This CAREER proposal is motivated by the current lack of diagnostic tools to dynamically monitor the mechanical state of the stented artery that is surgically implanted to allow proper cardiovascular flow. Multiphysics models will be developed and experiments carried out to study the mechanics of a stented artery with an in situ polymer sensor. The research effort is targeted towards development of: (1) a 3-D multiphysics framework for evaluating large strain pulsatile membrane sensors and (2) a coupled structure-sensor model to predict the correlation between sensor output and the varying stage of artery occlusion and rigidification.
The proposed work is anticipated to bridge the gap between vascular mechanical response and the current state of vascular health, and quantum jump the in situ health monitoring capability to enable critical clinician intervention through multifunctional sensors.
The main objective of this research is to develop an in situ sensor to determine the state of unhealth in stented arteries. Ultimately, an in situ sensor that preserves hollow lumen geometry would significantly impact medical diagnostics. Since arteries at various stages of disease have a unique constitutive response, the output of an in situ artery sensor would have corresponding distinct profiles (Fig. 1). Our objective was to build a multi-physics modeling framework to couple artery behavior with stent response and sensor output. We made a number of advancements in the areas of developing soft sensors, modeling shape memory polymers (as a potential stent material), and modeling the artery. These accomplishments are summarized below: Soft Sensors We first implemented an existing semi-structural model to describe finite deformations of an arterial segment and developed a method that accounts for planar fiber distributions with more than one distributed fiber property. We developed a general model for the electromechanical response of a tubular sensor sandwiched between a representative pulsatile pressure input (blood flow) and the artery wall. This mathematical model successfully describes the deformation of a tubular segment subject to quasi-static pulsatile flow and the corresponding sensor output. The model predicts the expected sensor output for a physiological pressure range and frequencies. Comparison of numerical results and experimental data for a soft polymer segment validated the electromechanical model. Damaged arteries will show a stiffened mechanical response in comparison to healthy arteries. Using published data for healthy and diseased arteries, we modeled the sensor output. Using this model, we conducted an extensive number of parameter studies that showed that a soft capacitive sensor based on a monolithic tube design would be insensitive to certain changes in deformation. Therefore, we proposed a nanostructured polymer sensor based on a double layer of buckled gold and silver particles. Our results showed that the double layer design had improved sensitivity but suffered from repeatability issues because of nanoscopic fractures. Soft Stents based on Shape Memory Polymers Soft stents based on shape memory polymers have been an active area of biomedical investigation for more than a decade. The main advantages being the inherent deployment capability and the reduced stiffness mismatch between tissue and stent. Notwithstanding these potential advantages, shape memory polymers have complex material behavior and are plagued by unpredictable time varying properties, which ultimately limits their commercial implementation in certain biomedical implants and devices. Not only is it a challenge to model their behavior there is a lack of understanding how chemistry and material synthesis inflluences those properties and ultimately performance. We therefore approached our research simultaneously from molecular and macroscopic perspectives. On the macroscale, we developed a modified thermoviscoelastic model for shape memory polymers by modifying an existing model. The model includes effects of thermal expansion, temperature dependent viscoelasticity and structural relaxation. Our results show that the modified model is able to capture the shape recovery (deployment) behavior of the SMP through the glass transition. This is accomplished with far fewer parameters (11 parameters) than has been formally proposed (17 parameters). At the molecular scale, we expect to see behavior that represents the characteristic temperature-dependent mechanical properties and cycling response of a shape memory polymer (SMP) material. Thus we considered different simulation setups to determine the required level of detail and determine the most important molecular mechanisms that contribute to the shape memory effect. Coarse-grained molecular dynamics simulations are an ideal tool to answer this question being at a scale between atomistic and continuum. In the context of multiscale modeling, the goal is to understand the molecular mechanisms of shape memory polymer behavior; this will facilitate in closing the "gap" between synthesis, processing, structure, and morphology at the micro-scale, and material and device performance at the macro-scale. Our results show that temperature-dependent behavior relevant to SMPs can be reasonably represented. This suggests that coarse-grained simulations can play an important role in furthering our understanding of the shape memory effect, developing multiscale models of thermomechanical behavior, and proposing routes for material development. Modeling Arteries The current state of the art in cardiovascular research suggests that a more accurate model for simulating arteries would be beneficial over a purely mechanical continuum model. In the last year of this grant, our efforts focused on developing a more sophisticated model of the artery that would incorporate the local biochemistry and electrophysiology. We developed an anisotropic chemo-mechanical model within a large deformation framework for the artery media layer incorporating smooth muscle effects (Fig. 2). The derived constitutive model captures the main characteristics of the artery wall (media layer) with active smooth muscle tone in isometric and isotonic tests. This model will serve as the mathematical basis for future work, which will study the electrical and chemo-mechanical coupled behaviors of the artery wall, such as the myogenic response and damage.
