The continuous actuation of liquid metal with a 3D-printed electrowetting device

Abstract

The ability of liquid metals (LMs) to recover from repeated stretching and deformation is a particularly attractive attribute for soft bioelectronics. In addition to their high electrical and thermal conductivity, LMs can be actuated, potentially enabling highly durable electro-mechanical and microfluidics systems for applications such as cooling, drug delivery, or reconfigurable electronics. In particular, continuous electrowetting (CEW) phenomena can actuate liquid metal at relatively low voltage and affordable power requirements for wearable systems (~ < 10 V, ~ 10 - 100 µW) by inducing a surface tension gradient across the LM. However, sustaining LM actuation remains challenging due to factors such as electrolyte depletion, polarity changes in multi-electrode systems, and limitations related to LM composition. Here, we demonstrate LM actuation in a circular conduit for prolonged durations of at least nine hours. We enabled sustained actuation by sequentially applying short, direct current (DC) pulses through a multi-electrode system based on the dynamics of LM actuation. As a proof of concept, we also demonstrated the ability of LM to transport electrically conducting, non-conducting, and magnetic materials within a microchannel and show the liquid metal actuation system can be potentially miniaturized to the size of a wearable device. We envision that with further miniaturization of the device architectures, our CEW platform can enable future integration of low-voltage electro-mechanical systems into a broad range of wearable form factors.

Supplementary information: The online version contains supplementary material available at 10.1007/s44258-025-00052-8.

Keywords: 3D printing; Cargo transport; Electrowetting; Liquid metal actuation.

Fig. 1

A 3D-printed device demonstrating liquid metal (LM) actuation via continuous electro-wetting (CEW) in a circular conduit, with potential applications in wearable electronics (A) A schematic illustration of the 3D-printed device demonstrating CEW in a circular channel integrated with a series of electrodes. B The top view of a microfluidic channel with electrodes actuating the liquid metal droplet towards the positive electrode upon excitation. C 3D rendering of the wearable prototype with CEW LM actuation in the form factor of a watch. The scale bar is 10 mm. The inset shows the cross-sectional schematic of the prototype. D (i) The uniform electric charge distribution along the surface of the liquid metal in the absence of an electric field. (ii) The induced surface tension gradient due to charge reconfiguration when an electric field is applied. (iii) The force balance on the liquid metal inside a 3D-printed microchannel when an electric potential is applied across the two ends of the channel. The motion of liquid metal is modulated by the surface tension force F_γ, electro-osmotic force F_e, friction force F_f, and viscous force F_η

Fig. 2

Real-time actuation of the liquid metal (LM) immersed in NaOH electrolyte, under applied electric field, as visualized using high-speed camera. A Schematic of a circular microchannel with a liquid metal under six graphite electrodes. Figures (B-F) are images of the LM at different timestamps when the LM makes a complete revolution in a clockwise direction. The channel is 2.5 mm wide, 6 mm deep and contains 2.5 mm long electrodes in a 0.4 M NaOH electrolyte. The scale bar is 5 mm

Fig. 3

Characterization of the electrical pulses and velocity of LM under different parameters. A The electrical potential difference between two adjacent electrodes when 8.7 V is supplied from the power source (B) Velocity of the LM at different voltages (C) Minimum voltage required for LM of different volumes (D) Velocity of LM in channels with different cross sectional area of electrodes for different commutation periods (E) Velocity of liquid metal when different volumes of LM are injected into the microchannel of varying channel width under varying commutation periods (F) The velocity of LM before (red) and after (blue) refilling electrolyte when a continuous potential difference is applied to the device. B, C and E are performed at different commutation periods of 0.1 s, 0.15 s and 0.2 s

Fig. 4

The characterization of the average and instantaneous velocity of the liquid metal. A The average velocity of liquid metal when a 0.10 s commutation period is applied on the device. The initial jump of the actuation is due to the excess charges on the surface of the liquid metal before reconfiguration. B The instantaneous velocity of liquid metal at a 0.10 s of time-period. C Average velocity of the liquid metal at a 0.15 s time which shows a larger deviation on the velocity at different time periods compared to 0.1 s. D Corresponding instantaneous velocity of the liquid metal showing a larger delay at the junction of the electrode. E Average velocity of the liquid metal at 0.20 s of the of the electrical actuation. F Instantaneous velocity of the liquid metal at 0.1 s of electrical actuation. The negative values indicate that the liquid metal moves in the reverse direction at the electrode junctions

Fig. 5

The motion of liquid metal can be utilized to achieve cargo transport of non-conductive, conductive, and magnetic composite materials (A) Transport of a glass bead as the non-conductive material (B) Transport of graphite chip as a conducting material and (C) figure showing the delivery of a magnetic composite. The scale bars are 5 mm. The inset on the figures shows a clearer view of how the LM is aligned with the different types of cargo in front

Fig. 6

A proof-of-concept demonstration of CEW in the wearable form factor of a watch. A The CEW device as worn on the wrist. The inset shows the complete device with the enclosed circular channel and the electrodes. The thin 32 Ga wires are connected to the electronics pack (B-D) Time series images of the top view of the device showing continuous actuation of the LM. E–G Time series images of the side view of the device showing continuous actuation of the LM. The position of the LM is highlighted in the dashed circle along with the direction of motion which can be tuned as needed. The scale bars are 5 mm

Fig. 7

Demonstration of CEW on our proof-of-concept device with alternate biocompatible electrolytes (A)(i-iii) Time series images of the bottom view of the device showing continuous actuation of the LM in phosphate buffered saline (B)(i-iii) Time series images of the bottom view of the device showing continuous actuation of the LM in 0.5 M aqueous NaCl solution. The scale bars are 5 mm