Engineering Tough Supramolecular Hydrogels with Structured Micropillars for Tunable Wetting and Adhesion Properties.

Soft-lithography is widely used to fabricate microstructured surfaces on plastics and elastomers for designable physical properties such as wetting and adhesions. However, it remains a big challenge to construct high-aspect-ratio microstructures on the surface of hydrogels due to the difficulty in demolding from the gel with low strength and stiffness. Demonstrated here is the engineering of tough hydrogels by soft-lithography to form well-defined micropillars. The mechanical properties of poly(acrylamide-co-methacrylic acid) hydrogels with dense hydrogen-bond associations severely depend on temperature, with Young's modulus increasing from 8.1 MPa at 15 °C to 821.8 MPa at -30 °C, enabling easy demolding at low temperatures. Arrays of micropillars are maintained on the surface of the gel, and can be used at room temperature when the gel restores soft and stretchable. The hydrogel also exhibits good shape-memory property, favoring tailoring the morphology with a switchable tilt angle of micropillars. Consequently, the hydrogel shows tunable wetting and adhesion properties, as manifested by varying contact angles and adhesion strengths. These surface properties can also be tuned by geometry and arrangement of micropillars. This facile strategy by harnessing tunable viscoelasticity of supramolecular hydrogels should be applicable to other soft materials, and broaden their applications in biomedical and engineering fields.

Polymerization-Induced Crystallization of Dopant Molecules: An Efficient Strategy for Room-Temperature Phosphorescence of Hydrogels.

Conventional hydrogels such as polyacrylamide and polyacrylic acid ones seldom exhibit phosphorescences at ambient conditions, which limit their applications as optical materials. We propose and demonstrate here a facile strategy to afford these hydrogels with room-temperature phosphorescence by polymerization-induced crystallization of dopant molecules that results in segregation and confinement of the gel matrix with carbonyl groups and thus clusterization-induced phosphorescence. As a model system, crown ethers (CEs) are dissolved in an aqueous solution of concentrated acrylamide that greatly increases the solubility of CEs. During the polymerization process, CEs crystallize to form large spherulites in the polyacrylamide hydrogel. The crystallization arises from the drastically reduced solubility of CEs after the conversion of monomers to polymers during the gel synthesis. The resultant composite hydrogel with a water content of 67 wt % exhibits extraordinary phosphorescence behavior yet maintains good stretchability and resilience. We found that the partial gel matrix is squeezed and confined by in situ-formed crystals, leading to carbonyl clusters and thus phosphorescence emission. The composite gel shows green phosphorescence with an emission peak at 512 nm and a lifetime of 342 ms. The afterglow emission is detectable by the naked eye for several seconds. This strategy has good universality, as validated in other hydrogels with different polymeric matrices and dopant molecules. The development of hydrogels with good mechanical and phosphorescent properties should merit the design of multifunctional soft machines with applications in biomedical and engineering fields.

Intrinsic Anti-Freezing and Unique Phosphorescence of Glassy Hydrogels with Ultrahigh Stiffness and Toughness at Low Temperatures.

Most hydrogels become frozen at subzero temperatures, leading to degraded properties and limited applications. Cryoprotectants are massively employed to improve anti-freezing property of hydrogels; however, there are accompanied disadvantages, such as varied networks, reduced mechanical properties, and the risk of cryoprotectant leakage in aqueous conditions. Reported here is the glassy hydrogel having intrinsic anti-freezing capacity and excellent optical and mechanical properties at ultra-low temperatures. Supramolecular hydrogel of poly(acrylamide-co-methacrylic acid) with moderate water content (≈50 wt.%) and dense hydrogen-bond associations is in a glassy state at room temperature. Since hydrogen bonds become strengthened as the temperature decreases, this gel becomes stronger and stiffer, yet still ductile, with Young's modulus of 900 MPa, tensile strength of 30 MPa, and breaking strain of 35% at -45 °C. This gel retains high transparency even in liquid nitrogen. It also exhibits unique phosphorescence due to presence of carbonyl clusters, which is further enhanced at subzero temperatures. Further investigations elucidate that the intrinsic anti-freezing property is related to a fact that most water molecules are tightly bound and confined in the glassy matrix and become non-freezable. This correlation, as validated in several systems, provides a roadmap to develop intrinsic anti-freezing hydrogels for widespread applications at extreme conditions.

Engineering viscoelastic mismatch for temporal morphing of tough supramolecular hydrogels.

Viscoelasticity is a generic characteristic of soft biotissues and polymeric materials, endowing them with unique time- and rate-dependent properties. Here, by spatiotemporally tailoring the viscoelasticity in tough supramolecular hydrogels, we demonstrate reprogrammable morphing of the gels based on differential viscoelastic recovery processes that lead to internal strain mismatch. The spatial heterogeneity of viscoelasticity is encoded through integrating dissimilar hydrogels or by site-specific treatment of a singular hydrogel. The temporal morphing behavior of tough gels, including a fast deformation process and then a slow shape-recovery process, is related to the kinetics of associative interactions and the entropic elasticity of supramolecular networks after pre-stretching and release, which takes place spontaneously in the absence of external stimuli. Such a kinetically driven morphing mechanism resolves the trade-off between the mechanical robustness and shape-changing speed in tough hydrogels with dense entanglements and physical associations, and should be applicable to other viscoelastic materials. A numerical theory for the temporal morphing of tough supramolecular gels has been formulated by dynamic coupling of viscoelastic recovery and mechanics of deformations, which is further implemented to predict the sophisticated morphed structures. Furthermore, magnetic particles are incorporated into the morphed tough hydrogels to devise versatile soft actuators and robots for specific applications.

Healable, Recyclable, and Multifunctional Soft Electronics Based on Biopolymer Hydrogel and Patterned Liquid Metal.

Recent years have witnessed the rapid development of sustainable materials. Along this line, developing biodegradable or recyclable soft electronics is challenging yet important due to their versatile applications in biomedical devices, soft robots, and wearables. Although some degradable bulk hydrogels are directly used as the soft electronics, the sensing performances are usually limited due to the absence of distributed conducting circuits. Here, sustainable hydrogel-based soft electronics (HSE) are reported that integrate sensing elements and patterned liquid metal (LM) in the gelatin-alginate hybrid hydrogel. The biopolymer hydrogel is transparent, robust, resilient, and recyclable. The HSE is multifunctional; it can sense strain, temperature, heart rate (electrocardiogram), and pH. The strain sensing is sufficiently sensitive to detect a human pulse. In addition, the device serves as a model system for iontophoretic drug delivery by using patterned LM as the soft conductor and electrode. Noncontact detection of nearby objects is also achieved based on electrostatic-field-induced voltage. The LM and biopolymer hydrogel are healable, recyclable, and degradable, favoring sustainable applications and reconstruction of the device with new functions. Such HSE with multiple functions and favorable attributes should open opportunities in next-generation electronic skins and hydrogel machines.

Multi-level encryption of information in morphing hydrogels with patterned fluorescence.

Fluorescent hydrogels have attracted tremendous attention recently in the field of information security due to the booming development of information technology. Along this line, it is highly desired to improve the security level of concealed information by the advancements of materials and encryption technologies. Here we report multi-level encryption of information in a bilayer hydrogel with shape-morphing ability and patterned fluorescence. This hydrogel is composed of a fluorescence layer containing chromophore units in the poly(acrylic acid) network and an active layer with UV-absorption agents in the poly(N-isopropylacrylamide-co-acrylic acid) network. The former layer exhibits tunable fluorescence tailored by UV light irradiation to induce unimer-to-dimer transformation of the chromophores, facilitating the write-in of information through photolithography. The latter layer is responsive to temperature, enabling morphing of the bilayer hydrogel. Therefore, the bilayer hydrogel encoded with patterned fluorescent patterns can deform into three-dimensional configurations at room temperature to conceal the information, which is readable only after successive procedures of shape recovery at an appropriate temperature and under UV light irradiation from the right direction. The combination of morphing materials and patterned fluorescence as a new avenue to improve the encryption level of information should merit the design of other smart materials with integrated functions for specific applications.

[Characterization and photoluminescence properties of an azomethin-zinc complex].

A Schiff base organic metal complex, Bis(salicylidene)-1,2-phenylenediam-ine Zinc(II) with high purity, was synthesized and purified by vacuum sublimation. Its structure, thermal stability and energy band structure were investigated by element analysis, FTIR spectra, TG-DTA curve, UV-Vis absorption spectra, fluorescece emission spectra and PL spectra. Experimental results showed that the complex is a thermally stable, polycrystalline material, with glass temperature and decomposition temperature being 183 and 449 degrees C, respectively. In its infrared spectrum, a high intensity band was at about 1 385 cm(-1). This band was typical of the conjugated C=N stretching vibration, which shifted to higher frequency in relation to the free ligand of salicylaldehyde with 1,2-phenylenediamine. The new bnd at 529 cm(-1) was assigned to Zn-O stretching vibration. Its UV absorption bands were at about 297 and 406 nm, and its tetrahydrofuran solution emitted intensive blue-green fluorescence at the peak wavelength of 508 nm. The absorption band at about 406 nm can be assigned to the intrinsic absorption of C=N. Its optical gap was about 2.62 eV, which was determined by the intrinsic absorption band edge of the complex in tetrahydrofuran solution. Under UV excitation at 365 nm, the complex in film emitted yellow-green fluorescence with the maximum emission peak at 562 nm and a full-width at half-maximum of 48.5 nm in PL spectra. Finally, yellow organic light-emitting devices using this complex as the emissive layer were fabricated and investigated.