Investigation of the phase separation of PNIPAM using infrared spectroscopy together with multivariate data analysis

Sustainable 3D printing by reversible salting-out effects with aqueous salt solutions

Achieving a simple yet sustainable printing technique with minimal instruments and energy remains challenging. Here, a facile and sustainable 3D printing technique is developed by utilizing a reversible salting-out effect. The salting-out effect induced by aqueous salt solutions lowers the phase transition temperature of poly(N-isopropylacrylamide) (PNIPAM) solutions to below 10 °C. It enables the spontaneous and instant formation of physical crosslinks within PNIPAM chains at room temperature, thus allowing the PNIPAM solution to solidify upon contact with a salt solution. The PNIPAM solutions are extrudable through needles and can immediately solidify by salt ions, preserving printed structures, without rheological modifiers, chemical crosslinkers, and additional post-processing steps/equipment. The reversible physical crosslinking and de-crosslinking of the polymer through the salting-out effect demonstrate the recyclability of the polymeric ink. This printing approach extends to various PNIPAM-based composite solutions incorporating functional materials or other polymers, which offers great potential for developing water-soluble disposable electronic circuits, carriers for delivering small materials, and smart actuators.

An Antioxidative and Active Shrinkage Hydrogel Integratedly Promotes Re-Epithelization and Skin Constriction for Enhancing Wound Closure

Delayed re-epithelization and weakened skin contractions are the two primary factors that hinder wound closure in large-scale acute or chronic wounds. However, effective strategies for targeting these two aspects concurrently are still lacking. Herein, an antioxidative active-shrinkage hydrogel (AHF@AS Gel) is constructed that can integratedly promote re-epithelization and skin constriction to accelerate large-scale acute and diabetic chronic wound closure. The AHF@AS Gel is encapsulated by antioxidative amino- and hydroxyl-modified C70 fullerene (AHF) and a thermosensitive active shrinkage hydrogel (AS Gel). Specifically, AHF relieves overactivated inflammation, prevents cellular apoptosis, and promotes fibroblast migration in vitro by reducing excessive reactive oxygen species (ROS). Notably, the AHF@AS Gel achieved ≈2.7-fold and ≈1.7-fold better re-epithelization in acute wounds and chronic diabetic wounds, respectively, significantly contributing to the promotion of wound closure. Using proteomic profiling and mechanistic studies, it is identified that the AHF@AS Gel efficiently promoted the transition of the inflammatory and proliferative phases to the remodeling phase. Notably, it is demonstrated that AS Gel alone activates the mechanosensitive epidermal growth factor receptor/Akt (EGFR/Akt) pathway and promotes cell proliferation. The antioxidative active shrinkage hydrogel offers a comprehensive strategy for acute wound and diabetic chronic wound closure via biochemistry regulation integrating with mechanical forces stimulation.

A solar/radiative cooling dual-regulation smart window based on shape-morphing kirigami structures

The energy efficiency of buildings has become a critical issue due to their substantial contribution to global energy consumption. Windows, in particular, are often the least efficient component of the building envelope, and conventional smart windows focus solely on regulating solar transmittance while overlooking radiative cooling. Although several recent designs achieved dual-control of solar and radiative cooling, these windows still face limitations in terms of durability, limited modulation ability and energy-saving performance. To address these challenges, we propose a novel dual-control smart window design consisting of a reconfigurable kirigami structure and polydimethylsiloxane-laminated thermochromic hydrogel coated with silver nanowires. In summer, the thermochromic hydrogel turns translucent to suppress the solar heat gain, while the high emissivity kirigami structure covers the exterior surface of the window, promoting radiative cooling. In winter, the hydrogel becomes transparent to allow for solar transmission. Additionally, the kirigami structure undergoes an out-of-plane structural change, opening towards the outside environment to expose the underlying low-emissivity silver nanowires and suppress heat radiation. Our design achieves a promising solar transmittance modulation ability of ∼24% and a good long-wave infrared emissivity regulation ability of 0.5. Furthermore, it exhibits significantly improved durability, which is nine times longer than the lifespan of conventional smart hydrogels. Our novel approach offers a promising solution for constructing energy-efficient and durable smart windows and outperforms existing state-of-the-art solar/radiative cooling dual-regulation smart windows in the literature.

Fabrication of a novel core–shell–shell temperature-sensitive magnetic composite with excellent performance for papain adsorption

Herein, a novel temperature-sensitive magnetic composite (Fe₃O₄@SiO₂@P(NIPAM-co-VI)/Cu²⁺) with a uniform core–shell–shell structure was successfully prepared via a layer-by-layer method. The resulting magnetic composite revealed good magnetic properties and remarkable affinity to papain with a maximum adsorption capacity of 199.17 mg g⁻¹. The adsorption equilibrium data fitted the pseudo-second-order kinetic and Freundlich models well, and the major thermodynamics parameters indicated that adsorption was an endothermic and spontaneous process. Fe₃O₄@SiO₂@P(NIPAM-co-VI)/Cu²⁺ could thermally protect papain, which is attributed to the reversible hydrophilic–hydrophobic transition of the composite at temperatures below and above the lower critical solution temperature. More importantly, the magnetic composite could be recycled at least six times without a remarkable loss in its adsorption capacity, and the process of adsorption and elution had no significant effect on the activity and structure of papain. This work could provide a novel separation method for papain without loss of its activity.

A novel core–shell–shell temperature-sensitive magnetic composite was designed. The composites showed excellent performance for papain adsorption and could thermally protect papain.

Fast photothermal poly(NIPAM-co-β-cyclodextrin) supramolecular hydrogel with self-healing through host-guest interaction for intelligent light-controlled switches

A graphene oxide/poly(N-isopropylacrylamide-co-β-cyclodextrin) (GO/poly(NIPAM-co-β-CD)) hydrogel has been synthesized through host-guest interaction between β-cyclodextrin (β-CD) and the isopropyl group of N-isopropylacrylamide (NIPAM). The product exhibits rapid responses to the stimuli of temperature and near-infrared (NIR) irradiation, self-healing properties, and excellent mechanical properties. The host-guest interaction serves as the main physical cross-linker, while a hydrogen bond between the hydroxyl group of β-CD, GO sheets and amide group of NIPAM acts as a secondary cross-linker. The volume phase transition temperature and NIR response rate of such a hydrogel are controlled by its contents of β-CD and GO. The obtained hydrogels showing excellent properties might be applied in remote contactless control devices in advanced smart technologies. Based on the excellent characteristics of the hydrogels, remote light-controlled switches have been designed, and more applications will be explored, such as intelligent light-controlled drivers and soft robots.

Poly(N,N-bis(2-methoxyethyl)acrylamide), a thermoresponsive non-ionic polymer combining the amide and the ethyleneglycolether motifs

Poly(N,N-bis(2-methoxyethyl)acrylamide) (PbMOEAm) featuring two classical chemical motifs from non-ionic water-soluble polymers, namely, the amide and ethyleneglycolether moieties, was synthesized by reversible addition fragmentation transfer (RAFT) polymerization. This tertiary polyacrylamide is thermoresponsive exhibiting a lower critical solution temperature (LCST)–type phase transition. A series of homo- and block copolymers with varying molar masses but low dispersities and different end groups were prepared. Their thermoresponsive behavior in aqueous solution was analyzed via turbidimetry and dynamic light scattering (DLS). The cloud points (CP) increased with increasing molar masses, converging to 46 °C for 1 wt% solutions. This rise is attributed to the polymers’ hydrophobic end groups incorporated via the RAFT agents. When a surfactant-like strongly hydrophobic end group was attached using a functional RAFT agent, CP was lowered to 42 °C, i.e., closer to human body temperature. Also, the effect of added salts, in particular, the role of the Hofmeister series, on the phase transition of PbMOEAm was investigated, exemplified for the kosmotropic fluoride, intermediate chloride, and chaotropic thiocyanate anions. A pronounced shift of the cloud point of about 10 °C to lower or higher temperatures was observed for 0.2 M fluoride and thiocyanate, respectively. When PbMOEAm was attached to a long hydrophilic block of poly(N,N-dimethylacrylamide) (PDMAm), the cloud points of these block copolymers were strongly shifted towards higher temperatures. While no phase transition was observed for PDMAm-b-pbMOEAm with short thermoresponsive blocks, block copolymers with about equally sized PbMOEAm and PDMAm blocks underwent the coil-to-globule transition around 60 °C.

HCl Gas Sensor Coating Based on Poly(N-isopropylacrylamide) Nanoparticles Prepared from Water-Methanol Binary Solvent

Poly(N-isopropylacrylamide) (PNIPAM) nanoparticles formed in water-methanol binary solvent were successfully deposited on a resonator surface at room temperature by exploiting the cononsolvency effect on the phase transition of PNIPAM aqueous solutions. Scanning electron microscopic observation revealed that the nanoparticles were secondary and made up of agglomerated primary spherical particles of about 10-nm diameter, buried in the film. The magnitude of the sensor response toward HCl gas was larger than that of the nanoparticle sensor prepared from pure water solvent, and the sensitivity to 1 ppm of HCl of sensor-coated nanoparticles based on the present method was 3.3 Hz/ppm. The recovery of the sensors was less than 90% at first cycle measurement, but had improved to almost 100% at the third cycle.

Rapid thermal responsive conductive hybrid cryogels with shape memory properties, photothermal properties and pressure dependent conductivity

Stimuli responsive cryogels with multi-functionality have potential application for electrical devices, actuators, sensors and biomedical devices. However, conventional thermal sensitive poly(N-isopropylacrylamide) cryogels show slow temperature response speed and lack of multi-functionality, which greatly limit their practical application. Herein we present conductive fast (2 min for both deswelling and reswelling behavior) thermally responsive poly(N-isopropylacrylamide) cryogels with rapid shape memory properties (3 s for shape recovery), near-infrared (NIR) light sensitivity and pressure dependent conductivity, and further demonstrated their applications as temperature sensitive on-off switch, NIR light sensitive on-off switch, water triggered shape memory on-off switch and pressure dependent device. These cryogels were first prepared in dimethyl sulfoxide below its melting temperature in ice bath and subsequently put into aniline or pyrrole solution to in situ deposition of conducting polyaniline or polypyrrole nanoparticles. The continuous macroporous sponge-like structure provides cryogels with rapid responsivity both in deswelling, reswelling kinetics and good elasticity. After incorporating electrically conductive polyaniline or polypyrrole nanoaggregates, the hybrid cryogels exhibit desirable conductivity, photothermal property, pressure dependent conductivity and good cytocompatibility. These multifunctional hybrid cryogels make them great potential as stimuli responsive electrical device, tissue engineering scaffolds, drug delivery vehicle and electronic skin.

Temperature-Switchable Agglomeration of Magnetic Particles Designed for Continuous Separation Processes in Biotechnology

The purpose of this work was the synthesis and characterization of thermally switchable magnetic particles for use in biotechnological applications such as protein purification and enzymatic conversions. Reversible addition-fragmentation chain-transfer polymerization was employed to synthesize poly(N-isopropylacrylamide) brushes via a "graft-from" approach on the surface of magnetic microparticles. The resulting particles were characterized by infrared spectroscopy and thermogravimetric analysis and their temperature-dependent agglomeration behavior was assessed. The influence of several factors on particle agglomeration (pH, temperature, salt type, and particle concentration) was evaluated. The results showed that a low pH value (pH 3-4), a kosmotropic salt (ammonium sulfate), and a high particle concentration (4 g/L) resulted in improved agglomeration at elevated temperature (40 °C). Recycling of particles and reversibility of the temperature-switchable agglomeration were successfully demonstrated for ten heating-cooling cycles. Additionally, enhanced magnetic separation was observed for the modified particles. Ionic monomers were integrated into the polymer chain to create end-group functionalized particles as well as two- and three-block copolymer particles for protein binding. The adsorption of lactoferrin, bovine serum albumin, and lysozyme to these ion exchange particles was evaluated and showed a binding capacity of up to 135 mg/g. The dual-responsive particles combined magnetic and thermoresponsive properties for switchable agglomeration, easy separability, and efficient protein adsorption.

Second order multivariate curve resolution of Fourier transform infrared spectroscopic data of the photo-induced crosslinking of thymine functionalized polymers

A meaningful characterization of the photo-induced curing process of materials based on styrene monomers functionalized with thymine and charged ionic groups was accomplished using FT-IR spectroscopy in combination with second-order multivariate calibration algorithms. The polymer composition as well as the irradiation dose effects on the photo-crosslinking of copolymer films was experimentally determined. Each FT-IR absorption spectra was decomposed into the contribution of individual species by means of chemometric algorithms. A second-order strategy involving a three-way array for each sample and analyzing all arrays simultaneously was used. Temperature and solvent frequently have a strong influence on the FT-IR peak producing shifts and trilinearity lost. A new methodology to properly pre-align the spectroscopic matrix data is used based on the decomposition of a three-way array via a suitably initialized and constrained PARAFAC model. The chemical reaction mechanism describing the underlying process in terms of identifiable steps was determined. Associated key parameters and equilibrium rate constants that characterize the interconversion and stability of diverse species were predicted. Additionally, it was possible to quantify all the species even in the presence of a non-calibrated compound.