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Highlights: This review discusses the potential of degradable metallic micromotors for a variety of biomedical and environmental applications. The design principles, fabrication techniques and degradation mechanisms of degradable metallic micromotors are reviewed in detail. Challenges and future directions for the development of degradable metallic micromotors for real-life applications are presented. Synthetic micromotor has gained substantial attention in biomedicine and environmental remediation. Metal-based degradable micromotor composed of magnesium (Mg), zinc (Zn), and iron (Fe) have promise due to their nontoxic fuel-free propulsion, favorable biocompatibility, and safe excretion of degradation products Recent advances in degradable metallic micromotor have shown their fast movement in complex biological media, efficient cargo delivery and favorable biocompatibility. A noteworthy number of degradable metal-based micromotors employ bubble propulsion, utilizing water as fuel to generate hydrogen bubbles. This novel feature has projected degradable metallic micromotors for active in vivo drug delivery applications. In addition, understanding the degradation mechanism of these micromotors is also a key parameter for their design and performance. Its propulsion efficiency and life span govern the overall performance of a degradable metallic micromotor. Here we review the design and recent advancements of metallic degradable micromotors. Furthermore, we describe the controlled degradation, efficient in vivo drug delivery, and built-in acid neutralization capabilities of degradable micromotors with versatile biomedical applications. Moreover, we discuss micromotors' efficacy in detecting and destroying environmental pollutants. Finally, we address the limitations and future research directions of degradable metallic micromotors. [ABSTRACT FROM AUTHOR]

Magnetic actuation techniques and microrobots have attracted considerable interest due to their potential applications in biomedicine. Interventional techniques have emerged as a minimally invasive approach to treat a wide range of vascular diseases. The current practice of interventional procedures is, however, limited by manual control of interventional devices and time-consuming procedures. Moreover, fluoroscopy is considered as an essential part of the procedure today despite posing many limitations for patients and physicians. Recently, various microrobotic solutions have been proposed for vascular interventions, including advances in magnetic navigation systems and magnetically steerable catheters and guidewires, which have shown potential benefits such as reduced radiation doses, improved access to difficult-to-reach and tortuous anatomy. This paper reviews the commercial magnetic actuation systems and magnetically actuated interventional microrobots that have been developed by academic research groups and medical companies worldwide, outlining their capability, applicability as well as limitations. We further address the challenges and future prospects of the research toward clinical acceptance of magnetic interventional technologies. [ABSTRACT FROM AUTHOR]

Abstract: The energy harvesting by utilizing the piezoelectric effect for the conversion of oscillatory mechanical energy to useful electrical energy has been promising for self-powered devices. The output power can be controlled by designing the size and shape of the constituents of the harvester. This study demonstrates the effect of Ti plate (elastic layer) thickness on the resonant frequency, neutral axis position, vibration amplitude and energy harvesting performance of the cantilever structured piezoelectric energy harvester (PEH). Here, the each harvester had the same dimensions of piezoelectric layer and the same proof mass position at the end of the cantilever while it had the different elastic layer thicknesses (70-300 μm). The analysis revealed that the output power showed the opposite trend in vibration amplitude with varying the elastic layer thickness. Among all of the PEHs, the configuration with the largest elastic layer thickness (300 μm) exhibited a maximum output power of 48 μW at 76 Hz under 0.2 g acceleration, despite of the smallest vibration amplitude and the highest resonant frequency. The outcomes suggest that the thickness ratio of the piezoelectric and elastic layers should be optimized to realize the best harvesting performance.Graphical Abstract: [ABSTRACT FROM AUTHOR]

Microprobes are used to repair neuronal injury by recording electrical signals from neuronal cells around the surface of the device. Following implantation into the brain, the immune response results in formation of scar tissue around the microprobe. However, neurons must be in close proximity to the microprobe to enable signal recording. A common reason for failure of microprobes is impaired signal recording due to scar tissue, which is not related to the microprobe itself. Therefore, the device-cell interface must be improved to increase the number of neurons in contact with the surface. In this study, we developed nanostructured SU-8 microprobes to support neuronal growth. Nanostructures of 200 nm diameter and depth were applied to the surface of microprobes, and the attachment and neurite outgrowth of PC12 cells on the microprobes were evaluated. Neuronal attachment and neurite outgrowth on the nanostructured microprobes were significantly greater than those on non-nanostructured microprobes. The enhanced neuronal attachment and neurite outgrowth on the nanostructured microprobes occurred in the absence of an adhesive coating, such as poly- l-lysine, and so may be useful for implantable devices for long-term use. Therefore, nanostructured microprobes can be implanted without adhesive coating, which can cause problems in vivo over the long term. [ABSTRACT FROM AUTHOR]

A novel cell-stimulation system was fabricated using 10 × 29 piezoelectric micromachined ultrasonic transducer (pMUT) arrays for targeted ultrasonic cell stimulation. Both the diameter of a single pMUT element and the edge-to-edge gap were 120 μm, and the size of a pMUT array was 2.27 × 6.84 mm, to be placed at the bottom of a Transwell. The measured resonance frequency of a single pMUT element was 1.48 ± 0.13 MHz and the measured acoustic intensity of the pMUT array was 0.15 ± 0.03 MPa at 1 mm away from the transducer. A pMUT array was mounted on a print circuit board (PCB), which was designed in accordance with the size of a 12-well Transwell. The Transwell was placed on the PCB and wire bonding was performed to electrically connect the PCB and pMUT arrays. After wiring, the PCB and pMUT arrays were coated with 2.6-μm thick parylene-C to ensure biocompatibility and waterproofing. PC12 cells were used for ultrasonic cell stimulation tests to examine the proposed all-in-one low-intensity pulsed ultrasound stimulation system. Various stimulation times and duty cycles were used simultaneously for cell proliferation in a confined cell culture environment. All stimulation groups showed increased cell proliferation rates, in the range 138-166%, versus the proliferation rate of the control group. [ABSTRACT FROM AUTHOR]

Two types of microfabricated acoustic transducers-the resonator array of isolated beams (RAIB) and the resonator array of coupled beams (RACB)-were built using MEMS technology. The frequency selectivity of the two types of transducers was qualitatively compared using the transfer functions measured with a scanning laser Doppler vibrometer (SLDV). Each RAIB beam operated as a band-pass filter in the proximity to the corresponding resonance frequency, thereby showing the frequency-selective feature of the basilar membrane (BM). However, the frequency selectivity of RACB was poor because the SiO membrane, which mechanically coupled the neighboring beams, suppressed the independent response of each beam. The influence of the mechanical coupling of the SiO membrane was simulated using lumped parametric models. Simulation results showed that the coupling stiffness of the SiO membrane may significantly undermine the frequency selectivity of the resonator array. The comparison of the frequency-selective performance revealed that RAIB is a better option than RACB for mimicking the tonotopy of BM. [ABSTRACT FROM AUTHOR]

We report an experimental assessment of the electrical performance of two piezoelectric beam arrays for artificial basilar membranes (ABMs): a continuous beam array (CBA) and a narrow-supports beam array (NSBA). Both arrays consist of piezoelectric beams of sequentially varying lengths that mimic the frequency selectivity of mammalian cochleae. The narrow supports of the NSBA resulted in lateral deformation of the beams, whereas the CBA beams were flat. The displacement and piezoelectric output of each beam were measured at the resonance frequency of each beam using a scanning laser-Doppler vibrometer (SLDV). Both ABM prototypes showed mechanical frequency selectivity that depended on the beam length. The CBA generated a piezoelectric output in the range 6.6-23.2 μV and exhibited electrical frequency separability, whereas the NSBA failed to generate sufficient electrical potential due to the lateral deformation of the piezoelectric beams. The CBA was found to be more effective as an ABM, with potential for use in cochlear implants. [ABSTRACT FROM AUTHOR]

Microelectromechanical system (MEMS) test sockets with attached tips were developed for a ball-grid-array integrated circuit (BGA IC) package, using MEMS fabrication technology in conjunction with Ni-Co electroplating and XeF isotropic silicon etching. The electroplating process was used to create metal tips and thick metal cantilevers, which increased the contact reliability and conductivity. XeF isotropic silicon etching was used to release the cantilever array, which reduced the process cost and increased the mechanical stability. A finite element analysis (FEA) simulation was conducted to verify the force-deflection relationships of five types of cantilevers with attached tips. With an input deflection of 40 μm, the measured forces ranged from 12.55 to 26.14 mN, and matched the FEA results. The path resistance between a tip and the appropriate electrode pad was less than 3.49 Ω for all cantilevers when the tips were connected to the corresponding positions with more than 5 mN of contact force. The cantilever design with a length of 430 um and a tip height of 60 um (L430D60) showed the best performance in terms of the force-deflection relationship, a well as resistance. Therefore, the fabricated MEMS test sockets with tips are potentially applicable to actual BGA IC package tests. [ABSTRACT FROM AUTHOR]

In this study, a one-dimensional beam array acoustic sensor was built using microelectromechanical system technology to achieve mechanical frequency selectivity. The acoustic sensor contained 16 beams of various lengths. The frequency selectivity was evaluated with a scanning laser Doppler vibrometer, while applying an alternating current having various frequencies with 2 volts amplitude and 0 volt offset. The beams formed separate band-pass filters in the proximity of the corresponding resonance frequencies in the range of 3 kHz to 13 kHz. The first resonance frequencies of the beams were calculated using finite element analysis to simulate the frequency response. In the finite element analysis models, mode shapes were studied to understand the effect of the beam deformation caused by the residual stress generated during the MEMS fabrication. The measured and simulated first resonance frequencies of the beams provided solid evidence of the tonotopicity of the sensor. [ABSTRACT FROM AUTHOR]

Here, we develop a flexible microrobot enhancing the steerability of a conventional guidewire. To improve steerability, a microrobot is attached to the tip of the guidewire and guided using an external magnetic field generated by an electromagnetic coil system. The flexible microrobot is fabricated via replica-molding and features a body made of polydimethylsiloxane (PDMS) and a single permanent magnet. As the robot is made of a deformable material, it can be steered using a low-intensity external magnetic field; the robot can potentially be guided into the coronary artery. To study steering performance, we employed mathematical modeling and a finite element model (FEM), and performed experiments under various magnetic fields. We found that a mathematical model using the Euler-Bernoulli beam could not precisely calculate the deformation angles. The FEM more accurately estimated those angles. The deformation angle can be controlled from 0 to 80° at a magnetic field intensity of 15 mT. The trackability at angles of 45 and 80° of the guidewire-based microrobot was confirmed in vitro using a two-dimensional blood vessel phantom. [ABSTRACT FROM AUTHOR]

We proposed a piezoelectric artificial basilar membrane (ABM) composed of a microelectromechanical system cantilever array. The ABM mimics the tonotopy of the cochlea: frequency selectivity and mechanoelectric transduction. The fabricated ABM exhibits a clear tonotopy in an audible frequency range (2.92-12.6 kHz). Also, an animal model was used to verify the characteristics of the ABM as a front end for potential cochlear implant applications. For this, a signal processor was used to convert the piezoelectric output from the ABM to an electrical stimulus for auditory neurons. The electrical stimulus for auditory neurons was delivered through an implanted intra-cochlear electrode array. The amplitude of the electrical stimulus was modulated in the range of 0.15 to 3.5 V with incoming sound pressure levels (SPL) of 70.1 to 94.8 dB SPL. The electrical stimulus was used to elicit an electrically evoked auditory brainstem response (EABR) from deafened guinea pigs. EABRs were successfully measured and their magnitude increased upon application of acoustic stimuli from 75 to 95 dB SPL. The frequency selectivity of the ABM was estimated by measuring the magnitude of EABRs while applying sound pressure at the resonance and off-resonance frequencies of the corresponding cantilever of the selected channel. In this study, we demonstrated a novel piezoelectric ABM and verified its characteristics by measuring EABRs. [ABSTRACT FROM AUTHOR]