Stimulation Modulates Adhesion and Mechanics of Hydrogel Adhesives

Programming hydrogel adhesion with engineered polymer network topology

Hydrogel adhesion that can be easily modulated in magnitude, space, and time is desirable in many emerging applications ranging from tissue engineering and soft robotics to wearable devices. In synthetic materials, these complex adhesion behaviors are often achieved individually with mechanisms and apparatus that are difficult to integrate. Here, we report a universal strategy to embody multifaceted adhesion programmability in synthetic hydrogels. By designing the surface network topology of a hydrogel, supramolecular linkages that result in contrasting adhesion behaviors are formed on the hydrogel interface. The incorporation of different topological linkages leads to dynamically tunable adhesion with high-resolution spatial programmability without alteration of bulk mechanics and chemistry. Further, the association of linkages enables stable and tunable adhesion kinetics that can be tailored to suit different applications. We rationalize the physics of polymer chain slippage, rupture, and diffusion at play in the emergence of the programmable behaviors. With the understanding, we design and fabricate various soft devices such as smart wound patches, fluidic channels, drug-eluting devices, and reconfigurable soft robotics. Our study presents a simple and robust platform in which adhesion controllability in multiple aspects can be easily integrated into a single design of a hydrogel network.

Stretchable Semi-Interpenetrating Carboxymethyl Guar Gum-Based Composite Hydrogel for Moisture-Proof Wearable Strain Sensor

Wearable strain sensors of conductive hydrogels have very broad application prospects in electronic skins and human-machine interfaces. However, conductive hydrogels suffer from unstable signal transmission due to environmental humidity and inherent shortcomings of their materials. Herein, we introduce a novel moisture-proof conductive hydrogel with high toughness (2.89 MJ m-3), mechanical strength (1.00 MPa), and high moisture-proof sensing performance by using dopamine-functionalized gold nanoparticles as conductive fillers into carboxymethyl guar gum and acrylamide. Moreover, the hydrogel can realize real-time monitoring of major and subtle human movements with good sensitivity and repeatability. In addition, the hydrogel-assembled strain sensor exhibits stable sensing signals after being left for 1 h, and the relative resistance change rate under different strains (25-300%) shows no obvious noise signal up to 99% relative humidity. Notably, the wearable strain sensing is suitable for wearable sensor devices with high relative humidity.

A double-network strategy for the tough tissue adhesion of hydrogels with long-term stability under physiological environment

Achieving tough and stable tissue adhesion under a physiological environment is of great significance for the clinical applications of hydrogel adhesives. The current tough hydrogel adhesives face challenges in the preservation of the maximal adhesion for a long time due to swelling. Here, we propose a double-network strategy for tough tissue adhesion by a hydrogel with long-term stability under a physiological environment. A double-network hydrogel consisting of a covalently crosslinked primary network with tunable hydrophilicity and a non-covalently crosslinked secondary network with functional groups is designed. The primary network exhibited hydrophobicity in the physiological environment, which could constrict the secondary network and limit the swelling of the entire hydrogel. The secondary network could form strong interlinks with tissue and provide large energy dissipation through the unzipping of its noncovalent crosslinks when separated by a force. The combination of the two networks resulted in a tough and stable tissue adhesion. A poly(N-isopropylacrylamide)/calcium alginate hydrogel synthesized based on this strategy realized an adhesion energy of 300-500 J m^-2 with porcine tissues, and the maximal adhesion could be maintained for over 1000 min after submerging in a PBS solution at 37 °C. The swelling behavior of the hydrogel and changes in mechanical properties under the physiological environment are studied, and its application in repairing the aorta wound is demonstrated.

Toward Plant Cyborgs: Hydrogels Incorporated onto Plant Tissues Enable Programmable Shape Control

Engineered living materials (ELMs) that incorporate living organisms and synthetic materials enable advanced functional properties. Here, we seek to create plant cyborgs by combining plants or plant tissues with stimuli-responsive polymeric materials. Plant tissues with integrated shape control may find applications in regenerative medicine, and the shape control of living plants enables another dimension of adaptability and response to environmental threats, which can be applied to next-generation precision farming. In this work, we develop chemistry to integrate stimuli-responsive poly(N-isopropylacrylamide) (PNIPAM) hydrogels with decellularized plant tissues assisted by 3D printing. We demonstrate programmable shape morphing in response to thermal cues and ultraviolet (UV) light. Specifically, by taking advantage of the extrusion-based 3D printing method, we deposit nanocomposite PNIPAM precursors onto silane-treated decellularized leaf surface with prescribed shapes and spatial control. When subjected to external stimuli, the strain mismatch generated between the swellable nanocomposite PNIPAM and nonswellable decellularized leaf enables folding and bending to occur. This strategy to integrate the plant tissues with stimuli-responsive hydrogels allows the control of leaf morphology, opening avenues for plant-based biosensors and soft actuators to enhance food security; such materials also may find applications in biomedicine as tissue-engineering scaffolds.

Dual-Sensing, Stretchable, Fatigue-Resistant, Adhesive, and Conductive Hydrogels Used as Flexible Sensors for Human Motion Monitoring

Hydrogel-based sensors serve as an ideal platform for developing personalized wearable electronics due to their high flexibility and conformability. However, the weak stretchability and inferior conductivity of hydrogels have severely restricted their large-scale application. Herein, a natural polymer-based conductive hydrogel integrated with favorable mechanical properties, good adhesive performance, and excellent fatigue resistance was fabricated via interpenetrating tannic acid (TA) into a chitosan (CS) cross-linked network in an acidic aqueous solution. The hydrogel was composed of a regular hierarchical porous structure, which was built by the hydrogen bonding between TA and CS. In addition, the hydrogels exhibited adjustable mechanical properties (maximum yield stress of 7000 Pa) and good stretchability (strain up to 320%). Benefiting from the abundant catechol groups of TA, the proposed hydrogels could repeatedly adhere to various material surfaces and could be easily peeled off without residue. Moreover, the hydrogel exhibited stable conductivity, high stretching sensitivity (gauge factor of 2.956), rapid response time (930 ms), and excellent durability (>300 cycles), which can be assembled as a strain sensor to attach to the human body for precise monitoring of human exercise behavior, distinguishing physiological signals, and recognizing speech. Furthermore, the prepared hydrogels also exhibited stable sensing performance to temperature. As a result, the hydrogels exhibited dual sensory performance for both temperature and strain deformation. It is anticipated that the incorporation of strain sensors and thermal sensors will provide theoretical guidance for developing multifunctional conductive hydrogels and pave a way for the versatile application of hydrogel-based flexible sensors in wearable devices and soft actuators.

Preparation of hydrogel nanocomposite functionalized silica microspheres and its application in mixed-mode liquid chromatography

Hydrogel is a kind of three-dimensional network structure polymer that can absorb water and swell in water. It has been widely used in many fields due to its flexible functionality. We proposed the design strategy of dual-network hydrogel assisted by a metal-organic-framework (MOF) and modified them on the surface of silica (with average particle diameter of 5 μm and average pore diameter of 76 Å). On the basis of effectively avoiding shortcomings such as osmotic pressure caused by swelling, abundant mesh types of composite material also improves the separation selectivity of the stationary phase. A variety of analytes such as nucleosides/bases, antibiotics, organic acids, carbohydrates, alkylbenzenes, polycyclic aromatic hydrocarbons, pesticides and anions can be selectively separated. The research on the retention behavior and the interaction mechanism proves that the column can be used in mixed mode liquid chromatography. By comparing with the optimized chromatographic conditions of commercial HILIC column and C18 column, this new type of stationary phase also has some significant advantages in the selective separation of mixed analytes. This new stationary phase also has excellent acid/base stability. The intraday relative standard deviation of their retention time under acidic conditions is 0.05%-0.26% (n = 10), and the intraday relative standard deviation under basic conditions is 0.11-0.14% (n = 10). After optimizing the chromatographic conditions, the efficiency of this new type of chromatographic column can reach 90,300 plates/m (sucrose). In short, a new strategy for applying hydrogel to liquid chromatography with high selectivity and chromatographic separation performance is proposed.

Self-Healing Silicone Elastomer with Stable and High Adhesion in Harsh Environments

Adhesive and self-healing elastomers are urgently needed for their convenience and intelligence in biological medicine, flexible electronics, intelligent residential systems, etc. However, their inevitable use in harsh environments results in further enhancement requirements of the structure and performance of adhesive and self-healing elastomers. Herein, a novel self-healing and high-adhesion silicone elastomer was designed by the synergistic effect of multiple dynamic bonds. It revealed excellent stretchability (368%) and self-healing properties at room temperature (98.1%, 5 h) and in a water environment (96.4% for 5 h). Meanwhile, the resultant silicone elastomer exhibited high adhesion to metal and nonmetal and showed stable adhesion in harsh environments, such as under acidic (pH 1) and alkaline (pH 12) environments, salt water, petroleum ether, water, etc. Furthermore, it was applied as a shatter-proof protective layer and a rust-proof coating, proving its significant potential in intelligent residential system applications.

Thermal-Regulated Adhesion Enhancement and Fast Switching within the Viscoelastic Glass Transition Zone of a Shape Memory Polymer

The effects of temperature and preload on adhesion of an epoxy-based shape memory polymer (ESMP) were investigated experimentally. The ESMP sheet and polydimethylsiloxane (PDMS) control sample were prepared by mold casting. The adhesion measurements were carried out on a home-built adhesion tester using a hemispherical glass indenter as the counter-surface. The reduced modulus was determined by the Oliver-Pharr method and Hertz theory. The viscoelastic energy dissipation was extracted, and the effective work of adhesion was calculated by the fitting method and JKR theory. In the vicinity of glass transition temperature (T_g), the viscoelastic enhancement factor (1 + f(v, T)) is as high as 955 because of the strong viscoelastic effect of the ESMP sheet. The adhesion strength of the ESMP sheet is about 589 kPa under a relatively smaller contact displacement condition (∼44.6 μm). The strong viscoelastic effect induces more viscoelastic energy dissipation that contributes to the effective work of adhesion and leads to strong preload dependence of the adhesion. The pull-off force F_pull-off is demonstrated to linearly depend on F_m^1/3E_r^2/3. In the sharp half glass transition zone (T > T_g), the viscoelasticity and rigidity rapidly decrease with the temperature increasing about 10 °C, leading to a 6-fold reduction in adhesion. The results indicate that the adhesion of the ESMP sheet can be significantly enhanced and meanwhile rapidly switched within the viscoelastic glass transition zone.

Highly stretchable, compressible, adhesive hydrogels with double network

In this work, a double network bovine serum albumin-polyacrylamide (BSA-PAM) adhesive hydrogel was fabricated, in which combination of physical interactions including hydrogen bonds and chain entanglements, and chemical covalent photo-crosslinking. The BSA-PAM hydrogel exhibited excellent mechanical and adhesive properties. The composite hydrogel not only demonstrated excellent tensile properties (maximum force elongation 1552%~2037%), but also displayed extremely high fatigue resistance even when subjected to compress strains of up to 85%. More importantly, the BSA-PAM hydrogel showed excellent adhesiveness to various substrates (90 kPa~150 kPa for glass and 9.74 kPa~35.09 kPa for pigskin). This work provided a facile way of fabricating tough, stretchable and adhesive BSA-PAM hydrogels.