Shape Memory Polymeric Materials Research Articles

Mechanically interlocked [c2]daisy chain backbone enabling advanced shape-memory polymeric materials

The incorporation of mechanically interlocked structures into polymer backbones has been shown to confer remarkable functionalities to materials. In this work, a [c2]daisy chain unit based on dibenzo-24-crown-8 is covalently embedded into the backbone of a polymer network, resulting in a synthetic material possessing remarkable shape-memory properties under thermal control. By decoupling the molecular structure into three control groups, we demonstrate the essential role of the [c2]daisy chain crosslinks in driving the shape memory function. The mechanically interlocked topology is found to be an essential element for the increase of glass transition temperature and consequent gain of shape memory function. The supramolecular host-guest interactions within the [c2]daisy chain topology not only ensure robust mechanical strength and good network stability of the polymer, but also impart the shape memory polymer with remarkable shape recovery properties and fatigue resistance ability. The incorporation of the [c2]daisy chain unit as a building block has the potential to lay the groundwork for the development of a wide range of shape-memory polymer materials.

Curvature tuning through defect-based 4D printing

Emerging 4D printing techniques have enabled the realization of smart materials whose shape or properties can change with time. Two important phenomena play important roles in the 4D printing of shape memory polymeric materials. First, the anisotropic deformation of the printed filaments due to residual stresses can be harnessed to create out-of-plane shape transformations. Second, the unavoidable formation of micro-defects during the printing processes often affects the programmability of the printed object. Here, we propose a design approach that harnesses these two effects occurring during fused deposition modeling to create tailor-made curved geometries from initially 2D flat disks. We first determined the size and distribution of the imperfections formed within printed structures by varying two printing parameters namely the printing speed and the number of printed materials. Spatially varying the printing speed and combining polylactic acid filaments with a softer material without shape memory properties allowed us to cover a variety of shapes from negative to positive values of the mean and Gaussian curvature. We propose an analytical model to calculate the magnitude of the maximum out-of-plane deformation from the anisotropic expansion factor of the constituting microstructures. Furthermore, we develop computational models to predict the complex shape-changing of thermally actuated 4D printed structures given the distribution of rationally introduced imperfections and we demonstrate the potential applications of such defect-based metamaterials in drug delivery systems.

Shape Memory Polymeric Materials for Biomedical Applications: An Update

Shape memory polymers (SMPs) are emerging smart materials that have the ability to change to a shape and revert to their permanent shape on application of external stimulus. SMPs can be divided into four types based on their permanent shape, elasticity origin, and temporary shape fixing mechanism: chemically cross-linked glassy thermosets, chemically cross-linked semi-crystalline, physically cross-linked glassy copolymers, and physically cross-linked block copolymers. The broad overview of recent developments on SMPs for biomedical applications proves their wide applicability in the field of general medicine, drug delivery, regenerative medicine, dentistry, neuromedicine, cancer therapy, orthopedics, and corrosion protection. Herein, a comprehensive analysis of SMPs and their composites with focus on their types, mechanism, functionality and biomedical applications is presented. Relevant biomedical technologies based on SMPs and their future advancement towards biomedical applications are also discussed.

Fabrication of thermoplastic polyurethane/polylactide shape-memory blends with tunable optical and mechanical properties via a bilayer structure design

Herein, the shape-memory polymeric materials (SMPs) with tunable morphological structures were fabricated via a bilayer structure design. The bilayers containing pure thermoplastic polyurethane (TPU) and pure polylactide (PLA) and pure TPU and co-continuous TPU/PLA blend (PTB) were produced by hot laminating, which transformed the sea-island structure in conventional blend into special co-continuous structures without compositions changing. The shape fixity and shape recovery ratios dramatically increased from 77.0% to 82.9% of the blend to 99.4% and 83.4% of TPU/PTB bilayer and 99.4% and 89.1% of TPU/PLA bilayer, even larger than those of the co-continuous structured blend. The enhanced phase continuity along the deformation direction and the strong shearing effect between two layers optimized the temporary shape fixation and the permanent shape recovery. Particularly, the bilayer assembly endowed the TPU/PLA bilayer with optical transparence, while the dispersed PLA resulted in an opacity of the TPU/PTB bilayer and the conventional blend. In addition, the TPU/PLA bilayer realized balanced tensile strength and ductility via maximumly optimizing the synergistic effect of two components in mechanical properties. This work provides a promising method to fabricate transparent SMPs with balanced mechanical performances, which can be extended to other material combinations.

A bioinspired and hierarchically structured shape-memory material.

Shape-memory polymeric materials lack long-range molecular order that enables more controlled and efficient actuation mechanisms. Here, we develop a hierarchical structured keratin-based system that has long-range molecular order and shape-memory properties in response to hydration. We explore the metastable reconfiguration of the keratin secondary structure, the transition from α-helix to β-sheet, as an actuation mechanism to design a high-strength shape-memory material that is biocompatible and processable through fibre spinning and three-dimensional (3D) printing. We extract keratin protofibrils from animal hair and subject them to shear stress to induce their self-organization into a nematic phase, which recapitulates the native hierarchical organization of the protein. This self-assembly process can be tuned to create materials with desired anisotropic structuring and responsiveness. Our combination of bottom-up assembly and top-down manufacturing allows for the scalable fabrication of strong and hierarchically structured shape-memory fibres and 3D-printed scaffolds with potential applications in bioengineering and smart textiles.

Body Temperature-Triggered Shape-Memory Effect via Toughening Sustainable Poly(propylene carbonate) with Thermoplastic Polyurethane: toward Potential Application of Biomedical Stents

Shape-memory polymeric materials triggered by body temperature were fabricated via toughening sustainable poly(propylene carbonate) (PPC) with thermoplastic polyurethane (TPU). With an addition of TPU through melt blending, the ductility of PPC was dramatically enhanced, leading to the increase of shape recoverability but a deterioration of shape fixity. Remarkably, the blend containing 50 wt % TPU (PT50) presented the optimal shape-memory effect (SME) with balanced shape recovery and shape fixation performances because of the formation of the co-continuous structure promoting the synergy between PPC and TPU. Moreover, the PT50 sample exhibited significant improvement in not only the shape recovery ratio (∼95.0% recovery) but also the recovery speed and recovery stress, which enabled it to achieve an excellent SME when applied in practical use. After processed into a spiral-like stent, PT50 still showed a fast response to 37 °C, giving an efficient self-expansion within only 20 s. Besides, the blood and cell compatibility testing results revealed the good biocompatibility of PT50, further demonstrating the great potential of this material for development of biomedical stents.

Strategy for Fabricating Multiple-Shape-Memory Polymeric Materials via the Multilayer Assembly of Co-Continuous Blends.

Shape-memory polymeric materials containing alternating layers of thermoplastic polyurethane (TPU) and co-continuous poly(butylene succinate) (PBS)/polycaprolactone (PCL) blends (denoted SLBs) were fabricated through layer-multiplying coextrusion. Because there were two well-separated phase transitions caused by the melt of PCL and PBS, both the dual- and triple-shape-memory effects were discussed. Compared with the blending specimen with the same components, the TPU/SLB multilayer system with a multicontinuous structure and a plenty of layer interfaces was demonstrated to have higher shape fixity and recovery ability. When the number of layers reached 128, both the shape fixity and recovery ratios were beyond 95 and 85% in dual- and triple-shape-memory processes, respectively, which were difficult to be achieved through conventional melt-processing methods. On the basis of the classic viscoelastic theory, the parallel-assembled TPU and SLB layers capable of maintaining the same strain along the deforming direction were regarded to possess the maximum ability to fix temporary shapes and trigger them to recover back to original ones through the interfacial shearing effect. Accordingly, the present approach provided an efficient strategy for fabricating outstanding multiple-shape-memory polymers, which may exhibit a promising application in the fields of biomedical devices, sensors and actuators, and so forth.

Design and development of a hybrid bioartificial water-induced shape memory polymeric material as an integral component for the anastomosis of human hollow organs

A large number of pathologies require the resection of the bowel and anastomoses to rejoin the two remaining stumps to regain lumen patency. Various materials have been used to rejoin one bowel end to the other such as catgut, stainless steel, and absorbable sutures. The present method for anastomosis surgery uses an entero-entero anastomosis (EEA) circular stapler with only a staple line. This method can have some drawbacks, such as intracellular fluid leakage and local inflammations. The aim of this study is to design and develop a novel bioartificial polymer with a ring shape made of polyvinyl alcohol (PVA) and gelatin (80/20 ratio (w/w)) loaded both directly with acetylsalicylic acid and with nanoparticles incorporating the same drug to reduce local inflammation even for a prolonged period of time. A physical method (8cycles freezing/thawing) was used to obtain a crosslinked bioartificial shape memory ring.Mechanical analysis showed a storage modulus having a comparable value with that of the human colon. HPLC analysis pointed out a sustained and prolonged release of the anti-inflammatory drug both immediately after anastomosis surgery and during healing period. Cell culture tests indicated the cytocompatibility of the bioartificial device. A shape memory of the hydrogel prepared in ring form was observed at 37°C after immersion in water. These bioartificial devices can represent a new approach to serve as a multifunctional anastomotic ring.

Design of well-defined shape memory networks with high homogeneity: towards advanced shape memory polymeric materials

A well-defined shape memory network with high homogeneity was engineered via step-growth thiol-norbornene photopolymerization.

Shape memory polymeric materials based on aliphatic polyesters

Shape memory polymers (SMPs) have the capacity of memorizing a temporary shape and recovering their initial shape in response to various external stimuli. As one of promising class of intelligent materials, it has attracted great attention because of its potential applications in many areas, such as biomaterials and biotechnology, automotive industry, and aerospace field. Aliphatic polymers play an important role in the molecular design of SMPs owing to its excellent biodegradability and biocompatibility, and suitable thermal transition temperature range. This review focuses on the recent progress of shape memory polymers based on the aliphatic polyesters such as poly(lactic acid) (PLA), poly ( p -dioxanone) (PPDO), poly(butylene succinate) (PBS), poly( ω -pentadecalactone) (PPDL) and poly( ɛ -caprolactone) (PCL) and so on. From the view of architecture design, different systems are classified by block copolymers, crosslinking networks and interpenetrating networks. The influence of architectural features on the shape memory behaviors is also discussed. The block copolymers have been identified a typical mature system in design of dual-shape material, and the crystalline aliphatic polyesters can be served as the netpiont or the molecular switch in a SMP, depending on its transition temperature range among the binary systems. The thermoplastic feature of block copolymers endows an easy processibility. The various architectures of crosslinking network meet the increasing demand of SMPs design. Constructing network with crystallizable polyester segment such as PCL may achieve SMPs in one-way and two-way mode, and the stress-free two-way mode can also be realized by an elaborate architecture with pre-programed treatment. Embedding two segments in one network may achieve a triple-shape memory effect, meanwhile, incorporating two segments in an interpenetrating polymer network may result in a triple-shape material with improved performance. Recently, the SMPs based on dynamic networks become the new trend, which may combine the self-healing with shape memory effect in one system, and both dynamic covalent bond and non-covalent bond have been concerned. In order to fulfill the increasing requirement of application, the SMPs with multi-shape memory effect, multi-responsibility and multi-functionality are highly expected.

Biodegradable Shape Memory Polymeric Material from Epoxidized Soybean Oil and Polycaprolactone

This article deals with the synthesis of plant oil-based shape memory materials from epoxidized soybean oil (ESO) and polycaprolactone (PCL). PolyESO/PCLs were synthesized by an acid-catalyzed curing in the presence of PCL. During the reaction, PCL scarcely reacted with ESO and the crystallinity of the PCL component decreased to form a semi-interpenetrating network structure. The incorporation of the PCL components improved the maximum stress and strain at break of ESO-based network polymer. The polyESO/PCL was gradually degraded by Pseudomonas cepasia lipase. Furthermore, the polyESO/PCLs exhibited excellent shape memory properties, and the strain fixity depended on the feed ratio of ESO and PCL. The shape memory-recovery behaviors were repeatedly practicable. The resulting materials are expected to contribute to the development of biodegradable intelligent materials.

Rubber-like electrically conductive polymeric materials with shape memory

This paper presents a heating-responsive shape memory polymeric material, which is not only rubber-like at room temperature and above its shape recovery temperature, but also electrically conductive. This polymeric material is made of silicone, melting glue (MG), and carbon black (CB). The influence of volume fractions of MG and CB on the elasticity, electrical resistivity, and shape memory effect of the polymeric material is systematically investigated. The feasibility of Joule heating for shape recovery is experimentally demonstrated with an electric power of 31 V.

It Is the Outside That Counts: Chemical and Physical Control of Dynamic Surfaces

Materials capable of dynamically controlling surface chemistry and topography are highly desirable. We have designed a system that is uniquely able to remotely control the presented functionality and geometry at a given time by using a functionalizable shape memory material. This was accomplished by incorporating controlled amounts of an azide-containing monomer into a shape memory polymeric material. These materials are capable of physically changing surface geometry over a broad range of length scales from >1 mm to 100 nm. Using copper-assisted click chemistry, they can be functionalized with a variety of molecules to yield different surfaces. Combining these features gave materials that can change both the presented geometry and functionality at tunable transition temperatures.

Rubber-like shape memory polymeric materials with repeatable thermal-assisted healing function

A thermo-responsive shape memory polymeric material is reported, which is not only rubber-like from above to below its shape recovery temperature, but also has a repeatable thermal-assisted healing function. The mechanisms behind both features are identified. It is found that while micron-sized inclusions play a part in the shape memory effect (SME), tangled polymer chains contribute to the rubber-like phenomenon and repeatable thermal-assisted healing function atop the SME. This tangling effect enables a perfect combination of high elasticity, SME and repeatable thermal-assisted healing to be simultaneously achieved.

Studies on the cycling, processing and programming of an industrially applicable shape memory polymer Tecoflex® (or TFX EG 72D)

The present investigations were undertaken to find out whether and how often cycling, processing and programming can be repeated, whether repeated programming affects the one way effect and how much irreversible strain the shape memory polymeric material accumulates at a particular temperature. The effect was investigated in dependence of different stress levels, and the effect of both recovery temperature and recovery time was considered. As a model material the commercially and industrially applicable amorphous shape memory polymer Tecoflex® was examined and subjected to 50 programming cycles. Tecoflex® is characterized by a glass transition temperature, Tg, of 74 °C, above which it looses all its strength. During tensile testing at 20 °C (T < Tg), stresses a steady increase to 26 MPa as strains approached the rupture strain of 25%. It is observed that at 60 °C (T < Tg, but near Tg) the material can be strained to more than 2500% before rupture occurs while stresses slowly increase to values less than 0.3 MPa. It turns out that programming, cooling, unloading and heating to trigger the one way effect causes an increase of irreversible strain that is associated with a corresponding decrease of the intensity of the one way effect during the first thermomechanical cycles.