Dachao Li

Snap-Through and Pull-In Instabilities of an Arch-Shaped Beam Under an Electrostatic Loading

The snap-through and pull-in instabilities of the micromachined arch-shaped beams under an electrostatic loading are studied both theoretically and experimentally. The pull-in instability that results in a system collision with an electrode substrate may lead to a system failure and, thus, limits the system maximum displacement. The beam/plate structure with a flat initial configuration under an electrostatic loading can only experience the pull-in instability. With the different arch configurations, the structure may experience either only the pull-in instability or the snap-through and pull-in instabilities together. As shown in our computation and experiment, those arch-shaped beams with the snap-through instability have the larger maximum displacement compared with the arch-shaped beams with only the pull-in stability and those with the flat initial configuration. The snap-through occurs by exerting a fixed load, and the structure experiences a discontinuous displacement jump without consuming power. Furthermore, after the snap-through jump, the structures are demonstrated to have the capacity to withstand further electrostatic loading without pull-in. Those properties of consuming no power and increasing the structure deflection range without pull-in is very useful in microelectromechanical systems design, which can offer better sensitivity and tuning range.

In vitro glucose measurement using tunable mid-infrared laser spectroscopy combined with fiber-optic sensor.

Because mid-infrared (mid-IR) spectroscopy is not a promising method to noninvasively measure glucose in vivo, a method for minimally invasive high-precision glucose determination in vivo by mid-IR laser spectroscopy combined with a tunable laser source and small fiber-optic attenuated total reflection (ATR) sensor is introduced. The potential of this method was evaluated in vitro. This research presents a mid-infrared tunable laser with a broad emission spectrum band of 9.19 to 9.77[Formula: see text](1024~1088 cm(-1)) and proposes a method to control and stabilize the laser emission wavelength and power. Moreover, several fiber-optic ATR sensors were fabricated and investigated to determine glucose in combination with the tunable laser source, and the effective sensing optical length of these sensors was determined for the first time. In addition, the sensitivity of this system was four times that of a Fourier transform infrared (FT-IR) spectrometer. The noise-equivalent concentration (NEC) of this laser measurement system was as low as 3.8 mg/dL, which is among the most precise glucose measurements using mid-infrared spectroscopy. Furthermore, a partial least-squares regression and Clarke error grid were used to quantify the predictability and evaluate the prediction accuracy of glucose concentration in the range of 5 to 500 mg/dL (physiologically relevant range: 30~400 mg/dL). The experimental results were clinically acceptable. The high sensitivity, tunable laser source, low NEC and small fiber-optic ATR sensor demonstrate an encouraging step in the work towards precisely monitoring glucose levels in vivo.

A MEMS differential viscometric sensor for affinity glucose detection in continuous glucose monitoring

Micromachined viscometric affinity glucose sensors have been previously demonstrated using vibrational cantilever and diaphragm. These devices featured a single glucose detection module that determines glucose concentrations through viscosity changes of glucose-sensitive polymer solutions. However, fluctuations in temperature and other environmental parameters might potentially affect the stability and reliability of these devices, creating complexity in their applications in subcutaneously implanted continuous glucose monitoring (CGM). To address these issues, we present a MEMS differential sensor that can effectively reject environmental disturbances while allowing accurate glucose detection. The sensor consists of two magnetically driven vibrating diaphragms situated inside microchambers filled with a boronic-acid based glucose-sensing solution and a reference solution insensitive to glucose. Glucose concentrations can be accurately determined by characteristics of the diaphragm vibration through differential capacitive detection. Our in-vitro and preliminary in-vivo experimental data demonstrate the potential of this sensor for highly stable subcutaneous CGM applications.

A continuous glucose monitoring device by graphene modified electrochemical sensor in microfluidic system

This paper presents a continuous glucose monitoring microsystem consisting of a three-electrode electrochemical sensor integrated into a microfluidic chip. The microfluidic chip, which was used to transdermally extract and collect subcutaneous interstitial fluid, was fabricated from five polydimethylsiloxane layers using micromolding techniques. The electrochemical sensor was integrated into the chip for continuous detection of glucose. Specifically, a single-layer graphene and gold nanoparticles (AuNPs) were decorated onto the working electrode (WE) of the sensor to construct a composite nanostructured surface and improve the resolution of the glucose measurements. Graphene was transferred onto the WE surface to improve the electroactive nature of the electrode to enable measurements of low levels of glucose. The AuNPs were directly electrodeposited onto the graphene layer to improve the electron transfer rate from the activity center of the enzyme to the electrode to enhance the sensitivity of the sensor. Glucose oxidase (GOx) was immobilized onto the composite nanostructured surface to specifically detect glucose. The factors required for AuNPs deposition and GOx immobilization were also investigated, and the optimized parameters were obtained. The experimental results displayed that the proposed sensor could precisely measure glucose in the linear range from 0 to 162 mg/dl with a detection limit of 1.44 mg/dl (S/N = 3). The proposed sensor exhibited the potential to detect hypoglycemia which is still a major challenge for continuous glucose monitoring in clinics. Unlike implantable glucose sensors, the wearable device enabled external continuous monitoring of glucose without interference from foreign body reaction and bioelectricity.

U-shaped fiber-optic ATR sensor enhanced by silver nanoparticles for continuous glucose monitoring

An implantable U-shaped fiber ATR sensor enhanced by silver nanoparticles on cylindrical surface was presented for continuous glucose monitoring to overcome the drawbacks of traditional glucose sensing technique based on enzyme electrodes. A U-shaped structure was addressed to increase effective optical length at limited implantable space to enhance the sensitivity of fiber ATR sensor. A novel method to fabricate silver nanoparticles on cylindrical surface of U-shaped fiber ATR sensor based on chemical reduction of its silver halide material directly without any preliminary nanoparticles synthesis and following covalent bond or self-assembly was proposed. Five glucose absorption wavelengths in the mid-infrared band were employed for specific glucose monitoring. The experimental results indicate that the sensitivity and resolution of the silver-nanoparticle-enhanced U-shaped fiber-optic ATR sensor are approximately three times those of a conventional one. The high sensitivity and low-noise performance makes it promising for in vivo glucose monitoring in the future clinical applications.

An Interstitial Fluid Transdermal Extraction System for Continuous Glucose Monitoring

A novel microfluidic system which is fabricated with five polydimethylsiloxane layers for interstitial fluid (ISF) extraction, collection, and measurement toward the application of continuous and real-time glucose monitoring is presented in this paper. The system consists of a micro vacuum generator for ISF transdermal extraction and fluid manipulation, micro chambers for the collection of ISF, micro pneumatic valves for fluid management, and a micro flow sensor for ISF volume measurement. Sequentially controlled by the pneumatic valves, the ISF extraction, collection, and volumetric measurement functions of the system were demonstrated using the stable vacuum generated by the integrated vacuum generator. Through low-frequency ultrasound pretreated full-thickness pig skin, the normal saline solution with different glucose concentrations was transdermally extracted, collected, and measured. The absolute error in the volume measurement of the transdermally extracted “ISF analog” was less than 0.05 μL. The microfluidic system makes it possible to realize the clinical application of continuous glucose monitoring based on ISF transdermal extraction technology.

Using skin impedance to improve prediction accuracy of continuous glucose monitoring system

The continuous blood glucose monitoring system using interstitial fluid (ISF) extracted by ultrasound and vacuum is proposed in this paper. The skin impedance measurement is introduced into the system to monitor the skin permeability variation. Low-frequency ultrasound is applied on skin surface to enhance the skin permeability by disrupting the lipid bilayers of the stratum corneum (SC), and then ISF is extracted out of skin continuously by vacuum. The extracted ISF is diluted and the concentration of glucose in it is detected by a biosensor and used to predict the blood glucose concentration. The skin permeability is variable during the extraction, and its variation affects the prediction accuracy. The skin impedance is an excellent indicator of skin permeability in that the lipid bilayers of the SC, which offer electrical resistance to the skin, retard transdermal transport of molecules. So the skin impedance measured during the extraction is transformed to skin conductivity to estimate correlation coefficient between skin conductivity and permeability. Skin conductivity correlates well with skin permeability. The method and experiment system mentioned above may be significative for improving the prediction accuracy of continuous blood glucose monitoring system.

Design, fabrication and testing of a micro-Venturi tube for fluid manipulation in a microfluidic system

In this paper, a micro-Venturi tube fabricated with polydimethylsiloxane (PDMS) is studied for interstitial fluid (ISF) transdermal extraction and fluid manipulation in a microfluidic system toward the application of continuous glucose monitoring. The fabrication structure parameters of the PDMS Venturi tube were theoretically analyzed and experimentally validated against the output vacuum efficiency of the Venturi structure. The optimization methods of the Venturi structure were also discussed. In addition, an optimized micro-Venturi structure was proposed and fabricated. A vacuum pressure of less than 86 kPa had been achieved when an external pressure of 240 kPa was applied to this optimized Venturi tube. Both experimental and mathematical results demonstrate the potential applicability of the micro-Venturi tube in ISF transdermal extraction and fluid manipulation.

A MEMS Dielectric Affinity Glucose Biosensor

Continuous glucose monitoring (CGM) sensors based on affinity detection are desirable for long-term and stable glucose management. However, most affinity sensors contain mechanical moving structures and complex design in sensor actuation and signal readout, limiting their reliability in subcutaneously implantable glucose detection. We have previously demonstrated a proof-of-concept dielectric glucose sensor that measured pre-mixed glucose-sensitive polymer solutions at various glucose concentrations. This sensor features simplicity in sensor design, and possesses high specificity and accuracy in glucose detection. However, lack of glucose diffusion passage, this device is unable to fulfill real-time in-vivo monitoring. As a major improvement to this device, we present in this paper a fully implantable MEMS dielectric affinity glucose biosensor that contains a perforated electrode embedded in a suspended diaphragm. This capacitive-based sensor contains no moving parts, and enables glucose diffusion and real-time monitoring. The experimental results indicate that this sensor can detect glucose solutions at physiological concentrations and possesses good reversibility and reliability. This sensor has a time constant to glucose concentration change at approximately 3 min, which is comparable to commercial systems. The sensor has potential applications in fully implantable CGM that require excellent long-term stability and reliability.

Measurement of glucose concentration by fiber-optic surface plasmon resonance sensor

A measurement method of glucose concentration based on fiber-optic surface plasmon resonance (FO-SPR) is proposed to achieve online, real-time detection of human blood glucose concentration. The end-reflection structure of FO-SPR sensor was simulated and the impact of different parameters on sensor performance was analyzed. Then the FO-SPR sensor was manufactured according to the optimized parameters. A glucose concentration measurement system with SPR sensor was set up. Glucose solutions with different concentrations were measured and the experiment results showed that the correlation coefficient of fitting curve between the glucose concentration and resonance wavelength was above 0.95 at the human blood glucose range of 0~200mg/dL. The measurement repeatability was also proved to be able to meet the requirements of blood glucose concentration detection in clinics.