No abstract

This study demonstrates the preparation of dual cross-linked hydrogels capable of selective, spontaneous de-cross-linking behavior triggered by head-to-tail depolymerization. A primary covalent structure of the materials was established on a thermoresponsive network containing pendant functional moieties that could induce noncovalent interaction when combined with a macro-cross-linker. The incorporation of the macro-cross-linker not only reinforced the entire structure while providing secondary physical cross-linking but also caused the resulting materials to show autonomous responses via depolymerization. As a result, transformation of the material was achieved without structural collapse, as the noncovalent network in the materials was rapidly, selectively, and completely removed through the depolymerization reaction of the polymer cross-linker when initiated by a trace (<0.01 wt %) stimulus. Thus, the macroscopic changes in the physical and chemical properties of the hydrogels were investigated. Furthermore, we used the proposed strategy to design actuating materials that exhibit reversible, programmed, large-scale behavior and finally demonstrated re-cross-linking by the addition of an extra macro-cross-linker after the de-cross-linking reaction.

Ag 2 Te is well-known as a silver ion conductor. In this compound, a phase transition occurs at around 420 K and silver ions jump to interstitial sites repeatedly at the point of starting the phase transition. We consider that the active movement of silver ions would have influence on the scatterings of both charge carriers and heat carrying phonons in Ag 2 Te. In order to evaluate the effect of the silver ion conduction on the thermoelectric properties of Ag 2 Te, the Seebeck coefficient, electrical resistivity, thermal conductivity and Hall coefficient of polycrystalline bulk samples of Ag 2 Te were examined in the temperature range from room temperature to 650 K. The electrical resistivity and the Seebeck coefficient dramatically changed at around 420 K due to the change of the carrier concentration before and after the phase transition. However, the carrier mobility and the lattice thermal conductivity showed no remarkable change at around the phase transition temperature. These results imply that the thermoelectric transport properties of Ag 2 Te were affected by the change of the crystal structure rather than the presence or absence of the movement of silver ions.

This study has demonstrated the design of stimuli-responsive double-network hydrogels that are formed by sequential polymerization and show chemical transformation by selective de-cross-linking without structural failure owing to chemical orthogonality. Each self-immolative and thermoresponsive network established together the double-network structure through a thiol−ene click reaction and radical polymerization. The hydrogel exhibited enhanced mechanical strength but chemically transformed through the selective de-cross-linking of specific network triggered by a molecular stimulus, which significantly alters physical properties of the material such as tunable toughness and lower critical solution temperature behavior. In addition, the material displayed a thermoresponsive, controlled release. Only after treatment with the stimulus did the hydrogel release cargo molecules on demand via de-cross-linking while maintaining the entire structure.

Over the past few decades, the photoacoustic (PA) effect has been widely investigated, opening up diverse applications, such as photoacoustic spectroscopy, estimation of chemical energies, or point-of-care detection. Notably, photoacoustic imaging (PAI) has also been developed and has recently received considerable attention in bio-related or clinical imaging fields, as it now facilitates an imaging platform in the near-infrared (NIR) region by taking advantage of the significant advancement of exogenous imaging agents. The NIR PAI platform now paves the way for high-resolution, deep-tissue imaging, which is imperative for contemporary theragnosis, a combination of precise diagnosis and well-timed therapy. This review reports the recent progress on NIR PAI modality, as well as semiconducting contrast agents, and outlines the trend in current NIR imaging and provides further direction for the prospective development of PAI systems.

Superabsorbent hydrogels are significant not only in materials science but also in industries and daily life, being used in diapers or soil conditioners as typical examples. The main feature of these materials is their capacity to hold considerable amount of water, which is strongly dependent on the cross-linking density. This study focuses on the preparation of hydrogels by reweighing the effect of cross-linking density on physical properties, which provides green fabrication of bilayered hydrogels that consist of homogeneous structural motifs but show programmed responses via sequential radical polymerization. In particular, when two hydrogel layers containing different cross-linking densities are joined together, an integrated linear bilayer shows heterogeneous deformation triggered by water. We monitor the linear hydrogel bilayer bending into a circle and engineer it by incorporating disperse dyes, changing colors as well as physical properties. In addition, we demonstrate an electric circuit switch using a patterned hydrogel. Anisotropic shape change of the polyelectrolyte switch closes an open circuit and lights a light-emitting diode in red. This proposed fabrication and engineering can be expanded to other superabsorbent systems and create smart responses in cross-linked systems for biomedical or environmental applications.

A sustainable biobased thermoset exhibiting shape-memory behavior and modular recycling capabilities has been developed herein. The prepared thermoset consists of naringenin and biocompatible polymer components. Naringenin, which has three phenolic moieties, has been converted to a multifunctional monomer containing glycidyl groups and readily formed a thermosetting network via epoxide ring opening reaction with a poly­(ethylene glycol) diacid under solvent-free conditions. The resulting material is malleable yet as strong as articular cartilage and selectively absorbs water when compared with n-dodecane oil. Moreover, the thermoset can be physically reused. After being crumpled, stretched, or coiled, the initial shape of the material is restored in response to heat or water. Furthermore, the material is amenable to chemical recycling in a bulk state via transesterification, and its components can be recovered on a molecular level after degradation under benign conditions, as was confirmed using a model compound.

Over the past twenty years, photoacoustics—also called optoacoustics—have been widely investigated and, in particular, extensively applied in biomedical imaging as an emerging modality. Photoacoustic imaging (PAI) detects an ultrasound wave that is generated via photoexcitation and thermoelastic expansion by a short nanosecond laser pulse, which significantly reduces light and acoustic scattering, more than in other typical optical imaging and renders high-resolution tomographic images with preserving high absorption contrast with deep penetration depth. In addition, PAI provides anatomical and physiological parameters in non-invasive manner. Over the past two decades, this technique has been remarkably developed in the sense of instrumentation and contrast agent materials. In this review, we briefly introduce state-of-the-art multiscale imaging systems and summarize recent progress on exogenous bio-compatible and -degradable agents that address biomedical application and clinical practice.