Shape Memory Polymer Foams Research Articles

Porous media properties of reticulated shape memory polymer foams and mock embolic coils for aneurysm treatment

BackgroundShape memory polymer (SMP) foams are being investigated as an alternative aneurysm treatment method to embolic coils. The goal of both techniques is the reduction of blood flow into the aneurysm and the subsequent formation of a stable thrombus, which prevents future aneurysm rupture. The purpose of this study is to experimentally determine the parameters, permeability and form factor, which are related to the flow resistance imposed by both media when subjected to a pressure gradient.MethodsThe porous media properties—permeability and form factor—of SMP foams and mock embolic coils (MECs) were measured with a pressure gradient method by means of an in vitro closed flow loop. We implemented the Forchheimer-Hazen-Dupuit-Darcy equation to calculate these properties. Mechanically-reticulated SMP foams were fabricated with average cell sizes of 0.7E-3 and 1.1E-3 m, while the MECs were arranged with volumetric packing densities of 11-28%.ResultsThe permeability of the SMP foams was an order of magnitude lower than that of the MECs. The form factor differed by up to two orders of magnitude and was higher for the SMP foams in all cases. The maximum flow rate of all samples tested was within the inertial laminar flow regime, with Reynolds numbers ranging between 1 and 35.ConclusionsThe SMP foams impose a greater resistance to fluid flow compared to MECs, which is a result of increased viscous and inertial losses. These results suggest that aneurysms treated with SMP foam will have flow conditions more favorable for blood stasis than those treated with embolic coils having packing densities ≤ 28%.

Low density biodegradable shape memory polyurethane foams for embolic biomedical applications

Low density shape memory polymer foams hold significant interest in the biomaterials community for their potential use in minimally invasive embolic biomedical applications. The unique shape memory behavior of these foams allows them to be compressed to a miniaturized form, which can be delivered to an anatomical site via a transcatheter process and thereafter actuated to embolize the desired area. Previous work in this field has described the use of a highly covalently crosslinked polymer structure for maintaining excellent mechanical and shape memory properties at the application-specific ultralow densities. This work is aimed at further expanding the utility of these biomaterials, as implantable low density shape memory polymer foams, by introducing controlled biodegradability. A highly covalently crosslinked network structure was maintained by use of low molecular weight, symmetrical and polyfunctional hydroxyl monomers such as polycaprolactone triol (PCL-t, Mn=900g), N,N,N0,N0-tetrakis(hydroxypropyl)ethylenediamine and tris(2-hydroxyethyl)amine. Control over the degradation rate of the materials was achieved by changing the concentration of the degradable PCL-t monomer and by varying the material hydrophobicity. These porous SMP materials exhibit a uniform cell morphology and excellent shape recovery, along with controllable actuation temperature and degradation rate. We believe that they form a new class of low density biodegradable SMP scaffolds that can potentially be used as “smart” non-permanent implants in multiple minimally invasive biomedical applications.

In Vitro Study of Transcatheter Delivery of a Shape Memory Polymer Foam Embolic Device for Treating Cerebral Aneurysms

In Vitro Study of Transcatheter Delivery of a Shape Memory Polymer Foam Embolic Device for Treating Cerebral Aneurysms

In vivo response to an implanted shape memory polyurethane foam in a porcine aneurysm model.

Cerebral aneurysms treated by traditional endovascular methods using platinum coils have a tendency to be unstable, either due to chronic inflammation, compaction of coils, or growth of the aneurysm. We propose to use alternate filling methods for the treatment of intracranial aneurysms using polyurethane-based shape memory polymer (SMP) foams. SMP polyurethane foams were surgically implanted in a porcine aneurysm model to determine biocompatibility, localized thrombogenicity, and their ability to serve as a stable filler material within an aneurysm. The degree of healing was evaluated via gross observation, histopathology, and low vacuum scanning electron microscopy imaging after 0, 30, and 90 days. Clotting was initiated within the SMP foam at time 0 (<1 h exposure to blood before euthanization), partial healing was observed at 30 days, and almost complete healing had occurred at 90 days in vivo, with minimal inflammatory response.

Virtual Treatment of Basilar Aneurysms Using Shape Memory Polymer Foam

Numerical simulations are performed on patient-specific basilar aneurysms that are treated with shape memory polymer (SMP) foam. In order to assess the post-treatment hemodynamics, two modeling approaches are employed. In the first, the foam geometry is obtained from a micro-CT scan and the pulsatile blood flow within the foam is simulated for both Newtonian and non-Newtonian viscosity models. In the second, the foam is represented as a porous media continuum, which has permeability properties that are determined by computing the pressure gradient through the foam geometry over a range of flow speeds comparable to those of in vivo conditions. Virtual angiography and additional post-processing demonstrate that the SMP foam significantly reduces the blood flow speed within the treated aneurysms, while eliminating the high-frequency velocity fluctuations that are present within the pre-treatment aneurysms. An estimation of the initial locations of thrombus formation throughout the SMP foam is obtained by means of a low fidelity thrombosis model that is based upon the residence time and shear rate of blood. The Newtonian viscosity model and the porous media model capture similar qualitative trends, though both yield a smaller volume of thrombus within the SMP foam.

Inorganic–Organic Shape Memory Polymer (SMP) Foams with Highly Tunable Properties

Thermoresponsive shape memory polymers (SMPs) are a class of smart materials that can return from a temporary to a permanent shape with the application of heat. Porous SMP foams exhibit unique properties versus solid, nonporous SMPs, enabling their utility in different applications, including some in the biomedical field. Reports on SMP foams have focused on those based on organic polymer systems. In this study, we have prepared inorganic-organic SMP foams comprising inorganic polydimethylsiloxane (PDMS) segments and organic poly(ε-caprolactone) PCL segments. The PCL segments served as switching segments to induce shape changing behavior whereas the length of the PDMS soft segment was systematically tuned. SMP foams were formed via the photochemical cure of acrylated (AcO) macromers AcO-PCL(40)-block-PDMS(m)-block-PCL(40)-OAc (m = 0, 20, 37, 66 and 130) using a revised solvent casting/particulate leaching (SCPL) method. By varying the PDMS segment length, PDMS-PCL foams having excellent shape memory behavior were obtained that exhibited highly tunable properties, including pore size, % porosity, compressive modulus, and degradation rate.

In-Situ Mechanical- and Functional-Behavior Characterization of a Shape-Memory Polymer for Sand-Control Applications

Summary Pumping a gravel slurry between a downhole screen and the rock matrix has been used to prevent production of undesirable solids or sand from the formation in traditional oil and gas completions. Recently developed expandable technology attempts to eliminate this operation by expanding metallic sand-control screens against the wellbore. However, one problem with this process is borehole irregularity. It is desirable to have a smart downhole-screen structure that is capable of self-expanding and conforming to the borehole surfaces in certain downhole conditions. Use of an advanced shape-memory-polymer (SMP) structure offers breakthrough performance for sand-control applications. Proper characterization and full understanding of the thermal, mechanical and functional behavior of the SMP structure in simulated downhole pressure, temperature, and fluid conditions are critical for successful applications. This study used a high-pressure/high-temperature (HP/HT) in-situ mechanical test system and test method to characterize the in-situ hot-wet mechanical and functional behavior of the SMP foam, including deployment and contact-pressure-development functional properties, compressive resistance, and time-dependent stress-relaxation and creep mechanical properties. These properties were determined during immersion in a fluid environment at various temperatures under ambient pressure and 3,000- to 5,000-psi hydrostatic pressure. Effects of the test temperature, fluid condition, and the hydrostatic pressure on mechanical and functional properties of the SMP foam were identified. Study results were used to establish the operational window of the SMP screen as a sand-control solution.

Porous inorganic–organic shape memory polymers

Thermoresponsive shape memory polymers (SMPs) are a type of stimuli-sensitive materials that switch from a temporary shape back to their permanent shape upon exposure to heat. While the majority of SMPs have been fabricated in the solid form, porous SMP foams exhibit distinct properties and are better suited for certain applications, including some in the biomedical field. Like solid SMPs, SMP foams have been restricted to a limited group of organic polymer systems. In this study, we prepared inorganic–organic SMP foams based on the photochemical cure of a macromer comprised of inorganic polydimethylsiloxane (PDMS) segments and organic poly(ε-caprolactone) (PCL) segments, diacrylated PCL40-block-PDMS37-block-PCL40. To achieve tunable pore size with high interconnectivity, the SMP foams were prepared via a refined solvent-casting/particulate-leaching (SCPL) method. By varying design parameters such as degree of salt fusion, macromer concentration in the solvent and salt particle size, the SMP foams with excellent shape memory behavior and tunable pore size, pore morphology, and modulus were obtained.

Shape memory polymer foams from emulsion templating

Shape memory polymers (SMPs) change their shape under a stimulus (thermal, chemical, light) and return from an imposed temporary shape to their permanent, original shape. SMPs usually contain “permanent” domains that determine the permanent shape (chemical or physical crosslinks) and “reversible” domains that determine the temporary shape, usually by heating above a glass transition temperature or a melting point (Tm). Compared to fully dense SMPs, SMP foams can undergo higher temporary deformations and can exhibit higher deformations when they recover. In this paper, SMP foams based upon (meth)acrylates with crystallizable long side-chains were synthesized through emulsion-templating within nanoparticle-stabilized high internal phase Pickering emulsions where the nanoparticles also served as crosslinking centers. The nature of the polymer backbone affected the nature of the crystalline phase for identical side chains. The SMP foams at room temperature maintained the temporary shape (a strain of 0.7) imposed above the Tm and exhibited good recovery upon reheating for all four compression–recovery cycles. While the methacrylate-based SMP exhibited a single-stage recovery, the acrylate-based SMP, with identical side-chains, exhibited a two-stage recovery that can be associated with the existence of two crystalline phases. The recovery behavior was described using Kelvin–Voigt units in series with the dependence of viscosity on temperature described using a WLF-like relationship.

Opacification of shape memory polymer foam designed for treatment of intracranial aneurysms.

Shape memory polymer (SMP) foam possesses structural and mechanical characteristics that make them very promising as an alternative treatment for intracranial aneurysms. Our SMP foams have low densities, with porosities as high as 98.8%; favorable for catheter delivery and aneurysm filling, but unfavorable for attenuating X-rays. This lack of contrast impedes the progression of this material becoming a viable medical device. This paper reports on increasing radio-opacity by incorporating a high-Z element, tungsten particulate filler to attenuate X-rays, while conserving similar physical properties of the original non-opacified SMP foams. The minimal amount of tungsten for visibility was determined and subsequently incorporated into SMP foams, which were then fabricated into samples of increasing thicknesses. These samples were imaged through a pig's skull to demonstrate radio-opacity in situ. Quantification of the increase in image contrast was performed via image processing methods and standard curves were made for varying concentrations of tungsten doped solid and foam SMP. 4% by volume loading of tungsten incorporated into our SMP foams has proven to be an effective method for improving radio-opacity of this material while maintaining the mechanical, physical and chemical properties of the original formulation.

Estimation of aneurysm wall stresses created by treatment with a shape memory polymer foam device

In this study, compliant latex thin-walled aneurysm models are fabricated to investigate the effects of expansion of shape memory polymer foam. A simplified cylindrical model is selected for the in-vitro aneurysm, which is a simplification of a real, saccular aneurysm. The studies are performed by crimping shape memory polymer foams, originally 6 and 8 mm in diameter, and monitoring the resulting deformation when deployed into 4-mm-diameter thin-walled latex tubes. The deformations of the latex tubes are used as inputs to physical, analytical, and computational models to estimate the circumferential stresses. Using the results of the stress analysis in the latex aneurysm model, a computational model of the human aneurysm is developed by changing the geometry and material properties. The model is then used to predict the stresses that would develop in a human aneurysm. The experimental, simulation, and analytical results suggest that shape memory polymer foams have potential of being a safe treatment for intracranial saccular aneurysms. In particular, this work suggests oversized shape memory foams may be used to better fill the entire aneurysm cavity while generating stresses below the aneurysm wall breaking stresses.

The effect of moisture absorption on the physical properties of polyurethane shape memorypolymer foams

The effect of moisture absorption on the glass transition temperature (Tg) and the stress/strain behavior of network polyurethane shape memory polymer (SMP)foams has been investigated. With our ultimate goal of engineering polyurethane SMPfoams for use in blood-contacting environments, we have investigated the effects ofmoisture exposure on the physical properties of polyurethane foams. To the best of ourknowledge, this study is the first to investigate the effects of moisture absorption at varyinghumidity levels (non-immersion and immersion) on the physical properties ofpolyurethane SMP foams. The SMP foams were exposed to differing humidity levels forvarying lengths of time, and they exhibited a maximum water uptake of 8.0% (bymass) after exposure to 100% relative humidity for 96 h. Differential scanningcalorimetry results demonstrated that water absorption significantly decreased theTg of the foam, with a maximum water uptake shifting theTg from67 to 5 °C. Samples that were immersed in water for 96 h and immediately subjected to tensile testingexhibited 100% increases in failure strains and 500% decreases in failure stresses; however,in all cases of time and humidity exposure, the plasticization effect was reversibleupon placing moisture-saturated samples in 40% humidity environments for 24 h.

Enhancing the Deformation of Shape Memory Sandwich Panels

Abstract: Composite sandwich panels fabricated using a thermosetting shape memory polymer matrix material and a corresponding thermoset shape memory polymer foam core offer the potential to demonstrate large, recoverable, deformations in otherwise stiff structures, under flexural loading. However, as with flexure of thin, fibre-reinforced shape memory matrix laminates, deflection is limited by fibre compression buckling because of the reduced shape memory matrix stiffness at elevated temperature. A hybrid matrix concept has been developed for sandwich panels loaded in flexure in a single direction. This concept uses a non-shape memory resin as the matrix for a fraction of the plies on the surface of the facesheet loaded in compression. It is predicted that, at the elevated temperatures required for the generation of deformation in the shape memory structural sandwich panel, the shape memory matrix and foam moduli will be substantially reduced, while the modulus of the non-shape memory resin will not. Thus, at elevated temperature this effectively leads to a shift of the neutral axis towards the non-shape memory surface, keeping the low stiffness shape memory matrix material in tension and extending the range of deformation prior to onset of fibre buckling. The experiments performed demonstrate that this hybrid matrix approach enables a three-fold increase in mid-span deformation prior to buckling of fibres in the compression surface plies. Furthermore, the force measured to attain the deformed geometry, at elevated temperature, only increases approximately 10–15%, while the magnitude of the force required remains very low. Thus, the hybrid matrix approach functions as predicted and enables the development of sandwich panel structural elements which can undergo large, recoverable deformations.

Cyclic compression behavior of epoxy shape memory polymer foam

Shape memory polymer foams have significant potential in aerospace and biomedical applications, however their thermomechanical behavior and damage response under cyclic deformation are not well understood. This paper investigates effects of pre-strain and hold period between compressive cycles on cyclic behavior of epoxy shape memory polymer foam with relative densities of 20%, 30%, or 40%. For each relative density, foams were cyclically deformed under conditions of strain control with periodic hold times (0, 40, and 300 s). During the hold time the sample was completely unconstrained; neither stress nor a strain was applied. A 17% relative density foam was periodically imaged at 12 μm resolution using micro-computed tomography scans to determine the mesostructural response to cyclic deformation of 40%, 60%, or 80% engineering strain. These scans provided global quantitative data as well as qualitative regional information on cyclic deformation mechanisms. Individual struts were isolated and tracked across the scans to qualitatively show local response to cyclic conditions. The experimental data are discussed in light of finite element modeling of the foam (published in another article) to better understand the cyclic deformation behavior.

Shape Memory Polymer Foams for Biomedical Devices

Shape Memory Polymer Foams for Biomedical Devices

Shape Memory Polymer Foams for Biomedical Devices

Shape memory polymers (SMPs) constitute a group of high performance smart materials that have recently gained widespread attention. Their potential role in clinical applications has only become recognized in the last 7-8 years. These materials were found to be biocompatible, non-toxic and non-mutagenic. Some simple SMP applications are already used in a clinical setting, whereas others are still in development. Recently developed polyurethane-based SMP foams combined with cold hibernated elastic memory (CHEM) processing, widen their potential medical applications. The glass transition temperature (Tg) of CHEM foams can be tailored for shape restoration/self-deployment of clinical devices when inserted in the human body. They can be miniaturized and deformed, inserted in the human body through small catheters, and subsequently can recover a larger predetermined shape once in satisfactory position. In this review paper CHEM foam materials and their potential medical applications are described. Their major advantages over other medical materials are delineated. Also, several important medical applications of CHEM foams are explained here as well.

Controlling the Physical Properties of Random Network Based Shape Memory Polymer Foams

AbstractShape memory polymers (SMPs) are increasingly being considered for use in minimally invasive medical devices. For safe deployment of implanted devices it is important to be able to precisely control the actuation temperature of the device. In this study we report the effect of varying monomer composition on the glass transitions/actuation temperatures (Tg) of novel low density shape memory foams. The foams were based on hexamethylenediisocyanate (HDI), triethanolamine (TEA) and tetrakis (2-hydroxyl propyl) ethylenediamine (HPED), and were produced via a combination of chemical and physical blowing process. The process for post-foaming cleaning was also varied. Foams were characterized by DSC, DMA, and for shape memory. No clear trends were observed for foam samples without cleaning, and this was attributed to process chemicals acting as plasticizers. In foams cleaned via washing and/or sonication, the Tg was observed to decrease for compositions that were higher in the TEA content. Also, no change in shape memory behavior was observed for varying compositions. This work demonstrates the ability to tailor actuation transition temperature while maintaining shape memory behavior for low density foams suitable for aneurysm treatment.

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

BackgroundInterventional 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.MethodsA 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.ResultsSpatial maps of the steady-state fluid velocity and temperature change inside and outside the laser-heated SMP foam device were acquired.ConclusionsThough 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.

Deformation of epoxy shape memory polymer foam: Part II. Mesoscale modeling and simulation

The present study focuses on how the relative density of an epoxy shape memory polymer foam affects mesostructural response to deformation. The modeled foam had relative densities of 20%, 30%, and 40%, with a glass transition temperature ( T g) near 85 °C as measured by dynamic mechanical analysis. Statistical analysis of the dynamic mechanical analyisis data showed that stiffness did not significantly vary across foam samples of the same relative density. The stress–strain response of the base polymer was obtained through tensile and compressive tests at T g +17 °C and fit to a hyperelastic material model using the ABAQUS material evaluation function. Meshes for the three relative densities were obtained through micro-CT scanning of actual foam, to accurately represent the mesostructure. Overall compression responses predicted by ABAQUS correlated well with experimental stress–strain responses. The simulations qualitatively showed a shift in mesostructure response to deformation between the 20% relative dense material and the materials with higher relative densities. Quantitatively, both the local maximum shear strain and tensile strain cumulative probability distributions within the material were analyzed and a lognormal function was fit to these distributions. The function had three parameters fit to the distribution. Two other parameters, the applied macroscale strain and the reference strain value, were considered in scaling the distributions. These lognormal distributions were then used to predict local strain distributions at applied compressive strain values larger than those simulated in the finite element modeling. The results are discussed in light of the potential local damage mechanisms in shape memory polymer foam.

Deformation of epoxy shape memory polymer foam. Part I: Experiments and macroscale constitutive modeling

Shape memory polymer foams have both shape memory properties and attributes of low density and compressibility of foams. Potential applications for these materials span from embolic sponges for biomedical uses to morphing wings on advanced airframes. This study focuses on how the relative density of the foam affects macroscale response to deformation. Epoxy shape memory foams with relative densities of 20, 30%, and 40% and a glass transition temperature ( T g) near 85 °C as measured by dynamic mechanical analysis were tested in compression. Micro-CT scans were used to characterize the micro-architecture for each relative density. Tensile test data showed a temperature dependence on the effect of relative density on tensile strain-to-failure. Compression tests demonstrated similar effects of relative density at different temperatures. Unconstrained shape recovery tests showed no effect of relative density on free strain recovery, while constrained stress recovery showed a strong effect of relative density. Relative density did have a slight effect on constrained cooling, which was demonstrated to be controlled by viscous relaxation followed by thermal stress relaxation. A model from the Gibson and Ashby on cellular solids was used to simulate the effect of relative density on the macroscale stress-strain properties. The prediction of the effect of relative density on modulus correlated well with the DMA data, and the compressive response was fit to each compression curve. However, it was necessary to modify the prediction for the densification strain and allow it to depend on temperature for the fits to correlate with experimental data at higher strains. The fitting parameters were fairly temperature independent once the material was above T g.