Advances in smart materials: Stimuli-responsive hydrogel thin films

This review highlights recent developments in the field of stimuli-responsive hydrogels, focusing primarily on thin films, with a thickness range between 100 nm to 10 μm. The theory and dynamics of hydrogel swelling is reviewed, followed by specific applications. Gels are classified based on the active stimulus—mechanical, chemical, pH, heat, and light—and fabrication methods, design constraints, and novel stimuli-responses are discussed. Often, these materials display large physiochemical reactions to a relatively small stimulus. Noteworthy materials larger than 10 μm, but with response times on the order of seconds to minutes are also discussed. Hydrogels have the potential to advance the fields of medicine and polymer science as useful substrates for “smart” devices. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1084–1099     

Stimuli-responsive properties of N-isopropylacrylamide-based ultrathin hydrogel films prepared by photo-cross-linking

To develop stimuli-responsive ultrathin polymer films on a solid substrate, a novel photo-cross-linkable polymer with both temperature- and pH-responsive properties was prepared. The photoreactive 4-aminobenzophenone (BP) was introduced onto the side groups of poly(N-isopropylaclylamide-co-2-carboxyisopropylaclylamide) [poly(NIPAAm-co-CIPAAm)]. This copolymer was designed for highly random sequences of comonomers. After the formation of spin-coated polymer films on a solid substrate, UV-light irradiation started the cross-linking reaction. The spin-coating processes and stability of the polymer films were quantitatively monitored by a quartz crystal microbalance (QCM), and the thickness was estimated using an atomic force microscope (AFM). These measurements confirmed the formation of a very plain polymer film, and the film thickness was precisely controlled by the concentration of the polymer solution used for spin coating. Moreover, the obtained films showed a high stability due to the cross-liking reaction and UV irradiation. Cyclic voltammetry using potassium ferricyanide revealed that the ions could permeate the photo-cross-linked ultrathin polymer films. The permeability of the ultrathin hydrogel films was dramatically changed by varying the pH and temperature of the aqueous media. These observations suggest that the preparation of isopropylacrylamide-based stimuli-responsive ultrathin hydrogel films is possible.     

Composition and structure of platinum-containing thin composite films prepared from silica sols

Chemical processes that can occur during aging of sols based on water–alcohol solutions of tetraethoxysilane (TEOS) and hexachloroplatinic acid and during formation of films from these sols are analyzed by the results of visible and UV spectroscopy and X-ray powder diffraction. The sizes and compositions of the platinum particles that are formed in xerogels and in thin films on the nanometer and submicron levels are estimated based on X-ray crystallography, energy dispersive spectral analysis, high-resolution scanning and transmission electron microscopy, and grazing incidence small-angle X-ray scattering studies.     

SnO2 thin films prepared by dip-coating from microwave synthesized colloidal suspensions

Tin(IV) oxide thin films have been prepared by dip-coating. The suspensions used for these depositions have been synthesized by microwave-induced thermohydrolysis of tin tetrachloride aqueous solutions in the presence of hydrochloric acid. Single or multiple depositions were tested, on glass substrates as well as on pure SiO2. The obtained thin films were characterized by optical microscopy, interferometric roughness measurements (Micromap), scanning electron microscopy, secondary ion mass spectroscopy, and scanning tunneling microscopy.     

Proton conductive thin films prepared by plasma polymerization

Proton conductive membranes were prepared as thin films of about 10 μm thickness by an ion beam assisted plasma polymerization process. Argon ions were generated in a high frequency plasma and accelerated towards a PTFE target where CF fragments were released as a consequence of the ion impact. Various sulfur components (SO₂, CF₃SO₃H or ClSO₃H) were added to achieve proton conductivity by the formation of sulfonic acid groups. The CF fragments combined with the sulfur components to form a coherent thin film on a substrate. Mass spectrometric investigations revealed, however, that sulfur oxygen compounds were extremely delicate towards reduction to sulfur carbon compounds like CS₂ or SCF₂. The best membrane conductivities (>10⁻⁴ S/cm) and highest ion exchange capacities (0.15 mmol/g) were achieved with chlorosulfonic acid involved in the plasma polymerization process. Ultra-thin layers of these of these plasma polymers (ca. 300 nm) were subsequently deposited onto Nafion^® membranes in order to suppress methanol permeation for a potential application in a direct methanol fuel cell (DMFC). The ratio of proton conductivity and methanol diffusion coefficient was employed for an assessment of the transport characteristics of the coated membrane. Diffusion coefficients were determined in a flow cell coupled to a mass spectrometer. The plasma polymer coating decreased both the methanol permeation and the proton conductivity. With a proton conductive plasma polymer coating the decrease of methanol diffusion could outweigh the loss of proton conductivity. Plasma coating offers a way to suppress methanol crossover in DMFCs and to maintaining the proton conductivity.     

自组装成膜法制TiO2薄膜

采用有机分子自组装(Self-Assembly)成膜技术将硅烷偶联剂[(CH3O)3Si(CH2)3SH]组装在普通的玻片表面, 得到二维有的单层有机膜, 并将膜端基(SH)原位氧化为磺酸基(SO3H)。利用该功能基(SO3H)的吸附性, 从四氯化钛的水溶液中淀积制得了TiO2薄膜。X射线光电子能谱(XPS)和原子力显微镜(AFM)等研究显示, TiO2薄膜是均匀和连续的, 具有良好的透明性。     

Thermo-reversible swelling of thin hydrogel films immobilized by low-pressure plasma

Thin films of graft copolymers consisting of poly(N-isopropylacrylamide) (PNiPAAm) or poly(N,N-diethylacrylamide) (PDEAAm) as polymer backbone and poly(ethyleneglycol) as side chains were cross-linked on fluoropolymer substrates by low-pressure plasma treatment. All immobilized polymers exhibit a lower critical solution temperature between 34 and 40 degrees C. The swelling and collapsing of the hydrogels was examined with temperature-dependent spectroscopic ellipsometry. Two time ranges of swelling were observed: a fast 'dynamic' and a slow 'equilibrium' swelling. The dynamic swelling occurs within minutes or less, whereas the equilibrium swelling needs several days to complete. The surface-bound hydrogels show a shift in the transition temperature toward lower temperatures compared with the behavior in solution. Full reversibility of the dynamic swelling/collapsing was found, but the temperature scan exhibits a hysteresis between heating and cooling cycles. The PNiPAAm-containing hydrogels show a sharper transition compared to the PDEAAm-containing hydrogels, which is almost linear over a wide temperature range.     

Characteristics of SnO2 thin films prepared by SILAR

SnO₂ thin films were deposited on glass substrates by using Successive Ionic Layer Adsorption and Reaction (SILAR) method at room temperature. The film thickness effect on characteristic parameters such as structural, morphological, optical and electrical properties of the films was studied. Also, the films were annealed in oxygen atmosphere (400 °C, 30 min) and characteristic parameters of the films were investigated. The X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) studies showed that all the films exhibited polycrystalline nature with tetragonal structure and were covered well on glass substrates. After the investigation of the crystalline and surface properties of the films, it was found that they were improving with increasing film thickness. Optical band gap decreased from 3.90 eV to 3.54 eV and electrical conductivity changed between 0.015–0.815 (Ω-cm)⁻¹ as the film thickness increased from 215 to 490 nm. The refractive index (n), optical static and high frequency dielectric constants (ɛ_o, ɛ_∞) values were calculated by using the optical band gap values as a function of the film thickness.     

Ordered micro/mesoporous composite prepared as thin films

A new synthesis method for preparation of thin films and powders consisting of zeolite beta nanocrystals embedded in ordered mesoporous silica matrix is described. The final structures possessing bimodal porosity, i.e., high degree of mesophase order and spatially defined microporous zeolite nanocrystals are obtained via simultaneous solvent evaporation of preformed silica/surfactant/ethanol/nanosized zeolite beta assemblies. The films were characterized with grazing-incident diffraction (GID), nitrogen sorption based on gravimetric measurements with quartz crystal microbalance (QCM) devices, and transmission electron microscopy (TEM). It is shown that the incorporation of beta nanocrystals in the mesoporous silica matrix and the mesophase order itself can be controlled through the variation of the fractional amounts of the zeolite nanoparticles and silica/surfactant solutions. The HR-TEM measurements showed that the nanosized Beta microporous crystals are separated and at the same time connected through an ordered mesostructured matrix.     

Core-shell type polymer microspheres prepared by domain fixing of block copolymer films

Well-defined poly[styrene(S)-b-isoprene(I)] diblock copolymers were prepared by sequential anionic addition. The crosslinking reactions of polyisoprene (PI) spherical domains of these block copolymers were carried out in a n-hexane solution of sulfur monochloride (S₂Cl₂). Electron micrograph of crosslinked products indicates the structure of core-shell type polymer microspheres. It is found from NMR and turbi+imetric measurements that the solubility of core-shell type polymer microspheres depends strongly on that of block chains comprising the shell portion. The particle size of these microspheres shows a narrow distribution.     

Fabrication and properties of thermosensitive organic/inorganic hybrid hydrogel thin films

We report on a facile method for fabricating thermosensitive organic/inorganic hybrid hydrogel thin films from a cross-linkable organic/inorganic hydrid copolymer, poly[ N-isopropylacrylamide- co-3-(trimethoxysilyl)propylmethacrylate] [P(NIPAm- co-TMSPMA)]. Fourier transform infrared (FT-IR) spectra confirmed the formation of hybrid hydrogel thin films after hydrolysis of the methoxysilyl groups (Si-O-CH 3) and subsequent condensation of the silanol groups (Si-OH). Atomic force microscopy (AFM) images revealed that the surface morphology of the hydrogel thin films depended on the supporting substrates. Microdomains were observed for the hydrogel thin films on a gold surface, which can be attributed to inhomogeneous network structures. The thermoresponsive swelling-deswelling behavior and the viscoelastic properties of the hydrogel thin films were investigated as a function of temperature (25-45 degrees C) by using a quartz crystal microbalance (QCM) operated in water. The high frequency shear modulus of the P(NIPAm- co-TMPSMA) hydrogel thin films was several hundred kilopascals.     

Photoelectrochemical laser imaging on anodically prepared α-PbO thin films

The influence of various potential windows on the synthesis of α-PbO and the uniformity of its formation over the entire surface of the lead electrode has been studied by scanning a laser beam over the entire surface and measuring the corresponding photocurrent using the photoelectrochemical laser imaging technique. This technique revealed that a very uniformly distributed highly (110) plane oriented thin film of α-PbO is obtained by potentiodynamic anodization of the lead electrode in an alkaline medium at 80 ^°C in the potential range of ?0.32 to +0.08 V vs SCE, giving the highest (73%) quantum yield at 480 nm, an open-circuit photopotential of 800 mV and a short-circuit photocurrent of 5.5 mA cm^?2. Gärtner analysis suggests that the highest photoactivity achieved with this material is due to the formation of a large space charge width (0.63 m) and diffusion length (0.36 m) of the minority carriers and absorption of almost 100% of the light (480 nm) within the space charge and diffusion regions.     

Charge‐mosaic membrane prepared from microspheres

A new type of charge‐mosaic membrane was prepared from poly(4‐vinyl pyridine) microspheres 250–350 nm in diameter as an anion‐exchange element and from linear poly(sodium styrene sulfonic acid) as a cation‐exchange element. The mosaic structure on the surface of the membrane and the continuous connection of the microspheres in the membrane were confirmed by scanning electron microscopy observations and the electrical conductivity of the membrane. The microspheres were continuously connected from one side to the other side of the membrane surface when the weight fraction of the microspheres increased more than 20–25 wt % in the membrane. The flux of KCl, the permeability coefficient of KCl, the volume flux, and the selectivity dialysis separation of the electrolyte (KCl) and nonelectrolyte (glucose and saccharose) of the membrane indicated that it had the characteristic properties of a charge‐mosaic membrane. Ion‐exchange membranes composed of each raw material of the charge‐mosaic membrane did not show these properties. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1251–1261, 2003     

Hybrid ZnO/polymer thin films prepared by RF magnetron sputtering

Zinc oxide/poly(acrylic acid) (ZnO/PAA) multilayered hybrid films with different layer thicknesses were prepared by radio frequency magnetron sputtering. Zinc peroxide was used as precursor materials for the preparation of ZnO layers, since the zinc peroxide decomposes to ZnO during the film deposition. The films have a high transmittance in the visible region and exhibit visible photoluminescence emission. The band gap energy of the films—determined by the Tauc relationship—decreases with increasing layer thickness (3.40–3.36 eV) due to the increasing crystalline size of the ZnO particles. The morphological investigations showed that a real layered hybrid film structure formed.     

PEC films prepared from Chitosan-Alginate coacervates

Chitosan-alginate polyelectrolyte complex (PEC) have been prepared in situ in beads and microspheres. This study examines the preparation of suitable chitosan-alginate coacervates for casting into homogeneous PEC films for potential applications in packaging, controlled release systems and wound dressings. Coacervation between chitosan and alginate was rapid, but the rate may be controlled with the addition of water miscible organic solvents. Compared with ethanol and PEG200, acetone was the more promising solvent moderator. Suspensions of fine, uniformly dispersed coacervates were produced by a dropwise addition of 0.25% w/v chitosan solution (solvent: 1: 1 v/v of 2% acetic acid and acetone) into 0.25% w/v sodium alginate solution in water under rapid agitation. The PEC films were transparent and flexible. They exhibited high permeability to water vapor, but resisted complete dissolution in 0.1 M HCI, distilled water and pH 7.4 phosphate buffer solution. Microscopic heterogeneity in the films could be reduced by immersion in aqueous media, but this was accompanied by modifications in the thickness, permeability and mechanical property of the films.     

Epitaxial ternary nitride thin films prepared by a chemical solution method

It is indispensable to use thin films for many technological applications. This is the first report of epitaxial growth of ternary nitride AMN2 films. Epitaxial tetragonal SrTiN2 films have been successfully prepared by a chemical solution approach, polymer-assisted deposition. The structural, electrical, and optical properties of the films are also investigated.     

Low-temperature preparation of highly conductive thin films from acrylic acid-stabilized silver nanoparticles prepared through ligand exchange

The preparation of AcA-stabilized Ag nanoparticles and its application to make highly conductive thin films are reported. The AcA-stabilized Ag nanoparticles were prepared through a ligand exchange of original oleylamine (OLA)-coated Ag nanoparticles with acrylic acid (AcA), which acted as both an antisolvent and a modifying ligand during the ligand exchange process. Efficiencies of the ligand exchange as well as the properties of Ag nanoparticles were analyzed using various techniques including TEM, FT-IR, XPS, TGA, and UV-vis methods. The thin films were fabricated by annealing spin-coated AcA-stabilized Ag nanoparticles. Further, the effects of annealing temperature, time, and film thickness on both the film morphology and electrical conductivity have been investigated. In this work, due to the low boiling temperature of stabilizer (AcA) and adjustment of annealing conditions, high electrical conductivity was obtained for the Ag thin films. For example, when annealing at 175 °C for 30 min, a 70 nm thick film showed a maximum electrical conductivity of 1.12 × 10(5) S cm(-1). A conductive layer on a flexible polymer substrate (e.g., PET) sheet has been successfully prepared by annealing a spin-coated film at 140 °C for 30 min. The combined advantages of long-term stability of the AcA-stabilized Ag nanoparticles, low annealing temperature, and high conductivity of the prepared thin films make this relatively simple method attractive for applications in flexible electronics.     

Interactions between temperature-sensitive hydrogel microspheres and granulocytes

Poly(N-isopropylacrylamide) (PNIPAM) has a low critical solution temperature (LCST) at 32°C in water and the hydrophilicity changes through the LCST. The microspheres whose surface was composed of PNIPAM exhibited phase transition behavior around 32°C. Therefore, the interactions between PNIPAM micropheres and granulocytes depended on the temperature. That is, the oxygen consumption and active oxygen production by cells in contact with PNIPAM-containing microspheres and adhesion of the microspheres to the cell surface were more enhanced above the LCST of PNIPAM than below it, whereas no significant temperature dependence of cell–microspheres interaction was observed in nonthermosensitive microsphere systems. It was suggested that the function of cells could be controlled with temperature using the temperature-sensitive microspheres.     

Chitosan based hydrogel microspheres as drug carriers

Chitosan/tripolyphosphate (CHIT/TPP) and chitosan/tripolyphosphate/chondroitin sulfate (CHIT/TPP/CHS) core-shell type microspheres were prepared by polyelectrolyte complexation in order to develop a biocompatible matrix for drug delivery. The continual method using a multi-loop reactor under sterile conditions was applied for microsphere preparation. All the types of microspheres produced were spherical in shape and had a porous structure. The mechanical resistance of the microspheres increased in the presence of CHS as the second polyanion, which toughened the microsphere shell structure. For a drug release application, the process of microsphere preparation was modified by dissolving ofloxacin (OFL), the fluoroquinolone antibiotic, in CHIT solution before complex formation. This study shows the difference in OFL release comparing the microspheres CHIT/TPP and CHIT/TPP/CHS and implies the potential to control this process.