A biological breadboard platform for cell adhesion and detachment studies

Peptide-Mediated Platelet Capture at Gold Micropore Arrays

Ordered spherical cap gold cavity arrays with 5.4, 1.6, and 0.98 μm diameter apertures were explored as capture surfaces for human blood platelets to investigate the impact of surface geometry and chemical modification on platelet capture efficiency and their potential as platforms for surface enhanced Raman spectroscopy of single platelets. The substrates were chemically modified with single-constituent self-assembled monolayers (SAM) or mixed SAMs comprised of thiol-functionalized arginine-glycine-aspartic acid (RGD, a platelet integrin target) with or without 1-octanethiol (adhesion inhibitor). As expected, platelet adhesion was promoted and inhibited at RGD and alkanethiol modified surfaces, respectively. Platelet adhesion was reversible, and binding efficiency at the peptide modified substrates correlated inversely with pore diameter. Captured platelets underwent morphological change on capture, the extent of which depended on the topology of the underlying substrate. Regioselective capture of the platelets enabled study for the first time of the surface enhanced Raman spectroscopy of single blood platelets, yielding high quality Raman spectroscopy of individual platelets at 1.6 μm diameter pore arrays. Given the medical importance of blood platelets across a range of diseases from cancer to psychiatric illness, such approaches to platelet capture may provide a useful route to Raman spectroscopy for platelet related diagnostics.

Remote control of reversible localized protein adsorption in microfluidic devices

We present a facilely prepared graphene oxide (GO)/ poly(dimethylsiloxane) (PDMS) composite by dispersing nanosized GO in PDMS. On the basis of the combination of photothermal effects of GO and grafted thermoresponsive polymer, poly(N-isopropylacrylamide) (PNIPAAm), an optical-driving approach for remote control of localized wettability is realized. And this method has been successfully applied in the spatially controlled reversible protein adsorption in microfluidic devices.

Mechanically driven accumulation of microscale material at coupled solid-fluid interfaces in biological channels

The accumulation of microscale materials at solid-fluid interfaces in biological channels is often the initial stage of certain growth processes, which are present in some forms of atherosclerosis. The objective of this work is to develop a relatively simple model for such accumulation, which researchers can use to qualitatively guide their analyses. Specifically, the approach is to construct rate equations for the accumulation at the solid-fluid interface as a function of the intensity of the shear stress. The accumulation of material subsequently reduces the cross-sectional area of the channel until the fluid-induced shear stress at the solid-fluid interface reaches a critical value, which terminates the accumulation rate. Characteristics of the model are explored analytically and numerically.

The interaction of vinculin with actin

Vinculin can interact with F-actin both in recruitment of actin filaments to the growing focal adhesions and also in capping of actin filaments to regulate actin dynamics. Using molecular dynamics, both interactions are simulated using different vinculin conformations. Vinculin is simulated either with only its vinculin tail domain (Vt), with all residues in its closed conformation, with all residues in an open I conformation, and with all residues in an open II conformation. The open I conformation results from movement of domain 1 away from Vt; the open II conformation results from complete dissociation of Vt from the vinculin head domains. Simulation of vinculin binding along the actin filament showed that Vt alone can bind along the actin filaments, that vinculin in its closed conformation cannot bind along the actin filaments, and that vinculin in its open I conformation can bind along the actin filaments. The simulations confirm that movement of domain 1 away from Vt in formation of vinculin 1 is sufficient for allowing Vt to bind along the actin filament. Simulation of Vt capping actin filaments probe six possible bound structures and suggest that vinculin would cap actin filaments by interacting with both S1 and S3 of the barbed-end, using the surface of Vt normally occluded by D4 and nearby vinculin head domain residues. Simulation of D4 separation from Vt after D1 separation formed the open II conformation. Binding of open II vinculin to the barbed-end suggests this conformation allows for vinculin capping. Three binding sites on F-actin are suggested as regions that could link to vinculin. Vinculin is suggested to function as a variable switch at the focal adhesions. The conformation of vinculin and the precise F-actin binding conformation is dependent on the level of mechanical load on the focal adhesion.

The interface between a cell and its substrate is strengthened by the formation of focal adhesions. In this study molecular dynamics simulations are used to explore the connectivity of one focal adhesion forming protein, vinculin, and the cytoskeletal filament, F-actin. The simulations demonstrate: (1) that vinculin can link along F-actin at these focal adhesions when it adopts an open conformation, (2) that the vinculin tail (Vt) can bind F-actin at its barbed-end preventing actin polymerization, (3) that vinculin can adopt two open conformations, and (4) that the second open conformation is necessary for vinculin to cap the actin filament. The results suggest that vinculin can act as a variable switch, changing its shape and the nature of its interaction with F-actin depending on the level of stress seen at a focal adhesion. Under the highest stress vinculin would adopt the open II conformation and link anywhere on F-actin, even its barbed-end. Under less stress vinculin could adopt the open I conformation and bind along F-actin. And under minimal stress vinculin could adopt its closed conformation. This variability allows for vinculin to truly function as the cell's mechanical reinforcing agent.

Large-area patterning of the tackiness of a nanocomposite adhesive by sintering of nanoparticles under IR radiation

We present a simple technique to switch off the tack adhesion in selected areas of a colloidal nanocomposite adhesive. It is made from a blend of soft colloidal polymer particles and hard copolymer nanoparticles. In regions that are exposed to IR radiation, the nanoparticles sinter together to form a percolating skeleton, which hardens and stiffens the adhesive. The tack adhesion is lost locally. Masks can be made from silicone-coated disks, such as coins. Under the masks, adhesive island regions are defined with the surrounding regions being a nontacky coating. When optimizing the nanocomposite's adhesive properties, the addition of the hard nanoparticles raises the elastic modulus of the adhesive significantly, but adhesion is not lost because the yield point remains relatively low. During probe-tack testing, the soft polymer phases yield and enable fibrillation. After heating under IR radiation, the storage modulus increases by a factor of 5, and the yield point increases nearly by a factor of 6, such that yielding and fibrillation do not occur in the probe-tack testing. Hence, the adhesion is lost. Loading and unloading experiments indicate that a rigid skeleton is created when the nanoparticles sinter together, and it fractures under moderate strains. This patterning method is relatively simple and fast to execute. It is widely applicable to other blends of thermoplastic hard nanoparticles and larger soft particles.

An intelligent digital microfluidic system with fuzzy-enhanced feedback for multi-droplet manipulation

The complexity of droplet hydrodynamics on a digital microfluidic (DMF) system eventually weakens its potential for application in large-scale chemical/biological micro-reactors. We describe here an intelligent DMF technology to address that intricacy. A wide variety of control-engaged droplet manageability is proposed and demonstrated through the operation of our modular DMF prototype, which comprises: (i) rigid profiling ability of different droplet's hydrodynamics under a real-time trajectory track of droplet-derived capacitance, permitting accurate and autonomous multi-droplet positioning without visual setup and heavy image signal processing; (ii) fuzzy-enhanced controllability saving up to 21% charging time when compared with the classical approach, enhancing the throughput, fidelity and lifetime of the DMF chip, while identifying and renouncing those weakened electrodes deteriorated over time, and (iii) expert manipulability of multi-droplet routings under countermeasure decisions in real time, preventing droplet-to-droplet or task-to-task interference. Altogether, this work exhibits the first modular DMF system with built-in electronic-control software-defined intelligence to enhance the fidelity and reliability of each droplet operation, allowing future manufacturability of a wide range of life science analyses and combinatorial chemical screening applications.

Directed cytoskeleton self-organization

The cytoskeleton architecture supports many cellular functions. Cytoskeleton networks form complex intracellular structures that vary during the cell cycle and between different cell types according to their physiological role. These structures do not emerge spontaneously. They result from the interplay between intrinsic self-organization properties and the conditions imposed by spatial boundaries. Along these boundaries, cytoskeleton filaments are anchored, repulsed, aligned, or reoriented. Such local effects can propagate alterations throughout the network and guide cytoskeleton assembly over relatively large distances. The experimental manipulation of spatial boundaries using microfabrication methods has revealed the underlying physical processes directing cytoskeleton self-organization. Here we review, step-by-step, from molecules to tissues, how the rules that govern assembly have been identified. We describe how complementary approaches, all based on controlling geometric conditions, from in vitro reconstruction to in vivo observation, shed new light on these fundamental organizing principles.

Passive control of cell locomotion using micropatterns: the effect of micropattern geometry on the migratory behavior of adherent cells

Directed cell migration is critical to a variety of biological and physiological processes. Although simple topographical patterns such as parallel grooves and three-dimensional post arrays have been studied to guide cell migration, the effect of the dimensions and shape of micropatterns, which respectively represent the amount and gradient of physical spatial cues, on cell migration has not yet been fully explored. This motivates a quantitative characterization of cell migration in response to micropatterns having different widths and divergence angles. The changes in the migratory (and even locational) behavior of adherent cells, when the cells are exposed to physical spatial cues imposed by the micropatterns, are explored here using a microfabricated biological platform, nicknamed the "Rome platform". The Rome platform, made of a biocompatible, ultraviolet (UV) curable polymer (ORMOCOMP), consists of 3 μm thick micropatterns with different widths of 3 to 75 μm and different divergence angles of 0.5 to 5.0°. The migration paths through which NIH 3T3 fibroblasts move on the micropatterns are analyzed with a persistent random walk model, thus quantifying the effect of the divergence angle of micropatterns on cell migratory characteristics such as cell migration speed, directional persistence time, and random motility coefficient. The effect of the width of micropatterns on cell migratory characteristics is also extensively investigated. Cell migration direction is manipulated by creating the gradient of physical spatial cues (that is, divergence angle of micropatterns), while cell migration speed is controlled by modulating the amount of them (namely, width of micropatterns). In short, the amount and gradient of physical spatial cues imposed by changing the width and divergence angle of micropatterns make it possible to control the rate and direction of cell migration in a passive way. These results offer a potential for reducing the healing time of open wounds with a smart wound dressing engraved with micropatterns (or microscaffolds).