Contactless power-transfer to electronic devices in solution

In a first step we will demonstrate that different electronic devices can be wirelessly powered in an aqueous solution under the influence of an external electric field. The induced polarization leads to an electron flow from the δ+ side of the object to the δ− end. When an electronic element is incorporated in the path of electron flow it is possible to operate the device without physical contact to the feeder electrodes. The first example, illustrated in Figure 2A, is the operation of a light-emitting diode. Three independent diodes are placed in water and exposed to the electric field established between the anode and the cathode of the electrochemical cell. As can be seen from the side view in Figure 2B, the polarization leads to the reduction of protons on the right connector of the LED, whereas oxygen evolution is observed on the left side. Due to the intrinsic stoichiometry of water electrolysis, the amount of generated hydrogen is twice as high as that of oxygen and this is reflected by the respective quantities of visible rising gas. The amplitude of the concomitant light emission can be tuned by adjusting the external electric field. However there is a threshold to overcome which is the difference in formal potential of the two involved redox reactions. Once the polarization voltage is higher than this value, there is a classic exponential Butler-Volmer dependence between the overpotential experienced by both sides of the bipolar object and the reaction rate (current) on these two sides. This means that the higher the polarization voltage the higher the current/bubble production and therefore also the light emission.

Figure 2 Examples of contactless powered electronic devices (A) Contactless simultaneous powering of three LEDs immersed in an aqueous solution and exposed to the electric field between the feeder anode and cathode (top view of the cell) (B) Side view of one LED immersed in water and exposed to an electric field. The induced polarization (δ− and δ+) leads to H 2 and O 2 evolution at the electrical input and output of the diode. The resulting local current powers the LED which then emits light (C) Electronic circuit of a temperature sensor (for details see Figure S2) immersed in water and exposed to the electric field between cathode and anode. The polarization leads to the generation of gases and a local electric current which powers the circuit. An integrated emitter allows communication with a receiver placed outside of the water reservoir. The emitted infrared signal encodes the information about the local temperature. (D) Contactless measurement of the temperature in the solution as a function of time. Inset: Received signal from the temperature sensor. The number of pulses is proportional to the temperature in the solution (see Figure S3). Red and green squares represent the initial and final temperature respectively measured with a conventional thermometer. Full size image

Besides LEDs, more sophisticated electronic circuits can also be powered based on this concept, as can be seen in Fig. 2C. It illustrates the operation of a complex electronic device capable of performing multiple tasks in a contactless way and in an aqueous environment. An electronic sensor powered by the bipolar current measures the local temperature and provides an analog signal to an embedded central processing unit. After signal treatment, a numerical signal is generated and transmitted from an embedded infrared emitter to a receiver located outside of the water reservoir. This signal can be converted into a measure of the temperature (Figure 2D), thus allowing to track for example the ohmic heating of the solution (for details see SI). These proof-of-principle experiments demonstrate that bipolar electrochemistry is able to deliver a stable power input for useful electronic devices. Technologies required to miniaturize such kind of devices are available36, allowing to design systems that run with currents in the μA range and/or with a power as low as tens of nW at 0.5 V. Thus one can imagine driving already relatively sophisticated circuits without causing significant Joule heating, because the currents through the solution will then be more than thousand times lower. In this way multiple functionalities could potentially be integrated, leading to remote controlled e-swimmers.

Propulsion of e-swimmers

In this section we show how multiple electric fields can be used to impart directional motion to e-swimmers. We recently reported levitation experiments with glassy carbon beads as swimmers10, but they did not allow for motion in more than one direction. In the present case we propose a concept for the dynamic control of the motion in several directions, independently of the cell geometry and to simultaneously integrate an electronic functionality.

A rocket-like swimmer, composed of an outer plastic shell (b), combined with two metal wires (a) that act as bipolar electrodes (Figure 3A, 3B), can be moved by controlling the kinetics of the gas bubble production. The tip of the rocket has a tapered opening through which gas bubbles can escape. If the rate of the bubble formation exceeds the release rate, gas will accumulate inside the swimmer and drive the swimmer up (see Figure S4). In contrast, decreasing the applied electric field will allow gas to be released faster than it is produced, thus causing the swimmer to sink. The balance of both forces controls the buoyancy-driven motion (see Figure S5 and Video 1 in SI).

Figure 3 Illustration of two different e-swimmer designs. (A) Scheme of the rocket swimmer; (B) Rocket swimmer; (C) Rocket swimmer with integrated LEDs; (D) Scheme of the rocket swimmer with integrated LEDs; (E) Scheme of the cube swimmer; (F) Cube swimmer; (G) Cube swimmer with integrated LEDs; (H) Scheme of the cube swimmer with integrated LEDs. a: metal wire; b: plastic cone; c: LED; d: polystyrene cube; e: polymer plates. Full size image

In order to not only control the motion in the vertical direction, but also along a horizontal axis, an alternative swimmer design has been adapted. It is based on a polymer cube (d) with two metal wires, one along the vertical axis and one along the horizontal axis. The latter wire is in addition modified at its both extremities with two tilted polymer plates (e) (Figures 3E, 3F). A platinum mesh is attached to the bottom of the vertical electrode to facilitate water reduction due to the electrocatalytic properties of platinum. Two orthogonal electric fields were applied using two sets of voltage generators. At the horizontal bipolar electrode, H 2 and O 2 are formed at the two extremities and since the volume of produced H 2 is twice that of O 2 , the resulting force will push the swimmer from the H 2 production side towards the O 2 production side34. The tilted plates increase the surface of interaction with the bubbles in order to generate an optimized action/reaction force for the horizontal translation. A movement in both horizontal directions can be achieved by inverting the polarity of the electric field (see Videos 2 and 3 in SI).

The hydrodynamic behavior in the vertical direction of the rocket and the cube swimmers is different. For the rocket swimmer the characteristic rising and sinking periods are directly controlled by the applied electric field (here from 0 V cm−1 to 2.9 V cm−1) and the diameter of the gas release hole. Figure S5B shows a certain phase shift between the changes in applied potential and the resulting change in height, which corresponds to the time necessary for the accumulation or release of gas. The vertical speed of the rocket swimmer increases linearly with time, because the equilibrium between gas production and release leads to a steady state amount of gas accumulated inside the rocket and therefore to a constant acceleration. On the other hand, the cube tends to constantly accumulate increasing amounts of bubbles underneath the swimmer as a function of time, which corresponds to a gradual increase in driving force and thus an increasing acceleration (see Figure S5C).

As the evolution of hydrogen and oxygen at the two opposite sides of the swimmer during the bipolar electrochemical experiment is intimately related to a shuttling of electrons through the object, a local electric current exists that can be used for the simultaneous operation of an electronic device. As proof-of-principle we have chosen LEDs ((c) in Figures 3D and 3H) to confer an additional functionality to the object, thus leading to the first example of e-swimmers based on this concept.

Figures 3C and 3G show such light emitting swimmers. The vertical velocity of the light emitting rocket increases again linearly with time (Figure 4A). For the light emitting cube swimmer, two LEDs with different colors were integrated in opposite working directions in order to track the polarity of the applied horizontal field (Figure 4B). When the polarity of the horizontal electric field is inverted during the upward motion, not only the direction of motion changes, but also the active LED (and color) changes (Figure 4C and Video 5 in SI). It is clear that the motion in the horizontal direction is less spectacular and slower than in the vertical direction which is due to the partial lack of buoyancy forces, but nevertheless it is possible to change the direction of motion by a change in polarity of the electric field.