Transcatheter placement of a low-profile biodegradable pulmonary valve made of small intestinal submucosa: A long-term study in a swine model

Can Heart Valve Decellularization Be Standardized? A Review of the Parameters Used for the Quality Control of Decellularization Processes

When a tissue or an organ is considered, the attention inevitably falls on the complex and delicate mechanisms regulating the correct interaction of billions of cells that populate it. However, the most critical component for the functionality of specific tissue or organ is not the cell, but the cell-secreted three-dimensional structure known as the extracellular matrix (ECM). Without the presence of an adequate ECM, there would be no optimal support and stimuli for the cellular component to replicate, communicate and interact properly, thus compromising cell dynamics and behaviour and contributing to the loss of tissue-specific cellular phenotype and functions. The limitations of the current bioprosthetic implantable medical devices have led researchers to explore tissue engineering constructs, predominantly using animal tissues as a potentially unlimited source of materials. The high homology of the protein sequences that compose the mammalian ECM, can be exploited to convert a soft animal tissue into a human autologous functional and long-lasting prosthesis ensuring the viability of the cells and maintaining the proper biomechanical function. Decellularization has been shown to be a highly promising technique to generate tissue-specific ECM-derived products for multiple applications, although it might comprise very complex processes that involve the simultaneous use of chemical, biochemical, physical and enzymatic protocols. Several different approaches have been reported in the literature for the treatment of bone, cartilage, adipose, dermal, neural and cardiovascular tissues, as well as skeletal muscle, tendons and gastrointestinal tract matrices. However, most of these reports refer to experimental data. This paper reviews the most common and latest decellularization approaches that have been adopted in cardiovascular tissue engineering. The efficacy of cells removal was specifically reviewed and discussed, together with the parameters that could be used as quality control markers for the evaluation of the effectiveness of decellularization and tissue biocompatibility. The purpose was to provide a panel of parameters that can be shared and taken into consideration by the scientific community to achieve more efficient, comparable, and reliable experimental research results and a faster technology transfer to the market.

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.

Over 400 Uses of An Intestinal Submucosal Extracellular Matrix Patch in a Congenital Heart Program

BACKGROUND

Repair of complex congenital heart disease frequently requires usage of a patch as an anatomical substitute. The study's aim is to evaluate the use, effectiveness and safety of utilizing small intestine submucosa extracellular matrix (SIS-ECM) patches in a congenital cardiac surgery program.

METHODS

This is a single-center, retrospective, cohort study of surgeries utilizing SIS-ECM between 2012-2019. The SIS-ECM data was categorized by usage and type (4-ply and 2-ply). All re-interventions/complications were reviewed by an independent surgeon, a practicing congenital heart surgeon and a pediatric cardiologist.

RESULTS

408 SIS-ECM patches were used in 309 patients (M/F=188/121; median age 8.5months). The usage consisted of 314 (77%) arterioplasties, 22 (5.4%) venoplasties, 63 (15.4%) intracardiac repairs, and 9 (2.2%) valve repairs. The most common usage was pulmonary artery repair (n=181; 44.4%). Median follow-up time was 3.9 years (range: 3days-7.4years). Ten (2.5%) patches required surgical (2 in first 30-days and 5 in 1^st year) and 27 (6.6%) required percutaneous re-interventions (2 in first 30-days and 22 in 1^st year). Between 4-ply (n=376) and 2-ply (n=32) SIS-ECM, rate of surgical (2.1% (n=8) vs 6.3% (n=2); p=0.18) or percutaneous re-interventions (6.4% (n=24) vs 9.4% (n=3); p=0.46) was not different. There were no deaths related to the SIS-ECM patch or reports of calcification.

CONCLUSIONS

SIS-ECM is a viable patch option that can be used in various cardiac and vascular reconstructive surgeries with low risk of failure and calcification. Long-term, positive outcomes may be maximized by consistent techniques and understanding appropriate applications of the patch.

De Novo Valve Tissue Morphology Following Bioscaffold Mitral Valve Replacement in a Juvenile Non-Human Primate Model

The utility of implanting a bioscaffold mitral valve consisting of porcine small intestinal submucosa (PSIS) in a juvenile baboon model (12 to 14 months old at the time of implant; n = 3) to assess their in vivo tissue remodeling responses was investigated. Our findings demonstrated that the PSIS mitral valve exhibited the robust presence of de novo extracellular matrix (ECM) at all explantation time points (at 3-, 11-, and 20-months). Apart from a significantly lower level of proteoglycans in the implanted valve’s annulus region (p < 0.05) at 3 months compared to the 11- and 20-month explants, there were no other significant differences (p > 0.05) found between any of the other principal valve ECM components (collagen and elastin) at the leaflet, annulus, or chordae tendinea locations, across these time points. In particular, neochordae tissue had formed, which seamlessly integrated with the native papillary muscles. However, additional processing will be required to trigger accelerated, uniform and complete valve ECM formation in the recipient. Regardless of the specific processing done to the bioscaffold valve, in this proof-of-concept study, we estimate that a 3-month window following bioscaffold valve replacement is the timeline in which complete regeneration of the valve and integration with the host needs to occur.

Next-generation tissue-engineered heart valves with repair, remodelling and regeneration capacity

Valvular heart disease is a major cause of morbidity and mortality worldwide. Surgical valve repair or replacement has been the standard of care for patients with valvular heart disease for many decades, but transcatheter heart valve therapy has revolutionized the field in the past 15 years. However, despite the tremendous technical evolution of transcatheter heart valves, to date, the clinically available heart valve prostheses for surgical and transcatheter replacement have considerable limitations. The design of next-generation tissue-engineered heart valves (TEHVs) with repair, remodelling and regenerative capacity can address these limitations, and TEHVs could become a promising therapeutic alternative for patients with valvular disease. In this Review, we present a comprehensive overview of current clinically adopted heart valve replacement options, with a focus on transcatheter prostheses. We discuss the various concepts of heart valve tissue engineering underlying the design of next-generation TEHVs, focusing on off-the-shelf technologies. We also summarize the latest preclinical and clinical evidence for the use of these TEHVs and describe the current scientific, regulatory and clinical challenges associated with the safe and broad clinical translation of this technology.

Porcine Small Intestinal Submucosa Mitral Valve Material Responses Support Acute Somatic Growth

Background: Conceptually, a tissue engineered heart valve would be especially appealing in the pediatric setting since small size and somatic growth constraints would be alleviated. In this study, we utilized porcine small intestinal submucosa (PSIS) for valve replacement. Of note, we evaluated the material responses of PSIS and subsequently its acute function and somatic growth potential in the mitral position. Methods and Results: Material and mechanical assessment demonstrated that both fatigued 2ply (∼65 μm) and 4ply (∼110 μm) PSIS specimens exhibited similar failure mechanisms, but at an accelerated rate in the former. Specifically, the fatigued 2ply PSIS samples underwent noticeable fiber pullout and recruitment on the bioscaffold surface, leading to higher yield strength (p < 0.05) and yield strain (p < 0.05) compared to its fatigued 4ply counterparts. Consequently, 2ply PSIS mitral valve constructs were subsequently implanted in juvenile baboons (n = 3). Valve function was longitudinally monitored for 90 days postvalve implantation and was found to be robust in all animals. Histology at 90 days in one of the animals revealed the presence of residual porcine cells, fibrin matrix, and host baboon immune cells but an absence of tissue regeneration. Conclusions: Our findings suggest that the altered structural responses of PSIS, postfatigue, rather than de novo tissue formation, are primarily responsible for the valve's ability to accommodate somatic growth during the acute phase (90 days) following mitral valve replacement. Impact Statement Tissue engineered heart valves (TEHVs) offer the potential of supporting somatic growth. In this study, we investigated a porcine small intestinal submucosa bioscaffold for pediatric mitral heart valve replacement. The novelty of the study lies in identifying material responses under mechanical loading conditions and its effectiveness in being able to function as a TEHV. In addition, the ability of the scaffold valve to support acute somatic growth was evaluated in the Baboon model. The current study contributes toward finding a solution for critical valve diseases in children, whose current prognosis for survival is poor.

Failure of decellularized porcine small intestinal submucosa as a heart valved conduit

OBJECTIVE

Decellularized extracellular matrix made from porcine small intestinal submucosa, commercially available as CorMatrix (CorMatrix Cardiovascular, Inc, Roswell, Ga) is used off-label to reconstruct heart valves. Recently, surgeons experienced failures and words of caution were raised. The aim of this study was to evaluate decellularized porcine small intestinal submucosa as right-sided heart valved conduit in a xenogeneic animal model.

METHODS

A pulmonary valve replacement was performed with custom-made valved conduits in 10 lambs and 10 sheep (1 month [3 lambs and 3 sheep], 3 months [3 lambs and 3 sheep], 6 months [4 lambs and 4 sheep]). Valve function was assessed after implantation and before the animal was put to death. Explanted conduits were inspected macroscopically and analyzed using immunohistochemistry and scanning electron microscopy. They also underwent mechanical testing and testing for biochemical composition.

RESULTS

All valved conduits were successfully implanted. Five sheep and 2 lambs died due to congestive heart failure within 2 months after surgery. In the animals that died, the valve leaflets were thickened with signs of inflammation (endocarditis in 4). Five sheep and 8 lambs (1 month: 6 out of 6 animals, 3 months: 4 out of 6 animals, 6 months: 3 out of 8 animals) survived planned follow-up. At the time they were put to death, 5 lambs had significant pulmonary stenosis and 1 sheep showed severe regurgitation. A well-functioning valve was seen in 4 sheep and 3 lambs for up to 3 months. These leaflets showed limited signs of remodeling.

CONCLUSIONS

Fifty percent of sheep and 20% of lambs died due to valve failure before the planned follow-up period was complete. A well-functioning valve was seen in 35% of animals, albeit with limited signs of tissue remodeling at ≤3 months after implantation. Further analysis is needed to understand the disturbing dichotomous outcome before clinical application can be advised.

Cell Sources for Tissue Engineering Strategies to Treat Calcific Valve Disease

Cardiovascular calcification is an independent risk factor and an established predictor of adverse cardiovascular events. Despite concomitant factors leading to atherosclerosis and heart valve disease (VHD), the latter has been identified as an independent pathological entity. Calcific aortic valve stenosis is the most common form of VDH resulting of either congenital malformations or senile “degeneration.” About 2% of the population over 65 years is affected by aortic valve stenosis which represents a major cause of morbidity and mortality in the elderly. A multifactorial, complex and active heterotopic bone-like formation process, including extracellular matrix remodeling, osteogenesis and angiogenesis, drives heart valve “degeneration” and calcification, finally causing left ventricle outflow obstruction. Surgical heart valve replacement is the current therapeutic option for those patients diagnosed with severe VHD representing more than 20% of all cardiac surgeries nowadays. Tissue Engineering of Heart Valves (TEHV) is emerging as a valuable alternative for definitive treatment of VHD and promises to overcome either the chronic oral anticoagulation or the time-dependent deterioration and reintervention of current mechanical or biological prosthesis, respectively. Among the plethora of approaches and stablished techniques for TEHV, utilization of different cell sources may confer of additional properties, desirable and not, which need to be considered before moving from the bench to the bedside. This review aims to provide a critical appraisal of current knowledge about calcific VHD and to discuss the pros and cons of the main cell sources tested in studies addressing in vitro TEHV.

Recent advances and challenges on application of tissue engineering for treatment of congenital heart disease

Congenital heart disease (CHD) affects a considerable number of children and adults worldwide. This implicates not only developmental disorders, high mortality, and reduced quality of life but also, high costs for the healthcare systems. CHD refers to a variety of heart and vascular malformations which could be very challenging to reconstruct the malformed region surgically, especially when the patient is an infant or a child. Advanced technology and research have offered a better mechanistic insight on the impact of CHD in the heart and vascular system of infants, children, and adults and identified potential therapeutic solutions. Many artificial materials and devices have been used for cardiovascular surgery. Surgeons and the medical industry created and evolved the ball valves to the carbon-based leaflet valves and introduced bioprosthesis as an alternative. However, with research further progressing, contracting tissue has been developed in laboratories and tissue engineering (TE) could represent a revolutionary answer for CHD surgery. Development of engineered tissue for cardiac and aortic reconstruction for developing bodies of infants and children can be very challenging. Nevertheless, using acellular scaffolds, allograft, xenografts, and autografts is already very common. Seeding of cells on surface and within scaffold is a key challenging factor for use of the above. The use of different types of stem cells has been investigated and proven to be suitable for tissue engineering. They are the most promising source of cells for heart reconstruction in a developing body, even for adults. Some stem cell types are more effective than others, with some disadvantages which may be eliminated in the future.

Evaluation of transventricular placement of porcine small intestinal submucosa stent valves in the pulmonary position in juvenile sheep model

OBJECTIVES

We assessed the transventricular placement of porcine small intestinal submucosa (SIS) stent valves in a juvenile sheep model at the 3-month follow-up evaluation.

METHODS

We constructed a pulmonary stent valve by suturing a porcine SIS bicuspid valve into a bell-shaped 'Z' nitinol stent and implanted 7 SIS stent valves transventricularly in the pulmonary position in 7 sheep. The function of the stent valves was assessed using a pulsatile flow simulation system in vitro. Haemodynamic, angiographic, echocardiographic, histologic and radiographic examinations were carried out before, immediately after implantation and 3 months after implantation.

RESULTS

All SIS stent valves were successfully implanted in the pulmonary position in 7 sheep. Angiographic, echocardiographic, haemodyamic and macroscopic studies confirmed firm anchoring and good positioning of the stents immediately after implantation and at 3-month follow-up. All stent valves had good function immediately after implantation and at the end of the protocol, with the exception of 1 stent valve with mild stenosis detected at the end of the protocol. All SIS valves were free of calcifications and thrombus formation, and all stents were intact with no fractures and migration based on postmortem examination and X-radiography.

CONCLUSIONS

We demonstrated successful implantation of porcine SIS stent valves in the pulmonary position in sheep with excellent valve function at the 3-month follow-up evaluation. Porcine SIS has potential superiority as a pulmonary stent bioprosthetic valve material, and the bell-shaped nitinol stent has potential superiority as a frame for pulmonary stent valves.

Tissue-Engineered Heart Valves: A Call for Mechanistic Studies

Heart valve disease carries a substantial risk of morbidity and mortality. Outcomes are significantly improved by valve replacement, but currently available mechanical and biological replacement valves are associated with complications of their own. Mechanical valves have a high rate of thromboembolism and require lifelong anticoagulation. Biological prosthetic valves have a much shorter lifespan, and they are prone to tearing and degradation. Both types of valves lack the capacity for growth, making them particularly problematic in pediatric patients. Tissue engineering has the potential to overcome these challenges by creating a neovalve composed of native tissue that is capable of growth and remodeling. The first tissue-engineered heart valve (TEHV) was created more than 20 years ago in an ovine model, and the technology has been advanced to clinical trials in the intervening decades. Some TEHVs have had clinical success, whereas others have failed, with structural degeneration resulting in patient deaths. The etiologies of these complications are poorly understood because much of the research in this field has been performed in large animals and humans, and, therefore, there are few studies of the mechanisms of neotissue formation. This review examines the need for a TEHV to treat pediatric patients with valve disease, the history of TEHVs, and a future that would benefit from extension of the reverse translational trend in this field to include small animal studies.

Biomaterial-driven in situ cardiovascular tissue engineering-a multi-disciplinary perspective

There is a persistent and growing clinical need for readily-available substitutes for heart valves and small-diameter blood vessels. In situ tissue engineering is emerging as a disruptive new technology, providing ready-to-use biodegradable, cell-free constructs which are designed to induce regeneration upon implantation, directly in the functional site. The induced regenerative process hinges around the host response to the implanted biomaterial and the interplay between immune cells, stem/progenitor cell and tissue cells in the microenvironment provided by the scaffold in the hemodynamic environment. Recapitulating the complex tissue microstructure and function of cardiovascular tissues is a highly challenging target. Therein the scaffold plays an instructive role, providing the microenvironment that attracts and harbors host cells, modulating the inflammatory response, and acting as a temporal roadmap for new tissue to be formed. Moreover, the biomechanical loads imposed by the hemodynamic environment play a pivotal role. Here, we provide a multidisciplinary view on in situ cardiovascular tissue engineering using synthetic scaffolds; starting from the state-of-the art, the principles of the biomaterial-driven host response and wound healing and the cellular players involved, toward the impact of the biomechanical, physical, and biochemical microenvironmental cues that are given by the scaffold design. To conclude, we pinpoint and further address the main current challenges for in situ cardiovascular regeneration, namely the achievement of tissue homeostasis, the development of predictive models for long-term performances of the implanted grafts, and the necessity for stratification for successful clinical translation.

Pulmonary Valve Replacement With Small Intestine Submucosa-Extracellular Matrix in a Porcine Model

BACKGROUND

Prosthetic materials available for pediatric pulmonary valve replacement (PVR) lack growth potential, inevitably leading to a size mismatch. Small intestine submucosa-derived extracellular matrix (SIS-ECM) has been suggested to possess regenerative properties. We aimed to investigate its function and potential to increase in size as a PVR in a piglet.

METHODS

An SIS-ECM trileaflet valved conduit was designed. Hanford minipigs, n = 6 (10-34 kg), underwent PVR with an intended survival of six months, with monthly echocardiograms evaluating valve size and function. The conduit was excised for histologic analysis.

RESULTS

Of the six, one was sacrificed at three months for midterm analysis, and one at month 3 due to endocarditis. The remaining four constituted the study cohort. The piglet weight increased by 186% (19.56 ± 10.22 kg to 56.00 ± 7.87 kg). Conduit size increased by 30% (1.42 ± 0.14 cm to 1.84 ± 0.14 cm; P < .01). The native right ventricular outflow tract increased by 43% and the native pulmonary artery by 84%, resulting in a peak gradient increase from 10.08 ± 2.47 mm Hg to 36.25 ± 18.80 mm Hg (P = .03). Additionally, all valves developed at least moderate regurgitation. Conduit histology showed advanced remodeling with myofibroblast infiltration, neovascularization, and endothelialization. The leaflets remodeled beginning at the base with the leaflet edge being less cellular. In addition to the known endocarditis, bacterial colonies were discovered within a leaflet in another.

CONCLUSIONS

The SIS-ECM valved conduit implanted into a piglet demonstrated cellular infiltration with vascular remodeling and an increase in diameter. Conduit stenosis was a result of slower rates of size increase than native tissue. Suboptimal leaflet performance requires design modifications.

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

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

On the Mechanics of Transcatheter Aortic Valve Replacement

Transcatheter aortic valves (TAVs) represent the latest advances in prosthetic heart valve technology. TAVs are truly transformational as they bring the benefit of heart valve replacement to patients that would otherwise not be operated on. Nevertheless, like any new device technology, the high expectations are dampened with growing concerns arising from frequent complications that develop in patients, indicating that the technology is far from being mature. Some of the most common complications that plague current TAV devices include malpositioning, crimp-induced leaflet damage, paravalvular leak, thrombosis, conduction abnormalities and prosthesis-patient mismatch. In this article, we provide an in-depth review of the current state-of-the-art pertaining the mechanics of TAVs while highlighting various studies guiding clinicians, regulatory agencies, and next-generation device designers.

CorMatrix valved conduit in a porcine model: long-term remodelling and biomechanical characterization

OBJECTIVES

Porcine small intestinal submucosa extracellular matrix (CorMatrix; CorMatrix Cardiovascular, Rosewell, GA) is a relatively novel tissue substitute used in cardiovascular applications. We investigated the biological reaction and remodelling of CorMatrix as a tri-leaflet valved conduit in a pig model. We hypothesized that CorMatrix maintains a durable architecture as a valved conduit and remodels to resemble surrounding tissues.

METHODS

We fashioned the valved conduit using a 7 × 10 cm 4-ply CorMatrix sheet and placed it in the thoracic aorta of seven landrace pigs for 3, 4, 5 and 6 months. Biodegradation, replacement by native tissue, strength and durability were examined by histology, immunohistochemistry and mechanical testing.

RESULTS

Four pigs, one per time frame, completed the study. The conduit lost its original architecture as a tri-leaflet valve due to cusp immobility, subsequent attachment to the wall segment and consequent maintenance of a thick arterial wall-like structure. Scaffold resorption was incomplete, with disorganized inconsistent spatial and temporal degradation even at 6 months. Fibrosis, scarring and calcification started at 4 months and chronic inflammation persisted. The partially remodelled scaffold did not resemble the aortic wall, suggesting impaired remodelling. Mechanical testing showed progressive weakening of the tissues over time, which were liable to breakage.

CONCLUSIONS

CorMatrix is biodegradable; however, it failed to remodel in a structured and anatomical fashion in an arterial environment. Progressive mechanical and remodelling failure in this scenario might be explained by the complexity of the conduit design and the host's chronic inflammatory response, leading to early fibrosis and calcification.

Small intestinal submucosa extracellular matrix (CorMatrix®) in cardiovascular surgery: a systematic review

Extracellular matrix (ECM) derived from small intestinal submucosa (SIS) is widely used in clinical applications as a scaffold for tissue repair. Recently, CorMatrix® porcine SIS-ECM (CorMatrix Cardiovascular, Inc., Roswell, GA, USA) has gained popularity for 'next-generation' cardiovascular tissue engineering due to its ease of use, remodelling properties, lack of immunogenicity, absorbability and potential to promote native tissue growth. Here, we provide an overview of the biology of porcine SIS-ECM and systematically review the preclinical and clinical literature on its use in cardiovascular surgery. CorMatrix® has been used in a variety of cardiovascular surgical applications, and since it is the most widely used SIS-ECM, this material is the focus of this review. Since CorMatrix® is a relatively new product for cardiovascular surgery, some clinical and preclinical studies published lack systematic reporting of functional and pathological findings in sufficient numbers of subjects. There are also emerging reports to suggest that, contrary to expectations, an undesirable inflammatory response may occur in CorMatrix® implants in humans and longer-term outcomes at particular sites, such as the heart valves, may be suboptimal. Large-scale clinical studies are needed driven by robust protocols that aim to quantify the pathological process of tissue repair.

New technologies for surgery of the congenital cardiac defect

The surgical repair of complex congenital heart defects frequently requires additional tissue in various forms, such as patches, conduits, and valves. These devices often require replacement over a patient's lifetime because of degeneration, calcification, or lack of growth. The main new technologies in congenital cardiac surgery aim at, on the one hand, avoiding such reoperations and, on the other hand, improving long-term outcomes of devices used to repair or replace diseased structural malformations. These technologies are: 1) new patches: CorMatrix® patches made of decellularized porcine small intestinal submucosa extracellular matrix; 2) new devices: the Melody® valve (for percutaneous pulmonary valve implantation) and tissue-engineered valved conduits (either decellularized scaffolds or polymeric scaffolds); and 3) new emerging fields, such as antenatal corrective cardiac surgery or robotically assisted congenital cardiac surgical procedures. These new technologies for structural malformation surgery are still in their infancy but certainly present great promise for the future. But the translation of these emerging technologies to routine health care and public health policy will also largely depend on economic considerations, value judgments, and political factors.

Microstructural manipulation of electrospun scaffolds for specific bending stiffness for heart valve tissue engineering

Biodegradable thermoplastic elastomers are attractive for application in cardiovascular tissue construct development due to their amenability to a wide range of physical property tuning. For heart valve leaflets, while low flexural stiffness is a key design feature, control of this parameter has been largely neglected in the scaffold literature where electrospinning is being utilized. This study evaluated the effect of processing variables and secondary fiber populations on the microstructure, tensile and bending mechanics of electrospun biodegradable polyurethane scaffolds for heart valve tissue engineering. Scaffolds were fabricated from poly(ester urethane) urea (PEUU) and the deposition mandrel was translated at varying rates in order to modify fiber intersection density. Scaffolds were also fabricated in conjunction with secondary fiber populations designed either for mechanical reinforcement or to be selectively removed following fabrication. It was determined that increasing fiber intersection densities within PEUU scaffolds was associated with lower bending moduli. Further, constructs fabricated with stiff secondary fiber populations had higher bending moduli whereas constructs with secondary fiber populations which were selectively removed had noticeably lower bending moduli. Insights gained from this work will be directly applicable to the fabrication of soft tissue constructs, specifically in the development of cardiac valve tissue constructs.

Reconstruction of pulmonary artery with porcine small intestinal submucosa in a lamb surgical model: Viability and growth potential

OBJECTIVES

This study investigated the time-dependent remodeling and growth potential of porcine small intestine submucosa as a biomaterial for the reconstruction of pulmonary arteries in a lamb model.

METHODS

Left pulmonary arteries were partially replaced with small intestine submucosal biomaterial in 6 lambs. Two animals each were humanely killed at 1, 3, and 6 months. Computed tomographic angiography, macroscopic examination of the implanted patch, and microscopic analysis of tissue explants were performed.

RESULTS

All animals survived without complications. Patency and arborization of the pulmonary arteries were detected 6 months after implantation. There was no macroscopic narrowing or aneurysm formation in the patch area. The luminal appearance of the patch was similar to the intimal layer of the adjacent native pulmonary artery. Scanning electron microscopy showed that the luminal surface of the patch was covered by confluent cells. Immunohistochemical examination confirmed endothelialization of the luminal side of the patch in all of the explanted patches. The presence of smooth muscle cells in the medial layer was confirmed at all time points; however, expression of elastin, growth of the muscular layer, and complete degradation of patch material were detectable only after 6 months. The presence of c-Kit-positive cells suggests migration of multipotent cells into the patch, which may play a role in remodeling the small intestine submucosal biomaterial.

CONCLUSIONS

Our data confirmed that remodeling and growth potential of the small intestine submucosal biomaterial are time dependent. Additional experiments are required to investigate the stability of the patch material over a longer period.

Substrates for cardiovascular tissue engineering

Cardiovascular tissue engineering aims to find solutions for the suboptimal regeneration of heart valves, arteries and myocardium by creating 'living' tissue replacements outside (in vitro) or inside (in situ) the human body. A combination of cells, biomaterials and environmental cues of tissue development is employed to obtain tissues with targeted structure and functional properties that can survive and develop within the harsh hemodynamic environment of the cardiovascular system. This paper reviews the up-to-date status of cardiovascular tissue engineering with special emphasis on the development and use of biomaterial substrates. Key requirements and properties of these substrates, as well as methods and readout parameters to test their efficacy in the human body, are described in detail and discussed in the light of current trends toward designing biologically inspired microenviroments for in situ tissue engineering purposes.

Development and evaluation of a novel artificial catheter-deliverable prosthetic heart valve and method for in vitro testing

BACKGROUND

This work presents a novel artificial prosthetic heart valve designed to be catheter or percutaneously deliverable, and a method for in vitro testing of the device. The device is intended to create superior characteristics in comparison to tissue-based percutaneous valves.

METHODS

The percutaneous heart valve (PhV) was constructed from state-of-the-art polymers, metals and fabrics. It was tested hydrodynamically using a modified left heart simulator (Lhs) and statically using a tensile testing device.

RESULTS

The PhV exhibited a mean transvalvular pressure gradient of less than 15 mmhg and a mean regurgitant fraction of less than 5 percent. It also demonstrated a resistance to migration of up to 6 N and a resistance to crushing of up to 25 N at a diameter of 19 mm. The PhV was crimpable to less than 24 F and was delivered into the operating Lhs via a 24 F catheter.

CONCLUSION

An artificial PhV was designed and optimized, and an in vitro methodology was developed for testing the valve. The artificial PhV compared favorably to existing tissue-based PhVs. The in vitro test methods proved to be reliable and reproducible. The PhV design proved the feasibility of an artificial alternative to tissue based PhVs, which in their traditional open-heart implantable form are known to have limited in vivo durability.

Approaches to heart valve tissue engineering scaffold design

Heart valve disease is a significant cause of mortality worldwide. However, to date, a nonthrombogenic, noncalcific prosthetic, which maintains normal valve mechanical properties and hemodynamic flow, and exhibits sufficient fatigue properties has not been designed. Current prosthetic designs have not been optimized and are unsuitable treatment for congenital heart defects. Research is therefore moving towards the development of a tissue engineered heart valve equivalent. Two approaches may be used in the creation of a tissue engineered heart valve, the traditional approach, which involves seeding a scaffold in vitro, in the presence of specific signals prior to implantation, and the guided tissue regeneration approach, which relies on autologous reseeding in vivo. Regardless of the approach taken, the design of a scaffold capable of supporting the growth of cells and extracellular matrix generation and capable of withstanding the unrelenting cardiovascular environment while forming a tight seal during closure, is critical to the success of the tissue engineered construct. This paper focuses on the quest to design, such a scaffold.

Catheter-based modification of heart valve diseases: from experimental to clinical application

Efforts to modify cardiac valve defects using catheter-based techniques are increasing at the present time. We present observations on cardiac valve morphology and disease and review the progress being made to address valve defects with these innovative methods. Some new procedures developed through animal experimentation have already been put to use in clinical practice, but the newness of these techniques and the small number of cases in which they have been applied to date precludes an evaluation of their long-term durability. Although at the present time cardiac surgery remains the standard for treating most cases of valve disease, in certain situations a catheter-based treatment might provide a reasonable alternative, even if only temporary, especially for individuals with serious disease who are not suitable candidates for surgery.

Biological matrices and bionanotechnology

A crucial step towards the goal of tissue engineering a heart valve will be the choice of scaffold onto which an appropriate cell phenotype can be seeded. Successful scaffold materials should be amenable to modification, have a controlled degradation, be compatible with the cells, lack cytotoxicity and not elicit an immune or inflammatory response. In addition, the scaffold should induce appropriate responses from the cells seeded onto it, such as cell attachment, proliferation and remodelling capacity, all of which should promote the formation of a tissue construct that can mimic the structure and function of the native valve. This paper discusses the various biological scaffolds that have been considered and are being studied for use in tissue engineering a heart valve. Also, strategies to enhance the biological communication between the scaffold and the cells seeded onto it as well as the use of bionanotechnology in the manufacture of scaffolds possessing the desired properties will be discussed.

Antibacterial activity within degradation products of biological scaffolds composed of extracellular matrix

Biological scaffolds composed of extracellular matrix (ECM) have been shown to be resistant to deliberate bacterial contamination in preclinical in vivo studies. The present study evaluated the degradation products resulting from the acid digestion of ECM scaffolds for antibacterial effects against clinical strains of Staphylococcus aureus and Escherichia coli. The ECM scaffolds were derived from porcine urinary bladder (UBM-ECM) and liver (L-ECM). These biological scaffolds were digested with acid at high temperatures, fractionated using ammonium sulfate precipitation, and tested for antibacterial activity in a standardized in vitro assay. Degradation products from both UBM-ECM and L-ECM demonstrated antibacterial activity against both S. aureus and E. coli. Specific ammonium sulfate fractions that showed antimicrobial activity varied for the 2 different ECM scaffold types. The results of this study suggest that several different low-molecular-weight peptides with antibacterial activity exist within ECM and that these peptides may help explain the resistance to bacterial infection provided by such biological scaffolds.

Percutaneous heart valve replacement: histology and calcification characteristics of biological valved stents in juvenile sheep

INTRODUCTION

Percutaneous techniques to replace the pulmonary valve are emerging as an alternative to congenital cardiac surgical procedures. Promising experimental and early clinical results have been reported so far, focusing on technical feasibility and valved stent function. The present study aimed to describe the micropathology after experimental percutaneous valve replacement.

METHODS

Self-expanding nitinol stents carrying a valved bovine jugular vein were transfemorally implanted into the pulmonary position of nine sheep. After 3 months of survival, macro- and micropathological examinations were carried out using standard staining techniques and immunohistochemistry. Additionally, calcification characteristics were determined by X-ray examinations and von Kossa stainings.

RESULTS

Six of nine animals survived the 3-month study time with good angiographic and echocardiographic function. All valves were grossly functional at the time of explantation. Slight fibrous overgrowth was seen at the inflow portions of two valved stents. No cuspal perforations or intracuspal hematomas were observed. Light microscopy proved the absence of cellular inflammatory infiltrates in any tissue samples. The myocardium directly proximal to the stent appeared structurally normal without calcification. The overall structure of the native pulmonary artery was well preserved with few mineral deposits spread diffusely throughout the wall distal to the stent. Massive calcification appeared in the bovine jugular-vein wall together with increased numbers of T lymphocytes. Neither calcific deposits in the cusps nor extrinsic mineralization was noted.

CONCLUSION

For the first time, micropathologic evaluation of percutaneously implanted heart valves is described. The results demonstrate that calcification of valved stents occurs in the wall portions without affecting the cusps. The cardiac structures in the vicinity had normal histology without inflammation.

Percutaneous Technique for Creation of Tricuspid Regurgitation in an Ovine Model

Experimental models of tricuspid regurgitation (TR) are needed to study the percutaneous placement of prosthetic atrioventricular valves. The purpose of this study was to develop an appropriate simple and reproducible percutaneous experimental model for creation of tricuspid regurgitation. Tricuspid regurgitation was successfully created through papillary muscle avulsion using a guide-wire loop in seven sheep with regurgitation documented on right ventricular angiograms and a significant increase in heart rate and right atrial pressures. Acute onset of tricuspid regurgitation was poorly tolerated in one animal that died. Autopsy examinations showed avulsion of one papillary muscle in four animals and two papillary muscles in three animals.

Percutaneous Pulmonary Valve Replacement: 3-Month Evaluation of Self-Expanding Valved Stents

PURPOSE

In a recent study our group established an acute animal model of percutaneous pulmonary valve replacement using self-expanding nitinol stents. The present study was performed to evaluate these valved stents over a 3-month period.

DESCRIPTION

Bovine jugular xenografts were sutured into nitinol stents. Transfemoral implantation in the pulmonary position using a modified commercially available application device (with a 22-French outer diameter) was evaluated in 9 sheep.

EVALUATION

Two sheep died shortly after successful valved stent implantation due to internal venous hemorrhage. Another 1 sheep died 2.5 months after the procedure due to vegetations on the neovalve leading to subtotal stenosis. All other animals survived the 3-month study time (n = 6). An orthotopic pulmonary valved stent position was achieved in 4 animals and a supravalvular position in 1. During the deployment procedure, rhythm disturbances occurred in all animals, and mean arterial blood pressure dropped from 83.9 +/- 26.0 mm Hg to 68.3 +/- 22.3 mm Hg (p = 0.006) (n = 5). The peak-to-peak transvalvular gradient was 5.1 +/- 4.0 mm Hg initially (n = 5), and 3.6 +/- 1.6 mm Hg at follow-up (n = 5). Three-month angiographic and echocardiographic follow-up confirmed competent neovalves without paravalvular leakages.

CONCLUSIONS

After 3 months of implantation, percutaneously implanted memory nitinol valved stents demonstrated good function in the sheep.

Prevention of device-related tissue damage during percutaneous deployment of tissue-engineered heart valves

BACKGROUND

Endovascular application of pulmonary heart valves has been recently introduced clinically. A tissue-engineering approach was pursued to overcome the current limitations of bovine jugular vein valves (degeneration and limited longevity). However, deployment of the delicate tissue-engineered valves resulted in severe tissue damage. Therefore the objective of this study was to prevent tissue damage during the folding and deployment maneuver.

MATERIAL AND METHODS

Porcine pulmonary heart valves, small intestinal submucosa, and ovine carotid arteries were obtained from a slaughterhouse. After dissection and antimicrobial incubation, the valves were trimmed (removal of sinus and most of the muscular ring) to fit into the deployment catheter. The inside (in-stent group, n = 6) or outside (out-stent group, n = 6) of a nitinol stent was covered by an acellular small intestinal submucosa, and the valves were sutured into the stent. The valves were folded, tested for placement in the deployment catheter, and decellularized enzymatically. Myofibroblasts were obtained from carotid artery segments and seeded onto the scaffolds. The seeded constructs were placed in a dynamic bioreactor system and cultured for 16 consecutive days. After endothelial cell seeding, the constructs were folded, deployed, and processed for histology and surface electron microscopy.

RESULTS

The valves opened and closed competently throughout the entire dynamic culture. Surface electron microscopy revealed an almost completely preserved tissue in the in-stent group. Stents covered with small intestinal submucosa on the outside, however, showed severe damage.

CONCLUSION

This study demonstrates that small intestinal submucosa covering of the inside of a pulmonary valved stent can prevent stent strut-related tissue damage.

Percutaneous therapies for valvular heart disease

Balloon valvuloplasty has been in clinical use for over 20 years, but the prospect of repairing and replacing cardiac valves via catheter-based techniques represents a truly recent development. This review introduces evolving technologies and their relevance to cardiovascular pathologists.

Percutaneous Valve Replacement: Significance of Different Delivery Systems In Vitro and In Vivo

BACKGROUND AND PURPOSE

Percutaneous heart valve replacement is an exciting growing field in cardiovascular medicine yet still with some major problems. Only sophisticated improvement of the instruments could make it a real alternative to conventional surgery. Therefore, the aim of this study was to evaluate different delivery devices for percutaneous heart valve replacement in vitro and in vivo.

METHODS

A catheter prototype designed by our group, and two commercially available devices for the delivery of esophageal stents and aortic endoprostheses, were tested. After in vitro experiments, an ovine animal model of transfemoral pulmonary valve implantation was established using biological valved self-expanding stents. Only the delivery device for aortic endografts (Medtronic, Talent, Santa Rosa, CA, USA) allowed fast in vitro procedures without material fatigue. This device was chosen for the in vivo tests.

RESULTS

Technical success was achieved in 9 of 10 animals (90%). One animal died after perforation of the ventricular wall. Orthotopic pulmonary placement was performed in 6 animals and intentional supravalvular valved stent placement in 3 animals.

CONCLUSIONS

An adequate in vitro model for this evolving field of interventional heart valve replacement is presented. Furthermore, the present study pinpoints the key characteristics that are mandatory for a delivery system in percutaneous pulmonary valve implantation. With regard to the delivery device's ductility observed during this "venous" study, an approach to transfemoral aortic valve implantation seems feasible.