Bioinspired Soft Elastic Metamaterials for Reconstruction of Natural Hearing

Abstract

Natural hearing which means hearing naturally like normal people is critical for patients with hearing loss to participate in life. Cochlear implants have enabled numerous severe hearing loss patients to hear voice functionally, while cochlear implant users can hardly distinguish different tones or appreciate music subject to the absence of rate coding and insufficient frequency channels. Here a bioinspired soft elastic metamaterial that reproduces the shape and key functions of the human cochlea is reported. Inspired by human cochlea, the metamaterials are designed to possess graded microstructures with high effective refractive index distributed on a spiral shape to implement position-related frequency demultiplexing, passive sound enhancements of 10 times, and high-speed parallel processing of 168-channel sound/piezoelectric signals. Besides, it is demonstrated that natural hearing artificial cochlea has fine frequency resolution up to 30 Hz, a wide audible range from 150-12 000 Hz, and a considerable output voltage that can activate the auditory pathway in mice. This work blazes a promising trail for reconstruction of natural hearing in patients with severe hearing loss.

Keywords: cochlear implants; elastic metamaterials; natural hearing; piezoelectric materials; soft materials.

Figure 1

Human natural hearing process and the design of BSEM. a) Natural hearing process of normal people. External sounds are transmitted to the brain through the outer ear, middle ear, inner ear, and auditory nerve pathway. b) The detailed structure of BSEM. BSEM can replace the defective cochlea to bridge the auditory pathway. c) Side view of a completed BSEM. d,e) Photographs of the metamaterials sample (without piezoelectric flakes) and enlarged branch microstructures, respectively. f,g) Photographs of piezoelectric flakes and acrylic sockets, respectively. The animal, car, piano, and people icons are designed by Freepik.

Figure 2

Theoretical analysis, simulation, and experimental characterization of rate coding and place coding. a) Schematic of a bioinspired soft elastic metamaterial. b) Calculated effective index and vibration distribution along the arc length L of the bioinspired metamaterial. The wave compression effect is observed accompanied by an increase in the effective index. c) Dispersion properties of the effective index at different locations along the propagation axis. d) A simulation model for metamaterials with piezoelectric flakes. e) Simulated displacement distribution in metamaterials when elastic waves are incident at 150 Hz. f) Simulated place‐pitch mapping of BSEM. g) Peak to peak voltage (V _pp) of 8 channels in BSEM under tone‐burst excitations in C major scale. The excitation intensity is calibrated as the sound with a loudness of 70 dB. h) Voltage gains of 4 channels in BSEM by comparing the output voltage of an unstructured sample.

Figure 3

Demonstration for distinguishing different tones and appreciating music. a) Experimental setup to simulate the hearing process of human. b,c) Original spectrograms of a piano song, and animal sounds, respectively. The frequency range is limited to 0–4 kHz. d,e) Spectrograms (processed by an 8‐channel CI) of a piano song, and animal sounds, respectively. f,g) Spectrograms (processed by 8 channels of BSEM) of a piano song, and animal sounds, respectively. h,i) Two frames in Movie S9 (Supporting Information). Isolated hind limbs repeated bouncing synchronizing amplified electrical stimulations from BSEM.

Figure 4

Validation for activation of auditory pathway by in vivo animal experiments. a) Experimental setup for ABR acquisition under direct electrical stimulations from BSEM. b) The ABR to tone bursts of 6 frequencies at direct electrical stimulations from BSEM. c) Sampling results with acoustic stimulation on normal mice and deaf mice (control). The right coordinates indicate the scale of the duration and amplitude of ABR in the whole figure. d) Results obtained when the cochlea was not stimulated at all, and obtained on deaf mice (control). e) The statistical results of ABR under direct electrical stimulations from BSEM at different frequencies. Each sampling is the summation and average of the evoked potentials induced by 300 electrical stimulation of the cochlea under each same stimulation condition, and the results were repeated 10 times.