Designing a Non-invasive Surface Acoustic Resonator for Ultra-high Sensitive Ethanol Detection for an On-the-spot Health Monitoring System

Surface acoustic wave (SAW) sensors–based on piezoelectric crystal resonators–are extremely sensitive to even very small perturbations in the external atmosphere, because the energy associated with the acoustic waves is confined to the crystal surface. In this study, we present a critical review of the recent researches and developments predominantly used for SAW-based organic vapor sensors, especially ethanol. Besides highlighting their potential to realize real-time ethanol sensing, their drawbacks such as indirect sensing, invasive, time initializing, and low reliability, are properly discussed. The study investigates a proposed YZ-lithium niobate piezoelectric substrate with interdigital transducers patterned on the surface. Design of the resonator plays an important role in improving mass sensitivity, particularly the sensing area. Accordingly, a tin dioxide (SnO₂) layer with a specific thickness is generated on the surface of the sensor because of its high affinity to ethanol molecules. To determine the values of sensor configuration without facing the practical problems and the long theoretical calculation time, it is shown that the mass sensitivity of SAW sensors can be calculated by a simple three-dimensional (3-D) finite element analysis (FEA) using a commercial finite-element platform. In design validation step, different concentrations of ethanol are applied to investigate the acoustic wave properties of the sensor. The FEA data are used to obtain the surface and bulk total displacements of the sensor and fast Fourier transform (FFT) on output spectrum. The sensor could develop into highly sensitive and fast responsive structure so that a positive intensity shift of 0.18e-2 RIU is observed when the sensor is exposed to 15 ppm ethanol. It is capable of continuously monitoring the ethanol gas whether as an ultra-high sensitive sensor or switching applications for medical and industrial purposes.     

SPR biosensing coupled to a digital microfluidic microstreaming system

This article reports on a proof-of-concept system composed of a droplet based surface plasmon resonance (SPR) system coupled to a surface acoustic wave (SAW) microfluidic plateform. It is now well established that surface based binding analyses such as SPR are highly influenced by the transport of analyte to the sensing surface. Further, obtaining reliable equilibrium in flow cells to realize quantification studies is not straightforward. An original solution compared to generally used pressure driven flows is then proposed to favourably cope with these issues. Efficiency of SAW microstreaming coupled to SPR biosensing is considered, in order to improve the accuracy of kinetic parameter estimation in mass transport limited regime and to realize reliable quantification studies. First, the droplet based SPR technique and its advantages are presented. Then, the integration of the microstreaming on the system is discussed. Streptavidin binding is then monitored in static mode and under SAW streaming mode.     

Printable microfluidic systems using pressure sensitive adhesive material for biosensing devices

Background

In biosensors with a fluid analyte, the integration of a microfluidic system, which guides the analyte into the sensing area, is critical. Quicker and economical ways to build up microfluidic systems will make point of care diagnostics viable. Printing is a low-cost technology that is increasingly used in emerging organic and flexible electronics and biosensors. In this paper, we present printed fluidic systems on flexible substrates made with pressure sensitive adhesive materials.

Methods

Printable pressure sensitive adhesive materials have been used for making microfluidic systems. Flexible substrates have been used, and two types of adhesive materials, one thermally dried and another UV curable, have been tested. Top sealing layer was laminated directly on top of the printed microfluidic structure. Flow tests were done with deionized water.

Results

Flow tests with deionized water show that both adhesive materials are suitable for capillary flow driven fluidic devices. Flow test using water as dielectric material was also done successfully on a printed electrolyte gated organic field effect transistor with an integrated microfluidic system.

General significance

Due to its ease of process and low cost, printed microfluidic system is believed to find more applications in biosensing devices. This article is part of a Special Issue entitled Organic Bioelectronics—Novel Applications in Biomedicine.     

Passive wireless MEMS microphones for biomedical applications

This paper introduces passive wireless telemetry based operation for high frequency acoustic sensors. The focus is on the development, fabrication, and evaluation of wireless, battery-less SAW-IDT MEMS microphones for biomedical applications. Due to the absence of batteries, the developed sensors are small and as a result of the batch manufacturing strategy are inexpensive which enables their utilization as disposable sensors. A pulse modulated surface acoustic wave interdigital transducer (SAW-IDT) based sensing strategy has been formulated. The sensing strategy relies on detecting the ac component of the acoustic pressure signal only and does not require calibration. The proposed sensing strategy has been successfully implemented on an in-house fabricated SAW-IDT sensor and a variable capacitor which mimics the impedance change of a capacitive microphone. Wireless telemetry distances of up to 5 centimeters have been achieved. A silicon MEMS microphone which will be used with the SAW-IDT device is being microfabricated and tested. The complete passive wireless sensor package will include the MEMS microphone wire-bonded on the SAW substrate and interrogated through an on-board antenna. This work on acoustic sensors breaks new ground by introducing high frequency (i.e., audio frequencies) sensor measurement utilizing SAW-IDT sensors. The developed sensors can be used for wireless monitoring of body sounds in a number of different applications, including monitoring breathing sounds in apnea patients, monitoring chest sounds after cardiac surgery, and for feedback sensing in compression (HFCC) vests used for respiratory ventilation. Another promising application is monitoring chest sounds in neonatal care units where the miniature sensors will minimize discomfort for the newborns.     

Covalent bound sensing layers on surface acoustic wave (SAW) biosensors

This paper reports on the development of immunosensors based on commercially available surface acoustic wave (SAW) devices working at 380 MHz. Approaches for coating the sensor surface with a sensing layer of receptive biomolecules are presented and discussed. It was found that the sensitivity strongly relates to the immobilization method. Additionally, the sensitivity can be influenced by the density of accessible biomolecules on the active sensing area. Usually, by most of the standard immobilization procedures, two-dimensional layers of receptive biomolecules are obtained. We present a three-dimensional layer, which provides a higher absolute amount of recognition molecules. A dextran layer is photoimmobilized to the sensor surface and the recognition molecules are covalently embedded into the dextran matrix. The feasibility of specific immunosensing is investigated using SAW sensors connected to a fluid handling system.     

Fluorescence sensing of intermolecular interactions and development of direct molecular biosensors

Molecular biosensors are devices of molecular size that are designed for sensing different analytes on the basis of biospecific recognition. They should provide two coupled functions - the recognition (specific binding) of the target and the transduction of information about the recognition event into a measurable signal. The present review highlights the achievements and prospects in design and operation of molecular biosensors for which the transduction mechanism is based on fluorescence. We focus on the general strategy of fluorescent molecular sensing, construction of sensor elements, based on natural and designed biopolymers (proteins and nucleic acids). Particular attention is given to the coupling of sensing elements with fluorescent reporter dyes and to the methods for producing efficient fluorescence responses.     

Detection of Escherichia coli O157:H7 with langasite pure shear horizontal surface acoustic wave sensors

The toxigenic Escherichia coli O157:H7 bacterium has been connected with hemorrhagic colitis and hemolytic uremic syndrome, which may be characterized by diarrhea, kidney failure and death. On average, O157:H7 causes 73,000 illnesses, 2100 hospitalizations and 60 deaths annually in the United States alone. There is the need for sensors capable of rapidly detecting dangerous microbes in food and water supplies to limit the exposure of human and animal populations. Previous work by the authors used shear horizontal surface acoustic wave (SH SAW) devices fabricated on langasite (LGS) Euler angles (0°, 22°, 90°) to successfully detect macromolecular protein assemblies. The devices also demonstrated favorable temperature stability, biocompatibility and low attenuation in liquid environments, suggesting their applicability to bacterial detection. In this paper, a biosensor test setup utilizing a small volume fluid injection system, stable temperature control and high frequency phase measurement was applied to validate LGS SH SAW biosensors for bacterial detection. The LGS SH SAW delay lines were fabricated and derivatized with a rabbit polyclonal IgG antibody, which selectively binds to E. coli O157:H7, in this case a non-toxigenic test strain. To quantify the effect of non-specific binding (negative control), an antibody directed against the trinitrophenyl hapten (TNP) was used as a binding layer. Test E. coli bacteria were cultured, fixed with formaldehyde, stained with cell-permeant nucleic acid stain, suspended in phosphate buffered saline and applied to the antibody-coated sensing surfaces. The biosensor transmission coefficient phase was monitored using a network analyzer. Phase responses of about 14° were measured for the E. coli detection, as compared to 2° due to non-specific anti-TNP binding. A 30:1 preference for E. coli binding to the anti-O157:H7 layer when compared to the anti-TNP layer was observed with fluorescence microscopy, thus confirming the selectivity of the antibody surface to E. coli.     

Microfluidic pH-sensing chips integrated with pneumatic fluid-control devices

This paper presents a microfluidic chip capable of performing precise continuous pH measurements in an automatic mode. The chip is fabricated using micro-electro-mechanical-systems (MEMS)-based techniques and incorporates polydimethylsiloxane (PDMS) microstructures, pH-sensing electrodes and pneumatic fluid-control devices. Through its enhanced microchannel design and use of pneumatic fluid-control devices, the microfluidic chip reduces the dead volume of the sample and increases the pumping rate. The maximum pumping rate of the developed micro-pump is 28 microL/min at an air pressure of 10 psi and a driving frequency of 10 Hz. The total sample volume consumed in each sensing operation is just 0.515 microL. As a result, the developed chip reduces the sample volume compared to conventional large-scale pH-sensing systems. The microfluidic chip employs the electrochemical sensing method to conduct precise pH level measurements. The sensing electrodes are fabricated by sputtering a layer of SiO(2)-LiO(2)-BaO-TiO(2)-La(2)O(3) (SLBTLO) onto platinum (Pt) electrodes and the pH value of the sample is evaluated by measuring the potential difference between the sensing electrodes and a reference electrode. Additionally, the integration of the microfluidic chip with a pneumatic fluid-control device facilitates automatic sample injection and a continuous sensing operation. The developed system provides a valuable tool with which to examine pH values in a wide range of biomedical and industrial applications.     

Integration of a surface acoustic wave biosensor in a microfluidic polymer chip

SAW devices based on horizontally polarized surface shear waves (HPSSW) enable label-free, sensitive and cost-effective detection of biomolecules in real time. It is known that small sampling volumes with low inner surface areas and minimal mechanical stress arising from sealing elements of miniaturized sampling chambers are important in this field. Here, we present a new approach to integrate SAW devices with sampling chamber. The sensor device is encapsulated within a polymer chip containing fluid channel and contact points for fluidic and electric connections. The chip volume is only 0.9 microl. The polymeric encapsulation was performed tailor-made by Rapid Micro Product Development 3Dimensional Chip-Size-Packaging (RMPD 3D-CSP), a 3D photopolymerisation process. The polymer housing serves as tight and durable package for HPSSW biosensors and allows the use of the complete chips as disposables. Preliminary experiments with these microfluidic chips are shown to characterise the performance for their future applications as generic bioanalytical micro devices.     

High efficiency Hall effect micro-biosensor platform for detection of magnetically labeled biomolecules

Detection of magnetically labeled biomolecules using micro-Hall biosensors is a promising method for monitoring biomolecular recognition processes. The measurement efficiency of standard systems is limited by the time taken for magnetic beads to reach the sensing area of the Hall devices. Here, micro-current lines were integrated with Hall effect structures to manipulate the position of magnetic beads via field gradients generated by localized currents flowing in the current lines. Beads were accumulated onto the sensor surface within seconds of passing currents through the current lines. Real-time detection of magnetic beads using current lines integrated with Hall biosensors was achieved. These results are promising in establishing Hall biosensor platforms as efficient and inexpensive means of monitoring biomolecular reactions for medical applications.     

Surface modification of an acoustic biosensor allowing the detection of low concentrations of cancer markers

Analyte detection with biosensors is strongly influenced by the preparation of the biosensor surface including choice of sensing layers and coupling methods for corresponding capture molecules. We investigated the influence of different coupling procedures, especially considering coupling chemistry and incubation times for reagents, by means of surface acoustic wave (SAW) biosensors. The effect on the signal response was tested in two subsequent protein assays. Our optimized coupling procedure allowed the detection of the breast cancer markers HER-2 (human epidermal growth factor receptor-2) and TIMP-1 (tissue inhibitor of metalloproteinase-1) below the respective clinical cutoff values of only a few nanograms per milliliter.     

Influence of intermediate aminodextran layers on the signal response of surface acoustic wave biosensors

Surface acoustic wave (SAW) devices based on horizontally polarized surface shear waves enable direct and label-free detection of proteins in real time. Binding reactions on the sensor surface are detected by determining changes in surface wave velocity caused mainly by mass adsorption or change of viscoelasticity in the sensing layer. Intermediate hydrogel layers have been proven to be useful to immobilize capture molecules or ligands corresponding to the analyte. However, the SAW signal response strongly depends on the morphology of the hydrogel due to different relative changes of its acoustomechanical parameters such as viscoelasticity and density. In this work five aminodextrans (AMD) and one diamino polyethylene glycol (DA-PEG) were used as intermediate hydrogel layers. Sensors with immobilized streptavidin and samples containing biotinylated bovine serum albumin were used to exemplify affinity assays based on immobilized capture molecules for protein detection. The effects of the three-dimensional AMDs and the two-dimensional (2D) DA-PEG on the SAW signal response were investigated. The signal height decreased with increasing molar mass and increasing amount of immobilized AMD. Consequently, thin hydrogel layers are ideal to obtain optimum signal responses in this type of assay, whereas it is not necessarily a 2D hydrogel that gives the best results.     

Evanescent wave biosensors

Optical biosensors, based on evanescent wave technology, are analytical devices that measure the interactions between biomolecules in real time, without the need for any labels. Specific ligands are immobilized to a sensor surface, and a solution of receptor or antibody is injected over the top. Binding is measured by recording changes in the refractive index, caused by the molecules interacting near the sensor surface within the evanescent field. Evanescent wave-based biosensors are being used to study an increasing number of applications in the life sciences, including the binding and dissociation kinetics of antibodies and receptor-ligand pairs, protein-DNA and DNA-DNA interactions, epitope mapping, phage display libraries, and whole cell- and virus-protein interactions. There are currently four commercially available avanescent wave biosensors on the market. This article describes the technology behind their sensing techniques, as well as the range of applications in which they are employed.     

Integration of biomolecules and nanomaterials: towards highly selective and sensitive biosensors

Significant efforts have been made toward the development of high-performance biosensors for various applications. Advances in nanotechnology have resulted in the development of highly sensitive electrochemical sensing devices. It is believed that highly sensitive and selective biosensors can be realized through the integration of biomolecules and nanomaterial-based sensor platforms. Numerous articles have described combining biomolecules as recognition elements with nanotechnology for the development of biosensors with enhanced selectivity and sensitivity. Recent advances in the development of biosensors through the integration of biomolecules with nanotechnology are reviewed in this article.     

Gravimetric biosensors based on acoustic waves in thin polymer films

Most gravimetric biosensors use thin piezoelectric quartz crystals, either as resonating crystals (quartz crystal microbalance, QCM), or as bulk/surface acoustic wave (SAW) devices. In the majority of these the mass response is inversely proportional to the crystal thickness which, at a limit of about 150 microns, gives inadequate sensitivity. A new system is described in which acoustic waves are launched in very thin (10 microns) tensioned polymer films to produce an oscillatory device. A theoretical equation for this system is almost identical to the well-known Sauerbrey equation used in the QCM method. Because the polymer films are so thin, a 30-fold increase in sensitivity is predicted and verified by adding known surface masses. Temperature sensitivity is a problem so a separate control sensor and careful temperature regulation are necessary. Preliminary results showing the real time binding of protein (IgG), a step towards immunosensor development, and the use of mass enhancing particles are presented. Inexpensive materials are used so disposable gravimetric biosensors may become feasible.     

Pulse mode shear horizontal-surface acoustic wave (SH-SAW) system for liquid based sensing applications

In this work, we describe a novel pulse mode shear horizontal-surface acoustic wave (SH-SAW) polymer coated biosensor that monitors rapid changes in both amplitude and phase. The SH-SAW sensors were fabricated on 36 degrees rotated Y-cut X propagating lithium tantalate (36 YX.LT). The sensitivity of the device to both mass loading and visco-elastic effects may be increased by using a thin guiding layer of cross-linked polymer. Two acoustic modes are excited by the electrodes in this crystalline direction. Metallisation of the propagation path of the 36 YX.LT devices allows the two modes to be discriminated. Successive polymer coatings resulted in the observation of resonant conditions in both modes as the layer thickness was increased. Using the 36 YX.LT devices, we have investigated the application of a novel pulse mode system by sensing a sequence of deposition and removal of a biological layer consisting of vesicles of the phospholipid POPC. A continuous wave system was used to verify the accuracy of the pulse mode system by sensing a series of poly(ethylene glycol) (PEG) solutions. The data clearly demonstrates the ability of the 36 YX.LT pulse mode system to provide rapid measurements of both amplitude and phase for biosensing applications.     

Potential diagnostic applications of biosensors: current and future directions

This review describes recent advances in biosensors of potential clinical applications. Biosensors are becoming increasingly important and practical tools in pathogen detection, molecular diagnostics, environmental monitoring, food safety control as well as in homeland defense. Electrochemical biosensors are particularly promising toward these goals arising due to several combined advantages including low-cost, operation convenience, and miniaturized devices. We review the clinical applications of electrochemical biosensors based on a few selected examples, including enzyme-based biosensors, immunological biosensors and DNA biosensors.     

Terahertz Microfluidic Sensing Using a Parallel-plate Waveguide Sensor

Refractive index (RI) sensing is a powerful noninvasive and label-free sensing technique for the identification, detection and monitoring of microfluidic samples with a wide range of possible sensor designs such as interferometers and resonators ^1,2. Most of the existing RI sensing applications focus on biological materials in aqueous solutions in visible and IR frequencies, such as DNA hybridization and genome sequencing. At terahertz frequencies, applications include quality control, monitoring of industrial processes and sensing and detection applications involving nonpolar materials.Several potential designs for refractive index sensors in the terahertz regime exist, including photonic crystal waveguides ³, asymmetric split-ring resonators ⁴, and photonic band gap structures integrated into parallel-plate waveguides ⁵. Many of these designs are based on optical resonators such as rings or cavities. The resonant frequencies of these structures are dependent on the refractive index of the material in or around the resonator. By monitoring the shifts in resonant frequency the refractive index of a sample can be accurately measured and this in turn can be used to identify a material, monitor contamination or dilution, etc.The sensor design we use here is based on a simple parallel-plate waveguide ^6,7. A rectangular groove machined into one face acts as a resonant cavity (Figures 1 and 2). When terahertz radiation is coupled into the waveguide and propagates in the lowest-order transverse-electric (TE₁) mode, the result is a single strong resonant feature with a tunable resonant frequency that is dependent on the geometry of the groove ^6,8. This groove can be filled with nonpolar liquid microfluidic samples which cause a shift in the observed resonant frequency that depends on the amount of liquid in the groove and its refractive index ⁹.Our technique has an advantage over other terahertz techniques in its simplicity, both in fabrication and implementation, since the procedure can be accomplished with standard laboratory equipment without the need for a clean room or any special fabrication or experimental techniques. It can also be easily expanded to multichannel operation by the incorporation of multiple grooves ¹⁰. In this video we will describe our complete experimental procedure, from the design of the sensor to the data analysis and determination of the sample refractive index.     

An interleukin-6 ZnO/SiO(2)/Si surface acoustic wave biosensor

A novel high sensitivity ZnO/SiO(2)/Si Love mode surface acoustic wave (SAW) biosensor for the detection of interleukin-6 (IL-6), is reported. The biosensors operating at 747.7MHz and 1.586GHz were functionalized by immobilizing the monoclonal IL-6 antibody onto the ZnO biosensor surface both through direct surface adsorption and through covalent binding on gluteraldehyde. The morphology of the IL-6 antibody-protein complex was studied using scanning electron microscopy (SEM), and the mass of the IL-6 protein immobilized on the surface was measured from the frequency shift of the SAW resonator biosensor. The biosensor was shown to have extended linearity, which was observed to improve with higher sensor frequency and for IL-6 immobilization through the monoclonal antibody. Preliminary results of biosensor measurements of low levels of IL-6 in normal human serum are reported. The biosensor can be fully integrated with CMOS Si chips and developed as a portable real time detection system for the interleukin family of proteins in human serum.