This process was repeated to build multiple layers, abbreviated as (PDAC/SPS)n, wherenrepresents the number of PDAC/SPS bilayers (BLs), and equals to 10, 20, 30, 40 or 50 with SPS as the topmost layer in each case. focal adhesion == Introduction == A major challenge in the field of tissue engineering is to optimize the surface characteristics to achieve a controlled or UPF-648 desired level of cell adhesion under physiological conditions. The cell adhesive property of the surfaces can be modulated through various physical, chemical and mechanical properties of the surface, individually or in combination. These properties include the hydrophobicity and hydrophilicity1, surface charge2, surface roughness or topography3,4, and stiffness5-7of the substrate. Layer-by-layer (LbL) assembled polyelectrolyte multilayer (PEM) thin films, introduced by Decher8, provide a versatile approach for altering physical, chemical or mechanical properties of a substrate UPF-648 to address this challenge. Over the past decade, LbL films have shown promise for various clinically relevant biological applications9. For example, cytophobic (cell resistive) LbL thin film coatings on implantable hydrogels for nerve repair applications4,10have been put forth as a possible method for controlling the growth of leptomeningeal fibroblasts, which hinder the progression of regenerating axons4. LbL films have also been applied to create three-dimensional cellular multilayers11, patterned co-cultures2, microarrays12, biosensors13, functional cell surfaces14, etc. Many of these applications capitalize around the tunability of the cell adhesive behavior around the thin films. Different deposition parameters, such as the type and composition of polyelectrolytes15-18, pH19-22and salt concentration19,23-30, during LbL fabrication influence the film swelling, hydration, and mobility of the polymer chains within the films. These factors affect the intrinsic properties of the LbL films, such as surface roughness, stiffness, degree of hydration, and thickness, which in-turn, alter the cytophobic or cytophilic characteristic of the surface15,16,19,21,31,32. Here, we show that increasing the number of bilayers of PDAC/SPS films from 10 to 20, corresponding to a film thickness of 37.6nm (~ 40nm) to 95.9nm (~ 100 nm), respectively, switches the films from a cytophilic to a cytophobic surface (Schematic 1). We demonstrate this effect with bone marrow mesenchymal stem cells (MSCs) and NIH3T3 fibroblasts. The thickness increases linearly as the number of bilayers increases, causing a shift to cytophobic behavior with concomitant decrease in cell spreading and adhesion (see Results and Discussion). A factor previously shown to influence the cell adhesion of linearly growing PEMs consisting of strong polyelectrolytes, i.e. high ionic strength of the UPF-648 deposition salts19which causes film swelling and hydration, was kept constant CREB4 in this study. Therefore, the salt concentration cannot explain the switch in the adhesive behavior observed as the number of bilayers (deposition cycles) increases. == Schematic 1. == Diagram showing multilayers composed of linearly growing strong polyelectrolytes i.e. PDAC and SPS, fabricated at a deposition ionic strength of 0.1M NaCl, exhibit increased cytophobicity as the number of bilayers increases, as shown in images (A) to (E). Bands with violet and blue colors represents positively charged PDAC and negatively charged SPS polyelectrolyte chains, respectively; and one set of violet/purple colored band represents ten bilayers of PDAC/SPS. Red, green and blue colors inside the cell structure represent actin filaments, focal adhesion contacts and nucleus of the cell, respectively. Image (F) illustrates a previous study19with a higher deposition ionic strength, the multilayers exhibit more cytophobicity due to swelling and hydration within the multilayer structure. The thickness band in image (F) represents a more loopy configuration of the polyelectrolytes with enhanced swelling and hydration within the multilayer19as compared to those in images (A-E). Cellular adhesion behavior in response to their physical surroundings (i.e. substrate) are modulated through mechanotransduction33-35. In order to sense mechanical states or changes in their surroundings, cells actively apply traction forces around the substrate through focal adhesion proteins and complexes. The mechanical response of the substrate from active probing is then transmitted through the pre-stressed cytoskeleton by actin filaments and other signaling molecules, finally reaching the inner nuclear membrane proteins34-36. The pre-stress is the pre-existing tensional stress borne by the cytoskeleton. The fidelity and velocity of this intracellular mechanical signaling is modulated through the pre-stress of the cytoskeletal filaments. It is suggested that remodeling of the focal adhesions plays a critical role.