Shape Memory Polymer Foams Research Articles

Shape memory polymer foams for cerebral aneurysm reparation: Effects of plasma sterilization on physical properties and cytocompatibility

Shape memory polyurethanes (SMPUs) represent promising candidate materials for aneurysm embolization, since they could enable clinical problems still associated with these clinical procedures to be overcome. In this work, we report on the characterization of physicochemical, thermomechanical and in vitro interface properties of two SMPU foams (Cold Hibernated Elastic Memory, CHEM), proposed as a material for embolization devices in minimally invasive procedures. Moreover, because device sterilization is mandatory for in vivo applications, effects on the properties of the foams after plasma sterilization were also evaluated. Both foams (CHEM 3520 and CHEM 5520) showed excellent shape recovery ability (recovery rate, R r , up to 99%) in conventional shape recovery tests, performed at constant heating rate. Transition temperatures ( T trans ), determined by tan δ peaks in dynamic mechanical analysis (DMA), were 32.2 and 45.1 °C, for CHEM 3520 and 5520, respectively. The value of T trans affects shape memory ability in the recovery test at 37 °C, which simulates the behavior after implantation of the device: in fact, R r was significantly higher for lower T trans foam ( R r ≈ 82% and R r ≈ 46%, respectively, for CHEM 3520 and CHEM 5520). After plasma sterilization performed by a Sterrad ® sterilization system, an increase in open porosity was observed: this is probably due to the sterilization cycle; however, no effects on shape recovery behavior were observed. Furthermore, plasma treatment had no significant effect on L929 cells in in vitro cytotoxicity tests, performed on cell culture medium extracts in contact with foams for up to 7 days. Moreover, direct cytocompatibility tests showed a good colonization and growth from L929 cells on CHEM foams, suggesting the effectiveness of an in vivo healing process. All these results seem to suggest that CHEM foams could be advantageously used for manufacturing devices for mini-invasive embolization procedures of aneurysms.

Thermo-mechanical behavior of epoxy shape memory polymer foams

Shape memory polymer foams have significant potential in biomedical and aerospaceapplications, but their thermo-mechanical behavior under relevant deformation conditionsis not well understood. In this paper we examine the thermo-mechanical behavior of epoxyshape memory polymer foams with an average relative density of nearly 20%. Thesefoams are deformed under conditions of varying stress, strain, and temperature.The glass transition temperature of the foam was measured to be approximately90 °C and compression and tensile tests were performed at temperatures ranging from 25 to125 °C. Various shape recovery tests were used to measure recovery properties under differentthermo-mechanical conditions. Tensile strain to failure was measured as a function oftemperature to probe the maximum recovery limits of the foam in both temperature andstrain space. Compression tests were performed to examine compressibility of the materialas a function of temperature; these foams can be compacted as much as 80% and stillexperience full strain recovery over multiple cycles. Furthermore, both tensile strain to failuretests and cyclic compression recovery tests revealed that deforming at a temperature of80 °C maximizes macroscopic strain recovery. Deformation temperatures above or below thisoptimal value lead to lower failure strains in tension and the accumulation ofnon-recoverable strains in cyclic compression. Micro-computed tomography (micro-CT)scans of the foam at various compressed states were used to understand foam deformationmechanisms. The micro-CT studies revealed the bending, buckling, and collapse of cellswith increasing compression, consistent with results from published numerical simulations.

Vascular Dynamics of a Shape Memory Polymer Foam Aneurysm Treatment Technique

The vascular dynamics of a shape memory polymer foam aneurysm treatment technique are assessed through the simulated treatment of a generic basilar aneurysm using coupled fluid dynamics and heat transfer calculations. The shape memory polymer foam, which expands to fill the aneurysm when heated, is modeled at three discrete stages of the treatment procedure. To estimate an upper bound for the maximum amount of thermal damage due to foam heating, a steady velocity is specified through the basilar artery, corresponding to a minimum physiological flow velocity over a cardiac cycle. During expansion, the foam alters the flow patterns within the aneurysm by shielding the aneurysm dome from a confined jet that issues from the basilar artery. The time scales for thermal damage to the artery walls and surrounding blood flow are computed from the temperature field. The flow through the post-treatment bifurcation is comprised of two counter-rotating vortex tubes that are located beneath the aneurysm neck and extend downstream into the outlet arteries. Beneath the aneurysm neck, a marked increase in the wall shear stress is observed due to the close proximity of the counter-rotating vortex tubes to the artery wall.

Medical applications of shape memory polymers

Shape memory polymers (SMP) are lightweight, have a high strain/shape recovery ability, are easy to process, and required properties can be tailored for variety of applications. Recently a number of medical applications have been considered and investigated, especially for polyurethane-based SMP. SMP materials were found to be biocompatible, non-toxic and non-mutagenic. The glass transition temperature (Tg) can be tailored for shape restoration/self-deployment of clinical devices when inserted in the human body. Newly developed SMP foams, together with cold hibernated elastic memory (CHEM) processing, further broaden their potential biomedical applications. Polyurethane-based SMP are described here and major advantages are identified over other medical materials. Some SMP applications are already used in a clinical setting, whereas others are still in development. Lately, several important applications are being considered for CHEM foams as self-deployable vascular and coronary devices. One example is the endovascular treatment of aneurysms.

Prototype laser-activated shape memory polymer foam device for embolic treatment of aneurysms

Conventional embolization of cerebral aneurysms using detachable coils is time-consuming and often requires retreatment. These drawbacks have prompted the development of new methods of aneurysm occlusion. We present the fabrication and laser deployment of a shape memory (SMP) polymer expanding foam device. Data acquired in an in vitro basilar aneurysm model with and without flow showed successful treatment, with the flow rate affecting foam expansion and the temperature at the aneurysm wall.

Thermomechanical Behavior of a Polyurethane Shape Memory Polymer Foam

Shape memory polymers (SMPs) have attracted great interest in recent years. The SMP foams are outstanding, owing to their high shape recovery ratio in compression. They can be used for, for instance, micro foldable vehicles, shape determination, and microtags. This article presents a study on the thermomechanical behavior of a polyurethane SMP foam, associated with these three applications. This includes four types of tests namely, compression test, free recovery test, constrained cooling test, and gripping test.

Endovascular Coiling Versus Neurosurgical Clipping for Patients With Aneurysmal Subarachnoid Hemorrhage

Endovascular Coiling Versus Neurosurgical Clipping for Patients With Aneurysmal Subarachnoid Hemorrhage

Shape fixity and shape recovery of polyurethane shape-memory polymer foams

The thermomechanical properties of polyurethane shape memory polymer (SMP) foams were investigated experimentally. The results obtained can be summarized as follows. (1) By cooling the foam after compressive deformation at high temperature, stress decreases and the deformed shape is fixed. Stress decreases markedly in the region of temperature below the glass transition temperature Ts during the cooling process. (2) By heating the shape-fixed foam under no load, the original shape is recovered. Strain is recovered markedly at the temperature region in the vicinity of Tg. (3) The ratio of shape fixity is 100 per cent and that of shape recovery 98 per cent. Neither ratio depends on the number of cycles. (4) Recovery stress increases by heating under constraint of the fixed shape. Recovery stress is about 80 per cent of the applied maximum stress. Relaxed stress at high temperature is not recovered. (5) The shape deformed at high temperature is maintained for six months under no load at Tg’60 K without depending on maximum strain, and the original shape is recovered by heating thereafter. (6) If the deformed shape is kept at high temperature, the original shape is not recovered. The factors influencing the shape irrecovery are the holding conditions of strain, temperature, and time.

Shape fixity and shape recovery of polyurethane shape-memory polymer foams

The thermomechanical properties of polyurethane shape memory polymer (SMP) foams were investigated experimentally. The results obtained can be summarized as follows. (1) By cooling the foam after compressive deformation at high temperature, stress decreases and the deformed shape is fixed. Stress decreases markedly in the region of temperature below the glass transition temperature Ts during the cooling process. (2) By heating the shape-fixed foam under no load, the original shape is recovered. Strain is recovered markedly at the temperature region in the vicinity of Tg. (3) The ratio of shape fixity is 100 per cent and that of shape recovery 98 per cent. Neither ratio depends on the number of cycles. (4) Recovery stress increases by heating under constraint of the fixed shape. Recovery stress is about 80 per cent of the applied maximum stress. Relaxed stress at high temperature is not recovered. (5) The shape deformed at high temperature is maintained for six months under no load at Tg’60 K without depending on maximum strain, and the original shape is recovered by heating thereafter. (6) If the deformed shape is kept at high temperature, the original shape is not recovered. The factors influencing the shape irrecovery are the holding conditions of strain, temperature, and time.

Influence of long-term storage in cold hibernation on strain recoveryand recovery stress of polyurethane shape memory polymer foam

In this paper, the effects of long-term storage in compressed coldhibernated elastic memory (CHEM) polyurethane foam, a kind of shape memorypolymer, are investigated experimentally. The foams were pre-strained at ahigh temperature, which was above the glass transition temperature, to 80%and 93.4%, respectively, and then cooled back to room temperature. Aftervarious periods of cold hibernation (up to two months), they were heated up atfixed length or against different constant loads. It is found that: (1) themaximum stress that the foam could exert at fixed length depends heavily onthe amount of pre-strain; (2) expansion rates of 380 and 1273% from thehibernated size against a 1 N load (pre-strained by 80 and 93.4%,respectively) are achievable. However, upon further increases in load, theexpansion is reduced dramatically. It appears that the tested CHEMpolyurethane foam retains its shape memory properties even after being storedin a compacted state for a long period. Complete strain recovery is attainablefor a hibernation period of up to two months.

105 ポリウレタン系形状記憶ポリマーフォームの変形特性

The thermomechanical properties of polyurethane-shape memory polymer foam were investigated. The main results are summarized as follows. (1) Strain is recovered at temperatures around the glass transition temperature during heating. The temperature at which strain is recovered decreases with an increase in maximum strain. (2) The rate of strain recovery is 99% and does not change under cyclic deformation. (3) The rate of strain fixity is 100%. (4) Stress diminished during cooling. Stress decreases markedly at temperatures below the glass transition temperature. (5) Because large deformation and large variation in volume are obtained in shape-memory polymer foam, shape-memory polymer foam has excellent functional performance for large elements of mechanics and structures.

Detecting powder mixture inhomogeneity for measurement errors : J.J.Walker, D.K.Rollins. (Iowa State University, Ames, Iowa, USA.) PowderTechnol., vol 92, no 1, 1997, 9–15

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 (Tg) 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 Tg +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.

Thermodynamic assessment of niobium-tungsten-carbon system : W.Huang, M.Selleby. (Royal Inst. of Technology, Stockholm, Sweden.) Z.Metallkunde, vol 88, no 1, 1997, 55–62

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 (Tg) 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 Tg +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.