Center for Advanced Smart Sensors and Structures
The College of Engineering and Applied Sciences,
Western Michigan University, Kalamazoo, MI
Sensors provide a link between the digital world of computers, modern communications systems and the “real” or analogue world in which we live in, making it possible for us to obtain real time information about our surroundings, especially in inaccessible and inhospitable environments. In present-day biotechnological applications, the analysis of biochemical products is of utmost importance. The complexity of interfacing a biochemical environment directly to an electronic device needs to be overcome as smaller and faster devices are highly desired for replacing time-consuming laboratory-analyses. The attractive properties of biochemical sensors such as high sensitivity and high selectivity along with low detection limits are extremely promising for biochemical sensing applications. Researchers in this interdisciplinary program work towards: (a) understanding the cellular and molecular biology/chemistry of binding proteins at the sensor surface, (Bio/Chem); (b) develop tools, techniques and protocols to non-intrusively collect relevant bio/chemical information using an array of micro/nano probes, sensors and analysis protocols (Micro/Nano); (c) characterize the electrical parameters, choose appropriate materials for sensors and develop better models to understand the interface between sensors and bio/chemical (Bio/Micro/Modeling/Materials/Chem); (d) design and develop microelectronic integrated circuits for sensor control at the local level and processing at local and remote levels with wireless communication and distributed computing capabilities (VLSI/INFO). The Center for Advanced Smart Sensors and Structures is dedicated to performing research in wide variety of areas related to smart sensors and structures. This center forms the nucleus for cross-disciplinary research and provides a mentoring source for doctoral and masters level students.
In this research work, a wireless humidity sensor was inkjet printed on a flexible polyethyleneterephthalate (PET) substrate film using silver (Ag) nanoparticle based ink. The printed sensor consisted of an interdigitated capacitor (IDC) and an inductive coil pair in planar form. The IDC of the LC resonant circuit was spin coated with a humidity sensitive polymer poly (2-hydroxyethylmethacrylate) (pHEMA) and placed inside a Caron 6030 humidity chamber. It was observed that the capacitance of the IDC was directly proportional to the relative humidity. This change in capacitance resulted in a shift in the resonant frequency of the LC sensor which was remotely measured through an inductive detection coil.
An efficient electrochemical biosensor for the detection of various chemical and biological species was successfully fabricated by incorporating gold (Au) interdigitated electrodes (IDE), with 5 μm width and spacing, on a glass substrate, using photolithography technique. Gold was chosen as the electrode material for this work due to its inertness and because of its known affinity for biomolecules, especially for its ability to bind to proteins. Also a flow cell, with inlet and outlet ports for the microfluidic chamber, was fabricated using an acrylic material with a reservoir volume of 78 μl. Analysis of the impedance based response of the two-terminal device successfully demonstrated the feasibility of the biosensor to distinguish among various concentrations of chemical substances like potassium chloride (KCl), lead sulphide (PbS), mercury sulphide (HgS) and cadmium sulphide (CdS) as well as some biological proteins such as mouse monoclonal IgG, sarcosine and D - proline at pico molar (pM) concentration levels.
This project addresses the challenges of fabricating miniaturized, low-cost, flexible sensors via high - throughput techniques which are expected to be used for applications in chemical and biological detection. The researchers aim at printing (Gravure, Inkjet and Screen), characterization and testing of carbon nanotubes, graphite and silver inks as electrodes for interdigitated electrodes on paper, glass and polyethyleneterephthalate (PET) substrates. An efficient electrochemical biosensor was successfully printed on a flexible PET substrate film using silver (Ag) nanoparticle based ink. The electrochemical impedance spectroscopy (EIS) response of the printed sensor for detecting low concentrations of biochemical species revealed a very high sensitivity at pico molar (pM) concentration levels of potassium chloride (KCl), lead sulphide (PbS), mercury sulphide (HgS), cadmium sulphide (CdS), sarcosine and D - proline. Fabricating arrays of organic thin film transistor (OTFT) structures on flexible substrates using traditional printing techniques are also part of this research study.
A capacitive type humidity sensor (Inter Digitated Capacitor (IDC)) was successfully printed on a polyethyleneterephthalate (PET) substrate by means of rotogravure printing using silver (Ag) nanoparticle based ink as metallization with dimensions of 200 µm electrode finger width and spacing. The fabricated device was spin coated with humidity sensitive hydrophilic polymer (Poly Methyl Methacrylate (PMMA)). The capacitive response of sensor towards Relative Humidity (%RH) was measured in the range of 40% RH to 80% RH. The capacitive response of the printed sensor towards humidity showed a maximum hysteresis of 8 % at 60% RH. The sensor showed a variation of only 0.8 % from the average value at 70% RH and 25°C.
Three Dimensional Localization using a Passive Wireless SAW Transponder
The aim of this project is to develop a real-time sensing system based on wireless SAW technology. This sensing system can be used to identify and track SAW tags in an indoor environment. A finite element analysis was employed to analyze the second order effects of a passive surface acoustic wave (SAW) transponder. The second order effects, which occur due to the electrode perturbation and wave reflection, were analyzed using a two-dimensional finite element analysis method. A 31-bit Pseudo Random (PN) code sequence was implemented to demonstrate the second order effects on different configurations of the SAW transponder.
Fully Printed Wireless LC sensor for Heavy Metal Detection
This research project focused on the successful development of a fully printed wireless LC sensor for the detection of toxic heavy metals. The sensor, consisting of an inductor, detection coil and interdigitated electrodes (IDE) in planar form, was fabricated using screen and gravure printing technologies on a flexible polyethyleneterephthalate (PET) substrate with silver (Ag) based ink as metallization. The capability of the printed LC sensor for detecting very low concentrations of toxic heavy metals was demonstrated. The wireless response of the printed LC sensor revealed a very high sensitivity at picomolar levels of cadmium sulphide (CdS) and lead sulphide (PbS).
Novel flexible strain gauge sensor fabricated using screen printing
A flexible strain gauge sensor was successfully designed and fabricated using screen printing on polyethylene terephthalate (PET) substrate using silver (Ag) based ink as metallization. The electromechanical characteristics of the printed strain gauge sensor were examined by subjecting the sensor to a 3-point bend test. A 1.89 % maximum change in the resistance was observed when the sensor was subject towards a displacement of 2 mm, for 10,000 cycles. This response of the sensor demonstrated the potential of the fabricated sensor to be used in sensing applications for safety measures.
Guided Shear Horizontal Surface Acoustic Wave (SAW) Sensor
In this project a portable, rapid detection 64° YX LiNbO3 SAW transducer was fabricated. Aluminum Nitride (AlN) layer was then deposited on the active area as acoustic wave guiding layer with 10 μm electrode width and spacing. Acrylic material was used to fabricate a flow cell with a 3 μl reservoir volume having inlet and outlet ports for the micro fluidic chamber. Structural studies and morphological analysis, conducted on fluid channeled between the delay-line interdigitated electrodes, revealed that the deposited AlN thin film layers have strong preferential c-axis orientation and is compact with grain dimensions of less than 80 nm respectively. Polyaniline nanofibers were polymerized and synthesized to obtain 50 nm average diameters. The nanofibers were deposited on the layered SAW device and were tested towards hydrogen (H2) gas, while operating at room temperature. The device demonstrated a large and reproducible response to different concentrations of the H2 gas making it an ideal candidate for H2 sensing at room temperature.
Solidly Mounted Thin Film Bulk Acoustic Resonator (SMFBAR)
This research study aims to develop a prototype device based on the SMFBAR technology that will measure prostate specific antigen (PSA) efficiently in a cost effective manner at very low concentration (pg/ml region). The SMFBAR based sensor is designed with a piezoelectric thin film sandwiched between the top and bottom electrodes, on top of an acoustic mirror structure also known as Bragg reflector layers The specific aims for this project include: (a) Fabricating chips capable of detecting the binding of antibodies and their interactions with PSA based on FBAR technology, (b) Evaluation of the chips for the detection of PSA and (c) Development of on-chip detection system for point-of-care testing based on CMOS-MEMS technology.
PMMA/64° YX-LiNbO3 Guided SH-SAW Based Immunosensing System
In this study, a poly methylmethacrylate (PMMA)/64° YX-LiNbO3 guided shear horizontal mode surface acoustic wave (SH-SAW) sensor was designed and fabricated for the detection of biological antigen-antibody interactions. Various thicknesses of PMMA guiding layer were spin coated on SH-SAW sensor. A test setup utilizing a small-volume flow cell with inlet and outlet ports for the microfluidic cell and employing polydimethylsiloxane (PDMS) based microfluidic channels, was also designed and fabricated using an acrylic material with a reservoir volume of 3 µl. The interactions between protein G and immunoglobulin G (IgG) solutions were measured and analyzed using the PMMA coated SH-SAW devices. The frequency shift of 64° YX-LiNbO3 device with 0.4 μm PMMA guiding layer was measured from 3 kHz to 68 kHz as the concentration of IgG was increased from 1 μg/ml to 10 μg/ml. The calculated sensitivity of the guided SH-SAW was found to be 7.3 kHz/(μg/ml). This result showed that this guiding SH-SAW device was suitable for the liquid sensing applications.
Development of Guided SH-SAW based Wireless Sensing Platform for Monitoring Protein Binding
In this study, a wireless biosensing platform was developed for the detection of protein binding. The system consisted of a layered ZnO/36° YX-LiTaO3 guided shear horizontal mode surface acoustic wave (SH-SAW) device and a microfluidic flow cell. The influence of the interactions between protein A and mouse IgG on the characteristics of the SH-SAW device was measured and analyzed for varying concentrations of mouse IgG. The experimental results demonstrated that the insertion loss of the device increased from -46.8 dB to -50.9 dB and the center frequency of the device decreased from 94.56 MHz to 94.49 MHz as the mouse IgG concentration was increased from 1 µM to 40 µM, respectively. This sensor platform enabled real time monitoring of protein binding within a microfluidic flow cell. The experimental results show that this guided SH-SAW based wireless sensing system is viable for biosensing applications.
Fully Printed Skin-Like Flexible Pressure Sensor
A flexible pressure sensor was gravure printed on a polyethyleneterephthalate (PET) with silver (Ag) ink as the metallization layer. Initially, an array of 4 bottom electrodes with dimensions of 4 cm × 0.5 cm and 0.5 cm spacing were gravure printed. A 4 cm × 4 cm polydimethylsiloxane (PDMS) layer was then screen printed on top of the electrodes to act as the dielectric layer. This was followed by the gravure printing of an array of 4 top electrodes, with similar dimensions as that of the bottom electrode, with a 90° rotation in angle when compared to the bottom electrodes resulting in a grid structure. Finally, a passivation layer of PDMS was screen printed on the top electrodes. The overall thickness of the printed pressure sensor was measured to be 180 µm. The printed flexible pressure sensor was tested by placing varying weights on top of the passivation layer. This caused the distance between the top and bottom electrodes to be reduced, thereby resulting in a change of capacitance based on the change in the overall thickness and dielectric constant of the PDMS dielectric layer. A 4 %, 24 %, 39 % and 41 % change in capacitance was observed as the weight increased from 2.2 kPa to 8.6 kPa to 23.5 kPa to 0.1 MPa, respectively.
A Field-Portable Potentiostat System with Full Onboard Function Generation
A handheld, field-portable potentiostat system was successfully designed and fabricated to perform a wide range of electrochemical impedance spectroscopy (EIS) experiments. The onboard function generation capabilities of this system produces reference signals to drive numerous types of voltammetry experiments and thus provides increased functionality over previously built potentiostat systems. The system produced is capable of AC signal generation ranging from extremely low frequencies up to 200 kHz and signal amplitudes from 15 mV to 3 V at the counter electrode. This system also has a 16-bit DC resolution to bias AC signals or to produce a wide range of DC waveforms with an operating range of ±10 V. The capability of the potentiostat system was demonstrated by performing EIS on varying concentrations of mercury sulphide (HgS), a toxic heavy metal. The EIS based response of the potentiostat system revealed a very high sensitivity at pico molar (pM) concentration levels of HgS.
Fully Printed Organic Thin Film Transistors (OTFT) Based Flexible Humidity Sensors
A flexible fully printed organic thin film transistor (OTFT) was successfully fabricated and employed as a humidity sensor. The bottom gated OTFT, with width to length ratio (w/l) ratio of 26, was printed on a flexible poly ethylene terephthalate (PET) substrate. Conductive gate and dielectric layers were printed using gravure printing technique. Source and drain electrodes were deposited by means of screen printing. An active layer of pentacene, a humidity sensitive material, was deposited using inkjet printing. The current-voltage characteristics of the fully printed device were studied from 25% RH to 90% RH. A percentage change of 2000 % at 90% RH, in the drain current (ID), was registered when compared to the ID at 25% RH.
Surface Enhanced Raman Spectroscopy (SERS) Based Optical Sensors
In this research, novel flexible surface enhanced Raman spectroscopy (SERS) substrates were successfully fabricated by inkjet and gravure printing a thin film of silver (Ag) nanoparticle ink, with 20~50 nm particle size, on a flexible polyethyleneterephthalate (PET). The feasibility of the fabricated SERS substrates for detecting toxic heavy metals such as mercury sulfide (HgS) and cadmium sulfide (CdS) were demonstrated. The SERS based response of the printed substrates produced an enhanced Raman signal when compared to target molecules adsorbed on bare PET. An enhancement factor of 5 orders of magnitude due to existence of hot spots between nanoparticles was obtained. This response demonstrated the feasibility of the novel SERS substrate to be used in applications for detection of toxic heavy metals.
A Novel Flexible Microfluidic Platform: Integration of Conventional Printed Circuit Board (PCB) Technology and Inkjet Printing
A novel microfluidic sensing platform to be used for the detection of various biochemicals was successfully developed. Silver (Ag) based ink, to be used as interdigitated electrodes (IDE), was printed using a Dimatix 2831 inkjet printer on flexible polyethylene terephthalate (PET) substrate. Polydimethylsiloxane (PDMS) based microfluidic channels were fabricated using master molds created with PCB technology. The printed PET substrate and PDMS were bonded using a laboratory corona treater. The dynamic impedance response of this system demonstrated percentage changes of 107 % and 28 % for 1 nM and 1 pM concentrations of potassium chloride (KCl), respectively when compared to deionized (DI) water at 1 mV applied potential.
Opto-Electrochemical Based Dual Detection of Heavy Metal Compounds Using a Novel Flow Cell
An efficient sensing system that detects heavy metal compounds, by employing opto-electrochemical based dual detection technique, has been successfully developed. A novel microfluidic flow cell consisting of an inlet and outlet port with a reservoir volume of 25 µl was designed and fabricated using acrylic material. An electrochemical sensor with gold (Au) interdigitated electrodes (IDE) on a glass substrate was used for the electrical impedance spectroscopy (EIS) of various heavy metal compounds. EIS performed on cadmium sulfide (CdS) and mercury sulfide (HgS) yielded picomolar (pM) concentration detection levels. Selective detection of heavy metal compounds was made possible based on optical signals produced in the Raman emission spectra.