable BHVs through and improved the general understanding of BHV failure mechanisms. The major technological hurdle for the lifespan of the BHV implant continues to be the durability of the constituent leaflet biomaterials, which if improved can lead to substantial clinical impact. In order to develop improved solutions for BHV biomaterials, it is critical to have a better understanding of the inherent biomechanical behaviors of the leaflet biomaterials, including chemical treatment technologies, the impact of repetitive mechanical loading, and the inherent failure modes. This review seeks to provide a comprehensive overview of these issues, with a focus on developing insight on the mechanisms of BHV function and failure. Additionally, this review provides a detailed summary of the computational biomechanical simulations that have been used to inform and develop a higher level of understanding of BHV tissues and their failure modes. Collectively, this information should serve as a tool not only to infer reliable and dependable prosthesis function, but also to instigate and facilitate the design of future bioprosthetic valves and clinically impact cardiology.

High compliance vascular grafts based on semi-interpenetrating networks

Current synthetic vascular grafts have poor patency rates in small diameter applications (<6 mm) due to intimal hyperplasia arising from a compliance mismatch between the graft and native vasculature. Enormous efforts have focused on improving biomechanical properties; however, polymeric grafts are often constrained by an inverse relationship between burst pressure and compliance. We have developed a new, semi-interpenetrating network (semi-IPN) approach to improve compliance without sacrificing burst pressure. The effects of heat treatment on graft morphology, fiber architecture, and resultant biomechanical properties are presented. In addition, biomechanical properties after equilibration at physiological temperature were investigated in relation to polyurethane microstructure to better predict in vivo performance. Compliance values as high as 9.2 ± 2.7 %/mmHg x 10-4 were observed for the semi-IPN graft while also maintaining high burst pressure, 1780 ± 230 mm Hg. The high compliance of these heat-treated poly(carbonate urethane) (PCU) and semi-IPN grafts is expected to improve long-term patency rates beyond even saphenous vein autografts by preventing intimal hyperplasia. The fundamental structure-property relationships gained from this work may also be utilized to advance biomedical device designs based on thermoplastic polyurethanes.

Tri-layered elastomeric scaffolds for engineering heart valve leaflets

Tissue engineered heart valves (TEHVs) that can grow and remodel have the potential to serve as permanent replacements of the current non-viable prosthetic valves particularly for pediatric patients. A major challenge in designing functional TEHVs is to mimic both structural and anisotropic mechanical characteristics of the native valve leaflets. To establish a more biomimetic model of TEHV, we fabricated tri-layered scaffolds by combining electrospinning and microfabrication techniques. These constructs were fabricated by assembling microfabricated poly(glycerol sebacate) (PGS) and fibrous PGS/poly(caprolactone) (PCL) electrospun sheets to develop elastic scaffolds with tunable anisotropic mechanical properties similar to the mechanical characteristics of the native heart valves. The engineered scaffolds supported the growth of valvular interstitial cells (VICs) and mesenchymal stem cells (MSCs) within the 3D structure and promoted the deposition of heart valve extracellular matrix (ECM). MSCs were also organized and aligned along the anisotropic axes of the engineered tri-layered scaffolds. In addition, the fabricated constructs opened and closed properly in an ex vivo model of porcine heart valve leaflet tissue replacement. The engineered tri-layered scaffolds have the potential for successful translation towards TEHV replacements.

Design and testing of a cyclic stretch and flexure bioreactor for evaluating engineered heart valve tissues based on poly(glycerol sebacate) scaffolds

Cyclic flexure and stretch are essential to the function of semilunar heart valves and have demonstrated utility in mechanically conditioning tissue-engineered heart valves. In this study, a cyclic stretch and flexure bioreactor was designed and tested in the context of the bioresorbable elastomer poly(glycerol sebacate). Solid poly(glycerol sebacate) membranes were subjected to cyclic stretch, and micromolded poly(glycerol sebacate) scaffolds seeded with porcine aortic valvular interstitial cells were subjected to cyclic stretch and flexure. The results demonstrated significant effects of cyclic stretch on poly(glycerol sebacate) mechanical properties, including significant decreases in effective stiffness versus controls. In valvular interstitial cell-seeded scaffolds, cyclic stretch elicited significant increases in DNA and collagen content that paralleled maintenance of effective stiffness. This work provides a basis for investigating the roles of mechanical loading in the formation of tissue-engineered heart valves based on elastomeric scaffolds.

Electrospun PGS:PCL microfibers align human valvular interstitial cells and provide tunable scaffold anisotropy

Tissue engineered heart valves (TEHV) can be useful in the repair of congenital or acquired valvular diseases due to their potential for growth and remodeling. The development of biomimetic scaffolds is a major challenge in heart valve tissue engineering. One of the most important structural characteristics of mature heart valve leaflets is their intrinsic anisotropy, which is derived from the microstructure of aligned collagen fibers in the extracellular matrix (ECM). In the present study, a directional electrospinning technique is used to fabricate fibrous poly(glycerol sebacate):poly(caprolactone) (PGS:PCL) scaffolds containing aligned fibers, which resemble native heart valve leaflet ECM networks. In addition, the anisotropic mechanical characteristics of fabricated scaffolds are tuned by changing the ratio of PGS:PCL to mimic the native heart valve's mechanical properties. Primary human valvular interstitial cells (VICs) attach and align along the anisotropic axes of all PGS:PCL scaffolds with various mechanical properties. The cells are also biochemically active in producing heart-valve-associated collagen, vimentin, and smooth muscle actin as determined by gene expression. The fibrous PGS:PCL scaffolds seeded with human VICs mimick the structure and mechanical properties of native valve leaflet tissues and would potentially be suitable for the replacement of heart valves in diverse patient populations.

Electrospun vascular grafts with improved compliance matching to native vessels

Coronary artery bypass grafting is one of the most commonly performed major surgeries in the United States. Autologous vessels such as the saphenous vein are the current gold standard for treatment; however, synthetic vascular prostheses made of expanded poly(tetrafluoroethylene) or poly(ethylene terephthalate) are used when autologous vessels are unavailable. These synthetic grafts have a high failure rate in small diameter (<4 mm) applications due to rapid reocclusion via intimal hyperplasia. Current strategies to improve clinical performance are focused on preventing intimal hyperplasia by fabricating grafts with compliance and burst pressure similar to native vessels. To this end, we have developed an electrospun vascular graft from segmented polyurethanes with tunable properties by altering material chemistry and graft microarchitecture. Relationships between polyurethane tensile properties and biomechanical properties were elucidated to select polymers with desirable properties. Graft thickness, fiber tortuosity, and fiber fusions were modulated to provide additional tools for controlling graft properties. Using a combination of these strategies, a vascular graft with compliance and burst pressure exceeding the saphenous vein autograft was fabricated (compliance = 6.0 ± 0.6%/mmHg × 10(-4) , burst pressure = 2260 ± 160 mmHg). This graft is hypothesized to reduce intimal hyperplasia associated with low compliance in synthetic grafts and improve long-term clinical success. Additionally, the fundamental relationships between electrospun mesh microarchitecture and mechanical properties identified in this work can be utilized in various biomedical applications.

Laser microfabricated poly(glycerol sebacate) scaffolds for heart valve tissue engineering

Microfabricated poly(glycerol sebacate) (PGS) scaffolds may be applicable to tissue engineering heart valve leaflets by virtue of their controllable microstructure, stiffness, and elasticity. In this study, PGS scaffolds were computationally designed and microfabricated by laser ablation to match the anisotropy and peak tangent moduli of native bovine aortic heart valve leaflets. Finite element simulations predicted PGS curing conditions, scaffold pore shape, and strut width capable of matching the scaffold effective stiffnesses to the leaflet peak tangent moduli. On the basis of simulation predicted effective stiffnesses of 1.041 and 0.208 MPa for the scaffold preferred (PD) and orthogonal, cross-preferred (XD) material directions, scaffolds with diamond-shaped pores were microfabricated by laser ablation of PGS cured 12 h at 160°C. Effective stiffnesses measured for the scaffold PD (0.83 ± 0.13 MPa) and XD (0.21 ± 0.03 MPa) were similar to both predicted values and peak tangent moduli measured for bovine aortic valve leaflets in the circumferential (1.00 ± 0.16 MPa) and radial (0.26 ± 0.03 MPa) directions. Scaffolds cultivated with fibroblasts for 3 weeks accumulated collagen (736 ± 193 μg/g wet weight) and DNA (17 ± 4 μg/g wet weight). This study provides a basis for the computational design of biomimetic microfabricated PGS scaffolds for tissue-engineered heart valves.

Constitutive modeling of coronary arterial media--comparison of three model classes

Accurate modeling of arterial elasticity is imperative for predicting pulsatile blood flow and transport to the periphery, and for evaluating the mechanical microenvironment of the vessel wall. The goal of the present study is to compare a recently developed structural model of porcine left anterior descending artery media to two commonly used typical representatives of phenomenological and structure-motivated invariant-based models, in terms of the number of model parameters, model descriptive and predictive powers, and requisite different test protocols for reliable parameter estimation. The three models were compared against 3D data of radial inflation, axial extension, and twist tests. Also checked are the models predictive capabilities to response data not used for estimation, including both tests outside the range of estimation database, as well as protocols of a different nature. The results show that the descriptive estimation error (model fit to estimation database), measured by the sum of squared residuals (SSE) between full 3D data and model predictions, was about twice as low for the structural (4.58%) model compared to the other two (9.71 and 8.99% for the phenomenological and structure-motivated models, respectively). Similar SSE ratios were obtained for the predictive capabilities. Prediction SSE at high stretch based on estimation of two low stretches yielded an SSE value of 2.81% for the structural model, and 10.54% and 7.87% for the phenomenological and structure-motivated models, respectively. For the prediction of twist from inflation-extension data, SSE values for the torsional stiffness was 1.76% for the structural model and 39.62 and 2.77% for the phenomenological and structure-motivated models. The required number of model parameters for the structural model is four, whereas the phenomenological model requires six to nine and the structure-motivated has four parameters. These results suggest that modeling based on the tissue structural features improves model reliability in describing given data and in predicting the tissue general response.

Finite element analysis of an accordion-like honeycomb scaffold for cardiac tissue engineering

Optimizing the function of tissue engineered cardiac muscle is becoming more feasible with the development of microfabricated scaffolds amenable to mathematical modeling. In the current study, the elastic behavior of a recently developed poly(glycerol sebacate) (PGS) accordion-like honeycomb (ALH) scaffold [Engelmayr et al., 2008. Nature Materials 7 (12), 1003-1010] was analyzed. Specifically, 2D finite element (FE) models of the ALH unit cell (periodic boundary conditions) and tessellations (kinematic uniform boundary conditions) were utilized to determine a representative volume element (RVE) and to retrospectively predict the elastic effective stiffnesses. An RVE of 90 ALH unit cells (≃3.18×4.03mm) was found, indicating that previous experimental uni-axial test samples were mechanically representative. For ALH scaffolds microfabricated from PGS cured 7.5h at 160°C, FE predicted effective stiffnesses in the two orthogonal material directions (0.081±0.012 and 0.033±0.005MPa) matched published experimental data (0.083±0.004 and 0.031±0.002MPa) within 2.4% and 6.4%. Of potential use as a design criterion, model predicted global strain amplifications were lower in ALH (0.54 and 0.34) versus rectangular honeycomb (1.19 and 0.74) scaffolds, appearing to be inversely correlated with previously measured strains-to-failure. Important in matching the anisotropic mechanical properties of native cardiac muscle, FE predicted ALH scaffolds with 50μm wide PGS struts to be maximally anisotropic. The FE model will thus be useful in designing future variants of the ALH pore geometry that simultaneously provide proper cardiac anisotropy and reduced stiffness to enhance heart cell-mediated contractility.

Three-dimensional quantitative micromorphology of pre- and post-implanted engineered heart valve tissues

There is a significant gap in our knowledge of engineered heart valve tissue (EHVT) development regarding detailed three-dimensional (3D) tissue formation and remodeling from the point of in vitro culturing to full in vivo function. As a step toward understanding the complexities of EHVT formation and remodeling, a novel serial confocal microscopy technique was employed to obtain 3D microstructural information of pre-implant (PRI) and post-implant for 12 weeks (POI) EHVT fabricated from PGA:PLLA scaffolds and seeded with ovine bone-marrow-derived mesenchymal stem cells. Custom scaffold fiber tracking software was developed to quantify scaffold fiber architectural features such as length, tortuosity, and minimum scaffold fiber-fiber separation distance and scaffold fiber orientation was quantified utilizing a 3D fabric tensor. In addition, collagen and cellular density of ovine pulmonary valve leaflet tissue were also analyzed for baseline comparisons. Results indicated that in the unseeded state, scaffold fibers formed a continuous, oriented network. In the PRI state, the scaffold showed some fragmentation with a scaffold volume fraction of 7.79%. In the POI specimen, the scaffold became highly fragmented, forming a randomly distributed short fibrous network (volume fraction of 2.03%) within a contiguous, dense collagenous matrix. Both PGA and PLLA scaffold fibers were observed in the PRI and POI specimens. Collagen density remained similar in both PRI and POI specimens (74.2 and 71.5%, respectively), though the distributions in the transmural direction appeared slightly more homogenous in the POI specimen. Finally, to guide future 2D histological studies for large-scale studies (since acquisition of high-resolution volumetric data is not practical for all specimens), we investigated changes in relevant collagen and scaffold metrics (collagen density and scaffold fiber orientation) with varying section spacing. It was found that a sectioning spacing up to 25 μm (for scaffold morphology) and 50 μm (for collagen density) in both PRI and POI tissues did not result in loss of information fidelity, and that sectioning in the circumferential or radial direction provides the greatest preservation of information. This is the first known work to investigate EHVT microstructure over a large volume with high resolution and to investigate time evolving in vivo EHVT morphology. The important scaffold fiber structural changes observed provide morphological information crucial for guiding future structurally based constitutive modeling efforts focused on better understanding EHVT tissue formation and remodeling.

On the biomechanical function of scaffolds for engineering load-bearing soft tissues

Replacement or regeneration of load-bearing soft tissues has long been the impetus for the development of bioactive materials. While maturing, current efforts continue to be confounded by our lack of understanding of the intricate multi-scale hierarchical arrangements and interactions typically found in native tissues. The current state of the art in biomaterial processing enables a degree of controllable microstructure that can be used for the development of model systems to deduce fundamental biological implications of matrix morphologies on cell function. Furthermore, the development of computational frameworks which allow for the simulation of experimentally derived observations represents a positive departure from what has mostly been an empirically driven field, enabling a deeper understanding of the highly complex biological mechanisms we wish to ultimately emulate. Ongoing research is actively pursuing new materials and processing methods to control material structure down to the micro-scale to sustain or improve cell viability, guide tissue growth, and provide mechanical integrity, all while exhibiting the capacity to degrade in a controlled manner. The purpose of this review is not to focus solely on material processing but to assess the ability of these techniques to produce mechanically sound tissue surrogates, highlight the unique structural characteristics produced in these materials, and discuss how this translates to distinct macroscopic biomechanical behaviors.

Characterization of the complete fiber network topology of planar fibrous tissues and scaffolds

Understanding how engineered tissue scaffold architecture affects cell morphology, metabolism, phenotypic expression, as well as predicting material mechanical behavior has recently received increased attention. In the present study, an image-based analysis approach that provides an automated tool to characterize engineered tissue fiber network topology is presented. Micro-architectural features that fully defined fiber network topology were detected and quantified, which include fiber orientation, connectivity, intersection spatial density, and diameter. Algorithm performance was tested using scanning electron microscopy (SEM) images of electrospun poly(ester urethane)urea (ES-PEUU) scaffolds. SEM images of rabbit mesenchymal stem cell (MSC) seeded collagen gel scaffolds and decellularized rat carotid arteries were also analyzed to further evaluate the ability of the algorithm to capture fiber network morphology regardless of scaffold type and the evaluated size scale. The image analysis procedure was validated qualitatively and quantitatively, comparing fiber network topology manually detected by human operators (n = 5) with that automatically detected by the algorithm. Correlation values between manual detected and algorithm detected results for the fiber angle distribution and for the fiber connectivity distribution were 0.86 and 0.93 respectively. Algorithm detected fiber intersections and fiber diameter values were comparable (within the mean +/- standard deviation) with those detected by human operators. This automated approach identifies and quantifies fiber network morphology as demonstrated for three relevant scaffold types and provides a means to: (1) guarantee objectivity, (2) significantly reduce analysis time, and (3) potentiate broader analysis of scaffold architecture effects on cell behavior and tissue development both in vitro and in vivo.

The role of organ level conditioning on the promotion of engineered heart valve tissue development in-vitro using mesenchymal stem cells

We have previously shown that combined flexure and flow (CFF) augment engineered heart valve tissue formation using bone marrow-derived mesenchymal stem cells (MSC) seeded on polyglycolic acid (PGA)/poly-L-lactic acid (PLLA) blend nonwoven fibrous scaffolds (Engelmayr, et al., Biomaterials 2006; vol. 27 pp. 6083-95). In the present study, we sought to determine if these phenomena were reproducible at the organ level in a functional tri-leaflet valve. Tissue engineered valve constructs (TEVC) were fabricated using PGA/PLLA nonwoven fibrous scaffolds then seeded with MSCs. Tissue formation rates using both standard and augmented (using basic fibroblast growth factor [bFGF] and ascorbic acid-2-phosphate [AA2P]) media to enhance the overall production of collagen were evaluated, along with their relation to the local fluid flow fields. The resulting TEVCs were statically cultured for 3 weeks, followed by a 3 week dynamic culture period using our organ level bioreactor (Hildebrand et al., ABME, Vol. 32, pp. 1039-49, 2004) under approximated pulmonary artery conditions. Results indicated that supplemented media accelerated collagen formation (approximately 185% increase in collagen mass/MSC compared to standard media), as well as increasing collagen mass production from 3.90 to 4.43 pg/cell/week from 3 to 6 weeks. Using augmented media, dynamic conditioning increased collagen mass production rate from 7.23 to 13.65 pg/cell/week (88.8%) during the dynamic culture period, along with greater preservation of net DNA. Moreover, when compared to our previous CFF study, organ level conditioning increased the collagen production rate from 4.76 to 6.42 pg/cell/week (35%). Newly conducted CFD studies of the CFF specimen flow patterns suggested that oscillatory surface shear stresses were surprisingly similar to a tri-leaflet valve. Overall, we found that the use of simulated pulmonary artery conditions resulted in substantially larger collagen mass production levels and rates found in our earlier CFF study. Moreover, given the fact that the scaffolds underwent modest strains (approximately 7% max) during either CFF or physiological conditioning, the oscillatory surface shear stresses estimated in both studies may play a substantial role in eliciting MSC collagen production in the highly dynamic engineered heart valve fluid mechanical environment.

A bilayered elastomeric scaffold for tissue engineering of small diameter vascular grafts

A major barrier to the development of a clinically useful small diameter tissue engineered vascular graft (TEVG) is the scaffold component. Scaffold requirements include matching the mechanical and structural properties with those of native vessels and optimizing the microenvironment to foster cell integration, adhesion and growth. We have developed a small diameter, bilayered, biodegradable, elastomeric scaffold based on a synthetic, biodegradable elastomer. The scaffold incorporates a highly porous inner layer, allowing cell integration and growth, and an external, fibrous reinforcing layer deposited by electrospinning. Scaffold morphology and mechanical properties were assessed, quantified and compared with those of native vessels. Scaffolds were then seeded with adult stem cells using a rotational vacuum seeding device to obtain a TEVG, cultured under dynamic conditions for 7 days and evaluated for cellularity. The scaffold showed firm integration of the two polymeric layers with no delamination. Mechanical properties were physiologically consistent, showing anisotropy, an elastic modulus (1.4 + or - 0.4 MPa) and an ultimate tensile stress (8.3 + or - 1.7 MPa) comparable with native vessels. The compliance and suture retention forces were 4.6 + or - 0.5 x 10(-4) mmHg(-1) and 3.4 + or - 0.3N, respectively. Seeding resulted in a rapid, uniform, bulk integration of cells, with a seeding efficiency of 92 + or - 1%. The scaffolds maintained a high level of cellular density throughout dynamic culture. This approach, combining artery-like mechanical properties and a rapid and efficient cellularization, might contribute to the future clinical translation of TEVGs.

Bioengineering challenges for heart valve tissue engineering

Surgical replacement of diseased heart valves by mechanical and tissue valve substitutes is now commonplace and enhances survival and quality of life for many patients. However, repairs of congenital deformities require very small valve sizes not commercially available. Further, a fundamental problem inherent to the use of existing mechanical and biological prostheses in the pediatric population is their failure to grow, repair, and remodel. It is believed that a tissue engineered heart valve can accommodate many of these requirements, especially those pertaining to somatic growth. This review provides an overview of the field of heart valve tissue engineering, including recent trends, with a focus on the bioengineering challenges unique to heart valves. We believe that, currently, the key bioengineering challenge is to determine how biological, structural, and mechanical factors affect extracellular matrix (ECM) formation and in vivo functionality. These factors are fundamental to any approach toward developing a clinically viable tissue engineered heart valve (TEHV), regardless of the particular approach. Critical to the current approaches to TEHVs is scaffold design, which must simultaneously provide function (valves must function from the time of implant) as well as stress transfer to the new ECM. From a bioengineering point of view, a hierarchy of approaches will be necessary to connect the organ-tissue relationships with underpinning cell and sub-cellular events. Overall, such approaches need to be structured to address these fundamental issues to lay the basis for TEHVs that can be developed and designed according to truly sound scientific and engineering principles.

Periodontal ligament cellular structures engineered with electrospun poly(DL-lactide-co-glycolide) nanofibrous membrane scaffolds

Periodontal tissue engineering is expected to overcome the limitations associated with the existing regenerative techniques for the treatment of periodontal defects involving alveolar bone, cementum, and periodontal ligament. Cell-based tissue engineering approaches involve the utilization of in vitro expanded cells with regenerative capacity and their delivery to the appropriate sites via biomaterial scaffolds. The aim of this study was to establish living periodontal ligament cell-containing structures on electrospun poly(DL-lactic-co-glycolic acid) (PLGA) nanofiber membrane scaffolds, assess their viability and characteristics, and engineer multilayered structures amenable to easy handling. Human periodontal ligament (hPDL) cells were expanded in explant culture and then characterized morphologically and immunohistochemically. PLGA nanofiber membranes were prepared by the electrospinning process; mechanical tensile properties were determined, surface topography, nanofiber size, and porosity status were investigated with SEM. Cells were seeded on the membranes at approximately 50,000 cell/cm(2) and cultured for 21 days either in expansion or in osteogenic induction medium. Cell adhesion and viability were demonstrated using SEM and MTT, respectively, and osteogenic differentiation was determined with IHC and immunohistomorphometric evaluation of osteopontin, osteocalcin, and bone sialoprotein marker expression. At days 3, 6, 9, and 12 additional cell/membrane layers were deposited on the existing ones and multilayered hybrid structures were established. Results indicate the feasibility of periodontal ligament cell-containing tissue-like structures engineering with PDL cells and electrospun nanofiber PLGA scaffolds supporting cell adhesion, viability and osteogenic differentiation properties of cells in hybrid structures amenable to macroscopic handling.

Microstructure and mechanics of collagen-fibrin matrices polymerized using ancrod snake venom enzyme

The relationship between microstructural features and macroscopic mechanical properties of engineered tissues was investigated in pure and mixed composite scaffolds consisting of collagen Type I and fibrin proteins containing embedded smooth muscle cells. In order to vary the matrix microstructure, fibrin polymerization in mixed constructs was initiated using either the blood-derived enzyme thrombin or the snake venom-derived enzyme ancrod, each at low and high concentrations. Microstructural features of the matrix were quantified by analysis of high resolution scanning electron micrographs. Mechanical properties of the scaffolds were assessed by uniaxial tensile testing as well as creep testing. Viscoelastic parameters were determined by fitting creep data to Burger's four-parameter model. Oscillatory dynamic mechanical testing was used to determine the storage modulus, loss modulus, and phase shift of each matrix type. Mixed composite scaffolds exhibited improved tensile stiffness and strength, relative to pure collagen matrices, as well as decreased deformation and slower relaxation in creep tests. Storage and loss moduli were increased in mixed composites compared with pure collagen, while phase shift was reduced. A correlation analysis showed that the number of fiber bundles per unit volume was positively correlated with matrix modulus, strength, and dynamic moduli, though this parameter was negatively correlated with phase shift. Fiber diameter also was negatively correlated with scaffold strength. This study demonstrates how microstructural features can be related to the mechanical function of protein matrices and provides insight into structure-function relationships in such materials. This information can be used to identify and promote desirable microstructural features when designing biomaterials and engineered tissues.

A novel flex-stretch-flow bioreactor for the study of engineered heart valve tissue mechanobiology

Tissue engineered heart valves (TEHV) have been observed to respond to mechanical conditioning in vitro by expression of activated myofibroblast phenotypes followed by improvements in tissue maturation. In separate studies, cyclic flexure, stretch, and flow (FSF) have been demonstrated to exhibit both independent and coupled stimulatory effects. Synthesis of these observations into a rational framework for TEHV mechanical conditioning has been limited, however, due to the functional complexity of tri-leaflet valves and the inherent differences of separate bioreactor systems. Toward quantifying the effects of individual mechanical stimuli similar to those that occur during normal valve function, a novel bioreactor was developed in which FSF mechanical stimuli can be applied to engineered heart valve tissues independently or in combination. The FSF bioreactor consists of two identically equipped chambers, each having the capacity to hold up to 12 rectangular tissue specimens (25 x 7.5 x 1 mm) via a novel "spiral-bound" technique. Specimens can be subjected to changes-in-curvature up to 50 mm(-1) and uniaxial tensile strains up to 75%. Steady laminar flow can be applied by a magnetically coupled paddlewheel system. Computational fluid dynamic (CFD) simulations were conducted and experimentally validated by particle image velocimetry (PIV). Tissue specimen wall shear stress profiles were predicted as a function of paddlewheel speed, culture medium viscosity, and the quasi-static state of specimen deformation (i.e., either undeformed or completely flexed). Velocity profiles predicted by 2D CFD simulations of the paddlewheel mechanism compared well with PIV measurements, and were used to determine boundary conditions in localized 3D simulations. For undeformed specimens, predicted inter-specimen variations in wall shear stress were on average +/-7%, with an average wall shear stress of 1.145 dyne/cm(2) predicted at a paddlewheel speed of 2000 rpm and standard culture conditions. In contrast, while the average wall shear stress predicted for specimens in the quasi-static flexed state was approximately 59% higher (1.821 dyne/cm(2)), flexed specimens exhibited a broad intra-specimen wall shear stress distribution between the convex and concave sides that correlated with specimen curvature, with peak wall shear stresses of approximately 10 dyne/cm(2). This result suggests that by utilizing simple flexed geometric configurations, the present system can also be used to study the effects of spatially varying shear stresses. We conclude that the present design provides a robust tool for the study of mechanical stimuli on in vitro engineered heart valve tissue formation.

Prediction of extracellular matrix stiffness in engineered heart valve tissues based on nonwoven scaffolds

The in vitro development of tissue engineered heart valves (TEHV) exhibiting appropriate structural and mechanical characteristics remains a significant challenge. An important step yet to be addressed is establishing the relationship between scaffold and extracellular matrix (ECM) mechanical properties. In the present study, a composite beam model accounting for nonwoven scaffold-ECM coupling and the transmural collagen concentration distribution was developed, and utilized to retrospectively estimate the ECM effective stiffness in TEHV specimens incubated under static and cyclic flexure conditions (Engelmayr Jr et~al. in Biomaterials 26(2):175-187 2005). The ECM effective stiffness was expressed as the product of the local collagen concentration and the collagen specific stiffness (i.e., stiffness/concentration), and was related to the overall TEHV effective stiffness via an empirically determined scaffold-ECM coupling parameter and measured transmural collagen concentration distributions. The scaffold-ECM coupling parameter was determined by flexural mechanical testing of polyacrylamide gels (i.e., ECM analogs) of variable stiffness and associated scaffold-polyacrylamide gel composites (i.e., engineered tissue analogs). The transmural collagen concentration distributions were quantified from fluorescence micrographs of picro-sirius red stained TEHV sections. As suggested by a previous structural model of the nonwoven scaffold (Engelmayr Jr and Sacks in J Biomech Eng 128(4):610-622, 2006), nonwoven scaffold-ECM composites did not follow a traditional rule of mixtures. The present study provided further evidence that the primary mode of reinforcement in nonwoven scaffold-ECM composites is an increase in the number fiber-fiber bonds with a concomitant increase in the effective stiffness of the spring-like fiber segments. Simulations of potential ECM deposition scenarios using the current model indicated that the present approach is sensitive to the specific time course of tissue deposition, and is thus very suitable for studies of ECM formation in engineered heart valve tissues.

Bioprosthetic heart valve heterograft biomaterials: structure, mechanical behavior and computational simulation

The present review surveys significant developments in the biomechanical characterization and computational simulation of biologically derived chemically cross-linked soft tissues, or 'heterograft' biomaterials, used in replacement bioprosthetic heart valve (BHV). A survey of mechanical characterization techniques, relevant mechanical properties and computational simulation approaches is presented for both the source tissues and cross-linked biomaterials. Since durability remains the critical problem with current bioprostheses, changes with the mechanical behavior with fatigue are also presented. Moreover, given the complex nature of the mechanical properties of heterograft biomaterials it is not surprising that most constitutive (stress-strain) models, historically used to characterize their behavior, were oversimplified. Simulations of BHV function utilizing these models have inevitably been inaccurate. Thus, more recent finite element simulations utilizing nonlinear constitutive models, which achieve greater model fidelity, are reviewed. An important conclusion of this review is the need for accurate constitutive models, rigorously validated with appropriate experimental data, in order that the design benefits of computational models can be realized. Finally, for at least the coming 20 years, BHVs fabricated from heterograft biomaterials will continue to be extensively used, and will probably remain as the dominant valve design. We should thus recognize that rational, scientifically based approaches to BHV biomaterial development and design can lead to significantly improved BHV, over the coming decades, which can potentially impact millions of patients worldwide with heart valve disease.

Cyclic flexure and laminar flow synergistically accelerate mesenchymal stem cell-mediated engineered tissue formation: Implications for engineered heart valve tissues

Bone marrow-derived mesenchymal stem cells (BMSCs) are relatively accessible and exhibit a pluripotency suitable for cardiovascular applications such as tissue-engineered heart valves (TEHVs). Recently, Sutherland et al. [From stem cells to viable autologous semilunar heart valve. Circulation 2005; 111(21): 2783-91] demonstrated that BMSC-seeded TEHV can successfully function as pulmonary valve substitutes in juvenile sheep for at least 8 months. Toward determining appropriate mechanical stimuli for use in BMSC-seeded TEHV cultivation, we investigated the independent and coupled effects of two mechanical stimuli physiologically relevant to heart valves-cyclic flexure and laminar flow (i.e. fluid shear stress)-on BMSC-mediated tissue formation. BMSC isolated from juvenile sheep were expanded and seeded onto rectangular strips of nonwoven 50:50 blend poly(glycolic acid) (PGA) and poly(l-lactic acid) (PLLA) scaffolds. Following 4 days static culture, BMSC-seeded scaffolds were loaded into a novel flex-stretch-flow (FSF) bioreactor and incubated under static (n=12), cyclic flexure (n=12), laminar flow (avg. wall shear stress=1.1505 dyne/cm(2); n=12) and combined flex-flow (n=12) conditions for 1 (n=6) and 3 (n=6) weeks. By 3 weeks, the flex-flow group exhibited dramatically accelerated tissue formation compared with all other groups, including a 75% higher collagen content of 844+/-278 microg/g wet weight (p<0.05), and an effective stiffness (E) value of 948+/-233 kPa. Importantly, collagen and E values were not significantly different from values measured for vascular smooth muscle cell (SMC) -seeded scaffolds incubated under conditions of flexure alone [Engelmayr et al. The independent role of cyclic flexure in the early in vitro development of an engineered heart valve tissue. Biomaterials 2005; 26(2): 175-87], suggesting that BMSC-seeded TEHV can be optimized to yield results comparable to SMC-seeded TEHV. We thus demonstrated that cyclic flexure and laminar flow can synergistically accelerate BMSC-mediated tissue formation, providing a basis for the rational design of in vitro conditioning regimens for BMSC-seeded TEHV.

Guidance of engineered tissue collagen orientation by large-scale scaffold microstructures

The tensile strength and stiffness of load-bearing soft tissues are dominated by their collagen fiber orientation. While microgrooved substrates have demonstrated a capacity to orient cells and collagen in monolayer tissue culture, tissue engineering (TE) scaffolds are structurally distinct in that they consist of a three-dimensional (3-D) open pore network. It is thus unclear how the geometry of these open pores might influence cell and collagen orientation. In the current study we developed an in vitro model system for quantifying the capacity of large scale ( approximately 200 microm), geometrically well-defined open pores to guide cell and collagen orientation in engineered tissues. Non-degradable scaffolds exhibiting a grid of 200 microm wide rectangular pores (1:1, 2:1, 5:1, and 10:1 aspect ratios) were fabricated from a transparent epoxy resin via high-resolution stereolithography. The scaffolds (n=6 per aspect ratio) were surface modified to support cell adhesion by covalently grafting GRGDS peptides, sterilized, and seeded with neonatal rat skin fibroblasts. Following 4 weeks of static incubation, the resultant collagen orientation was assessed quantitatively by small angle light scattering (SALS), and cell orientation was evaluated by laser confocal and scanning electron microscopy. Cells adhered to the struts of the pores and proceeded to span the pores in a generally circumferential pattern. While the cell and collagen orientations within 1:1 aspect ratio pores were effectively random, higher aspect ratio rectangular pores exhibited a significant capacity to guide global cell and collagen orientation. Preferential alignment parallel to the long strut axis and decreased spatial variability were observed to occur with increasing pore aspect ratio. Intra-pore variability depended in part on the spatial uniformity of cell attachment around the perimeter of each pore achieved during seeding. Evaluation of diamond-shaped pores [Sacks, M.S. et al., 1997. J. Biomech. Eng. 119(1), 124-127] suggests that they are less sensitive to initial conditions of cell attachment than rectangular pores, and thus more effective in guiding engineered tissue cell and collagen orientation.