[Show abstract][Hide abstract] ABSTRACT: This paper presents an aptamer-based graphene nanosensor capable of detecting small molecules. To address difficulties in direct detection of small molecules associated with their low electric charges, we use a competitive sensing approach as demonstrated with dehydroepiandrosterone sulfate (DHEA-S) as a target analyte, which is a small molecular steroid hormone with important applications in clinical diagnostics. A DHEA-S aptamer is captured by a complementary short DNA probe immobilized on the graphene and released upon exposure to DHEA-S in solution due to the binding between DHEA-S and the aptamer. The aptamer release is detected by measuring the change in the conductivity of graphene. Experimental results show that the time rate of aptamer release from the graphene is inversely proportional to DHEA-S concentration in solution. Thus, the nanosensor can potentially enable label-free, specific and quantitative measurement of DHEA-S and other small molecules.
Micro Electro Mechanical Systems (MEMS), 2014 IEEE 27th International Conference on, San Francisco, CA; 01/2014
[Show abstract][Hide abstract] ABSTRACT: This paper presents an integrated magnetic micropump that uses in-plane compliance-based check valves and a magnetically actuated membrane. The device, which allows for simple fabrication and system integration with other functional elements, consists of two functional layers both fabricated from poly(dimethylsiloxane) (PDMS). The upper PDMS layer provides a compliant membrane with an electroplated thin-film permalloy strip for actuation, while the lower PDMS layer incorporates microfluidic components including the microchannels, pump chamber, and a pair of check valves for flow regulation. The PDMS check valves, each having a compliant flap in contact with a stiff stopper to allow for unidirectional fluid flow with minimized leakage, are located at the inlet and outlet of the pump chamber, respectively. As such, the unidirectional flow at a controlled volumetric rate can be readily generated in accordance with the pumping actions. Systematic characterization of the micropump has been performed by studying the dependence of its pumping flow rate on the driving frequency of magnetic actuation, and the back pressure. Experimental results show that this micropump is capable of generating fluid flow of 0.15 μL/min at the frequency of 2 Hz, corresponding to a volume resolution of 1 nL per stroke, and working reliably against a maximum back-pressure of 550 Pa, demonstrating the potential application of this micropump for various integrated lab-on-a-chip systems.
[Show abstract][Hide abstract] ABSTRACT: Isolation of cells from heterogeneous biological samples is critical in both basic biological research and clinical diagnostics. Affinity-based methods, such as those that recognise cells by binding antibodies to cell membrane biomarkers, can be used to achieve specific cell isolation. Microfluidic techniques have been employed to achieve more efficient and effective cell isolation. By employing aptamers as surface-immobilised ligands, cells can be easily released and collected after specific capture. However, these methods still have limitations in cell release efficiency and spatial selectivity. This study presents an aptamer-based microfluidic device that not only achieves specific affinity cell capture, but also enables spatially selective temperature-mediated release and retrieval of cells without detectable damage. The specific cell capture is realised by using surface-patterned aptamers in a microchamber on a temperature-control chip. Spatially selective cell release is achieved by utilising a group of microheater and temperature sensor that restricts temperature changes, and therefore the disruption of cell-aptamer interactions, to a design-specified region. Experimental results with CCRF-CEM cells and sgc8c aptamers have demonstrated the specific cell capture and temperature-mediated release of selected groups of cells with negligible disruption to their viability.
IET Nanobiotechnology 01/2014; 8(1):2-9. · 1.00 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A continuous glucose monitor with a differential dielectric sensor implanted within the subcutaneous tissue that determines the glucose concentration in the interstitial fluid is presented. The device, created using microelectromechanical systems (MEMS) technology, consists of sensing and reference modules that are identical in design and placed in close proximity. Each module contains a microchamber housing a pair of capacitive electrodes residing on the device substrate and embedded in a suspended, perforated polymer diaphragm. The microchambers, enclosed in semi-permeable membranes, are filled with either a polymer solution that has specific affinity to glucose or a glucose-insensitive reference solution. To accurately determine the glucose concentration, changes in the permittivity of the sensing and the reference solutions induced by changes in glucose concentration are measured differentially. In vitro characterization demonstrated the sensor was capable of measuring glucose concentrations from 0 to 500 mg dL(-1) with resolution and accuracy of ~1.7 μg dL(-1) and ~1.74 mg dL(-1), respectively. In addition, device drift was reduced to 1.4% (uncontrolled environment) and 11% (5 °C of temperature variation) of that from non-differential measurements, indicating significant stability improvements. Preliminary animal testing demonstrated that the differential sensor accurately tracks glucose concentration in blood. This sensor can potentially be used clinically as a subcutaneously implanted continuous monitoring device in diabetic patients.
[Show abstract][Hide abstract] ABSTRACT: 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.
Journal of Microelectromechanical Systems 06/2013; 23(1):14-20. · 2.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Genotyping of single nucleotide polymorphisms (SNPs) allows diagnosis of human genetic disorders associated with single base mutations. Conventional SNP genotyping methods are capable of providing either accurate or high-throughput detection, but are still labor-, time-, and resource-intensive. Microfluidics has been applied to SNP detection to provide fast, low-cost, and automated alternatives, although these applications are still limited by either accuracy or throughput issues. To address this challenge, we present a MEMS-based SNP genotyping approach that uses solid-phase-based reactions in a single microchamber on a temperature control chip. Polymerase chain reaction (PCR), allele specific single base extension (SBE), and desalting on microbeads are performed in the microchamber, which is coupled with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) to analyze the SBE product. Experimental results from genotyping of the SNP on exon 1 of the HBB gene, which causes sickle cell anemia, demonstrate the potential of the device for rapid, accurate, multiplexed and high-throughput detection of SNPs.
Sensors and Actuators A Physical 06/2013; 195:175–182. · 1.84 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Surface enhanced Raman spectroscopy (SERS) has been used in a variety of biological applications due to its high sensitivity and specificity. Here, we report a SERS-based biosensing approach for quantitative detection of biomolecules. A SERS substrate bearing gold-decorated silicon nanopillars is functionalized with aptamers for sensitive and specific detection of target molecules. In this study, TAMRA-labeled vasopressin molecules in the picomolar regime (1 pM to 1 nM) are specifically captured by aptamers on the nanostructured SERS substrate and monitored by using an automated SERS signal mapping technique. From the experimental results, we show concentration-dependent SERS responses in the picomolar range by integrating SERS signal intensities over a scanning area. It is also noted that our signal mapping approach significantly improves statistical reproducibility and accounts for spot-to-spot variation in conventional SERS quantification. Furthermore, we have developed an analytical model capable of predicting experimental intensity distributions on the substrates for reliable quantification of biomolecules. Lastly, we have calculated the minimum needed area of Raman mapping for efficient and reliable analysis of each measurement. Combining our SERS mapping analysis with an aptamer functionalized nanopillars substrate is found to be extremely efficient for detection of low-abundance biomolecules.
[Show abstract][Hide abstract] ABSTRACT: 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.
Journal of Micromechanics and Microengineering 05/2013; 23(5):55020. · 1.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This paper presents an elastomeric polymer microchip for mechanically tunable cell trapping. This cell trapping approach utilizes the excellent deformability of the polymer to modulate the characteristics of cell trapping, which allows a predetermined number of cells, ranging from single cells to multiple cells, to be captured in each cell trap. The microchip consists of two elastomeric polymer sheets, one of which bears microstructures to physically capture cells. Experimental results demonstrate that the dimensions of cell capture microstructures can be modulated effectively by applying external force to construct arrays of different numbers of cells on the microchip.
Micro Electro Mechanical Systems (MEMS), 2013 IEEE 26th International Conference on; 01/2013
[Show abstract][Hide abstract] ABSTRACT: This paper presents label-free characterization of temperature-dependent biomolecular affinity binding on solid surfaces using a microcantilever-based device. The device consists of a Parylene cantilever one side of which is coated with a gold film and functionalized with molecules as an affinity receptor to a target analyte. The cantilever is located in a poly(dimethylsiloxane) (PDMS) microfluidic chamber that is integrated with a transparent indium tin oxide (ITO) resistive temperature sensor on the underlying substrate. The ITO sensor allows for real-time measurements of the chamber temperature, as well as unobstructed optical access for reflection-based optical detection of the cantilever deflection. To test the temperature-dependent binding between the target and receptor, the temperature of the chamber is maintained at a constant setpoint, while a solution of unlabeled analyte molecules is continuously infused through the chamber. The measured cantilever deflection is used to determine the target–receptor binding characteristics. We demonstrate label-free characterization of temperature-dependent binding kinetics of the platelet-derived growth factor (PDGF) protein with an aptamer receptor. Affinity binding properties including the association and dissociation rate constants as well as equilibrium dissociation constant are obtained, and shown to exhibit significant dependencies on temperature.
Sensors and Actuators B Chemical 01/2013; 176:653–659. · 3.84 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present an overview of our efforts to integrate aptamers and microfluidic devices, including manipulation of biomolecules and cells using aptamers, and isolation of target-binding nucleic acids. The aptamer-based devices for target manipulation are capable of specific analyte extraction and enrichment as well as isocratic elution, and can be coupled to biodetection systems for highly sensitive analyte detection. The microfluidic devices for isolation of target-binding nucleic acids demonstrate the potential for integrated selection of aptamers having predefined binding characteristics against a broad spectrum of practically important biological analytes.
Nano/Micro Engineered and Molecular Systems (NEMS), 2013 8th IEEE International Conference on; 01/2013
[Show abstract][Hide abstract] ABSTRACT: Cell manipulation has important applications in biological research and clinical diagnostics. We integrate microfabrication with physical modulation to enable selective and flexible cell manipulation, such as isolation, trapping and recovery of cells. We use the strong temperature dependence of affinity binding between aptamers and cells to enable specific cell capture and temperature-mediated release of selected cells. We also exploit the large compliance of elastomers to create an array of cell-trapping microstructures, whose dimensions can be mechanically modulated to capture a predetermined number of cells. Thus, enhanced utility and flexibility for practical applications can be attained, as demonstrated by specific capture and temperature-mediated release of CCRF-CEM cells, as well as tunable trapping of MCF-7 cells.
Nano/Micro Engineered and Molecular Systems (NEMS), 2013 8th IEEE International Conference on; 01/2013
[Show abstract][Hide abstract] ABSTRACT: This paper investigates the light-driven migration of the multi-cellular microorganism Dictyostelium discoideum as a potential bio-actuation mechanism in microsystems. As a platform for slug migration we use microscale confinements, which consist of intersecting microchannels fabricated from solidified agar–water solution. The agar surface provides necessary moisture to the slugs during the experiment while remaining sufficiently stiff to allow effective slug migration. The movements of the slugs in the microchannels are driven and guided by phototaxis via controlling light transmitted through optical fibers. The microchannels impose geometrical confinements on the migrating slugs, improving the spatial precision of the migration. We demonstrate that slugs that form in a microchamber can be driven to migrate through the microchannels, as well as steered to a particular direction at microchannel intersections. Our experimental results indicate that slug movements can be more effectively controlled in microchannels, and potentially useful for bio-actuation applications.
Sensors and Actuators A Physical 12/2012; 188:312–319. · 1.84 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Objective: We describe miniaturized differential glucose sensors based on affinity binding between glucose and a synthetic polymer. The sensors possess excellent resistance to environmental disturbances and can potentially allow wireless measurements of glucose concentrations within interstitial fluid in subcutaneous tissue for long-term, stable continuous glucose monitoring (CGM). Methods: The sensors are constructed using microelectromechanical systems (MEMS) technology and exploit poly(N-hydroxy-ethyl acrylamide-ran-3-acrylamidophenylboronic acid) (PHEAA-ran-PAAPBA), a glucose-binding polymer with excellent specificity, reversibility, and stability. Two sensing approaches have been investigated, which respectively, use a pair of magnetically actuated diaphragms and perforated electrodes to differentially measure the glucose-binding-induced changes in the viscosity and permittivity of the PHEAA-ran-PAAPBA solution with respect to a reference, glucose-unresponsive polymer solution. Results: In vivo characterization of the MEMS affinity sensors were performed by controlling blood glucose concentrations of laboratory mice by exogenous glucose and insulin administration. The sensors experienced an 8-30 min initialization period after implantation and then closely tracked commercial capillary glucose meter readings with time lags ranging from 0-15 min during rapid glucose concentration changes. Clarke error grid plots obtained from sensor calibration suggest that, for the viscometric and dielectric sensors, respectively, approximately 95% (in the hyperglycemic range) and 84% (ranging from hypoglycemic to hyperglycemic glucose concentrations) of measurement points were clinically accurate, while 5% and 16% of the points were clinically acceptable. Conclusions: The miniaturized MEMS sensors explore differential measurements of affinity glucose recognition. In vivo testing demonstrated excellent accuracy and stability, suggesting that the devices hold the potential to enable long-term and reliable CGM in clinical applications.
Journal of diabetes science and technology 11/2012; 6(6):1436-44.
[Show abstract][Hide abstract] ABSTRACT: Isolation of cells from heterogeneous mixtures is critically important in both basic cell biology studies and clinical diagnostics. Cell isolation can be realized based on physical properties such as size, density and electrical properties. Alternatively, affinity binding of target cells by surface-immobilized ligands, such as antibodies, can be used to achieve specific cell isolation. Microfluidics technology has recently been used in conjunction with antibody-based affinity isolation methods to capture, purify and isolate cells with higher yield rates, better efficiencies and lower costs. However, a method that allows easy release and collection of live cells from affinity surfaces for subsequent analysis and detection has yet to be developed. This paper presents a microfluidic device that not only achieves specific affinity capture and enrichment, but also enables non-destructive, temperature-mediated release and retrieval of cells. Specific cell capture is achieved using surface-immobilized aptamers in a microchamber. Release of the captured cells is realized by a moderate temperature change, effected via integrated heaters and a temperature sensor, to reversibly disrupt the cell-aptamer interaction. Experimental results with CCRF-CEM cells have demonstrated that the device is capable of specific capture and temperature-mediated release of cells, that the released cells remain viable and that the aptamer-functionalized surface is regenerable.
Lab on a Chip 08/2012; 12(18):3504-13. · 5.70 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present an innovative microfluidic approach to transcranial delivery of small quantities of drugs in brief time pulses for neurobiological studies. The approach is based on a two-stage process of consecutive drug dispensing and delivery, demonstrated by a device featuring a fully planar design in which the microfluidic components are integrated in a single layer. This 2-D configuration offers ease in device fabrication and is compatible to diverse actuation schemes. A compliance-based and normally closed check valve is used to couple the microchannels that are responsible for drug dispensing and delivery. Brief pneumatic pressure pulses are used to mobilize buffer and drug solutions, which are injected via a cannula into brain tissue. Thus, the device can potentially allow transcranial drug delivery and can also be potentially extended to enable transdermal drug delivery. We have characterized the device by measuring the dispensed and delivered volumes under varying pneumatic driving pressures and pulse durations, the standby diffusive leakage, and the repeatability in the delivery of multiple pulses of drug solutions. Results demonstrate that the device is capable of accurately dispensing and delivering drug solutions 5 to 70 nL in volume within time pulses as brief as 50 ms, with negligible diffusive drug leakage over a practically relevant time scale. Furthermore, testing of pulsatile drug delivery into intact mouse brain tissue has been performed to demonstrate the potential application of the device to neurobiology.
Journal of Microelectromechanical Systems 02/2012; 21(1). · 2.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a microchip for isolation of aptamers that bind to target ligands at prespecified temperatures. The device uses integrated resistive heaters and sensors to control the temperatures of (poly)dimethylsiloxane (PDMS) microchambers for temperature-specific selection and bead-based amplification of aptamers. Aptamers are isolated from a randomized DNA library at specified temperatures, and amplified onto microbeads using bead-based polymerase chain reaction (PCR). As a proof of concept, the device was used to isolate aptamers for human immunoglobulin E (IgE), with the enriched pool of candidate aptamers exhibiting a much higher and temperature-dependent affinity for the target protein. The procedure was performed with significantly less reagent use in a much shorter time period (4 hours) than with conventional devices (up to 2 days).
Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS) 01/2012;
[Show abstract][Hide abstract] ABSTRACT: This paper presents an integrated, MEMS-based approach to the detection of single nucleotide polymorphisms (SNPs). This SNP detection approach aims to eliminate complicated fluidic control operations by using a single-chamber microfluidic device, which incorporates polymerase chain reaction (PCR) and single base extension (SBE) on microbeads. Experimental results demonstrate that the device is capable of performing bead-based PCR, chemical elution, thermal elution, bead-based SBE, and on-chip desalting, which together potentially allow rapid, simple, accurate, multiplexed and highly efficient detection of SNPs.
Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS) 01/2012;
[Show abstract][Hide abstract] ABSTRACT: We present a microfluidic approach to characterizing temperature-dependent biomolecular interactions. Solvated L-arginine vasopressin (AVP) and its immobilized RNA aptamer (spiegelmer) were allowed to achieve equilibrium binding in a microchip at a series of selected temperatures. Unbound AVP were collected and analyzed with matrix-assisted laser desorption∕ionization mass spectrometry (MALDI-MS), yielding melting curves that reveal highly temperature-dependent zones in which affinity binding (36-45 °C) or dissociation (25-33 °C and 50-65 °C) occurs. Additionally, temperature-dependent binding isotherms were constructed; from these, thermodynamic quantities involved in binding were extracted. The results illustrated a strong change in heat capacity of interaction for this system, suggesting a considerable thermodynamic influence controlling vasopressin-spiegelmer interaction.