Thermo/hydration responsive shape memory polymers with enhanced hydrophilicity for biomedical applications

Shape memory polymers (SMPs) have thermomechanical property drastically altering around the glass-transition temperature (Tg), temporary shape fixity and shape recovery property, and so on. To understand the mechanisms of such unique properties and design products including SMPs optimally, a mathematical model which is simple yet can represent these properties reasonably and accurately is necessary. Especially, it is important for the model to be able to consider the effect of temperature and strain history on the properties because the properties are considered to depend on not only the temperature and the strain themselves but also their history. Therefore, in this paper first the effect of the temperature and strain history on the shape fixity and shape recovery properties was examined. Then, a micromechanical model of SMP was developed to describe this effect. This model consisted of a lot of unit elements having four springs, two dash pots, one latch, and one thermal expansion element. Different Tg was assigned to each element because Tg depends on local condition inside of SMP and distributed in general. It was found from the experiment that the temperature and strain history applied to SMP when the shape is gradually frozen has an influence on the shape recovery process. With respect to the model, it was shown that the proposed micromechanical model can capture the thermomechanical behaviors of SMP and, especially, the temperature and strain histories dependence on the shape recovery process quite well.

Thermally-induced two-way shape memory polymers: Mechanisms, structures, and applications

PurposeIntracranial aneurysms (ICAs) pose a serious clinical risk due to their potential for rupture, leading to subarachnoid hemorrhage, high morbidity, and mortality. This study aims to develop a proof-of-concept device for the targeted delivery of shape memory polymers (SMPs)-based embolic devices to improve aneurysm occlusion and reduce recurrence.MethodsA novel system was designed combining a radial compression fixture and an electronic device for Joule heating and electrolytic detachment (ED). Three SMP geometries (5 mm, 6.5 mm spherical, and patient-specific) were evaluated for the shape recovery and thermal responses. In-vitro testing was performed using 6.5 mm and patient-specific geometries in PDMS aneurysm phantoms under physiological relevant conditions utilizing ovine blood.ResultsControlled activation of the SMPs at currents of 400 mA achieved reproducible shape recovery ratios (SRRs) up to 75.71%, with detachment occurring at <100 mA. Surface temperatures remained below 45 °C. In-vitro deployment resulted in aneurysm sac occlusion of 90.32% (patient-specific) and 94.12% (idealized), without evidence of thermal damage or gas accumulation. Flow visualization confirmed reduced bubble entry into the aneurysm sac post-deployment.ConclusionThis study demonstrates the feasibility of a targeted delivery system for patient-specific ICA treatment using SMPs. While further refinement and in-vivo validation are required, these findings highlight the potential of SMPs as durable embolization devices capable of conforming to complex aneurysm geometries and providing more effective occlusion compared to current methods.

Biostable shape memory polymers that remain stable in physiological conditions are beneficial for user-defined shape recovery in response to a specific stimulus. For potential commercialization and biocompatibility considerations, biomaterial synthesis must be simple and scalable. Hence, a library of biostable and cytocompatible shape memory polymers with tunable thermomechanical properties based on hard segment content was synthesized using a solvent-free method. Polymer surface chemistry, thermomechanical and shape memory properties, and biostability were assessed. We also investigated the effects of processing methods on thermomechanical and shape memory properties. All polymers showed high glass transition temperatures (>50 °C), which indicates that their temporary shape could be preserved after implantation. Polymers also demonstrate high shape fixity (73-80%) and shape recovery (93-95%). Minimal mass loss (<5%) was observed in accelerated oxidative (20% H2O2) and hydrolytic (0.1 M NaOH) media. Additionally, minimal shape recovery (∼0%) occurred in programmed samples with higher hard segment content that were stored in degradation media. After 40 days of storage in media, programmed samples recovered their primary shapes upon heating to temperatures above their transition temperature. Annealing to above the polymer melting point and solvent casting of polymers improved shape memory and thermal properties. To enable their potential use as biomaterial scaffolds, fiber formation of synthesized polyurethanes was compared with those of samples synthesized using a previously reported solvent-based method. The new method provided polymers that can form fibrous scaffolds with improved mechanical and shape memory properties, which is attributed to the higher molecular weight and crystalline content of polymers synthesized using the new, solvent-free approach. These biostable segmented polyurethanes could be coupled with a range of components that respond to specific stimuli, such as enzymes, magnetic field, pH, or light, to enable a specific shape change response, which could be coupled with drug and/or bioactive material delivery in future work.

Indentation of shape memory polymers: Characterization of thermomechanical and shape recovery properties

Shape memory polymer (SMP) systems exhibiting semicrystalline- elastomer blends, such as thermoplastic polyurethane and polylactic acid have been well studied, but their use in biomedical shape memory applications has been limited by their high activation temperature. SMPs are capable of deformation and recovery through the activation of an external stimuli, such as temperature. Critical criteria for SMPs used in biomedical applications is achieving a stimulus temperature close to 37°C while still experiencing sufficient shape recovery. A polymer’s glass transition temperature has been well defined as the SMP system’s activation temperature and therefore should be decreased to achieve a decreased activation temperature. In this work, a well-known, biocompatible plasticizer, polyethylene oxide (PEO), was added to thermoplastic polyurethane (TPU) – polylactic acid (PLA) SMP blends to observe the plasticizing effect on the structural, thermal, mechanical, and shape memory properties of the polymer blends. Additionally, the geometry of the fabricated SMP samples was tailored to further enhance the shape memory effect through a bowtie honeycomb structure. Our results suggest that the addition of PEO into the SMP system may be an effective method for decreasing the polymer’s glass transition temperature through the alteration of the polymer chain structure. With the addition of 30% PEO, the glass transition temperature of the TPU/PLA blend was successfully decreased from 62.4°C to 34.6°C while achieving 86.5% shape recovery when activated at 37°C, which is only a 5% decrease in shape recovery when activated at 50°C. These results suggest that the addition of a biocompatible plasticizer may overcome the limitation of employing temperature activated SMP systems in biomedical applications, and enhances the potential of these materials for reconfigurable structures, energy dissipation systems, and SHM in civil engineering applications.

With variable stiffness properties, shape memory polymers (SMPs) become the most promising materials of morphing skins. In this article, based on hydro‐epoxy resin/carboxyl‐terminated butadiene acrylonitrile/maleic anhydride matrix, composite morphing skins are prepared by doping conductive carbon black (CB). By adjusting the content of CB, in‐plane and out‐of‐plane properties of skins can be adjusted. Because of the in‐plane tensile stress acting on the skin during flight, tensile test is required. The results indicate that in‐plane tensile strength of skin decreases as the CB content increases. In electrical resistivity test, the electronic resistivity of skin decreases sharply at a low‐percolation threshold (1.9 wt%) and then maintains a low and steady level. As for out‐of‐plane properties, thermo‐ and electro‐active shape memory test are studied. All of the selected typical composite skins have 100% thermo‐driven shape recovery rate. Some skins have 100% thermo‐driven shape fixity rate and fast electro‐active shape recovery speed. When the CB content is 2.6 or 3 wt%, the skin can recover its out‐of‐plane deformed shape completely at 150 V. Benefit from easy control and remote drive, the composite SMP skins are proved to be a promising candidate material for morphing skins.

Shape memory polymers (SMPs) are evolving intensively in smart materials research. SMPs can undergo large deformations, hold a temporary shape and then recover their original shape upon exposure to a particular external stimulus. The emerging development of fibre reinforced shape memory polymer composites (SMPCs) has improved SMPs’ mechanical properties to be comparable to modern fibre reinforced composites. Moreover, the inclusion of nanoparticles has enhanced the diversity in activation mechanisms and improved structural properties and durability.&#13;\n&#13;\nToday, the SMPCs are generally viewed as promising substances for engineered applications that have raised intensive interest among the scientists and engineers. Interestingly, stimulation by light enables unique and advanced shape memory effects (SMEs) in SMPCs that can fulfill the sophisticated demands of engineering. The ability to vary the light wavelength, intensity, periodicity, polarization and the irradiated position on the SMP component enables wavelength selective, reversible, sequential and multiple SMEs. Furthermore, light travel long distances in a vacuum, can be focussed onto a certain area, is safe to humans and, can be a control signal carrier and most notably, can be sent through optical fibres to extremely inaccessible areas.&#13;\n&#13;\nIncorporation of metallic, carbon and organic based photothermal fillers into a SMP matrix is a convenient proven method to prepare light activated shape memory polymer composites (LASMPCs). These photothermal fillers convert electromagnetic radiation into thermal energy that triggers the SMP matrix. The current study comprehensively reviewed the performances, mechanisms, challenges and limitations of the existing LASMPC systems and associated optical technologies. Predominantly this research was intended to develop reinforced LASMPC systems and allied optical technologies applicable for large scale structural engineering applications.&#13;\n&#13;\nIn this study, styrene and bisphenol A epoxy based thermosets and polylactic acid (PLA) based thermoplastic SMPs were used as polymer matrix materials. Carbon fibre, glass fibre, multiwalled carbon nanotube (MWCNT) and rare earth organic complexes of Nd(TTA)3Phen and Yb(TTA)3Phen were used as reinforcement and photothermal fillers. The samples were produced by means of vacuum bagging, mould casting and 4D printing. Morphological investigations were conducted to identify the microstructure and manufacturability of the LASMPCs. The samples’ mechanical and physical properties were tested by standard test methods for polymer matrix composites. Dynamic mechanical analysis was carried out to determine the thermomechanical characteristics. The photothermal effect was investigated through thermal imaging and associated shape memory behaviours were studied through shape fixing and recovery experiments. A finite element analysis was performed with ABAQUS to simulate the photothermal heating behaviour.&#13;\n&#13;\nThe current study established a material development framework for LASMPC materials and presented apt manufacturing methods for structural and large scale engineering applications. Different modes of light diffusion such as scattered low intensity light, focussed laser beams and light delivery through fibre optics were proved for structural LASMPCs. Sequential and selective wavelength activation of structural LASMPCs are presented. The FEA simulation provided additional understanding on light exposed LASMPCs’ 3-dimensional heating behaviour. Embedding a D-shaped optical fibre in an LASMPC created a single unit smart actuator, which was remotely activated by light. It was proven that flexible radiation shields can reduce the unanticipated shape recovery of LASMPCs components and protect the LASMPCs from polymer degradation due to radiation.&#13;\n&#13;\nThe improvements to their structural properties and innovative remote activation methodologies have justified LASMPCs’ aptness for space engineering applications. This study has proven its successful completion by three selected case studies of space applications. An LASMPC solar panel array model was sequentially deployed in four recovery steps. In a vacuum, a space habitat structure model was compressed to a third of its volume and then recovered its original shape. A 4D printed boom structure model was tested, proving customized LASMPC structures for sophisticated space equipment fabrication. The deployable LASMPC structures can reduce the occupied room in spacecraft. The improvements to the structural properties of LASMPCs while keeping the SME, understanding on thermomechanical and photothermal behaviours, remote actuation through optical fibres, radiation shields and the insight to space applications are the most significant novel outcomes of this study. This thesis opens up windows of opportunity for the scientific and engineering communities to provide innovative solutions to realize the sophisticated large scale engineering demands in aerospace and space applications and beyond.

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 181361, “Advancement in Openhole Sand-Control Applications With Shape-Memory Polymer,” by Xiuli Wang and Gbenga Osunjaye, Baker Hughes, a GE company, prepared for the 2016 SPE Annual Technical Conference and Exhibition, Dubai, 26–28 September. The paper has not been peer reviewed. Conventional openhole sand-control techniques include standalone screens, expandable screens, and openhole gravel packing (OHGP). In some cases, the uniformity, size, and level of compressive strength of the formation dictate the use of an OHGP completion. However, OHGP is not always feasible or practical for some reservoirs and geographic locations. Shape-memory-polymer (SMP) technology can fill the gaps that exist with current OHGP techniques. This paper summarizes a technology using SMP to provide downhole sand control in openhole environments. Introduction The SMP conformable sand-control system consists of an engineered polymer material compacted onto a retainer cartridge (outer shroud, plain Dutch-weave mesh, and inner shroud). The assembly is then installed onto a perforated base pipe by mechanical means to provide a simple modular system for openhole sand-control applications. SMP Principle and Engineered Behaviors. The SMP typically has two different states: the glass state at low temperatures and the rubber state at higher temperatures. The point of change of state (the transition from the glass state to the rubber state), and the associated temperature, is referred to as the glass transit temperature (Tg). The engineered SMP has a defined Tg—the point where shape recovery starts. The initial SMP is first heated to the Tg of the polymer. Then, a mechanical force is applied to compact it to a smaller diameter and onto a retainer cartridge. The condensed SMP is then cooled below its Tg, and the force is removed. The SMP retains its compacted size until the surrounding temperature of the SMP reaches the Tg, at which point shape recovery begins. The bottomhole temperature (BHT) of the candidate reservoirs needs to be lower than the Tg to ensure that the SMP is rigid under the downhole conditions during the production phase. After the SMP sand-control system is installed in a wellbore, to expand the SMP under a BHT that is lower than the Tg, an activation fluid must be introduced. This activation fluid serves as a catalyst and temporarily reduces the Tg below the BHT so the SMP will be recovered to its initial shape. After the SMP is deployed, the activation fluid is flushed out. The SMP retains its recovered shape that conforms to the wellbore, and it is ready for production.

The glass transition temperatures of epoxy-based shape memory polymers (SMPs), which contain a flexibilizer at various contents of 0%, 5%, 10% and 15% respectively, are determined through DMA tests. The shape memory effect of such materials is investigated through shape recovery experiments. Experimental results show that the content of flexibilizer has much influence on the shape memory effect of epoxy-based SMP. A shape recovery equation is developed based on the results of shape recovery experiment. Numerical calculations show that the developed shape recovery equation well predicts the shape recovery behaviors of epoxy-based SMP.

Shape memory polymers (SMPs) are employed in a wide range of fields because to their appealing properties, which include low density, low production costs, and ease of processing. In the present work, SMP composites were developed using ultrasonic mixing of carbon fiber (CF) and amorphous epoxy to enhance the properties of epoxy and shape memory effect. The prepared composites were tested for mechanical and shape memory properties. Flexural strength and tensile strength of pristine epoxy enhanced after dispersing 1% CF which are about 12.7% and 16.2%, respectively, higher than pristine epoxy. Fracture toughness of epoxy improves after adding 1% CF which is about 94.05% higher compared to pristine epoxy. UV–vis-NIR showed reduction in energy bandgap. Shape fixity and shape recovery ratios for pristine epoxy was 98.49% and 93.40%, respectively. These ratios decrease to 90.52% and 87.86%, respectively with addition of 1.5 wt.% CF. However, recovery time improves from 57 s to 33 s with addition of 1.5 wt.% CF in epoxy. Shape recovery rate enhances with CF reinforcement in epoxy composite.

Shape memory property and mechanism of the knitting-fabric reinforced epoxy composites subjected to tensile loading

Shape memory polymers (SMPs) is a type of smart polymers. Shape-memory polymers (SMPs) allow for the development of stimuli-responsive polymer-based materials that can undergo substantial recoverable deformation in response to an external stimulus. Shape-memory effects (SMEs) of these SMP-based materials for task-specific applications are picking the interest of academic and industrial researchers. Significant progress has been gained in the field of shape memory polymers (SMPs) in recent years, with the discovery of new compositions for property tweaking, the discovery of new processes for shape fixing and recovery, and the initiation of phenomenological modelling. The current review will provide a brief account of the history of SMPs, their qualities, their structure and mechanism, classification, biodegradable SMPs and use of SMPs in various fields. Recent advancements in shape memory polymer composites (SMPCs) and shape-memory polymer blends (SMPBs) are also emphasized. This review explains distinct classification criteria of SMPs with a view of polymer type and structure and external stimulation.

Data from comprehensive thermomechanical tests of shape memory polymers are reported, with specimens tested up to 75% strain and between 30—120°C temperatures. The data is analyzed and key observations are drawn. The stress/strain behavior during loading at temperatures above glass transition for the Veriflex TM shape memory polymer tested was linear and did not show much variation with the actual temperature. When the polymer is cooled with end constraints, thermally-induced tensile stresses developed, but only after the temperature reduced below glass transition and the material stiffened. When the constraints were then released, 97—98% of the original strain was locked in. Reheating the shape memory polymer beyond the glass transition temperature resulted in shape recovery (shape memory effect). When the polymer was reheated while constraining the strain, the full recovery stress developed was about the stress that the polymer was initially loaded to during deformation at high temperature. Examining the Young's modulus at elevated and low temperature showed that the loading modulus at high temperatures is around 60 times lower than the low temperature unloading modulus of the stretched sample, and around 3000 times lower than the low temperature loading modulus of an unstretched sample.

Ultra-high stretchability and shape fixation rate shape memory polyurethanes based on cyclic polytetrahydrofuran molecular rings