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Invited speakers

Wilhelm Huck (Radboud University, Institute for molecules and materials) (10.30-11.15)
Cells and gels: matrix elasticity and spreading dynamics regulate cell behaviour
The mechanical properties of the extracellular matrix (ECM) have emerged as an important microenvironmental cue regulating cell spreading and cell fate decisions. Studies on mechanically tuneable synthetic substrates, such as polyacrylamide hydrogels coated with collagen or other ECM proteins, have demonstrated a strong correlation between hydrogel stiffness and cell adhesion, spreading, proliferation and differentiation. In this talk, I will show results of epidermal stem cell differentiation on a range of soft substrates as well as on controlled cellular patterns that limit collective behaviour, and dissociate the different chemical, mechanical and topological parameters involved in dictating cell phenotype. A crucial aspect that has received little attention thus far is the dynamic nature of how complex systems such as cells adapt to their environment. Complex networks could well share some characteristics even if the system as a whole is in a different state, and therefore the cellular response to mechanical properties might not be known when only looking at single observables as cell spreading, cell shape or lineage selection, at a single point in time. I will discuss unpublished findings, where wel demonstrate how hMSCs adapt to different substrates via different trajectories, resulting in steady state behaviour that is similar in certain aspects, but leads to fundamentally different cell states.
Indrani Chakraborty (University of Leiden) (11.15-11.45)
Colloidal Joints with Designed Motion Range and Tunable Joint Flexibility
The miniaturization of machines towards the micron- and nanoscales requires the development of joint-like elements that enable and constrain motion. We present a facile method to create colloidal joints, that is, anisotropic colloidal particles that control the motion range of bonded particles. We demonstrate quantitatively that we can control the flexibility of these colloidal joints by tuning the DNA linker concentration in the bond area. We show that the colloidal shape controls the range of motion that these colloidal joints enable, due to the finite-sized patch of bonded linkers that cannot cross regions of high curvature. Finally we demonstrate the potential of the colloidal joints for programmable bottom-up self-assembly by creating flexible colloidal molecules and colloidal polymers. The reconfigurability and motion constraint offered by our colloidal joints make them promising building blocks for the development of switchable materials and nanorobots.
Bas Overvelde (Amolf) (13.30-14.15)
Embracing Compliance and Instabilities to Achieve Function in Mechanical Metamaterials and Devices
The use of soft materials has led to the development of soft devices that have the potential to be more robust, adaptable, and safer for human interaction than traditional rigid systems. State-of-the-art developments push these robotic systems towards applications such as soft rehabilitation and diagnostic devices, exoskeletons for gait assistance, grippers that can handle diverse objects, and electronics that can be embedded in the human body. Furthermore, compliance has found its way into the design of metamaterials. Applications of compliance in metamaterials range from tunable auxetic behavior, stiffness, optical properties and phononic and acoustic band-gap behavior to tunable surface properties such as the drag coefficient, and chemistry. These examples illustrate the potential of using compliance to create new and improved functionality in structural and robotic applications. While the geometrical non-linearities and instabilities that arise when using soft or flexible materials directly complicate the design process and fall outside the scope of traditional engineering, exactly these non-linearities make the systems inherently capable of rich behavior. As such, to bring these systems closer to application and to uncover their true potential, we need to gain a better understanding of the principles that govern their behavior. In this talk I will focus on the design of non-linear structures and devices - including origami-inspired reconfigurable architected materials and soft actuators that harness instabilities - that exhibit a nontrivial relation between input and output (i.e. loading and response). I propose computational and relatively simple experimental techniques that allow us to effectively explore the design space, and that lead to an understanding of the relation between shape and function in these compliant systems. I will also briefly touch upon "Soft Robotic Matter", our future research direction that focuses on the role that feedback between sensing and actuation could play in these systems, and how we plan on incorporating this feedback in artificial materials.
Natalia Domeradzka (Wageningen) (14.15-14.45)
Cross-linking and bundling of self-assembled protein-based polymer fibrils via heterodimeric coiled coils
Previously, we developed triblock protein polymers that form fibrillar hydrogels at low protein polymer concentrations (denoted C2-SH48-C2). We here demonstrate that the structure of these hydrogels can be tuned via heterodimeric coiled coils that cross-link and bundle the self-assembled protein-polymer fibrils. We fused well-characterized, 47 amino acids-long heterodimeric coiled coil "linkers" (DA or DB) to the C-terminus of the triblock polymer. The resulting C2-SH48-C2-DA and C2-SH48-C2-DB polymers, were successfully produced as secreted proteins in Pichia pastoris, with titers of purified protein in the order of g L-1 of clarified broth. Atomic force microscopy showed that fibrils formed by either C2-SH48-C2-DA or C2-SH48-C2-DB alone already displayed extensive bundling, apparently as a result of homotypic (DA/DA and DB/DB) interactions. For fibrils prepared from protein polymers having no linkers, plus a small fraction of polymers containing either DA or DB linkers, no cross-linking and bundling was observed. At these same low concentrations of linkers, fibrils containing both the DA and the DB linkers did show cross-linking and bundling as a consequence of heterodimer formation. This work shows that coiled coil modules can be employed to control bundling of supramolecular fibrils, which is promising for the further development of materials that mimic the extracellular matrix.
Anupam Pandey (University of Twente) (15.45-16.15)
Liquid drops attract or repel by the inverted cheerios effect
We study the interaction between liquid drops on a soft substrate. When millimetric liquid drops run down a soft, vertical surface under gravity, we observe A deviation of their trajectories from straight line as two drops approach each other. Remarkably, if the wall is very thick drops always attract and coalesce, whereas drops on a thin layer repel each other. We experimentally determine the force-distance curves of this interaction, and perform a free energy minimisation to reveal the underlying mechanism of drop-drop. Importantly, we show that the interplay between capillarity and bulk elasticity of the soft substrate governs the nature of the interaction force.
Viktoria Wollrab (Amolf) (16.15-16.45)
Self-organization of active motor-driven actin-polymer networks
Active cell shape transformations during migration and division are driven by an active polymer gel that is based on a network of actin filaments. Actin filaments possess a structural polarity and interact via passive crosslink proteins and active myosin motor proteins. It is still unclear how interactions of actin filaments and motors on the molecular scale result in collective force generation at the scale of the cell. To elucidate the microscopic basis of the out-of-equilibrium properties of this key biological system we reconstitute biomimetic model systems from purified proteins. We observe a rich variety of self-organized patterns and dynamics that emerge from an initially random state. Interestingly, we find that myosin motor activity results in polarity sorting of actin filaments. Due to directed myosin motion, these networks efficiently generate stresses which lead to contraction. We show that specific motor-filament interaction nucleates these polarity sorted structures while crosslinking stabilizes them and allows their growth. This mechanism might also be relevant for contractility and pattern formation in cells.