A self-healing nanocomposite double network bacterial nanocellulose/gelatin hydrogel for three dimensional printing.

Extrusion-based three-dimensional (3D) printing of gelatin is important for additive manufactured tissue engineering scaffolds, but gelatin's thermal instability has remained an ongoing challenge. The gelatin tends to suddenly collapse at mild temperatures, which is a significant limitation for using it at physiological temperature of 37 °C. Hence, fabrication of a thermo-processable gelatin hydrogel adapted for extrusion-based additive manufacturing is still a challenge. To achieve this, a self-healing nanocomposite double-network (ncDN) gelatin hydrogel was fabricated with high thermo-processability, shear-thinning, mechanical strength, self-healing, self-recovery, and biocompatibility. To do this, amino group-rich gelatin was first created by combining gelatin with carboxyl methyl chitosan. Afterwards, a self-healing ncDN gelatin hydrogel was formed via an in-situ formation of imine bonds between the blend of gelatin/carboxyl methyl chitosan (Gel/CMCh) and dialdehyde-functionalized bacterial nanocellulose (dBNC). dBNC plays as nanofiber cross-linkers capable of simultaneously crosslinking and reinforcing the double networks of Gel/CMCh through formation of dynamic 3D imine bonds. Based on our findings, our self-healing ncDA gelatin hydrogel displayed great potential as a promising ink for additive manufactured tissue engineering scaffolds.

Starch-g-Acrylic Acid/Magnetic Nanochitin Self-Healing Ferrogels as Flexible Soft Strain Sensors.

Mechanically robust ferrogels with high self-healing ability might change the design of soft materials used in strain sensing. Herein, a robust, stretchable, magneto-responsive, notch insensitive, ionic conductive nanochitin ferrogel was fabricated with both autonomous self-healing and needed resilience for strain sensing application without the need for additional irreversible static chemical crosslinks. For this purpose, ferric (III) chloride hexahydrate and ferrous (II) chloride as the iron source were initially co-precipitated to create magnetic nanochitin and the co-precipitation was confirmed by FTIR and microscopic images. After that, the ferrogels were fabricated by graft copolymerisation of acrylic acid-g-starch with a monomer/starch weight ratio of 1.5. Ammonium persulfate and magnetic nanochitin were employed as the initiator and crosslinking/nano-reinforcing agents, respectively. The ensuing magnetic nanochitin ferrogel provided not only the ability to measure strain in real-time under external magnetic actuation but also the ability to heal itself without any external stimulus. The ferrogel may also be used as a stylus for a touch-screen device. Based on our findings, our research has promising implications for the rational design of multifunctional hydrogels, which might be used in applications such as flexible and soft strain sensors, health monitoring, and soft robotics.

A self-healing magneto-responsive nanocellulose ferrogel and flexible soft strain sensor.

Crosslinks are the building blocks of hydrogels and play an important role in their overall properties. They may either be reversible and dynamic allowing for autonomous self-healing properties, or permanent and static resulting in robustness and mechanical strength. Hence, a combination of crosslinks is often required to engineer the 3D network of hydrogels with both autonomous self-healing and required robustness for strain sensing application; however, this complicates the fabrication of such hydrogels. The facile, yet versatile, approach used in this study is to forgo the use of extra crosslinks and instead rely solely on the properties of magnetic nanocellulose to fabricate a tough, stretchy, yet magneto-responsive, ionic conductive ferrogel for strain sensing. The ferrogel also gives stimuli-free and autonomous self-healing capacity, as well as the ability to monitor real-time strain under external magnetic actuation. The ferrogel also functions as a touch-screen pen. Based on our findings, this study has the potential to advance the rational design of multifunctional hydrogels, with applications in soft and flexible strain sensors, health monitoring and soft robotics.

A 3D printable dynamic nanocellulose/nanochitin self-healing hydrogel and soft strain sensor.

Presented here is the synthesis of a 3D printable nano-polysaccharide self-healing hydrogel for flexible strain sensors. Consisting of three distinct yet complementary dynamic bonds, the crosslinked network comprises imine, hydrogen, and catecholato-metal coordination bonds. Self-healing of the hydrogel is demonstrated by macroscopic observation, rheological recovery, and compression measurements. The hydrogel was produced via imine formation of carboxyl methyl chitosan, oxidized cellulose nanofibers, and chitin nanofibers followed by two subsequent crosslinking stages: immersion in tannic acid (TA) solution to create hydrogen bonds, followed by soaking in FeIII solution to form catecholato-metal coordination bonds between TA and FeIII. The metal coordination bonds were critical to imparting conductivity to the hydrogel, a requirement for flexible strain sensors. The hydrogel exhibits excellent shear-thinning and dynamic properties with high autonomous self-healing (up to 89%) and self-recovery (up to 100%) at room temperature without external stimuli. Furthermore, it shows good printability, biocompatibility, and strain sensing ability.

Dynamic nanocellulose hydrogels: Recent advancements and future outlook.

Nanocellulose is of great interest in material science nowadays mainly because of its hydrophilic, renewable, biodegradable, and biocompatible nature, as well as its excellent mechanical strength and tailorable surface ready for modification. Currently, nanocellulose is attracting attention to overcome the current challenges of dynamic hydrogels: robustness, autonomous self-healing, and self-recovery (SELF) properties simultaneously occurring in one system. In this regard, this review aims to explore current advances in design and fabrication of dynamic nanocellulose hydrogels and elucidate how incorporating nanocellulose with dynamic motifs simultaneously improves both SELF and robustness of hydrogels. Finally, current challenges and prospects of dynamic nanocellulose hydrogels are discussed.

Dynamic Nanohybrid-Polysaccharide Hydrogels for Soft Wearable Strain Sensing.

Electroconductive hydrogels with stimuli-free self-healing and self-recovery (SELF) properties and high mechanical strength for wearable strain sensors is an area of intensive research activity at the moment. Most electroconductive hydrogels, however, consist of static bonds for mechanical strength and dynamic bonds for SELF performance, presenting a challenge to improve both properties into one single hydrogel. An alternative strategy to successfully incorporate both properties into one system is via the use of stiff or rigid, yet dynamic nano-materials. In this work, a nano-hybrid modifier derived from nano-chitin coated with ferric ions and tannic acid (TA/Fe@ChNFs) is blended into a starch/polyvinyl alcohol/polyacrylic acid (St/PVA/PAA) hydrogel. It is hypothesized that the TA/Fe@ChNFs nanohybrid imparts both mechanical strength and stimuli-free SELF properties to the hydrogel via dynamic catecholato-metal coordination bonds. Additionally, the catechol groups of TA provide mussel-inspired adhesion properties to the hydrogel. Due to its electroconductivity, toughness, stimuli-free SELF properties, and self-adhesiveness, a prototype soft wearable strain sensor is created using this hydrogel and subsequently tested.

Rational Design of Mussel-Inspired Hydrogels with Dynamic Catecholato-Metal Coordination Bonds.

Nature has often been the main source of inspiration for designing smart functional materials. As an example, mussels can attach to almost any wet surfaces, for example, wood, rocks, metal, etc., due to the presence of catechols containing amino acid 3,4-dihydroxyphenyl-l-alanine (DOPA). Fabrication of mussel-inspired hydrogels using dynamic catecholato-metal coordination bonds has recently been in the limelight because of the hydrogels' ease of gelation, interesting self-healing, self-recovery, adhesiveness, and pH-responsiveness, as well as shear-thinning and mechanical properties. Mussel inspired hydrogels take advantage of catechols, for example, DOPA in the blue mussel, to undergo catecholatometal gelation through coordination chemistry. This review explores the latest developments in the fabrication of such hydrogels using catecholato-metal coordination bonds, and discusses their potential applications in sensors, flexible electronics, tissue engineering, and wound dressing. Moreover, current challenges and prospects of such hydrogels are discussed. The main focus of this paper is on providing a deeper understanding of this growing field in terms of chemistry, physics, and associated properties.

Dynamic Mussel-Inspired Chitin Nanocomposite Hydrogels for Wearable Strain Sensors.

It is an ongoing challenge to fabricate an electroconductive and tough hydrogel with autonomous self-healing and self-recovery (SELF) for wearable strain sensors. Current electroconductive hydrogels often show a trade-off between static crosslinks for mechanical strength and dynamic crosslinks for SELF properties. In this work, a facile procedure was developed to synthesize a dynamic electroconductive hydrogel with excellent SELF and mechanical properties from starch/polyacrylic acid (St/PAA) by simply loading ferric ions (Fe3+) and tannic acid-coated chitin nanofibers (TA-ChNFs) into the hydrogel network. Based on our findings, the highest toughness was observed for the 1 wt.% TA-ChNF-reinforced hydrogel (1.43 MJ/m3), which is 10.5-fold higher than the unreinforced counterpart. Moreover, the 1 wt.% TA-ChNF-reinforced hydrogel showed the highest resistance against crack propagation and a 96.5% healing efficiency after 40 min. Therefore, it was chosen as the optimized hydrogel to pursue the remaining experiments. Due to its unique SELF performance, network stability, superior mechanical, and self-adhesiveness properties, this hydrogel demonstrates potential for applications in self-wearable strain sensors.

Dynamic plant-derived polysaccharide-based hydrogels.

Plant-derived polysaccharides are widely used to fabricate hydrogels because of their ease of gelation and functionalization, plus exceptional biological properties. As an example, nanocellulose is a suitable candidate to fabricate hydrogels for tissue engineering applications due to its enhanced mechanical and biological properties. However, hydrogels are prone to permanent failure whilst under load without the ability to reform their networks once damaged. Recently, considerable efforts are being made to fabricate dynamic hydrogels via installation of reversible crosslinks within their networks. In this paper, we review the developments in the design of dynamic hydrogels from plant-derived polysaccharides, and discuss their applications in tissue engineering, sensors, bioelectronics devices, etc. The main goal of the paper is to elucidate how the network design of hydrogels can influence their dynamic properties: self-healing and self-recovery. Complementary to this, current challenges and prospects of dynamic plant-derived hydrogels are discussed.

Dynamic Hydrogels and Polymers as Inks for Three-Dimensional Printing.

Developing rationally designed dynamic hydrogels and polymers as inks for 3D printing is in the limelight today. They would enable us to precisely fabricate complex structures in high resolutions and modular platforms with smart functions (e.g., self-healing and self-recovery), as well as tunable mechanical, chemical, and biological properties. In this paper, we explore recent developments in dynamic hydrogels and polymers as inks for 3D printing and discuss their properties and applications in tissue engineering, biomedicine, soft robotics, and sensors. The main scope of the paper is to give a deeper understanding of the field in terms of chemistry, physics, and associated properties. Moreover, the challenges and prospects of hydrogel/polymer inks will be discussed. We envisage that 3D printed dynamic hydrogels and polymers will provide unprecedented opportunities in designing and fabricating smarter structures.