Poly Ethylene Oxide Chains Research Articles

Spectroscopic Behavior of Some A3B Type Tetrapyrrolic Complexes in Several Organic Solvents and Micellar Media

The paper presents spectral studies of some unsymmetrical A3B tetrapyrrolic, porphyrin-type complexes with Cu(II) and Zn(II) in different solvents and micellar media aimed at estimating their properties in connection with the living cell. The results indicate that the position of the absorption and emission peaks is mostly influenced by the central metal ion and less by the environmental polarity or the peripheric substituents of the porphyrinic core. The comparison between the overall absorption and emission spectra of the compounds in methanol or cyclohexane vs. direct and reverse Triton X micellar systems, respectively, suggests for all compounds the localization at the interface between the polyethylene oxide chains and the tert-octyl-phenyl etheric residue of the Triton X-100 molecules. These findings could be important when testing the compounds embedded in liposomes or other delivery systems to the targeted cell.

The lateral diffusion of polymer single chain at oil-water interfaces and hydrophobic interaction

The lateral diffusion of individual polymer chain of polyethylene oxide (PEO) at oil-water interfaces was studied using fluorescence correlation spectroscopy (FCS). The diffusion coefficient of PEO was systematically under variable salt concentration of NaCl in the aqueous phase and it was found that the diffusion coefficient of the adsorbed polymer chain was very much determined by the viscosity of the oil phase. This fact indicates an enhanced hydrophobic interaction of the PEO chain with the oil phase.

Structure, morphology and ionic conductivity of solid polymer electrolyte

Abstract Polyethylene oxide (PEO) complexed with potassium iodide (KI) is synthesized to investigate the ionic conductivity of alkaline based polymer electrolytes. The structural and morphological characterizations of the nanocomposite polymer electrolytes are performed by X-ray diffractometry (XRD), atomic force microscopy (AFM), differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) measurements. The ionic conductivity increases with the increase of KI concentration up to about 20 wt.%. The effect of nanosized ceria (CeO 2 ∼ 10 nm) fillers on ionic conductivity in PEO–KI polymer electrolyte is also carried out, keeping PEO to KI wt.% ratio 80:20 and 85:15. The result reveals that the addition of ceria nanoparticles enhances the conductivity by two orders of magnitude. The presence of ceria at the highest concentration induces the same molecular environment within PEO chain as that of undoped PEO. Temperature dependence of ionic conductivity follows Arrhenius mechanism.

Rheological properties of physical gel formed by hydrophobically modified urethane ethoxylate (HEUR) associative polymers in methanol–water mixtures

We investigated the effects of methanol on the two rheological properties, dynamic modulus and flow behaviour, for an aqueous solution of hydrophobically ethoxylated urethane (HEUR). When the added methanol constitutes 0–10 mol% of the sample, the gel relaxation time shortens; when it constitutes 20 mol% of the sample, the distribution of relaxation times broadens. Relaxation of the physical gel formed by a transient network is directly related to the lifetime of the crosslink points, i.e. flower micelles. We speculate that methanol addition shortens the relaxation time by changing the hydrophobic interactions in the flower micelles. The changed hydrophobic interactions then affect not only the relaxation time but also the shape of the HEUR-chain molecular associating structures which in turn affects the mechanical spectrum. Under constant shear flow, shear thickening increases with increasing methanol concentration, and the increase in stress under constant shear flow shows unusual behaviour. A possible contributing factor to this behaviour may be the non-cosolvency of methanol with polyethyleneoxide (PEO). At some critical concentration, methanol in PEO aqueous solution becomes a poor solvent, which then affects the properties of the PEO chains in the transient networks of HEUR aqueous solution. The rheological properties of the transient networks clearly affect the properties of both the crosslink points and the chains. In short, methanol addition induces complicated changes in gel mechanical properties.

Pluronic–lysozyme conjugates as anti-adhesive and antibacterial bifunctional polymers for surface coating

This paper describes the preparation and characterization of polymer–protein conjugates composed of a synthetic triblock copolymer with a central polypropylene oxide (PPO) block and two terminal polyethylene oxide (PEO) segments, Pluronic F-127, and the antibacterial enzyme lysozyme attached to the telechelic groups of the PEO chains. Covalent conjugation of lysozyme proceeded via reductive amination of aldehyde functionalized PEO blocks (CHO-Pluronic) and the amine groups of the lysine residues in the protein. SDS-PAGE gel electrophoresis together with MALDI-TOF mass spectrometry analysis revealed formation of conjugates of one or two lysozyme molecules per Pluronic polymer chain. The conjugated lysozyme showed antibacterial activity towards Bacillus subtilis. Analysis with a quartz crystal microbalance with dissipation revealed that Pluronic–lysozyme conjugates adsorb in a brush conformation on a hydrophobic gold-coated quartz surface. X-ray photoelectron spectroscopy indicated surface coverage of 32% by lysozyme when adsorbed from a mixture of unconjugated Pluronic and Pluronic–lysozyme conjugate (ratio 99:1) and of 47% after adsorption of 100% Pluronic–lysozyme conjugates. Thus, bifunctional brushes were created, possessing both anti-adhesive activity due to the polymer brush, combined with the antibacterial activity of lysozyme. The coating having a lower degree of lysozyme coverage proved to be more bactericidal.

Conductivity enhancement induced by casting of polymer electrolytes under a magnetic field

We recently presented a procedure for orienting the polyethylene-oxide (PEO) helices in a direction perpendicular to the film plane by casting the polymer electrolytes (PE) under a magnetic field (MF). Here we study the influence of magnetic fields of different strengths and configurations on the structural properties and ionic conductivity of concentrated LiCF 3SO 3 (LiTf) and LiAsF 6:P(EO) pristine and composite polymer electrolytes containing γ-Fe 2O 3 nanoparticles. Some data of LiI:P(EO) system are shown for comparison. We suggest that the effect of type of salt (LiI, LiTf and LiAsF 6) on the structure–conductivity relationship of the polymer electrolytes cast under magnetic field is closely connected to the crystallinity of the PEO–LiX system. It was found that the higher the content of the crystalline phase and the size of spherulites in the typically cast salt-polymer system, the stronger the influence of the magnetic field on the conductivity enhancement when the electrolyte is cast and dried under MF. Casting of the PE from a high-dielectric-constant solvent results in disentanglement of the PEO chains, which facilitates even more the perpendicular orientation of helices under applied MF. The enhancement of ionic conductivity was appreciably higher in the PEs cast under strong NdFeB magnets than under SmCo. Both bulk (intrachain) and grain-boundary conductivities increase when a MF is applied, but the improvement in the grain-boundary conductivity – associated with ion-hopping between polymer chains – is more pronounced. For LiAsF 6:(PEO) 3 at 65 °C, the interchain conductivity increased by a factor of 75, while the intrachain conductivity increased by a factor of 11–14. At room temperature, the SEI resistance of these PEs, cast under NdFeB HMF, decreased by a factor of up to 7, as compared to the typically cast polymer electrolytes. The effect of MF on orientation is observed directly down to the molecular level by 7Li nuclear magnetic resonance measurements.

Resolution of a nonionic surfactant oligomeric mixture by means of DOSY with inverse micelle assistance

DOSY is a recognized, efficient technique in the analysis of mixtures. It relies on the differences in self-diffusion coefficients, which are determined by the molecular size. Nowadays, efforts are directed towards devising matrices able to interact with the components of the mixture with differential affinity, and therefore capable to interfere with the diffusion processes and to display resolving power towards species of close, or even equal molecular weight, like isomers. Usually, commercial nonionic surfactants are mixtures of oligomeric species, since the head group, which is a short polyoxyehtylene chain, is somewhat polydisperse. The embedment of Igepal CA-520, 5 polyoxyethylene iso-octylphenyl ether, in an inverse microemulsion led to the separation of (1)H signals of the various oligomeric components. This ensued from the differential partitioning between the oil and the surface of the inverse micelles, which depends on the ethyleneoxide number (EON) of the head groups. Thus, it was possible to ascertain that the length distribution of the polyethyleneoxide chains is ingood agreement with the Poisson distribution theoretically predicted for the polymerization of ethylene oxide. The DOSY spectrum contributed to the assignment of the signals and afforded the partition degree, between the two environments, for each individual oligomeric species, providing further insight into nonionic inverse microemulsions, at present widely employed reaction media in the nanotechnological syntheses.

Synthesis of silver nanoparticles within intercalated clay/polymer nanocomposite via in situ electron transfer reaction

In this study, we present a new process for the synthesis of silver nanoparticles using intercalated clay/polymer nanocomposites via in situ electron transfer reduction process. Clay minerals are first intercalated by amphiphilic urethane precursor chains (UAN) and nano-dispersed at hydrophobic monomer (styrene). AgNO 3 crystals were added and mixed with UAN/styrene mixture to dissociate it not only by forming complex with silicate layers of clay minerals but also by making complex with polyethylene oxide chains of UAN chains. Radicals formed by UV-radiation caused reduction of dissolved Ag + to silver nanoparticles (in situ electron transfer reaction) and copolymerization of clay/UAN/styrene mixture (radical polymerization) simultaneously, resulting in direct conversion of the solution to silver nanoparticle dispersed clay/polymer nanocomposite film without additional reduction process. Size and morphology of silver nanoparticles formed within silicate layers depended on the surface property of clay minerals, which could be confirmed by XRD and TEM measurements.

Extracting a Single Polyethylene Oxide Chain from a Single Crystal by a Combination of Atomic Force Microscopy Imaging and Single-Molecule Force Spectroscopy: Toward the Investigation of Molecular Interactions in Their Condensed States

A thiol-labeled single polyethylene oxide chain has been pulled out of its single crystal and the corresponding extraction force obtained quantitatively by a good combination of atomic force microscopy (AFM) imaging and AFM-based single-molecule force spectroscopy (SMFS). Our study extends the AFM-based SMFS to the investigation of polymer interactions in their condensed states (e.g., in polymer single crystals).

Nisin adsorption to polyethylene oxide layers and its resistance to elution in the presence of fibrinogen

The adsorption and elution of the antimicrobial peptide nisin at silanized silica surfaces coated to present pendant polyethylene oxide chains was detected in situ by zeta potential measurements. Silica microspheres were treated with trichlorovinylsilane to introduce hydrophobic vinyl groups, followed by self assembly of the polyethylene oxide–polypropylene oxide–polyethylene oxide (PEO–PPO–PEO) triblock surfactant Pluronic® F108, or an F108 derivative with nitrilotriacetic acid end groups. Triblock-coated microspheres were γ-irradiated to covalently stabilize the PPO-surface association. PEO layer stability was evaluated by triblock resistance to elution by SDS, and layer uniformity was evaluated by fibrinogen repulsion. Introduction of nisin to uncoated or triblock-coated microspheres produced a significant positive change in surface charge (zeta potential) as a result of adsorption of the cationic peptide. In sequential adsorption experiments, the introduction of fibrinogen to nisin-loaded triblock layers caused a decrease in zeta potential that was consistent with partial elution of nisin and/or preferential location of fibrinogen at the interface. This change was substantially more pronounced for uncoated than triblock-coated silica, indicating that the PEO layer offers enhanced resistance to nisin elution.

Determination of primary bond scissions by mass spectrometric analysis of ultrasonic degradation products of poly(ethylene oxide-block-propylene oxide) copolymers

Ultrasonic degradation of poly(ethylene oxide-block-propylene oxide) copolymers consisting of a hydrophilic and a hydrophobic portion was studied with the aim to determine the location of bonds involved in the initial scission of the copolymers. LC-APCI-IT-MS and LC-APCI-orbitrap-MS were used for the detailed structural analysis of degradation products. The results indicated that initial bond scissions occurred principally at the boundary regions between backbones of polyethylene oxide (PEO) and polypropylene oxide (PPO) chains. Further structural analysis revealed the presence of oxygen adducts in the degradation products. Comparison with a thermal degradation carried out in helium atmosphere, one can conclude that the oxygen adducts are formed by radical reaction with water or dissolving oxygen molecules. The study demonstrated that chemical reactions as well as physical bond stress scissions are involved in the ultrasonic degradation of the copolymers.

Synthesis and characterization of mesoporous zirconium oxide thin films

Zirconium oxide (ZrO2) has generated wide interest because of its potential in applications such as solid oxide fuel cells, catalysts and optical devices. In these applications, it is important to control the grain size of the material and increase the surface area by introducing porosity with tailored pore size. This paper presents a synthesis route for the preparation of mesoporous zirconia using spin-coating method combined with block copolymer templating evaporation induced self assembly (EISA). The hybrid mesophase was formed by zirconium oxychloride precursor ZrOCl2·8H2O and Brij 700 poly-ethylene oxide based block copolymer template C18H37(OCH2CH2). FT-IR and Raman measurements of the hybrid mesophase provided information on the possible intermolecular interactions between the precursor and the copolymer template. The results indicate a weak ion-dipole interaction between the inorganic precursor and the organic template after the solvent evaporation, possibly directly between the zirconium and the oxygen of the poly-ethylene oxide chain. The removal of the organic block copolymer by heat treatment resulted in mesoporous zirconia with pore size of ~4-8 nm and crystalline grain size of ~10-17 nm. The morphology in thin films depended significantly on the solvent quality and the block copolymer concentration. Tailoring the surface morphology and the grain size resulted in super-hydrophilic mesoporous zirconia thin films in contrast to water contact angle of 50° on conventional tetragonal zirconium oxide.

Synthesis and evaluation of heparin immobilized “side-on” to polystyrene microspheres coated with end-group activated polyethylene oxide

Thiol (–SH) groups were introduced into unfractionated heparin by reaction of carboxyl groups in its uronic acid residues with 3,3′dithiobis(propanoic)hydrazide. Thiolated heparin derivatives were then linked to pyridyl disulfide-activated polyethylene oxide–polypropylene oxide–polyethylene oxide triblocks, which had previously been coated onto the surfaces of 1.15 μm polystyrene microspheres. The heparin immobilization reaction was monitored spectrophotometrically as colored pyridine-2-thione was released. In addition, the zeta potentials of uncoated, triblock-coated, and heparin-containing microsphere suspensions were recorded to demonstrate the successful attachment of heparin on the surface. However this “side-on” attachment of heparin to pendant, polyethylene oxide chains did not significantly increase the anticoagulant or anti-Factor Xa activity of microsphere suspensions.

Ultrafiltration Membranes Incorporating Amphiphilic Comb Copolymer Additives Prevent Irreversible Adhesion of Bacteria

We examined the resistance to bacterial adhesion of a novel polyacrylonitrile (PAN) ultrafiltration membrane incorporating the amphiphilic comb copolymer additive, polyacrylonitrile-graft-polyethylene oxide (PAN-g-PEO). The adhesion of bacteria (E. coli K12) and the reversibility of adhered bacteria were tested with the novel membrane, and the behavior was compared to a commercial PAN ultrafiltration membrane. Under static (no flow) bacterial adhesion tests, we observed no bacterial adhesion to the PAN/PAN-g-PEO membrane at all ionic strengths tested, even with the addition of calcium ions. In contrast, significant adhesion of bacterial cells was observed on the commercial PAN membrane, with increased cell adhesion at higher ionic strengths and in the presence of calcium ions. Under crossflow filtration conditions, initial bacterial deposition rate increased with ionic strength and with addition of calcium ions for both membranes, with generally lower bacterial deposition rate with the PAN/PAN-g-PEO membrane. However, deposited bacteria were readily removed (between 97 and 100%) from the surface of the PAN/PAN-g-PEO membrane upon increasing the crossflow and eliminating the permeate flow (i.e., no applied transmembrane pressure), suggesting reversible adhesion of bacteria. In contrast, bacterial adhesion on the commercial PAN membrane was irreversible, with approximately 50% removal of adhered bacteria at moderate ionic strengths (10 and 30 mM) and less than 25% removal at high ionic strength (100 mM). The resistance to bacterial adhesion of the PAN/PAN-g-PEO membrane was further analyzed via measurement of interaction forces with atomic force microscopy (AFM). No adhesion forces were detected between a carboxylated colloid probe and the PAN/PAN-g-PEO membrane, while the probe exhibited strong adhesion to the commercial PAN membrane, consistent with the bacterial adhesion tests. The exceptional resistance of the PAN/PAN-g-PEO membrane to bacterial adhesion is attributable to steric repulsion imparted by the dense brush layer of polyethylene oxide (PEO) chains.

Patterning block copolymer thin films by deep X-ray lithography

Developing fast, cheap and reliable micro and nanofabrication technologies for block copolymer thin films is a key issue for exploiting the wide potential applications of this class of materials. We have used a synchrotron source of high energy photons (hard X-rays) for developing a lithographic tool that allows direct writing of block-copolymer thin films. We have exposed films prepared by a tri-block copolymer, Pluronic F127, to increasing doses of radiation to evaluate the effect of high energy X-rays on the samples. The as-deposited films show a crystalline structure due to the crystallization of polyethylene oxide chains in Pluronic F127; the exposure to low doses causes a phase change from crystalline to amorphous, as is shown by infrared spectroscopy. Another effect of the exposure to X-rays of the block copolymer films is the surface roughness reduction which has been observed by atomic force microscopy, At higher doses the X-rays erase the film from the substrate allowing the formation of patterned polymeric structures. Deep X-ray lithography has proved to be a very effective tool to pattern block copolymer films by a direct, top-down method.

Surface adsorption‐induced conformational ordering and crystallization of polyethylene oxide

AbstractThe conformational ordering and crystallization of polyethylene oxide (PEO) in the presence of KBr surface were studied with in situ Fourier transform infrared spectroscopy (FTIR). KBr was chosen because of its low absorption on IR, which allows adding large amounts KBr into PEO matrix without sacrificing IR signal significantly. The presence of KBr enhances conformational order well above the melting temperature of PEO, which can further accelerate or decelerate the crystallization process. Low concentrations of KBr powder in the PEO melt promotes crystallization process, whereas high concentration of KBr restricts large portion of PEO chains on KBr surfaces, which hinder the diffusion and rearrangement of conformation and consequently slow down the crystallization process. Acceleration of crystallization requires a synergetic effect between the adsorbed chains with ordered conformation and the free chains with a fast diffusion rate, where the former and the later are responsible to lower the nuclei barrier and to maintain the low activation energy of diffusion, respectively. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 106–112, 2010

Responsive Pore Size Properties of Composite NF Membranes Based on PVDF Graft Copolymers

Uncharged nanofiltration (NF) membranes can potentially perform size-based separations of small molecules. Thin film composite (TFC) membranes, prepared by coating a porous support membrane with a thin layer of poly(vinylidene fluoride)-graft-poly(oxyethylene) methacrylate (PVDF-g-POEM), were shown previously to have such capability, as well as exceptional fouling resistance and high pure water permeability (Macromolecules 2004, 37, 7663–7668). The selectivity and permeability of these membranes arises from the microphase separation of the PVDF backbone and polyethylene oxide (PEO) side chains into bicontinuous hydrophobic and hydrophilic phases, the PVDF domains providing mechanical integrity, and the PEO domains acting as “nanochannels” whose dimensions control molecular transport through the membrane. This study explores the influence of process and solution parameters on the permeability and selectivity of PVDF-g-POEM TFC NF membranes. By controlling the degree of swelling of the hydrated PEO chains filling the channels, the effective pore size of the membrane is shown to be tunable. Membrane permeability to water and to organic dye molecules of approximately 1 nm diameter is found to increase when the solvent quality of the feed for the PEO chains is reduced by raising the temperature, pressure, or ionic strength, or by the addition of ethanol. This property holds promise for applications in the biochemical, pharmaceutical, and food industries for low-cost, high-throughput fractionation of molecules.

Mesoscopic dynamics of polymer chains in high strain rate extensional flows

We take a step towards accessing the physics of viscoelastic liquid breakup in high speed, high strain rate flows by performing Brownian dynamics computations of dilute uniaxial, equibiaxial, and ellipsoidal polymeric extensional flows. Our computational implementation of the bead-spring model, when tailored to the DNA molecule, consistently with recent works of Larson and co-workers, is shown: (a) to predict a coil-stretch transition at Deborah number D e ≈ 0.5 , and (b) to reproduce the experimental longest relaxation time. Furthermore, after adapting the model parameters to represent the polyethylene oxide (PEO) chain (for M = 1 0 6 Da), we find it possible to reproduce our own experimental data of the longest relaxation time, the transient extensional viscosity of dilute solutions at small Deborah numbers, and a coil-stretch transition at Deborah number D e ≈ 0.5 . Extended to large Deborah numbers, the model predicts that polymer stretching is controlled by: (a) the randomness of the initial conditions that, in combination with rapid kinematically imposed compression, lead to the formation of initially frozen chain-folds, and (b) the speed with which thermo-kinematic processes relax these folds. The slowest fold relaxation occurs during uniaxial extension. As expected, the introduction of stretching along a second direction enhances the efficiency of fold relaxation mechanisms. Even for Deborah numbers (based on the chain longest relaxation time) of the order of one thousand, there is a large variation in the time a polymer needs in order to extend fully, and the effects of Brownian motion cannot be ignored. The computed Trouton ratios and polymer contributions to the total stress as functions of Hencky strain provide information about the relative importance of elastic effects during polymeric liquid stretching. At high strain rates, the steady state elastic stresses increase linearly with the Deborah number, resembling at that stage an anisotropic Newtonian fluid (constant extensional viscosity).

Synthesis and Characterization of Some Amino Acid Derived Schiff Bases Bearing Nonionic Species as Corrosion Inhibitors for Carbon Steel in 2N HCl

A novel series of nonionic amino acid Schiff-Bases were synthesized and characterized using different spectroscopic tools to elucidate their chemical structures. The surface and thermodynamic properties of these compounds were studied using classical measurements including surface and interfacial tension and emulsification tendency. The surface parameters of these compounds including surface tension, critical micelle concentration (CMC), effectiveness (πcmc), efficiency (Pc20), maximum surface excess (Γmax), and minimum surface area (Amin) showed their good surface activity. Their thermodynamic parameters of adsorption and micellization including free energy change of micellization and adsorption showed their tendency toward adsorption at the interfaces and also micelle formation at lower concentrations. The synthesized compounds were also evaluated as corrosion inhibitors for carbon steel at different doses (400, 200, 100, 50, and 25 ppm) in 2 N HCl using gravimetric technique (weight loss). The results showed that these inhibitors are characterized by very high corrosion inhibition efficiency ranged between 99.93% and 97.98% and low corrosion rates varied between 0.09 mpy and 0.17 mpy for higher doses (400 ppm). The efficiency of inhibition was decreased by increasing the exposure time. The most efficient corrosion inhibitor of the synthesized compounds was the inhibitor which contains polyethylene oxide chain length of 23 EO units and alkyl chain length of 12 methylene groups. The effect of the hydrophobic and hydrophilic chain length in the inhibitor molecules was discussed and rationalized with their inhibition efficiency. The tendency of these inhibitors toward complexation with the transition metals was also discussed in order to explain their higher efficiency.

Biodegradable dendritic positron-emitting nanoprobes for the noninvasive imaging of angiogenesis

A biodegradable positron-emitting dendritic nanoprobe targeted at alpha(v)beta(3) integrin, a biological marker known to modulate angiogenesis, was developed for the noninvasive imaging of angiogenesis. The nanoprobe has a modular multivalent core-shell architecture consisting of a biodegradable heterobifunctional dendritic core chemoselectively functionalized with heterobifunctional polyethylene oxide (PEO) chains that form a protective shell, which imparts biological stealth and dictates the pharmacokinetics. Each of the 8 branches of the dendritic core was functionalized for labeling with radiohalogens. Placement of radioactive moieties at the core was designed to prevent in vivo dehalogenation, a potential problem for radiohalogens in imaging and therapy. Targeting peptides of cyclic arginine-glycine-aspartic acid (RGD) motifs were installed at the terminal ends of the PEO chains to enhance their accessibility to alpha(v)beta(3) integrin receptors. This nanoscale design enabled a 50-fold enhancement of the binding affinity to alpha(v)beta(3) integrin receptors with respect to the monovalent RGD peptide alone, from 10.40 nM to 0.18 nM IC(50). Cell-based assays of the (125)I-labeled dendritic nanoprobes using alpha(v)beta(3)-positive cells showed a 6-fold increase in alpha(v)beta(3) receptor-mediated endocytosis of the targeted nanoprobe compared with the nontargeted nanoprobe, whereas alpha(v)beta(3)-negative cells showed no enhancement of cell uptake over time. In vivo biodistribution studies of (76)Br-labeled dendritic nanoprobes showed excellent bioavailability for the targeted and nontargeted nanoprobes. In vivo studies in a murine hindlimb ischemia model for angiogenesis revealed high specific accumulation of (76)Br-labeled dendritic nanoprobes targeted at alpha(v)beta(3) integrins in angiogenic muscles, allowing highly selective imaging of this critically important process.