Nonequilibrium critical dynamics of the two-dimensional Ashkin-Teller model at the Baxter line.

We investigate the short-time universal behavior of the two-dimensional Ashkin-Teller model at the Baxter line by performing time-dependent Monte Carlo simulations. First, as preparatory results, we obtain the critical parameters by searching the optimal power-law decay of the magnetization. Thus, the dynamic critical exponents θ_{m} and θ_{p}, related to the magnetic and electric order parameters, as well as the persistence exponent θ_{g}, are estimated using heat-bath Monte Carlo simulations. In addition, we estimate the dynamic exponent z and the static critical exponents ß and ν for both order parameters. We propose a refined method to estimate the static exponents that considers two different averages: one that combines an internal average using several seeds with another, which is taken over temporal variations in the power laws. Moreover, we also performed the bootstrapping method for a complementary analysis. Our results show that the ratio ß/ν exhibits universal behavior along the critical line corroborating the conjecture for both magnetization and polarization.

TGF-ß1 activation in human hamstring cells through growth factor binding peptides on polycaprolactone surfaces.

The administration of soluble growth factors (GFs) to injured tendons and ligaments (T/L) is known to promote and enhance the healing process. However, the administration of GFs is a complex, expensive and heavily-regulated process and only achieved by employing supraphysiological GF concentrations. In addition, for proper healing, specific and spatial immobilization of the GFs (s) is critical. We hypothesized that biomaterials functionalized with GF-binding peptides can be employed to capture endogenous GFs in a spatially-controlled manner, thus overcoming the need for the exogenous administration of supraphysiological doses of GFs. Here we demonstrate that the modification of films of polycaprolactone (PCL) with transforming growth factor ß1 (TGF-ß1)-binding peptides allows GFs to be captured and presented to the target cells. Moreover, using a TGF-ß reporter cell line and immunocytochemistry, we show that the GFs retained their biological activity. In human primary tendon cells, the immobilized TGF-ß1 activated TGF-ß target genes ultimately lead to a 2.5-fold increase in total collagen matrix production. In vivo implantation in rats clearly shows an accumulation of TGF-ß1 on the polymer films functionalized with the TGF-ß1-binding peptide when compared with the native films. This accumulation leads to an increase in the recruitment of inflammatory cells at day 3 and an increase in the fibrogenic response and vascularization around the implant at day 7. The results herein presented will endow current and future medical devices with novel biological properties and by doing so will accelerate T/L healing. STATEMENT OF SIGNIFICANCE: Our study describes the possibility to deliver hTGF-ß1 to human derived hamstring cells using a non-covalent bioactive strategy. The significance of our results in vivo with our functionalized biomaterial with TGF-ß1-binding peptides lies in the fact that these materials can now be employed to capture endogenous TGF-ß1 in a spatially-controlled manner, overcoming the need for exogenous administration of supra-physiological TGF-ß1 doses. Our method is different from current solutions that rely on global TGF-ß1 administration, soaking the devices with TGF-ß1, etc. Therefore we believe that our method is a significant change from current state-of-the-art in the types of devices that are used for ligament/tendon repair and that following our method can endow current and future medical devices with TGF-ß1 binding properties.

Statistics, distillation, and ordering emergence in a two-dimensional stochastic model of particles in counterflowing streams.

In this paper, we propose a stochastic model which describes two species of particles moving in counterflow. The model generalizes the theoretical framework that describes the transport in random systems by taking into account two different scenarios: particles can work as mobile obstacles, whereas particles of one species move in the opposite direction to the particles of the other species, or particles of a given species work as fixed obstacles remaining in their places during the time evolution. We conduct a detailed study about the statistics concerning the crossing time of particles, as well as the effects of the lateral transitions on the time required to the system reaches a state of complete geographic separation of species. The spatial effects of jamming are also studied by looking into the deformation of the concentration of particles in the two-dimensional corridor. Finally, we observe in our study the formation of patterns of lanes which reach the steady state regardless of the initial conditions used for the evolution. A similar result is also observed in real experiments involving charged colloids motion and simulations of pedestrian dynamics based on Langevin equations, when periodic boundary conditions are considered (particles counterflow in a ring symmetry). The results obtained through Monte Carlo simulations and numerical integrations are in good agreement with each other. However, differently from previous studies, the dynamics considered in this work is not Newton-based, and therefore, even artificial situations of self-propelled objects should be studied in this first-principles modeling.

Global persistence exponent of the double-exchange model.

We obtained the global persistence exponent for a continuous spin model on the simple cubic lattice with double-exchange interaction by using two different methods. First, we estimated the exponent theta(g) by following the time evolution of probability P(t) that the order parameter of the model does not change its sign up to time t[P(t) approximately t(-theta(g)]. Afterwards, that exponent was estimated through the scaling collapse of the universal function L(theta(g)(z)P(t) for different lattice sizes. Our results for both approaches are in very good agreement with each other.