Recent Advances in Hydrogels

The physicochemical properties of stimuli-responsive polymers can be easily adjusted using changes in their external environment. This attribute made them to capture the attention of many researchers as this facilitates their recovery from the bulk sample after the separation process. Researchers have used stimuli-responsive materials such as pH-responsive, thermo-responsive and light-responsive polymers in separation science. They have utilized them during adsorption studies, aqueous two-phase systems and membrane systems. These applications of various types of stimuli-responsive polymers are well-articulated in this review. The advantages and limitations of some of the stimuli-responsive polymers are also summarised in this paper. In addition, the challenges and future prospects of stimuli-responsive polymer are also explored in detail in this review. 

Stimuli-responsive polymers and their applications in separation science

The impact of COVID-19 has rendered medical technology an important factor to maintain social stability and economic increase, where biomedicine has experienced rapid development and played a crucial part in fighting off the pandemic. Conductive hydrogels (CHs) are three-dimensional (3D) structured gels with excellent electrical conductivity and biocompatibility, which are very suitable for biomedical applications. CHs can mimic innate tissue’s physical, chemical, and biological properties, which allows them to provide environmental conditions and structural stability for cell growth and serve as efficient delivery substrates for bioactive molecules. The customizability of CHs also allows additional functionality to be designed for different requirements in biomedical applications. This review introduces the basic functional characteristics and materials for preparing CHs and elaborates on their synthetic techniques. The development and applications of CHs in the field of biomedicine are highlighted, including regenerative medicine, artificial organs, biosensors, drug delivery systems, and some other application scenarios. Finally, this review discusses the future applications of CHs in the field of biomedicine. In summary, the current design and development of CHs extend their prospects for functioning as an intelligent and complex system in diverse biomedical applications.

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Biocompatible Conductive Hydrogels: Applications in the Field of Biomedicine

Thermal stiffening materials that are naturally soft but adaptively self-strengthen upon heat are intriguing for load-bearing and self-protection applications at elevated temperatures. However, to simultaneously achieve high modulus change amplitude and high mechanical strength at the stiffened state remains challenging. Herein, entropy-mediated polymer-mineral cluster interactions are exploited to afford thermal stiffening hydrogels with a record-high storage modulus enhancement of 13,000 times covering a super wide regime from 1.3 kPa to 17 MPa. Such a dramatic thermal stiffening effect is ascribed to the transition from liquid-liquid to solid-liquid phase separations, and at the molecular level, driven by enhanced polymer-cluster interactions. The hydrogel was further processed into sheath-core fibers and smart fabrics, which demonstrated self-strengthening and self-powered sensing properties by co-weaving another liquid metal fiber as both the joule heater and triboelectric layer.

Entropy-Mediated Polymer-Cluster Interactions Enable Dramatic Thermal Stiffening Hydrogels for Mechanoadaptive Smart Fabrics.

Construction and Function of Robust and Moist Bilayer Chitosan-Based Hydrogel Wound Dressing

A light-sensitive moiety, azobenzene, is incorporated into polyacrylamide (PAM) to yield copolymers with upper critical solution temperatures (UCST) in aqueous solutions. The thermoresponsiveness of the copolymers can be tuned not only by light irradiation, which causes the transition between the trans- and cis-forms of the azobenzene, but also by addition of α-cyclodextrin (α-CD), which preferentially complexes with the trans-form of azobenzene. The UCST transition is a combined effect of hydrogen bonding between the amide groups of the PAM chains and the hydrophobic interaction between azobenzene units, the latter of which can be influenced by the photoisomerization of azobenzene and the host–guest interaction between azobenzene and α-CD. The strategy of incorporating switchable hydrophobicity may inspire designs of UCST polymers, which may have potential applications as drug delivery devices and catalyst carriers.

Temperature-, Light-, and Host-Molecule-Responsive Polymers with UCST Behavior for Aqueous Sensing Applications

Biomimetic Self-Deformation of Polymer Interpenetrating Network with Stretch-Induced Anisotropicity

Thermo-responsive shape memory hydrogels generally achieve shape fixation at low temperatures, and shape recovery at high temperatures. However, these hydro-gels usually suffer from poor mechanical properties. Herein, we present a unique poly(acrylic acid)/calcium acetate shape memory hydrogel with cold-induced shape recovery performances as ultrastrong artificial muscles. Since the acetate groups could form aggregate at high temperatures and thus induce the association of the hydrogel network, the hydrogel can be fixed into a temporary shape upon heating and recover to its original shape in a cold environment. Moreover, a programmable shape recovery process is realized by adjusting the shape fixing time. In addition, the unique shape memory process enables the application demonstration as bio-inspired artificial muscles with an ultrahigh work density of 45.2 kJ m−3, higher than that of biological muscles (∼8 kJ m−3). 

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Cold-induced shape memory hydrogels for strong and programmable artificial muscles

The interaction between the aggregation-induced emissive (AIE) luminogens (AIEgen) and the polymer is the key scientific question in the design of functional AIE hydrogels. In this study, we report the AIE behavior of a poly(acrylic acid) (PAAc) and poly(acrylamide) (PAAm) interpenetrating network (IPN) hydrogel doped with tetra-cationic tetraphenylethene (TCTPE). The cationic AIEgen can bind with PAAc through ionic interaction, while PAAc and PAAm chains can associate with each other through hydrogen-bonds (H-bonds). These two interactions can restrict the intramolecular rotation and thus activate the luminescence of the AIEgen. The PAAc-PAAm H-bonds can be broken by increasing temperature or pH, restoring the intramolecular rotation of the AIEgen and quenching the fluorescence of the hydrogel. Therefore, the TCTPE-doped IPN hydrogel is designed as temperature- and pH-sensitive displayers which can record information imprinted by photo-printing or iono-printing with good switchability and reversibility. Another application of this TCTPE-doped hydrogel is demonstrated as a luminescent soft actuator, which has fast shape deformation and editable fluorescence pattern. The above results reveal a pathway to tune the emission behavior through tuning polymer-polymer and polymer-AIEgen interactions, which may inspire new design strategies of aggregation-induced emissive polymers and broaden their applications.

Lighted up by hydrogen-bonding: luminescence behavior and applications of AIEgen-doped interpenetrating network hydrogel

Small dop of comonomer, giant shift of cloud point: Thermo-responsive behavior and mechanism of poly(methylacrylamide) copolymers with an upper critical solution temperature

Jianlei Lu

Papers

Temperature-, Light-, and Host-Molecule-Responsive Polymers with UCST Behavior for Aqueous Sensing Applications

Biomimetic Self-Deformation of Polymer Interpenetrating Network with Stretch-Induced Anisotropicity

Cold-induced shape memory hydrogels for strong and programmable artificial muscles

Lighted up by hydrogen-bonding: luminescence behavior and applications of AIEgen-doped interpenetrating network hydrogel

Small dop of comonomer, giant shift of cloud point: Thermo-responsive behavior and mechanism of poly(methylacrylamide) copolymers with an upper critical solution temperature