A Highly Mechanical, Conductive, and Cryophylactic Double Network Hydrogel for Flexible and Low-Temperature Tolerant Strain Sensors

Strong, tough, and elastic poly(vinyl alcohol)/polyacrylamide DN hydrogels based on the Hofmeister effect for articular cartilage replacement

Traditional hydrogels are usually weak and brittle, which limit their application in articular cartilage replacement because cartilage is generally strong, tough, and elastic in nature. Therefore, it is highly desirable to construct hydrogels to mimic the mechanical properties of the native articular cartilage. Herein, in this work, poly(vinyl alcohol)/polyacrylamide (PVA/PAM) DN hydrogels were prepared by in situ polymerization, which were then treated with Hofmeister series ions (Cit3-, SO42-, and Cl-) to achieve H-PVA/PAM DN hydrogels. Among the three Hofmeister ions, the DN hydrogel treated with Cit3- (named PVA/PAM-Cit) showed the densest microstructure and the highest crystallinity degree. In this context, PVA/PAM-Cit exhibited a tensile strength of 18.9 ± 1.6 MPa, a compressive strength of 102.3 ± 7.9 MPa, a tensile modulus of 10.6 ± 2.1 MPa, a compressive modulus of 8.9 ± 0.8 MPa, and a roughness of 66.2 ± 4.2 MJ m-3, respectively, which were the highest strength and modulus, and the second highest toughness when compared with those of the reported PVA and PVA based DN hydrogels so far. It also showed an extreme high elasticity, which could maintain a stress of 99.2% after 500 cycles of fatigue testing. Additionally, PVA/PAM-Cit can promote the adhesion, spreading and proliferation of chondrocytes. These results verify that such a strong, tough, and elastic hydrogel could be a novel candidate material for articular cartilage replacement.

Dual Network Hydrogel with High Mechanical Properties, Electrical Conductivity, Water Retention and Frost Resistance, Suitable for Wearable Strain Sensors

With the progress of science and technology, intelligent wearable devices have become more and more popular in our daily life. Hydrogels are widely used in flexible sensors due to their good tensile and electrical conductivity. However, traditional water-based hydrogels are limited by shortcomings of water retention and frost resistance if they are used as the application materials of flexible sensors. In this study, the composite hydrogels formed by polyacrylamide (PAM) and TEMPO-Oxidized Cellulose Nanofibers (TOCNs) are immersed in LiCl/CaCl2/GI solvent to form double network (DN) hydrogel with better mechanical properties. The method of solvent replacement give the hydrogel good water retention and frost resistance, and the weight retention rate of the hydrogel was 80.5% after 15 days. The organic hydrogels still have good electrical and mechanical properties after 10 months, and can work normally at −20 °C, and has excellent transparency. The organic hydrogel show satisfactory sensitivity to tensile deformation, which has great potential in the field of strain sensors.

Investigation of Soft Matter Nanomechanics by Atomic Force Microscopy and Optical Tweezers: A Comprehensive Review

Soft matter exhibits a multitude of intrinsic physico-chemical attributes. Their mechanical properties are crucial characteristics to define their performance. In this context, the rigidity of these systems under exerted load forces is covered by the field of biomechanics. Moreover, cellular transduction processes which are involved in health and disease conditions are significantly affected by exogenous biomechanical actions. In this framework, atomic force microscopy (AFM) and optical tweezers (OT) can play an important role to determine the biomechanical parameters of the investigated systems at the single-molecule level. This review aims to fully comprehend the interplay between mechanical forces and soft matter systems. In particular, we outline the capabilities of AFM and OT compared to other classical bulk techniques to determine nanomechanical parameters such as Young's modulus. We also provide some recent examples of nanomechanical measurements performed using AFM and OT in hydrogels, biopolymers and cellular systems, among others. We expect the present manuscript will aid potential readers and stakeholders to fully understand the potential applications of AFM and OT to soft matter systems.

Herbal molecule-mediated dual network hydrogels with adhesive and antibacterial properties for strain and pressure sensing

Multifunctional integration is the focus of hydrogel-based flexible sensors, and formation of a dual network (DN) could shed light on the fabrication of hydrogels with multifunctionality and enhanced properties. In this study, a DN hydrogel was fabricated by the self-assembly of herbal molecule glycyrrhizic acid (GA) as the first hydrogel network and subsequent photocrosslinking of methacrylated sodium alginate (SA-MA) to form the second network. Profiting from the good compatibility between the two hydrogel networks, the obtained DN hydrogels with a homogeneous porous microstructure were endowed with remarkably enlarged stretching (114.5%) and compression (74.4%) strains. In addition, they were demonstrated to display excellent bacteriostatic activity (>99.9%) against Escherichia coli and Staphylococcus aureus owing to the synergetic antibacterial effect of GA and SA-MA. The DN hydrogels as strain sensors possessed high sensitivity (GF = 1.39), linear sensing (R ² > 0.99), rapid response (180 ms), and good stability (1300 times) for human motion detection. Besides, the DN hydrogels could also be used to conduct pressure sensing such as application of heavy weights and even human pulses. All results suggest that the developed DN hydrogels have great potential in serving as epidermal and implantable flexible sensors for human health monitoring.

A Review on Thermal Properties of Hydrogels for Electronic Devices Applications

Hydrogels, as a series of three-dimensional, crosslinked, hydrophilic network polymers, exhibit extraordinary properties in softness, mechanical robustness and biocompatibility, which have been extensively utilized in various fields, especially for electronic devices. However, since hydrogels contain plenty of water, the mechanical and electrochemical properties are susceptible to temperature. The thermal characteristics of hydrogels can significantly affect the performance of flexible electronic devices. In this review, recent research on the thermal characteristics of hydrogels and their applications in electronic devices is summarized. The focus of future work is also proposed. The thermal stability, thermoresponsiveness and thermal conductivity of hydrogels are discussed in detail. Anti-freezing and anti-drying properties are the critical points for the thermal stability of hydrogels. Methods such as introducing soluble ions and organic solvents into hydrogels, forming ionogels, modifying polymer chains and incorporating nanomaterials can improve the thermal stability of hydrogels under extreme environments. In addition, the critical solution temperature is crucial for thermoresponsive hydrogels. The thermoresponsive capacity of hydrogels is usually affected by the composition, concentration, crosslinking degree and hydrophilic/hydrophobic characteristics of copolymers. In addition, the thermal conductivity of hydrogels plays a vital role in the electronics applications. Adding nanocomposites into hydrogels is an effective way to enhance the thermal conductivity of hydrogels.

Compliant, Tough, Anti-Fatigue, Self-Recovery, and Biocompatible PHEMA-Based Hydrogels for Breast Tissue Replacement Enabled by Hydrogen Bonding Enhancement and Suppressed Phase Separation

Although hydrogel is a promising prosthesis implantation material for breast reconstruction, there is no suitable hydrogel with proper mechanical properties and good biocompatibility. Here, we report a series of compliant and tough poly (hydroxyethyl methacrylate) (PHEMA)-based hydrogels based on hydrogen bond-reinforcing interactions and phase separation inhibition by introducing maleic acid (MA) units. As a result, the tensile strength, fracture strain, tensile modulus, and toughness are up to 420 kPa, 293.4%, 770 kPa, and 0.86 MJ/m³, respectively. Moreover, the hydrogels possess good compliance, where the compression modulus is comparable to that of the silicone breast prosthesis (~23 kPa). Meanwhile, the hydrogels have an excellent self-recovery ability and fatigue resistance: the dissipative energy and elastic modulus recover almost completely after waiting for 2 min under cyclic compression, and the maximum strength remains essentially unchanged after 1000 cyclic compressions. More importantly, in vitro cellular experiments and in vivo animal experiments demonstrate that the hydrogels have good biocompatibility and stability. The biocompatible hydrogels with breast tissue-like mechanical properties hold great potential as an alternative implant material for reconstructing breasts.