Hydrophobic Association Hydrogels Research Articles

Starch/ionic liquid/hydrophobic association hydrogel with high stretchability, fatigue resistance, self-recovery and conductivity for sensitive wireless wearable sensors

Conductive hydrogels have been widely used in wearable electronics due to their flexible, conductive and adjustable properties. With ever-growing demand for sustainable and biocompatible sensing materials, biopolymer-based hydrogels have drawn significant attention. Among them, starch-based hydrogels have a great potential for wearable electronics. However, it remains challenging to develop multifunctional starch-based hydrogels with high stretchability, good conductivity, excellent durability and high sensitivity. Herein, amylopectin and ionic liquid were introduced into a hydrophobic association hydrogel to endow it with versatility. Benefiting from the synergistic effect of amylopectin and ionic liquid, the hydrogel exhibited excellent mechanical properties (the elongation of 2540 % with a Young's modulus of 12.0 kPa and a toughness of 1.3 MJ·m−3), self-recovery, good electrical properties (a conductivity of 1.8 S·m−1 and electrical self-healing), high sensitivity (gauge factor up to 26.85) and excellent durability (5850 cycles). The above properties of the hydrogel were closely correlated to its internal structure from hydrophobic association, H-bonding and electrostatic interaction, and can be regulated by changing the component contents. A wireless wearable sensor based on the hydrogel realized accurate and stable monitoring of joint motions and expression changes. This work demonstrates a kind of promising biopolymer-based materials as candidates for high-performance flexible wearable sensors.

Hydrophobic Association Hydrogel Enabled by Multiple Noncovalent Interactions for Wearable Bioelectronics in Amphibious Environments

Hydrophobic Association Hydrogel Enabled by Multiple Noncovalent Interactions for Wearable Bioelectronics in Amphibious Environments

Synthesis of a polyacrylamide hydrogel modified with a reactive carbamate surfactant: Characterization, swelling behavior, and mathematical models

Hydrogels are polymeric materials that can absorb and retain large amounts of water. They have various applications in agriculture, horticulture, oil recovery, and water management. However, conventional hydrogels have limited swelling (Q), equilibrium swelling (Q∞), and water retention capacities (DQ) under different environmental conditions. In this study, we synthesized a series of hydrophobically associated hydrogels by the aqueous polymerization of acrylamide, acrylic acid, 2-acrylamide-2-methylpropanesulfonic acid, and a novel amphiphilic hydrophobic monomer (RCS-10). Further, we investigated the effects of the RCS-10 content on Q, the mechanical properties, and water retention capacity (DQ) of the hydrogels. We also examined the influence of temperature, pH, salt type and concentration, and urea concentration on Q, finding that an increase in the RCS-10 content decreased Q and increased the DQ of the hydrogels. The hydrogels exhibited high Q in distilled water, low-salt solutions, and low-urea (1 wt%) solutions. In the latter, the Q∞ was as high as 768.99 g/g. The hydrogels also showed excellent cyclic swelling performance and maintained high Q∞ (759.99 g/g) after six cycles. Crucially, the hydrophobic association hydrogels demonstrated superior swelling and water retention properties compared to conventional hydrogels. Thus, the hydrogels have applications in water management in various fields including agriculture and horticulture.

Multi-Physically Cross-Linked Hydrogels for Flexible Sensors with High Strength and Self-Healing Properties.

Excellent mechanical properties and self-healing properties are very important for the practical application of hydrogel flexible sensors. In this study, acrylic acid and stearyl methyl acrylate were selected as monomers to synthesize hydrophobic association hydrogels, and multi-physically cross-linked hydrogels were synthesized by adding ferric chloride and polyvinyl alcohol to introduce ion interaction and a hydrogen bond cross-linking network. The hydrogels were characterized by FTIR, XRD and SEM, and the mechanical properties and self-healing properties were tested using a universal testing machine. It was confirmed that the strength of the hydrogel was significantly improved with the addition of ferric chloride and polyvinyl alcohol, and the hydrogel still showed good self-healing properties. Further testing of its application as a conductive sensor has demonstrated sensitive and stable motion sensing capabilities. This provides an important reference for high-performance hydrogel sensors with both high strength and self-healing properties.

Flexible MXene‐Based Hydrogel Enables Wearable Human–Computer Interaction for Intelligent Underwater Communication and Sensing Rescue

AbstractConductive hydrogels have recently attracted extensive attention in the field of smart wearable electronics. Despite the current versatility of conductive hydrogels, the balance between mechanical properties (tensile properties, strength, and toughness) and electrical properties (electrical conductivity, sensitivity, and stability) still faces great challenges. Herein, a simplified method for constructing hydrophobic association hydrogels with excellent mechanical and electrical properties is proposed. The prepared conductive hydrogels exhibit high tensile properties (≈1224%), high linearity in the whole‐strain–range (R2 = 0.999), and a wide strain sensing range (2700%). The conductive hydrogel can realize more than 1000 cycles of sensing under 500% tensile strain. As an application demonstration, an underwater communication device is assembled in combination with polydimethylsiloxane/Triton X‐100 film coating, which successfully transmits underwater signals and provides warning of potential hazards. This study provides a new research method for developing underwater equipment with excellent mechanical properties and sensing properties.

Arrested Phase Separation Enables High-Performance Keratoprostheses.

Corneal transplantation is impeded by donor shortages, immune rejection, and ethical reservations. Pre-made cornea prostheses (keratoprostheses) offer a proven option to alleviate these issues. Ideal keratoprostheses must possess optical clarity and mechanical robustness, but also high permeability, processability, and recyclability. Here, it is shown that rationally controlling the extent of arrested phase separation can lead to optimized multiscale structure that reconciles permeability and transparency, a previously conflicting goal by common pore-forming strategies. The process is simply accomplished by hydrothermally treating a dense and transparent hydrophobic association hydrogel. The examination of multiscale structure evolution during hydrothermal treatment reveals that the phase separation with upper miscibility gap evolves to confer time-dependent pore growth due to slow dynamics of polymer-rich phase which is close to vitrification. Such a process can render a combination of multiple desired properties that equal or surpass those of the state-of-the-art keratoprostheses. In vivo tests confirm that the keratoprosthesis can effectively repair corneal perforation and restore a transparent cornea with treatment outcomes akin to that of allo-keratoplasty. The keratoprosthesis iseasy to access and convenient to carry, and thus would be an effective temporary substitute for a corneal allograft in emergency conditions.

Synthesis and characterization of a hydrophobic association hydrogel for drug delivery

This work reports the synthesis, characterization, and evaluation of a hydrophobic association hydrogel containing vinyl alginate (SA), acrylamide, and the hydrophobic molecule N,N'-(disulfanediylbis(4,1-phenylene))diacrylamide (SPDAAm) for release of drug very slightly water-soluble in water. The SPDAAm molecule was synthetized with acrylic acid and 4-Aminophenyl disulfide and further characterized by H1 NMR spectroscopy. The hydrogel was prepared with a crosslinking chemistry using vinyl SA and acrylamide in an aqueous surfactant solution at a concentration less than the (critical micellar concentration) CMC. The introduction of SPDAAm in the hydrogel increases the swelling degree owing to hydrophobic association. Furthermore, SPDAAm gives a pH-dependent and retardant effect for caffeine (CFN) release. At a pH of 7.4, the CFN release mechanism is controlled by swelling, macromolecular relaxation of the polymer, and erosion of the hydrogel. The hydrogel with SPDAAm shows acceptable biocompatibility and has potential applications as a biomaterial in controlled release of poorly water-soluble molecules, like CFN.

High‐strength, ultrastretchable hydrophobic association hydrogel reinforced by tailored modified carboxymethyl cellulose

AbstractA new pathway to construct high‐strength, ultrastretchable hydrogels based on tailored modified carboxymethyl cellulose (TMCMC)and hydrophobic association(HPA) system was investigated. TMCMC was prepared from degradation of carboxymethyl cellulose (CMC)/N,N′‐methylene bisacrylamide (MBA) chemical cross‐linked hydrogels. The residual double bonds of TMCMC, confirmed by 1H‐NMR and FTIR analysis, reacted with acrylamide in HPA to form TMCMC/HPA hydrogels, and the homogeneous and fine spatial network structure of TMCMC/HPA hydrogel could be observed by scanning electron microscopy. Rheological analysis revealed that the coexistence of physical and chemical crosslinking in TMCMC/HPA hydrogels. Further studies showed that TMCMC/HPA hydrogel with MBA content of 0.5 wt% has outstanding mechanical properties, and its fracture stress, elongation and tensile strength reach 1.17 MPa, 3717.05% and 15.68 MJ·m−3, respectively. Moreover, the hydrogel displayed good swelling resistance and stable strain electrical signal response.

Length Effects of Short Alkyl Side Chains on Phase-Separated Structure and Dynamics of Hydrophobic Association Hydrogels

Length Effects of Short Alkyl Side Chains on Phase-Separated Structure and Dynamics of Hydrophobic Association Hydrogels

Tough and self‐healing hydrophobic association hydrogels with cationic surfactant

AbstractHydrophobic association (HA) hydrogels with outstanding mechanical, rheological and recovery properties were successfully synthesized by micellar copolymerization of acrylamide with lauryl methacrylate. The synthesis occurred at room temperature and the synthesis condition was moderate by using the redox initiator system of Ammonium persulfate ‐ sodium bisulfite as initiators. Cationic surfactant (dodecyl trimethyl ammonium bromide) was utilized to form micelles with hydrophobe, served as physical cross‐linking points in the 3D networks of hydrogels. The HA hydrogels showed a high tensile strength of 181 kPa, superior stretchability of 2300% and excellent toughness of 2.16 MJ m−3. Moreover, they owned extraordinary self‐recovery under different conditions. It is hopeful that the hydrogels with superior mechanical strength and self‐healing properties would be applied to the fields of biomedicine and engineering. Meanwhile, based on above materials, HA hydrogels could also be synthesized with the combination of hydrophobic association and other synergistic effects, such as latex particles, electrostatic effect and nanoparticles.

Flexible strain sensors with rapid self-healing by multiple hydrogen bonds

Hydrogels have been applied in flexible strain sensors due to good conductivity, stretchability and sensitivity to strain. However, gaining rapid and effective self-healing hydrogels for the durability of flexible strain sensors is still a challenging. Herein, we report a tough, conductive hydrogel that can rapidly self-healing at room temperature. The hydrogel is fabricated by incorporating polystyrene-co-poly (N,N-dimethylacrylamide) (P (St-co-DMAA)) microspheres prepared by emulsion polymerization into hydrophobic association hydrogels networks composed of polyacrylamide-co-poly (hexadecyl methacrylate) (PAAm-co-PHMA), sodium dodecyl sulfate (SDS) and sodium chloride (NaCl). The hydrogel possesses a dual dynamic crosslinking comprising of multiple hydrogen bonds and hydrophobic association. The multiple hydrogen bonds formed between the outer regions of microspheres and PAAm chains provide rapid self-healing of hydrogels and contribute to re-association of hydrophobic chains and further improve self-healing efficiency. (99% Self-healing efficiency at room temperature for 2 h). At the same time, the dual crosslinking also gives the hydrogel good mechanical properties and fatigue resistance. In addition, the existence of sodium ions and chloride ions also endowed the hydrogel high sensitive deformation-dependent conductivity, make the hydrogel able to sensitive sensing of large and small strain signals (e.g. finger bending and breathing). And the sensing ability of hydrogel can be also recovered quickly after self-healing. Therefore, this strategy will expand the application range of a new generation of hydrogels such as self-healing conductors, robotic electronic skin and strain sensors, etc.

Highly stretchable, self-healing, and 3D printing prefabricatable hydrophobic association hydrogels with the assistance of electrostatic interaction

Self-healing and 3D printing prefabricatable physically crosslinked hydrogels were prepared by copolymerization of butyl acrylate, 2-(dimethylamino)ethyl methacrylate, and methacrylic acid, followed by soaking in water.

Toughening Mechanism of Hydrophobic Association Hydrogels Reinforced by Latex Particles

AbstractIn this investigation, the copolymer poly(n‐butyl acrylate)‐b‐PEG‐b‐poly(n‐butyl acrylate) hydrogel is synthesized by atom transform radical polymerization and chosen as a simplified hydrophobic association hydrogel model. After adding the latex particles, hydrophobic poly(n‐butyl acrylate) segments can self‐assemble in an aqueous solution, forming a hydrogel (LP‐Gel) model with network structure. Structures of this model system are investigated by small angle X‐ray scattering. Dynamics are investigated through a combination of linear viscoelastic and creep measurements. It is found that introduction of the latex particles increases the crosslinking density and the association energy compared to the particles‐free system. This simple LP‐Gel model reveals the toughening mechanism of latex particle‐modified hydrophobic association hydrogels.

Mechanically robust hydrophobic association hydrogel electrolyte with efficient ionic transport for flexible supercapacitors

Hydrogel electrolytes are attracting increasing interest and have made great progress in flexible supercapacitors (SCs). However, the unsatisfactory ionic conductivities and mechanical properties of traditional hydrogel electrolytes greatly limit their use in the flexible SCs, which is hard to run a long-term operation and repeated-deformation for energy devices. Here a hydrophobic association hydrogel electrolyte (HA-GPE) for flexible supercapacitors was successfully prepared by a facile and effective polymerization process. The stable hydrophobic association physical crosslinking design can disperse external stress effectively, provides HA-GPE robust mechanical properties with a tensile strength of 431.9 kPa, an elongation at break of 1150.2% and a compressive stress of 4.8 MPa, much better than those hydrogel electrolytes recently reported. While methodical molecular design enables abundant ion-conductive amine (–NH2) and ethylene oxide (EO) functional groups distributed in HA-GPE network, together with the interconnected porous network structures, allowing for more efficient Li+ ions transport, bring the ionic conductivity achieves as high as 31.1 mS cm−1 at room temperature, which is 3 times higher than the corresponding liquid electrolyte (10.5 mS cm−1). The flexible supercapacitor using this HA-GPE as the electrolyte and the activated carbon as the electrode exhibits excellent electrochemical performance with the specific capacitance of 130.3 F g−1 at 0.5 A g−1 and capacitance retention of 90.1% after 5000 cycles, superior to that of supercapacitor with liquid electrolyte and commercial separator. Additionally, the device associated with the HA-GPE possesses outstanding electrochemical stability under both bending states and durability test. All these results suggest that the HA hydrogel electrolyte is a promising candidate for portable electronics.

Swelling behavior of hydrophobic association hydrogels prepared from octylphenol polyoxyethylene (7) acrylate and sodium dodecylsulfate

Hydrophobic association hydrogels (HA-gels) were successfully synthesized from octylphenol polyoxyethylene (7) acrylate (OP-7-AC) and sodium dodecylsulfate (SDS) and their swelling behaviors were studied. Swelling test indicates that the swelling ratio and swelling rate of hydrogel go down first then go up with increased content of SDS. HA-gels exhibited a special “swelling overshoot” phenomenon because of the dissolution of non-gel compositions and surplus SDS. Diffusion of water molecules or the extension of network chains played a major role in the beginning of swelling process of xerogel. With OP-7-AC content increased, swelling ratio of hydrogel and xerogel increased and the “swelling overshoot” became more obvious. At the beginning of swelling, the gel loss increased sharply and then tended to be constant. The gel loss increased with the increase in SDS and OP-7-AC content. The results indicate that SDS and OP-7-AC contents have great effects on the swelling ratio and stability of HA-gels.

Tough hydrophobic association hydrogels with self-healing and reforming capabilities achieved by polymeric core-shell nanoparticles

The application range of hydrogels can be greatly widened by improving their mechanical properties. It is still a great challenge to develop hydrogels with good mechanical properties, reliable self-healing properties and remolding ability at the same time. Inspired by biological soft tissue with excellent mechanical properties and self-healing properties, here, a facile method to fabricate poly (styrene-acrylic acid) (P(S-AA)) core-shell nanoparticles with plenty of carboxyl groups on their surface, and their enhancement to hydrophobic association hydrogels was reported. Under stress, the dynamic physical bonds including hydrogen bonding between polymer chains and P(S-AA) core-shell nanoparticles (NPs), and entanglement of hydrophobic chains were destroyed to effectively dissipate energy, and uniform hydrogel network leads to smooth stress-transfer, which makes the core-shell nanoparticles composite hydrophobic association hydrogels (MHA gels) excellent mechanical properties, such as excellent mechanical properties, toughness and ductility, and good self-healing properties as well. These features make the MHA gels have great potential in biomedical applications such as tissue engineering, articular cartilage and artificial skin.

Robust and anti-fatigue hydrophobic association hydrogels assisted by titanium dioxide for photocatalytic activity.

Currently, robust and functional hydrogels have attracted extensive attention due to their potential applications in wastewater treatment, farmland water conservation and other fields. Herein, a series of hydrophobic association hydrogels assisted by titanium dioxide (TiO2) was fabricated via one-pot in situ photo-induced polymerization. TiO2 nanoparticles could act as both photo-initiators and physical crosslinking points. The TiO2-assisted hydrophobic association hydrogels exhibited a high tensile strength of 306 kPa, superior compression strength of 2.17 MPa and excellent fatigue resistance. Simultaneously, the incorporation of TiO2 endowed the hydrogel with photocatalytic capacity for dye wastewater treatment based on the inherent nature of TiO2. The results indicated that the hydrogels contributed to the degradation of various ionic dyes including methylene blue, rhodamine B and bromophenol blue, and the removal of methylene blue achieved a rate of 96.63%. Significantly, the hydrogel could be repeatedly utilized and the removal rate showed no evident decrease after five cycles, indicating that the hydrogels could be powerful candidates as photocatalysts for dye wastewater treatment.

The effect of methyl group on the mechanical properties of hydrophobic association hydrogel

ABSTRACTHydrophobic association hydrogels were fabricated via micellar copolymerization of acrylamide and hydrophobic monomers lauryl (meth)acrylate (LA or LMA) in an aqueous solution of sodium dodecyl sulfate. The effect of methyl groups of hydrophobic monomers on the crosslinking network structure and mechanical behavior of the gels was investigated on the basis of rubber elastic theory. It was found that the LMA‐gel exhibited higher effective crosslink density and elastic modulus. The methyl groups of hydrophobic monomers limited the flexibility of the methacrylate backbone in the association domain, which resulted in the increment of chains constraints. With the increase of stretch rate, the dissipated energy of LMA‐gel increased more highly than that of LA‐gel. In addition, the methyl group hindered the movement of polymer chains, leading to the lower recovery efficiency of dissipated energy for LMA‐gel. In contract, the LA‐gel exhibited a rapid response to external force, and possessed better elasticity and self‐recovery property. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 1505–1512

Effects of high-concentration salt solutions and pH on swelling behavior of physically and chemically cross-linked hybrid hydrophobic association hydrogels with good mechanical strength

ABSTRACTNew physically and chemically cross-linked hybrid hydrophobic association hydrogels (Hybrid HA-gels) were prepared by radical copolymerization. The effect of salinity on the swelling behavior of Hybrid HA-gels was studied. Hybrid HA-gels, like ampholytic hydrogels, displayed antipolyelectrolyte behavior in high-concentration NaCl and MgCl2 solutions. According to the analysis of experimental data, the antipolyeletrolyte behavior of Hybrid HA-gels should be attributed to the rapid hydrolysis of amide groups on polymer, and the hydrolysis reaction can be promoted by high concentrations of salt solutions. Also, the results of swelling tests verified also the reversibility of complexing action of Ca2+ with carboxylate groups within the Hybrid HA-gels. In addition, the effect of pH values on the swelling behavior of the Hybrid HA-gels was also studied in detail.

Tough, sticky and remoldable hydrophobic association hydrogel regulated by polysaccharide and sodium dodecyl sulfate as emulsifiers

Hydrophobic association hydrogels have been extensively studied during the past decades. However, the fracture stress of hydrophobic association hydrogels obtained with anionic surfactants (such as sodium dodecyl sulfate) achieved hundreds of Pascal. In this investigation, combined surfactants consisting of polysaccharide (gum arabic) and sodium dodecyl sulfate were utilized to stabilize hydrogels with high fracture stress of more than 1 MPa. Moreover, the hydrogels exhibited excellent self-healing capacity and remoldable behavior without any stimulation. Simultaneously, the hydrogels demonstrated an adhesive behavior for various solid substrates including polytetrafluoroethylene, plastics, rubbers, glasses, metals and woods. The hydrogel with toughness, self-healing, stickiness and remoldable properties would be expected to be applied in the medical fields, such as wound dressing, medical electrodes, tissue adhesives and portable equipment.