A Flexible Tubular Pump Using Electrowetting

We introduce sub-millimeter diameter flexible tubes capable of pumping liquids using electrowetting on a dielectric (EOWD). The pumps can power wearables thanks to their fiber format but can also enable applications in other areas like microfluidic dispensing.<br/><br/>EWOD describes the manipulation of droplets on dielectric surfaces using electric fields. Traditionally, EWOD is used to move, merge, and split droplets on a flat surface, primarily in lab-on-a-chip applications. Here we use EWOD droplet actuation for the first time in a tubular geometry, with a series of electrodes on the outside of the tube. The tube can pump any two alternating immiscible liquids, one being conductive and the other insulating. We demonstrate pumping with colored water and silicone oil. The droplets are moved step by step (from one electrode to the next), which allows for µl-precision control of the volume pumped. The pumps described here operate at 600 V. The actuating electrodes are not in direct contact with the pumping liquid avoiding electrically induced chemical reactions which could shorten the device lifetime.<br/><br/>Teflon or glass tubes with 100 µm wall thickness and 0.9 mm inner diameter serve both as the structure of the pump and as the dielectric. Indium tin oxide (ITO) ring electrodes of width 1.8 mm and spaced by 200 µm are sputtered on the outside of the tube and connected in three electrical phases. The ITO was patterned with sacrificial ethyl cellulose, which is inkjet printed on the tube using a custom rotating setup. The inside of the tube was dip coated with Cytop, ensuring low friction and high droplet contact angles. A central 50 µm diameter copper wire grounds the droplets inside the pump.<br/><br/>Since the tube and the electrodes are transparent, we can directly observe the droplet actuation and change in contact angle. We first placed a single water droplet inside the otherwise air-filled tube. Applying 800 V to an adjacent ring electrode creates an electrical field between the grounded droplet inside and the active electrode outside the dielectric tube. As a result, we observed the droplet change its contact angle and move within ~40 ms underneath the active electrode. The droplet stops moving once the overlap area between the electrode and the droplet is maximized. Activating the next electrode moves the droplet to that electrode. Every third ring electrode is connected to the same phase. When switching between the three phases at 24 Hz, the droplet reached continuous speeds of 80 mm/s.<br/><br/>We then filled the pump with alternating oil and water droplets, ensuring there is one water droplet every three electrodes. Applying a voltage to all electrodes of one phase actuates every adjacent water droplet. Our 0.9 mm diameter pump with 10 simultaneously actuated droplets (30 electrodes) moved liquids at 9 ml/min when switching between phases at 1.5 Hz. We report further data on pressure, which for this pump is estimated at 2 kPa.<br/><br/>We demonstrated EWOD pumping in a flexible tube geometry, achieving high-speed droplet movement and continuous pumping.