Auxetic patch material thickens during systole and improves pump function in a finite element model of myocardial infarct repair

Introduction: : Patch reinforcement of the infarcted heart has been shown to counteract infarct expansion and left ventricular (LV) functional degradation. However, traditional patch plasty also restricts diastolic filling, reducing cardiac output. These disadvantages can be minimized with strategic modification of standard patch materials [1, 2], suggesting further functional improvements could be possible through a broader exploration of patch materials. The objective of this study is to examine the potential advantages of a patch graft with auxetic properties, having a negative Poisson’s ratio (𝛎), to more closely mimic the dynamics of healthy heart and enhance post-infarct LV pump function.

Materials and

Methods: : A finite element model of canine left-ventricle biomechanics during a full cardiac cycle was originally developed by Estrada and colleagues [3] using the FEBio software suite [4, 5], and used to study the effects of surgical patch reinforcement in the context of acute ischemic injury (Figure 1A). For the current study, the model was modified to simulate epicardial implantation of Auxetic Patch materials, in addition to a Meiotic (i.e., non-auxetic) Patch, and compared to unpatched Infarct and healthy Baseline conditions, assuming the same pressure loading time course in all cases. All patch materials assumed a neo-Hookean constitutive law with an elastic modulus of 100 kPa, and the Poisson’s ratio was prescribed as 𝛎=0.4 for meiotic material, and 𝛎=-0.2 for auxetic material. In addition, a heterogeneous patch material was simulated comprising a layer of auxetic material covered by an outer layer of meiotic material, as described below. Radial strain vs. time was examined in a central element of the injury/patch region, and pressure-volume loops were also generated for each condition to assess pump function. Informed by the in silico modeling results, a porcine feasibility study (n=1) of acute patch repair for chronic MI was performed using both a standard Gore-Tex cardiovascular patch (WL Gore, Newark, DE) and a patch with an auxetic foam layer (Auxadyne, Keystone Heights, FL) attached to a spandex backing for suturing.

Results, Conclusions, and Discussions:: Relative to end-diastole, the Baseline model exhibited positive radial strain due to systolic wall thickening, whereas the Infarct and Meiotic Patch models showed negative radial strain due to adverse wall thinning through the cardiac cycle (Figure 1B). The Homogeneous Auxetic Patch graft exhibited radial thickening during systole (Figure 1B), driven by tension on the patch from the neighboring contracting myocardium; but the patch thickness expanded away from the LV cavity and therefore, based on the pressure-volume relationship (Figure 1C), this did not improve ventricular pump function compared to a standard meiotic patch graft. These results were corroborated during in vivo testing of a simple auxetic foam patch (Figure 1D), in which P-V loops showed a reduction in volume during auxetic patch implantation compared to unpatched infarct, but no significant benefit of the auxetic compared to the meiotic Gore-Tex patch (Figure 1E), validating the finite element model. Alternatively, a Heterogeneous Auxetic Patch design with a stiff outer reinforcement layer also exhibited systolic thickening (Figure 1B) that was inwardly-directed by the outer boundary constraint and restored the LV pressure-volume relationship toward baseline function (Figure 1C), without adversely affecting fiber stress in the border zone or remote myocardium compared to the meiotic patch (not shown). This feasibility study demonstrates the potential to harness auxetic mechanical properties for improving LV pump function in the setting of myocardial infarct, motivating further exploration and optimization of auxetic patch materials for surgical repair of injured myocardium.

Acknowledgements and/or References (Optional):: Acknowledgements
Funding from NIH/NIBIB R21-EB028520; NIH/NHLBI F31-HL146033; i3-Genesis Award from Mount Sinai Innovation Partners, and expert technical support from Drs. Tomoki Sakata and Kiyotake Ishikawa.

References
1. Fomovsky, G. M., Clark, S. A., Parker, K. M., Ailawadi, G. & Holmes, J. W. Anisotropic Reinforcement of Acute Anteroapical Infarcts Improves Pump Function. Circ. Heart Fail. 5, 515–522 (2012).
2. Fomovsky, G. M., Macadangdang, J. R., Ailawadi, G. & Holmes, J. W. Model-Based Design of Mechanical Therapies for Myocardial Infarction. J Cardiovasc. Transl. Res. 4, 82–91 (2011).
3. Estrada, A. C., Yoshida, K., Clarke, S. A. & Holmes, J. W. Longitudinal Reinforcement of Acute Myocardial Infarcts Improves Function by Transmurally Redistributing Stretch and Stress. J. Biomech. Eng. 142, (2020).
4. Maas, S. A., Ellis, B. J., Ateshian, G. A. & Weiss, J. A. FEBio: Finite Elements for Biomechanics. J. Biomech. Eng. 134, (2012).
5. Maas, S. A., Ateshian, G. A. & Weiss, J. A. FEBio: History and Advances. Annu. Rev. Biomed. Eng. 19, 279–299 (2017).