Tannic acid modified antifreezing gelatin organohydrogel for low modulus, high toughness, and sensitive flexible strain sensor

Recent advances in tannic acid-based gels: Design, properties, and applications

With the flourishing of mussel-inspired chemistry, the fast-growing development for environmentally friendly materials, and the need for inexpensive and biocompatible analogues to PDA in gel design, TA has led to its gradual emergence as a research focus due to its remarkable biocompatible, renewable, sustainable and particular physicochemical properties. As a natural building block, TA can be used as a substrate or crosslinker, ensuring versatile functional polymeric networks for various applications. In this review, the design of TA-based gels is summarized in detail (i.e., different interactions such as: metal coordination, electrostatic, hydrophobic, host-guest, cation-π and π-π stacking interactions, hydrogen bonding and various reactions including: phenol-amine Michael and Schiff base, phenol-thiol Michael addition, phenol-epoxy ring opening reaction, etc.). Subsequently, TA-based gels with a variety of functionalities, including mechanical, adhesion, conductive, self-healing, UV-shielding, anti-swelling, anti-freezing, shape memory, antioxidant, antibacterial, anti-inflammatory and responsive properties are introduced in detail. Then, a summary of recent developments in the use of TA-based gels is provided, including bioelectronics, biomedicine, energy, packaging, water treatment and other fields. Finally, the difficulties that TA-based gels are currently facing are outlined, and an original yet realistic viewpoint is provided in an effort to spur future development.

Antibacterial and antioxidative hydrogel dressings based on tannic acid-gelatin/oxidized sodium alginate loaded with zinc oxide nanoparticles for promoting wound healing

Wound infection resulting in delayed wound healing and wound deterioration remains a clinical challenge. Recently, multifunctional hydrogel dressing was a promising strategy which has attracted wide attention in preventing wound infection and promoting wound healing. In this study, a hybrid hydrogel made of gelatin (GL), tannic acid (TA), oxidized sodium alginate (OSA), and zinc oxide nanoparticles (ZnO NPs) was prepared mainly by double network cross-linking approach, named tannic acid-gelatin/oxidized sodium alginate/zinc oxide (TA-GL/OSA/ZnO). The composite hydrogels exhibited improved mechanical properties, which provided by TA modified the structure of GL network, Schiff base reaction between GL and OSA, and the strengthening effect of ZnO NPs. Meanwhile, the composite hydrogel showed high antibacterial activity against Staphylococcus aureus (S. aureus) (97.8 % ± 0.9 %) and Escherichia coli (E. coli) (96.6 % ± 1.2 %), attributed to the synergistic effect of TA and ZnO NPs. Furthermore, benefiting from the good antioxidative properties of TA, the sustain-released Zn2+ with the good capability to kill bacteria, and promoting the regeneration of skin epithelial tissues in BALB/c mice constantly, the multifunctional hydrogel had a significant therapeutic effect on wound healing and broad application prospects.

Microbial Polysaccharides as Functional Components of Packaging and Drug Delivery Applications

This review examines microbial polysaccharides' properties relevant to their use in packaging and pharmaceutical applications. Microbial polysaccharides are produced by enzymes found in the cell walls of microbes. Xanthan gum, curdlan gum, pullulan, and bacterial cellulose are high-molecular-weight substances consisting of sugar residues linked by glycoside bonds. These polysaccharides have linear or highly branched molecular structures. Packaging based on microbial polysaccharides is readily biodegradable and can be considered as a renewable energy source with the potential to reduce environmental impact. In addition, microbial polysaccharides have antioxidant and prebiotic properties. The physico-chemical properties of microbial polysaccharide-based films, including tensile strength and elongation at break, are also evaluated. These materials' potential as multifunctional packaging solutions in the food industry is demonstrated. In addition, their possible use in medicine as a drug delivery system is also considered.

Monolithic DNApatite: An Elastic Apatite with Sub-Nanometer Scale Organo-Inorganic Structures

Hydroxyapatite (HA) exhibits outstanding biocompatibility, bioactivity, osteoconductivity, and natural anti-inflammatory properties. Pure HA, ion-doped HA, and HA-polymer composites are investigated, but critical limitations such as brittleness remain; numerous efforts are being made to address them. Herein, the novel self-crystallization of a polymeric single-stranded deoxyribonucleic acid (ssDNA) without additional phosphate ions for synthesizing deoxyribonucleic apatite (DNApatite) is presented. The synthesized DNApatite, DNA1Ca2.2(PO4)1.3OH2.1, has a repetitive dual phase of inorganic HA crystals and amorphous organic ssDNA at the sub-nm scale, forming nanorods. Its mechanical properties, including toughness and elasticity, are significantly enhanced compared with those of HA nanorod, with a Young's modulus similar to that of natural bone.

Dual detection of human motion and glucose in sweat with polydopamine and glucose oxidase doped self-healing nanocomposite hydrogels

The detection of human motion and sweat composition are important for human health or sports training, so it is necessary to develop flexible sensors for monitoring exercise processes and sweat detection. Mussel secretion of adhesion proteins enables self-healing of byssus and adhesion to surfaces. We prepared Au nanoparticles@polydopamine (AuNPs@PDA) nanomaterials based on mussel-inspired chemistry and compounded them with polyvinyl alcohol (PVA) hydrogels to obtain PVA/AuNPs@PDA self-healing nanocomposite hydrogels. Dopamine (DA) was coated on the surface of AuNPs to obtain AuNPs based composite (AuNPs@PDA) and the AuNPs@PDA was implanted into the PVA hydrogels to obtain nanocomposite hydrogel through facile freeze-thaw cycle. Glucose oxidase (GOD) was added to the hydrogel matrix to achieve specific detection of glucose in sweat. The obtained hydrogels exhibit high deformability (573.7 %), excellent mechanical strength (550.3 KPa) and self-healing properties (85.1 %). The PVA/AuNPs@PDA hydrogel sensors exhibit quick response time (185.0 ms), wide strain sensing range (0-500 %), superior stability and anti-fatigue properties in motion detection. The detection of glucose had wide concentration detection range (1.0 μmol/L-200.0 μmol/L), low detection limits (0.9 μmol/L) and high sensitivity (24.4 μA/mM). This work proposes a reference method in dual detection of human exercise and sweat composition analysis.

High-Saline-Enabled Hydrophobic Homogeneous Cross-Linking for Extremely Soft, Tough, and Stretchable Conductive Hydrogels as High-Sensitive Strain Sensors

Design of admirable conductive hydrogels combining robust toughness, soft flexibility, desirable conductivity, and freezing resistance remains daunting challenges for meeting the customized and critical demands of flexible and wearable electronics. Herein, a promising and facile strategy to prepare hydrogels tailored to these anticipated demands is proposed, which is prepared in one step by homogeneous cross-linking of acrylamide using hydrophobic divinylbenzene stabilized by micelles under saturated high-saline solutions. The influence of high-saline environments on the hydrogel topology and mechanical performance is investigated. The high-saline environments suppress the size of hydrophobic cross-linkers in micelles during hydrogel polymerization, which weaken the dynamic hydrophobic associations to soften the hydrogels. Nevertheless, the homogeneous cross-linked networks ensure antifracture during ultralarge deformations. The obtained hydrogels show special mechanical performance combining extremely soft deformability and antifracture features (Young's modulus, 5 kPa; stretchability, 10200%; toughness, 134 kJ m-2; and excellent anticrack propagation). The saturated-saline environments also endow the hydrogels with desirable ion conductivity (106 mS cm-1) and freezing resistance (<20 °C). These comprehensive properties of the obtained hydrogels are quite suitable for flexible electronic applications, which is demonstrated by the high sensitivity and durability of the derived strain sensors.

Organohydrogels with cellulose nanofibers enhanced supramolecular interactions toward high performance self-adhesive sensing pads

Gel materials with tailored functions and tissue-like properties have gained significant interest in emerging applications, including tissue engineering scaffolds, flexible electronics, and soft robotics. In this work, we developed a stretchable, flexible, adhesive, and conductive organohydrogel through physical cross-linking of the poly (N-[tris (hydroxymethyl) methyl] acrylamide-co-acrylamide) (denoted as P(THMA-AM)) network in the presence of cellulose nanofiber (CNF), sodium chloride, and glycerol. The gel matrix is rich in intermolecular interactions, including hydrogen bonding and ionic interactions, which contribute to a highly compact and cohesive structure without the requirement of any chemical crosslinkers. Moreover, the plasticizing effect of glycerol can mitigate the self-entanglement of CNFs, enhancing their mobility and ultimately conferring the organohydrogel with exceptional stretchability and flexibility. The resulting organohydrogel exhibited superior mechanical properties, self-adhesion, and ionic conductivity, making it an excellent candidate for strain-sensing applications, particularly in distinguishing and monitoring human movements.

Fabrication of Eco-Friendly Wearable Strain Sensor Arrays via Facile Contact Printing for Healthcare Applications

Wearable flexible strain sensors with spatial resolution enable the acquisition and analysis of complex actions for noninvasive personalized healthcare applications. To provide secure contact with skin and to avoid environmental pollution after usage, sensors with biocompatibility and biodegradability are highly desirable. Herein, wearable flexible strain sensors composed of crosslinked gold nanoparticle (GNP) thin films as the active conductive layer and transparent biodegradable polyurethane (PU) films as the flexible substrate are developed. The patterned GNP films (micrometer- to millimeter-scale square and rectangle geometry, alphabetic characters, and wave and array patterns) are transferred onto the biodegradable PU film via a facile, clean, rapid and high-precision contact printing method, without the need of a sacrificial polymer carrier or organic solvents. The GNP-PU strain sensor with low Young's modulus (≈17.8 MPa) and high stretchability showed good stability and durability (10 000 cycles) as well as degradability (42% weight loss after 17 days at 74 °C in water). The GNP-PU strain sensor arrays with spatiotemporal strain resolution are applied as wearable eco-friendly electronics for monitoring subtle physiological signals (e.g., mapping of arterial lines and sensing pulse waveforms) and large-strain actions (e.g., finger bending).

Abundant tannic acid modified gelatin/sodium alginate biocomposite hydrogels with high toughness, antifreezing, antioxidant and antibacterial properties

The acidity of high tannic acid (TA) content solution can destroy the structure of protein, such as gelatin (G). This causes a big challenge to introduce abundant TA into the G-based hydrogels. Here, the G-based hydrogel system with abundant TA as hydrogen bonds provider was constructed by a "protective film" strategy. The protective film around the composite hydrogel was first formed by the chelation of sodium alginate (SA) and Ca2+. Subsequently, abundant TA and Ca2+ were successively introduced into the hydrogel system by immersing method. This strategy effectively protected the structure of the designed hydrogel. After treatment with 0.3 w/v TA and 0.06 w/v Ca2+ solutions, the tensile modulus, elongation at break and toughness of G/SA hydrogel increased about 4-, 2-, and 6-fold, respectively. Besides, G/SA-TA/Ca2+ hydrogels exhibited good water retention, anti-freezing, antioxidant, antibacterial properties and low hemolysis ratio. Cell experiments showed that G/SA-TA/Ca2+ hydrogels possessed good biocompatibility and could promote cell migration. Therefore, G/SA-TA/Ca2+ hydrogels are expected to be used in the field of biomedical engineering. The strategy proposed in this work also provides a new idea for improving the properties of other protein-based hydrogels.

Chitosan-reinforced gelatin composite hydrogel as a tough, anti-freezing, and flame-retardant gel polymer electrolyte for flexible supercapacitors

Hydrogel-based electrolytes for flexible solid-state supercapacitors (SSCs) have received significant attention due to their mechanical robustness and stable electrochemical performance over a wide temperature range. However, achieving flame retardancy in such SSCs at subzero temperatures to increase their practical utility remains challenging. Furthermore, there is a need for sustainable and bio-friendly SSCs that use natural polymer-based hydrogel electrolytes. This study reports a novel approach for developing a chitosan-reinforced anti-freezing ionic conductive gelatin hydrogel to meet these demands. Immersion of chitosan-containing gelatin hydrogels in salt solutions caused chitosan precipitation, resulting in composite hydrogels. The precipitated chitosan contributes to the reinforcement of the gelatin hydrogel network, resulting in a high mechanical toughness of up to 3.81 MJ/m³, a fracture energy of 26 kJ/m², anti-freezing properties (below -30 °C), and excellent flame retardancy without softening. Furthermore, the hydrogel exhibits excellent electrochemical performance, with an ionic conductivity ranging from 72 mS/cm at room temperature (26 °C) to 39 mS/cm at -30 °C. The proposed hydrogel exhibits potential for use in SSC as a gel polymer electrolyte. This study demonstrates a novel strategy for controlling the mechanical, thermal, and electrochemical characteristics of flexible supercapacitors using biological macromolecules.

Tannic acid: a versatile polyphenol for design of biomedical hydrogels

Tannic acid (TA), a natural polyphenol, is a hydrolysable amphiphilic tannin derivative of gallic acid with several galloyl groups in its structure. Tannic acid interacts with various organic, inorganic, hydrophilic, and hydrophobic materials such as proteins and polysaccharides via hydrogen bonding, electrostatic, coordinative bonding, and hydrophobic interactions. Tannic acid has been studied for various biomedical applications as a natural crosslinker with anti-inflammatory, antibacterial, and anticancer activities. In this review, we focus on TA-based hydrogels for biomaterials engineering to help biomaterials scientists and engineers better realize TA's potential in the design and fabrication of novel hydrogel biomaterials. The interactions of TA with various natural or synthetic compounds are deliberated, discussing parameters that affect TA-material interactions thus providing a fundamental set of criteria for utilizing TA in hydrogels for tissue healing and regeneration. The review also discusses the merits and demerits of using TA in developing hydrogels either through direct incorporation in the hydrogel formulation or indirectly via immersing the final product in a TA solution. In general, TA is a natural bioactive molecule with diverse potential for engineering biomedical hydrogels.