A review on recent advances in gel adhesion and their potential applications

Boron nitride microfiber reinforced polyacrylic acid hydrogels with excellent self-adhesion, fast pH response, and strain sensitivity

Hydrogels are widely utilized in the sensor field, but their inadequate adhesion presents a significant obstacle. Herein, a new multifunctional BNMFs/PAA composite hydrogel was prepared via the incorporation of one-dimensional porous boron nitride microfibers (BNMFs) and polyacrylic acid (PAA) hydrogels. BNMFs, as a reinforcing filler, play a very important role in enhancing the properties of the composite hydrogels. In particular, the porous micrometer structure plays a unique role in improving the adhesion properties of PAA hydrogels. The steric hindrance and the rich hydroxyl functional groups coming from BNMFs are key factors for the excellent adhesion of the composite hydrogels. The composite hydrogels show strong adhesion to various substrate materials. For iron plates and biological tissues, the adhesion energy can reach 1377 J m-2 and 317 J m-2, respectively. In addition, the developed BNMFs/PAA composite hydrogels exhibit excellent mechanical properties. The fracture strain of the composite hydrogels is increased by 2.4 times compared to pure PAA hydrogels. The hydrogen bonds formed between BNMFs and PAA are conducive to the mechanical properties of the BNMFs/PAA composite hydrogels. Meanwhile, BNMFs as fillers play a role in carrying and dissipating force. Furthermore, the BNMFs/PAA composite hydrogels have excellent strain and pH response characteristics. This is because the crosslinking network of the composite hydrogels becomes loose after the addition of BNMFs, resulting in rapid ion transport pathways. Therefore, the developed BNMFs/PAA composite hydrogels will have broad application prospects in the fields of motion monitoring, intelligent skin and biological adhesives.

Reversibly Sticking Metals and Graphite to Hydrogels and Tissues

We have discovered that hard, electrical conductors (e.g., metals or graphite) can be adhered to soft, aqueous materials (e.g., hydrogels, fruit, or animal tissue) without the use of an adhesive. The adhesion is induced by a low DC electric field. As an example, when 5 V DC is applied to graphite slabs spanning a tall cylindrical gel of acrylamide (AAm), a strong adhesion develops between the anode (+) and the gel in about 3 min. This adhesion endures after the field is removed, and we term it as hard-soft electroadhesion or EA[HS]. Depending on the material, adhesion occurs at the anode (+), cathode (-), or both electrodes. In many cases, EA[HS] can be reversed by reapplying the field with reversed polarity. Adhesion via EA[HS] to AAm gels follows the electrochemical series: e.g., it occurs with copper, lead, and tin but not nickel, iron, or zinc. We show that EA[HS] arises via electrochemical reactions that generate chemical bonds between the electrode and the polymers in the gel. EA[HS] can create new hybrid materials, thus enabling applications in robotics, energy storage, and biomedical implants. Interestingly, EA[HS] can even be achieved underwater, where typical adhesives cannot be used.

Biomimetic Antibacterial Gelatin Hydrogels with Multifunctional Properties for Biomedical Applications

A facile novel approach of introducing dopamine and [2-(methacryloyloxy) ethyl] dimethyl-(3-sulfopropyl) ammonium hydroxide via dopamine-triggered in situ synthesis into gelatin hydrogels in the presence of ZnSO4 is presented in this study. Remarkably, the resulting hydrogels showed 99.99 and 100% antibacterial efficiency against Gram-positive and Gram-negative bacteria, respectively, making them the highest performing surfaces in their class. Furthermore, the hydrogels showed adhesive properties, self-healing ability, antifreeze properties, electrical conductivity, fatigue resistance, and mechanical stability from -100 to 80 °C. The added multifunctional performance overcomes several disadvantages of gelatin-based hydrogels such as poor mechanical properties and limited thermostability. Overall, the newly developed hydrogels show significant potential for numerous biomedical applications, such as wearable monitoring sensors and antibacterial coatings.

Plant-inspired adhesive and injectable natural hydrogels: in vitro and in vivo studies

The development of alternative therapeutic treatments based on the use of medicinal and aromatic plants, such as Juniper communis L., has aroused interest in the medical field to find new alternatives to conventional therapeutic treatments, which have shown problems related to bacterial resistance, high costs, or sustainability in their production. The present work describes the use of hydrogels based on sodium alginate and carboxymethyl cellulose, with combinations of juniperus leaves and berry extracts, in order to characterize their chemical characteristics, antibacterial activity, tissue adhesion test, cytotoxicity in the L929 cell line, and their effects on an in vivo model in mice to maximize the use of these materials in the healthcare field. Overall, an adequate antibacterial potential against S. aureus, E. coli and P. vulgaris was obtained with doses above 100 mg.mL-1 of hydrogels. Likewise, low cytotoxicity in hydrogels combined with extracts has been identified according to the IC50 value at 17.32 µg.mL-1, compared to the higher cytotoxic activity expressed by the use of control hydrogels with a value at 11.05 µg.mL-1. Moreover, in general, the observed adhesion was high to different tissues, showing its adequate capacity to be used in different tissue typologies. Furthermore, the invivo results have not shown erythema, edema, or other complications related to the use of the proposed hydrogels. These results suggest the feasibility of using these hydrogels in biomedical applications given the observed safety.

Thermosensitive P(AAc-co-NIPAm) hydrogels display enhanced toughness and self-healing via ion-ligand interactions

Hydrogels containing thermosensitive polymers such as poly(N-isopropylacrylamide) (P(NIPAm)) may contract during heating and show great promise in fields ranging from soft robotics to thermosensitive biosensors. However, these gels often exhibit low stiffness, tensile strength, and mechanical toughness, limiting their applicability. Through copolymerization of P(NIPAm) with poly(Acrylic acid) (P(AAc)) and introduction of ferric ions (Fe³⁺ ) that coordinate with functional groups along the P(AAc) chains, we here introduce a thermoresponsive hydrogel with significantly enhanced mechanical extensibility, strength, and toughness. Using both experimentation and constitutive modeling, we find that increasing the ratio of m(AAc):m(NIPAm) in the prepolymer decreases strength and toughness but improves extensibility. In contrast, increasing Fe³⁺ concentration generally improves strength and toughness with little decrease in extensibility. Due to reversible coordination of the Fe³⁺ bonds, these gels display excellent recovery of mechanical strength during cyclic loading and self-healing ability. While thermosensitive contraction imbued by the underlying P(NIPAm) is reduced slightly with increased Fe³⁺ concentration, the temperature transition range is widened and shifted upwards towards that of human body temperature (between 30 and 40°C), perhaps rendering these gels suitable as in vivo biosensors. Finally, these gels display excellent adsorptive properties with a variety of materials, rendering them possible candidates in adhesive applications. This article is protected by copyright. All rights reserved.

A review of recent advances in metal ion hydrogels: mechanism, properties and their biological applications

The mechanisms, common properties and biological applications of different types of metal ion hydrogels are summarized.

Peptide Hydrogel with Antibacterial Performance Induced by Rare Earth Metal Ions

Metal ion-induced peptide assembly is an interesting field. As compared to traditional antibacterial Ag⁺, rare earth metal ions possess the advantage of antibacterial performance with photostability and low toxicity. Herein, a new peptide Fmoc-FFWDD-OH was designed and synthesized, which could form a stable hydrogel induced by rare earth metal ions, including Tb³⁺, Eu³⁺, and La³⁺. The mechanical properties were characterized by rheological measurements, and they exhibited elasticity-dominating properties. Transmission electron microscopy (TEM) images showed a large number of nanoscale fiber structures formed in the hydrogel. Circular dichroism (CD) spectra, Fourier transform infrared (FT-IR) spectra, ThT assays, and X-ray diffraction (XRD) pattern illustrated the formation mechanism of the fiber structure. The rare earth ion-induced peptide hydrogel was proved to possess good antibacterial performance on Escherichia coli (E. coli) with excellent biocompatibility. The introduction of rare earth metal ions may have some potential applications in the biological antibacterial and medical fields.

Drosera-Inspired Dual-Actuating Double-Layer Hydrogel Actuator

Drosera is a small insectivorous plant whose antennae can fold up, encircle, and prey. The rapid movement of the antennae is achieved by the synergistic effect of a double-layer structure with the antennae contracts on the front and expands on the back. In this work, a drosera-inspired dual-actuating double-layer hydrogel actuator is proposed, in which the temperature-responsive poly(N, N-diethyl acrylamide) (PDEAAm) layer acts as the main actuation layer and a moisture-responsive poly(acrylamide) (PAAm) layer acts as the auxiliary actuation layer. In a water environment with low temperature, both the PAAm and PDEAAm layers absorb water and expand with a swelling property. When the temperature exceeds the lower critical solution temperature of PDEAAm, the PDEAAm layer undergoes a hydrophilic-hydrophobic transition and shrinks rapidly. Therefore, the synergistic effect of the double-layer hydrogel enables the double-layer hydrogel to achieve a large bending angle at high temperature. In addition, when designing and fabricating shape-patterned double-layer hydrogels, complex shape changes can be achieved. Due to the physical and chemical properties, the actuator can be used to grab, transport, and release objects. This drosera-inspired double-layer hydrogel actuator has high practical value, which may provide new insights for the design and manufacture of artificial intelligence materials.

Advances and Novel Perspectives on Colloids, Hydrogels, and Aerogels Based on Coordination Bonds with Biological Interest Ligands

This perspective article shows new advances in the synthesis of colloids, gels, and aerogels generated by combining metal ions and ligands of biological interest, such as nucleobases, nucleotides, peptides, or amino acids, among other derivatives. The characteristic dynamism of coordination bonds between metal center and biocompatible-type ligands, together with molecular recognition capability of these ligands, are crucial to form colloids and gels. These supramolecular structures are generated by forming weak van der Waals bonds such as hydrogen bonds or π–π stacking between the aromatic rings. Most gels are made up of nano-sized fibrillar networks, although their morphologies can be tuned depending on the synthetic conditions. These new materials respond to different stimuli such as pH, stirring, pressure, temperature, the presence of solvents, among others. For these reasons, they can trap and release molecules or metal ions in a controlled way allowing their application in drug delivery as antimicrobial and self-healable materials or sensors. In addition, the correct selection of the metal ion enables to build catalytic or luminescent metal–organic gels. Even recently, the use of these colloids as 3D-dimensional printable inks has been published. The elimination of the solvent trapped in the gels allows the transformation of these into metal–organic aerogels (MOAs) and metal–organic xerogels (MOXs), increasing the number of possible applications by generating new porous materials and composites useful in adsorption, conversion, and energy storage. The examples shown in this work allow us to visualize the current interest in this new type of material and their perspectives in the short-medium term. Furthermore, these investigations show that there is still a lot of work to be done, opening the door to new and interesting applications.

A review of the properties and applications of bioadhesive hydrogels

Due to their outstanding properties, bioadhesive hydrogels have been extensively studied by researchers in recent years.