PNIPAAm Chains Research Articles

Temperature-dependent behavior of poly(N-isopropylacrylamide) brushes via neutron reflectometry

Poly(N-isopropylacrylamide) (PNIPAAm)-modified interfaces are promising for various biomedical applications, however, the PNIPAAm brush structure must be investigated at various temperatures to enable the design of functional PNIPAAm interfaces. In this study, PNIPAAm brush structures with various densities and chain lengths are prepared via atom-transfer radical polymerization and analyzed using neutron reflectometry. The volume fraction of PNIPAAm exhibits temperature-dependent shrinkage; slight PNIPAAm shrinkage is observed below the lower critical solution temperature (LCST) of PNIPAAm, whereas large shrinkage occurs across the LCST of PNIPAAm. The volume of water in the PNIPAAm brush is 40% even when it is dehydrated at 37 °C and is slightly extended relative to its dry state. The dense PNIPAAm brush is thicker than the diluted PNIPAAm brush because the densely packed PNIPAAm chain tends to form an extended structure. A higher volume fraction of D2O is observed in the diluted PNIPAAm brush than in the dense PNIPAAm brush because the former can accommodate a larger volume of D2O. These results indicate that neutron reflectometry can provide essential information on the PNIPAAm brush configurations with various chain lengths and densities at different temperatures. These findings are applicable to the design of further functional PNIPAAm brush interfaces for biomedical applications.

Reversibly Adhesive, Anti-Swelling, and Antibacterial Hydrogels for Tooth-Extraction Wound Healing.

Oral wound treatment faces challenges due to the complex oral environment, thus, sealing the wound quickly becomes necessary. Although some materials have achieved adhesion and sterilization, how to effectively solve the contradiction between strong adhesion and on-demand removal remains a challenge. Herein, a reversibly adhesive hydrogel is designed by free radical copolymerization of cationic monomer [2-(acryloyloxy) ethyl] trimethylammonium chloride (ATAC), hydrophobic monomer ethylene glycol phenyl ether acrylate (PEA) and N-isopropylacrylamide (NIPAAm). The cationic quaternary ammonium salts provide electrostatic interactions, the hydrophobic groups provide hydrophobic interactions, and the PNIPAAm chain segments provide hydrogen bonding, leading to strong adhesion. Therefore, the hydrogel obtains an adhesion strength of 18.67KPa to oral mucosa and can seal wounds fast within 10s. Furthermore, unlike pure PNIPAAm, the hydrogel has a lower critical solution temperature of 40.3°C due to the contribution of ATAC and PEA, enabling rapid removal with 40°C water after treatment. In addition, the hydrogel realizes excellent anti-swelling ratio (≈80%) and antibacterial efficiency (over 90%). Animal experiments prove that the hydrogel effectively reduces inflammation infiltration, promotes collagen deposition and vascular regeneration. Thus, hydrogel as a multi-functional dressing has great application prospects in oral wound management.

Temperature and photosensitive PVDF-g-PNIPAAm/GPE-PDA@ZnO composite membranes with efficient dyes separation capability and light-cleaning function

In this paper, PVDF-g-PNIPAAm/GEP-PDA@ZnO composite membranes that can undergo light-induced structural response and photodegradation to achieve self-cleaning have been designed and applied to the separation and purification of dye wastewater. The membranes were prepared by blending graphene (GPE)-polydopamine (PDA)-coated zinc oxide (ZnO) core–shell particles into the PVDF-g-PNIPAAm matrix with a non-solvent induced phase separation technique. The GPE and ZnO are tightly bonded by highly adhesive PDA to form a “strawberry-like” core–shell structure. GPE absorbs light energy and converts it into heat, resulting in changes in PNIPAAm chain segments to expand the pore size for self-cleaning. While the membrane is warming up, GPE further promotes the activation of active groups on the surface of PDA@ZnO at high temperature, resulting in unique and excellent photothermal catalytic activity. As a result, the membrane achieves both self-cleaning and pollutant degradation, which results in an FRR of 99.6% after light cleaning. Moreover, the GPE-PDA@ZnO nanoparticle-modified PVDF-g-PNIPAAm membranes exhibit excellent cycling stability with a degradation flux recovery rate of more than 97% even after multiple UV cleaning. Thus, PVDF-g-PNIPAAm/GPE-PDA@ZnO composite membranes extend the versatility of PVDF-based membranes and show extraordinary potential for application in wastewater treatment.

Temperature and photo sensitive PVDF-g-PNIPAAm/BN@PDA-Ag nanocomposite membranes with superior wasterwater separation and light-cleaning capabilities

Traditional water treatment membranes always face the problem of membrane contamination in the practical application. Those contaminants deposited on the membrane surface and inside the membrane pores make the filtration process more difficult. Herein, the temperature and photo sensitive nanocomposite membranes with superior wasterwater separation and light-cleaning capabilities were prepared by a non-solvent induced phase separation method, which combined the thermo-responsive property of poly(N-isopropylacrylamide) (PNIPAAm) grafted polyvinylidene fluoride (PVDF) and the high photothermal conversion character of core–shell structured BN@PDA-Ag nanosheets. The membranes can be heated to high temperature within a short period of light exposure due to the fast photothermal conversion capability of Ag and the excellent thermal conductivity of boron nitride (BN). The increase in temperature led to the contraction of PNIPAAm chains and bigger pore size. The microstructure, filtration, and cleaning performance of the nanocomposite membranes can be controlled by illumination, which allowed PVDF-g-PNIPAAm/BN@PDA-Ag membranes to achieve the effect of first interception and then cleaning. The membranes showed retention rates of 82, 99 and 95 % for BSA, oil–water emulsions and various dyes with flux recovery rates of > 90 % after light cleaning. This work provides a simpler, more efficient membrane cleaning strategy with superior potential for dealing with membrane contamination.

Nano-Superfine Polymer Fibers by Electrospinning and Their Thermal-Switching Function

Intelligent or smart polymer materials are those which are either “active” or responsive to a variety of environmental stimuli, such as changes in pH, temperature, ionic strength or electric field, or the presence of a specific chemical substrate. A thermally sensitive polymer, poly(N-isopropylacrylamide) (PNIPAAm), has been of great interest because aqueous solutions of PNIPAAm undergo fast, reversible switching of hydrophilicity/hydrophobicity around 32 °C at their lower critical solution temperature (LCST) of about 29–38 °C (different with variations in their structure). PNIPAAm chains show an expanded conformation in water below the LCST because of strong hydration and change to the compact forms above the LCST by sudden dehydration; this is the reversible switching effect of hydrophilic to hydrophobic. These switchable materials may have wide applications in functional textiles, controllable drug release and special fibers. Electrospinning is a simple, convenient, and versatile technique for generating fibers with diameters that range from several micrometers to tens of nanometers. Herein, we report an electrospinning method to prepare nano PNIPAAm fiber and their thermal response was characterized. Based on the results, the diameters of the PNIPAAm nanofibers within the respective system were around 200 nm and beads-less; they were successfully prepared by an electrospinning method in all cases and the reversible switching with LCST (32 °C) feature was observed clearly in the fibers.

Rhodamine-containing double-network hydrogels for smart window materials with tunable light transmittance, low-temperature warning, and deformation sensing

Smart window materials have attracted much attention because of their energy savings ability, light passage control in response to external stimuli, but most studies focus on the tunability of a single property, specifically, optical transmittance. Here, we report a rhodamine (Rh) mechanophore-functionalized PNIPAAm-based double-network (DN) hydrogel with the potential to act as multifunctional smart window materials that have the combination of a transmittance regulation ability, freezing−/compression-induced mechanochromic and mechanofluorescent properties. Benefiting from naturally reversible hydrophilic/hydrophobic phase transitions of PNIPAAm chains, DN hydrogels exhibit an excellent temperature-induced optical transmittance change between transparency and opaqueness. Because of the covalent incorporation of Rh mechanophores into the first network within DN hydrogels, the gels can display visualizable mechanochromism or mechanofluorescence upon freezing or compression. This is attributed to the generation of the colored and fluorescent ring-opened Rh spirolactams via mechanoactivation. This work is the first example of thermochromic smart window materials with a visualized low-temperature warning and damage sensing abilities and provides a new strategy to fabricate multifunctional smart window materials.

Thermoresponsive Nanostructures: From Mechano-Bactericidal Action to Bacteria Release.

Overuse of antibiotics can increase the risk of notorious antibiotic resistance in bacteria, which has become a growing public health concern worldwide. Featured with the merit of mechanical rupture of bacterial cells, the bioinspired nanopillars are promising alternatives to antibiotics for combating bacterial infections while avoiding antibacterial resistance. However, the resident dead bacterial cells on nanopillars may greatly impair their bactericidal capability and ultimately impede their translational potential toward long-term applications. Here, we show that the functions of bactericidal nanopillars can be significantly broadened by developing a hybrid thermoresponsive polymer@nanopillar-structured surface, which retains all of the attributes of pristine nanopillars and adds one more: releasing dead bacteria. We fabricate this surface through coaxially decorating mechano-bactericidal ZnO nanopillars with thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm) brushes. Combining the benefits of ZnO nanopillars and PNIPAAm chains, the antibacterial performances can be controllably regulated between ultrarobust mechano-bactericidal action (∼99%) and remarkable bacteria-releasing efficiency (∼98%). Notably, both the mechanical sterilization against the live bacteria and the controllable release for the pinned dead bacteria solely stem from physical actions, stimulating the exploration of intelligent structure-based bactericidal surfaces with persistent antibacterial properties without the risk of triggering drug resistance.

A highly selective ATP-responsive biomimetic nanochannel based on smart copolymer

ATP-sensitive potassium (KATP) channels couple intracellular metabolism to the electrical activity by regulating K+ flux across the plasma membrane, thus playing an important role in both normal and pathophysiology. To understand the mechanism of ATP regulating biological ion channels, developing an ATP-responsive artificial nanochannel is an appealing but challenging topic because KATP channel is a heteromultimer of two subunits (potassium channel subunit (Kir6.x) and sulfonylurea receptor (SUR)) and exhibit dynamic functions with adjustability and reversibility. Inspired by the structure of KATP channels, we designed a smart copolymer modified nanochannel that may address the challenge. In the tricomponent poly(N-isopropylacrylamide) (PNIPAAm, PNI)-based copolymer system, phenylthiourea was used to bind the phosphate units of nucleotides and phenylboronic acid was introduced to combine the pentose ring of the nucleoside unit. Besides, a –COOH group with electron-withdrawing property was added into the phenylthiourea units, which may promote the hydrogen-bond-donating ability of thiourea. Specially, the smart copolymer not only provided static binding sites for recognition but also translated the recognition of ATP into their dynamic conformational transitions by changing the hydrogen-bonding environments surrounding PNIPAAm chains, thus achieving the gating function of nanochannel, which resembled the integration and coordination of Kir6.x and SUR units in active KATP. The ATP-regulated ion channel exhibited excellent stability and reversibility. This study is the first example showing how to learn from nature to assemble the ATP-responsive artificial nanochannel and demonstrate the possible mechanism of ATP gating.

Effect of concentration and ionic strength on the lower critical solution temperature of poly(N-isopropylacrylamide) investigated by small-angle X-ray scattering

ABSTRACT Thermoresponsive polymers, with special emphasis on poly(N-isopropylacrylamide) (PNIPAAM) have been the focus of several investigations due to its potential applications in many fields of physical and polymer chemistry. PNIPAAM has a “lower critical solution temperature” at 32°C. The LCST can be finely tuned by copolymerization with hydrophobic or hydrophilic comonomers, and with a change of physical chemical parameters, such as ionic strength of the solution. We investigated the effect of polymer concentration on the LCST in two different solution environment, in pure water and in 50 mM NaCl solution with small-angle X-ray scattering (SAXS) of relatively low molecular mass polymers. We showed that the radius of gyration of the pNIPAAM chains increases with the addition of NaCl to the system due to the low level of specific adsorption of chloride ions on the polymer, whilst increasing the temperature causes a shrinkage of the polymer chains. Furthermore, by increasing the temperature the attractive interaction between the individual polymer chains is enhanced which turns into aggregation at the LCST.

A temperature-responsive composite for adaptive microwave absorption

Smart microwave absorption (MA) materials with responsiveness to environment are highly desired in modern communication systems, while designing and fabricating high-performance microwave absorbers with adaptivity to working in dynamic service environment is still a challenge. Here we report a temperature-responsive composite composing covalently bonded poly(N-isopropylacrylamide) (PNIPAAm) and graphene aerogel (GA)for adaptive MA. The dynamic change of hydrogen bonds actuate the conformation change of PNIPAAm chains when the temperature varies below and above the critical transition temperature of 34.8℃, which drives the reduced graphene oxide sheets in GA to delaminate and restacking simultaneously. As a result, the electrical, dielectric and thermal properties of the composite become adaptively switchable. More important, the composite is capable of achieving an effective MA within quite different frequency bands under stimuli of temperature changes. Typically, at 20℃, the effective absorbing band (below −10 dB) ranges from 8.5 to 17.5 GHz, and the minimum reflection loss (RLmin) reaches −32 dB, while when the temperature increases to 50℃, the effective absorbing band shifts to 4.5 ~ 14.0 GHz with the same RLmin. This temperature-responsive composite has great potential of being used as smart devices, especially the advanced electromagnetic protection equipment in the modern communication systems.

Life-Inspired Endogenous Dynamic Behavior of Lipid Droplet-like Microcompartments in Artificial Adipocyte-like Structures

Chemical systems that can replicate cellular behaviors are gaining increasing attention and are being used to study various biological processes. Here, a protein- or amylose-based assembly at an oi...

Thermo- and pH-sensitive glycosaminoglycans derivatives obtained by controlled grafting of poly(N-isopropylacrylamide)

Poly(N-isopropyl acrylamide) grafted heparin and chondroitin sulfate were synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization. The copolymers were characterized by NMR, IR, SEC, DLS, SLS and NTA methods. High grafting densities were reached for both glycosaminoglycans. The temperature, pH and polymer concentration affected the low critical solution temperatures values. The increased pNIPAAm chain length, grafting density and concentration led to the sharp phase transition at 35 °C. Spherical nanogels were formed around this temperature. Terminal dodecyl trithiocarbonate groups of the copolymers were reduced to thiols that allowed formation of sensitive nanogels with sharp phase transitions induced by pNIPAAm chains. The copolymers showed no toxicity to the ocular cells and they provided the prolonged release of dexamethasone phosphate at 37 °C. These copolymers are interesting alternatives for ocular drug delivery.

Soft lithographic fabrication of free-standing ceramic microcomponents using poly(N-isopropylacrylamide) brushes grafted poly(dimethylsiloxane) micromolds

In order to fabricate intact and precise ceramic microcomponents applied in microelectromechanical systems (MEMS), thermo-responsive poly(N-isopropylacrylamide) (PNIPAAm) brushes are grafted onto poly(dimethylsiloxane) (PDMS) micromolds by visible light-induced polymerization. The application of PDMS-Br@PNIPAAm micromolds in soft lithographic fabrication of ceramic microcomponents is demonstrated by producing alumina green microcomponents. The PDMS-Br@PNIPAAm micromolds are hydrophobic with a water contact angle of 129.7 ± 0.3° at 15 °C. During the molding process, the micromolds are changed to hydrophilic with a water contact angle of 81.3 ± 0.4° at 4 °C, which could improve the flowability of aqueous alumina slurry by forming intermolecular hydrogen bonding with PNIPAAm chains. During the demolding process, the micromolds are hydrophobic as the intermolecular hydrogen bonding is broken after drying. Consequently, free-standing alumina green microcomponents with microchannels of 73.7 ± 0.8 μm wide are fabricated by microtransfer molding, a maximum aspect ratio of 3.6 could be achieved. The lateral and longitudinal shrinkages of the microchannels are 9.8 ± 0.6% and 11.5 ± 0.4% respectively. The PDMS-Br@PNIPAAm micromolds can be reused more than 13 times. It is believed that moisture-sensitive PDMS-Br@PNIPAAm micromolds can be effectively used to fabricate ceramic microcomponents with high aspect ratios for MEMS.

Photo- and Thermosensitive Polymer Membrane with a Tunable Microstructure Doped with Graphene Oxide Nanosheets and Poly(N-isopropylacrylamide) for the Application of Light-Cleaning.

Traditional polymer membranes exhibit a constant structure that makes adjustment of the filtration process difficult, such as flux changing and contaminant cleaning. Inspired by the automatically closing behavior of leaf stomata under strong light, we prepared a membrane with thermo- and photosensitivities, whose microstructure, as well as filtration properties, could be controlled by adjusting the light condition. The membrane was fabricated by the immersion phase inversion method with a casting solution of polyvinylidene fluoride-g-poly(N-isopropylacrylamide) (PVDF-g-PNIPAAm) and graphene oxide (GO) nanosheets. Additionally, the membrane could be heated to a high temperature in a short time under illumination, causing shrinkage of its PNIPAAm chains and expansion of its membrane pores. On the basis of the reversible photoinduced structural transformation, the membrane exhibited a high water gating ratio under the switching of light on/off. Moreover, we proposed a novel and simple method to clear the contaminant from the pores of the membrane via light, which we named "light-cleaning". Light-cleaning had a flux recovery rate of 99.2%, substantially higher than that of back-washing (62%). This work not only extends the controllability and functionality of the polymer membrane but also develops a new membrane cleaning system.

Thermally Driven Interfacial Switch between Adhesion and Antiadhesion on Gas Bubbles in Aqueous Media.

It is greatly important to understand the gas bubble behaviors and realize their reliable manipulation. There still remain many challenges in the capture, transport, and release of gas bubbles at a preferred location intelligently. Herein, we provide a simple approach to manufacture a composite film with poly(N-isopropylacrylamide) (PNIPAAM) and polypropylene that exhibits smart, reversible, and reliable regulation of gas bubble adhesive behaviors (high and low adhesion) by controlling the temperature (above or below the lower critical solution temperature). By adjusting the composite surface temperature, thermally driven interface switching between intermolecular and intramolecular hydrogen bonding of the PNIPAAM chains resulted in low and high adhesion of air bubbles in an aqueous medium. Gas bubbles could be precisely captured, directionally transported, and precisely released at any preferred location.

Characterization of novel thermoresponsive poly(butylene adipate-co-terephthalate)/poly(N-isopropylacrylamide) electrospun fibers

In this work, electrospun fibers of poly(butylene adipate-co-terephthalate) (PBAT) and poly(N-isopropylacrylamide) (PNIPAAm) blends, PBAT/PNIPAAm, with different mass ratios, were obtained. For characterization of their morphological, structural, thermal and wettability properties, scanning electron microscopy (SEM), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), thermogravimetry (TG), differential scanning calorimetry (DSC) and drop water contact angle (DWCA) measurements were carried out. The optimum conditions determined for electrospinning were: voltage of 25 kV, flow rate of 1.3 mL·h−1 and working distance of 15 cm. The most important result was achieving PBAT/PNIPAAm electrospun fibers with a thermoresponsive behavior and a sudden response to temperature at PNIPAAm’s LCST (lower critical solution temperature). From SEM analysis, the higher the content of PNIPAAm in the blend, the higher the average diameter of the electrospun fibers was; also, the higher content of PNIPAAm in the blend allowed the fibers to be rounded with no presence of beads. From ATR-FTIR analysis, no formation of strong intermolecular interactions, such as hydrogen bonding, between the PBAT and PNIPAAm chains was observed. From TG and DSC analyses, the thermal stability of the PBAT/PNIPAAm fibers remained unchanged in respect to the electrospinning. DWCA measurements for PBAT/PNIPAAm 70/30 and 50/50 fibers, in response to temperature, indicated a gap of 50° to 60° at PNIPAAm’s LCST, evidencing their high thermosensitivity. With these results, PBAT/PNIPAAm electrospun fibers with 30% or more in mass ratio of PNIPAAm might find a potential application for the cell adhesion/detachment field.

Bulk poly(N-isopropylacrylamide) (PNIPAAm) thermoresponsive cell culture platform: toward a new horizon in cell sheet engineering.

Although a conventional method of utilizing thermoresponsive grafted poly(N-isopropylacrylamide) (PNIPAAm) enables the harvest of a healthy confluent cell-to-cell junction preserved cell sheet while limiting the use of the trypsin enzyme, the absolute necessity in the delicate control of a sensitive nm-scale PNIPAAm chain length inevitably decelerates the advancement of cell sheet engineering. In this study, we demonstrate, for the first time, a thermoresponsive cell culture platform composed only of a 'bulk' form of a PNIPAAm hydrogel with the Young's modulus being increased up to the MPa scale. The surface roughness of the bulk PNIPAAm hydrogel initially modulated by the cross-linker concentration was altered from the nm- to μm-scale in response to a change in temperature above/below the low critical solution temperature (LCST) of 32 °C. The appropriate control of the surface roughness allowed the stable attachment (above the LCST) and easy detachment (below the LCST) of diverse cells and enabled the harvest of cell sheets composed of cell lines (C2C12 and NIH3T3) or even primary cells (human umbilical vein endothelial cells and keratinocytes). During their incubation at 37 °C, the cell lines were able to be attached on every surface of the prepared PNIPAAm cell culture platforms, whereas the primary cells were found to be only attached on a surface having a roughness below ∼30 nm. Furthermore, in the aspect of cell sheet detachment at the incubation temperature of 20 °C, the cell sheets composed of cell lines were fully detached from the surface of the platform having a roughness of ∼10 μm or higher, while the cell sheets composed of primary cells were entirely detached from the surface with a roughness of ∼19 μm or higher. Based on such behaviors of the diverse cells at a given surface roughness, this study further suggests a universal thermoresponsive cell culture platform which allows the harvest of all types of cells from cell lines to primary cells in a desired shape. Our suggested universal cell culture platform could play a powerful and versatile role in accelerating the advancement of cell sheet engineering.

Facile Fabrication of Hierarchically Thermoresponsive Binary Polymer Pattern for Controlled Cell Adhesion.

A versatile platform allowing capture and detection of normal and dysfunctional cells on the same patterned surface is important for accessing the cellular mechanism, developing diagnostic assays, and implementing therapy. Here, an original and effective method for fabricating binary polymer brushes pattern is developed for controlled cell adhesion. The binary polymer brushes pattern, composed of poly(N-isopropylacrylamide) (PNIPAAm) and poly[poly(ethylene glycol) methyl ether methacrylate] (POEGMA) chains, is simply obtained via a combination of surface-initiated photopolymerization and surface-activated free radical polymerization. This method is unique in that it does not utilize any protecting groups or procedures of backfilling with immobilized initiator. It is demonstrated that the precise and well-defined binary polymer patterns with high resolution are fabricated using this facile method. PNIPAAm chains capture and release cells by thermoresponsiveness, while POEGMA chains possess high capability to capture dysfunctional cells specifically, inducing a switch of normal red blood cells (RBCs) arrays to hemolytic RBCs arrays on the pattern with temperature. This novel platform composed of binary polymer brush pattern is smart and versatile, which opens up pathways to potential applications as microsensors, biochips, and bioassays.

Microfluidic formation of proteinosomes.

Herein we describe a novel microfluidic method for the generation of proteinosome micro-droplets, based on bovine serum albumin and glucose oxidase conjugated to PNIPAAm chains. The size of such water-in-oil droplets is regulated via control of the input reagent flow rate, with generated proteinosome populations exhibiting narrower size distributions than those observed when using standard bulk methodologies. Importantly, proteinosomes transferred from an oil to an aqueous-environment remain intact, become fully hydrated and exhibit an increase in average size. Moreover, functional proteinosomes prepared via microfluidics exhibit lower Km values and higher enzymatic activities than proteinosomes produced by bulk methodologies.

Clarification of the inner microenvironments in poly(N-isopropylacrylamide) hydrogels in macrogel and microgel forms using a fluorescent probe technique

Temperature dependence of cross-linked poly(N-isopropylacrylamide) (PNIPAAm) gels in particle microgel and macrogel forms with different densities of crosslinking was studied by fluorescence measurements of 8-anilino-1-naphthalene sulfonic acid (ANS) doped into the hydrogels. The experiments for each sample were carried out for some weeks, by making sure that equilibrium was established at each temperature. The fluorescence intensity ratio of ANS at 460 and 540 nm was found to be useful as a measure of the polarity of the environment around ANS molecules in a gel. The results indicate that the environment of PNIPAAm chains in macrogel and microgel forms of different crosslinking densities are the same. Fluorescence depolarization of ANS first clarified that, after volume phase transition, the collapsed state of PNIPAAm gels is unlike a polymer film, but has free spaces where ANS can rotate freely because of the water molecules escaping from the PNIPAAm chains. The information of these free spaces produced by desorption of water molecules would be useful in the processing and utilization of PNIPAAm particle gels.