Highly Swellable, Dual-Responsive Hydrogels Based on PNIPAM and Redox Active Poly(ferrocenylsilane) Poly(ionic liquid)s: Synthesis, Structure, and Properties

Advances in Membrane Separation for Biomaterial Dewatering

Biomaterials often contain large quantities of water (50-98%), and with the current transition to a more biobased economy, drying these materials will become increasingly important. Contrary to the standard, thermodynamically inefficient chemical and thermal drying methods, dewatering by membrane separation will provide a sustainable and efficient alternative. However, biomaterials can easily foul membrane surfaces, which is detrimental to the performance of current membrane separations. Improving the antifouling properties of such membranes is a key challenge. Other recent research has been dedicated to enhancing the permeate flux and selectivity. In this review, we present a comprehensive overview of the design requirements for and recent advances in dewatering of biomaterials using membranes. These recent developments offer a viable solution to the challenges of fouling and suboptimal performances. We focus on two emerging development strategies, which are the use of electric-field-assisted dewatering and surface functionalizations, in particular with hydrogels. Our overview concludes with a critical mention of the remaining challenges and possible research directions within these subfields.

Thermo-Electro-Responsive Redox-Copolymers for Amplified Solvation, Morphological Control, and Tunable Ion Interactions

Electro-responsive metallopolymers can possess highly specific and tunable ion interactions, and have been explored extensively as electrode materials for ion-selective separations. However, there remains a limited understanding of the role of solvation and polymer-solvent interactions in ion binding and selectivity. The elucidation of ion-solvent-polymer interactions, in combination with the rational design of tailored copolymers, can lead to new pathways for modulating ion selectivity and morphology. Here, we present thermo-electrochemical-responsive copolymer electrodes of N-isopropylacrylamide (NIPAM) and ferrocenylpropyl methacrylamide (FPMAm) with tunable polymer-solvent interactions through copolymer ratio, temperature, and electrochemical potential. As compared to the homopolymer PFPMAm, the P(NIPAM0.9-co-FPMAm0.1) copolymer ingressed 2 orders of magnitude more water molecules per doping ion when electrochemically oxidized, as measured by electrochemical quartz crystal microbalance. P(NIPAM0.9-co-FPMAm0.1) exhibited a unique thermo-electrochemically reversible response and swelled up to 83% after electrochemical oxidation, then deswelled below its original size upon raising the temperature from 20 to 40 °C, as measured through spectroscopic ellipsometry. Reduced P(NIPAM0.9-co-FPMAm0.1) had an inhomogeneous depth profile, with layers of low solvation. In contrast, oxidized P(NIPAM0.9-co-FPMAm0.1) displayed a more uniform and highly solvated depth profile, as measured through neutron reflectometry. P(NIPAM0.9-co-FPMAm0.1) and PFPMAm showed almost a fivefold difference in selectivity for target ions, evidence that polymer hydrophilicity plays a key role in determining ion partitioning between solvent and the polymer interface. Our work points to new macromolecular engineering strategies for tuning ion selectivity in stimuli-responsive materials.

Recent advances in poly(ionic liquid)s for biomedical application

Poly(ionic liquid)s (PILs) are polymers containing ions in their side-chain or backbone, and the designability and outstanding physicochemical properties of PILs have attracted widespread attention from researchers. PILs have specific characteristics, including negligible vapor pressure, high thermal and chemical stability, non-flammability, and self-assembly capabilities. PILs can be well combined with advanced analytical instruments and technology and have made outstanding contributions to the development of biomedicine aiding in the continuous advancement of science and technology. Here we reviewed the advances of PILs in the biomedical field in the past five years with a focus on applications in proteomics, drug delivery, and development. This paper aims to engage pharmaceutical and biomedical scientists to full understand PILs and accelerate the progress from laboratory research to industrialization.

Stimuli-responsive DNA-based hydrogels for biosensing applications

The base sequences of DNA are endowed with the rich structural and functional information and are available for the precise construction of the 2D and 3D macro products. The hydrogels formed by DNA are biocompatible, stable, tunable and biologically versatile, thus, these have a wide range of promising applications in bioanalysis and biomedicine. In particular, the stimuli-responsive DNA hydrogels (smart DNA hydrogels), which exhibit a reversible and switchable hydrogel to sol transition under different triggers, have emerged as smart materials for sensing. Thus far, the combination of the stimuli-responsive DNA hydrogels and multiple sensing platforms is considered as biocompatible and is useful as the flexible recognition components. A review of the stimuli-responsive DNA hydrogels and their biosensing applications has been presented in this study. The synthesis methods to prepare the DNA hydrogels have been introduced. Subsequently, the current status of the stimuli-responsive DNA hydrogels in biosensing has been described. The analytical mechanisms are further elaborated by the combination of the stimuli-responsive DNA hydrogels with the optical, electrochemical, point-of-care testing (POCT) and other detection platforms. In addition, the prospects of the application of the stimuli-responsive DNA hydrogels in biosensing are presented.

Colloidal dispersion of poly(ionic liquid)/Cu composite particles for protective surface coating against SAR-CoV-2

Herein, we report a waterproof anti-SARS-CoV-2 protective film prepared by spray-coating of an aqueous colloidal dispersion of poly(ionic liquid)/copper (PIL/Cu) composite nanoparticles onto a substrate. The PIL dispersion was prepared by suspension polymerization of 3-dodecyl-1-vinylimdiazolium bromide in water at 70°C. The copper acetate salt was added into the PIL nanoparticle dispersion and in situ reduced into copper nanoparticles anchoring onto the PIL nanoparticles. Despite being waterborne, the PIL in bulk is intrinsically insoluble in water and the formed coating is stable in water. The formed surface coating by PIL/copper composite nanoparticles was able to deactivate SARS-CoV-2 virions by 90.0% in 30 minutes and thus may effectively prevent the spread of SARS-CoV-2 through surface contact. This method may provide waterborne dispersions for a broad range of antivirus protective surface coatings for both outdoor and indoor applications.

ROMP Synthesis of Side-Chain Ferrocene-Containing Polyelectrolyte and Its Redox-Responsive Hydrogels Showing Dramatically Improved Swelling with β-Cyclodextrin

A new side-chain ferrocene (Fc)-containing polyelectrolyte has been synthesized by controlled ring-opening metathesis polymerization of a water-soluble Fc-containing norbornene-based quaternary ammonium salt, as well as the corresponding covalently cross-linked polyelectrolyte hydrogel. In order to provide Fc-containing supramolecular polyelectrolyte hydrogels whose swelling property is largely improved by host-guest interaction, a covalently cross-linked polyelectrolyte hydrogel is soaked into the β-CD aqueous solution to form β-CD@Fc supramolecular polyelectrolyte hydrogel, or alternatively the quaternary ammonium salt supramolecular monomer is first formed, then copolymerized with a crosslinking agent to fabricate the supramolecular hydrogel with better water absorption ability. All the Fc-containing hydrogels exhibited good redox-responsiveness with swelling-shrinking behaviors by chemically reversibly adjusting the disassembly/assembly of β-CD@Fc inclusion complexes. This is the first example of side-chain Fc-containing polycationic supramolecular hydrogels possessing swelling-shrinking properties based on the splitting/combining of β-CD and Fc units, and potential applications are expected as controlled drug delivery and actuators.

Model-based design and synthesis of ferrocene containing microgels

Modelling and synthesis go hand in hand to efficiently engineer copolymer microgels with various architectures: core–shell structures (with ferrocene mainly in the core or in the shell) and also microgels with homogeneous comonomer distribution.

Poly(ionic liquid) composites

This review highlights recent advances in the development of poly(ionic liquid)-based composites for diverse materials applications.

Preparation of an Interpenetrating Network of a Poly(ampholyte) and a Cholesteric Polymer and Investigation of Its Hydrochromic Properties

A new water-responsive photonic coating based on a hygroscopic amphoteric poly(ampholyte) has been developed. The material consists of an interpenetrating network between the poly(ampholyte) and a cholesteric liquid crystalline polymer that reflects light. Swelling of this hybrid material upon contact with water causes a red-shift of the reflection band. As both cation and anion are incorporated in the ionic network, this coating possesses a high stability of its water responsiveness after prolonged and/or repeated exposure to water, even if the water contains dissolved ions. In addition, optimization of the water response of the coatings is demonstrated by changing the composition of the base cholesteric mixture, and color patterns were prepared through selective UV exposure.

Recent Progress in Macromolecule-Anchored Hybrid Gold Nanomaterials for Biomedical Applications

Gold nanoparticles (AuNPs), with elegant thermal, optical, or chemical properties due to quantum size effects, may serve as unique species for therapeutic or diagnostic applications. It is worth mentioning that their small size also results in high surface activity, leading to significantly impaired stability, which greatly hinders their biomedical utilizations. To overcome this problem, various types of macromolecular materials are utilized to anchor AuNPs so as to achieve advanced synergistic effect by dispersing, protecting, and stabilizing the AuNPs in polymeric-Au hybrid self-assemblies. In this review, the most recent development of polymer-AuNP hybrid systems, including AuNPs@polymeric nanoparticles, AuNPs@polymeric micelle, AuNPs@polymeric film, and AuNPs@polymeric hydrogel are discussed with respect to their different synthetic strategies. These sophisticated materials with diverse functions, oriented toward biomedical applications, are further summarized into several active domains in the areas of drug delivery, gene delivery, photothermal therapy, antibacterials, bioimaging, etc. Finally, the possible approaches for future design of multifunctional polymer-AuNP hybrids by combining hybrid chemistry with biological interface science are proposed.

Cargo shuttling by electrochemical switching of core-shell microgels obtained by a facile one-shot polymerization

Controlling and understanding the electrochemical properties of electroactive polymeric colloids is a highly topical but still a rather unexplored field of research. This is especially true when considering more complex particle architectures like stimuli-responsive microgels, which would entail different kinetic constraints for charge transport within one particle. We synthesize and electrochemically address dual stimuli responsive core-shell microgels, where the temperature-responsiveness modulates not only the internal structure, but also the microgel electroactivity both on an internal and on a global scale. In detail, a facile one-step precipitation polymerization results in architecturally advanced poly(N-isopropylacrylamide-co-vinylferrocene) P(NIPAM-co-VFc) microgels with a ferrocene (Fc)-enriched (collapsed/hard) core and a NIPAM-rich shell. While the remaining Fc units in the shell are electrochemically accessible, the electrochemical activity of Fc in the core is limited due to the restricted mobility of redox active sites and therefore restricted electron transfer in the compact core domain. Still, prolonged electrochemical action and/or chemical oxidation enable a reversible adjustment of the internal microgel structure from core-shell microgels with a dense core to completely oxidized microgels with a highly swollen core and a denser corona. The combination of thermo-sensitive and redox-responsive units being part of the network allows for efficient amplification of the redox response on the overall microgel dimension, which is mainly governed by the shell. Further, it allows for an electrochemical switching of polarity (hydrophilicity/hydrophobicity) of the microgel, enabling an electrochemically triggered uptake and release of active guest molecules. Hence, bactericidal drugs can be released to effectively kill bacteria. In addition, good biocompatibility of the microgels in cell tests suggests suitability of the new microgel system for future biomedical applications.

Probing hidden colloidal transitions with the assistance of surface plasmons

With the assistance of surface plasmons, a vesicle intermediate of Au@PNIPAM clusters is revealed during the cooling cycle, which is due to the co-aggregation of free PNIPAM beads and Au@PNIPAM aggregates in the heating cycle.

Design of Robust Thermal and Anion Dual-Responsive Membranes with Switchable Response Temperature

In this study, poly(ionic liquids/ N-isopropylacrylamide) (PIL/NIPAM) modified poly(ether sulfone) microporous membranes were prepared using a pore-filling method. Due to the anion-sensitive wettability of the PIL and the thermal-sensitive phase transformation of PNIPAM, the permeability of the modified membranes showed robust anion and thermal dual-responsive behaviors. In addition, the response temperature of the membranes could be adjusted precisely from 30 to 55 °C by anion exchange, which was attributed to the cooperative interaction of the PIL and PNIPAM. The switchable response temperature and the dual-responsive performances of the membranes were demonstrated by measuring the water fluxes under various conditions. The results indicated that the membrane permeabilities increased when exchanging the counteranions (CAs) from hydrophilic to hydrophobic ones; the thermal response behaviors were also obvious, and the sensitivity increased when increasing the hydrophobicity of the CA (the fluxes could be adjusted from 0 to 3800 mL/m² mmHgh by controlling the temperature and CAs). At last, filtration tests were designed with the membranes, and the results indicated that by controlling the temperature and/or CA species, three different poly(ethylene glycol) molecules could be easily separated according to their molecule sizes in a single step.

Fabrication of Thermoresponsive Polymer-Functionalized Cellulose Sponges: Flexible Porous Materials for Stimuli-Responsive Catalytic Systems

In this present work, a thermoresponsive and recyclable catalytic system was prepared by grafting poly( N-isopropylacrylamide)- co-poly(glycidyl methacrylate) (PNIPAM- co-PGM) to a cellulose sponge, which was reinforced by polydopamine (PDA) and (3-aminopropyl)triethoxysilane (APTMS). Au nanoparticles (Au NPs) were loaded via in situ reduction of HAuCl₄ with PDA. Fourier transform infrared, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, and thermogravimetric analysis results revealed that the Au NPs (<10 nm) were homogenously dispersed on the surface of the sponge. Catalytic experiments with sponges prepared without PNIPAM- co-PGM demonstrated an increased reaction rate when the temperature of the reaction medium was elevated. However, in the presence of PNIPAM-i co-PGM in the sponges, the reaction rate was decreased when the reaction temperature was higher than the lower critical solution temperature of the polymer. The sponge could be conveniently separated from the reactions and reused up to 22 cycles.

Fast formation of a supramolecular ion gel/solvoplastic elastomer with excellent stretchability

This study describes a simple yet efficient approach for the preparation of an ionic gel that is also elastomeric in its solid-state bulk form. A series of poly(2-(diethylamino)ethyl methacrylate-co-lauryl methacrylate) P(DMAEMA-co-LMA) copolymers were synthesized first by radical polymerization. Quaternization of the PDMAEMA component in tetrahydrofuran enables the formation of supramolecular network, giving rise to an ion gel. An elastomer with an elongation at break of over 600% was obtained from the gel. The elastomer, connected by supramolecular ionic cross-links, is solvoplastic in certain solvents. The simple yet efficient approach of the formation of ion-gel and the dried elastomer allows fast preparation of both gel-like and solid-state elastic materials for various applications where recyclability is required.

Facile fabrication of thermo/redox responsive hydrogels based on a dual crosslinked matrix for a smart on-off switch

Stimuli-responsive or "smart" soft materials have raised considerable attention due to their ability to spontaneously respond to external environmental variations and have a great potential for wide applications. Herein, a thermo/redox responsive hydrogel is facilely constructed based on a dual crosslinked matrix: the primary chemical crosslinked copolymer is composed of thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) and poly(ionic liquid), and the secondary physical crosslinking component is generated by the ionic coordination between iron ions and carboxyl groups in the poly(ionic liquid). The non-covalent ion coordination crosslinking is introduced into a covalently crosslinked network, which further strengthens the soft PNIPAM matrix and enhances the mechanical performances of the hydrogels. The excellent thermosensitivity of PNIPAM and the good conductive property of poly(ionic liquid) provide the hydrogel with an attractive performance as a thermo-responsive switch. Moreover, the trapped iron ions in the network endow the hydrogels with redox-responsiveness, which could be reversibly chemically oxidized and reduced. The mechanical strength of hydrogels could also be tuned by the crosslinked capacity of iron ions within the gel matrix between the strong binding of the oxidized state (Fe3+) and poor coordination of the reduced state (Fe2+). These stimuli-responsive hydrogels have the potential to be used as smart materials for stimuli-responsive devices.

Synthesis and Characterization of pH and Thermo Dual-Responsive Hydrogels with a Semi-IPN Structure Based on N-Isopropylacrylamide and Itaconamic Acid

A series of semi-interpenetrating polymer network (semi-IPN) hydrogels were synthesized and investigated in this study. Linear copolymer poly(N-isopropylacrylamide-co-itaconamic acid) p(NIPAM-co-IAM), which is formed by copolymerization of N-isopropylacrylamide (NIPAM) and itaconamic acid (IAM, 4-amino-2-ethylene-4-oxobutanoic acid), was introduced into a solution of NIPAM to form a series of pH and thermo dual-responsive p(NIPAM-co-IAM)/pNIPAM semi-IPN hydrogels by free radical polymerization. The structural, morphological, chemical, and physical properties of the linear copolymer and semi-IPN hydrogels were investigated. The semi-IPN hydrogel showed high thermal stability according to thermal gravimetric analyzer (TGA). Scanning electronic microscopy (SEM) images showed that the pore size was in the range of 119~297 µm and could be controlled by the addition ratio of the linear copolymer in the semi-IPN structure. The addition of linear copolymer increased the fracture strain from 57.5 ± 2.9% to 91.1 ± 4.9% depending on the added amount, while the compressive modulus decreased as the addition increased. Moreover, the pH and thermo dual-responsive properties were investigated using differential scanning calorimetry (DSC) and monitoring the swelling behavior of the hydrogels. In deionized (DI) water, the equilibrium swelling ratio of the hydrogels decreased as the temperature increased from 20 °C to 50 °C, while it varied in various pH buffer solutions. In addition, the swelling and deswelling rates of the hydrogels also significantly increased. The results indicate that the novel pH-thermo dual-responsive semi-IPN hydrogels were synthesized successfully and may be a potential material for biomedical, drug delivery, or absorption application.

Thermal Switching of Thermoresponsive Polymer Aqueous Solutions

Thermal switches are of great importance to thermal management in a wide variety of applications. However, traditional thermal switches suffer from being large and having slow transition rates. To overcome these limitations, we took advantage of abrupt second-order phase transitions in thermoresponsive polymer aqueous solutions to enable fast thermal switching. While thermoresponsive polymers have been widely studied for biomedical applications, their thermal switching capability has not been studied. In this work, we used poly(N-isopropylacrylamide) (PNIPAM) as a model system to demonstrate abrupt thermal conductivity changes of thermoresponsive polymer aqueous solutions across their transition temperatures by using a powerful approach, the transient thermal grating technique, which has high sensitivity. We observed a thermal switching ratio up to 1.15 in dilute PNIPAM aqueous solutions (up to 0.025 g/mL) across the transition. This work may provide new opportunities to engineer thermal switches using second-order phase transitions of thermoresponsive polymer aqueous solutions or abrupt higher-order phase transitions in general.

Controllable thermal and pH responsive behavior of PEG based hydrogels and applications for dye adsorption and release

A series of controllable thermal and pH dual-responsive copolymeric hydrogels (PMA) were prepared by a one-pot reaction with poly(ethylene glycol) methyl ether acrylate (PEGA), 2-methoxyethyl acrylate (MEA) and acroleic acid (AA). The hydrogels exhibited good mechanical properties and a sensitive response to pH and temperature. Besides, the Lower Critical Solution Temperature (LCST) of the hydrogels can be adjusted from 37 °C to 58 °C by changing the content of AA. The hydrogels also showed excellent selective adsorption properties. The maximum adsorption quantity of organic cationic dye brilliant green and methylene blue were 0.49 mg mg⁻¹ and 0.42 mg mg⁻¹ respectively, much better than previous reports. Furthermore, using the thermal and pH responsibility, the PMA hydrogels can release the adsorbed molecules with control. Nearly 95% of carriers could be released at pH 4.01 and 65 °C over 8 h. The regeneration ability makes the materials easy to reuse many times. Due to these properties, these dual-responsive hydrogels have great potential applications in various fields for adsorption, drug delivery, release and tissue engineering.

Controllable thermal and pH responsive hydrogels not only showed good stimuli-response and mechanical properties, but also have excellent adsorption properties.

Engineering Biodegradable and Biocompatible Bio-ionic Liquid Conjugated Hydrogels with Tunable Conductivity and Mechanical Properties

Conventional methods to engineer electroconductive hydrogels (ECHs) through the incorporation of conductive nanomaterials and polymers exhibit major technical limitations. These are mainly associated with the cytotoxicity, as well as poor solubility, processability, and biodegradability of their components. Here, we describe the engineering of a new class of ECHs through the functionalization of non-conductive polymers with a conductive choline-based bio-ionic liquid (Bio-IL). Bio-IL conjugated hydrogels exhibited a wide range of highly tunable physical properties, remarkable in vitro and in vivo biocompatibility, and high electrical conductivity without the need for additional conductive components. The engineered hydrogels could support the growth and function of primary cardiomyocytes in both two dimentinal (2D) and three dimensional (3D) cultures in vitro. Furthermore, they were shown to be efficiently biodegraded and possess low immunogenicity when implanted subcutaneously in rats. Taken together, our results suggest that Bio-IL conjugated hydrogels could be implemented and readily tailored to different biomedical and tissue engineering applications.

Thermoresponsive Semi-IPN Hydrogel Microfibers from Continuous Fluidic Processing with High Elasticity and Fast Actuation

Hydrogels with rapid and strong response to external stimuli and possessing high elasticity and strength have been considered as platform materials for numerous applications, e.g., in biomaterials engineering. Thermoresponsive hydrogels based on semi-interpenetrating polymer networks (semi-IPN) featuring N-isopropylacrylamide with copolymers of poly(N-isopropylacrylamide-co-hydroxyethyl methacrylate) p(NIPAM-HEMA) chains are prepared and described. The copolymer was characterized by FTIR, NMR, and GPC. The semi-IPN structured hydrogel and its responsive properties were evaluated by dynamic mechanical measurements, SEM, DSC, equilibrium swelling ratio, and dynamic deswelling tests. The results illustrate that the semi-IPN structured hydrogels possess rapid response and high elasticity compared to conventional pNIPAM hydrogels. By using a microfluidic device with double coaxial laminar flow, we succeeded in fabricating temperature responsive ("smart") hydrogel microfibers with core-shell structures that exhibit typical diameters on the order of 100 μm. The diameter of the fibers can be tuned by changing the flow conditions. Such hydrogel fibers can be used to fabricate "smart" devices, and the core layer can be potentially loaded with cargos to incorporate biological function in the constructs. The platforms obtained by this approach hold promise as artificial "muscles", and also "smart" hydrogel carriers providing a unique biophysical and bioactive environment for regenerative medicine and tissue engineering.

Engineering highly swellable dual-responsive protein-based injectable hydrogels: the effects of molecular structure and composition in vivo

Highly swellable, dual-responsive bovine serum albumin (BSA)-based injectable hydrogels that exhibit sol-to-gel phase transitions in response to the physiological pH and temperature have been developed.

A new thermoresponsive polymer of poly(N-acryloylsarcosine methyl ester) with a tunable LCST

A thermoresponsive polymer of the tertiary amide-based polyacrylamide, PNASME, was synthesized and its tunable thermoresponse was investigated.