Stimuli-responsive gels that exhibit a volume change in response to an environmental change have attracted much attention as smart soft materials for sensors, actuators, drug delivery systems and other applications. We proposed a novel strategy for designing molecularly stimuli-responsive gels, “molecule-responsive gels”, which undergo changes in the volume in response to a target molecule. The strategy utilizes molecular complexes as dynamic crosslinks that reversibly associate and dissociate in response to a target molecule. Two types of molecule-responsive gels have been developed using various molecular complexes as dynamic crosslinks: “biomolecule-crosslinked gel” and “molecule-imprinted gel.” The biomolecule-crosslinked gels swell and molecule-imprinted gels shrink in the presence of a target molecule because their crosslinking densities change by dissociation and association of molecular complexes as dynamic crosslinks. This review focuses on molecule-responsive gels that show swelling/shrinking changes in response to a target molecule such as glucose, antigen, tumor marker and endcrine disrupting chemicals. Furthermore, molecule-responsive gel nanoparticles, thin layers, micro-valves for various applications such as DDS, sensor and microfluidics are described in the review.

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Physical properties of polymer gels are determined by various structural parameters. Although molecular models predicting these properties have been proposed, the systematical understanding is inhibited by inevitable heterogeneity of polymer network structure. Recently, we successfully developed the Tetra-PEG gel with homogeneous network structure and well-controlled network structures. In this review, we introduce our recent studies on the dependence of the structural parameters (connectivity and polymer concentration) on the elastic modulus using the Tetra-PEG gel as a model system. Elastic moduli of the polymer gels prepared around the overlapping concentration is predicted by the phantom network model. The model predicting the elastic modulus shifts from the phantom to the affine network models with an increase in the preparation concentration. Based on the experimental results of the tearing test, uniaxial stretching and swelling test, the elasticity of the Tetra-PEG gel is mainly governed by the chemical crosslinks, and the fluctuation of crosslinks is suppressed increasing the concentration. Conventional molecular models considering the effect of the entanglement, however, cannot predict these results. Further modification of the existing molecular models are desired.

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Several years passed since 3-D printers attracted people’s attention as a new tool that provides the designed 3-D objects with the modeling method suitable for variable materials. The factors behind this rapid growth of 3-D printer movement are the reduction of development cost due to the expired patent about the fused deposition modeling (FDM), the rise of free 3-D modeling software, and sharing of the information about 3-D printers through the internet. Now we can design and manufacture goods personally, that is, individually. On the other hand, we have been originally developing the 3-D gel printer named SWIM-ER in different background from common 3-D printers at the same time. In this article, we will consider the advantages of gels in comparison with hard materials such as a plastic, metal and ceramic used in common 3-D printers, discuss the outlook of 3-D printed gel model utilizing its characteristics, and introduce the appearance of tough hydrogel as one of the reasons why we started to develop the SWIM-ER. Finally, we would like to show our progress of gel ink for the SWIM-ER and the 3-D gel model shaped in the form of a finger. We are convinced that the SWIM-ER will be a novel tool that promotes the next manufacturing.

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The field of metal-organic frameworks (MOFs) has been burgeoning in the past decade, which consist of metal ion as a node and organic ligand as a frame to form an infinite periodic structure with intrinsic micropore. Combination of metal ions and organic ligands can provide a plethora of MOFs with various sizes, shapes, or properties of the micropore. This prominent feature of MOFs has prompted many chemists and physicists to explore MOFs in diverse research fields, especially in catalysts and adsorbents. Recently, several examples of new type of network polymers have been emerged by the aid of MOFs. In this review, we focus on the development of such new type of network polymers, including “hybrid networks” of organic polymer and MOF.

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Biomarkers are important indicators for various diseases. The development of easy method for detecting biomarkers is required for preservation and early diagnosis. In this context, we developed a biomarker detection system using supramolecular gels. Generally, it is very difficult to recognize a specific biomarker by supramolecular assemblies. To realize the selective sensing of a specific biomarker, we hybridized supramolecular gels with enzymes. The enzymes embedded in the supramolecular gels could work and induce the decomposition of gel fibers and gel－sol transition, in response to a specific biomarker. Furthermore, encapsulation of the signal amplification system for improvement of detection sensitivity in the gels allows us to detect biomarkers in human blood plasma at the diagnostic concentration level.

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