Robust, tough and anti-fatigue cationic latex composite hydrogels based on dual physically cross-linked networks

Local Mechanism Governs Global Reinforcement of Nanofiller-Hydrogel Composites

We reveal the mechanism for the strong reinforcement of attractive nanofiller-hydrogel composites. Measuring the linear viscoelastic properties of hydrogels containing filler nanoparticles, we show that a significant increase of the modulus can be achieved at unexpectedly low volume fractions of nanofillers when the filler-hydrogel interactions are attractive. Using three-dimensional numerical simulations, we identify a general microscopic mechanism for the reinforcement, common to hydrogel matrices of different compositions and concentrations and containing nanofillers of varying sizes. The attractive interactions induce a local increase in the gel density around the nanofillers. The effective fillers, composed of the nanofillers and the densified regions around them, assemble into a percolated network, which constrains the gel displacement and enhances the stress coupling throughout the system. A global reinforcement of the composite is induced as the stresses become strongly coupled. This physical mechanism of reinforcement, which relies only on attractive filler-matrix interactions, provides design strategies for versatile composites that combine low nanofiller fractions with an enhanced mechanical strength.

Tough and self-healing hydrogels based on transient crosslinking by nanoparticles

In this investigation, transient crosslinking was constructed to obtain a hydrogel with excellent mechanical and self-healing properties. Firstly, core-shell particles with hydrophilic amino groups were prepared by emulsion polymerization and subsequently dispersed into hydrophobic association polyacrylamide hydrogels. Transient crosslinking was constructed through hydrogen bonding between core-shell particles and polyacrylamide. As a result, the hydrogels exhibited a tensile strength of 1.4 MPa and self-healing efficiency of 98% at 24 h. Furthermore, reconstruction of the transient crosslinking was confirmed from rheological measurements. Therefore, the essential reinforcement principle based on transient crosslinking would open a novel strategy to obtain hydrogels with superior toughness and self-healing properties.

Novel Hydrogel Material with Tailored Internal Architecture Modified by “Bio” Amphiphilic Components—Design and Analysis by a Physico-Chemical Approach

Nowadays, hydrogels are found in many applications ranging from the industrial to the biological (e.g., tissue engineering, drug delivery systems, cosmetics, water treatment, and many more). According to the specific needs of individual applications, it is necessary to be able to modify the properties of hydrogel materials, particularly the transport and mechanical properties related to their structure, which are crucial for the potential use of the hydrogels in modern material engineering. Therefore, the possibility of preparing hydrogel materials with tunable properties is a very real topic and is still being researched. A simple way to modify these properties is to alter the internal structure by adding another component. The addition of natural substances is convenient due to their biocompatibility and the possibility of biodegradation. Therefore, this work focused on hydrogels modified by a substance that is naturally found in the tissues of our body, namely lecithin. Hydrogels were prepared by different types of crosslinking (physical, ionic, and chemical). Their mechanical properties were monitored and these investigations were supplemented by drying and rehydration measurements, and supported by the morphological characterization of xerogels. With the addition of natural lecithin, it is possible to modify crucial properties of hydrogels such as porosity and mechanical properties, which will play a role in the final applications.

A highly sensitive strain sensor based on a silica@polyaniline core-shell particle reinforced hydrogel with excellent flexibility, stretchability, toughness and conductivity

Hydrogel-based flexible strain sensors for personal health monitoring and human-machine interaction have attracted wide interest among researchers. In this paper, hydrophobic association and nanocomposite conductive hydrogels were successfully prepared by introducing polyaniline coated silica (SiO2@PANI) core-shell particles into an acrylamide-lauryl methacrylate (P(AM/LMA)) copolymer matrix. The hydrophobic interaction between the SiO2@PANI core-shell particles and the hydrophobic LMA in the P(AM/LMA) chains induced the hydrogels with outstanding mechanical properties. Furthermore, the polyaniline on the SiO2 surface and the inorganic salt formed a conductive network, which synergistically enhanced the conductivity of the hydrogels. The obtained hydrogels integrate high tensile strength (1398 kPa), ultra-stretchability (>1000%), wonderful strain sensitivity (gauge factor = 10.407 at 100-1100% strain), quick response (300 ms), and excellent durability (>300 cycles) due to the hydrophobic association and nanocomposite effect. The prepared SiO2@PANI-P(AM/LMA) hydrogel shows high stress sensitivity to detect human movements and displays a broad application prospect in flexible strain-sensor field.

Physicomechanical, rheological and in vitro cytocompatibility properties of the electron beam irradiated blend hydrogels of tyramine conjugated gum tragacanth and poly (vinyl alcohol)

In the present study, preparation of blend hydrogels of tyramine conjugated gum tragacanth and poly (vinyl alcohol) was carried out by electron beam irradiation, and modification of hydrogel properties with poly (vinyl alcohol) was demonstrated. Gel content, swelling behavior, pore size and mechanical and rheological properties of hydrogels prepared at 14, 28 and 56 kilogray (kGy) with different ratios of polymers were investigated. Gel content increased from 67 ± 2% for pure tyramine conjugated gum tragacanth hydrogel to >92% for blend hydrogels. However, the corresponding equilibrium swelling degree decreased from 35.21 ± 1.51 to 9.14 ± 1.66 due to the higher crosslink density of blend hydrogel. The mechanical strength of the hydrogels with interconnected pores increased significantly in the presence of poly (vinyl alcohol) and increasing irradiation dose up to 28 kGy with a twenty-fold enhancement of stress fracture and excellent elastic recovery in cyclic compression analysis. The equilibrium swelling degree of blend hydrogel containing 3% w/v tyramine conjugated gum tragacanth and 2% w/v poly (vinyl alcohol) prepared at 28 kGy was 16.59 ± 0.81. The biocompatibility of hydrogels was tested in the presence of rabbit bone marrow mesenchymal stem cells. The viability of cells exposed to hydrogel extract was >92% after 7 days of culture and indicated hydrogel biocompatibility with potential biomedical applications.

Conductive Hydrogels-A Novel Material: Recent Advances and Future Perspectives

A conductive hydrogel is a kind of polymer material having substantial potential applications with various properties, including high toughness, self-recoverability, electrical conductivity, transparency, freezing resistance, stimuli responsiveness, stretchability, self-healing, and strain sensitivity. Herein, according to the current research status of conductive hydrogels, properties of conductive hydrogels, preparation methods of different conductive hydrogels, and their application in different fields, such as sensor and actuator fabrication, biomedicine, and soft electronics, are introduced. Furthermore, the development direction and application prospects of conductive hydrogels are proposed.

Fabrication of novel MXene (Ti3C2)/polyacrylamide nanocomposite hydrogels with enhanced mechanical and drug release properties

A highly stretchable nanocomposite (NC) hydrogel was fabricated via in situ free radical polymerization of acrylamide. In particular, an exfoliated two-dimensional MXene (Ti₃C₂) nanosheet was utilized as a crosslinker instead of traditional organic crosslinkers. The exfoliated Ti₃C₂ nanosheets were confirmed by atomic force microscopy (AFM) and dynamic light scattering (DLS) measurements. Compared with traditional organic crosslinked N,N-methylene bisacrylamide (BIS)/polyacrylamide (PAM) hydrogels (fracture strength of 32.0 kPa and elongation of 109.6%), the synthesized Ti₃C₂/PAM NC hydrogels exhibited greatly improved mechanical properties with fracture strengths of 66.5 to 102.7 kPa, compressive strengths of 400.6 to 819.4 kPa and elongations at break of 2158.6% to 3047.5% as the Ti₃C₂ content increases from 0.0145% to 0.0436%. The enhanced mechanical performances can be attributed to the honeycomb-like fine structure with uniform pores as well as more flexible polymer chains in NC hydrogel networks. When loaded with drugs, Ti₃C₂/PAM NC hydrogels exhibited good sustained-release performance, higher drug loading amounts (97.5-127.7 mg g^-1) and higher percentage releases (62.1-81.4%), greatly superior to those of the BIS/PAM hydrogel (46.4 mg g^-1, 45.0%). Our work reveals the application of MXene materials in the fabrication of NC hydrogels with enhanced mechanical and drug release behaviors.

Effect of size of latex particles on the mechanical properties of hydrogels reinforced by latex particles

Herein, cationic latex particles (CL) of different particle sizes were introduced as a cross-linking center to enhance the mechanical properties of the hydrophobically-associated hydrogels (P(AAm-co-HMA)-CL). Firstly, cationic polymethylmethacrylate (PMMA) latex particles were synthesized via soap-free emulsion polymerization. Subsequently, P(AAm-co-HMA)-CL hydrogels with outstanding mechanical properties were prepared using acrylamide as the monomer, hexadecyl methacrylate as the hydrophobic molecule, and CL as the cross-linking center. The size of CL had a significant effect on the mechanical properties and self-recovery properties of the P(AAm-co-HMA)-CL hydrogels. The hydrogel with larger CL size exhibited low mechanical properties due to weak hydrophobic interactions. In contrast, the hydrogel with small CL size displayed excellent mechanical properties due to an effective entanglement of the hydrophobic chains with the smaller size CL, which significantly affects the mechanical properties of the hydrogel. As a result, the maximum fracture stress and fracture strain of the hydrogel were up to 1.47 MPa and 2847%, respectively. This study can have a profound impact on the development of the technology of toughening hydrogels with latex particles.

Tailoring the self-assembly of linear alkyl chains for the design of advanced materials with technological applications

The self-assembly of n-alkyl chains at the bulk or at the interface of different types of materials and substrates has been extensively studied in the past. The packing of alkyl chains is driven by Van der Waals interactions and can generate crystalline or disordered domains, at the bulk of the material, or self-assembled monolayers at an interface. This natural property of alkyl chains has been employed in recent years to develop a new generation of materials for technological applications. These studies are dispersed in a variety of journals. The purpose of this article was to discuss some selected examples where these advanced properties arise from a process involving the self-assembly of alkyl chains. We included a description of electronic devices and new-generation catalysts with properties derived from a controlled two-dimensional (2D) or three-dimensional (3D) self-assembly of alkyl chains at an interface. Then, we showed that controlling the crystallization of alkyl chains at the bulk can be used to generate a variety of advanced materials such as superhydrophobic coatings, shape memory hydrogels, hot-melt adhesives, thermally reversible light scattering (TRLS) films for intelligent windows and form-stable phase change materials (FS-PCMs) for the storage of thermal energy. Finally, we discussed two examples where advanced properties derive from the formation of disordered domains by physical association of alkyl chains. This was the case of photoluminescent nanocomposites and materials used for reversible optical storage.

Selective Adsorption and Separation of Organic Dyes by Poly(acrylic acid) Hydrogels Formed with Spherical Polymer Brushes and Chitosan

Direct discharge of industry organic dyes has caused serious environmental pollution. In this study, a series of double network poly(acrylic acid) (PAA) hydrogels were fabricated with spherical polymer brushes (SPBs) and chitosan (CS) as crosslinker. Neutral spherical polyelectrolyte brushes of polystyrene–poly-N-isopropylacrylamide (PNIPAM@PS) in which poly(N-isopropylacrylamide) (PNIPAM) arms were grafted on polystyrene (PS) nanospheres, were employed as macro-crosslinkers. The innumerable hydrogen bonds both between the highly entangled PAA chains and between PNIPAM and the PAA chains composed the first network of the hydrogels. The electrostatic interactions between CS and the PAA chains formed the second network of the hydrogels. These double network hydrogels, named PNIPAM@PS/CS/PAA, achieve good compressive performance and a low swell ratio because of their compact structure through plentiful hydrogen bonding and electrostatic interactions. The hydrogel could absorb cationic dyes from water with high separation efficiency and selectivity due to the electrostatic interaction between the carboxy groups and dye molecules. The adsorption process fitted a pseudo-second-order kinetic model and Langmuir isotherm model very well. Moreover, the hydrogel can separate cationic dyes from mixed dye solutions through electrostatic interactions. After being loaded with silver nanoparticles, the obtained silver@hydrogel exhibited a good capacity for the photocatalytic degradation towards different dyes. The hydrogels are promising for dye-containing wastewater treatment.

Highly tough, anti-fatigue and rapidly self-recoverable hydrogels reinforced with core-shell inorganic-organic hybrid latex particles

The introduction of SiO₂ particles as crosslinking points into hydrogels has been recognized as a suitable way for toughening hydrogels, due to their versatile functionalization and large specific surface area. However, chemically linked SiO₂ nanocomposite hydrogels often exhibited negligible fatigue resistance and poor self-recoverable properties due to the irreversible cleavage of covalent bonds. Here, we proposed a novel strategy to improve stretchability, fatigue resistance and self-recoverable properties of hydrogels by using SiO₂-g-poly(butyl acrylate) core-shell inorganic-organic hybrid latex particles as hydrophobic crosslinking centers for hydrophobic association. The obtained hydrogel could distribute the surrounding applied stress by disentanglement of the hybrid latex particles from hydrophobic segments. Based on this strategy, the formulated hydrogels showed an excellent tensile strength of 1.48 MPa, superior stretchability of 2511% and remarkable toughness of 12.62 MJ m^-3. Moreover, the hydrogels owned extraordinary anti-fatigue, rapid self-recovery and puncture resistance properties. Therefore, this strategy provided a novel pathway for developing advanced soft materials with potential applications in biomedical engineering, such as tendons, muscles, cartilages, etc.