Artificial Hair Cell Research Articles

Data-Driven Sparse Sensor Placement Optimization on Wings for Flight-By-Feel: Bioinspired Approach and Application.

Flight-by-feel (FBF) is an approach to flight control that uses dispersed sensors on the wings of aircraft to detect flight state. While biological FBF systems, such as the wings of insects, often contain hundreds of strain and flow sensors, artificial systems are highly constrained by size, weight, and power (SWaP) considerations, especially for small aircraft. An optimization approach is needed to determine how many sensors are required and where they should be placed on the wing. Airflow fields can be highly nonlinear, and many local minima exist for sensor placement, meaning conventional optimization techniques are unreliable for this application. The Sparse Sensor Placement Optimization for Prediction (SSPOP) algorithm extracts information from a dense array of flow data using singular value decomposition and linear discriminant analysis, thereby identifying the most information-rich sparse subset of sensor locations. In this research, the SSPOP algorithm is evaluated for the placement of artificial hair sensors on a 3D delta wing model with a 45° sweep angle and a blunt leading edge. The sensor placement solution, or design point (DP), is shown to rank within the top one percent of all possible solutions by root mean square error in angle of attack prediction. This research is the first to evaluate SSPOP on a 3D model and the first to include variable length hairs for variable velocity sensitivity. A comparison of SSPOP against conventional greedy search and gradient-based optimization shows that SSPOP DP ranks nearest to optimal in over 90 percent of models and is far more robust to model variation. The successful application of SSPOP in complex 3D flows paves the way for experimental sensor placement optimization for artificial hair-cell airflow sensors and is a major step toward biomimetic flight-by-feel.

Artificial basilar membrane/hair cell integrated acoustic system for keyword spotting in noisy environments inspired by human cochlea

We report a novel speech recognition method using a noise-robust acoustic sensor system that integrates a spatially frequency-separating sensor with a nonlinear amplification algorithm, mimicking the cochlea’s basilar membrane and hair cells. The multichannel piezoelectric artificial basilar membrane (ABM) sensor detects specific sound frequencies with high sensitivity over 0.2–6 kHz. The signal processing model of the Artificial Hair Cell inspired by the signal transduction mechanism of inner hair cells, simultaneously enhances the frequency selectivity of ABM sensors and improves noise robustness. In a 0 dB SNR noisy environment, it effectively detected the voice signal with a maximum SNR of 57 dB. Furthermore, we converted the frequency-separated signals for speech sounds in various noisy environments into heatmap images and utilized them as input for a CNN-based speech recognition algorithm. Consequently, our system demonstrated noise-robust recognition performance with 94 % accuracy, even in noisy environments.

Artificial Hair Cell Sensor Based on Nanofiber-Reinforced Thin Metal Films.

Engineering artificial mechanosensory hair cells offers a promising avenue for developing diverse biosensors spanning applications from biomedicine to underwater sensing. Unfortunately, current artificial sensory hair cells do not have the ability to simultaneously achieve ultrahigh sensitivity with low-frequency threshold detection (e.g., 0.1 Hz). This work aimed to solve this gap by developing an artificial sensory hair cell inspired by the vestibular sensory apparatus, which has such functional capabilities. For device characterization and response testing, the sensory unit was inserted in a 3D printed lateral semicircular canal (LSCC) mimicking the environment of the labyrinth. The sensor was fabricated based on platinum (Pt) thin film which was reinforced by carbon nanofibers (CNFs). A Pi-shaped hair cell sensor was created as the sensing element which was tested under various conditions of simulated head motion. Results reveal the hair cell sensor displayed markedly higher sensitivity compared to other reported artificial hair cell sensors (e.g., 21.47 mV Hz-1 at 60°) and low frequency detection capability, 0.1 Hz < f < 1.5 Hz. Moreover, like the LSCC hair cells in biology, the fabricated sensor was most sensitive in a given plane of rotational motion, demonstrating features of directional sensitivity.

Bioinspired Artificial Hair Sensors for Flight-by-Feel of Unmanned Aerial Vehicles: A Review

Flight-by-feel is an emerging approach to flight control that uses distributed arrays of pressure, strain, and flow sensors to guide aircraft. Among these, hair-type flow sensors have received the least attention yet hold some advantages over conventional sensors. This paper reviews hair-like flow microsensors developed since 2013, focusing on developments in design, construction, and application. Hair-like flow sensors can be found in artificial cochleae, submersible navigation, terrestrial robots, and, rarely but increasingly, on aircraft. In this survey, we categorize hair-like flow sensors into three types (long whisker-like hairs, ultrasensitive microscale hairs, and short trichoid-like hairs), and primarily cover sensors that may be suitable for use on aircraft. The recent progress in flow-based flight control using distributed sensing is also discussed, along with the optimization of sensor placement and the potential for flight-by-feel in sixth-generation military and civilian aircraft designs. This survey aims to provide a consolidated account of the history and state-of-the-art of artificial hair-cell flow sensors, motivate consideration of flight-by-feel as a viable flight control paradigm, and define avenues for future research. As engineering and biological science continue to converge, we hope that researchers in both fields find this survey an inspirational and useful resource.

Self-sensing active artificial hair cells inspired by the cochlear amplifier, Part II: Experimental validation

Mimicking the nonlinear compressive behavior of the mammalian cochlear amplifier that results in the compression of high-intensity sounds and amplification of faint stimuli can lead to transformative improvements in the dynamic range, sharpness of the response, and threshold of sound detection in cochlear implants to aid individuals with hearing loss. Furthermore, it can enhance the dynamic properties of sensors. This research on developing self-sensing artificial hair cells (AHCs) validates the phenomenological control algorithm established in Part I of the paper to achieve a cochlea-like response from the quadmorph AHCs. As the beam is excited, the voltage of the piezoelectric layers is measured and used to generate a control voltage. Consequently, the controller applies cubic damping to the AHC, while reducing linear damping near its first natural frequency to replicate the biological cochlea’s function. Experimental results validate the model built in Part I of the paper and the work is extended to implement a multi-channel AHC. The system works independent of external sensors and offers significant advantages over previous generations of AHCs such as the ability to embed AHCs in a limited space and to combine several AHCs in an array without the need for external feedback measurement devices.

An Ultrahighly Pressure Sensitive Electronic Fish Skin for Underwater Wave Sensing

Underwater flexible sensors have a future for wide application, which is promising for attaching them to underwater creatures to monitor vital signals and biomechanical analysis of their motion and perceive tiny environmental disturbances. However, the pressure waves induced by biological swimming are extremely weak and susceptible to undercurrents, making them difficult to sense. Here, we report an ultrahighly sensitive biomimetic electronic fish skin designed by embedding an artificial pseudocapacitive-based hair cell into a simulated canal neuromast encapsulation structure, in which the artificial hair cell, as the key sensitive unit, is assembled from hybrid film electrodes and polyurethane-acidic electrolyte foam. Such a film is prepared by inter-cross-linking MXene and holey reduced graphene oxide with the assistance of l-cysteine, effectively increasing the interfacial capacitance and alleviating the oxidation issues of MXene. Meanwhile, the acidic foam with high porosity shows great compressibility to adapt to a high-pressure underwater environment. Consequently, the device exhibits ultrahighly sensitivity (maximum sensitivity ∼173688 kPa-1) over a wide range of depths (0-100 m) and remains stable after 10000 repeated tests. As an example case, the device is integrated as a motion monitoring system to identify the minor disturbances triggered by instantaneous postural changes of fish. The electronic fish skin is expected to demonstrate enormous potentials in flow field monitoring, ocean current detecting, and even seismic waves warning.

Amplitude control for an artificial hair cell undergoing an Andronov-Hopf bifurcation

The dynamics of an artificial hair cell is analyzed by considering the bifurcation behavior of a dominant mode model. It is shown that this model undergoes an Andronov-Hopf bifurcation similar to its biological counterpart, i.e., the hair cell in the mammalian cochlea. Consequently, this dynamical behavior is exploited to control the amplitude of the deflection of the artificial hair cell. In particular feedforward control with disturbance injection is designed based on an approximation of the oscillation using an envelope model to achieve a constant deflection amplitude. The approach is evaluated in numerical simulations.

Morphological Fabrication of Equilibrium and Auditory Sensors through Electrolytic Polymerization on Hybrid Fluid Rubber (HF Rubber) for Smart Materials of Robotics.

The development of auditory sensors and systems is essential in smart materials of robotics and is placed at the strategic category of mutual communication between humans and robots. We designed prototypes of the rubber-made equilibrium and auditory sensors, mimicking hair cells in the saccule and the cochlea at the vestibule of the human ear by utilizing our previously proposed technique of electrolytic polymerization on the hybrid fluid rubber (HF rubber). The fabricated artificial hair cells embedded with mimicked free nerve endings and Pacinian corpuscles, which are well-known receptors in the human skin and have already been elucidated effective in the previous study, have the intelligence of equilibrium and auditory sensing. Moreover, they have a voltage that is generated from built-in electricity caused by the ionized particles and molecules in the HF rubber due to piezoelectricity. We verified the equilibrium and auditory characteristics by measuring the changes in voltage with inclination, vibration over a wide frequency range, and sound waves. We elucidated experimentally that the intelligence has optimum morphological conditions. This work has the possibility of advancing the novel technology of state-of-the-art social robotics.

Self-sensing active artificial hair cells inspired by the cochlear amplifier, Part I: Theoretical and numerical realization

The mammalian cochlear amplifier displays a unique nonlinear characteristic of amplifying or compressing the acoustic stimuli based on their level. Designing control algorithms for artificial hair cells (AHCs) with active sensing capabilities could mimic such dynamics. The present work focuses on developing a novel self-sensing active AHC scheme that implements the amplification/compression function of the AHCs and extends the authors’ previous designs for future cochlear implants or sensor design applications. The AHC functions near a Hopf bifurcation and varies the output piezoelectric voltage by a power-law relationship with the input. It is the first time that a self-sensing AHC is modeled as a quadmorph cantilever controlled by a phenomenological cubic damping controller. The AHC’s control signal is supplied to a pair of its piezoelectric layers and the output of the AHC is the sensed voltage of the second piezoelectric pair. This is in contrast to the previous studies where the AHC’s tip-velocity was measured with external sensors. The requirement of permanent external sensors in the system was a strict limitation that is eliminated in this work. An in-depth study of this novel AHC is conducted in this paper and AHC’s implementation is examined experimentally in Part II of the paper.

Development of Ultrasensitive Biomimetic Auditory Hair Cells Based on Piezoresistive Hydrogel Nanocomposites.

With an ageing population, hearing disorders are predicted to rise considerably in the following decades. Thus, developing a new class of artificial auditory system has been highlighted as one of the most exciting research topics for biomedical applications. Herein, a design of a biocompatible piezoresistive-based artificial hair cell sensor is presented consisting of a highly flexible and conductive polyvinyl alcohol (PVA) nanocomposite with vertical graphene nanosheets (VGNs). The bilayer hydrogel sensor demonstrates excellent performance to mimic biological hair cells, responding to acoustic stimuli in the audible range between 60 Hz to 20 kHz. The sensor output demonstrates stable mid-frequency regions (∼4-9 kHz), with the greatest sensitivity as high frequencies (∼13-20 kHz). This is somewhat akin to the mammalian auditory system, which has remarkable sensitivity and sharp tuning at high frequencies due to the "active process". This work validates the PVA/VGN sensor as a potential candidate to play a similar functional role to that of the cochlear hair cells, which also operate over a wide frequency domain in a viscous environment. Further characterizations of the sensor show that increasing the sound amplitude results in higher responses from the sensor while taking it to the depth drops the sensor outputs due to attenuation of sound in water. Meanwhile, the acoustic pressure distribution of sound waves is predicted through finite element analysis, whereby the numerical results are in perfect agreement with experimental data. This proof-of-concept work creates a platform for the future design of susceptible, flexible biomimetic sensors to closely mimic the biological cochlea.

Cochlear amplifier inspired two-channel active artificial hair cells

The mammalian auditory system exhibits rich characteristics that has inspired research for decades. Some of these key characteristics include frequency selectivity of the basilar membrane (BM), nonlinear amplification of low-level stimuli, and compressive nonlinearity that enables the ear to sense over a large range of sound pressure levels (0–120 dB in humans). Cochlear outer hair cells (OHCs) play a vital role in the nonlinear amplification and compression of input signals and this behavior is referred to as the cochlear amplifier. Damage to OHCs due to aging, diseases, and environmental conditions often results in a sensorineural hearing loss. As a result, there is a need to develop better hearing prosthesis that transduce sound-induced vibrations to electrical signals by mimicking OHCs. In the present work, active artificial hair cells (AHCs) made of piezoelectric cantilever beams are developed to mimic the nonlinear behavior of the OHCs. In literature, AHC based prosthesis operate over a spectrum by developing an array of AHCs with different fundamental frequencies. The present work extends the frequency bandwidth of each AHC to sense over multiple resonant frequencies, unlike conventional single-channel AHCs. Therefore, a single active multi-frequency resonant AHC is presented to minimize the need for more artificial hair cells to cover the desired bandwidth of interest in an array format. Herein, the response of a 26 mm AHC to a base excitation is controlled by applying a phenomenological feedback control law that tunes the system close to a Hopf bifurcation near the first and second natural frequencies. The experimental results show a power relation of about one-third, between the output and input of the AHC. This is very similar to the compressive nonlinearity seen in the mammalian cochlea. The present effort is a continuation of the authors’ previous studies in bio-inspired nonlinear control of AHCs, which can lead to an implantable cochlear device, acoustic, or flow sensor in future.

Active, artificial hair cells for biomimetic sound detection based on active cantilever technology.

We aim at building and studying artificial hair cells (AHC) based on MEMS technology to understand the extraordinary sound perception of the human ear and build a sensor system with similar properties. These perception properties, i.e. detecting six orders of sound pressure level and simultaneously frequency differences of only 3-5 Hz, are obtained mainly due to the sophisticated biological sensors in the inner ear, called hair cells, which convert the acoustic waves into electric signals. They amplify weak inputs and compress larger ones, known as compressive nonlinearity, thus enabling this impressive dynamic range, typically not captured by current engineering solutions. We tackle this demand by building artificial hair cells on the basis of smart, self-actuated and self-sensing mechanical resonator beams with suitable actuation feedback. Thereby, we take advantage of the fact that the compressive nonlinearity arises naturally in dynamical systems tuned to a bifurcation point. This tuning is achieved by an appropriate feedback loop inspired by physiological models. Initial results on the detection properties of a single AHC will be shown demonstrating amplification and a decreased width of the resonance peak.

Design and modeling of a novel high sensitive MEMS piezoelectric vector hydrophone

In this paper, a novel micro electromechanical systems (MEMS) piezoelectric hydrophone with the ability to detect the direction of the sound in two dimensions was designed and analyzed. Piezoelectric hydrophones are widely used today. These devices constitute the main part of the sonar systems. Sonars are used in marine vessels and for transportation of marine military equipment, such as submarines and battleships. Hydrophones work by converting received sound pressure to electrical signals. The idea of the present paper for designing hydrophones is taken from sea creatures and use artificial hair cell structure. This structure not only has the advantages of piezoelectric sensors such as being active and having optimal sensitivity, but it is also able to detect the direction of the sound and work at low frequencies, the performance of the sensor has been improved compared with the previous works (Ito et al. in Sens Actuators, 2008; Choi et al. in Sens Actuators, 2010; Guan et al. in Microsyst Technol, 2011; Sens Actuators, 2012; Zhang et al. in Design of a monolithic integrated three-dimensional MEMS bionic vector hydrophone, 2014), in a way that its sensitivity is ź 191 dB in the frequency range of below 10.4 kHz (0 dB re 1 V/μPa).

Artificial hair cells inspired by active hair bundle motility

Hair cells are specialized sensory cells found in the auditory and vestibular systems of many vertebrates. Many of these cells display an active hair bundle motility that amplifies the hair cell’s response to small stimuli, increases frequency sensitivity, and produces an amplitude compression that allows the cell to transduce a large range of input amplitudes. These features are important in an animal’s ability to detect and understand complex stimuli in its environment. The hair bundle’s nonlinear amplification results from combination of a nonlinear stiffness produced by opening and closing ion channels and an adaptation motor that brings the hair bundle close to a dynamic instability. This article presents the models and designs for a bio-inspired, artificial hair cell inspired by active hair bundle motility. These sensors are based on cantilevered beams with a nonlinear stiffness and a piezoelectric actuator to mimic the adaptation motor. Artificial hair cells can serve as bio-inspired microphones hydrophones, accelerometers, or other dynamic sensors. By mimicking hair bundle motility, these sensors could detect smaller stimuli, have enhanced frequency resolution, and transduce a wider range of input levels compared to traditional sensors.

From Biological Cilia to Artificial Flow Sensors: Biomimetic Soft Polymer Nanosensors with High Sensing Performance.

We report the development of a new class of miniature all-polymer flow sensors that closely mimic the intricate morphology of the mechanosensory ciliary bundles in biological hair cells. An artificial ciliary bundle is achieved by fabricating bundled polydimethylsiloxane (PDMS) micro-pillars with graded heights and electrospinning polyvinylidenefluoride (PVDF) piezoelectric nanofiber tip links. The piezoelectric nature of a single nanofiber tip link is confirmed by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). Rheology and nanoindentation experiments are used to ensure that the viscous properties of the hyaluronic acid (HA)-based hydrogel are close to the biological cupula. A dome-shaped HA hydrogel cupula that encapsulates the artificial hair cell bundle is formed through precision drop-casting and swelling processes. Fluid drag force actuates the hydrogel cupula and deflects the micro-pillar bundle, stretching the nanofibers and generating electric charges. Functioning with principles analogous to the hair bundles, the sensors achieve a sensitivity and threshold detection limit of 300 mV/(m/s) and 8 μm/s, respectively. These self-powered, sensitive, flexible, biocompatibale and miniaturized sensors can find extensive applications in navigation and maneuvering of underwater robots, artificial hearing systems, biomedical and microfluidic devices.

A study of active artificial hair cell models inspired by outer hair cell somatic motility

The cochlea displays an important, nonlinear amplification of sound-induced oscillations. In mammals, this amplification is largely powered by the somatic motility of the outer hair cells. The resulting cochlear amplifier has three important characteristics useful for hearing: an amplification of responses from low sound pressures, an improvement in frequency selectivity, and an ability to transduce a broad range of sound pressure levels. These useful features can be incorporated into designs for active artificial hair cells, bio-inspired sensors for use as microphones, accelerometers, or other dynamic sensors. The sensor consists of a cantilever beam with piezoelectric actuators. A feedback controller applies a voltage to the actuators to mimic the outer hair cells’ somatic motility. This article describes three control laws for an active artificial hair cell inspired by models of the outer hair cells’ somatic motility. The first control law is based on a phenomenological model of the cochlea while the second and third models incorporate physiological aspects of the biological cochlea to further improve sensor performance. Simulations show that these models qualitatively reproduce the key aspects of the mammalian cochlea, namely, amplification of oscillations from weak stimuli, higher quality factors, and a wider input dynamic range.

Flow field sensing with bio-inspired artificial hair cell arrays

The hair cell is a biological sensor that uses microscopic hair-like structures to detect delicate motions of surrounding fluid. Inspired by this principle, an artificial hair cell (AHC) sensory method based on bio-membrane transducers is developed for airflow sensing. One-dimensional arrays built from modular AHC units measure local velocity at different points in a flow profile. Each of the AHC units uses thinly extruded glass fibers as mechanical receptors of air velocity. Hair vibrations are converted to current via hydrogel-supported (lipid bilayers) by virtue of their mechanosensitive properties. The AHC outputs are combined into one channel, requiring a demultiplexing operation to recover individual hair cell information. This is achieved by tuning each AHC hair length to a unique frequency response and recovering individual sensor information based on the frequency content of the signal. The method is entitled tuned frequency response encoding (TFRE). When several AHC units are excited simultaneously by an airflow, the resulting signal is a superposition of each sensor's individual response. The excitation at each sensor is reconstructed from the frequencies that appear in the combined output. This technique was inspired by how organisms use hair cells with tuned responses to mechanically process flow stimuli. It takes advantage of a novel AHC's high signal-to-noise ratio (compared to other membrane-based AHCs) and linear output response to flow velocity. Initial tests with linear arrays of three AHCs show success in estimating the shape of the velocity profile from an air source that varies in position and intensity. However, temporal variations in some cases in membrane size affect sensitivity properties and make accurate flow velocity estimation difficult. Nevertheless, under stable conditions, the measured velocity profiles match closely with theoretical predictions. The implementation of the array sensing method demonstrates the sensory capability of bilayer-based AHC arrays, but highlights the difficulties of achieving consistent performance with biomolecular materials.

Bio-inspired piezoelectric artificial hair cell sensor fabricated by powder injection molding

A piezoelectric artificial hair cell sensor was fabricated by the powder injection molding process in order to make an acoustic vector hydrophone. The entire process of powder injection molding was developed and optimized for PMN–PZT ceramic powder. The artificial hair cell sensor, which consists of high aspect ratio hair cell and three rectangular mechanoreceptors, was precisely fabricated through the developed powder injection molding process. The density and the dielectric property of the fabricated sensor shows 98% of the theoretical density and 85% of reference dielectric property of PMN–PZT ceramic powder. With regard to homogeneity, three rectangular mechanoreceptors have the same dimensions, with 3 μm of tolerance with 8% of deviation of dielectric property. Packaged vector hydrophones measure the underwater acoustic signals from 500 to 800 Hz with −212 dB of sensitivity. Directivity of vector hydrophone was acquired at 600 Hz as analyzing phase differences of electric signals.

Artificial fish skin of self-powered micro-electromechanical systems hair cells for sensing hydrodynamic flow phenomena.

Using biological sensors, aquatic animals like fishes are capable of performing impressive behaviours such as super-manoeuvrability, hydrodynamic flow 'vision' and object localization with a success unmatched by human-engineered technologies. Inspired by the multiple functionalities of the ubiquitous lateral-line sensors of fishes, we developed flexible and surface-mountable arrays of micro-electromechanical systems (MEMS) artificial hair cell flow sensors. This paper reports the development of the MEMS artificial versions of superficial and canal neuromasts and experimental characterization of their unique flow-sensing roles. Our MEMS flow sensors feature a stereolithographically fabricated polymer hair cell mounted on Pb(Zr(0.52)Ti(0.48))O3 micro-diaphragm with floating bottom electrode. Canal-inspired versions are developed by mounting a polymer canal with pores that guide external flows to the hair cells embedded in the canal. Experimental results conducted employing our MEMS artificial superficial neuromasts (SNs) demonstrated a high sensitivity and very low threshold detection limit of 22 mV/(mm s(-1)) and 8.2 µm s(-1), respectively, for an oscillating dipole stimulus vibrating at 35 Hz. Flexible arrays of such superficial sensors were demonstrated to localize an underwater dipole stimulus. Comparative experimental studies revealed a high-pass filtering nature of the canal encapsulated sensors with a cut-off frequency of 10 Hz and a flat frequency response of artificial SNs. Flexible arrays of self-powered, miniaturized, light-weight, low-cost and robust artificial lateral-line systems could enhance the capabilities of underwater vehicles.

Developing an active artificial hair cell using nonlinear feedback control

The hair cells in the mammalian cochlea convert sound-induced vibrations into electrical signals. These cells have inspired a variety of artificial hair cells (AHCs) to serve as biologically inspired sound, fluid flow, and acceleration sensors and could one day replace damaged hair cells in humans. Most of these AHCs rely on passive transduction of stimulus while it is known that the biological cochlea employs active processes to amplify sound-induced vibrations and improve sound detection. In this work, an active AHC mimics the active, nonlinear behavior of the cochlea. The AHC consists of a piezoelectric bimorph beam subjected to a base excitation. A feedback control law is used to reduce the linear damping of the beam and introduce a cubic damping term which gives the AHC the desired nonlinear behavior. Model and experimental results show the AHC amplifies the response due to small base accelerations, has a higher frequency sensitivity than the passive system, and exhibits a compressive nonlinearity like that of the mammalian cochlea. This bio-inspired accelerometer could lead to new sensors with lower thresholds of detection, improved frequency sensitivities, and wider dynamic ranges.