AB-polymer networks based on oligo( -caprolactone) segments showing shape-memory properties

Development of tissue-engineered tracheal scaffold with refined mechanical properties and vascularisation for tracheal regeneration

Introduction: Attempted tracheal replacement efforts thus far have had very little success. Major limiting factors have been the inability to efficiently re-vascularise and mimic the mechanical properties of native tissue. The major objective of this study was to optimise a previously developed collagen-hyaluronic acid scaffold (CHyA-B), which has shown to facilitate the growth of respiratory cells in distinct regions, as a potential tracheal replacement device. Methods: A biodegradable thermoplastic polymer was 3D-printed into different designs and underwent multi-modal mechanical assessment. The 3D-printed constructs were incorporated into the CHyA-B scaffolds and subjected to in vitro and ex vivo vascularisation. Results: The polymeric backbone provided sufficient strength to the CHyA-B scaffold, with yield loads of 1.31-5.17 N/mm and flexural moduli of 0.13-0.26 MPa. Angiogenic growth factor release (VEGF and bFGF) and angiogenic gene upregulation (KDR, TEK-2 and ANG-1) was detected in composite scaffolds and remained sustainable up to 14 days. Confocal microscopy and histological sectioning confirmed the presence of infiltrating blood vessel throughout composite scaffolds both in vitro and ex vivo. Discussion: By addressing both the mechanical and physiological requirements of tracheal scaffolds, this work has begun to pave the way for a new therapeutic option for large tracheal defects.

Universal Strain Energy-Mediated Dynamic Porosity in Physically Networked Elastomers and Their Applications

Mechanical and physical properties of porous polymers are highly dependent on the arrangement of their internal pores, which once synthesized are widely considered static. However, here we introduce an unconventional dynamic porosity strategy in physically networked elastomer polymers, irrespective of their chemistry. This strategy allows for an omnidirectional and reversible reconfiguration of porosity in response to applied mechanical deformations, even at ambient conditions. In particular, the normal contact pressure between human fingers (just 0.62 MPa) applied on thin elastomer films results in a permanent reversion of the pores to a denser and more solid state. The porous-to-solid transition leads to a 3 order of magnitude reduction in pore density and up to a 22% relative volumetric shrinkage of the films, resulting in an opaque-to-transparent transition (OTT) that acts as a visual indication of porosity state (porous vs nonporous). It is shown that the pore reversion pressure onset is dependent on the average pore-to-pore distance that is controllable through process-specific parameters. Furthermore, the porosity transition is reversible for multiple cycles when the through-plane compression activation is coupled with an in-plane stretch (ε = 700%). A strain energy-mediated thermodynamic model is successfully implemented to confirm the effects of mechanical deformations on pore reversion and generation. Finally, applications of the newfound dynamic porosity concept are exploited for pressure indication, on-demand modulation of materials' mechanical and thermal characteristics, and flexible photomasks.

Self-Healing of Electrical Damage in Polymers

Polymers are widely used as dielectric components and electrical insulations in modern electronic devices and power systems in the industrial sector, transportation, and large appliances, among others, where electrical damage of the materials is one of the major factors threatening the reliability and service lifetime. Self-healing dielectric polymers, an emerging category of materials capable of recovering dielectric and insulating properties after electrical damage, are of promise to address this issue. This paper aims at summarizing the recent progress in the design and synthesis of self-healing dielectric polymers. The current understanding to the process of electrical degradation and damage in dielectric polymers is first introduced and the critical requirements in the self-healing of electrical damage are proposed. Then the feasibility of using self-healing strategies designed for repairing mechanical damage in the healing of electrical damage is evaluated, based on which the challenges and bottleneck issues are pointed out. The emerging self-healing methods specifically designed for healing electrical damage are highlighted and some useful mechanisms for developing novel self-healing dielectric polymers are proposed. It is concluded by providing a brief outlook and some potential directions in the future development toward practical applications in electronics and the electric power industry.

Sequence-Rearranged Cocrystalline Polymer Network with Shape Reconfigurability and Tunable Switching Temperature

Switching temperature (T_sw) is a key parameter governing the service condition of shape memory polymers (SMPs). However, tuning T_sw of SMPs often requires sophisticated synthesis or intricate processing. Herein, we report a simple yet effective strategy to prepare the SMPs with tunable T_sw and good reconfigurability by using the cocrystalline polyesters as the reversible phase. The cocrystallizable copolyesters with rearranged sequences were prepared by the transesterification of mixed polyester diols and then photo-cross-linked to achieve the SMP networks. Cocrystallization of copolymer blocks endows the SMP networks tunable melting point and relatively high crystallinity, affording the network good shape fixing and recovery ability at body temperature. Besides, the dynamic nature of transesterification, that enables the network to have good shape reconfigurability, allows for the easy processing of SMPs with complicated shapes. The reconfigurable SMPs capable of actuating at the body temperature show great potential for use as biomedical devices.

Temperature-Controlled Orientation of Proteins on Temperature-Responsive Grafted Polymer Brushes: Poly(butyl methacrylate) vs Poly(butyl acrylate): Morphology, Wetting, and Protein Adsorption

Poly( n-butyl methacrylate) (PBMA) or poly( n-butyl acrylate) (PBA)-grafted brush coatings attached to glass were successfully prepared using atom-transfer radical polymerization "from the surface". The thicknesses and composition of the PBMA and PBA coatings were examined using ellipsometry and time-of-flight secondary ion mass spectrometry (ToF-SIMS), respectively. For PBMA, the glass-transition temperature constitutes a range close to the physiological limit, which is in contrast to PBA, where the glass-transition temperature is around -55 °C. Atomic force microscopy studies at different temperatures suggest a strong morphological transformation for PBMA coatings, in contrast to PBA, where such essential changes in the surface morphology are absent. Besides, for PBMA coatings, protein adsorption depicts a strong temperature dependence. The combination of bovine serum albumin and anti-IgG structure analysis with the principal component analysis of ToF-SIMS spectra revealed a different orientation of proteins adsorbed to PBMA coatings at different temperatures. In addition, the biological activity of anti-IgG molecules adsorbed at different temperatures was evaluated through tracing the specific binding with goat IgG.

Biodegradable Shape Memory Polymers in Medicine

Shape memory materials have emerged as an important class of materials in medicine due to their ability to change shape in response to a specific stimulus, enabling the simplification of medical procedures, use of minimally invasive techniques, and access to new treatment modalities. Shape memory polymers, in particular, are well suited for such applications given their excellent shape memory performance, tunable materials properties, minimal toxicity, and potential for biodegradation and resorption. This review provides an overview of biodegradable shape memory polymers that have been used in medical applications. The majority of biodegradable shape memory polymers are based on thermally responsive polyesters or polymers that contain hydrolyzable ester linkages. These materials have been targeted for use in applications pertaining to embolization, drug delivery, stents, tissue engineering, and wound closure. The development of biodegradable shape memory polymers with unique properties or responsiveness to novel stimuli has the potential to facilitate the optimization and development of new medical applications.

Two-year performance study of porous, thermoset, shape memory polyurethanes intended for vascular medical devices

The long-term shape-recovery behavior of shape memory polymers has often been shown to be dependent on the length of time the material has been stored in the secondary shape. Typically, recovery performance and shape fixity will decrease with increased time in the secondary shape. In medical materials, a shelf-life is crucial to establish as it sets the upper threshold for device performance in a clinical setting, and a reduction in shape recovery would limit the development of SMP medical devices. Here, we present a two-year study of strain recovery, strain fixity, and shape recovery kinetics for passively and actively actuated SMPs intended for vascular devices. While kinetic experiments using immersion DMA indicate slight material relaxation and a decrease in the time to recovery, these changes are not found for bulk recovery experiments. The results indicate that a two-year shelf-life for these SMPs is very reasonable, as there is no change in the recovery kinetics, strain recovery, or strain fixity associated with this aging time. Further, a thermal accelerated aging test is presented for more rapid testing of the shape memory behavior of these SMPs and is compared with the real time aging results, indicating that this test is a reasonable indicator of the two-year behavior.

Preparation, Characterization, and Mechanism for Biodegradable and Biocompatible Polyurethane Shape Memory Elastomers

Thermally induced shape memory is an attractive feature of certain functional materials. Among the shape memory polymers, shape memory elastomers (SMEs) especially those with biodegradability have great potential in the biomedical field. In this study, we prepared waterborne biodegradable polyurethane SME based on poly(ε-caprolactone) (PCL) oligodiol and poly(l-lactic acid) (PLLA) oligodiol as the mixed soft segments. The ratio of the soft segments in polyurethanes was optimized for shape memory behavior. The thermally induced shape memory mechanism of the series of polyurethanes was clarified using differential scanning calorimeter (DSC), X-ray diffraction (XRD), and small-angle X-ray scattering (SAXS). In particular, the in situ SAXS measurements combined with shape deformation processes were employed to examine the stretch-induced (oriented) crystalline structure of the polyurethanes and to elucidate the unique mechanism for shape memory properties. The polyurethane with optimized PLLA crystalline segments showed a diamond-shape two-dimensional SAXS pattern after being stretched, which gave rise to better shape fixing and shape recovery. The shape memory behavior was further tested in 37 °C water. The biodegradable polyurethane comprising 38 wt % PCL segments and 25 wt % PLLA segments and synthesized at a relatively lower temperature by the waterborne procedure showed ∼100% shape recovery in 37 °C water. The biodegradable polyurethane SME also demonstrated good endothelial cell viability as well as low platelet adhesion/activation. We conclude that the waterborne biodegradable polyurethane SME possesses a unique thermally induced shape memory mechanism and may have potential applications in making shape memory biodegradable stents or scaffolds.

Shape Memory Actuation of Janus Nanoparticles with Amphipathic Cross-Linked Network

Preparation of nanoscale Janus particles that can respond to external stimulation and, at the same time, be prepared using an easily achievable method presents a significant challenge. Here, we have demonstrated the shape memory of Janus nanoparticles (SMJNPs) with a multifunctional combination of Janus nanostructure and a shape memory effect, composed of a well-defined amphipathic sucrose-poly(ε-caprolactone) cross-linked network. A sudden negative pressure method was first used to prepare the Janus-shaped nanoparticles (temporary shape), which can switch their shape and wettability. The Janus-shaped nanoparticle is an amphipathic structure composed of hydrophilic and hydrophobic parts. Moreover, in response to temperature, the nanoparticle can recover their nanosphere state via a shape memory process. The novel Janus nanoparticles with the shape memory property also show a great potential for application such as drug delivery.

Multiresponsive Shape Memory Blends and Nanocomposites Based on Starch

Smart multiresponsive bionanocomposites with both humidity- and thermally activated shape-memory effects, based on blends of ethylene-vinyl acetate (EVA) and thermoplastic starch (TPS) are designed. Thermo- and humidity-mechanical cyclic experiments are performed in order to demonstrate the humidity- as well as thermally activated shape memory properties of the starch-based materials. In particular, the induced-crystallization is used in order to thermally activate the EVA shape memory response. The shape memory results of both blends and their nanocomposites reflect the excellent ability to both humidity- and thermally activated recover of the initial shape with values higher than 80 and 90%, respectively.

Shape-memory surfaces for cell mechanobiology

Shape-memory polymers (SMPs) are a new class of smart materials, which have the capability to change from a temporary shape 'A' to a memorized permanent shape 'B' upon application of an external stimulus. In recent years, SMPs have attracted much attention from basic and fundamental research to industrial and practical applications due to the cheap and efficient alternative to well-known metallic shape-memory alloys. Since the shape-memory effect in SMPs is not related to a specific material property of single polymers, the control of nanoarchitecture of polymer networks is particularly important for the smart functions of SMPs. Such nanoarchitectonic approaches have enabled us to further create shape-memory surfaces (SMSs) with tunable surface topography at nano scale. The present review aims to bring together the exciting design of SMSs and the ever-expanding range of their uses as tools to control cell functions. The goal for these endeavors is to mimic the surrounding mechanical cues of extracellular environments which have been considered as critical parameters in cell fate determination. The untapped potential of SMSs makes them one of the most exciting interfaces of materials science and cell mechanobiology.

Soft memory in a ferroelectric nanoparticle-doped liquid crystal

A small quantity of BaTiO(3) ferroelectric nanoparticles (FNP) was doped in a liquid crystal (LC), and the LC + FNP hybrid was found to exhibit a nonvolatile electromechanical memory effect in the isotropic phase. The permanent dipole moment of the FNPs causes the LC molecule to form short-range pseudonematic domains surrounding the FNPs. The FNP-induced short-range orders become more prominent in the isotropic phase when the global nematic order is absent. These short-range domains, being anisotropic in nature, interact with an external electric field, exhibiting a Fréedericksz-type transition. When the field is turned off, these domains stay oriented, showing a hysteresis effect due to the absence of any long-range order and restoring forces in the isotropic phase. The hysteresis graph for this memory effect shows a significant pretransitional behavior on approaching the nematic phase from the isotropic phase.

Design of super-elastic biodegradable scaffolds with longitudinally oriented microchannels and optimization of the channel size for Schwann cell migration

We newly designed super-elastic biodegradable scaffolds with longitudinally oriented microchannels for repair and regeneration of peripheral nerve defects. Four-armed poly(ε-caprolactone-co-D,L-lactide)s (P(CL-co-DLLA)s) were synthesized by ring-opening copolymerization of CL and DLLA from terminal hydroxyl groups of pentaerythritol, and acryloyl chloride was then reacted with the ends of the chains. The end-functionalized P(CL-co-DLLA) was crosslinked in a cylindrical mold in the presence of longitudinally oriented silica fibers as the templates, which were later dissolved by hydrofluoric acid. The elastic moduli of the crosslinked P(CL-co-DLLA)s were controlled between 10^-1 and 10² MPa at 37 °C, depending on the composition. The scaffolds could be elongated to 700% of their original size without fracture or damage ('super-elasticity'). Scanning electron microscopy images revealed that well-defined and highly aligned multiple channels consistent with the mold design were produced in the scaffolds. Owing to their elastic nature, the microchannels in the scaffolds did not collapse when they were bent to 90°. To evaluate the effect of the channel diameter on Schwann cell migration, microchannels were also fabricated in transparent poly(dimethylsiloxane), allowing observation of cell migration. The migration speed increased with channel size, but the Young's modulus of the scaffold decreased as the channel diameter increased. These findings may serve as the basis for designing tissue-engineering scaffolds for nerve regeneration and investigating the effects of the geometrical and dimensional properties on axonal outgrowth.

High performance shape memory polymer networks based on rigid nanoparticle cores

Smart materials that can respond to external stimuli are of widespread interest in biomedical science. Thermal-responsive shape memory polymers, a class of intelligent materials that can be fixed at a temporary shape below their transition temperature (T(trans)) and thermally triggered to resume their original shapes on demand, hold great potential as minimally invasive self-fitting tissue scaffolds or implants. The intrinsic mechanism for shape memory behavior of polymers is the freezing and activation of the long-range motion of polymer chain segments below and above T(trans), respectively. Both T(trans) and the extent of polymer chain participation in effective elastic deformation and recovery are determined by the network composition and structure, which are also defining factors for their mechanical properties, degradability, and bioactivities. Such complexity has made it extremely challenging to achieve the ideal combination of a T(trans) slightly above physiological temperature, rapid and complete recovery, and suitable mechanical and biological properties for clinical applications. Here we report a shape memory polymer network constructed from a polyhedral oligomeric silsesquioxane nanoparticle core functionalized with eight polyester arms. The cross-linked networks comprising this macromer possessed a gigapascal-storage modulus at body temperature and a T(trans) between 42 and 48 degrees C. The materials could stably hold their temporary shapes for > 1 year at room temperature and achieve full shape recovery <or= 51 degrees C in a matter of seconds. Their versatile structures allowed for tunable biodegradability and biofunctionalizability. These materials have tremendous promise for tissue engineering applications.

Dynamic chemical devices: modulation of contraction/extension molecular motion by coupled-ion binding/pH change-induced structural switching

Dynamic chemical devices involve morphological or constitutional modifications in molecular or supramolecular systems, induced by internal or external physical or chemical triggers. Reversible changes in shape result in molecular motions and define motional dynamic devices presenting mechanical-like actions of various types. Suitably designed polyheterocyclic strands such as compounds 1-5 wrap into helical conformations. The binding of lead(II) ions to the coordination subunits contained in the strand leads to complete uncoiling and yields a polymetallic complex presenting a fully extended shape. The addition of a cryptand complexing agent that strongly binds lead(II) ions and releases them under protonation allows a reversible pH-modulation of lead(II) levels in the medium, which in turn induces coiling/uncoiling of the molecular strand. This system thus represents a motional dynamic device which performs a mechano-chemical process, realizing alternating extension/contraction motions triggered by ion binding. It achieves a linear motor-type of action of very large stroke amplitude fueled by ionic processes.