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

Tough and elastic hydrogels based on robust hydrophobicity-assisted metal ion coordination for flexible wearable devices

Flexible wearable sensors that combine excellent flexibility, high elasticity, sensing capabilities, and outstanding biocompatibility are gaining increasing attention. In this study, we successfully develop a robust and elastic hydrogel-based flexible wearable sensor by modulating molecular structures combined with metal ion coordination. We leverage three N-acryloyl amino acid monomers, including N-acryloyl glycine (AG), N-acryloyl alanine (AA), and N-acryloyl valine (AV) with different hydrophobic groups adjacent to the carboxyl group, to copolymerize with acrylamide (AM) in the presence of Zr4+ for hydrogel preparation in one step (P(AM3-AG/AA/AV0.06)-Zr0.034+ hydrogels). Our investigation reveals that the P(AM3-AV0.06)-Zr0.034+ hydrogel with the most hydrophobic side group demonstrates superior mechanical properties (1.1 MPa tensile stress, 3566 kJ m-3 toughness and 1.3 kJ m-2 fracture energy) and resilience to multiple tensile (30% strain, 500 cycles) and compression cycling (50% strain, 500 cycles). Moreover, the P(AM3-AV0.06)-Zr0.034+ hydrogel exhibits good biocompatibility and high conductivity (1.1 S m-1) and responsivity (GF = 16.21), and is proved to be suitable as a flexible wearable sensor for comprehensive human activity monitoring.

Colorimetric Sensors Based on Poly(acrylic Acid)/TiO2 Nanocomposite Hydrogels for Monitoring UV Radiation Exposure

In recent years, there has been an open debate on proper sun exposure to reduce the risk of developing skin cancer. The mainly encountered issue is that general guidelines for UV radiation exposure could not be effective for all skin types. The implementation of customized guidelines requires a method by which to measure the UV dose as a result of daily exposure to sunlight, ideally with an inexpensive, easy-to-read sensor. In this work, we present the characterization of nanocomposite hydrogel materials acting as colorimetric sensors upon exposure to UV light. The sensor was prepared using a poly(acrylic acid) (PAA) hydrogel matrix in which TiO2 nanoparticles and methylene blue (MB) were integrated. Raman mapping was used to determine the network structure of the hydrogel and its water distribution. The TiO2 nanoparticles dispersed in the PAA matrix maintain their photoactivity and catalyze a reaction by which methylene blue is converted into leuko-methylene. The conversion causes a discoloration effect that is visible to the naked eye and can therefore be used as an indicator of UV radiation exposure. Moreover, it was possible to tune the discoloration rate to the limit exposure of each skin type, simply by changing the ratio of titanium dioxide to dye. We obtained a response time ranging from 30 min to 1.5 h. Future work will be dedicated to the possibility of scaling up this range and to improve the sensor wearability; however, our study paves the way to the realisation of sensors suitable for public use, which could help us find a solution to the challenge of balancing sufficient UV exposure to prevent Vitamin D deficiency with excessive UV exposure that could ultimately cause skin cancer.

Dual-Stimuli Cooperative Responsive Hydrogel Microactuators Via Two-Photon Lithography

With the development of bionics as well as materials science, intelligent soft actuators have shown promising applications in many fields such as soft robotics, sensing, and remote manipulation. Microfabrication technologies have enabled the reduction of the size of responsive soft actuators to the micron level. However, it is still challenging to construct microscale actuators capable of responding to different external stimuli in complex and diverse conditions. Here, this work demonstrates a dual-stimuli cooperative responsive hydrogel microactuator by asymmetric fabrication via femtosecond laser direct writing. The dual response of the hydrogel microstructure is achieved by employing responsive hydrogel with functional monomer 2-(dimethylamino)ethyl methacrylate. Raman spectra of the hydrogel microstructures suggest that the pH and temperature response of the hydrogel is generated by the changes in tertiary amine groups and hydrogen bonds, respectively. The asymmetric hydrogel microstructures show opposite bending direction when being heated to high temperature or exposed to acid solution, and can independently accomplish the grasp of polystyrene microspheres. Moreover, this work depicts the cooperative response of the hydrogel microactuator to pH and temperature at the same time. The dual-stimuli cooperative responsive hydrogel microactuators will provide a strategy for designing and fabricating controllable microscale actuators with promising applications in microrobotics and microfluidics.

Nanocarbon-Enforced Anisotropic MusCAMLR for Rapid Rescue of Mechanically Damaged Skeletal Muscles

Mechanical damages to skeletal muscles could be detrimental to the active work hours and lifestyle of athletes, mountaineers, and security personnel. In this regard, the slowness of conventional treatment strategies and drug-associated side effects greatly demand the design and development of novel biomaterials, which can rescue such mechanically damaged skeletal muscles. To accomplish this demand, we have developed a musculoresponsive polymer-carbon composite for assisting myotubular regeneration (MusCAMLR). The MusCAMLR is enforced to attain anisotropic muscle-like characteristics while incorporating a smartly passivated nanoscale carbon material in the PNIPAM gel under physiological conditions as a stimulus, which is not achieved by the pristine nanocarbon system. The MusCAMLR establishes a specific mechanical interaction with muscle cells, supports myotube regeneration, maintains excellent mechanical similarity with the myotube, and restores the structural integrity and biochemical parameters of mechanically damaged muscles in a delayed onset muscle soreness (DOMS) rat model within a short period of 72 h. Concisely, this study discloses the potential of smartly passivated nanocarbon in generating an advanced biomaterial system, MusCAMLR, from a regularly used polymeric hydrogel system. This engineered polymer-carbon composite reveals its possible potential to be used as a nondrug therapeutic alternative for rescuing mechanically damaged muscles and probably can be extended for therapy of various other diseases including muscular dystrophy.

Composite Nanoarchitectonics of Photoactivated Titania-Based Materials with Anticancer Properties

The synthesis of titania-based composite materials with anticancer potential under visible-light irradiation is the aim of this study. In specific, titanium dioxide (TiO₂) nanoparticles (NPs) chemically modified with silver were embedded in a stimuli-responsive microgel (a crosslinked interpenetrating network (IP) network that was synthesized by poly (N-Isopropylacrylamide) and linear chains of polyacrylic acid sodium salt, forming composite particles. The ultimate goal of this research, and for our future plans, is to develop a drug-delivery system that uses optical fibers that could efficiently photoactivate NPs, targeting cancer cells. The produced Ag-TiO₂ NPs, the microgel and the composite materials were characterized through X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), micro-Raman spectroscopy, ultraviolet-visible spectroscopy (UV-Vis), dynamic light scattering (DLS) and transmission electron microscopy (TEM). Our results indicated that Ag-TiO₂ NPs were successfully embedded within the thermoresponsive microgel. Either Ag-TiO₂ NPs or the composite materials exhibited high photocatalytic degradation efficiency on the pollutant rhodamine B and significant anticancer potential under visible-light irradiation.

First events in the coil-to-globule transition of PVME in water: An ultrafast temperature jump - time-resolved elastic light scattering study

HYPOTHESIS

The coil-to-globule transition is an essential phenomenon in protein and polymer solutions. Late stages of such transitions, >1 µs, have been thoroughly studied. Yet, the initial ones are a matter of speculations. Here, we present the first observation of a sub-nanosecond stage of the coil-to-globule transition of poly (vinyl methyl ether), PVME, in water.

EXPERIMENTS

The detection of an early stage of the coil-to-globule transition has been possible thanks to a novel experimental approach - time-resolved elastic light scattering study, following an ultrafast temperature jump. We identified a molecular process active in the observed stage of the transition with use of broadband dielectric spectroscopy.

FINDINGS

In the experiment's time window, from a few ps to around 600 ps, we observed an increase in the light scattering intensity 300-400 ps after the temperature jump that heated the sample above its lower critical solution temperature (LCST). The observed time coincides with the time of segmental relaxation of PVME, determined by broadband dielectric spectroscopy in the temperature range of the LCST of the PVME/water mixture. This coincidence strongly suggests that the observed herein stage of coil-to-globule transition is the rapid formation of local nuclei along the polymer chain. Those nuclei may grow and aggregate in later stages of the process, which are out of our experimental time window.

Engineering Tough Metallosupramolecular Hydrogel Films with Kirigami Structures for Compliant Soft Electronics

A simple and effective approach is demonstrated to fabricate tough metallosupramolecular hydrogel films of poly(acrylic acid) by one-pot photopolymerization of the precursor solution in the presence of Zr⁴⁺ ions that form coordination complexes with the carboxyl groups and serve as the physical crosslinks of the matrix. Both as-prepared and equilibrated hydrogel films are transparent, tough, and stable over a wide range of temperature, ionic strength, and pH. The thickness of the films can be easily tailored with minimum value of ≈7 μm. Owing to the fast polymerization and gelation process, kirigami structures can be facilely encoded to the gel films by photolithographic polymerization, affording versatile functions such as additional stretchability and better compliance of the planar films to encapsulate objects with sophisticated geometries that are important for the design of soft electronics. By stencil printing of liquid metal on the hydrogel film with a kirigami structure, the integrated soft electronics shows good compliance to cover curved surfaces and high sensitivity to monitor human motions. Furthermore, this strategy is applied to diverse natural and synthetic macromolecules containing carboxyl groups to develop tough hydrogel films, which will open opportunities for the applications of hydrogel films in biomedical and engineering fields.

Volume fraction determination of microgel composed of interpenetrating polymer networks of PNIPAM and polyacrylic acid

Interpenetrated polymer network microgels, composed of crosslinked networks of poly(N-isopropylacrylamide) and polyacrylic acid (PAAc), have been investigated through rheological measurements at four different amounts of PAAc. Both PAAc content and crosslinking degree modify particle dimensions, mass and softness, thereby strongly affecting the volume fraction and the system viscosity. Here the volume fraction is derived from the flow curves at low concentrations by fitting the zero-shear viscosity with the Einstein-Batchelor equation which provides a parameterkto shift weight concentration to volume fraction. We find that particles with higher PAAc content and crosslinker are characterized by a greater value ofkand therefore by larger volume fractions when compared to softer particles. The packing fractions obtained from rheological measurements are compared with those from static light scattering for two PAAc contents revealing a good agreement. Moreover, the behaviour of the viscosity as a function of packing fraction, at room temperature, has highlighted an Arrhenius dependence for microgels synthesized with low PAAc content and a Vogel-Fulcher-Tammann dependence for the highest investigated PAAc concentration. A comparison with the hard spheres behaviour indicates a steepest increase of the viscosity with decreasing particles softness. Finally, the volume fraction dependence of the viscosity at a fixed PAAc and at two different temperatures, below and above the volume phase transition, shows a quantitative agreement with the structural relaxation time measured through dynamic light scattering indicating that interpenetrated polymer network microgels softness can be tuned with PAAc and temperature and that, depending on particle softness, two different routes are followed.

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.

Glass and Jamming Rheology in Soft Particles Made of PNIPAM and Polyacrylic Acid

The phase behaviour of soft colloids has attracted great attention due to the large variety of new phenomenologies emerging from their ability to pack at very high volume fractions. Here we report rheological measurements on interpenetrated polymer network microgels composed of poly(N-isopropylacrylamide) (PNIPAM) and polyacrylic acid (PAAc) at fixed PAAc content as a function of weight concentration. We found three different rheological regimes characteristic of three different states: a Newtonian shear-thinning fluid, an attractive glass characterized by a yield stress, and a jamming state. We discuss the possible molecular mechanisms driving the formation of these states.

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.

Development of Smart Composites Based on Doped-TiO2 Nanoparticles with Visible Light Anticancer Properties

In this study, the synthesis of smart, polymerically embedded titanium dioxide (TiO2) nanoparticles aimed to exhibit photo-induced anticancer properties under visible light irradiation is investigated. The TiO2 nanoparticles were prepared by utilizing the sol gel method with different dopants, including nitrogen (N-doped), iron (Fe-doped), and nitrogen and iron (Fe,N-doped). The dopants were embedded in an interpenetrating (IP) network microgel synthesized by stimuli responsive poly (N-Isopropylacrylamide-co-polyacrylicacid)-pNipam-co-PAA forming composite particles. All the types of produced particles were characterized by X-ray powder diffraction, micro-Raman, Fourier-transform infrared, X-ray photoelectron, ultra-violet-visible spectroscopy, Field Emission Scanning Electron, Transmission Electron microscopy, and Dynamic Light Scattering techniques. The experimental findings indicate that the doped TiO2 nanoparticles were successfully embedded in the microgel. The N-doped TiO2 nano-powders and composite particles exhibit the best photocatalytic degradation of the pollutant methylene blue under visible light irradiation. Similarly, the highly malignant MDA-MB-231 breast cancer epithelial cells were susceptible to the inhibition of cell proliferation at visible light, especially in the presence of N-doped powders and composites, compared to the non-metastatic MCF-7 cells, which were not affected.