Sublinear drag regime at mesoscopic scales in viscoelastic materials.

Stressed soft materials commonly present viscoelastic signatures in the form of power-law or exponential decay. Although exponential responses are the most common, power-law time dependencies arise peculiarly in complex soft materials such as living cells. Understanding the microscale mechanisms that drive rheologic behaviors at the macroscale shall be transformative in fields such as material design and bioengineering. Using an elastic network model of macromolecules immersed in a viscous fluid, we numerically reproduce those characteristic viscoelastic relaxations and show how the microscopic interactions determine the rheologic response. The macromolecules, represented by particles in the network, interact with neighbors through a spring constant k and with fluid through a non-linear drag regime. The dissipative force is given by γvα, where v is the particle's velocity, and γ and α are mesoscopic parameters. Physically, the sublinear regime of the drag forces is related to micro-deformations of the macromolecules, while α ≥ 1 represents rigid cases. We obtain exponential or power-law relaxations or a transitional behavior between them by changing k, γ, and α. We find that exponential decays are indeed the most common behavior. However, power laws may arise when forces between the macromolecules and the fluid are sublinear. Our findings show that in materials not too soft not too elastic, the rheological responses are entirely controlled by α in the sublinear regime. More specifically, power-law responses arise for 0.3 ⪅ α ⪅ 0.45, while exponential responses for small and large values of α, namely, 0.0 ⪅ α ⪅ 0.2 and 0.55 ⪅ α ⪅ 1.0.

Double power-law viscoelastic relaxation of living cells encodes motility trends.

Living cells are constantly exchanging momentum with their surroundings. So far, there is no consensus regarding how cells respond to such external stimuli, although it reveals much about their internal structures, motility as well as the emergence of disorders. Here, we report that twelve cell lines, ranging from healthy fibroblasts to cancer cells, hold a ubiquitous double power-law viscoelastic relaxation compatible with the fractional Kelvin-Voigt viscoelastic model. Atomic Force Microscopy measurements in time domain were employed to determine the mechanical parameters, namely, the fast and slow relaxation exponents, the crossover timescale between power law regimes, and the cell stiffness. These cell-dependent quantities show strong correlation with their collective migration and invasiveness properties. Beyond that, the crossover timescale sets the fastest timescale for cells to perform their biological functions.

Microrheology of cells with magnetic force modulation atomic force microscopy.

We propose a magnetic force modulation method to measure the stiffness and viscosity of living cells using a modified AFM apparatus. An oscillating magnetic field makes a magnetic cantilever oscillate in contact with the sample, producing a small AC indentation. By comparing the amplitude of the free cantilever motion (A0) with the motion of the cantilever in contact with the sample (A1), we determine the sample stiffness and viscosity. To test the method, the frequency-dependent stiffness of 3T3 fibroblasts was determined as a power law k(s)(f) = α + ß(f/f¯)(γ) (α = 7.6 × 10(-4) N m(-1), ß = 1.0 × 10(-4) N m(-1), f¯ = 1 Hz, γ = 0.6), where the coefficient γ = 0.6 is in good agreement with rheological data of actin solutions with concentrations similar to those in cells. The method also allows estimation of the internal friction of the cells. In particular we found an average damping coefficient of 75.1 µN s m(-1) for indentation depths ranging between 1.0 µm and 2.0 µm.

Comparison of the viscoelastic properties of cells from different kidney cancer phenotypes measured with atomic force microscopy.

The viscoelastic properties of human kidney cell lines from different tumor types (carcinoma (A-498) and adenocarcinoma (ACHN)) are compared to a non-tumorigenic cell line (RC-124). Our methodology is based on the mapping of viscoelastic properties (elasticity modulus E and apparent viscosity η) over the surface of tens of individual cells with atomic force microscopy (AFM). The viscoelastic properties are averaged over datasets as large as 15000 data points per cell line. We also propose a model to estimate the apparent viscosity of soft materials using the hysteresis observed in conventional AFM deflection-displacement curves, without any modification to the standard AFM apparatus. The comparison of the three cell lines show that the non-tumorigenic cells are less deformable and more viscous than cancerous cells, and that cancer cell lines have distinctive viscoelastic properties. In particular, we obtained that E(RC-124) > E(A-498) > E(ACHN) and η(RC-124) > Î·(A-498) > Î·(ACHN).

Lactobacillus GG administered in oral rehydration solution to children with acute diarrhea: a multicenter European trial.

BACKGROUND: The probiotic Lactobacillus GG is effective in promoting a more rapid recovery of acute, watery diarrhea in children with rotavirus enteritis. Very limited information is available, however, on the potential role of such agents in non-rotaviral diarrheal episodes. Furthermore, no evidence is available concerning the efficacy of Lactobacillus GG administered in the oral rehydration solution during oral rehydration therapy. A multicenter trial was conducted to evaluate the efficacy of Lactobacillus GG administered in the oral rehydration solution to patients with acute-onset diarrhea of all causes. METHODS: Children 1 month to 3 years of age with acute-onset diarrhea were enrolled in a double-blind, placebo-controlled investigation. Patients were randomly allocated to group A, receiving oral rehydration solution plus placebo, or group B, receiving the same preparation but with a live preparation of Lactobacillus GG (at least 10(10) CFU/250 ml). After rehydration in the first 4 to 6 hours, patients were offered their usual feedings plus free access to the same solution until diarrhea stopped. RESULTS: One hundred forty children were enrolled in group A, and 147 in group B. There were no differences at admission between the groups in age, sex, previous types of feeding, previous duration of diarrhea, use of antibiotics, weight, height, weight-height percentile, prevalence of fever, overall status, degree of dehydration, and percentage of in- versus outpatients. Duration of diarrhea after enrollment was 71.9 +/- 35.8 hours in group A versus 58.3 +/- 27.6 hours in group B (mean +/- SD; P = 0.03). In rotavirus-positive children, diarrhea lasted 76.6 +/- 41.6 hours in group A versus 56.2 +/- 16.9 hours in groups B (P < 0.008). Diarrhea lasted longer than 7 days in 10.7% of group A versus 2.7% of group B patients (P < 0.01). Hospital stays were significantly shorter in group B than in group A. CONCLUSIONS: Administering oral rehydration solution containing Lactobacillus GG to children with acute diarrhea is safe and results in shorter duration of diarrhea, less chance of a protracted course, and faster discharge from the hospital.

[Acute infectious diarrhea in children]. / Diarreia aguda infecciosa na criança.

Diarrhoea is defined either clinically or pathophysiologically. Some epidemiological issues are considered in a global as well as in a national perspective. The main etiologies and pathophysiological mechanisms are referred. Some important natural defenses against intestinal infection are pointed out. Clinical manifestations are briefly described. The main therapeutical guidelines are discussed. Breast milk is referred to as one of the most important preventive measures against acute infectious diarrhoea.

[Anomalous insertion of mitral papillary muscles in obstructive hypertrophic myocardiopathy. Report of 2 cases]. / Insercão anómala de músculos papilares mitrais na miocardiopatia hipertrófica obstrutiva. A propósito de dois casos.

Hypertrophic obstructive cardiomyopathy is a pathologic entity whose features are complex and diverse. Its surgical treatment generally requires myectomy. When the left ventricle outflow gradient is also due to mitral valve abnormalities, mitral valve replacement has been considered as the treatment of choice for the relief of the obstruction. We report two cases in which transesophageal echocardiography has allowed a good definition of this complex area of the heart and a better diagnosis and treatment of its components. Its use led to the diagnosis of abnormal papillary muscles inserted directly in the anterior leaflet of the mitral valve, with severe left ventricle outflow obstruction. Surgical treatment consisted of septal myectomy and excision of the muscles, thus preventing the need for mitral replacement. Systolic anterior motion of the mitral valve subsided and the outflow gradients became negligible. This new approach for the treatment of hypertrophic obstructive cardiomyopathy will be of benefit for patients in whom the anomalous insertion of the papillary muscles are recognized before surgery, thus preventing mitral valve replacement.