Poly Acrylic Acid Network Research Articles

A Skin Stress Shielding Platform Based on Body Temperature-Induced Shrinking of Hydrogel for Promoting Scar-Less Wound Healing.

Stress concentration surrounding wounds drives fibroblasts into a state of high mechanical tension, leading to the delay of wound healing, exacerbating pathological fibrosis, and even causing tissue dysfunction. Here, an innovative skin stress-shielding hydrogel wound dressing is reported that makes the wound sites shrink as a response to body temperature and then remolds the stress micro-environment of wound sites to reduce the formation of skin scars. Composed of a modified natural temperature-sensitive polymer cross-linked with polyacrylic acid networks, this hydrogel wound dressing has demonstrated a substantial decrease in scar area for full-thickness wounds in rat models. The physical forces exerted by the wound dressing are instrumental in attenuating the activation and transduction of fibroblasts within the wound sites, thereby mitigating the excessive deposition of the extracellular matrix (ECM). Notably, the wound dressing significantly down-regulates the expression of transforming growth factor-β1(TGF-β1) and collagen I, while concurrently exerting a dramatic inhibitory effect on the integrin-focal adhesion kinase (FAK)/phosphorylated-FAK (p-FAK) signaling pathway. Collectively, the fabrication of functional hydrogels with a stress-shielding profile is a new route for achieving scar-less wound healing, thus offering immense potential for improving clinical outcomes and restoring tissue integrity.

Sodium carboxymethyl cellulose and MXene reinforced multifunctional conductive hydrogels for multimodal sensors and flexible supercapacitors

With the growing demand for eco-friendly materials in wearable smart electronic devices, renewable, biocompatible, and low-cost hydrogels based on natural polymers have attracted much attention. Cellulose, as one of the renewable and degradable natural polymers, shows great potential in wearable smart electronic devices. Multifunctional conductive cellulose-based hydrogels are designed for flexible electronic devices by adding sodium carboxymethyl cellulose and MXene into polyacrylic acid networks. The multifunctional hydrogels possess excellent mechanical property (stress: 310 kPa; strain: 1127 %), toughness (206.67 KJ m−3), conductivity (1.09 ± 0.12 S m−1) and adhesion (82.19 ± 3.65 kPa). The multifunctional conductive hydrogels serve as strain sensors (Gauge Factor (GF) = 5.79, 0–700 % strain; GF = 14.0, 700–900 % strain; GF = 40.36, 900–1000 % strain; response time: 300 ms; recovery time: 200 ms) and temperature sensors (Temperature coefficient of resistance (TCR) = 2.5755 °C−1 at 35 °C- 60 °C). The sensor detects human activities with clear and steady signals. A distributed array of flexible sensors is created to measure the magnitude and distribution of pressure and a hydrogel-based flexible touch keyboard is also fabricated to recognize writing trajectories, pressures and speeds. Furthermore, a flexible hydrogel-based supercapacitor powers the LED and exhibits good cyclic stability over 15,000 charge-discharge cycles.

Notch-insensitive, underwater adhesive, and self-healing ionic skins

Ionic skins (I-skins) mimic the sensing function of natural skin based on ionic conduction have essential applications in wearable sensors for human motion detections. However, the fragile, low electrical sensitivity, lack of adhesive and self-healing performances still limits their practical applications and service life. Herein, we reported an ionic skin which was fabricated by the ionic liquid, acrylic acid and Fe 3+ through the one-pot method. The polyacrylic acid network is cross-linked by intermolecular hydrogen bonds and the complexation of Fe 3+ , where the ionic liquid can bind through reversible electrostatic interactions. Such design endows the resultant hydrogel with superior tensile properties (tensile strength 341 kPa, strain at break 2042%), good notch insensitivity, air/underwater dual adhesive property for the surface of various solid materials (including glass, iron, copper and aluminum) and excellent healability (self-healing efficiency as high as 80.5% at room temperature). In addition, the I-skin exhibits ultra-high electrical sensitivity (GF = 13.12) and an extensive strain monitoring range (5%–300%). Not only can it be used as expression and human motion detector, but also can realize “information programming”. The rich application scenarios and functions prove its potential to be used as wearable sensors. • Physical cross-linking hydrogel prepared by one-pot method. • Not-insensitive, air/underwater adhesive. • Mechanical/electrical dual self-healing. • Wearable strain sensors for human motion detection. • Electrical signal conversion through Morse code.

Lignin reinforced hydrogels with fast self-recovery, multi-functionalities via calcium ion bridging for flexible smart sensing applications

Hydrogels have found applications in many different fields. However, poor mechanical properties, such as low elasticity and lack of rapid recovery under large deformation, can severely limit their applications. In this study, we developed lignin reinforced hydrogels made of calcium ion containing ternary polymers (lignosulfonate (LS), alginate (Alg), and polyacrylic acid (PAA)). The resultant hydrogel has excellent elasticity, rapid self-recovery, and multi-functionalities. The covalent PAA network acts as the elastic scaffold of hydrogel, while calcium bridging networks of LS, Alg, and PAA, as well as the strong hydrogen bonding network in the system, function as sacrifice bonds to dissipate energy and transfer stress. The PAA/LS/Alg/Ca hydrogels exhibit rapid and durable elastic recovery ability under large deformation with the highest compressive stress of 835 kPa (95% strain), highest tensile fracture stress of 357 kPa, and highest tensile strain of 1144%. In addition, these tough hydrogels show UV resistance, self-healing, antifreeze, and excellent electro-conductivity. When assembled into a strain sensor, stable and reliable electrical responses with 375 ms response time are demonstrated. The PAA/LS/Alg/Ca hydrogel strain sensors can monitor human movements with responsive and accurate physiological signals. These results support the conclusion that the PAA/LS/Alg/Ca hydrogel strain sensors have great application potential in flexible wearable electronics and smart devices.

Poly acrylic acid stabilized magnetic nanoemulsions for visual defect detection: Effect of pH on detection sensitivity and colloidal stability

Magnetic field assisted tunable colour contrast in magnetic nanoemulsions is exploited for visual detection of defects in ferromagnetic components. We demonstrate an enhanced defect detection sensitivity by stabilizing oil-in-water magnetic nanoemulsion with poly acrylic acid (PAA) at varying pH. In this detection technique, the variation of magnetic permeability, in the vicinity of the defects, causes magnetic flux leakage, which results in a distinct colour contrast on the magnetic nanoemulsion sensor due to the formation of anisotropic periodic structures, parallel to the external field direction, where the inter-droplet spacing within the linear chains depends on the effective magnetic flux. By exploiting this colour and magnetic flux dependence, severity of the defects is assessed by observing the colour contrast. Our studies indicate that defect detection sensitivity decreases with increasing pH due to globule-to-coil conformational transition of the interfacial PAA molecules. With increasing pH, the PAA network swells due to mobile counter-ions induced osmotic pressure and electrostatic repulsion between the charged monomer units, leading to a solvated coil-like configuration, which is confirmed from the increase in hydrodynamic diameter and zeta potential. The inter-droplet force measurements show an increase in decay length by ~3.5 times, when pH is increased from ~1.5 to ~4.0. The lower decay length at pH ~ 1.5 indicates the formation of linear chain-like structures with smaller inter-droplet spacing near the defect edges, where the magnetic flux leakage is maximum. This results in a fewer number of randomly oriented droplets at the defect centre, thereby enhancing the defect detection sensitivity at lower pH. However, nanoemulsions stabilized at pH > 3 show increased colloidal stability, as compared to pH ~ 1.5. Our studies indicate that nanoemulsions at pH ~ 3.5 exhibit an optimum colloidal stability with good defect detection sensitivity. The obtained results are beneficial for developing high sensitivity, reliable and reusable magnetic nanoemulsion sensors for naked eye visualization of defects in ferromagnetic components.

Bioinspired Strategy to Reinforce Hydrogels via Cooperative Effect of Dual Physical Cross‐Linkers

AbstractMany biological tissues including cartilage and tendons are composed of polymeric hydrogels, which exhibit high toughness and rapid self‐recovery. However, developing hydrogels with both high toughness and rapid recovery remains a challenge. Inspired by the nacre of abalone shell, two non‐covalent cross‐linkers (Al3+ and diethylenetriamine [DETA]) with different relaxation times are introduced into a polyacrylic acid network. Compared with mono cross‐linked hydrogels, the dual physical cross‐linked hydrogel exhibits both high toughness (work of extension at fracture up to 8.0 MJ m−3) and rapid self‐recovery ability without loss of extensibility. Strong carboxylate‐DETA ionic cross‐links with longer relaxation time endow the hydrogels with high strength and help to localize the reformation of carboxylate‐Al3+ coordinate bonds; weak carboxylate‐Al3+ coordinate bonds with shorter relaxation time dissociate and reform to dissipate energy. Therefore, the hydrogels can dissipate massive energy effectively without any residual strain. More notably, the toughness and hysteresis of hydrogels can be completely recovered in 20 min. This finding unravels a new path to reinforce the hydrogels by the cooperation of different non‐covalent interactions, which can be applied in more material systems.

Oxygen reduction reaction induced pH-responsive chemo-mechanical hydrogel actuators.

We describe and characterize elementary designs for electrochemical micro- and macro-scale chemomechanical hydrogel actuators. The actuation of a pH-sensitive cross-linked polyacrylic acid (PAA) hydrogel is driven in the model devices through the oxygen reduction reaction (ORR) occurring at the electrodes of an embedded Au mesh micro-electrochemical array. Proton consumption by the ORR at the cathode of the embedded electrochemical cell leads to the formation of a localized pH gradient that in turn drives the strain response in the composite actuators. The dynamics result from the ionization of the carboxylic acid moieties of the PAA network in the high pH region, yielding an osmotic pressure that drives a volumetric expansion due to water imbibition. This system actuates both stably and reversibly; when the electrochemically-induced ORR is halted, the localized pH gradient dissipates due to diffusive mixing, which in turn relaxes the induced strains. Two approaches to the fabrication of hydrogel actuators were examined in this work. The first method adopted a design based on small flagella (∼0.2 mm × 1.5 mm × 60 μm, width × length × height) in which the actuating PAA structures are molded atop a set of fixed electrodes mounted on a supporting substrate. These hydrogel actuators show fast, large-amplitude, and largely reversible responses in the ORR mediated chemomechanical dynamics. We also investigated larger hydrogel actuators (∼4.5 mm × 11 mm × 1 mm, width × length × height), based on an autonomous design that embeds an open mesh stretchable micro-electrode array within the hydrogel. The significant and design-dependent impacts of mass transfer on the chemomechanical dynamics are evidenced in each case, a feature examined to elucidate more efficient mesoscopic design rules for actuators of this form.

A new approach to improve antifouling property of PVDF membrane using in situ polymerization of PAA functionalized TiO 2 nanoparticles

In this study, polyacrylic acid (PAA) and titanium dioxide nanoparticles were used for increasing antifouling characteristic of polyvinylidene fluoride membrane. Two different methods were applied for immobilization of TiO 2 on PVDF membrane surface and compared their ability in flux recovery ratio of modified membranes. One method was self-assembling of TiO 2 on surface of PAA grafted membrane and another was initially functionalization of TiO 2 by acrylic acid monomers and next in situ grafting polymerization of this blend solution, which called “grafting from” technique. SEM images show that TiO 2 nanoparticles were dispersed uniformly and completely in “grafting from” method rather than self-assembling procedure. In addition, FT-IR spectroscopy confirmed the presence of PAA and TiO 2 nanoparticles on the membrane surface. Antifouling properties were tested by using whey solution. Better resistance to fouling was observed for membranes prepared from functionalized TiO 2 due to high grafting yield and low agglomeration of TiO 2 in this method. Repeating the fouling process indicates that stability of nanoparticles on membrane surface during operation and cleaning process is high because of covalent attachment of TiO 2 to PAA network.

Tuning the stop bands of inverse opal hydrogels with double network structure by controlling the solvent and pH

Polyacrylic acid (PAA) and polyacrylamide (PAAm) double network (DN) hydrogels with high mechanical strength (about 1.5 MPa) are obtained when two kinds of monomer solutions of 4M AA with 5 mol% crosslinker and 4M AAm with 0.1 mol% crosslinker are used for the optimal preparation. Their high mechanical strength can be maintained even at high water content (above 50%) and at external stimuli (solvent and pH). This optimized DN hydrogel is used to develop the PAA/PAAm inverse opal hydrogel with DN structure by twice infiltration-polymerization and colloidal templating. Its photonic stop band can be tuned by controlling the solvent and pH. It first shows a small red-shift (about 20 nm), and then a large blue-shift (about 180 nm) with the increased ethanol content. For pH response, the DN inverse opal hydrogel has a large stop-band shift of about 140 nm when the pH increases from 1.2 to 5.6. Moreover, the DN inverse opal hydrogel also shows rapid recovery ability without hysteresis phenomenon in strong acidic environment, good reproducibility and durability. The interaction between the independently crosslinked PAA network and PAAm network plays a significant role in determining the response performance.

Controlled immobilization of chondroitin sulfate in polyacrylic acid networks

Novel semi-interpenetrating polymer networks (semi-IPNs) of chondroitin sulfate (ChS) and acrylic acid (AA) were prepared with the aim of obtaining a hydrogel for use as a colon-specific drug carrier. By controlling the concentrations of cross-linking agent, diethylene glycol dimethacrylate (DEGDA), as well as the reaction solvent, high swelling percentages were obtained (approx. 1600%). However, the highest sol percent obtained for these hydrogels was approx. 70%, and most of the chondroitin sulfate remained soluble and could be extracted. Therefore, an alternative approach was adopted: methacrylate-grafted ChS (ChSMA) was synthesized and then co-polymerized with acrylic acid (AA) at a molar ratio of 1:5 with various concentrations of AA. The sol content of these ChSMA-AA hydrogels was reduced to approx. 20%, and the cross-linking densities were almost 100-fold higher than those of the semi-IPNs. FT-IR spectra showed that the H-bonding interactions between ChS and PAA and the spectra of the semi-IPNs were similar to that of PAA itself after sol extraction. In contrast, the FT-IR spectra of ChSMA-AA remained intact after sol extraction. Ketoprofen was used as a model drug to test the sustained release behavior of these hydrogels.

An energy-absorbing polyelectrolyte gel matrix composite material

A novel polyacrylic acid gel matrix composite material functionalized by hydrophobic nanoporous silica particulates is developed. When subjected to a high pressure, water molecules can be released from the polyacrylic acid network and forced into the nanopores. Due to the exposure of the large pore surface to the nonwetting liquid, the system free energy increases significantly. As the pressure is removed, the water remains in the confining nanoenvironment, and thus the material is energy absorbing. The energy absorption efficiency is much higher than that of many conventional protective materials.

Characterization of Palladium Nanoparticles Adsorpt on Polyacrylic Acid Particles as Hydrogenation Catalyst

Palladium particles can be prepared in sizes of a few nanometers from Pd(OAc)2in the presence of a block copolymer in aromatic unpolar solvents with suitable reducing agents like NaBH4 or LiAlH4. After mixing the organic metal solution with a polyacrylic acid (PAA) dispersion of defined concentration and following a crosslinking process with an epoxide, reactive membranes with a well-defined pore diameter were obtained. The catalytic activity of the prepared palladium particles incorporated in the crosslinked PAA network has been proven by the gas phase hydrogenation of propyne in a special membrane reactor.