Cell Sourcing and Culture Conditions for Fibrin-Based Valve Constructs

Sandwich-like electro-conductive polyurethane-based gelatin/soybean oil nanofibrous scaffolds with a targeted release of simvastatin for cardiac tissue engineering

Cardiac tissue engineering (CTE) is a promising way for the restoration of injured cardiac tissue in the healthcare system. The development of biodegradable scaffolds with appropriate chemical, electrical, mechanical, and biological properties is an unmet need for the success of CTE. Electrospinning is a versatile technique that has shown potential applications in CTE. Herein, four different types of multifunctional scaffolds, including synthetic-based poly (glycerol sebacate)-polyurethane (PGU), PGU-Soy scaffold, and a series of trilayer scaffolds containing two outer layers of PGU-Soy and a middle (inner) layer of gelatin (G) as a natural and biodegradable macromolecule without simvastatin (S) and with simvastatin (GS), an anti-inflammatory agent, were fabricated in the sandwich-like structure using electrospinning technique. This approach offers a combination of the advantages of both synthetic and natural polymers to enhance the bioactivity and the cell-to-cell and cell-to-matrix intercommunication. An in vitro drug release analysis was performed after the incorporation of soybean oil (Soy) and G. Soy is used as a semiconducting material was introduced to improve the electrical conductivity of nanofibrous scaffolds. The physicochemical properties, contact angle, and biodegradability of the electrospun scaffolds were also assessed. Moreover, the blood compatibility of nanofibrous scaffolds was studied through activated partial thromboplastin time (APTT), prothrombin time (PT), and hemolytic assay. The results showed that all scaffolds exhibited defect-free morphologies with mean fiber diameters in the range of 361 ± 109 to 417 ± 167 nm. A delay in blood clotting was observed, demonstrating the anticoagulant nature of nanofibrous scaffolds. Furthermore, rat cardiomyoblast cell lines (H9C2) were cultured on scaffolds for 7 days, and the morphology and cell arrangement were monitored. Data indicated an appropriate cytocompatibility. Of note, in the PGU-Soy/GS nanofibrous scaffold, a high survival rate was indicated compared to other groups. Our findings exhibited that the simvastatin-loaded polymeric system had positive effects on cardiomyoblasts attachment and growth and could be utilized as a drug release carrier in the field of CTE.

Mechanical and matrix effects of short and long-duration exposure to beta-aminopropionitrile in elastase-induced model abdominal aortic aneurysm in mice

Objective

Evaluate the mechanical and matrix effects on abdominal aortic aneurysms (AAA) during the initial aortic dilation and after prolonged exposure to beta-aminopropionitrile (BAPN) in a topical elastase AAA model.

Methods

Abdominal aortae of C57/BL6 mice were exposed to topical elastase with or without BAPN in the drinking water starting 4 days before elastase exposure. For the standard AAA model, animals were harvested at 2 weeks after active elastase (STD2) or heat-inactivated elastase (SHAM2). For the enhanced elastase model, BAPN treatment continued for either 4 days (ENH2b) or until harvest (ENH2) at 2 weeks; BAPN was continued until harvest at 8 weeks in one group (ENH8). Each group underwent assessment of aortic diameter, mechanical testing (tangent modulus and ultimate tensile strength [UTS]), and quantification of insoluble elastin and bulk collagen in both the elastase exposed aorta as well as the descending thoracic aorta.

Results

BAPN treatment did not increase aortic dilation compared with the standard model after 2 weeks (ENH2, 1.65 ± 0.23 mm; ENH2b, 1.49 ± 0.39 mm; STD2, 1.67 ± 0.29 mm; and SHAM2, 0.73 ± 0.10 mm), but did result in increased dilation after 8 weeks (4.3 ± 2.0 mm; P = .005). After 2 weeks, compared with the standard model, continuous therapy with BAPN did not have an effect on UTS (24.84 ± 7.62 N/cm2; 18.05 ± 4.95 N/cm2), tangent modulus (32.60 ± 9.83 N/cm2; 26.13 ± 9.10 N/cm2), elastin (7.41 ± 2.43%; 7.37 ± 4.00%), or collagen (4.25 ± 0.79%; 5.86 ± 1.19%) content. The brief treatment, EHN2b, resulted in increased aortic collagen content compared with STD2 (7.55 ± 2.48%; P = .006) and an increase in UTS compared with ENH2 (35.18 ± 18.60 N/cm2; P = .03). The ENH8 group had the lowest tangent modulus (3.71 ± 3.10 N/cm2; P = .005) compared with all aortas harvested at 2 weeks and a lower UTS (2.18 ± 2.18 N/cm2) compared with both the STD2 (24.84 ± 7.62 N/cm2; P = .008) and ENH2b (35.18 ± 18.60 N/cm2; P = .001) groups. No differences in the mechanical properties or matrix protein concentrations were associated with abdominal elastase exposure or BAPN treatment for the thoracic aorta. The tangent modulus was higher in the STD2 group (32.60 ± 9.83 N/cm2; P = .0456) vs the SHAM2 group (17.99 ± 5.76 N/cm2), and the UTS was lower in the ENH2 group (18.05 ± 4.95 N/cm2; P = .0292) compared with the ENH2b group (35.18 ± 18.60 N/cm2). The ENH8 group had the lowest tangent modulus (3.71 ± 3.10 N/cm2; P = .005) compared with all aortas harvested at 2 weeks and a lower UTS (2.18 ± 2.18 N/cm2) compared with both the STD2 (24.84 ± 7.62 N/cm2; P = .008) and ENH2b (35.18 ± 18.60 N/cm2; P = .001) groups. Abdominal aortic elastin in the STD2 group (7.41 ± 2.43%; P = .035) was lower compared with the SHAM2 group (15.29 ± 7.66%). Aortic collagen was lower in the STD2 group (4.25 ± 0.79%; P = .007) compared with the SHAM2 group (12.44 ± 6.02%) and higher for the ENH2b (7.55 ± 2.48%; P = .006) compared with the STD2 group.

Conclusions

Enhancing an elastase AAA model with BAPN does not affect the initial (2-week) dilation phase substantially, either mechanically or by altering the matrix content. Late mechanical and matrix effects of prolonged BAPN treatment are limited to the elastase-exposed segment of the aorta.

Clinical Relevance

This paper explores the use of short- and long-term exposure to beta-aminopropionitrile to create an enhanced topical elastase abdominal aortic aneurysm model in mice. Readouts of aneurysm severity included loss of mechanical stability and vascular extracellular matrix composition reminiscent of what is seen in the course of human disease. Additionally, we show that the thoracic aorta, unlike the findings below the renal arteries, is not damaged in our animal model.

Fibrous heart valve leaflet substrate with native-mimicked morphology

Tissue-engineered heart valves are a promising alternative solution to prosthetic valves. However, long-term functionalities of tissue-engineered heart valves depend on the ability to mimic the trilayered, oriented structure of native heart valve leaflets. In this study, using electrospinning, we developed trilayered microfibrous leaflet substrates with morphological characteristics similar to native leaflets. The substrates were implanted subcutaneously in rats to study the effect of their trilayered oriented structure on in vivo tissue engineering. The tissue constructs showed a well-defined structure, with a circumferentially oriented layer, a randomly oriented layer and a radially oriented layer. The extracellular matrix, produced during in vivo tissue engineering, consisted of collagen, glycosaminoglycans, and elastin, all major components of native leaflets. Moreover, the anisotropic tensile properties of the constructs were sufficient to bear the valvular physiological load. Finally, the expression of vimentin and α-smooth muscle actin, at the gene and protein level, was detected in the residing cells, revealing their growing state and their transdifferentiation to myofibroblasts. Our data support a critical role for the trilayered structure and anisotropic properties in functional leaflet tissue constructs, and indicate that the leaflet substrates have the potential for the development of valve scaffolds for heart valve replacements.

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.

Valve Tissue Engineering with Living Absorbable Threads

Tissue engineering (TE) depends on the population of scaffolds with appropriate cells, arranged in a specific physiological direction using a variety of techniques. Here, a novel technique of creating "living threads" is described based on thin (poly(ε-caprolactone) fibers of different diameters (23-243 μm). The fibers readily attract human mesenchymal stem cells (MSCs), which are firmly adhered. These versatile fibers can be used to produce dimensional shapes identical in shape to the cup-like structure of a normal human valve, while preserving the specific orientation of both the cells and the fibers. The MSCs on leaflets and the cells cultured in flask shown similar epitopes expression when analyzed by fluorescence activated cell sorting. Together, these characteristics have important functional implications as living absorbable fibers can be a valuable resource in TE of living tissues, including heart valves.

The potential applications of fibrin-coated electrospun polylactide nanofibers in skin tissue engineering

Fibrin plays an important role during wound healing and skin regeneration. It is often applied in clinical practice for treatment of skin injuries or as a component of skin substitutes. We prepared electrospun nanofibrous membranes made from poly(l-lactide) modified with a thin fibrin nanocoating. Fibrin surrounded the individual fibers in the membrane and also formed a thin fibrous mesh on several places on the membrane surface. The cell-free fibrin nanocoating remained stable in the cell culture medium for 14 days and did not change its morphology. On membranes populated with human dermal fibroblasts, the rate of fibrin degradation correlated with the degree of cell proliferation. The cell spreading, mitochondrial activity, and cell population density were significantly higher on membranes coated with fibrin than on nonmodified membranes, and this cell performance was further improved by the addition of ascorbic acid in the cell culture medium. Similarly, fibrin stimulated the expression and synthesis of collagen I in human dermal fibroblasts, and this effect was further enhanced by ascorbic acid. The expression of beta₁-integrins was also improved by fibrin, and on pure polylactide membranes, it was slightly enhanced by ascorbic acid. In addition, ascorbic acid promoted deposition of collagen I in the form of a fibrous extracellular matrix. Thus, the combination of nanofibrous membranes with a fibrin nanocoating and ascorbic acid seems to be particularly advantageous for skin tissue engineering.

Cyclic Stretch and Perfusion Bioreactor for Conditioning Large Diameter Engineered Tissue Tubes

A cyclic stretch and perfusion bioreactor was designed to culture large diameter engineered tissue tubes for heart valve applications. In this bioreactor, tubular tissues consisting of dermal fibroblasts in a sacrificial fibrin gel scaffold were placed over porated latex support sleeves and mounted in a custom bioreactor. Pulsatile flow of culture medium into the system resulted in cyclic stretching as well as ablumenal, lumenal, and transmural flow (perfusion). In this study, lumenal remodeling, composition, and mechanical strength and stiffness were compared for tissues cyclically stretched in this bioreactor on either the porated latex sleeves or solid latex sleeves, which did not permit lumenal or transmural flow. Tissues cyclically stretched on porated sleeves had regions of increased lumenal remodeling and cellularity that were localized to the columns of pores in the latex sleeve. A CFD model was developed with COMSOL Multiphysics(®) to predict flow of culture medium in and around the tissue, and the predictions suggest that the enhanced lumenal remodeling was likely a result of elevated shear stresses and transmural velocity in these regions. This work highlights the beneficial effects of increased nutrient transport and flow stimulation for accelerating in vitro tissue remodeling.

Effects of Intermittent and Incremental Cyclic Stretch on ERK Signaling and Collagen Production in Engineered Tissue

Intermittent cyclic stretching and incrementally increasing strain amplitude cyclic stretching were explored to overcome the reported adaptation of fibroblasts in response to constant amplitude cyclic stretching, with the goals of accelerating collagen production and understanding the underlying cell signaling. The effects of constant amplitude, intermittent, and incremental cyclic stretching regimens were investigated for dermal fibroblasts entrapped in a fibrin gel by monitoring the extracellular signal-regulated kinase (ERK1/2) and p38 pathways, collagen transcription, and finally the deposited collagen protein. Activation of ERK1/2, which has been shown to be necessary for stretch-induced collagen transcription, was maximal at 15 min and decayed by 1 h. ERK1/2 was reactivated by an additional onset of stretching or by an increment in the strain amplitude 6 h after the initial stimulus, which was approximately the lifetime of activated p38, a known ERK1/2 inhibitor. While both intermittent and incremental regimens reactivated ERK1/2, only incremental stretching increased collagen production compared to samples stretched with constant amplitude, resulting in a 37% increase in collagen per cell after 2 weeks. This suggests that a regimen with small, frequent increments in strain amplitude is optimal for this system and should be used in bioreactors for engineered tissues requiring high collagen content.

Multiple-Step Injection Molding for Fibrin-Based Tissue-Engineered Heart Valves

Heart valves are elaborate and highly heterogeneous structures of the circulatory system. Despite the well accepted relationship between the structural and mechanical anisotropy and the optimal function of the valves, most approaches to create tissue-engineered heart valves (TEHVs) do not try to mimic this complexity and rely on one homogenous combination of cells and materials for the whole construct. The aim of this study was to establish an easy and versatile method to introduce spatial diversity into a heart valve fibrin scaffold. We developed a multiple-step injection molding process that enables the fabrication of TEHVs with heterogeneous composition (cell/scaffold material) of wall and leaflets without the need of gluing or suturing components together, with the leaflets firmly connected to the wall. The integrity of the valves and their functionality was proved by either opening/closing cycles in a bioreactor (proof of principle without cells) or with continuous stimulation over 2 weeks. We demonstrated the potential of the method by the two-step molding of the wall and the leaflets containing different cell lines. Immunohistology after stimulation confirmed tissue formation and demonstrated the localization of the different cell types. Furthermore, we showed the proof of principle fabrication of valves using different materials for wall (fibrin) and leaflets (hybrid gel of fibrin/elastin-like recombinamer) and with layered leaflets. The method is easy to implement, does not require special facilities, and can be reproduced in any tissue-engineering lab. While it has been demonstrated here with fibrin, it can easily be extended to other hydrogels.

Mechanical boundary conditions bias fibroblast invasion in a collagen-fibrin wound model

Because fibroblasts deposit the collagen matrix that determines the mechanical integrity of scar tissue, altering fibroblast invasion could alter wound healing outcomes. Anisotropic mechanical boundary conditions (restraint, stretch, or tension) could affect the rate of fibroblast invasion, but their importance relative to the prototypical drivers of fibroblast infiltration during wound healing--cell and chemokine concentration gradients--is unknown. We tested whether anisotropic mechanical boundary conditions affected the directionality and speed of fibroblasts migrating into a three-dimensional model wound, which could simultaneously expose fibroblasts to mechanical, structural, steric, and chemical guidance cues. We created fibrin-filled slits in fibroblast-populated collagen gels and applied uniaxial mechanical restraint along the short or long axis of the fibrin wounds. Anisotropic mechanical conditions increased the efficiency of fibroblast invasion by guiding fibroblasts without increasing their migration speed. The migration behavior could be modeled as a biased random walk, where the bias due to multiple guidance cues was accounted for in the shape of a displacement orientation probability distribution. Taken together, modeling and experiments suggested an effect of strain anisotropy, rather than strain-induced fiber alignment, on fibroblast invasion.

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.

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.

Cardiac fibroblasts support endothelial cell proliferation and sprout formation but not the development of multicellular sprouts in a fibrin gel co-culture model

A primary impediment to cardiac tissue engineering lies in the inability to adequately vascularize the constructs to optimize survival upon implantation. During normal angiogenesis, endothelial cells (ECs) require a support cell to form mature patent lumens and it has been demonstrated that pericytes, vascular smooth muscle cells and mesenchymal stem cells (MSCs) are all able to support the formation of mature vessels. In the heart, cardiac fibroblasts (CFs) provide important electrical and mechanical functions, but to date have not been sufficiently studied for their role in angiogenesis. To study CFs role in angiogenesis, we co-cultured different concentrations of various cell types in fibrin hemispheres with appropriate combinations of their specific media, to determine the optimal conditions for EC growth and sprout formation through DNA analysis, flow cytometry and immunohistology. ECs proliferated best when co-cultured with CFs and analysis of immunohistological images demonstrated that ECs formed the longest and most numerous sprouts with CFs as compared to MSCs. However, ECs were able to produce more multicellular sprouts when in culture with the MSCs. Moreover, these effects were dependent on the ratio of support cell to EC in co-culture. Overall, CFs provide a good support system for EC proliferation and sprout formation; however, MSCs allow for more multicellular sprouts, which is more indicative of the in vivo process.

Tubular heart valves from decellularized engineered tissue

A novel tissue-engineered heart valve (TEHV) was fabricated from a decellularized tissue tube mounted on a frame with three struts, which upon back-pressure cause the tube to collapse into three coapting "leaflets." The tissue was completely biological, fabricated from ovine fibroblasts dispersed within a fibrin gel, compacted into a circumferentially aligned tube on a mandrel, and matured using a bioreactor system that applied cyclic distension. Following decellularization, the resulting tissue possessed tensile mechanical properties, mechanical anisotropy, and collagen content that were comparable to native pulmonary valve leaflets. When mounted on a custom frame and tested within a pulse duplicator system, the tubular TEHV displayed excellent function under both aortic and pulmonary conditions, with minimal regurgitant fractions and transvalvular pressure gradients at peak systole, as well as well as effective orifice areas exceeding those of current commercially available valve replacements. Short-term fatigue testing of one million cycles with pulmonary pressure gradients was conducted without significant change in mechanical properties and no observable macroscopic tissue deterioration. This study presents an attractive potential alternative to current tissue valve replacements due to its avoidance of chemical fixation and utilization of a tissue conducive to recellularization by host cell infiltration.

Combating Adaptation to Cyclic Stretching By Prolonging Activation of Extracellular Signal-Regulated Kinase

In developing implantable tissues based on cellular remodeling of a fibrin scaffold, a key indicator of success is high collagen content. Cellular collagen synthesis is stimulated by cyclic stretching but is limited by cellular adaptation. Adaptation is mediated by deactivation of extracellular signal-regulated kinase (ERK); therefore inhibition of ERK deactivation should improve mechanically stimulated collagen production and accelerate the development of strong engineered tissues. The hypothesis of this study is that p38 mitogen activated protein kinase (p38) activation by stretching limits ERK activation and that chemical inhibition of p38/isoforms with SB203580 will increase stretching-induced ERK activation and collagen production. Both p38 and ERK were activated by 15 minutes of stretching but only p38 remained active after 1 hour. After an effective dose of inhibitor was identified using cell monolayers, 5 M SB203580 was found to increase ERK activation by two-fold in cyclically stretched fibrin-based tissue constructs. When 5 M SB203580 was added to the culture medium of constructs exposed to three weeks of incremental amplitude cyclic stretch, 2.6 fold higher stretching-induced total collagen was obtained. In conclusion, SB203580 circumvents adaptation to stretching induced collagen production and may be useful in engineering tissues where mechanical strength is a priority.

Decellularized tissue-engineered heart valve leaflets with recellularization potential

Tissue-engineered heart valves (TEHV) have been proposed as a promising solution for the clinical needs of pediatric patients. In vivo studies have shown TEHV leaflet contraction and regurgitation after several months of implantation. This has been attributed to contractile cells utilized to produce the extracellular matrix (ECM) during TEHV culture. Here, we utilized such cells to develop a mature ECM in a fibrin-based scaffold that generates commissural alignment in TEHV leaflets and then removed these cells using detergents. Further, we evaluated recellularization with potentially noncontractile cells. A tissue-engineered leaflet model was developed with mechanical anisotropy and tensile properties comparable to an ovine pulmonary valve leaflet. No change in tensile properties occurred after decellularization using 1% sodium dodecyl sulfate and 1% Triton detergent treatment. Cell removal was verified by DNA quantitation and western blot analysis for cellular proteins. Histological and scanning electron microscope imaging showed no significant change in the ECM organization and microstructure. We further tested the recellularization potential of decellularized leaflets by seeding human mesenchymal stem cells (hMSC) on the surface of the leaflets and evaluated them at 1 and 3 weeks in two culture conditions. One medium (M1) was chosen to maintain the MSC phenotype while a second medium (M2) was used to potentially differentiate cells to an interstitial cell phenotype. Cellular quantitation showed that the engineered leaflets were recellularized to the highest concentration with M2 followed by M1, with minimum cell invasion of decellularized native leaflets. Histology showed cellular invasion throughout the thickness of the leaflets in M2 and partial invasion in M1. hMSC stained positive for MSC markers, but also for α-smooth muscle actin in both media at 1 week, with no presence of MSC markers at 3 weeks with the exception of CD90. These results show that engineered leaflets, while having similar tensile properties and collagen content compared to native leaflets, have better recellularization potential.

Encapsulation of cardiomyocytes in a fibrin hydrogel for cardiac tissue engineering

Culturing cells in a three dimensional hydrogel environment is an important technique for developing constructs for tissue engineering as well as studying cellular responses under various culture conditions in vitro. The three dimensional environment more closely mimics what the cells observe in vivo due to the application of mechanical and chemical stimuli in all dimensions ¹. Three-dimensional hydrogels can either be made from synthetic polymers such as PEG-DA ² and PLGA ³ or a number of naturally occurring proteins such as collagen ⁴, hyaluronic acid ⁵ or fibrin ^6,7. Hydrogels created from fibrin, a naturally occurring blood clotting protein, can polymerize to form a mesh that is part of the body's natural wound healing processes ⁸. Fibrin is cell-degradable and potentially autologous ⁹, making it an ideal temporary scaffold for tissue engineering.

Here we describe in detail the isolation of neonatal cardiomyocytes from three day old rat pups and the preparation of the cells for encapsulation in fibrin hydrogel constructs for tissue engineering. Neonatal myocytes are a common cell source used for in vitro studies in cardiac tissue formation and engineering ⁴. Fibrin gel is created by mixing fibrinogen with the enzyme thrombin. Thrombin cleaves fibrinopeptides FpA and FpB from fibrinogen, revealing binding sites that interact with other monomers ¹⁰. These interactions cause the monomers to self-assemble into fibers that form the hydrogel mesh. Because the timing of this enzymatic reaction can be adjusted by altering the ratio of thrombin to fibrinogen, or the ratio of calcium to thrombin, one can injection mold constructs with a number of different geometries ^11,12. Further we can generate alignment of the resulting tissue by how we constrain the gel during culture ¹³.

After culturing the engineered cardiac tissue constructs for two weeks under static conditions, the cardiac cells have begun to remodel the construct and can generate a contraction force under electrical pacing conditions ⁶. As part of this protocol, we also describe methods for analyzing the tissue engineered myocardium after the culture period including functional analysis of the active force generated by the cardiac muscle construct upon electrical stimulation, as well as methods for determining final cell viability (Live-Dead assay) and immunohistological staining to examine the expression and morphology of typical proteins important for contraction (Myosin Heavy Chain or MHC) and cellular coupling (Connexin 43 or Cx43) between myocytes.

Strategies for tissue engineering cardiac constructs to affect functional repair following myocardial infarction

Tissue-engineered cardiac constructs are a high potential therapy for treating myocardial infarction. These therapies have the ability to regenerate or recreate functional myocardium following the infarction, restoring some of the lost function of the heart and thereby preventing congestive heart failure. Three key factors to consider when developing engineered myocardial tissue include the cell source, the choice of scaffold, and the use of biomimetic culture conditions. This review details the various biomaterials and scaffold types that have been used to generate engineered myocardial tissues as well as a number of different methods used for the fabrication and culture of these constructs. Specific bioreactor design considerations for creating myocardial tissue equivalents in vitro, such as oxygen and nutrient delivery as well as physical stimulation, are also discussed. Lastly, a brief overview of some of the in vivo studies that have been conducted to date and their assessment of the functional benefit in repairing the injured heart with engineered myocardial tissue is provided.

Mechanical properties of tissue-engineered vascular constructs produced using arterial or venous cells

There is a clinical need for better blood vessel substitutes, as current surgical procedures are limited by the availability of suitable autologous vessels and suboptimal behavior of synthetic grafts in small caliber arterial graft (<5  mm) applications. The aim of the present study was to compare the mechanical properties of arterial and venous tissue-engineered vascular constructs produced by the self-assembly approach using cells extracted from either the artery or vein harvested from the same human umbilical cord. The production of a vascular construct comprised of a media and an adventitia (TEVMA) was achieved by rolling a continuous tissue sheet containing both smooth muscle cells and adventitial fibroblasts grown contiguously in the same tissue culture plate. Histology and immunofluorescence staining were used to evaluate the structure and composition of the extracellular matrix of the vascular constructs. The mechanical strength was assessed by uniaxial tensile testing, whereas viscoelastic behavior was evaluated by stepwise stress-relaxation and by cyclic loading hysteresis analysis. Tensile testing showed that the use of arterial cells resulted in stronger and stiffer constructs when compared with those produced using venous cells. Moreover, cyclic loading demonstrated that constructs produced using arterial cells were able to bear higher loads for the same amount of strain when compared with venous constructs. These results indicate that cells isolated from umbilical cord can be used to produce vascular constructs. Arterial constructs possessed superior mechanical properties when compared with venous constructs produced using cells isolated from the same human donor. This study highlights the fact that smooth muscle cells and fibroblasts originating from different cell sources can potentially lead to distinct tissue properties when used in tissue engineering applications.

Tissue engineering on matrix: future of autologous tissue replacement

Tissue engineering aims at the creation of living neo-tissues identical or close to their native human counterparts. As basis of this approach, temporary biodegradable supporter matrices are fabricated in the shape of a desired construct, which promote tissue strength and provide functionality until sufficient neo-tissue is formed. Besides fully synthetic polymer-based scaffolds, decellularized biological tissue of xenogenic or homogenic origin can be used. In a second step, these scaffolds are seeded with autologous cells attaching to the scaffold microstructure. In order to promote neo-tissue formation and maturation, the seeded scaffolds are exposed to different forms of stimulation. In cardiovascular tissue engineering, this "conditioning" can be achieved via culture media and biomimetic in vitro exposure, e.g., using flow bioreactors. This aims at adequate cellular differentiation, proliferation, and extracellular matrix production to form a living tissue called the construct. These living autologous constructs, such as heart valves or vascular grafts, are created in vitro, comprising a viable interstitium with repair and remodeling capabilities already prior to implantation. In situ further in vivo remodeling is intended to recapitulate physiological vascular architecture and function. The remodeling mechanisms were shown to be dominated by monocytic infiltration and chemotactic host-cell attraction leading into a multifaceted inflammatory process and neo-tissue formation. Key molecules of these processes can be integrated into the scaffold matrix to direct cell and tissue fate in vivo.

Fibrin degradation enhances vascular smooth muscle cell proliferation and matrix deposition in fibrin-based tissue constructs fabricated in vitro

Completely biological tissue replacements can be fabricated by entrapping cells in a molded fibrin gel. Over time, the fibrin is degraded and replaced with cell-produced extracellular matrix. However, the relationship between fibrin degradation and matrix deposition has not been elucidated. We developed techniques to quantify fibrin degradation products (FDP) and examine plasmin activity in the conditioned medium from fibrin-based constructs. Fibrin-based tissue constructs fabricated with vascular smooth muscle cells (vSMC) were cultured for 5 weeks in the presence of varied concentrations of the fibrinolysis inhibitor -aminocaproic acid and cellularity, and deposited collagen and elastin were measured weekly. These data revealed that increasing concentrations of -aminocaproic acid led to delayed and diminished FDP production, lower vSMC proliferation, and decreased collagen and elastin deposition. FDP were shown to have a direct biological effect on vSMC cultures and vSMC within the fibrin-based constructs. Supplementing construct cultures with 250 or 500μg/mL FDP led to 30% higher collagen deposition than the untreated controls. FDP concentrations as high as 250μg/mL were estimated to exist within the constructs, indicating that FDP generation during remodeling of the fibrin-based constructs exerted direct biological activity. These results help explain many of the positive outcomes reported with fibrin-based tissue constructs in the literature, as well as demonstrate the importance of regulating plasmin activity during their fabrication.

Initial fiber alignment pattern alters extracellular matrix synthesis in fibroblast-populated fibrin gel cruciforms and correlates with predicted tension

Human dermal fibroblasts entrapped in fibrin gels cast in cross-shaped (cruciform) geometries with 1:1 and 1:0.5 ratios of arm widths were studied to assess whether tension and alignment of the cells and fibrils affected ECM deposition. The cruciforms of contrasting geometry (symmetric vs. asymmetric), which developed different fiber alignment patterns, were harvested at 2, 5, and 10 weeks of culture. Cruciforms were subjected to planar biaxial testing, polarimetric imaging, DNA and biochemical analyses, histological staining, and SEM imaging. As the cruciforms compacted and developed fiber alignment, fibrin was degraded, and elastin and collagen were produced in a geometry-dependent manner. Using a continuum mechanical model that accounts for direction-dependent stress due to cell traction forces and cell contact guidance with aligned fibers that occurs in the cruciforms, the mechanical stress environment was concluded to influence collagen deposition, with deposition being the greatest in the narrow arms of the asymmetric cruciform where stress was predicted to be the largest.

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.

Vascular smooth muscle cells as a valvular interstitial cell surrogate in heart valve tissue engineering

BACKGROUND

Vascular smooth muscle cells (VSMCs) are a potential autologous cell source for aortic valve tissue engineering, but have a phenotype that differs from that of valvular interstitial cells in vivo. We hypothesized that combining basic fibroblast growth factor (bFGF), epidermal growth factor (EGF), or platelet-derived growth factor (PDGF) with transforming growth factor beta-1 (TGF-beta1) would achieve a valvular interstitial cell-like phenotype of VSMCs.

METHODS

VSMC phenotype was assessed by immunofluorescence, proliferation was measured by the tetrazolium reduction (MTT) assay, and extracellular matrix gene expression was determined by real-time polymerase chain reaction.

RESULTS

Combinations of growth factors that included PDGF showed the greatest increases in proliferation. Immunofluorescence for alpha-smooth muscle actin demonstrated an inverse correlation between proliferation and a myofibroblast-like phenotype, while combinations of TGF-beta1+ EGF+bFGF (TEF) and TGF-beta1+EGF+PDGF (TEP) induced the greatest change of alpha-smooth muscle actin expression compared to untreated controls. Finally, TEP treatment showed an increase in versican, fibronectin, and type I collagen mRNA expression, while decreasing matrix metalloproteinase 1 expression.

CONCLUSIONS

Combination of TGF-beta1 with EGF and PDGF induces VSMC proliferation and expression of extracellular matrix constituents found in the aortic valve. In vitro preconditioning of VSMCs provides a potentially viable surrogate cell source for developing a valve graft.

Cell-induced alignment augments twitch force in fibrin gel-based engineered myocardium via gap junction modification

A high-potential therapy for repairing the heart post-myocardial infarction is the implantation of tissue-engineered myocardium. While several groups have developed constructs that mimic the aligned structure of the native myocardium, to date no one has investigated the particular functional benefits conferred by alignment. In this study we created myocardial constructs in both aligned and isotropic configurations by entrapping neonatal rat cardiac cells in fibrin gel. Constructs were cultured statically for 2 weeks, and then characterized. Histological staining showed spread cells that express typical cardiac cell markers in both configurations. Isotropic constructs had higher final cell and collagen densities, but lower passive mechanical properties than aligned constructs. Twitch force associated with electrical pacing, however, was 181% higher in aligned constructs, and this improvement was greater than what would be expected from merely aligning the cells in the isotropic constructs in the force measurement direction. Our hypothesis was that this was due to improved gap junction formation/function facilitated by cell alignment, and further analyses of the twitch force data, as well as Western blot results of connexin 43 expression and phosphorylation state, support this hypothesis. Regardless of the specific mechanism, the results presented in this study underscore the importance of recapitulating the anisotropy of the native tissue in engineered myocardium.

Regulation of valvular interstitial cell calcification by components of the extracellular matrix

Understanding the interactions between extracellular matrix (ECM) components and valvular interstitial cells (VICs) is relevant to both treating heart valve disease and designing heart valve tissue engineering scaffolds, yet the VIC-ECM relationship has not been well characterized. Thus, the aim of this study was to characterize VIC-ECM interactions, paying specific attention to whether ECM composition affected the in vitro calcification of VICs. Our results show that the number and size of calcific nodules formed in VIC cultures, as well as the expression of the mineralization markers alkaline phosphatase (ALP) and CBFa1, were highly dependent upon the composition of the culture surface. VICs cultured on certain ECM components, that is, collagen and fibronectin, were resistant to calcification, even upon treatment with mineralization-inducing growth factors. Meanwhile, cultures of VICs on fibrin, laminin, and heparin coatings had a high number of calcified nodules, although only VICs on fibrin expressed significantly elevated levels of ALP and CBFa1. Nodule composition analysis revealed the presence of multiple types of mineralization. Although apoptotic and necrotic cells were more concentrated in nodules, these nodules did contain a strong majority population of viable cells. Characterizing this ECM-dependence of VIC calcification will help us to identify appropriate biomaterial environments for heart valve tissue engineering as well as elucidate mechanisms of valvular disease.

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.

Fibrin: a versatile scaffold for tissue engineering applications

Tissue engineering combines cell and molecular biology with materials and mechanical engineering to replace damaged or diseased organs and tissues. Fibrin is a critical blood component responsible for hemostasis, which has been used extensively as a biopolymer scaffold in tissue engineering. In this review we summarize the latest developments in organ and tissue regeneration using fibrin as the scaffold material. Commercially available fibrinogen and thrombin are combined to form a fibrin hydrogel. The incorporation of bioactive peptides and growth factors via a heparin-binding delivery system improves the functionality of fibrin as a scaffold. New technologies such as inkjet printing and magnetically influenced self-assembly can alter the geometry of the fibrin structure into appropriate and predictable forms. Fibrin can be prepared from autologous plasma, and is available as glue or as engineered microbeads. Fibrin alone or in combination with other materials has been used as a biological scaffold for stem or primary cells to regenerate adipose tissue, bone, cardiac tissue, cartilage, liver, nervous tissue, ocular tissue, skin, tendons, and ligaments. Thus, fibrin is a versatile biopolymer, which shows a great potential in tissue regeneration and wound healing.

2D and 3D collagen and fibrin biopolymers promote specific ECM and integrin gene expression by vascular smooth muscle cells

Collagen Type I and fibrin are polymeric proteins commonly used in the field of regenerative medicine as the foundational matrix of engineered tissues. We examined the response of vascular smooth muscle cells (VSMC) to both two-dimensional (2D) substrates as well as three-dimensional (3D) matrices of these biopolymers. Pure collagen Type I, pure fibrin and composite matrices consisting of 1:1 mixtures of collagen and fibrin were studied. Relative gene expression of three ECM molecules (collagen Type I and III, and tropoelastin) and three integrin subunits (integrins alpha1, beta1 and beta3) was determined over 7 days in culture using quantitative RT-PCR. Expression of all of these marker genes was up-regulated in 3D matrices, relative to 2D substrates. Tropoelastin, integrin alpha1 and integrin beta1 were highest in collagen matrices, while collagen III and integrin beta3 expression were highest in pure fibrin, and collagen I expression was highest in the collagen-fibrin composite materials. Both the compositional and temporal expression patterns of these specific ECM-related genes were suggestive of a wound healing response. These results illuminate the short-term responses of VSMC to 2D and 3D biopolymer matrices, and have relevance to tissue engineering and cardiovascular biology.

Comparison of 2D fiber network orientation measurement methods

The mechanical properties of tissues, tissue analogs, and biomaterials are dependent on their underlying microstructure. As such, many mechanical models incorporate some aspect of microstructure, but a robust protocol for characterizing fiber architecture remains a challenge. A number of image-based methods, including mean intercept length (MIL), line fraction deviation (LFD), and Fourier transform methods (FTM), have been applied to microstructural images to describe material heterogeneity and orientation, but a performance comparison, particularly for fiber networks, has not been conducted. In this study, we constructed 40 two-dimensional test images composed of simulated fiber networks varying in fiber number, orientation, and anisotropy index. We assessed the accuracy of each method in measuring principal direction (theta) and anisotropy index (alpha). FTM proved to be the superior method because it was more reliable in measurement accuracy (Deltatheta = 2.95 degrees +/- 6.72 degrees , Deltaalpha = 0.03 +/- 0.02), faster in execution time, and flexible in its application. MIL (Deltatheta = 6.23 degrees +/- 10.68 degrees , Deltaalpha = 0.08 +/- 0.06) was not significantly less accurate than FTM but was much slower. LFD (Deltatheta = 9.97 degrees +/- 11.82 degrees , Deltaalpha = 0.24 +/- 0.13) consistently underperformed. FTM results agreed qualitatively with fibrin gel SEM micrographs, suggesting that FTM can be used to obtain image-based statistical measurements of microstructure.

Functional tissue-engineered valves from cell-remodeled fibrin with commissural alignment of cell-produced collagen

Heart valve replacements composed of living tissue that can adapt, repair, and grow with a patient would provide a more clinically beneficial option than current inert replacements. Bioartificial valves were produced by entrapping human dermal fibroblasts within a fibrin gel. Using a mold design that presents appropriate mechanical constraints to the cell-induced fibrin gel compaction, gross fiber alignment (commissure-to-commissure alignment in the leaflets and circumferential alignment in the root) and the basic geometry of a native aortic valve were obtained. After static incubation on the mold in complete medium supplemented with transforming growth factor beta 1, insulin, and ascorbate, collagen fibers produced by the entrapped cells were found to coalign with the fibrin based on histological analyses. The resultant tensile mechanical properties were anisotropic. Ultimate tensile strength and tensile modulus of the leaflets in the commissural direction were 0.53 and 2.34 MPa, respectively. The constructs were capable of withstanding backpressure commensurate with porcine aortic valves in regurgitation tests (330 mmHg) and opened and closed under physiological pressure swings of 10 and 20 mmHg, respectively. These data support proof of principle of using cell-remodeled fibrin gel to produce tissue-engineered valve replacements.

Tissue-to-cellular level deformation coupling in cell micro-integrated elastomeric scaffolds

In engineered tissues we are challenged to reproduce extracellular matrix and cellular deformation coupling that occurs within native tissues, which is a meso-micro scale phenomenon that profoundly affects tissue growth and remodeling. With our ability to electrospin polymer fiber scaffolds while simultaneously electrospraying viable cells, we are provided with a unique platform to investigate cellular deformations within a three dimensional elastomeric fibrous scaffold. Scaffold specimens micro-integrated with vascular smooth muscle cells were subjected to controlled biaxial stretch with 3D cellular deformations and local fiber microarchitecture simultaneously quantified. We demonstrated that the local fiber geometry followed an affine behavior, so that it could be predicted by macro-scaffold deformations. However, local cellular deformations depended non-linearly on changes in fiber microarchitecture and ceased at large strains where the scaffold fibers completely straightened. Thus, local scaffold microstructural changes induced by macro-level applied strain dominated cellular deformations, so that monotonic increases in scaffold strain do not necessitate similar levels of cellular deformation. This result has fundamental implications when attempting to elucidate the events of de-novo tissue development and remodeling in engineered tissues, which are thought to depend substantially on cellular deformations.

Cyclic distension of fibrin-based tissue constructs: evidence of adaptation during growth of engineered connective tissue

Tissue engineering provides a means to create functional living tissue replacements. Here, we examine the effects of 3 weeks of cyclic distension (CD) on fibrin-based tubular tissue constructs seeded with porcine valve interstitial cells. CD with circumferential strain amplitude ranging from 2.5% to 20% was applied to evaluate the effects of CD on fibrin remodeling into tissue. We hypothesized that during long-term CD cells adapt to cyclic strain of constant strain amplitude (constant CD), diminishing tissue growth. We thus also subjected constructs to CD with strain amplitude that was incremented from 5% to 15% over the 3 weeks of CD [incremental CD (ICD)]. For constant CD, improvement occurred in construct mechanical properties and composition, peaking at 15% strain: ultimate tensile strength (UTS) and tensile modulus increased 47% and 45%, respectively, over statically incubated controls (to 1.1 and 4.7 MPa, respectively); collagen density increased 29% compared with controls (to 27 mg/ml). ICD further improved outcomes. UTS increased 98% and modulus increased 62% compared with the largest values with constant CD, and collagen density increased 34%. Only in the case of ICD was the ratio of collagen content to cell number greater (70%) than controls, consistent with increased collagen deposition per cell. Studies with human dermal fibroblasts showed similar improvements, generalizing the findings, and revealed a 255% increase in extracellular signal-regulated kinase signaling for ICD vs. constant CD. These results suggest cell adaptation may limit conventional strategies of stretching with constant strain amplitude and that new approaches might optimize bioreactor operation.

Viscoelastic property measurement in thin tissue constructs using ultrasound

We present a dual-element concave ultrasound transducer system for generating and tracking of localized tissue displacements in thin tissue constructs on rigid substrates. The system is comprised of a highly focused PZT-4 5-MHz acoustic radiation force (ARF) transducer and a confocal 25-MHz polyvinylidene fluoride imaging transducer. This allows for the generation of measurable displacements in tissue samples on rigid substrates with thickness values down to 500 microm. Impulse-like and longer duration sine-modulated ARF pulses are possible with intermittent M-mode data acquisition for displacement tracking. The operations of the ARF and imaging transducers are strictly synchronized using an integrated system for arbitrary waveform generation and data capture with a shared timebase. This allows for virtually jitter-free pulse-echo data well suited for correlation-based speckle tracking. With this technique we could faithfully capture the entire dynamics of the tissue axial deformation at pulse-repetition frequency values up to 10 kHz. Spatio-temporal maps of tissue displacements in response to a variety of modulated ARF beams were produced in tissue-mimicking elastography phantoms on rigid substrates. The frequency response was measured for phantoms with different modulus and thickness values. The frequency response exhibited resonant behavior with the resonance frequency being inversely proportional to the sample thickness. This resonant behavior can be used in obtaining high-contrast imaging using magnitude and phase response to sinusoidally modulated ARF beams. Furthermore, a second order forced harmonic oscillator (FHO) model was shown to capture this resonant behavior. Based on the FHO model, we used the extended Kalman filter (EKF) for tracking the apparent modulus and viscosity of samples subjected to dc and sinusoidally modulated ARF. The results show that the stiffness (apparent modulus) term in the FHO is largely time-invariant and can be estimated robustly using the EKF. On the other hand, the damping (apparent viscosity) is time varying. These findings were confirmed by comparing the magnitude response of the FHO (with parameters obtained using the EKF) with the measured ones for different thin tissue constructs.

Mechanical and failure properties of extracellular matrix sheets as a function of structural protein composition

The goal of this study was to determine how alterations in protein composition of the extracellular matrix (ECM) affect its functional properties. To achieve this, we investigated the changes in the mechanical and failure properties of ECM sheets generated by neonatal rat aortic smooth muscle cells engineered to contain varying amounts of collagen and elastin. Samples underwent static and dynamic mechanical measurements before, during, and after 30 min of elastase digestion followed by a failure test. Microscopic imaging was used to measure thickness at two strain levels to estimate the true stress and moduli in the ECM sheets. We found that adding collagen to the ECM increased the stiffness. However, further increasing collagen content altered matrix organization with a subsequent decrease in the failure strain. We also introduced collagen-related percolation in a nonlinear elastic network model to interpret these results. Additionally, linear elastic moduli correlated with failure stress which may allow the in vivo estimation of the stress tolerance of ECM. We conclude that, in engineered replacement tissues, there is a tradeoff between improved mechanical properties and decreased extensibility, which can impact their effectiveness and how well they match the mechanical properties of native tissue.