Responsive Swelling Behavior Research Articles

Light- and Solvent-Responsive Bilayer Hydrogel Actuators with Reversible Bending Behaviors.

Light-responsive hydrogel systems have gained significant attention due to their unique ability to undergo controlled and reversible swelling behavior in response to light stimuli. Combining light-responsive hydrogels with nonresponsive polymers offers a unique self-folding feature that can be used in soft robotic actuator designs. However, simple formulation of such systems with rapid response time is still a challenging task. Herein, we demonstrate a simple but versatile bilayer polymeric design combining light-responsive spiropyran-polyacrylamide (SP-PAAm) with polyacrylamide (PAAm) hydrogels. The photochromic spiropyran in our polymer design is a closed-ring, hydrophobic compound and turns into an open-ring, hydrophilic merocyanine isomer under light irradiation. The swelling degree of SP-PAAm and PAAm hydrogels was evaluated using LED lights with different wavelengths and solvent media (e.g., water, ethanol, DMF, and DMSO). We observed that SP-PAAm hydrogels reached a swelling ratio of ∼370% with the illumination of the blue LED in the DMF medium. By combining light-responsive SP-PAAm hydrogels with nonresponsive PAAm, a proof-of-concept demonstration was performed to demonstrate the applicability of our fabricated platforms. Although fabricated one-armed bilayer hydrogels possessed self-folding ability with a folding angle of ∼40° in 30 min, the four-armed bilayer platforms demonstrated more efficient and rapid folding behavior and reached a folding angle of ∼75° in ∼15 min. Given their simplicity and efficiency, we believe that such polymeric designs may offer new avenues for the fields of polymeric actuators and soft robotic systems.

Nanofiber-based flexible actuator with high transparency and large-scale deformation for biomimetic and intelligent control

In this study, a novel flexible PVP/PVA-co-PE composite film with excellent transparency and large-scale bending deformation capability was prepared by a simple impregnation process. Due to the similar refractive index between the filling phase of PVP and PVA-co-PE nanofibers, the pore layer of the nanofibrous was replaced by PVP after impregnation, thus endowing the composite film with excellent optical transparency. In addition, the excellent hygroscopic response of PVP promoted the rapid hygroscopic swelling behavior in response to external moisture, and thus achieved complex bending deformation of the PVP/PVA-co-PE composite film. Experimental results show that the optimum optical transparency of the composite film can reach 82.53%, and the maximum bending angle of the composite film can reach 180° within 1.2 s of contact with moisture stimulation. Based on the excellent optical transparency and actuation performance, the PVP/PVA-co-PE composite film was prepared into smart windows, Venus flytrap and bionic reptiles, which promoted its application and development in the field of intelligent bionic and smart control systems.

Microneedles coated with composites of phenylboronic acid-containing polymer and carbon nanotubes for glucose measurements in interstitial fluids

A microneedle (MN) sensor coated with a sensing composite material was proposed for measuring glucose concentrations in interstitial fluid (ISF). The sensing composite material was prepared by blending a polymer containing glucose-responsive phenylboronic acid (PBA) moieties (i.e., polystyrene-block-poly(acrylic acid-co-acrylamidophenylboronic acid)) with conductive carbon nanotubes (CNTs). The polymer exhibited reversible swelling behavior in response to glucose concentrations, which influenced the distribution of the embedded CNTs, resulting in sensitive variations in electrical percolation, even when coated onto a confined surface of the MN in the sensor. We varied the ratio of PBA moieties and the loading amount of CNTs in the sensing composite material of the MN sensor and tested them in vitro using an ISF-mimicking gel with physiological glucose concentrations to determine the optimal sensitivity conditions. When tested in animal models with varying blood glucose concentrations, the MN sensor coated with the selected sensing material exhibited a strong correlation between the measured electrical currents and blood glucose concentrations, showing accuracy comparable to that of a glucometer in clinical use.

Single-Step Fabrication of a Dual-Sensitive Chitosan Hydrogel by C-Mannich Reaction: Synthesis, Physicochemical Properties, and Screening of its Cu2+ Uptake

Uncovering the value of waste materials is one of the keys to sustainability. In this current work, valorization of chitosan was pursued to fabricate a novel modified chitosan functional hydrogel using a process-efficient protocol. The fabrication proceeds by a one-pot and single-step C-Mannich condensation of chitosan (3% w/v), glutaraldehyde (20 eq.), and 4-hydroxycoumarin (40 eq.) at 22 °C in 3% v/v acetic acid. The Mannich base modified chitosan hydrogel (CS-MB) exhibits a dual-responsive swelling behavior in response to pH and temperature that has not been observed in any other hydrogel systems. Combining the pre-defined optimal swelling pH (pH = 4) and temperature (T = 22 °C), the CS-MB was screened for its Cu2+ adsorption capacity at this condition. The CS-MB achieved an optimal adsorption capacity of 12.0 mg/g with 1.2 g/L adsorbent dosage after 36 h with agitation. The adsorption of Cu2+ on the surface of CS-MB was verified by EDS, and an overview of the adsorption sites was exhibited by FT-IR. The simply fabricated novel CS-MB hydrogel under investigation presents a unique response to external stimuli that exhibits a promise in heavy metal removal from aqueous media.

Multi-material stimuli-responsive hydrogels with optically induced actuation

A vision for soft, autonomous materials entails synthesis of multiple senses in multifunctional materials where material response requires sensitivity to external stimuli. Stimuli-responsive hydrogels are of particular interest for optically induced mechanical response due to the ability to transform external stimuli into large, reversible shape change. Specifically, temperature-responsive hydrogels are broadly used and can be designed to achieve deformation through the photothermal effect as a result of surface plasmonic resonance of gold nanoparticles. Here, a multi-material stimuli-responsive hydrogel network with embedded gold nanoparticles is demonstrated in a unit cell pattern with anisotropic swelling behavior in response to visible light. Reversible, anisotropic swelling leads to bending motion that contributes to the development of soft, autonomous materials.

Enhanced intestinal stability and pH sensitive release of quercetin in GIT through gellan gum hydrogels

The objective of the work was to improve the intestinal stability and pH sensitive release of quercetin in the gastrointestinal tract. To achieve this aim, novel quercetin loaded pH sensitive hydrogel beads of gellan gum were developed through an ionic cross-linking technique using calcium chloride (CaCl2) as cross-linking agents. Field emission scanning electron microscopy (FESEM) was used to characterize the morphological appearance of the prepared hydrogel beads. Drug-polymer and drug-CaCl2 interaction were studied by using infrared spectroscopy. The important finding of the study was the formation of stable hydrogel beads at a very low concentration of CaCl2 (0.3 % w/v). There was a drug polymer interaction in addition to polymer and CaCl2 interaction that is confirmed by infrared spectroscopy. All the formulation shows good entrapment efficiencies within the range of 58.56–93.71 %. The prepared beads exhibited a pH responsive swelling behavior with maximum swelling at the simulated intestinal fluid (pH 7.4) (SIF). Hydrogel beads also showed a pH responsive release behavior and sustained release of drug at SIF. Moreover, the intestinal stability of quercetin was improved when encapsulated in gellan gum hydrogel beads in comparison to the solution containing quercetin.

Effect of xylan sulfate on the responsive swelling behavior of poly(methacrylatoethyl trimethyl ammonium chloride)-based composite hydrogels

Xylan sulfate, one important xylan derivative, was prepared using microfluidic reactor system and incorporated in situ under the polymerization of methacroylcholine chloride (DMC), resulting in composite hydrogels containing variable amount of xylan sulfate. The introduction of xylan sulfate imparts enhanced temperature-swelling responsiveness to the hydrogels, and substantially increases their water swelling ratio up to > 200 g/g, which is twice higher than the neat polyDMC hydrogel. Such swelling behavior may relate to the electrostatic interaction between the –SO3− in xylan sulfate and the –NH4+ in polyDMC. This interaction significantly improves the formation of porous architecture and the tensile toughness of the hydrogels. The composite hydrogels exhibit responsive swelling behavior at different salt, pH, and solvent conditions due to osmotic pressure decline, charge shielding and salting-out effect. They load 2.2 times amount of tetracycline hydrochloride and show 50.3% slower release rate than the neat polyDMC hydrogel, which shows great potential for drug delivery applications.

Reduced graphene oxide decorated superporous polyacrylamide based interpenetrating network hydrogel as dye adsorbent

Herein, nano-CaCO3 is utilized as exfoliating pore forming agent to enhance the structural porosity of reduced graphene oxide based hybrid polyacrylamide (PAAm) hydrogel for the application of dye removal. The structural incorporation of microcrystalline chains (MCC) to the hydrogel structure is adopted to form stable interpenetrating polymer network, whereas; in situ reduction is performed onto the leached nanocomposite hydrogel to enhance the dye removal behavior for all dyes. The prepared MCC/poly (AAm-co-NaAc) nanocomposite hydrogel is morphologically and structurally characterized with SEM, TEM, XRD and FTIR. Thermal and viscoelastic behaviors of the porous nanocomposite hydrogel are evaluated through TGA and rheometer. Surface pattern of nano-CaCO3 leached and unleached nanocomposite hydrogels are explored through AFM measurements, while superporous feature of the nanocomposite hydrogel is analyzed through BET adsorption isotherm and mercury porosimetry. Moreover, pH responsive swelling behavior of the nanocomposite hydrogel is well studied along with detail investigation towards removal of dye pollutants (MB/RhB) with respect to contact time and solution temperature.

Graphene quantum dot decorated magnetic graphene oxide filled polyvinyl alcohol hybrid hydrogel for removal of dye pollutants

Herein, a novel and simple coagulation route is adopted for quick preparation of graphene quantum dots (GQDs) integrated magnetic graphene oxide (GO/Fe3O4/GQD) by using ethanol-HCl as destabilizing agent. The ultra-high surface area of GQDs with layered graphene like structure makes them more sensitive to environmental changes as well as eco-friendly (cell viability ~ 86.7%) candidate for dye removal application. The as-prepared nanohybrid filler is then in situ incorporated within PEG-1500 induced macroporous polyvinyl alcohol/carboxymethyl cellulose hydrogel beads. The prepared PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel is characterized with SEM, TEM, XRD and FTIR. Surface topography and roughness of the nanocomposite hydrogel is evaluated through AFM microscopy, while pore distribution in the nanocomposite hydrogel is explored through BET adsorption isotherm and mercury porosimetry. Moreover, pH responsive swelling behavior of the nanocomposite hydrogel is well studied along with detail investigation towards removal of cationic pollutants (MB/RhB) with respect to contact time and solution temperature.

Stimuli-Responsive Polysaccharide Hydrogels for Biomedical Applications: a Review

This review aims to present methods for obtaining polysaccharide hydrogels, their properties and sensitivity to environmental stimuli, as well as their potential applications in biomedicine. Living systems respond to external stimuli by adapting themselves to changing conditions. Hydrogels are a class of materials with 3D networks of polymers that can absorb high amounts of water or biological fluids while remaining insoluble under physiological conditions compared with general absorbent materials, with their characteristic being dependent on network structure and the external environment. Stimuli-responsive hydrogels have the ability to respond to changes in their external environment. They can exhibit dramatic changes in their swelling behavior, network structure, permeability, and mechanical strength in response to variations in temperature, pH, glucose, electric field, light, etc. However, such changes are reversible; therefore, hydrogels can convert to their initial state as soon as the trigger is removed. Because of compatibility with living tissues, hydrogels can be used in different biomedical purposes. The application of stimuli-responsive polysaccharide hydrogels in the biomedical field has become increasingly popular with many research groups and industries. In addition to their ability to undergo large reversible transitions in their swelling behavior due to small physiological or environmental changes, they are also often highly biocompatible and versatile and possess a high storage capacity for the immobilization of biomolecules.

In-situ-Investigation of Enzyme Immobilization on Polymer Brushes.

Herein, we report on the use of a combined setup of quartz-crystal microbalance, with dissipation monitoring and spectroscopic ellipsometry, to comprehensively investigate the covalent immobilization of an enzyme to a polymer layer. All steps of the covalent reaction of the model enzyme glucose oxidase with the poly(acrylic acid) brush by carbodiimide chemistry, were monitored in-situ. Data were analyzed using optical and viscoelastic modeling. A nearly complete collapse of the polymer chains was found upon activation of the carboxylic acid groups with N-(3-Dimethylaminopropyl)-N′-ethylcarbodiimide and N-Hydroxysuccinimide. The reaction with the amine groups of the enzyme occurs simultaneously with re-hydration of the polymer layer. Significantly more enzyme was immobilized on the surface compared to physical adsorption at similar conditions, at the same pH. It was found that the pH responsive swelling behavior was almost not affected by the presence of the enzyme.

Stimuli responsive biopolymer (chitosan) based blend hydrogels for wound healing application

Stimuli responsive chitosan (CS) and poly (N-vinyl-2-pyrrolidone) (PVP) have attained hydrogel properties in the presence of 74% neutralized poly acrylic acid (PAA) which can be exploited for wound healing applications. The FTIR spectra confirmed the presence of all specific functional groups and the developed interactions in the hydrogels. The thermal analysis explained that the hydrogel samples are thermally more stable than individual chitosan and PVP. The antimicrobial analysis revealed that all the samples show antibacterial activity against E. coli and the biodegradation analysis is performed to confirm the hydrogels degradation. The hydrogels showed enhanced responsive swelling behavior against different media depending upon the amount of PVP. The %age swelling in water is decreased with the increase in the amount of PVP. The most considerable swelling behavior is observed against pH, as they manifested low swelling at acidic pH and high swelling at neutral pH while at pH 8, the prominent values are obtained. This distinctive behavior of hydrogels and their biocompatibility made them pertinent to drug delivery and their release profile is examined spectrophotometrically using silver sulfadiazine (antibiotic for burnt wounds) showed 91.2% of drug release for a period of 1 h in phosphate buffer saline (PBS) in a consistent and controlled manner.

Modification of gum ghatti via grafting with acrylamide and analysis of its flocculation, adsorption, and biodegradation properties

In this work, an environmentally friendly gum ghatti-crosslinked-polyacrylamide (Gg-cl-PAAM) hydrogel was synthesized from gum ghatti (Gg) and acrylamide (AAM) using a microwave-assisted grafting technique, and tested for use in water purification applications as an adsorbent and flocculent. The Gg-cl-PAAM was characterized using SEM, FTIR, and TGA, and displayed pH responsive swelling behavior, with maximum swelling (2117%) observed in solution with neutral pH. The flocculation characteristics of Gg-cl-PAAM were tested in clay solutions as a function of pH, temperature, and the polymer mass loading, showing that the best performance is obtained at neutral pH at 40°C. The adsorption capacities of Gg-cl-PAAM for the removal different dyes such as brilliant green (BG), rhodamine B (RhB), congo red (CR), and methyl orange (MO) were tested, revealing that the adsorption of all dyes followed the Langmuir isotherm model, with qm values of 523.62mgg−1 for BG, 421.60mgg−1 for RhB, 179.09mgg−1 for CR, and 173.69mgg−1 for MO. Finally, the environmentally friendly nature of Gg-cl-PAAM was examined using the soil-burial composting method, which demonstrated 93% degradation of the Gg-cl-PAAM hydrogel within 60days.

Photo-crosslinked mechanically strong PCL4-PDMAEM hydrogels

In this study, a series of thermo-responsive hydrogels based on N′N-dimethylaminoethyl methacrylate (DMAEM) and methacrylate functionalized poly(caprolactone) (PCL4-IEMA) were prepared by bulk photo-polymerization method. Poly(caprolactone) (PCL) was modified with 2-isocyanatoethylmethacrylate (IEMA) to form a macro-crosslinker due to its excellent biocompatibility to generate high-strength smart hydrogels with pH and temperature responsivity. The synthesized macro-crosslinker and PCL4-PDMAEM hydrogel were characterized by 1H NMR, GPC and FT-IR analyses. Sol-gel transitions were examined by oscillatory measurements in Rheometer under UV light. Swelling kinetics as a function of time, swelling ratios at different pH values and temperatures were studied. pH responsive swelling behavior of PCL4-PDMEAM hydrogels was demonstrated with a highest swelling value at pH = 2. Temperature responsivity was also proved with a LCST around 40 °C between the shrunken and swollen state. Young modulus of the sample was also calculated around 0.3 MPa. The results indicated that using PCL4-IEMA as a macro-crosslinker is an effective way to obtain mechanically strong - tough - hydrogels by means of dissipating the applied energy through flexible crosslink units. In addition, swelling and mechanical properties of the hydrogels could be tailored by varying the amount of the crosslinker.

Development of visible-light responsive and mechanically enhanced "smart" UCST interpenetrating network hydrogels.

An interpenetrating polymer network (IPN), chlorophyllin-incorporated environmentally responsive hydrogel was synthesized and exhibited the following features: enhanced mechanical properties, upper critical solution temperature (UCST) swelling behavior, and promising visible-light responsiveness. Poor mechanical properties are known challenges for hydrogel-based materials. By forming an interpenetrating network between polyacrylamide (PAAm) and poly(acrylic acid) (PAAc) polymer networks, the mechanical properties of the synthesized IPN hydrogels were significantly improved compared to hydrogels made of a single network of each polymer. The formation of the interpenetrating network was confirmed by Fourier Transform Infrared Spectroscopy (FTIR), the analysis of glass transition temperature, and a unique UCST responsive swelling behavior, which is in contrast to the more prevalent lower critical solution temperature (LCST) behaviour of environmentally responsive hydrogels. The visible-light responsiveness of the synthesized hydrogel also demonstrated a positive swelling behavior, and the effect of incorporating chlorophyllin as the chromophore unit was observed to reduce the average pore size and further enhance the mechanical properties of the hydrogel. This interpenetrating network system shows potential to serve as a new route in developing "smart" hydrogels using visible-light as a simple, inexpensive, and remotely controllable stimulus.

Generation of pH responsive fluorescent nano capsules through simple steps for the oral delivery of low pH susceptible drugs

pH responsive nano capsules are promising as it can encapsulate low pH susceptible drugs like insulin and guard them from the hostile environments in the intestinal tract. The strong acidity of the gastro-intestinal tract and the presence of proteolytic enzymes are the tumbling blocks for the design of drug delivery vehicles through oral route for drugs like insulin. Nano capsules are normally built over templates which are subsequently removed by further steps. Such processes are complex and often lead into deformed and collapsed capsules. In this study, we choose calcium carbonate (CaCO3) nano particles to serve as template. Over CaCO3 nanoparticles, silica layers were built followed by polymethacrylic acid chains to acquire pH responsiveness. During the polymerization process of the methacrylic acid, the calcium carbonate core particles were dissolved leading to the formation of nano hollow capsules having a size that ranges from 225 to 246 nm and thickness from 19 to 58 nm. The methodology is simple and devoid of additional steps. The nano shells exhibited 80% release of the loaded model drug, insulin at pH 7.4 while at pH 2.0 the capsules nearly stopped the release of the drug. Polymethacrylic acid shows pH responsive swelling behavior that it swells at intestinal pH (7.0–7.5) and shrinks at gastric pH (∼2.0) thus enabling the safe unloading of the drug from the nano capsules.

CO2‐responsive swelling behavior and metal‐ion adsorption properties in novel histamine‐conjugated polyaspartamide hydrogel

ABSTRACTNovel polyaspartamide copolymers containing histamine pendants (PHEA‐HIS) were prepared from polysuccinimide, which is the thermal polycondensation product of aspartic acid, via a successive ring‐opening reaction using histamine (HIS) and ethanolamine (EA). The prepared water‐soluble copolymer was then crosslinked by reacting it with hexamethylene diisocyanate in order to provide a hydrogel with both good gel strength and reversible CO2 absorption characteristics. PHEA‐HIS gel is also pH‐sensitive and eligible to coordinate to metal ions such as Pb2+, Cu2+, and Ni2+ due to the imidazole units in its structure. The CO2‐responsive swelling behavior, metal‐ion adsorption, and morphology of the crosslinked gels were investigated. The approach described here results is a promising hydrogel with potential for a variety of industrial and biomedical applications including CO2 capture, CO2‐responsive and switchable sensors, and smart drug delivery systems. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43305.

PH responsive graft copolymers of chitosan

Grafting suitable polymers onto chitosan can produce cationic or polyampholyte polymers or hydrogels that are potential smart biomedical materials. Chitosan-graft-[poly(diethylamino)ethyl methacrylate] has been prepared in three different physical forms as linear free chains in solution, chemical gels crosslinked with glutaraldehyde, and poly(diethylamino)ethyl methacrylate] grafted onto chitosan tripolyphosphate gel beads. In addition to chemical structure, the graft copolymers were characterized with respect to their dissolution and swelling behavior in aqueous solution. It has been established that solubility of the products is controlled by the grafting yield. While pH sensitive polymers, which collapse at a given pH value are obtained at lower grafting yields, hydrogels form at higher grafting yields with pH responsive swelling behavior. Glutaraldehyde crosslinked chitosan-graft-[poly(diethylamino)ethyl methacrylate] gels and chitosan tripolyphosphate gel beads grafted with poly[(diethylamino)ethyl methacrylate] exhibit pH sensitive swelling with highest equilibrium swelling capacity at pH=1.2.

Effect of Polyethylene Glycol on Bis(2‐hydroxyethyl) terephthalate‐Based Polyurethane/Alginate pH‐Sensitive Blend for Oral Protein Delivery

ABSTRACTPolyurethanes (PUs) are synthesized by reacting two polyols, namely bis(2‐hydroxyethyl) terephthalate (BHET) and polyethylene glycol (PEG), taken in four different ratios and hexamethylene diisocyanate, where the polyol content is slightly higher than the stoichiometrically required amount. BHET is obtained by the glycolysis of polyethylene terephthalate waste. The synthesized PUs are blended with sodium alginate in the ratio of 7:3 and then cross‐linked by calcium chloride to get a pH‐sensitive protein (bovine serum albumin, BSA) carrier. X‐ray diffraction patterns of different PU compositions indicate the presence of amorphous phase. The hydroxyl value of OH‐terminated PUs is found to be increasing with increasing amount of PEG in the composition. PEGylated PU shows higher water uptake and protection against hemolysis with a rising amount of PEG in the PU. Fourier transform infrared spectroscopy of the blend shows characteristic peaks for both PU and alginate distinctly, indicating good dispersion of both the polymers. pH responsive swelling behavior in different pH media corresponding to the gastrointestinal tract (pH 1.2 and 7.4) is investigated for all the prepared blends. The minimum degree of swelling is observed for PU with the maximum amount PEG at pH 1.2, whereas the degree of swelling of the same PU significantly increased at pH 7.4. BSA encapsulation efficiency of the blends increases with an increase in PEG content in the PU and varies within the range of 88–99%. The release profile of BSA at pH 7.4 is found to be similar as observed in the swelling study.

Effect of Deacetylation on Characterization of pH Stimulus Responsive Chitosan-Acrylamide Hydrogels Using Radiation

Smart hydrogels which can change their swelling behavior and other properties in response to environmental stimuli such as temperature, pH, solvent composition and electric fields, have attracted great interest as chemical sensor material and controlled release system. The pH stimulus responsive hydrogels were synthesized by gamma-irradiation graft copolymerization of chitosan-acrylamide. In this research the influence of deacetylation process on the hydrogels characterization were investigated by measuring grafting efficiency, gel fraction, swelling degree, and crosslink density. Evidence of grafting was confirmed by FTIR spectroscopy. X-ray diffraction showed reduction in the crystallinity of chitosan with different deacetylation process also after the graft copolymerization reaction. The results showed that decreasing acetyl group of chitosan increase the grafting efficiency, gel fraction and swelling degree. While crystallinity decreased. The hydrogels indicated pH-dependent swelling behaviour.