Polyacrylamide Microgels Research Articles

Optimization of synthesis conditions and calibration of water-swellable polyacrylamide microgels

Precipitation Polymerization (PP) and Inverse Emulsion Polymerization (IEP) are the main procedures widely used for the synthesis of microgels from acrylamide derivate monomers, in both academic and industrial spheres. Here, these procedures were optimized for pure acrylamide monomer to synthetize a “model” polyacrylamide microgel targeting a minimized size of approximately 200 nm in radius. For this neutral and non-responsive monomer, the role of the co-solvent in PP and co-surfactant in IEP was discussed. This work demonstrates the crucial step of purification and calibration on microgels physical properties in suspension. In particular, the filtration step has a significant impact on their rheological behaviour, where the flow properties requiring a complex description become analysable with a standard Carreau–Yasuda model. In summary, this paper provides a systematic discussion on the role of each step in the elaboration of model microgel, as a routine checklist to be considered before further physical-chemical studies.

High-throughput viscoelastic characterization of cells in hyperbolic microchannels.

Extensive research has demonstrated the potential of cell viscoelastic properties as intrinsic indicators of cell state, functionality, and disease. For this, several microfluidic techniques have been developed to measure cell viscoelasticity with high-throughput. However, current microchannel designs introduce complex stress distributions on cells, leading to inaccuracies in determining the stress-strain relationship and, consequently, the viscoelastic properties. Here, we introduce a novel approach using hyperbolic microchannels that enable precise measurements under a constant extensional stress and offer a straightforward stress-strain relationship, while operating at a measurement rate of up to 100 cells per second. We quantified the stresses acting in the channels using mechanical calibration particles made from polyacrylamide (PAAm) and found that the measurement buffer, a solution of methyl cellulose and phosphate buffered saline, shows strain-thickening following a power law up to 200 s-1. By measuring oil droplets with varying viscosities, we successfully detected changes in the relaxation times of the droplets and our approach could be used to get the interfacial tension and viscosity of liquid-liquid droplet systems from the same measurement. We further applied this methodology to PAAm microgel beads, demonstrating the accurate recovery of Young's moduli and the near-ideal elastic behavior of the beads. To explore the influence of altered cell viscoelasticity, we treated HL60 human leukemia cells with latrunculin B and nocodazole, resulting in clear changes in cell stiffness while relaxation times were only minimally affected. In conclusion, our approach offers a streamlined and time-efficient solution for assessing the viscoelastic properties of large cell populations and other microscale soft particles.

Smartphone-Based Fluorescent Profiling of Quaternary MicroRNAs in Urine for Rapid Diagnosis of Urological Cancers Using a Multiplexed Isothermal Exponential Amplification Reaction.

Urological cancers such as bladder or prostate cancer represent one of the most malignant tumors that accounts for an extremely high mortality. However, conventionally standard diagnostics for urological cancers are hardly available in low-resource settings. We developed herein a hand-held fluorescent imaging platform by integrating a multiplexed isothermal exponential amplification reaction (EXPAR) with a microgel-enriched methodology for sensitive profiling of quaternary microRNAs (miRNAs) in urine and quick diagnosis of urological cancers at the early stage. The target miRNA mixtures in the urine underwent four parallel EXPARs without cross-reactivity, followed by surface concentration and hybridization by the encoded polyacrylamide microgels. This mix-and-read strategy allowed for one-pot analysis of several key miRNAs simultaneously and provided 5-fold enhancement in fluorescent detection sensitivities compared to the individual EXPAR-based assays. Four urinary miRNAs (let-7a, miRNA-155, -223, and -143) could be quantitatively determined in a wide linear range from 50 fM to 30 nM, with the limits of detection at femtomolar levels. Using a smartphone-based imaging microreader, healthy and cancerous cohorts with prostate, bladder, and renal cell cancers could be discriminated in 30 min with the accuracy >83% using linear discriminant analysis. The developed detection platform has proven to be a portable, noninvasive, and useful complement to the toolbox for miRNA-based liquid biopsies, which holds immense potential and advantage for regular and large-scale applications in early cancer diagnosis.

Dye degradation study by incorporating Cu-doped ZnO photocatalyst into polyacrylamide microgel

Dye degradation study by incorporating Cu-doped ZnO photocatalyst into polyacrylamide microgel

Electrochemically Initiated Synthesis of Polyacrylamide Microgels and Core-shell Particles.

Herein, we developed a simple procedure for synthesizing micrometer-sized microgel particles as a suspension in an aqueous solution and thin films deposited as shells on different inorganic cores. A sufficiently high constant potential was applied to the working electrode to commence the initiator decomposition that resulted in gelation. Under hydrodynamic conditions, this initiation allowed preparing different morphology microgels at room temperature. Importantly, neither heating nor UV-light illumination was needed to initiate the polymerization. Moreover, thin films of the cross-linked gel were anchored on different core substrates, including silica and magnetic nanoparticles. Scanning electron microscopy and transmission electron microscopy imaging confirmed the microgel particles’ and films’ irregular shape and porous structure. Energy-dispersive X-ray spectroscopy indicated that the core coating with the microgel film was successful. Dynamic light scattering measured the micrometer size of gel particles with different combinations of acrylic monomers. Thermogravimetric analysis and the first-derivative thermogravimetric analysis revealed that the microgels’ thermal stability of different compositions was different. Fourier-transform infrared and 13C NMR spectroscopy showed successful copolymerization of the main, functional, and cross-linking monomers.

Influence of Lipase Immobilization Mode on Ethyl Acetate Hydrolysis in a Continuous Solid-Gas Biocatalytic Membrane Reactor.

Solid-gas biocatalysis was performed in a specially designed continuous biocatalytic membrane reactor (BMR). In this work, lipase from Candida rugosa (LCR) and ethyl acetate in vapor phase were selected as model enzyme and substrate, respectively, to produce acetic acid and ethanol. LCR was immobilized on functionalized PVDF membranes by using two different kinds of chemical bond: electrostatic and covalent. Electrostatic immobilization of LCR was carried out using a membrane functionalized with amino groups, while covalent immobilization was carried out using membrane, with or without surface-immobilized polyacrylamide (PAAm) microgels, functionalized with aldehyde groups. These biocatalytic membranes were tested in a solid-gas BMR and compared in terms of enzyme specific activity, catalytic activity, and volumetric reaction rate. Results indicated that lipase covalently immobilized is more effective only when the immobilization is mediated by microgels, showing catalytic activity doubled with respect to the other system with covalently bound enzyme (4.4 vs 2.2 μmol h-1). Enzyme immobilized by ionic bond, despite a lower catalytic activity (3.5 vs 4.4 μmol h-1), showed the same specific activity (1.5 mmol·h-1·g-1ENZ) of the system using microgels, due to a higher enzyme degree of freedom coupled with an analogously improved enzyme hydration. Using the optimized operating conditions regarding immobilized enzyme amount, ethyl acetate, and molar water flow rate, all three BMRs showed continuous catalytic activity for about 5 months. On the contrary, the free enzyme (in water/ethyl acetate emulsion) at 50 °C was completely inactive and at 30 °C (temperature optimum) has a specific activity 2 orders of magnitude lower (8.4 × 10-2 mmol h-1 g-1) than the solid-gas biocatalytic membrane reactor. To the best of our knowledge, this is the first example of solid-gas biocatalysis, working in the gaseous phase in which a biocatalytic membrane reactor, with the enzyme/substrate system lipase/ethyl acetate, was used.

Abstract 35: 3D in vitro system for immuno-oncology: Real-time imaging of drug delivery, tumoroids, and immune cell activity

Abstract A Liquid Like Solid (LLS) 3D culture system in a convenient microtiter plate format enables long-duration culture of patient derived microtumors (>30 days), in situ confocal imaging, 3D cytotoxic drug studies, inclusion of T cells, and measurements of T cell migration, infiltration, and killing. Introduction: Cancer is a disease in 3-dimensions and there is a desperate need for new tools and infrastructure to study immuno-oncology treatment methods in 3D. Fabrication of microtumors using 3D printing in LLS media comprised of soft granular microgels (3-5 μm crosslinked polyacrylamide (PAA) microgel particles: 7.5% PAA) facilitate precise arrangement of detailed assemblies of extra-cellular matrix components and cells (1), including: epithelial cells (breast ATCC MCF10A), stem cells (bone marrow MSC), cancer cells (breast ATCC MCF7, prostate ATCC-PC3, osteosarcoma ATCC-MG63, melanoma ATCC-A375, primary glioblastoma, and osteosarcoma), fibroblasts (ThermoFisher dermal fibroblasts C0045C), endothelial cells (ThermoFisher HUVECs C0035C), and CD4+/CD8+ T cells (PBMC and Jurkat E6-1). Methods: Long term culture was enabled through the design, development, and validation of a modular perfusion system that uses passive negative pressure within a 96-well microtiter plate format to transport liquid growth medium, drugs (doxorubicin and puromycin), antibodies (aPD1 J43 clone, aCD3, aCD28), growth factors, FBS, and metabolic waste without disturbing the spatial organization and positioning of the experiments (i.e. 3D orientation is preserved and the packed bed of microgel particles remain solid). The perfusion velocities are precisely controlled to between 1-100 nm/s by setting the negative pressure, and complete exchange of liquid media can be tailored from hours to days. Results: Real-time imaging in these 3D assays is performed using in situ confocal microscopy under controlled perfusion. Cell motility, adhesion, and dynamic rearrangement of fibroblasts and endothelial cells within a 3D co-culture of microtumors evolved over the first 72 hours. Immunohistochemistry with Ki-67 and PCNA staining indicated active cell proliferation of the tumoroids after 28 days of continuous culture. Tracking of activated CD8+ T cells revealed super-diffusive motion in the presence of 3D tumors within a range of 250 µm. Activated T cell migration speeds have been measured to be between 1.3 and 2.0 μm/s in the 3D LLS media, and preliminary estimates of T cell migration forces are on the order of 1 nN. Conclusions: This integrated system of 3D bioprinting, perfusion culture plates, and confocal microscopy enables in situ 3D studies of cancer biology, immunotherapy, and drug treatment regimens and provides unique insights and measurements of immune cell invasion dynamics in 3D microtumors.

Microfluidic model systems used to emulate processes occurring during soft particle filtration

Cake layer formation in membrane processes is an inevitable phenomenon. For hard particles, especially cake porosity and thickness determine the membrane flux, but when the particles forming the cake are soft, the variables one has to take into account in the prediction of cake behavior increase considerably. In this work we investigate the behavior of soft polyacrylamide microgels in microfluidic model membranes through optical microscopy for in situ observation both under regular flow and under enhanced gravity conditions. Particles larger than the pore are able to pass through deformation and deswelling. We find that membrane clogging time and cake formation is not dependent on the applied pressure but rather on particle and membrane pore properties. Furthermore, we found that particle deposits subjected to low pressures and low g forces deform in a totally reversible fashion. Particle deposits subjected to higher pressures only deform reversibly if they can re-swell due to capillary forces, otherwise irreversible compression is observed. For membrane processes this implies that when using deformable particles, the pore size is not a good indicator for membrane performance, and cake formation can have much more severe consequences compared to hard particles due to the sometimes-irreversible nature of soft particle compression.

Colloidal crystals of compliant microgel beads to study cell migration and mechanosensitivity in 3D.

Tissues are defined not only by their biochemical composition, but also by their distinct mechanical properties. It is now widely accepted that cells sense their mechanical environment and respond to it. However, studying the effects of mechanics in in vitro 3D environments is challenging since current 3D hydrogel assays convolve mechanics with gel porosity and adhesion. Here, we present novel colloidal crystals as modular 3D scaffolds where these parameters are principally decoupled by using monodisperse, protein-coated PAAm microgel beads as building blocks, so that variable stiffness regions can be achieved within one 3D colloidal crystal. Characterization of the colloidal crystal and oxygen diffusion simulations suggested the suitability of the scaffold to support cell survival and growth. This was confirmed by live-cell imaging and fibroblast culture over a period of four days. Moreover, we demonstrate unambiguous durotactic fibroblast migration and mechanosensitive neurite outgrowth of dorsal root ganglion neurons in 3D. This modular approach of assembling 3D scaffolds from mechanically and biochemically well-defined building blocks allows the spatial patterning of stiffness decoupled from porosity and adhesion sites in principle and provides a platform to investigate mechanosensitivity in 3D environments approximating tissues in vitro.

Novel microreactors of polyacrylamide (PAM)[sbnd]CdS microgels for admirable photocatalytic H2 production under visible light

Cadmium sulfide, with its narrow band gap, can be used as a photocatalyst in the visible light region for the splitting of water, but its limited number of active sites and tendency to agglomerate are problematic for producing high yields of hydrogen. Therefore, an inverse emulsion polymerization method was used to fabricate polyacrylamide (PAM) microgels as a substrate to immobilize CdS nanoparticles (PAM-CdS). The PAM microgels not only immobilized the CdS nanoparticles, but also prevented aggregation. NCd bonds in the PAM-CdS microgels facilitated electron transfer from the PAM to the CdS resulting in more electrons participating in the H2 production process. The electrostatic interactions between the PAM and CdS also hindered the recombination of electron-hole pairs. These PAM-CdS microgels exhibit admirable photocatalytic H2 production performance with a H2 production rate of up to 5.21 mmol h−1 g−1.

New insights into the viscosity features of cationic polyacrylamide microgels in aqueous solutions

ABSTRACTIncreasing attention has recently been paid to the rheological behavior of microgel colloids and to the physical forces that control their behavior. Here, based on a series of cationic microgels that were synthesized by inverse microemulsion polymerization, the physical forces were explored by viscosity analysis of swollen microgels with different crosslinking densities and cationic contents. The results indicate that within a wide concentration range, the viscosity curves for these cationic microgels perfectly correspond to the Krieger–Dougherty model as modified by Tan et al. In particular, the specific volume in this model decreases at the critical overlapping concentration and reveals the interaction intensity between neighboring microgels. Furthermore, the viscosity index in the Herschel–Bulkley equation indicates that the interaction domain among microgels undergoes a transfer from an electroviscous effect to osmotic deswelling with increasing concentrations. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46297.

Standardized microgel beads as elastic cell mechanical probes.

Cell mechanical measurements are gaining increasing interest in biological and biomedical studies. However, there are no standardized calibration particles available that permit the cross-comparison of different measurement techniques operating at different stresses and time-scales. Here we present the rational design, production, and comprehensive characterization of poly-acrylamide (PAAm) microgel beads mimicking size and overall mechanics of biological cells. We produced mono-disperse beads at rates of 20-60 kHz by means of a microfluidic droplet generator, where the pre-gel composition was adjusted to tune the beads' elasticity in the range of cell and tissue relevant mechanical properties. We verified bead homogeneity by optical diffraction tomography and Brillouin microscopy. Consistent elastic behavior of microgel beads at different shear rates was confirmed by AFM-enabled nanoindentation and real-time deformability cytometry (RT-DC). The remaining inherent variability in elastic modulus was rationalized using polymer theory and effectively reduced by sorting based on forward-scattering using conventional flow cytometry. Our results show that PAAm microgel beads can be standardized as mechanical probes, to serve not only for validation and calibration of cell mechanical measurements, but also as cell-scale stress sensors.

Development of a Novel Immobilization Method by Using Microgels to Keep Enzyme in Hydrated Microenvironment in Porous Hydrophobic Membranes.

Using colloidal polyacrylamide (PAAm) microgels as carriers, a novel strategy for covalent immobilization of enzymes maintained in hydrated microenvironment on/in a macroporous surface-functionalized hydrophobic polyvinylidene fluoride (PVDF) membrane is developed. The PAAm microgels are synthesized by inverse miniemulsion polymerization, and first the parameters are investigated which are suited to obtain particles in the desired size range, 100-200 nm, with narrow size distribution. Amino functions are then imparted to the microgels applying the Hofmann reaction. The modification is confirmed by Fourier-transform infrared spectroscopy analysis, ninhydrin test, and elemental analysis. In addition, functionalized microgels are characterized by dynamic light scattering. The amino-functionalized PAAm microgels are then immobilized on pre-modified PVDF membrane having aldehyde functionalities on the surface. Afterward, unreacted aldehyde groups still present on the membrane where quenched by ethanolamine and the enzyme lipase from Candida rugosa (LCR) is subsequently immobilized on the microgels loaded PVDF membrane via glutaraldehyde cross-linking, exploiting the free amino groups on immobilized microgels. Catalytic efficiency of LCR immobilized by this strategy is evaluated using para-nitrophenyl palmitate as substrate and compared with LCR directly immobilized on PVDF membrane without microgels. Results show that LCR immobilized by means of microgels exhibits better performance with a 2.3-fold higher specific biocatalytic activity.

Water‐free synthesis of temperature‐sensitive polyacrylamide microgels and pore modeled oil recovery performance

ABSTRACTPolymer gel treatments can improve sweep efficiency and reduce water production during oil recovery operations. In this article, a novel suspension polymerization method was developed to synthesize a temperature‐sensitive microgel. The microgel was prepared by suspension polymerization above the melting point of the monomer in a nonpolar solvent without water. Dry microspheres were obtained, which can be readily used without post‐treatment. Two different crosslinkers were employed in the suspension polymerization synthesis to give the particles thermally responsive aqueous swelling properties. After entering pore channels, gel particles expand to engineered size to realize flow profile changes within in a reservoir formation. When dispersed into water under lower temperatures (ambient to 40 °C), the original dry particles can swell about 18 times their original size. Exposure to a harsher environment (e.g., 80 °C) resulted in cleavage of the labile crosslinking agent and some chain cleavage gave a further size expansion. A Millipore film filtration model system was adopted to evaluate pore occlusion performance of the gel particles. It was found that the nuclear pores were effectively sealed by swollen microgels only when gel particle sizes were similar to or smaller than the membrane pores and interpore separation distance. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44581.

Size‐controlled gold nanoparticles inside polyacrylamide microgels

ABSTRACTGold nanoparticles (AuNPs) of different sizes were synthesized into a crosslinked network of polyacrylamide (PAAm) microgels. PAAm was prepared by means of semicontinuous inverse heterophase polymerization under monomer‐starving conditions with a z‐average particle size of 384 ± 18 nm. AuNPs with controlled size were obtained by a reduction reaction of Au+3 to Au0 from a gold(III) chloride trihydrate (HAuCl4) solution inside microgel the crosslinked network (AuNP‐PAAm). The reduction reaction was verified for 2 h by ultraviolet–visible spectroscopy (UV–vis). AuNP–PAAm exhibited a particle size between 288 ± 12 and 230 ± 15 nm at HAuCl4 concentrations of 0.4 and 1.3 mM, respectively. The AuNP–PAAms were observed by transmission electron microscopy, and their sizes were determined to be 19 ± 2 nm (1.3 mM) and 17 ± 2 nm (1.1 mM). With UV–vis spectroscopy, we detected the formation of AuNPs at a wavelength of 552 nm, and with X‐ray diffraction analysis, we proved that the crystal arrangement was face‐centered cubic. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43560.

Effect of polyacrylamide on rheology of fresh cement pastes

In this paper, we study the effects of polyacrylamide (PAM) on both aqueous solutions and fresh cement pastes. Our solution viscosity and hydrodynamic radii measurements in various solutions suggest that calcium ions lead to a cross-linking of anionic PAM and to the formation of PAM micro-gels. Our adsorption measurements show that the majority of these micro-gels adsorb on cement grains whereas our rheological measurements on cement pastes suggest that these micro-gels are able to adsorb simultaneously on several cement grains and bridge them increasing therefore the macroscopic yield stress of the suspension. Finally, we show that the contribution of PAM to yield stress is very sensitive to shear history, as the paste structure seems to progressively lose its ability to rebuild with each shearing or re-mixing cycle. We suggest that this feature could find its origin in the progressive flattening of the micro-gels at the surface of the cement grains.

Cationic microgel emulsion with a high solid content by a multistep addition method in inverse microemulsion polymerization

ABSTRACTThe multistep addition of a monomer and initiator was developed to successfully synthesize cationic polyacrylamide microgels with solid contents (SCs) greater than 35% and cationic monomer concentrations of 0–40 mol % by inverse microemulsion polymerization. Two feed methods, three‐step nonuniform addition and five‐step uniform addition, were implemented to obtain microgel emulsions with 37% SC. The former addition method was designed according to the solubilization limit of the microemulsion before step polymerization, and that of the latter was a constant based on the remaining surfactant weight in the reactor. The product properties in the intermediate processes of these two methods were compared by dynamic light scattering and viscosity measurement. The results show that the products here were translucent microemulsions instead of milky ones when they were synthesized by a semicontinuous polymerization. Also, the particle sizes of these two methods were almost the same; this indicated that the oscillation phenomenon in continuous polymerization at a high SC was avoided. With the former feed method, the risk and operation cost in the synthesis process could be cut down greatly. Moreover, the viscosity of the cationic microgel emulsion conformed to the Krieger–Dougherty equation with a greater value of intrinsic viscosity than that of a hard‐sphere system because of an electroviscous effect. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40585.

Influence of the host matrix of the enzyme in the performance of amperometric biosensors

Abstract In this work we present a comparative study of four amperometric biosensors of phenol prepared by means of immobilization of tyrosinase in organic and inorganic matrices. The immobilization within organic matrices consisted on enzyme entrapped in both polyacrylamide microgels, and polyvinylimidazole microparticles. On the other hand, two inorganic matrices were investigated: a laponite clay and the calcium phosphate cement, brushite. The enzyme was absorbed on these inorganic matrices and subsequently was cross-linked by glutaraldehyde. The detection of phenolic compounds was performed in aqueous and organic media by direct electrochemical reduction of the enzymatic product, o-quinone, at −0.1 V versus SCE. The large differences found in biosensors performance are attributed to the environment surrounding the enzyme, or the biomaterial layers used in the fabrication of the biosensor. Sensors based on inorganic matrices provided the best results in detection of monophenols and catechol in aqueous solution, being the biosensor of brushite the most sensitive device ( S = 46 A M −1 cm −2 ) with the lowest detection limit (DL = 3 nM). On organic media (acetonitrile, dioxane, and ethanol), the optimum catechol detection was achieved with both, biosensors based on brushite ( S = 3.3 A M −1 cm −2 , DL = 40 nM) and also with biosensors prepared with polyvinylimidazole microparticles (S = 12.5 mA M −1 cm −2 , DL = 8 μM).

A green synthesis of CTAB–PTP/PAM microhydrogel and its application in oxidation of DBT

Polyacrylamide (PAM) microgels covered with hexadecyltrimethylammonium (CTAB)-peroxotungstophosphate (PTP) complexes were synthesized and characterized by scanning electron microscopy and Fourier transform infrared. The results indicate the CTAB–PTP/PAM composite microsphere with surface-wrinkling morphology and PAM microgel core/CTAB–PTP shell structure. The surface-wrinkling structure changed with the amount of CTAB–PTP supported on the PAM microgels. The results based on CTAB–PTP/PAM composite microspheres used in oxidation of dibenzothiophene demonstrated that the resulting microspheres have high catalytic performance and could be reused three times with a small decrease in activity. The proposed method is commonly significant for construction of reusable catalyst used in diphase catalysis.

Silicon dioxide hollow microspheres with porous composite structure: Synthesis and characterization

In this paper, a strategy for hollow porous silica microspheres with ideally flower structure is presented. SiO2/PAM hybrid composite microspheres with porous were synthesized by the reaction that the porous polyacrylamide (PAM) micro-gels immersed in tetraethoxysilane (TEOS) anhydrous alcohol solution and water in a moist atmosphere, with ammonium hydroxide as a catalyst. The SiO2 hollow microspheres with porous were obtained after calcination of the composite microspheres at 550°C for 4h. The morphology, composition, and crystalline structure of the microspheres were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermo-gravimetric analysis (TGA), Fourier transform infrared spectroscopy (FI-IR), and X-ray diffraction (XRD), N2 absorption analysis, respectively. The results indicated that the obtained hollow porous SiO2 microspheres were a perfect flower structure.