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.</p

Development of Tissue Engineered Scaffolds for Cardiovascular Repair and Replacement in Pediatric Patients

Congenital Heart Diseases (CHD) are abnormalities present in the heart and great vessels at birth. It is one of the most frequently diagnosed congenital disorders, affecting approximately 40,000 live birth each year in the United States. The incidence of new CHD patients and the relative distribution of defects have not changed over time and remain a birth rate function. Out of the new patients found to have CHD each year, an estimated 2,500 patients have a defect that requires a substitute, non-native valve, or conduit artery to replace structures that are congenitally absent or hypoplastic. Materials in current use for conduit and valve replacement involve varying degrees of stiffness and flexibility, durability, calcification, susceptibility to infection, thrombosis, and a lack of growth potential for the replacement. Biomaterials developed using tissue engineering principles could overcome the limitations encountered with current strategies. This research aims to develop potentially superior valves and conduits using acellular xenograft tissues that are physically cross- linked to protect the Extracellular Matrix (ECM) from rapid degradation. The hypothesis is that such a replacement graft would allow cellular ingrowth of host cells and potentially enable regenerative growth and remodeling of the graft. A decellularization protocol was developed, and the most effective crosslinker protecting the extracellular matrix structure was identified. The decellularized scaffolds crosslinked with Penta galloyl glucose (PGG) were analyzed in-vitro for stability and mechanical properties, in subcutaneous rat and in valve replacement in sheep-models to determine the biocompatibility and functionality of the developed scaffolds. Tissue-engineered scaffolds prepared from decellularized PGG treated tissues were found to have mechanical properties comparable to that of native tissues, while being more resistant to enzymatic degradation. Subcutaneous implantation of scaffolds demonstrated their biocompatibility and superior resistance to calcification compared to currently available glutaraldehyde fixed tissues. The tissue-engineered conduits and valves implanted in large animal models also demonstrated adequate implant functionality, cellular infiltration, implant remodeling, and growth of the implants. PGG treated decellularized xenografts could be an effective replacement option for pediatric patients, reducing the need for reoperations required with current devices

Comparison of heart valve flow dynamics assessment between echocardiography and pulse duplication

Published ThesisHeart valve surgery and valvular heart disease still pose a significant threat to patients worldwide. The aortic valve doesn't remain healthy and has largely been the focus of innovation and the development of replacement heart valves. Improving the ability of blood to flow througha prosthetic valve while minimizing the load on the heart is regarded as one of the performance objectives of prosthetic heart valves. In order to meet valvular performance objectives and to assess whether potential prosthetic heart valves meets hydrodynamic performance, testing simulated under in vivo flow conditions is necessary. Pulse duplication is widely accepted as a valid method to determine the performance of heart valves during their development. Few specialised centres exist to perform pulse duplication tests accurately and in accordance to the required ISO and FDA standards for cardiovascular implants. Real-time patient data of prosthetic heart valves is however not obtained with pulse duplication but with echocardiography. Modern day pulse duplicators come equipped with viewing chambers that can allow for echocardiographic measurements. Therefore, the aim of this study was to perform pulse duplication and echocardiography simultaneously on five different prosthetic heart valves using a commercial ViVitro pulse duplicator system. METHODS A hydrodynamic evaluation was performed on five prosthetic heart valves (i) Medtronic-Hall mechanical valve (tilting disc), (ii) Carbomedics mechanical valve (bileaflet), (iii) Glycar mechanical valve (Glycar), (iv) Edwards Perimount (tissue valve), (v) ViVitro reference (ViVitro) using pulse duplication and echocardiography. All the valves were inserted in the aortic position of the pulse duplicator and echocardiographic measurements was performed simultaneously. Each of the valves were tested at 5 different testing conditions by varying the stroke volume and beats per minute. The study concludes with a comparison between the pulse duplicator data and the echocardiography data acquired. RESULTS Pulse duplication: -The Glycar valve had the largest pressure drop across the valve at the lowest CO (3.6 L/min) of 17.15 mmHg, although it increased steadily at a slower rate than the other four valves. The Glycar and tissue valve had the highest EOA of 1.885 cm2 and 1.884 cm2 respectively at a peak CO of 9.6 L/min. The bi-leaflet valve had the highest EOA of 2.002 cm2 (CO 3.6 L/min), however the EOA deteriorated as the CO increased resulting in an EOA of 1.572 cm2 at a CO of 9.6L/min. The tissue valve had the largest RF for all testing conditions, ranging from 16.3% (CO 8.0 L/min) to 25.6% (4.9 L/min) where the bi-leaflet valve had the lowest (0.72% - 3.42%). Echocardiography: -The Glycar valve had the lowest overall pressure drop for all CO. The pulse duplicator pressure drop results were more consistent than three echocardiography results measured on the pulse duplicator. The bileaflet and Glycar valves EOA showed better consistency across the CO range than the ViVitro, tissue and tilting disk valves. The data showed that no definite correlation between all the valves exists between echocardiography and pulse duplication for EOA. However, a correlation for pressure drop between the pulse duplicator and echocardiographic data was demonstrated for both the tissue and bi-leaflet valve

Degradation of engineered polyurethane heart valves in a mechanically demanding environment with variable mixing of polyester and polycarbonate soft segments

Valvular heart disease (VHD) is a major source of morbidity and mortality leading to approximately 290,000 valve replacement surgeries worldwide each year. Current replacement prosthetics include mechanical and bioprosthetic heart valves, which are burdened by chronic anticoagulation therapy and tissue degeneration, respectively, as well as an inability to grow and remodel. Tissue engineered heart valves (TEHVs) have been proposed to overcome these limitations by providing a scaffold that is designed to be gradually replaced by autologous functional tissue. As such, TEHVs should degrade at a rate matching new tissue formation to achieve proper function and avoid structural failure. Biodegradable polyurethane elastomers are suitable candidates for TEHVs and offer tunable degradability based on soft segment chemistry. Polyester soft segments in poly(ester urethane)urea (PEUU) generate faster degradation than polycarbonate soft segments in poly(carbonate urethane)urea (PCUU). These biodegradable polyurethanes can be electrospun into fully assembled, fibrous TEHVs. The objectives of this study were to evaluate the in vitro degradation profile of three polyurethane soft segment mixing strategies and the effects of a mechanically demanding environment on the degradation rate. Equal ratios of faster-degrading polyester and slower-degrading polycarbonate segments were mixed into polyurethanes using three strategies: 1) soft segment mixing during synthesis to form poly(ester carbonate urethane)urea, 2) physical blending of PEUU and PCUU polymers during solvation to form a single solution, and 3) electrospinning from two independent streams of PEUU and PCUU solutions. These mixing strategies varied the chemical composition of the polymer chains and electrospun fibers between groups. Electrospun TEHVs from each mixing strategy were subjected to accelerated degradation in a pulse duplicator with enzymatic solution for two weeks. Relative degradation rates were quantified based on scaffold mass and thickness loss, macro- and microscopic structural changes, and viscosity reduction. Additionally, biaxial mechanical compliance was monitored throughout degradation and initial scaffold blood compatibility was assessed. Soft segment mixed TEHVs had the most degradation while co-spun TEHVs degraded very little. Additionally, mechanical strength was maintained for each mixing strategy throughout degradation. Findings of this study are instrumental in efficiently designing TEHVs where tunable degradation is critical to match the in vivo tissue formation rate

A novel polymeric prosthetic heart valve: design, manufacture, and testing

In this thesis a flexible leaflet polymeric prosthetic aortic heart valve was designed, manufactured, and tested. The prosthesis was designed with the aim of overcoming the need for anticoagulant therapy, which is required for current mechanical prostheses; while also having lifelong durability, which current bioprosthetic heart valves are not able to achieve. Inspired by the anisotropic architecture of collagen in the natural valve, a shortlist of polystyrene based block copolymers (BCPs), which can be processed to yield mechanically anisotropic materials, was proposed. The shortlist was evaluated based upon processability, biostability, ex vivo haemocompatibility, and a novel material performance index comprising the flexural modulus and the cyclic fatigue stress predicted by fracture mechanics methods. Polystyrene-block-polyethylene-polypropylene-block-polystyrene with 22 mol% polystyrene (SEPS22) was selected for further testing and use in the design. Haemocompatibility and calcification of the BCPs was assessed against reference materials. In measures of coagulation and thrombogenicity the BCPs were better than polyester, but worse than expanded polytetrafluoroethylene and pericardium graft materials. In measures of inflammation, the BCPs and polytetrafluoroethylene were better than polyester and pericardium. A durable heparin coating gave SEPS22 superior haemocompatibility compared to all the reference materials. The BCPs calcified less than pericardium, but calcification still accelerated failure. The technique of injection moulding discs of the BCP from a point was used to create a novel biaxial structure of cylindrical polystyrene domains. A combination of modelling and bench-scale injection moulding was used to select a point from which the prosthetic heart valve injection tool cavity should be filled. By simultaneously injecting at a point at the centre of the free edge of each leaflet, a bioinspired orientation was produced. Based upon hydrodynamic testing, a spherical form leaflet design was selected. The hydrodynamic performance of the complied with the ISO 5840 standard for cardiac valve prostheses, but the fatigue performance was inadequate due to the leaflets being thinner than specified due to manufacturer error. Fatigue prediction and finite element analysis were used to conjecture that correctly manufactured polymeric valves could theoretically reach the ISO limit, indicating that there is potential for polymeric prostheses to overcome the issues of durability and need for anticoagulation

Development of a prosthetic heart valve with inbuilt sensing technology, to aid in continuous monitoring of function under various stenotic conditions

In spite of technological advances in the design of prosthetic heart valves, they are still often subject to complications after implantation. One of the common complications is valve stenosis, which involves the obstruction of the valve orifice caused by biological processes. The greatest challenge in diagnosing the development of valve failure and complications is related to the fact that the valve is implanted and isolated. To continuously monitor the state of the valve and its performance would be of great benefit but practically can only be achieved by instrumenting the implanted valve. In this thesis, we explore the development of a prosthetic valve with inbuilt sensing technology to aid in continuous monitoring of valve function under various stenotic conditions. 22mm polyurethane valves were designed via dipcoating. A custom made mock circulatory system was designed and hydrodynamic testing of the polyurethane valves under different flow rates were performed with Effective orifice area (EOA) and Transvalvular Pressure Gradient (TVPG) being the parameters of interest. Valves were subjected to varying levels of obstruction to investigate the effect obstruction has on the pressure gradient across the valves. Similar tests were performed on a Carpentier Edwards SAV 2650 model bioprosthetic valve for comparison. Polyurethane valves were then instrumented with strain gauges to measure peak to peak strain difference, in response to varying levels of obstructions. All the polyurethane valves exhibited good hydrodynamic performance with EOA (>1cm2) under baseline physiological conditions. It was also discovered that pressure difference across the valves was directly proportional to the flow rate. The pressure difference also demonstrated a slow increase during the initial stages of simulated stenosis and a sudden increase as the obstruction became severe. This provides further evidence to support the ideal that stenosis is a slow progressive disease which may not present symptoms until severe. The peak to peak strain differences also tend to decrease as the severity of the obstruction was increased. The peak to peak strain difference is indicative of the pressures within the valve (intravalvular pressure). The results suggest that directly monitoring the pressures within the valve could be a useful diagnostic tool for detecting valve stenosis. Future works involves miniaturisation of the sensors and also the incorporation of telemetry into the sensor design.In spite of technological advances in the design of prosthetic heart valves, they are still often subject to complications after implantation. One of the common complications is valve stenosis, which involves the obstruction of the valve orifice caused by biological processes. The greatest challenge in diagnosing the development of valve failure and complications is related to the fact that the valve is implanted and isolated. To continuously monitor the state of the valve and its performance would be of great benefit but practically can only be achieved by instrumenting the implanted valve. In this thesis, we explore the development of a prosthetic valve with inbuilt sensing technology to aid in continuous monitoring of valve function under various stenotic conditions. 22mm polyurethane valves were designed via dipcoating. A custom made mock circulatory system was designed and hydrodynamic testing of the polyurethane valves under different flow rates were performed with Effective orifice area (EOA) and Transvalvular Pressure Gradient (TVPG) being the parameters of interest. Valves were subjected to varying levels of obstruction to investigate the effect obstruction has on the pressure gradient across the valves. Similar tests were performed on a Carpentier Edwards SAV 2650 model bioprosthetic valve for comparison. Polyurethane valves were then instrumented with strain gauges to measure peak to peak strain difference, in response to varying levels of obstructions. All the polyurethane valves exhibited good hydrodynamic performance with EOA (>1cm2) under baseline physiological conditions. It was also discovered that pressure difference across the valves was directly proportional to the flow rate. The pressure difference also demonstrated a slow increase during the initial stages of simulated stenosis and a sudden increase as the obstruction became severe. This provides further evidence to support the ideal that stenosis is a slow progressive disease which may not present symptoms until severe. The peak to peak strain differences also tend to decrease as the severity of the obstruction was increased. The peak to peak strain difference is indicative of the pressures within the valve (intravalvular pressure). The results suggest that directly monitoring the pressures within the valve could be a useful diagnostic tool for detecting valve stenosis. Future works involves miniaturisation of the sensors and also the incorporation of telemetry into the sensor design

Toward Growth-Accommodating Polymeric Heart Valves with Graphene-Network Reinforcement

Graphene is a 2D material well known for its high intrinsic strength of 100 GPa and Young’s modulus of 1 TPa. Because of its 2D nature, the most promising avenues to utilize graphene as a mechanical material include incorporating it as reinforcement in a nanocomposite and creating free-standing foams and aerogels. However, the current techniques are not well-controlled – the reinforcing graphene particles are often discontinuous and randomly dispersed – making it difficult to accurately model and predict the resulting material properties. Here we aim to develop a framework for a new class of nanocomposites reinforced not by discrete nanoparticles, but by a continuous 3D graphene network. These 3D graphene networks were formed by chemical vapor deposition of graphene on periodic metallic microlattices, thereby providing mechanical reinforcement for the lattices. To assist in the lattice design, analytical models were derived for the mechanical properties of core/shell composite lattices and experimentally validated through compression testing of polymer lattices coated with electroless Ni-P. The models and experiments showed good agreement at lower shell thicknesses, while there was divergence at higher thicknesses, likely due to fabrication imperfections. The analytical models were also applied to hollow metallic lattices coated with graphene and compared to experimental data. The results showed that the models are plausible and suggest that graphene has a significant strengthening effect on the microlattices. These studies represent a paradigm shift in the design and fabrication of nanocomposites as one may now precisely prescribe the placement of the reinforcing nanomaterials. On a broader scale, this work also lays the framework for using a 2D material to span 3D space, enabling further exploration of 2D material properties and applications. One potential application area for a graphene-reinforced polymer composite is in prosthetic heart valves. The tissue of a human heart valve leaflet is heavily reinforced with networks of collagen and elastin fibers. One could similarly incorporate a graphene network as reinforcement within the polymeric leaflets of a prosthetic valve. One promising application of polymeric valves is in growth-accommodating implants for pediatric patients. Here we aim to develop a polymeric valved conduit that can be expanded by transcatheter balloon dilation to match a child’s growth. We designed the valve, characterized and selected materials, fabricated the devices and performed benchtop in vitro testing. The first generation of an expandable biostable valved conduit displayed excellent hydrodynamic performance before and after permanent balloon dilation from 22 to 25 mm. The second generation has shown the potential for a greater dilation from 12 to 24 mm. These results demonstrate concept feasibility and motivate further development of a polymeric balloon-expandable device to replace valves in children and avoid reoperations

Haemodynamic alterations after percutaneous valve implantation

Many patients who suffer from aortic valve dysfunction are too weak to be eligible for valve replacement via surgery, due to co-morbidities and old age. Transcatheter aortic valve (TAV) implantation has been developed as an alternative to surgery, enabling replacement of the dysfunctional valve percutaneously. However, the inability to remove the native leaflets leads to the bioprosthetic being held in place inside a pseudo-cylindrical structure. The passive nature of heart valves means the surrounding fluid environment’s dynamics are critical in producing optimum performance, and would ideally be returned to the healthy, physiological state. The association of TAVs with thrombotic events, such as strokes, has not yet been fully explained. A pulse duplicator and particle image velocimetry were used to model and characterise the flow fields of a healthy, physiological aortic root and valve, which was then compared to those resulting from a number of typical post- surgical outcomes, identifying the fluid mechanisms promoted by the root geometry to optimise the ejection and closing phases of the cardiac cycle, and revealing the importance of an optimal integration of valve and root architecture, and characterising common post-surgical environments. The same techniques were then used to examine the flow dynamics of the region following TAV implantation, revealing the effect of TAV alignment with its hosts’ commissures, and how the presence of native leaflets, commonly omitted from in vitro TAV testing, affect the valve performance. Slow and stagnant flow was observed within the sinuses due to the native leaflets, whilst global valve performance was broadly unaffected, and omission of the native leaflets resulted in improved haemodynamic performance. A model of coronary arteries was incorporated into the benchtop simulation, revealing increase of flow in the upper coronary sinuses, but flow at the base of all sinuses remained very slow following TAV implantation. The elucidation of this stagnation, associated with thrombotic events, provides an explanation for the increased levels of thrombotic-associated pathologies following TAV implantation