Development and understanding of highly mechanically robust and electronically conducting hydrogels are extremely important for ever-increasing energy-based applications.Conventional mixing/blending of conductive additives with hydrophilic polymer network prevents both high mechanical strength and electronic conductivity to be presented in polymer hydrogels.Here,we proposed a double-network(DN)engineering strategy to fabricate PVA/PPy DN hydrogels,consisting of a conductive PPy-PA network via in-situ ultrafast gelation and a tough PVA network via a subsequent freezing/thawing process.The resultant PVA/PPy hydrogels exhibited superior mechanical and electrochemical properties,including electrical conductivity of~6.8 S/m,mechanical strength of~0.39 MPa,and elastic moduli of~0.1 MPa.Upon further transformation of PVA/PPy hydrogels into supercapacitors,they demonstrated a high capacitance of~280.7 F/g and a cycle life of 2000 galvanostatic charge/discharge cycles with over 94.3%capacity retention at the current density of 2 mA/cm2 and even subzero temperatures of−20℃.Such enhanced mechanical performance and electronic conductivity of hydrogels are mainly stemmed from a synergistic combination of continuous electrically conductive PPy-PA network and the two interpenetrating DN structure.This in-situ gelation strategy is applicable to the integration of ionic-/electrical-conductive materials into DN hydrogels for smart-soft electronics,beyond the most commonly used PEDOT:PSS-based hydrogels.
Booming sophisticated robotics and prosthetics put forward high requirements on soft conductive materials that can bridge electronics and biology,those soft conductive materials should imitate the mechanical properties of biological tissues and build information transmission networks.Until now,it remains a great challenge to handle the trade-off among ease of preparation,high conductivity,processability,mechanical adaptability,and external stimuli responsiveness.Herein,a kind of readily prepared and processed multifunctional MXene nanocomposite hydrogel is reported,which is prepared via the fast gelation of cationic monomer initiated by delaminated MXene sheets.The gelation time can be adjusted(several seconds to minutes)based on the MXene loadings.By adjusting the MXene ratio,the resulting nanocomposites are ultrastretchable(>5000%),three-dimensional(3D)printable,and show outstanding mechanical and electrical self-healing.As expected,the integration of multifunctional systems onto various substrates(e.g.,gloves and masks)is further demonstrated via 3D printing and could achieve diverse sensory capabilities toward strain,pressure,and temperature,showing great prospects as smart flexible electronics.
Inflammatory skin disorders can cause chronic scarring and functional impairments,posing a significant burden on patients and the healthcare system.Conventional therapies,such as corticosteroids and nonsteroidal anti-inflammatory drugs,are limited in efficacy and associated with adverse effects.Recently,nanozyme(NZ)-based hydrogels have shown great promise in addressing these challenges.NZ-based hydrogels possess unique therapeutic abilities by combining the therapeutic benefits of redox nanomaterials with enzymatic activity and the water-retaining capacity of hydrogels.The multifaceted therapeutic effects of these hydrogels include scavenging reactive oxygen species and other inflammatory mediators modulating immune responses toward a pro-regenerative environment and enhancing regenerative potential by triggering cell migration and differentiation.This review highlights the current state of the art in NZ-engineered hydrogels(NZ@hydrogels)for anti-inflammatory and skin regeneration applications.It also discusses the underlying chemo-mechano-biological mechanisms behind their effectiveness.Additionally,the challenges and future directions in this ground,particularly their clinical translation,are addressed.The insights provided in this review can aid in the design and engineering of novel NZ-based hydrogels,offering new possibilities for targeted and personalized skin-care therapies.
Amal George KurianRajendra K.SinghVarsha SagarJung‑Hwan LeeHae‑Won Kim
We developed a fluorescent double network hydrogel with ionic responsiveness and high mechanical properties for visual detection.The nanocomposite hydrogel of laponite and polyacrylamide serves as the first network,while the ionic cross-linked hydrogel of terbium ions and sodium alginate serves as the second network.The double-network structure,the introduction of nanoparticles and the reversible ionic crosslinked interactions confer high mechanical properties to the hydrogel.Terbium ions are not only used as the ionic cross-linked points,but also used as green emitters to endow hydrogels with fluorescent properties.On the basis of the “antenna effect” of terbium ions and the ion exchange interaction,the fluorescence of the hydrogels can make selective responses to various ions(such as organic acid radical ions,transition metal ions) in aqueous solutions,which enables a convenient strategy for visual detection toward ions.Consequently,the fluorescent double network hydrogel fabricated in this study is promising for use in the field of visual sensor detection.
Owing to their low flexibility,poor processability and a lack of responsiveness,inorganic materials are usually non-ideal for constructing a living organism.Hence,to date,lifelike materials with structural hierarchies and adaptive properties usually rely on light and soft organic molecules,although few exceptions have been acquired using two-dimensional(2D)inorganic nanosheets.Herein,with a systematic study on the gelation behavior of carbon-based 0D quantum dots,1D nanotubes,and 3D fullerenes,we find that acidified 1D carbon nanotubes(CNTs)can serve as an alternative building block for fabricating purely inorganic biomimetic soft materials.The as-prepared CNT gels exhibit not only a pH-or photothermal-triggered mechanical and tribological adaptivity,which allows them to simulate the behavior of sea cucumbers,peacock mantis shrimps,and mammalian muscles or cortical bones,but also a unique damping property that is similar to spider’s cuticular pad.Their high elasticity,effective lubrication,excellent biocompatibility,and controllable friction and wear also allow them to function as a new type of smart lubricants,whose tribological properties can be regulated either by its internal pH changes or spatiotemporally by near-infrared(NIR)light irradiations,free of any toxic and flammable base oils or additives.
Wearable pressure sensors made from conductive hydrogels hold significant potential in health monitoring.However,limited pressure range(Pa to hundreds of kPa)and inadequate antibacterial properties restrict their practical applications in diagnostic and health evaluation.Herein,a wearable high-performance pressure sensor was assembled using a facilely prepared porous chitosan-based hydrogel,which was constructed from commercial phenolphthalein particles as a sacrificial template.The relationship between the porosity of hydrogels and sensing performance of sensors was systematically explored.Herein,the wearable pressure sensor,featuring an optimized porosity of hydrogels,exhibits an ultrawide sensing capacity from 4.83 Pa to 250 k Pa(range-to-limit ratio of 51,760)and high sensitivity throughout high pressure ranges(0.7 kPa~(-1),120–250 kPa).The presence of chitosan endows these hydrogels with outstanding antibacterial performance against E.coli and S.aureus,making them ideal candidates for use in wearable electronics.These features allow for a practical approach to monitor full-range human motion using a single device with a simple structure.
Semi-interpenetrating(semi-IPN)hydrogels formed by the continuous interpenetration of cross-linked polymer network and linear non-crosslinked polymer with multifunctionality are widely used in biomedical and other fields.However,the negative impact of linear polymer on the homogeneity of the cross-linked network often leads to a decrease in the mechanical properties of semi-IPN hydrogels and severely limits their applications.Herein,a bioinspired hydrogen-bonding induced phase separation strategy is presented to construct the tough semi-IPN polyvinylpyrrolidone/polyacrylamide hydrogels(named PVP/PAM hydrogels),including the linear polymer polyvinylpyrrolidone(PVP)and cross-linked polyacrylamide(PAM)network.The resultant PVPx/PAM hydrogels exhibit unique phase separation induced by the hydrogen bonding between PVP and PAM and affected by the amount of substance of PVP.Meanwhile,the phase separation of PVPx/PAM hydrogels results in excellent mechanical properties with a strain of 2590%,tensile strength of 0.28 MPa and toughness of 2.17 MJ/m^(3).More importantly,the hydrogen bonding between PVP and PAM firstly disrupts to dissipate energy under external forces,so the PVPx/PAM hydrogels exhibit good self-recovery properties and outperform chemically cross-linked PAM hydrogels in impact resistance and damping applications.It is believed that the PVPx/PAM hydrogels with hydrogen-bonding induced phase separation possess more potential application prospects.
Thermochromic smart windows have gained increasing popularity in light modulation and energy management in buildings.However,the fabrication of flexible thermochromic smart windows with high luminous transmittance(Tlum),tailorable critical temperature(τc),strong solar modulation ability(ΔTsol),and long-term durability remains a huge challenge.In this study,hydrogel-based thermochromic smart windows are fabricated by sandwiching thermochromic hydrogels of polyallylamine hydrochloride,polyacrylic acid,and carbonized polymer dots(CPDs)complexes between two pieces of transparent substrates.Benefiting from the incorporation of nanosized CPDs,the thermochromic hydrogel has an ultrahigh Tlum of~98.7%,a desirableτc of~24.2℃,aΔTsol of~89.3%and a rapid transition time of~3 s from opaque state to transparent state.Moreover,the thermochromic hydrogel exhibits excellent anti-freezing ability,tight adhesion toward various substrates,and excellent self-healing capability.The self-healing capability enables the fabrication of large-area smart windows by welding multiple hydrogel pieces.The smart windows retain their original thermochromic properties after being stored under ambient conditions for at least 147 days or undergoing 10,000 uninterrupted heating/cooling cycles.The model houses with smart windows can achieve a temperature reduction of 9.2℃,demonstrating the excellent indoor temperature modulation performance of the smart windows.
Yuting WangXu FangSiheng LiNi AnHongyu PanJunqi Sun
Conventional photonic devices exhibit static optical properties that are design-dependent,including the material’s refractive index and geometrical parameters.However,they still possess attractive optical responses for applications and are already exploited in devices across various fields.Hydrogel photonics has emerged as a promising solution in the field of active photonics by providing primarily deformable geometric parameters in response to external stimuli.Over the past few years,various studies have been undertaken to attain stimuli-responsive photonic devices with tunable optical properties.Herein,we focus on the recent advancements in hydrogel-based photonics and micro/nanofabrication techniques for hydrogels.In particular,fabrication techniques for hydrogel photonic devices are categorized into film growth,photolithography(PL),electron-beam lithography(EBL),and nanoimprint lithography(NIL).Furthermore,we provide insights into future directions and prospects for deformable hydrogel photonics,along with their potential practical applications.
Hydrogels have been extensively studied for applications in various fields, such as tissue engineering and soft robotics, as determined by their mechanical properties. The mechanical design of hydrogels typically focuses on the modulus, toughness, and deformability. These characteristics play important roles and make great achievements for hydrogel use. In recent years, a growing body of research has concentrated on the fatigue property of hydrogels, which determines their resistance to crack propagation in the networks during cyclic mechanical loads for applications. However, knowledge of hydrogel fatigue behavior remains notably deficient. Here, we present a brief overview of the fatigue behavior of hydrogels, encompassing the general experimental methods to measure the fatigue property and fundamental theoretical calculation models. Then, we highlight multiple strategies to enhance the fatigue resistance of hydrogels. Finally, we present our perspectives on fatigue-resistant hydrogels, outstanding challenges and potential directions for future research.