PNIPAM Network Research Articles

Carbon nanotube–cesium tungsten oxide–poly(N-isopropylacrylamide) hydrogel-based optical writing display

Studies have attempted to improve thermal conductivity by complexing polymers. Via the complexing of photothermal materials with a heat-sensitive polymer, heat can be generated internally, thus resulting in efficient changes in the properties of the heat-sensitive polymer. This study fabricated an optical writing display to locally induce heat generation in poly(N-isopropylacrylamide) (pNIPAM), which is a heat-responsive material, using the photothermal effect of a near-infrared (NIR) absorbing material. Cesium tungsten oxide (CTO), an NIR-absorbing material, exhibits excellent properties due to its photothermal properties. By mixing carbon nanotubes (CNTs), the photothermal effect caused by NIR rays was strengthened and was more effectively controlled by providing thermal conductivity characteristics. The CNT–CTO hybrid absorbed the NIR laser, converted it into heat, and transmitted it to the adjacent pNIPAM network. When pNIPAM received heat, a hydrophilic–hydrophobic phase transition occurred, and a white pattern was observed owing to a change in the refractive index inside the hydrogel. Upon stopping the application of the NIR laser, rapid cooling occurred due to the high thermal conductivity of the CNTs, and the hydrophobic–hydrophilic phase transition was quickly achieved and restored. Therefore, based on the results of this study, optical writing displays using heat-responsive polymers containing photothermal particles and NIR lasers are expected to be applicable for information transmission in extreme environments, such as aviation, space, and the military, owing to the ability of lasers to travel in a straight line.

UV-Crosslinked Poly(N-isopropylacrylamide) Interpenetrated into Chitosan Structure with Enhancement of Mechanical Properties Implemented as Anti-Fouling Materials.

High-performance properties of interpenetration polymer network (IPN) hydrogels, based on physically crosslinked chitosan (CS) and chemically crosslinked poly(N-isopropylacrylamide) (PNiPAM), were successfully developed. The IPN of CS/PNiPAM is proposed to overcome the limited mechanical properties of the single CS network. In this study, the viscoelastic behaviors of prepared materials in both solution and gel states were extensively examined, considering the UV exposure time and crosslinker concentration as key factors. The effect of these factors on gel formation, hydrogel structures, thermal stabilities of networks, and HeLa cell adhesion were studied sequentially. The sol-gel transition was effectively demonstrated through the scaling law, which agrees well with Winter and Chambon's theory. By subjecting the CS hydrogel to the process operation in an ethanol solution, its properties can be significantly enhanced with increased crosslinker concentration, including the shear modulus, crosslinking degree, gel strength, and thermal stability in its swollen state. The IPN samples exhibit a smooth and dense surface with irregular pores, allowing for much water absorption. The HeLa cells were adhered to and killed using the CS surface cationic charges and then released through hydrolysis by utilizing the hydrophilic/hydrophobic switchable property or thermo-reversible gelation of the PNiPAM network. The results demonstrated that IPN is a highly attractive candidate for anti-fouling materials.

Exceptionally Fast Temperature-Responsive, Mechanically Strong and Extensible Monolithic Non-Porous Hydrogels: Poly(N-isopropylacrylamide) Intercalated with Hydroxypropyl Methylcellulose.

Exceptionally fast temperature-responsive, mechanically strong, tough and extensible monolithic non-porous hydrogels were synthesized. They are based on divinyl-crosslinked poly(N-isopropyl-acrylamide) (PNIPAm) intercalated by hydroxypropyl methylcellulose (HPMC). HPMC was largely extracted after polymerization, thus yielding a 'template-modified' PNIPAm network intercalated with a modest residue of HPMC. High contents of divinyl crosslinker and of HPMC caused a varying degree of micro-phase-separation in some products, but without detriment to mechanical or tensile properties. After extraction of non-fixed HPMC, the micro-phase-separated products combine superior mechanical properties with ultra-fast T-response (in 30 s). Their PNIPAm network was highly regular and extensible (intercalation effect), toughened by hydrogen bonds to HPMC, and interpenetrated by a network of nano-channels (left behind by extracted HPMC), which ensured the water transport rates needed for ultra-fast deswelling. Moreover, the T-response rate could be widely tuned by the degree of heterogeneity during synthesis. The fastest-responsive among our hydrogels could be of practical interest as soft actuators with very good mechanical properties (soft robotics), while the slower ones offer applications in drug delivery systems (as tested on the example of Theophylline), or in related biomedical engineering applications.

Sponge-like, semi-interpenetrating self-sensory hydrogel for smart photothermal-responsive soft actuator with biomimetic self-diagnostic intelligence

Actuators based on hydrogels are good candidates for biomimetic soft robots due to their excellent mechanical properties and flexibility. Despite great efforts in hydrogel-based actuators, their actuation process still differs from the “recognition-judgment-execution” process that conscious intelligent beings go through owing to the lack of perceptive capacity. Herein, to imitate the intelligent actuation process of conscious organisms, a fast photo-thermal responsive and self-sensory soft actuator is proposed based on sponge-like hydrogel with semi-interpenetrating (semi-IPN) polymer network of thermosensitive poly (N-isopropylacrylamide) (PNIPAM) and electronically conductive polyaniline (PANI). The proposed actuator exhibits excellent photo-thermal responsiveness enabling remotely triggered actuation controlled by near infrared light (NIR), covering all azimuths with ultrafast response rate (within 2 s) and recover rate (within 1 s). Meanwhile, the conformational transition of PNIPAM networks during the course of actuation also lead to accompanying migration of the interpenetrated PANI chains and significant variation of local conductivity (>15%), endowing the actuator with self-sensing capacity to monitor its deformations. Eventually, a smart self-adaptive actuator is designed inspired by neuromuscular in intelligent organisms, which can actively perceive external environmental stimuli and consciously regulate its own shape transformation by monitoring and analyzing electrical signal. This sensory actuating hydrogel can provide insights into the development of artificial intelligent soft robotics with higher levels of autonomy and complexity.

Using Excimer Laser for Manufacturing Stimuli Responsive Membranes.

A 248 nm KrF excimer laser can be used to manufacture temperature and pH-responsive polymer-based membranes for controlled transport applications. This is done by a two-step approach. In the first step, well-defined/shaped and orderly pores are created on commercially available polymer films by ablation by using an excimer laser. The same laser is used subsequently for energetic grafting and polymerization of a responsive hydrogel polymer inside the pores fabricated during the first step. Thus, these smart membranes allow controllable solute transport. In this paper, determination of appropriate laser parameters and grafting solution characteristics are illustrated to obtain the desired membrane performance. Fabrication of membranes with 600 nm to 25 μm pore sizes by using the laser through different metal mesh templates is discussed first. Laser fluence and the number of pulses need to be optimized to obtain the desired pore size. Mesh size and film thickness primarily control the pore sizes. Typically, pore size increases with increasing fluence and the number of pulses. Larger pores can be created by using higher fluence at a given laser energy. The vertical cross-section of the pores turns out to be inherently tapered due to the ablative action of the laser beam. The pores created by laser ablation can be grafted with PNIPAM hydrogel by using the same laser to perform a bottom-up grafting-from type pulsed laser polymerization (PLP) in order to achieve the desired transport function controlled by temperature. For this purpose, a set of laser frequencies and pulse numbers need to be determined to obtain the desired hydrogel grafting density and the extent of cross-linking, which ultimately provide controlled transport by smart gating. In other words, on-demand switchable solute release rates can be achieved by controlling the cross-linking level of the microporous PNIPAM network. The PLP process is extremely fast (few seconds) and provides higher water permeability above the lower critical solution temperature (LCST) of the hydrogel. Experiments have shown high mechanical integrity for these pore-filled membranes, which can sustain pressures up to 0.31 MPa. The monomer (NIPAM) and cross-linker (mBAAm) concentrations in the grafting solution need to be optimized in order to control the network growth inside the support membrane pores. The cross-linker concentration typically has a stronger effect on the temperature responsiveness. The pulsed laser polymerization process described can be extended to different unsaturated monomers, which can be polymerized by the free radical process. For example, poly(acrylic acid) can be the grafted to provide pH responsiveness to membranes. As for the effects of thickness, a decreasing trend is observed in the permeability coefficient with increasing thickness. Furthermore, the film thickness has little or no effect on PLP kinetics. The experimental results have shown that membranes manufactured by excimer laser are excellent choices for applications where flow uniformity is the prime requirement, as they possess uniform pore sizes and distribution.

Temperature-Responsive Double-Network Cooling Hydrogels

Skin inflammation can occur due to the immune response of the body, infections, and injuries and lead to an increase of skin temperature. Cooling inflamed skin can relieve pain and efficiently dissipate heat. Evaporative cooling hydrogels are hence promising materials for preparing flexible dressings for cooling inflamed skin because of the high latent heat of water vaporization (∼2450 J·g–1 at 100 °C and 1 atm). We fabricate here a double-network hydrogel formed with poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) and poly(N-isopropylacrylamide) (PNIPAM). Water can be released by the PNIPAM network above ∼32 °C, which is close to human’s skin temperature, and provides heat dissipation and cooling through evaporation. The network formed with PAMPS delays the release of water from the hydrogels. Large and prolonged cooling performance of a skin model and enhanced mechanical properties are obtained with the proposed design. Compared to other reported hydrogels for passive cooling, our material demonstrates a superior cooling power with lower temperature. This design of the hydrogel can be adapted for cooling other structures such as photovoltaic panels or roofs of buildings.

Highly Efficient Switchable Underwater Adhesion in Channeled Hydrogel Networks

AbstractThe ability to switch adhesion strength is a highly desirable property for adhesives applied in a wet environment. The major challenges involve the presence of a water layer between the substrate and adhesive, and the incorporation of efficient switching mechanisms. Despite the recent progresses in devising such systems, there exist several intrinsic limitations in the current strategies, such as high residual adhesion, the use of solid–liquid transition, or thin film configurations. Herein, a channeled poly(N‐isopropylacrylamide) (PNIPAm) hydrogel containing bio‐inspired dopamine‐comonomers is reported, which undergoes temperature‐controlled reversible switching of underwater adhesion on both hydrophilic and hydrophobic surfaces. The introduction of microscopic channels inside the hydrogel, achieved by removing a sacrificial agarose network, greatly facilitates water removal from the interface and thus promotes underwater adhesive strength. On glass, the maximum adhesive stress of the channeled hydrogel can reach six times that of hydrogels without channels. Additionally, high switching efficiency and low residual adhesion can be achieved by the thermal phase transition of the PNIPAm network, also demonstrated by the capture and release of lightweight, irregular, fragile, and biological objects using the hydrogel. The channeling strategy provides implications for designing future underwater adhesive systems for, e.g., soft robotics or biomedical applications.

Fast Transport and Transformation of Biomacromolecular Substances via Thermo-Stimulated Active "Inhalation-Exhalation" Cycles of Hierarchically Structured Smart pNIPAM-DNA Hydrogels.

Although smart hydrogels hold great promise in biosensing and biomedical applications, their response to external stimuli is governed by the passive diffusion-dependent substance transport between hydrogels and environments and within the 3D hydrogel matrices, resulting in slow response to biomacromolecules and limiting their extensive applications. Herein, inspired by the respiration systems of organisms, an active strategy to achieve highly efficient biomolecular substance transport through the thermo-stimulated "inhalation-exhalation" cycles of hydrogel matrices is demonstrated. The cryo-structured poly(N-isopropylacrylamide) (pNIPAM)-DNA hydrogels, composed of functional DNA-tethered pNIPAM networks and free-water-containing macroporous channels, exhibit thermally triggered fast and reversible shrinking/swelling cycles with high-volume changes, which drive the formation of dynamic water stream to accelerate the intake of external substances and expelling of endogenous substances, thus promoting the functional properties of hydrogel systems. Demonstrated by catalytic DNAzyme and CRISPR-Cas12a-incorporating hydrogels, significantly enhanced catalytic efficiency with up to 280% and 390% is achieved, upon the introduction of active "inhalation-exhalation" cycles, respectively. Moreover, remotely near-infrared (NIR)-triggering of "inhalation-exhalation" cycles is achieved after the introduction of NIR-responsive MXene nanosheets into the hydrogel matrix. These hydrogel systems with enhanced substance transport and transformation properties hold promise in the development of more effective biosensing and therapeutic systems.

Thermoresponsive bilayer hydrogel with switchable bending directions as soft actuator

Shaping-deforming thermoresponsive hydrogel actuators have showed great potential toward comprehensive application in artificial intelligence including soft robots and advanced electronics. Despite research progress in designing hydrogels with complexed molecular and hierarchy structures, integrating feasible manufacturing process and switchable shape transformation remained a challenge. Herein, we reported a straightforward strategy to develop a novel actuator by asymmetric composed bilayer hydrogel. Poly(N-isopropylacrylamide) (PNIPAM) and poly(vinyl alcohol) PVA have been applied to fabricate PNIPAM/(PNIPAM/PVA) bilayer hydrogel in which water shrinkage speed could be tuned by PVA crystallinity. Interpenetration of PVA into PNIPAM network promised a faster water losing speed than PNIPAM while a slowed down water diffusion was observed when PVA crystalized during freezing/thawing cycle. Consequently, the hydrogel exhibits two opposite bending behavior under the same temperature-driven process. In addition, the hydrogen bond interaction between PVA and PNIPAM endows the bilayer hydrogel with excellent interfacial adhesion, which prevents delamination after several freezing/thawing cycles and shrinkage/swelling cycles. Approaches in this study points to a future direction in designing and fabricating intelligent materials for several scenarios including soft robotics and biomedical devices. • The novel actuator with switchable bending direction was fabricated through asymmetric composed bilayer hydrogel. • Interfacial adhesion was enhanced by hydrogen bonding between PVA and PNIPM. • Bending speed and direction could be tuned by adjusting PVA crystallinity through freezing/thawing cycles.

Reversible and Remote Thermoregulation of a Recyclable DNAzyme/pNIPAM Microgel Catalyst Formed via a Microfluidic Device

AbstractThe regulation of the catalytic activities of DNAzymes is of great significance for their biosensing and biomedical applications. Herein, thermoregulation of the catalytic activity of DNAzyme is achieved through the immobilization of DNAzymes into the crosslinked polymeric matrix of poly‐N‐isopropylacrylamide (pNIPAM) microgels prepared with a microfluidic device. At a temperature lower than the lower critical solution temperature (LCST) of pNIPAM, the immobilized DNAzymes exhibit lower catalytic performance due to the restricted entry of DNA substrates into the polymeric networks; while at a temperature higher than the LCST, the hydrophilic DNAzyme structures are exposed to exhibit high catalytic activity due to the shrinking of the pNIPAM networks. Results show a more than 60% of increase in catalytic activity by raising the temperature from 29 to 34 °C and reversible regulation of the DNAzyme catalytic activity is achieved by cyclic raising and lowering of the solution temperature. Through a simple heating‐centrifugation process, the DNAzyme/pNIPAM microgels can be recycled for further applications. Moreover, by introducing photothermal MXene nanosheets, the remote control of the catalytic activity of the DNAzyme by near‐infrared light irradiation is achieved. These DNAzyme/pNIPAM microgels as low‐cost, stable, reversible, and recyclable biocatalyst may have potential in biosensing and biomedical applications.

Link between Morphology, Structure, and Interactions of Composite Microgels.

We combine small-angle scattering experiments and simulations to investigate the internal structure and interactions of composite poly(N-isopropylacrylamide)–poly(ethylene glycol) (PNIPAM–PEG) microgels. At low temperatures the experimentally determined form factors and the simulated density profiles indicate a loose internal particle structure with an extended corona that can be modeled as a starlike object. With increasing temperature across the volumetric phase transition, the form factor develops an inflection that, using simulations, is interpreted as arising from a conformation in which PEG chains are incorporated in the interior of the PNIPAM network. This gives rise to a peculiar density profile characterized by two dense, separated regions, at odds with configurations in which the PEG chains reside on the surface of the PNIPAM core. The conformation of the PEG chains also have profound effects on the interparticle interactions: Although chains on the surface reduce the solvophobic attraction typically experienced by PNIPAM particles at high temperatures, PEG chains inside the PNIPAM network shift the onset of attractive interaction at even lower temperatures. Our results show that by tuning the morphology of the composite microgels, we can qualitatively change both their structure and their mutual interactions, opening the way to explore new collective behaviors of these objects.

Thermoresponsive metalloprotein-based hybrid hydrogels for the reversible and highly selective removal of lead(ii) from water

A thermoresponsive hybrid hydrogel PNIPAM-co-PbrRP was synthesized by incorporating peptide PbrRP into the PNIPAM network. By simply changing the temperature across LCST, it can enrich nanomolar Pb(II) from natural water samples with high efficiency.

A model for inhomogeneous large deformation of photo-thermal sensitive hydrogels

The current study develops a 3D constitutive model for photo-thermal sensitive hydrogels based on free energy decomposition. The hydrogel under study is PNIPAM network with copper chlorophyllin nanoparticle agents attached to the network. The effect of light intensity is considered as a rise in temperature since chlorophyllin nanoparticle agents absorb light irradiation and convert it to heat. Moreover, it is necessary to consider the effect of dissociation of these agents on the hydrogel’s free energy function; therefore, a term is added to the free energy function. After introducing the model, some problems, including the free swelling and uniaxial loading problems, are studied, and the obtained results are compared with experimental data to validate the model. The results of the model are in good agreement with experiments, which confirms the validity of the model. Next, to develop a numerical tool to study problems with complicated boundary conditions, the model is implemented in ABAQUS by developing a user-defined UHYPER subroutine, and several practical problems are studied. For example, the deformation of a bilayer made of a sensitive hydrogel attached to a neutral elastomer and the behavior of a self-folding structure is investigated with respect to temperature and light intensity changes. Thereafter, the problem of coexistent phases in a rod due to the light irradiation is investigated. The obtained results confirm the performance of the presented model for use in complicated boundary value problems.

Cellulose nanocrystals reinforced highly stretchable thermal-sensitive hydrogel with ultra-high drug loading

Hydrogels often have poor mechanical properties which limit their application in load-bearing tissues such as muscle and cartilage. In this work, a near-infrared light-triggered stretchable thermal-sensitive hydrogel with ultra-high drug loading was developed by a combination of natural polymeric nanocrystals, a network of synthetic thermo-responsive polymer, and magnetic Fe3O4 nanoparticles. The hydrogels comprise cellulose nanocrystals (CNCs) decorated with Fe3O4 nanoparticles (Fe3O4/CNCs) dispersed homogeneously in poly(N-isopropylacrylamide) (PNIPAm) networks. The composite hydrogels exhibit an extensibility of 2200%. Drug loading of vancomycin (VCM) reached a high value of 10.18 g g−1 due to the dispersion of Fe3O4/CNCs and the interactions between the CNCs and the PNIPAm network. Importantly, the hydrogels demonstrated a thermo-response triggered by NIR, with the temperature increasing from 26 to 41 °C within 60 s. The hydrogels have high biocompatibility evidenced by cell proliferation tests, illustrating that these hydrogels are promising as dressings for wound closure, and wound healing.

Tuning the Thermoresponsive Behavior of Surface-Attached PNIPAM Networks: Varying the Crosslinker Content in SI-ATRP.

The synthesis and thermoresponsive properties of surface-attached poly(N-isopropylacrylamide)-co-N,N′-methylene bisacrylamide (PNIPAM-co-MBAM) networks are investigated. The networks are formed via SI-ARGET-ATRP (“grafting-from”) on thiol-based initiator-functionalized gold films. This method is reliable, well controlled, fast, and applicable to patterned surfaces (e.g., nanopores) for networks with dry thicknesses >20 nm. Surface-attached PNIPAM-co-MBAM gels are swollen below their volume phase transition temperature but above collapse without complete expulsion of water (retain ∼50 vol %). The swelling/collapse transition is studied using complementary SPR and QCMD techniques. The ratio between swollen and collapsed heights characterizes the thermoresponsive behavior and is shown to not depend on network height but to vary with MBAM content. The higher the proportion of the crosslinker, the lower the magnitude of the phase transition, until all responsiveness is lost at 5 mol % MBAM. The temperature range of the transition is broadened for more crosslinked PNIPAM-co-MBAM gels but remains centered around 32 °C. Upon reswelling, less crosslinked networks display sharp transitions, while for those containing ≥3 mol % MBAM, transitions remain broad. This tunable behavior persists for gels on nanostructured gold surfaces. Investigating PNIPAM-co-MBAM networks on gold plasmonic nanowell arrays is a starting point for expanding their scope as thermo-controlled nanoactuators.

Synthesis, characterization and in vitro cytotoxicity studies of poly-N-isopropyl acrylamide gel nanoparticles and films

In this work, we show synthesis that leads to thermoreponsive poly-N-isopropyl acrylamide (pNIPAM) nanogels with sizes below 100 nm, irrespectively of the surfactant to crosslinker ratio. We also show that in many environments the temperature induced pNIPAM collapse at Lower Critical Solution Temperature (LCST) of 32.5 °C is accompanied by gel nanoparticles' aggregation. Thus, the proper information on the nanoparticle (NP) structure and deswelling can be obtained only if the routinely measured hydrodynamic radius is supplemented by information on the molecular weight, which can be obtained from the intensity of scattered light. We measured the dynamics and reversibility of the deswelling and subsequent aggregation processes.Furthermore, we show that the highly concentrated pNIPAM gel NPs reversibly form bulk hydrogel networks of varied interconnected porous structure. We show, that in case of drying pNIPAM gel NPs above the LCST, it is possible to obtain films with 20-fold increase in storage modulus (G′) compared to hydrogel networks measured at room temperature. They exhibit temperature hysteresis behavior around LCST of 32.5 °C similar to pNIPAM films. Finally, we show that these hydrogel films, lead to extended proliferation of cells across three different types: fibroblast, endothelial and cancer cells. Additionally, none of the films exhibited any cytotoxic effects. Our study brings new insights into physicochemical characterization of pNIPAM gel NPs and networks behavior in realistic conditions of in vitro measurements, especially by means of dynamic light scattering as well as final unique properties of both gel NPs and formed porous films for possible tissue engineering applications.

Design Rationale for Stimuli-Responsive, Semi-interpenetrating Polymer Network Hydrogels-A Quantitative Approach.

Stimuli-responsive semi-interpenetrating polymer network (semi-IPN) hydrogels form an important class of polymers for their tunable properties via molecular design. They are widely investigated for a diverse range of applications including drug delivery, sensors, actuators, and osmotic agents. However, in-depth studies on some of the critical design principles affecting diffusion/leaching of linear polymer from semi-IPN hydrogels are lacking. Herein, for the first time, by preparing a series of model semi-IPN hydrogels based on thermally responsive poly (N-isopropyl acrylamide) (PNIPAM) network and linear poly(sodium acrylate) (PSA), a systematic and quantitative study concerning linear polymer chain retention and swelling/deswelling kinetics is reported. The study shows that PSA retention is significantly affected not only by PSA molecular weight and concentration, but also by polymerization temperature, which could be linked to homogeneity and internal morphology of the hydrogel. Surprisingly, there is no obvious influence of crosslinking density of PNIPAM network toward PSA retention, while faster swelling and deswelling at higher crosslinking density are observed in terms of swelling rate constant and deswelling activation energy. These findings offer new insights on the factors affecting structural and physicochemical properties of such semi-IPN hydrogels, which should in turn serve as a general guideline for materials design.

Temperature-Responsive Poly(N-Isopropylacrylamide) Nanogels: The Role of Hollow Cavities and Different Shell Cross-Linking Densities on Doxorubicin Loading and Release.

Smart polymers with extraordinary characteristics are studied in drug-delivery applications. In the current study, temperature-responsive hybrid core-shell nanoparticles were synthesized by precipitation polymerization of N-isopropylacrylamide and vinyl-modified silica nanoparticles. These temperature-responsive hybrid core-shells were prepared with different cross-linking densities by using 2, 4, and 8 mol % of N,N-methylene bisacrylamide (MBA). Hydrolysis of the silica cores of the hybrid core-shells resulted in hollow poly(N-isopropylacrylamide) (PNIPAM) nanogels. Functionalization of silica nanoparticles with vinyl-containing silane modifier of 3-(trimethoxysilyl) propyl methacrylate (MPS) in two different contents was proven by Fourier transform infrared spectroscopy. Preparation of the hybrid PNIPAM nanogels and etching of the silica cores were studied using thermogravimetric analysis and also electron microscopy imaging. Sensitivity of the PNIPAM nanogel samples to temperature was studied using ultraviolet-visible (UV-vis) spectroscopy. In addition, dynamic light scattering was used for investigation of the squeezing and expansion of the hybrid and hollow samples against variation of temperature. The UV-vis spectroscopy results display higher absorption intensities in higher contents of MPS modifier and MBA cross-linker. The swelling content of the nanogels with hollow cavities was higher than that of the hybrid samples. The hybrid nanogels with 2 and 8 wt % silica content and different cross-linking densities and also their hollow nanoparticles were used for loading and release of doxorubicin (DOX). The release characteristics of the DOX-loaded nanogels were studied at different temperatures using UV-vis spectroscopy. The DOX release was higher at temperatures lower than the gel collapse temperature of the PNIPAM network. Although the nanogels with hollow cavities displayed higher loading capacities, the release percentage was higher for the hybrid PNIPAM nanogels, which was confirmed by the experimental release profiles and mathematical models. The most appropriate fitting of the DOX release data from the PNIPAM nanogel samples was observed for the Korsmeyer-Peppas model. Cytotoxicity studies on HeLa cell line showed that drug-loaded hollow samples showed higher toxicity due to loading of a higher amount of DOX.

PNIPAM-based colloidal photonic crystals above phase transition temperature and its application in naked-eye glucose-detection

The colloidal photonic crystals (CPCs) self-assembled by poly (N-isopropylacrylamide)/poly (acrylic acid) (PNIPAM/PAA) interpenetrating nanogels (IPN-BAC) crosslinked with N,N′-Bis(acryloyl)cystamine (BAC) can maintain highly ordered structures and striking structural colors at a pH range of 4.3–9.5, even as the temperature is above its phase transition temperature (PTT) of around 33 °C. However, the CPCs self-assembled with IPN-BIS nanogels crosslinked with N,N′-methylenebisacrylamide (BIS) are quickly disrupted as temperature is increased up to its PTT. Temperature-dependent FT-IR spectral variations and DSC curve clearly illustrate that the IPN-BIS nanogels occur to dramatically collapse while the IPN-BAC nanogels undergo the slight dehydration and a continuous phase transition upon heating, because the COOH groups in the IPN-BAC nanogels are more uniformly dispersed in the PNIPAM network than those in IPN-BIS nanogels. Due to strong water absorption of COO, IPN-BAC nanogels show negligible deswelling behaviors even as temperature is increased above its PTT. The results provide the first demonstrated example of the robust thermo/pH-sensitive photonic crystals at the temperature well above its PTT.

Molecular mechanisms driving the microgels behaviour: A Raman spectroscopy and dynamic light scattering study

Responsive microgels based on poly(N-isopropylacrylamide) (PNIPAM) exhibit peculiar behaviours due to the competition between the hydrophilic and hydrophobic interactions of the constituent networks. The interpenetration of poly-acrylic acid (PAAc), a pH-sensitive polymer, within the PNIPAM network, to form Interpenetrated Polymer Network (IPN) microgels, affects this delicate balance and the typical Volume-Phase Transition (VPT) leading to complex behaviours whose molecular nature is still completely unexplored.Here we investigate the molecular mechanism driving the VPT and its influence on particle aggregation for PNIPAM/PAAc IPN microgels by the combined use of Dynamic Light Scattering and Raman Spectroscopy. Our results highlight that PNIPAM hydrophobicity is enhanced by the interpenetration of PAAc promoting interparticle interactions, a crossover concentration is found above which aggregation phenomena become relevant. Moreover we find that, at variance with PNIPAM, for IPN microgels a double-step molecular mechanisms occurs upon crossing the VPT, the first involving the coil-to-globule transition typical of PNIPAM and the latter associated to PAAc steric hindrance.