Shape Memory Polymer Material Research Articles

Regulation of Glass Transition Temperature in Thermo‐Responsive Epoxy Shape Memory Polymers: The Roles of Chemical Crosslinking Density and Chain Flexibility

ABSTRACTShape memory polymers (SMPs) are novel kinds of smart materials whose shape can be changed in external conditions such as heat stimulus. Due to their excellent structural versatility and high elastic strain, they have important applications in the field of biomaterials. Here, a series of amorphous shape memory polymer materials are prepared and their shape memory properties and glass transition temperature (Tg) are also investigated. A special kind of epoxy polymer, whose shape can be changed at around physiological temperature, is synthesized by regulating the crosslinking density and introducing flexible aliphatic epoxy chains. The shape memory function of this epoxy polymer is studied by deformation test. In addition, to explore their application in the field of biomedical materials, the biocompatibility of the materials are evaluated, including the effects on the erythrocyte hemolysis rate and cell activity. The experimental results show that this kind of epoxy polymer has good shape memory function and biocompatibility.

Design of thermally programmable 3D shape memory polymer-based devices tailored for endovascular treatment of intracranial aneurysms

Despite recent technological advancements in endovascular embolization devices for treating intracranial aneurysms (ICAs), incomplete occlusion and aneurysm recanalization remain critical challenges. Shape memory polymer (SMP)-based devices, which can be manufactured and tailored to patient-specific aneurysm geometries, possess the potential to overcome the suboptimal treatment outcome of the gold standard: endovascular coiling. In this work, we propose a highly porous patient-specific SMP embolic device fabricated via 3D printing to optimize aneurysm occlusion, and thus, improve the long-term efficacy of endovascular treatment. To facilitate device deployment at the aneurysm via Joule-heating, we introduce a stable, homogeneous coating of poly-pyrrole (PPy) to enhance the electrical conductivity in the SMP material. Using an in-house pulse width modulation circuit, we induced Joule-heating and characterized the shape recovery of the PPy-coated SMP embolic devices. We found that the employed PPy coating enables enhanced electrical and thermal conductivity while only slightly altering the glass transition temperature of the SMP material. Additionally, from a series of parametric studies, we identified the combination of catalyst concentration and pyrrole polymerization time that yielded the shape recovery properties ideal for ICA endovascular therapy. Collectively, these findings highlight a promising material coating for a future coil-free, personalized shape memory polymer (SMP) embolic device, designed to achieve long-lasting, complete occlusion of aneurysms.

Ag-S coordination strategy for high recovery driving stress in recyclable shape memory polymers

To address the issue of low strength and recovery stress in recyclable shape memory polymer (SMP) materials with dynamic covalent bonds in practical applications, we propose a method based on Ag-S coordination interactions to improve dynamic covalent bonds, thereby preparing recyclable SMP with high recovery stress and large recovery strain. The introduction of in situ reduced silver nanoparticles (AgNPs) in the SMP system play multiple roles, serving as additional physical crosslinking points at the microscale and coordinating with disulfide bonds in the system to construct a multi-level dynamic network structure. This endows the material with exceptional mechanical properties (tensile strength of 25.93 MPa and fracture strain of 1466.83 %) and recovery driving stress (5.59 MPa), while maintaining more than 99 % shape recovery and fixity rate. Furthermore, due to the outstanding photothermal performance of AgNPs, the composite material has acquired remote photothermal driving capabilities. The proposed Ag-S coordination enhancement offers a new approach for SMP materials that achieve both large actuation distance and high recovery stress, holding significant potential for applications in soft robotics and actuators.

Investigation of thermomechanical behaviors of acrylate-based shape memory polymers by molecular dynamics simulation

Shape memory polymers (SMPs) have received a lot of attention lately due to their potential use as actuators in various applications, including robotics and biomedical device. SMPs are a better choice than other smart materials due to their remarkable ability to recover from significant deformations and their ability to be stimulated remotely. Here, we investigate the mechanical and recovery behaviors of SMPs composed of acrylate-based monomers and crosslinkers with varying crosslink densities, using full-atomistic molecular dynamics (MD) simulations. Physical parameters, such as density and glass transition temperature, are determined for systems with different crosslinking densities. Tensile loading is applied to characterize the mechanical characteristics and shape memory behaviors. Furthermore, the shape-memory behavior of the system with highest crosslinking density is investigated to understand the effects of recovery temperature and deformation temperature on shape recovery. Our findings shed light on the critical structural elements that determine shape recovery ability, giving insights for advanced SMP material design and development. This study adds to our fundamental understanding of SMP behavior and has ramifications for a wide range of technological applications.

Fabrication of Shape Memory Polymer Endovascular Thrombectomy Device for Treating Ischemic Stroke.

Stroke is the second result for death and ischemic stroke constitutes most of all stroke cases. Ischemic stroke takes place when blood clot or embolus blocks cerebral vessel and interrupts blood flow, which often leads to brain damage, permanent disability, or death. There is a 4.5-h (golden hour) treatment window to restore blood flow prior to permanent neurological impairment results. Current stroke treatments consist mechanical system or thrombolytic drug therapy to disrupt or dissolve thrombus. Promising method for stroke treatment is mechanical retrieving of thrombi employing device deployed endovascularly. Advent of smart materials has led to research fabrication of several minimally invasive endovascular devices that take advantage of new materials capabilities. One of these capabilities is shape memory, is capability of material to store temporary form, then activate to primary shape as subjected to stimuli. Shape memory polymers (SMPs) are employed as good materials for thrombectomy device fabrication. Therefore, current review presents thrombectomy device development and fabrication with SMPs. Design, performance, limitations, and in vitro or in vivo clinical results of SMP-based thrombectomy devices are identified. Review also sheds light on SMP's future outlook and recommendations for thrombectomy device application, opening a new era for advanced materials in materials science.

Multiscale Transfer Printing via Shape Memory Polymer with High Adhesion and Modulus Switchability.

Flexible electronics have gained significant attention as an innovative solution to meet the growing need for information collection from the human body and the environment. However, a critical challenge lies in the need for a transfer printing technique that can fabricate rigid and brittle devices on flexible organic substrates. Here, we develop a multiscale transfer printing technique using an ultraviolet-curable shape memory polymer (SMP) that serves as both the stamp and the receiver substrate. The SMP demonstrates exceptional mechanical performance with toughness at room temperature and remarkable flexibility near its glass transition temperature. The SMP material exhibits an impressive shape recovery ratio and remarkable adhesion switchability. We demonstrate robust transfer printing of diverse objects with different materials and morphologies and in situ transfer of multiscale metallic structures. In addition, the in situ fabricated transparent hyperthermia patches with embedded metal grids are demonstrated, offering potential application in the field of sensors, wearable devices, and electronic skin.

Analysis of mechanical and shape memory properties of biocompatible shape memory polymers with different Fe3O4 contents

Shape memory polymers (SMPs) can recall their original shape when exposed to environmental factors like heat, light, or magnetic fields. These SMPs are extensively used in medical applications, notably in stents, due to their shape memory and biocompatibility after implantation in the human body. However, conventional stents require balloons for expansion, limiting their flexibility. To address this, the study developed an SMP material that can regain its original shape without the need for a balloon. To effectively induce self-response, magnetically responsive Fe3O4 nanoparticles at concentrations of 10%, 15%, and 20% were integrated, and their dispersion was verified using scanning electron microscopy. The study also utilized differential scanning calorimetry to measure the glass transition temperature, crystallization temperature, and melting point of the synthesized polymers. The confirmation of uniformly dispersed nanoparticles revealed that the synthesized polymers exhibited a high shape memory ratio (84%–93%). Shape memory ratio represents the ability to return from a temporarily deformed state to the initial, original shape. Therefore, the biocompatible, magnetic-responsive SMP developed in this study has the potential to be utilized in various medical devices as an advanced shape-memory material.

Estimation of constitutive parameters for shape memory polymer thermo-mechanical cycle by an integrated simulation and inverse optimization method

In this work, a method for inverse optimization of shape memory polymer material parameters is proposed to determine the intrinsic model parameters more quickly and accurately for simulated experimental test analysis. First, the basic mechanical properties of the material are characterized experimentally and the parameters of the generalized Maxwell model based on SMP material are fitted. Then, a joint simulation inversion optimization method is built in the ISIGHT-MATLAB-ABAQUS integrated environment to optimize the time domain Prony series parameters using downhill simplex optimization algorithm. The results show that the average errors of the stress curves before and after the optimization of the cooling solid phase and the experimental results are 0.31 MPA and 0.23 MPA, respectively, and the average errors of the strain curves in the warming recovery phase are 1.70% and 0.40%, respectively. After the inversion optimization, the average errors of the stress curves are optimized by 25.8% and the average errors of the strain curves are optimized by 76.5%. This method makes the simulation results more stable with the experimental results, which can effectively reduce the experimental cost and promote the shape memory property of SMP to be widely used in engineering.

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.

Effects of iron oxide and carbon nanotube contents on mechanical properties and shape memory behavior of biocompatible and biodegradable shape memory polymers

AbstractThe shape memory polymers (SMPs) is a potential alternative material for conventional shape memory alloys (SMAs) in various fields. Previous studies have focused on the application of SMPs materials as an alternative to SMAs. However, considering problems such as the poor mechanical properties, its application has been limited. This study developed a biodegradable, magnetically responsive SMPs material that can be used for medical implants with shape memory behavior under magnetic force and improved mechanical properties by adding nanoparticles. Moreover, effect of the added nanoparticle was investigated due to contents ratio. Nanocomposite was synthesized with polycaprolactone which has shape memory properties, Fe3O4 that generates heat under magnetic field, and carbon nanotubes with reinforcing effect. The morphology analysis confirmed the homogeneous dispersion of the Fe3O4/Carbon‐nanotubes (CNTs) in polymer matrix. The tensile test shows that tensile strength (14.9–17.6 MPa) increased when Fe3O4 (0–20 wt%) was included and that it did not increase significantly when CNTs (0–0.8 wt%) were included. The shape memory test result confirmed that developed composite had a shape memory under a magnetic field. It was confirmed that even if Fe3O4 was incorporated into polycaprolactone, did not lose its exothermic properties under magnetic field. The developed polymer showed high shape memory ratio (91%–98%) in all the nanoparticle content rates. With regard to biodegradability, weight loss occurred over time in the phosphate‐buffered saline solution. Therefore, the biocompatible, biodegradable, magnetic response shape memory polymer developed in this study might applied to various medical implant as advanced shape memory materials.

Effects of recycling on polystyrene shape memory polymers for in-situ resource utilization

One-way, thermoplastic shape memory polymers (SMPs) used as actuators for self-folding origami are typically single-use materials that would be decommissioned upon completion of mission objectives. As a result, there exists an abundance of unutilized, single-use SMP waste. In-situ resource utilization (ISRU) and recycling offer solutions for the use and integration of sustainable SMP material infrastructures on Earth and for long-duration space missions. Unfortunately, mechanical recycling causes degradation of material properties. Therefore, it is imperative to quantify the effects of recycling on SMP properties. Herein, we utilize a thermo-mechanical recycling method applied to polystyrene SMPs. After recycling, we conduct Fourier transform infrared spectroscopy, differential scanning calorimetry, and dynamic mechanical analysis to investigate changes to the chemical structure, viscoelastic properties, and shape recovery response of the polymer. The results indicate negligible changes to the viscoelastic and shape recovery properties of the recycled material from one to six recycling sequences (extrusions) when compared to the non-recycled material. The most evident form of deterioration occurred in the physical appearance of the material. Otherwise, the shape recovery performance and thermo-mechanical properties remained consistent after recycling. Therefore, the recovery characteristics (recovery ratio, recovery time, and actuation stress) do not change significantly after six recycling sequences, making this material viable for ISRU applications in space environments.

4D printing of PLA/PCL composites with tunable shape memory properties via printing and programming parameters

4D printing of biocompatible shape memory polymer materials is of great significance in the biomedical field. The addition of PCL material into PLA matrix is capable of modulating the elastic modulus of the material while remaining a reduced degradation rate. However, the influence of 4D printing parameters and shape programming parameters on their shape memory properties had not been systematically investigated. Here, we examine the effects of 4D printing process parameters and shape programming parameters on the shape memory properties of PLA/PCL composites. We first explore the effects of key printing parameters on the print quality and shape memory properties of pure PLA materials, and select composites with a PCL mass fraction of 15% to fully characterize their thermodynamic properties. The effects of programming stress and programming temperature on the shape memory properties of the PCL15 composites are analyzed. Finally, the artificial neural network algorithm is employed to optimize its programming parameters, and brings 5.76% enhancement of shape recovery ratio compared with that before optimization. This work provides a systematic study of the shape memory properties of 4D printed PLA/PCL composites, paving the way for promoting their application in the biomedical industry.

Adjustable indentation and vibration isolation performances of nacre-like metamaterial

ABSTRACT Along with the living environment, organisms have evolved structures that adapt to specific environments and have better mechanical properties. Bioinspired materials learn from nature and improve their mechanical properties by imitating the structure of living organisms. Based on the 4D printed shape memory polymer and the bioinspired design method, this research proposes a soft and hard phase hybrid bioinspired metamaterial with shape memory effect and programmable mechanical properties. Compared with traditional nacre-like materials, bioinspired materials have adjustable characteristics of mechanical properties, impact resistance, and low-frequency vibration isolation. First, based on the constitutive relation of SMP (Shape memory polymer) material and its numerical simulation, an intelligent bioinspired metamaterial is designed. Subsequently, the mechanical properties and vibration isolation behavior and adjustability performance of multi-scale bioinspired metamaterials are explained by experiments. Finally, the adjustable functional mechanism of the deformation and vibration isolation of the bioinspired metamaterial is described. The research of these bioinspired metamaterials has broad application prospects in the fields of impact protection and low-frequency vibration absorption.

A Modified Constitutive Model for Shape Memory Polymers Based on Nonlinear Thermo-Visco-Hyperelasticity with Application to Multi-Physics Problems

Shape memory polymers (SMPs) as a class of smart material have provided diverse attributes recently used in different applications. Raising the usage of SMPs, especially in more sensitive environments, such as the human body or similar high-risk circumstances, highlights the necessity of more accurate simulations. Suitable constitutive modeling is the foundation of an accurate simulation. Not only should such modeling consider precise details to diminish errors, but also it must provide a robust and powerful procedure to calibrate the material parameters. To achieve these goals, in this paper a modified constitutive model for SMPs based on the concept of internal state variables and rational thermodynamics is proposed in large deformation. Taking its basis from the nonlinear hyperelasticity and viscoelasticity, the model can provide a more accurate prediction of SMPs response. In comparison to other available constitutive models based on viscoelastic approach, the number of material parameters is smaller. Furthermore, performing a new approach for material parameters extraction, two different SMP materials were calibrated. The proposed model’s capability was assessed by comparing the model outputs with experimental results in diverse conditions such as different temperature rates and applied stretch ratios. The user-friendly implementation process of this model in multi-physics software based on the finite element method can be counted as another advantage of the proposed model. Hence, to simulate smart systems containing SMP elements, three multi-physics analyses in various fields and conditions were performed, and the importance of conducting such multi-physics phenomena has been discussed.

3D Printing photo-induced lignin nanotubes/polyurethane shape memory composite

The excellent flexibility, biocompatibility, and shape memory properties of polyurethane (PU) have attracted great attention to fabricate devices for application in the fields of electronics and biomedical, etc. However, it still consists a challenge to fabricate shape memory polymer (SMP) materials with complicated shapes in response to remote stimuli precisely and instantly. In this paper, we achieved ultraviolet light responsive SMP composites by direct ink writing (DIW) 3D printing shape memory polyurethane (SMPU) with introduction of the laboratory prepared lignin nanotube (LNT) as UV absorbent. We prepared lignin nanotube with uniform morphology by dissolving and then controllable molecular assembling way during dialysis in deionized water. The 3D printed SMP composite of PU/LNT was found to reach the shape recovery rate of 99% under UV radiation (370 nm) for 15 min. In addition, the tensile strength of the 3D printed SMP composite of PU/LNT is increased to 16.4 MPa, which is 131% higher than that of the pristine PU (7.11 MPa). It is worth noting that the glass transition temperature (Tg) of the 3D printed SMP composite of PU/LNT changed steadily from 47 °C to 33 °C by increasing the LNT content to 9%, which endows the composites with much wider application feasibility. The exploration on photo responsive 3D printed SMP composites with accurate shape recovery rate provides new candidates to improve the development of biomaterials and intelligent robots.

3D fabrication of Shape-Memory polymer networks based on coumarin Photo-Dimerization

Photo-crosslinked shape-memory polymer (SMP) materials were obtained by the copolymerization of methacrylates bearing 2-ethylhexyl plasticizing group (EHMA) and photoactive coumarin unit (CoumMA). The photo-dimerization of coumarin units ensured the crosslinking of P(EHMA-co-CoumMA) and further allowed the fabrication by stereolithography of well-defined butterfly with ribbed wings to illustrate the potential of promising coumarin chemistry in additive fabrication. The influence of dye ratio on the optical and the mechanical properties of the transparent P(EHMA-co-CoumMA) films as well as the shape memory properties of the films in unfolding and twisting deformations were detailed and demonstrate the opportunity of coumarin-bearing photoprintable polymers for 4D printing.

Programmable, biodegradable composite scaffolds with variable pore morphology for minimal invasive bone repair

Tissue engineered biocompatible scaffolds could mimic the natural bone structure for cell adhesion and proliferation, however, patients suffer from large volume implantation. In this study, a porous composite biocompatible bone scaffold based on shape memory polymer materials (SMP) was developed from poly(ε-caprolactone) (PCL) diol, hexamethylene diisocyanate (HDI) and hydroxyapatite (HA). The prepared shape memory composite scaffold presented good pore connectivity, high porosity, good mechanical and biological properties. Additionally, after programming to a temporary shape, the scaffolds could recover to the initial shape, filling the site of bone defect. Moreover, HA improved the mechanical properties of scaffolds, promoted cell adhesion and influencing the shape recovering speed. During in vitro mineralization, HA could promote the formation and deposition of bone-like hydroxyapatite and accelerate the repair of damaged tissues, no inflammatory problem occurred after the in vivo implantations. These programmable scaffolds could be applied in minimal invasive bone repair.

Programmable micro-transfer-printing for heterogeneous material integration

Micro-transfer printing is an emerging assembly technique to deterministically transfer ink comprising micro-/nano-objects from a donor substrate to a receiver substrate. This technique has generated increasing interest over the last decade due to its unique capability of integrating diverse heterogeneous materials into various structures and layouts. The growing number of applications enabled by micro-transfer printing includes flexible electronics, sensors, photovoltaics, and micro-LED displays. This work presents a micro-transfer printing approach, which relies on thermally induced adhesion modulation of shape memory polymer materials. An individually addressable micro-fabricated resistive heater array is used to locally deliver the heat for transfer pixel actuation. Selectively transferring chips from a closely packed microchip donor array (pitch: 100 µm, size: 50 × 50 µm2) is demonstrated. The transfer head can be dynamically configured to assemble micro-objects in arbitrary patterns, allowing digital manufacturing, object sorting, or in-line assembly correction of defects. These capabilities, together with the simple and robust structure of the transfer head, can enable high process scalability and flexibility for heterogeneous material integration.

Shape memory, thermal conductivity, and mechanical property of polylactic acid and natural rubber composites reinforced by an inorganic thermal conductive network

AbstractWith the development and application of thermotropic shape memory polymer materials in the biomedical field, it is very crucial for these materials to improve their thermal conductivity efficiency and shape memory recovery rate by endowing them a high thermal conductivity. In our work, a PLA/NR/T‐ZnOw/GR shape memory composite was prepared by melt blending with tetrapod zinc oxide whisker (T‐ZnOw) and graphene (GR) as the double inorganic fillers and polylactic acid (PLA) and natural rubber (NR) as the polymer matrix. The synergism of T‐ZnOw and GR constructs a three‐dimensional (3D) thermal conductive network, making the thermal conductivity of PLA/NR/5wt%T‐ZnOw/2.5wt%GR increase to 1.215 W∙(m∙K)−1. The addition of T‐ZnOw reduces the agglomeration of GR in the PLA/NR matrix and the interfacial thermal resistance between the fillers and the matrix. It also improves the heat transfer rate, resulting in an excellent shape memory of PLA/NR/T‐ZnOw/GR with a morphology recovery rate of ca. 96%, a fixed rate of 95% and high thermal stability. The presented biodegradable shape memory composite with the 3D thermal conductive network is promising in the fields of medical treatment and intelligent sensors.

4D structures for the short-time building of emergency shelters

The increasing number of internal displaced people due to conflicts or natural disasters over the years represents the need to create structural solutions for first emergency shelter, which are currently supplied in tarpaulin and structures improvised by the locals. Innovative technologies like 4D printing could present an alternative, being possible to develop 3D structures with smart materials that react with environmental stimulus, such as water, light, heat and others. Unlike 3D printing, which is static, the factor ‘time’ adds a dimension to the 4D, making the structures self-assembling, multifunctional or self-repairing. Given this, the aim of this paper is to present research for the application of 4D-printed structures, for crises, to speed up and ease the assembly process, replacing the structures currently used with one that integrates the emergency kit. It also presents a review on emergency shelters, as well as on 3D and 4D printing processes, clarifying its definition and introducing a case study demonstrative of the process, by printing with shape memory filaments, specifically poly lactic acid and shape memory polymers. A concept was developed and validated, evaluating the possibility of applying 4D printing process in the development of structures that are easy to transport and assemble in emergencies, using the beneficial characteristics of shape memory polymer material. Therefore, the simulation of the concept and the improvement of material properties in terms of specific strength and toughness, which can be achieved using composite materials, are indicated as future recommendations. Moreover, the concept must be evaluated from a structural point of view.