Outperforming piezoelectric ultrasonics with high-reliability single-membrane CMUT array elements.

It has long been hypothesized that capacitive micromachined ultrasound transducers (CMUTs) could potentially outperform piezoelectric technologies. However, challenges with dielectric charging, operational hysteresis, and transmit sensitivity have stood as obstacles to these performance outcomes. In this paper, we introduce key architectural features to enable high-reliability CMUTs with enhanced performance. Typically, a CMUT element in an array is designed with an ensemble of smaller membranes oscillating together to transmit or detect ultrasound waves. However, this approach can lead to unreliable behavior and suboptimal transmit performance if these smaller membranes oscillate out of phase or collapse at different voltages. In this work, we designed CMUT array elements composed of a single long rectangular membrane, with the aim of improving the output pressure and electromechanical efficiency. We compare the performance of three different modifications of this architecture: traditional contiguous dielectric, isolated isolation post (IIP), and insulated electrode-post (EP) CMUTs. EPs were designed to improve performance while also imparting robustness to charging and minimization of hysteresis. To fabricate these devices, a wafer-bonding process was developed with near-100% bonding yield. EP CMUT elements achieved electromechanical efficiency values as high as 0.95, higher than values reported with either piezoelectric transducers or previous CMUT architectures. Moreover, all investigated CMUT architectures exhibited transmit efficiency 2-3 times greater than published CMUT or piezoelectric transducer elements in the 1.5-2.0 MHz range. The EP and IIP CMUTs demonstrated considerable charging robustness, demonstrating minimal charging over 500,000 collapse-snap-back actuation cycles while also mitigating hysteresis. Our proposed approach offers significant promise for future ultrasonic applications.

Miniature Deformable MEMS Mirrors for Ultrafast Optical Focusing.

Here, we introduce ultrafast tunable MEMS mirrors consisting of a miniature circular mirrored membrane, which can be electrostatically actuated to change the mirror curvature at unprecedented speeds. The central deflection zone is a close approximation to a parabolic mirror. The device is fabricated with a minimal membrane diameter, but at least double the size of a focused optical spot. The theory and simulations are used to predict maximum relative focal shifts as a function of membrane size and deflection, beam waist, and incident focal position. These devices are demonstrated to enable fast tuning of the focal wavefront of laser beams at ≈MHz tuning rates, two to three orders of magnitude faster than current optical focusing technologies. The fabricated devices have a silicon membrane with a 30-100 µm radius and a 350 nm gap spacing between the top and bottom electrodes. These devices can change the focal position of a tightly focused beam by ≈1 mm at rates up to 4.9 MHz and with response times smaller than 5 µs.

Transparent capacitive micromachined ultrasound transducer linear arrays for combined realtime optical and ultrasonic imaging.

Transparent ultrasound transducers could enable many novel applications involving both ultrasonics and optics. Recently, we reported transparent capacitive micromachined ultrasound transducers (CMUTs) and demonstrated through-illumination photoacoustic imaging. This work presents the feasibility of transparent CMUTs for combined ultrasound imaging and through-array white-light imaging with a miniature camera placed behind the array. Transparent CMUT devices are fabricated with an adhesive wafer bonding technique and provide high transparency up to 90% in visible wavelengths. Fabricated linear arrays have a central operating frequency of 9 MHz with 128 active elements. Realtime plane-wave imaging is performed for ultrasound imaging, and lateral and axial resolutions of, respectively, 234 and 338 µm are achieved. Transparent CMUT has demonstrated a high transmit sensitivity of 1.4 kPa/V per channel with a 100 VDC bias voltage. The signal-to-noise ratio for a beamformed image of wire targets is determined to be 28.4 dB. To the best of our knowledge, this is the first report of combined realtime optical and ultrasonic imaging with transparent arrays. This technology may enable one to visually see what is being scanned and scan what one sees without co-registration errors. Future applications could include multi-modality probes for interventional and surgical procedures.

Enrichment and ratiometric detection of circulating tumor cells using PSMA- and folate receptor-targeted magnetic and surface-enhanced Raman scattering nanoparticles.

The presence of circulating tumor cells (CTCs) in a patient's bloodstream is a hallmark of metastatic cancer. The detection and analysis of CTCs is a promising diagnostic and prognostic strategy as they may carry useful genetic information from their derived primary tumor, and the enumeration of CTCs in the bloodstream has been known to scale with disease progression. However, the detection of CTCs is a highly challenging task owing to their sparse numbers in a background of billions of background blood cells. To effectively utilize CTCs, there is a need for an assay that can detect CTCs with high specificity and can locally enrich CTCs from a liquid biopsy. We demonstrate a versatile methodology that addresses these needs by utilizing a combination of nanoparticles. Enrichment is achieved using targeted magnetic nanoparticles and high specificity detection is achieved using a ratiometric detection approach utilizing multiplexed targeted and non-targeted surface-enhanced Raman Scattering Nanoparticles (SERS-NPs). We demonstrate this approach with model prostate and cervical circulating tumor cells and show the ex vivo utility of our methodology for the detection of PSMA or folate receptor over-expressing CTCs. Our approach allows for the mitigation of interference caused by the non-specific uptake of nanoparticles by other cells present in the bloodstream and our results from magnetically trapped CTCs reveal over a 2000% increase in targeted SERS-NP signal over non-specifically bound SERS-NPs.