Acrylate monomer polymerization triggered by iron oxide magnetic nanoparticles and catechol containing microgels.

Phenol and its derivatives are the most used polymerization inhibitors for vinyl-based monomers. Here, we reported a novel catalytic system composed of mussel inspired adhesive moiety, catechol, in combination with iron oxide nanoparticles (IONPs) to generate hydroxyl radical (•OH) at pH 7.4. Catechol-containing microgel (DHM) was prepared by copolymerizing dopamine methacrylamide (DMA) and N-hydroxyethyl acrylamide (HEAA), which generated superoxide (•O2-) and hydrogen peroxide (H2O2) as a result of catechol oxidation. In the presence of IONPs, the generated reactive oxygen species were further converted to •OH, which initiated free radical polymerization of various water-soluble acrylate-based monomers including neutral (acrylamide, methyl acrylamide, etc.), anionic (2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt), cationic ([2-(methacryloyloxy)ethyl]trimethylammonium chloride), and zwitterionic (2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide) monomers. Compared with the typical free radical initiating systems, the reported system does not require the addition of extra initiators for polymerization. During the process of polymerization, a bilayer hydrogel was formed in situ and exhibited the ability to bend during the process of swelling. The incorporation of IONPs significantly enhanced magnetic property of the hydrogel and the combination of DHM and IONPs also improved the mechanical properties of these hydrogels.

Bilayer Hydrogel Actuators with High Mechanical Properties and Programmable Actuation via the Synergy of Double-Network and Synchronized Ultraviolet Polymerization Strategies.

Bilayer hydrogel actuators, consisting of an actuating layer and a functional layer, show broad applications in areas such as soft robotics, artificial muscles, drug delivery and tissue engineering due to their inherent flexibility and responses to stimuli. However, to achieve the compatibility of good stimulus responses and high mechanical properties of bilayer hydrogel actuators is still a challenge. Herein, based on the double-network strategy and using the synchronous ultraviolet (UV) polymerization method, an upper critical solution temperature (UCST)-type bilayer hydrogel actuator was prepared, which consisted of a poly(acrylamide-co-acrylic acid)[MC] actuating layer and an agar/poly(N-hydroxyethyl acrylamide-co-methacrylic acid)[AHA] functional layer. The results showed that the tensile stress/strain of the bilayer hydrogel actuator was 1161.21 KPa/222.07%. In addition, the UCST of bilayer hydrogels was ~35 °C, allowing the bilayer hydrogel actuator to be curled into an "◎" shape, which could be unfolded when the temperature was 65 °C, but not at a temperature of 5 °C. Furthermore, hydrogel actuators of three different shapes were designed, namely "butterfly", "cross" and "circle", all of which demonstrated good actuating performances, showing the programmable potential of bilayer hydrogels. Overall, the bilayer hydrogels prepared using double-network and synchronous UV polymerization strategies realized the combination of high mechanical properties with an efficient temperature actuation, which provides a new method for the development of bilayer hydrogel actuators.

Fabrication of flexible accelerated-wound-healing chitosan/dopamine-based bilayer hydrogels for strain sensors.

Flexible conductive hydrogels have great potential for healthcare and human motion sensing. However, it is difficult to simultaneously achieve conductive hydrogel epidermal sensors with reliable adhesion capabilities and excellent sensing properties, as well as accelerated wound healing performance in wearable hydrogels. Here, an epidermal sensor with excellent adhesion (0.6 kPa) and tensile strain (218.0 %) properties was assembled from an easy-to-prepare bilayer antimicrobial hydrogel, which effectively accelerates wound healing, as well as for human motion sensing. The upper hydrogel layer was composed of PVA, which could effectively enhance the mechanical properties of the bilayer hydrogel. The lower hydrogel layer consisted of polyacrylamide (PAm) and chitosan-dopamine (CC-DA). PAm with good adhesion properties adhered effectively to the skin surface. CC-DA not only had adhesion properties, but also has good antibacterial effects. It inhibited the growth of bacteria, which assisted in wound healing and infection prevention. Therefore, the design of the bilayer hydrogel combined the mechanical enhancement of PVA with the adhesion properties and antimicrobial effect of PAm and CC-DA to provide better wound repair. In addition, the double-layer hydrogel with good electrical conductivity (1.65 S·m-1) could sensitively monitor the tiny electrophysiological signals emitted by the human body during exercise rehabilitation training.

Heterogeneity Regulation of Bilayer Polysaccharide Hydrogels for Integrating pH- and Humidity-Responsive Actuators and Sensors.

Bilayer hydrogel-based actuators have attracted much interest because inhomogeneous structures are easily constructed in hydrogels. We used three kinds of polysaccharides, including anionic carboxymethyl cellulose (CMC), cationic chitosan (CS), and amphoteric carboxymethyl chitosan (CMCS), as both structure-constructing units and actuation-controlling units in this work to fabricate physically crosslinked poly(vinyl alcohol) bilayer hydrogels. The spatial heterogeneity was tuned by changing the types and concentrations of polysaccharides in different layers, to regulate pH- and humidity-driven actions of bilayer hydrogels. Based on the distortion of the ionic channel during the humidity-motivated deformation of bilayer hydrogels, a two-in-one flexible device integrating a humidity-driven actuator and humidity-responsive sensor was then developed, which could detect the alterations of environmental humidity in real time. Moreover, good tensile toughness and interfacial bonding as well as the strain-resistance effect endowed the bilayer hydrogels with the capability of identifying human motion as a strain sensor, unlocking more application scenarios. This work provides an overall insight into the heterogeneity regulation of bilayer hydrogels using polysaccharides as stimulus-responsive units and also proposes an interesting strategy of manufacturing hydrogel-based flexible devices with both actuating and sensing capabilities.

Versatile and Simple Strategy for Preparing Bilayer Hydrogels with Janus Characteristics.

Bilayer hydrogels are attracting tremendous attention for their capability to integrate several different functions on the two sides of the gel, that is, imparting the gel with Janus characteristics, which is highly desired in many engineering and biomedical applications including soft actuators, hydrogel patches, and wearable electronics. However, the preparation process of the bilayer materials usually involves several complicated steps and is time-consuming, while the interfacial bonding is another main concern. Here, a simple and versatile method is proposed to obtain bilayer hydrogels within just one step based on the method of introducing viscosity contrast of the precursors for different layers. The bilayer structure can be well maintained during the whole preparation process with a constrained interfacial molecular exchange to ensure the strong bonding strength. The key requirements for forming distinct bilayer structures in situ are studied and discussed in detail. Bilayer hydrogels with different chemical designs are prepared via this strategy to tailor the good distribution of desired functions for soft actuators, wound healing patches, and wearable electronics. We believe that the strategy illustrated here will provide new insights into the preparation and application of bilayer materials.

A novel assessment of microstructural and mechanical behaviour of bilayer silica-reinforced nanocomposite hydrogels as a candidate for artificial cartilage.

The complex structure of healthy articular cartilage facilitates the joint withstanding the imposed pressures and retaining interstitial fluid to lessen stresses on its soft tissue, while easing the locomotion and minimising friction between cartilage mates. Avascular nature of this tissue results in unrecoverable damaged lesions and severe pain over time. Polymeric hydrogels are promising candidate materials for the replacement of the damaged cartilage. Hence, a tough bilayer nanocomposite acrylamide-acrylic acid hydrogel reinforced with silica nanoparticles (SNPs) was designed and synthesised. The mechanical characterisations showed a significant increase in compressive strength up to 1.4 MPa and doubled elastic modulus (240 kPa) by utilising only 0.6 wt% SNPs compared to the non-reinforced hydrogel. The optimum amounts of monomers and SNPs resulted in the compression of samples up to 85% strain without failure. Viscoelastic responses improved as the stress relaxation lessened to half in all nanocomposite hydrogels. Diffusion rate theory was applied, and the results showed to what extent elastic modulus results in an improvement in stress relaxation. The proposed hydrogel formulation exhibited the poroelastic relaxation occurred before viscoelastic relaxation at the time elapses under stress relaxation tests. SEM images showed uniform funnel-like porosity with 570 µm thick lubricious layer, which is an important feature to retain interstitial fluid. Energy-dispersive X-ray spectroscopy was conducted to characterise the elemental composition within the polymeric macrostructure.

Tough Interfacial Adhesion of Bilayer Hydrogels with Integrated Shape Memory and Elastic Properties for Controlled Shape Deformation.

The weak adhesion between two hydrogel layers may lead to the delamination of bilayer hydrogels or low force transfer efficiency during deformation. Here, tough interfacial adhesive bilayer hydrogels with rapid shape deformation and recovery were prepared by simple attachment-heating of two gel layers. The bilayer hydrogels, composed of a shape memory gel (S-gel) and an elastic gel (E-gel), exhibited extremely tough interfacial adhesion between two layers (Γ âˆ¼ 2200 J/m2). The shape deformation and shape recovery of the bilayer hydrogels, tuned by "heating-stretching" mode and "stretching-heating-stretching" mode, were rapid (<5 s) and no delamination between two gel layers was detected during shape deformation. Based on the fast shape deformation and recovery, the bilayer hydrogels could mimic the flower and hand, and a gel gripper could be fabricated to catch the object in the hot water. This work provides a simple method to prepare tough adhesive bilayer hydrogels with controlled shape deformation.

Mussel-Inspired Tough Hydrogel with In Situ Nanohydroxyapatite Mineralization for Osteochondral Defect Repair.

Repairing osteochondral defects is a considerable challenge because it involves the breakdown of articular cartilage and underlying bone. Traditional hydrogels with a homogenized single-layer structure cannot fully restore the function of osteochondral cartilage tissue. In this study, a mussel-inspired hydrogel with a bilayer structure is developed to repair osteochondral defects. The hydrogel is synthesized by simultaneously polymerizing two layers using a one-pot method. The resulting upper and lower gelatin methacryloyl-polydopamine hydrogel layers are used as cartilage and subchondral bone repair layers, respectively. Polydopamine-induced hydroxyapatite in situ mineralization takes place in the lower layer to mimic the structure of subchondral bone. The bilayer hydrogel exhibits good mechanical properties for the synergistic effect of covalent and noncovalent bonds, as well as nanoreinforcement of mineralized hydroxyapatite. To improve the tissue-inducibility of hydrogels, transforming growth factor ß3 is immobilized in the upper layer to induce cartilage regeneration, while bone morphogenetic protein 2 is immobilized in the lower layer to induce bone regeneration. Bone and cartilage repair performance of the hydrogel is examined by implantation into a full-thickness cartilage defect of a rabbit knee joint. The bilayer-structure hydrogel promotes regeneration of osteochondral tissue, thus providing a new option for repair of osteochondral defects.