Interfacing microfabricated and nanomaterial-based sensors with a modular environmental monitoring system C. K. Harnett Department of Electrical and Computer Engineering University of Louisville Louisville, KY USA
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Abstract—Microelectromechanical systems (MEMS) and nanomaterial-based sensors are increasingly important in environmental monitoring. A new modular software and hardware platform called “SALAMANDER” (Serial Amphibious Linear Arrays of Micro And Nano Devices for Environmental Research) provides an electronic interface for these devices as well as conventional sensors. The SALAMANDER platform enables environmental researchers and students to assemble an array of aquatic sensors at high spatial density, for fundamental studies of sediment transport. Specifications are given for integrating new sensor types such as sample collectors and chemical sensors for determining the composition of sediment in this multi-year project. Keywords - MEMS sensors, nanomaterials, environmental sensors, wireless sensor networks, educational sensor systems, distributed sensors
I. INTRODUCTION Microelectromechanical systems (MEMS) and nanomaterial-based sensors offer new low-power, high sensitivity options for detecting environmental conditions. Their small size and low cost make it possible to deploy large numbers of sensors in a small area, for data collection at higher spatial resolution than ever before. However, interfacing these small sensors alongside conventional environmental sensors in a large-scale data collection system remains a challenge. Common options for this task include logging data to an onboard memory chip that must be recovered from the field, connecting all sensors to a local wired network with a single access point, providing each microsensor with its own wireless interface and power source, or a hybrid approach in which local wired networks are connected to wireless gateways that serve multiple sensors. The modular software and hardware platform in this report takes the hybrid approach. The system is called “SALAMANDER,” for Serial Amphibious Linear Arrays of Micro And Nano Devices for Environmental Research. An individual SALAMANDER consists of linked underwater sensors connected to an above-water wireless node. Because they are robust with respect to damage or disruption of individual devices, wireless sensor networks are ideal
Figure 1: Set of wireless nodes with local wired networks of underwater sensors.
communication systems for this type of distributed sampling project. However, radio waves do not propagate far in water, moist soil, and similar conductive media. A local wired network can instead sample conditions in the medium and broadcast it out through an above-water wireless circuit, as in groundwater-sampling “javelins” or “pylons” [1,2]. We combine this general approach, shown in Fig. 1, with a modular sensor attachment system that lets end users customize the network to their own experiments.
II.
DESIGN FOR DIVERSE USER GROUPS
The SALAMANDER platform is designed for a wide range of users, from students to environmental researchers. These users are concerned with collecting data from the network as quickly as possible. Therefore, an important project objective is for these diverse user groups to customize the sensor arrays without having to modify sensor housings or software. Modular construction allows users to assemble a
high spatial density sensor array for fundamental and applied studies of sediment transport. This report focuses on incorporation of newly-developed MEMS/nano sensors into the system using a standard electrical and mechanical interface, and a software interface that recognizes sensors as they are added. These design considerations enable users with a wide range of expertise to field-assemble a customized sensor system which automatically reports sensor depth, type, and individual calibration coefficients along with sensor data. Another user group consists of MEMS/nanofabrication researchers who produce new types of sensors, for instance resistive sensors that respond to water headspace vapors adsorbing onto nanomaterials [3]. These groups can make their sensors compatible with the network by providing an analog output, a set of calibration coefficients, and electrical contacts to power the sensor on and off. III. SENSOR OVERVIEW Temperature, pressure, flow rate and optical turbidity sensors are currently integrated into a data collection system designed for a two-week battery life [4]. Within this distributed system, flow rate sensors (Fig. 2) are coupled with sensors that can identify sediment density and ambient conditions over a stream cross-section to produce a detailed spatiotemporal profile of sediment flux in watersheds. Fig. 2 shows six different sensors connected to one wireless node.
Figure 2: Flexible bend sensor for flow detection. Inset: a SALAMANDER with six attached sensors.
Because optical turbidity (water cloudiness) determines the amount of light reaching plants and other organisms in water bodies, optical measurements alone provide valuable quantitative information about the effects of local sediment mitigation projects [5], even without providing detailed information on water chemistry. However, much more information about the origin of the sediment and effect on water quality becomes available with chemical sensors alongside the physical sensors discussed above. IV.
INTERFACING MICRO AND NANOSENSORS
The pressure sensors discussed above are resistive transducers which convert the deflection of a silicon membrane into an analog voltage. This is a classic bulkmicromachined MEMS device. Because the membrane is over a sealed atmospheric-pressure cavity, a pressure reference is available to determine stream depth, which can more than double during storms. The flow sensors may also be viewed as a microsensor. Although microfabrication techniques are not necessary to produce them, the devices sense changes in electrical resistance as microscale conductive paths are formed and broken during bending of a composite material that contains conductive granules in an insulating matrix. Both pressure and flow sensors operate on the resistive sensing principle, which means they dissipate power during measurement. In this battery-operated system, it is essential to conserve power, so the sensors are powered up and allowed to stabilize immediately before a measurement, then are shut down while other sensors are polled. This and other methods of power conservation should be considered when designing new MEMS devices and nanosensors for the system. Each sensor in the network can be supplied with a maximum of ~50 mA at 5V, and can interface with a dedicated 8-bit analog-to-digital (A-to-D) converter that reads a signal within the range of 0 to 5V. The converter chip has four channels. Any channels unused for A-to-D conversion can instead be used as addressable switches for turning the sensor on and off, or reversing the direction of current through the device. Each sensor is supplied with a ground, power, and data connection through a inch pipe fitting. Two types of micro- and nanomaterial-based chemical sensors compatible with the system include three-dimensional microelectrodes for collection of trace metals by electroplating (Fig. 3), and resistive chemical sensors based on chemical adsorption onto nanomaterials. Resistive nanomaterial-based sensors interface with the electronics by similar methods to the pressure and flow sensors discussed above, for in-situ measurements of chemical concentrations in the water. Encapsulating the resistive chemical sensors with a vaporpermeable membrane allows concentration and detection of chemicals in gases dissolved in the water. The three-dimensional microelectrodes in Fig. 3 offer a large specific surface area for collecting trace metals by electroplating. In-situ detection of individual chemicals is possible through electrochemical detection [6] as metals are deplated off the electrodes by application of a reversed
voltage. However, for an extremely low-power application, the microelectrodes may be considered a sample archiving device. In this format, the microelectrode chip is collected from the field during a battery-maintenance visit and analyzed at a laboratory using highly sensitive techniques such as mass spectroscopy, or even isotopic composition analysis. MEMS fabrication techniques make it possible to integrate hundreds of sets of three-dimensional electrodes with individual pins on a multiplexer chip, so that a small current (nA range) can be applied to successive electrodes for time-stamped archiving of trace metals. At the laboratory, each set of electrodes can be heated to the melting point by applying a higher electric current (~40 mA) which is not observed to damage neighboring devices. Because the electrodes protrude from the surface, there is little thermal contact between adjacent collectors, which is advantageous for desorbing materials from individual collectors into analysis equipment.
V.
CONCLUSIONS
Microfabricated and nanomaterial-based sensors provide new capabilities for a low-power environmental sensor network, especially in the area of chemical sensing. The sensors described here, and other “lab on a chip” chemical sensors will produce a new platform for monitoring water quality at the resolution of individual watersheds, and improve our ability to model and understand the origin and fate of sediment in the streams. Ongoing efforts in automating sensor spatial location [7] and calibration, combined with incorporation of new sensor types, are directed at an open, reconfigurable system for diverse user groups to determine the nature and quantity of sediments transported in watersheds.
ACKNOWLEDGMENTS Sarah M. Courtney and Evgenia Moiseeva contributed to construction of sensors pictured here, and Dr. James Fox provided useful discussions on sensor specifications for environmental applications. REFERENCES [1]
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Figure 3: Top: scanning electron micrographs of 250-micron diameter pop-up electrode cages fabricated at the University of Louisville. (Left: closed cage, right: opened cage constructed of a single continuous filament). Bottom: schematic of an addressable metal sample collector array for archival storage of trace element samples. Metals plate onto inner red electrodes.
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