A synthetic leaflet heart valve with improved opening characteristics

A parametric study on pulse duplicator design and valve hemodynamics

BACKGROUND

In vitro assessment is mandatory for artificial heart valve development. This study aims to investigate the effects of pulse duplicator features on valve responsiveness, conduct a sensitivity analysis across valve prosthesis types, and contribute on the development of versatile pulse duplicator systems able to perform reliable prosthetic aortic valve assessment under physiologic hemodynamic conditions.

METHODS

A reference pulse duplicator was established based on literature. Further optimization process led to new designs that underwent a parametric study, also involving different aortic valve prostheses. These designs were evaluated on criteria such as mean pressure differential and pulse pressure (assessed from high-fidelity pressure measurements), valve opening and closing behavior, flow, and regurgitation. Finally, the resulting optimized setup was tested under five different hemodynamic settings simulating a range of physiologic and pathologic conditions.

RESULTS

The results show that both, pulse duplicator design and valve type significantly influence aortic and ventricular pressure, flow, and valve kinematic response. The optimal design comprised key features such as a compliance chamber and restrictor for diastolic pressure maintenance and narrow pulse pressure. Additionally, an atrial reservoir was included to prevent atrial-aortic interference, and a bioprosthetic valve was used in mitral position to avoid delayed valve closing effects.

CONCLUSION

This study showed that individual pulse duplicator features can have a significant effect on valve's responsiveness. The optimized versatile pulse duplicator replicated physiologic and pathologic aortic valve hemodynamic conditions, serving as a reliable characterization tool for assessing and optimizing aortic valve performance.

Polymeric Heart Valves Will Displace Mechanical and Tissue Heart Valves: A New Era for the Medical Devices

The development of a novel artificial heart valve with outstanding durability and safety has remained a challenge since the first mechanical heart valve entered the market 65 years ago. Recent progress in high-molecular compounds opened new horizons in overcoming major drawbacks of mechanical and tissue heart valves (dysfunction and failure, tissue degradation, calcification, high immunogenic potential, and high risk of thrombosis), providing new insights into the development of an ideal artificial heart valve. Polymeric heart valves can best mimic the tissue-level mechanical behavior of the native valves. This review summarizes the evolution of polymeric heart valves and the state-of-the-art approaches to their development, fabrication, and manufacturing. The review discusses the biocompatibility and durability testing of previously investigated polymeric materials and presents the most recent developments, including the first human clinical trials of LifePolymer. New promising functional polymers, nanocomposite biomaterials, and valve designs are discussed in terms of their potential application in the development of an ideal polymeric heart valve. The superiority and inferiority of nanocomposite and hybrid materials to non-modified polymers are reported. The review proposes several concepts potentially suitable to address the above-mentioned challenges arising in the R&D of polymeric heart valves from the properties, structure, and surface of polymeric materials. Additive manufacturing, nanotechnology, anisotropy control, machine learning, and advanced modeling tools have given the green light to set new directions for polymeric heart valves.

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.

Proposed percutaneous aortic valve prosthesis made of cryogel

Transcatheter heart valves are promising for high-risk patients. Generally, their leaflets are made of pericardium stented in a Nitinol basket. Despite their relative success, they are associated with significant complications such as valve migration, implantation risks, stroke, coronary obstruction, myocardial infraction, acute kidney injury (which all are due to the release of detached solid calcific pieces in to the blood stream) and expected issues existing with tissue valves such as leaflet calcification. This study is an attempt to fabricate the first ever polymeric percutaneous valves made of cryogel following the geometry and mechanical properties of porcine aortic valve to address some of the above-mentioned shortcomings. A novel, one-piece, tricuspid percutaneous valve, consisting of leaflets made entirely from the hydrogel, polyvinyl alcohol cryogel reinforced by bacterial cellulose natural nanocomposite, attached to a Nitinol basket was developed and demonstrated. Following the natural geometry of the valve, a novel approach was applied based on the revolution about an axis of a hyperboloid shape. The geometry was modified based on avoiding sharp warpage of leaflets and removal of the central opening orifice area of the valve when valve is fully closed using the finite element analysis. The modified geometry was replaced by a cloud of (control) points and was essentially converted to Bezier surfaces for further adjustment. A cavity mold was then designed and fabricated to form the valve. The fabricated valve was sewn into the Nitinol basket which is covered by Dacron cloth. The models presented in this study merit further development and revisions for both aortic and mitral positions.

Characterization of mechanical properties of pericardium tissue using planar biaxial tension and flexural deformation

Flexure is an important mode of deformation for native and bioprosthetic heart valves. However, mechanical characterization of bioprosthetic leaflet materials has been done primarily through planar tensile testing. In this study, an integrated experimental and computational cantilever beam bending test was performed to characterize the flexural properties of glutaraldehyde-treated bovine and porcine pericardium of different thicknesses. A strain-invariant based structural constitutive model was used to model the pericardial mechanical behavior quantified through the bending tests of this study and the planar biaxial tests previously performed. The model parameters were optimized through an inverse finite element (FE) procedure in order to describe both sets of experimental data. The optimized material properties were implemented in FE simulations of transcatheter aortic valve (TAV) deformation. It was observed that porcine pericardium TAV leaflets experienced significantly more flexure than bovine when subjected to opening pressurization, and that the flexure may be overestimated using a constitutive model derived from purely planar tensile experimental data. Thus, modeling of a combination of flexural and biaxial tensile testing data may be necessary to more accurately describe the mechanical properties of pericardium, and to computationally investigate bioprosthetic leaflet function and design.

Polyetheretherketone (PEEK) for medical applications

Polyetheretherketone (PEEK) is a polyaromatic semi-crystalline thermoplastic polymer with mechanical properties favorable for bio-medical applications. Polyetheretherketone forms: PEEK-LT1, PEEK-LT2, and PEEK-LT3 have already been applied in different surgical fields: spine surgery, orthopedic surgery, maxillo-facial surgery etc. Synthesis of PEEK composites broadens the physicochemical and mechanical properties of PEEK materials. To improve their osteoinductive and antimicrobial capabilities, different types of functionalization of PEEK surfaces and changes in PEEK structure were proposed. PEEK based materials are becoming an important group of biomaterials used for bone and cartilage replacement as well as in a large number of diverse medical fields. The current paper describes the structural changes and the surface functionalization of PEEK materials and their most common biomedical applications. The possibility to use these materials in 3D printing process could increase the scientific interest and their future development as well.

A Novel Design of a Polymeric Aortic Valve

Introduction

In this paper we propose a novel method for developing a polymeric heart valve that could potentially offer an optimum solution for a heart valve substitute. The valve design proposed will provide superior hydrodynamic performance and excellent structural integrity. A full description of the design process is given together with an analysis of the hemodynamic performance using a 2-way strongly coupled Fluid Structure Interaction (FSI).

Method

A polymeric tri-leaflet heart valve is designed based on a patient's sinus of Valsalva (SOV) geometry. The design strategy aims to improve valve hemodynamic performance as well as valve durability by avoiding stress concentrations in the leaflets and reducing the maximum stress level. The valve dynamics and stress levels are also validated by comparing the predicted data to existing experimental and numerical data.

Results

The stress distribution in the valve structure is fully characterized throughout the simulation and Von Mises stress is found to be up to 5.32 Mpa during diastole. The results show that an effective orifice area (EOA) and a pressure drop of 3.22 cm^2, and 3.52 mmHg, respectively, can be achieved using the proposed design.

Conclusions

The optimized valve demonstrates high hemodynamic performance with no sign of damaging stress concentration in the entire cardiac cycle.

Computational modeling of cardiac valve function and intervention

In the past two decades, major advances have been made in the clinical evaluation and treatment of valvular heart disease owing to the advent of noninvasive cardiac imaging modalities. In clinical practice, valvular disease evaluation is typically performed on two-dimensional (2D) images, even though most imaging modalities offer three-dimensional (3D) volumetric, time-resolved data. Such 3D data offer researchers the possibility to reconstruct the 3D geometry of heart valves at a patient-specific level. When these data are integrated with computational models, native heart valve biomechanical function can be investigated, and preoperative planning tools can be developed. In this review, we outline the advances in valve geometry reconstruction, tissue property modeling, and loading and boundary definitions for the purpose of realistic computational structural analysis of cardiac valve function and intervention.

A parametric study on mathematical formulation and geometrical construction of a stentless aortic heart valve

This study presents a novel methodology for constructing an accurate geometrical model of a stentless aortic heart valve replacement (AVR). The main objective is to propose an optimized AVR model that can be used as an ideal scaffold for tissue engineering applications or a biocompatible prosthesis. Current techniques available for creating heart valve geometry, including leaflets, are very complicated and are not precise, due to the extensive use of various complicated parameters. This paper introduces an alternative design procedure that uses limited and effective numbers of controlling parameters to construct the whole valve including the sinus of valsalva. In doing so the hyperbolic curves for multithickness leaflets are used and a 3D elliptical formulation is incorporated for the surface geometry of the sinus of valsalva. Still, the feasibility and the precision of the mathematical method are established by performing standard deviation analysis on the constructed surfaces. The surface fitting residuals are found as low as error 0.2351 mm with standard deviation of 8.83e-5 over the commissural lines. Preliminary validation to the proposed AVR model performance is achieved by testing the generated AVR model under quasi static condition while obtaining the mesh independent setup. The numerical model showed a rapid response of the leaflets to the transvalvular pressure where adequate values of stress are measured over the commissural lines.

Recent Advances in Polymeric Heart Valves Research

Heart valve replacement is fast becoming a routine surgery worldwide, and heart valve prostheses are today considered among the most widely used cardiovascular devices. Mechanical and bioprostheses have been the traditional choices to the replacement surgeries. However, such valves continue to expose patients to risks including thrombosis, infection and limited valve durability. In recent years, advances in polymer science give rise to an important new class of artificial heart valve made predominantly of polyurethane-based materials, which show improved biocompatibility and biostability. These polymeric heart valves have demonstrated excellent hemodynamic performance and good durability with excellent fatigue stress resistance. Advancements in the designs and manufacturing methods also suggested improved in the durability of polymeric heart valves. Animal studies with these valves have also shown good biocompatibility with minimal calcification of the valve leaflets. With these promising progresses, polymeric heart valves could be a viable alternative in the heart valve replacement surgeries in the near future.

Prosthetic aortic heart valves: modeling and design

Although heart valve replacement is among the most common cardiovascular surgical procedures, their outcome is often difficult to predict. One of the reasons is the design and choice of the materials used for the fabrication of the prostheses. This review paper describes the use of modeling techniques in prosthetic heart valve (HV) design and aims at the justification and development of a polymer based trileaflet mechanical heart valve (MHV). The closing/opening phase behavior of the bileaflet MHV was investigated. The potential problem of valve failure due to crack propagation in the brittle pyrolytic carbon leaflet was also discussed. These studies suggest that although bileaflet MHV performs satisfactorily, there are justifications for improvement. Since the native aortic HV is trileaflet and made of anisotropic and hyperelastic tissue, one possible approach to a better MHV design is based on our ability to closely mimic the natural geometry and biomaterial properties.

Design and simulation of a poly(vinyl alcohol)—bacterial cellulose nanocomposite mechanical aortic heart valve prosthesis

In this study, a polymeric aortic heart valve made of poly(vinyl alcohol) (PVA)—bacterial cellulose (BC) nanocomposite is simulated and designed using a hyperelastic non-linear anisotropic material model. A novel nanocomposite biomaterial combination of 15 wt % PVA and 0.5 wt % BC is developed in this study. The mechanical properties of the synthesized PVA—BC are similar to those of the porcine heart valve in both the principal directions. To design the geometry of the leaflets an advance surfacing technique is employed. A Galerkin-based non-linear finite element method is applied to analyse the mechanical behaviour of the leaflet in the closing and opening phases under physiological conditions. The model used in this study can be implemented in mechanical models for any soft tissues such as articular cartilage, tendon, and ligament.

Leaflet geometry extraction and parametric representation of a pericardial artificial heart valve

Reverse engineering technology was used to reconstruct the complex leaflet geometry of a commercial pericardial valve in our study. Results show that the three-dimensional computer-aided design model of the leaflet surface can be rendered by fitting the surface either to cloud points or by a group of B-splines fitted to a set of cloud points that had been obtained by the process of laser-scanning digitizing. However, an acceptable smooth surface is usually not guaranteed and additional manipulation is required. An alternative method is introduced in this paper, which involves the fitting of an equation to the leaflet geometry to create a smooth surface. The geometrical profile of a pericardial artificial heart valve was scanned using a laser digitizing system. The leaflet profile is represented as a set of cloud points. A quadric surface is fitted to a set of unique points, which were located on the set of cloud points. A mathematical equation is obtained by solving a least-squares fit. The geometry of the fitted leaflet surface has been proven to be closely represented by an elliptical hyperboloid. The quadratic equations of the leaflet curvatures, calculated along both the circumferential and the radial directions, resulted in simple hyperbolic curvatures. The advantages of using elliptical hyperboloid geometry for the leaflet surface are discussed and compared with other types of conicoid geometries. The concepts of parametric representation of the leaflet geometry and parametric design for leaflets are discussed. A smooth surface without inflection points and with an adjustable surface area suitable for a series of stent sizes with incremented diameters is created by this method of a single parametric design. Finally, a generic method to apply the geometry extraction and parametric representation to most pericardial heart valve prostheses was discussed. The application to valves with natural shape was introduced, challenges were identified, and a technical solution was proposed.

Measurement and reconstruction of the leaflet geometry for a pericardial artificial heart valve

This paper describes the measurement and reconstruction of the leaflet geometry for a pericardial heart valve. Tasks involved include mapping the leaflet geometries by laser digitizing and reconstructing the 3D freeform leaflet surface based on a laser scanned profile. The challenge is to design a prosthetic valve that maximizes the benefits offered to the recipient as compared to the normally operating naturally-occurring valve. This research was prompted by the fact that artificial heart valve bioprostheses do not provide long life durability comparable to the natural heart valve, together with the anticipated benefits associated with defining the valve geometries, especially the leaflet geometries for the bioprosthetic and human valves, in order to create a replicate valve fabricated from synthetic materials. Our method applies the concept of reverse engineering in order to reconstruct the freeform surface geometry. A Brown & Shape coordinate measuring machine (CMM) equipped with a HyMARC laser-digitizing system was used to measure the leaflet profiles of a Baxter Carpentier-Edwards pericardial heart valve. The computer software, Polyworks was used to pre-process the raw data obtained from the scanning, which included merging images, eliminating duplicate points, and adding interpolated points. Three methods, creating a mesh model from cloud points, creating a freeform surface from cloud points, and generating a freeform surface by B-splines are presented in this paper to reconstruct the freeform leaflet surface. The mesh model created using Polyworks can be used for rapid prototyping and visualization. To fit a freeform surface to cloud points is straightforward but the rendering of a smooth surface is usually unpredictable. A surface fitted by a group of B-splines fitted to cloud points was found to be much smoother. This method offers the possibility of manually adjusting the surface curvature, locally. However, the process is complex and requires additional manipulation. Finally, this paper presents a reverse engineered design for the pericardial heart valve which contains three identical leaflets with reconstructed geometry.

Design and manufacture of a polyvinyl alcohol (PVA) cryogel tri-leaflet heart valve prosthesis

Although current artificial heart valves are life sustaining medical devices, improvements are still necessary to address deficiencies. Bioprosthetic valves have a compromised fatigue life, while mechanical valves have better durability but are prone to thromboembolic complications. A novel, one-piece, tricuspid valve, consisting of leaflets, stent and sewing ring, made entirely from the hydrogel, polyvinyl alcohol cryogel (PVA-C), has been developed and demonstrated. This valve has three thin leaflets attached to a cylindrical stent. In order to approximate the complex shape of the surface of the natural heart valve leaflets, two different geometries have been proposed: revolution about an axis of a hyperboloid shape and revolution about an axis of an arc subtending (joining) two straight lines. The parameters of both geometries were examined based on a compromise between avoiding sharp curvature of leaflets and minimization of the central opening of the valve when closed. The revolution of an arc subtending two straight lines was selected as the preferred geometry since it has the benefit of a smaller central opening when the value of the maximum curvature for the leaflets is the same for each valve geometry. A cavity mold has been designed and constructed to form the PVA-C heart valve. The three leaflets were formed and integrated into the stent and sewing ring in a single process. Prototype heart valves were manufactured in the mold from a solution of PVA and water, by controlled freezing and thawing cycles.

Compression-induced changes on physical structures and calcification of the aromatic polyether polyurethane composite

It is generally accepted that stress causes calcification in both bio-prosthetic and polyurethane heart valves. However, simple uni-axially- and bi-axially-stretched samples did not yield a feasible model for the elaboration of the stress-induced calcification. In this study, heat compaction combined with the incorporation of polyethylene has been explored. Specimens of polyurethane were solution cast onto a porous bi-axially-drawn ultra-high-molecular-weight polyethylene film and then heat compacted under a pressure of 18 MPa at a chosen temperature for 1.5 h. The heat-compaction-induced calcification and physical changes of the polyurethane composite were evaluated using a 28-day in vitro calcification model and Attenuated Total Reflection-Fourier Transform-Infrared (ATR-FT-IR) spectroscopy. The calcification results indicated that heat-compaction-induced calcification was double that achieved without heat compaction. Heat-compacted polyurethane composite showed higher affinity to calcium ions than the non-heat compacted sample. The ATR-FT-IR results showed that the heat-compaction-induced physical changes include distortions of polymeric molecules and permanent changes of microstructures. The distortions of polymeric molecules could be deteriorated in contact with different media. The relaxation of the stressed structures of the polyether moiety might serve as a calcium trap and a heterogeneous nucleation site for calcification. The permanent changes of microstructures resulted from high distortions also served as affinity sites attracting calcification.

A synthetic fiber-reinforced stentless heart valve

There is strong evidence that failure of bioprosthetic and synthetic valves occurs as a consequence of high tensile and bending stresses, acting on the leaflets during opening and closing. In stented prostheses, whether synthetic or biological, the absence of contraction of the aortic base causes the leaflets to be subjected to an unphysiological degree of flexure, which is also related to calcification. However, a stentless synthetic valve, which has a flexible aorta base, can be a good alternative for stented synthetic valves. Moreover, fiber-reinforcement is assumed to lead to a decrease of tears and perforation as a result of reduced stresses in the weaker parts of the leaflets in their closed configuration. The manufacturing method for a stentless, fiber-reinforced, synthetic valve is presented. Prototypes are tested in a pulse duplicator system. The results show that the mean systolic pressure difference is very low, while the high regurgitation (up to 26%) is probably caused by a too small coaptation area of the leaflets.

A three-dimensional mechanical analysis of a stentless fibre-reinforced aortic valve prosthesis

Failure of bioprosthetic and synthetic three-leaflet valves has been shown to occur as a consequence of high tensile and bending stresses, acting on the leaflets during opening and closing. Moreover, in the stented prostheses, whether synthetic or biological, the absence of contraction of the aortic base, due to the rigid stent, causes the leaflets to be subjected to an unphysiological degree of flexure, which is related to calcification. It is shown that the absence of the stent, which gives a flexible aortic base and leaflet attachment, and leaflet fibre-reinforcement result in reduced stresses in the weaker parts of the leaflets in their closed configuration. It is postulated that this leads to a decrease of tears and perforations, which may result in a improved long-term behaviour. The effect of a flexible leaflet attachment and aortic base of a synthetic valve is investigated with a finite element model. Different fibre-reinforced structures are analysed with respect to the stresses that are likely to contribute to the failure of fibre-reinforced prostheses and compared with the results obtained for a stented prosthesis. Results show that for the stentless models a reduction of stresses up to 75% is obtained with respect to stented models with the same type of reinforcement.

New polyurethane heart valve prosthesis: design, manufacture and evaluation

In light of the thrombogenicity of mechanical valves and the limited durability of bioprosthetic valves, alternative designs and materials are being considered for prosthetic heart valves. A new tri-leaflet valve, made entirely from polyurethane, has been developed. The valve comprises three thin polyurethane leaflets (approximately 100 microns thick) suspended from the inside of a flexible polyurethane frame. The closed leaflet geometry is elliptical in the radial direction and hyperbolic in the circumferential direction. Valve leaflets are formed and integrated with their support frame in a single dip coating operation. The dipping process consistently gives rise to tolerably uniform leaflet thickness distributions. In hydrodynamic tests, the polyurethane valve exhibits pressure gradients similar to those for a bioprosthetic valve (St Jude Bioimplant), and levels of regurgitation and leakage are considerably less than those for either a bi-leaflet mechanical valve (St Jude Medical) or the bioprosthetic valve. Six out of six consecutively manufactured polyurethane valves have exceeded the equivalent of 10 years function without failure in accelerated fatigue tests. The only failure to date occurred after the equivalent of approximately 12 years cycling, and three valves have reached 527 million cycles (approximately 13 years equivalent). The simplicity of valve manufacture, combined with promising results from in vitro testing, indicate that further evaluation is warranted.

In vitro function and durability assessment of a novel polyurethane heart valve prosthesis

While flexible-leaflet, central-flow prosthetic heart valves promise relief from anticoagulation therapy, they continue to be restricted by inadequate durability. In consequence, a novel trileaflet valve, made entirely from polyurethane, has been developed. A batch of 6 consecutively manufactured polyurethane valves was subjected to hydrodynamic function and accelerated fatigue testing. Computerized data acquisition and control systems have been introduced to improve valve testing methodologies. In terms of hydrodynamic function, the polyurethane valve demonstrates transvalvular pressure gradients similar to those for a bioprosthetic valve (Carpentier-Edwards) and levels of retrograde flow significantly less than those for either the bioprosthetic valve or a bileaflet mechanical valve (St Jude Medical). The equivalent of 10 years of cycling without failure has been exceeded by all 6 polyurethane valves in accelerated fatigue tests with 2 valves remaining intact after 674 million cycles (equivalent to approximately 17 years) in continuing tests. Highspeed photography revealed considerable differences in leaflet motion between valves cycled at accelerated and physiological rates.

The influence of open leaflet geometry on the haemodynamic flow characteristics of polyurethane trileaflet artificial heart valves

In vitro velocity data were obtained downstream of two versions of the Leeds polyurethane trileaflet heart valve in a simulated pulsatile flow regime using laser Doppler velocimetry. The main difference between the two valves studied was the manufacturing method used to create the valves. The film-fabricated valve was constructed from solvent-cast sheets of polyurethane, thermally formed into the correct leaflet geometry. The dip-cast valve used a stainless steel mould which was dipped into a polyurethane solution to produce the valve leaflets. Significant differences were visible between the fully open leaflet shape of each valve. The distribution of mean axial velocity and Reynolds normal stress (RNS) was shown to be dependent on the shape of the fully open valve orifice. For the film-fabricated valves, flow recirculation and high values of RNS were present downstream of the frame posts. The maximum value of RNS obtained downstream of the film-fabricated valve at peak systole was 147 N/m2. Results for the dip-cast valve showed a more uniform distribution of mean axial velocity and RNS resulting from the more circular central orifice produced by the dip-cast leaflets. The maximum value of RNS obtained downstream of the dip-cast valve at peak systole was 109 N/m2. These results demonstrate the effect of the open valve geometry on the flow characteristics downstream of trileaflet valves and that minor changes to the open leaflet geometry can significantly affect the flow characteristics and the possibility of flow-related blood damage occurring in vivo.

Finite element stress analysis of a new design of synthetic leaflet heart valve

This paper presents a parametric finite element analysis of the stresses in the leaflets of a new design of polyurethane heart valve in the closed position. The alpharabola geometry of the valve has previously been reported by Leat and Fisher (1) and has been shown to demonstrate good opening characteristics. The effects of variations in leaflet offset parameter, g, length, h, and local thickening have been determined for a valve where the frame is assumed rigid. A spherical leaflet geometry has also been analysed for comparative purposes. Results have shown that the alpharabola leaflet geometry can reduce the maximum principal tensile stress to 60 per cent of that for a spherical valve of the same mesh density.

In vitro determination of the curvatures and bending strains acting on the leaflets of polyurethane trileaflet heart valves during leaflet motion

Leaflet tears originating from the free leaflet edge and calcification around the commissural region are common modes of failure exhibited by explanted bioprosthetic trileaflet heart valves. These may be a result of the cyclic bending and high levels of curvature that affect the leaflets within these areas during normal valve operation. These high levels of curvature occur in a short time period (approximately 20 ms) during rapid leaflet opening and to a lesser degree during leaflet closure. The curvatures that occur at the free leaflet edge of two designs of polyurethane trileaflet heart valve were determined in vitro at various stages during a cardiac cycle using a high-speed video camera (1000 frames/s). Significant deformations at the free leaflet edge were observed and bending radii as low as 0.55 +/- 0.125 mm (mean +/- standard deviation) were present during leaflet opening, 0.76 +/- 0.24 mm during leaflet closure and 1.01 +/- 0.27 mm while the valve was fully open during peak systole. The values of curvature were used to determine the values of bending strain and bending stress acting at the free leaflet edge using thin shell bending theory. The calculated values of bending strain were a maximum during the leaflet flexure associated with valve opening. These high levels of bending strain, which occur for short periods of time, are likely to be an important determinant of the valve's durability. It has been shown that the method of manufacture significantly influenced the level of bending strain in the valve leaflets. Valves manufactured using a dip-casting technique resulted in open leaflet bending strains up to 31 per cent lower than valves manufactured from solvent-cast sheets of polyurethane.

Influence of the compliance of the pump housing and cannulas of a paracorporeal pneumatic ventricular assist device on transient pressure characteristics

The dependence of transient pressure characteristics of a ventricular assist device (VAD) on the compliance of its housing and cannulas was investigated in a mock circulation. The peak rate of change of pressure (dP/dtmax) values was greater in the cannulas than other compartments and was associated with valve closure-induced pressure oscillations. When cannula compliance was increased from 0.0057 to 0.0129 cm3/mm Hg, these values decreased by approximately 20%, and outflow cannula pressure oscillation frequency decreased from 17.5 Hz by 35%. This trend was also apparent in the inflow. A VAD housing compliance increase from 0.0162 to 0.0483 cm3/mm Hg caused a dP/dtmax decrease of 30% in both the blood chamber and the outflow cannula. The effect of this change on the inflow was weaker implying that housing absorbs the energy associated with outflow deceleration more effectively than the inflow. These findings suggest that increasing VAD housing and cannulas compliance can improve hydrodynamic performance.

The influence of manufacturing methods on the function and performance of a synthetic leaflet heart valve

There is considerable interest in polyurethane synthetic leaflet heart valves for both ventricular assist devices and direct implantation in the body. Two different manufacturing methods, thermal forming, and dip casting, of the leaflets have been investigated. There were only small differences in the hydrodynamic function of the valves made by the two methods. However, the durability of dip cast valves was far superior to the thermally formed film fabricated leaflets, with all of the dip cast valves reaching 160 million cycles without failure. This study indicates that a correctly designed and manufactured polyurethane synthetic leaflet heart valve has the potential for long-term implantation in the body.