Dynamically Tunable Friction via Subsurface Stiffness Modulation

Currently soft robots primarily rely on pneumatics and geometrical asymmetry to achieve locomotion, which limits their working range, versatility, and other untethered functionalities. In this paper, we introduce a novel approach to achieve locomotion for soft robots through dynamically tunable friction to address these challenges, which is achieved by subsurface stiffness modulation (SSM) of a stimuli-responsive component within composite structures. To demonstrate this, we design and fabricate an elastomeric pad made of polydimethylsiloxane (PDMS), which is embedded with a spiral channel filled with a low melting point alloy (LMPA). Once the LMPA strip is melted upon Joule heating, the compliance of the composite structure increases and the friction between the composite surface and the opposing surface increases. A series of experiments and finite element analysis (FEA) have been performed to characterize the frictional behavior of these composite pads and elucidate the underlying physics dominating the tunable friction. We also demonstrate that when these composite structures are properly integrated into soft crawling robots inspired by inchworms and earthworms, the differences in friction of the two ends of these robots through SSM can potentially be used to generate translational locomotion for untethered crawling robots.

Time-resolved microwell cell-pairing array reveals multiple T cell activation profiles

Time-controlled stimulation of lymphocytes arrayed in microwells upon contact with opposing surface covered by a monolayer of antigen presenting cells.

Modeling and Design of a Cushioning Groove Considering a Slanting Impact

The cushioning groove is a simple means to limit end stop speeds of small devices moving in some fluid, for instance, of spools in switching valves. A shallow groove limited by one or two edges is placed on one or both sides of the moving element. When the groove meets its opposing contact surface the fluid pressed out by the motion causes an increased pressure in the groove which provides the cushioning effect. It overcomes fluid stiction problems which are frequently encountered in squeeze gap type cushioning if the system is under high fluid pressure. The elementary cushioning groove concept assumes that the groove edges are exactly parallel to the contacting surface. In this paper, the performance of the cushioning groove in case of some slanting of the groove edges to the opposing surface is studied by means of a mathematical model. Slanting reduces the cushioning force and causes a resulting torque to the moving system due to an asymmetric pressure. Insufficient cushioning becomes more likely and, in turn, a repelling motion.

Solvent Soaking and Drying of Carbon Nanotube Forests for Enhanced Contact Area and Thermal Interface Conductance

Forests comprised of nominally vertically aligned carbon nanotubes (CNTs), having outstanding thermal and mechanical properties, are excellent candidates for thermal interface materials (TIMs). However, the thermal performance of CNT forest TIMs has been limited by the presence of high thermal contact resistances at the CNT tip interface. The high thermal contact resistance at the CNT tip interface stems from two sources: (1) the relatively weak van der Waals type bonding, which impedes phonon transport, and (2) low contact area. In this work we will show that common solvents, such as water, can be applied to the CNT forest to increase the contact area and reduce the contact resistance by an average of 75%. Specifically, there are two likely mechanisms that can increase the contact area when a CNT forest is wet with a fluid and compressed in an interface. The first is relaxing the van der Waals interactions between contacting CNTs within the forest, consequently decreasing the stiffness of the forest and allowing it to better conform to the opposing surface. The second is the pulling of CNT tips through capillary interactions into contact with the opposing surface as the solvent evaporates. By measuring the thermal resistance of CNT TIMs before and after soaking in variety of solvents the capacity of each mechanism for reducing the contact resistance is explored.