The industrial and biomedical applications of hydrogels made from either natural or synthetic sources are strongly limited by their poor mechanical properties. A normal structure (NS) hydrogel breaks under low stress because there are very few energy dissipation mechanisms to slow crack propagation. In addition, as their crosslinking points are distributed irregularly and the polymer chains between the crosslinking points have different lengths, the stress cannot be evenly distributed between the polymer chains, and crack initiation is facile. Many efforts have been focused on increasing the mechanical strength of hydrogels, [1] but the robustness still remains unsatisfactory. In recent years, three kinds of novel hydrogels with unique structures and high mechanical strength have been developed. [2] Topological (TP) gels have figure-ofeight crosslinkers that can slide along polymer chains.[3] The gel swells to about 500 times its original weight and can be stretched to nearly 20 times its original length. The nanocomposite (NC) hydrogel is made from specific polymers with a water-swellable inorganic clay. [4] Most of the macromolecules are grafted onto nanoparticles, indicating that the nanoparticle clay acts as a highly multifunctional crosslinking agent. We believe that the high mechanical strength of this material has its origin in the very high functionality of the rigid crosslinked points and the lack of short chains between crosslinked components, as every active chain has to stretch between nanoparticles. The extension degree of a chain before breakage is controlled by the relationship between its relaxed end-to-end distance and its contour length, which is low for short chains. When a short chain in an NS hydrogel breaks, its load is thrown onto just one or two other adjacent chains, which dramatically increases their load. Hence, multiple chain fractures occur, causing voids and microcracks. However, in an NC hydrogel with large, rigid crosslinking points, the load from a single broken chain will be spread over many other chains, and the material is less likely to form the microcracks and voids responsible for initiating bulk failure. Gong et al. have reported a new method of obtaining strong and tough hydrogels by making double-network (DN) materials with a high molar ratio of the second network to the first network. In this case, the first network is highly crosslinked and the second network is loosely crosslinked.[5] These DN hydrogels demonstrate extremely high mechanical strength. By adding a third component to a DN gel, either a weakly crosslinked network or noncrosslinked linear chains, gels with high-strength and low-frictional coefficients were obtained. [6] Macromolecular microspheres (MMSs) have become an important structure in polymeric materials. The hydrogel microspheres on the micro-or nanoscale are known as microgels or nanogels, respectively. They are usually environmentally sensitive and are mainly used in drug delivery and other biomedical applications. [7] However, it is difficult to...

Shape memory hydrogels have promising applications in a wide variety of fields. Here we report the facile fabrication of a novel type of shape memory hydrogels physically cross-linked with both stronger and weaker hydrogen bonding (H-bonding). Strong multiple H-bonding formed between poly(vinyl alcohol) (PVA) and tannic acid (TA) leads to their coagulation when they are physically mixed at an elevated temperature and easy gelation at room temperature. The amorphous structure and strong H-bonding endow the PVA-TA hydrogels with excellent mechanical properties, as indicated by their high tensile strengths (up to 2.88 MPa) and high elongations (up to 1100%). The stronger H-bonding between PVA and TA functions as the "permanent" cross-link and the weaker H-bonding between PVA chains as the "temporary" cross-link. The reversible breakage and formation of the weaker H-bonding imparts the PVA-TA hydrogels with excellent temperature-responsive shape memory. Wet and dried hydrogel samples with a deformed or elongated shape can recover to their original shapes when immersed in 60 °C water in a few seconds or at 125 °C in about 2.5 min, respectively.

SUMMARYAnthocyanin pigmentation is an important consumer trait in peach (Prunus persica). In this study, the genetic basis of the blood-flesh trait was investigated using the cultivar Dahongpao, which shows high levels of cyanidin-3-glucoside in the mesocarp. Elevation of anthocyanin levels in the flesh was correlated with the expression of an R2R3 MYB transcription factor, PpMYB10.1. However, PpMYB10.1 did not co-segregate with the blood-flesh trait. The blood-flesh trait was mapped to a 200-kb interval on peach linkage group (LG) 5. Within this interval, a gene encoding a NAC domain transcription factor (TF) was found to be highly up-regulated in blood-fleshed peaches when compared with non-red-fleshed peaches. This NAC TF, designated BLOOD (BL), acts as a heterodimer with PpNAC1 which shows high levels of expression in fruit at late developmental stages. We show that the heterodimer of BL and PpNAC1 can activate the transcription of PpMYB10.1, resulting in anthocyanin pigmentation in tobacco. Furthermore, silencing the BL gene reduces anthocyanin pigmentation in blood-fleshed peaches. The transactivation activity of the BL-PpNAC1 heterodimer is repressed by a SQUAMOSA promoter-binding protein-like TF, PpSPL1. Low levels of PpMYB10.1 expression in fruit at early developmental stages is probably attributable to lower levels of expression of PpNAC1 plus the presence of high levels of repressors such as PpSPL1. We present a mechanism whereby BL is the key gene for the blood-flesh trait in peach via its activation of PpMYB10.1 in maturing fruit. Partner TFs such as basic helix-loop-helix proteins and NAC1 are required, as is the removal of transcriptional repressors.

Functionalized graphene has been considered as one of the most important materials for preparing polymer nanocomposites due to its unique physical structure and properties. To increase the interfacial interaction between polymer component and graphene oxide (GO) sheets, in situ grafting polymerization initiated by a free radical initiator immobilized on GO sheets is a better choice. We report a facile and effective strategy for preparing graphene peroxide (GPO) via the radiation-induced peroxidation of GO. The formation of peroxides on GO is proven by iodometric measurement and other characterizations. Using GPO as a polyfunctional initiating and cross-linking center, we obtained GO composite hydrogels exhibiting excellent mechanical properties, namely, very high tensile strength (0.2-1.2 MPa), extremely high elongations (2000-5300%), and excellent resilience. This work provides new insight into the fabrication of GO/polymer nanocomposites to fulfill the excellent mechanical properties of graphene.

Anisotropic tough hydrogels are of great importance in biomedical fields. Tough poly(vinyl alcohol) (PVA) hydrogels with anisotropic porous structure and mechanical properties are obtained with a facile directional freezing-thawing (DFT) technique. The PVA gels have an aligned porous structure, with long aligned channels in the direction parallel to the freezing direction and pores with similar sizes in the perpendicular direction. The degree of crystallinity of the freeze-dried PVA hydrogels increases with number of DFT cycles, and it can reach 55.8%. The PVA hydrogels have excellent mechanical properties, as exhibited by the high tensile strengths (0.3-1.2 MPa), medium moduli (0.03-0.10 MPa) and high fracture energies (160-420 J m À2 ) of the gels with solid contents of 10-12%. More importantly, the gels exhibit significant anisotropy in the mechanical properties, and their tensile strengths, moduli and fracture energies are higher in the perpendicular direction than those in the parallel direction. Anisotropic mechanical behaviors can also be found in the cyclic tensile tests of the PVA hydrogels. The anisotropic mechanical properties of the DFT PVA hydrogels could be attributed to the oriented arrangement of crystalline regions along the direction perpendicular to the direction of freezing.

Super-strong and tough poly(vinyl alcohol)/poly(acrylic acid) hydrogels based on hydrogen bonding are prepared by the strategy of immersing and cold-drawing.

Novel hydrogels with excellent mechanical properties have prompted applications in biomedical and other fields. The reported tough hydrogels are usually fabricated by complicated chemical and/or physical methods. To develop more facile fabrication methods is very important for the practical applications of tough hydrogels. We report a very simple yet novel method for fabricating tough hydrogels that are totally physically cross-linked by cooperative hydrogen bonding between a pre-existing polymer and an in situ polymerized polymer. In this work, tough hydrogels are prepared by heating aqueous acrylamide (AAm) solution in the presence of poly(Nvinylpyrrolidone) (PVP) but without any chemical initiators or covalent bonding cross-linking agents. Mechanical tests of the asprepared and swollen PVP-in situ-PAAm hydrogels show that they exhibit very high tensile strengths, high tensile extensibility, high compressive strengths, and low moduli. Comparative synthesis experiments, DSC characterization, and molecular modeling indicate that the formation of strong cooperative hydrogen bonding between the pre-existing PVP and the in situ formed PAAm chains contributes to the gel formation and the toughening of the hydrogels. The unique microstructure of the gels with evenly distributed flexible cross-linking sites and long polymer chains attached to them endow the hydrogels with an excellent mechanism of distributing the applied load.

Convenient patterning and precisely programmable shape deformations are crucial for the practical applications of shape deformable hydrogels. Here, a facile and versatile computer‐assisted ion inkjet printing technique is described that enables the direct printing of batched, very complicated patterns, especially those with well‐defined, programmable variation in cross‐linking densities, on one or both surfaces of a large‐sized hydrogel sample. A mechanically strong hydrogel containing poly(sodium acrylate) is first prepared, and then digital patterns are printed onto the hydrogel surfaces by using a commercial inkjet printer and an aqueous ferric solution. The complexation between the polyelectrolyte and ferric ions increases the cross‐linking density of the printed regions, and hence the gel sample can undergo shape deformation upon swelling/deswelling. The deformation rates and degrees of the hydrogels can be conveniently adjusted by changing the printing times or the different/gradient grayscale distribution of designed patterns. By printing appropriate patterns on one or both surfaces of the hydrogel sheets, many complex 3D shapes are obtained from shape deformations upon swelling/deswelling, such as cylindrical shell and forsythia flower (patterns on one surface), ding (patterns on both surfaces), blooming flower (different/gradient grayscale distributive patterns on one surface), and non‐Euclidean plates (different/gradient grayscale distributive patterns on both surfaces).