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High element density and strict constraints of the element's size have significantly limited the design and fabrication of 2-D ultrasonic arrays, especially fully sampled 2-D arrays. Recently, 3-D printing technology has been one of the most rapidly developing fields. Along with the great progress of 3-D printing technology, complex and detailed 3-D structures have become readily available with a short iteration cycle, which allows us to reduce the complexity of routing and helps to ameliorate assembly problems in 2-D ultrasound array fabrication. In this work, we designed and fabricated 2-D ultrasound arrays for an array of applications with a pitch-shifting interposer, which allowed us to fit different array designs with the same circuit design and significantly reduce the requirements in routing and connection for 2-D array fabrication at frequencies from 4 to 10 MHz. Results demonstrated that this design would make 2-D arrays more available and affordable.

This paper developed an ultrasound concave 2D ring array transducer for retinal visual stimulation of the brain to restore vision in blind people. Theory, simulation, and experiment were presented.

Implantable medical electronics (IMEs) are now becoming increasingly prevalent for diagnostic and therapeutic purposes. Despite extensive efforts, a primary challenge for IMEs is reliable wireless power and communication to provide well-controlled, therapeutically relevant effects. Ultrasonic energy transfer and communication (UETC) employing traveling ultrasound waves to transmit energy has emerged as a promising wireless strategy for IMEs. Nevertheless, conventional UETC systems are rigid, bulky, and based on toxic lead-based piezoelectric materials, raising efficiency and safety concerns. Here, we present a novel transcutaneous UETC system based on a two-dimensional flexible lead-free piezoelectric array (f-LFPA) that hybridizes high-performance (piezoelectric coefficient

Ultrasound neuromodulation is an emerging technology. A significant amount of effort has been devoted to investigating the feasibility of noninvasive ultrasound retinal stimulation. Recent studies have shown that ultrasound can activate neurons in healthy and degenerated retinas. Specifically, high-frequency ultrasound can evoke localized neuron responses and generate patterns in visual circuits. In this review, we recapitulate pilot studies on ultrasound retinal stimulation, compare it with other neuromodulation technologies, and discuss its advantages and limitations. An overview of the opportunities and challenges to develop a noninvasive retinal prosthesis using high-frequency ultrasound is also provided.

There is uncertainty about deep vein thrombosis standard treatment as thrombus stiffness alters each case. Here, we investigated thrombus' stiffness of different compositions and ages using shear wave elastography (SWE). We then studied the effectiveness of ultrasound-thrombolysis on different thrombus compositions.Shear waves generated through mechanical shaker and traveled along thrombus of different hematocrit (HCT) levels, whereas 18-MHz ultrasound array used to detect wave propagation. Thrombus' stiffness was identified by the shear wave speed (SWS). In thrombolysis, a 3.2 MHz focused transducer was applied to different thrombus compositions using different powers. The thrombolysis rate was defined as the percentage of weight loss.The estimated average SWS of 20%, 40%, and 60% HCT thrombus were 0.75 m/s, 0.44 m/s, and 0.32 m/s, respectively. For Thrombolysis, the percentage weight loss at 8 MPa Negative pressure for the same HCT groups were 23.1%, 35.29%, and 39.66% respectively.SWS is inversely related to HCT level and positively related to thrombus age. High HCT thrombus had higher weight loss compared to low HCT. However, the difference between 20% and 40% HCT was more significant than between 40% and 60% HCT in both studies. Our results suggest that thrombus with higher SWS require more power to achieve the same thrombolysis rate as thrombus with lower SWS.Characterizing thrombus elastic property undergoing thrombolysis enables evaluation of ultrasound efficacy for fractionating thrombus and reveals the appropriate ultrasound parameters selection to achieve a certain thrombolysis rate in the case of a specific thrombus stiffness.

Currently, blindness cannot be cured and patients’ living quality can be compromised severely. Ultrasonic (US) neuromodulation is a promising technology for the development of noninvasive cortical visual prosthesis. We investigated the feasibility of transcranial focused ultrasound (tFUS) for noninvasive stimulation of the visual cortex (VC) to develop improved visual prosthesis. tFUS was used to successfully evoke neural activities in the VC of both normal and retinal degenerate (RD) blind rats. Our results showed that blind rats showed more robust responses to ultrasound stimulation when compared with normal rats. ( ${p} , two-sample t-test). Three different types of ultrasound waveforms were used in the three experimental groups. Different types of cortical activities were observed when different US waveforms were used. In all rats, when stimulated with continuous ultrasound waves, only short-duration responses were observed at “US on and off” time points. In comparison, pulsed waves (PWs) evoked longer low-frequency responses. Testing different parameters of PWs showed that a pulse repetition frequency higher than 100 Hz is required to obtain the low-frequency responses. Based on the observed cortical activities, we inferred that acoustic radiation force (ARF) is the predominant physical mechanism of ultrasound neuromodulation.

Ultrasound-induced energy transfer (UET) represents an emerging technology that can be integrated into implantable medical systems for wireless power and information communication applications. However, the spatial–temporal resolution, directivity, and versatility of traditional piezo-ultrasound systems are generally limited. Here, we describe the novel design and implementation of a hybrid-induced energy transfer strategy using photoacoustic (PA) and piezo-ultrasound (PU) technology in a 3D twining wireless implant, which shows multi-mode transmission advantages, with high power, better resolution, and flexible directivity. This new ultrasonic system exploits a focused ring piezo-transducer and a miniaturized fiber-photoacoustic converter as the hybrid acoustic source and a (K,Na)NbO3-based lead-free linear piezo-array as the harvester. Complying with FDA regulations, hybrid-induced multifunctionality, such as stable powering (∼21.3 mW cm−2) and the high-resolution signal communication (signal-to-noise ratio: ∼22.5 dB) of a two-dimensional image, was demonstrated ex vivo using 12 mm-thick porcine tissue. The 3D twining design also ensures that the device can achieve stable energy harvesting without operational difficulties, a unique advantage that can facilitate its future clinical application.

Ultrasound single-beam acoustic tweezer system has attracted increasing attention in the field of biomechanics. Cell biomechanics play a pivotal role in leukemia cell functions. To better understand and compare the cell mechanics of the leukemia cells, herein, we fabricated an acoustic tweezer system in-house connected with a 50-MHz high-frequency cylinder ultrasound transducer. Selected leukemia cells (Jurkat, K562, and MV-411 cells) were cultured, trapped, and manipulated by high-frequency ultrasound single beam, which was transmitted from the ultrasound transducer without contacting any cells. The relative deformability of each leukemia cell was measured, characterized, and compared, and the leukemia cell (Jurkat cell) gaining the highest deformability was highlighted. Our results demonstrate that the high-frequency ultrasound single beam can be utilized to manipulate and characterize leukemia cells, which can be applied to study potential mechanisms in the immune system and cell biomechanics in other cell types.

Electronic visual prostheses, or biomimetic eyes, have shown the feasibility of restoring functional vision in the blind through electrical pulses to initiate neural responses artificially. However, existing visual prostheses predominantly use wired connections or electromagnetic waves for powering and data telemetry, which raises safety concerns or couples inefficiently to miniaturized implant units. Here, we present a flexible ultrasound-induced retinal stimulating piezo-array that can offer an alternative wireless artificial retinal prosthesis approach for evoking visual percepts in blind individuals. The device integrates a two-dimensional piezo-array with 32-pixel stimulating electrodes in a flexible printed circuit board. Each piezo-element can be ultrasonically and individually activated, thus, spatially reconfigurable electronic patterns can be dynamically applied via programmable ultrasound beamlines. As a proof of concept, we demonstrate the ultrasound-induced pattern reconstruction in ex vivo murine retinal tissue, showing the potential of this approach to restore functional, life-enhancing vision in people living with blindness.

Current treatments for meibomian gland dysfunction have several limitations, creating a necessity for other advanced treatment options. The purpose of this study is to determine the effectiveness of focused ultrasound stimulation for the treatment of dry eye disease caused by meibomian gland dysfunction. An in vivo study of nine Dutch Belted rabbits was conducted with focused ultrasound stimulation of the meibomian glands. A customized line-focused ultrasonic transducer was designed for treatment. Fluorescein imaging, Schirmer’s test, and Lipiview II ocular interferometer were used to quantify outcomes from three aspects: safety, tear production, and lipid layer thickness. Both tear secretion and lipid layer thickness improved following ultrasound treatment. Five to 10 min after the ultrasound treatment, the mean values of lipid layer thickness increased from 55.33 ± 11.15 nm to 95.67 ± 22.77 nm ( p

Flame-retardant and thermal management structures have attracted great attention due to the requirement of high-temperature exposure in industrial, aerospace, and thermal power fields, but the development of protective fire-retardant structures with complex shapes to fit arbitrary surfaces is still challenging. Herein, we reported a rotation-blade casting-assisted 3D printing process to fabricate nacre-inspired structures with exceptional mechanical and flame-retardant properties, and the related fundamental mechanisms are studied. 3-(Trimethoxysilyl)propyl methacrylate (TMSPMA) modified boron nitride nanoplatelets (BNs) were aligned by rotation-blade casting during the 3D printing process to build the “brick and mortar” architecture. The 3D printed structures are more lightweight, while having higher fracture toughness than the natural nacre, which is attributed to the crack deflection, aligned BN (a-BNs) bridging, and pull-outs reinforced structures by the covalent bonding between TMSPMA grafted a-BNs and polymer matrix. Thermal conductivity is enhanced by 25.5 times compared with pure polymer and 5.8 times of anisotropy due to the interconnection of a-BNs. 3D printed heat-exchange structures with vertically aligned BNs in complex shapes were demonstrated for efficient thermal control of high-power light-emitting diodes. 3D printed helmet and armor with a-BNs show exceptional mechanical and fire-retardant properties, demonstrating integrated mechanical and thermal protection.

Ultrasound energy transfer (UET) is developed and integrated into various bioelectronics with diagnostic, therapeutic, and monitoring capabilities. However, existing UET platforms generally enable one function at a time due to the single ultrasound channel architecture, limiting the full potential of bioelectronics that requires multicontrol modes. Here, a multichannel piezo-ultrasound implant (MC-PUI) is presented that integrates a hybrid waterborne acoustic metastructure (HWAM), multiple piezo-harvesters, and a miniaturized circuit with electronic components for selective wireless control via ultrasound frequency switching. The HWAM that utilizes both a 3D-printed air-diffraction matrix and a half-lambda Fabry-Perot resonator is optimized to provide the advantage of ultrasound selectivity at megahertz frequencies. Complying with U.S. Food and Drug Administration regulations, frequency-controlled multifunctional operations, such as wireless charging (≈11.08 µW) at 3.3 MHz and high-sensitivity wireless switch/control (threshold ≈0.55 MPa) of micro-light-emitting diode/motor at 1 MHz, are demonstrated ex vivo using porcine tissue and in vivo in a rat. The developed MC-PUI enhances UET versatility and opens up a new pathway for wireless implant design.

Mechanical properties of biological tissues are significant biomarkers for diagnosing various diseases. Assessing the viscoelastic properties of multi-layer tissues has remained challenging for a long time. Some shear wave models have been proposed to estimate thin-layer tissues' viscoelasticity recently. However, the potential applications of these models are highly restricted since few biological tissues are single-layered. Here we proposed a multi-layer model for layer-specific viscoelasticity estimation of biological tissues. Integrating the theoretical model and ultrasonic micro-elastography imaging system, the viscoelasticity of both layers was assessed. Dual-layer phantoms and ex vivo porcine eyes were used to verify the proposed model. Results obtained from the mechanical test and shear wave rheological model using bulk phantoms were provided as validation criteria. The representative phantom had two layers with elastic moduli of 1.6 ± 0.2 kPa and 18.3 ± 1.1 kPa, and viscosity moduli of 0.56 ± 0.16 Pa·s and 2.11 ± 0.28 Pa·s, respectively. The estimated moduli using the proposed model were 1.3 ± 0.2 kPa and 16.20 ± 1.8 kPa, and 0.80 ± 0.31 Pa·s and 1.87 ± 0.67 Pa·s, more consistent with the criteria (one-tailed t-test, p < 0.1). By contrast, other methods, including the group velocity method and single-layer Rayleigh-Lamb model, generate significant errors in their estimates. For the ex vivo porcine eye, the estimated viscoelasticity was 23.2 ± 8.3 kPa and 1.0 ± 0.4 Pa·s in the retina, and 158.0 ± 17.6 kPa and 1.2 ± 0.4 Pa·s in the sclera. This study demonstrated the potential of the proposed method to significantly improve accuracy and expand clinical applications of shear wave elastography.

Currently, blindness cannot be cured and patients' living quality can be compromised severely. Ultrasonic neuromodulation is a promising technology for the development of non-invasive cortical visual prosthesis. We investigated the feasibility of transcranial focused ultrasound (tFUS) for noninvasive stimulation of the visual cortex to develop improved visual prosthesis. tFUS was used to successfully evoke neural activities in the visual cortex (VC) of both normal and retinal degenerate (RD) blind rats. Our results showed that blind rats showed more robust responses to ultrasound stimulation compared to normal rats. (p

Piezoelectric composites are considered excellent core materials for fabricating various ultrasonic devices. For the traditional fabrication process, piezoelectric composite structures are mainly prepared by mold forming, mixing, and dicing-filing techniques. However, these techniques are limited on fabricating shapes with complex structures. With the rapid development of additive manufacturing (AM), many research fields have applied AM technology to produce functional materials with various geometric shapes. In this study, the Mask-Image-Projection-based Stereolithography (MIP-SL) process, one of the AM (3D-printing) methods, was used to build BaTiO3-based piezoelectric composite ceramics with honeycomb structure design. A sintered sample with denser body and higher density was achieved (i.e., density obtained 5.96 g/cm3), and the 3D-printed ceramic displayed the expected piezoelectric and ferroelectric properties using the complex structure (i.e., piezoelectric constant achieved 60 pC/N). After being integrated into an ultrasonic device, the 3D-printed component also presents promising material performance and output power properties for ultrasound sensing (i.e., output voltage reached 180 mVpp). Our study demonstrated the effectiveness of AM technology in fabricating piezoelectric composites with complex structures that cannot be fabricated by dicing-filling. The approach may bring more possibilities to the fabrication of micro-electromechanical system (MEMS)-based ultrasonic devices via 3D-printing methods in the future.

Thermoelectric generators (TEGs) provide a unique solution for energy harvesting from waste heat, presenting a potential solution for green energy. However, traditional rigid and flexible TEGs cannot work on complex and dynamic surfaces. Here, we report a stretchable TEG (S-TEG) (over 50% stretchability of the entire device) that is geometrically suitable for various complex and dynamic surfaces of heat sources. The S-TEG consists of hot-pressed nanolayered p-(Sb2Te3) and n-(Bi2Te3)-type thermoelectric couple arrays and exploits the wavy serpentine interconnects to integrate all units. The internal resistance of a 10 × 10 array is 22 ohm, and the output power is ∼0.15 mW/cm2 at ΔT = 19 K on both developable and nondevelopable surfaces, which are much improved compared with those of existing S-TEGs. The energy harvesting of S-TEG from the dynamic surfaces of the human skin offers a potential energy solution for the wearable devices for health monitoring.

Purpose To quantitatively investigate both optic nerve head (ONH) and peripapillary sclera (PPS) biomechanical properties of porcine eyes through an ultrasonic elastography imaging system in response to both increasing and decreasing intraocular pressure (IOP). Methods The Young’s modulus of the ONH and PPS were assessed using our high resolution ultrasonic imaging system which utilized a mechanical shaker to induce shear waves and an off-axis aligned 40 MHz needle transducer to track micron-level displacement along the direction of wave propagation. In this study, imaging on a total of 8 ex vivo porcine eyes preloaded with IOPs from 6 mmHg to 30 mmHg was performed. To have a better understanding of the effect of varying IOP on biomechanics, both increasing and decreasing IOPs were investigated. Results The increase of the Young’s modulus of ONH (92.4 ± 13.9 kPa at 6 mmHg to 224.7 ± 71.1 kPa at 30 mmHg) and PPS (176.8 ± 14.3 kPa at 6 mmHg to 573.5 ± 64.4 kPa at 30 mmHg) following IOP elevation could be observed in the reconstructed Young’s modulus of the shear wave elasticity (SWE) imaging while the B-mode structural images remained almost unchanged. In addition, for the same IOP level, both ONH and PPS have a tendency to be stiffer with decreasing IOP as compared to increasing IOP. Conclusions Our results demonstrate the feasibility of using our ultrasonic elastography system to investigate the stiffness mapping of posterior eye with high resolution in both increasing and decreasing IOPs, making this technique potentially useful for glaucoma.

Focused ultrasound (FUS) is a rapidly developing stimulus technology with the potential to uncover novel mechanosensory dependent cellular processes. Since it is noninvasive, it holds great promise for future therapeutic applications in patients used either alone or as a complement to boost existing treatments. For example, FUS stimulation causes invasive but not noninvasive cancer cell lines to exhibit marked activation of calcium signaling pathways. Here, we identify the membrane channel PANNEXIN1 (PANX1) as a mediator for activation of calcium signaling in invasive cancer cells. Knockdown of PANX1 decreases calcium signaling in invasive cells, while PANX1 overexpression enhances calcium elevations in non-invasive cancer cells. We demonstrate that FUS may directly stimulate mechanosensory PANX1 localized in endoplasmic reticulum to evoke calcium release from internal stores. This process does not depend on mechanosensory stimulus transduction through an intact cytoskeleton and does not depend on plasma membrane localized PANX1. Plasma membrane localized PANX1 however plays a different role in mediating the spread of intercellular calcium waves via ATP release. Additionally, we show that FUS stimulation evokes cytokine/chemokine release from invasive cancer cells, suggesting that FUS could be an important new adjuvant treatment to improve cancer immunotherapy.

Objective: The choroidal vessels, which supply oxygen and nutrient to the retina, may play a pivotal role in eye disease pathogenesis such as diabetic retinopathy and glaucoma. In addition, the retrobulbar circulation that feeds the choroid shows an important pathophysiologic role in myopia and degenerative myopia. Owing to the light-absorbing retinal pigment epithelium (RPE) and optically opaque sclera, choroidal and retrobulbar vasculature were difficult to be observed using clinically accepted optical coherence tomography angiography (OCT-A) technique. Here, we have developed super-resolution ultrasound microvessel imaging technique to visualize the deep ocular vasculature. Methods: An 18-MHz linear array transducer with compounding plane wave imaging technique and contrast agent – microbubble was implemented in this study. The centroid intensity of each microbubble was detected using image deconvolution algorithm with spatially variant point spread function, and then accumulated in successive frames in order to reconstruct microvasculature. The image deconvolution technique was first evaluated in a simulation study and experimental flow phantoms. The performance was then validated on normal rabbit eyes in vivo . Results: The image deconvolution based super-resolution ultrasound microvessel imaging technique shows good performance on either simulation study or flow phantoms. In vivo rabbit eye study indicated that the micron-level choroidal and retrobulbar vessels around the optic nerve head were successfully reconstructed in multiple 2D views and 3D volume imaging. Conclusions: Our results demonstrate the capability of using super-resolution ultrasound microvessel imaging technique to image the microvasculature of the posterior pole of the eye. This efficient approach can potentially lead to a routinely performed diagnostic procedure in the field of ophthalmology.

Age-related macular degeneration (AMD) is a progressive retinal disease and becomes the leading cause of blindness. It is well established that early detection is the key to preservation of functional vision. However, it is very difficult to diagnose AMD in very early stages, before structural changes are evident. Consequently, investigating the biomechanical properties of the retina maybe essential for understanding its physiological function. In this study, we present a shear wave-based quantitative method for estimating the elasticity of the posterior eye using shaker-based optical coherence elastography. This technique has been developed and validated on both a homogeneous phantom and a healthy rabbit in vivo. The shear wave speed from the ganglion side to the photoreceptor side of the rabbit eye is 4.1 m/s, 4.9 m/s, and 6.7 m/s, respectively. In addition, the most stiff sclera region has an average shear wave speed of 9.1 m/s. The results demonstrate the feasibility of using this technique to quantify biomechanical properties of the posterior eye and its potential translation to the clinical study. Impact statement Herein, we propose a potentially clinical applicable shaker-based optical coherence elastography (OCE) technique to characterize the biomechanical properties of the posterior eye, including different layers of the retina. Compared with either acoustic radiation force OCE or air-puff OCE, the newly developed method can induce sufficient shear wave propagation at the posterior eye with high resolution and large field of view.

Bioresorbable vascular scaffold (BVS) has recently been spotlighted for its unique characteristics of absorbing into blood vessels and eventually disappearing. Although intravascular ultrasound (IVUS) is the most common guiding tool for stent deployment, the echogenicity of BVS struts has changed as the center of stent lumen and scanning rotation is not concentric, which may cause a critical erroneous measurement in practice. This study investigated the physical conditions for dimming the stent brightness in IVUS images using a finite-difference method (FDM) to numerically solve acoustic wave propagation through nonhomogeneous medium. The dimmed brightness is caused by an angled rectangular cross section of a strut and its similar acoustic impedance with water. Imaging frequency is not a major cause. However, the angle between the acoustic beam and the BVS surface is the major cause of the dimmed brightness. As a solution, an approach using a frequency compounding method with signal polarity comparator was proposed to recover the reduced brightness without sacrificing spatial resolutions. Based on the simulation study, the signal level from BVS can be attenuated down by 17 dB when the angle between the acoustic beamline and the surface of BVS is more than 45°. With the proposed frequency compounding approach, the reduced signal can be recovered by 6 dB. In the experimental BVS IVUS imaging, strut brightness was reduced by 18 dB with an angled strut position and recovered by 5 dB with the proposed frequency compounding method. A pig coronary was imaged to demonstrate the performance of the proposed method.

The mechanosensitivity of the optic nerve head (ONH) plays a pivotal role in the pathogenesis of glaucoma. Characterizing elasticity of the ONH over changing physiological pressure may provide a better understanding of how changes in intraocular pressure (IOP) lead to changes in the mechanical environment of the ONH. Optical coherence elastography (OCE) is an emerging technique that can detect tissue biomechanics noninvasively with both high temporal and spatial resolution compared with conventional ultrasonic elastography. We describe a confocal OCE system in measuring ONH elasticity in vitro, utilizing a pressure inflation setup in which IOP is controlled precisely. We further utilize the Lamb wave model to fit the phase dispersion curve during data postprocessing. We present a reconstruction of Young’s modulus of the ONH by combining our OCE system with a Lamb wave model for the first time. This approach enables the quantification of Young’s modulus of the ONH, which can be fit using a piecewise polynomial to the corresponding IOP.

Endoscopic dual-modality photoacoustic (PA) and ultrasound (US) imaging has the capability of providing morphology and molecular information simultaneously. An ultrasonic transducer was applied for detecting PA signals and performing US imaging which determines the sensitivity and performance of a dual-modality PA/US system. In our study, a miniature single element 32-MHz lead magnesium niobate-lead titanate (PMN-PT) epoxy 1–3 composite based ultrasonic transducer was developed. A miniature endoscopic probe based on this transducer has been fabricated. Using the dual modality PA/US system with a PMN-PT/epoxy 1–3 composite based ultrasonic transducer, phantom and in vivo animal studies have been conducted to evaluate the performance. The preliminary results show enhanced bandwidths of the new ultrasonic transducer and improved signal-to-noise ratio of PA and US images of rat colorectal wall compared with PMN-PT and lead zirconate titanate (PZT) composite based ultrasonic transducers. Keywords: Photoacoustic imaging, Ultrasound imaging, Endoscopic imaging, Gastrointestinal, Ultrasonic transducer, PMN-PT epoxy 1-3 composite, Composite based ultrasonic transducer

Stretchable and twistable sensors attract much attention due to their excellent flexibility, stability, and sensitivity for most wearable devices. Star nasal moles in nature possess a flexible and highly sensitive nose that is composed of dozens of epidermal appendages or rays densely covered with small organs, which provides us inspiration for fabricating highly sensitive sensors. Herein, we present a strategy to build a star-nose-inspired sensor with stretchable and twistable structure, where the nose’s ray-like parallel polyimide films were fabricated by electrospinning and chemical silver coating. The sensor is finally encapsulated in PDMS and its sensitivity is significantly improved due to the design of bio-inspired parallel films. The bio-inspired sensor finally exhibits ultra-light, ultra-thin, and high sensitivity (Gauge Factor (GF) ~ 4000) characteristics, ensuring a perfect fit for human skin to achieve stretch, twist, and pressure detection functions. At last, an anisotropic sensor for omnidirectionally sensing human motions was built by crosswise assembling two of functional layers. This study highlights interesting possibilities for bioinspired sensors, with integrated light, thin, stretchable, twistable, and anisotropic features for wearable devices for monitoring human movement and personal health.

Ultrasonic driven wireless charging technology has recently attracted much attention in the next generation bio-implantable systems; however, most developed ultrasonic energy harvesters are bulky and rigid and cannot be applied to general complex surfaces. Here, a flexible piezoelectric ultrasonic energy harvester (PUEH) array was designed and fabricated by integrating a large number of piezoelectric active elements with multilayered flexible electrodes in an elastomer membrane. The developed flexible PUEH device can be driven by the ultrasonic wave to produce continuous voltage and current outputs on both planar and curved surfaces, reaching output signals of more than 2 Vpp and 4 μA, respectively. Potential applications of using the flexible PUEH to charge energy-storage devices and power commercial electronics were demonstrated. Its low attenuation performance was also evaluated using the in vitro test of transmitting power through pork tissue, demonstrating its potential use in the next generation of wirelessly powered bio-implantable micro-devices.

Atherosclerosis and its thrombotic complications plague developed countries. The rupture of vulnerable atherosclerotic plaques contributes to acute cardiovascular events and sudden cardiac deaths. Historically, coronary angiography has proved an invaluable tool for the detection and treatment of coronary stenoses which may result in myocardial ischemia; however, the method lacks the capacity to provide thorough information about properties of the lesion (i.e. whether it is lipid-rich, fibrotic, or calcified). The recent advances in electronics, biomaterials and microfabrication techniques have enabled novel multimodality catheters for the assessment of atherosclerotic plaques, such as the integration of intravascular ultrasound with photoacoustic microscopy, or optical coherence tomography, and the utilization of stretchable electrodes for electrochemical impedance spectroscopy. These pave the way for identification of complexity and composition of potentially unstable plaques as well as investigations of stenosis severity, plaque formation and remodeling, in both humans and studied animal models. However, up to date, an effective real-time detection of the atherosclerotic lesions prone to rupture which could be ready for clinical trials, remains an unmet challenge. In this context, this review highlights existing and newly-emerged intravascular sensors to assess metabolically and mechanically unstable plaques. Advantages and limitations, as well as further development and potential clinical applications will be thoroughly discussed.