Strain rate dependent nanostructure of hydrogels with reversible hydrophobic associations during uniaxial extension.

An energy dissipation mechanism during deformation is required to impart toughness to hydrogels. Here we describe how in situ small angle X-ray scattering (SAXS) provides insight into possible energy dissipation mechanisms for a tough hydrogel based on an amphiphilic copolymer where nanoscale associations of the hydrophobic moieties act as effective crosslinks. The mechanical properties of the hydrogels are intimately coupled with the nanostructure that provides reversible crosslinks and evolves during deformation. As the extension rate increases, more mechanical energy is dissipated from rearrangements of the crosslinks. The scattering is consistent with hopping of hydrophobes between the nanoscale aggregates as the primary rearrangement mechanism. This rearrangement changes the network conformation that leads to non-affine deformation, where the change in the nanostructure dimension from SAXS is less than 15% of the total macroscopic strain. These nanostructure changes are rate dependent and correlated with the relaxation time of the hydrogel. At low strain rate (0.15% s-1), no significant change of the nanostructure was observed, whereas at higher strain rates (1.5% s-1 and 8.4% s-1) significant nanostructure anisotropy occurred during extension. These differences are attributed to the ability for the network chains to rearrange on the time scale of the deformation; when the characteristic time for extension is longer than the average segmental relaxation time, no significant change in nanostructure occurs on uniaxial extension. These results illustrate the importance of strain rate in the mechanical characterization and consideration of relaxation time in the design of tough hydrogels with reversible crosslinks.

Quantitative Rheometry of Thin Soft Materials Using the Quartz Crystal Microbalance with Dissipation.

In the inertial limit, the resonance frequency of the quartz crystal microbalance (QCM) is related to the coupled mass on the quartz sensor through the Sauerbrey expression that relates the mass to the change in resonance frequency. However, when the thickness of the film is sufficiently large, the relationship becomes more complicated and both the frequency and damping of the crystal resonance must be considered. In this regime, a rheological model of the material must be used to accurately extract the adhered film's thickness, shear modulus, and viscoelastic phase angle from the data. In the present work we examine the suitability of two viscoelastic models, a simple Voigt model ( Physica Scripta 1999, 59, 391-396) and a more realistic power-law model ( Langmuir 2015, 31, 4008-4017), to extract the rheological properties of a thermoresponsive hydrogel film. By changing temperature and initial dry film thickness of the gel, the operation of QCM was traversed from the Sauerbrey limit, where viscous losses do not impact the frequency, through the regime where the QCM response is sensitive to viscoelastic properties. The density-shear modulus and the viscoelastic phase angle from the two models are in good agreement when the shear wavelength ratio, d/λ n, is in the range of 0.05-0.20, where d is the film thickness and λ n is the wavelength of the mechanical shear wave at the nth harmonic. We further provide a framework for estimating the physical properties of soft materials in the megahertz regime by using the physical behavior of polyelectrolyte complexes. This provides the user with an approximate range of allowable film thicknesses for accurate viscoelastic analysis with either model, thus enabling better use of the QCM-D in soft materials research.

Shape Memory of Microscale and Nanoscale Imprinted Patterns on a Supramolecular Polymer Compound.

Surface memory effects for micropattern and nanopattern are demonstrated for shape memory compounds composed of mixtures of the zinc salt of a sulfonated poly(ethylene-co-propylene-co-ethylidene norbornene) ionomer and three different low molar mass fatty acids (FAs): lauric acid (LA), stearic acid (SA), and zinc stearate (ZnSt). This work shows the ability to tune the surface pattern switching temperature (Tc ) by simply varying the FA melting point. The melting point of the FA in the ionomer compound is depressed from that of the pure FA due to strong dipolar interactions between the ionomer and the FAs. Surface pattern memory and recovery are shown for compounds with 20 wt% LA, SA, or ZnSt, where Tc = 50, 80, and 100 °C, respectively. Recovery efficiencies for micropatterns are better than 92% for all three compounds and 73% for a nanopattern for the ionomer/ZnSt compound.

Tailor-Made Fluorinated Copolymer/Clay Nanocomposite by Cationic RAFT Assisted Pickering Miniemulsion Polymerization.

Fluorinated polymers in emulsion find enormous applications in hydrophobic surface coating. Currently, lots of efforts are being made to develop specialty polymer emulsions which are free from surfactants. This investigation reports the preparation of a fluorinated copolymer via Pickering miniemulsion polymerization. In this case, 2,2,3,3,3-pentafluoropropyl acrylate (PFPA), methyl methacrylate (MMA), and n-butyl acrylate (nBA) were copolymerized in miniemulsion using Laponite-RDS as the stabilizer. The copolymerization was carried out via reversible addition-fragmentation chain transfer (RAFT) process. Here, a cationic RAFT agent, S-1-dodecyl-S'-(methylbenzyltriethylammonium bromide) trithiocarbonate (DMTTC), was used to promote polymer-Laponite interaction by means of ionic attraction. The polymerization was much faster when Laponite content was 30 wt % or above with 1.2 wt % RAFT agent. The stability of the miniemulsion in terms of zeta potential was found to be dependent on the amount of both Laponite and RAFT agent. The miniemulsion had particle sizes in the range of 200-300 nm. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) analyses showed the formation of Laponite armored spherical copolymer particles. The fluorinated copolymer films had improved surface properties because of polymer-Laponite interaction.

Human immunodeficiency virus antibodies and the vaccine problem.

Despite the great advances made in controlling human immunodeficiency virus type 1 (HIV-1) infection with antiretroviral drug treatment, a safe and efficacious HIV vaccine has yet to be developed. Here, we discuss why clinical trials and vaccine development for HIV have so far been disappointing, with an emphasis on the lack of protective antibodies. We review approaches for developing appropriate HIV immunogens and the stimulation of long-lasting B-cell responses with antibody maturation. We conclude that candidate reagents in the pipeline for HIV vaccine development are unlikely to be particularly effective. Although the major funders of HIV vaccine research and development are placing increasing emphasis on clinical product development, a genuine breakthrough in preventing HIV infection through vaccines is more likely to come from novel immunogen research.

Overcoming confinement limited swelling in hydrogel thin films using supramolecular interactions.

The thin film behavior of poly(N-isopropylacrylamide-stat-2-(N-ethylperfluorooctane sulfonamido)ethyl acrylate) (NIPAAm-stat-FOSA) based hydrogels containing 5 mol% FOSA was elucidated using quartz crystal microbalance with dissipation (QCM-D) in combination with spectroscopic ellipsometry (SE) through examination of the lower critical solution temperature (LCST) and temperature dependent swelling for (dry) thicknesses ranging from 10 nm to 121 nm. For all thin films measured, the LCST was shown to slightly increase (>3 °C) in comparison to that of the bulk sample. However for these films, the increase in LCST was statistically identical, irrespective of thickness. Surprisingly, the volumetric swelling of the hydrogel in thin films, even at temperatures less than the LCST, was similar (within 20%) to the volumetric swelling of the bulk hydrogel, despite the expected significant decrease associated with the hydrogel being constrained by the substrate as predicted by one dimensional Flory-Rehner theory. We attribute this enhancement in swelling compared to theoretical expectations to the ability of the hydrophobic crosslinks to re-arrange under stress, which provides a mechanism to alleviate the decreased dimensionality imposed by the substrate; this mechanism is consistent with a large hysteresis in the swelling when cycling between 35 °C and 5 °C. Unlike the LCST, the swelling ratio increases with decreasing film thickness. At low temperatures (below the LCST), the volume swelling ratio increased from 3.9 to 4.9, while at temperatures above the LCST the swelling ratio increased from 1.5 to 2.5 when the film thickness decreased from 121 nm to 10 nm. The combination of facile processing through solution casting without the need for additional crosslinking chemistry and limited thickness dependent variation of swelling and LCST behavior in these physically crosslinked hydrogels makes these materials attractive for applications requiring thermoresponsive soft coatings.

Consolidation and Forced Elasticity in Double-Network Hydrogels.

This paper discusses two observations that are unique with respect to the mechanics of double network (DN) hydrogels, forced elasticity driven by water diffusion and consolidation, which are analogous to the so-called Gough-Joule effects in rubbers. A series of DN hydrogels were synthesized from 2-acrylamido-2-methylpropane sulfuric acid (AMPS), 3-sulfopropyl acrylate potassium salt (SAPS) and acrylamide (AAm). Drying of AMPS/AAm DN hydrogels was monitored by extending the gel specimens to different stretch ratios and holding them until all the water evaporated. At high extension ratios, the gels underwent plastic deformation. Water diffusion measurements performed on AMPS/AAm DN hydrogels that were dried at different stretch ratios indicated that the diffusion mechanism deviated from Fickian behavior at extension ratios greater than two. Study of the mechanical behavior of AMPS/AAm and SAPS/AAm DN hydrogels during tensile and confined compression tests showed that despite their large water content, DN hydrogels can retain water during large-strain tensile or compression deformations.

Fabrication of Tough Double-Network Hydrogels from Highly Cross-Linked Brittle Neutral Networks Using Alkaline Hydrolysis.

This paper describes a simple method to synthesize tough hydrogels from a highly cross-linked neutral network. It was found that applying alkaline hydrolysis to a highly cross-linked hydrogel synthesized from acrylamide (AAm) can increase its swelling ratio dramatically. Double-network (DN) hydrogels synthesized from polymerization of loosely cross-linked AAm networks inside a highly cross-linked AAm gel were not tough. However, repeating the same recipes with a second polymerization step to synthesize a DN hydrogel from a hydrolyzed highly cross-linked AAm gel resulted in tough hydrogels. Those gels exhibited finite tensile behavior similar to that of conventional DN hydrogels. Moreover, craze-like patterns were observed during tensile loading of a DN hydrogel synthesized from a hydrolyzed highly cross-linked first network and a loosely cross-linked second network. The patterns remained in the gel even after strain hardening at high stretch ratios. The craze-like pattern formation was suppressed by increasing the concentration of cross-linking monomer in the second polymerization step. Crack propagation in DN hydrogels synthesized using hydrolysis was also studied by applying a tensile load on notched specimens.

A facile method for preparing sticky, hydrophobic polymer surfaces.

Textured surfaces consisting of nanometer- to micrometer-sized lightly sulfonated polystyrene ionomer (SPS) particles were prepared by rapid evaporation of the solvent from a dilute polymer solution-cast onto silica. The particle textured ionomer surfaces were prepared by either spin-coating or solution-casting ionomer solutions at controlled evaporation rates. The effects of the solvent used to spin-coat the film, the molecular weight of the ionomer, and the rate of solvent evaporation on the surface morphology of cast films were investigated. The surface morphologies were consistent with a spinodal decomposition mechanism, where the surface first existed as a percolated-like structure and then ripened into droplets if molecular mobility was retained for sufficient time. The SPS particles or particle aggregates were robust and resisted separation from the surface even after annealing at 120 °C for 1 week. The water contact angles on as-prepared surfaces were relatively low, ∼90°, due to the polar groups in the ionomer, but when the surface was modified by chemical vapor deposition of 1H,1H,2H,2H-perfluorooctyltrichlorosilane, the surface contact angles increased to ∼109° on smooth surfaces and up to ∼140° on the textured surfaces. Although the surfaces were hydrophobic, the contact angle hysteresis was relatively high and water droplets stuck to these surfaces even when the surface was turned upside down.

An in vitro assessment of the physical properties of novel Hyflex nickel-titanium rotary instruments.

AIM: To determine several properties including torsional and fatigue limits, as well as torque during canal preparation, of Hyflex, a rotary instrument manufactured from so-called controlled memory nickel-titanium alloy. METHODOLOGY: The instruments were tested in vitro using a special torque bench that permits both stationary torque tests according to ISO3630-1 and fatigue limit determination, as well as measurement of torque (in Ncm) and apical force (in N) during canal preparation. Fatigue limit (in numbers of cycles to failure) was determined in a 90°, 5 mm radius block-and-rod assembly. Simulated canals in plastic blocks were prepared using both a manufacturer-recommended single-length technique as well as a generic crown-down approach. anova with Bonferroni post hoc procedures was used for statistical analysis. RESULTS: Torque at failure ranged from 0.47 to 1.38 Ncm, with significant differences between instrument sizes (P < 0.0001). Fatigue life ranged from 260 to 2565, with the shortest and longest lifespan for instruments size 20, .04 taper and size 25, .08 taper, respectively. Torque during canal preparation was significantly higher for small instruments used in the single-length technique but lower for the size 40, .04 taper, compared to a crown-down approach. No instrument fractured; 82% of the instruments used were plastically deformed; however, only 37% of these remained deformed after a sterilization cycle. CONCLUSIONS: Hyflex rotary instruments are bendable and flexible and have similar torsional resistance compared to instruments made of conventional NiTi. Fatigue resistance is much higher, and torque during preparation is less, compared to other rotary instruments tested previously under similar conditions.

Infection of human cells by an endogenous retrovirus of pigs.

The possible use of pig organs and tissues as xenografts in humans is actively being considered in biomedical research. We therefore examined whether pig endogenous retrovirus (PERV) genomes can be infectiously transmitted to human cells in culture. Two pig kidney cell lines spontaneously produce C-type retrovirus particles. Cell-free retrovirus produced by the PK-15 kidney cell line (PERV-PK) infected pig, mink and human kidney 293 cell lines and co-cultivation of X-irradiated PK-15 cells with human cells resulted in a broader range of human cell infection, including human diploid fibroblasts and B- and T-cell lines. Kidney, heart and spleen tissue obtained from domestic pigs contained multiple copies of integrated PERV genomes and expressed viral RNA. Upon passage in human cells PERV-PK could rescue a Moloney retroviral vector and acquired resistance to lysis by human complement.

Kaposi's sarcoma-associated herpesvirus infects endothelial and spindle cells.

Kaposi's sarcoma (KS), a vascular tumour that contains characteristic spindle cells forming slit-like spaces, may have an infectious aetiology. Recently, sequences of a new human herpesvirus, KSHV/HHV-8, have been identified in both HIV-associated and classical KS. We sought to identify the target cell of this virus in KS tumour tissue. Using PCR in situ hybridization (PCR-ISH) we show that KSHV/HHV-8 is present in the flat endothelial cells lining vascular spaces of KS lesions as well as in typical KS spindle cells. These findings show that KSHV/HHV-8 is present in the cell types thought to represent neoplastic cells in these lesions.

Brittle-to-Ductile Transition of Sulfonated Polystyrene Ionomers.

This study examines the brittle-to-ductile transition of sulfonated polystyrene ionomers (SPS) with different counterions. The polystyrene precursor was unentangled and had two ionic groups per chain on average. Thus, its terminal relaxation time was comparable to the lifetime of the associating ionic groups. Three types of ionomer samples were used to tune the association lifetime: (1) fully neutralized SPS with different alkali-metal counterions, (2) fully neutralized SPS with mixed sodium and cesium counterions, and (3) partially neutralized SPS with sodium or cesium counterions. For all three systems, the brittle-to-ductile transition could be represented by a diagram of two Weissenberg numbers, Wi and WiR, defined with respect to the terminal and Rouse relaxation times, respectively. A flowable region existed at sufficiently low Wi, independent of WiR. At higher Wi, a brittle-to-ductile transition of the ionomer melt occurred above a critical value of WiR. To achieve ductility during the application of rapid elongational flow, the Rouse-type motions should be sufficiently slow relative to the rate of ion-dissociation, so that the strain-induced breakup of the ionic cross-links would not cause very strong chain retraction that may further lead to the macroscopic fracture.

T cells reactive to an inducible heat shock protein induce disease in toxin-induced interstitial nephritis.

T cells reactive against immunodominant regions of inducible heat shock proteins (HSPs) have been identified in the chronic inflammatory lesions of several experimental autoimmune diseases. Since HSPs are known to be induced by a number of renal tubular epithelial cell toxins associated with chronic interstitial nephritis, we investigated the relevance of HSP expression and T cell reactivity to HSP70 in a model of progressive inflammatory interstitial nephritis. Chronic administration of cadmium chloride (CdCl2) to SJL/J mice induces HSP70 expression in renal tubular cells 4-5 wk before the development of interstitial mononuclear cell infiltrates. CdCl2 also induces HSP70 expression in cultured tubular epithelial cells from SJL/J mice. CD4+, TCR-alpha/beta+ T cell lines specific for an immunodominant HSP peptide are cytotoxic to heat stressed or CdCl2-treated renal tubular cells. Such HSP-reactive T cells mediate an inflammatory interstitial nephritis after adoptive transfer to CdCl2-treated mice at a time when immunoreactive HSP70 is detectable in the kidneys, but before the development of interstitial mononuclear cell infiltrates. T cells isolated from the nephritic kidneys of mice treated with CdCl2 for 13 wk are also cytotoxic to heat shocked or cadmium-treated tubular cells. These kidney-derived T cells additionally induced interstitial nephritis after passive transfer, indicating their pathogenic significance. Our studies strongly support a role for HSP-reactive T cells in CdCl2-induced interstitial nephritis and suggest that the induction of HSPs in the kidney by a multitude of "non-immune" events may initiate or facilitate inflammatory damage by HSP-reactive lymphocytes.

Varying ratios of wavelengths in dual wavelength LED photomodulation alters gene expression profiles in human skin fibroblasts.

BACKGROUND AND OBJECTIVE: LED photomodulation has been shown to profoundly influence cellular behavior. A variety of parameters with LED photomodulation can alter cellular response in vitro. The effects of one visible and one infrared wavelength were evaluated to determine the optimal ratio to produce a net increase in dermal collagen by altering the ratio of total energy output of each wavelength. The ratio between the two wavelengths (590 and 870 nm) was shifted in 25% increments. STUDY DESIGN/MATERIALS AND METHODS: Human skin fibroblasts in culture were exposed to a 590/870 nm LED array with total combined energy density fixed at 4.0 mW/cm.. The ratio of 590/870 nm tested parameters were: 100/0%, 75/25%, 50/50%, 25/75%, and 0/100%. These ratios were delivered using pulsed duty cycle of exposure (250 milliseconds "on" time/100 milliseconds "off" time/100 pulses) for a total energy fluence of 0.1 J/cm.. Gene expression was examined using commercially available extra cellular matrix and adhesion molecule RT PCR Arrays (SA Biosciences, Frederick, MD) at 24 hours post-exposure. RESULTS: Different expression profiles were noticed for each of the ratios studied. Overall, there was an average (in an 80 gene array) of 6% expression difference in up or downregulation between the arrays. The greatest increase in collagen I and decrease in collagenase (MMP-1) was observed with 75/25% ratio of 590/870 nm. The addition of increasing proportions of IR wavelengths causes alteration in gene expression profile. The ratios of the wavelengths caused variation in magnitude of expression. CONCLUSIONS: Cell metabolism and gene expression can be altered by simultaneous exposure to multiple wavelengths of low energy light. Varying the ratios of specific wavelength intensity in both visible and near infrared light therapy can strongly influence resulting fibroblast gene expression patterns.

Monoclonal antibody analysis of keratin expression in epidermal diseases: a 48- and 56-kdalton keratin as molecular markers for hyperproliferative keratinocytes.

The polypeptide composition of epidermal keratin varies in disease. To better understand the biological meaning of these variations, we have analyzed keratins from a number of human epidermal diseases by the immunoblot technique using AE1 and AE3 monoclonal antikeratin antibodies. The results reveal a continuous spectrum of keratin expression ranging from one closely resembling the normal in vivo pattern to one almost identical to cultured epidermal keratinocytes. Specifically, a 50-kilodalton (kd) (AE1-positive) and a 58-kd (AE3-positive) keratin are present in all diseases, supporting the concept that they represent "permanent" markers for keratinocytes. A 56.5-kd (AE1) and a 65-67-kd (AE3) keratin, previously shown to be markers for keratinization, are expressed only by lesions retaining a keratinized morphology. A 48-kd (AE1) and a 56-kd (AE3) keratin are present in all hyperproliferative (para- or nonkeratinized) disorders, but not in normal abdominal epidermis or in ichthyosis vulgaris which is a nonhyperproliferative disease. These two keratins have previously been found in various nonepidermal keratinocytes undergoing hyperproliferation, suggesting that these keratins are not epidermis-specific and may represent markers for hyperproliferative keratinocytes in general. In various epidermal diseases, there is a reciprocal expression of the (keratin) markers for hyperproliferation and keratinization, supporting the mutual exclusiveness of the two cellular events. Moreover, our results indicate that, as far as keratin expression is concerned, cultured human epidermal cells resemble and thus may be regarded as a model for epidermal hyperplasia. Finally, the apparent lack of any major, disease-specific keratin changes in the epidermal disorders studied so far implies that keratin abnormalities probably represent the consequence, rather than the cause, of these diseases.

How does HIV cause AIDS?

Many questions have been posed about acquired immunodeficiency syndrome (AIDS) pathogenesis. Is human immunodeficiency virus (HIV) both necessary and sufficient to cause AIDS? Is AIDS essentially an autoimmune disease, triggering apoptosis, or is virus infection the cause of T helper lymphocyte depletion? What is the significance of HIV tropism and the role of macrophages and dendritic cells in AIDS? Is there viral latency and why is there usually a long period between infection and AIDS? Is HIV variation a crucial aspect of its pathogenesis and, if so, do virulent strains emerge? Although this article provides few definitive answers, it aims to focus commentary on salient points. Overall, it is increasingly evident that both the tropism and burden of HIV infection correlate closely with the manifestations of disease.

Molecular mimicry of human cytokine and cytokine response pathway genes by KSHV.

Four virus proteins similar to two human macrophage inflammatory protein (MIP) chemokines, interleukin-6 (IL-6), and interferon regulatory factor (IRF) are encoded by the Kaposi's sarcoma-associated herpesvirus (KSHV) genome. vIL-6 was functional in B9 proliferation assays and primarily expressed in KSHV-infected hematopoietic cells rather than KS lesions. HIV-1 transmission studies showed that vMIP-I is similar to human MIP chemokines in its ability to inhibit replication of HIV-1 strains dependent on the CCR5 co-receptor. These viral genes may form part of the response to host defenses contributing to virus-induced neoplasia and may have relevance to KSHV and HIV-I interactions.

Epitopes of the CD4 antigen and HIV infection.

The CD4 (or T4) surface antigen of human T lymphocytes is an important part of the receptor for the human immunodeficiency virus (HIV). After binding to the receptor, the HIV may enter the T cell and induce the formation of syncytia. In an attempt to identify the receptor site more closely, monoclonal antibodies (Mab's) to CD4 were tested for their ability to block HIV infection in a syncytium formation assay, and the CD4 epitopes so identified were mapped by antibody cross-blocking. The antibodies that showed strong inhibition of HIV fell into two main families while a third group of Mab's blocked syncytia formation weakly or not at all. Several different isolates of HIV as well as the laboratory strain CBL1 grown in CEM cells were used to induce the syncytia. The data indicate that only some epitopes of CD4 are important for virus binding and imply that the virus-binding site for CD4 is conserved in different isolates of HIV with substantially divergent env gene sequences. Preliminary studies of patients suggest that polymorphism of these epitopes does not play a role in determining susceptibility to infection.

Productive infection and cell-free transmission of human T-cell leukemia virus in a nonlymphoid cell line.

Human T-cell leukemia virus (HTLV), American PL isolate, was transmitted by cocultivation and by cell-free filtrates to a nonlymphoid human osteogenic sarcoma (HOS) cell line, designated HOS/PL, but not to nine other lines bearing receptors for HTLV. HOS and HOS/PL cells are not dependent on interleukin-2 and do not express interleukin-2 receptors that are recognized by anti-Tac monoclonal antibody. HTLV released by the Japanese MT2 cell line was also transmitted to HOS cells. The infected HOS cells release substantial titers of progeny HTLV which is antigenically indistinguishable from parental virus and is able to transform T cells.