Delhaas, T. | bioRxiv Science

bioRxiv2023

Evaluating flash freezing for preservation of rat abdominal aorta for delayed biomechanical characterization

Most studies investigating arterial stiffening use animal rather than human arteries. This is because human tissue becomes available in small amounts and at irregular times, which complicates planning of experimental work. Suitable tissue preservation methods for delayed biomechanical testing prevents the need for testing fresh tissue and alleviates some of the logistical challenges of human ex vivo studies. Therefore, the present study aimed to investigate whether the existing method of flash freezing and subsequent cryostorage provides is suitable for delaying the characterization of arterial biomechanics. Fresh and flash frozen abdominal aortas (n=16 and 14, respectively) were quasi- statically and dynamically tested using a biaxial testing set-up with dynamic pressurization capabilities. The acquired biomechanical data was modeled using a constituent-based quasi-linear viscoelastic modeling framework, deriving directional stiffness parameters, individual constituent biomechanical contributions, and viscoelastic stiffening under dynamic pressurization conditions. Flash freezing reduced arterial wall thickness, increased circumferential stiffness, as well as reduced viscoelastic stiffening at higher pressures. These findings reflected those in the modeled contribution of collagen to arterial biomechanics, showing increased collagen load bearing at higher pressures. However, despite the above mentioned detectable changes, flash freezing did not alter the mechanical relation between elastin and collagen, maintaining a non-linear response to pressurization and stretch. Flash freezing may thus be suitable for studies requiring delayed characterization of passive arterial biomechanics, assuming care is taken to ascert that the impact of flash freezing on study groups can be approached as a systematic error.

bioengineering↗

bioRxiv2022

Biomechanical characterisation of thoracic ascending aorta with preserved pre-stresses

Mechanical properties of an aneurysmatic thoracic aorta are potential markers of future growth and remodeling and can help to estimate risk of rupture. Aortic geometries obtained from routine medical imaging do not display wall stress distribution and mechanical properties. Mechanical properties for a given vessel may be determined from medical images at different physiological pressures using inverse finite element analysis. However, without considering pre-stresses, the estimation of mechanical properties will lack accuracy. In the present paper, we propose and evaluate a mechanical parameter identification technique, which recovers pre-stresses by determining the zero-pressure configuration of the aortic geometry. We first validated the method on a cylindrical geometry and subsequently applied it to a realistic aortic geometry. Verification of the assessed parameters was performed using synthetically generated reference data for both geometries. The method was able to estimate the true mechanical properties with an accuracy ranging from 98% to 99%. Author summaryCurrent clinical guidelines recommend to perform surgical intervention on thoracic ascending aortas with diameters greater than 55 mm. However, diameter sizes solely are not indicative of the degenerative changes in the vessel wall. Patient-specific tissue mechanical properties give better insight into the degenerative changes caused due to the variations in mass densities of the vessel wall constituents. Inverse finite element analysis can be performed on aortic geometries derived from medical images to determine tissue mechanical properties. Performing inverse finite element analysis on geometries obtained directly from medical images leads to incorrect estimation of vessel wall stresses and therefore, mechanical properties. Mechanical properties estimation is incorrect because inverse analysis does not take into account the pre-stressed nature of the geometry. In this study, we propose a novel approach to non-invasively estimate in vivo mechanical properties of the thoracic ascending aorta by performing inverse finite element analysis, which considers the vessel wall pre-stresses.

bioengineering↗