We consider a model of self-avoiding walk on a lattice with on-site repulsion and an attraction for every vertex of the walk visited on the surface to study force-induced desorption of a linear polymer chain adsorbed on an attractive surface and use the exact enumeration technique for analyzing how the critical force for desorption f c (T ) depends on the temperature. The curve f c (T )gives the boundary separating the adsorbed phase from the desorbed phase. Our results show that * Electronic address: pramod@justice.com, yashankit@yahoo.com, ysingh@bhu.ac.in When a long flexible polymer chain interacts with an impenetrable surface its conformational properties are strongly modified in comparison with its bulk properties [1, 2]. This is due to a subtle competition between the lowering of internal energy near an attractive surface and the loss of entropy due to constraints imposed by the impenetrable surface. For a strongly attractive surface, the polymer chain sticks to the surface, and for weak attraction it prefers to stay away from the surface. Thus there is a transition from the state when chain is mostly attached to the surface (adsorbed) to the state of detachment (desorbed) when the temperature is increased. The transition between theses two states is marked by a transition temperature T a with the adsorbed phase for T < T a and the desorbed phase for T > T a .A model of self avoiding-walk on a lattice with on-site repulsion and attraction energy for every vertex of the walk visited on the surface provides an adequate model for understanding the adsorption-desorption transition [3, 4, 5]. We extend this model of self-avoiding walk to study the force-induced desorption of a linear polymer chain adsorbed on an attractive surface and calculate the critical force f c (T ) for desorption as a function of temperature.Response of a polymeric chain to externally applied force can be measured experimentally by using techniques like optical or magnetic tweezers [6] and atomic force microscope [7].We consider self-avoiding walks (SAW s) that start from a point on an impenetrable surface and experience a force f in a direction perpendicular to the surface at the other end.The applied force, because of its direction, favours desorption and one expects a critical force, f c (T ), for desorption. At a given T when the applied force f is less than f c (T ) the polymer will be adsorbed while for f > f c (T ) the polymer will be desorbed. The curve f c (T ), therefore, gives the boundary that separates the desorbed phase from the adsorbed phase in the (f, T ) plane.Let z = 0 represents the surface and walks start from a point (origin) on the surface. In case of two dimensions (2-d) the surface is a line whereas in case of three dimensions (3-d) the surface is a plane. Since the surface is impenetrable, walks are restricted to half of the space (z ≥ 0) only. We enumerated all SAW s upto a certain length on a square lattice in 2-d and on a cubic lattice in 3-d. Let C N (N s , h) be the number of SAW s of N vertex (a vertex of...

We use the lattice model of directed walks to investigate the conformational as well as the adsorption properties of a semiflexible homopolymer chain immersed in a good solvent in two and three dimensions. To account for the stiffness in the chain we have introduced energy barrier for each bend in the walk and have calculated the persistent length as a function of this energy. For the adsorption on an impenetrable surface perpendicular to the preferred direction of the walk we have solved the model exactly and have found the critical value of the surface attractions for the adsorption in both two and three dimensions.We have also enumerated all the possible walks on square and cubic lattices for the number of steps N ≤ 30 for two-dimensions and N ≤ 20 for three dimensions and have used ratio method for extrapolation. The transition located using this method is in excellent agreement with the results found from the analytical method.

We describe the fabrication and use of an in vitro wounding device that denudes cultured epithelium in patterns designed to leave behind strips or islands of cells sufficiently narrow or small to ensure that all remaining cells become rapidly activated and then migrate, dedifferentiate and proliferate in near synchrony. The design ensures that signals specific to regenerating cells do not become diluted by quiescent differentiated cells that are not affected by wound induced activation. The device consists of a flat circular disk of rubber engraved to produce alternating ridges and grooves in patterns of concentric circles or parallel lines. The disk is mounted at the end of a pneumatically controlled piston assembly. Application of controlled pressure and circular or linear movement of the disk on cultures produced highly reproducible wounding patterns. The near synchronous regenerative activity of cell bands or islands permitted the collection of samples large enough for biochemical studies to sensitively detect alterations involving mRNA for several early response genes and protein phosphorylation in major signaling pathways. The method is versatile, easy to use and reproducible, and should facilitate biochemical, proteomic and genomic studies of wound induced regeneration of cultured epithelium.

A long flexible neutral polymer chain immersed in a poor solvent and interacting with an impenetrable attractive surface exhibits a phase known as surface attached globule (SAG) in addition to other adsorbed and desorbed phases. In the thermodynamic limit, the SAG phase has the same free energy per monomer as the globular phase, and the transition between them is a surface transition. We have investigated the phase diagrams of such a chain in both two-and three-dimensions and calculated the distribution of monomers in different domains of the phase diagram.

The equilibrium thermodynamic properties of a linear polymer chain confined to a space between two impenetrable walls (lines) at a distance D under various solvent conditions have been studied using series analysis and exact enumeration technique. We have calculated the end-to-end distance of polymer chain, which shows a nonmonotonic behavior with inter wall separation D. The density distribution profile shows a maxima at a particular value of (D=)D*. Around this D*, our results show that the collapse transition occurs at higher temperature as compared to its bulk value of 2d and 3d. The variation of theta-temperature with D shows a re-entrance behavior. We also calculate the force of compression exerted by the walls (lines) on the polymer.