Polymer fluctuation lubrication in hydrogel gemini interfaces.

Interfacial sliding speed and contact pressure between the sub-units of particulate soft matter assemblies can vary dramatically across systems and with dynamic conditions. By extension, frictional interactions between particles may play a key role in their assembly, global configuration, collective motion, and bulk material properties. For example, in tightly packed assemblies of microgels - colloidal microspheres made of hydrogel - particle stiffness controls the fragility of the glassy state formed by the particles. The interplay between particle stiffness and shear stress is likely mediated by particle-particle normal forces, highlighting the potential role of hydrogel-hydrogel friction. Here we study friction at a twinned "Gemini" interface between hydrogels. We construct a lubrication curve that spans four orders of magnitude in sliding speed, and find qualitatively different behaviour from traditional lubrication of engineering material surfaces; fundamentally different types of lubrication occur at the hydrogel Gemini interface. We also explore the role played by polymer solubility and hydrogel-hydrogel adhesion in hydrogel friction. We find that polymer network elasticity, mesh size, and single-chain relaxation times can describe friction at the gel-gel interface, including a transition between lubrication regimes with varying sliding speed.

Effect of systemic high dose vitamin C therapy on forearm blood flow reactivity during endotoxemia in healthy human subjects.

OBJECTIVE: Acute inflammation induced by administration of Escherichia coli lipopolysaccharide endotoxin (LPS) reduces plasma concentrations of vitamin C and impairs vascular endothelium-derived nitric oxide (NO) bioactivity. We tested the hypothesis that systemically administered high dose vitamin C restores the endogenous anti-oxidant potential and improves NO-dependent vasodilatation in the forearm vasculature. DESIGN & SETTING: 36 male subjects were enrolled in this balanced, placebo controlled cross-over study. Forearm blood flow (FBF) reactivity to acetylcholine (ACh) and glyceryl-trinitrate (GTN), a sensitive test for endothelial function, was assessed at baseline and 4h after LPS-administration (20 IU/kg i.v). The effect of two different doses of intravenous vitamin C (Vitamin C-Injektopas®), 320 mg/kg and 480 mg/kg over 2h, or placebo on forearm vascular function was studied after LPS. MAIN RESULTS: LPS caused transient flu-like symptoms, decreased plasma vitamin C concentrations and reduced the ACh-dependent increase in FBF by up to 76%. Vitamin C at a mean plasma concentration of 3.2 or 4.9 mmol/L restored the response to ACh compared to baseline. CONCLUSION: High dose systemic vitamin C recovers LPS-induced endothelium-dependent vasodilation in the forearm resistance vasculature. This provides a rationale for a further clinical study of the systemic vitamin C effect under inflammatory conditions.

Static or breakloose friction for lubricated contacts: the role of surface roughness and dewetting.

We present experimental data for the static or breakloose friction for lubricated elastomer contacts, as a function of the time of stationary contact. Due to fluid squeeze-out from the asperity contact regions, the breakloose friction force increases continuously with the time of stationary contact. We consider three different cases: (a) PDMS rubber balls against flat smooth glass surfaces, (b) PDMS cylinder ribs against different substrates (glass, smooth and rough PMMA and an inert polymer) and (c) application to syringes. Due to differences in the surface roughness and contact pressures the three systems exhibit very different time dependences of the breakloose friction. In case (a) for rough surfaces the dry contact area A is a small fraction of the nominal contact area A0, and the fluid squeeze-out is fast. In case (b) the dry contact area is close to the nominal contact area, A/A0 ≈ 1, and fluid squeeze-out is very slow due to percolation of the contact area. In this case, remarkably, different fluids with very different viscosities, ranging from 0.005 Pa s (water­glycerol mixture) to 1.48 Pa s (glycerol), give very similar breakloose friction forces as a function of the time of stationary contact. In case (c) the contact pressure and the surface roughness are larger than in case (b), and the squeeze-out is very slow so that even after a very long time the area of real contact is below the percolation threshold. For all cases (a)­(c), the increase in the breakloose friction is mainly due to the increase in the area of real contact with increasing time, because of the fluid squeeze-out and dewetting.

Adhesion: role of bulk viscoelasticity and surface roughness.

We study the adhesion between smooth polydimethylsiloxane (PDMS) rubber balls and smooth and rough poly(methyl methacrylate) (PMMA) surfaces, and between smooth silicon nitride balls and smooth PDMS surfaces. From the measured viscoelastic modulus of the PDMS rubber we calculate the viscoelastic contribution to the crack-opening propagation energy γeff(v,T) for a wide range of crack tip velocities v and for several temperatures T. The Johnson-Kendall-Roberts (JKR) contact mechanics theory is used to analyze the ball pull-off force data, and γeff(v,T) is obtained for smooth and rough surfaces. We conclude that γeff(v,T) has contributions of similar magnitude from both the bulk viscoelastic energy dissipation close to the crack tip, and from the bond-breaking process at the crack tip. The pull-off force on the rough surfaces is strongly reduced compared to that of the flat surface, which we attribute mainly to the decrease in the area of contact on the rough surfaces.

Elastic contact mechanics: percolation of the contact area and fluid squeeze-out.

The dynamics of fluid flow at the interface between elastic solids with rough surfaces depends sensitively on the area of real contact, in particular close to the percolation threshold, where an irregular network of narrow flow channels prevails. In this paper, numerical simulation and experimental results for the contact between elastic solids with isotropic and anisotropic surface roughness are compared with the predictions of a theory based on the Persson contact mechanics theory and the Bruggeman effective medium theory. The theory predictions are in good agreement with the experimental and numerical simulation results and the (small) deviation can be understood as a finite-size effect. The fluid squeeze-out at the interface between elastic solids with randomly rough surfaces is studied. We present results for such high contact pressures that the area of real contact percolates, giving rise to sealed-off domains with pressurized fluid at the interface. The theoretical predictions are compared to experimental data for a simple model system (a rubber block squeezed against a flat glass plate), and for prefilled syringes, where the rubber plunger stopper is lubricated by a high-viscosity silicon oil to ensure functionality of the delivery device. For the latter system we compare the breakloose (or static) friction, as a function of the time of stationary contact, to the theory prediction.

Molecular phylogeny and taxonomy of medically important fungi.

Some recent advances in study of molecular evolution and taxonomy of human pathogens are discussed. In systemic Onygenales as well as in Chaetothyriales, pathogenic species are phylogenetically intermingled with non-pathogenic taxa. When a teleomorph of Coccidioides immitis is eventually found, it is predicted to resemble Uncinocarpus, a genus otherwise comprising environmental species. In the dermatophytes, Trichophyton and Microsporum are paraphyletic, whereas Epidermophyton is polyphyletic. On the basis of 18S and ITS rDNA sequencing data, Exophiala anamorphs (black yeasts) are confirmed to be closely related to the ascomycete genus Capronia. The related neurotropic species Cladophialophora bantiana is remarkable in consistently having introns in its 18S rDNA gene.

Phylogenetic analysis of ten black yeast species using nuclear small subunit rRNA gene sequences.

The nuclear small subunit rRNA genes of authentic strains of the black yeasts Exophiala dermatitidis, Wangiella dermatitidis, Sarcinomyces phaemuriformis, Capronia mansonii, Nadsoniella nigra var. hesuelica, Phaeoannellomyces elegans, Phaeococcomyces exophialae, Exophiala jeanselmei var. jeanselmei and E. castellanii were amplified by PCR and directly sequenced. A putative secondary structure of the nuclear small subunit rRNA of Exophiala dermatitidis was predicted from the sequence data. Alignment with corresponding sequences from Neurospora crassa and Aureobasidium pullulans was performed and a phylogenetic tree was constructed using the neighbor-joining method. The obtained topology of the tree was confirmed by bootstrap analysis. Based upon this analysis all fungi studied formed a well-supported monophyletic group clustering as a sister group to one group of the Plectomycetes (Trichocomaceae and Onygenales). The analysis confirmed the close relationship postulated between Exophiala dermatitidis, Wangiella dermatitidis and Sarcinomyces phaeomuriformis. This monophyletic clade also contains the telemorph species Capronia mansonii thus confirming the concept of a teleomorph connection of the genus Exophiala to a member of the herpotrichiellaceae. However, Exophiala castellanii did not belong to this clade. Therefore, this species is not the anamorph of Capronia mansonii as it was postulated.

Nutrient uptake by the hindlimb of growing pigs treated with porcine somatotropin and insulin.

The shift in nutrient partitioning induced by porcine somatotropin (pST) is accompanied by a decrease in insulin sensitivity for whole-body glucose uptake. The relative contribution of metabolic changes in the hindlimb was investigated in eight pigs (55 kg) that had received recombinant pST (120 micrograms/kg) or excipient (control) for 7 d. Uptake of metabolites by the hindlimb was measured under basal conditions and during hyperinsulinemic-euglycemic clamps at low [14 ng/(kg.min)] and high [360 ng/(kg.min)] insulin infusion rates. Dextrose infusion rate required to maintain euglycemia was used as an index of whole-body glucose uptake in response to exogenous insulin. Effects of pST on hindlimb and whole-body glucose uptake were evident only at physiological levels of insulin (basal and low insulin infusion rate). During the low rate of insulin infusion, dextrose infusion rate was 79% lower for pST-treated pigs and glucose uptake by the hindlimb was 59% lower compared with control pigs. The decrease in glucose uptake by the hindlimb was entirely accounted for by the estimated reduction in glucose utilization by adipose tissue of the hindlimb. Glucose:oxygen quotients were reduced during basal (57%) and low insulin infusion (63%) with pST treatment, indicating a change in the pattern of substrate utilization. This is consistent with the concept that pST directs nutrients away from adipose and towards muscle growth by altering the response of tissues to homeostatic signals such as insulin.

Porcine somatotropin affects the dietary lysine requirement and net lysine utilization for growing pigs.

This study was conducted to determine the effects of exogenous porcine somatotropin (pST) on the dietary lysine requirement and efficiency of absorbed lysine utilization for pigs during the 20- to 60-kg phase of growth. Seventy-two crossbred pigs (20 +/- 0.7 kg body wt) received daily intramuscular injections of either excipient (0 dose) or pST (150 micrograms/kg body wt) and were fed diets in which protein and lysine concentrations ranged from 6.4 to 23.5 g/100 g diet and from 0.40 to 1.48 g/100 g diet, respectively. Nutrient density was altered to compensate for reduced feed intake with pST, but diets were approximately isocaloric. Rate and efficiency of gain and whole-body protein accretion rate exhibited a dose-response improvement (P < 0.01) to increases in dietary protein for both excipient and pST-treated pigs. Pigs receiving pST grew more rapidly and more efficiently than control counterparts (P < 0.01). Treatment with pST improved the rate of protein accretion (P < 0.01) at all but the lowest level of dietary protein. The net lysine utilization for lysine accretion and maintenance was 0.46 for control pigs and 0.57 for pigs receiving pST, a 24% improvement in the efficiency. Thus, treatment with pST increased the maximum rate of protein accretion as well as the partial efficiency with which dietary lysine is used for protein accretion. Consequently, only a 9% increase in dietary lysine was required to maximize protein deposition in pST-treated pigs, because the metabolic efficiency of lysine utilization was improved.

Dose-dependent effects of exogenous porcine somatotropin on the yield, distribution, and proximate composition of carcass tissues in growing pigs.

Dose-dependent effects of porcine somatotropin (pST) on mass, distribution, and proximate composition of carcass tissues were investigated in 46 growing pigs. Barrows, weighing 30 +/- 1 kg, were assigned to five treatment groups to receive 0, 50, 100, 150, or 200 micrograms of recombinant pST/kg BW per day until pigs individually reached the 90-kg slaughter weight. Left carcass sides were fabricated into whole-sale cuts that were separated into muscle, adipose tissue, bone, and skin. Despite the reduction in dressing percentage, total muscle mass of the side was significantly increased by 3.9 to 5.7 kg (28 to 36%) by the lowest and highest doses of pST, respectively, whereas adipose tissue mass was decreased by 4.4 to 8.6 kg (38 to 74%). Bone mass was increased by 8 to 27% (P < .05), and skin mass was increased by 16 to 38% (P < .01) across the dose range. Distribution of carcass weight among the wholesale cuts was altered by pST toward lower proportions in the belly, jowl, and fat trimmings and greater proportions in the four lean cuts. Porcine somatotropin substantially reduced lipid concentration in all muscle groups in a dose-dependent manner, resulting in increased protein and moisture concentrations (P < .05). Adipose tissue lipid concentrations were reduced to an even greater extent at each dose. The progressive increase in muscle mass observed with pST doses > 50 micrograms/kg BW was less than the associated decrease in adipose tissue mass, indicating that a dose range of 50 to 100 micrograms/kg BW per day may be optimum for improving carcass value.

Influence of genotype and sex on the response of growing pigs to recombinant porcine somatotropin.

The dose-dependent effects of porcine somatotropin (pST) on growth performance and composition of carcass gain were investigated in 150 growing pigs. The experiment involved two genotypes (barrows from the Pig Improvement Company [PIC] and a University of Nebraska [NEB] gene pool line) and two sexes (PIC barrows and boars). At 30 kg, pigs were randomly assigned within each genotype and sex subclass to receive daily i.m. injections of 50, 100, 150, or 200 micrograms of pST/kg BW or an equivalent volume of an excipient. A diet (3.5 Mcal of DE/kg) supplemented with crystalline amino acids and containing 22.5% CP was available on an ad libitum basis until pigs were slaughtered at approximately 90 kg live weight. Excipient-treated PIC barrows exhibited faster and more efficient growth (P less than .001) and a higher capacity for carcass protein accretion (P less than .001) but similar rates of lipid deposition compared to excipient-treated NEB barrows. Within the PIC genotype, control boars grew at a rate similar to that of barrows, but they were more efficient (P less than .05) and deposited more carcass protein (P less than .05) and less lipid (P less than .001). Carcass protein accretion rate increased (P less than .001) up to approximately 150 micrograms of pST.kg BW-1.d-1, whereas lipid deposition decreased (P less than .001) with each incremental dose of pST. Although differences between PIC boars and barrows for all criteria were negated with increasing pST dose, they were maintained between the two genotypes. Polynomial regressions suggested that a slightly higher pST dose was required to optimize the feed:gain ratio compared with rate of gain and that the dose (micrograms per kilogram BW per day) was a function of the genotype and sex (feed:gain: 185, 170, and 155; rate of gain: 155, 155, and 125 for NEB barrows, PIC barrows, and PIC boars, respectively).