Soft artificial electroreceptors for noncontact spatial perception

Abstract

Elasmobranch fishes, such as sharks, skates, and rays, use a network of electroreceptors distributed on their skin to locate adjacent prey. The receptors can detect the electric field generated by the biomechanical activity of the prey. By comparing the intensity of the electric fields sensed by each receptor in the network, the animals can perceive the relative positions of the prey without making physical contact. Inspired by this capacity for prey localization, we developed a soft artificial electroreceptor that can detect the relative positions of nearby objects in a noncontact manner by sensing the electric fields that originate from the objects. By wearing the artificial receptor, one can immediately receive spatial information of a nearby object via auditory signals. The soft artificial electroreceptor is expected to expand the ways we can perceive space by providing a sensory modality that did not evolve naturally in human beings.

Fig. 1.. A soft artificial electroreceptor (SAER).

(A and B) Rays use a network of electroreceptors distributed on their skin to locate nearby prey. (C) Each receptor, consisting of an ionic conductive hydrogel-filled canal, can detect the electric field generated by the biomechanical activity of prey animals. By comparing the intensity of the electric fields sensed by each receptor in the network, rays can estimate the relative position of nearby prey without making physical contact. (D) A wearable SAER was developed to support human spatial perception in daily life. (E and F) Same as the ray, the SAER can locate the relative positions of nearby objects by comparing the intensity of the electric fields sensed by each hydrogel electric field receiver.

Fig. 2.. Electroreception in rays for noncontact prey localization.

(A) Electroreceptors of an ocellate river stingrays (i.e., P. motoro) distributed in the skin around the mouth and gills. Scale bar, 3 cm. (B) The experimental setup used to characterize electroreception in the ray. Two electrodes and an odor-delivery tube were buried under the sand. (C) Swimming trajectory of the ray when a food odor was injected into the center of the tank via the odor-delivery tube. The ray randomly swam in an attempt to find prey. (D) The time that the ray spent within 7.5 cm from each electrode. The prey-searching behavior of the ray was not biased toward a specific direction. (E) When prey-simulating electric fields were generated via the left electrode, the ray hovers around the electrode in an attempt to find prey. (F) The ray stayed 44 times longer near the left electrode than the right electrode. Error bars represent SD (n = 5). Photo credit: Won Jun Song and Younghoon Lee, Seoul National University.

Fig. 3.. Fabrication of a SAER with high mechanical reliability.

(A) Conductive hydrogels were covalently anchored to an elastomer during 3D printing for rapid and elaborate fabrication of a SAER with high mechanical reliability. (B) In the absence of grafting agent activation, delamination occurs between the hydrogels and the elastomer during the printing process due to poor interfacial bonding. Scale bar, 250 μm. (C) Grafting agent activation allows radicals in the elastomer to participate in free-radical polymerization of the hydrogel during printing, thus forming a tough interface between the hydrogel and elastomer layers. To enhance the contrast between the hydrogel and elastomer, fluorescent dyes were added to the hydrogel. (D) The SAER was highly transparent to visible light of all colors because the rapidly fabricated hydrogel/elastomer interface scatters a negligible amount of visible light. Scale bar, 1 cm. (E) The hydrogel layer on the elastomer could be stretched more than 400% without delamination. To enhance the contrast between the hydrogel and elastomer, fluorescent dyes were added to the hydrogel. Scale bar, 1 cm. Photo credit: Won Jun Song and Younghoon Lee, Seoul National University.

Fig. 4.. Sensing capability of the SAER.

(A and B) Schematic diagrams illustrating the sensing mechanism of the SAER. Objects usually have static charges on their surfaces because contact with its surroundings causes the object to be charged through contact electrification (A). The electric fields originating from the object induce voltage in the hydrogel electric field receiver of the SAER (B). By measuring the voltage across the external load connected between the receiver and ground, the intensity and polarity of the electric field can be estimated. (C) The finite element method (FEM) was used to investigate voltage induction as a function of distance between the object and the receiver. (D) Equivalent circuit of the object-SAER system for theoretical analysis. (E) The induced voltage closely traced the position of the object. (F to I), The induced voltage was measured while varying the external load resistance (F), oscillating frequency of the object (G), object and charging materials (H), and initial distance between the SAER and the object (I). Error bars represent SD (n = 3). PP, polypropylene; PI, polyimide; PTFE, polytetrafluoroethylene.

Fig. 5.. Through-wall sensing capability of the SAER.

(A) Under an external electric field, a dielectric material becomes polarized and transmits the electric field. (B and C) The induced voltage was measured as the thickness (B) and relative permittivity (C) of the dielectric barrier were varied. (D) When an external electric field is applied, excess charges accumulate on the surface of a grounded conductor, preventing transmission of the electric field. (E and F) The induced voltage was measured as the thickness (E) and lithium chloride (LiCl) concentration (F) of the hydrogel conductive shield were varied. (G) A SAER with two hydrogel electric field receivers was placed in front of an oscillating pendulum. Light-emitting diodes (LEDs) were programmed to be turned on when the induced voltage exceeded a threshold voltage. Scale bar, 4 cm. (H) A dielectric barrier was placed between the oscillating pendulum and the receiver on the left side. (I) A conductive shield was placed between the oscillating pendulum and the receiver on the left side. To enhance the contrast between the hydrogels and background, fluorescent dyes were added to the hydrogel electric field receivers. (J) A SAER can detect human movements in real time. Scale bar, 25 cm. (K) The SAER successfully detects the movement of a person, even when a 3-cm-thick wooden wall is blocking between the person and a hydrogel electric field receiver. Error bars represent SD (n = 3). Photo Credit: Won Jun Song and Younghoon Lee, Seoul National University.

Fig. 6.. A wearable SAER for noncontact spatial perception.

(A) Without the conductive shield, data line connected to the receiver can cause distortion of the voltage signal. (B) When the data line is covered with a hydrogel conductive shield, the voltage signal from each electric field receiver became independent of the object direction. “L,” “R,” “F,” and “B” as “left,” “right,” “front,” and “back,” respectively. (C) The wearable SAER was attached to the hollow of the hand. Scale bar, 2 cm. (D) By comparing the intensity of the electric fields sensed by each hydrogel receiver (Rx), the SAER can be used to locate the relative position of the object. (E and F) By wearing the SAER, a person can perceive spatial information related to a dynamic (E) and static (F) object via auditory signals. Error bars represent SD (n = 3). Photo credit: Won Jun Song and Younghoon Lee, Seoul National University.