Poly (N-isopropylacrylamide) microgel-based optical devices for sensing and biosensing

Functional aligned mesenchymal stem cell sheets fabricated using micropatterned thermo-responsive cell culture surfaces

Mesenchymal stem cells (MSCs) are frequently applied for cell transplantation and regenerative therapy because they secrete diverse therapeutic cytokines that prompt immuno-stimulatory and tissue repair processes. Furthermore, cultured MSC sheets exhibit enhanced cytokine secretion compared to their MSC suspensions, and represent a durable, versatile format for tissue engineering as singular, multi-layered, or multi-cell type sandwiched, transplantable constructs. Tissue engineered implants with various cellular orientations have been reported. In this study, patterned, temperature-responsive culture surfaces were used to prepare oriented MSC sheets. Patterned culture surfaces were fabricated by grafting polyacrylamide (PAAm) onto commercial poly(N-isopropylacrylamide) (PNIPAAm)-modified plastic via photopolymerization using a stripe-patterned photomask. Patterned surfaces were characterized using x-ray photoelectron spectroscopy, fluorescently labelled fibronectin and albumin adsorption assays, wetting (contact angle) measurements, atomic force microscopy, and scanning electron microscopy. Striped grafted patterns of PAAm were fabricated on the PNIPAAm-coated culture substrates, and PAAm polymerized within the PNIPAAm overlayer. Cell-aligned MSC sheets were then produced from MSC culture on this patterned surface, secreting higher amounts of therapeutic cytokines (vascular endothelial growth factor, hepatocyte growth factor, and transforming growth factor-β) than non-aligned MSC control sheets. In addition, aligned MSC sheets maintained enhanced cell multi-potent differentiation capabilities. New, aligned MSC sheets might exhibit improved functional properties for cell sheet transplant therapies.

Effective cell sheet preparation using thermoresponsive polymer brushes with various graft densities and chain lengths

Various cell sheets have been used as effective and useful cellular tissues in tissue engineering and regenerative therapy. Poly(N-isopropylacrylamide) (PNIPAAm)-modified surfaces have been investigated for effective cell sheet preparation. In this study, the effective PNIPAAm graft density and chain length of PNIPAAm brushes for various cell types were investigated. The PNIPAAm brush-grafted glass was prepared via silanization and subsequent atom transfer radical polymerization (ATRP). The density of the PNIPAAm brushes was modulated by changing the ATRP initiator and co-adsorber composition, while the PNIPAAm brush length was modulated by changing the monomer concentration in the ATRP. The hydrophilicity of the PNIPAAm brushes increased with increasing PNIPAAm brush length because long PNIPAAm brushes tended to hydrate. Fibronectin adsorption increased with decreasing PNIPAAm brush concentration because the exposed hydrophobic co-adsorber in the dilute PNIPAAm brush enhanced the adsorption of fibronectin. The cell-sheet fabrication ability was investigated using six types of PNIPAAm brushes. An endothelial cell sheet was fabricated using a dense, short PNIPAAm brush. NIH/3T3 sheets can be fabricated using three types of PNIPAAm brushes: dense-long PNIPAAm brushes, moderately dense-short PNIPAAm brushes, and dilute-long PNIPAAm brushes. MDCK cell sheets could not be prepared using the PNIPAAm brushes. A549 cell sheets were prepared using a dense-short PNIPAAm brush and moderately dense-short PNIPAAm brushes. These results indicate that the optimal PNIPAAm brush conditions for cell sheet preparation vary depending on cell type. Thus, modulation of PNIPAAm brush density and length is an effective approach for preparing target cell sheets.

Exploring guest molecule uptake in pH-responsive polyelectrolyte microgels via Monte Carlo simulations

Understanding the interactions of guest molecules like proteins and nanoparticles with microgels is fundamental for using microgels as nanocarriers. However, understanding and predicting the system properties becomes increasingly difficult as the systems become more complex. In this study, we systematically investigated the uptake of these guest molecules in a pH-responsive polyelectrolyte microgel modeled as a bead-spring network using Monte Carlo simulations. To narrow down the complexity of the systems, we modeled the guest molecules as simple charged beads. The simulations included the variation of (i) guest molecule charge, (ii) size, and (iii) number, as well as the influence of (iv) the addition of salt. The effect of these parameters on the ionization, swelling, and guest molecule uptake was investigated. The uptake of guest molecules with higher charges enhanced the ionization of the microgel at low pH. The strongest effect was observed for beads with charge z = +15. For higher guest molecule charges, the polymer chains could not fully wrap around the guest molecules, to provide enough microgel charges to fully compensate for the repulsive interactions between the guest beads. In general, the uptake of guest molecules leads to a collapse of the microgel due to attractive electrostatic interactions. With the increasing size of the guest molecules, their excluded volume increases, and the microgel swells with their uptake. Adding monovalent salt slightly decreases the uptake at low ionization of the network due to electrostatic screening. The presence of salt ions with higher valency further decreases the uptake of guest molecules into a fully ionized microgel.

PNIPAM Mesoglobules in Dependence on Pressure

Poly(N-isopropylacrylamide) (PNIPAM) in aqueous solution forms mesoglobules above its cloud point temperature Tcp. While these are small and compact at atmospheric pressure, they are large and water-rich at high pressure. To identify the transition between these states, we employed optical microscopy and carried out isothermal pressure scans. Using very small angle neutron scattering, we determined the size and water content of the mesoglobules in pressure scans at different temperatures above Tcp. We observe a distinct transition at pressures of 35-55 MPa with the transition pressure depending on temperature. While the transition is smooth at high temperatures, i.e., far away from the coexistence line, it is abrupt at low temperatures, i.e., close to the coexistence line. Hence, at high temperatures, the swelling of the mesoglobules dominates, whereas at low temperatures, the coalescence of mesoglobules prevails. Subsequently decreasing the pressure results in a gradual deswelling of the mesoglobules at high temperature. In contrast, at low temperatures, small and compact mesoglobules form, but the large aggregates persist. We conclude that, on the time scale of the experiment, the disintegration of the large swollen aggregates into small and compact mesoglobules is only partially possible. Erasing the history by cooling the sample at the maximum pressure into the one-phase state does not result in qualitative changes for the behavior with the only difference that Fewer mesoglobules are formed when the pressure is decreased again. The newly identified transition line separates the low-pressure from the high-pressure regime.

A Novel Approach for the Synthesis of Responsive Core-Shell Nanogels with a Poly(N-Isopropylacrylamide) Core and a Controlled Polyamine Shell

Microgel particles can play a key role, e.g., in drug delivery systems, tissue engineering, advanced (bio)sensors or (bio)catalysis. Amine-functionalized microgels are particularly interesting in many applications since they can provide pH responsiveness, chemical functionalities for, e.g., bioconjugation, unique binding characteristics for pollutants and interactions with cell surfaces. Since the incorporation of amine functionalities in controlled amounts with predefined architectures is still a challenge, here, we present a novel method for the synthesis of responsive core-shell nanogels (dh < 100 nm) with a poly(N-isopropylacrylamide) (pNIPAm) core and a polyamine shell. To achieve this goal, a surface-functionalized pNIPAm nanogel was first prepared in a semi-batch precipitation polymerization reaction. Surface functionalization was achieved by adding acrylic acid to the reaction mixture in the final stage of the precipitation polymerization. Under these conditions, the carboxyl functionalities were confined to the outer shell of the nanogel particles, preserving the core's temperature-responsive behavior and providing reactive functionalities on the nanogel surface. The polyamine shell was prepared by the chemical coupling of polyethyleneimine to the nanogel's carboxyl functionalities using a water-soluble carbodiimide (EDC) to facilitate the coupling reaction. The efficiency of the coupling was assessed by varying the EDC concentration and reaction temperature. The molecular weight of PEI was also varied in a wide range (Mw = 0.6 to 750 kDa), and we found that it had a profound effect on how many polyamine repeat units could be immobilized in the nanogel shell. The swelling and the electrophoretic mobility of the prepared core-shell nanogels were also studied as a function of pH and temperature, demonstrating the successful formation of the polyamine shell on the nanogel core and its effect on the nanogel characteristics. This study provides a general framework for the controlled synthesis of core-shell nanogels with tunable surface properties, which can be applied in many potential applications.

Moisture changes inside hydrogel particles during their drying process investigated with fluorescence lifetime imaging

The properties of hydrogels and microgels, i.e. hydrogel particles, depend strongly on their water content. Based on our previously developed method to access the local water content in microgels, we performed fluorescence lifetime microscopy measurements at different stages of drying poly(N-isopropylacrylamide) (PNIPAM) microgels under ambient conditions. For this purpose, the red-emitting dye ATTO 655 was covalently attached to the microgels. Its emission is quenched by water molecules due to an energy transfer from the first excited state of the dye to a vibrational level of the water molecules. The quenching constant or, equivalently, the fluorescence lifetime, gives direct access to the local water concentration. We measured the fluorescence lifetime after spin-coating, reswelling and at different times of subsequent drying to follow the changes of water content during this process. We found that the microgels are not totally dry after spin coating, but drying them to their equilibrium moisture under ambient temperature and humidity conditions requires several hours. Additionally, we determined the moisture inside microgels in equilibrium at different air humidities. In summary, the method allows for a detailed investigation of the moisture inside hydrogels and gives straight-forward access to in situ and operando measurements of hydrogel systems.

Keratin-PNIPAM Hybrid Microgels: Preparation, Morphology and Swelling Properties

Combinations of synthetic polymers, such as poly(N-isopropylacrylamide) (PNIPAM), with natural biomolecules, such as keratin, show potential in the field of biomedicine, since these hybrids merge the thermoresponsive properties of PNIPAM with the bioactive characteristics of keratin. This synergy aims to produce hybrids that can respond to environmental stimuli while maintaining biocompatibility and functionality, making them suitable for various medical and biotechnological uses. In this study, we exploit keratin derived from wool waste in the textile industry, extracted via sulfitolysis, to synthesize hybrids with PNIPAM microgel. Utilizing two distinct methods-polymerization of NIPAM with keratin (HYB-P) and mixing preformed PNIPAM microgels with keratin (HYB-M)-resulted in hybrids with 20% and 25% keratin content, respectively. Dynamic light scattering (DLS) and transmission electron microscopic (TEM) analyses indicated the formation of colloidal systems with particle sizes of around 110 nm for HYB-P and 518 nm for HYB-M. The presence of keratin in both systems, 20% and 25%, respectively, was confirmed by spectroscopic (FTIR and NMR) and elemental analyses. Distinct structural differences were observed between HYB-P and HYB-M, suggesting a graft copolymer configuration for the former hybrid and a complexation for the latter one. Furthermore, these hybrids demonstrated temperature responsiveness akin to PNIPAM microgels and pH responsiveness, underscoring their potential for diverse biomedical applications.

bFGF-releasing biodegradable nanoparticles for effectively engrafting transplanted hepatocyte sheet

Hepatic tissue engineering has been applied for the treatment of intractable liver diseases, and hepatocyte sheets are promising for this purpose. However, hepatocyte sheets have poor survival after transplantation because of their high metabolic activity. In this study, we aimed to develop basic fibroblast growth factor (bFGF)-releasing nanoparticles to prolong the survival of hepatocyte sheets after transplantation. The nanoparticles were prepared by electrospraying a bFGF-dispersed poly(D,l-lactide-co-glycolide) emulsion. bFGF-loaded PLGA nanoparticles can be developed by optimizing the applied electrospray voltage and the oil:water ratio of the emulsion. The prepared nanoparticles exhibited prompt release at the initial duration and continuous gradual release at the subsequent duration. Hepatocyte sheet engraftment was evaluated by transplanting hepatocyte sheets containing the prepared nanoparticles into rats. The hepatocyte sheets with the prepared nanoparticles exhibited longer survival than those without the bFGF nanoparticles or solution owing to the local and continuous release of bFGF from the nanoparticles and the subsequent enhanced angiogenesis at the transplantation site. These results indicated that the prepared bFGF-releasing nanoparticles can enhance the efficiency of hepatocyte sheet transplantation. The developed bFGF-releasing nanoparticles would be useful for the transplantation of cellular tissue with post-transplantation survival challenges.

Network architecture dependent mechanical response in temperature responsive collagen-PNIPAM composites

Collagen is the most abundant protein in the mammalian extracellular matrix. In-vitro collagen-based materials with specific mechanical properties are important for various bio-medical and tissue-engineering applications. Here, we study the reversible mechanical switching behaviour of a bio-compatible composite formed by collagen networks seeded with thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) microgel particles, by exploiting the swelling/de-swelling of the particles across the lower critical solution temperature (LCST). Interestingly, we find that the shear modulus of the system reversibly enhances whenever the diameter of the microgel particles is changed from that corresponding to the polymerization temperature of the composite, irrespective of swelling or, de-swelling. However, the degree of such enhancement significantly depends on the temperature-dependent collagen network architecture quantified by the mesh size of the network. Furthermore, confocal imaging of the composite during the temperature switching reveals that the reversible clustering of microgel particles above LCST plays a crucial role in the observed switching response.

Buckling and Interfacial Deformation of Fluorescent Poly(N-isopropylacrylamide) Microgel Capsules

Hollow microgels are fascinating model systems at the crossover between polymer vesicles, emulsions, and colloids as they deform, interpenetrate, and eventually shrink at higher volume fraction or when subjected to an external stress. Here, we introduce a system consisting of microgels with a micrometer-sized cavity enabling a straightforward characterization in situ using fluorescence microscopy techniques. Similarly to elastic capsules, these systems are found to reversibly buckle above a critical osmotic pressure, conversely to smaller hollow microgels, which were previously reported to deswell at high volume fraction. Simulations performed on monomer-resolved in silico hollow microgels confirm the buckling transition and show that the presented microgels can be described with a thin shell model theory. When brought to an interface, these microgels, that we define as microgel capsules, strongly deform and we thus propose to utilize them to locally probe interfacial properties within a theoretical framework adapted from the Johnson-Kendall-Roberts (JKR) theory. Besides their capability to sense their environment and to address fundamental questions on the elasticity and permeability of microgel systems, microgel capsules can be further envisioned as model systems mimicking anisotropic responsive biological systems such as red blood and epithelial cells thanks to the possibility offered by microgels to be synthesized with custom-designed properties.

Two-dimensional colloidal crystal of soft microgel spheres: Development, preparation and applications

Two-dimensional (2D) colloidal crystals are ordered monolayer arrays of colloidal sphere particles assembled on the substrates or at phase interfaces. Owing to their unique periodic structure and fascinating properties, 2D colloidal crystals have aroused considerable interest because of their potential applications. Among them, 2D colloidal crystals self-assembled from soft microgel spheres stand out particularly. The 2D colloidal crystals of soft microgel spheres combine the advantages of monolayer colloidal crystals and sensitive microgels, which have a good application prospect in biomedical area. In this article, we provide a systematic overview of 2D colloidal crystals of soft microgel spheres related to their development, preparation and applications. First, various preparation methods of 2D colloidal crystal of microgels are introduced, including dip-coating, drop-coating, spin-coating, interface assembly, surface reaction-assisted assembly, and so forth. Second, representative biomedical applications consisting of optical sensor, drug delivery, antibacterial coating, cell culture, and colloidal template are also exemplified to show the high performance of 2D colloidal crystals of soft microgel spheres. In addition, we also present prospects of future developments of 2D microgel colloidal crystals.

Magnetic response of CoFe2O4 nanoparticles confined in a PNIPAM microgel network

The paper addresses coupling of magnetic nanoparticles (MNPs) with the polymer matrix of temperature-sensitive microgels and their response to magnetic fields. Therefore, CoFe₂O₄@CA (CA = citric acid) NPs are embedded within N-isopropylacrylamid (NIPAM) based microgels. The volume phase transition (VPT) of the magnetic microgels and the respective pure microgels is studied by dynamic light scattering and electrophoretic mobility measurements. The interaction between MNPs and microgel network is studied via magnetometry and AC-susceptometry using a superconducting quantum interference device (SQUID). The data show a significant change of the magnetic properties by crossing the VPT temperature (VPTT). The change is related to the increased confinement of the MNP due to the shrinking of the microgels. Modifying the microgel with hydrophobic allyl mercaptan (AM) affects the swelling ability and the magnetic response, i.e. the coupling of MNPs with the polymer matrix. Modeling the AC-susceptibility data results in an effective size distribution. This distribution represents the varying degree of constraint in MNP rotation and motion by the microgel network. These findings help to understand the interaction between MNPs and the microgel matrix to design multi responsive systems with tunable particle matrix coupling strength for future applications.

Lab-on-fiber sensing system based on responsive Fabry-Perot optical resonance cavities prepared throughin situconstruction strategy

For decades, lab-on-fiber (LOF) sensing systems have become an emerging optical sensing platform due to the features of small size and light weight. Herein, a simple and efficientin situconstruction strategy was reported for the preparation of LOF sensing platform based on the integration of responsive Fabry-Perot optical resonance cavity on optical fibers. The responsive Fabry-Perot optical resonance cavity with thermal poly(N-isopropylacrylamide) polymer brush layer sandwiched by two silver layers was constructed on the end surface of the optical fiber through combiningin situsurface-initiated polymerization and metal film deposition techniques. Owing to the thermo-responsiveness of the intermediate layer, the as-prepared LOF sensing system shows a sensitive response towards the environmental temperature. Importantly, the as-prepared LOF sensing system also possesses excellent repeatability and rapid response rate. Together with the features of high sensitivity, excellent repeatability and rapid response rate, we believe such LOF sensing system will provide a foundation for the future applications of medical diagnosis,in vivodetection and public security.

Chemical-Physical Behaviour of Microgels Made of Interpenetrating Polymer Networks of PNIPAM and Poly(acrylic Acid)

Microgels composed of stimuli responsive polymers have attracted worthwhile interest as model colloids for theorethical and experimental studies and for nanotechnological applications. A deep knowledge of their behaviour is fundamental for the design of new materials. Here we report the current understanding of a dual responsive microgel composed of poly(N-isopropylacrylamide) (PNIPAM), a temperature sensitive polymer, and poly(acrylic acid) (PAAc), a pH sensitive polymer, at different temperatures, PAAc contents, concentrations, solvents and pH. The combination of multiple techniques as Dynamic Light Scattering (DLS), Raman spectroscopy, Small Angle Neutron Scattering (SANS), rheology and electrophoretic measurements allow to investigate the hydrodynamic radius behaviour across the typical Volume Phase Transition (VPT), the involved molecular mechanism and the internal particle structure together with the viscoelastic properties and the role of ionic charge in the aggregation phenomena.

Importance of pH in Synthesis of pH-Responsive Cationic Nano- and Microgels

While cationic microgels are potentially useful for the transfection or transformation of cells, their synthesis has certain drawbacks regarding size, polydispersity, yield, and incorporation of the cationic comonomers. In this work, a range of poly(N-isopropylacrylamide) (PNIPAM) microgels with different amounts of the primary amine N-(3-aminopropyl)methacrylamide hydrochloride (APMH) as the cationic comonomer were synthesized. Moreover, the pH-value during reaction was varied for the synthesis of microgels with 10 mol% APMH-feed. The microgels were analyzed by means of their size, thermoresponsive swelling behavior, synthesis yield, polydispersity and APMH-incorporation. The copolymerization of APMH leads to a strong decrease in size and yield of the microgels, while less than one third of the nominal APMH monomer feed is incorporated into the microgels. With an increase of the reaction pH up to 9.5, the negative effects of APMH copolymerization were significantly reduced. Above this pH, synthesis was not feasible due to aggregation. The results show that the reaction pH has a strong influence on the synthesis of pH-responsive cationic microgels and therefore it can be used to tailor the microgel properties.

On the swelling behavior of poly(N-Isopropylacrylamide) hydrogels exposed to perfluoroalkyl acids

Per- and polyfluoroalkyl substances (PFAS) have rapidly accumulated in the environment due to their widespread use prior to commercial discussion in the early 21st century, and their slow degradation has magnified concerns of their potential toxicity. Monitoring their distribution is, therefore, necessary to evaluate and control their impact on the health of exposed populations. This investigation evaluates the capability of a simple polymeric detection scheme for PFAS based on crosslinked, thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) hydrogels. Surveying swelling perturbations induced by several hydrotropes and comparable hydrocarbon analogs, tetraethylammonium perfluorooctane sulfonate (TPFOS) showed a significantly higher swelling ratio on a mass basis (65.5 ± 8.8 at 15°C) than any of the other analytes tested. Combining swelling with the fluorimetric response of a solvachromatic dye, nile red, revealed the fluorosurfactant to initiate observable aggregation (i.e., its critical aggregation concentration) at 0.05 mM and reach saturation (i.e., its charge neutralization concentration) at 0.5 mM. The fluorosurfactant was found to homogeneously distribute throughout the polymer matrix with energy dispersive X-ray spectroscopy, marking the swelling response as a peculiar nexus of fluorinated interfacial positioning and delocalized electrostatic repulsion. Results from the current study hold promise for exploiting the physiochemical response of PNIPAM to assess TPFOS's concentration.

Stiffness Tomography of Ultra-Soft Nanogels by Atomic Force Microscopy

The softness of nanohydrogels results in unique properties and recently attracted tremendous interest due to the multi-functionalization of interfaces. Herein, we study extremely soft temperature-sensitive ultra-low cross-linked (ULC) nanogels adsorbed to the solid/water interface by atomic force microscopy (AFM). The ultra-soft nanogels seem to disappear in classical imaging modes since a sharp tip fully penetrates these porous networks with very low forces in the range of steric interactions (ca. 100 pN). However, the detailed evaluation of Force Volume mode measurements allows us to resolve their overall shape and at the same time their internal structure in all three dimensions. The nanogels exhibit an extraordinary disk-like and entirely homogeneous but extremely soft structure-even softer than polymer brushes. Moreover, the temperature-sensitive nanogels can be switched on demand between the ultra-soft and a very stiff state.

Multistimuli-Responsive Optical Hydrogel Nanomembranes to Construct Planar Color Display Boards for Detecting Local Environmental Changes

Planar metal-insulator-metal (MIM) optical cavities are attractive for biochemical and environmental sensing applications, as they offer a cost-effective cavity platform with acceptable performances. However, localized detection and scope of expansion of applicable analytes are still challenging. Here, we report a stimuli-responsive color display board that can exhibit local spectral footprints, for locally applied heat and alcohol presence. A thermoresponsive, optically applicable, and patternable copolymer, poly(N-isopropylacrylamide-r-glycidyl methacrylate), is synthesized and used with a photosensitive cross-linker to produce a responsive insulating layer. This layer is then sandwiched between two nanoporous silver membranes to yield a thermoresponsive MIM cavity. The resonant spectral peak is blue-shifted as the environmental temperature increases, and the dynamic range of the resonant peak is largely affected by the composition and structure of the cross-linker and the copolymer. The localized temperature increase of silk particles with gold nanoparticles by laser heating can be measured by reading the spectral shift. In addition, a free-standing color board can be transferred onto a curved biological tissue sample, allowing us to simultaneously read the temperature of the tissue sample and the concentration of ethanol. The stimuli-responsive MIM provides a new way to optically sense localized environmental temperature and ethanol concentration fluctuations.

Formation and Stability of Smooth Thin Films with Soft Microgels Made of Poly(N-Isopropylacrylamide) and Poly(Acrylic Acid)

In this work, soft microgels of Poly(N-Isopropylacrylamide) (PNIPAm) at two different sizes and of interpenetrated polymer network (IPN) composed of PNIPAm and Poly(Acrylic Acid) (PAAc) were synthesized. Then, solutions of these different types of microgels have been spin-coated on glass substrates with different degrees of hydrophobicity. PNIPAm particles with a larger diameter form either patches or a continuous layer, where individual particles are still distinct, depending on the dispersion concentration and spin speed. On the other, PNIPAm particles with a smaller diameter and IPN particles form a continuous and smooth film, with a thickness depending on the dispersion concentration and spin-speed. The difference in morphology observed can be explained if one considers that the microgels may behave as colloidal particles or macromolecules, depending on their size and composition. Additionally, the microgel size and composition can also affect the stability of the depositions when rinsed in water. In particular, we find that the smooth and continuous films show a stimuli-dependent stability on parameters such as temperature and pH, while large particle layers are stable under any condition except on hydrophilic glass by washing at 50 °C.

Synthesis and structure of temperature-sensitive nanocapsules

The transport and systematic release of functional agents at specific areas are key challenges in various application fields. These make the development of micro- and nanocapsules, which allow for uptake, storage, and triggered release, of high interest. Hollow thermoresponsive microgels, cross-linked polymer networks with a solvent-filled cavity in their center, are promising candidates as triggerable nanocapsules, as they can adapt their size and shape to the environment. Their shell permeability can be controlled by temperature, while the cavity can serve as a storage place for guest species. Here, we present the synthesis and structural characterization of temperature-responsive microgels, which are deswollen at room temperature and swell upon moderate cooling, to facilitate potential encapsulation experiments. We present microgels made from poly(N-isopropylacrylamide-co-diacetone acrylamide), p(NIPAM-co-DAAM), possessing a volume phase transition temperature below room temperature. Their colloidal stability in the deswollen state can be enhanced by adding a swollen polymer shell made of poly(N-isopropylacrylamide), pNIPAM, as periphery. The synthesis of hollow double-shell microgels comprising a cavity surrounded by an inner p(NIPAM-co-DAAM) shell and an outer pNIPAM shell is established. The inner network enables the control of the shell permeability: the network is deswollen at room temperature and swells upon moderate cooling. The outer network guarantees for steric stability at room temperature. Light scattering techniques are employed for the characterization of the microgels. Form factor analysis reveals that the cavity of the nanocapsules persists at all swelling states, making it an ideal site for the storage of guest species.

Development of stimuli-responsive intelligent polymer micelles for the delivery of doxorubicin

Doxorubicin is still used as a first-line drug in current therapeutics for numerous types of malignant tumours (including lymphoma, transplantable leukaemia and solid tumour). Nevertheless, to overcome the serious side effects like cardiotoxicity and myelosuppression caused by effective doses of doxorubicin remains as a world-class puzzle. In recent years, the usage of biocompatible polymeric nanomaterials to form an intelligently sensitive carrier for the targeted release in tumour microenvironment has attracted wide attention. These different intelligent polymeric micelles (PMs) could change the pharmacokinetics process of drugs or respond in the special microenvironment of tumour site to maximise the efficacy and reduce the toxicity of doxorubicin in other tissues and organs. Several intelligent PMs have already been in the clinical research stage and planned for market. Therefore, related research remains active, and the latest nanotechnology approaches for doxorubicin delivery are always in the spotlight. Centring on the model drugs doxorubicin, this review summarised the mechanisms of PMs, classified the polymers used in the application of doxorubicin delivery and discussed some interesting and imaginative smart PMs in recent years.

Universal Antibacterial Surfaces Fabricated from Quaternary Ammonium Salt-Based PNIPAM Microgels

Because of the excellent film-forming ability of poly(N-isopropylacrylamide) (PNIPAM) microgel and high-efficient bactericidal property of quaternary ammonium salt (QAS), QAS-based PNIPAM (QAS-PNIPAM) microgels are synthesized and employed to modify the surface of a range of commonly used materials including metal, plastic, and elastomer. Bacterial culture is carried out on such QAS-PNIPAM microgel-modified surfaces to examine the viability of the attached bacteria. It is found that the bactericidal efficiency is nearly 100% on the modified surfaces of all the studied materials. We attribute the high-efficient bactericidal performance of QAS-PNIPAM microgel film to the QAS component rather than the topography of the microgel film itself. In addition, the microgel film is robust and shows great integrity even after culture of the bacteria and repeated rinses, and the cell experiment demonstrates that this microgel film is cyto-compatible. Therefore, such a simple, versatile method of preparing antibacterial films paves the way for future bactericidal applications.

Precise regulation of particle size of poly(N-isopropylacrylamide) microgels: Measuring chain dimensions with a "molecular ruler"

HYPOTHESIS

Poly(N-isopropylacrylamide) microgels are used extensively in the design of drug carriers, surfaces for control of cell adhesion, and optical devices. Particle size is a key factor and has a significant influence in many areas.

EXPERIMENTS

In this work, precise control of the particle size of poly(N-isopropylacrylamide) microgels was achieved by controlling the separation distance of the poly(N-isopropylacrylamide) chains. Dibromoalkanes of different size were used as an adjustable "molecular ruler" to measure molecular dimensions in poly(N-isopropylacrylamide) nanoaggregates at the critical crosslinking temperature.

FINDINGS

We find that the chain separation distance decreases as the temperature increases with a sharp decrease over the 55-to-65 °C interval. Based on the observed relationships between chain separation and crosslinker, the particle size of poly(N-isopropylacrylamide) microgels can be regulated by changing the length of the "molecular ruler" (crosslinker) at the same temperature. Furthermore, for partly crosslinked poly(N-isopropylacrylamide) microgels that contain free crosslinkable sites, the particle size can be reduced still more by further crosslinking ("re-crosslinking") with crosslinkers of different size. It is shown that the particle size can be regulated by adjusting the length of "molecular ruler" and the degree of crosslinking. This work provides a "molecular level" method for precise control of poly(N-isopropylacrylamide) microgel particle size.

Triggered drug release from hybrid thermoresponsive nanoparticles using near infrared light

Aim: Developing hybrid poly(N-isopropylacrylamide)-based nanogels decorated with plasmonic hollow gold nanoparticles for on-demand drug delivery and their physico-chemical characterization, bupivacaine loading and release ability upon light irradiation, and in vitro cell viability. Materials & methods: Hollow gold nanoparticles were prepared by galvanic replacement reaction; poly(N-isopropylacrylamide)-based nanogels were synthesized via precipitation polymerization and their electrostatic coupling was accomplished using poly(allylamine hydrochloride) as cationic polyelectrolyte linker. Results & conclusion: Colloidal stability of the resulted hybrid nanovectors was demonstrated under physiological conditions together with their fast response and excellent heating efficiency after light stimulation, indicating their potential use as triggered drug-delivery vectors. Moreover, their influence on cell metabolism and cell cycle under subcytotoxic doses were studied showing excellent cytocompatibility.

Degradable and Thermosensitive Microgels with Tannic Acid as the Sole Cross-Linker

Poly(N-isopropylacrylamide) (PNIPAM)-tannic acid (TA) microgels were successfully prepared via surfactant-free emulsion polymerization (SFEP) at 70 °C in aqueous solution using N-isopropylacrylamide (NIPAM) as the monomer and a natural polyphenol macromolecule, TA, as the sole cross-linker. The cross-linking network of the PNIPAM-TA microgels was confirmed to contain both physical cross-linking structures formed via hydrogen-bonding interactions between TA and PNIPAM chains and chemical cross-linking structures formed via capturing the radicals of propagating polymer chains by catechol and pyrogallol groups of TA. Furthermore, TA was applied to modify the surface of hydrophobic Fe₃O₄ nanoparticles, leading to hydrophilic Fe₃O₄@TA composite nanoparticles, which were successfully used as the cross-linker to fabricate PNIPAM-Fe₃O₄@TA organic-inorganic hybrid microgels. The obtained PNIPAM-TA and PNIPAM-Fe₃O₄@TA organic-inorganic hybrid microgels had a uniform spherical shape with a relatively narrow size distribution and exhibited thermosensitive behavior and pH-tunable degradation. The PNIPAM-TA microgels were stable in the pH range of 1.3-11.1 but underwent complete degradation with pH above 11.4. The PNIPAM-Fe₃O₄@TA hybrid microgels were partially degraded at pH values of 1.3 and 2.1, stable in the pH range of 3.1-11.1, and underwent complete degradation at pH above 11.4. The partial degradation of PNIPAM-Fe₃O₄@TA organic-inorganic hybrid microgels under strong acidic conditions was attributed to the disintegration of Fe₃O₄ nanoparticles. The complete degradation of both microgels at pH above 11.4 was attributed to the hydrolysis of ester groups of TA under strong alkali conditions.

Tuning the Structure and Properties of Ultra-Low Cross-Linked Temperature-Sensitive Microgels at Interfaces via the Adsorption Pathway

The structure of poly(N-isopropylacrylamide) (PNIPAM) microgels adsorbed onto a solid substrate is investigated in the dry and hydrated states by means of atomic force microscopy (AFM). We compare two different systems: a regularly cross-linked microgel containing 5 mol % cross-linker and ultra-low cross-linked microgels (ULC) prepared without a dedicated cross-linker. Furthermore, we compare three different adsorption processes: (i) in situ adsorption from solution, (ii) spin-coating, and (iii) Langmuir-Blodgett deposition from an oil-water interface. The results demonstrate that the morphology and the temperature-induced collapse of microgels adsorbed onto a solid substrate are very different for ultra-low cross-linked microgels as compared to regularly cross-linked microgels, despite the fact that their general behavior in solution is very similar. Furthermore, the morphology of ULC microgels can be controlled by the adsorption pathway onto the substrate. Absorbed ULC microgels are strongly deformed when being prepared either by spin-coating or by Langmuir-Blodgett deposition from an oil-water interface. After rehydration, the ULC microgels cannot collapse as entire objects, instead small globules are formed. Such a strong deformation can be avoided by in situ adsorption onto the substrate. Then, the ULC microgels exhibit half-ellipsoidal shapes with a smooth surface in the collapsed state similar to the more cross-linked microgels. As ULC microgels can be selectively trapped either in a more particle-like or in a more polymer-like behavior, coatings with strongly different topographies and properties can be prepared by one and the same ultra-low cross-linked microgel. This provides new opportunities for the development of smart polymeric coatings.

Hyperspectral infrared laser polarimetry for single-shot phase-amplitude imaging of thin films

We recently presented a novel laser-based infrared (IR) spectroscopic phase-amplitude polarimeter for sub-decisecond and sub-mm² measurements of organic thin films [Opt. Lett.44, 4387 (2019)OPLEDP0146-959210.1364/OL.44.004387]. Here we report on the hyperspectral-imaging capabilities of this device. The single-shot polarimeter employs a broadly tunable mid-IR (1318-1765  cm^-1) quantum cascade laser (QCL) and a four-channel beam-division design for simultaneous phase and amplitude measurements. Fast QCL tuning speeds of up to 1500  cm^-1/s enable hyperspectral mapping of large sample areas (50×50  mm²) within several tens of minutes, achieving 120 μm spatial and <0.5  cm^-1 spectral resolution. We apply the instrument for imaging both the heterogeneous chemical and structural properties of sub-100 nm thin polymer and fatty-acid films. Our polarimeter opens up new applications regarding laterally resolved IR analyses of complex thin films.

Optical Detection of Fe3+ Ions in Aqueous Solution with High Selectivity and Sensitivity by Using Sulfasalazine Functionalized Microgels

A highly selective and sensitive optical sensor was developed to colorimetric detect trace Fe³⁺ ions in aqueous solution. The sensor was the sulfasalazine (SSZ) functionalized microgels (SSZ-MGs), which were fabricated via in-situ quaternization reaction. The obtained SSZ-MGs had hydrodynamic radius of about 259 ± 24 nm with uniform size distribution at 25 °C. The SSZ-MG aqueous suspensions can selectively and sensitively response to Fe³⁺ ions in aqueous solution at 25 °C and pH of 5.6, which can be quantified by UV-visible spectroscopy and also easily distinguished by the naked eye. Job’s plot indicated that the molar binding ratio of SSZ moiety in SSZ-MGs to Fe³⁺ was close to 1:1 with an apparent association constant of 1.72 × 10⁴ M⁻¹. A linear range of 0–12 μM with the detection limit of 0.110 μM (0.006 mg/L) was found. The obtained detection limit was much lower than the maximum allowance level of Fe³⁺ ions in drinking water (0.3 mg/L) regulated by the Environmental Protection Agency (EPA) of the United States. The existence of 19 other species of metal ions, namely, Ag⁺, Li⁺, Na⁺, K⁺, Ca²⁺, Ba²⁺, Cu²⁺, Ni²⁺, Mn²⁺, Pb²⁺, Zn²⁺, Cd²⁺, Co²⁺, Cr³⁺, Yb³⁺, La³⁺, Gd³⁺, Ce³⁺, and Bi³⁺, did not interfere with the detection of Fe³⁺ ions.

Incorporation of short, charged peptide tags affects the temperature responsiveness of positively-charged elastin-like polypeptides

Elastin-like polypeptides (ELPs) are recombinant protein domains exhibiting lower critical solution temperature (LCST) behavior. This LCST behavior is controlled not only by intrinsic factors including amino acid composition and polypeptide chain length but also by non-ELP fusion domains. Here, we report that the presence of a composite non-ELP sequence that includes both His and T7 tags or a short Ser-Lys-Gly-Pro-Gly (SKGPG) sequence can dramatically change the LCST behavior of a positively-charged ELP domain. Both the His and T7 tags have been widely used in recombinant protein design to enable affinity chromatography and serve as epitopes for protein detection. The SKGPG sequence has been used to improve the expression of ELPs. Both the composite tag and the SKGPG sequence are <15% of the total length of the ELP fusion proteins. Despite the small size of the composite tag, its incorporation imparted pH-sensitive LCST behavior to the positively-charged ELP fusion protein. This pH sensitivity was not observed with the incorporation of the SKGPG sequence. The pH sensitivity results from both electrostatic and hydrophobic interactions between the composite tag and the positively-charged ELP domain. The hydrophobicity of the composite tag also alters the ELP interaction with Hofmeister salts by changing the overall hydrophobicity of the fusion protein. Our results suggest that incorporation of short tag sequences should be considered when designing temperature-responsive ELPs and provide insights into utilizing both electrostatic and hydrophobic interactions to design temperature-responsive recombinant proteins as well as synthetic polymers.

pH-Switchable IFT variations and emulsions based on the dynamic noncovalent surfactant/salt assembly at the water/oil interface

Additional HCl can facilely control the dynamic noncovalent interaction between anionic surfactant sodium dodecyl benzene sulfonate (SDBS) and additional organic matter, 4,4'-oxydianiline (ODA), at the water/oil interface. At low HCl concentration (ODA/HCl molar ratio (r) = 1 : 1.5, [ODA] = 250 mg L-1), the ODA+ ions effectively enhanced the SDBS ability to reduce the water/oil interfacial tension (IFT) by about two orders of magnitude, while the (SDBS)2/ODA2+ gemini-like surfactants could be constructed at a relatively high HCl concentration (r = 1 : 4, [ODA] = 250 mg L-1), which could largely reduce the IFT to 1.19 × 10-3 mN m-1. Molecular simulation was employed to explore the interfacial activity of ODAn+ (ODA+/ODA2+) ions and the SDBS/ODAn+ interaction. The control experiments used another three surfactants to verify the proposed model. The pH-switchable gradual protonation of amino groups in ODA molecules determined the SDBS/ODA interfacial assembly, which was responsible for the reversal of IFT variations and the related emulsion behaviors.

Numerical Design and Experimental Realization of a PNIPAM-Based Micro Thermosensor

Stimuli-responsive polymers are capable of responding to external stimuli and therefore have been widely used for sensing. However, such applications are often based on naïve designs and cannot achieve the desired performance. In this study, we created a micro thermosensor with temperature-sensitive poly( N-isopropylacrylamide) (PNIPAM) hydrogel and temperature-insensitive poly(ethylene glycol) diacrylate (PEGDA) hydrogel using stop-flow lithography. The thermosensor is a bihydrogel microparticle consisting of a NIPAM-rich section and a NIPAM-poor section. Since the sensor is similar to a bimetallic strip in structure, its deformation can be easily identified to indicate temperature. To gain better control over the sensor performance, a numerical model capable of predicting the thermal behavior of the sensor was also developed. The model simulated the mass transfer and polymerization reaction during the fabrication process to determine the distributions of PNIPAM and PEGDA in the sensor. The information was then applied to predict the sensor deformation at various temperatures. We have used the model to access the effects of sensor geometry and fabrication temperature on the performance of the sensor. The sensor made under the guidelines from the numerical model has a working range between 16 and 55 °C. Except at very large deformation, the thermal response of the microsensor measured in experiments follows closely the numerical prediction. We believe such a numerical model can also be used for developing other applications involving stimuli-responsive polymers such as shape-evolving microparticles and origami-based microstructures. With the small size, ease of use, low manufacturing cost, good biocompatibility, and broad sensing range near physiological condition, the PNIPAM-based micro thermosensor should have strong potential to be used for bio-related applications and in a confined environment.

Gel-like ionic complexes for antimicrobial, hemostatic and adhesive properties

Ion-specific effects offer a great opportunity to construct intelligent macromolecular systems with diverse architectures, on-demand controlled release behaviors and interfacial responsiveness.

A Time-Efficient Dip Coating Technique for the Deposition of Microgels onto the Optical Fiber Tip

The combination of responsive microgels and Lab-on-Fiber devices represents a valuable technological tool for developing advanced optrodes, especially useful for biomedical applications. Recently, we have reported on a fabrication method, based on the dip coating technique, for creating a microgels monolayer in a controlled fashion onto the fiber tip. In the wake of these results, with a view towards industrial applications, here we carefully analyze, by means of both morphological and optical characterizations, the effect of each fabrication step (fiber dipping, rinsing, and drying) on the microgels film properties. Interestingly, we demonstrate that it is possible to significantly reduce the duration (from 960 min to 31 min) and the complexity of the fabrication procedure, without compromising the quality of the microgels film at all. Repeatability studies are carried out to confirm the validity of the optimized deposition procedure. Moreover, the new procedure is successfully applied to different kinds of substrates (patterned gold and bare optical fiber glass), demonstrating the generality of our findings. Overall, the results presented in this work offer the possibility to improve of a factor ~30 the fabrication throughput of microgels-assisted optical fiber probes, thus enabling their possible exploitation in industrial applications.

Computational investigation of microgels: synthesis and effect of the microstructure on the deswelling behavior

We present computer simulations of a realistic model of microgels. Unlike the regular network frameworks usually assumed in the simulation literature, we model and simulate a realistic and efficient synthesis route, mimicking cross-linking of functionalized chains inside a cavity. This model is inspired, e.g., by microfluidic fabrication of microgels from macromolecular precursors and is different from standard polymerization routes. The assembly of the chains is mediated by a low fraction of interchain crosslinks. The microgels are polydisperse in size and shape but globally spherical objects. In order to deeply understand the microgel structure and eventually improve the synthesis protocol we characterize their conformational properties and deswelling kinetics, and compare them with the results found for microgels obtained via underlying regular (diamond-like) structures. For the same molecular weight, monomer concentration and effective degree of cross-linking, the specific microstructure of the microgel has no significant effect on the locus of the volume phase transition (VPT). However, it strongly affects the deswelling kinetics, as revealed by a consistent analysis of the domain growth during the microgel collapse. Though both the disordered and the regular networks exhibit a similar early growth of the domains, an acceleration is observed in the regular network at the late stage of the collapse. Similar trends are found for the dynamic correlations coupled to the domain growth. As a consequence, the fast late processes for the domain growth and the dynamic correlations in the regular network are compensated, and the dynamic correlations follow a power-law dependence on the growing length scale that is independent of the microgel microstructure.

Highly Selective and Sensitive Detection of Pb2+ in Aqueous Solution Using Tetra(4-pyridyl)porphyrin-Functionalized Thermosensitive Ionic Microgels

Tetra(4-pyridyl)porphyrin (TPyP)-functionalized thermosensitive ionic microgels (TPyP5-MGs) were synthesized by a two-step quaternization method. The obtained TPyP5-MGs have a hydrodynamic radius of about 189 nm with uniform size distribution and exhibit thermosensitive character. The TPyP5-MG microgel suspensions can optically respond to trace Pb²⁺ ions in aqueous solution with high sensitivity and selectivity over the interference of other 19 species of metal ions (Yb³⁺, Gd³⁺, Ce³⁺, La³⁺, Bi³⁺, Ba²⁺, Zn²⁺, Ni²⁺, Co²⁺, Mn²⁺, Cr³⁺, K⁺, Na⁺, Li⁺, Al³⁺, Cu²⁺, Ag⁺, Cd²⁺, and Fe³⁺) by using UV-visible spectroscopy. The sensitivity of TPyP5-MGs toward Pb²⁺ can be further improved by increasing the solution temperature. The limit of detection for TPyP5-MG microgel suspensions in the detection of Pb²⁺ in aqueous solution at 50 °C is about 25.2 nM, which can be further improved to be 5.9 nM by using the method of higher order derivative spectrophotometry and is much lower than the U. S. EPA standard for the safety limit of Pb²⁺ ions in drinking water. It is further demonstrated that the TPyP5-MG microgel suspensions have a potential application in the detection of Pb²⁺ in real world samples, which give consistent results with those obtained by elemental analysis.