Stable electroosmotically driven actuators

Abstract

We have previously presented "nastic" actuators based on electroosmotic (EO) pumping of fluid in microchannels using high electric fields for potential application in soft robotics. In this work we address two challenges facing this technology: applying EO to meso-scale devices and the stability of the pumping fluid. The hydraulic pressure achieved by EO increases with as 1/d 2 , where d is the depth of the microchannel, but the flow rate (which determines the stroke and the speed) is proportional to nd, where n is the number of channels. Therefore to get high force and high stroke the device requires a large number of narrow channels, which is not readily achievable using standard microfabrication techniques. Furthermore, for soft robotics the structure must be soft. In this work we present a method of fabricating a three-dimensional porous elastomer to serve as the array of channels based on a sacrificial sugar scaffold. We demonstrate the concept by fabricating small pumps. The flexible devices were made from polydimethylsiloxane (PDMS) and comprise the 3D porous elastomer flanked on either side by reservoirs containing electrodes. The second issue addressed here involves the pumping fluid. Typically, water is used for EO, but water undergoes electrolysis even at low voltages. Since EO takes place at kV, these systems must be open to release the gases. We have recently reported that propylene carbonate (PC) is pumped at a comparable rate as water and is also stable for over 30 min at 8 kV. Here we show that PC is, however, degraded by moisture, so future EO systems must prevent water from reaching the PC.

Bubble‐free electrokinetic flow with propylene carbonate

“…Previously, we reported [47] that pumping performance drops after PC is exposed to air: the current rises and the velocity decreases. This occurs after even a brief exposure to room air and is irreversible, but becomes significant for actuator performance only after several hours exposure.…”

Development of a bubble-based single cell picking system

“…Then, it was dried on the hot plate at 65 • C for about 5 min. After air plasma treatment (Plasma Cleaner, YZD08-2C; Tangshan Yanzhao Technology, China), the PDMS block containing gas channels was bonded with the 10 µm thick porous membrane [51], and then placed in an ultrasonic cleaner with the same 20% ethanol solution for at least 20 min to remove residual sacrificial sugar particles [52] in the membrane. Subsequently, this double-layer PDMS chip was placed on a plate heater at 65 • C for at least 10 min, and punched to form both inlet and outlet holes.…”

Fabrication of a Miniature Paper-Based Electroosmotic Actuator

“…Typically, water is used in EO pumps due to its high polarity, but in water, electrolysis causes depletion of the pumping liquid, changes in pH, and evolution of gas bubbles, resulting in irreproducible pumping, low actuation pressure, and device failure. In this paper, we used propylene carbonate (PC) as the pumping liquid because it exhibits bubble-free operation up to kV [ 8 , 56 ]. The zeta potential for both water and PC in PDMS is negative [ 8 ]: fluid moves from the positive electrode toward the negative electrode, indicating that the mobile charges at the walls are positive (cations).…”

“…One approach to increasing the number of pumping channels and reducing their diameter is to pack conventional microfluidic channels with beads [ 48 ] or porous monoliths [ 47 , 48 , 57 , 58 , 59 , 60 ]. Another approach is to employ pre-formed microporous structures, such as mullite [ 50 ], porous glass frits [ 44 ], or porous elastomeric sponges [ 56 ].…”