A Capacitive Micromachined Ultrasonic Transducer-Based Resonant Sensor Array for Portable Volatile Organic Compound Detection with Wireless Systems.

The development of portable volatile organic compound (VOC) sensors is essential for home healthcare and workplace safety because VOCs are environmental pollutants that may critically affect human health. Here, we report a compact and portable sensor platform based on a capacitive micromachined ultrasonic transducer (CMUT) array offering multiplex detection of various VOCs (toluene, acetone, ethanol, and methanol) using a single read-out system. Three CMUT resonant devices were functionalized with three different layers: (1) phenyl-selective peptide, (2) colloids of single-walled nanotubes and peptide, and (3) poly(styrene-co-allyl alcohol). As each device exhibited different sensitivities to the four VOCs, we performed principal component analysis to achieve selective detection of all four gases. For the simultaneous detection of VOCs using CMUT sensors, the changes in the resonant frequencies of three devices were monitored in real time, but using only a single oscillator through an electrically controlled relay to achieve compactness. In addition, by devising a wireless system, measurement results were transmitted to a smartphone to monitor the concentration of VOCs. We used multiple sensors to obtain a larger number of fingerprints for pattern recognition to enhance selectivity but interfaced these sensors with a single read-out circuit to minimize the footprint of the overall system. The compact CMUT-based sensor array based on a multiplex detection scheme is a promising sensor platform for portable VOC monitoring.

Capacitive Micromachined Ultrasonic Transducer (CMUT)-based Biosensor for Detection of Low Concentration Neuropeptide.

Accurate detection of neuropeptides in cerebrospinal fluid (CSF) plays an important role in both indepth studies and early diagnosis of neurological diseases. Here, we report a biosensor based on Capacitive Micromachined Ultrasonic Transducer (CMUT) which is capable of detecting low concentrations (pg $\sim $ ng/ml) of a neuropeptide involved with the progression of Alzheimer's diseases, somatostatin (SST). A 10-MHz CMUT was fabricated and utilized as a physical resonant sensor which detects the change in the concentration of analyte through the mass-loading mechanism. The resonant plate was sequentially coated with protein G and antibodies to provide specificity to SST; Cysteine-tagged protein G layer enables controlled immobilization of antibodies in a welloriented manner. The change in the resonant frequency of the CMUT sensor was measured after incubating the sensor in various concentrations of SST. The significant shifts in the resonant frequency were observed for SST concentrations in the range of 10 pg/ml $\sim 1$ ng/ml. Compared to the previously reported biosensors developed for SST detection, our sensor shows discernable responses for SST that are $\sim 6$ orders of magnitude lower in concentration. Thus, this work demonstrates the potential of the CMUT resonant sensor as a promising biosensor platform for detection of neuropeptides involved with neurodegenerative diseases that often exist in low concentrations in CSF.

Polypyrrole/Agarose Hydrogel-Based Bladder Volume Sensor with a Resistor Ladder Structure.

Chronic monitoring of bladder activity and urine volume is essential for patients suffering from urinary dysfunctions. However, due to the anatomy and dynamics of the bladder, chronic and precise monitoring of bladder activity remains a challenge. Here, we propose a new sensing mechanism that measures the bladder volume using a resistive ladder network with contact switches. Instead of measuring the impedance between the electrode continuously, the proposed sensor provides a digitized output ('on' or 'off') when the bladder volume reaches a certain threshold value. We present simple proof-of-concept sensors which compare the discrete-mode operation to the continuous-mode operation. In addition, by using multiple pairs of this contact-mode switch in a resistor ladder structure, we demonstrate monitoring of the bladder volume in four discrete steps using an idealized balloon and an ex vivo pig's bladder. We implemented the resistive ladder network using a conductive polypyrrole/agarose hydrogel composite which exhibits a Young's modulus comparable to that of the bladder wall. Compared to the continuous-mode operation, the proposed sensing mechanism is less susceptible to drift due to material degradation and environmental factors.