Phase transition of aqueous solutions of poly(N-isopropylacrylamide) and poly(N-isopropylmethacrylamide)

Molecular Dynamics Simulations of Concentrated and Dilute Aqueous Solutions of Poly(N-Isopropylacrylamide) Using a Modified OPLS-AA Model

We perform molecular dynamics (MD) simulations of aqueous poly(N-isopropylacrylamide) (PNIPAM) solutions using the OPLS-AA model and its modified version in combination with the TIP4P/2005 water model. The original OPLS-AA model underestimates the demixing temperature of concentrated solutions and the coil-globule transition temperature in dilute solutions, whereas the modified model reproduces both. It was reported that the transition from the globule state to the coil state never occurred in MD simulations with the original OPLS-AA model. This problem is avoided by using the modified model because of the fast dynamics near the coil-globule transition temperature, which is higher for the modified model than for the original model by ∼60 K. The distribution functions of two structural order parameters calculated from well equilibrated trajectories of the modified model clearly show that the coil-globule transition is not a discontinuous first-order transition, as suggested in previous MD studies, but a continuous transition as predicted from mean field theory. We also find that the effect of polymer-polymer hydrogen bonding on the dynamics of aggregated polymer chains is less significant than assumed in experimental studies.

Multi-stimuli-responsive polymers enabled by bio-inspired dynamic equilibria of flavylium chemistry

As part of a complex equilibria network with other chemical species, flavyliums, the chromophoric component of anthocyanins, hold great potential for use in functional polymers. This study presents the successful syntheses of polymers containing two distinct flavylium-structures, generated via post-modification of a parent polymer synthesised using reversible addition-fragmentation chain transfer (RAFT) polymerisation. The selective modification of acetophenone moieties enabled precise tuning of the polymers' properties, which are strongly influenced by the markedly different chemical characteristics of flavyliums and the other species in equilibria with them. The synthesised flavylium-containing polymers exhibit multi-stimuli responsiveness to variations in solvent, pH, light, and temperature, thereby introducing intricacy and viable functionality to the polymer system. The surface activity and critical aggregation concentrations (CAC) of the synthesised polymers were studied using profile analysis tensiometry (PAT), revealing distinct aggregation and self-assembly behaviours. Fractal-like aggregates formed by the flavylium-containing polymers were investigated using cryogenic electron microscopy (Cryo-EM) and small-angle X-ray scattering (SAXS). This research bridges the colourful dynamic equilibria of flavylium chemistry with polymer chemistry, paving the pathway for further investigations into flavylium-polymer interactions and the development of tuneable material properties of responsive polymers.

Thermoresponsive Behaviors of Poly(N-methacryloyl glycinamide) and Poly(N-acryloyl glycinamide): Effect of Methacrylation

Poly(N-acryloyl glycinamide) (PNAGA) proves to be an interesting and useful polymer with an upper critical solution temperature (UCST) behavior in water due to intra- and intermolecular hydrogen bonding. Its methacrylamide counterpart, poly(N-methylacryloyl glycinamide) (PNMAGA), has a different UCST behavior, which is easier to dissolve in water. In this work, PNMAGA and PNAGA were synthesized by RAFT polymerization and free radical polymerization, and their solution properties in dilute aqueous media have been studied and compared in detail to elucidate the effects of temperature, polymer concentration, molecular weight, and chain end. The direct comparison provides a better understanding of the UCST behaviors. The presence of an extra methyl group on the repeating unit helps the polymer to dissolve better and eliminates the need for special thermal treatment to obtain a complete dissolution. Infrared spectroscopy and X-ray diffraction analysis show variation in hydrogen bonds between the two polymers and their respective monomers, providing insights into the structural origin of their different solution properties.

A responsive living material prepared by diffusion reveals extracellular enzyme activity of cyanobacteria

Stimuli-responsive engineered living materials (ELMs) can respond to environmental or biochemical cues and have broad utility in biological sensors and machines, but have traditionally been limited to biocompatible scaffolds. This is because they are typically made by mixing cells into a precursor solution before crosslinking. Here, we demonstrate a diffusion mechanism for incorporating cells of the cyanobacterium Synechococcus elongatus sp. PCC 7942 (S. elongatus) into nanoclay-poly-N-isopropylacrylamide (NC-PNIPAm), a hydrogel with a cytotoxic precursor, by exploiting its temperature-dependent shape-morphing behavior. Subsequent growth of S. elongatus caused a decrease in the bending curvature and stiffness (local Young's modulus) of NC-PNIPAm due to partial degradation by an unannotated enzyme. Creation and observation of this cyanobacteria-hydrogel ELM showcases a method for diffusing cells into a hydrogel as well as characterizing an extracellular enzyme.

Nano- and meso-scale aggregation of poly(N-isopropylacrylamide) below the lower critical solution temperature: A wide-angle dynamic light scattering study

Poly-N-isopropylacrylamide (PNIPAm), a thermorresponsive polymer, highly soluble in water below its lower critical solution temperature (LCST), is widely used in biomedical applications like drug delivery. Being able to measure PNIPAm size and aggregation state in solution quickly, inexpensively, and accurately below the LCST is critical when stoichiometric particle or molecular ratios are required. Dynamic light scattering (DLS) is probably the most widely available, and inexpensive nanoparticle sizing technique, but there are limitations with respect to sample polydispersity. Here, we first investigated factors governing the ability of DLS to accurately measure PNIPAm size in solution at 25 °C as part of a quality study of five different molecular weight, commercially sourced PNIPAm. All samples were polydisperse and accurate particle size distribution (PSD) data was only obtained from distribution fitting, being consistent and accurate down to ∼ 0.1 wt%. In water at 1 wt%, Rh, extracted from distribution fitting: 12.4 ± 0.6 nm (55 kDa), 10.0 ± 0.22 nm (38 kDa), 6.2 ± 0.15 nm (28.5 kDa), and 9.7 ± 0.14 nm (20-25 kDa) were significantly higher than that expected for single PNIPAm chains in solution. Measurements in different buffers of varying pH (7.4-5.0) yielded similar sizes (Rh of 6-15 nm) and polydispersity indicating that these were stable aggregates. These aggregates could be broken down with Triton-X but not with sodium dodecyl sulphate, ultrasound, or by heating above the LCST and then cooling. We suggest that this nanoscale aggregation and increased polydispersity was caused a variety of factors including by solid-state aging during prolonged storage (>5 years) induced by water adsorption, and/or manufacturing processes. Stirring was found to produce larger, meso-scale (Rh > 150 nm), soluble aggregates and the rate of formation of these meso-particles was linear with stirring time (with a concomitant linear decrease in the faction of original nanoscale aggregates). Meso-particle formation was not correlated with MW, but was inversely correlated to polymer concentration suggesting that aggregation was driven by adsorption at air/liquid interfaces rather than solution phase collisions. In conclusion, PNIPAm particle size and distribution was highly dependent on multiple factors including source, storage conditions, and exposure to air-water interfaces. Standard wide angle DLS is however an effective and rapid method for identifying and quantifying PNIPAm aggregation.

Sulfobetaine/Alginate/Chitosan Supported Hybrid N‐Isopropylacrylamide Hydrogels: Composition‐Dependent Diffusion/Compression Properties and Theophylline/Diclofenac Sodium/Ciprofloxacin Release Kinetics

Poly(N‐isopropylacrylamide) (N), poly([3‐(methacryloylamino)propyl] dimethyl(3‐sulfopropyl) ammonium hydroxide) (SB) and SB/N hydrogels were prepared using N,N′‐methylenebisacrylamide as crosslinker, while their hybrid semi‐/full‐IPNs N1A, N1C, N1CA, SB/N1A, SB/N1C, and SB/N1CA were synthesized in the presence of Alginate (A)/Chitosan (C). All the hydrogels were evaluated by taking into account their appearances, compression strengths and swelling behaviors in the ranges of pH 1.2–9.0 and temperature 4°C–40°C. The compressive moduli of N and SB/N hydrogels increased from ~10 to 80 kPa by changing composition (from N, SB/N to N1CA, SB/N1CA), swelling solution (from DDW to PBS) and temperature (from 25° to 37°C). The release profiles of diclofenac sodium (DFNa), theophylline (Thp), and ciprofloxacin (CIP) from N, SB/N, and their semi‐/full‐IPNs were investigated at pH 1.2 and pH 7.4, mimicking gastric and intestinal fluids. Higuchi, Peppas, and Weibull models were used, to describe the mechanisms of DFNa, Thp, and CIP releases from the hybrid IPNs of N and SB/N. The values of n (> 0.45) and β (> 0.75) at 37°C for Peppas and Weibull equations showed that DFNa and CIP releases from SB/N hybrids, which are more hydrophilic than IPNs of N, are mainly controlled by swelling/relaxation process.

Thermoresponsiveness Across the Physiologically Accessible Range: Effect of Surfactant, Cross-Linker, and Initiator Content on Size, Structure, and Transition Temperature of Poly(N-isopropylmethacrylamide) Microgels

The influence of surfactant, cross-linker, and initiator on the final structure and thermoresponse of poly(N-isopropylmethacrylamide) (pNIPMAM) microgels was evaluated. The goals were to control particle size (into the nanorange) and transition temperature (across the physiologically accessible range). The concentration of the reactants used in the synthesis was varied, except for the monomer, which was kept constant. The thermoresponsive suspensions formed were characterized by dynamic light scattering, small-angle X-ray scattering, atomic force microscopy, and rheology. Increasing surfactant, sodium dodecyl sulfate content, produced smaller microgels, as expected, into the nanorange and with greater internal entanglement, but with no change in phase transition temperature (LCST), which is contrary to previous reports. Increasing cross-linker, N,N-methylenebis acrylamide, content had no impact on particle size but reduced particle deformability and, again contrary to previous reports of decreases, progressively increased the LCST from 39 to 46 °C. The unusual LCST trends were confirmed using different rheological techniques. Initiator, potassium persulfate, content was found to weakly influence the outcomes. An optimized content was identified that provides functional nanogels in the 100 nm (swollen) size range with controlled LCST, just above physiological temperature. The study contributes chemistry-derived design rules for thermally responsive colloidal particles with physiologically accessible LCST for a variety of biomedical and soft robotics applications.

Molecularly Engineered Supramolecular Thermoresponsive Hydrogels with Tunable Mechanical and Dynamic Properties

Synthetic supramolecular polymers and hydrogels in water are emerging as promising biomaterials due to their modularity and intrinsic dynamics. Here, we introduce temperature sensitivity into the nonfunctionalized benzene-1,3,5-tricarboxamide (BTA-EG4) supramolecular system by incorporating a poly(N-isopropylacrylamide)-functionalized (BTA-PNIPAM) moiety, enabling 3D cell encapsulation applications. The viscous and structural properties in the solution state as well as the mechanical and dynamic features in the gel state of BTA-PNIPAM/BTA-EG4 mixtures were investigated and modulated. In the dilute state (c ∼μM), BTA-PNIPAM acted as a chain capper below the cloud point temperature (Tcp = 24 °C) but served as a cross-linker above Tcp. At higher concentrations (c ∼mM), weak or stiff hydrogels were obtained, depending on the BTA-PNIPAM/BTA-EG4 ratio. The mixture with the highest BTA-PNIPAM ratio was ∼100 times stiffer and ∼10 times less dynamic than BTA-EG4 hydrogel. Facile cell encapsulation in 3D was realized by leveraging the temperature-sensitive sol-gel transition, opening opportunities for utilizing this hydrogel as an extracellular matrix mimic.

Tuning Structural Organization via Molecular Design and Hierarchical Assembly to Develop Supramolecular Thermoresponsive Hydrogels

The cellular microenvironment is composed of a dynamic hierarchical fibrillar architecture providing a variety of physical and bioactive signals to the surrounding cells. This dynamicity, although common in biology, is a challenge to control in synthetic matrices. Here, responsive synthetic supramolecular monomers were designed that are able to assemble into hierarchical fibrous structures, combining supramolecular fiber formation via hydrogen bonding interactions, with a temperature-responsive hydrophobic collapse, resulting in cross-linking and hydrogel formation. Therefore, amphiphilic molecules were synthesized, composed of a hydrogen bonding ureido-pyrimidinone (UPy) unit, a hydrophobic alkyl spacer, and a hydrophilic oligo(ethylene glycol) tail. The temperature responsive behavior was introduced by functionalizing these supramolecular amphiphiles with a relatively short poly(N-isopropylacrylamide) (PNIPAM) chain (M n ∼ 2.5 or 5.5 kg/mol). To precisely control the assembly of these monomers, the length of the alkyl spacer between the UPy moiety and PNIPAM was varied in length. A robust sol-gel transition, with the dodecyl UPy-PNIPAM molecule, was obtained, with a network elasticity enhancing over 2000 times upon heating above room temperature. The UPy-PNIPAM compounds with shorter alkyl spacers were already hydrogels at room temperature. The sol-gel transition of the dodecyl UPy-PNIPAM hydrogelator could be tuned by the incorporation of different UPy-functionalized monomers. Furthermore, we demonstrated the suitability of this system for microfluidic cell encapsulation through a convenient temperature sol-gel transition. Our results indicate that this novel thermoresponsive supramolecular system offers a modular platform to study and guide single-cell behavior.

Dynamic Behavior of Poly(N-isopropylmethacrylamide) in Neat Water and in Water/Methanol Mixtures

We investigate the collective dynamics of thermoresponsive polymer poly(N-isopropylmethacrylamide) (PNIPMAM) in aqueous solution and in water/methanol mixtures in the one-phase region. In neat water, the polymer concentration c is varied in a wide range around the overlap concentration c*, that is estimated at 23 g L-1. Using dynamic light scattering (DLS), two decays ("modes") are consistently observed in the intensity autocorrelation functions for c = 2-150 g L-1 with relaxation rates which are proportional to the square of the momentum transfer. Below c*, these are attributed to the diffusion of single chains and to clusters from PNIPMAM that are formed due to hydrophobic interactions. Above c*, they are assigned to the diffusion of the chain segments between overlap points and to long-range concentration fluctuations. From the temperature-dependent behavior of the overall scattering intensities and the dynamic correlation lengths of the fast mode, the critical temperatures and the scaling exponents are determined. The latter are significantly lower than the static values predicted by mean-field theory, which may be related to the presence of the large-scale inhomogeneities. The effect of the cosolvent methanol on the dynamics is investigated for polymer solutions having c = 30 g L-1 and methanol volume fractions in the solvent mixtures of up to 60 vol %. The phase diagram was established by differential scanning calorimetry. The slow mode detected by DLS becomes significantly weaker as methanol is added, i.e., the solutions become more homogeneous. Beyond the minimum of the coexistence line, which is located at 40-50 vol % of methanol, the dynamics is qualitatively different from the one at lower methanol contents. Thus, going from the water-rich to the methanol-rich side of the miscibility gap, the change of interaction of the PNIPMAM chains with the two solvents has a severe effect on the collective dynamics.

Responsive Acrylamide-Based Hydrogels: Advances in Interpenetrating Polymer Structures

Hydrogels, composed of hydrophilic homopolymer or copolymer networks, have structures similar to natural living tissues, making them ideal for applications in drug delivery, tissue engineering, and biosensors. Since Wichterle and Lim first synthesized hydrogels in 1960, extensive research has led to various types with unique features. Responsive hydrogels, which undergo reversible structural changes when exposed to stimuli like temperature, pH, or specific molecules, are particularly promising. Temperature-sensitive hydrogels, which mimic biological processes, are the most studied, with poly(N-isopropylacrylamide) (PNIPAm) being prominent due to its lower critical solution temperature of around 32 °C. Additionally, pH-responsive hydrogels, composed of polyelectrolytes, change their structure in response to pH variations. Despite their potential, conventional hydrogels often lack mechanical strength. The double-network (DN) hydrogel approach, introduced by Gong in 2003, significantly enhanced mechanical properties, leading to innovations like shape-deformable DN hydrogels, organic/inorganic composites, and flexible display devices. These advancements highlight the potential of hydrogels in diverse fields requiring precise and adaptable material performance. In this review, we focus on advancements in the field of responsive acrylamide-based hydrogels with IPN structures, emphasizing the recent research on DN hydrogels.

Study of the Thermal Phase Transition of Poly(N,N-diethylacrylamide-co-N-ethylacrylamide) Random Copolymers in Aqueous Solution

N-alkyl-substituted polyacrylamides exhibit a thermal coil-to-globule transition in aqueous solution driven by an increase in hydrophobic interactions with rising temperature. With the aim of understanding the role of N-alkyl substituents in the thermal transition, this study focuses on the molecular interactions underlying the phase transition of poly(N,N-diethylacrylamide-co-N-ethylacrylamide) random copolymers. Poly(N,N-diethylacrylamide) (PDEAm), poly(N-ethylacrylamide) (PNEAm), and their random copolymers were synthesized by free radical polymerization and their chemical structure characterized spectroscopically. It was found that the values of the cloud-point temperature increased with PNEAm content, and particle aggregation processes took place, increasing the negative charge density on their surface. The cloud-point temperature of each copolymer decreased with respect to the theoretical values calculated assuming an absence of interactions. It is attributed to the formation of intra- and interchain hydrogen bonding in aqueous solutions. These interactions favor the formation of more hydrophobic macromolecular segments, thereby promoting the cooperative nature of the transition. These results definitively reveal the dominant mechanism occurring during the phase transition in the aqueous solutions of these copolymers.

Advances in injectable hydrogels for radiation-induced heart disease

Radiological heart damage (RIHD) is damage caused by unavoidable irradiation of the heart during chest radiotherapy, with a long latency period and a progressively increasing proportion of delayed cardiac damage due to conventional doses of chest radiotherapy. There is a risk of inducing diseases such as acute/chronic pericarditis, myocarditis, delayed myocardial fibrosis and damage to the cardiac conduction system in humans, which can lead to myocardial infarction or even death in severe cases. This paper details the pathogenesis of RIHD and gives potential targets for treatment at the molecular and cellular level, avoiding the drawbacks of high invasiveness and immune rejection due to drug therapy, medical device implantation and heart transplantation. Injectable hydrogel therapy has emerged as a minimally invasive tissue engineering therapy to provide necessary mechanical support to the infarcted myocardium and to act as a carrier for various bioactive factors and cells to improve the cellular microenvironment in the infarcted area and induce myocardial tissue regeneration. Therefore, this paper combines bioactive factors and cellular therapeutic mechanisms with injectable hydrogels, presents recent advances in the treatment of cardiac injury after RIHD with different injectable gels, and summarizes the therapeutic potential of various types of injectable hydrogels as a potential solution.

Unraveling the Role of Deep Eutectic Solvents with Varying Hydrogen-Bond Acceptors on the Thermoresponsive Polymer Poly(N-isopropylacrylamide)

Deep eutectic solvents (DESs) have emerged as promising tools for crafting polymeric materials across diverse domains. This study delves into the impact of a series of DESs on the phase behavior of poly(N-isopropylacrylamide) (PNIPAM) in aqueous environments, presenting compelling insights into their performance. Specifically, we explore the conformational phase behavior of PNIPAM in the presence of four distinct lactic acid (LA)-based DESs: LA-betaine (LA-BET), LA-proline (LA-PRO), LA-choline chloride (LA-CC), and LA-urea (LA-U). By maintaining a consistent hydrogen-bond donor (HBD) while varying the hydrogen-bond acceptor (HBA), we unravel how different DES compositions modulate the phase transition behavior of PNIPAM. Our findings underscore the profound influence of DESs comprising LA as the HBD and diverse HBAs-BET, PRO, CC, and U on the thermoresponsive behavior of PNIPAM. Employing spectroscopic techniques such as ultraviolet-visible (UV-vis) spectroscopy, steady-state fluorescence, Fourier transform infrared spectroscopy (FTIR), dynamic light scattering (DLS), ζ-potential, and transmission electron microscopy (TEM), we elucidate the preferential interactions between the HBA groups within DESs and the hydration layer of PNIPAM. Notably, temperature-dependent DLS analyses reveal a discernible decrease in the lower critical solution temperature (LCST) of PNIPAM with increasing DES concentration, ultimately disrupting the hydrogen-bond interactions and resulting in early hydrophobic collapse of the polymer, which can be clearly seen in the TEM micrographs. Furthermore, the formation of polymer composites within the mixed system leads to notable alterations in the physiochemical properties of PNIPAM, as evidenced by shifts in its LCST value in the presence of DESs. This perturbation disrupts hydrogen-bond interactions, inducing hydrophobic collapse of the polymers, a phenomenon vividly captured in TEM micrographs. In essence, our study sheds new light on the pivotal role of varying HBA groups within DESs in modulating the conformational transitions of PNIPAM. These insights not only enrich our fundamental understanding but also hold immense promise for the development of smart polymeric systems with multifaceted applications spanning bioimaging, biomedical science, polymer science, and beyond.

Precision Loading and Delivery of Molecular Cargo by Size-Controlled Coacervation of Gold Nanoparticles Functionalized with Elastin-like Peptides

Thermoresponsive elastin-like peptides (ELPs) have been extensively investigated in biotechnology and medicine, but little attention has been paid to the process by which coacervation causes ELP-decorated particles to aggregate. Using gold nanoparticles (AuNPs) functionalized with a cysteine-terminated 96-repeat of the VPGVG sequence (V96-Cys), we show that the size of the clusters that reversibly form above the ELP transition temperature can be finely controlled in the 250 to 930 nm range by specifying the concentration of free V96-Cys in solution and using AuNPs of different sizes. We further find that the localized surface plasmon resonance peak of the embedded AuNPs progressively red-shifts with cluster size, likely due to an increase in particle-particle contacts. We exploit this fine control over size to homogeneously load precise amounts of the dye Nile Red and the antibiotic Tetracycline into clusters of different hydrodynamic diameters and deliver cargos near-quantitatively by deconstructing the aggregates below the ELP transition temperature. Beyond establishing a key role for free ELPs in the agglomeration of ELP-functionalized particles, our results provide a path for the thermally controlled delivery of precise quantities of molecular cargo. This capability might prove useful in combination photothermal therapies and theranostic applications, and to trigger spatially and temporally uniform responses from biological, electronic, or optical systems.

Interactions between Water and a Hydrophobic Polymer

To investigate the mechanisms of interactions between a hydrophobic polymer and water, molecular dynamics calculations and Raman spectroscopic measurements of cis-1,4-polyisoprene (PI)-water systems were performed. The results show that PI in water undergoes a coil-globule transition at around 248 K. The transition is attributed to changes in the density and diffusivity of water. The volume expansion of the supercooled liquid water induces the coil structure of PI. The phase separation of PI from water with an increase in the self-diffusion coefficient of water molecules results in the globule structure of PI. The self-diffusion coefficient of free water with PI is larger than that of pure water because PI has an effect to decrease the hydrogen-bonding strength of water. The result suggests that the effects of the coexisting water are important factors governing the physical and chemical properties of hydrophobic polymers.

Effect of End Groups on the Cloud Point Temperature of Aqueous Solutions of Thermoresponsive Polymers: An Inside View by Flory-Huggins Theory

In an effort to gain insight into the origin of the effects of end groups on the cloud point temperature (Tcp) as a function of the polymer molar mass of thermoresponsive polymers with lower critical solution behavior in dilute aqueous solutions, we use the Flory-Huggins (FH) theory amended for end groups. The theory was applied to available experimental data sets of poly(N-isopropylacrylamide) (PNIPAM), poly(4-vinylbenzyl methoxytris(oxyethylene) ether) (PTEGSt), and poly(α-hydro-ω-(4-vinylbenzyl)tetrakis(oxyethylene) ether) (PHTrEGSt). The theory relates the variations in TcpM,ϕcp for different end groups to the effective FH χ parameter of the end groups and explains the qualitative notion that the influence of the end groups is related to the hydrophobicity/hydrophilicity of the end groups relative to that of the so called intrinsic TcpM,ϕcp response of a polymer without end groups. The limits to the applicability of the FH theory are established, and a set of possible theoretical improvements is considered. The ultimate scrutiny of the simple FH theory and suggested improved theories must await the measurement of truly thermodynamic cloud points; the available cloud points are merely estimations of the thermodynamic cloud point, for which the deviation to the true cloud point cannot be established with sufficient accuracy.

Glycosaminoglycan-mimetic infernan grafted with poly(N-isopropylacrylamide): Toward a thermosensitive polysaccharide

Glycosaminoglycans (GAGs) are essential constituents of the cell surface and extracellular matrix, where they are involved in several cellular processes through their interactions with various proteins. For successful tissue regeneration, developing an appropriate matrix supporting biological activities of cells in a similar manner than GAGs remains still challenging. In this context, this study aims to design a thermosensitive polysaccharide that could further be used as hydrogel for tissue engineering applications. For this purpose, infernan, a marine bacterial exopolysaccharide (EPS) endowed with GAG-mimetic properties was grafted with a thermosensitive polymer, poly(N-isopropylacrylamide) (pNIPAM). Eight grafted polysaccharides were obtained by varying EPS/pNIPAM molar ratio and the molecular weight of pNIPAM. Their physicochemical characteristics and their thermosensitive properties were determined using a multi-technique, experimental approach. In parallel, molecular dynamics and Monte Carlo simulations were applied at two different scales to elucidate, respectively, the molecular conformation of grafted infernan chain and their ability to form an infinite network undergoing a sol-gel transition near the percolation, a necessary condition in hydrogel formation. It comes out from this study that thermosensitive infernan was successfully developed and its potential use in tissue regeneration as a hydrogel scaffold will further be assessed.

Green synthesis of aryl‐(4‐oxo‐1,2‐dihydroquinazolin‐4‐yl‐methylene) pyrazole‐TiO2 nanoparticles as dyes removable for waste water treatment

Facile synthesis for true intermediate (E)‐ and Z‐2‐(((5‐bromo‐1‐(3‐chloropyridin‐2‐yl)‐1H‐pyrazol‐3‐yl)methylene)amino)‐5‐chloro‐3‐methylbenzoic acid precursors and 2‐(5‐bromo‐1‐(3‐chloropyridin‐2‐yl)‐1H‐pyrazol‐3‐yl)‐6‐chloro‐3,8‐dimethylquinazolin‐4(3H)‐one (QPP) as electron donor/electron acceptor to be working as efficient dye sensitizers. These new derivatives have a wide‐band gap photo‐catalysts semiconductor and are performing to spread to the visible‐light region to give more stability and efficiency towards biomedicine for different diseases. In the present work, the authors are suggesting the design and synthesis of new structures of QPP molecule with extended visible light absorptivity due to the extended π‐π/n‐π conjugations, to stimulate TiO2 nanoparticles in visible‐light region. The physicochemical characterizations confirmed the successful synthesis of QPP, TiO2, and QPP II/TiO2 samples with the proposed structures. Fixing of QPP on the TiO2 surface is refining the optical properties of TiO2 with enhancing the charge separation and generating the efficient antenna in the visible region. Furthermore, the QPP II/TiO2 sample achieved a threefold enhancement in the observed rate constant of the photo‐degradation of rhodamine B dye when compared to the bare TiO2. Finally, the effect of the scavengers was investigated by to be the most reactive species, and the mechanism of the enhancement was suggested for investigation of the proposed structures in various photo‐catalytic and biomedical applications.

Overcoming Water Diffusion Limitations in Hydrogels via Microtubular Graphene Networks for Soft Actuators

Hydrogel-based soft actuators can operate in sensitive environments, bridging the gap of rigid machines interacting with soft matter. However, while stimuli-responsive hydrogels can undergo extreme reversible volume changes of up to ≈90%, water transport in hydrogel actuators is in general limited by their poroelastic behavior. For poly(N-isopropylacrylamide) (PNIPAM) the actuation performance is even further compromised by the formation of a dense skin layer. Here it is shown, that incorporating a bioinspired microtube graphene network into a PNIPAM matrix with a total porosity of only 5.4% dramatically enhances actuation dynamics by up to ≈400% and actuation stress by ≈4000% without sacrificing the mechanical stability, overcoming the water transport limitations. The graphene network provides both untethered light-controlled and electrically powered actuation. It is anticipated that the concept provides a versatile platform for enhancing the functionality of soft matter by combining responsive and 2D materials, paving the way toward designing soft intelligent matter.

Temperature-Responsive Nanoporous Membranes from Self-Assembly of Poly(N-isopropylacrylamide) Hairy Nanoparticles

Nanoporous membranes play a critical role in numerous separations on laboratory and industrial scales, ranging from water treatment to biotechnology. However, few strategies exist that allow for the preparation of mechanically robust nanoporous membranes whose separation properties can be easily tuned. Here, we introduce a new family of tunable nanoporous membranes based on nanoparticles decorated with temperature-responsive polymer brushes. We prepared mechanically robust membranes from hairy nanoparticles (HNPs) carrying PNIPAM polymer brushes. We assembled the HNPs into thin films through pressure-driven deposition of nanoparticle suspensions and measured the permeability and filtration cutoff of these membranes at different temperatures. The membrane pore diameter at room temperature varied between 10 and 30 nm depending on the polymer length. The water permeability of these membranes could be controlled by temperature, with the effective pore diameter increasing by a factor of 3-6 (up to 100 nm) when the temperature was increased to 60 °C. The size selectivity of these membranes in the filtration of nanoparticles could also be attenuated by temperature. Molecular dynamics computer simulations of a coarse-grained HNP model show that temperature-sensitive pores sizes are consistent with our experimental results and reveal the polymer configurations responsible for the observed filtration membrane permeability. We expect that these membranes will be useful for separations and in the preparation of responsive microfluidic devices.

Tailoring thermoresponsiveness of biocompatible polyethers: copolymers of linear glycerol and ethyl glycidyl ether

Linear polyglycerol is known as a highly hydrophilic and biocompatible polymer that is currently considered for numerous medical applications. Derived from this well-known structure, the synthesis of highly biocompatible, thermoresponsive polyether copolymers via statistical anionic ring-opening copolymerization of ethyl glycidyl ether (EGE) and ethoxy ethyl glycidyl ether (EEGE) is described. Subsequent deprotection of the acetal groups of EEGE yields copolymers of linear glycerol (linG) and EGE, P(linG-co-EGE). These copolymers showed monomodal and narrow molecular weight distributions with dispersities Đ ≤ 1.07. The microstructure was investigated via in situ¹H NMR kinetics experiments, revealing reactivity ratios of r_EEGE = 1.787 ± 0.007 and r_EGE = 0.560 ± 0.002, showing a slightly favored incorporation of EEGE over EGE. Due to the deliberate incorporation of rather hydrophobic EGE units into the water soluble linPG, tunable thermoresponsive behavior is achieved with cloud point temperatures T_cp between 9.0-71.4 °C. Besides the commonly utilized method turbidimetry, temperature-dependent ¹H NMR measurements were used for more accurate and reproducible results. The change of the hydrodynamic radii r_H of the copolymers and their aggregates upon reaching T_cp was investigated via DOSY NMR spectroscopy. To explore possible biomedical applications, as an example, the cell viability and immunology of an exemplary P(linG-co-EGE) copolymer sample was investigated. Since both, cell viability and immunology are comparable to the gold standard PEG, the herein presented copolymers show high potential as biocompatible and thermoresponsive alternatives to PEG for biomedical applications.

Supramolecular temperature responsive assembly of polydopamine reduced graphene oxide

Graphene oxide (GO) and reduced graphene oxide (rGO) colloidal systems can directly respond to environmental stimuli such as pH, ionic strength, and light by themselves, but not to temperature. Here we show that surface modification of rGO with polydopamine (PDA) leads to a temperature-responsive composite material, even though neither rGO nor PDA have intrinsic temperature responsiveness. Reducing GO with dopamine results in rGO/PDA flakes with hydrophilic PDA clusters attached to hydrophobic rGO domains, which mimics the amphiphilic structure of temperature responsive poly(N-isopropylacrylamide) (PNIPAM). The rGO/PDA flakes self-assemble at temperature higher than 30 °C, causing flake aggregation and precipitation in suspensions with concentration of 0.05 g L^-1, which is reversible upon cooling, shaking, and re-heating. A solution-to-gelation transition occurs upon heating suspensions with concentration of 10 g L^-1. Nacre-like films and porous monoliths are obtained by drying rGO/PDA suspensions at different concentrations. Films and porous monoliths obtained by drying suspensions that are previously self-assembled through heat have more compact structures compared to those obtained with suspensions that are not heated. Overall, this work introduces the concept of supramolecular temperature responsive assembly of nanomaterials (STRAN), i.e., that temperature response can be introduced in nanomaterials by combining non-responsive components that function cooperatively in supramolecules, whose interactions with solvents can be modulated by temperature changes, mimicking what happens in macromolecular systems such as PNIPAM. STRAN could be applied to nanomaterials beyond GO to develop responsive systems whose self-assembly in suspension and architectural features realized upon drying can be controlled by temperature.

In situ single particle characterization of the themoresponsive and co-nonsolvent behavior of PNIPAM microgels and silica@PNIPAM core-shell colloids

Poly(N-isopropylacrylamide) (PNIPAM) microgels and PNIPAM colloidal shells attract continuous strong interest due to their thermoresponsive behavior, as their size and properties can be tuned by temperature. The direct single particle observation and characterization of pure, unlabeled PNIPAM microgels in their native aqueous environment relies on imaging techniques that operate either at interfaces or in cryogenic conditions, thus limiting the observation of their dynamic nature. Liquid Cell (Scanning) Transmission Electron Microscopy (LC-(S) TEM) imaging allows the characterization of materials and dynamic processes such as nanoparticle growth, etching, and diffusion, at nanometric resolution in liquids. Here we show that via a facile post-synthetic in situ polymer labelling step with high-contrast marker core-shell Au@SiO₂ nanoparticles (NPs) it is possible to determine the full volume of PNIPAM microgels in water. The labelling allowed for the successful characterization of the thermoresponsive behavior of PNIPAM microgels and core shell silica@PNIPAM hybrid microgels, as well as the co-nonsolvency of PNIPAM in aqueous alcoholic solutions. The interplay between electron beam irradiation and PNIPAM systems in water resulted in irreversible shrinkage due to beam induced water radiolysis products, which in turn also affected the thermoresponsive behavior of PNIPAM. The addition of 2-propanol as radical scavenger improved PNIPAM stability in water under electron beam irradiation.

Thermoresponsive and co-nonsolvency behavior of poly(N-vinyl isobutyramide) and poly(N-isopropyl methacrylamide) as poly(N-isopropyl acrylamide) analogs in aqueous media

Sets of the nonionic polymers poly(N-vinyl isobutyramide) (pNVIBAm) and poly(N-isopropyl methacrylamide) (pNIPMAm) are synthesized by radical polymerization covering the molar mass range from about 20,000 to 150,000 kg mol−1, and their thermoresponsive and solvent-responsive behaviors in aqueous solution are studied. Both polymers feature a lower critical solution temperature (LCST) apparently of the rare so-called type II, as characteristic for their well-studied analogue poly(N-isopropyl acrylamide) (pNIPAm). Moreover, in analogy to pNIPAm, both polymers exhibit co-nonsolvency behavior in mixtures of water with several co-solvents, including short-chain alcohols as well as a range of polar aprotic solvents. While the cloud points of the aqueous solutions are a few degrees higher than those for pNIPAm and increase in the order pNIPAm < pNVIBAm < pNIPMAm, the co-nonsolvency behavior becomes less pronounced in the order pNIPAm > pNVIBAm > pNIPMAm. Exceptionally, pNIPMAm does not show co-nonsolvency in mixtures of water and N,N-dimethylformamide.

Graphical Abstract

Shape-Shifting Thermoresponsive Block Copolymer Nano-Objects

In this Feature Article, we review our recent progress in the design of shape-shifting thermoresponsive diblock copolymer nano-objects, which are prepared using various hydroxyl-functional (meth)acrylic monomers (e.g. 2‑hydroxypropyl methacrylate, 4‑hydroxybutyl acrylate or hydroxybutyl methacrylate) to generate the thermoresponsive block. Unlike traditional thermoresponsive polymers such as poly(N-isopropylacrylamide), there is no transition between soluble and insoluble polymer chains in aqueous solution. Instead, thermally driven transitions between a series of copolymer morphologies (e.g. spheres, worms, vesicles or lamellae) occur on adjusting the aqueous solution temperature owing to a subtle change in the partial degree of hydration of the permanently insoluble thermoresponsive block. Such remarkable self-assembly behavior is unprecedented in colloid science: no other amphiphilic diblock copolymer or surfactant system undergoes such behavior at a fixed chemical composition and concentration. Such shape-shifting nano-objects are characterized by transmission electron microscopy, dynamic light scattering, small-angle X-ray scattering, rheology and variable temperature ¹H NMR spectroscopy. Potential applications for this fascinating new class of amphiphiles are briefly considered.

Charge-Selective Aggregation Behavior of Thermoresponsive Polyelectrolytes Having Low Charge Density in Aqueous Solutions of Organic Counterions

The aggregation behavior of thermoresponsive polyelectrolytes with low charge density in aqueous solutions of organic counterions was investigated. We synthesized two thermoresponsive polyelectrolytes: anionic poly(N-isopropylacrylamide-co-(3-sulfopropyl)acrylamide potassium) (P-NIP-SPAK) and cationic poly(N-isopropylacrylamide-co-(3-acrylamidepropyl)trimethylammonium chloride) (P-NIP-AAPTAC). The polyelectrolytes remained soluble in their aqueous solutions even above the lower critical soluble temperature of P-NIP owing to the strong hydration property of the ionic groups. The aggregation occurred when organic counterions were added to the solution. In these solution systems, the concentration of counterions exceeds those of ionic groups introduced into the polyelectrolytes. The aggregation behavior is attributed to the salting-out effect of counterions accommodated near the polyelectrolyte surface by electrostatic interaction. This aggregation behavior was utilized for the charge-selective recognition of amino acids. P-NIP-SPAK aggregated only when basic amino acids were added under acidic conditions, whereas P-NIP-AAPTAC aggregated only when acidic amino acids were added under basic conditions. The results herein demonstrate that P-NIP-SPAK and P-NIP-AAPTAC have the potential to be used as charge-selective polymer sensors for amino acids without having to strictly control the experimental conditions.

Harnessing the polymer-particle duality of ultra-soft nanogels to stabilise smart emulsions

Micro- and nanogels are widely used to stabilise emulsions and simultaneously implement their responsiveness to the external stimuli. One of the factors that improves the emulsion stability is the nanogel softness. Here, we study how the softest nanogels that can be synthesised with precipitation polymerisation of N-isopropylacrylamide (NIPAM), the ultra-low crosslinked (ULC) nanogels, stabilise oil-in-water emulsions. We show that ULC nanogels can efficiently stabilise emulsions already at low mass concentrations. These emulsions are resistant to droplet flocculation, stable against coalescence, and can be easily broken upon an increase in temperature. The resistance to flocculation of the ULC-stabilised emulsion droplets is similar to the one of emulsions stabilised by linear pNIPAM. In contrast, the stability against coalescence and the temperature-responsiveness closely resemble those of emulsions stabilised by regularly crosslinked pNIPAM nanogels. The reason for this combination of properties is that ULC nanogels can be thought of as colloids in between flexible macromolecules and particles. As a polymer, ULC nanogels can efficiently stretch at the interface and cover it uniformly. As a regularly crosslinked nanogel particle, ULC nanogels protect emulsion droplets against coalescence by providing a steric barrier and rapidly respond to changes in external stimuli thus breaking the emulsion. This polymer-particle duality of ULC nanogels can be exploited to improve the properties of emulsions for various applications, for example in heterogeneous catalysis or in food science.

Thermo-Responsive Polymer-siRNA Conjugates Enabling Artificial Control of Gene Silencing around Body Temperature

PURPOSE

Controlling small interfering RNA (siRNA) activity by external stimuli is useful to exert a selective therapeutic effect at the target site. This study aims to develop a technology to control siRNA activity in a thermo-responsive manner, which can be utilized even at temperatures close to body temperature.

METHODS

siRNA was conjugated with a thermo-responsive copolymer that was synthesized by copolymerization of N-isopropylacrylamide (NIPAAm) and hydrophilic N,N-dimethylacrylamide (DMAA) to permit thermally controlled interaction between siRNA and an intracellular gene silencing-related protein by utilizing the coil-to-globule phase transition of the copolymer. The composition of the copolymer was fine-tuned to obtain lower critical solution temperature (LCST) around body temperature, and the phase transition behavior was evaluated. The cellular uptake and gene silencing efficiency of the copolymer-siRNA conjugates were then investigated in cultured cells.

RESULTS

The siRNA conjugated with the copolymer with LCST of 38.0°C exhibited ~ 11.5 nm of the hydrodynamic diameter at 37°C and ~ 9.8 nm of the diameter at 41°C, indicating the coil-globule transition above the LCST. In line with this LCST behavior, its cellular uptake and gene silencing efficiency were enhanced when the temperature was increased from 37°C to 41°C.

CONCLUSION

By fine-tuning the LCST behavior of the copolymer that was conjugated with siRNA, siRNA activity could be controlled in a thermo-responsive manner around the body temperature. This technique may offer a promising approach to induce therapeutic effects of siRNA selectively in the target site even in the in vivo conditions.

Novel ABA block copolymers: preparation, temperature sensitivity, and drug release

A new PEGylated macroiniferter was prepared based on the polycondensation reaction of polyethylene oxide (PEO), methylene diphenyl diisocyanate (MDI), and 1,1,2,2-tetraphenyl-1,2-ethanediol (TPED). The macroiniferter consists of PEO end groups and readily reacts with acrylamides (such as N-isopropylacrylamide, NIPAM) and forms ABA block copolymers (PEO-PNIPAM-PEO). This approach of making amphiphilic ABA block copolymers is robust, versatile, and useful, particularly for the development of polymers for biomedical applications. The resulting amphiphilic PEO-PNIPAM-PEO block copolymers are also temperature sensitive, and their phase transition temperatures are close to human body temperature and therefore they have been applied as drug carriers for cancer treatment. Two PEO-PNIPAM-PEO polymers with different molecular weights were prepared and selected to make temperature-sensitive micelles. As a result of the biocompatibility of these micelles, cell viability tests proved that these micelles have low toxicity toward cancer cells. The resultant polymer micelles were then used as drug carriers to deliver the hydrophobic anticancer drug doxorubicin (DOX), and the results showed that they exhibit significantly higher cumulative drug release efficiency at higher temperatures. Moreover, after loading DOX into the micelles, cellular uptake experiments showed easy uptake and cell viability tests showed that DOX-loaded micelles possess a better therapeutic effect than free DOX at the same dose.

Tailoring a UCST-LCST-pH Multiresponsive Window through a Single Polymer Complex of Chitosan-Hyaluronic Acid

Multistimuli-responsive polymers are important for controlled release. Owing to the fact that these polymers are derived from vinyl-based monomers, their decoration with other molecules is limited. Polysaccharides, especially chitosan (CS) and hyaluronic acid (HA), are pH-responsive biopolymers, whose chemical structures contain reactive functional groups for feasible chemical modifications to obtain add-on functions. The present work demonstrates the introduction of polymers with upper critical solution temperature (UCST) and lower critical solution temperature (LCST) performances onto CS and HA, respectively. By simply varying the mole ratio between the CS-containing UCST polymer and the HA-containing LCST polymer along with adjusting the pH, a polymer system with a UCST-LCST-pH multiresponsive window can be obtained. This multiresponsive window enables us to control the encapsulation and release with repeatability as evidenced from a model study on lysozyme. The present work, for the first time, shows a simple approach to obtain multiresponsive biodegradable polymers through the formation of a single polymer complex to tailor a specific multiresponsive window.

Effect of variations in manufacturing and material properties on the self-folding behaviors of hydrogel and elastomer bilayer structures

The stimuli-responsive self-folding structure is ubiquitous in nature, for instance, the mimosa folds its leaves in response to external touch or heat, and the Venus flytrap snaps shut to trap the insect inside. Thus, modeling self-folding structures has been of great interest to predict the final configuration and understand the folding mechanism. Here, we apply a simple yet effective method to predict the folding angle of the temperature-responsive nanocomposite hydrogel/elastomer bilayer structure manufactured by 3D printing, which facilitates the study of the effect of the inevitable variations in manufacturing and material properties on folding angles by comparing the simulation results with the experimentally measured folding angles. The defining feature of our method is to use thermal expansion to model the temperature-responsive nanocomposite hydrogel rather than the nonlinear field theory of diffusion model that was previously applied. The resulted difference between the simulation and experimentally measured folding angle (i.e., error) is around 5%. We anticipate that our method could provide insight into the design, control, and prediction of 3D printing of stimuli-responsive shape morphing (i.e., 4D printing) that have potential applications in soft actuators, robots, and biomedical devices.

Synthesis of Thermoresponsive PNIPAM-Grafted Cellulose Sulfates for Bioactive Multilayers via Layer-by-Layer Technique

The robust thermoresponsive and bioactive surfaces for tissue engineering by combining poly-N-isopropylacrylamide (PNIPAM) and cellulose sulfate (CS) remain highly in demand but not yet realized. Herein, PNIPAM-grafted cellulose sulfates (PCSs) with diverse degrees of substitution ascribed to sulfate groups (DS_S) are synthesized for the first time. Higher sulfated PCS2 generally forms larger aggregates than lower sulfated PCS1 at their cloud point temperatures (TCP) of around 33 °C, whereas PCS1 leads to larger aggregates at body temperature (37 °C). Via the layer-by-layer (LbL) technique, biocompatible polyelectrolyte multilayers (PEMs) composed of PCSs as polyanions in combination with poly-l-lysine (PLL) or quaternized chitosan (QCHI) as polycations were fabricated. The resulting surfaces contained a more intermingled structure of polyanions with both polycations, while higher sulfated cellulose derivatives (CS2 and PCS2) displayed greater stability. Studies on toxicity and biocompatibility of PEM using 3T3 mouse fibroblasts showed a lower cytotoxicity of PEM with PCS2 and CS2 than PCS1 and CS1. Furthermore, the PEM using PCS2 particularly in combination with QCHI demonstrated excellent biocompatibility that is promising for new bioactive, thermoresponsive coatings on biomaterials and substrata for culturing adhesion-dependent cells.

Entropy-Driven Thermo-gelling Vitrimer

Thermo-gelling polymers have been envisioned as promising smart biomaterials but limited by their weak mechanical and thermodynamic stabilities. Here, we propose a new thermo-gelling vitrimer, which remains at a liquid state because of the addition of protector molecules preventing the crosslinking, and with increasing temperature, an entropy-driven crosslinking occurs to induce the sol-gel transition. Moreover, we find that the activation barrier in the metathesis reaction of vitrimers plays an important role, and experimentally, one can use catalysts to tune the activation barrier to drive the vitrimer to form an equilibrium gel at high temperature, which is not subject to any thermodynamic instability. We formulate a mean-field theory to describe the entropy-driven crosslinking of the vitrimer, which agrees quantitatively with computer simulations and paves the way for the design and fabrication of novel vitrimers for biomedical applications.

Distinctly different solvation behaviors of poly(N,N-diethylacrylamide) gels in water/acetone and water/DMSO mixtures

The solvation behaviors and intermolecular interactions of a poly(N,N-diethylacrylamide) (PDEA) gel network in water/DMSO and water/acetone mixtures have been investigated by variable-temperature high-resolution ¹H MAS NMR. Unlike decreasing volume phase transition temperature (VPTT) of the typical thermosensitive poly(N-isopropylacrylamide) (PNIPAM) gel induced by both acetone and DMSO in a water-rich region, distinct phase transition behaviors are revealed for the PDEA gel. That is, acetone is found to increase the VPTT of PDEA directly in the water-rich region while DMSO is also found to increase the VPTT of PDEA at a very low concentration but then decrease the VPTT as the concentration further increases. The above distinctly different VPTT shifts of PDEA are attributed to the different polymer-cosolvent interactions in water/acetone and water/DMSO systems. DMSO molecules with a strong water affinity are preferentially excluded by the PDEA gel network, and can promote the volume phase transition by favoring the collapse of the PDEA gel network, while acetone molecules preferentially adsorbed on the polymer interact with PDEA via non-specific van der Waals interaction, which favors the swollen state of the PDEA gel.

External Stimuli-Responsive Characteristics of Poly(N,N'-diethylacrylamide) Hydrogels: Effect of Double Network Structure

Swelling experiments and NMR spectroscopy were combined to study effect of various stimuli on the behavior of hydrogels with a single- and double-network (DN) structure composed of poly(N,N'-diethylacrylamide) and polyacrylamide (PAAm). The sensitivity to stimuli in the DN hydrogel was found to be significantly affected by the introduction of the second component and the formation of the double network. The interpenetrating structure in the DN hydrogel causes the units of the component, which is insensitive to the given stimulus in the form of the single network (SN) hydrogel, to be partially formed as globular structures in DN hydrogel. Due to the hydrophilic PAAm groups, temperature- and salt-induced changes in the deswelling of the DN hydrogel are less intensive and gradual compared to those of the SN hydrogel. The swelling ratio of the DN hydrogel shows a significant decrease in the dependence on the acetone content in acetone-water mixtures. A certain portion of the solvent molecules bound in the globular structures was established from the measurements of the 1H NMR spin-spin relaxation times T2 for the studied DN hydrogel. The time-dependent deswelling and reswelling kinetics showed a two-step profile, corresponding to the solvent molecules being released and absorbed during two processes with different characteristic times.

A Review on Novel Channel Materials for Particle Image Velocimetry Measurements-Usability of Hydrogels in Cardiovascular Applications

Particle image velocimetry (PIV) is an optical and contactless measurement method for analyzing fluid blood dynamics in cardiovascular research. The main challenge to visualization investigated in the current research was matching the channel material’s index of refraction (IOR) to that of the fluid. Silicone is typically used as a channel material for these applications, so optical matching cannot be proven. This review considers hydrogel as a new PIV channel material for IOR matching. The advantages of hydrogels are their optical and mechanical properties. Hydrogels swell more than 90 vol% when hydrated in an aqueous solution and have an elastic behavior. This paper aimed to review single, double, and triple networks and nanocomposite hydrogels with suitable optical and mechanical properties to be used as PIV channel material, with a focus on cardiovascular applications. The properties are summarized in seven hydrogel groups: PAMPS, PAA, PVA, PAAm, PEG and PEO, PSA, and PNIPA. The reliability of the optical properties is related to low IORs, which allow higher light transmission. On the other hand, elastic modulus, tensile/compressive stress, and nominal tensile/compressive strain are higher for multiple-cross-linked and nanocomposite hydrogels than single mono-cross-linked gels. This review describes methods for measuring optical and mechanical properties, e.g., refractometry and mechanical testing.

Toward Plant Cyborgs: Hydrogels Incorporated onto Plant Tissues Enable Programmable Shape Control

Engineered living materials (ELMs) that incorporate living organisms and synthetic materials enable advanced functional properties. Here, we seek to create plant cyborgs by combining plants or plant tissues with stimuli-responsive polymeric materials. Plant tissues with integrated shape control may find applications in regenerative medicine, and the shape control of living plants enables another dimension of adaptability and response to environmental threats, which can be applied to next-generation precision farming. In this work, we develop chemistry to integrate stimuli-responsive poly(N-isopropylacrylamide) (PNIPAM) hydrogels with decellularized plant tissues assisted by 3D printing. We demonstrate programmable shape morphing in response to thermal cues and ultraviolet (UV) light. Specifically, by taking advantage of the extrusion-based 3D printing method, we deposit nanocomposite PNIPAM precursors onto silane-treated decellularized leaf surface with prescribed shapes and spatial control. When subjected to external stimuli, the strain mismatch generated between the swellable nanocomposite PNIPAM and nonswellable decellularized leaf enables folding and bending to occur. This strategy to integrate the plant tissues with stimuli-responsive hydrogels allows the control of leaf morphology, opening avenues for plant-based biosensors and soft actuators to enhance food security; such materials also may find applications in biomedicine as tissue-engineering scaffolds.

Role of chain architecture in the solution phase assembly and thermoreversibility of aqueous PNIPAM/silyl methacrylate copolymers

Stimuli-responsive polymers functionalized with reactive inorganic groups enable creation of macromolecular structures such as hydrogels, micelles, and coatings that demonstrate smart behavior. Prior studies using poly(N-isopropyl acrylamide-co-3-(trimethoxysilyl)propyl methacrylate) (P(NIPAM-co-TMA)) have stabilized micelles and produced functional nanoscale coatings; however, such systems show limited responsiveness over multiple thermal cycles. Here, polymer architecture and TMA content are connected to the aqueous self-assembly, optical response, and thermo-reversibility of two distinct types of PNIPAM/TMA copolymers: random P(NIPAM-co-TMA), and a 'blocky-functionalized' copolymer where TMA is localized to one portion of the chain, P(NIPAM-b-NIPAM-co-TMA). Aqueous solution behavior characterized via cloud point testing (CPT), dynamic light scattering (DLS), and variable-temperature nuclear magnetic resonance spectroscopy (NMR) demonstrates that thermoresponsiveness and thermoreversibility over multiple cycles is a strong function of polymer configuration and TMA content. Despite low TMA content (≤2% mol), blocky-functionalized copolymers assemble into small, well-ordered structures above the cloud point that lead to distinct transmittance behaviors and stimuli-responsiveness over multiple cycles. Conversely, random copolymers form disordered aggregates at elevated temperatures, and only exhibit thermoreversibility at negligible TMA fractions (0.5% mol); higher TMA content leads to irreversible structure formation. This understanding of the architectural and assembly effects on the thermal cyclability of aqueous PNIPAM-co-TMA can be used to improve the scalability of responsive polymer applications requiring thermoreversible behavior, including sensing, separations, and functional coatings.

Release of bioactive compounds from delivery systems by stimuli-responsive approaches; triggering factors, mechanisms, and applications

Recent advances in emerging nanocarriers and stimuli-responsive (SR) delivery systems have brought about a revolution in the food and pharmaceutical industries. SR carriers are able to release the encapsulated bioactive compounds (bioactives) upon an external trigger. The potential of releasing the loaded bioactives in site-specific is of great importance for the pharmaceutical industry and medicine that can deliver the cargo in an appropriate condition. For the food industry, release of encapsulated bioactives is considerably important in processing or storage of food products and can be used in their formulation or packaging. There are various stimuli to control the favorite release of bioactives. In this review, we will shed light on the effect of different stimuli such as temperature, humidity, pH, light, enzymatic hydrolysis, redox, and also multiple stimuli on the release of encapsulated cargo and their potential applications in the food and pharmaceutical industries. An overview of cargo release mechanisms is also discussed. Furthermore, various alternatives to manipulate the controlled release of bioactives from carriers and the perspective of more progress in these SR carriers are highlighted.

Stimuli-Responsive Hybrid Metal Nanocomposite - A Promising Technology for Effective Anticancer Therapy

Cancer is one of the most challenging, life-threatening illnesses to cure, with over 10 million new cases diagnosed each year globally. Improved diagnostic cum treatment with common side-effects are warranting for successful therapy. Nanomaterials are recognized to improve early diagnosis, imaging, and treatment. Recently, multifunctional nanocomposites attracted considerable interest due to their low-cost production, and ideal thermal and chemical stability, and will be beneficial in future diagnostics and customized treatment capacity. Stimuli-Responsive Hybrid Metal Nanocomposites (SRHMNs) based nanocomposite materials pose the on/off delivery of bioactive compounds such as medications, genes, RNA, and DNA to specific tissue or organs and reduce toxicity. They simultaneously serve as sophisticated imaging and diagnostic tools when certain stimuli (e.g., temperature, pH, redox, ultrasound, or enzymes) activate the nanocomposite, resulting in the imaging-guided transport of the payload at defined sites. This review in detail addresses the recent advancements in the design and mechanism of internal breakdown processes of the functional moiety from stimuli-responsive systems in response to a range of stimuli coupled with metal nanoparticles. Also, it provides a thorough understanding of SRHMNs, enabling non-invasive interventional therapy by resolving several difficulties in cancer theranostics.

Ca2+-Chelation-Induced Fabrication of Multistimuli-Responsive Charged Nanogels from Phospholipid-Polymer Conjugates and Use for Drug/Protein Loading

Thermoresponsive phospholipid-poly(N-isopropylacrylamide) (PL-PNIPAM) conjugates were synthesized via reversible addition fragmentation chain transfer polymerization mediated by a phospholipid-modified trithiocarbonate. Temperature triggered the micellization of the PL-PNIPAM conjugate to form phosphate group-decorated micelles in the aqueous solution. Driven by the chelation of phospholipids and Ca²⁺, the PL-PNIPAM conjugate and Ca²⁺ ions formed size-tunable nanoclusters at a temperature beyond the lower critical solution temperature. To fabricate cross-linked nanogels, NIPAM was copolymerized with N-succinimidyl acrylate (NSA) to obtain the PL-P(NIPAM-co-NSA) conjugate bearing pendent cross-linkable functionalities. Subsequently, the size-controllable nanogels containing disulfide linkages were generated at 37 °C by cross-linking the PL-P(NIPAM-co-NSA)/Ca²⁺ nanoclusters with cystamine through modulation of Ca²⁺ concentrations. These negatively charged nanogels demonstrate temperature/pH/reduction triple responsiveness. The nanogels can be efficiently loaded with doxorubicin (DOX) and proteins with various isoelectric points. The DOX-loaded nanogels exhibited a temperature/pH/reduction triple-responsive release profile. The immobilized RNase A, BSA, and GOx retained the protein bioactivity. The release of RNase A-loaded nanogels possesses a temperature-responsive profile. The immobilization of Lys and cytochrome C in nanogels inhibited protein bioactivity. However, the addition of NaCl triggered the recovery of bioactivity. These multistimuli-responsive nanogels can provide a versatile platform applicable in biotechnology and drug/protein delivery.