Design and fabrication of functional hydrogels through interfacial engineering

Recent Progress in Self-Healable Hydrogel-Based Electroluminescent Devices: A Comprehensive Review

Flexible electronics have gained significant research attention in recent years due to their potential applications as smart and functional materials. Typically, electroluminescence devices produced by hydrogel-based materials are among the most notable flexible electronics. With their excellent flexibility and their remarkable electrical, adaptable mechanical and self-healing properties, functional hydrogels offer a wealth of insights and opportunities for the fabrication of electroluminescent devices that can be easily integrated into wearable electronics for various applications. Various strategies have been developed and adapted to obtain functional hydrogels, and at the same time, high-performance electroluminescent devices have been fabricated based on these functional hydrogels. This review provides a comprehensive overview of various functional hydrogels that have been used for the development of electroluminescent devices. It also highlights some challenges and future research prospects for hydrogel-based electroluminescent devices.

Thermoresponsive ionogels with switchable adhesion in air and aqueous environments induced by LCST phase behavior

The rapid development of wearable devices is in urgent demand for materials with switchable adhesion both in air and aqueous environments. Herein, we report a thermoresponsive ionogel with switchable adhesion against various substrates both in air and aqueous environments. The switchable adhesion of ionogels is realized by a phase separation induced collapse of the polymer network and the subsequent extrusion of ionic liquids (ILs) on ionogel surfaces. The hydrophobic poly(butyl acrylate) (PBA) network and ILs endow the ionogels with excellent water-resistance ability, which enables the application of ionogels in aqueous environments. As a result, the adhesion strength of ionogels against rubber can reach an on/off ratio of 75-fold (45 kPa versus 0.6 kPa) and 7.7-fold (21 kPa versus 2.7 kPa) in air and aqueous environments, respectively. By varying the ratio of two structurally similar ILs in their blends, the responsive temperature of ionogels can be tuned within a wide temperature range from 32 °C to 100 °C. Furthermore, we show a demonstration of an underwater on demand capture and release by taking advantage of the switchable adhesion of ionogels. These nonvolatile ionogels with tunable responsive temperatures and high on/off adhesion strength ratio both in air and aqueous environments show broad applications in the fields related to wearable devices, soft robots and submersible sensors.

Shape and stiffness memory ionogels with programmable pressure-resistance response

Flexible pressure sensors usually require functional materials with both mechanical compliance and appropriate electrical performance. Most sensors based on materials with limited compressibility can hardly balance between high sensitivity and broad pressure range. Here, we prepare a heterophasic ionogel with shape and stiffness memory for adaptive pressure sensors. By combining the microstructure alignment for stiffness changing and shape memory micro-inclusions for stiffness fixing, the heterophasic ionogels reveal tunable compressibility. This controllable pressure-deformation property of the ionogels results in the pressure sensors' programmable pressure-resistance behavior with tunable pressure ranges, varied detection limits, and good resolution at high pressure. Broad pressure ranges to 220 and 380 kPa, and tunable detection limit from 120 to 330 and 950 Pa are realized by the stiffness memory ionogel sensors. Adaptive detection is also brought out to monitor tiny pressure changes at low stiffness and distinguish different human motions at high stiffness. Using shape and stiffness memory materials in pressure sensors is a general design to achieve programmable performance for more complex application scenarios.

Development of Conductive Hydrogels for Fabricating Flexible Strain Sensors

Conductive hydrogels can be prepared by incorporating various conductive materials into polymeric network hydrogels. In recent years, conductive hydrogels have been developed and applied in the field of strain sensors owing to their unique properties, such as electrical conductivity, mechanical properties, self-healing, and anti-freezing properties. These remarkable properties allow conductive hydrogel-based strain sensors to show excellent performance for identifying external stimuli and detecting human body movement, even at subzero temperatures. This review summarizes the properties of conductive hydrogels and their application in the fabrication of strain sensors working in different modes. Finally, a brief prospectus for the development of conductive hydrogels in the future is provided.

Bioactive Coatings on Titanium: A Review on Hydroxylation, Self-Assembled Monolayers (SAMs) and Surface Modification Strategies

Titanium (Ti) and its alloys have been demonstrated over the last decades to play an important role as inert materials in the field of orthopedic and dental implants. Nevertheless, with the widespread use of Ti, implant-associated rejection issues have arisen. To overcome these problems, antibacterial properties, fast and adequate osseointegration and long-term stability are essential features. Indeed, surface modification is currently presented as a versatile strategy for developing Ti coatings with all these challenging requirements and achieve a successful performance of the implant. Numerous approaches have been investigated to obtain stable and well-organized Ti coatings that promote the tailoring of surface chemical functionalization regardless of the geometry and shape of the implant. However, among all the approaches available in the literature to functionalize the Ti surface, a promising strategy is the combination of surface pre-activation treatments typically followed by the development of intermediate anchoring layers (self-assembled monolayers, SAMs) that serve as the supporting linkage of a final active layer. Therefore, this paper aims to review the latest approaches in the biomedical area to obtain bioactive coatings onto Ti surfaces with a special focus on (i) the most employed methods for Ti surface hydroxylation, (ii) SAMs-mediated active coatings development, and (iii) the latest advances in active agent immobilization and polymeric coatings for controlled release on Ti surfaces.

A sandcastle worm-inspired strategy to functionalize wet hydrogels

Hydrogels have been extensively used in many fields. Current synthesis of functional hydrogels requires incorporation of functional molecules either before or during gelation via the pre-organized reactive site along the polymer chains within hydrogels, which is tedious for polymer synthesis and not flexible for different types of hydrogels. Inspired by sandcastle worm, we develop a simple one-step strategy to functionalize wet hydrogels using molecules bearing an adhesive dibutylamine-DOPA-lysine-DOPA tripeptide. This tripeptide can be easily modified with various functional groups to initiate diverse types of polymerizations and provide functional polymers with a terminal adhesive tripeptide. Such functional molecules enable direct modification of wet hydrogels to acquire biological functions such as antimicrobial, cell adhesion and wound repair. The strategy has a tunable functionalization degree and a stable attachment of functional molecules, which provides a tool for direct and convenient modification of wet hydrogels to provide them with diverse functions and applications.

Fabrication of Elastic Macroporous Polymers with Enhanced Oil Absorbability and Antiwaxing Performance

Porous polymers are of great interest in potential energy storage and environmental remediation applications. However, traditional fabrication methods are either time-consuming or energy-consuming and deteriorate the mechanical strength of polymer materials. In this study, polymerization-induced phase separation was used to realize the template-free fabrication of superflexible macroporous polymers. Since the solvent is also used as a porogen, this method can be widely used to synthesize several porous polymers by carefully choosing the solvent and monomer. Compared to nonstructured polymers, the prepared macroporous polymers exhibited enhanced mechanical strength, superflexibility, multicompressibility, and bending properties. Along with hydrophobicity/oleophilicity and macroporous structures, the as-prepared porous polymers demonstrated controllable oil absorbability and release; furthermore, after infusing with lubrication liquid, these materials can be used as antiwaxing materials. The elastic porous polymers prepared using this simple and universal method show great potential for various applications, including controlled drug release, antiwaxing, and lubrication.

Tirapazamine-embedded polyplatinum(iv) complex: a prodrug combo for hypoxia-activated synergistic chemotherapy

A polyprodrug complex containing oxygen depleting chemodrugs and hypoxia-activated antitumor agents can serve as a promising drug delivery system for synergistic chemotherapy.

Recent Advances in Bioinspired Gel Surfaces with Superwettability and Special Adhesion

Engineering surface wettability is of great importance in academic research and practical applications. The exploration of hydrogel-based natural surfaces with superior properties has revealed new design principles of surface superwettability. Gels are composed of a cross-linked polymer network that traps numerous solvents through weak interactions. The natural fluidity of the trapped solvents confers the liquid-like property to gel surfaces, making them significantly different from solid surfaces. Bioinspired gel surfaces have shown promising applications in diverse fields. This work aims to summarize the fundamental understanding and emerging applications of bioinspired gel surfaces with superwettability and special adhesion. First, several typical hydrogel-based natural surfaces with superwettability and special adhesion are briefly introduced, followed by highlighting the unique properties and design principles of gel-based surfaces. Then, the superwettability and emerging applications of bioinspired gel surfaces, including liquid/liquid separation, antiadhesion of organisms and solids, and fabrication of thin polymer films, are presented in detail. Finally, an outlook on the future development of these novel gel surfaces is also provided.

Diffusion-Freezing-Induced Microphase Separation for Constructing Large-Area Multiscale Structures on Hydrogel Surfaces

Hydrogels with multiscale structured surface have attracted significant attention for their valuable applications in diverse areas. However, current strategies for the design and fabrication of structured hydrogel surfaces, which suffer from complicated manufacturing processes and specific material modeling, are not efficient to produce structured hydrogel surfaces in large area, and therefore restrict their practical applications. To address this problem, a general and reliable method is reported, which relies on the interplay between polymer chain diffusion and the subsequent freezing-induced gelation and microphase separation processes. The basic idea is systematically analyzed and further exploited to manufacture gel surfaces with gradient structures and patterns through the introduction of temperature gradient and shape control of the contact area. Moreover, the formed micro/nanostructured surfaces are exemplified to work as capillary systems and thus can uplift the liquid spontaneously indicating the potential application for anti-dehydration. It is believed that the proposed facile and large-area fabrication method can inspire the design of materials with various functionalized surfaces.

Rational design of nano-architecture composite hydrogel electrode towards high performance Zn-ion hybrid cell

In this paper, we developed a novel Zn-ion hybrid cell based on a graphene-conducting polymer composite hydrogel (capacitor-type) cathode and a zinc metal (battery-type) anode. The pseudocapacitive-type cathode materials can effectively boost the capacity of Zn-ion hybrid cell compared to that of electrical double layer cathode materials. In particular, the composite hydrogel with rational designed three-dimensional (3D) nano-architecture combining 3D porous nanostructure with hydrogel, can significantly enlarge the active interfaces between the electrode and electrolyte. According to our experiments, the 3D graphene@PANI composite hydrogel electrode exhibits a large capacity of 154 mA h g-1, a superior rate capability and excellent capacity retention of 80.5% after 6000 charge-discharge cycles in a Zn-ion hybrid cell. The outstanding electrochemical properties demonstrate that the 3D nanostructure composite hydrogel materials can effectively promote the material utilization, transport of charges, and reduce the degradation of conducting polymers, leading to a highly efficient, fast and stable electrochemical process. Based on our results, Zn-ion hybrid cells based on a composite hydrogel electrode could be an extremely promising candidate for next generation electrochemical energy storage devices.

Surface-Selective Grafting of Crosslinking Layers on Hydrogel Surfaces via Two Different Mechanisms of Photopolymerization for Site-Controllable Release

This study reports an effective method for controlling substance-release sites of hydrogel. Glycidyl methacrylate, which contains two functional groups, namely, double-bond acrylate and epoxide, is photografted on a hydrogel surface through hydrogen abstraction photopolymerization due to the existence of a hydrogen donor, such as an amine, in the hydrogel matrix. The remaining epoxide group crosslinks the polymer chain of polyglycidyl methacrylate. Substance release of hydrogel is changed due to the altered surface texture of hydrogel. Rate and site-controlled substance release are achieved by controlling the thickness and site of surface grafting and the extent of epoxide ring opening. This study may provide a novel method for achieving hydrogel function or modified performance of other biomaterials to meet biological activity requirements.

Hydrogels Based on Dynamic Covalent and Non Covalent Bonds: A Chemistry Perspective

Hydrogels based on reversible covalent bonds represent an attractive topic for research at both academic and industrial level. While the concept of reversible covalent bonds dates back a few decades, novel developments continue to appear in the general research area of gels and especially hydrogels. The reversible character of the bonds, when translated at the general level of the polymeric network, allows reversible interaction with substrates as well as responsiveness to variety of external stimuli (e.g., self-healing). These represent crucial characteristics in applications such as drug delivery and, more generally, in the biomedical world. Furthermore, the several possible choices that can be made in terms of reversible interactions generate an almost endless number of possibilities in terms of final product structure and properties. In the present work, we aim at reviewing the latest developments in this field (i.e., the last five years) by focusing on the chemistry of the systems at hand. As such, this should allow molecular designers to develop a toolbox for the synthesis of new systems with tailored properties for a given application.

General Strategy to Fabricate Highly Filled Microcomposite Hydrogels with High Mechanical Strength and Stiffness

Conventional synthetic hydrogels are intrinsically soft and brittle, which severely limits the scope of their applications. A variety of approaches have been proposed to improve the mechanical strength of hydrogels. However, a facile and ubiquitous strategy to prepare hydrogels with high mechanical strength and stiffness is still a challenge. Here, we report a general strategy to prepare highly filled microcomposite hydrogels with high mechanical performance using an ultrasonic assisted strategy. The microparticles were dispersed in the polymer network evenly, resulting in homogeneous and closely packed structures. The as-prepared hydrogels with extraordinary mechanical performance can endure compressive stress up to 20 MPa (at 75% strain) and exhibit high stiffness (elastic modulus is around 18 MPa). By using our comprehensive strategy, different hydrogels can enhance their mechanical strength and stiffness by doping various microparticles, leading to a much wider variety of applications.