Computational biomechanics of acute myocardial infarction and its treatment

The intramyocardial injection of biomaterials is an emerging therapy for myocardial infarction. Computational methods can help to study the mechanical effect s of biomaterial injectates on the infarcted heart s and can contribute to advance and optimise the concept of this therapy. The distribution of polyethylene glycol hydrogel injectate delivered immediately after the infarct induction was studied using rat infarct model. A micro-structural three-dimensional geometrical model of the entire injectate was reconstructed from histological micro graphs. The model provides a realistic representation of biomaterial injectates in computational models at macroscopic and microscopic level. Biaxial and compression mechanical testing was conducted for healing rat myocardial infarcted tissue at immediate (0 day), 7, 14 and 28 days after infarction onset. Infarcts were found to be mechanically anisotropic with the tissue being stiffer in circumferential direction than in longitudinal direction. The 0, 7, 14 and 28 days infarcts showed 443, 670, 857 and 1218 kPa circumferential tensile moduli. The 28 day infarct group showed a significantly higher compressive modulus compared to the other infarct groups (p= 0.0055, 0.028, and 0.018 for 0, 7 and 14 days groups). The biaxial mechanical data were utilized to establish material constitutive models of rat healing infarcts. Finite element model s and genetic algorithms were employed to identify the parameters of Fung orthotropic hyperelastic strain energy function for the healing infarcts. The provided infarct mechanical data and the identified constitutive parameters offer a platform for investigations of mechanical aspects of myocardial infarction and therapies in the rat, an experimental model extensively used in the development of infarct therapies. Micro-structurally detailed finite element model of a hydrogel injectate in an infarct was developed to provide an insight into the micromechanics of a hydrogel injectate and infarct during the diastolic filling. The injectate caused the end-diastolic fibre stresses in the infarct zone to decrease from 22.1 to 7.7 kPa in the 7 day infarct and from 35.7 to 9.7 kPa in the 28 day infarct. This stress reduction effect declined as the stiffness of the biomaterial increased. It is suggested that the gel works as a force attenuating system through micromechanical mechanisms reducing the force acting on tissue layers during the passive diastolic dilation of the left ventricle and thus reducing the stress induced in these tissue layers

Development of a tissue-regenerative vascular graft: Structural and Mechanical Aspects

In attempt to prevent graft failure, the tissue-regeneration field offered the porous vascular scaffolds as promising solution for the lack of endothelialization seen in the small-calibre synthetic vascular graft. Another cause of graft failure was reported to be the mechanical mismatch between the graft and the host vessel. This study concerned the investigation and optimization of structural designs of tissue-regenerative vascular grafts, comprising ingrowth permissible porous polyurethane (PPU) foam and knitted reinforcement wire mesh, with the aim of providing vascular prostheses that mimic arterial mechanics. A 3D geometry of a knitted eight-loop wire mesh was imported into Abaqus CAE® 6.8-2 and assembled with a PPU tube geometry such that the wire mesh acted as external reinforcement (EX) or embedded reinforcement (EM) to the PPU tube. A 45°-section assembly was meshed using 8-node linear brick elements. Nitinol (NITI) and polyurethane (PU) material models were used for the knitted mesh. Material parameters obtained in experimental tests were implemented in hyperfoam (PPU), shape memory alloy (NITI) and linear elastic (PU) constitutive models. The luminal grafts surfaces were subjected to uniformly distributed pressure load ramping from 0 to 200mmHg. Models were compared in terms of predicted maximum stress and strain, wall compression, strain energy, radial displacement and compliance. The predicted radial compliance ranged between 1.2 and 15.6%/100mmHg in the reinforced grafts, compared to 106.4 and 65.1%/100mmHg for the non-reinforced grafts. The maximum stress in the Nitinol remained safe at 33 % of stress associated with start of austenite-martensite phase transformation (i.e. 483MPa). The maximum stress and strain values detected in the PPU tube indicated recoverable elastic deformation. The reinforcement enhanced the mechanical performance of the graft without affecting its tissue-regenerating characteristics, as the predicted maximum wall compression indicated that the reduction in size of pore windows would still allow ingrowth of capillaries and arterioles