The human body is an engineered system that utilizes an extensive range of interconnected processes to perform tasks. One category of processes relevant to mechanical engineering is thermal science, which includes both heat transfer and fluid flow. There are a great many organs and interconnecting vessels where such processes occur. Here, a selected group of thermal/fluid processes are set forth and subjected to synergistic analysis by numerical simulation and experimentation.
In this thesis, two case studies are described. One of these involves the heat transfer and fluid flow in association with the removal of plaque from partially blocked arteries. This work necessarily involves the assessment of the temperature rise created by the frictional interaction of the plaque-removal tool and the surface of the plaque material. This process takes place in the presence of flowing blood. The work involved in vitro and in vivo experiments, the latter with respect to a cadaver. The experimentation also involves means for determining the geometric configurations of arteries before and after plaque removal. The geometric configuration of the artery is of importance due to its effects on the flow of blood through the vessel.
The experimental findings were post-processed and subsequently employed as input information to appropriate numerical simulation models. In the case of heat transfer, a bioheat model was employed to evaluate local temperatures in human tissue. In turn, the local-temperature information was employed to evaluate a thermal tissue injury model. The outcome of that evaluation indicated that temperatures produced by a plaque-removal device would not create tissue necrosis.
The efficacy of plaque removal as a means for ameliorating cardiovascular insufficiency was investigated by means of numerical simulation. The geometric configurations of both a highly stenosed blood vessel and subsequently debulked vessel were determined by means of intravenous ultrasound (IVUS) imaging. These images were reconstituted to create solid models which were subsequently utilized as the basis of numerical simulation. The simulation took account of heart-induced pressure pulsations which were employed as input information to the time-dependent, three-dimensional Navier-Stokes equations. The ratio of the volumetric rates of blood flow passing through the originally stenosed vessel and the subsequently debulked vessel enabled the formulation of a metric to describe the efficacy of the plaque removal therapy. For the posterior tibial artery, an increase of 2.5 fold in the volumetric flow rate was discovered.
The second case study investigates the thermal effects of the presence of rechargeable implants in the body. It is well established that implants may be sources of heat generation. In the situation to be considered here, the implant does not contain a long-lived internal battery to power its functional activities. Rather, the needed power is provided by means of a transformer whose primary (antenna) is situated externally. The secondary of the transformer is contained within the implant. Both the antenna and the implant generate heat. Both of these heat sources may increase the tissue temperature in the subcutaneous zone and may also increase the temperature in the deeper tissue.
To determine the magnitude of the rate of heat generation by these heat sources, a unique experimental facility was created and implemented. The facility was designed to accommodate antennas and implants of varying geometrical characteristics. A calorimetric method was employed as the means for the determination of the magnitude of the energy transferred from the heat generating devices to their respective environments. Experiments were performed for both the case in which the axes of the implant and the antenna are collinear and for the case in which the axes are misaligned. The criticality of the alignment issue stems from the fact that patients, rather than medical professionals, are required to perform the alignment task. It was found that alignment is, in fact, a major factor with regard to the rates of heat generation and the concomitant temperature elevation of the tissue neighboring these devices.
Attention was focused on neuromodulation implants and their related antennas. For the study, the leading therapeutic neuromodulation devices were employed. For each of these, independent evaluations of heat generation rates were performed. The obtained information provided critical inputs to enable a reliable numerical simulation activity to be performed. The purpose of the simulation was to obtain temporal and spatial variations of temperatures within the relevant tissue beds. The temperature results were then utilized to assess the likelihood of tissue necrosis resulting from excessive temperatures imposed over lengthy durations.
It was found that there were substantial differences in the outcomes of the various neuromodulation devices. In particular, issues of safety were discovered for at least two of the evaluated devices. It was also found that misalignment aggravated safety issues which were of marginal concern when the implant and its antenna were perfectly aligned.