10 Integrated Electronic Noses and Microsystems for Chemical...

10

Integrated Electronic Noses and Microsystems for Chemical

Analysis

Julian W. Gardner, Marina Cole

Abstract

There is considerable interest in the miniaturization and mass production of electro-nic noses through the exploitation of recent advances in the emergent field of micro-systems technology. In this chapter we explore the future outlook for integrated elec-tronic noses by first reviewing the different types ofmicrofluidic components that havebeen reported in the literature, such as microchannels, microvalves, andmicropumps.Next, we describe recent efforts to develop microelectronic noses based upon the in-tegration of sensor arrays and smart interfaces. Finally we report upon work in therelated field of micro total analysis systems, in which, for example, a micro gas chro-matograph or a micro mass spectrometer are being fabricated; these physically-basedmicroinstruments may be regarded as a type of micronose and thus in competitionwith integrated electronic micronoses.

10.1

Introduction

The integration of gasmicrosensors and signal processing circuitry is a subject of ever-increasing importance in the chemical sensor community. It offers lower unit costthrough batch production of wafers, smaller device size, better reproducibility, super-ior signal conditioning by less noise generated in the transmission of the sensor sig-nals to the processing electronics, and an improved limit of detection for the wholesensing system. The full integration of gas microsensors and signal processing circui-try has been brought a step nearer with reports of an increase in sensor reproducibilityby the integration of arrays of sensors onto the same substrate [1–4], improvements insensor sensitivity through advances in individual microsensor technologies [5, 6] andfinally the development of novel gas-sensitive materials, for example see Gardner andBartlett [7], and Attard et al. [8]. Many commercial (non-chemical) sensors have beenrealized in recent years through the integration of both the electronic signal processingcircuitry and the sensing part on the same silicon die. Some examples include pressuresensors, ultrasound sensors and gas flow sensors, proximity and temperature sensors

Handbook of Machine Olfaction: Electronic Nose Technology.Edited by T.C. Pearce, S.S. Schiffman, H.T. Nagle, J.W. GardnerCopyright ª 2003 WILEY-VCH Verlag GmbH Co. KGaA, WeinheimISBN: 3-527-30358-8

231231

[9–12]. For a review of silicon sensors, readers are referred to Gardner et al. [13]. Thefield of gas sensors is covered in Chapter 9 of this book where a detailed description ofthe design of the single-chip multisensor system comprising four different sensors, aswell as driving and signal-conditioning circuitry, can be found.Recent years have also seen substantial efforts in the development of smart and

intelligent sensor technology. The main advantages of intelligent sensors are theirimproved performance and reliability – achieved through the addition of self-testingand self-diagnostic functions [14]. Emphasis has also been given to the development ofapplication-specific integrated circuits (ASICs) for intelligent sensors. Taner andBrignell [15] have studied the advantages of ASIC technology, which enables intelli-gent devices to deal both with systematic variation in sensor parameters and providesgood solutions for sensor communications.Parallel with the integration of microsensors and signal processing electronics, and

the realization of smart sensor interfaces, sampling and fluid-handling techniqueshave been rapidly developing. Micro flow sensors, micropumps and microvalvesstarted emerging in the late 1980s marking the beginning of the field of micro-fluidics. So far, life sciences and chemistry have been the main application areasof microfluidics in the liquid phase. Considering that sample handling is a criticalarea, which has an enormous influence on the performance of e-noses (see Chap-ter 3), microfluidics should have a significant impact on the future development ofsuperior, integrated electronic nose (e-nose) systems. Microfluidic technology com-bined with smart silicon sensor arrays could lead to the development of cheap,and possibly disposable devices, particularly important in medical applicationssuch as chemical and biological assays.In this chapter, a review of different solid-state sensor systems and smart sensor

interfaces for e-noses will be given together with an overview of existing microfabri-cated components for fluid handling, such as microvalves, micropumps, and micro-

Fig. 10.1 Basic components that make up an integrated e-nose or

chemical analysis system

10 Integrated Electronic Noses and Microsystems for Chemical Analysis232

channels. A block diagram of the basic components that make up an integrated e-nosesystem is shown in Fig. 10.1 and will be described below. The integration of differentcomponents into microsystems and microinstruments will also be discussed, and thefuture outlook concerning sensor arrays, biological assay devices, and neuromorphicsystems will be briefly outlined. Microsystems for chemical analysis based on gaschromatography, mass spectrometry, and optical spectrometry techniques will alsobe reviewed. Finally, a future outlook is given of e-noses and microsystems for che-mical analysis.

10.2

Microcomponents for Fluid Handling

In the early 1990s, microfluidics was established as a general term for the researchdiscipline dealing with fluid transport phenomena on the micrometer scale and fluidiccomponents, devices, and systems built with microfabrication technologies. The ma-jor applications of microfluidics are in the fields of medical diagnostics, genetic se-quencing, drug discovery, and proteomics. This section focuses on microcomponentsfor fluid handling, such as microchannels, microchambers, microvalves and micro-pumps that could be applicable to the development of integrated e-noses and micro-systems for chemical rather than biological analysis.Advancements in photolithography turned the possibility ofminiaturizing analytical

systems into reality. Initially, only simple channels and reservoirs could be made byphotolithography on glass or silicon wafers, and electro-osmosis was the only way tomove liquids. Over the last 10 years, the fabrication of new microfluidic components,such as valves, pressure systems, metering systems, reaction chambers, and detectionsystems, has led towards the development of more complex manufacturing technol-ogies, e.g. deep reactive ion etching (DRIE), andmultiplayer processes such as the five-layer polysilicon Sandia process) and hence the possibility for lab-on-a-chip prototypes[16]. Apart from their use in research, microfluidic devices also have significant com-mercial potential. In 1999, the Systems Planning Corporation, Arlington, VA, releaseda market study on microelectromechanical systems (MEMS) that projected a micro-fluidics market of 3 to 4.5 billion euros by 2003. 30% of this total is split equally be-tween sensors and lab-on-a-chip applications. Another microsystems market studycompleted in 1996 by a task force of the European Commission’s Network of Excel-lence in Multi-functional Microsystems (NEXUS) forecasted a market of Q 2.8 billionfor microstructure-based disposable assay devices alone by 2002.

10.2.1

Microchannels and Mixing Chambers

Microchannels are essential components of microfluidic systems. They provide theconnections between pumps, valves and sensors [17], and they are used as separationcolumns for different types of gas or liquid chromatographs [18, 19]. They also act as

10.2 Microcomponents for Fluid Handling 233233

Fig. 10.2 Scanning electron microscope (SEM)

images of several types of microchannels, fabricated

with: (a) bulk micromachining and wafer bonding,

(b) surface micromachining, and (c) buried channel

technology. From Boer et al. [21]


heat exchangers in the cooling of electronic chips [20]. Commonmethods used for thefabrication of microchannels include bulk etching with wafer-to-wafer bonding, bulketching with sealing using low-pressure chemical vapour deposition (LPCVD) materi-als, conventional surface micromachining of channels, imprinting plastic substrates,X-ray LIGA (German acronym: Lithographie, Galvanoformung, Abformung) technol-ogy and DRIE, and channel forming from UV photodefinable SU-8 photoresist usingLIGA (sometimes called UV LIGA). Figure 10.2 shows scanning electron micrograph(SEM) images of several types of microchannels fabricated with bulk micromachiningand wafer bonding (Fig. 10.2a), surface micromachining (Fig. 10.2b), and buried chan-nel technology (BCT) (Fig. 10.2c). One problem with wafer-to-wafer techniques, suchas anodic bonding or direct fusion bonding, is the possible creation of wafer-to-wafermisalignments and the formation of microvoids at the bonding interface, which mayaffect the functional performance of the device. Another difficulty is that electroniccircuitry (e.g. CMOS) cannot be incorporated on the same substrate because of thehigh process temperatures and voltages needed to perform anodic bonds. The useof surface micromachining obviates the need for accurate wafer alignment. In thisapproach, structural parts are embedded in layers of a suitable sacrificial materialon the surface of a substrate. Dissolving the sacrificial material forms a completemicrochannel. By this method microchannels can be fabricated in various differentpassive materials, e.g. silicon nitride, polysilicon, metal, polymer, and silicon diox-ide. One major disadvantage of this technique has been that the vertical dimensionof such channels is restricted by the maximum sacrificial layer thickness that can bedeposited and etched within an acceptable time period. Researchers at the Universityof Twente have proposed BCT as an alternative to conventional bulk and surface mi-cromachining [21]. BCT allows the fabrication of complete microchannels in a singlewafer with only one lithographic mask, and processing on one side of the wafer. Themicrostructures are constructed by trench etching, coating of the sidewalls of thetrench, removal of the coating at the bottom of the trench, and finally etching intothe bulk of the silicon substrate. This method for the fabrication of these deviceswas derived from the SCREAM (single-crystal reactive etching and metallization) pro-cess [22]. The structure can be sealed by the deposition of a suitable layer that closes thetrench. Using the above procedure it is possible to construct cavities, reaction cham-bers or crossing channels. A spiral-shaped channel with a length of 10 m and a dia-meter of 30 lmwas also developed by the same research group for possible applicationas a separation element in gas chromatography.The method of imprinting plastic substrates involves the low-temperature pattern-

ing of plastic substrates using either small diameter wire or a micromachined silicontemplate. Silicon templates have also been used as a negative master tool for fabrica-tion of polymer microchannels and mixers by hot embossing and microinjectionmolding [23]. Templates have also been made in metals and, more recently, dia-mond. Figure 10.3 shows a micrograph of a mixer, which has been replicated on ahot embossed polymethylmethacrylate wafer. The advantage of this method is thatthe resultant channels are robust, easy to fabricate at low cost, and compatiblewith biological fluids (unlike silicon); it also allows the integration of other microflui-dic elements and the sensors but not electronic circuitry. Another technology that


Fig. 10.3 Micrograph of

an embossed mixer (smallest

channel dimensions:

50 lm � 50 lm). From

Greschke et al. [22]

Fig. 10.4 Process steps for fabricating the MWlCs(a) etch channel, (b) form porous silicon (PS), (c)

under-etch PS. From Tjerkstra et al. [24]


could be used is LIGA technology. The principal advantage of the LIGA process is thatmicrodevices can be fabricated with a height-to-width aspect ratio of up to 200, typicallyseveral millimeters in height and 10 lm in width, but the microchannels fabricatedusing this technology are usually made from plastics, metals, and ceramics rather thansilicon. The process is also very expensive because it requires a synchrotron source.Finally, channels can be made using thick UV LIGA. Using this method only thesidewalls of the channel can be formed, so to seal them some method of bondingis required. The advantage is that the thickness of the wall can be easily controlledand a high aspect ratio achieved.Releasing electrochemically formed porous silicon from the bulk silicon substrate

by under-etching at increased current density is another technique that can be used forthe microfabrication of microchannels, in particular multi-walled microchannels(MWlCs) [24]. Figure 10.4 shows the main processing steps for the formation of aMWlC by etching channels in a p-type silicon wafer using LPCVD silicon-rich siliconnitride as the mask material. Figure 10.5 shows an SEM image of a MWlC containingtwo porous layers. To create more robust devices, i.e. to increase the strength of thestructure, microchannels can be fabricated with a porous silicon membrane sus-pended halfway across an etched cavity surrounded by silicon nitride walls.Most of the above methods allow for some integration with sensors, but external

integration with the electronic circuitry is typically used, e.g. hybrid packaging ormulti-chip modules. In order to allow direct integration of sensors, actuators, andother electronics with the microchannels, Rasmussen et al. have proposed two meth-ods for the fabrication of microchannels using the standard CMOS process and simpleand inexpensive post-processing steps [25]. In the first method, shallowmicrochannelsof the order of 0.4 lm are realized by removing surface layers incorporated in a stan-dard CMOS integrated circuit process. Larger channels with depth of 30 to 300 lm canbe fabricated through the second method that employs bulk micromachining techni-ques. Both methods offer the possibility to create a complete smart microfluidic sys-

Fig. 10.5 SEM photograph of a

MWlC containing two porous

layers. From Tjerkstra et al. [24]


tem that possesses integratedmicrofluidic elements, sensors and electronics as shownin Fig. 10.6.

10.2.2

Microvalves

Microvalves are one of the most important building blocks of microfluidic systemsused for fluid flow control. They can be classified in two categories: active valves(with an actuator) and passive check valves (without an actuator).

10.2.2.1 Active Microvalves

An active microvalve consists of a device body that contains the fluid under pressure, avalve seat to modify the fluid flow, and an actuator to control the position of the valveseat. The first reported microvalve was designed as an injection valve for use in inte-grated gas chromatography [26]. It had a silicon valve seat and a nickel diaphragmactuated by an external solenoid. Following this first design, a large number of micro-valves have been designed and reported, and they can be classified on the basis of theactuation method employed. These methods include pneumatic, thermopneumatic,thermomechanic, piezoelectric, electrostatic, electromagnetic, electrochemical, andcapillary force microvalves. Figure 10.7 shows some of these actuating devices.Pneumatic valves have a membrane structure as the valve seat. Although pneumatic

actuation is based upon a very simple principle it requires an external pressure source,which makes the pneumatic valves unsuitable for most compact applications. A lowspring constant is also an important parameter and in order to achieve it thin mem-branes or corrugated membranes have to be designed. Soft elastic materials, such as

Fig. 10.6 Microscope photo-

graphs of a fabricated chip for

surface-micromachining shallow

channels before etch. From

Rasmussen et al. [25]


silicon rubber or Parylene, can be used to realize the low spring constant [27],whereashard materials such as silicon and glass are problematic.Thermopneumatic valves utilize a sealed pressure cavity filled with a liquid. Actuation

is based on the change in volume of the sealed liquid. The phase change from liquid togas or from solid to liquid can also be used if a larger volume expansion is required.The disadvantage of these types of valves is the incompatibility of the technology, sincethe liquid has to be primed, filled, and sealed individually. Thin films of solid paraffinmaterial could be used as an alternative [28] since they could be integrated in the batchfabrication.Thermomechanic microvalves utilize the principle of the conversion of thermal energy

directly into mechanical stress. There are three types of thermomechanic actuators:solid-expansion [29], shape-memory alloy [30], and bimetallic actuators. A bimetallicvalve as shown in Fig. 10.7a, was introduced by Jerman [31] and was one of the firstcommercialized microfluidic components. The valve was designed for a gas flow con-troller, and its actuator consists of a central boss, a circular diaphragm made of bime-tallic materials, and a circular heater. The temperature, controlled by the heater, of thebimetallic structure on the diaphragm was used to vary the force applied to the boss bythe diaphragm so that the gas flow can be adjusted. The valve was designed to controlthe gas volumetric flow-rate from 0 to 90 mL min�1.Piezoelectric microvalves utilize the piezoelectric effect as an actuation principle. The

thin-film piezoelectric actuators do not deliver sufficient force/displacement for valve

Fig. 10.7 Schematics and principles of operation of (a) bimetallic

(from Jerman [31]), (b) electrostatically actuated (from Shoji and Esashi

[41]), and (c) electromagnetic active microvalves (from Yanagisawa et

al. [37])


application, so all reported piezoelectric microvalves use external stack-type piezoelec-tric actuators, bimorph piezocantilevers, or bimorph piezodisks. Bimorph actuatorsare used if larger deflections are required. They either consist of two bonded piezo-electric layers, with anti-parallel polarization [32], or of a piezoelectric layer bonded to anon-piezoelectric layer [33].Electrostatically actuated microvalves have typically been designed with a valve seat in

the form of a cantilever. Figure 10.7b shows a cantilever structure fabricated by surfacemicromachining. The valve seat could also have a form of flexible metal bridge. Such adesign of microvalve was fabricated by Shikida and Sato [34] in order to allow an ade-quate gas flow-rate under low pressure. The valve is suitable for rarefied gas controlsystems but it requires a large actuating voltage of 100 V. In order for a large deflectionto be achieved with a relatively small drive voltage, a combination of the electrostaticforce, buckling effect, and pneumatic force were used. This arrangement was utilizedfor a bistable valve in an implantable drug delivery system [35]. Vandelli et al. havedesigned a microvalve array for fluid flow control with the valve membrane andone electrode made out of a polysilicon layer, whereas the other electrode was bulksilicon fabricated using surface micromachining. The valves were arranged in anarray so that the flow rate could be controlled digitally [36].Electromagnetic microvalve with a valve cap made out of a soft magnetic Ni-Fe alloy

supported by a spring is shown in Fig. 10.7c [37]. It moves vertically in the magneticfield gradient applied by the external electromagnet. This valve was designed as a flowregulator for a high vacuum application. A microvalve activated by a combination ofelectromagnetic and electrostatic forces has also been fabricated [38]. The structureconsists of a gas flow inlet having a counter electrode, a deflectable membrane coatedwith a metal conductor and two permanent magnets. Current pulses of 200 mA and avoltage of 30 V were typically applied for electromagnetic and electrostatic actuation.The response time was below 0.4 ms.There are also some reported examples of so-called electrochemical and capillary-

force valves. Electrochemical valves are actuated gas bubbles generated by the electro-lysis reaction, where the pressure inside the bubble is proportional to the surface ten-sion and the radius of curvature. Capillary-force valves use capillary-force actuationwhere the surface tension and the capillary force can be controlled actively or passivelyby different means: electrocapillary, thermocapillary, and passive capillary. The use ofsome of these effects are described in two published papers [39, 40].

10.2.2.2 Passive Microvalves (Check Valves)

This type of microvalve is typically designed for use in micropumps where a very smallleakage under reverse applied pressure and a large reverse-to-forward flow resistanceratio is required. The dimensions of check valves are small in comparison with thevalves with integrated or external actuators. A review paper by Shoji and Esashi de-scribes a wide variety of check valve structures [41]. A typical cantilever-type structureis shown in Fig. 10.8. Oosterbroek et al. have reported on the design, simulation, andrealization of in-plane operating passive microvalves [42]. Figure 10.9 shows func-tional, art, and SEM impressions of a 2 � 5 high-density in-plane check valve array.


10.2.3

Micropumps

Micropumps can be classified into two categories: mechanical pumps with movingparts and non-mechanical without moving parts. Mechanical pumps can be furtherdivided into three major groups: reciprocating, peristaltic, and valve-less rectificationpumps. Non-mechanical pumps are mainly used to move liquids and constituents inliquids, which need to avoid any moving mechanical parts. Electrohydrodynamic,electro-osmotic, or ultrasonic effects are normally employed in the operation ofnon-mechanical micropumps.

10.2.3.1 Mechanical Micropumps

A reciprocating-type micropump consists of a pump pressure chamber with a flexiblemembrane driven by an actuator unit and passive microvalves (check valves). Twomain conditions have to be satisfied in order for reciprocating micropumps to func-tion correctly. First, the minimum compression ratio of micropumps (the ratio be-tween the stroke volume and the dead volume that causes serious constraints inmost micropump designs) has to be determined and secondly the pump pressurehas to be high. For the gas micropumps that are of interest in integrated e-nose ap-

Fig. 10.8 Schematic of cantilever-type passive (check-valve)

microvalve. From Shoji and Esashi [41]

Fig. 10.9 (a) Functional (b) art, and (c) SEM impressions of a

2 � 5 high-density in-plane check valve array. From Osterbroek

et al. [42]


plications, the criterion for the minimum compression ratio e is given by Richter et al.[43]:

e >p0

p0 � jDpcritj

� �1=c

�1

where p0 is atmospheric pressure, Dpcrit is the critical pressure required for the ope-ning of the check valve, and c is the adiabatic coefficient (for air c takes a value of 1.4).(The above equation could be simplified at small critical pressures and low pumpfrequencies where there is isothermal behaviour.)The maximum output pressure of the micropump depends directly on the available

force of the actuator used. The different types of actuators that have been employed todate are piezoelectric, pneumatic, electrostatic, and thermopneumatic. The first piezo-electric micropump (Fig. 10.10), developed and reported in 1988 by van Lintel et al.[44], was made in silicon and used a piezoelectric disk. Since then, various differenttypes of micropumps have been designed in order to satisfy the two requirementsmentioned above. Over the years the dead volume of the pump chamber has becamesmaller, and the check valves and the pump membranes have been made out of softermaterials with low spring constants. An example of a self-priming micropump [45]with a small dead volume able to pump gas is shown in Fig. 10.11. Here the deadvolume was minimised by minimising the dead volume of the valve unit down to500 nL, hence increasing the compression ratio to 0.111. In another development[46] van Lintel’s original design was improved to achieve a compression ratio of1.16. Another way to achieve improvements in the micropump design is throughthe design of flexible check-valves. The valves, in the forms of cantilever, havebeen used with integrated electrostatic [47] or bimorph piezoelectric disks [48] as ac-tuators. Materials that are more flexible than silicon, such as polyimide, polyester, andparylene, have also been used in the design of flexible check-valves. A thermo-pneu-matically driven micropump was fabricated using the LIGA process [49]. In this thepump case is made by injection moulding of polysulfone (PSU) and the pump cham-ber is covered by a polyimide membrane. A similar design was fabricated by plasticinjection moulding and uses a polyester valve [50] whereas the design reported in [51]has a pump membrane made out of silicone rubber, and the disk valve from parylene

Fig. 10.10 Piezoelectric disk reciprocating micropump. From van Lintel et al. [44]


deposited through CVD process. Maximum flow rates reported for reciprocating typemicropumps range from 4 to 13 000 lL min�1.Functionality of peristaltic micropumps is based on the peristaltic motion of the pump

chambers, and theoretically, peristaltic pumps should have three or more pump cham-bers with reciprocating membranes. These types of pumps do not require passivevalves for the flow rectification, nor do they require a high chamber pressure, sothe two main conditions that have to be satisfied are a large stroke volume and a largecompression ratio. Three types of actuation principles have been employed in reportedperistaltic micropumps: piezoelectric, electrostatic and thermopneumatic along withseveral types of fabrication processes, such as bulk micromachining, surface micro-machining, plastic moulding, and a combination of bulk micromachining and anodicbonding. Maximum reported flow rates range from 3 to 30 000 lL min�1 (for air).Examples include a surface micromachined pump with electrostatic actuators [52],a thermopneumatically driven micropump having three active pressure chamberswith flexible membranes [53], a micropump with curved pump chambers and a flex-ible plastic membrane with electrostatic actuation [54]. A new pumping principlecalled dual-diaphragm pump, which consists of two actuating membranes in thepump chamber, is reported by Cabuz et al. [55]. This type of pump is able topump up to the maximum reported flow-rate 30 mL min�1 of air.Valveless rectification micropumps are similar to check-valve pumps except that, in-

stead of check-valves, diffusers/nozzles are used for the flow rectification. In orderto optimize the valveless pump designs, the stroke volume has to be maximized whilethe dead volume has to be minimized. The first piezoelectric micropump using noz-zle/diffuser elements instead of check-valves [56] was presented in 1993. The originalvalve was fabricated in brass using precision machining. The same research group in1997 presented the first valveless diffuser pump [57] fabricated using DRIE fabricationprocess shown in Fig. 10.12. The maximum pump pressure was 74 kPa and the max-imum pump volumetric flow-rate was 2.3 mL min�1 for water. Two different thermo-plastic replication methods for the fabrication of valveless pumps: hot embossing andinjection moulding have also been tested [58]. Deep precision-milled brass mouldinserts and deep microelectroformed nickel mould inserts defined from DRIE siliconwafers were used for these designs. Figure 10.13 shows the design of the precision-

Fig. 10.11 Check-valve self-priming micropump. From Linneman et al. [45]


milled brass mould insert pump. Tested pumps had a maximum volumetric flow-rateof 1.2 mL min�1. Nguyen and Huang [59] have demonstrated the design of miniaturevalveless pumps based on a printed circuit board technique. The pump could be op-erated as a single diffuser/nozzle pump (or a peristaltic pump) and has a maximumflow-rate of 3 mL min�1.

Fig. 10.12 Fabrication process of the DRIE diffuser

pump. From Olsson et al. [57]

Fig. 10.13 (a) Top layout view of single-chamber

diffuser pump, and cross-sectional views of (b) a

single-chamber pump unit, and (c) two pump units

stacked in parallel arrangement for high pump

flow. From Olsson et al. [58]


10.2.3.2 Nonmechanical Micropumps

Nonmechanical pumps are based on non-mechanical pumping principles and candriven by capillary-force, thermal, chemical, electrical or magnetic means. Micro-pumps using electrohydrodynamic and electrokinetic effects have been reported.Electrohydrodynamic (EHD) actuation is based on electrostatic forces acting on di-

electric fluids, such as organic solutions. Two main types of electrohydrodynamicmicropumps have been reported: the EHD induction [60] and the EHD injectionpumps [61].Electrokinetic micropumps use the electrical field for pumping conductive fluid. They

can be divided into two categories: pumps based on the electrophoresis effect andpumps based on the electro-osmosis effect. Electrophoresis can be described as themotion of charged particles under an electric field in a fluid relative to the unchargedfluid molecules. The velocity of the charged species is proportional to the applied elec-trical field. Electrophoresis pumps have their application in processes such as theseparation of large molecules such as DNA or proteins [62]. The separation, per-formed in microchannels, is called capillary electrophoresis. Electro-osmosis is theeffect of pumping fluid in a channel under an applied electric field. Changing theapplied electric field or the pH of the solution that affects the potential arisingfrom the charge on the channel wall can control the electro-osmotic flow velocity.In microanalysis systems, the electro-osmosis effect is used for delivering buffer solu-tions [63] and, when combined with the electrophoretic effect, for separating out dif-ferent molecules. The drawbacks of electro-osmosis are that it cannot be used whenseveral interconnected channels are required for sample processing, and it is not com-patible with high-ionic-strength buffers.Information on various other types of micropumps, such as surface-tension driven

pumps, magnetohydrodynamic pumps, and ferrofluidic magnetic pumps can also befound in the literature.

10.3

Integrated E-Nose Systems

10.3.1

Monotype Sensor Arrays

The performance of integrated e-nose systems largely depends upon the performanceof the sensor array used. The integration of the sensor array on to the same substrateoffers a reduction in sensor variation and also improves device reliability. Other ad-vantages include reduced fabrication cost, smaller dimensions and lower power con-sumption. The majority of sensor arrays reported to date are monotype and can bedivided into several categories based upon either the sensor material or type em-ployed, such as conducting polymer, tin oxide, quartz resonator, surface acousticwave (SAW) and FET sensor arrays. Readers are referred to Chapter 4 for a full de-scription of the different types of chemical sensors.

10.3 Integrated E-Nose Systems 245245

Conducting polymers are very attractive for gas-sensing applications because of theirease of deposition, the large variety of available polymer combinations, and their abilityto operate at room temperature [64, 65]. Neaves andHatfield [66, 67] reported on one ofthe first ASIC chips for conducting polymer sensor arrays. In this, the integrated sen-sor array consists of 64 parallel gold electrodes (forming 32 resistive sensors) and aground plane fabricated on a ceramic substrate. Each conducting polymer sensor con-sists of a two-layer composite: the poly(pyrrole) as the base polymer and a secondpolymer grown electrochemically onto this base. Figure 10.14 shows a micrographof a resistance-measuring ASIC used in the final design of the integrated nose systemfor interrogation of 32 conducting polymer resistors. Another sensor array consistingof five conducting polymers in a microbridge configuration has been developed atWarwick University [68]. The microbridge arrangement was used in order to reducethe temperature dependence of the discrete polymer elements. The array, which hasbeen fabricated in a standard 0.8 lmCMOS technology, is shown in Fig. 10.15. P-typesilicon (or metal electrodes) is used to form resistors for either electrochemical deposi-tion of polymers or spray coating of carbon-black polymer compositematerials, such asthose reported by Lewis [69] (Caltech). These polymer materials can also be depositedon co-fired ceramic substrates, glass slides, and silicon. Thesematerials have also beenused for other micromachined gas sensor arrays. Zee and Judy [70] reported two typesof devices, bulk micromachined and patterned thick-film sensors. The microma-chined, so-called ‘wells’, have been designed to contain the liquid volume during de-position. This type of design permits good reproducibility in the deposition and createslarger exposure areas for sensing while minimizing the chip area. It also allows for theintegration of on-chip electronics for signal conditioning and processing. Figure 10.16shows photographs of micromachined gas sensor arrays with polymer carbon blackcomposite materials.

Fig. 10.14 Photomicrograph

of a resistance-measuring ASIC

used in the design of the inte-

grated nose system for interro-

gation of 32 conducting polymer

resistors. From Neaves and

Hatfield [67]


A novel ChemFET sensor array reported by Covington et al. [71] also uses carbon-black composite polymer materials. A linear dependence to toluene concentration andsensitivity of up to 2.8 lVppm�1 was observed. The device comprises an array of fourn-type enhanced MOSFET sensors, fabricated by the Institute of Microtechnology(IMT, University of Neuchatel, Switzerland). Briand et al. [72] first reported on thesedevices as catalytic field-effect gas sensors in 2000. In this case, three of the MOSFETshad their gate covered with thin catalytic metals and were used as gas sensors, whilethe fourth one had a standard gate covered with nitride and acted as a reference. Sen-sitivities to the gases hydrogen and ammonia were tested.Most of the reported integrated gas array sensors are based on tin oxide technology.

Gardner et al. [73] reported on an array of six MOS odour sensors on single siliconsubstrate with six separate integrated heaters in 1995. A sensor array reported byDas et al. [74] consists of four integrated thick-film tin-oxide gas sensors. The arraywas fabricated on a single substrate and the sensor responses to different concentra-tions of various alcohols and alcoholic beverages were reported. Another micro-machined tin oxide gas sensor array composed of three different devices on thesame rectangular membrane and working at different temperatures was used forthe detection of NO2, CO, and toluene [75]. A sensor array consisting of 40 monolithicsensor elements with different sensitivities achieved by gradient techniques was usedfor halitosis analysis [76]. Forty-one parallel platinum strips partitioned the surface ofthe device into 40 gas-sensitive segments (SnO2 and WO3 were used). Four heatingelements were based on the reverse side and the sensitivity of the array to malodourcomponents was tested.Flexural plate wave (FPW) sensors and SAW devices have both been used as the

elements for analytical sensor systems. In both sensors acoustic waves are generatedwithin a piezoelectric substrate that has usually been coated with a chemically sensitivefilm. A pair of interdigital transducers is normally used to generate and receive acous-tic waves. The difference between these two types of devices is that the active region ofthe FPW sensor is the membrane of thickness much smaller than the acoustic wave-

Fig. 10.15 Photograph of a

five-element CMOS gas sensor

[68]


length. SAW sensor arrays have often been used and reported in gas analysis instru-ments [77, 78] but rarely as integrated arrays. Baca et al. [79], from the Sandia NationalLaboratories (USA), have reported on the development of a GaAs monolithic surfaceacoustic wave integrated circuit (Fig. 10.17) aimed at chemical sensing applications. Aprototype instrument describing an integrated array of six polymer-coated FPWs usedtogether with an absorbent pre-concentrator is reported by Cai et al. [80]. Each FPWmembrane is a layered composite, 5-lm thick, consisting of a silicon nitride layer, apolished layer of p-doped polysilicon, and a ZnO piezoelectric layer attached periph-erally to the silicon substrate. The whole system is shown in Fig. 10.18. Responses tothermally desorbed samples of individual organic solvent vapours and binary and tern-ary vapour mixtures are reported. Another example of an integrated FPW sensor arrayhas been recently reported by Cunningham et al. [81] of the Draper Laboratory. Theyhave designed a chemical-vapour detection and biosensor array based on microfabri-cated silicon resonators (FPW sensors) coated with thin-film polymer sorption layers.The devices were fabricated on silicon-on-insulator (SOI) wafers and the work was aninitial step towards the development of a large multi-element FPW array with severalhundred devices operating within a single silicon chip.Bulk acoustic wave sensors, in particular the thickness mode quartz-crystal micro-

balances (QCM), have also been used in e-nose applications but not as integratedmicrosensor arrays. A monolithic sensor array based on six elements integrated onthe same quartz crystal designed for monitoring agricultural emissions was reported

Fig. 10.16 Photograph of gas

sensors with polymer-carbon

black films deposited: (a) on a

custom built low-temperature

co-fired ceramic substrate and

(b) on a micromachined chip

attached to the ceramic sub-

strate. From Zee and Judy [70]


by Boeker et al. [82]. The dimensions of the quartz substrate are 12 mm � 20 mm edgelength and 168 lm thickness with resonant frequency of 10 MHz.Optical sensor arrays using image processing are another attractive technique for

application in e-nose systems (see Chapter 8). A fibre-optic bead-based sensor arrayhas been designed at Tufts University and employed to discriminate between differ-ent odours [83]. The system incorporates high-density arrays of micrometer-scale op-tical fibres, with polymer beads doped with fluorescent dyes placed at the end of eachfibre. The binding of vapor molecules to the polymers changes the light emitted fromthe dyes, forming a colour signature. A similar technique has been used for the char-acterization of multicomponent monosaccharide solutions. In this, a chip-based sen-sor array composed of individually addressable polystyrene-poly(ethylene glycol) andagarose microspheres has been used. The microspheres are arranged in anisotropi-cally etched cavities that are designed to serve as miniaturized reaction vessels andanalysis chambers (Fig. 10.19). Identification of analytes takes place through colori-metric and fluorescence changes to receptor and indicator molecules, which are cova-lently attached to termination sites on the polymeric microspheres [84]. Photomechan-

Fig. 10.17 Micrograph of a

monolithic GaAs SAW integrated

circuit. From Baca et al. [79]

Fig. 10.18 Photograph showing an integrated

array of six polymers coated flexural plate wave

sensors (FPWs) with an absorbent pre-concen-

trator (PCT). From Cai et al. [80]


ical chemical microsensors reported by Datskos et al. [85] should also bementioned. Inthis work it was demonstrated that photo-induced bending of microcantilevers de-pends on the number of absorbed molecules on their surface. The authors claimthat by choosing different wavelengths tuneable chemical selectivity could beachieved. Apart from identification, a real time visualisation of gas/odour flow hasalso been studied (see Chapter 16 on odour tracing). A portable homogeneous gassensor array was used to visualize the flow of a target gas, and the direction of thegas source was estimated using a real-time image-processing algorithm [86].Finally, silicon-based microelectrode arrays for chemical analysis have been re-

ported. An array consisting of various electrode shapes and sizes designed andused for a systematic study on some aspects of electrochemical sensing (i.e. influenceof electrode geometry) was reported by Schoning et al. [87]. Sensor arrays with differ-ent electrode geometries have been studied at Warwick University for organic crystals,metal oxide, and polymer resistive devices [88–90] and offer certain functional im-provements, such as faster responses or higher common-mode rejection ratios.

10.3.2

Multi-type Sensor Arrays

A study on the advantage of hybrid modular systems over monosystems, aimed at thepossibility of achieving optimum discrimination power of an e-nose system, has beenconducted by Ulmer et al. [91] at Tuebingen University. The system used for thiscomparative study consisted of 16QCMs and MOSs. The results suggest that when-ever high reliability and a high degree of reproducibility and separation power arerequired in the analysis of a complex gas matrix hybrid modular systems shouldbe used. Another hybrid instrument, designed by Dyer and Gardner [92] at WarwickUniversity, employs both resistive and piezoelectric sensors in arrays with improved

Fig. 10.19 Microspheres ar-

ranged in anisotropically etched

cavities designed as miniaturi-

zed reaction vessels and analysis

chambers. From Goodey et al.

[84]


dynamic characteristics, and agrees with the above findings. High-precision program-mable interface circuitry was developed for this system and a resolution of 0.05% wasachieved. Recently, two examples of multi-type sensor arrays have been reported. Thefirst one consisting of four different sensors designed at ETH Zurich has been de-scribed in Chapter 9. The second system is a result of collaboration between Cam-bridge and Warwick Universities. In this, an integrated smart sensor has been devel-oped consisting of two types of devices, chemo-resistive gas sensors and microcalor-imeteric devices with active microFET heaters and temperature sensors on an SOImembrane [93]. The smart SOI sensors can operate at temperatures up to 350 8Cand offer excellent, uniform thermal distribution over the sensing area.A method for selecting an optimum sensor array has been suggested by Chaudry et

al. [94]. A step-wise elimination procedure, which ranks the inclusion of sensors in anarray according to their contribution to the overall sensitivity and selectivity values, wasadopted in this study. Various other techniques could be used to optimize sensor arrayresponse through either smart sensor interfacing [95] or smart signal processing (i.e.adaptive thresholding for improving selectivity or signal processing for improving gassensor response time using analogue VLSI) [96, 97]. A combination of microfluidictechnology, sensor arrays, smart sensor interfacing and signals processing shouldresult in the development of superior e-nose systems and they may, perhaps, be com-parable to the conventional chemical analysis microsystems currently being devel-oped. These micro, total analysis systems (lTAS) are described in the next section.

10.4

Microsystems for Chemical Analysis

10.4.1

Gas Chromatographs

Chromatography is a popular analytical tool commonly employed by chemists to ana-lyze liquid and gas mixtures. Figure 10.20 illustrates the basic components of a typicalgas chromatograph (GC) [98], namely, a carrier gas bottle, an injection port, a longseparation column through which the gas components pass down, a detector, anda data processing system. The components in the gas mixture are separated out be-cause the column is either coated or packed with a stationary-phase film that absorbsthe different components to differing degrees. Consequently, the components traveldown the tube at different rates depending on their specific sorptive properties, and

Fig. 10.20 Basic set-up of a gas chromatography system used to analyze gas mixtures

10.4 Microsystems for Chemical Analysis 251251

hence are partitioned out. Ideally, the components will be totally separated out in timewhen they emerge from the column and hence can be measured by a single detector.The ideal graph is illustrated in Fig. 10.21 with five major components clearly visible.This technique is widely used and a description can be found of it in most analyticalchemistry books. However, GC systems tend to be bulky, fragile and expensive itemsof equipment with limited sensitivity.Gas chromatography has been used in olfaction to help analyze complex odours with

only limited success. They can be used to separate out fairly large concentrations ofcertain organic components for which specific coatings (stationary phases) exist. GCsare also used as the front-end of an olfactometer with a person sniffing the output,instead of the sensor, and recording the specific notes as they emerge. This so-calledGC olfactometer can help organoleptic panels identify the presence of certain notes incomplex odors. In fact, there is some evidence that the human olfactory system gen-erates its own spatio-temporal sorption patterns in the olfactory mucosa and so is itselfa type of GC [99].The first attempts to make a micromachined version of a GC were initially reported

as long ago as 1975 by Terry at Stanford University (USA). The separation column wasmade from the isotropic wet etching of a silicon wafer. Figure 10.22 shows a cross-section of the device reported later in 1979 [100] with a pyrex glass lid. The systemincluded a sample injector (silicon valve) and integrated thermal conductivity sensorbut not the air supply. From 1975 to 1998 this research group further developed themicro GC and a recent review of the field has been published by Kolesar et al. [101].Figure 10.23 shows a photograph of a micromachined GC column that is 10 lm deep,300 lm wide, and 0.9 m to 1.5 m in length. In this case copper phthalocyanine has

Fig. 10.21 Ideal gas chromatograph in which all the com-

ponents of a chemical mixture are separated out and appear

as distinct peaks in the time trace

Fig. 10.22 Cross-section of a

micromachined GC unit sho-

wing an integrated thermal

conductivity sensor at the end of

the silicon column. From Terry et

al. [100]


been sputtered down to act as the stationary phase sensitive to reactive gases. Themicro column was shown to separate mixtures of ammonia and nitrogen dioxidein air.In 1997 a Japanese group led by Hannoe et al. [102] (Japanese Integrated Informa-

tion and Energy Systems Laboratories) reported on the use of an ultrasonic etchingtechnique to produce the micro channel, again with a pyrex lid, but this time a PCTFE-sputtered thin-film coating. The GCmicro column has the dimensions of 10 lmdeep,100 lm across and 2 m long. Then Wiranto et al. [103], an Australian group, isotro-pically etched a GC column again with a pyrex lid; this time the column was 20 lmdeep, 200 lm wide and only 125 cm long.The problem associated with the etching of deep channels was solved in the late

1990s with the advent of the DRIE process, and so it is now possible to make microGC columns more accurately and with superior properties. Perhaps the most sophis-ticated system is that being developed by Matzke et al. [104] of the Sandia Laboratoriesusing a plasma-etched (DRIE Bosch process) pyrex lid. The GC column is part of whatis referred to as the ChemLab and Fig. 10.24a shows the schematic arrangement of thischip. The columns are now 200 to 400 lm deep with width of 10, 40 and 80 lm andlengths of only 10, 30 and 100 cm. The group plans to microfabricate a pre-concen-trator and pump thus making the entire instrument on a chip as shown in Fig. 10.24a.It is also possible to try and simplify the integration process through the combinationof electrophoresis to pump the mobile phase, and chromatography to separate withstationary phases, a method called micro capillary electrochromatography [105]. How-ever this technique is mainly suitable for a liquid mobile phase, and requires a highvoltage supply which are incompatible with standard integrated circuit processes.

Fig. 10.23 SEM of the cross-section of a silicon micromachined GC column [101]


Fig. 10.24 (a) Schematic layout of the ChemLab (Sandia Laborato-

ries, USA). The chip is envisaged to be the size of a dime coin,

(b) commercial portable ‘micro’ GC called the Chrompack and used

widely for environmental gas analysis


GC is a useful analytical tool for chemists and there are a number of companies thatmake portable micro GC with somemicromachined parts in them – but still about thesize of a computer tower case. For example, the company ASTmake a battery-operatedunit that contains two micro GC columns with a silicon micromachined thermal con-ductivity sensor. Similar units are made by MTI (see Fig. 10.24b) and by ChrompackInternational; these so-called Chrompack units are widely used to analyze the air fororganic pollutants [106]; modifications to this basic unit by Tuan et al. [107] have alsobeen reported that seek to enhance its basic performance. However, all of these microGCs have some major drawbacks as regards analyzing complex odours. Firstly, thetime it takes for the odorant components to travel down the columns and partitioncan be tens or even hundreds of seconds and, secondly, the separation for some im-portant classes of odours is relatively poor. However, there are two other analyticaltools used by chemists alongside the GC, namely, the mass spectrometer and theoptical spectrometer that may be regarded as complementary techniques. We shallnow discuss them in turn.

10.4.2

Mass Spectrometers

The composition of a liquid (or vapour) can be analyzed using a mass spectrometer(MS). Figure 10.25 shows the general layout of an MS in which the sample is injectedin to themobile phase (normally helium gas) and themolecules ionized [108]. The ionsare first accelerated in a vacuum by applying a voltage and finally separated by a mag-netic field according to the ratio of their mass to charge. The number of ions is countedfor each particular mass (the ions are usually singly charged species) using an iongauge and this is commonly referred to as the abundance. The magnetic sectorcan be replaced by either a quadrupole electrostatic lens or a time-of-flight elementto produce a more compact unit. Indeed a quadrupole mass spectrometer is now mar-keted by Agilent Technologies Inc. (USA) as the Chemical Sensor (Agilent 4440) and

Fig. 10.25 Layout of a magnetic sector mass spectrometer.

From Gardner and Bartlett [108]


comprises a headspace autosampler connected up to a quadrupole mass spectrometerunit and a PC for data analysis (see Fig. 10.26). This unit has been used to analyzevarious odorant problems and Fig. 10.27 shows the mass spectra for a complex odourgenerated from the headspace of a bacterial sample and covers amass range from 45 to550 Daltons [109]. As can be seen, themass spectra for natural odours is complex and apattern recognition system is needed to analyze the differences. In this example, alinear technique such as discriminant function analysis was able to resolve the differ-ences between the growth phases.The MS, like the GC, is a fairly large, heavy, and expensive instrument. Recent

efforts have been made to miniaturize parts of the MS, such as the quadrupolelens and the sampling orifice, using various micromachining techniques. For exam-ple, Fig. 10.28a shows a miniature quadrupole lens system produced by Syms et al.[110] in 1996 together with a more recent version reported by Friedhoff [111] in

Fig. 10.26 Photograph of the

Chemical Sensor (a quadrupole

mass spectrometer sold by Agi-

lent Technologies)

Fig. 10.27 Mass spectra for the headspace of the bacteria E. coli

when in two of its phases of growth. From de Matos et al. [109]


Fig. 10.28 Micro mass spectrometer: (a) schematic parts of a quadrupole

electrostatic lens, (b) photograph of a micro quadrupole lens (from Syms et al.

[110]), and (c) mass spectrum from a micro mass spectrometer


1999 (Fig. 10.28b). These relatively crude microsystems are capable of separating out asmall number of different light masses as shown in Fig. 10.28c.Further advances are being made in the development of micro-injection ports for

micro-MS instruments but the challenges associated with making a miniature ion-source detector and a vacuum system are significant. Nevertheless an integratedmicrofluidic-tandem MS has been reported by Figeys et al. [112] for the analysis ofprotein and peptide masses (in solution). This instrument examines the highermasses of 500 to 1,000 Daltons and is aimed at analyzing biological systems at theprotein level rather than odours – by definition, odours have lower weights otherwisethey are not volatile.Finally, it should also be noted that there are clearly many examples in which mo-

lecules of the same mass have quite different odors. For example, the position of aketone group in undecanone causes the smell to change from fruity to rue-like. Simi-lar changes occur when comparing cis and trans isomers of unsaturated compounds.The other examples of this phenomenon may be found in Chapter 1. Consequently,there is no simple mass-activity relationship for odours. The situation is further com-plicated by the fact that the ionization of a single fragile odorantmolecule can lead to itsfragmentation and so the mass spectrum is more complicated. Consequently, themass spectra should really be considered as a chemical signature rather than an ac-curate measure of themass content in the original complex odour, and of course, theremay not be a unique mapping between smell and mass signature. The combination ofa GC followed by an MS instrument is a powerful and sensitive analytical tool but isclearly an extremely large and expensive unit. Making a micro GC-MS would be theultimate challenge!

10.4.3

Optical Spectrometers

Molecules have characteristic modes of mechanical vibration and rotation, and thesecan be detected by looking at the amount of light at different frequencies that is ab-sorbed by the molecules. The technique is called optical spectroscopy and the mole-cules are usually analyzed using light in the UV to IR range. Although the technique isgenerally much less sensitive to odorant molecules than GC or MS, micro spectro-meter integrated circuits are being developed rapidly for the telecommunications in-dustry. For example, Fig. 10.29 shows the principle in which light from a fibre-optic issplit by a deflection grating in to its various frequencies, and these are detected using a256-element CCD array [113]. The combination of these technologies with a micro-fluidic system could lead to a low-cost solution for the screening of simple odours.However, that will require improvements in both the sensitivity of the optical sensorarray and the width of the frequency spectrum.


Fig. 10.29 Optical micro spectrometer integrated circuit chip:

(a) schematic and (b) actual device. From Gardner et al. [113]


10.5

Future Outlook

A nose in its totality comprises a sampling system (i.e. sniffing mechanism), and afluid flow system as well as a distributed sensor array and complex signal processingarchitecture. In this chapter we have described some of the current efforts towardsmaking a micro nose that integrates the sampling and fluidic system with the actualsensing system. It is clear that the integration of a sampling system will improve thereliability and performance of an electronic nose, as it is also evident that the creationof multi-type sensor arrays enhances its dynamic range. The latter may also be furtherextended through the use of biological materials within the sensing elements; howeverthe lack of stability of most biological materials exposed to the environment suggeststhat such bio-electronic noses could only really be used for a very short period of time.This lack of stability creates the need for a micro cassette that holds a sequence ofbiosensors that can be employed at the appropriate time, somewhat analogous to aphotographic film cassette. Based on an analogy with the human olfactory system,the cassette would need to be wound on every 20 or so days.The production of a reliable sampling microsystem will also enable the use of the

dynamical part of the sensor signal, which has been shown to be very useful [114].However, miniaturization of the system is essential so that the time-constants asso-ciated with the physical transport of the odour around the channels and chambers aremuch smaller than the response times of the sensors themselves. This permits thedifferent rate kinetics of the chemical sensors for the different analytes to be ob-served, and thus used to help the discrimination process. The micro channels, microchambers and micro pumps will permit the delivery of odours extremely quickly andreproducibly to the sensor array (or mass filter), and so this should permit the creationof a new generation of dynamical micronoses.Nevertheless, the technological advances that permit the creation of such a physical

embodiment of e-noses will not, in our opinion, be sufficient to solve the more com-plex odour problems. It is difficult to visualize a mass/optical spectrum from a mass/optical spectrometer (miniaturized or not) resolving subtle differences in the head-space of such as cheeses and beverages. Instead, the spatio-temporal information gen-erated by GC-based and/or sensor-based micronoses will require quite different typesof signal processing algorithms from the customary the principal components analy-sis, discriminant function analysis and neural networking methods described in ear-lier chapters. The types of nonlinear dynamical filters that will be required may well beneuromorphic algorithms similar to those used in our human olfactory systems. Con-sequently, the future emphasis will turn from the construction of the miniature hard-ware towards the identification of suitable dynamical models, which could either bedata-driven or parametric. Of course this generic approach is challenging and will leadto integrated noses whose cost may be unacceptable in some application fields. Forinstance, the most likely competitor to e-noses in the medical domain may be dispo-sable biochips that seek specific proteins or protein sequences. It is unlikely that ageneric micronose can compete with such a low-cost screening method. However,there are other biomedical applications in which it is possible to use an e-nose to


screen for whole viable micro-organisms (see Chapter 18) because a protein biochiplacks such a capability.

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