Thermoresponsive Structures Research Articles

Swift 4D printing of thermoresponsive shape-memory polymers using vat photopolymerization

Shape-memory polymers (SMPs) are smart materials that have gained significant attention in recent years owing to their widespread application in smart structures and devices. Digital light processing (DLP), a vat-photopolymerization-based technique, is a significantly faster technology for printing a complete layer in a single step. The current study reports a facile and fast method for the 3D printing of SMP-based smart structures using a DLP 3D printer and a customized resin. A liquid crystal (LC, RM257) was combined with the resin to introduce shape-memory properties. The combination of LCs in photocurable resin provides the opportunity to directly 3D-print thermoresponsive structures, avoiding the complexity of SMP resin preparation. The structures were printed with different geometries, and the shape-memory response was measured. Lattice structures were fabricated and programmed to obtain tunable mechanical properties. Furthermore, the strain-sensing response was measured to demonstrate the utility of these lattice structures as smart patches for joint-movement sensing. The SMPs can be prepared conveniently and can potentially be used for various applications, such as smart tools, toys, and meta-material sensors.

Digital light processing of poly(ε-caprolactone)-based resins into porous shape memory scaffolds

Digital light processing (DLP) is a promising 3D printing technique with excellent resolution, enabling the fabrication of complex structures to serve the tissue engineering (TE) field. However, its full potential in this regard remains insufficiently explored by the scarcity of ‘smart’ photoresponsive biodegradable and biocompatible polymers. By combining biodegradable shape memory polymers with DLP printing, complex thermo-responsive structures can be developed, allowing minimally invasive TE. The current study focuses on the development of biodegradable and biocompatible photosensitive resins consisting of poly(ε-caprolactone) acrylate-endcapped urethane-based polymers (AUPs) to create thermo-responsive porous scaffolds with a shape shift close to body temperature. The molar mass of the AUP along with the photo-initiator concentration are varied, enabling the investigation of the influence of these parameters on the resin properties and the DLP processability. The newly developed resins exhibit excellent properties in terms of viscosity, photocrosslinking kinetics, polymer fraction, and swelling capacity. Compression tests demonstrated that the porous scaffolds exhibit a compressive modulus around 75 kPa, making them suitable to serve a variety of soft TE applications. Additionally, the 3D printed scaffolds are biocompatible as indicated by a cell viability and metabolic activity exceeding 85%. Finally, the scaffolds’ shape memory properties are evidenced both qualitatively as quantitatively via Dynamic Mechanical Analysis (DMA) experiments. These experiments indicate a shape shift around body temperature and shape fixity and shape recovery values superior to 94%, rendering them excellent candidates for, among others, minimally invasive TE strategies.

Thermoresponsive Reflective Scattering of Meso-Scale Phase Separation Structures of Uniaxially Orientation-Ordered Liquid Crystals and Reactive Mesogens.

Polymer network liquid crystals (PNLCs) capable of thermoresponsive change in reflective scattering were fabricated using a self-organization technique called photopolymerization-induced phase separation. These PNLCs exhibit nonscattering states at temperatures τ below the nematic-to-isotropic (NI) phase transition temperature τNI but reflective scattering states at τ values above τNI. The magnitude of change of optical clarity is 80% and of solar transmittance is 20% in PNLCs with a thickness of 50 μm. The microscopic structures consist of wavelength- or meso-scale phase separation domains of liquid crystals (LCs) and polymerized reactive mesogens (RMs) in which cyanobiphenyl (CB) groups are thermoresponsively transformed between uniaxially orientation-co-ordered and disordered states. Such thermoresponsive structures were fabricated by employing the CB groups as mesogenic bodies, which were expected to mutually associate due to their physicochemical structures. Cross-linkers stabilized the meso-scale domains and made the PNLCs durable through repeated temperature changes. Polarizing optical microscopy (POM) and scanning electron microscopy showed meso-scale composites that reflectively scatter visible and near-infrared light. POM and Fourier-transform infrared spectroscopy at different temperatures suggest that the orientation order of the CB groups changes in the LC phase in response to temperature but remains ordered in the RM phase. Such a thermoresponsive change in the orientation order produces the switchability in meso-scale nonuniformity and consequently in reflective light scattering. The thermoresponsive PNLCs are not only effective as energy-saving smart windows but also advantageous at stages of manufacture, installation, and operation.

Modeling and characterization of shape memory properties and decays for 4D printed parts using stereolithography

The integration of shape memory materials into additive manufacturing has added a new dimension of time to conventional 3D printing and enabled innovative product designs with high tailorability and adaptability. To date, most studies on shape memory effects mainly adopt experimental approaches to characterize the material responsiveness to various stimulation conditions considering a single thermomechanical loading cycle. The information regarding the cyclic shape memory behaviors as well as the potential additive manufacturing-induced impacts on the achieved shape memory performance is limited. In this paper, the shape memory behaviors of the stereolithography printed thermo-responsive structures are theoretically modeled by jointly considering the influences from both the printing process and the shape memory process. The cyclic shape memory effects are analytically characterized and experimentally validated using methacrylate copolymers under iterative thermomechanical loadings. Meanwhile, case studies are presented to provide insights into shape memory behaviors upon the impacts of various levels of critical process parameters. The results indicate an exceptional prediction accuracy of 96.24% and 95.73% for the established shape fixity and recovery models, respectively. It is also observed that the printing process parameters, including layer thickness and scan speed, have considerable impacts on the shape memory performance of the printed parts.

Thermoresponsive Structure and Dye Encapsulation of Micelles Comprising Bolaamphiphilic Quercetin Polyglycoside.

A bolaamphiphile is a special member of amphiphilic molecules, which contains a hydrophobic skeleton and two water-soluble groups on both ends. Bolaamphiphiles form thermally stable associates in water under lower concentration than those of typical monoheaded amphiphiles, indicating the potential usefulness of bolaamphiphiles as the component of nanomaterials. However, the structural diversity of bolaamphiphiles is limited at this moment. We recently developed the synthesis of quercetin-3-O-polyglycoside (QP) as a new entry of bolaamphiphiles via a one-pot polymerization using sugar-based cyclic sulfite initiated by quercetin skeleton. Herein, we show the bolaamphiphilic properties of QP in detail. The micellization behaviors of QP are systematically investigated through comparison with those of quercetin (Que) and isoquercitrin (IQ) to evaluate the roles of glycone on the micellization of quercetin derivatives. The morphology of the micelles bearing QP is observed by cryo-transmission electron microscopy (cryo-TEM), suggesting the formation of bolaamphiphile-specific giant ribbon-like micelles in addition to spherical micelles. The thermoresponsive micellization behaviors are also discussed through the critical micelle concentration (CMC) values, the dynamic light scattering analyses at various temperatures, and thermal hysteresis of the micellizations. It is indicated that the polysaccharide chains integrated on the surface of micelles would serve as a steric protecting group to endow the micelles with kinetic stability. These results will shed light on natural glycoside skeletons to design a new class of micelles for advanced health applications in near future.

Directed Printing and Reconfiguration of Thermoresponsive Silica-pNIPAM Nanocomposites.

Printing of polymeric composites into desired patterns and shapes has revolutionized small-scale manufacturing processes. However, high-resolution printing of adaptive materials that change shape in response to external stimuli remains a significant technical challenge. The article presents a new approach of printing thermoresponsive poly(N-isopropylacrylamide) into macroscopic structures that dynamically reconfigure in response to heating and cooling cycles. The printing process is performed using an external laser source, which enables thermal cross-linking of the polymer ink consisting of monomer, cross-linker, initiator, and inorganic nanoparticles. It is shown that the addition of silica nanoparticles enhances the mechanical properties of poly(N-isopropylacrylamide) while maintaining its thermoresponsiveness at micrometer-scale resolution, which otherwise is not feasible by extrusion-based three-dimensional printing techniques. It is demonstrated that spatial reconfiguration of the printed monolayers upon increasing temperature is governed by the local geometry, which enables mimicking the reconfiguration of plant leaves in a natural environment. The study lays a foundation for developing a new fabrication platform to print thermoresponsive structures that may find applications in biomedical implants, sensors, and other multi-responsive materials.

Impact of polypeptide sequence on thermal properties for diblock, random, and alternating copolymers containing a stoichiometric mixture of glycine and valine

This paper describes the experimental elucidation for the impact of polypeptide sequence on the solution structure and the thermo-responsivity, having considered the corresponding theoretical prediction using a binary pattern of hydrophobic and hydrophilic units. We prepare diblock, random, and alternating copolymers consisting of a stoichiometric amount of valine as a hydrophobic unit and glycine as a hydrophilic unit. Through the thermo-responsivity measurements of aqueous polymer solutions, it is found that the UV spectra of diblock copolypeptide exhibit a cloud point upon heating, which indicate that the polymer adopts a random coil structure at low temperature. On the other hand, the alternating peptide shows an upper critical solution temperature (UCST) behavior indicating a globule-to-coil transition upon heating. The opposite behavior of alternating peptide to that of diblock copolypeptide is found to be dependent on not the chirality of repeating units but the alternating binary pattern of hydrophobic and hydrophilic units. The random copolypeptide exhibits not only coil-to-globule transition at low temperature but also globule-to-coil transition at high temperature owing to the presence of both segregated and alternating polymer sequences in the polymer. The thermo-responsive structures of the polymers in an aqueous media are also discussed by determining the thermodynamic parameters for the transitions.

RNA thermometers are common in α- and γ-proteobacteria

Expression of many rhizobial small heat-shock genes is controlled by the ROSE element, a thermoresponsive structure in the 5'-untranslated region of the corresponding mRNAs. Using a bioinformatics approach, we found more than 20 new potential ROSE-like RNA thermometers upstream of small heat-shock genes in a wide variety of alpha- and gamma-proteobacteria. Northern blot analyses revealed heat-inducible transcripts of the representative candidate Caulobacter crescentus CC2258, Escherichia coli ibpA and Salmonella typhimurium ibpA genes. Typical sigma(32)-type promoters were mapped upstream of the potential RNA thermometers by primer extension. Additional translational control was demonstrated in a lacZ reporter system and by site-directed mutagenesis. RNA secondary structure predictions strongly suggest that the Shine-Dalgarno sequence in the RNA thermometers is masked at low temperatures. Combining two regulatory modules, a sigma(32) promoter and a ROSE-type RNA thermometer, provides a novel stringent mechanism to control expression of small heat-shock genes.

Temperature-controlled Structural Alterations of an RNA Thermometer

Thermoresponsive structures in the 5'-untranslated region of mRNA are known to control translation of heat shock and virulence genes. Expression of many rhizobial heat shock genes is regulated by a conserved sequence element called ROSE for repression of heat shock gene expression. This cis-acting, untranslated mRNA is thought to prevent ribosome access at low temperature through an extended secondary structure, which partially melts when the temperature rises. We show here by a series of in vivo and in vitro approaches that ROSE is a sensitive thermometer responding in the physiologically relevant temperature range between 30 and 40 degrees C. Point mutations predicted to disrupt base pairing enhanced expression at 30 degrees C. Compensatory mutations restored repression, emphasizing the importance of secondary structures in the sensory RNA. Only moderate inducibility of a 5'-truncated ROSE variant suggests that interactions between individual stem loops coordinate temperature sensing. In the presence of a complementary oligonucleotide, the functionally important stem loop of ROSE was rendered susceptible to RNase H treatment at heat shock temperatures. Since major structural rearrangements were not observed during UV and CD spectroscopy, subtle structural changes involving the Shine-Dalgarno sequence are proposed to mediate translational control. Temperature perception by the sensory RNA is an ordered process that most likely occurs without the aid of accessory factors.

Electrophysiological responses to thermal stimuli in peripheral nerves of the snail Helix pomatia L.

1. In Helix pomatia single units of the inner lip nerves and the pedal nerves were recorded during thermal stimulation of the lip region and the epithelium of the sole, respectively. 2. 13.3% of all single units recorded from the inner lip nerves and 11.7% of the pedal nerve fibres proved to be sensitive to thermal stimuli. 3. These results suggest that peripheral thermoresponsive structures contribute to the control of behavior leading to avoidance of unfavorable ambient temperatures.