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.
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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)