applications such as cell scaffolds, soft tissue substitutes and bioactuators. [ 1,3 ] However, their load-bearing applications are often limited by the poor mechanical performances. [ 4,5 ] Conventional hydrogels do not exhibit high mechanical properties because of uneven crosslinking and weak interaction among the chains. Recently, several high mechanical hydrogels have been developed and investigated as potential soft tissue replacements, [ 6,7 ] but few of them exhibit a combination of high mechanical properties including stiffness, toughness, tensile and compressive strengths, anti-fatigue as well as mechanical recoverability. [ 8,9 ] The well-known slide-ring [ 10 ] and tetra-PEG [ 11 ] hydrogels are designed to have ideally homogeneous networks to eliminate the intrinsic defects to a maximum extent, eventually leading to the enhanced mechanical properties. Other strategies are advanced to increase the functionalities [ 12 ] between two crosslink points to toughen the hydrogels, such as inorganic nanocomposite hydrogels, [13][14][15] macromolecular microsphere composite hydrogels, [ 16 ] graphene [ 17,18 ] or its oxide [ 19 ] composite hydrogels and micro- [ 20 ] or nano-structure [ 21 ] hydrogels. All of these gels have high compressive strength and large elongation, but their tensile strength (ranging from 190 kPa to 600 kPa) and modulus are not satisfi ed enough; furthermore, their high elastic properties show no hysteresis behavior, thus lacking a mechanism for mechanical energy dissipating. As a result, the mechanical properties are observably reduced when the gels contain a defect. [ 9 ] In order to solve these problems, a variety of mechanical energy dissipation mechanisms are implemented into the network to toughen the hydrogels. For instance, the PAMPS/PAAm double network (DN) gels can sacrifi ce the rigid chemical bond of the fi rst network to dissipate energy to achieve MPa magnitude order of tensile and compressive stress but the fatigue resistance is low, and the preparation process is relatively complicated. [22][23][24][25] Suo and his co-workers [ 9 ] reported synthesis of a hydrogel with high stretchability and astonishing fracture energies of 9,000 J/m 2 by forming interpenetrating polymer networks of covalently crosslinked PAAm and ionically crosslinked zipper-like High strength hydrogels were previously constructed based on dipole-dipole and hydrogen bonding reinforcement. In spite of the high tensile and compressive strengths achieved, the fracture energy of the hydrogels strengthened with sole noncovalent bondings was rather low due to the lack in energy dissipating mechanism. In this study, combined dipole-dipole and hydrogen bonding interactions reinforced (DHIR) hydrogels are synthesized by onestep copolymerization of three feature monomers, namely acrylonitrile (AN, dipole monomer), acrylamide (AAm, H-bonding monomer), and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS, anionic monomer) in the presence of PEGDA575, a hydrophilic crosslinker. The electrostatic repulsion from PAM...

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Inspired by stimuli-responsive remarkable changes in consistency (hardening, softening, autolysis) of sea cucumbers, we synthesized a supramolecular polymer(SP) hydrogel directly by photoinitiated aqueous polymerization of N-acryloyl 2-glycine monomer bearing one amide and one carboxyl group on the side chain. The SP hydrogels doped with Ca(2+) demonstrated excellent mechanical properties-high tensile strength (∼1.3 MPa), large stretchability (up to 2300%), high compressive strength (∼10.8 MPa), and good toughness (∼1000 J m(-2)) due to cooperative hydrogen bonding interactions from amide and carboxyl together with Ca(2+) cross-linking. Responding to the change in pH and Ca(2+) concentration, the hydrogels could modulate their network stability and mechanical properties: at pH3.0 and higher Ca(2+) content, the hydrogel formed low swelling network which was stiff and stable; in alkaline or neutral buffer with lower content of or without Ca(2+), the hydrogel formed a highly swollen transient network, which was soft and eventually autolyzed. The reversible multiple noncovalent bonds enabled the hydrogels to achieve thermoplasticity, self-healability, and reusability. Notably, distinct formulations of hydrogels could be welded together under heating to form a gradient hydrogel. In vitro cytotoxicity assay and subcutaneous implantation indicated that the SP hydrogels were biocompatible and autolytic in vivo. The SP hydrogels may find applications as temporary biodevices for intestinal drug delivery or for injectable filling in assisting suturing small vessels.

Nanomaterials that integrate functions of imaging and gene delivery have been of great interest due to their potential use in simultaneous diagnosis and therapy. Herein, polycation-b-polysulfobetaine block copolymer, poly[2-(dimethylamino) ethyl methacrylate]-b-poly[N-(3-(methacryloylamino) propyl)-N,N-dimethyl-N-(3-sulfopropyl) ammonium hydroxide] (PDMAEMA-b-PMPDSAH) grafted luminescent carbon dots (CDs) were prepared via surface-initiated atom transfer radical polymerization (ATRP) and investigated as a multifunctional gene delivery system (denoted as CD-PDMA-PMPD) in which the CD cores acted as good multicolor cell imaging probes, the cationic PDMAEMA acted as a DNA condensing agent, and the outer shell of zwitterionic PMPDSAH block protected the vector against nonspecific interactions with serum components. As revealed by the fluorescent spectrum study, the photoluminescent attributes, especially the tunable emission property, were well inherited from the parent CDs. The CD-PDMA-PMPD could condense plasmid DNA into nanospheres with sizes of approximate 50 nm at a proper complex ratio, posing little cytotoxicity at higher ratios. It was shown that the hybrid vector exhibited significantly suppressed BSA protein adsorption and superior hemocompatibility compared to those of the widely used PEI25k. In the in vitro transfection assay, an increased serum concentration from 10 to 50% caused a dramatic drop in PEI25k transfection performance, whereas the transfection efficiency of CD-PDMA-PMPD was well maintained; CD-PDMA80-PMPD40 showed 13 and 28 times higher transfection efficiencies than PEI25k at 30 and 50% serum concentration, respectively. Intriguingly, the carbon dots in the transfected cells displayed excitation-dependent fluorescent emissions, portending that this polycation-polyzwitterion modified CD will be a promising theranostic vector with excellent stealth performance.

In this study, ion-responsive hydrogen bonding strengthened hydrogels (termed as PVV) were synthesized by one-pot copolymerization of 2-vinyl-4,6-diamino-1,3,5-triazine (VDT), 1-vinylimidazole (VI), and polyethylene glycol diacrylate. The diaminotriazine-diaminotriazine (DAT-DAT) H-bonding interaction and copolymerization of VI contributed to a notable increase in comprehensive performances including tensile/compressive strength, elasticity, modulus, and fracture energy. In addition, introducing mM levels of zinc ions could further increase the mechanical properties of PVV hydrogels and fix a variety of temporary shapes due to the strong coordination of zinc with imidazole. The release of zinc ions from the hydrogel contributed to an antibacterial effect, without compromising the shape memory effect. Remarkably, a multiwalled hydrogel tube (MWHT) fixed with Zn(2+) demonstrated much higher flexural strengths and a more sustainable release of zinc ions than the solid hydrogel cylinder (SHC). A Zn(2+)-fixed MWHT was implanted subcutaneously in rats, and it was found that the Zn(2+)-fixed MWHT exhibited anti-inflammatory and wound healing efficacies. The reported high strength hydrogel with integrated functions holds potential as a tissue engineering scaffold.

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A hydrogen-bonded and calcium ion crosslinked hydrogel, termed as PVDT-PAA, was synthesized by one-step photo-polymerization of 2-vinyl-4,6-diamino-1,3,5-triazine (VDT), acrylic acid (AA), and polyethylene glycol diacrylate (PEGDA, Mn=4,000). Combined physical crosslinkings from inter-diaminotriazine and coordination of Ca 2+ with carboxyls contributed to a significant enhancement in the mechanical properties of PVDT-PAA hydrogels. Furthermore, reversible Ca 2+ crosslinking imparted shape memory function to the hydrogel which were able to firmly memorize multiform shapes and return to the initial state in response to Ca 2+ . Interestingly, the PVDT-PAA hydrogels with weaker H-bonding interaction demonstrated a sharp volume change phenomenon induced by Ca 2+ . This volume change could be utilized to trigger unharmful cell detachment from hydrogel surface supposedly due to Ca 2+ -induced marked variation of 2 mechanotransduction between cells and substrate interface. This H-bonding and ion-crosslinking strategy opens a new opportunity for designing and constructing multifunctional high strength hydrogels for the biomedical applications. Graphic AbstractDiaminotriazine hydrogen bonding reinforced and Ca 2+ -crosslinked high strength shape memory hydrogels are fabricated. Ca 2+_ induced dramatic volume shrinkage is utilized to trigger the unharmful cell detachment.

In the present study, high-strength photoresponsive hydrogels were prepared by the photoinitiated copolymerization of acrylamide (AAm, hydrophilic hydrogen bonding monomer), 2-vinyl-4,6-diamino-1,3,5-triazine (VDT, hydrophobic hydrogen bonding monomer), and spiropyran-containing monomer (SPAA) in the presence of cross-linker poly(ethylene glycol) diacrylate (PEG575DA, Mn = 575). The double hydrogen bondings from AAm-AAm and diaminotriazine-diaminotriazine contributed to the considerable enhancement in tensile and compressive properties of the hydrogels, which showed an excellent ability to resist a variety of external forces. Fifteen minutes of UV (365 nm) irradiation led to the detachment of adhered cells due to the increased surface hydrophilicity caused by the isomerization of spiropyran moieties. Furthermore, repeated attachment/detachment of cells was realized by the alternate illumination of visible and UV light. Reverse gene transfection was carried out successfully by anchoring the PVDT/pDNA complex nanoparticles on the gel surface through hydrogen bonding between diaminotriazine motifs prior to cell seeding. Importantly, fibronectin (FN) modification combined with supplementing PVDT/pDNA complex nanoparticles after the first cycle of reverse gene transfection, so-called sandwich gene transfection, further increased the gene transfection level. A short time of UV light exposure could result in the nonharmful detachment of gene-modified cells from the gel surface. This high-strength photosensitive hydrogel holds potential as a reusable soft-wet platform for cell harvesting as well as gene transfection operation at higher efficiency.