A comprehensive numerical simulation has been performed to inter-relate the fluid mechanics of the formation of co-axial jets and their development downstream of the plane of jet emergence. The fluid flow was modeled as being turbulent, with corresponding Reynolds numbers in the jet-formation section of 10000, 20000, and 50000. The model was based on a fully developed pipe flow encountering a double-pipe arrangement and splitting between the two pipes. Subsequent to flow development in the central pipe and in its enveloping annulus, the flow exited into a large free space and became a co-axial free jet. The simulations were performed for geometric parameters which varied the relative cross sections of the central pipe and of the annulus and also varied the length of the jet-formation section. The overall pressure drop responsible for both the jet formation and for the subsequent free-jet development was found to be due to friction in contradistinction to inertial losses. At the jet exit, the velocity profile is discontinuous because of the intrusive presence of the walls of the pipes which bound the jet-formation section. These intrusions cause a double-humped velocity profile which disappeared with decreasing downstream distance from the jet origin. At sufficient downstream distances, the width of the jet was found to be independent of both the geometrical parameters and the Reynolds number.

Objectives: A synergistic experimental and numerical investigation has provided quantitative information on the response of surrogate human tissue temperatures to misalignment of the implant and antenna of neuromodulation devices during recharging.Materials and Methods: The experimental phase of the work provided information on the rates of heat transfer from the implant and the antenna to their respective surroundings. The heat transfer data were used as input to a biothermal model from which tissue temperature distributions were obtained.Results: It was found that misalignment increases tissue temperatures compared with those for the aligned case for all of the investigated devices. These increases ranged from 0.5 degrees C to 5.3 degrees C.Conclusion: Notwithstanding these increases, the lowest temperatures were attained by the Restore Ultra device for all operating conditions. The temperature levels achieved by the Precision Plus and Eon Mini devices were found to be greater than those for the Restore Ultra but their relative rankings depend on the thermal boundary conditions and the duration of the recharging period. The foregoing rank ordering was validated by a sensitivity study in which the heat transfer data inputted to the numerical simulation was varied systematically. The aforementioned comparisons correspond with identical recharging periods for all of the devices.

University of Minnesota Ph.D. dissertation. December 2011. Major: Mechanical Engineering. Advisor: Eph Sparrow. 1 computer file (PDF);ix, 145 pages.

In the design of medical devices, the use of numerical simulation, either with or without complementary experimentation, may lead to a more competent product. The experimentation in question may either be performed in vitro or in vivo. This paper conveys a case study in which the two methodologies, numerical simulation and in vitro experimentation used in tandem, enabled the evaluation of safety issues related to a heat-generating implant. The numerical simulation was implemented by means of ANSYS finite-element software employed in the transient mode. The experimental work provided information necessary for the execution of the simulation and, therefore, was performed as the first phase of the research. The implant is of the type that is equipped with a short-lived battery that requires intermittent recharging. The recharging is accomplished by means of an antenna that is externally mounted on the skin surface. The antenna is the primary of a transformer, and the implant contains the secondary of the transformer. During the recharging period of the battery, heat is generated in both the antenna and the implant. By the symbiotic use of the experimental results and the numerical simulation, time-dependent temperatures were determined in the tissue that is situated in the neighborhood of the implant and the antenna. These temperatures were evaluated from the standpoint of possible tissue damage.

The advent of rechargeable biomedical implants for neuromodulation has introduced the practice of recharging implantable batteries through a patient’s skin. Long-term operation of such implants is achieved by periodically recharging the implant’s battery by means of a magnetic field produced by an antenna situated on the surface of the skin. During recharging periods, heat is generated within both the implant and the antenna. The heat flowing from these components into adjacent tissue creates the possibility of tissue temperature elevations that may be unsafe. This issue was investigated by means of a synergistic combination of experimentation and numerical simulation. The experiments measured the rates at which heat generated within the components flowed into their respective surroundings. This information was utilized as input information to the numerical simulations. The simulation model consisted of four tissue layers plus the skin-surface-mounted antenna. Two realistic external thermal environments were considered for the simulations. Both the experimentation and the simulations were performed for three leading neuromodulation devices: Precision Plus, Eon Mini, and Restore Ultra. In the presence of convective/radiative heat losses in a 20°C environment, the maximum tissue temperature during recharging never exceeded 39°C for the Restore Ultra, but exceeded 41°C for the Precision Plus and the Eon Mini. In an adiabatic environment, similar findings were observed; the temperatures associated with the Precision Plus and Eon Mini exceeded 41°C, while the tissue temperatures near the Restore Ultra maintained values less than 41°C. The work reported here deals with devices that have not previously been investigated.

The two major fluid flow systems of the human body, blood circulation and respiration, experience timewise pulsations. The variations of the fluid velocity during a pulsation/respiration cycle give rise to transitions in the flow regime during the course of a cycle. At the lowest fluid velocity encountered in the cycle, it is likely that the flow is laminar. As the velocity increases, the laminar regime may transist into a regime called transitional intermittent. Further increases in velocity may lead either to the fully developed intermittent regime or to the fully developed turbulent regime. Once the velocity attains a maximum and begins to decrease, the process of laminarization may be initiated wherein a succession of flow regimes may occur in opposite order to that described in the foregoing. The current capabilities of numerical simulation are limited to a single, user-specified flow regime, either laminar or turbulent. Consequently, the successive spontaneous flow regime transitions encountered in human-body fluid flows have been heretofore beyond the reach of biomedical investigators. Indeed, a thoroughgoing literature review failed to unearth any biomedical-oriented publications in which flow regime transitions have been taken into account. The present investigation is aimed at applying, for the first time, a flow transition model previously developed for steady flows to unsteady flows. The flows to be considered are timewise periodic, with amplitudes, periods, and mean values appropriate to blood flows in large arteries. Special consideration will be given to the magnitudes of the wall shear stresses that are created by such flows, since the accumulation of plaque depends decisively on the shear. The work will also take account of variations in the flow geometry.

The magnetic-field-driven heat generation in neuromodulation systems consisting of implanted and skin-surface-mounted components gives rise to the potential of discomfort, cell damage, and possible necrosis. The skin-surface-mounted component, commonly termed the antenna, serves the function of the primary of a transformer, and the implant is the secondary. Heating occurs in both of these components during the recharging of a battery situated in the implant. Previously reported experimental data for the heat generation characteristics of three commercially available neurostimulation systems has been enhanced by further experiments carried out as part of this investigation. A numerical simulation of the temperature distribution in tissue beds adjacent to the implant and the antenna has been performed here. The aggregated data have been used as input information to a bio-heat-transfer model which yields both the spatial and temporal variations of the temperature field. It was found that during long-duration recharging periods, the temperature of the tissue rises in response to the heat generation. This information enables the identification of the magnitude and location in the tissue of the hot-spot temperature. The temporal temperature variation at the hot spot was employed in conjunction with a tissue-damage integral to identify the possibility of cell damage and/or necrosis. It was found that two of the three investigated neuromodulation systems did not give rise to temperature levels that may cause tissue damage. However, the third of the systems caused temperatures of sufficient elevation so that for recharging periods on the order of 2h, necrosis was found to be likely in situations where heat transfer is suppressed at the surface of the skin.

In vitro and cadaver experiments, coupled with numerical simulations, were performed to assess the possibility that orbital atherectomy might cause thermal damage of tissue. The experiments involved debulking operations on a surrogate artery and on the plaque-lined posterior tibial artery of a cadaver. Temperatures and coolant flow rates measured during these experiments enabled a numerical simulation of the debulking of a plaque-lined artery in a living human. The temperature variations from the numerical simulations were used to evaluate a thermal injury index. The resulting values of the index were found to be several orders of magnitude below the threshold value for thermal injury. It is concluded that it is extremely unlikely that the use of an orbital debulking device, the Diamondback 360°™ (Cardiovascular Systems, Inc.), can lead to thermal injury of the artery wall.