Magnetic resonance flow velocity and temperature mapping of a shape memory polymer foam device

BioMedical Engineering OnLine, Dec 2009

Background Interventional medical devices based on thermally responsive shape memory polymer (SMP) are under development to treat stroke victims. The goals of these catheter-delivered devices include re-establishing blood flow in occluded arteries and preventing aneurysm rupture. Because these devices alter the hemodynamics and dissipate thermal energy during the therapeutic procedure, a first step in the device development process is to investigate fluid velocity and temperature changes following device deployment. Methods A laser-heated SMP foam device was deployed in a simplified in vitro vascular model. Magnetic resonance imaging (MRI) techniques were used to assess the fluid dynamics and thermal changes associated with device deployment. Results Spatial maps of the steady-state fluid velocity and temperature change inside and outside the laser-heated SMP foam device were acquired. Conclusions Though non-physiological conditions were used in this initial study, the utility of MRI in the development of a thermally-activated SMP foam device has been demonstrated.

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Magnetic resonance flow velocity and temperature mapping of a shape memory polymer foam device

BioMedical Engineering OnLine Magnetic resonance flow velocity and temperature mapping of a shape memory polymer foam device Ward Small IV 2 Erica Gjersing 1 Julie L Herberg 2 Thomas S Wilson 2 Duncan J Maitland 0 2 0 Department of Biomedical Engineering, Texas A&M University, College Station , Texas, 77843 , USA 1 Department of Chemical Engineering and Materials Science, University of California , Davis, California, 95616 , USA 2 Lawrence Livermore National Laboratory , Livermore, California, 94550 , USA Background: Interventional medical devices based on thermally responsive shape memory polymer (SMP) are under development to treat stroke victims. The goals of these catheterdelivered devices include re-establishing blood flow in occluded arteries and preventing aneurysm rupture. Because these devices alter the hemodynamics and dissipate thermal energy during the therapeutic procedure, a first step in the device development process is to investigate fluid velocity and temperature changes following device deployment. Methods: A laser-heated SMP foam device was deployed in a simplified in vitro vascular model. Magnetic resonance imaging (MRI) techniques were used to assess the fluid dynamics and thermal changes associated with device deployment. Results: Spatial maps of the steady-state fluid velocity and temperature change inside and outside the laser-heated SMP foam device were acquired. Conclusions: Though non-physiological conditions were used in this initial study, the utility of MRI in the development of a thermally-activated SMP foam device has been demonstrated. - Background Shape memory polymers (SMPs) are a class of polymeric materials that can be fabricated into a primary shape, deformed into a stable secondary shape, and controllably actuated to recover the primary shape. The basis for the shape memory effect has been previously described in detail [1]. Although there is wide chemical variation in these materials, they can be grouped into categories with high physical similarity based on the method of actuation, which can be achieved thermally, through photoinduced reaction, or by introduction of an external plasticizer [2]. For SMPs that are actuated thermally, such as those in the present work, raising the temperature of the polymer above its characteristic glass transition temperature (Tg) results in a decrease in the elastic modulus from that of the glassy state (~109 Pa) to that of an elastomer (~106 to 107 Pa) [3] as the primary shape is recovered. Upon cooling, the original modulus is nearly completely recovered and the primary form is stabilized [4]. Encouraged by the shape memory behavior and biocompatibility [5,6], many biomedical applications for SMPbased active devices have emerged [2]. In particular, researchers are developing various interventional medical devices based on thermally responsive SMP. Such catheter-delivered devices include expandable stents [7,8], microactuators for retrieving blood clots in ischemic stroke patients [9,10], and embolic coils [11] and foams [12,13] for filling aneurysms. When the Tg of the SMP is above body temperature (37°C), an external heating mechanism such as laser (photothermal) [9,14] or electroresistive [10,14] heating is needed. Safe and effective device actuation requires limiting the thermal impact to the surrounding blood and tissue, posing a key challenge in SMP interventional device development. Another development consideration relevant for implantable devices (e.g., stent or embolic device) is the effect of the deployed device on the blood flow. Since changes in the hemodynamics and temperature induced by the intervention ultimately govern its safety and efficacy, there is a need to understand these changes and their physiological impact. A first step in the device development process is to investigate fluid velocity and temperature changes following device deployment in a simplified in vitro model. Though the results of such an investigation do not necessarily provide a direct assessment of the physiological impact in an actual clinical procedure, they may be used to modify device properties (e.g., foam density), adjust heating parameters (e.g., laser power), and validate computational models which can be extended to simulate physiological conditions. The non-invasive methods of nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) have been extensively used for chemical, material, biological, and medical applications. In MRI methods, a magnetic field gradient is used in conjunction with an NMR experiment to spatially encode spectral signatures based on numerous contrast parameters. These signatures may include structure (chemical shift), dynamics (relaxation times), or velocity (diffusion and flow). Spatial maps of fluid flow and temperature obtained by MRI can provide insight into the impact of the intervention. Our team has previously reported the use of various tools to study the fluid and thermal dynamic (...truncated)


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Ward Small, Erica Gjersing, Julie L Herberg, Thomas S Wilson, Duncan J Maitland. Magnetic resonance flow velocity and temperature mapping of a shape memory polymer foam device, BioMedical Engineering OnLine, 2009, pp. 42, 8, DOI: 10.1186/1475-925X-8-42