In-vitro characterization of flexible biomaterials for use within a novel polymeric heart valve

Heart valves are essential for blood circulation. Valvular heart disease (VHD) is a major global health issue, affecting over 40.5 million people. The primary treatment is surgical valve replacement, but current options have limitations. Mechanical heart valves (MHVs) can last over 40 years but require lifelong anticoagulation, while bioprosthetic heart valves (BHVs) avoid anticoagulation but typically become dysfunctional within 10-20 years [1].

The Wheatley Heart Valve (WHV) is a novel polymeric valve designed to combine MHV durability with BHV biocompatibility. Its unique S-shaped leaflet design enables manufacture from a single, flat polymer sheet, thereby making a range of materials and manufacturing methods possible. Mechanical and surface characterization of the candidate biomaterials are crucial steps towards future optimization of the WHV design and manufacture process.

This study has produced a series of Lubrizol Carbothane™ TPU flat sheets using different manufacturing methods. These sheets have then been subjected to tensile tests and surface contact angle (SCA) testing.

2.1 Materials Carbothane™ TPU grades AC 4095A polymer pellets were obtained from The Lubrizol Corporation. The pellets were dissolved in N-N Dimethylacetamide (DMAC) at 20% and 30% (w/v). Flat sheets were then either 1) cast in glass petri dishes or 2) spin-coated on 4” silicon wafers, with the 30% solution first spun at 500 rpm for 40s and then 8000 rpm for 20s. All samples were left to cure for 24 hrs in a vacuum oven at 60°C. Finally, samples were laser cut to the desired shape prior to testing. For SCA testing, Polydimethylsiloxane (PDMS), banana leaves, and glass slides, were used as control samples.

2.2 Equipment For tensile testing, a Biomomentum Mach-1 Mechanical Tester was used, with a 0-250N load cell. Data was processed using Python (3.14), R (4.4.1), Microsoft Excel, GraphPad Prism (10.1.2), and Kinovea v2023.1.2.

2.3 Isotropy Tensile Testing 2 groups of 45 x 9 mm flat sheet cast AC 4095 A samples were cut (n=3) perpendicular to each other and arbitrarily assigned as the longitudinal and transverse axis. Samples were preconditioned (10 cycles: ramp speed 7.5 mm/s, pulled 1.5 mm) and then pulled 10 mm along the x-axis at 7.5 mm/s. The gauge length was 30 mm. Sample thicknesses were measured at 3 points per sample with a digital micrometer. Young’s Modulus (YM), E was calculated using equation 1. The thicknesses and YMs were each compared with a Welches t-test.

E= σ/(ε ), σ=F/A Where F is the force applied, A is cross-sectional area, σ is stress, ε is strain.

3.1 Isotropy Tensile Testing The mean thickness of the transverse and longitudinal axis groups (n=3) were 251 (SD = 7.0) µm and 230 (SD = 11.3) µm, respectively. There was no significant difference between the group thicknesses (p=0.0823, Welches t-test). Figure 1 shows the YM of each group; there was no significant difference between the longitudinal and transverse tests (61.87 MPa v 57.85 MPa, p>0.05).

3.2 Surface Contact Angle

The mean SCA of the tested sample groups are shown in Table 1.

The tensile testing showed that the flat-cast samples were isotropic. There were slight, insignificant, differences in YM between the transverse axis and the longitudinal axis. This difference may reflect the difference in thickness between the samples, introduced by variability in the manufacturing method. In future research, it may be beneficial to introduce anisotropy to better match the native valve.

SCA is an indicator of potential biocompatibility, which is useful for long-term, blood contacting and implantable heart valves. The SCA testing showed the AC 4095 A material to be hydrophobic, similar to the PDMS control. The spin coated AC 4095 A samples were more hydrophobic than the flat cast ones, consistent with Wenzel’s model, as the surface appeared smoother [2]. This study reproduced the Janus wettability results of banana leaves [3], providing a useful control for potential future studies of leaflet design research. Further investigation into droplet deformation could explore how material topography and isotropy are linked.

Future work will investigate the impact and potential benefits of different manufacturing methods and possible coatings on the mechanical behavior and haemocompatibility of the valve leaflets. The spin coated isotropy will be investigated. These data will be used in in-silico models of the valve to help inform design optimization [4].

The AC 4095A flat cast material is isotropic and AC 4095 A is hydrophobic. These data collected have provided a baseline which can be taken forward to define the optimum leaflet design in the Wheatley valve.

Keywords

Flexible biomaterials, Polymeric heart valve