Effects of concentration of hydrophobic component and swelling in saline solutions on mechanical properties of a stretchable hydrogel.

An elastic biopolymer, resilin possesses exceptional qualities such as high stretchability and resilience. Such attributes are utilized in nature by many species for mechanical energy storage to facilitate movement. The properties of resilin are attributed to the balanced combination of hydrophilic and hydrophobic segments. To mimic the properties of resilin, we developed a hydrogel system composed of hydrophilic acrylic acid (AAc) and methacrylamide (MAM) chains and hydrophobic poly(propylene glycol diacrylate) (PPGDA) chains. The gel was produced through free-radical polymerization in 0.8 M NaCl solutions using KPS as an initiator. In these gels, AAc and MAM can form hydrogen bonds, whereas the association between PPGDA chains can lead to hydrophobic domains. The PPGDA concentration affects the level of hydrogen bonding and gel mechanical properties. Tensile experiments revealed that the elastic modulus increased with a higher PPGDA concentration. Retraction experiments demonstrated increased velocity and acceleration when released from a stretched state with increasing PPGDA concentration. Swelling and deswelling of gels in saline solutions led to a change in mechanical properties and retraction behavior. This study shows that the stretchability and resilience of these hydrogels can be adjusted by changing the concentration of hydrophobic components.

Gels of Semiconducting Polymers in Benign Solvents.

Gels of semiconducting polymers have many potential applications, including biomedical devices and sensors. Here, we report a self-assembled gel system consisting of isoindigo-based semiconducting polymers with galactose side chains in benign, alcohol-based solvents. Because of the carbohydrate side chains, the modified isoindigo polymers are soluble in alcohols. We obtained thermoreversible gels in 1-propanol using these polymers and di-Fmoc-l-lysine, a molecular gelator. The polymers and molecular gelators have been selected in such a way that they do not have significant physical interactions. The molecular gelator self-assembled to form a fibrous structure that confines the polymer chains in the interstitial spaces of the fibers. The polymer chains formed local aggregations and increased the shear moduli of the gels significantly. Bulky galactose side chains and the less planar nature of the polymer backbone hindered the formation of long-range assembled structures of the polymers. However, the dispersion of polymers throughout the gel samples resulted in a percolated structure in the dried gel films. The bulk electrical conductivity of dried gels confirmed the presence of such percolated structures. Our results demonstrated that carbohydrate-containing conjugated polymers can be combined with molecular gelators to obtain gels in eco-friendly solvents.

Effects of Salt on Phase Behavior and Rheological Properties of Alginate-Chitosan Polyelectrolyte Complexes.

Oppositely charged polyelectrolytes often form polyelectrolyte complexes (PECs) due to the association through electrostatic interactions. Obtaining PECs using natural, biocompatible polyelectrolytes is of interest in the food, pharmaceutical, and biomedical industries. In this work, PECs were prepared from two biopolymers, positively charged chitosan and negatively charged alginate. We investigate the changes in the structure and properties of PECs by adding sodium chloride (salt doping) to the system. The shear modulus of PECs can be tuned from ∼10 to 104 Pa by changing the salt concentration. The addition of salt led to a decrease in the water content of the complex phase with increasing shear modulus. However, at a very high salt concentration, the shear modulus of the complex phase decreased but did not lead to the liquid coacervate formation, typical of synthetic polyelectrolytes. This difference in phase behavior has likely been attributed to the hydrophobicity of chitosan and long semiflexible alginate and chitosan chains that restrict the conformational changes. Large amplitude oscillatory shear experiments captured nonlinear responses of PECs. The compositions of the PECs, determined as a function of salt concentration, signify the preferential partitioning of salt into the complex phase. Small-angle X-ray scattering of the salt-doped PECs indicates that the Kuhn length and radius of the alginate-chitosan associated structure qualitatively agree with the captured phase behavior and rheological data. This study provides insights into the structure-property as a function of salt concentration of natural polymer-based PECs necessary for developing functional materials from natural polyelectrolytes.

Using molecular straps to engineer conjugated porous polymer growth, chemical doping, and conductivity.

Controlling network growth and architecture of 3D-conjugated porous polymers (CPPs) is challenging and therefore has limited the ability to systematically tune the network architecture and study its impact on doping efficiency and conductivity. We have proposed that π-face masking straps mask the π-face of the polymer backbone and therefore help to control π-π interchain interactions in higher dimensional π-conjugated materials unlike the conventional linear alkyl pendant solubilizing chains that are incapable of masking the π-face. Herein, we used cycloaraliphane-based π-face masking strapped monomers and show that the strapped repeat units, unlike the conventional monomers, help to overcome the strong interchain π-π interactions, extend network residence time, tune network growth, and increase chemical doping and conductivity in 3D-conjugated porous polymers. The straps doubled the network crosslinking density, which resulted in 18 times higher chemical doping efficiency compared to the control non-strapped-CPP. The straps also provided synthetic tunability and generated CPPs of varying network size, crosslinking density, dispersibility limit, and chemical doping efficiency by changing the knot to strut ratio. For the first time, we have shown that the processability issue of CPPs can be overcome by blending them with insulating commodity polymers. The blending of CPPs with poly(methylmethacrylate) (PMMA) has enabled them to be processed into thin films for conductivity measurements. The conductivity of strapped-CPPs is three orders of magnitude higher than that of the poly(phenyleneethynylene) porous network.

Correction: Temperature- and strain-dependent transient microstructure and rheological responses of endblock-associated triblock gels of different block lengths in a midblock selective solvent.

Correction for 'Temperature- and strain-dependent transient microstructure and rheological responses of endblock-associated triblock gels of different block lengths in a midblock selective solvent' by Rosa Maria Badani Prado et al., Soft Matter, 2022, 18, 7020-7034, https://doi.org/10.1039/D2SM00567K.

Temperature- and strain-dependent transient microstructure and rheological responses of endblock-associated triblock gels of different block lengths in a midblock selective solvent.

Endblock associative ABA gels in midblock selective solvents are attractive due to their easily tunable mechanical properties. Here, we present the effects of A- and B-block lengths on the rheological properties and microstructure of ABA gels by considering three low and one high polymer concentrations. The triblock polymer considered is poly(methyl methacrylate)-poly(n-butyl acrylate)-poly(methyl methacrylate) [PMMA-PnBA-PMMA] and the midblock solvent is 2-ethyl-1-hexanol. The gelation temperature has been found to be strongly dependent on the B-block (PnBA) length, as longer B-blocks facilitate network formation resulting in higher gelation temperature even with lower polymer chain density. Longer A-blocks (PMMA chains) make the endblock association stronger and significantly increase the relaxation time of gels. Temperature-dependent microstructure evolution for the gels with high polymer concentration reveals that the gel microstructure does not change significantly after the gel formation takes place. The dynamic change of microstructure in an applied strain cycle was captured using RheoSAXS experiments. The microstructure orients with the applied strain and the process is reversible in nature, indicating no significant A-block pullout. Our results provide new understandings regarding the temperature and strain-dependent microstructural change of ABA gels in midblock selective solvents.

Multicolor Nitrogen-Doped Carbon Quantum Dots for Environment-Dependent Emission Tuning.

Carbon quantum dots (CQDs) have potential applications in many fields such as light-emitting devices, photocatalysis, and bioimaging due to their unique photoluminescence (PL) properties and environmental benignness. Here, we report the synthesis of nitrogen-doped carbon quantum dots (NCQDs) from citric acid and m-phenylenediamine using a one-pot hydrothermal approach. The environment-dependent emission changes of NCQDs were extensively investigated in various solvents, in the solid state, and in physically assembled PMMA-PnBA-PMMA copolymer gels in 2-ethyl-hexanol. NCQDs display bright emissions in various solvents as well as in the solid state. These NCQDs exhibit multicolor PL emission across the visible region upon changing the environment (solutions and polymer matrices). NCQDs also exhibit excitation-dependent PL and solvatochromism, which have not been frequently investigated in CQDs. Most CQDs are nonemissive in the aggregated or solid state due to the aggregation-caused quenching (ACQ) effect, limiting their solid-state applications. However, NCQDs synthesized here display a strong solid-state emission centered at 568 nm attributed to the presence of surface functional groups that restrict the π-π interaction between the NCQDs and assist in overcoming the ACQ effect in the solid state. NCQD-containing gels display significant fluorescence enhancement in comparison to the NCQDs in 2-ethyl hexanol, likely because of the interaction between the polar PMMA blocks and NCQDs. The application of NCQDs-Gel as a solid/gel state fluorescent display has been presented. This research facilitates the development of large-scale, low-cost multicolor phosphor for the fabrication of optoelectronic devices, sensing, and bioimaging applications.

Microencapsulated phase change material via Pickering emulsion stabilized by cellulose nanofibrils for thermal energy storage.

A phase change material (PCM) has an ability to store and release a large amount of energy in a wide range of temperature by the latent heat of fusion upon melting and crystallization. Microencapsulation may protect PCM from undesirable reaction and leaching. Herein, we report the microencapsulation of n-hexadecane via oil-in-water Pickering emulsions stabilized by non-chemically modified cellulose nanofibrils (CNF). The maximum size of PCM-CNF microcapsules was 12 ± 3.4 µm in diameter. The surface coverage of the microcapsule by CNF was as high as 67%, consistent with porous morphology of the freeze-dried microcapsules. With 59% PCM loading, the PCM-CNF microcapsule exhibited 132.5 and 141.1 J/g as stored and released thermal energy, respectively. The microcapsule slurry showed a reversible change in storage modulus by one order of magnitude across the transition temperature of n-hexadecane. Combined results demonstrate the successful microencapsulation of PCM via CNF-based Pickering emulsions for a sustainable thermal energy storage material.

Physicochemical and Antimicrobial Properties of Thermosensitive Chitosan Hydrogel Loaded with Fosfomycin.

Thermosensitive chitosan hydrogels-renewable, biocompatible materials-have many applications as injectable biomaterials for localized drug delivery in the treatment of a variety of diseases. To combat infections such as Staphylococcus aureus osteomyelitis, localized antibiotic delivery would allow for higher doses at the site of infection without the risks associated with traditional antibiotic regimens. Fosfomycin, a small antibiotic in its own class, was loaded into a chitosan hydrogel system with varied beta-glycerol phosphate (ß-GP) and fosfomycin (FOS) concentrations. The purpose of this study was to elucidate the interactions between FOS and chitosan hydrogel. The Kirby Bauer assay revealed an unexpected concentration-dependent inhibition of S. aureus, with reduced efficacy at the high FOS concentration but only at the low ß-GP concentration. No effect of FOS concentration was observed for the planktonic assay. Rheological testing revealed that increasing ß-GP concentration increased the storage modulus while decreasing gelation temperature. NMR showed that FOS was removed from the liquid portion of the hydrogel by reaction over 12 h. SEM and FTIR confirmed gels degraded and released organophosphates over 5 days. This work provides insight into the physicochemical interactions between fosfomycin and chitosan hydrogel systems and informs selection of biomaterial components for improving infection treatment.

Structural, optical, photocatalytic, and optoelectronic properties of Zn2SnO4 nanocrystals prepared by hydrothermal method.

Zn2SnO4 (ZTO) nanocrystals are extensively studied in various fields. However, size-dependent ZTO nanocrystals are still challenging to understand their structural, optical, photocatalytic, and optoelectronic properties. ZTO nanocrystals are synthesized by a facile hydrothermal reaction method. The structural properties of the synthesized ZTO nanocrystals are studied by x-ray diffraction and transmission electron microscope. The sizes of the ZTO nanocrystals are controlled by the pH values of the precursor and the molar ratios of the Zn:Sn in the starting materials. ZTO nanocrystals with the small size of 6 nm and large size of 270 nm are obtained by our method. The Eu3+ ions are doped into ZTO nanocrystals to probe size-dependent Eu doping sites, which shows significant potential applications in light emitting diode phosphors. Moreover, the photocatalytic activity of ZTO nanocrystals on rhodamine (RhB) decoloration are investigated, and the results show that 6 nm ZTO nanocrystals show better performance in the photocatalytic decoloration of RhB compared to 270 nm nanocrystals. Most importantly, we design and fabricate optoelectronic devices to detect IR light based on our nanocrystals and a self-prepared NIR cyanine dye. The device based on small sized ZTO nanocrystals exhibits better device performance under 808 nm IR light compared to that of the large sized ZTO nanocrystals. We believe this work represents ZTO size-dependent properties in term of structural, optical, photocatalytic, and optoelectronic properties as a multifunctional material.

Autoclaving of Poloxamer 407 hydrogel and its use as a drug delivery vehicle.

With antibiotic-resistant bacteria becoming increasingly prevalent, biomaterials capable of targeted, in situ drug delivery are urgently needed. The synthetic polymer Poloxamer 407 (P407) is of particular interest due to its thermoreversible gelation. Clinical use of P407 typically involves sterilization via autoclaving, but the effects of these extreme environmental conditions on hydrogel water content, rheological properties and efficacy as a drug delivery vehicle remain unknown. The aim of this study was to investigate the effects of autoclaving on the properties of P407 hydrogel. Autoclaving reduced hydrogel water content due to evaporation, thus increasing the polymer weight fraction of the hydrogels. In contrast, except for a reduction in gelation temperature following autoclaving, autoclaved hydrogels had similar rheological properties as nonautoclaved hydrogels. In vitro, autoclaving did not hinder the hydrogel's efficacy as a carrier for vancomycin antibiotic, and P407 (with and without vancomycin) had a bactericidal effect on planktonic Staphylococcus aureus. An in vivo pilot study using P407 to deliver bacteriophage highlighted the need for additional understanding of the functionality of the hydrogel for surgical applications. In conclusion, P407 hydrogel water content and gelation temperature were reduced by autoclave sterilization, while other rheological properties and the efficacy of the biomaterial as a delivery vehicle for vancomycin in vitro were unaffected.

Achieving High-Speed Retraction in Stretchable Hydrogels.

Hydrogels mimicking elastomeric biopolymers such as resilin, responsible for power-amplified activities in biological species necessary for locomotion, feeding, and defense have applications in soft robotics and prosthetics. Here, we report a bioinspired hydrogel synthesized through a free-radical polymerization reaction. By maintaining a balance between the hydrophilic and hydrophobic components, we obtain gels with an elastic modulus as high as 100 kPa, stretchability up to 800%, and resilience up to 98%. Such properties enable these gels to catapult projectiles. Furthermore, these gels achieve a retraction velocity of 16 m s-1 with an acceleration of 4 × 103 m s-2 when released from a stretched state, and these values are comparable to those observed in many biological species during a power amplification process. By utilizing and tuning the simple synthetic strategy used here, these gels can be used in soft robotics, prosthetics, and engineered devices where power amplification is desired.

Rheological properties and failure of alginate hydrogels with ionic and covalent crosslinks.

Polysaccharide-based hydrogels are being used in a wide variety of applications ranging from tissue engineering to food products due to their biocompatibility and the ease of gel formation. In real-life applications, hydrogels can undergo large strain deformation, which may result in structural damage leading to failure. Here, we report the nonlinear rheological properties and failure behavior of alginate hydrogels, a class of polysaccharide hydrogels, synthesized via ionic and covalent crosslinking. Gels with ionic crosslinks or ionic alginate hydrogels are prepared by addition of Ca2+ ions in the aqueous solution of sodium alginate, and the covalently crosslinked alginate gels or chemical alginate hydrogels are obtained via amidation reactions. Because of their structural differences, ionic and chemical alginate hydrogels display different scattering profiles captured by using small angle X-ray scattering (SAXS) technique. Both ionic and chemical alginate hydrogels exhibit strain stiffening behavior when subjected to large amplitude oscillatory shear (LAOS) and the strain-stiffening behavior is accompanied by negative normal stress. A custom-built cavitation rheometer has been utilized to probe the local failure behavior of these gels. The cavitation rheometry captures different defect growth or fracture mechanism in ionic versus chemical alginate hydrogels, even if these two types of gels have a similar linear elastic modulus. Based on the critical pressure for gel fracture, we have provided an estimate of the critical energy release rate.

A quality by design approach to develop topical creams via hot-melt extrusion technology.

The advantages of hot-melt extrusion technology (HME) over conventional techniques to develop topical semisolids have been established. However, this technique is not widely used for semisolid production. Therefore, the aim of this novel work was to develop creams using the melt extrusion technology while applying Quality by Design (QbD) principles to study the effects of the extrusion process parameters on the product characteristics. The model drug selected was hydrocortisone acetate. A 23 factorial design was considered for the factor influence study, which resulted in eight formulations to be extruded. Of the process parameters considered, the temperature of zone 2 had a significant influence on the work of adhesion of the creams. A similar permeation profile was obtained for all the formulations with the formulations following a diffusion based drug release mechanism. The results from the size distribution graph indicated stable cream formulations. In conclusion, this technology coupled with a design of experiments approach could be utilized to study how the extrusion process parameters could be modified to develop consistent topical creams with ideal product characteristics.

Pluronic-based dual-stimuli sensitive polymers capable of thermal gelation and pH-dependent degradation for in situ biomedical application.

Thermo-sensitive hydrogels are considered ideal for applications in the biomedical fields for their biocompatibility, flexibility, tissue-like water content, and reversible gelation property. By adjusting sufficient hydrophilic-hydrophobic balance in block copolymer structure, thermogel's critical gelation temperature can be modified to be near the physiological temperature, which makes it an appealing candidate for in situ gel depot. In this study, we report successful syntheses of novel multiple block copolymer compounds, denoted as dual-stimuli sensitive polymers (DSSPs), by copolymerizing Pluronic® P104 (7,100 Da) and 2,2-bis(aminoethoxy)propane (BAP) using diisocyanate linkers, L-lysine ethyl ester diisocyanate (DSSP-1) and 1,6-hexamethylene diisocyanate (DSSP-2). Through effective elongation of polymer chain lengths (DSSP-1: 41,760 Da, DSSP-2: 41,230 Da), Pluronic® P104's reversible thermal gelation properties were enhanced, as demonstrated by lowered critical gelation temperatures (DSSP-1: 36°C, DSSP-2: 38.7°C; 15 wt.%) that is near the physiological temperature. Furthermore, integration of acid-labile BAP allowed rapid pH-dependent degradation of the polymer, which was displayed by gel permeation chromatography (GPC) and release profiles of nile red and irinotecan from polymeric micelles and gels, respectively.

Investigation of failure behavior of a thermoplastic elastomer gel.

Gels are increasingly being used in many applications, and it is important to understand how these gels fail subjected to mechanical deformation. Here, we investigate the failure behavior of a thermoplastic elastomer gel (TPEG) consisting of poly(styrene)-poly(isoprene)-poly(styrene) in mineral oil, in tensile mode, under constant stress, and in fracture tests, where the fracture initiates from a predefined crack. In these gels, the poly(styrene) endblocks associate to form spherical aggregates, as captured using SAXS. Shear-rheology experiments indicate that the poly(isoprene) midblocks connecting these aggregates are loosely entangled. The relaxation behavior of these gels has been captured by time-temperature superposition of frequency sweep data and stress-relaxation experiments. The relaxation process in these gels involves endblock pullout from the aggregates and subsequent relaxation of the chains. An unfavorable enthalpic interaction between the endblock and mineral oil results in a significantly large relaxation time. These gels display rate dependent mechanical properties, likely due to the midblock entanglements. Fracture and creep failure tests provide insights into the gel failure mechanism. Creep experiments indicate that these gels fail by a thermally activated process. Fracture experiments capture the energy release rate as a function of crack-tip velocity. The critical energy release rate is estimated by incorporating the friction force the polystyrene chains are subjected to, as those are pulled out of aggregates, and the enthalpic cost to overcome unfavorable interaction between poly(styrene) and mineral oil. Our results provide further insights to the failure behavior of the self-assembled TPEGs.

Development of poloxamer gel formulations via hot-melt extrusion technology.

Poloxamer gels are conventionally prepared by the "hot" or the "cold" process. But these techniques have some disadvantages such as high energy consumption, requires expensive equipment and often have scale up issues. Therefore, the objective of this work was to develop poloxamer gels by hot-melt extrusion technology. The model drug selected was ketoprofen. The formulations developed were 30% and 40% poloxamer gels. Of these formulations, the 30% poloxamer gels were selected as ideal gels. DSC and XRD studies showed an amorphous nature of the drug after extrusion. It was observed from the permeation studies that with increasing poloxamer concentration, a decrease in drug permeation was obtained. Other studies conducted for the formulations included in-vitro release studies, texture analysis, rheological studies and pH measurements. In conclusion, the hot-melt extrusion technology could be successfully employed to develop poloxamer gels by overcoming the drawbacks associated with the conventional techniques.

Probing Gelation and Rheological Behavior of a Self-Assembled Molecular Gel.

Molecular gels have been investigated over the last few decades; however, mechanical behavior of these self-assembled gels is not well understood, particularly how these materials fail at large strain. Here, we report the gelation and rheological behavior of a molecular gel formed by self-assembly of a low molecular weight gelator (LMWG), di-Fmoc-l-lysine, in 1-propanol/water mixture. Gels were prepared by solvent-triggered technique, and gelation was tracked using Fourier transform infrared (FTIR) spectroscopy and shear rheology. FTIR spectroscopy captures the formation of hydrogen bonding between the gelator molecules, and the change in IR spectra during the gelation process correlates with the gelation kinetics results captured by rheology. Self-assembly of gelator molecules leads to a fiber-like structure, and these long fibers topologically interact to form a gel-like material. Stretched-exponential function can capture the stress-relaxation data. Stress-relaxation time for these gels have been found to be long owing to long fiber dimensions, and the stretching exponent value of 1/3 indicates polydispersity in fiber dimensions. Cavitation rheology captures fracture-like behavior of these gels, and critical energy release rate has been estimated to be of the order 0.1 J/m2. Our results provide new understanding of the rheological behavior of molecular gels and their structural origin.

Anisotropic Nanoparticles Contributing to Shear-Thickening Behavior of Fumed Silica Suspensions.

Rheological characteristics of a concentrated suspension can be tuned using anisotropic particles having various shapes and sizes. Here, the role of anisotropic nanoparticles, such as surface-functionalized multiwall carbon nanotubes (MWNTs) and graphene oxide nanoplatelets (GONPs), on the rheological behavior of fumed silica suspensions in poly(ethylene glycol) (PEG) is investigated. In these mixed-particle suspensions, the concentrations of MWNTs and GONPs are much lower than the fumed silica concentration. The suspensions are stable, and hydrogen-bonded PEG solvation layers around the particles inhibit their flocculation. Fumed silica suspensions over the concentration range considered here display shear-thickening behavior. However, for a larger concentration of MWNTs and with increasing aspect ratios, the shear-thickening behavior diminishes. In contrast, a distinct shear-thickening response has been observed for the GONP-containing suspensions for similar mass fractions (MFs) of MWNTs. For these suspensions, shear thickening is achieved at a lower solid MFs compared to the suspensions consisting of only fumed silica. A significant weight reduction of shear-thickening fluids that can be achieved by this approach is beneficial for many applications. Our results provide guiding principles for controlling the rheological behavior of mixed-particle systems relevant in many fields.

Molecular Insights into Gelation of Di-Fmoc-l-Lysine in Organic Solvent-Water Mixtures.

Despite significant interest in molecular gels due to their intriguing structure formation through self-assembly and their stimuli-responsive behavior, our understanding of the gel formation mechanism of a low-molecular-weight gelator (LMWG) is incomplete. Here, we report a combined experimental and computational study on a LMWG, di-Fmoc-l-lysine, that has two aromatic moieties and multiple hydrogen bond donors and acceptors. Gelation in various organic solvent-water mixtures was obtained through the solvent-triggered technique. We show that an approach based on approximate cohesive energy density derived from density functional theory (DFT) calculations can capture the experimental solubility trend of LMWGs in different organic solvents. Furthermore, DFT calculations indicate parallel and helical structures to be the preferred structural motifs for gelator dimers. We believe that these motifs can potentially lead to fiber formation as observed with microscopy. Our work provides a relatively simple yet effective approach to quantify interactions between solvents and complex gelators that can help rationalize solubility and gelation behavior.