Magnetic field sensors using VO2/Fe3O4nanoparticle devices.

We combined the metal-insulator transition (MIT) properties of VO2and the magnetic properties of Fe3O4to realize a magnetometer with very large nonlinearity and switching characteristics. VO2, Fe3O4nanoparticles, and a conductive binder (silver paint) were mixed and drop-casted onto two-terminal gap junction devices. The device's current-voltage characteristics exhibited current-switching behavior related to MIT in VO2which changed with the external magnetic field. The magnetoresistance and magnetostriction in Fe3O4both contributed to the field sensitivity of the sensor. Sensitivities as high as 1 A nT-1(or 50.8 V T-1with a current bias) were observed near the MIT voltage. The resulting minimum detectable signal was 20 pT/SQRT(Hz).

Whole virus detection using aptamers and paper-based sensor potentiometry.

Paper-based sensors, microfluidic platforms and electronics have attracted attention in the past couple of decades because they are flexible, can be recycled easily, environmentally friendly and inexpensive. Here, we report a paper-based potentiometric sensor to detect the whole Zika virus with a minimum sensitivity of 0.26 nV/Zika and a minimum detectable signal (MDS) of 2.4x107 Zika. Our paper sensor works very similar to a P-N junction where a junction is formed between two different regions with different electrochemical potentials on the paper. These two regions with slightly different ionic contents, ionic species and concentrations, produce a potential difference given by the Nernst equation. Our paper sensor consists of 2-3 × 10 mm segments of paper with conducting silver paint contact patches on two ends. The paper is dipped in a buffer solution containing aptamers designed to bind to the capsid proteins on Zika. We then added the Zika (in its own buffer) to the region close to one of the silver paint contacts. The Zika virus (40 nm diameter with 43 kDa or 7.1 × 10-20 gm weight) became immobilized in the paper's pores and bonded with the resident aptamers creating a concentration gradient. Atomic force microscopy and Raman spectroscopy were carried out to verify that both the aptamer and Zika become immobilized in the paper. The potential measured between the two silver paint contacts reproducibly became more negative upon adding the Zika. We also showed that a liquid crystalline display (LCD) powered by the sensor can be used to read the sensor output.

Bio-mimetic synthetic cell hydrogel magnetometer.

We present a bio-inspired hydrogel magnetometer where the cell potential (V oc) between two hydrogels is used to measure an external magnetic field. Ferromagnetic particles located in the hydrogels move in response to the external field and change the V oc (sensitivity ~ 3.7 V T-1). As the field becomes larger than a critical field B c (~38 mT), these particles puncture the hydrogel boundary shorting out the concentration gradient region and abruptly reducing the V oc (sensitivity ~ 23.5 V T-1). In this regime, the V oc behaves similar to the neuron firing. In subsequent measurement cycles, the particles remain in punctured holes and the sensor behaviour is neuron-like with lower sensitivity (~20 V T-1). V oc also changes as a function of pressure (8 mV kPa-1) and temperature (2 mV K-1). After 4 h, the ionic concentration gradient diminishes in the device, and similar to biological cell fatigue, V oc decreases and can be recharged with many different techniques.

Remote Microwave and Field-Effect Sensing Techniques for Monitoring Hydrogel Sensor Response.

This paper presents two novel techniques for monitoring the response of smart hydrogels composed of synthetic organic materials that can be engineered to respond (swell or shrink, change conductivity and optical properties) to specific chemicals, biomolecules or external stimuli. The first technique uses microwaves both in contact and remote monitoring of the hydrogel as it responds to chemicals. This method is of great interest because it can be used to non-invasively monitor the response of subcutaneously implanted hydrogels to blood chemicals such as oxygen and glucose. The second technique uses a metal-oxide-hydrogel field-effect transistor (MOHFET) and its associated current-voltage characteristics to monitor the hydrogel's response to different chemicals. MOHFET can be easily integrated with on-board telemetry electronics for applications in implantable biosensors or it can be used as a transistor in an oscillator circuit where the oscillation frequency of the circuit depends on the analyte concentration.

Multiscale mechanics of hierarchical structure/property relationships in calcified tissues and tissue/material interfaces.

This paper presents a review plus new data that describes the role hierarchical nanostructural properties play in developing an understanding of the effect of scale on the material properties (chemical, elastic and electrical) of calcified tissues as well as the interfaces that form between such tissues and biomaterials. Both nanostructural and microstructural properties will be considered starting with the size and shape of the apatitic mineralites in both young and mature bovine bone. Microstructural properties for human dentin and cortical and trabecular bone will be considered. These separate sets of data will be combined mathematically to advance the effects of scale on the modeling of these tissues and the tissue/biomaterial interfaces as hierarchical material/structural composites. Interfacial structure and properties to be considered in greatest detail will be that of the dentin/adhesive (d/a) interface, which presents a clear example of examining all three material properties, (chemical, elastic and electrical). In this case, finite element modeling (FEA) was based on the actual measured values of the structure and elastic properties of the materials comprising the d/a interface; this combination provides insight into factors and mechanisms that contribute to premature failure of dental composite fillings. At present, there are more elastic property data obtained by microstructural measurements, especially high frequency ultrasonic wave propagation (UWP) and scanning acoustic microscopy (SAM) techniques. However, atomic force microscopy (AFM) and nanoindentation (NI) of cortical and trabecular bone and the dentin-enamel junction (DEJ) among others have become available allowing correlation of the nanostructural level measurements with those made on the microstructural level.

Piezoresistive pressure sensors in the measurement of intervertebral disc hydrostatic pressure.

BACKGROUND CONTEXT: An implantable, freestanding, minimally invasive, intervertebral disc pressure sensor would vastly improve the knowledge of spinal biomechanics and the understanding of spinal disease. Additionally, it would improve clinical indications for surgical interventions in disc-related pathology. Adaptation of current commercially available materials, technology, and microfabrication techniques may now make the production of such a device feasible. PURPOSE: To determine if piezoresistive pressure sensor (PPS) technology could be applied as the functional sensing element in an intervertebral disc microsensor. METHODS: Commercially available PPS chips were modified, producing sensor chips measuring 0.8 cm(2) by 0.3 cm with an internal sensing element measuring 0.15 cm(2) by 0.1cm. A needle-mounted pressure sensor functionally identical to those used in discography procedures was also tested in parallel as a control. Both sensors were calibrated for hydrostatic pressure using a purpose-built pressure chamber and then tested in human functional spinal units. Methods were developed to implant the sensor and measure the intervertebral disc pressure in response to axial compressive loads. RESULTS: Modified commercially available PPS elements were functionally adapted to measure intervertebral disc pressures. Both the PPS and the needle-mounted sensor measured a linear increase in hydrostatic disc pressure with applied axial load. Fluctuations between the slopes of the output versus load curves were observed in the PPS sensor experimental trials. These fluctuations were attributed to the large size of our working model and its impact on the hydrostatic and mechanical properties of the disc. CONCLUSIONS: It is hypothesized that future miniaturization of this working model will eliminate mechanical disruption within the disc and the fluctuations in the slope of sensor output that this induces. It should be possible to construct an implantable sensor for the intervertebral disc. This may provide valuable clinical and physiological data.

Near-field microwave microscope and electron-spin-resonance detection: ruby crystal surface.

Microwave photons can image a surface by using near-field geometry with spatial resolution close to the nanometer-length scale. We detected electron-spin resonance (ESR) on ruby surfaces by using microwave photons at the S-band frequency (3.73 GHz). The spatial locations of the electron-spin centers were pinpointed with localized incident microwave photons generated by using evanescent microwave microscopy (EMM). We show that the EMM probe is capable of resolving 20,000 spin transitions, compared with the approximately 10(10) minimum detectable spins of the conventional continuous-wave ESR spectrometer. This represents roughly a 6-order-of-magnitude enhancement in sensitivity. Our ultimate goal is to achieve the minimum detectable spin transition of a single electron and nanometer-level spatial resolution by using microfabricated atomic force microscopy-EMM probes.