Cross-linked Polyacrylamide Research Articles

An Explicit Model to Extract Viscoelastic Properties of Cells From AFM Force-Indentation Curves

Atomic force microscopy (AFM) has become one of the most used techniques for quantifying the mechanical properties of soft materials such as living cells. AFM force-indentation curves are conventionally fitted with a Hertzian model to extract elastic properties. The elastic properties solely are, however, insufficient to describe the mechanical properties of cells. Hence, extending the quantification to assess, in addition, the viscous behavior, is necessary for a more adequate characterization of the mechanical properties. Here, we expand the analysis capabilities to describe the viscoelastic behavior of the probed materials while using the same conventional AFM force-indentation curves. Our model gives an explicit relation of force and indentation and extracts physically meaningful cell mechanical parameters. We first validated the model on simulated forceindentation curves of a viscoelastic half-infinite space. Then, we applied the fitting model to the force-indentation curves of two hydrogels with different crosslinking mechanisms, polyacrylamide and agarose. We demonstrated that the viscoelastic properties extracted using our fitting model reflected adequately the distinct nature of these hydrogels. Finally, we characterized HeLa cells in two different cell cycle phases, interphase and mitosis and showed that mitotic cells have a higher apparent elasticity and a lower apparent viscosity when compared to interphase cells. Our study provides a simple and rapid method, which can be directly integrated into the standard AFM framework, for extracting the viscoelastic properties of materials. This facilitates the exploration of advanced material properties in general and the understanding of complex biophysical processes in particular.

Self-healing and mechanical performance of dynamic glycol chitosan hydrogel nanocomposites.

The application of functional self-healing and mechanically robust hydrogels in bioengineering, drug delivery, soft robotics, etc., is continuously growing. However, fabricating hydrogels that simultaneously possess good mechanical and self-healing properties remains a challenge. Developing robust hydrogel formulations for the encapsulation and release of hydrophobic substances is a major challenge especially in some pharmaceutical treatments where the many of drugs show incompatibility with the hydrophilic hydrogel matrices. Schiff base hydrogels have been developed using a benzaldehyde multifunctional amphiphilic polyacrylamide crosslinker in conjunction with glycol chitosan. The polymeric crosslinker was synthesized by a two-step reaction using aqueous Cu-RDRP to give an ABA telechelic copolymer of N,N-dimethyl acrylamide (DMAc) and N-hydroxyethyl acrylamide (HEAm) from a bifunctional PEG. The polymer was then modified by post functionalization leading to a multifunctional benzaldehyde crosslinker that was shown to be capable of self-assembly into aggregates in aqueous media serving as a possible candidate for the entrapment of hydrophobic substances. Aqueous solutions of the crosslinker spontaneously formed hydrogels when mixed with glycol chitosan due to the in situ formation of imine bonds. Hydrogels were characterized while additional comparisons were made with a commonly used bifunctional PEG crosslinker. The effect of introducing partially reduced graphene oxide (GO) nanosheets was also examined and led to enhancements in both mechanical properties (2.0 fold increase in modulus and 1.4 fold increase in strain) and self-healing efficiencies (>99% from 60% by rheology) relative to the pristine polymer hydrogels.

Synthesis and absorption properties of hybrid polyacrylamide hydrogels

Hydrogel absorbents were synthesized based on cross-linking of Polyacrylamide grafted on Dextran or Dextran sulfate sodium salt. It was found that the swelling capacity of hydrogels depends on cross-linker concentration as well as Dextran component. FT-IR studies of hydrogels were carried out to analyze its structure. Methylene blue dye as model cationic medical drug was applied for the absorption/desorption study. Hybrid hydrogels prove the mechanism of absorption by the formation of charged polyelectrolyte/dye complexes. The kinetic model of pseudo-first order was applied to determine the rate constant k of the reaction between hydrogels and dye in the solution.

A wide temperature-tolerant hydrogel electrolyte mediated by phosphoric acid towards flexible supercapacitors

Conventional hydrogel electrolytes tend to deteriorate significantly or even deactivate at high or low operating temperatures, which has become the main obstacle to the development of current temperature-tolerance supercapacitors. Herein, a groundbreaking temperature-tolerant strategy was firstly proposed that a novel hydrogel electrolyte with wide operating temperature was successfully prepared. Phosphoric acid (PA) and water as mixed solvents were utilized to dissolve chitosan (CS) in the chemical crosslinking polyacrylamide (PAAm) network to obtain tough and adhesive CS-PAAm hydrogels. Meanwhile, based on the existence of phosphoric acid molecules, the CS-PAAm hydrogels showed extremely high conductivity and wide range of temperature-tolerance from −60 °C to 100 °C. Surprisingly, the adhesiveness and toughness remained almost unchanged. Then, CS-PAAm hydrogels as electrolytes were successfully coupled with activated carbon electrodes to construct supercapacitors, which presented excellent flexibility and electrochemical stability over a wide temperature range from −60 °C to 100 °C. Therefore, it is foreseeable that this simple and effective strategy would provide novel insight and opportunity for a new generation of flexible energy storage devices with wide temperature-tolerance.

Preparation of polyacrylamide/nanosheets LYH:Eu nanocomposite film and enhanced photoluminescence

Abstract The Polymer/inorganic nanocomposite (PNC) film of HTTA modified layered rare-earth hydroxide nanosheets cross-linked polyacrylamide (HTTA-LYH:Eu-PAM) was successfully synthesized. Because of the antenna effect between the ligand and luminescence center, the ultra-thin LYH:Eu nanosheets revealed highly photoluminescence property after modified by HTTA. The HTTA-LYH:Eu-PAM composite film prepared by spin coating method exhibited high light transmittance, and high quantum efficiency.

Swelling of polyacrylamide-based hydrogel in aqueous solutions of low-molecular salts

The swelling kinetics of cross-linked polyacrylamide was studied depending on the composition of the external solution. It was shown that the polymer gel swelling during the transition from water to a potassium or sodium chloride solution and vice versa sharply decreases, then gradually increases. The swelling of gel in a solution of sodium chloride is higher than that of potassium chloride. In the equilibrium swollen gel, the concentration of salts in the gel phase is higher than in the external solution and increases with the transition from potassium chloride to sodium chloride.

Ultra-Stretchable, durable and conductive hydrogel with hybrid double network as high performance strain sensor and stretchable triboelectric nanogenerator

Abstract Hydrogels with integrated attributes of stretchability, conductivity, transparency and robustness have been emerging because of their promising applications in wearable devices, human health monitoring, advanced intelligent systems and energy harvesting. In this paper, we developed stretchable and conductive hydrogels via hybrid double networks approach by combination of rigid physically cross-linked gelatin, tough chemically cross-linked polyacrylamide (PAM) and poly(3,4-ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT: PSS) as conducting component. The double networks can be further interlocked by using physical entanglements and abundant dynamic hydrogen bonds, contributing to the improved mechanical properties and self-recovery ability. A transparent and wearable strain sensor is fabricated by sandwiching the hydrogels with two layers of adhesive polyurethane (PU) tape, exhibiting good sensitivity (gauge factor (GF) = 1.58), ultra-wide sensing range of 0-2850% strain, short response time of 200 ms and superior durability and reproducibility (1200 cycles), which endows the sensitive monitors with effective discernibility for detecting intricate human motions. Importantly, the hydrogel-based device can act as a highly stretchable (300% strain) triboelectric nanogenerator (STENG) for efficient energy harvesting, giving a short circuit current (ISC) of 26.9 μA, open circuit voltage (VOC) of 383.8 V and short-circuit transferred charge (QSC) of 92 nC. The integrated abilities of strain sensing and energy harvesting promise the hydrogels for high performance self-powered wearable devices and stretchable power sources.

Microrheological approach for the viscoelastic response of gels

In this paper I present a simple and self-consistent framework based on microrheology that allows one to obtain the mechanical response of viscoelastic fluids and gels from the motion of probe particles immersed on it. By considering a non-markovian Langevin equation, I obtain general expressions for the mean-squared displacement and the time-dependent diffusion coefficient that are directly related to the memory kernels and the response function, and which allow one to obtain estimates for the complex shear modulus and the complex viscosity of the material. The usefulness of such approach is demonstrated by applying it to describe experimental data on chemically cross-linked polyacrylamide through its sol-gel transition.

Direct synthesis of c-axis-oriented HZSM-5 zeolites in polyacrylamide hydrogel

c-Axis-oriented HZSM-5 zeolites were directly synthesized by introducing crosslinked polyacrylamide (C-PAM) hydrogels into ZSM-5 synthesis gels. The growth orientation of the crystals as well as the acidity of ZSM-5 could be modulated by the C-PAM content in the synthesis gel. The crystal growth along the a-axis and b-axis was obviously restricted, whereas c-axis growth was distinctly promoted with increasing C-PAM content, which was probably due to the C-PAM having stronger interactions with the {010} and {100} facets than the {101} facet. Sample Z-2, which was prepared with 2% C-PAM exhibited the shortest b-axis length elongated prism structure and strong acid sites. Furthermore, Z-2 exhibited excellent catalytic performance in the methanol-to-olefin (MTO) reaction showing the highest methanol conversion (100%) and the light olefin selectivity (TOS) (81.2%), as well as the longest lifetime (>23 h).

Swelling and Dissolution Transitions of DNA- and “Bis”-Cross-Linked Polyacrylamide

We examine the swelling and dissolution of hydrogels cross-linked by a combination of physical and chemical junctions, focusing on the technologically important DNA- and “bis”-cross-linked polyacry...

High-efficient and synergetic antibacterial nanocomposite hydrogel with quaternized chitosan/Ag nanoparticles prepared by one-pot UV photochemical synthesis.

Hydrogel dressings have significant advantages such as absorption of tissue exudate, maintenance of proper moist environment, and promotion of cell proliferation. However, facile preparation method and high-efficient antibacterial hydrogel dressings are still a great challenge. In this study, a facile approach to prepare antibacterial nanocomposite hydrogel dressing to accelerate healing was explored. The hydrogels consisted of quaternized chitosan and chemically cross-linked polyacrylamide, as well as silver nanoparticles (AgNPs) stabilized by chitosan. The synthesis of the hydrogels including the formation of AgNPs and polymerization of acrylamide was accomplished simultaneously under UV irradiation in 1 hour without adding initiator. The hydrogels showed favorable tensile strength of ∼100 kPa with elongation at break over 1000% and shear modulus of ∼104 Pa as well as suitable swelling ratio, which were appropriate for wound dressing. The combination of quaternized chitosan and AgNPs exhibited high-efficient and synergetic antibacterial performance with low cytotoxicity. In vivo animal experiments showed that the hydrogel can effectively prevent wound infection and promote wound healing. This study provides a facile method to produce antibacterial hydrogel wound dressing materials.

Eco-friendly interpenetrating network hydrogels integrated with natural soil colloid as a green and sustainable modifier for slow release of agrochemicals

Natural soil colloid, an important component of natural soil, has high specific surface area and strong adsorbability. Although natural soil colloids are ubiquitous in natural subsurface environments, they have not been utilized to synthesize composite materials for any practical application. In this work, a series of eco-friendly interpenetrating polymer network hydrogels integrated with natural soil colloid were synthesized via the ionic crosslinking of alginate and covalent cross-linking of polyacrylamide (PAM) in the aqueous dispersion of natural soil colloid. The natural soil colloid was extracted from the natural soil by a simple, green and sustainable process, without any formation of pollution. The swelling characteristic experiment showed that the soil colloid in polymeric matrix accelerated water absorption by the hydrogels. Hydrogels released 34.4% of pesticide and 56.9% of fertilizer loaded during 54 and 30.5 h at the slowest release rates, respectively. The release data indicated that the hydrogels could release the fertilizer and the pesticide in a slow manner. These findings show that natural soil colloid can be used as a green and sustainable modifier to improve the slow release of hydrogels. Therefore, the synthesized hydrogels integrated with natural soil colloid show great potential as carriers of agrochemicals in modern agriculture and horticulture.

Anisotropic Double-Network Hydrogels via Controlled Orientation of a Physical Sacrificial Network

We report a method to create anisotropic double-network (DN) hydrogels, through the controlled orientation of a physical sacrificial network. A cross-linked polyacrylamide hydrogel is synthesized from a solution containing a semirigid anionic polyelectrolyte. Subsequently, the gel is stretched to orient the semirigid polyelectrolyte, which does not relax in the stretched state because of the high contour length in comparison to the mesh size of the polyacrylamide network. The polyelectrolyte is then physically cross-linked with a multivalent cation, ZrCl2O, to fix the anisotropy. Anisotropy was visualized by observing birefringence and quantified by small-angle X-ray scattering. By comparing the scattering in the oriented direction versus perpendicular to the oriented direction, a structural anisotropy factor was calculated. Uniaxial tensile testing was performed on samples of varying prestretch, both parallel and perpendicular to the stretching direction. Young’s modulus, fracture stress, fracture strain, and work of extension were characterized, and the resulting mechanical anisotropy was compared to the structural anisotropy factor. We find that the anisotropy of Young’s modulus and fracture stress is directly controlled by the anisotropy of the sacrificial network, while fracture strain and work of extension show little influence from structural anisotropy. The results of this work demonstrate that prestretching of a physical sacrificial network is a controllable and simple method to create anisotropic DN hydrogels.

Study on the association behavior of synthesized hydrophobically associating polymer microspheres

Cross-linked polyacrylamide microspheres are widely used as in-depth flooding agents in petroleum development due to their unique properties of thickening, salt tolerance, shear resistance, etc. However, these materials still encounter major challenges in high-salinity reservoirs. In this work, a kind of hydrophobically associating polymer microspheres called P(AM-S) was synthesized via an inverse suspension polymerization technique with 2-Acrylamidohexadecanesulfonic acid (S) as a hydrophobic monomer. The association behavior and mechanism of P(AM-S) were investigated by hydrodynamic size, morphology, microstructure, fluorescence analyses. The results revealed that the critical association concentration (CAC) of the synthesized P(AM-S) was 5000 mg/L. When the microsphere concentration reached the CAC, the microspheres aggregated to form a sheet-like region. The average particle size and apparent viscosity of the microspheres both decreased first and then increased with increasing NaCl concentration, showing excellent salt tolerance. This type of material is expected to be applied in high-salinity reservoirs.

Complex formation in the Fe(II)-Fe(III)-acrylamide–water system and chemical models

This paper investigates complex formation in the Fe(II)-Fe(III)-acrylamide-water system. Dependences of oxidative potentials on the concentration and model parameters of iron acrylamide complexes are discussed. Effect of reaction pH on polymerization of acrylamide and applications of polyacrylamide crosslinking reactions are described. At low pH and high temperatures, formation of water-insoluble cross-linked polymers is possible due to creation of imide bridges between the macromolecules (–CO–NH–CO–). Calculations of the formation constants according to their chemical and physical properties such as pH, temperature and ionic strength of the solution are outlined. Chemical models of acrylamide coordination compounds with Fe(III) and Fe(II) at 298.16 K and energy J = 0.5 are shown in this article. A chemical model of the equilibria numerical values of the complexes for oxidized and reduced forms of the metal, the number of protons they contain, the number of coordinated ligands, hydroxyl groups (g, p, s, l and k), are calculated by comparing the slopes of the experimental curves and partial derivative oxidative potentials of concentration variables. Dependences of the Fe(II) and Fe(III) complexes formation on pH for the systems of Fe(II)-Fe(III)-acrylamide-H2O have been determined.

Preparation of anionic functional microspheres and study on their adsorption properties for methylene blue

Organic contaminants have a destructive influence on the environment. Dye is a kind of serious organic pollutant. The aim of this project was to explore the adsorption of methylene blue (MB) to purify wastewater. Cross-linked polyacrylamide (CPAM) microspheres were acted as the base material, ammonium persulfate (Aps) as the initiator, sodium styrene sulfonate (SSS) as anionic functional monomer, graft-polymerization functional microspheres (CPAM-g-PSSS) were prepared. The chemical structures and physicochemical properties of functional microspheres were characterized by FT-IR, zeta potential, scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). On this basis, the adsorption performance and mechanism functional microspheres CPAM-g-PSSS for MB by CPAM-g-PSSS were explored and analyzed. The experimental results show that the redox system with ammonium persulfate as the initiator could effectively initiate graft-polymerization of SSS onto CPAM microspheres. Under suitable conditions, functional microspheres with graft degree of 12.36 g/100 g can be prepared. The functional microspheres had good adsorption performance for MB at alkaline conditions, and the adsorption capacity can reach 55 mg/g.

3D hierarchical porous nitrogen-doped carbon/Ni@NiO nanocomposites self-templated by cross-linked polyacrylamide gel for high performance supercapacitor electrode

Three-dimensional nitrogen-doped carbon network incorporated with nickel@nickel oxide core-shell nanoparticles composite (3D NC/Ni@NiO) has been facilely prepared, self-templated by the cross-linked polyacrylamide aerogel precursor containing NiCl2. Characterizations reveal that the Ni@NiO nanoparticles distribute homogeneously in the 3D nitrogen-doped carbon matrix and the composite is of hierarchical porous structure. When used as supercapacitor electrode in a three-electrode system, the 3D NC/Ni@NiO exhibits enhanced electrical conductivity and excellent electrochemical performance, presenting a high specific capacitance (389F g−1 at 5 mV s−1), good rate capability (276 F g−1 at 100 mV s−1) and outstanding cycling performance (with the capacitance retention of 70.2% after 5000 charge-discharge cycles). This is due to the synergistic effects of conductive metallic nickel, pseudocapacitive nickel oxide as well as in situ nitrogen doping of carbon network. Moreover, an asymmetric supercapacitor (ASC) was fabricated with NC/Ni@NiO as positive electrode and active carbon as negative electrode. The ASC device exhibits a maximum energy density of 19.4 W h kg−1 at a power density of 700 W kg−1 and shows good cycling stability (73.8% capacity retention after 3000 cycles), indicating that it has great promise for practical energy storage and conversion application.

Preparation of pectin-based dual-crosslinked network as a binder for high performance Si/C anode for LIBs

The modification of pectin polysaccharide through grafting with polyacrylamide and crosslinking is newly proposed as a powerful candidate for a water-soluble binder of high-capacity Si anodes in lithium ion batteries. The grafting of pectin with polyacrylamide enhances adhesion in the electrode and contributes to the good wettability of the carbonate electrolyte. Herein, dual-crosslinking of pectin-g-polyacrylamide was achieved by the ionic crosslinking of pectin with divalent calcium ions and the chemical crosslinking of polyacrylamide with a bisacrylamide. As a result, the dual-crosslinked binder improves further the cycling performance of the Si/C composite anode with a 1.2 mg cm−2 loading, which retains a specific capacity of 729 mAh g−1 after 300 cycles. In contrast, the Si/C electrode containing dual-crosslinked alginate with polyacrylamide shows a specific capacity of 515 mAh g−1 after 300 cycles.

Highly integrated sulfur cathodes with strong sulfur/high-strength binder interactions enabling durable high-loading lithium–sulfur batteries

Abstract The development of high-sulfur-loading Li–S batteries is a key prerequisite for their commercial applications. This requires to surmount the huge polarization, severe polysulfide shuttling and drastic volume change caused by electrode thickening. High-strength polar binders are ideal for constructing robust and long-life high-loading sulfur cathodes but show very weak interfacial interaction with non-polar sulfur materials. To address this issue, this work devises a highly integrated sulfur@polydopamine/high-strength binder composite cathodes, targeting long-lasting and high-sulfur-loading Li–S batteries. The super-adhesion polydopamine (PD) can form a uniform nano-coating over the graphene/sulfur (G–S) surface and provide strong affinity to the cross-linked polyacrylamide (c-PAM) binder, thus tightly integrating sulfur with the binder network and greatly boosting the overall mechanical strength/conductivity of the electrode. Moreover, the PD coating and c-PAM binder rich in polar groups can form two effective blockades against the effusion of soluble polysulfides. As such, the 4.5 mg cm−2 sulfur-loaded G–S@PD-c-PAM cathode achieves a capacity of 480 mAh g−1 after 300 cycles at 1 C, while maintaining a capacity of 396 mAh g−1 after 50 cycles at 0.2 C when the sulfur loading rises to 9.1 mg cm−2. This work provides a system-wide concept for constructing high-loading sulfur cathodes through integrated structural design.

Elastin-Based Thermoresponsive Shape-Memory Hydrogels.

A shape-memory hydrogel is a programmable hydrogel material that can store specific shapes and execute functions in response to stimuli. In this report, we developed shape-memory hydrogels by creating double-network polymeric structures using a physically cross-linking elastin-like polypeptide (ELP) and a chemically cross-linking polyacrylamide (PAM). We synthesized the hydrogel matrix by polymerizing the acrylamide mixed in an ELP solution. We exploited the lower critical solution temperature transition of the ELP to enable the hydrogel to hold a new desired shape at an elevated temperature of 55 °C. The original shape of the hydrogel can then be recovered by lowering the temperature to 20 °C. The shape-memory hydrogels we developed exhibit ultrafast functionality and high repeatability. Taking advantage of the temperature-induced shape-memory capability, we also demonstrate practical functions such as gripping an object and connecting two tubes. Our materials with effective temperature-driven shape-memory functionality will be useful for developing novel materials for biomedical applications in the future.