Founder's Award, 25th Annual Meeting of the Society for Biomaterials, perspectives. Providence, RI, April 28-May 2, 1999. Tissue heart valves: current challenges and future research perspectives

A novel mono-physics particle-based approach for the simulation of cardiovascular fluid-structure interaction problems

BACKGROUND AND OBJECTIVE

Fluid-structure interaction (FSI) is required in the study of several cardiovascular engineering problems were the mutual interaction between the pulsatile blood flow and the tissue structures is essential to establish the biomechanics of the system. Traditional FSI methods are partitioned approaches where two independent solvers, one for the fluid and one for the structure, are asynchronously coupled. This process results into high computational costs. In this work, a new FSI scheme which avoids the coupling of different solvers is presented in the framework of the truly incompressible smoothed particle hydrodynamics (ISPH) method.

METHODS

In the proposed FSI method, ISPH particles contribute to define both the fluid and structural domains and are solved together in a unified system. Solid particles, geometrically defined at the beginning of the simulation, are linked through spring bounds with elastic constant providing the material Young's modulus. At each iteration, internal elastic forces are calculated to restore the springs resting length. These forces are added in the predictor step of the fractional-step procedure used to solve the momentum and continuity equations for incompressible flows of all particles.

RESULTS

The method was validated with a benchmark test case consisting of a flexible beam immersed in a channel. Results showed good agreement with the system coupling approach of a well-established commercial software, ANSYS®, both in terms of fluid-dynamics and beam deformation. The approach was then applied to model a complex cardiovascular problem, consisting in the aortic valve operating function. The valve dynamics during opening and closing phases were compared qualitatively with literature results, demonstrating good consistency.

CONCLUSIONS

The method is computationally more efficient than traditional FSI strategies, and overcomes some of their main drawbacks, such as the impossibility of simulating the correct valve coaptation during the closing phase. Thanks to the incompressibility scheme, the proposed FSI method is appropriate to model biological soft tissues. The simplicity and flexibility of the approach also makes it suitable to be expanded for the modelling of thromboembolic phenomena.

Postfunctionalization of biological valve leaflets with a polyphenol network and anticoagulant recombinant humanized type III collagen for improved anticoagulation and endothelialization

Almost all commercial bioprosthetic heart valves (BHVs) are crosslinked with glutaraldehyde (GLUT); however, issues such as immune responses, calcification, delayed endothelialization, and especially severe thrombosis threaten the service lifespan of BHVs. Surface modification is expected to impart GLUT-crosslinked BHVs with versatility to optimize service performance. Here, a postfunctionalization strategy was established for GLUT-crosslinked BHVs, which were firstly modified with metal-phenolic networks (MPNs) to shield the exposed calcification site, and then anticoagulant recombinant humanized type III collagen (rhCOLIII) was immobilized to endow them with long-term antithrombogenicity and enhanced endothelialization properties. The postfunctionalization coating exhibited promising mechanical properties and resistance to enzymatic degradation capability resembling that of GLUT-crosslinked porcine pericardium (GLUT-PP). With the introduction of meticulously tailored rhCOLIII, the anti-coagulation and re-endothelialization properties of TA/Fe-rhCOLIII were significantly improved. Furthermore, the mild inflammatory response and reduced calcification were evidenced in TA/Fe-rhCOLIII by subcutaneous implantation. In conclusion, the efficacy of the proposed strategy combining anti-inflammatory MPNs and multifunctional rhCOLIII to improve anticoagulation, reduce the inflammatory response, and ultimately achieve rapid reendothelialization was supported by both ex vivo and in vivo experiments. Altogether, the current findings may provide a simple strategy for enhancing the service function of BHVs after implantation and show great potential in clinical applications.

Comparison of long-term outcomes of bioprosthetic and mechanical aortic valve replacement in patients younger than 65 years

OBJECTIVES

The objectives of this study were to compare rates of mortality and reoperations for patients aged younger than 65 years who underwent surgical aortic valve replacement (AVR). AVR with a bioprosthetic valve (BV) is increasing among younger patients, however evidence to inform the choice between BV or mechanical valve is limited.

METHODS

We performed a retrospective cohort study using linked hospital and mortality data from Australia, for 3969 AVR patients between 2003 and 2018. We compared outcomes for valves in inverse probability of treatment-weighted cohorts, stratified according to age (18-54 years; 55-64 years). We used weighted Cox regression models to estimate hazard ratios (HRs) and weighted cumulative incidence function for subdistribution hazards, for follow-up intervals: 0 to 10 and >10 to 15 years.

RESULTS

Among patients aged 55 to 64 years, there was no difference in mortality at 0 to 10 years. However, at >10 to 15 years, mortality was higher among BV recipients (HR, 1.56; 95% CI, 1.01-2.42). There was no difference among patients aged 18 to 54 years. Reoperation rates for patients aged 55 to 64 years did not differ according to valve type at 0 to 10 years, but were higher for BV than mechanical valve at >10 to 15 years (HR, 2.87; 95% CI, 1.69-4.86). For patients aged 18 to 54 years, reoperation rates were consistently higher for BV at both time intervals (HR, 2.54 [95% CI, 1.03-6.25] and HR, 4.48 [95% CI, 2.15-9.32], respectively).

CONCLUSIONS

Patients aged 55 to 64 years who received a BV had a higher risk of mortality beyond 10 years. Rates of reoperations were higher among patients implanted with a BV in the entire cohort. Further investigation of long-term outcomes among patients with a BV is necessary. Continuous long-term monitoring of BV technologies will ensure evidence-based decision-making and regulation.

Sacrificial scaffold-assisted direct ink writing of engineered aortic valve prostheses

Heart valve disease has become a serious global health problem, which calls for numerous implantable prosthetic valves to fulfill the broader needs of patients. Although current three-dimensional (3D) bioprinting approaches can be used to manufacture customized valve prostheses, they still have some complications, such as limited biocompatibility, constrained structural complexity, and difficulty to make heterogeneous constructs, to name a few. To overcome these challenges, a sacrificial scaffold-assisted direct ink writing approach has been explored and proposed in this work, in which a sacrificial scaffold is printed to temporarily support sinus wall and overhanging leaflets of an aortic valve prosthesis that can be removed easily and mildly without causing any potential damages to the valve prosthesis. The bioinks, composed of alginate, gelatin, and nanoclay, used to print heterogenous valve prostheses have been designed in terms of rheological/mechanical properties and filament formability. The sacrificial ink made from Pluronic F127 has been developed by evaluating rheological behavior and gel temperature. After investigating the effects of operating conditions, complex 3D structures and homogenous/heterogenous aortic valve prostheses have been successfully printed. Lastly, numerical simulation and cycling experiments have been performed to validate the function of the printed valve prostheses as one-way valves.

Soft Biomimetic Approach for the Development of Calcinosis-Resistant Glutaraldehyde-Fixed Biomaterials for Cardiovascular Surgery

Pathological aseptic calcification is the most common form of structural valvular degeneration (SVD), leading to premature failure of heart valve bioprostheses (BHVs). The processing methods used to obtain GA-fixed pericardium-based biomaterials determine the hemodynamic characteristics and durability of BHVs. This article presents a comparative study of the effects of several processing methods on the degree of damage to the ECM of GA-fixed pericardium-based biomaterials as well as on their biostability, biocompatibility, and resistance to calcification. Based on the assumption that preservation of the native ECM structure will enable the creation of calcinosis-resistant materials, this study provides a soft biomimetic approach for the manufacture of GA-fixed biomaterials using gentle decellularization and washing methods. It has been shown that the use of soft methods for preimplantation processing of materials, ensuring maximum preservation of the intactness of the pericardial ECM, radically increases the resistance of biomaterials to calcification. These obtained data are of interest for the development of new calcinosis-resistant biomaterials for the manufacture of BHVs.

Pediatric pulmonary valve replacements: Clinical challenges and emerging technologies

Congenital heart diseases (CHDs) frequently impact the right ventricular outflow tract, resulting in a significant incidence of pulmonary valve replacement in the pediatric population. While contemporary pediatric pulmonary valve replacements (PPVRs) allow satisfactory patient survival, their biocompatibility and durability remain suboptimal and repeat operations are commonplace, especially for very young patients. This places enormous physical, financial, and psychological burdens on patients and their parents, highlighting an urgent clinical need for better PPVRs. An important reason for the clinical failure of PPVRs is biofouling, which instigates various adverse biological responses such as thrombosis and infection, promoting research into various antifouling chemistries that may find utility in PPVR materials. Another significant contributor is the inevitability of somatic growth in pediatric patients, causing structural discrepancies between the patient and PPVR, stimulating the development of various growth-accommodating heart valve prototypes. This review offers an interdisciplinary perspective on these challenges by exploring clinical experiences, physiological understandings, and bioengineering technologies that may contribute to device development. It thus aims to provide an insight into the design requirements of next-generation PPVRs to advance clinical outcomes and promote patient quality of life.

Mechanical versus Bioprosthetic Aortic Valve Replacement in Middle-Aged Adults: A Systematic Review and Meta-Analysis

BACKGROUND

Mechanical prostheses and bioprosthetic prostheses have their own advantages and disadvantages. Mechanical ones are recommended for younger patients (<50 years old), and bioprosthetic ones are recommended for older patients (>70 years old). There is still debate regarding which kind of prosthesis is better for middle-aged patients (50 to 70 years old) receiving aortic valve replacement (AVR). To solve this problem, we conducted this meta-analysis. Given that only one randomized controlled trial (RCT) study was included, we conducted a subgroup analysis of RCT and propensity score matching (PSM) retrospective studies to reduce the bias.

METHODS

We systematically searched articles related to clinical outcomes of mechanical and bioprosthetic prostheses in middle-aged patients receiving AVR in the PubMed, Cochrane Library, and China National Knowledge Infrastructure (CNKI) databases. The published date was up to 1 October 2022. Studies were excluded if not only middle-aged patients were included, or if they lacked direct comparisons between mechanical and bioprosthetic prostheses.

RESULTS

In total, 22 studies with 32,298 patients were included in the final analysis. The results show that patients aged between 50 and 70 receiving AVR with mechanical prostheses achieved better long-term survival and fewer reoperations and valve-related events but suffered more with bleeding events. No significant difference could be found in terms of early mortality and long-term cardiac death. The same results could be observed in the subgroup analysis of RCT and PSM retrospective studies.

CONCLUSION

Both mechanical and bioprosthetic prostheses are beneficial to middle-aged patients undertaking AVR procedures. However, mechanical prostheses show better clinical outcomes in long-term survival and comorbidities. Individual recommendation is still necessary.

Mechanisms and Drug Therapies of Bioprosthetic Heart Valve Calcification

Valve replacement is the main therapy for valvular heart disease, in which a diseased valve is replaced by mechanical heart valve (MHV) or bioprosthetic heart valve (BHV). Since the 2000s, BHV surpassed MHV as the leading option of prosthetic valve substitute because of its excellent hemocompatible and hemodynamic properties. However, BHV is apt to structural valve degeneration (SVD), resulting in limited durability. Calcification is the most frequent presentation and the core pathophysiological process of SVD. Understanding the basic mechanisms of BHV calcification is an essential prerequisite to address the limited-durability issues. In this narrative review, we provide a comprehensive summary about the mechanisms of BHV calcification on 1) composition and site of calcifications; 2) material-associated mechanisms; 3) host-associated mechanisms, including immune response and foreign body reaction, oxidative stress, metabolic disorder, and thrombosis. Strategies that target these mechanisms may be explored for novel drug therapy to prevent or delay BHV calcification.

A comparative study of different tissue materials for bioprosthetic aortic valves using experimental assays and finite element analysis

BACKGROUND AND OBJECTIVE

Extracting the mechanical behaviors of bioprosthetic aortic valve leaflets is necessary for the appropriate design and manufacture of the prosthetic valves. The goal of this study was to opt a proper tissue for the valve leaflets by comparing the mechanical properties of the equine, porcine, and donkey pericardia with those of the bovine pericardium and human aortic valve leaflets.

METHODS

After tissue fixation in glutaraldehyde, the mechanical behaviors of the pericardial tissues were experimentally evaluated through computational methods. The relaxation tests were performed along the tissue fiber direction. The Mooney-Rivlin model was utilized to describe the hyperelastic behavior of the tissues at the ramp portion. The viscous behaviors at the hold portion were extracted using the Fung quasi-linear viscoelastic (QLV) model. Furthermore, the extracted parameters were used in the modeling of the bovine, equine, porcine, and donkey pericardia through finite element analysis (FEA).

RESULTS

Based on the results, relaxation percentages of the equine, donkey, and bovine pericardia were greater than that of the porcine pericardium and similar to the native human aortic valve leaflets. Indeed, the equine and donkey pericardia were found more viscous and less elastic than the porcine pericardium. Compared with the porcine pericardium, the mechanical properties of the equine and donkey pericardia were rather closer to those of the native human leaflets and bovine pericardium. The computational analysis demonstrated that the donkey pericardium is preferable over other types of pericardium due to the low stress on the leaflets during the systolic and diastolic phases and the large geometric orifice area (GOA).

CONCLUSION

The donkey pericardium might be a good candidate valve leaflet material for bioprosthetic aortic valves.

A Biohybrid Material With Extracellular Matrix Core and Polymeric Coating as a Cell Honing Cardiovascular Tissue Substitute

Objective

To investigate the feasibility of a hybrid material in which decellularized pericardial extracellular matrix is functionalized with polymeric nanofibers, for use as a cardiovascular tissue substitute.

Background

A cardiovascular tissue substitute, which is gradually resorbed and is replaced by host's native tissue, has several advantages. Especially in children and young adults, a resorbable material can be useful in accommodating growth, but also enable rapid endothelialization that is necessary to avoid thrombotic complications. In this study, we report a hybrid material, wherein decellularized pericardial matrix is functionalized with a layer of polymeric nanofibers, to achieve the mechanical strength for implantation in the cardiovascular system, but also have enhanced cell honing capacity.

Methods

Pericardial sacs were decellularized with sodium deoxycholate, and polycaprolactone-chitosan fibers were electrospun onto the matrix. Tissue-polymer interaction was evaluated using spectroscopic methods, and the mechanical properties of the individual components and the hybrid material were quantified. In-vitro blood flow loop studies were conducted to assess hemocompatibility and cell culture methods were used to assess biocompatibility.

Results

Encapsulation of the decellularized matrix with 70 μm thick matrix of polycaprolactone-chitosan nanofibers, was feasible and reproducible. Spectroscopy of the cross-section depicted new amide bond formation and C–O–C stretch at the interface. An average peel strength of 56.13 ± 11.87 mN/mm2 was measured, that is sufficient to withstand a high shear of 15 dynes/cm2 without delamination. Mechanical strength and extensibility ratio of the decellularized matrix alone were 18,000 ± 4,200 KPa and 0.18 ± 0.03% whereas that of the hybrid was higher at 20,000 ± 6,600 KPa and 0.35 ± 0.20%. Anisotropy index and stiffness of the biohybrid were increased as well. Neither thrombus formation, nor platelet adhesion or hemolysis was measured in the in-vitro blood flow loop studies. Cellular adhesion and survival were adequate in the material.

Conclusion

Encapsulating a decellularized matrix with a polymeric nanofiber coating, has favorable attributes for use as a cardiovascular tissue substitute.

Biomechanics of mitral valve leaflets: Second harmonic generation microscopy, biaxial mechanical tests and tissue modeling

Collagen fibers are the main load carrier in the mitral valve (MV) leaflets. Their orientation and dispersion are an important factor for the mechanical behavior. Most recent studies of collagen fibers in MVs lack either entire thickness study or high transmural resolution. The present study uses second harmonic generation (SHG) microscopy in combination with planar biaxial mechanical tests to better model and examine collagen fibers and mechanical properties of MV leaflets. SHG in combination with tissue clearing enables the collagen fibers to be examined through the entire thickness of the MV leaflets. Planar biaxial mechanical tests, on the other hand, enable the characterization of the mechanical tissue behavior, which is represented by a structural tissue model. Twelve porcine MV leaflets are examined. The SHG recording shows that the mean fiber angle for all samples varies on average by ±12° over the entire thickness and the collagen fiber dispersion changes strongly over the thickness. A constitutive model based on the generalized structure tensor approach is used for the associated tissue characterization. The model represents the tissue with three mechanical parameters plus the mean fiber direction and the dispersion, and predicts the biomechanical response of the leaflets with a good agreement (average r²=0.94). It is found that the collagen structure can be represented by a mean direction and a dispersion with a single family of fibers despite the variation in the collagen fiber direction and the dispersion over the entire thickness of MV leaflets. STATEMENT OF SIGNIFICANCE: Despite its prominent role in the mechanical behavior of mitral valve (MV) leaflets, the collagen structure has not yet been investigated over the entire thickness with high transmural resolution. The present study quantifies the detailed through thickness collagen fiber structure and examines the effects of its variation on MV tissue modeling. This is important because the study evaluates the assumption that the collagen fibers can be modeled with a representative single fiber family despite the variation across the thickness. In addition, the current comprehensive data set paves the way for quantifying the disruption of collagen fibers in myxomatous MV leaflets associated with disrupted collagen fibers.

Poly-2-methyl-2-oxazoline–modified bioprosthetic heart valve leaflets have enhanced biocompatibility and resist structural degeneration

Bioprosthetic heart valves (BHV) fabricated from glutaraldehyde-fixed heterograft tissue, such as bovine pericardium (BP), are widely used for treating heart valve disease, a group of disorders that affects millions. Structural valve degeneration (SVD) of BHV due to both calcification and the accumulation of advanced glycation end products (AGE) with associated serum proteins limits durability. We hypothesized that BP modified with poly-2-methyl-2-oxazoline (POZ) to inhibit protein entry would demonstrate reduced accumulation of AGE and serum proteins, mitigating SVD. In vitro studies of POZ-modified BP demonstrated reduced accumulation of serum albumin and AGE. BP-POZ in vitro maintained collagen microarchitecture per two-photon microscopy despite AGE incubation, and in cell culture studies was associated with no change in tumor necrosis factor-α after exposure to AGE and activated macrophages. Comparing POZ and polyethylene glycol (PEG)–modified BP in vitro, BP-POZ was minimally affected by oxidative conditions, whereas BP-PEG was susceptible to oxidative deterioration. In juvenile rat subdermal implants, BP-POZ demonstrated reduced AGE formation and serum albumin infiltration, while calcification was not inhibited. However, BP-POZ rat subdermal implants with ethanol pretreatment demonstrated inhibition of both AGE accumulation and calcification. Ex vivo laminar flow studies with human blood demonstrated BP-POZ enhanced thromboresistance with reduced white blood cell accumulation. We conclude that SVD associated with AGE and serum protein accumulation can be mitigated through POZ functionalization that both enhances biocompatibility and facilitates ethanol pretreatment inhibition of BP calcification.

Tissue response, macrophage phenotype, and intrinsic calcification induced by cardiovascular biomaterials: Can clinical regenerative potential be predicted in a rat subcutaneous implant model?

The host immune response to an implanted biomaterial, particularly the phenotype of infiltrating macrophages, is a key determinant of biocompatibility and downstream remodeling outcome. The present study used a subcutaneous rat model to compare the tissue response, including macrophage phenotype, remodeling potential, and calcification propensity of a biologic scaffold composed of glutaraldehyde-fixed bovine pericardium (GF-BP), the standard of care for heart valve replacement, with those of an electrospun polycarbonate-based supramolecular polymer scaffold (ePC-UPy), urinary bladder extracellular matrix (UBM-ECM), and a polypropylene mesh (PP). The ePC-UPy and UBM-ECM materials induced infiltration of mononuclear cells throughout the thickness of the scaffold within 2 days and neovascularization at 14 days. GF-BP and PP elicited a balance of pro-inflammatory (M1-like) and anti-inflammatory (M2-like) macrophages, while UBM-ECM and ePC-UPy supported a dominant M2-like macrophage phenotype at all timepoints. Relative to GF-BP, ePC-UPy was markedly less susceptible to calcification for the 180 day duration of the study. UBM-ECM induced an archetypical constructive remodeling response dominated by M2-like macrophages and the PP caused a typical foreign body reaction dominated by M1-like macrophages. The results of this study highlight the divergent macrophage and host remodeling response to biomaterials with distinct physical and chemical properties and suggest that the rat subcutaneous implantation model can be used to predict in vivo biocompatibility and regenerative potential for clinical application of cardiovascular biomaterials.

Biological Equivalence of GGTA-1 Glycosyltransferase Knockout and Standard Porcine Pericardial Tissue Using 90-Day Mitral Valve Implantation in Adolescent Sheep

Objective

There is growing interest in the application of genetically engineered reduced antigenicity animal tissue for manufacture of bioprosthetic heart valves (BHVs) to reduce antibody induced tissue calcification and accelerated structural valve degeneration (SVD). This study tested biological equivalence of valves made from Gal-knockout (GalKO) and standard porcine pericardium after 90-day mitral valve implantation in sheep.

Methods

GalKO (n = 5) and standard (n = 5) porcine pericardial BHVs were implanted in a randomized and blind fashion into sheep for 90-days. Valve haemodynamic function was measured at 30-day intervals. After explantation, valves were examined for pannus, vegetation, inflammation, thrombus, and tissue calcification.

Results

Nine of 10 recipients completed the study. There was no difference between study groups for haemodynamic performance and no adverse valve-related events. Explanted BHVs showed mild pannus integration and minimal thrombus, with no difference between the groups. Limited focal mineral deposits were detected by x-ray. Atomic spectroscopy analysis detected tissue calcium levels of 1.0 µg/mg ± 0.2 for GalKO BHVs and 1.9 µg/mg ± 0.9 for standard tissue BHVs (p = 0.4), considered to be both low and equivalent.

Conclusions

This is the first demonstration of biological equivalence between GalKO and standard pig pericardium. The GalKO mutation causes neither intrinsic detrimental biological nor functional impact on BHV performance. Commercial adaptation of GalKO tissue for surgical or transcatheter BHVs would remove the clinical disparity between patients producing anti-Gal antibody and BHVs containing the Gal antigen. GalKO BHVs may reduce accelerated tissue calcification and SVD, enhancing patient choices, especially for younger patients.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s13239-021-00585-0.

Simulating the time evolving geometry, mechanical properties, and fibrous structure of bioprosthetic heart valve leaflets under cyclic loading

Currently, the most common replacement heart valve design is the 'bioprosthetic' heart valve (BHV), which has important advantages in that it does not require permanent anti-coagulation therapy, operates noiselessly, and has blood flow characteristics similar to the native valve. BHVs are typically fabricated from glutaraldehyde-crosslinked pericardial xenograft tissue biomaterials (XTBs) attached to a rigid, semi-flexible, or fully collapsible stent in the case of the increasingly popular transcutaneous aortic valve replacement (TAVR). While current TAVR assessments are positive, clinical results to date are generally limited to <2 years. Since TAVR leaflets are constructed using thinner XTBs, their mechanical demands are substantially greater than surgical BHV due to the increased stresses during in vivo operation, potentially resulting in decreased durability. Given the functional complexity of heart valve operation, in-silico predictive simulations clearly have potential to greatly improve the TAVR development process. As such simulations must start with accurate material models, we have developed a novel time-evolving constitutive model for pericardial xenograft tissue biomaterials (XTB) utilized in BHV (doi: 10.1016/j.jmbbm.2017.07.013). This model was able to simulate the observed tissue plasticity effects that occur in approximately in the first two years of in vivo function (50 million cycles). In the present work, we implemented this model into a complete simulation pipeline to predict the BHV time evolving geometry to 50 million cycles. The pipeline was implemented within an isogeometric finite element formulation that directly integrated our established BHV NURBS-based geometry (doi: 10.1007/s00466-015-1166-x). Simulations of successive loading cycles indicated continual changes in leaflet shape, as indicated by spatially varying increases in leaflet curvature. While the simulation model assumed an initial uniform fiber orientation distribution, anisotropic regional changes in leaflet tissue plastic strain induced a complex changes in regional fiber orientation. We have previously noted in our time-evolving constitutive model that the increases in collagen fiber recruitment with cyclic loading placed an upper bound on plastic strain levels. This effect was manifested by restricting further changes in leaflet geometry past 50 million cycles. Such phenomena was accurately captured in the valve-level simulations due to the use of a tissue-level structural-based modeling approach. Changes in basic leaflet dimensions agreed well with extant experimental studies. As a whole, the results of the present study indicate the complexity of BHV responses to cyclic loading, including changes in leaflet shape and internal fibrous structure. It should be noted that the later effect also influences changes in local mechanical behavior (i.e. changes in leaflet anisotropic tissue stress-strain relationship) due to internal fibrous structure resulting from plastic strains. Such mechanism-based simulations can help pave the way towards the application of sophisticated simulation technologies in the development of replacement heart valve technology.

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.

Surgical pericardial heart valves: 50 Years of evolution

Valve disease carries a huge burden globally and the number of heart valve procedures are projected to increase from the current 300 000 to 800 000 annually by 2050. Since its genesis 50 years ago, pericardial heart valve has moved leaps and bounds to ever more ingenious designs and manufacturing methods with parallel developments in cardiology and cardiovascular surgical treatments. This feat has only been possible through close collaboration of many scientific disciplines in the fields of engineering, material sciences, basic tissue biology, medicine and surgery. As the pace of change continues to accelerate, we ask the readers to go back with us in time to understand developments in design and function of pericardial heart valves. This descriptive review seeks to focus on the qualities of pericardial heart valves, the advantages, successes and failures encapsulating the evolution of surgically implanted pericardial heart valves over the past five decades. We present the data on comparison of the pericardial heart valves to porcine valves, discuss structural valve deterioration and the future of heart valve treatments.

Comparison on the properties of bovine pericardium and porcine pericardium used as leaflet materials of transcatheter heart valve

BACKGROUND

In order to obtain the smaller delivery diameter, porcine pericardium had been used as a substitute material of bovine pericardium for the leaflet materials of transcatheter heart valve (THV). However, the differences between them had not been fully studied. Therefore, this study compared the microstructure, biochemical and mechanical properties of two materials and hydrodynamics of THV made by the two materials in detail.

METHODS

In this study, firstly, the microstructure of pericardium was analyzed by staining and scanning electron microscope; secondly, the biochemical properties of pericardium after different processes were compared by heat shrinkage temperature test, free amino and carboxyl concentration test, enzyme degradation test, subcutaneous implantation calcification analysis in rats; finally, the mechanical properties were evaluated by uniaxial tensile test before and after the pericardium being crimped, and then, the hydrodynamics of THV was studied according to the ISO5840 standard.

RESULTS

Compared with bovine pericardium, after the same process, porcine pericardium showed a looser and tinier fiber bundle, a similar free carboxyl concentration, a lower resistance to enzyme degradation, a significantly lower calcification, bearing capacity and damage after being crimped, a better hydrodynamic and adaption with lower cardiac output and deformation of implantation position. Meanwhile the dehydration process of pericardium almost had preserved all the biochemical advantages of two materials.

CONCLUSION

In this study, porcine and bovine pericardium showed some significant differences in biochemical, mechanical properties and hydrodynamics. According to the results, it was presumed that the thinner porcine pericardium might be more suitable for THV of right heart system. Meanwhile, more attention should be taken for the calcification of THV made by the bovine pericardium.

In vivo recellularization of xenogeneic vascular grafts decellularized with high hydrostatic pressure method in a porcine carotid arterial interpose model

Autologous vascular grafts are widely used in revascularization surgeries for small caliber targets. However, the availability of autologous conduits might be limited due to prior surgeries or the quality of vessels. Xenogeneic decellularized vascular grafts from animals can potentially be a substitute of autologous vascular grafts. Decellularization with high hydrostatic pressure (HHP) is reported to highly preserve extracellular matrix (ECM), creating feasible conditions for recellularization and vascular remodeling after implantation. In the present study, we conducted xenogeneic implantation of HHP-decellularized bovine vascular grafts from dorsalis pedis arteries to porcine carotid arteries and posteriorly evaluated graft patency, ECM preservation and recellularization. Avoiding damage of the luminal surface of the grafts from drying significantly during the surgical procedure increased the graft patency at 4 weeks after implantation (P = 0.0079). After the technical improvement, all grafts (N = 5) were patent with mild stenosis due to intimal hyperplasia at 4 weeks after implantation. Neither aneurysmal change nor massive thrombosis was observed, even without administration of anticoagulants nor anti-platelet agents. Elastica van Gieson and Sirius-red stainings revealed fair preservation of ECM proteins including elastin and collagen after implantation. The luminal surface of the grafts were thoroughly covered with von Willebrand factor-positive endothelium. Scanning electron microscopy of the luminal surface of implanted grafts exhibited a cobblestone-like endothelial cell layer which is similar to native vascular endothelium. Recellularization of the tunica media with alpha-smooth muscle actin-positive smooth muscle cells was partly observed. Thus, we confirmed that HHP-decellularized grafts are feasible for xenogeneic implantation accompanied by recellularization by recipient cells.

Increased utilization of bioprosthetic aortic valve technology:Trends, drivers, controversies and future directions

Introduction: Bioprosthetic valves (BPV) implanted surgically or by transcatheter valve implantation (TAVI) comprise an overwhelming majority of substitute aortic valves implanted worldwide.Areas Covered: Prominent drivers of this trend are: 1) BPV patients have generally better outcomes than those with a mechanical valve, and remain largely free of anticoagulation and its consequences; 2) BPV durability has improved over the years; and 3) the expanding use of TAVI and valve-in-valve (VIV) procedures permitting interventional management of structural valve degeneration (SVD). Nevertheless, key controversies exist: 1) optimal anticoagulation regimens for surgical and TAVI BPVs; 2) the incidence, mechanisms and mitigation strategies for SVD; 3) the use of VIV for treatment of SVD, and 4) valve selection recommendations for difficult cohorts, (e.g. patients 50-70 years, patients <50, childbearing age women). This communication reviews trends in and drivers of BPV utilization, current controversies, and future directions affecting BPV use.Expert Opinion: Long-term data are needed in several areas related to aortic BPV use, including anticoagulation/antiplatelet therapy, especially following TAVI. TAVI and especially VIV durability and optimal use warrant will benefit greatly from long-term data. Certain populations may benefit from such high-quality data on multi-year outcomes, particularly younger patients.

Inflammatory immune response in recipients of transcatheter aortic valves

Objective

Transcatheter aortic valve implantation (TAVI) is rapidly replacing cardiac surgery due to its minimal invasiveness and practicality. Midterm immunological studies on the biocompatibility of galactose-alpha-1,3-galactose (α-Gal)–carrying bioprosthetic heart valves for TAVI are not available. In this study we investigated whether bioprosthetic heart valves employed for TAVI augment an α-Gal–specific antibody-dependent and antibody-independent immune response 3 months after TAVI implantation.

Methods

This prospective observational study included 27 patients with severe aortic valve stenosis undergoing TAVI and 10 patients with severe mitral valve regurgitation treated with a transcatheter MitraClip (Abbott Laboratories, Abbott Park, Ill) procedure. Blood samples were drawn before and 90 days after treatment at a routine checkup. Serum samples were analyzed using enzyme-linked immunosorbent assay. Serum concentrations of α-Gal–specific immunoglobulin (Ig) G, IgG subclasses and IgE, complement factor 3a, NETosis-specific citrullinated H3, and the systemic inflammation markers soluble suppression of tumorigenicity and interleukin 33 were evaluated.

Results

Three months after TAVI, we found significantly increased serum concentrations of α-Gal–specific IgG3, complement factor complement factor 3a, citrullinated H3 levels, and soluble suppression of tumorigenicity (P = .002, P = .001, P = .025, and P = .039, respectively). Sensitization of α-Gal–specific IgE antibodies occurred in 55% of all patients after TAVI.

Conclusions

Our results indicate that TAVI elicits a midterm, specific humoral immune response against α-Gal and causes an unspecific humoral inflammation compared with patients undergoing MitraClip implantation. This observation will lead to a better understanding of postintervention morbidity and the long-term durability of bioprostheses and indicates that caution is appropriate when designing implantation strategies for younger patients.

Fifty years of the pericardial valve: Long-term results in the aortic position

It is now 50 years since the development of the first pericardial valve in 1971. In this time significant progress has been made in refining valve design aimed at improving the longevity of the prostheses. This article reviews the current literature regarding the longevity of pericardial heart valves in the aortic position. Side by side comparisons of freedom from structural valve degeneration are made for the valves most commonly used in clinical practice today, including stented, stentless, and sutureless valves. Strategies to reduce structural valve degeneration are also discussed including methods of tissue fixation and anti-calcification, ways to minimise mechanical stress on the valve, and the role of patient prosthesis mismatch.

Speckle tracking echocardiography in severe patient-prosthesis mismatch

BACKGROUND

Although aortic valve replacement (AVR) when successfully performed boasts low mortality rates in selected patients, prosthesis-patient mismatch (PPM) can be found in the majority of these individuals. Limited research is available supporting the benefit of two-dimensional speckle tracking echocardiography (2D-STE) in patients with severe PPM. This study sought to assess myocardial strain using 2D-STE to determine the relationship between subclinical left ventricular (LV) dysfunction and aortic PPM in patients undergoing AVR with preserved LV ejection fraction.

MATERIAL AND METHODS

We retrospectively examined all consecutive patients with isolated AVR who presented to our center from 2005 to 2018. The data of 1086 patients were analyzed. Severe PPM was defined as an indexed effective orifice area of 0.65 cm²/m² or less. As a result of the detailed assessment, 54 patients meeting the eligibility criteria were included in the study. Baseline data were collected and compared between the two groups of patients with severe PPM (n = 27) and those with normofunctional aortic prosthesis valve as a control group (n = 27). All patients underwent baseline echocardiography. Global longitudinal strain (GLS) and global circumferential strain (GCS) were evaluated by 2D-STE.

RESULTS

When compared with controls, patients with severe PPM had significantly decreased GLS (18.6 ± 2.9 vs. 21.4 ± 2.1; p < 0.01) and GCS (17.2 ± 3.6 vs. 21.7 ± 2.1; p < 0.01) values.

CONCLUSION

In addition to standard clinical and echocardiographic parameters, GLS and GCS suggest subclinical dysfunction and have incremental value in patients with severe PPM.

Multiscale Characterization of Isotropic Pyrolytic Carbon Used for Mechanical Heart Valve Production

Usage of pyrolytic carbon (PyC) to produce mechanical heart valves (MHVs) has led to heart valve replacement being a very successful procedure. Thus, the mechanical properties of employed materials for MHV production are fundamental to obtain the required characteristics of biocompatibility and wear resistance. In this study, two deposition methods of PyC were compared through a multiscale approach, performing three-point bending tests and nanoindentation tests. Adopted deposition processes produced materials that were slightly different. Significant differences were found at the characteristic scale lengths of the deposited layers. Setting changes of the deposition process permitted obtaining PyC characterized by a more uniform microstructure, conferring to the bulk material superior mechanical properties.

Poly(ethylene glycol)-Based Coatings for Bioprosthetic Valve Tissues: Toward Restoration of Physiological Behavior

Bioprosthetic valves (BPVs) have a limited lifespan in the body necessitating repeated surgeries to replace the failed implant. Early failure of these implants has been linked to various surface properties of the valve. Surface properties of BPVs are significantly different from physiological valves because of the fixation process used when processing the xenograft tissue. To improve the longevity of BPVs, efforts need to be taken to improve the surface properties and shield the implant from the bodily interactions that degrade it. Toward this goal, we evaluated the use of hydrogel coatings to attach to the BPV tissue and impart surface properties that are close to physiological. Hydrogels are well characterized for their biocompatibility and highly tunable surface characteristics. Using a previously published coating method, we deposited hydrogel coatings of poly(ethylene glycol)diacrylate (PEGDA) and poly(ethylene glycol)diacrylamide (PEGDAA) atop BPV samples. Coated samples were evaluated against the physiological tissue and uncoated glutaraldehyde-fixed tissue for deposition of hydrogel, surface adherence, mechanical properties, and fixation properties. Results showed both PEGDA- and PEGDAA-deposited coatings were nearly continuous across the valve leaflet surface. Further, the PEGDA- and PEGDAA-coated samples showed restoration of physiological levels of protein adhesion and mechanical stiffness. Interestingly, the coating process rather than the coating itself altered the material behavior yet did not alter the cross-linking from fixation. These results show that the PEG-based coatings for BPVs can successfully alter surface properties of BPVs and help promote physiological characteristics without interfering with the necessary fixation.

Effectiveness of distal arterial bypass with porcine decellularized vascular graft for treating diabetic lower limb ischemia

BACKGROUND

Application of tissue engineered vascular grafts for small-diameter artery reconstruction has been a much anticipated advance in vascular surgery. The aim of this study is to assess the effectiveness of small-diameter decellularized vascular grafts in below-knee bypass surgery for diabetic lower extremity ischemia.

METHODS

Three patients with diabetic lower limb ischemia were admitted to the Department of Vascular Surgery, Xuanwu Hospital, Capital Medical University between May, 2010 and June, 2010. Decellularized porcine arteries with modified surface were implanted in the lower extremity for below-knee arterial revascularization. Imaging examination was performed for assessment of graft mechanical stability and patency at 1 month and 6 months after implantation.

RESULTS

At 6 months after implantation, all three grafts were patent with no stenosis or aneurysm formation of the grafts were found on imaging assessment with primary patency rate of 100% (3/3) both at 1 month and 6 months after graft insertion.

CONCLUSION

Decellularized vascular graft with surface modification for the small-diameter artery reconstruction had good clinical results after 6 months follow-up in three patients with diabetic lower limb ischemia.

Possible Link Between the ABO Blood Group of Bioprosthesis Recipients and Specific Types of Structural Degeneration

Background Pigs/bovines share common antigens with humans: α-Gal, present in all pigs/bovines close to the human B-antigen; and AH-histo-blood-group antigen, identical to human AH-antigen and present only in some animals. We investigate the possible impact of patients' ABO blood group on bioprosthesis structural valve degeneration (SVD) through calcification/pannus/tears/perforations for patients ≤60 years at implantation. Methods and Results This was a single-center study (Paris, France) that included all degenerative bioprostheses explanted between 1985 and 1998, mostly porcine bioprostheses (Carpentier-Edwards second/third porcine bioprostheses) and some bovine bioprostheses. For the period 1998 to 2014, only porcine bioprostheses with longevity ≥13 years were included (total follow-up ≥29 years). Except for blood groups, important predictive factors for SVD were prospectively collected (age at implantation/longevity/number/site/sex/SVD types) and analyzed using logistic regression. All variables were available for 500 explanted porcine bioprostheses. By multivariate analyses, the A group was associated with an increased risk of: tears (odds ratio[OR], 1.61; P=0.026); pannus (OR, 1.5; P=0.054), pannus with tears (OR, 1.73; P=0.037), and tendency for lower risk of: calcifications (OR, 0.63; P=0.087) or isolated calcification (OR, 0.67; P=0.17). A-antigen was associated with lower risk of perforations (OR 0.56; P=0.087). B-group patients had an increased risk of: perforations (OR, 1.73; P=0.043); having a pannus that was calcified (OR, 3.0, P=0.025). B-antigen was associated with a propensity for calcifications in general (OR, 1.34; P=0.25). Conclusions Patient's ABO blood group is associated with specific SVD types. We hypothesize that carbohydrate antigens, which may or may not be common to patient and animal bioprosthetic tissue, will determine a patient's specific immunoreactivity with respect to xenograft tissue and thus bioprosthesis outcome in terms of SVD.

Inhibition of PP2A enhances the osteogenic differentiation of human aortic valvular interstitial cells via ERK and p38 MAPK pathways

AIMS

To investigate the role of PP2A in calcified aortic valve disease (CAVD).

MATERIALS AND METHODS

The expressions of PP2A subunits were detected by real-time polymerase chain reaction (RT-PCR) and western blot in aortic valves from patients with CAVD and normal controls, the activities of PP2A were analyzed by commercial assay kit at the same time. Aortic valve calcification of mice was evaluated through histological and echocardiographic analysis. ApoE^-/- mice and ApoE^-/- mice injected intraperitoneally with PP2A inhibitor LB100 were fed a high-cholesterol diet for 24 weeks. Immunofluorescent staining was used to locate the cell-type in which PP2A activity was decreased, the PP2A activity of valvular interstitial cells (VICs) treated with osteogenic induction medium was assessed by western blot and commercial assay kit. After changing the activity of VICs through pharmacologic and genetic intervention, the osteoblast differentiation and mineralization were assessed by western blot and Alizarin Red staining. Finally, the mechanism was clarified by using several specific inhibitors.

KEY FINDINGS

PP2A activity was decreased both in calcified aortic valves and human VICs under osteogenic induction. The PP2A inhibitor LB100 aggravated the aortic valve calcification of mice. Furthermore, PPP2CA overexpression inhibited osteogenic differentiation of VICs, whereas PPP2CA knockdown promoted the process. Further study revealed that the ERK/p38 MAPKs signaling pathways mediated the osteogenic differentiation of VICs induced by PP2A inactivation.

SIGNIFICANCE

This study demonstrated that PP2A plays an important role in CAVD pathophysiology, PP2A activation may provide a novel strategy for the pharmacological treatment of CAVD.

In vitro Neo-Genesis of Tendon/Ligament-Like Tissue by Combination of Mohawk and a Three-Dimensional Cyclic Mechanical Stretch Culture System

Tendons and ligaments are pivotal connective tissues that tightly connect muscle and bone. In this study, we developed a novel approach to generate tendon/ligament-like tissues with a hierarchical structure, by introducing the tendon/ligament-specific transcription factor Mohawk (MKX) into the mesenchymal stem cell (MSC) line C3H10T1/2 cells, and by applying an improved three-dimensional (3D) cyclic mechanical stretch culture system. In our developed protocol, a combination of stable Mkx expression and cyclic mechanical stretch synergistically affects the structural tendon/ligament-like tissue generation and tendon related gene expression. In a histological analysis of these tendon/ligament-like tissues, an organized extracellular matrix (ECM), containing collagen type III and elastin, was observed. Moreover, we confirmed that Mkx expression and cyclic mechanical stretch, induced the alignment of structural collagen fibril bundles that were deposited in a fibripositor-like manner during the generation of our tendon/ligament-like tissues. Our findings provide new insights for the tendon/ligament biomaterial fields.

Interfacial Coating Method for Amine-Rich Surfaces using Poly(ethylene glycol) Diacrylate Applied to Bioprosthetic Valve Tissue Models

Bioprosthetic heart valve implants are beset by calcification and failure due to the interactions between the body and the transplant. Hydrogels can be used as biological blank slates that may help to shield implants from these interactions; however, traditional light-based hydrogel polymerization is impeded by tissue opacity and topography. Therefore, new methods must be created to bind hydrogel to implant tissues. To address these complications, a two-step surface-coating method for bioprosthetic valves was developed. A previously developed bioprosthetic valve model (VM) was used to investigate and optimize the coating method. Generally, this coating is achieved by first reacting surface amine groups with an NHS-PEG-acrylate while also allowing glucose to absorb into the bulk. Then, glucose oxidase, poly(ethylene glycol) diacrylate (PEGDA), and iron ions are added to the system to initiate free-radical polymerization that bonds the PEGDA hydrogel to the acrylates sites on the surface. Results showed a thin (∼8 μm), continuous coating on VM samples that is capable of repelling protein adhesion (2% surface fouling versus 20% on uncoated samples) and does not significantly affect the surface mechanical properties. Based on this success, the coating method was translated to glutaraldehyde-fixed valve tissue samples. Results showed noncontinuous but evident coating on the surface, which was further improved by adjusting the coating solution. These results demonstrate the feasibility of the proposed two-step surface coating method for modifying the surface of bioprosthetic valve replacements.

Surgically implanted aortic valve bioprostheses deform after implantation: insights from computed tomography

OBJECTIVE

Little is known about the prevalence and degree of deformation of surgically implanted aortic biological valve prostheses (bio-sAVRs). We assessed bio-sAVR deformation using multidetector-row computed tomography (MDCT).

METHODS

Three imaging databases were searched for patients with MDCT performed after bio-sAVR implantation. Minimal and maximal valve ring diameters were obtained in systole and/or diastole, depending on the acquired cardiac phase(s). The eccentricity index (EI) was calculated as a measure of deformation as (1 - (minimal diameter/maximal diameter)) × 100%. EI of < 5% was considered none or trivial deformation, 5-10% mild deformation, and > 10% non-circular. Indications for MDCT and implanted valve type were retrieved.

RESULTS

One hundred fifty-two scans of bio-sAVRs were included. One hundred seventeen measurements were performed in systole and 35 in diastole. None or trivial deformation (EI < 5%) was seen in 67/152 (44%) of patients. Mild deformation (EI 5-10%) was seen in 59/152 (39%) and non-circularity was found in 26/152 (17%) of cases. Overall, median EI was 5.5% (IQR 3.4-7.8). In 77 patients, both systolic and diastolic measurements were performed from the same scan. For these scans, the median EI was 6.5% (IQR 3.4-10.2) in systole and 5.1% (IQR3.1-7.6) in diastole, with a significant difference between both groups (p = 0.006).

CONCLUSIONS

Surgically implanted aortic biological valve prostheses show mild deformation in 39% of cases and were considered non-circular in 17% of studied valves.

KEY POINTS

• Deformation of surgically implanted aortic valve bioprostheses (bio-sAVRs) can be adequately assessed using MDCT. • Bio-sAVRs show at least mild deformation (eccentricity index > 5%) in 56% of studied cases and were considered non-circular (eccentricity index > 10%) in 17% of studied valves. • The higher deformity rate found in bio-sAVRs with (suspected) valve pathology could suggest that geometric deformity may play a role in leaflet malformation and thrombus formation similar to that of transcatheter heart valves.

Immersogeometric fluid-structure interaction modeling and simulation of transcatheter aortic valve replacement

The transcatheter aortic valve replacement (TAVR) has emerged as a minimally invasive alternative to surgical treatments of valvular heart disease. TAVR offers many advantages, however, the safe anchoring of the transcatheter heart valve (THV) in the patients anatomy is key to a successful procedure. In this paper, we develop and apply a novel immersogeometric fluid-structure interaction (FSI) framework for the modeling and simulation of the TAVR procedure to study the anchoring ability of the THV. To account for physiological realism, methods are proposed to model and couple the main components of the system, including the arterial wall, blood flow, valve leaflets, skirt, and frame. The THV is first crimped and deployed into an idealized ascending aorta. During the FSI simulation, the radial outward force and friction force between the aortic wall and the THV frame are examined over the entire cardiac cycle. The ratio between these two forces is computed and compared with the experimentally estimated coefficient of friction to study the likelihood of valve migration.

Mechanical considerations for polymeric heart valve development: Biomechanics, materials, design and manufacturing

The native human heart valve leaflet contains a layered microstructure comprising a hierarchical arrangement of collagen, elastin, proteoglycans and various cell types. Here, we review the various experimental methods that have been employed to probe this intricate microstructure and which attempt to elucidate the mechanisms that govern the leaflet's mechanical properties. These methods include uniaxial, biaxial, and flexural tests, coupled with microstructural characterization techniques such as small angle X-ray scattering (SAXS), small angle light scattering (SALS), and polarized light microscopy. These experiments have revealed complex elastic and viscoelastic mechanisms that are highly directional and dependent upon loading conditions and biochemistry. Of all engineering materials, polymers and polymer-based composites are best able to mimic the tissue-level mechanical behavior of the native leaflet. This similarity to native tissue permits the fabrication of polymeric valves with physiological flow patterns, reducing the risk of thrombosis compared to mechanical valves and in some cases surpassing the in vivo durability of bioprosthetic valves. Earlier work on polymeric valves simply assumed the mechanical properties of the polymer material to be linear elastic, while more recent studies have considered the full hyperelastic stress-strain response. These material models have been incorporated into computational models for the optimization of valve geometry, with the goal of minimizing internal stresses and improving durability. The latter portion of this review recounts these developments in polymeric heart valves, with a focus on mechanical testing of polymers, valve geometry, and manufacturing methods.

Some Effects of Different Constitutive Laws on FSI Simulation for the Mitral Valve

In this paper, three different constitutive laws for mitral leaflets and two laws for chordae tendineae are selected to study their effects on mitral valve dynamics with fluid-structure interaction. We first fit these three mitral leaflet constitutive laws and two chordae tendineae laws with experimental data. The fluid-structure interaction is implemented in an immersed boundary framework with finite element extension for solid, that is the hybrid immersed boundary/finite element(IB/FE) method. We specifically compare the fluid-structure results of different constitutive laws since fluid-structure interaction is the physiological loading environment. This allows us to look at the peak jet velocity, the closure regurgitation volume, and the orifice area. Our numerical results show that different constitutive laws can affect mitral valve dynamics, such as the transvalvular flow rate, closure regurgitation and the orifice area, while the differences in fiber strain and stress are insignificant because all leaflet constitutive laws are fitted to the same set of experimental data. In addition, when an exponential constitutive law of chordae tendineae is used, a lower closure regurgitation flow is observed compared to that of a linear material model. In conclusion, combining numerical dynamic simulations and static experimental tests, we are able to identify suitable constitutive laws for dynamic behaviour of mitral leaflets and chordae under physiological conditions.

Generation of cattle knockout for galactose-α1,3-galactose and N-glycolylneuraminic acid antigens

Two well‐characterized carbohydrate epitopes are absent in humans but present in other mammals. These are galactose‐α1,3‐galactose (αGal) and N‐glycolylneuraminic acid (Neu5Gc) which are introduced by the activities of two enzymes including α(1,3) galactosyltransferase (encoded by the GGTA1 gene) and CMP‐Neu5Gc hydroxylase (encoded by the CMAH gene) that are inactive in humans but present in cattle. Hence, bovine‐derived products are antigenic in humans who receive bioprosthetic heart valves (BHVs) or those that suffer from red meat syndrome. Using programmable nucleases, we disrupted (knockout, KO) GGTA1 and CMAH genes encoding for the enzymes that catalyse the synthesis of αGal and Neu5Gc, respectively, in both male and female bovine fibroblasts. The KO in clonally selected fibroblasts was detected by polymerase chain reaction (PCR) and confirmed by Sanger sequencing. Selected fibroblasts colonies were used for somatic cell nuclear transfer (SCNT) to produce cloned embryos that were implanted in surrogate recipient heifers. Fifty‐three embryos were implanted in 33 recipients heifers; 3 pregnancies were carried to term and delivered 3 live calves. Primary cell cultures were established from the 3 calves and following molecular analyses confirmed the genetic deletions. FACS analysis showed the double‐KO phenotype for both antigens confirming the mutated genotypes. Availability of such cattle double‐KO model lacking both αGal and Neu5Gc offers a unique opportunity to study the functionality of BHV manufactured with tissues of potentially lower immunogenicity, as well as a possible new clinical approaches to help patients with red meat allergy syndrome due to the presence of these xenoantigens in the diet.

Preservation strategies for decellularized pericardial scaffolds for off-the-shelf availability

Decellularized biological scaffolds hold great promise in cardiovascular surgery. In order to ensure off-the-shelf availability, routine use of decellularized scaffolds requires tissue banking. In this study, the suitability of cryopreservation, vitrification and freeze-drying for the preservation of decellularized bovine pericardial (DBP) scaffolds was evaluated. Cryopreservation was conducted using 10% DMSO and slow-rate freezing. Vitrification was performed using vitrification solution (VS83) and rapid cooling. Freeze-drying was done using a programmable freeze-dryer and sucrose as lyoprotectant. The impact of the preservation methods on the DBP extracellular matrix structure, integrity and composition was assessed using histology, biomechanical testing, spectroscopic and thermal analysis, and biochemistry. In addition, the cytocompatibility of the preserved scaffolds was also assessed. All preservation methods were found to be suitable to preserve the extracellular matrix structure and its components, with no apparent signs of collagen deterioration or denaturation, or loss of elastin and glycosaminoglycans. Biomechanical testing, however, showed that the cryopreserved DBP displayed a loss of extensibility compared to vitrified or freeze-dried scaffolds, which both displayed similar biomechanical behavior compared to non-preserved control scaffolds. In conclusion, cryopreservation altered the biomechanical behavior of the DBP scaffolds, which might lead to graft dysfunction in vivo. In contrast to cryopreservation and vitrification, freeze-drying is performed with non-toxic protective agents and does not require storage at ultra-low temperatures, thus allowing for a cost-effective and easy storage and transport. Due to these advantages, freeze-drying is a preferable method for the preservation of decellularized pericardium. STATEMENT OF SIGNIFICANCE: Clinical use of DBP scaffolds for surgical reconstructions or substitutions requires development of a preservation technology that does not alter scaffold properties during long-term storage. Conclusive investigation on adverse impacts of the preservation methods on DBP matrix integrity is still missing. This work is aiming to close this gap by studying three potential preservation technologies, cryopreservation, vitrification and freeze-drying, in order to achieve the off-the-shelf availability of DBP patches for clinical application. Furthermore, it provides novel insights for dry-preservation of decellularized xenogeneic scaffolds that can be used in the routine clinical cardiovascular practice, allowing the surgeon the opportunity to choose an ideal implant matching with the needs of each patient.

Development and characterization of bladder acellular matrix cross-linked by dialdehyde carboxymethyl cellulose for bladder tissue engineering

In order to address the disadvantage of rapid degradation and serious immune response of bladder acellular matrix (BAM) tissues in clinical application, in this study, oxidized carboxymethyl cellulose (DCMC) was developed to replace glutaraldehyde (GA), a most common synthetic crosslinking reagent in clinical practice, to fix BAM tissues for lower cytotoxicity. The aim of this work was to evaluate feasibility of DCMC as a crosslinking reagent for BAM fixation and developing DCMC fixed-BAM (D-BAM) tissues for tissue engineering. For the preparation of DCMC, the results showed that when DCMC was prepared using a specific concentration of sodium periodate solution (the mass ratio of NaIO₄/CMC is 1 : 1) and a specific reaction time (4 hours), its cytotoxicity was the lowest and its fixation effect was better. The critical crosslinking characteristics and cytocompatibility of optimum D-BAM tissues were also investigated. The results demonstrated that DCMC-fixation (especially 30 mg ml⁻¹ DCMC-fixation) not only formed stable cross-linking bonds but also preserved well the original ultrastructure of the BAM tissues, which simultaneously increased the mechanical strength and capacity of the enzymatic hydrolytic resistance. The DCMC-fixation could also reduce the expression of α-Gal in BAM tissues and preserve the useful growth factors such as GAGs, KGF and TGF-β in bladder tissues. In addition, 30 mg ml⁻¹ D-BAM tissues had excellent cytocompatibility. Moreover, it could stimulate the secretion of PDGF and EGF from seeded bladder transitional epithelial cells (BTECs), which is a critical feature for further re-epithelialization. Its anti-calcification ability was also prominent, which is necessary in bladder repair. The present studies demonstrated that DCMC could be a potential biological crosslinking agent for BAM fixation due to its excellent crosslinking effects, and the D-BAM tissues were suitable to be used as a substitute for the bladder due to their resistance to enzymatic degradation, anticalcification and cytocompatibility.

To address the disadvantage of rapid degradation and serious immune response of bladder acellular matrix tissues in clinical application, oxidized carboxymethyl cellulose was developed to replace commonly used glutaraldehyde, to fix BAM tissues for lower cytotoxicity.

The effect of heparin hydrogel embedding on glutaraldehyde fixed bovine pericardial tissues: Mechanical behavior and anticalcification potential

Heart valve diseases remain common in industrialized countries. Bioprosthetic heart valves, introduced as free of anticoagulation therapy alternatives to mechanical substitutes. Still they suffer from long term failure due to calcification. Different treatment methods introduced to inhibit calcification, have so far been limited in success. Glycosaminoglycans (GAGs) possess properties including high negative charge, anticoagulation and anti-inflammatory activity that make them a potential solution for calcification problem. In this study, heparin hydrogel was prepared and characterized both chemically and mechanically. After that, heparin hydrogel embedded bovine pericardial tissues, fixed with glutaraldehyde, were produced and tested for their mechanical behavior and anticalcifcation potential in vitro using the constant composition model. In the calcification experiments, tissues were divided into three groups: a) Controls without treatment, b) Hydrogel treated tissues and c) Tissues with raw heparin dissolved in the calcification solution. The results showed that embedding of tissue with hydrogel had no stiffening effect on its mechanical behavior. Calcification assessment showed a significant efficacy on inhibition of calcium phosphate deposition of hydrogel treated (second group) in comparison to untreated tissues (control, first group). Calcification inhibition potential was very similar in both the second and raw heparin (third group). Histological data confirmed the obtained results, suggesting that heparin treatment is a promising anticalcification agent.

Impact of Clinically Relevant Elliptical Deformations on the Damage Patterns of Sagging and Stretched Leaflets in a Bioprosthetic Heart Valve

After implantation of a transcatheter bioprosthetic heart valve its original circular circumference may become distorted, which can lead to changes in leaflet coaptation and leaflets that are stretched or sagging. This may lead to early structural deterioration of the valve as seen in some explanted transcatheter heart valves. Our in vitro study evaluates the effect of leaflet deformations seen in elliptical configurations on the damage patterns of the leaflets, with circular valve deformation as the control. Bovine pericardial tissue heart valves were subjected to accelerated wear testing under both circular (N = 2) and elliptical (N = 4) configurations. The elliptical configurations were created by placing the valve inside custom-made elliptical holders, which caused the leaflets to sag or stretch. The hydrodynamic performance of the valves was monitored and high resolution images were acquired to evaluate leaflet damage patterns over time. In the elliptically deformed valves, sagging leaflets experienced more damage from wear compared to stretched leaflets; the undistorted leaflets of the circular valves experienced the least leaflet damage. Free-edge thinning and tearing were the primary modes of damage in the sagging leaflets. Belly region thinning was seen in the undistorted and stretched leaflets. Leaflet and fabric tears at the commissures were seen in all valve configurations. Free-edge tearing and commissure tears were the leading cause of valve hydrodynamic incompetence. Our study shows that mechanical wear affects heart valve pericardial leaflets differently based on whether they are undistorted, stretched, or sagging in a valve configuration. Sagging leaflets are more likely to be subjected to free-edge tear than stretched or undistorted leaflets. Reducing leaflet stress at the free edge of non-circular valve configurations should be an important factor to consider in the design and/or deployment of transcatheter bioprosthetic heart valves to improve their long-term performance.

Engineering biologically extensible hydrogels using photolithographic printing

Biomaterials for tissue engineering that recapitulate the mechanical response and biological function of native tissue are highly sought after to lessen the burden of damaged or diseased tissue. Poly(ethylene glycol) diacrylate (PEGDA) hydrogels are a popular candidate because of their favorable bioactive properties. However, their mechanical behavior is very dissimilar to that of biological tissue, which behaves in a mechanically anisotropic, nonlinear, and viscoelastic fashion. It has been previously shown that PEGDA hydrogels can be patterned in alternating linear strips of different stiffnesses to generate anisotropic behavior, but these constructs still have a linear stress-strain response. In this study, we imparted nonlinear mechanical properties to PEGDA hydrogels by fabricating composite hydrogel constructs consisting of a stiff sinusoidal reinforcement embedded into a softer base matrix. This was achieved by polymerizing low molecular weight (MW) PEGDA hydrogel precursor into a stiff sinusoidal shape and then polymerizing this construct into a high MW precursor. Samples were generated with different relative stiffness between the two components and a range of sinusoid periodicities to assess the tunability of the resulting stress-strain curve. Tensile testing indicates that the sinusoidal patterning gives rise to nonlinear stress-strain behavior. Varying the relative stiffness was shown to tune the slope of the linear region of the stress-strain curve, and varying periodicity was shown to affect the length of the toe region of this curve. We conclude that composite hydrogels with stiff sinusoidally-patterned reinforcements display mechanical properties more similar to those of biological tissue than uniform or linearly-patterned hydrogels.

STATEMENT OF SIGNIFICANCE

Hydrogel biomaterials are a popular candidate for engineering constructs that can mimic the properties of native tissue for disease modeling and tissue-engineering applications. Studies have shown that poly(ethylene) glycol diacrylate (PEGDA) hydrogels can be fabricated to display many biological aspects of native tissue. However, they are unable to recapitulate fundamental mechanical properties of such tissue, such as anisotropy and nonlinearity. Photolithographic techniques have been employed to generate anisotropic linear PEGDA hydrogels via patterned reinforcement. The present study indicates that such techniques can be modified to generate PEGDA constructs with a sinusoidal reinforcement that display a strongly nonlinear response to tensile loading. This work sets the stage for more intricate patterning for providing increased control over hydrogel mechanical response.

Responsive and Adaptive Micro Wrinkles on Organic-Inorganic Hybrid Materials

A buckling induced wrinkling is a general phenomenon in daily life, which is induced by mechanical instability at the interface of multi-layered systems. Variety of applications have been proposed for wrinkles in nano to micrometer periodicity on the surface of soft materials. In recent decades, researchers are trying to use wrinkles for variety of sophisticated applications such as micro pattern fabrication, control of wettability, templating/directing substrate for elongated nano materials or virus, size-selective adsorption/desorption of functional objects, cells or microorganisms, delamination induced material fabrication such as micro-rolls, substrates for stretchable electronics, valves for microfluidic devices and soft actuators. Herein, recent advances on the fabrication and application of micro-wrinkles are reviewed.