We use three-dimensional computer simulations to examine the free swimming of an elastic plate plunging sinusoidally in a viscous fluid with a Reynolds number of 250. We find that the free swimming velocity is maximized when the swimmer is driven near the first natural frequency leading to larger swimmer deformations, and that the free swimming velocity is nearly linearly related to the trailing edge displacement. The maximum swimmer performance is found at a non-resonance frequency. The maximum performance takes place when the swimmer exhibits a deformation pattern in which the transverse displacement of the swimmer's center of mass is minimized, which in turn reduces viscous losses.

An electrodeposition process for void-free bottom-up filling of sub-millimeter scale through silicon vias (TSVs) with Cu is detailed. The 600 μm deep and nominally 125 μm diameter metallized vias were filled with Cu in less than 7 hours under potentiostatic control. The electrolyte is comprised of 1.25 mol/L CuSO 4 −0.25 mol/L CH 3 SO 3 H with polyether and halide additions that selectively suppress metal deposition on the free surface and side walls. A brief qualitative discussion of the procedures used to identify and optimize the bottom-up void-free feature filling is presented.

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Using computational modeling, we design a microscopic swimmer made of a bilayered responsive hydrogel capable of swimming in a viscous fluid when actuated by a periodically applied stimulus. The gel has an X-shaped geometry and two bonded layers, one of which is responsive to environmental changes and the other which is passive. When the stimulus is turned on, the responsive layer swells and causes the swimmer to deform. We demonstrate that when such stimulus-induced deformations occur periodically the gel swimmer effectively propels forward through the fluid. We show that the swimming speed depends on the relative stiffness of the two gel layers composing the swimmer, and we determine the optimal stiffness ratio that maximizes the swimming speed.A s robotic swimmers become ever smaller and approach the microscale realm, researchers have developed a variety of clever methods to generate propulsion of miniature objects submerged in an aqueous solution. Such microscale swimmers could use biocatalytic propulsors, biomimetic nanowires that beat like synthetic flagella, responsive soft materials, and other approaches to propel themselves through a viscous fluid. 1−9 Further advances in microswimmer development could yield highly maneuverable and controllable robots that can be targeted to specific locations and autonomously perform complex tasks 10−13 and therefore can be effectively utilized in such applications as drug delivery, biosensing, micromanufacturing, and microsurgery. 14−19 A critical requirement of synthetic microswimmers is their ability to generate self-propelling motion in a fluid environment dominated by viscous forces without using complex mechanical machinery employed by larger macroscopic swimming devices. In this respect, soft responsive hydrogels that are typically biocompatible are especially attractive for designing active microscopic devices. Responsive hydrogels can generate a large amplitude mechanical motion controlled by chemical reactions 20,21 or in response to various external environmental changes 22,23 that include changes in temperature, pH, electric and magnetic fields, and light. 24−27 In other words, the motion of hydrogel swimmers can be directly controlled by changing their environment. The response time of hydrogels depends on the diffusion rate of the solvent into the swelling gel network and is proportional to the squared size of the network. Thus, micrometer-sized gels can exhibit response times on the order of seconds 28,29 and even faster, 30,31 similar to fast switching liquid-crystal elastomers, 32,33 which makes these materials suitable for applications requiring rapid periodical actuation.In our study, we use computational modeling to design an efficient autonomous microswimmer that is actuated using a responsive hydrogel and features a simple, easy-to-implement design. Our simulations show that an on/off periodic application of an external stimulus on the gel swimmer can lead to a rapid self-propelled motion caused by periodic swimmer deformations. More specifically, ...

Stimuli-sensitive hydrogels are an exciting class of materials with widespread potential for use in engineering and biomedical applications. The design of advanced functional devices using hydrogels requires an in-depth understanding of the physics and behaviour of such materials. While theoretical tools exist, they are often limited to simple cases. Thus, computational methods are necessary to model the complex unsteady physics of hydrogels with high fidelity. Mesoscale modelling is an emerging approach that enables simulations of polymeric structures at length and time scales in between those of molecular dynamics and continuum methods. In this feature article, we review various computational approaches to model responsive hydrogels and specifically focus on dissipative particle dynamics (DPD), a particle-based mesoscale method. We discuss several approaches for modelling cross-linked polymer networks in DPD, and describe recent applications of DPD to modelling hydrogel systems.